Femtosecond Time-Resolved Four-Wave Mixing Applied to the Investigation of Excited State Dynamics

Vinu V. Namboodiri

A thesis submitted in partial fulﬁlment of the requirements for the degree of

Doctor of Philosophy in Physics

Approved by the thesis Committee:

(Prof. Dr. Arnulf Materny)

(Prof. Dr. Ulrich Kleinekathöfer)

(PD Dr. Michael Schmitt)

Date of Defence: May 18, 2010 School of Engineering and Science

Femtosecond Time-Resolved Four-Wave Mixing Applied to the Investigation of Excited State Dynamics

Dedicated to my family, friends and Sreeappan

Abstract

Laser spectroscopy in the frequency domain has, very early, developed into a powerful tool for the analysis of structural properties of molecules. The development of ultrafast lasers added a new dimension to conventional spectroscopy by rendering time resolved measurements possible. The high time resolution offered by picosecond and femtosecond laser pulses enabled the real time observation of extremely fast processes, such as vibrations and rotations of molecules. With pulses of duration about 100 fs, it is now possible to monitor processes such as internal conversion, vibrational relaxation, and many other processes occurring in the excited electronic states which leads to reactive or energy transfer pathways. The work presented in this thesis fo- cuses on the study of molecular dynamics in these excited electronic states using time- resolved four-wave mixing (FWM) techniques. It is demonstrated that, by combining the FWM process with an excitation pulse, it is possible to study molecular dynamics in the excited states of gaseous and condensed phase samples. The advantages of the four-wave mixing technique over the commonly used time-resolved ﬂuorescence and pump-probe techniques are also discussed. The pump-FWM method is applied to simple molecules in the gas phase as well as to complex molecular systems where internal conversion processes dominate the ultrafast dynamics. The thesis also presents preliminary studies on the effect of the surface enhancement effect of the nonlinear optical process coherent anti-Stokes Raman scattering (CARS) in presence of colloidal metal particles.

vii

Contents

1 Introduction 1

2 Theory 5 2.1 Femtosecond Time Resolved Spectroscopy ...... 5 2.2 Perturbation theory for time resolved spectroscopy ...... 5 2.2.1 First order polarisation ...... 8 2.2.2 Second order polarization ...... 10 2.2.3 Third order polarization ...... 11 2.3 Pump-Probe Spectroscopy ...... 13 2.4 Coherent anti-Stokes Raman Scattering (CARS) ...... 15 2.5 Degenerate Four Wave Mixing (DFWM) ...... 18

3 Experimental Setup 21 3.1 Femtosecond Laser System ...... 21 3.2 Optical Parametric Ampliﬁers ...... 23 3.3 Pulse Characterization ...... 23 3.4 Experimental Setup ...... 24 3.5 Phase Matching in Four-Wave Mixing ...... 26

4 Excited State Dynamics in Molecular Iodine 29 4.1 Molecular states of Iodine ...... 30 4.2 Experimental ...... 32 4.3 Pump-Degenerate Four-Wave Mixing in Iodine ...... 33 4.4 Summary ...... 39

5 Excited State Dynamics in β-Carotene 41 5.1 Introduction ...... 41 5.2 β-Carotene Photophysics ...... 42 5.2.1 S2 State ...... 42 5.2.2 S1 State ...... 43

ix x

5.2.3 Vibrational Relaxation ...... 44 5.3 pump-Four Wave Mixing (pump-FWM) on β-Carotene ...... 45 5.4 Experimental Setup for pump-FWM ...... 46 5.5 Molecular Dynamics Using pump-DFWM ...... 47 5.5.1 Kinetic Analysis ...... 51 5.5.2 Comparison between results from two-pulse pump-probe and pump-DFWM experiments ...... 52 5.6 Ground and Excited State Dynamics Using pump-CARS ...... 56 5.7 Summary ...... 58

6 Surface Enhanced Coherent Anti-Stokes Raman Scattering 59 6.1 Surface enhanced CARS (SE-CARS) ...... 61 6.1.1 Experimental ...... 62 6.1.2 Preparation of Silver sol ...... 62 6.1.3 CARS Experiment ...... 63 6.2 Discussion on SE-CARS ...... 65 6.3 Summary ...... 67

7 Conclusion and Outlook 69

References 73

Acknowledgment 79

List of Publications 81

Appendix 83 1 Introduction

Optical spectroscopy is the most suitable experimental technique for understanding molecular structure and dynamics due to its speed and sensitivity. A careful study of the processes, such as absorption, emission, scattering, linear and circular dichroism, that occur when a light field interacts with a molecule can provide us with important information about the energy, concentration, conformation, and dynamics of the molecule. The invention of the laser, which can provide intense, coherent and tun- able radiation, revolutionized the field of spectroscopy. The increase in power of the radiation provided by lasers along with the ability to produce short pulses gave rise to a completely new nonlinear optical effects, where the absorbed radiation power depends nonlinearly on the incident power. Thereafter, nonlinear interaction of light with matter became an important field of study, which found applications in various fields including spectroscopy. Development of ultrafast laser systems brought about another breakthrough in the field of spectroscopy. Time resolved spectroscopy using picosecond (10−12) and femtosecond (10−15) lasers provided a new window for spectroscopists to investigate molecular dynamics, such as vibrations and rotations of molecules. Ahmed Zewail was awarded Nobel prize for his pioneering work in femtochemistry, which demonstrated the use of femtosecond pulses for real time probing of chemical reaction dynamics. The observation of coherent dynamics resulting from the excitation of molecular systems by femtosecond lasers is the heart of femtochemistry. The new insight into the dynamical aspects of molecules in short time scales brought great advances in the field of molecular physics. Several nonlinear optical techniques were then used along with ultra short pulses to probe different aspects of molecular dynamics occurring in pico and femtosecond time scales. The high peak intensities of ultrafast lasers often lead to different optical nonlin- earities. Among different nonlinear optical processes, third order nonlinear optical effects, due to the many degrees of freedom they offer, have proven to be ideal candi-

1 2 Introduction dates for time resolved spectroscopy to address different aspects of dynamics occuring in molecules [1, 2]. Most commonly used third order nonlinear optical processes are the pump-probe and four wave mixing (FWM). The pump-probe technique was used extensively by Zewail for his works on the ultrafast dynamics of the chemical bond [3]. Their works showed that the key to obtaining spectral and structural information in ultrafast time resolved measurements is based on the generation and detection of wavepackets [3,4]. Transient absorption techniques was the primary technique used to gain information about the ultrafast primary processes in photosynthesis and vision [5–10]. In recent years four wave mixing spectroscopy has rapidly been developed to study the dynamics of molecules in condensed, liquid and vapor phases. Four wave mixing technique using ultrashort pulses can be used in a variety of different forms such as coherent anti-Stokes Raman scattering (CARS), degenerate four wave mixing (DFWM), transient grating (TG), photon echo (PE) etc. to address different aspects of molecular dynamics. Coherent anti-Stokes Raman scattering (CARS) was first used by the groups of Lauberau, Zinth and Kaiser to monitor vibrational dynamics of molecules occurring in a femtosecond time scale [11–13]. Materny and co-workers time resolved femtosecond CARS and DFWM processes to observe wavepacket dynamics occuring on the ground and excited states of iodine vapour [14, 15]. By varying the time delay of one of the incident pulses and keeping the other two overlapped in time, they were able to observe wavepacket dynamics in both the ground and excited electronic states. Zewail and co-workers used pump- degenerate four wave mixing (DFWM) to investigate molecular dynamics. Here the pump pulse initiates the molecular dynamics and the DFWM process is used as a probe [16]. Dietzek et. al. used femtosecond time resolved four wave mixing for studying excited state dynamics in different systems [17, 18]. Brown et. al. used off resonant transient grating (TG) technique for the study of molecular dynamics in gas- phase systems ranging from single atoms to large polyatomic molecules [19]. Ultrafast energy transfer and solvation dynamics has been extensively investigated by Fleming et. al. using photon echo (PE) techniques [6,20,21]. Femtosecond nonlinear techniques such as PE and TG have also been used to investigate primary processes in photosynthesis [22–24]. Femtosecond stimulated Raman technique using a narrow band pump laser and a broad band probe laser has gained much interest in recent years. It can give vibrational structural information with high temporal and spectral resolution [25–28]. The work presented in this thesis is divided into two parts. One part of the work uses time resolved four wave mixing techniques using femtosecond laser pulses to investigate molecular dynamics taking place in the excited states of simple and complex 3 molecules. The degenerate four wave mixing technique with an additional pump pulse is applied to investigate the vibrational dynamics occurring in the excited states of the diatomic system iodine. Iodine is a much investigated molecule whose excited state properties are well characterized. Using iodine as a model system, the applicability of such time resolved nonlinear techniques in investigating higher lying excited states can be verified. The technique will then be extended to a more complex molecule. The photosynthetic pigment molecule β-carotene was chosen as the other system for investigation. Studies on the excited states of this molecule are important due to the important role it plays in plant photosynthesis [29]. Time resolved third order nonlinear optical processes are used in order to understand population and vibrational dynamics occurring in the excited states of β-carotene. The population dynamics after photo excitation is probed using a pump-DFWM scheme, where the pump pulse promotes population into the excited state and the subsequent flow of population is monitored using a DFWM process as probe. The results are then compared with those of a simple pump-probe technique, which shows that the pump-DFWM has clear advantages in disentangling different pathways of population decay. The second part of the work is based on the principle of surface enhancement of the Raman process near rough/colloidal metal surfaces. Even though the surface enhanced Raman scattering (SERS) effect is widely used in a variety of fields, the mechanism behind the effect is still unclear. There are two popular theories, which explain the mechanism of enhancement and both of them are not able to explain all the SERS effects observed. The electromagnetic effect explains the enhancement as due to an increased field density near rough surfaces, whereas the chemical theory assumes the formation of a charge transfer state between the molecule and the metal. Nonlinear optical processes can also be enhanced in presence of colloidal metal surfaces. Pyridine is a well investigated molecule, which shows a SERS effect when adsorbed onto colloidal silver particles. The SERS effect in pyridine is identified to be due to the formation of a charge transfer state between the molecule and metal (chemical effect). Also it is well known that time resolved coherent anti-Stokes Raman scattering (CARS) can give information about the structural dynamics taking place in a molecule. Thus, by combining time resolved CARS along with the surface enhancement effect of pyridine adsorbed onto colloidal silver particles, information can be obtained about the nature of the charge transfer state formed between the molecule and metal and its dynamics can be studied. The work presented in this thesis uses the enhancement of coherent anti-Stokes Raman scattering from pyridine adsorbed onto colloidal silver particles in an attempt to understand more about the mechanism behind the enhancement effect. 4 Introduction

The chapters of this thesis are organized as follows: The second chapter describes the theory necessary to understand time resolved third order nonlinear optical processes. The principles of pump-probe spectroscopy, coherent anti-Stokes Raman scattering and degenerate four wave mixing are also described in detail. The third chapter describes the setup used for the experiments presented in this thesis. The fourth chapter describes the time resolved FWM experiments applied to investigate the excited state dynamics in molecular iodine. The ﬁfth chapter deals with the excited state dynamics of β-carotene explored using pump-DFWM, pump-CARS and pump-probe techniques. The sixth chapter describes the preliminary experiments on the surface enhancement of coherent anti-Stokes Raman scattering. The last chapter summarizes the results of the experiments and presents the future prospects. 2 Theory

2.1 Femtosecond Time Resolved Spectroscopy

The investigation of ultrafast molecular dynamics was made possible by the development of ultrashort pulses. The short pulse duration and high peak power offered by femtosecond lasers makes them ideally suited for exploring elementary molecular dynamics using nonlinear optical spectroscopic techniques. Third order nonlinear optical processes involving multiple femtosecond pulses offer many degrees of freedom to explore different aspects of molecular dynamics. By employing multiple femtosecond laser pulses of different frequencies and wave vectors, different molecular transitions can be manipulated and important information about molecular dynamics such as vibrational relaxation times, population life times, decoherence times etc. can be extracted. This chapter reviews the theoretical framework used to describe time resolved nonlinear optical processes using perturbation theory with special attention on third order nonlinear optical processes such as pump-probe spectroscopy and four wave mixing spectroscopy [1,2,30,31]. Different nonlinear coherent spectroscopic techniques used in this project to study molecular dynamics are then presented.

2.2 Perturbation theory for time resolved spectroscopy

The theory of time resolved FWM processes is formulated based on third order perturbation theory, which is used to calculate the third order polarization, P(3)(t) responsible for the signal generation. In a semiclassical picture, the electromagnetic ﬁeld is treated classically and the matter is described quantum mechanically either using the wave function approach or using the density operator approach. The formal theory is outlined assuming a system, which as two states (a and b) with energies Ea and Eb interacting with three laser ﬁelds sequentially. Let H0 be the hamiltonian of the system

5 6 Theory in the absence of any external perturbations. Ã ! Ha 0 H0 = (2.1) 0 Hb

The off-diagonal elements are assumed to be zero so that the system is stationary. The FWM process is driven by the interaction of the system with three laser pulses with electric ﬁelds given by. −i(k·r−ωt) Ei(t) = Ei fi(ti)e + c.c. (2.2)

Where c.c. denotes complex conjugate and f (t) is the pulse envelope given by a Gaus- sian centered around time ti and gives the time delay between the pulses. In presence of a radiation ﬁeld, the Hamiltonian becomes H = H0 + V(t) where V(t) = −µ · Ei(t) is the interaction matrix and µ is the transition dipole moment. Ã ! 0 −µ E∗(t) V(t) = ab i (2.3) −µbaEi(t) 0

The time evolution of the system in presence of the radiation ﬁeld is governed by the time dependent Schrödinger equation

∂ ih¯ ψ(t) = [H + V(t)]ψ(t) (2.4) ∂t 0

According to perturbation theory, the solution to eq. 2.4 is found by expanding the wave function as a sum of wave functions ψ(n)(t) of different orders (n), each produced by the interaction with a radiation field. ψ(0)(t) is the unperturbed wave function. The first order correction to the unperturbed wave function, ψ(1)(t), is created by the interaction with the first laser pulse at time zero. The second order correction, ψ(2)(t), is created by the interaction of the second laser pulse with the first order wave function and so forth. The final wave function at time t, after n interactions with the external field can be written as

ψ(t) = ψ(0)(t) + ψ(1)(t) + ψ(2)(t) + ψ(2)(t) + ··· = ∑ ψ(n)(t) (2.5) n

The interaction of the material with the radiation field generates a polarization in the material, which serves as the source for the signal, which is detected. In general, for low intensity radiation from conventional sources such as light bulb or flash lamp, the polarization induced in the medium by the radiation is linearly proportional to the 2.2 Perturbation theory for time resolved spectroscopy 7 electric field strength.

P = e0χE (2.6) where e0 is the permittivity of free space and χ is the linear susceptibility which is, in general, a complex quantity. The imaginary part of χ determines the linear absorption and its real part determines the refractive index of the material. Thus, information about the optical characteristics of a material is contained in χ and the linear optical properties of a material can be described using eq. 2.6

When the intensity of the radiation is strong, as in the case of a laser, the relationship between the polarization and electric ﬁeld deviates from linearity and nonlinear terms must be introduced to describe the nonlinear optical effects.

