Third-order materials

Compared to materials for second-order nonlinear devices, the third-order materials are even further away from being ready for device applications. The relevant issues for third order materials are ... 

The search of third-order materials should not just be limited to conjugated structures. But only with an improved microscopic understanding of optical nonlinearities, can the scope, in any useful way, be broadened to include other classes of molecular materials. Incorporation of polarizable heavy atoms may be a viable route to increase Y. A suitable example is iodoform (CHI ) which has no ir-electron but has a value (3J ) comparable to" that of bithiophene... 

The current review will present selected results on materials which appear to this author to have promise in NLO applications. Most references will be recent. The review will follow a format in which materials for linear optics are first reviewed very briefly, followed by a discussion of second order, then third order materials. 

Both second-order and third-order materials have technological applications because of their ability to convert low-frequency light to high-frequency light. However, the efficiency with which they are able to accomplish this feat decreases dramatically from second order to third order. Even the second-order process is small compared to the first-order process. From the design point, one faces a dilemma avoid the symmetry constraint and live with the low efficiency of third-order materials or adhere to the symmetry constraints and reap the benefit of better conversion. In Chapter 12, more about synthetic strategies as they relate to producing nonlinear optical materials will be covered. 

Third-order materials which exhibit cubic effects such as third harmonic generation (THG) or four-wave mixing (FWM). These materials have no restrictions on their symmetry. 

Commonly employed techniques for third-order materials are THG, degenerate four-wave mixing (DFWM), and z-scan. These techniques have been thoroughly compared elsewhere. ... 

F. Kajzar, J. Messier, J. M. Nunzi and P. Raimond, Third Order Materials,... 

Another important class of photonic materials is polysilanes (Fig. 49.15) [217-222]. In polysilanes the delocalization of (T orbitals of the Si atom along the polymer backbone plays a significant role in their NLO properties. Their physical and chemical properties can be tailored by the choice of an appropriate substituent, which also determines the conformation of the polymer and its solubility. What is of more interest is their transparency in their NLO properties, values of a series of polysilanes are given in Table 49.9. Tables 49.10-49.13 contain y and values of some other third-order materials, namely metallophthalocyanine, bis-metallophthalocyanine, metallonaphthalocyanines, fuller-enes, and j8-carotenes. Although these molecules are not considered as polymers, they are macromolecules that have drawn a considerable amount of attention as third-order NLO materials. For example, the highly stable icosa-... 

The Two-Level Model for y and Its Limitations. Generally, essential-states-model calculations have been very successful in explaining and predicting the second-order response of noncentrosymmetric materials, but considerable work remains for understanding third-order materials. As a starting point, consider again the two-level model, which can be expressed for third-harmonic generation as(139)... 

This loss in scaling was attributed to a decrease in electron delocalization due to a loss of planar structure that occurred in the polymer sample as a result of the interchain spacers. Other studies have also indicated that the planarity of the polymer is a dominating factor in deciding the third-order nonlinear response of the polymer (159). Further investigation of the effect of the spacers and the effect of the substitution on third-order materials is needed. 

Interrelations Between Different Sets of Third-Order Material Constants... 

Unfortunately, in maity cases it is not easily possible to put these interrelationships into praetiee. Even though the number of independent coordinates of the various third-, forth-, fifth-, and sixth-rank third-order material tensors is limited by material syinmetiy, the number of independent quantities to be determined experimentally is, in general, quite high. It further turns out that many experiments yield only linear combinations of several independent constants, which could possibly be resolved oidy by experiments of a different type. The importance of this latter kind of problems has been underestimated for a long time. It was investigated in great detail by Hraska (1993, 1994, 1995). Relations like (6.33) can be put to full use only if complete sets of constants are available for a specific ciystal which is rarely the case. 

Third-order nonlinear optical effects are also of interest to physicists and communications engineers. Relatively little work has been directed at developing third-order materials until recently and only a few studies have concentrated on LB films. A good example of one such study is that performed by Saito and TsutsuF in which... 

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