Liquid Crystals for Fast Infrared Laser Switching and Optical Limiting Application
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Mat. Res. Soc. Symp. Proc. Vol. 479 0 1997 Materials Research Society
times con and the decay time r off for the laser induced refractive index change depend on the geometry of interaction such as sample thickness and laser beam size, as well as the thermal conductivities of the liquid crystal and the enclosing materials. A rule of thumb is that for a characteristics interaction length of 15 jtm, C's are on the order of 50 -tsec for thermal diffusion perpendicular to the liquid crystal axis, and are about 100 jtsec for thermal diffusion along the director axis. In terms of the nonlinear coefficient n2 for the index changes, defined by An = n 2 I [ I is the laser intensity in Watts/cm 2 ], the magnitude of n2 is on the order of 10-6 cm 2 /Watt for cw laser. For laser pulse shorter than the characteristic response times, e.g. a microsecond laser pulse, n2 is on the order of 10-7 cm 2 /Watt [I]. Again, we note that these 'response' times will vary significantly, depending on the actual laser-liquid crystal interaction geometry, and the temporal duration of the laser. The theory and practice of laser induced molecular reorientation in liquid crystals have been standardized for many years [1]. In the liquid crystalline phase, because of the large birefringence and easy susceptibility of the molecular orientation to external fields, the intensity of the laser needed to create substantial reorientation and refractive index change is rather modest. Typically, the reorientation nonlinearity n 2 is about 10-4 cm 2 /Watt for pure nematic liquid crystal. With the use of suitable dye dopants[3] and/or applied bias fields [1,4], n2 can be enhanced by more than two orders of magnitude. The The onset times 'con is inversely proportional to the laser intensity and can be as short as tens of nanosecond for laser intensity on the order of 102 MW/cm 2 . The decay time r off depends on the square of the sample thickness, and is typically about 30 milliseconds for a 25 micron thick sample. The decay time can be shortened by using liquid crystals of low viscosity and large elastic constants. EXPERIMENTAL RESULTS:For the infrared regime, particularly in the mid-to far-IR regime, these response times and requirements on the laser intensities are quite adequate for practical applications. Accordingly, we have performed some preliminary studies on optical switching and wave mixing effects using some typical lasers spanning this spectral regime We have previously demonstrated [5] that self-defocusing caused by thermal indexing efects can be used for optical limiting, and optical beam amplification. By sandwitching the nematic film between two ZnSe prisms, c.f. Figure 1 and making use of the negative thermal index change, we have also demonstrated Transmission ->Total Internal Reflection [T ->TIR] switching effect [6]; switching times as short as 0.5 ms were obtained with a low power CO 2 laser [square laser pulse of several ms in duration; 0.3 Watt power, -10 Watts/cm 2 in intensity]. A drawback of aligned nematic films is their polariza
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