(1) (2) 2 (3) 3 P = e0[χ E + χ E + χ E + ··· ] (2.7)

The macroscopic polarization is related to the dipole moment µ by

P(t) = Nhψ(t)|µ|ψ(t)i (2.8) where N is the number of molecules per unit volume and hψ|µ|ψi is the polarization per molecule. The perturbation expansion of ψ(t) in eq. 2.5 can be applied to the above equation to get the ﬁeld induced macroscopic polarization

P(t) = hψ(t)|µ|ψ(t)i = P(0)(t) + P(1)(t) + P(2)(t) + ··· (2.9)

Identifying the terms of same order in the perturbation expansion of polarization (eq. 2.9) and wave function (eq. 2.5) we can get the following relations for the different orders of polarization

P(0)(t) = hψ(0)(t)|µ|ψ(0)(t)i (2.10a) P(1)(t) = hψ(0)(t)|µ|ψ(1)(t)i + c.c. (2.10b) P(2)(t) = hψ(0)(t)|µ|ψ(2)(t)i + c.c. + hψ(1)(t)|µ|ψ(1)(t)i (2.10c) P(3)(t) = hψ(0)(t)|µ|ψ(3)(t)i + c.c. + hψ(1)(t)|µ|ψ(2)(t)i + c.c. (2.10d)

Thus, using the perturbation theory polarization of different orders can be determined using eq. 2.10. The term hψ(0)(t)|µ|ψ(0)(t)i appearing in eq. 2.10a is the perma- nent dipole moment. Terms of the form hψ(1)(t)|µ|ψ(1)(t)i represent populations and terms hψ(0)(t)|µ|ψ(1)(t)i which appear in higher order polarizations represent coher- 8 Theory ences. Fluorescence experiments, for example, rely on population terms contributing to the macroscopic polarization of the sample, which appear as a new source term in Maxwell’s equations resulting in the ﬂuorescence signal. Macroscopic polarisation of the sample resulting from coherence terms is the basis of coherent nonlinear optical spectroscopy. In experiments, which measures coherence, the signal is coherent and is emitted in a direction different from that of the incident beams and can be detected background free. In the following, the sequential interaction of three laser pulses resulting in third order polarization will be analyzed in detail for the two level system described above. Different four wave mixing processes that can be used for the measurement of population/coherence dynamics in the sample will also be discussed.

2.2.1 First order polarisation

Consider the interaction of the ﬁrst laser pulse with frequency ω1 and wave vector k1 at time t1 with the two level system. Assuming that before excitation there is no excited state amplitude, the initial state of the two level system can be written as Ã ! ψ ψ(0)(0) = a (2.11) 0

The ﬁrst order perturbation theory can be used to derive the ﬁrst order wave function

Z t i i i (1) − H0(t−t1) − H0t1 (0) ψ (t) = − e h¯ V(t1)e h¯ ψ (0)dt1 h¯ 0 Ã i !Ã ! Z t − Ha(t−t1) ∗ i e h¯ 0 0 −µabE1 (t1) = − i − H (t−t1) h¯ 0 0 e h¯ b −µbaE1(t1) 0 Ã !Ã ! − i H t e h¯ a 1 0 ψ (0) × a − i H t 0 e h¯ b 1 0 Ã ! 0 = R t i i (2.12) i − h¯ Hb(t−t1) − h¯ Hat1 − h¯ 0 e [−µbaE1(t1)]e ψa(0)dt1

Since the ﬁrst element is zero, we can write the equation for ﬁrst order process as

Z t i i i (1) − Hb(t−t1) − Hat1 ψ (t) = − e h¯ [−µbaE1(t1)]e h¯ ψa(0)dt1 (2.13) h¯ 0

Equation RHS of 2.13 can be interpreted in the following way: Reading from right to left, the initial wave function ψa(0) evolves from time zero to t = t1 under the 2.2 Perturbation theory for time resolved spectroscopy 9

influence of the ground state Hamiltonian Ha. At time t1, the electric field E1(t1) interacts with the transition dipole moment creating an excited state amplitude. The excited state amplitude then evolves from t1 to t under the influence of the excited state Hamiltonian Hb. Integration over t1 accounts for all instances of time possible for the interaction with the radiation field. A pictorial representation of the process, known as Feynman diagram, is shown in fig. 2.1a. The time increases from right to left. The time of interaction with the photon t1 can be anywhere between 0 and t. The time evolution of the system before and after interaction with the photon is also represented by the corresponding Hamiltonians. The first order polarization, which

i i --H() t t - H t h b 1 h a 1 ba ab t e t1 e 0 t1 E t k w ,k1 -mba × 1() 1 y a (0) w 1, 1 aa t aa 1 (a) (b)

Figure 2.1: Two possible pictorial representations of the first order process: (a) Feynman diagram for the first order process. The time increases from right to left. (b) Double sided Feynman diagrams showing two possible contributions to the first order process. See text for details

is responsible for the linear optical effects, can now be calculated by substituting the expression for ψ(1)(t) from eq. 2.13 in eq. 2.10b

P(1)(t) = hψ(0)(t)|µ|ψ(1)(t)i + c.c. (2.14)

Another useful and more informative representation of the process, known as double sided Feynman diagram, is shown in fig. 2.1b. This diagram shows the two possible contributions to the first order polarisation. The diagram can be interpreted as follows. The two vertical lines represent the bra (right) and ket (left) sides and time increases from bottom to top. The interaction with a photon is represented by a wavy arrow labeled with the frequency and wave vector of the photon. Arrows pointed towards the vertical lines represent absorption of a photon and those pointing away represent emission of a photon. The labels like a a and a b appearing between the vertical lines represent populations (if both the labels are the same - analogous to diagonal elements in the density matrix formalism) or coherences (if the labels are different - off-diagonal elements in the density matrix formalism). The system always starts from a ground state population denoted by the label a a (corresponding to the density matrix element ρaa denoting the ground state population) at the very bottom of the 10 Theory diagram. An absorption process on the left/right side increases the label (a→b) on the corresponding side and an emission process decreases the label (b→a). Thus, from fig. 2.1b it can be interpreted that the interaction with a single photon creates a coherence, which contributes to the first order polarization. The two parts in fig. 2.1b correspond to two contributions to the first order polarization eq. 2.14. One is the complex conjugate of the other and the only difference is in the side (bra or ket) of photon interaction. The difference between the photon interaction on the bra (right) side and ket (left) side is more apparent when one uses the density matrix formalism to describe optical processes. The right/left interaction becomes important when there are more than one photon interactions at different times. The time delay between left/right interaction becomes important when relaxation processes, such as population relaxation and dephasing, are involved. Different time orderings of the left/right interaction lead to different propagation intervals for the diagonal (population) versus off-diagonal (coherence) elements of the density matrix. Depending upon the time delay between these interactions, the process is given a different name and different physical inter- pretation. These aspects will be more clear when second and third order processes are considered in the following sections.

2.2.2 Second order polarization

Consider the interaction of the second laser pulse with frequency ω2 and wave vector k2 at time t2, t2 > t1. Following the same method used in the previous section for ﬁrst order polarisation, the second order wavefunction obtained using second order perturbation theory can be written as: µ ¶ 2 Z t Z t i 2 i (2) − Hx(t−t2) ψ (t) = − e h¯ [−µxbE2(t2)] h¯ 0 0 (2.15) i i − Hb(t2−t1) − Hat1 × e h¯ [−µbaE1(t1)]e h¯ ψa(0)dt1dt2

This expression also can be interpreted in the same manner as the first order wave function in the previous section. After interaction of the first pulse at time t1, the excited state amplitude generated evolves under the action of the Hamiltonian Hb until time t2. After the interaction of the second pulse at time t2, the subsequent evolution is shown to be under the action of the Hamiltonian Hx. Hx can be either Ha or Hb depending upon the different possibilities of the second interaction. The double sided Feynman diagrams in fig. 2.2 shows the different possibilities (pathways) of 2.2 Perturbation theory for time resolved spectroscopy 11 generation of second order polarization.

k k w2 , 2 w 2, 2 bb aa bb aa

k k ba w2 , 2 ba w 2, 2 ab ab

t1 aa aa aa aa k k k w 1, 1 w 1, 1 w ,k w1 , 1 (a) (b) (c) 1 1 (d)

Figure 2.2: Double sided Feynman diagrams showing different scenarios of second order process. (c) and (d) are the complex conjugates of (a) and (b) respectively. See text for details

The second pulse can either add to the excited state amplitude created by the first pulse as shown in diagrams 2.2a and 2.2c or can return the excited state amplitude back to the ground state as shown in diagrams 2.2b and 2.2d. Thus, the interaction of the first pulse creates a coherence that contributes to the first order polarisation (in the density matrix formalism, it creates the off diagonal density matrix element

ρab) and the interaction of the second pulse converts the coherence to an excited state population or recreates the ground state population (diagonal elements of the density matrix ρaa or ρbb). The second order polarization P(2)(t), which is responsible for the second order nonlinear optical effects can be calculated using eqs. 2.15 and 2.10c. P(2)(t) vanishes for isotropic media and hence the lowest order of optical nonlinearity observable in all media irrespective of symmetry is the third order.

2.2.3 Third order polarization

The interaction of the third and ﬁnal pulse, ω3, k3 with matter at time t3 (t3 > t2 > t1) creates a third order polarization, which acts as the source for the four wave mixing signal. The action of the third pulse can also be analyzed using the same procedure followed in the previous two sections. The third order wave function can be derived 12 Theory using the third order perturbation theory and is given by µ ¶ 3 Z t Z t Z t i 3 2 i (3) − H (t−t3) ψ (t) = − e h¯ a/b [−µE3(t3)] h¯ 0 0 0 i − H (t3−t2) (2.16) × e h¯ a/b [−µE2(t2)] i i − Hb(t2−t1) − Hat1 × e h¯ [−µE1(t1)]e h¯ ψa(0)dt1dt2dt3

This equation can also be interpreted in the same way as the ﬁrst and second order wave functions. The third order polarization, which is the source of the FWM signal, resulting from the interaction of three pulses can be written as

P(3)(t) = hψ(0)(t)|µ|ψ(3)(t)i + c.c. + hψ(1)(t)|µ|ψ(2)(t)i + c.c. (2.17)

The first term hψ(0)(t)|µ|ψ(3)(t)i is responsible for processes like coherent anti-Stokes Raman scattering (CARS) and the second term is responsible for processes like stimulated emission and excited state absorption Like in the second order case, several possible scenarios (pathways) can be considered for the interaction of the third pulse, which results in the third order polarization. There are eight possible pathways, which result in the third order polarization. Four of them are shown in the double sided Feynman diagrams in fig. 2.3. The other four contributions are the complex conjugates of these diagrams. Depending upon whether the field interactions are on the left/right side, third order processes are given different names and physical interpretations. The first diagram

R1 can be classified as resonance Raman process. The first two interactions are both on the left (ket) side and hence there is no excited state population when the third interaction takes place (In the diagram, after the first two interactions the labels are both a indicating a ground state population). Hence the signal (ωs, ks) is generated from a virtual process of the Raman type. In diagrams R2 and R3 the first two interactions excites both the bra and ket side resulting in a first order population in the excited state. The interaction of the third pulse with this population results in stimulated emission. By carefully choosing the frequencies, wave vectors and timing of the three pulses in a FWM experiment, different four wave mixing schemes can be realized. Each of these different schemes probes different sample properties (populations/coherences). A detailed account of all different possiblities is given in [2]. Three of the time resolved third order processes used in this work are described in the following sections. 2.3 Pump-Probe Spectroscopy 13

R1 R2

aa k aa ws ,k s t ws , s t ab k ab w 3, 3 t3 t3 k w ,k w 2, 2 aa 3 3 bb t2 t2 k ba ba w 2, 2

t1 t1 aa k aa k w 1, 1 w 1, 1

R3 R4 aa k aa k t ws , s t ws , s k ab ab w 3, 3 t t 3 3 w ,k bb aa 3 3 t k 2 t w 2, 2 k 2 w 2, 2 ab ab t1 t1 aa k aa k w 1, 1 w 1, 1

Figure 2.3: Feynman diagrams showing contributions to the third order polarization. The wavy arrow in grey color represents the signal resulting from the third order polarization. R1 corresponds to the resonance Raman process, R2 and R3 can be classiﬁed as stimulated emission and R4 corresponds to the CARS process. The complex conjugates of these diagrams also contribute to the third order polarization

2.3 Pump-Probe Spectroscopy

In a pump-probe experiment, a pump pulse with frequency ωpu and wave vector kpu excites the sample. The events after excitation are probed by a time delayed probe pulse of frequency ωpr and wave vector kpr. The optical path traversed by the two pulses is varied to change the delay between the two pulses. The modiﬁcation of the characteristics (spectrum, intensity, phase) of transmitted probe pulse is measured by varying the delay between the pump and probe pulses. The probe pulse characteristics carries information about the changes induced in the sample by the pump pulse. This technique can be used to measure changes in the absorption spectrum of a sample after perturbation by the pump pulse. Changes in the absorption spectrum 14 Theory can involve new transitions appearing or increase/decrease in the optical density. New electronic states prepared by the pump pulse can cause changes in the spectral characteristics of the probe pulse and the variation in the intensity of the probe pulse with time will yield the dynamics of the new state prepared by the pump pulse. Con- sider a system with three electronic levels a, b and c. The scheme of the pump-probe experiment to measure the formation of the third state c is given in ﬁg. 2.4a. The term in the third order polarization eq. 2.17 responsible for excited state absorption is hψ(1)(t)|µ|ψ(2)(t)i. The pump frequency is chosen to be resonant with the transition

R1 R2 cc cc t t

wpr bc wpr bc

t3 t3 bb bb wpr wpr t2 t2

ba wpu ab V t wpu t c 1 aa 1 aa wpu wpu Epr (t 3 ) R R Vb 3 4 wpu cc aa t t

wpr bc wpr ba Epu (t 1 ) E (t ) V pu 2 a t3 t3

wpu ac ca t2 t2

ab wpr wpr ba t t 1 aa 1 aa wpu wpu (a) (b)

Figure 2.4: (a)Pump-probe scheme to probe the formation of the third state in the three level system (b)Double sided Feynman diagrams showing contributions to the third order polarization in a three level system. The formation of the third state is shown in diagrams R1 − R3. R4 corresponds to the regeneration of the ground state. Each of these diagram has a complex conjugate and is not shown here

between states a and b and the probe frequency is resonant with the transition between states b and c. Figure 2.4b shows the double sided Feynman diagrams of possible processes that result in a population of the third state c. From ﬁg. 2.4a it can be seen that the interaction of the pump pulse produces a ﬁrst order population in the excited (1) b state. This wave packet (φ (t)) begins to evolve on the potential surface Vb. The 2.4 Coherent anti-Stokes Raman Scattering (CARS) 15

probe pulse arrives after a time delay ∆t = t2 − t1. When the duration of the probe pulse is much shorter than the dynamics on the potential surface Vb, the wave packet can be assumed to be frozen in time ∆t. Thus, the wave packet seen by the probe pulse can be assumed to be φ(1)(∆t). The action of the probe pulse can be understood as a ﬁrst-order spectroscopy on the frozen state φ(1)(∆t). Different time delays between the pump and probe pulses result in different wave packets φ(1)(∆t) seen by the probe pulse. The quantity that is measured in pump-probe experiment is the intensity of the spectral components of the probe pulse before the interaction of the pump pulse

Ipr(ν, 0) and after the interaction of the pump pulse for various time delays Ipr(ν, ∆t). The absorption of the probe pulse can be quantiﬁed using the Lambert-Beer law.

−α(ω)lN(∆t) Ipr(ω, ∆t) = Ipr(ω, 0) · 10 (2.18) where, α(ω) is the absorption coefﬁcient at frequency ω, N(∆t) is the population of the absorbing species and l is the tickness of the sample. The quantity that is measured is the optical density (OD) deﬁned as

Ipr(ω, 0) OD(ω, ∆t) = log = α(ω)lN(∆t) (2.19) Ipr(ω, ∆t)

For each time delay and each spectral component ω, the OD is proportional to α(ω). Thus the absorption spectrum of the sample excited by the pump pulse at time ∆t can be measured by measuring the OD for various frequency components ω. On the other hand, for a particular frequency component ω, varying the time ∆t gives the time variation of the absorbing population N(∆t). Thus, the population dynamics of the absorbing species can be determined by measuring OD for a particular ω varying ∆t. The pump-probe spectroscopy using femtosecond laser pulses is particularly useful in studies of ultrafast light driven processes like early events in photosynthesis and vision, observation of transition states in chemical path etc.

