Resonant conditions, nonlinear optics

The third-harmonic generation method has the advantage that it probes purely electronic nonlinearity. Therefore, orientational and thermal effects as well as other dynamic nonlinearities derived from excitations under resonance condition are eliminated (7). The THG method, however, does not provide any information on the time-response of optical nonlinearity. Another disadvantage of the method is that one has to consider resonances at oj, 2w and 3o> as opposed to degenerate four wave mixing discussed below which utilizes the intensity dependence of refractive index and where only resonances at a) and 2a) manifest. 

Equations (9.21) arise in various physical contexts. In particular, they describe the process of second harmonic generation in nonlinear optics (see, e.g. (8)). In fact, they can be applied to any process in which two classical oscillating modes (waves) transform into one and other under the resonance condition 2uja wb. 

A medium with saturable absorption can also be used for the nonlinear medium. In this case, the resonance condition is always met as far as the wavelength is concerned, but the feedback from the second mirror, which is required for resonant transmission, is not obtained until sufficient light is present inside the cavity to bleach the nonlinear medium. Devices of this type can show differential gain and optical bistability but not optical limiting. 

Excitation conditions At about 10cm"1 and farther above the 0-0 transition, the excitation wavelength does not seem to influence the fluorescence spectrum. This observation points to a rapid thermalization of the exciton population.87 In contrast, the excitation power has an important nonlinear effect Above a threshold stimulated emission is observed on the 0-1400 line, at 23692 cm"1.88 The crystal behaves as an optical resonator, with laser effects, which explains the sharpness and the intensity of this line in Fig. 2.17.89... 

According to a review on Kerr-like optical nonlinearities in liquid crystal [149], these experiments show that the photonic response may be much faster if short intense laser pulses are used instead of low-power cw laser radiation. This may open new aspects and perspectives in research and applications. In [150], the necessary conditions for the observation of resonance non-linearities in dye-doped CLCs are summarized ... 

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