Nanocrystalline silicon carbide ͑SiC͒ thin films were deposited by plasma enhanced chemical vapor deposition technique at different deposition temperatures (T d) ranging from 80 to 575°C and different gas flow ratios ͑GFRs͒. While diethylsilane was used as the source for the preparation of SiC films, hydrogen, argon and helium were used as dilution gases in different concentrations. The effects of T d , GFR and dilution gases on the structural and optical properties of these films were investigated using high resolution transmission electron microscope ͑HRTEM͒, micro-Raman, Fourier transform infrared ͑FTIR͒ and ultraviolet-visible optical absorption techniques. Detailed analysis of the FTIR spectra indicates the onset of formation of SiC nanocrystals embedded in the amorphous matrix of the films deposited at a temperature of 300°C. The degree of crystallization increases with increasing T d and the crystalline fraction (f c) is 65%Ϯ2.2% at 575°C. The f c is the highest for the films deposited with hydrogen dilution in comparison with the films deposited with argon and helium at the same T d. The Raman spectra also confirm the occurrence of crystallization in these films. The HRTEM measurements confirm the existence of nanocrystallites in the amorphous matrix with a wide variation in the crystallite size from 2 to 10 nm. These results are in reasonable agreement with the FTIR and the micro-Raman analysis. The variation of refractive index ͑n͒ with T d is found to be quite consistent with the structural evolution of these films. The films deposited with high dilution of H 2 have large band gap (E g) and these values vary from 2.6 to 4.47 eV as T d is increased from 80 to 575°C. The size dependent shift in the E g value has also been investigated using effective mass approximation. Thus, the observed large band gap is attributed to the presence of nanocrystallites in the films.

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The polarization-sensitive degenerate four-wave mixing technique is employed to study semiconductor quantum-well microcavities in the low-density limit. Frequency-domain and time-domain measurements are carried out to study exciton-polariton modes in long-͑high-Q) and short-͑low-Q) photon lifetime systems. In the high-Q system, normal-mode splitting is observed in the linear reflection as well as the degenerate fourwave mixing spectrum. In the low-Q system, though the linear reflection spectrum shows normal-mode splitting, the spectrum of the degenerate four-wave mixing is found to have a maximum only at the bare exciton resonance. Our experimental results are well reproduced by a weakly interacting boson model which accounts for the anharmonicity in the system due to exciton-exciton correlation and the phase-space filling effect. The four-wave mixing results from the high-Q system are in good agreement with the cavity polariton scattering treatment for the third-order nonlinear response of the microcavity confined excitons. The polarization sensitive frequency-domain degenerate four-wave-mixing measurements from the high-Q system enable us to obtain the ratio of the anharmonic parameters. The frequency and time-domain measurements show that the strong coupling between the exciton and photon changes the nonlinear optical response qualitatively. Incoherent effects like the excitation-induced dephasing are suppressed in the high-Q system, whereas they are more pronounced in the low-Q system. We also find differences in the signal decay rates for different polarization configurations, which is attributed to inhomogeneous broadening in the low-Q system.

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