2.4 Coherent anti-Stokes Raman Scattering (CARS)

CARS is a four wave mixing process based on a stimulated Raman process. The process involves three input lasers, usually known as the pump, Stokes, and probe lasers. The frequencies of the pump and Stokes lasers ωp and ωS are chosen such that

ωp − ωS = Ωv is in resonance with a vibrational transition. The pump and Stokes 16 Theory pulses are chosen to be time coincident and the simultaneous interaction of these two ﬁelds results in a coherent excitation of the vibrational mode Ωv. If the probe pulse (ωpr) is sent in during the coherence time of the excited vibrational mode, a coherent pulse is generated at the anti-Stokes frequency ωaS=ωpr+Ωv. The scheme of the CARS process is shown in ﬁg. 2.5 along with the relevant Feynman diagram. By varying the time delay between the time coincident pump-Stokes pair and the probe pulse, dynamics of the excited vibrational mode can be studied. Terms of the form

b¢ k ws , s aa b t b¢ a wpr t Dt k waS w 3, 3 t k a¢ a D w 2, 2 wp wS t0 a ba ¢ t0 Wv a aa wp ,k p

Figure 2.5: Energy level scheme describing CARS process and the corresponding Feynman diagram

hψ(0)(t)|µ|ψ(3)(t)i in the third order polarization (eq. 2.17) are responsible for the CARS process. This polarization induced in the sample acts as the source for the CARS signal. To ﬁnd the signal intensity, this polarization is introduced as the source term in the inhomogeneous Maxwell’s equation · ¸ ω2 4πω2 ∇2 + e(ω) E = − P(3)(ω) (2.20) c2 c2

This equation is applicable to all FWM processes in general. The solution to this equation gives the intensity of the FWM signal. The signal intensity, in general for any FWM process, is given by [1, 32, 33]

Ã !2 sin ∆kl I ∝ χ(3) 2 I I I l2 2 (2.21) FWM | | 1 2 3 ∆kl 2

(3) Where χ is the third order nonlinear susceptibility. Ii=1,2,3 are the intensities of the input lasers and l is the interaction length. ∆k is the phase mismatch factor which is described below. The frequencies and wave vectors involved in the FWM process 2.4 Coherent anti-Stokes Raman Scattering (CARS) 17 must satisfy the energy conservation and momentum conservation laws given in the following equations.

ωFWM = ω1 − ω2 + ω3 (2.22a)

kFWM = k1 − k2 + k3 (2.22b)

Equation 2.22b is known as the phase matching condition. The phase mismatch factor is deﬁned as ∆k = |k1 − k2 + k3 − kFWM|. From eq. 2.21 it can be seen that the dependence of signal intensity on ∆k is through the square of a sinc function. Hence, the signal intensity falls off rapidly as the phase mismatch ∆k deviates from 0. The sinc function can be maximized if ∆k = 0. This is achieved by selecting a particular wave vector geometry for the incoming beams known as folded-BOXCARS geometry, which is described in detail in section 3.5. The spectral dependence of χ(3) makes the FWM signal sensitive to a speciﬁc transition. By selecting different frequencies for the incident beams different resonance conditions can be achieved which results in a resonance enhancement of χ(3).

As described above, in CARS a Raman resonance is achieved by selecting ωp and (3) ωS such that ωp − ωS = Ωv. In this case, the component of χ , which has a Raman resonant denominator, will be enhanced, which is responsible for the CARS signal. The energy and momentum conservation in the particular case of CARS can be written as

ωaS = (ωp − ωs) + ωpr = Ωv + ωpr (2.23a)

kaS = kp − kS + kpr = kv + kpr (2.23b)

Here, Ωv is the frequency and kv is the phonon vector of the excited vibrational mode. Due to the phase matching condition, the CARS signal is always emitted in a well defined direction. This allows us to filter out the signal spatially, which then can be detected free from other parasitic light. CARS offers various advantages over conventional Raman scattering. Generally, spontaneous Raman scattering has a very low quantum yield whereas the CARS signal is much more intense than the spontaneous Raman signal. When the sample is fluorescent, a precise detection of the Raman signal can be difficult due to the fluorescence background. Since the CARS signal is always well directed and is on the high energy side of the exciting laser frequencies, it can be easily separated and can be detected with excellent sensitivity. These advantages make CARS a useful spectroscopic 18 Theory tool. By combining the CARS process with an additional excitation pulse, it can also be used to study excited state dynamics of samples. The application of this technique for β-carotene is detailed in chapter 5

2.5 Degenerate Four Wave Mixing (DFWM)

The basic principle of DFWM is similar to the CARS process except that all the input fields have the same frequency ω. The signal frequency in this case is also ω. The intensity of the DFWM signal is governed by eq. 2.21. DFWM also has to satisfy the energy conservation and phase matching conditions given by eq. 2.22. The origin of the DFWM signal can be understood using a transient grating picture. The fields of two beams, traveling in slightly different directions, interfere in the sample creating a grating by spatially modulating its optical properties. The absorption properties (amplitude grating) or refractive index (phase grating) of the sample can be modified depending upon the experimental conditions. The grating constant d is given by

λ d = (2.24) 2 sin(φ/2) where, φ is the angle between the two interacting beams creating the grating. The incoming third beam is scattered from this grating in a direction given by the Bragg condition. The modulation amplitude depends on the population life time and starts to decrease after the laser interaction. The intensity of the diffracted pulse will depend upon the modulation amplitude and its dynamics can be studied by varying the time delay between the time coincident pulses and the third pulse. The scheme for DFWM for a two level system is shown in ﬁg 2.6. In time resolved DFWM experiments, two of

g¢ g¢ DFWM Dt

w1 w2 w3 ws IP DT g g (a) (b)

Figure 2.6: (a) Energy level scheme describing DFWM process. (b) Scheme for using DFWM to study excited samples. IP is the initial pump used to excite the sample before the DFWM process. 2.5 Degenerate Four Wave Mixing (DFWM) 19 the pulses, generally called the pump pulses of the DFWM process, are time coincident and the third pulse, commonly called the probe pulse, is delayed with respect to the other two. When one of the pulses precedes the other two, the DFWM signal reveals the overall dephasing time T2. The first pulse creates a macroscopic polarization in the sample and at a certain time delay, the electric field of one of the other two pulses interferes with this polarization creating a grating. The trailing edge of the third pulse, which arrives simultaneously with the other pulse then probes this grating resulting in the DFWM signal. In this case, the decay of the macroscopic polarization, which decays with the dephasing time T2, is probed by the other two pulses. When the third pulse follows the other two time coincident pulses, the grating created by the first two pulses is probed by the third pulse. In this case, the DFWM signal reflects the population decay time T1. The DFWM process also can be used to study the behavior of excited states by combining it with an additional excitation pulse. This scheme is known as pump-DFWM. An initial pump (IP) pulse is used to excite the sample before the DFWM pulses and the relaxation of the excited sample can be studied by varying the time delay (∆T) between the IP pulse and the DFWM pulses. In contrast to the normal DFWM case, the pump-DFWM offers an added flexibility in choosing the pump wavelength as it is independent from the DFWM process.

3 Experimental Setup

This chapter details the experimental realization of time resolved four wave mixing (FWM) spectroscopy. The ﬁrst section provides a description of the laser system gen- erating the femtosecond pulses required for the experiments. Pulse characterization, wavelength tuning and pulse sequencing techniques are described in the subsequent sections. The special arrangement of beams used for fulﬁlling the phase matching condition is also described in detail in the last section.

3.1 Femtosecond Laser System

The femtosecond laser pulses required for the experiments are generated by a com- mercial laser system, Clark-MXR Inc., CPA-2010. The output pulse characteristics of the laser are given in table 3.1.

Model Clark-MXR Inc., CPA 2010 Central Wavelength 775 nm Pulse Width ≈150 fs Output Power 1 W Repetition Rate 1 KHz

Table 3.1: Output pulse characteristics of CPA-2010 laser system.

The operation of the laser system is based on chirped pulse amplification (CPA), which is described in detail in literature [32, 34, 35]. The CPA-2010 laser system contains a SErF fiber oscillator, a pulse stretcher, a regenerative Ti:Sapphire amplifier, pulse compressor, and an Nd:YAG laser arranged in a bi-level optical layout design as schematically shown in fig. 3.1 The SErF fiber laser is an active fiber ring laser pumped by an all solid-state laser diode operating at ≈ 980 nm. The SErF produces ≈ 100 fs pulses of wavelegth centered around 775 nm. These pulses are then taken to the pulse stretcher where they are stretched to pulses of duration ≈ 200 ps. The

21 22 Experimental Setup

SErFFiber 25Khz Oscillator ~775nm

Pulse Stretcher

DiodeLaser

BottomLevel

Regenerative Amplifier

ORC-1000Nd:Y AG Pulse PumpLaser Compressor (1KHz)

TopLevel Output

Figure 3.1: CPA-2010 Laser system schematic layout

stretched pulses are then amplified by the Ti:Sapphire regenerative amplifier pumped by a frequency doubled Nd:YAG laser (ORC-1000, Clark-MXR) operating at 1 kHz repetition rate. Pulses from the stretcher are coupled in and out of the regenerative amplifier using a Pockels cell, which is triggered by the Nd:YAG laser. The pulses en- tering the regenerative amplifier take several round trips through the active medium of the amplifier in order to achieve maximum amplification. The amplified pulses are then coupled into a pulse compressor where the pulses are compressed to about 150 fs using a holographic transmission grating. The final output pulses have an energy of 1 mJ at a repetition rate of 1 kHz and a pulse width of around 150 fs. 3.2 Optical Parametric Amplifiers 23

3.2 Optical Parametric Ampliﬁers

Two multi-pass optical parametric ampliﬁers (OPA; Light Conversion TOPAS) are used in order to generate different wavelengths required for the experiment. The 775 nm output from the CPA-2010 laser system is divided into two parts using a 50:50 beam splitter and are used to pump the two OPAs. The pump pulse, on interaction with the nonlinear crystal within the OPA, is converted to signal and idler pulses by difference- frequency generation. The frequencies of the pump (ωp), signal (ωs) and idler (ωi) pulses satisfy the relation ωp = ωs + ωi. The difference frequency generation can thus generate a large range of IR wavelengths. Visible wavelengths in the range 450- 700 nm can be generated by using processes such as second harmonic generation of the signal (or idler) (2ωs/i), sum frequency generation of signal (or idler) with the pump (ωp + ωs/i). Thus the visible wavelengths along with the IR wavelengths from the signal and idler give a very broad wavelength tuning range spanning from 450 nm to 2700 nm. An additional frequency doubling unit can be used to generate UV pulses which will again broaden the wavelength tuning range from 270 nm to 2700 nm. The output pulses from the OPAs are affected by dispersion due to the nonlinear processes involved in the generation of visible wavelengths. The dispersion can be compensated by using a standard two-pass double prism arrangement [34]. The duration of the output pulse from the OPA after compression will be 80 to 100 fs with an energy of several micro-joules.

3.3 Pulse Characterization

The temporal profile of the pulses is characterized using an autocorrelator (Mini, APE GmbH). In an autocorrelator, the pulse to be measured is split into two parts. One part goes through an optical delay line in order to introduce temporal delay between the pulses. The two parts are then spatially overlapped on a thin nonlinear crystal to generate sum frequency. The intensity of the sum frequency signal is recorded varying the time delay between the pulses. The trace obtained contains information about the temporal profile of the pulse. The autocorrelation trace is fitted with an appropriate envelope function (such as Gaussian or Sech2) to extract the pulse width. The spectral profile of the pulses were recorded using a monochromator and a CCD detector, which is described in detail in the next section 24 Experimental Setup

3.4 Experimental Setup

A schematic diagram of the setup used for the time resolved four wave mixing experiments described in this thesis is shown in fig. 3.2. The output of the CPA laser system is divided into two equal parts and fed into TOPAS1 and TOPAS2. The outputs from the TOPASs are compressed using prism compressors to about 80 fs. The output of TOPAS2 is used as the initial pump for the the pump-FWM experiment. The output of TOPAS1 is split into three parts using beam splitters and is used for the FWM process. All the pulses are taken through computer controlled linear translation stages (MICOS GmbH) fitted with retro-reflecting mir- rors. Using this arrangement, the path length that each beam travels can be adjusted precisely to introduce a time delay as small as ≈5 fs between pulses. The beams are then made parallel to each other using an alignment mask and focused non-collinearly onto the sample using a lens of focal length 15 cm. A second lens is used to collect and collimate the laser beams along with the signal. The direction of the signal is different from that of the input lasers and can thus be easily separated using another phase mask. The spatially separated signal is then focused on to the entrance slit of a monochromator, which is equipped with a multi channel CCD and a photomultiplier tube (PMT). The peltier-cooled CCD can be used for broad band detection and the PMT can be used for single channel detection. Both CCD and PMT were used for the experiments on β-carotene presented in chapter 5. For all other experiments only broad band detection was used. Since the signal resulting from femtosecond laser interaction is spectrally broad, detection using a CCD is more advantageous as it enables to record intensity variations as a function of spectral position and time delay. For detection using PMT, a gated box-car averager was used to reduce noise. For an efficient FWM signal, perfect overlap of the input pulses has to be ensured both spatially and temporally. Spatial overlap of the pulses is necessary to achieve phase matching and is described in detail in the next section. The temporal overlap of the pulses is ensured by making each of the input beam to traverse the same optical path length. The temporal overlap of the different input beams can be checked using different methods. One is a cross correlation setup based on sum frequency mixing in a BBO crystal. Two pulses are spatially overlapped on to the BBO crystal using a focusing lens and the path length traversed by one of the pulses is varied until the sum frequency signal is generated from the BBO crystal. A maximum sum frequency signal is obtained when the temporal overlap of the two pulses is perfect. Another method is based on optical Kerr effect. Two pulses whose polarizations are rotated by 45◦ with 3.4 Experimental Setup 25

Mask Signal

Spectrometer Signal CCD/PMT Lens

Lens Sample Computer

Lens Mask

DFWM3

g BS50/50 DFWM2

DelaySta DFWM1 BS30/70

InitialPump

Prism Compressor BC

Clark-MXR

AS2 CPA-2010

TOPAS1

TOP

BS50/50

Figure 3.2: Experimental Scheme employed for time resolved four wave mixing experiments. The circles shows the beam arrangement at the input and output ends of the sample, BC stands for Berek Compensator, which is used for polarization rotation 26 Experimental Setup respect to each other are spatially overlapped onto a highly nonlinear medium such as CS2. The electric ﬁeld of one of the pulses induces an anisotropic refractive index (birefringence) in the medium which causes a rotation of the polarization of the other beam. The rotation of the polarization is maximum when the two pulses overlap in time perfectly.

3.5 Phase Matching in Four-Wave Mixing

One of the important conditions that has to be satisﬁed for the efﬁcient generation of a multi wave mixing signal is the phase matching condition [1,2,34,36,37]. The phase matching condition ensures that the microscopic signal contribution from molecules located at different position add up to a macroscopic detectable signal. In FWM, the phase matching requires the sum of wavevectors of the beams involved to cancel each other. In general, the phase matching condition can be written as

±~k1 ±~k2 ±~k3 ±~ks = 0 (3.1) where, ~k1, ~k2, ~k3 are the wave vectors of the incident lasers and ~ks is the wave vector of the FWM signal. Thus, the direction of the FWM signal is determined by the phase matching condition. Since h¯~k is the momentum associated with a photon of wave vector ~k, the relation 3.1 can also be interpreted as the momentum conservation for the FWM process. Phase matching ensures that, within the interaction region of the lasers, the signal generated from one point is in phase with the signal generated from another point resulting in constructive interference and an intense signal. Phase matching in FWM can be ensured by using a speciﬁc beam geometry such that the vector sum of the incident wave vectors and wave vector of the signal satisfy eq. 3.1. For the experiments described in this thesis, a special beam geometry known as folded-BOXCARS geometry [38, 39] is used in order to achieve phase matching. The main advantage of this conﬁguration is that the signal can be spatially separated from the other beams and can be detected without any background. Figure 3.3 shows arrangement of beams in the folded-BOXCARS geometry for the CARS experiment.

