Frequency conversion materials

Of the many studies of applications of L-B films, we shall mention at last the development of non-linear optical materials. A molecule such as 4-nitro-aniline has a very large hyperpolarizability (section 7.2) and can be used in principle for frequency conversion in solution if a strong electric field is used to provide an orientation of the dipoles. There is no need for such an external field if similar molecules are used in an L-B film, and hyperpolarizability values far in excess of those of commonly used inorganic crystals have been obtained. Figure 8.24 shows two examples of the molecules used in... 

Second order materials will be used in optical switching (modulation), frequency conversion (SHG, wave mixing) and electro-optic applications, especially EO modulators (SLM). The applications all rely on the manifestation of... 

Three basic questions must be answered to ensure success in the search for an optimized nonlinear crystal for a particular application What are the most important optical properties which determine the crystal s figure of merit for the intended application What is the best methodology for characterizing those optical properties so that materials of interest can be identified efficiently Where in "materials space" can crystals with such properties be found with the highest probability Answers to these questions will be discussed in the context of a program to find improved frequency conversion crystals for high power lasers. 

In recent years there has been a growing interest in the search for materials with large macroscopic second-order nonlinearities [20-22] because of their practical utility as frequency doublers, frequency converters and electro-optic modulators [23] by means of second-harmonic generation, parametric frequency conversion (or mixing) and the electro-optic (EO) effect. They are described by X (2w w, u)), 0, w), respectively. In order to optimize... 

One subject that attracted much attention is the nonlinear optical properties of these semiconductor nanoclusters [17], The primary objective is to find materials with exceptional nonlinear optical response for possible applications such as optical switching and frequency conversion elements. When semiconductors such as GaAs are confined in two dimensions as ultrathin films (commonly referred to as multiple quantum well structures), their optical nonlinearities are enhanced and novel prototype devices can be built [18], The enhancement is attributed mostly to the presence of a sharp exciton absorption band at room temperature due to the quantum confinement effect. Naturally, this raises the expectation on three-dimensionally confined semiconductor nanoclusters. The nonlinearity of interest here is the resonant nonlinearity, which means that light is absorbed by the sample and the magnitude of the nonlinearity is determined by the excited state... 

Applications of second order nonlinear optical materials include the generation of higher (up to sixth) optical harmonics, the mixing of monochromatic waves to generate sum or difference frequencies (frequency conversion), the use of two monochromatic waves to amplify a third wave (parametric amplification) and the addition of feedback to such an amplifier to create an oscillation (parametric oscillation). 

In last years one observes a fast progress in synthesis and elaboration of non-centrosymmetric functionalized polymers for applications primarily in electrooptic modulation and frequency conversion. These materials possess large second order nonlinear optical susceptibility x and can be easily processed into good optical quality thin films for travelling wave applications. Essentially four types of polymeric structures have been developed, as shown in Fig. 1 ... 

The laser-based methods make use of different types of laser sources. Many of them, such as the solid-state laser or the semiconductor laser, are under continuous development because of the discovery of new lasing materials or the optimization of frequency conversion techniques. The type of the analytical application and the nature of the required information determine the choice of a special laser source, particularly pulsed or continuous-wave (CW) operating mode. Pulsed laser sources with fixed wavelength are usually used for LA, whereas both tunable pulsed and CW laser sources are preferred for element-specific detection or plasma diagnostics. 

Third-order and higher odd-order processes can be observed with electric dipole interactions in materials with any symmetry. They are used most commonly in materials that have a center of symmetry, such as gases, liquids, and some solids, since in these materials they are the lowest-order nonzero nonlinearities allowed by electric dipole transitions. Fourth-order and higher even-order processes involving electric dipole interactions are allowed only in crystals with no center of symmetry, and, although they have been observed, they are relatively inefficient and are seldom used for frequency conversion. 

Harmonic Generation. Nonlinear frequency conversion, especially harmonic generation, is important for the development of sources of coherent light at frequencies not otherwise accessible by efficient lasers, in particular the blue, violet, and UV frequencies needed for dense data storage. Harmonic generation is also useful for nonlinear spectroscopic studies, which probe excited states in materials (34). 

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