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Modified Z-Scan Techniques for Investigations of Nonlinear Chiroptical Effects

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Modified Z-Scan Techniques for Investigations of Nonlinear Chiroptical Effects Modified Z-scan techniques for investigations of nonlinear chiroptical effects Przemyslaw P. Markowicza), Marek Samoca,c), John Cerneb), Paras N. Prasada,b), Andrea Puccid) and Giacomo Ruggerid) a) Institute for Lasers, Photonics and Biophotonics b) Department of Physics State University of New York at Buffalo, Buffalo, NY 14260, USA c) Laser Physics Centre, Australian National University, Canberra, ACT 0200, Australia d) INSTM UdR Pisa c/o Dipartimento di Chimica e Chimica Industriale, Via Risorgimento 35, 56126 Pisa, Italy [email protected], [email protected], [email protected], [email protected], [email protected], [email protected] Abstract: We present simple modifications of the classic Z-scan technique for the investigations of nonlinear chiroptical effects, i.e. nonlinear circular birefringence and two-photon circular dichroism. Two methods for studying these effects: a “polarimetric Z-scan” and a “polarization modulated Z- scan” are described in detail. These techniques were applied to estimate the order of magnitude of the effects for several different materials. 2004 Optical Society of America OCIS codes: (190.4710) Optical nonlinearities in organic materials; References 1. P. N. Prasad. Introduction to biophotonics (Wiley & Sons, Hoboken, New Jersey, 2003). 2. I. Tinoco, Jr., "Two-photon circular dichroism," J. Chem. Phys. 62, 1006-1009 (1975). 3. E. A. Power, "Two-photon circular dichroism," J. Chem. Phys. 63, 1348-1350 (1975). 4. G. Wagniere, "Optical-activity of higher-order in a medium of randomly oriented molecules," J. Chem. Phys. 77, 2786-2792 (1982). 5. S. Kielich, "Nonlinear optical activity in liquids," Acta Physica Polonica 35, 861-862 (1969). 6. D. V. Vlasov and V. P. Zaitsev, "Experimental observation of nonlinear optical activity," Pis'ma Zh. Eksp. Teor. Fiz. 14, 171-175 (1971). 7. R. Cameron and G. C. Tabisz, "Observation of two-photon optical rotation by molecules," Mol. Phys. 90, 159-164 (1997). 8. F. Hache, H. Mesnil, M. C. Schanne-Klein, "Nonlinear circular dichroism in a liquid of chiral molecules: A theoretical investigation," Phys. Rev. B 60, 6405-6411 (1999). 9. H. Mesnil and F. Hache, "Experimental evidence of third-order nonlinear dichroism in a liquid of chiral molecules," Phys. Rev. Lett. 85, 4257-4260 (2000). 10. H. Mesnil, M. C. Schanne-Klein, F. Hache, M. Alexandre, G. Lemercier, C. Andraud, "Experimental observation of nonlinear circular dichroism in a pump-probe experiment," Chem. Phys. Lett. 338, 269-276 (2001). 11. H. Mesnil, M. C. Schanne-Klein, F. Hache, M. Alexandre, G. Lemercier, C. Andraud, "Wavelength dependence of nonlinear circular dichroism in a chiral ruthenium-tris(bipyridyl) solution," Phys. Rev. A 66, 013802 (2002). 12. J. Sztucki and W. Strek, "Two-photon circular dichroism in lanthanide(III) complexes," J. Chem. Phys. 85, 5547-5550 (1986). 13. K. E. Gunde and F. S. Richardson, "Fluorescence-detected two-photon circular dichroism of Gd3+ in trigonal Na3[Gd(C4H4O5)3].2NaClO4.6H2O," Chem. Phys. 194, 195-206 (1995). 14. M. Sheikh-bahae, A. A. Said, T. Wei, D. J. Hagan, E. W. v. Stryland, "Sensitive measurement of optical nonlinearities using a single beam," IEEE J. Quantum Electron. 26, 760-769 (1990). 15. A. Pucci, S. Nannizzi, G. Pescitelli, L. D. Bari, G. Ruggeri, "Chiroptical properties of terthiophene chromophores dispersed in oriented and unoriented polyethylene films," Macrom. Chemistry and Physics 205, 786-794 (2004). 16. D. H. Turner, I. Tinoco, M. Maestre, "Fluorescence detected circular-dichroism," J. Am. Chem. Soc. 96, 4340-4342 (1974). 17. X. Xie and J. D. Simon, "Picosecond circular dichroism spectroscopy: A Jones matrix analysis," J. Opt. Soc. Am. B 7, 1673-1684 (1990). #5140 - $15.00 US Received 27 August 2004; revised 6 October 2004; accepted 8 October 2004 (C) 2004 OSA 18 October 2004 / Vol. 12, No. 21 / OPTICS EXPRESS 5209 1. Introduction Optical isomers, i.e., molecules existing in two mirror image forms that cannot be superimposed by simple rotation (the phenomenon of chirality), commonly appear in all living organisms and their existence has very important consequences in biology and in biophotonics [1]. The properties of these molecules involve a slight difference in the interaction with left and right handed circularly polarized light. Such differences should also apply to nonlinear optical processes. There is already a great body of theoretical and experimental research on second-order nonlinear optical phenomena in chiral media. The studies of the combination of chirality with third-order (cubic) optical nonlinearity can further broaden our knowledge about light-matter interaction and may be of great practical interest as third-order effects are more general, requiring no special symmetry conditions. Theoretical description of the interaction of chiral molecules with high-intensity light has been carried out for quite some time [2-4] but the