In this arrangement, the three beams involved in the CARS process, pump (~kpu), probe

(~kpr) and Stokes (~ks) are aligned parallel to each other and are made to pass through three corners of a rectangular box with the help of a phase mask (shown in panel A of ﬁg. 3.3). The beams are then focused on to the sample using a lens. The interaction volume is deﬁned by the overlapping region of the three pulses. The direction of the 3.5 Phase Matching in Four-Wave Mixing 27

kS kS kaS

kpu kpr kpu kpr

(A)PhaseMask (B)WaveVectorDiagram

k Pin-hole aS

Lens

kpu k S kaS Lens k Sample pr

kpr

kpr kS

kS kpu k pu (C)BOXCARSbeamgoemetry:SideView

Figure 3.3: Folded-BOXCARS arrangement of beams in FWM spectroscopy. (A) Phase mask used to arrange the beams in the folded-BOXCARS geometry. The three beams involved in the FWM process pass through the holes parallel to each other. (B) Wave vector diagram illustrating the phase matching in folded-BOXCARS arrangement. The wave vectors satisfy the relation ~kpu −~kS +~kpr −~kaS = 0. (C) Side view of the beam arrangement in folded-BOXCARS geometry

CARS signal generated can be determined from the wave vector diagram shown in Panel B. The phase matching condition for the CARS process is given by

~kpu −~kS +~kpr = ~kaS (3.2)

The The signal, in this case, is directed along the fourth corner of the box, well separated from the three input beams. A second lens is used to collimate the output beams and a second phase mask can be used to separate the signal from the other beams and can be detected without any background. This conﬁguration is particularly useful for 28 Experimental Setup

DFWM experiments where all the lasers involved in the process have the same wavelength. The wave vector diagram corresponding to the DFWM case is similar to the one shown in panel B except that now all the beams have the same wavelength. The phase matching condition in the case of DFWM can be written as

kDFWM = kpu1 − kpu2 + kpr (3.3)

For pump-FWM experiments described in chapters 4 and 5, an additional pump pulse is introduced along with the FWM beams. The central hole of the phase mask is used for the additional pump beam in such cases. 4 Excited State Dynamics in Molecular Iodine

Femtosecond time resolved four-wave mixing techniques, in the form of degenerate four-wave mixing (DFWM), coherent anti-Stokes Raman scattering (CARS), transient grating (TG), photon echo (PE), etc. has been used by a number of groups for the investigation of different aspects of molecular dynamics in different systems [13–16, 19, 21]. Four wave mixing in gas phase also has been a subject of intense investigation. First observation of molecular rotation using a third order nonlinear process was achieved by Heritage et. al. [40]. Feyer et. al. used transient grating techniques extensively to investigate gas phase dynamics [41, 42]. Materny and co-workers applied time resolved DFWM and CARS to investigate iodine vapour. This chapter presents the application of DFWM along with an additional pump pulse in order to probe vibrational dynamics occurring in the excited electronic states of a simple diatomic molecular system - iodine. In the next chapter, the technique is extended to complex molecules. The applicability of pump-four-wave mixing (pump-FWM) in order to gain information about the dynamics occurring in the excited states of gas phase systems was demonstrated by Motzkus et. al. [16]. They showed that degenerate four-wave mixing using femtosecond pulses with an additional pump pulse can be incorporated in different temporal pulse schemes to study transition state dynamics of chemical reactions in gas phase. Using a pump-CARS experiment, Siebert et. al. achieved a mode selective monitoring of vibrational dynamics occurring in the ground electronic state of β-carotene. Molecular dynamics in iodine in the gas phase has been extensively investigated in the group of Kiefer. Schmitt et. al. used electronically resonant CARS and DFWM to study the vibrational dynamics of molecular iodine in the gas phase [14,43]. By varying the time delay of one of the incident pulses while keeping the other two incident pulses time coincident they showed that vibrational and rotational dynamics can be studied for both the ground and excited electronic states.

29 30 Excited State Dynamics in Molecular Iodine

The experiments presented in this chapter demonstrate the effectiveness of the pump-DFWM experiment in probing molecular dynamics occurring in the higher lying ion-pair states of iodine. Previously, multiphoton excitation and a pump-probe technique using vacuum UV pulses were used to monitor ion pair state dynamics [44, 45]. However, in these studies only states of odd symmetry could be accessed from the ground state. Since the ground state of iodine is of even parity, only excited states of odd parity can be accessed from the ground state due to selection rules. Here, this problem is overcome by using a pump-DFWM experiment. Furthermore, by varying the time delay between pulses, dynamics occurring in different states can be accessed. The experimental scheme of pump-DFWM is similar to that of a pump-probe scheme, in which the probe laser is replaced by a DFWM process. For the investigations of iodine, an initial pump is used to excite the molecular system from the ground (X) state to the B state. The subsequent dynamics is probed by a DFWM process, which is resonant with the B to ion-pair state. A brief description of the electronic states of iodine is presented in the following.

4.1 Molecular states of Iodine

1 + 3 + The spectroscopic properties of the ground X ∑g and excited B Π0u states of iodine has been studied extensively both experimentally and theoretically [46–49]. Most of the time resolved investigations were focused on these two states of iodine. The aim of this work is to to gain information about the ion pair states lying above the B state. Iodine has 20 ion pair states dissociating in to I+ and I− ions and 23 valence states dissociating into neutral atoms [46]. The ion-pair states are divided into 3 clusters of − 1 + 3 six states each and a g − u pair. The ﬁrst group, which dissociate into I ( S) + I ( P2) ions, lies around 40000 cm−1 above the ground state. The states in the second group − 1 + 3 3 are characterized by their dissociation into I ( S) + I ( P1, P0) ions and lies at around 47000 cm−1 above the X state and the third group dissociates into I−(1S) + I+(1D) ions and lies at around 52,000 cm−1. The potential energy diagram showing the energy positions of these states is shown in ﬁg. 4.1. The transitions between different states are governed by selection rules and their strength by the Franck-Condon factors. The 4.1 Molecular states of Iodine 31

E 50

b(1g )

3 -1

10 cm )

´ 30

20 B

Potentialenergy(

10 X

0 2 3 4 5 6 o Internucleardistance(A )

Figure 4.1: Potential energy curves for the electronic states of iodine relevant for the pump-DFWM experiment.

allowed transition for a one photon process is given by

g ⇔ u (4.1a) ∆Ω = 0, ±1 (4.1b) 0+ ⇔ 0+ , 0− ⇔ 0− (4.1c)

+ The ion-pair states relevant for the present study are β(1g) and E(0g ). These two states have been characterized in the frequency domain by two photon absorption and double resonance techniques [50, 51]. 32 Excited State Dynamics in Molecular Iodine

4.2 Experimental

The general aspects of the experimental setup are described in chapter 3. The folded- BOXCARS beam geometry used for phase matching is especially useful for gas phase samples where the sample density is low. The exact phase matching given by the folded-BOXCARS beam geometry gives a stronger signal even when the sample density is low and the signal is spatially separated from the input beams as discussed in section 3.5. The scheme of the pump-DFWM experiment is shown in ﬁg. 4.2.

+ E(0g )

b(1g ) Dt

wDFWM

DFWM

B DT IP

Figure 4.2: Experimental scheme of pump-DFWM in iodine. IP denotes the initial pump pulse. ∆T denotes the time delay between the initial pump and the DFWM process and ∆t is the time delay within the DFWM process.

Here, an initial pump is used to excite the molecular system to the B state. The wavelength of the initial pump is chosen to be 600 nm which is resonant with the X → B transition in iodine. The wavelength of the DFWM pulses is varied between 380 nm and 410 nm, which is resonant with the ion-pair states belonging to the first tier. The sample for investigation, iodine is taken in a cuvette, which is heated to around 60◦C. In the experimental configuration there are two time windows that can be varied in order to investigate the molecular dynamics in different electronic states. By choosing DFWM pulses to be time coincident and by varying the time delay ∆T between the initial pump and the DFWM process, dynamics of the B state can be accessed. In the other case, two pump pulses of the DFWM are chosen to be time coincident. The time delay ∆T between the initial pump and the time coincident DFWM pulses is fixed and the timing of the third DFWM pulse is varied in order to investigate the ion-pair state 4.3 Pump-Degenerate Four-Wave Mixing in Iodine 33 dynamics. The results of these two scenarios are discussed in detail in the following section.

4.3 Pump-Degenerate Four-Wave Mixing in Iodine

The pump-DFWM technique is similar to be pump-probe technique, where the probe pulse is replaced by a DFWM process. The excitation of population to the B state by the initial pump (IP) pulse creates a vibrational wave packet in the B state. The evolution of this wave packet is then probed by the DFWM process. Different time ordering of the pulses in the pump-DFWM process gives access to dynamics occuring in different molecular states. In the first case, all the DFWM pulses are temporally overlapped and the time delay ∆T between the initial pump and the DFWM process is varied. Figure 4.3. shows the DFWM transient as a function of the delay time ∆T. For negative time delays (∆T<0), the DFWM pulses precede the IP pulse. Thus, there is no excited state population seen by the DFWM pulses. Since the DFWM pulses are not in resonance with any transition from the ground state, the signal in this case is a weak non-resonant DFWM signal from the ground state. Hence, for the case when ∆T < 0 only a constant non-resonant DFWM signal from the ground state is observed. For positive delay times, the DFWM pulses arrive after the IP pulse. The IP pulse, being resonant with the X → B transition, creates a wave packet, which evolves in time in the excited B state potential surface. The time evolution of this wave packet is probed by the time coincident DFWM pulses. Since the DFWM pulses are resonant with the transition between B and ion-pair states, the DFWM signal will be resonantly enhanced and a strong signal can be observed. The transient shows an oscillatory behavior with a period of 318 fs, which corresponds to the period of the wave packet oscillation in the B state. The fast Fourier transform (FFT) of the transient is shown in fig. 4.3b. The FFT shows sharp peaks around 105 cm−1, which corresponds to the spacing between vibrational levels in the B state excited by the 600 nm initial pump pulse. The second harmonic corresponding to ∆ν=2 also can be seen in the FFT peak- ing around 209 cm−1. Thus, varying the time delay between the initial pump pulse and time coincident DFWM pulses, the spacing between vibrational levels accessed by the 600 nm initial pump pulse can be determined. In order to access the dynamics occurring in the ion-pair states, two of the three DFWM pulses (hereafter referred to as the pump pulses) were kept temporally overlapped and the time delay, ∆T, between the IP pulse and the time coincident pump pulses is fixed. The delay time is fixed at 42 ps so that the pump pulses of the DFWM 34 Excited State Dynamics in Molecular Iodine

Intensity

0 10 20 30 40

Delay time, T / ps

(a)

105.8

103.7 FFT

209

0 100 200 300 400

-1

Wavenumber / cm (b)

Figure 4.3: (a) DFWM sigal recorded as a function of the delay time ∆T between the initial pump and the time coincident DFWM pulses. (b) shows the corresponding FFT spectrum of the transient in (a). The peaks aroud 105 cm−1 correspond to the spacing of the vibrational levels in the B state accessed by the 600 nm pump pulse

process arrive at the sample 42 ps after the IP pulse. The time delay, ∆t between the third DFWM pulse (hereafter referred to as the probe pulse) and the time coincident pump pulses is varied. Figure 4.4 shows the transient recorded as a function of delay time ∆t. Figure 4.4a shows the DFWM transient recorded for negative delay times (∆t<0). This corresponds to the case when the probe pulse precedes the pump pulses of the DFWM process. The transient shows an oscillatory behaviour arising from the beating between different vibrational modes. The FFT of the transient shown in ﬁg. 4.4b shows new peaks at 98.9 and 197 cm−1 apart from the peaks at 105 and 209 cm−1 observed in the previous case. The appearance of the new peaks can be explained as follows: The experimental scenario for the case when ∆t<0 is depicted in ﬁg. 4.5. 4.3 Pump-Degenerate Four-Wave Mixing in Iodine 35

t<0 98.9 t<0

105

FFT Intensity

197 209

0 100 200 300 400

-40 -30 -20 -10 0

-1

Delay time, t / ps Wavenumber / cm (a) (b)

t>0 t>0 98.4

FFT

105 Intensity

197

0 100 200 300 400 0 10 20 30 40

-1

Delay time, t / ps Wavenumber / cm (c) (d)

Figure 4.4: DFWM transient for IP wavelength 600 nm and DFWM wavelength 400 nm. (a) and (c): DFWM signal recorded as a function of the delay time ∆t between the DFWM pump pulses and the DFWM probe pulse for negative and positive delay times, respectively. (b) and (d): FFT spectrum corresponding to the transients in (a) and (c). The peaks around 98 cm−1 correspond to the spacing of the vibrational levels in the ion-pair state accessed by the 400 nm DFWM pulse.

Here, the DFWM probe pulse, being resonant with the transition between B and ion- pair states, creates a wave packet in the ion-pair state. This situation is shown in the energy ladder diagram ﬁg. 4.5(b) and its corresponding Feynman diagram 4.5(d). From the Feynman diagram it can be seen that the interaction of the pulses proceeds through an intermediate population in the ion-pair state, which is indicated in red. The evolution of this population in the ion-pair states is then probed by the following pulse giving rise to the DFWM signal. Thus, the new peaks at 98.9 and 197 cm−1 arise from the ion pair states and the line at 98.9 cm−1 corresponds to the spacing between the vibrational levels in the ion-pair state accessed by the 400 nm pulse from the B state. The peak at 197 cm−1 is the second harmonic of this frequency. 36 Excited State Dynamics in Molecular Iodine

Ion-pairstate Ion-pairstate

Dt Dt

DFWM DFWM

B State B State (a) (b)

BB BB IB IB BB II Dt IB IB BB BB

Figure 4.5: Pictorial representation of the processes responsible for the DFWM signal for the case ∆t<0. The Feyman diagram can be understood in the same manner as described in chapter 2. The connected dotted lines in (c) and (d) indicate that those two pulses are in fact time coincident. They are shown separated for better understanding of the diagrams.

The energy ladder diagram fig. 4.5(a) and its corresponding Feynman diagram 4.5(a) shows another contribution to the DFWM signal for the case ∆t<0. Here, the interaction of the pulses creates an intermediate population in the B state, which is indicated in red. This population is then probed giving rise to the DFWM signal. However, the population in the B state is not a static population. The IP pulse creates a wave packet, which is evolving in the B state and this transient population is resulting in the signal. Hence, the transient nature of the B state population can also be observed in the transient, which corresponds to be peaks at 105 and 209 cm−1 in the FFT. Figure 4.4c shows the DFWM transient recorded for positive delay times (∆t>0). This corresponds to the case when the probe pulse succeeds the pump pulses of the DFWM process. As in the previous case, the transient shows an oscillatory behaviour arising from the beating between different vibrational modes. The FFT of the transient shown in fig. 4.4d shows contributions from both the B state and ion-pair state vibrational dynamics. As in the previous case, the contributions to the DFWM signal for ∆t>0 can be analyzed using Feynman diagrams shown in fig 4.6. From diagrams 4.3 Pump-Degenerate Four-Wave Mixing in Iodine 37

Ion-pairstate

Dt Dt Dt

B State (a) (b) (c)

BB BB BB IB IB IB

BB Dt II Dt BB

IB IB BI BB BB BB (d) (e) (f)

Figure 4.6: Pictorial representation of the processes responsible for the DFWM signal for the case ∆t>0.

