Optical wave mixing

Most studies in photorefractive crystals and polymers employ visible and near-infrared (around 1 pm) lasers, primarily owing to the fact that the underlying photorefractive effects require resonant excitation of photocharges and space-charge fields. In the near-infrared regime, in particular, the communication wavelength channel (1.55 pm), there have been recent studies on the electro-optical and nonlinear optical responses of liquid crystals. 

From many perspectives, liquid crystals are particularly suited for applications in this regime. First, they are nonabsorptive and possess large dielectric anisotropy (A8=8g-8o l) over the entire near-UV to infrared spectrum and beyond (400 mn-20 pm cf. Chapter 3). Secondly, the scattering loss a in this regime is an order of magnitude smaller than in the visible region, since the principal mechanism for scattering loss are director axis fluctuations [a 1/A, ( 2)]. 

Accordingly, unlike most nonhnear optical materials, liquid crystals enable optical wave mixings to occur with high efficiency in the visible as well as infrared regime. In this section, we discuss two exemplary wave mixing processes in hquid crystals stimulated orientational scatterings and optical phase conjugation processes. 

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The materials scientist must develop a material within which, for example, optical wave mixing can occur, with a controllable magnitude and response time. Most optical materials encountered on a day-to-day basis do not exhibit significant non-linear optical properties. The relationship between the polarisation of a material, P, and the electric field, E, generated by incident light is given by the general expression ... 

Abstract—"TYm origins and the dynamics of optical nonlinearities in nematic liquid crystal films, namely, laser-induced molecular reorien-tational and thermal refractive index changes, are analyzed in the context of optical wave mixings. Theoretical expressions for the basic non-linearities, the rise and decay time, diffraction efficiencies, and other pertinent parameters involved in the dynamic grating formation are derived. Experimental results obtained with visible and infrared laser pulses are analyzed. Some newly observed novel nonlinear processes are also reported. 

Linear and nonlinear optical properties of liquid crystals in their mesophases have been studied in several contexts, in both fundamental and application-oriented pursuits. In the context of nonlinear optical processes, they have recently received considerable renewed interests as a result of the newly discovered extraordinarily large optical nonlinearity due to the laser-induced molecular reorientation, and a renewed effort explicitly at the large thermal index effect in liquid crystals. In the last few years, several groups [2]-[10] have looked at the optical nonlinearity in the mesophases of liquid crystals and the associated nonlinear processes. A brief review of some of these nonlinear optical processes and the fundamental mechanisms in both the liquid crystal and the isotropic phases has recently appeared [1]. In this paper, therefore, we will concentrate only on optical wave mixing processes that are relevant to this Special Issue. 

I. C. Khoo and R. Normandin, Nanosecond laser induced optical wave mixing and ultrasonic wave generation in the nematic phase of liquid crystals, Opt. Lett., vol. 9, pp. 285-287, 1984 see also I. G. Khoo and R. Normandin, Nanosecond laser induced ultrasonic waves and erasable permanent gratings in smectic liquid crystal, J. Appl. Phys., vol. 55, pp. 1416-1418, 1984. 

The main non-display applications of liquid crystals can be subdivided into two classes. The first exploits their anisotropic optical properties in spatial light modulators or their nonlinear optical properties in optical wave mixing etc. Spatial light modulators are usually based on the ferroelectric SmC phase aligned in a thin film. Liquid crystal spatial light modulators may soon find advanced applications such as the storage of... 

For a more detailed discussion of stimulated scatterings as nonlinear optical wave mixing processes, the reader is referred to Chapter 11. 

Figure 8.13. (a) Schematic depiction of the optical wave mixing configivalion to stu orientation photorefractivity of nematic liquid crystal under ac bias, (b) Schematic showing various space charge fields involved in the orientational photorefractive effect in nematic liquid crystals. 

Nondegenerate Optical Wave Mixing Harmonic Generations... 

Nondegenerate optical wave mixing, where the frequencies of the incident and the generated waves are different, are due to nonlinear polarization of the form given by Equation (11.35). For example, the incident frequencies cdi, CO2, and CO3 can be combined to create new frequencies 0)4 = cdi CO2 0)3 involving sums or differences. These wave mixing processes are sometimes termed sum-difference frequency generations. 

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