experimental evidence is scarce. As early as in 1969, Kielich discussed the theory of nonlinear optical rotation [5] and there are indications that this phenomenon has been observed [6,7]. In 1975, Tinoco [2] and Power [3] published independent papers on two- photon circular dichroism. Later, Wagniere [4] predicted that optical activity of higher order should be observable in various third-order optical nonlinearity related experiments. Recently, a French group [8-11] has provided extensive experimental and theoretical treatment of the case of chiral contribution to absorption saturation which may be considered as an effective χ(3) process in the sense that it is an intensity-dependent absorption. Nonlinear circular dichroism has also been reported for lanthanides [12,13]. In principle, a multitude of nonlinear optical phenomena can arise from the handedness of light-matter interaction. We limit the scope of this paper to two phenomena which may be treated either as the chirality-induced modifications of the cubic nonlinear optical properties or, alternatively, as the nonlinear part of the usual chiroptical properties, i.e. the circular birefringence (optical activity) and circular dichroism. For fully degenerate third-order nonlinear optical phenomena defined by the nonlinear refractive index, n2, and the nonlinear absorption coefficient, β, one can introduce their chiral counterparts as δn2,c and δβc where the δ symbol stands for the difference between the value of the parameter for right- and left- handed circular polarization. On the other hand, the nonlinear circular birefringence δn2,c can be treated as derived from the optical rotation strength coefficient [α] which can be considered intensity dependent, with the coefficient d[α]/dI being a measure of the nonlinearity. One can show that d[α]/dI = π/λ δn2,c . In a similar way, the nonlinear dichroism which is related to the imaginary part of the complex susceptibility, is δβc = d(δε)/dI, where δε is the linear circular dichroism expressed as the difference of absorption coefficients (ε) for right and left handed circularly polarized light. The estimation of the order of magnitude of the nonlinear chiroptical effects has been attempted in several theoretical papers. Wagniere [4] predicted that the chiral modulation of NLO effects (i.e. the relative difference between the values of a nonlinear coefficient for the left-handed and right-handed circularly polarized light) may be of the order of 10-2. Estimates given by other authors are between 10-4 and 10-2. Thus, the effects should be measurable using standard modulation techniques. A convenient and ingeniously simple technique for measuring intensity dependent effects like nonlinear absorption and refraction is that of Z- scan [14]. We propose here two modifications of this technique for detection of nonlinear chiroptical effects: the “polarimetric Z-scan” and the “polarization modulated Z-scan”. 2. Polarimetric Z-scan The “polarimetric” Z-scan is presented in Fig. 1. As in a simple polarimeter, placing an optically active sample between crossed polarizers results in appearance of light transmission. By rotating the analyzer by angle φ one can achieve the minimum transmission condition. However, as the optically active sample is translated along the z axis, the changing light intensity leads to reappearance of the transmission which peaks at the z=0 position, i.e. where #5140 - $15.00 US Received 27 August 2004; revised 6 October 2004; accepted 8 October 2004 (C) 2004 OSA 18 October 2004 / Vol. 12, No. 21 / OPTICS EXPRESS 5210 the intensity is the highest. This simple principle can be used to determine the nonlinear optical rotation, however, the appearance of increased transmission as the sample is near the focal plane of the lens may also be attributed to other effects causing depolarisation of the light transmitted through the sample. A necessary check that the light polarization plane has indeed been rotated is performing the Z-scan at analyzer positions rotated slightly off the crossing position. polarizerOpt. active sample analyzer D lens lens z Fig. 1. Principle of polarimetric Z-scan. Figure 2 shows an example of such a set of results obtained using 100 fs pulses at 650 nm (1 kHz repetition rate) and a solution of sucrose as a sample. The existence of the intensity dependent rotation of the polarization plane is confirmed by the characteristic “heterodyne” effect which is due to the light transmitted through the analyzer acting as a local oscillator when mixing with the transmission due to the nonlinear polarization rotation. The theoretical lines in the figures were calculated using the Malus law dφ P = P + P sin2Ω+ I(z) T S 0 dI where PT and P0 are the transmitted and incident power, Ω is the angle of rotation of the analyzer from the low intensity minimum position, dφ/dI is the nonlinearity of the optical rotation strength of the substance in the cell, and I(z) is the cell-position-dependent intensity calculated knowing the Rayleigh length of the beam, zR A small background transmitted power PS is due to scattering.
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