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Single molecule spectroscopy using tunable lasers MAX PLANCK INSTITUTE FOR THE SCIENCE OF LIGHT, ERLANGEN, GERMANY

The fluorescence excitation spectra of single organic molecules in a solid state crystal are measured at cryogenic temperatures using a single frequency tunable laser light source based on optical parametric oscillator technology. This laser exhibits promising features as a light source for spectroscopy applications, including a broad tuning range from 450 to 650 nm, narrow linewidth < 1 MHz and mode-hop-free tuning over > 25 GHz. This applications note presents the experimental setup, measured spectra and discusses the applicability of this kind of laser for high-resolution spectroscopy.

Introduction more detailed description of the experi- of the dye or even of the pump laser Investigations of molecular processes mental methods is given, e.g. in [3]. used, when switching between wave- and their characteristics play an import- length ranges. Diode lasers are typically ant role in many scientific disciplines, Suitable excitation lasers restricted to small tuning ranges and low like medicine, biology, chemistry, and Single molecules in a solid state crystal output powers in the visible wavelength physics. Fluorescence spectroscopy can be regarded as nearly ideal two-lev- range. Optical parametric oscillators of single molecules gives fundamental el systems with natural linewidths in the (OPO) offer an attractive alternative: The insights into their properties and the in- range of 10 - 50 MHz. Due to imperfec- wavelength coverage of OPOs can be fluence of their surroundings [1,2]. How- tions in the crystal the transitions of indi- designed according to the experimental ever, studying single molecules in a host vidual molecules are inhomogeneously requirements, OPOs are solid-state sys- material is a challenging task with high distributed over more than 1 THz. With a tems which do not rely on consumables demands on the detectors and on the narrow-band laser tuned to an individ- such as dyes, and they deliver relatively laser light sources [3]. This report exam- ual transition single molecules can be high output powers. ines the applicability of a new tunable isolated from the ensemble. Therefore, continuous-wave (cw) laser light source the laser is a crucial component for the In this applications note, the commercially for various spectroscopy methods. excitation of single molecules: It requires available cw tunable OPO, C-WAVE, was a linewidth below the natural linewidth used. This OPO covers the wavelength Experiment of the molecules and a mode-hop-free ranges 450 – 650 nm (frequency doubled The experimental setup is illustrated in tunability over a broad spectral range to SHG option) and 900 – 1300 nm. The output Fig. 1. A frequency-tunable laser beam address many molecules. Furthermore power is in the 500 mW range and the line- is focused onto a sample containing the flexibility in choosing the center width is below 1 MHz. C-WAVE can be swept dibenzanthanthrene (DBATT) molecules wavelength for different kinds of mole- mode-hop-free over more than 25 GHz. hosted in a naphthalene crystal. The cule-host combinations, an output pow- Changes of the center frequency of several laser beam is tuned to resonances of er well above 200 mW, and low intensity GHz as well as large variations of the wave- the DBATT molecules, leading to their noise are other relevant parameters. length are fully computer controlled. The excitation and subsequent fluorescence. Commonly, dye lasers, Ti:Sa lasers, or good beam profile allows high coupling The fluorescence light is filtered and tunable diode lasers are used for such efficiencies into optical fibers and therefore measured with a single photon detector, experiments. Ti:Sa lasers are restricted a simple integration into existing setups. SPCM-AQR from Perkin Elmer. A High- to the wavelength ranges 700 – 1000 Finesse WS/6-200 wavemeter monitors nm and 350 – 500 nm when frequen- the frequency of the excitation laser. A cy-doubled. Dye lasers require a change Figure 1. Experimental setup: Confocal microscope including a cryostat. Figure 2. Fluorescence intensity of DBATT molecules hosted in a naphtha- A fiber guides the laser light to the experimental setup where it is cleaned lene crystal versus excitation frequency. Inset: Schematics of the inhomo- up with filters and focused onto the sample. The red-shifted fluorescence geneous broadening of DBATT molecules with a linewidth of 1 THz. light is collected, filtered, and analyzed with an avalanche photodiode (APD).

Results Summary For additional information, Fluorescence spectra of DBATT mole- The presented setup enables the please contact: cules have been measured at a center measurement of fluorescence exci- Dr. Tobias Utikal, Max Planck Institute wavelength of 618 nm and over a range tation spectra of single molecules in for the Science of Light, Erlangen, Ger- of 1 THz by stitching several mode- transparent host materials. Both laser many, [email protected], hop-free frequency sweeps. Figure 2 light sources, Ti:Sa lasers and C-WAVE, or shows a zoom in one of the mode-hop- are well suited for measuring narrow Dr. Niklas Waasem, Hübner GmbH & free scans. The molecules’ redshifted spectral features over wide frequency Co. KG, Kassel, Germany, niklas.waas- fluorescence signal is measured with ranges. A combination of these laser [email protected] the APD while the laser frequency is sources allows a nearly complete wave- scanned. The measurement shows length coverage from 450 – 1300 nm several narrow spectral features corre- with fully automatic wavelength control sponding to individual DBATT molecules and no need to change laser media or [4]. With C-WAVE it is also possible to perform realignments. This makes the lock the laser frequency with waveme- presented setup a user friendly, flexible ter-precision to one single molecule and sensitive spectrometer for charac- resonance in order to study its pho- terizing single molecules, color centers to-physics and photon dynamics. and semiconductor quantum dots.

References:

1 M. Orrit and J. Bernard, Single Pentacene Mole- 2 A. Maser et al., Few-photon coherent nonlinear 4 F. B. Zhelezko et al., Spectroscopic characteristics cules Detected by Fluorescence Excitation in a optics with a single molecule, Nature Photon 10, of single dibenzanthanthrene molecules isolated p-Terphenyl Crystal, Phys. Rev. Lett. 65, 2716 450. (2016). in a low-temperature naphthalene matrix, Appl. (1990). Spectrosc. 66, 334 (1999). 3 G. Wrigge et al., Efficient coupling of photons to a single molecule and the observation of its reso- nance fluorescence, Nature Phys. 4, 60 (2008).

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