4.6(a) and 4.6(c) and their corresponding Feynman diagrams 4.6(d) and 4.6(f) it can be seen that the interaction of the first two pulses creates a B state population, which is indicated in red. The third pulse probes this B state population resulting in the DFWM signal. In diagram 4.6(b) and its corresponding Feynman diagram 4.6(e) the interaction of the first two pulses results in a population in the ion-pair state. The third pulse stimulates emission from the ion-pair state, which is the DFWM signal. Thus, here also contribution from both B and ion-pair states can be observed in the DFWM signal. By varying the wavelength of the DFWM pulses, different vibrational levels in the ion-pair state can be accessed. Figure 4.7 shows the DFWM transients obtained for a DFWM wavelength 380 nm. Since the energy of the DFWM pulse is now higher than in the previous case, this pulse can access higher vibrational levels of the ion-pair state. The transients for ∆t<0 and ∆t>0 are shown in fig. 4.7 along with the corresponding FFT specta. It can be seen from the FFT that the vibrational spacing in the ion-pair state for ∆t<0 is now 93 cm−1 and 92.5 cm−1 for ∆t>0. This is less than the vibrational spacing obtained when the DFWM wavelength was 400 nm. In this case, the higher 38 Excited State Dynamics in Molecular Iodine energy DFWM pulse excites higher vibrational levels of the ion-pair states. Due to the anharmonicity of the potential, the spacing between vibrational levels decreases for higher vibrational levels. This reduction is reflected in the FFT of the transients. The B state vibrational contribution can also be seen in the FFT.

t<0 t<0

104

107

FFT Intensity

213 185

0 100 200 300 400

-40 -30 -20 -10 0

-1

Delay time, t / ps Wavenumber / cm (a) (b)

t>0 t>0

FFT Intensity

186

105

0 10 20 30 40 0 100 200 300 400

-1

Delay time, t / ps

Wavenumber / cm (c) (d)

Figure 4.7: DFWM transient for an IP wavelength 600 nm and a DFWM wavelength 380 nm. (a) and (c): DFWM signal recorded as a function of the delay time ∆t between the DFWM pump pulses and the DFWM probe pulse for negative and positive delay times, respectively. (b) and (d): FFT spectrum corresponding to the transients in (a) and (c). The peaks around 93 cm−1 correspond to the spacing of the vibrational levels in the ion-pair state accessed by the 380 nm DFWM pulse.

Figure 4.8 shows the transients and their corresponding FFTs when the DFWM wavelength is tuned to 410 nm. Here, the DFWM pulses are of lower energy than in the previous cases and hence excite low lying vibrational levels compared to the previous two cases. Since the spacing between vibrational levels is larger for low lying vibrational levels, the FFT of the transient shows larger vibrational spacings for the ion-pair state. 4.4 Summary 39

t<0

100

104

FFT Intensity

200

0 100 200 300 400 -40 -30 -20 -10 0

-1

Delay time, t / ps Wavenumber / cm (a) (b)

t>0 t>0

100

FFT Intensity

107

200

0 100 200 300 400 0 10 20 30 40

-1

Delay time, t / ps Wavenumber / cm (c) (d)

Figure 4.8: DFWM transient for an IP wavelength 600 nm and a DFWM wavelength 410 nm. (a) and (c): DFWM signal recorded as a function of the delay time ∆t between the DFWM pump pulses and the DFWM probe pulse for negative and positive delay times, respectively. (b) and (d): FFT spectrum corresponding to the transients in (a) and (c). The peaks around 100 cm−1 correspond to the spacing of the vibrational levels in the ion-pair state accessed by the 410 nm DFWM pulse.

4.4 Summary

The experiments presented in this chapter demonstrate the potential of the four wave mixing technique with an additional pump pulse in retrieving dynamical information about the excited states of molecular iodine. Since the four wave mixing techniques does not rely on ﬂuorescence, they can be applied to investigate non-ﬂuorescing excited states also. The coherent and well directed signal from this third order nonlinear process allows for background free detection of the signal. Also, by varying the time delay between various pulses involved in the process, dynamical information from different excited states can be obtained. In the next chapter, the pump-four wave 40 Excited State Dynamics in Molecular Iodine mixing is extended to investigate the excited state dynamics of the complex molecule β-carotene. 5 Excited State Dynamics in β-Carotene

5.1 Introduction

The previous chapter demonstrated the advantages of using four-wave mixing with an additional pump pulse in obtaining information about the excited state dynamical properties of the simple system iodine. In this chapter, the technique is extended to a complex molecule, β-Carotene. The molecular dynamics in β-carotene are a subject of intense investigation in ultrafast spectroscopy due to the important role played by these molecules in plant photosynthesis. In photosynthesis, plants harvest the sunlight and use its energy to drive their metabolic reaction [52]. In a photosynthetic system, carotenoids along with chlorophylls are the important pigments responsible for the process of photosynthesis. Carotenoids absorb sunlight in the visible region of the spectrum that is not covered by the chlorophylls. The excitation energy is then transferred to chlorophylls in a femtosecond time scale with very high efficiency. Early experiments done in carotenoid photophysics established the involvement of carotenoid excited states in the energy transfer from carotenoids to chlorophylls [53]. Most of the recent studies on the excited state dynamics of carotenoids are aimed at understanding the molecular properties of carotenoids in this high efficiency ultrafast energy transfer process. Also, the discovery that the absorbing state of carotenoids is not the lowest excited state [54] sparked much interest in the study of carotenoid excited states. Until the advent of ultrashort lasers, studies on excited state dynamics of carotenoids were limited due to the lack of spectroscopic techniques to follow fast processes. In the late 1980s Wasielewski et.al. [55] measured, for the first time, the lifetime of the first excited singlet state of β-carotene and other carotenoids using a picosecond laser. Since then, a wealth of experiments have been done by several research groups us-

41 42 Excited State Dynamics in β-Carotene ing femtosecond/picosecond time resolved spectroscopic techniques in order to explore carotenoid excited state dynamics and energy transfer between carotenoids and chlorophylls [56–58]. The review article by Polívka and Sundström [29] summarizes the current knowledge of excited state dynamics of carotenoids. This chapter demonstrates the use of FWM spectroscopy for the investigation of molecular dynamics taking place in β-carotene after exciting the system with an initial pump (IP) pulse. The analysis of the results using a kinetic model gives time constants for different relaxation dynamics taking place after photo-excitation. The sensitivity and advantages of the pump-DFWM technique are then compared with that of simple pump-probe techniques.

5.2 β-Carotene Photophysics

The molecular structure of carotenoids, shown in fig. 5.1a, contains a linear chain of alternating single and double carbon bonds that form a conjugated π-electron system. This is responsible for most of the spectroscopic properties of carotenoids. The absorption spectrum and a simplified energy level scheme of β-carotene are shown in figs.

5.1b and 5.1c, respectively. The C2h symmetry of the conjugated chain of β-carotene gives rise to states with the following symmetries: the ground state of carotenoids is − − + of 1Ag symmetry and the S1 and S2 states are of symmetries 2Ag and 1Bu , respectively. According to selection rules, a one photon transition from S0 to the ﬁrst excited singlet state (S1) is forbidden [54]. The properties of the excited states of β-carotene are described in the following sections.

5.2.1 S2 State

The strong absorption of carotenoids in the blue-green region (fig. 5.1b) is characterized by the transtion from the ground state (S0) to the second excited singlet state (S2). This transition usually exhibits a characteristic three peak structure corresponding to the lowest three vibrational levels of the S2 state. Steady state absorption and fluorescence studies have located the energies of S1 and S2 states of carotenoids in various environments [59–61]. Making use of the relationship between absorption intensity and fluorescence life time, the radiative life time of the S2 state is calculated to be in the nanosecond range. But, the quantum yield measured for the emission from this state resulted in values in the sub-picosecond range [62, 63]. The population, after excitation to the S2 state, undergoes a fast internal conversion (IC) to the S1 state. Thus, 5.2 β-Carotene Photophysics 43

(a)

{S }

25 1 +

S (1 B )

2 u

* -

20 -1 S (1B )

u cm

1 -

S (2 A )

1 g

10 Absorbance (arb. units) (arb. Absorbance Energy /10

300 350 400 450 500 550 1 -

S (1 A )

0 g Wavelength (nm)

(b) (c)

Figure 5.1: (a) Structure of all-trans β-carotene.(b) Shows the absorption spectrum of β-carotene in the blue-green range. (c) shows the energy level scheme of the ﬁrst two excited states of β-carotene. The arrow indicates the transition corresponding to the strong absorption in the blue green region shown in (b)

a proper understanding of the S2 dynamics is possible only through ultrafast time resolved techniques. Ultrafast time resolved spectroscopic techniques such as ﬂuores- cence up-conversion and transient absorption applied to carotenoids revealed that the life time of the S2 state is in the range of 100-300 fs and depends on both conjugation length and solvent properties [59, 63].

5.2.2 S1 State

Though the S1 state is the lowest excited state of β-carotene, a one photon transition between the S0 state and S1 is symmetry forbidden and hence this state is commonly called a dark state. The S1 state is populated through internal conversion from the S2 state. Due to the forbidden nature of the S0 → S1 transition, it took a long time to determine the energy of this state, which required progress in experimental techniques. 44 Excited State Dynamics in β-Carotene

Developments in ﬂuorescence techniques helped the determination of the S1 state energy [64]. Later, the resonance Raman technique was used by Sashima et. al. for the determination of the S1 energy of β-carotene [60]. A number of experimental techniques has been used by several groups for the determination of S1 lifetime. Wasielewski et.al. [55] determined, for the ﬁrst time, the life time of the optically inaccessible S1 state using picosecond transient absorption spectroscopy. The technique was then extended to study the dependence of conjugation length and solvent properties on the life time of the S1 state. The results suggested that, for carotenoids, the S1 life time varied from 282 ps to 9 ps with increase in conjugation length [65]. In most carotenoids, the S1 life time was seen to be not dependent on the solvent properties [29, 66]. For β-carotene, the S1 life time is determined to be in the range 9-11 ps.

5.2.3 Vibrational Relaxation

Due to the ultrafast nature of the carotenoid excited states, studies on the vibrational relaxation within electronic states were limited until the development of femtosecond time resolved Raman techniques. Even though the S2 state is the easily accessible state in carotenoids, vibrational relaxation in this state is less understood than the other states due its extremely fast vibrational relaxation. On the other hand, study of vibrational relaxation in the S1 state is more easier due to a strongly allowed absorption from the S1 state to a higher lying state SN. Using a time resolved transient grating technique, Siebert et. al. showed that the vibrational cooling in the S1 state of β-carotene takes place in 700 fs [67]. Time resolved Raman spectroscopy was used by

McCamant et. al. to study the vibrational relaxation in the S1 state of β-carotene [68]. The results obtained from the different time-resolved techniques described in the previous sections have helped to come to a rather detailed understanding of the complex molecular processes after light excitation. In short, the complete picture of β- carotene photodynamics can be summarized as follows: after excitation in to the S2 state, the population quickly decays, by internal conversion (IC), in to the higher vibrational states of the low lying S1 state within 150 ± 50 fs [29]. Vibrational relaxation in the S1 state takes place on a 600 fs time scale [69]. From the S1 state, the population decays back into the ground state in about 8 ± 1 ps [29]. The participation of the 1 − 1 + 1 − dark 1 Bu state in the internal conversion (IC) from 1 Bu state to 2 Ag state has been debated recently. The experiments performed by Cerullo [5] and Yoshizawa [69, 70] 1 − point the IC to occur in a two-stage pathway via the 1 Bu state. But investigations by 5.3 pump-Four Wave Mixing (pump-FWM) on β-Carotene 45

Wohlleben [71], Kukura [72] and Lustres [73] discount this fact.

5.3 pump-Four Wave Mixing (pump-FWM) on β-Carotene

In order to contribute to the already known facts about β-carotene photodynamics, we have recently performed further experiments using the pump-FWM technique. The powerful nature of the pump-FWM technique has been demonstrated in many experiments. Motzkus et al. replaced the probe in a pump-probe scheme by a degenerate FWM (DFWM) process without internal temporal resolution and applied this technique to unimolecular and bimolecular systems in gas phase [16]. They demonstrated the method’s advantages in high sensitivity and background-free detection particularly suitable for states that do not allow for fluorescence detection. Using femtosecond time resolved four wave mixing, Dietzek et. al. achieved a detailed mapping of the relaxation dynamics within the excited state manifold of magnesium octaethylpor- phyrin [17, 18]. Scaria et. al. demonstrated the use of pump-DFWM for investigating molecular dynamics in higher electronic states of gas phase samples [74, 75]. A mode selective monitoring of the population dynamics was achieved by Siebert et al. by using coherent anti-Stokes Raman scattering (CARS) as a probe in a pump probe scheme [76]. Here, the behavior of the different vibrational modes during IC is monitored by tuning the stimulated Raman resonance of the CARS probe to the frequency of these modes. Using pump-DFWM spectroscopy Hornung et al. studied the wave packet dynamics occurring on the ground S0 as well as the excited S1 states of β- carotene with 16 fs time resolution [77]. These authors made use of the broad spectral bandwidth associated with the short pulses for monitoring the vibrational dynamics. However, no information on the population dynamics (IC, VR) were presented. This chapter demonstrates the use of FWM spectroscopy for the investigation of the dynamics taking place in β-carotene after excitation with an IP pulse. The sensitivity of the FWM technique to the population dynamics in the system of interest makes it a valuable tool to explore the dynamics even in this complex system. A combination of an IP pulse and the DFWM technique was used to follow the population flow in β-carotene. Based on the results of this experiment, a model for the population flow in β-carotene is deduced, which fits well to the already known model. The results are compared with a simple and commonly used two pulse pump-probe experiment and the advantages and disadvantages of both the techniques are compared. Further more, experiments were also performed to monitor the vibrational dynamics of the ground and excited states of β-carotene using a pump-CARS scheme. Here, the wavelength 46 Excited State Dynamics in β-Carotene difference between the pump and Stokes pulse (stimulated Raman resonance) in the CARS scheme was tuned to about 1500 cm−1 to excite different Raman modes in the region.

5.4 Experimental Setup for pump-FWM

The general aspects of the experimental setup used for four-wave experiments is described in chapter 3. The scheme employed for pump-DFWM and pump-CARS experiments is shown in ﬁg. 5.4 For the pump-DFWM experiment, the output of one of the

S2 FWM

Dt S1

InitialPump S0

Figure 5.2: Experimental methodology employed for the investigation of molecular dynamics in β- carotene. An initial pump laser excites the molecular system. The subsequent processes are probed by an FWM process resonant with the S1–Sn transition. For pump-DFWM all pulses in the FWM process have the same frequency and for pump-CARS, the frequency difference between pump and Stokes lasers is tuned to 1500 cm−1.

OPAs served as the IP pulse. The output of the other OPA was split into three equal parts, which formed the three FWM beams. For the experiments, the wavelength of the IP was chosen to be 470 nm, which is in resonance with the electronic absorption transition between ground state S0 and excited state S2. The wavelength of the DFWM pulses (probe) was chosen to be 570 nm. This is in resonance with the transition from the excited S1 state to a higher lying state, Sn (known from the transient absorption 5.5 Molecular Dynamics Using pump-DFWM 47

of the S1 state) [5, 71] and at the same time comes close to the resonance conditions for a two-photon transition from the ground state S0 to the Sn0 state (known from the transient absorption of the S2 state) [5, 78]. The temporal resolution in this scheme of measurement was determined to be 160 fs by cross-correlation between IP and FWM pulses. A slightly different arrangement was used for the pump-CARS experiment. Here, in order to generate three different wavelengths, the output of the CPA was split up to pump three optical parametric ampliﬁers (one TOPAS and two non-collinear optical parametric ampliﬁers (NOPAs)). The output pulses of the OPAs were then compressed in double-pass two-prism arrangements. After compression the pulses were 20–30 fs long for the NOPA output while the output pulses of TOPAS had temporal widths of 70–80 fs. The output of one of the NOPAs was equally split giving rise to the pump and the probe pulses of the CARS process. The output of the TOPAS served as Stokes pulse. The other NOPA generated the IP pulse. For the CARS process, the wavenumber difference between the pump and Stokes pulses was set to 1500 cm−1.

