| Summary: | 碩士 === 國立交通大學 === 光電工程學系 === 100 === While the fundamental physical properties of indium nitride (InN) has been intensely studied since the first demonstration of its narrow bandgap (~0.6 eV), its nonlinear properties are relatively not well known. Metal nanomaterials have attracted great attention in the fields of plasmonics and metamaterials due to the excellent characteristics of large nonlinearity and tunability of size-dependent resonant wavelength. In particular, metal nanomaterials have the high third-order nonlinear susceptibility and the interaction with light induces large change in their refractive index and absorption. We applied the Z-scan technique to investigate the nonlinear optical properties of InN and 3-dimensional Au nanoparticle supercrystals.
In the experiment of the InN film, we successfully measured the nonlinear optical parameters at two wavelengths; the nonlinear refractive index n_2=(5.84±0.2)×?Q10?R^(-11) and nonlinear absorption coefficient β=-(2.12±0.06)×?Q10?R^(-6) at 800 nm and n_2=(1.86±0.1)×?Q10?R^(-10), β=(1.65±0.01)×?Q10?R^(-5) at 1550 nm, near the bandgap of InN. The opposite sign of nonlinear absorption coefficients is explained by the competition of different absorption processes such as the band filling effect at 800 nm and the band-gap renormalization effect at 1550 nm. In the measurement of 3D Au nanoparticle structures with a high repetition rate (80 MHz) laser, accumulated thermal effect elevated the sample temperature above the melting temperature, which is lower than that of Au film, and prevents the determination of the nonlinear coefficients. With a low repetition rate laser (1 kHz), however, the Z-scan signal-like responses were obtained only when the laser power is over a threshold value. Large peak intensity permanently damaged the samples and the morphology changes depend on the conditions of lasers such as pulsewidth and repetition rate.
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