Optically induced space-charge fields, dc voltage, and extraordinarily large nonlinearity in dye-doped nematic liquid crystals

Abstract

We have observed extraordinarily large optical nonlinearity in Methyl Red-doped nematic liquid-crystal film. Grating diffraction can be generated with an optical intensity as low as 40 microW/cm(2) , and a refractive-index change coefficient of more than 6 cm(2)/ W is obtained. The effect is attributed to formation of an optically induced dc space-charge field and to the resulting reorientation of the highly birefringent nematic director axis.

Electrically tunable, optically induced dynamic and permanent gratings in dye-doped liquid crystals

“…Above the cut-off frequency, we expect the enhancement to be reduced. This was, indeed, observed in the ac field and dc field effect on the transient grating as reported previously [1], [14] IV. PERMANENT GRATING AND THE AC FIELD EFFECT ON …”

“…As reported in earlier studies [14], [28], [29] of methyl-reddoped nematic liquid crystals (MRNLC), these films exhibit unusually large static space charge fields and photo-voltages. Under an optical intensity of 1 mW/cm , photo voltages of several millivolts are produced.…”

“…The orientation of molecules toward planar alignment indicates the strong contribution from phototransformed, adsorbed molecules in spite of the initial, strong homeotropic anchoring on both surfaces of the cells. Large dye concentrations and, therefore, significant adsorption on the surfaces is most likely to be responsible for this light-induced alignment [11]- [14]. The final configuration of molecules was stable, showed no changes with time for months and could not be erased by uniform illumination, an applied field, or heating.…”

“…Doping liquid crystals with dyes increases their light sensitivity and lowers the intensity threshold for molecular reorientation [14], resulting in the largest optically induced refractive index changing coefficient known to date. In doped liquid crystals, there are typically several mechanisms involved in the re- R. S. Cudney is with the División de Física Aplicada, CICESE, Ensenada, B.C., C.P.…”

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

“…In some dye-doped systems, surface alignment effects play an even more dominant role, leading to n 2 values 1 cm 2 /Watt [63,64]. Nevertheless, the requirement of a strong DC bias [> 1 Volt/µm], and the rather long response times make these effect unsuitable for practical switching applications.…”