Optical harmonic generation

Terhune R W, Maker P D and Savage C M 1962 Optical harmonic generation in calcite Phys. Rev. Lett. 8 404... 

The compounds K5Nb3OFi8 and Rb5Nb3OFi8 display promising properties for their application in electronics and optics. The compounds can be used as piezoelectric and pyroelectric elements due to sufficient piezo- and pyroelectric coefficients coupled with very low dielectric permittivity. In addition, the materials can successfully be applied in optic and optoelectronic systems due to their wide transparency range. High transparency in the ultraviolet region enables use of the materials as multipliers of laser radiation frequencies up to the second, and even fourth optical harmonic generation. 

Figure 2. Dipole array pictures ofsome situations in nonlinear optical harmonic generation. (See text for further discussion.)...

There have also been reports on the preparation of polar materials by a photo-electro-poling technique that combines the optically induced quadrupolar depletion of chromophores in the direction of the light electric vector with an additional field-induced orientation of dipolar chro-mophores. The latter allows the preparation of cold electrets, which are interesting for nonlinear optical applications, such as optical harmonic generation, wave mixing, etc. ... 

The optical frequency chains discussed in the previous section are difficult to build. Many lasers and optical harmonic generators have to be phase-locked and frequency-stabilized, and the whole setup can easily fill a large laboratory. Furthermore, each of these chains is restricted to a single optical frequency, which is linked to the cesium clock, just like the present frequency standard. 

The interfaces in general, and particularly with solid substrates break the head-to-tail symmetry of a liquid crystal phase and induce polar orientational order. The symmetry is reduced to the conical group Coov The latter allows a finite value of the second-order nonlinear susceptibility X2 responsible for the second optical harmonic generation [11]. This phenomenon has been observed in experiments on generation of the second harmonic in a ultrathin nematic layers on a solid substrate as shown in Fig. 10.9. 

Optical harmonic generation is one of the earliest and most developed area in the field of nonlinear optics. Not surprisingly, investigations of nonlinear optical effects in liquid crystals started also with the study of harmonic generation. The aim of the first studies was to observe second harmonic generation (SHG) in cholesteric liquid crystals. 

The fastest optical nonlinearity originates from the electronic response. To lowest order, )f = pP)/eo and = py)leQ, where P and /are molecular hyperpolarizabilities. As in the case of nonlinear susceptibilities, hyperpolarizabilities describe a variety of processes. Optical harmonic generation will be considered separately. 

Optical harmonic generation (second and third order) Nonlinear coefficients, orientation 26,27... 

BETHUNE D.S., (I988), "Optical Harmonic Generation and Mixing in... 

---