Nonlinear spectroscopy of chiral molecules: new spectroscopic techniques for studying left-right mirror asymmetry of biological molecules

Abstract

The methods of nonlinear optics are widely applied to the study of changes in structure of large biomolecules.1'2 The information obtained by these methods may be used for the investigation of relationship between structural changes of the biomolecules and their specific activity. Molecular left-right asymmetry is an origin of nonvanishing even order optical nonlinear susceptibilities of non-racemic solutions of chiral molecules. New nonlinear optical spectroscopic schemes based on three-and five-wave interactions in chiral homogeneous isotropic media (symmetry forbidden in centrosymmetric racemic solutions) are investigated with the aim to develop a novel technique to clearly distinguish between left and right mirror isomers of biological molecules and to measure their individualnonlinearopticalhyperpolarizabilities, and vibrational spectra.3Though second harmonic generation (SHG) due to quadratic dipolar nonlinearity is still symmetryforbidden in optically active liquids yet we have succeeded in experimental observation, for the first time, of noncollinear SHG due to five-mixing process according to a scheme 2 cv = 3cv -w, governed by a fourth order susceptibility tensor 4"• SHG signal is found to be well collimated and quasi phase-matched, it shows theoretically predicted polarization, spectral and temporal properties. 1.INTRODUCTION.An optically active isotropic liquid (for instance, non-racemic solution of mirror-asymmetric molecules) lacks the center of inversion and, hence, possesses non-vanishing even order dipolar nonlinear optical susceptibilities:(2)D (4)D 3-5 . .(2)D ... .x , x etc . Optical sum-frequency generation due to x in an optically active arabinose solutions was observed for the first time by Rentzepis et al.6 in a non-collinear, phase-mismatched geometry. However, second harmonic generation (SHG) due to dipolar quadratic nonlinearity is still forbidden in such liquids because of permutationsymmetryreason4.Some time ago, one of us (N.I.K.) has shown that higher order nonlinearity (namely, fourth-order one) does allow the SHG process to occur in phase-matched non-collinear scheme5. Is presented hereafter a Theory of interaction of strong laser field with mirror-asymmetric isotropic mediums, theory of five-wave interaction in chiral homogeneous isotropic media and new experimental results on observation of five-wave mixing 2w= 3w -cv 0-8194-1 155-8/93/$6.00 SPIE Vol. 1922 Laser Study of Macroscopic Biosystems (1992)129 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/16/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx

Effects of permanent dipole moments in high-order optical nonlinearity

“…This system involves two different lasers, one of which is used to generate two counterpropagating modes. A summary of the characteristics of each beam is given in table 3 and a more extensive description of the process can be found in [5][6][7]. Table 3.…”

“…this six-wave interaction can be represented as O (5) (ω ; −ω , ω, ω, ω, ω), the parameters within parentheses successively represent the harmonic signal photon, the harmonic pump photon, the two photons of one fundamental frequency input and, finally, the two photons of the other (counterpropagating) fundamental input. The beam parameters are fully characterized in table 5.…”

“…This problem has already been solved [17] and we will apply these results for the particular case of a two-level molecular system. Then, O (5) can be written as follows, dividing the contributions from the secular and non-secular parts: (3.15) where the sum labelled as 'non-secular, π {1, 2, 3, 4, 5, 6}' is subject to the condition table 6, are equivalent to those in [17], but are expressed more concisely here. Table 6.…”

“…Here we focus on a particular five-wave mixing process studied in previous works [5][6][7], in those cases without consideration of secularity or the features associated with two-level response. This system involves two different lasers, one of which is used to generate two counterpropagating modes.…”

“…The aim of this work is to expedite analysis of the involvement in high-order optical nonlinearity of the permanent dipole moments in molecular systems, focusing attention on processes involving up to six optical modes. Experimental studies of such high-order interactions [1][2][3][4][5][6][7][8][9] have recently attracted great interest, mainly for the following reasons: (a) experiments of this type offer dynamical information that is otherwise unavailable [10][11][12][13][14][15][16]; (b) they can afford mechanisms for the generation of even harmonics in media that are macroscopically isotropic [8,17,18]; (c) they offer considerable scope for the exploitation of resonances; and (d), unlike second harmonic generation (SHG) or sum-frequency generation (SFG), they present a flexibility of geometry. The latter characteristic can be usefully exploited for the fullest characterization of the salient optical nonlinearity.…”