The wavelength of the IP pulse was chosen to be resonant with the S0-to-S2 transition, while the exciting lasers of the CARS process were in electronic resonance with the

S1-to-Sn absorption. The sample for the investigation, all-trans-β-carotene, was obtained from Aldrich and was used as received. The solvents benzene, n-hexane, and toluene were used without further puriﬁcation. For all the measurements neat and pure solvents were used. The β-carotene solution was contained in a 1 mm cuvette. No sample degrada- tion was observed for the pulse energies used in these experiments.

5.5 Molecular Dynamics Using pump-DFWM

The pump-DFWM technique is used to follow the relaxation channels for the different molecular states of β-carotene after excitation by the pump pulse to the S2 state. The technique is similar to a pump-probe scheme where the probe pulse is replaced by a DFWM process. Here, the DFWM pulses where time coincident and the transients are recorded by varying the time delay between the initial pump (IP) pulse and the DFWM pulses. Figure 5.3 shows two transients recorded for β-carotene dissolved in benzene to an optical density (OD) of 0.30–0.35 at an IP wavelength of 470 nm. The energy of the IP pulse was 50 nJ. The sharp peak observed at ∆T = 0 for the transient in ﬁg. 5.3a is due to a "coherent artifact", which is assigned to a non-resonant contribution. When all lasers overlap temporally, a variety of signal spots could be seen arising 48 Excited State Dynamics in β-Carotene from different nonlinear processes, which due to their intensity and short lifetime (they vanished for ∆T 6= 0) can be assigned to non-resonant processes in the solvent. Due to their intensity and similar phase matching geometry, such contributions also were leaking through the detection pin hole causing the spike-like signal. For strong resonant signals due to the higher population in the S1 state, this spike-like signal was not observed any more (see ﬁg. 5.3b). Neglecting this artifact, at ∆T = 0 a fast rise is observed followed by an exponential decay. This exponential decay is seen to decrease to a value below the initial signal level for delay times in the range 18–20 ps. Finally, the signal recovers to the original level for later delay times and thereafter remains constant. In the following, the behavior of the transient with the delay time ∆T is discussed in detail.

Intensity (arb. units) Intensity (arb. units)

0 20 40 60 80 0 20 40 60 80

Delay Time, T (ps) Delay Time, T (ps) (a) (b)

Figure 5.3: DFWM signal recorded as a function of the delay time, ∆T between the initial pump and the DFWM beams for β-carotene dissolved in benzene. The sample had an optical density of 0.3–0.35 at 470 nm. The energy of the IP pulse was 50 nJ. Panels (a) and (b) demonstrate the effect of a change of the initially excited S2 population. The transient shown in panel (a) was obtained with lower IP power. The spike-like signal at time zero is due to the non-resonant coherent artifact. This signal contribution is covered by the stronger resonant signal in panel (b).

The energy level scheme in ﬁg. 5.4 shows the different contributions to the pump- DFWM signal. For the case ∆T < 0, the DFWM pulses arrive before the IP. The DFWM wavelength (570 nm) is not in resonance with the ground state absorption, but is close to resonance with a two-photon transition to the Sn0 state. Thus, the signal observed is a combination of the electronically non-resonant DFWM signal from the ground state and a two-photon resonant signal between the ground and Sn0 state. At ∆T = 0 the IP pulse arrives and takes a fraction of the ground state population into the S2 state where 5.5 Molecular Dynamics Using pump-DFWM 49

it undergoes a fast internal conversion (IC) to the S1 state. The S1 state of β-carotene

DFWM

IC DFWM

Figure 5.4: Energy level diagram of β-carotene showing different signal contributions to the pump- DFWM signal. 2P denotes the two-photon resonant contribution, NR-DFWM is the non-resonant signal contribution and DFWM is the resonant contribution. See text for details.

possesses a broad absorption peak at around 560 nm depending on the solvent. Since the DFWM laser pulses are in resonance with the transient S1 state absorption to the higher lying Sn state, resonance enhancement results in an intense signal from the S1 state. This can be seen in the transient as a rise of signal intensity after time zero. Since the IP takes a fraction of population away from the ground state, there will be a decrease in the DFWM signal contribution from the ground state caused by the bleaching of the ground state population. Therefore, the signal observed for ∆T > 0 is a combination of the resonant excited state signal (dominating for small T) and the reduced ground state signal. The signal then decays as the population in the S1 state is transferred back to the electronic ground state through another IC process. The interesting part of the transient is seen at delay times within the range 18–20 fs. During this time the signal is seen to go below its starting level (∆T < 0) and then rises back to its original level. This can be explained as follows. As mentioned above, the ground state signal for delay times ∆T < 0 consists of two components. One is the non-resonant DFWM signal from the ground state and the other is the two-photon resonant signal between the ground and Sn0 state. Since the two photon process is a nearly resonant process of the same order as the standard DFWM process having the 50 Excited State Dynamics in β-Carotene same phase matching conditions, its contribution to the overall ground state signal will be more than that of the non-resonant DFWM process. After photoexcitation, the relaxation from the S1 state takes population back to the ground state, but to higher vibrational states of the ground state from where neither single nor two-photon resonance conditions to a higher electronic state are fulfilled. Hence, the signal is only due to a weaker non-resonant DFWM process. This results in the signal intensity going below the starting level (no more signal from the S1 state and not enough signal from the bleached S0 state). The overall signal is seen to recover as the population in the hot vibrational levels of the ground state relaxes. Therefore, an analysis of the overall transient will give access to the IC times between the different singlet states as ∗ well as the population relaxation in the hot S0 state. Similar experiments were performed for β-carotene dissolved in toluene and n- hexane. Nearly the same transient was observed for toluene (fig. 5.5a). The transients recorded for β-carotene dissolved in n-hexane to an OD of 0.4–0.45 showed a slightly different picture. This is shown in fig. 5.5b. The IP pulse in this experiment possessed

0 20 40 60 80 Intensity (arb. units) (arb. Intensity units) (arb. Intensity

0 20 40 60 80 0 20 40 60 80

Delay Time, (ps) Delay Time, (ps) (a) (b)

Figure 5.5: (a): DFWM signal recorded as a function of the delay time, ∆T between the initial pump and the DFWM beams for β-carotene dissolved in toluene. (b): DFWM signal recorded as a function of the delay time, ∆T between the initial pump and the DFWM beams for β-carotene dissolved in n-hexane. The inset is a magniﬁed version to clearly show the low intensity part of the transient.

an energy of 50 nJ. From the transient it can be seen that the downward going signal in hexane is not so pronounced as in the case of benzene. The inset in Fig. 5.5b shows a more detailed view of the transient to have a clear picture of the downward signal. From the inset it is clear that the dip is also present for transients in n-hexane but not as 5.5 Molecular Dynamics Using pump-DFWM 51 strong as in benzene. However, the dip observed in toluene has a similar nature as in benzene transient. This difference could be ascribed to the different solvent properties.

5.5.1 Kinetic Analysis

The pump-DFWM transients were analyzed by ﬁtting them to a ﬁrst order kinetic model assuming the relaxation scheme (rig. 5.6)

hν k21=1/τ21 k10=1/τ10 ∗ k00=1/τ00 S0 −→ S2 −−−−−→ S1 −−−−−→ S0 −−−−−→ S0 (5.1)

where k21 and k10 are the S2 → S1 and S1 → S0 internal conversion rates and k00 is the

Figure 5.6: Relaxation scheme assumed for developing the kinetic model. rate of vibrational relaxation in the ground state. The time dependent populations in different states are obtained by solving the rate equations assuming N0(t) + N1(t) + ∗ N2(t) + N0 (t) = 1 and instantaneous population of the S2 state by the initial pump pulse, N0(0) = 1 − N2(0):

dN 2 = −k .N (5.2) dt 21 2 dN 1 = k .N − k .N (5.3) dt 21 2 10 1 dN∗ 0 = k .N − k .N∗ (5.4) dt 10 1 00 0 dN 0 = k .N∗ (5.5) dt 00 0 52 Excited State Dynamics in β-Carotene

The time dependent populations in different states are

−k21t N2(t) = N2(0)e (5.6) k21 N (t) = N (0) (e−k21t − e−k10t) (5.7) 1 2 (k − k ) h 10 21 i ∗ −k21t −k10t −k00t N0 (t) = N2(0) a1e + a2e + a3e (5.8) h i −k21t −k10t −k00t N0(t) = N0(0) + N2(0) 1 − b1e − b2e − b3e (5.9) where

k21k10 a1 = (5.10) (k10 − k21)(k00 − k21) k21k10 a2 = (5.11) (k21 − k10)(k00 − k10) k21k10 a3 = b3 = (5.12) (k21 − k00)(k10 − k00) k10k00 b1 = (5.13) (k10 − k21)(k00 − k21) k21k00 b2 = (5.14) (k21 − k10)(k00 − k10)

The DFWM intensity is proportional to the square of the population. The time depen- dant pump-DFWM signal is calculated as

∗ 2 2 2 S(t) = S0(t) + (N0 + N0 ) + R1.N0 + R2.N1 (5.15)

where, R1 is the enhancement factor for the two photon process in the ground state and R2 is the enhancement factor for the DFWM process in the S1 state. The variable parameters in eq. 5.15 are N2(0), k21, k10, k00, R1 and R2. The transients obtained from the experiment were fit to eq. 5.15 to extract the parameters. Figure 5.7 shows the best fit to the transient shown in fig. 5.3b. The parameters obtained from fitting the experimentally obtained data are listed in table 5.1. The values are in excellent agreement with values already obtained using other experimental techniques [29].

5.5.2 Comparison between results from two-pulse pump-probe and pump- DFWM experiments

A well established and relatively simple method for studying excited state dynamics of molecular systems is the two-pulse pump-probe technique. The probe step can e.g. 5.5 Molecular Dynamics Using pump-DFWM 53

Experimental Transient

Kinetic Model Fit

Intensity (arb. units) (arb. Intensity

0 50 100 150 200

Delay Time, T (ps)

Figure 5.7: DFWM signal recorded as a function of the delay time, ∆T between the initial pump and the DFWM beams for β-carotene dissolved in benzene. The black curve is the experimental transient. The best ﬁt obtained using the kinetic model eq. 5.15 is shown in red.

Parameter Value Known value [29]

τ21 0.25 ps 0.1-0.3 ps τ10 8.9 ps 9-11 ps τ00 12.7 ps 8-15 ps R1 4.7 (rel. units) - R2 313 (rel. units) -

Table 5.1: Parameters obtained from ﬁtting the experimentally obtained pump-DFWM transient using eq. 5.15.

involve the excitation of fluorescence or it measures the transient absorption (mostly employing white light continua). Carotenoids have been extensively studied using the latter pump-probe approach. A detailed review of carotenoid excited states studied using different spectroscopic techniques is given in [29]. This section discussses the results obtained from pump-probe and pump-DFWM experiments performed on β- carotene in order to highlight some advantages of using a DFWM process as probe as well as to compare the information gained from the different investigations of the molecular dynamics. Due to the high-order nonlinearity of the DFWM probe process, a N2|µ|8 dependence of the DFWM signal intensity arises compared to the N|µ|2 dependence of 54 Excited State Dynamics in β-Carotene pump-probe signal. Here, N is the number density of the sample under investigation and µ is the transition dipole moment [16, 79]. Due to the higher-order nonlinear dependence on both N and µ, the DFWM process is sensitive to dynamical changes of majority species and transitions with high transition probability, respectively. In the present case, β-Carotene is known to have strong transition dipole moments for both the two-photon absorption of the electronic ground state and the transient absorption of the excited S1 state. Hence, the pump-DFWM process is probing dynamics with a high state selectivity masking contributions arising from weak transitions (e.g. transitions starting from hot vibrational states). This can be of advantage, since a contribution of too many processes to the resulting signal might make it impossible to filter out the important information. On the other hand, e.g. the observation of transient absorption will also have an intrinsic selectivity, which for the same probe wavelengths filters out contributions, which still can be seen in the DFWM probing. While absorption only occurs when a population transfer from one state to the other takes place, a DFWM signal can exist even without electronic resonance, i.e. prereso- nance conditions are sufficient to enhance the DFWM signals. In the present case this means that for the chosen probe wavelength exclusively transient absorption starting from the S1 state can be observed. Therefore, a combination of these techniques helps to obtain more reliable data than the use of only one method. In order to directly compare the pump-DFWM experiment discussed above with pump-transient absorption results, we have performed a two-pulse pump-probe experiment with the IP as pump laser and one of the DFWM pulses as probe. In this experiment, we have measured the absorption of the probe laser as a function of the delay time between pump and probe pulses. Figure 5.8 shows the pump-probe transient recorded for β-carotene dissolved in benzene (the sample used also for the pump- DFWM experiment). As indicated above, the wavelengths were the same as that used for the pump-DFWM experiment, i.e. the pump wavelength was 470 nm and the probe wavelength 570 nm. It is apparent from the two transients 5.3 and 5.8 that the pump- DFWM transient for this wavelengths combination provides more information than that of the pump-probe transient. Besides that, the quality of the DFWM signal is con- siderably higher compared to the transient absorption signal. This is due to the fact that for the transient absorption a small change of the relatively high probe laser intensity has to be measured while the DFWM signal is practically free of background. ∗ The pump-probe experiment does not see contributions by the hot (S0) and relaxed ground state (S0) but only the S1 state dynamics. The time constant extracted for the

S1-to-S0 IC is found to be approximately 9 ps, which is in agreement with the result 5.5 Molecular Dynamics Using pump-DFWM 55

Intensity (arb. units) (arb. Intensity

0 10 20 30 40 50

Delay Time (ps)

Figure 5.8: Pump-probe signal recorded as a function of the delay time, between the pump (470 nm) and the probe pulses (570 nm) for β-carotene dissolved in benzene. The black curve is the experimental transient absorption signal while the red curve is the exponential ﬁt used to obtain the S1-to-S0 internal conversion (IC) time constant.

obtained from the corresponding pump-DFWM experiment (see Table 5.1). 56 Excited State Dynamics in β-Carotene

5.6 Ground and Excited State Dynamics Using pump-CARS

The ground state Raman spectrum of β-carotene is characterized by peaks at 1522 cm−1 (C=C stretch), 1160 cm−1 (C-C stretch), and 1007 cm−1 (methyl rock). The transient

Raman spectrum of the excited S1 state was recorded by Hashimoto and Koyama [80]. −1 The S1 state possesses major lines at 1793 cm (central carbon double bond stretch), 1530 cm−1 (terminal carbon double bond stretch), and 1230 cm−1 (carbon single bond stretch). As mentioned before, Hornung et al. used pump-DFWM employing short pulses to observe the dynamics of the S0 and S1 states [77]. In the experiments discussed in the following, we are using a different technique namely the pump-CARS (coherent anti-Stokes Raman scattering) scheme to monitor the dynamics of these states. A pump-CARS scheme was employed in earlier experiments by Siebert et. al. [76] to study the behavior of different modes during internal conversion. However, in these studies the wavelength of the CARS probe was in resonance with the ground S0 state to S2 state transition. Therefore no S1 dynamics were observed in their experiment.

