Principles and Applications of Nonlinear Optical Materials

Nonlinear optics is attracting increasing attention around the world because of its applications in telecommunications and possibilities for optical information storage and computing. Optical fibre communications show that optics is already the method of choice for many purposes, owing to its wide bandwidth and freedom from electromagnetic interference. This is certainly obvious to those of us whose cities have had their streets dug up to lay new fibre-optic cables! To the existing advantages of optics, nonlinear optics adds further improvements in efficiency and versatility. A simple example is in amplification of optical signals. Fibre-optic cables have such low absorption that they can transmit signals over many kilometres, but eventually the signals need to be amplified. At present, this is done by converting the weak optical signal to an electronic one, amplifying that electronically, and then converting the strong electronic signal into a strong optical signal again. It would obviously be more efficient if the light beam could be amplified directly, say by a laser beam in a suitable medium. Such a process comes into the realm of nonlinear optics.

The purpose of this chapter is to explain what nonlinear optical phenomena are and how they are characterized. This provides the background for the following chapter describing how the phenomena are used in devices. It also establishes concepts, notation and units for subsequent chapters.

In linear optics, photons are regarded as noninteracting and it has been the noninteracting nature of photons that has made them particularly useful in the communication of information. Nonlinear optical effects become significant at high intensities of light where the photons are observed as interacting. In linear optics, it has been the communication aspect which has driven much of modern research in optoelectronics to the point where the technology of light generation and confinement has reached a stage where high intensities light can be maintained over long interaction lengths and significant nonlinear optical effects can be observed and utilized. Nonlinear optics offers the intriguing possibility that interacting photons can be employed in several ways including: the generation of coherent light at new frequencies where there are no convenient laser sources; the generation of ultrashort pulses of light; the propagation of ultrashort pulses without distortion from linear dispersion; and in processing of information. It is an interesting thought that in nonlinear optics the situation is the reverse of superconductivity where a usually highly interacting fermion, the electron, is induced to adopt noninteracting photon-like qualities.

It is hard to exaggerate the technological importance of semiconductors; as a group of materials they have found extensive application in electronics, where the key feature is the ability to alter radically their electronic properties by employing small amounts of dopants. Their optical properties have also attracted considerable interest, and semiconductors have been shown to be very useful in opto-electronics, where they are widely employed in the detection and generation of light. As a consequence their linear optical properties have been extensively studied, and since the invention of the laser in 1960 their nonlinear optical properties have also attracted a great deal of interest. In particular, the large magnitude of the nonlinear optical properties at photon energies close to the band-gap energy has generated considerable attention; with the demonstration of optical bistability, all-optical switching and optical computing are causing excitement.

The bandwidth of an optical-fibre based telecommunications system is limited by the rate at which information can be encoded and decoded, at each end of the link. It is this electronic `bottleneck’ that has stimulated research into alternative technologies for ultrafast information processing. Devices based on nonlinear optical effects have attracted a great deal of research interest because of their potential ultrafast response, < 1 ps, and the possibility of performing all-optical processing functions, where conversion of the signal into an electrical form is no longer required (Frieberg and Smith, 1987).

The field of nonlinear optics, as we know it today, was born as a direct consequence of the invention of the laser in the early sixties. Nonlinear effects in electromagnetism had been observed as early as the late nineteenth century by Kerr, Röntgen, Kundt, and Pockels, and later by Raman in 1927 when he discovered spontaneous scattering of light into new wavelengths in passing through a transparent medium. However, it was not until 1961 that the first observation of coherent nonlinear optical effects was made by Franken et al. (1961), who demonstrated second-harmonic generation of light in the crystal of quartz. This discovery propelled the field of modern nonlinear optics and initiated intensive research in materials science and crystal technology. In a short period following this discovery, several other nonlinear optical phenomena including parametric amplification and frequency mixing were identified, and many important concepts such as phase-matching were quickly developed. During this period, much effort was also expended at the fundamental level on studies of crystal optics and the understanding of the most important aspects of nonlinear interactions of light with matter. With the rapid parallel progress in laser technology and the availability of higher-intensity laser sources in new spectral regions, an increasing number of nonlinear optical experiments became viable and many new nonlinear optical techniques were developed. Today, nonlinear optics is a vast area, and undoubtedly one of the most important areas, of physics, with a diverse range of applications in many other areas of science. The aim of this chapter is to provide an insight into this subject and review some of the most recent progress made in the development of new nonlinear optical materials and devices. The early part of the chapter is concerned with a brief discussion on the physical origin of some of the important nonlinear optical effects and techniques used for the exploitation of these effects, such as phase-matching. This section will also contain a review of crystal optics and a discussion of nonlinear susceptibilities. In the later part, we describe several important nonlinear materials that have recently been developed, discuss their use in frequency-conversion devices, and outline their potential for future nonlinear optical applications.

The special feature of molecular crystals that distinguishes them from ionic materials or semiconductors is that the constituent molecules retain most of their identity. A knowledge of the molecular properties, combined with crystallographic data specifying the orientation of the molecules in the crystal lattice, should therefore provide a basis for understanding the optical properties of the crystal. Molecular quantum theory, confirmed by measurements on well-separated molecules in vapours or dilute solution, provides a reasonably accurate guide to the strength of the molecular nonlinearities, and many molecules have now been synthesized specifically for the purpose of investigating their potentially powerful nonlinear properties. The prediction of crystal structure, which would also be necessary if materials for nonlinear optics applications were to be designed completely from theory, is not at present possible, although certain general guiding principles are useful in creating molecular structures that are likely to crystallize in the non-centrosymmetric forms required for second-order materials (Zyss and Chemla, 1987).

The superior mechanical properties of materials consisting of chains of repeating chemical groups, polymers, has led to their increasing use since the beginning of this century. The enormous versatility in organic synthetic methods has been brought to bear on the synthesis of materials whose combined processibility, cost and durability have no rival. These features will always be the driving force behind the further development of polymers for increasingly specialized tasks. Thus the term `speciality polymer’ has entered the language to indicate some added function synthetically incorporated to augment the expected mechanical excellence of the material.

Langmuir-Blodgett (LB) films are exceedingly thin films (2.5-1000 nm) which, under optimum circumstances, have a highly organized layer structure at a molecular level. Although LB films have been known since the 1930s (Gaines, 1966), only about twenty years ago was it recognized that the order at the molecular level could possibly be exploited for applications in opto-electronics, microelectronics and molecular electronics. As a consequence the last twenty years have seen intense research activity on LB films (Prasad and Williams, 1990; Ulman, 1991a). Great progress has been made but as yet no commercial device has been produced. Research, however, still proceeds vigorously on a number of fronts towards this objective.