Optical materials harmonic generation

Organic Materials for Optical Second Harmonic Generation... 

The unique properties of liquid crystals have also provided opportunity for study of novel nonlinear optical processes. An example involves the ability to modify the pitch of cholesteric liquid crystals. Because a pseudo-wave vector may be associated with the period of pitch, a number of interesting Umklapp type phasematching processes (processes in which wave vector conservation is relaxed to allow the vector addition to equal some combination of the material pseudo-wave vectors rather than zero) are possible in these pseudo-one-dimensional media. Shen and coworkers have investigated these employing optical third harmonic generation (5.) and four-wavemixing (6). 

Non-linear optical interactions occur in materials with high optical intensities and have been used to produce coherent light over a wide range of frequencies from the far infra-red to the ultra-violet. The three wave mixing process is of particular interest as it can be used for optical parametric amplification and optical second harmonic generation (SHG) and occurs in non-centrosymmetric materials. 

Laser lights with short wavelengths are important in optical data processing, and conjugated polymers are expected to be useful materials for their generation. Optical third-harmonic generation measurements have been performed for poly(phenylacetylenes) " The magnitudes of the third-order susceptibility were 7 x 10 esu for poly(phenylacetylene) and in the order of 10 -10" esu for its derivatives . ... 

Xu, C. Kondo, T. Sakakura, H. Kumata, K. Takahashi, Y. Ito. R. Optical third harmonic generation in layered perovskite-type material (CioH2iNH3)2Pbl4. Solid State Cornmun. 1991, 79, 245. 

Certain materials, such as many phthalocyanines [45], can only be deposited during each upstroke (Z-type deposition) and others only during each insertion (X-type). The type of deposition followed by a particular material affects the properties of the resulting LB films due to symmetry restrictions for second-order physical effects such as piezoelectricity or optical second-harmonic generation. The bilayer unit (equivalent to the unit cell in a crystal) is symmetric for Y-type deposition but non centric for Z- and X-type modes, as indicated in Figure 12.12. Unfortunately, several researchers have found that Z- and X-mode multilayers are often temporally unstable, although the zwitterionic dyes of Ashwell prove to be one of the exceptions [46]. 

Ferroelectric crystals (especially oxides in the form of ceramics) are important basic materials for technological applications in capacitors and in piezoelectric, pyroelectric, and optical devices. In many cases their nonlinear characteristics turn out to be very useful, for example in optical second-harmonic generators and other nonlinear optical devices. In recent decades, ceramic thin-film ferroelectrics have been utilized intensively as parts of memory devices. Liquid crystal and polymer ferroelectrics are utilized in the broad field of fast displays in electronic equipment. 

Tungsten Bronze-Type Family Ba2NaNb 0i (BNN) (LB Ni/mhcr 66-o7j. This crystal is ferroelectric below about 580 °C. The crystal structure is modulated below 300 °C. This material is utilized for optical second-harmonic generation and in optical parametric osdllators (Fig. 4.5-38). 

The most important effect based on the first hyperpolarizability P is the frequency doubling (SHG), which is explained in more detail in the following Section 13.3.3.1. The coefficient P provides information about the efficiency of a nonlinear optical material in generating such a second harmonic. 

Twieg, R. and Jain, K., Organic materials for optical second harmonic generation, in Nonlinear Optical Properties of Organic and Polymeric Materials (ACS Symp. Sen, 233), Williams, D. J., Ed., American Chemical Society, Washington, D.C., 1983, 57. 

Only certain types of crystalline materials can exhibit second harmonic generation (61). Because of symmetry considerations, the coefficient must be identically equal to zero in any material having a center of symmetry. Thus the only candidates for second harmonic generation are materials that lack a center of symmetry. Some common materials which are used in nonlinear optics include barium sodium niobate [12323-03-4] Ba2NaNb O lithium niobate [12031 -63-9] LiNbO potassium titanyl phosphate [12690-20-9], KTiOPO beta-barium borate [13701 -59-2], p-BaB204 and lithium triborate... 

Materials for Frequency Doubling. Second-order NLO materials can be used to generate new frequencies through second harmonic generation (SHG), sum and difference frequency mixing, and optical parametric oscillation (OPO). The first, SHG, is given in equation 3. 

Barium sodium niobium oxide [12323-03-4] Ba2NaNb 02, finds appHcation for its dielectric, pie2oelectric, nonlinear crystal and electro-optic properties (35,36). It has been used in conjunction with lasers for second harmonic generation and frequency doubling. The crystalline material can be grown at high temperature, mp ca 1450°C (37). 

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. 

Lamberth, C. Murphy, D. M. Mingos, D. M. P. Second Harmonic Generation Properties of Some Coordination Compounds Based on Pentadionato and Polyene Ligands. In Organic Materials for Nonlinear Optics II Harm, R. A., Bloor, D., Eds. Royal Society of Chemistry London, 1991 pp 183-189. 

The first and third order terms in odd powers of the applied electric field are present for all materials. In the second order term, a polarization is induced proportional to the square of the applied electric field, and the. nonlinear second order optical susceptibility must, therefore, vanish in crystals that possess a center of symmetry. In addition to the noncentrosymmetric structure, efficient second harmonic generation requires crystals to possess propagation directions where the crystal birefringence cancels the natural dispersion leading to phase matching. 

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