A substantial amount of work has been carried out in the area of nanocomposite materials for optical applications. Composites are typically constructed by embedding an optically functional phase into a processable, transparent matrix material. By doing so, the optical properties can be utilized in more technologically important forms such as films and fibers. This review covers many areas of optical composite research to date. Composites with second- and third-order nonlinearities and laser amplification properties are discussed with examples from the recent literature. Other composites, including transparent magnets, may be made using similar structures. The principles used to construct these composites may have important technological applications soon and are therefore summarized in this review.

Nanocomposite Materials for Optical Applications

Passive glass waveguides made by ion-exchange technique are potential candidates for integrated optics (IO) applications. The authors review the advantages in the technology as applied to fabrication of useful single-mode structures. Progress in process, fabrication, modeling, and waveguide performance using different monovalent cation systems and glass compositions is described with emphasis on fiber-compatible single-mode structures prepared from molten baths. It is shown how a systematic study allows a correlation between the process parameters and the waveguide characteristics so desirable in assuring reproducible characteristics of the devices. >

Ion-exchanged glass waveguides: a review

The authors review progress in the application of nonlinear optical phenomena to integrated optics. Essentially all integrated optics devices such as directional couplers, Mach-Zehnder interferometers, prism couplers, etc., can be used for all-optical signal processing when at least one of the waveguiding media exhibits an intensity-dependent refractive index. Experimental and theoretical results on the operating characteristics and figures of merit of such devices, as well as a new class of phenomena based on large nonlinear changes in waveguide properties will be summarized. Also discussed are measurement techniques for evaluating waveguide nonlinearities. Progress in the implementation of other nonlinear third order interactions, such as degenerate four-wave mixing and nonlinear spectroscopy, will also be reviewed. >

Third order nonlinear integrated optics

We review dispersion relations, which relate the real part of the optical susceptibility (refraction) to the imaginary part (absorption). We derive and discuss these relations as applied to nonlinear optical systems. It is shown that in the nonlinear case, for self-action effects the correct form for such dispersion relations is nondegenerate, i.e. it is necessary to use multiple frequency arguments. Nonlinear dispersion relations have been shown to be very useful as they usually only require integration over a limited frequency range (corresponding to frequencies at which the absorption changes), unlike the conventional linear Kramers-Kronig relation which requires integration over all absorbing frequencies. Furthermore, calculation of refractive index changes using dispersion relations is easier than a direct calculation of the susceptibility, as transition rates (which give absorption coefficients) are, in general, far easier to calculate than the expectation value of the optical polarization. Both resonant (generation of some excitation that is long lived compared with an optical period) and nonresonant ‘instantaneous’ optical nonlinearities are discussed, and it is shown that the nonlinear dispersion relation has a common form and can be understood in terms of the linear Kramers-Kronig relation applied to a new system consisting of the material plus some ‘perturbation’. We present several examples of the form of this external perturbation, which can be viewed as the pump in a pump-probe experiment. We discuss the two-level saturated atom model and bandfilling in semiconductors among others for the resonant case. For the nonresonant case some recent work is included where the electronic nonlinear refractive coefficient,n2, is determined from the nonlinear absorption processes of two-photon absorption, Raman transitions and the a.c. Stark effect. We also review how the dispersion relations can be extended to give alternative forms for frequency summation which, for example, allows the real and imaginary parts ofχ(2) to be related.

Kramers-Kronig relations in nonlinear optics

We have observed a darkening effect and the formation of permanent volume gratings in CdSSe semiconductor-doped glasses, both phenomena being certainly due to the same photochemical mechanism. This allowed us to reconcile contradictory results concerning the speed of their nonlinear response. We also studied the frequency dependence of X (3) below the gap and the intensity dependence of the reflectivity of these phase-conjugate mirrors. Our results can be interpreted in terms of the band-filling model with Boltzmann statistics. Finally, frequency- and. time-resolved luminescence measurements helped us to interpret various results.

New results on optical phase conjugation in semiconductor-doped glasses

A pump–probe Mach–Zehnder interferometer was used to investigate the nonlinear-optical properties of CdSxSe1-x-doped glass channel waveguides. The technique is used in situ and may be used to determine the suitability of the material under investigation for all-optical switching. The saturation and dynamical properties of the material are readily determined by using the technique, as are thermal effects. Semiconductor-plasma theory was used to predict the nonlinear optical properties, and reasonable agreement with experiment was obtained.

Optical nonlinearities in CdSxSe 1- x-doped glass waveguides

Fluence‐induced changes in absorption of a semiconductor‐doped‐glass directional coupler have resulted in nonlinear optical switching on a picosecond time scale. The functional dependence on the incident fluence confirms that the switching is dominated by absorption saturation.

Picosecond switching induced by saturable absorption in a nonlinear directional coupler

The properties of planar waveguides and directional couplers fabricated by potassium/sodium ion exchange in semiconductor‐doped glasses are described.

Semiconductor‐doped glass ion‐exchanged waveguides

We show that large changes are introduced into the operational characteristics of nonlinear directional couplers by both loss and saturation of the nonlinearity, especially the latter. Specific numerical examples are given.

Effects of saturation and loss on nonlinear directional couplers

We have measured nonlinear-optical effects in ion-exchanged channel waveguides fabricated in semiconductor-doped glass. The nonlinearity is manifested as optically induced saturable absorption displaying a resonant enhancement. The relaxation time of the nonlinearity was found to be tens of picoseconds. The relaxation time also exhibited wavelength dependence. Waveguide transmission and pulse-probe experiments using a picosecond mode-locked dye laser were used to carry out these measurements.

T. J. Cullen

Papers

Optical nonlinearities in CdSxSe 1- x-doped glass waveguides

Picosecond switching induced by saturable absorption in a nonlinear directional coupler

Semiconductor‐doped glass ion‐exchanged waveguides

Effects of saturation and loss on nonlinear directional couplers

Nonlinear-optical effects in ion-exchanged semiconductor-doped glass waveguides