Third-harmonic generation , nonlinear optics

Third Harmonic Generation Nonlinear Optical Materials... 

Fig. 8. Examples of some of the donor-acceptor substituted TEEs prepared for the exploration of structure-property relationships in the second- and third-order nonlinear optical effects of fully two-dimensionally-conjugated chromophores. For all compounds, the second hyperpolarizability y [10 esu], measured by third harmonic generation experiments in CHCI3 solution at a laser frequency of either A = 1.9 or 2.1 (second value if shown) pm is given in parentheses. n.o. not obtained...

Schaller, R. D., Johnson, J. C Wilson, K R., Lee, L. F., Haber, L. H. and Saykally, R. J. (2002) Nonlinear chemical imaging nanomicroscopy from second and third harmonic generation to multiplex (broad-bandwidth) sum frequency generation near-freld scanning optical microscopy. 

Geisler, T. Pedersen, K. Petersen, J. C. Third-harmonic Generation in Substituted Oligo-phenylene Vinylenes and Organic Square Planar Nickel Complexes. In Notions and Perspectives of Nonlinear Optics Keller, O., Eds. World Scientific Singapore, 1996 pp 580-585. 

The unique properties of liquid crystals have also provided opportunity for study of novel nonlinear optical processes. An example involves the ability to modify the pitch of cholesteric liquid crystals. Because a pseudo-wave vector may be associated with the period of pitch, a number of interesting Umklapp type phasematching processes (processes in which wave vector conservation is relaxed to allow the vector addition to equal some combination of the material pseudo-wave vectors rather than zero) are possible in these pseudo-one-dimensional media. Shen and coworkers have investigated these employing optical third harmonic generation (5.) and four-wavemixing (6). 

Shcheslavskiy, V., Petrov, G. I., and Yakovlev, V. V. 2003. Nonlinear optical susceptibility measurements in solutions using third-harmonic generation at interfaces. Appl. Phys. Lett. 82 3982-84. 

In summary, nonlinear optical imaging methods, such as TPM, allow for an order of improvement in the signal simply by utilization of 10 times shorter pulses. The enhancement is even more dramatic for higher-order nonlinearities, such as third harmonic generation where two orders of magnitude increase is expected. These improvements, however, rely on successful measurement and elimination of phase distortions. 

Written by an international panel of experts, this volume begins with a comparison of nonlinear optical spectroscopy and x-ray crystallography. The text examines the use of multiphoton fluorescence to study chemical phenomena in the skin, the use of nonlinear optics to enhance traditional optical spectroscopy, and the multimodal approach, which incorporates several spectroscopic techniques in one instrument. Later chapters explore Raman microscopy, third-harmonic generation microscopy, and nonlinear Raman microspectroscopy. The text explores the promise of beam shaping and the use of a broadband laser pulse generated through continuum generation and an optical pulse shaper. 

In addition to the blueshifling of the optical absorbance of Q-state semiconductor particles, a linear optical effect, there are nonlinear optical effects demonstrated by Q-state semiconductors. Two types have been observed by MCs prepared in organized films. One is the third harmonic generation or frequency tripling (44). A... 

The two important consequences of the third-order optical nonlinearities represented by x are third-harmonic generation and intensity dependence of the refractive index. Third-harmonic generation (THG) describes the process in which an incident photon field of frequency (oj) generates, through nonlinear polarization in the medium, a coherent optical field at 3a>. Through x interaction, the refractive index of the nonlinear medium is given as n = nQ+n I where n describes intensity dependence of the refractive index ana I is the instantaneous intensity of the laser pulse. There is no symmetry restriction on the third-order processes which can occur in all media including air. 

The third-harmonic generation method has the advantage that it probes purely electronic nonlinearity. Therefore, orientational and thermal effects as well as other dynamic nonlinearities derived from excitations under resonance condition are eliminated (7). The THG method, however, does not provide any information on the time-response of optical nonlinearity. Another disadvantage of the method is that one has to consider resonances at oj, 2w and 3o> as opposed to degenerate four wave mixing discussed below which utilizes the intensity dependence of refractive index and where only resonances at a) and 2a) manifest. 

This paper is a tutorial overview of the techniques used to characterize the nonlinear optical properties of bulk materials and molecules. Methods that are commonly used for characterization of second- and third-order nonlinear optical properties are covered. Several techniques are described briefly and then followed by a more detailed discussion of the determination of molecular hyperpolarizabilities using third harmonic generation. 

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