Second harmonic generation by femtosecond pulses in nematic liquid crystal

Generation of harmonics and supercontinuum in nematic liquid crystals

“…Detailed analysis of the characteristics and explanation of the physical mechanisms of generation of such supercontinua is beyond the scope of this paper; we plan to perform it in subsequent studies. We should only note that a similar phe-nomenon was observed previously by Enikeeva et al [7], who pumped an NLC cell by a solid-state femtosecond laser with a wavelength of ~800 nm. They observed a wide spectral background (luminescence) in the range from 350 to 500 nm when studying the second-harmonic generation (l » 400 nm) and suggested a multiphoton mechanism of luminescence excitation.…”

“…However, spectral supercontinua were observed in our studies in the case of NLC pumping by cw IR lasers, while pumping by a femtosecond laser gave rise to only harmonics. A possible reason for this difference is that a higher power and, simultaneously, shorter wavelength femtosecond laser was used in [7]; under these conditions multiphoton excitation is more likely to occur. In addition, the spectral supercontinuum obtained by us is much wider and barely overlaps the known fluorescence spectra of cyanobiphenyl-based NLCs, because it lies in the longer wavelength region (from 500 to 1100 nm).…”

“…Nevertheless, much attention is paid to the development of new practical methods implying the use of nonlinear optical properties of LCs, design of real devices on their basis (for example, miniature or microscopic laser frequency converters integrated with an optical fibre), and study of their limiting parameters. To date, experiments aimed at implementing nonlinear optical conversion processes in NLCs, related to both quadratic [4,5,7] and cubic [8 - 10] nonlinearities, have been performed using complicated and cumbersome laser systems having relatively low efficiency. New optical frequencies were generated using precisely aligned bulky focusing optics and cells with NLC samples.…”

Harmonic generation in a nematic liquid crystal

“…These factors also include influences that can be combined with one term-selfaction, which was first demonstrated in [2]. In laser irradiation, self-actions [2,[6][7][8][9], which deform the NLC and change the symmetry in the bulk, may include the light-induced Fredericks transition [10,11], thermo-mechanical [12], and thermo-orientation phenomena [13]. As a result, the generation of even harmonics seems to be divided into two consecutive stages: first, bulk deformation occurs, as a rule, leading to changes in the uniform direction of the optical axis (director) of the NLC in space and removing central symmetry, then nonlinear generation, as in solid crystals, occurs even harmonics.…”

“…Nonlinear optical phenomena in liquid crystals which are associated with the bulk conversion of the radiance frequency (harmonic generation, sum-frequency and differencefrequency generation) are phenomena that may eventually be of great applied value [6,7,14]. The features of the frequency conversion in LC mostly include the high efficiency of the process, which can reach several percent [7][8][9] at thicknesses of 10-100 microns.…”