The dynamics of the S1 state is important on account of the role it plays in the energy transfer from the β-carotene to chlorophyll molecules in light harvesting systems. The CARS probe is tuned to approximately 1500 cm−1 so as to excite different vibrational modes in this region. The wavelengths of the lasers responsible for the CARS process are in resonance with the S1-to-Sn transition. In order to distinguish the contributions from the S0 and S1 states, the dynamics were recorded in the absence and presence of the IP. In the measurements performed using this scheme, the pump and Stokes pulses of the stimulated Raman excitation within the CARS process were kept temporally overlapped and fixed. The CARS probe pulse was varied relative to the pump/Stokes pulse pair. The experiments were performed to realize two scenarios. In the first case the dynamics were recorded in the absence of the IP. The transient was recorded as a function of delay time, ∆t between the probe pulse and the fixed pump/Stokes pair. This is shown in panel (a) of fig. 5.9a. Since the CARS process is not in resonance with the ground state absorption, the signal obtained is the non resonant CARS signal from the ground state. For ∆t < 0 no signal is observed. At ∆t = 0 a coherent spike is observed followed by modulations in the anti-Stokes signal for later times. The CARS signal decays in a few picoseconds. The peak at time zero due to the coherent artifact is not shown at full intensity in the plot in order to show the fine structure of the transient at later times more clearly. The broad spectral bandwidth of the laser pulses results in the excitation of different modes in the ground state of β-carotene. The 5.6 Ground and Excited State Dynamics Using pump-CARS 57 temporal structure seen on the CARS transients is due to the beating of the different modes. In order to analyze the data, a fast Fourier transform (FFT) of the transient was performed. The FFT is shown in panel (b) of fig. 5.9a. The FFT spectrum shows a line at 365 cm−1, which corresponds to the frequency difference between the ground state Raman lines at 1522 and 1160 cm−1. The transients in this case thus reveal the dynamics of the ground state of β-carotene.

(a) (a) Intensity Intensity

0.0 0.5 1.0 1.5 2.0 2.5 3.0 0.0 0.5 1.0 1.5 2.0 2.5 3.0

Delay Time, t / ps Delay Time, t / ps

365 365 (b) (b)

269

FFT FFT

0 100 200 300 400 500 600 700 0 100 200 300 400 500 600 700

-1 -1

Wavenumber / cm Wavenumber / cm (a) (b)

Figure 5.9: CARS transients obtained for the ground and excites states of β-carotene. (a) CARS signal recorded in the absence of the initial pump (ground state). Panel (a) shows the transient and panel (b) shows the corresponding FFT spectrum. The latter shows a peak at approximately 365 cm−1, which corresponds to the frequency difference between the ground state Raman lines at 1522 and 1160 cm−1. (b) CARS signal recorded in the presence of the initial pump (excited state). Panel (a) shows the transient and panel (b) shows the corresponding FFT spectrum. The FFT shows peaks at approximately 365 and −1 269 cm , which correspond to the beating between vibrational modes in the ground S0 and excited S1 states, respectively.

In the second scenario the transients were recorded in the presence of the IP. For the experiment, the IP was made to arrive 1.5 ps earlier at the sample relative to the pump/Stokes pair. The IP takes the population to the S2 state from where it undergoes a fast (of the order of 150±50 fs) IC to the S1 state. This makes sure that part of the population is in the S1 state. The dynamics were recorded as in the previous case by varying the probe from negative to positive time delays. The transient recorded is 58 Excited State Dynamics in β-Carotene shown in panel (a) of Fig. 5.9b. Here, it should be considered that the CARS process is in resonance with the S1-to-Sn transition. The FFT spectrum of the transient for positive delay times is shown in panel (b) of Fig. 5.9b. The spectrum shows peaks at 269 and 365 cm−1. The line at 365 cm−1 clearly belongs to the ground state as observed in the previous case. However, the new peak at 269 cm−1, which was not present in the absence of the IP, results from the beating between the modes 1793 and 1530 cm−1 belonging to the excited S1 state. In this scheme of measurement the ground state dynamics as well as excited state dynamics are observed.

5.7 Summary

The versatility of the pump-FWM (four-wave mixing) technique enables to observe the dynamics occurring on different potential energy surfaces even in complex molecular systems. For the speciﬁc example of β-carotene, by using the pump-CARS (coherent anti-Stokes Raman) technique, the dynamics of the ground S0 state as well as the excited S1 state of β-carotene could be monitored. The fast Fourier transform spectra of these transients with and without IP show frequencies, which correspond to the beating between the different modes seen in the Raman spectra of the excited S1 as well as ground S0 states and exclusively ground S0 state, respectively. The pump-DFWM technique can be used such that an interesting combination of different dynamics can be observed and directly compared as demonstrated above. The parallel observation of excited state population and depletion and ground state depletion and re-population makes the calibration of the different signal levels seen in the transient easier. Processes, which might be not accessible by a S1 state probing alone, will be visible in the ground state recovery. The simple kinetic model assumed for the relaxation scheme is able to give time constants, which are in agreement with the known values. A comparison of the data received from our pump-DFWM experiment with those published elsewhere is given in table 5.1. The assumption of the ∗ population and decay of the hot ground state, S0, is perfectly explaining the observed transients. Our observations support the ﬁndings by Savolainen et. al. [81]. 6 Surface Enhanced Coherent Anti-Stokes Raman Scattering

Surface enhanced Raman Scattering (SERS) has become one of the most widely used techniques since its ﬁrst observation by Fleischmann et. al. in 1974 from the molecule pyridine adsorbed on to roughened silver electrodes. Thereafter, the effect has been demonstrated with a variety of molecules and with a number of metals. Most commonly used metals are coin metals such as Silver, Gold, Copper etc. Metal surfaces containing several coupled microscopic domains are seen to give the highest enhancement for the Raman spectra. Such surfaces are termed as SERS active systems. Besides the metal surfaces, metal colloids consisting of isolated metal particles of nanometer size in aqueous media are also identiﬁed to be good candidates for enhancing the Raman spectra of adsorbed molecules. However, the most intense SERS effect is observed from aggregated metal colloids consisting of large group of aggregated metal nanoparticles. Even though the use of SERS effect is widespread, the mechanism behind the effect is still not clearly understood. There are two theories describing the enhancement effect. One is the electromagnetic theory (EM theory) of enhancement and the other is the chemical enhancement theory [82]. According to the electromagnetic theory, the surface enhanced Raman effect is observed in surfaces having small metal features and gratings that can couple plasmon like electromagnetic resonances to electromagnetic plane waves. Such surfaces can absorb a photon and store the electromagnetic energy into the surface plasmon which is localized in a direction perpendicular to the surface and delocalized in the direction parallel to the surface. This results in an increase in the electromagnetic energy density near the surface which results in an increase in the scattered Raman intensity. The enhancement is the greatest when the plasmon

59 60 Surface Enhanced Coherent Anti-Stokes Raman Scattering frequency of the metal is in resonance with the incident radiation. EM theory predicts enhancement factors of the order of 1010 for the SERS effect Even though the EM theory could explain the SERS effect in general, it could not explain certain aspects of the effect such as the difference between the SERS effect seen in CO and N2 each having identical Raman cross sections but different SERS intensities. Also, the EM theory predicts a uniform enhancement of the ordinary Ra- man spectrum whereas the Raman spectrum observed in experiments shows different enhancement factors for different Raman modes [82]. In order to explain such observations, an alternate model was developed to explain the SERS effect, which is known as chemical enhancement theory. According to the chemical enhancement theory, the SERS effect involves a charge transfer process between the metal surface and the molecule adsorbed on it. The adsorption of the molecules on the metal surface results in the development of an absorbing state due to the broadening and shifting of free molecular energy states upon adsorption. This shift is considered to be due to the interactions between molecules and metal surface and molecules themselves. Thus, while the HOMO-LUMO transition in the free molecule is out of resonance with the laser, once the molecule is adsorbed onto a metal surface, the surface absorption can be in resonance or near resonance with the laser frequency. This results in resonance enhancement of Raman scattering due to the coupling of the systems. The enhancement factor caused by the chemical effect is still controversial. Results from experiments shows that the enhancement factor due to the chemical effect is less than that due to the EM effect. Nonlinear optical processes are also seen to be enhanced when using rough surfaces [83]. As described in chapter 2, the polarization P(r, t) induced by an intense electromagnetic radiation in a medium can be expresses as a power series in the electric ﬁeld strength E(r, t):

P(r, t) = χ(1)E(r, t) + χ(2)E(r, t)2 + χ(3)E(r, t)3 + ··· (6.1)

When the local field E(r, t) is enhanced due to the presence of rough metal spheres or metal colloids, huge enhancement of nonlinear optical effects can also be expected. This is because nonlinear optical effects involve the interaction of multiple laser fields with the matter and if any of the incident fields is within the plasmon resonance, the enhancement of the field results in a huge enhancement of the nonlinear optical effect. This principle can be applied to coherent anti-Stokes Raman scattering (CARS) which is a third order nonlinear process involving three incident fields, pump (ωp), Stokes 6.1 Surface enhanced CARS (SE-CARS) 61

(ωS) and probe (ωpr) as described in chapter 2. The experiments described in this chapter are aimed at understanding the mechanism of enhancement of the CARS signal from molecules adsorbed onto colloidal nanoparticles of coin metals such as silver and gold. For the chemical enhancement, the formation of a charge transfer state is assumed. This state acts as new excited electronic state in the CARS interaction. Therefore, similar to the experiments described in the other chapters of this thesis, the dynamics involving excited system states playes a central role here. If any of the three input ﬁelds for the CARS process is within the plasmon resonance of the metal nanoparticle, an enhanced CARS signal can be observed. The advantage of using the CARS technique is that, it can give structural information about the molecule investigated. Hence, important information about structural changes occurring in the molecule, including the above mentioned formation of a charge transfer state suggested by the chemical theory of enhancement, due to adsorption onto the metal surface can be explored. Moreover, using femtosecond pulses for CARS excitation enables to perform time resolved measurements to investigate the molecular dynamics of the adsorbed species. In the frame of the Ph.D. project only preliminary experiments have been performed, which are described in the following.

6.1 Surface enhanced CARS (SE-CARS)

Chew et. al. theoretically predicted the possibility of surface enhanced coherent anti- Stokes Raman scattering (SE-CARS) from molecules located near colloidal spheres [84]. The calculated results for benzene based on the electromagnetic theory of enhancement gave maximum enhancement factors of about 1010. The enhancement factor was seen to depend critically on the excitation proﬁle and fall off sharply with increase in the distance between the metal particle and molecule. Surface enhancement of CARS was observed experimentally by Liang et. al. using a nanosecond laser [85]. In their experiment, samples used were organic solvents such as benzene and toluene adsorbed onto colloidal silver particles prepared in N,N- dimethylformamide. The CARS signal scattered at right angles to the input lasers were detected and it showed enhancement of up to two orders of magnitude. Scanning the pump wavelength (ωp) over the absorption spectrum of the mixture of silver colloid with sample showed that the enhancement of CARS signal occurs for a very narrow range of wavelenghts (≈6 nm) around the absorption peak of the mixture. The CARS signal detected in the forward direction was weaker due to the scattering of the signal 62 Surface Enhanced Coherent Anti-Stokes Raman Scattering by the siver particles. The experiments described in this chapter is an extension of this experiment using femtosecond pulses.

6.1.1 Experimental

The sample used for testing the SE-CARS was pyridine due to its well known SERS properties. The chemical and electromagnetic effects responsible for SERS in pyridine has been well investigated and it has been shown that the chemical effect due to the formation of a pyridine/silver charge transfer complex has a strong effect on the SERS intensities shown by pyridine [86–88]. Thus, by choosing pyridine as the test sample, the effect and properties of additional resonances created by the charge transfer states, as predicted by the chemical effect theory, can be investigated.

6.1.2 Preparation of Silver sol

For the preparation of silver sol two previously known methods were used.

Procedure 1

This procedure for the preparation of silver sol is based on the well known Lee Meisel

Method [89]. In this method, 9 mg of AgNO3 was dissolved in 100 ml of distilled water and the solution was brought to boiling under vigorous stirring. A solution of 1% sodium citrate (1 ml) was added drop wise very slowly under dark conditions. The mixture was then kept boiling for 1 hour. The Ag-sol prepared by this procedure had a greenish yellow color with an absorption maximum around 437 nm (the gred curve in ﬁg. 6.1 ) and the sol was stable for a few weeks if stored under dark conditions. This method is limited to the production of aqueous solution of silver sols and another major limitation of this method is the low reproducibility of silver sols.

Procedure 2

In this procedure, a quick and more reproducible method reported by Hiramatsu et. al. [90] was used for the silver sol preparation. In this method, 21 mg of hydroxylamine hydrochloride was dissolved in 5 ml of distilled water and the solution was then quickly and dropwise added to the solution of 17 mg of silver nitrate dissolved in 90 ml of water, which resulted in Ag-sols in a few seconds. The absorption spectra of the colloids prepared in this way showed a maximum around 409 nm. 6.1 Surface enhanced CARS (SE-CARS) 63

Figure 6.1: Absorption spectra of the silver colloids prepared using the Lee Meisel Method (red) and the Hiramatsu method (green).

The silver sols prepared were tested for their SERS activity with pyridine using a Raman microscope setup. The excitation laser for the Raman measurements was the 514 nm output of an Ar-ion laser with a power of ≈ 20 mW. The Raman spectra of pyridine recorded with and without adding silver colloid are shown in ﬁg. 6.2. The black curve shows the Raman spectra obtained for 10−2 M pyridine without adding any silver sol. Large enhancement (4 orders of magnitude) of the Raman spectra can be seen for the same sample with the addition of silver colloid.

6.1.3 CARS Experiment

The experimental setup used for the SE-CARS experiments is the same folded-BOXCARS arrangement of beams as described in chapter 3. The CARS signal was collected both in the phase matched forward direction and non-phase matched 90◦ sideward direction. The presence of enhanced CARS signal in the sideward direction is ascribed to the relaxation of the CARS phase matching condition (eq. 3.2) due to the optical inho- mogeneity of the sample media. This effect is different from the partially coherent anti- Stokes Raman scattering (PCARS) described by Hamaguchi and co-workers [91, 92]. The detection part of the experiment is shown in ﬁg 6.3. The lowest concentration of pyridine that gave detectable CARS signal was 2.5 M. The pump and probe pulses for the CARS process is derived from the same laser and was varied from 490 nm to 550 nm. In each case, the Stokes wavelength was chosen 64 Surface Enhanced Coherent Anti-Stokes Raman Scattering

Pyridine + Ag-sol

Pyridine

Intensity (arb. units)

750 1000 1250

-1

Wavenumber (cm )

Figure 6.2: Raman spectrum of 10−2 M pyridine recorded without (black) and with (red) the addition of silver colloid

Mask

Signal Lens Spectrometer CCD/PMT Lens Sample

Computer FlipMirror

Lens Mask

Figure 6.3: Detection schemes employed for the SE-CARS experiments. Both the forward directed signal and 90◦ scattered signal are detected.

such that the frequency difference between the pump and Stokes lasers is 1000 cm−1, so as to excite the strongest Raman modes of pyridine at 1002 cm−1 and 1035 cm−1. All Raman modes within about 1000±300 cm−1 can be easily excited due to the broad spectrum of the femtosecond lasers (FWHM ≈ 4 nm). For the measurement of the resonant CARS spectra of pyridine, the pump and Stokes lasers were kept overlapped 6.2 Discussion on SE-CARS 65 in time while the probe laser was delayed by 1 ps. This was done to avoid the non- resonant contribution to the CARS spectra at time-zero. The CARS spectra obtained from 2.5 M pyridine for different concentrations of silver colloid are shown in ﬁg. 6.4. The largest enhancement factor observed was 10 when using a pump wavelength of

Pure Pyridine 2.5M

20 % Silver Colloid

10 % Silver Colloid

5 % Silver Colloid

Intensity units) (arb.

