Second frequence-doubling phenomena

One of the more interesting applications of non-linear optical effects is the generation of the second harmonic. This phenomenon results when a laser beam passes through a material having second-order NLO properties (hence, composed by non-centrosymmetric molecules) the light emitted has a frequency double that of the incident radiation (or the wavelength has been halved). 

Figure 2-2. A non-linear optical material, ammonium dihydrogen phosphate, displaying second-harmonic generation, the frequency doubling of light (infrared to blue). The origin of this physical phenomenon is entirely dependent on ionic displacement or molecular charge-transfer (see color plate...

The phenomenon of frequency doubling or second-harmonic generation can be visualized as follows. If the applied electric field is of frequency m, it can be represented as sinort. and the quadratic terms are seen to have a 2o) dependence. 

The first major pole is contributed by the output L-C filter. It represents a second order pole which exhibits a Q phenomenon, which is typically ignored, and a -40dB/decade rolloff above its corner frequency. The phase plot will quickly begin to lag starting at a frequency of 1/lOth the corner frequency, and will reach the full 180 degrees of lag at 10 times the corner frequency. The location of this double pole is found from... 

The role of specific interactions was not recognized for a long time. An important publication concerning this problem was the work by Liebe et al. [17], where a fine non-Debye behavior of the complex permittivity (v) was discovered in the submillimeter frequency range. The new phenomenon was described as the second Debye term with the relaxation time T2, which was shown to be very short compared with the usual Debye relaxation time td (note that td and 12 comprise, respectively, about 10 and 0.3 ps). A physical nature of the processes, which determines the second Debye term, was not recognized nor in Ref. [17], nor later in a number works—for example, in Refs. 54-56, where the double Debye approach by Liebe et al. was successfully confirmed. 

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