MULTIPLE TIME SCALES OPTICAL NONLINEARITIES OF LIQUID CRYSTALS FOR OPTICAL-TERAHERTZ-MICROWAVE APPLICATIONS (Invited Review)

Abstract

Abstract-We provide a critical account of the dynamics of laser induced refractive index changing mechanisms in nematic liquid crystals which may be useful for all-optical switching and modulation applications in the visible as well as the Terahertz and long-wavelength regime. In particular, the magnitude and response times of optical Kerr effects associated with director axis reorientation, thermal and order parameter changes, coupled flow-reorientation effects and individual molecular electronic responses are thoroughly investigated and documented, along with exemplary experimental demonstrations. Emphases are placed on identifying parameter sets that will enable all-optical switching with much faster response times compared to their conventional electro-optics counterparts.

MAXWELL STRESS INDUCED FLOW-DEFORMATION AND OPTICAL NONLINEARITIES IN LIQUID CRYSTALS (Invited Paper)

“…Figure 4(b). As reported previously [6,30], these simulations are borne out in experiments studies with sub-microseconds (λ = 750 nm) laser pulses. Figures 5(a) and 5(b) show exemplary transmitted laser pulse shapes for two different input laser energies, clearly showing the switching characteristics discussed above.…”

“…In electrode-free optical activated transmission switching, the incident light changes or randomizes the director axis orientation by virtue of its intensity [2][3][4][5][6][29][30][31]. In the present case, the Maxwell Stress from the focused laser beam causes radial flows-reorientation, cf.…”

“…is proportionally smaller, just as those effective nonlinearities [2][3][4][5][6] arising from other mechanisms [6] produced by laser pulse that is shorter than the response time.…”

“…Liquid crystals are arguably the most nonlinear and electro-optics active materials owing to their unique physical properties and extreme sensitivities to external fields [2][3][4][5][6]. Generally we can classify the mechanisms for index change in liquid crystals into two distinct types: (i) those originating from individual molecules, and (ii) collective crystalline responses.…”

“…We emphasize again here that Maxwell Stress is only one of several nonlinear effects that could be generated by such intense pulsed lasers in transparent liquid crystals. As in most organic materials, although they are transparent in terms of single-photon transitions, the constituent molecules of BPLC do undergo two-or multi-photon absorptions due to the high intensities of the laser [6][7][8][9][10][11][12]. Nonradiative processes following photo-absorptions invariably result in significant thermal heating and deformation of the BPLC lattice to create large refractive index changes [35,39] These thermal effects are clearly in play in our experiment; we have observed that upon longer exposure to the picosecond pulse trains, the transmitted beams all exhibit severe thermal blooming appearance in the transmitted beam, often culminating in the creation of visible bubbles in the liquid crystals.…”