700 800 900 1000 1100 1200 1300 1400

-1

Wavenumber / cm

Figure 6.4: Sruface enhanced CARS spectra obtained from pyridine. The CARS spectra from pyridine alone is shown in black clolor. The other curves are for different concentrations of silver colloids

550 nm. No enhancement of the CARS spectra was observed for the other wavelengths used. Also, no enhancement effect was observed in the 90◦ detection scheme in contrast to the earlier observations made when using nanosecond laser pulses [85]. The enhancement effect was seen to depend critically on a number of parameters, which will be discussed in detail in the following section.

6.2 Discussion on SE-CARS

The observed enhancement of CARS spectra from pyridine is seen to have a sensitive dependence on a variety of parameters.

• Dependence on excitation wavelength:

In the experiments performed by varying the pump laser from 490 nm to 550 nm, no enhancement effect was observed except when the pump laser was tuned to 550 nm. The wavelength of the Stokes laser in this case was set to 580 nm corresponding to the strongest Raman modes of pyridine. The wavelength of the 66 Surface Enhanced Coherent Anti-Stokes Raman Scattering

CARS signal was then centered around 522 nm. These wavelengths are completely away from the resonance frequency of surface plasmons which is around centered around 420 nm. Also, the reason for not observing any enhancement of CARS signal for other wavelengths is not yet clear. However, it can be argued that the enhancement effect is seen when the CARS signal frequency is closer to the plasmon resonance of silver particles than the pump laser.

• Dependence on silver sol concentration:

The enhancement effect is seen to depend critically on the concentration of the silver colloid. Figure 6.4 shows the enhancement obtained for different concentrations of the silver colloid added to pyridine. For a low concentration of silver colloid (5%) a small enhancement results. The effect is maximum for a particular concentration of colloid after which addition of colloids resulted in a decrease in the enhancement factor. The decrease in the enhancement factor for higher colloid concentration is thought to be due to the increase of scattering of the CARS signal by the large number of colloidal silver particles.

• Dependence on silver sol preparation:

The effect of surface enhancement is seen to have very sensitive dependence on the method of preparation of the colloids. This was also observed in conventional Raman experiments. The temperature conditions, light conditions, and speed of addition of reagents for the preparation of colloids affected the SERS activity of the colloids. Even colloids prepared using the same method did not yield SERS effect sometimes. This results in a poor repeatability of the experiments using silver colloids.

• Dependence on the laser intensity:

The stability of the colloids was seen to be affected by the intensity of the laser pulses. Large fluctuations are observed in the CARS signal due to the random motion of the colloidal particles within the laser focus. Thus, it is required to accumulate the signal over a long time to average out the fluctuation effects. However, exposing the silver colloid to the intense femtosecond laser pulses af- fects the stability of the colloid. These two counter acting effects poses a major difficulty in the reliable performance of the experiment. 6.3 Summary 67

6.3 Summary

From the results of the experiments presented in this chapter, it can be inferred that surface enhancement of coherent anti-Stokes Raman scattering is possible. While the conventional SERS experiments involving a single laser beam gave enhancement factors of up to four orders of magnitude, the enhancement factor observed for the CARS process, which involves the interaction of three laser beams, was just a factor of 10. Also, the sensitive dependence of the process on a number of different parameters makes the experiment difﬁcult. Since the colloids prepared in the same way is seen to give different SERS activity, a detailed investigation is necessary to understand the properties of colloids that can give enhancement. The performance of the experiment can be improved by using a reproducible method for the production of silver colloids and by improving sensitivity of detection. By improving the detection sensitivity, the intensity of the input lasers can be reduced and by this, the effect of laser intensity on the silver colloid can be reduced.

7 Conclusion and Outlook

This thesis presents the results of four wave mixing (FWM) experiments done in two different scenarios. In one part, two different femtosecond time resolved techniques, pump-degenerate four-wave mixing (pump-DFWM) and pump-coherent anti-Stokes Raman scattering (pump-CARS), are used to elucidate the excited state dynamics of a simple diatomic system iodine and a complex system β-carotene. The other experiment is based on the principle of surface enhancement of optical effects in presence of colloidal metal particles. The pump-DFWM experiments on iodine demonstrate the potential of FWM techniques applied to the investigation of excited state dynamics. In the case of iodine, vibrational dynamics in the higher lying ion-pair states can be observed using a DFWM with an additional pump pulse. Different time ordering of the input pulses gave access to dynamics occurring in different excited states of molecular iodine. The technique is superior to fluorescence based techniques as the signal does not depend on fluorescence and can be applied to non-fluorescing states as well. The technique was then extended to a more complex poly atomic system, β-carotene. In β-carotene, the DFWM process is used as probe to follow the population relaxation pathways of β-carotene after excitation into the S2 state by an initial pump pulse. The DFWM pulses are kept temporally overlapped and the time delay between the initial pump pulse and the DFWM pulses is varied in order to gain information about the relaxation pathways of the excited population. The results show that the population after excitation into the S2 state is quickly transferred to the S1 state in about 0.25 ps though an internal conversion. From the S1 state the population is transferred to the hot vibrational states of the ground (S0) state in about 8.9 ps through another internal conversion. The relaxation of the hot vibrational states of the ground state takes place in about 12.7 ps. The main advantage of the pump-DFWM is that it is able to probe the population dynamics in different electronic states by varying the time delay between the initial pump and the DFWM process, which in other cases would require the use

69 70 Conclusion and Outlook of different experiments. Here, by choosing the wavelength of the DFWM process to be in resonance with the S1 → Sn transition of β-carotene (fig. 5.4), the population dynamics in the S1 state is observed. At the same time the DFWM wavelength is in resonance with S0 → Sn0 state, which probes the population recovery of the ground state. This combination makes it easier to observe the vibrational relaxation occurring in the ground state of β-carotene. Processes, which might be not accessible by a S1 state probing alone, will be visible in the ground state recovery. A simple three state model is assumed to fit the pump-DFWM transient (figs. 5.3 and 5.7) which was used to determine the time constants. The time constants extracted using this model are in perfect agreement with the time constants already obtained using different experimental techniques. The results of the pump-DFWM experiments are then compared with the results of a simple and commonly used two pulse pump- probe experiment. In this scheme, a pump pulse used to excite the molecules to the

S2 state and a probe pulse resonant with the S0 → Sn transition is used to probe the subsequent population dynamics. The only information that can be extracted from this experiment (fig. 5.8) is the time constant for the S1 → S0 internal conversion, which is obtained as 9 ps. Similarly, by using the pump-CARS (coherent anti-Stokes Raman) technique, the dynamics of the ground S0 state as well as the excited S1 state of β-carotene could be monitored. The fast Fourier transform spectra of these transients with and without IP show frequencies, which correspond to the beating between the different modes seen in the Raman spectra of the excited S1 as well as ground S0 states and exclusively ground S0 state, respectively. Thus, the pump-DFWM technique can be used such that an interesting combination of different dynamics can be observed and directly compared as demonstrated in chapters 4 and 5. The simultaneous observation of vibrational dyamics occurring in the excited B and ion-pair states of iodine and the parallel observation of excited state population and depletion and ground state depletion and re-population in β-carotene shows the powerfulness of pump-four wave mixing techniques in proving a wealth of information about molecular dynamics occuring in different states at the same time. These experiments demonstrate the versatility of the pump-four-wave mixing techniques to observe the dynamics occurring on different potential energy surfaces even in complex molecular systems. The results of the experiments on the surface enhancement effect of coherent anti- Stokes Raman scattering (CARS) showed that the CARS signal can be enhanced in the presence of colloidal metal particles. However, the enhancement factor observed 71 is much less than that observed for the conventional Raman scattering. Besides that, the effect is seen to be sensitively dependent on the method of preparation of the colloids, the wavelengths of the laser pulses involved and theie intensity. All these factors, along with the fluctuation of the CARS signal due to the random motion of the colloidal particles in the laser focus rendered the observation of enhancement effect difficult in most cases. The subject of this thesis can be expanded in a number of ways. Firstly, the techniques used for studying the excited state dynamics of β-carotene can be extended to other carotenoid molecules of different conjugation lengths and in different molecular environments. This study will be very useful for understanding the light harvesting function of carotenoids. Extending the study further including carotenoids and chlorophylls can throw light into the nature energy transfer between these two molecules and the factors affecting the efficiency of this energy transfer. A knowledge about the factors affecting the energy transfer efficiency will be useful for the construction of artificial light harvesting systems with better efficiency. Secondly, the results from the studies of surface enhanced CARS show that a better knowledge of the physical and chemical properties of colloidal metal particles is needed to characterize their ability to give enhancement. In order to overcome the problems due to the random motion of colloidal particles within the laser focus, al- ternative techniques such as tip enhancement or use of rough metal surfaces can be used.

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With the completion of this thesis, an incredibly enriching phase of my life comes to an end . I would like to thank everybody who directly or indirectly helped me ﬁnish this project successfully. In one way this PhD thesis is a testimony to my love of Physics, yet this work could not have been possible without the personal and practical support of numerous people. I am most indebted to Prof. Dr. Arnulf Materny for being a patient supervisor and for his consistent support, guidance, good humor, and never failing kindness. I could learn a lot of experimental tricks, and minute details of experimental physics from him. Besides research, the barbecues at his home, were unforgettable experiences for me. I thank Arnulf and his wife for those lovely moments. I owe a huge debt of gratitude to my colleagues Dr. Günter Flachenecker, and my own brother Mahesh V. Namboodiri for all their help, advice and insights. Günter, it has been a pleasure working with you. Thanks a lot for your funny remarks and jokes, which made life in the femtosecond lab a lot more enjoyable. Special thanks Dr. Abraham Scaria and Dr. Jakow Konradi for helping me familiarize with the femtosecond lab. Experiments and discussions with Abraham has played a huge part in the successful completion of this thesis. The help, assistance and hospitality I received from my colleagues in the Raman lab Patrice Donfack, Rasha Hassanein, Dr. Animesh Kumar Ojha, Pinkie Eravuchira and Malte Sackmann are very much appreciated. The long discussions and beer sessions with Patrice were of immense joy and fun. Rasha has not only been a good colleague, but also a wonderful neighbour. She always remembered us whenever she prepared some nice Egyptian dishes. I also thank Gabriel Ivan Cava Diaz, Dr. Mohamed El- Khouly, Dr. Ajay K Singh and Khadga Karki for their valuable discussions during the course of this work. At this juncture, I would like to thank Dr. Torsten Balster who was always there whenever I had problems with the measurement software. I also thank, Bernd von der

79 80 Acknowledgment

Kammer who helped me to solve issues related to chemistry and computers. I have got to know a lot of wonderful people during my time at Jacobs University. Jörg Liebers, Carsten Olbrich, Robert Schulz, Sidhant Bom, Catalin Chimerel, Adrian Negrean, Malte Oppermann, Thomas Ponat, Eugene Yakimovich, Praveen PVK, I owe a lot for making Jacobs a wonderful place to be in. My Indian friends in Jacobs, Binit Lukose, Dr. Rami Reddy Vennapusa, Srikanth Kudithipudi, Arun, Sanjay and Abhishek, they made my home away from home. I can never forget the visits I made with them to the restaurant "Melody" to have hot and spicy pizza and auﬂauf. My heartfelt thanks to all my CUSAT mates for your support and nice times together. Manu Punnen John, Rajesh & Gayathri, Manesh & Anu, Sivaji & Maryam, Kishore, Abraham & Denny, Jyotsna & Lee, Rajesh & Sheeba and Sandeep thanks for being there, whenever needed. I would also like to express my deep gratitude to my teachers who taught me all these years but only a few of whom I mentioned here formally by name. I am deeply indebted Prof. K Sankara Narayanan, Prof. C. J. George, from MES Kalladi College, Mannarkkad, Prof. Dr. T. Ramesh Babu, Prof. Dr. V. P. N. Nampoori from CUSAT for showing the beauty of physics and guiding me through. These acknowledgments would not be complete without thanking my family for their support throughout the ups and downs of my life. My father K. V. Vasudevan Namboodiri, my mother Bhadra Antharjanam were my inspiration to go forward and conquer heights in my life. I don’t know how to thank my better half Deepa. Her love, care and constant support were instrumental in the timely completion of this thesis. The presence of my brother, Mahesh, in the same lab was a great relief which made me feel at peace even at difﬁcult situations.

Bremen, May 19, 2010 List of Publications

• V. Namboodiri, M. Namboodiri, G. Flachenecker and A. Materny, Two-photon process in femtosecond time resolved four-wave-mixing spectroscopy: β-carotene, J. Chem. Phys (Submitted)

• V. Namboodiri, A. Scaria, M. Namboodiri and A. Materny, Investigation of molecular dynamics in β-carotene using femtosecond pump-FWM spectroscopy, Las. Phys., 19(2):154-161, 2009

• A. Scaria, V. Namboodiri, J. Konradi and A. Materny, Ultrafast vibrational dynamics observed in higher electronic excited states of iodine using pump-UV DFWM spectroscopy, Phys. Chem. Chem. Phys., 10(7):983-989, 2008.

• A. Scaria, J. Konradi, V. Namboodiri and A. Materny, A comparison of the selective excitation of molecular modes in gas and liquid phase using femtosecond pulse shaping, J.Raman Spectrosc., 39(6):739-749, 2008

• J. Konradi, A. Gaal, A. Scaria, V. Namboodiri and A. Materny, Inﬂuence of electronic resonances on mode selective excitation with tailored laser pulses, J. Phys. Chem. A, 112(7): 1380-1391, 2008.

• A. Scaria, V. Namboodiri, J. Konradi and A. Materny, Vibrational dynamics of excited electronic states of molecular iodine, J. Chem. Phys., 127(14):144305(1-6), 2007

• J. Konradi, A. Scaria, V. Namboodiri and A. Materny, Application of feedback- controlled pulse shaping for control of CARS spectra: The role of phase and amplitude modulation, J.Raman Spectrosc., 38(8):1006-1021, 2007.

• A. Scaria, J. Konradi, V. Namboodiri, M. Sackmann and A. Materny, Femtosecond CARS on molecules exhibiting ring puckering vibration in gas and liquid phase, Chem. Phys. Lett., 433(1-3):19-27, 2006.

81 82 List of Publications

• A. Santhi, V. Namboodiri, P. Radhakrishnan and VPN. Nampoori, Spectral dependence of third order nonlinear optical susceptibility of zinc phthalocyanine, J. Appl. Phys., 100(5):053109(1-5), 2006.

• A. Santhi, V. Namboodiri, P. Radhakrishnan and VPN. Nampoori, Simultaneous determination of nonlinear optical and thermo-optic parameters of liquid samples, Appl. Phys. Lett., 89(23):231113(1-3), 2006.

• A. Santhi, V. Namboodiri, J. Kesavayya, P. Radhakrishnan and VPN. Nampoori, Dual beam thermal lens and z-scan studies of the thermo-optical properties of some nonlinear materials. Proc. SPIE., 5710:91, 2005.

• V. Namboodiri, A. Scaria, VPN. Nampoori, V. M. Nandakumaran and P. Rad- hakrishnan, Refractive index measurement using multimode ﬁbers with long period grating, Proc. SPIE., 5459:415, 2004.

Curriculum Vitae

Personal Details

Name Vinu V. Namboodiri Date of Birth 31-05-1979 Place of Birth Kerala, India Parents K. V. Vasudevan Namboodiri Bhadra Antharjanam Marital Status Married Wife Deepa Mundayoor

Education

1996-1999 Bachelor of Science in Physics University of Calicut, Kerala, India 1999-2001 Master of Science in Physics Cochin University of Science and Technology, Kerala, India Master Thesis : A Review of Quantum Cryptography

Research

2003-2006 Junior Research Fellow at the International School of Photonics Cochin University of Science and Technology, Kerala, India 2006-2010 Research Student (Jacobs University Bremen) PhD Thesis: Femtosecond Time-Resolved Four-Wave Mixing Applied to the Investigation of Excited State Dynamics