Nonlinear Optical Properties Of Hydrogenagted Nanocrystalline Silicon Thin Films

With the size of semiconductor integrated devices approaching the limit of quantum tunneling gradually, Moore’s law is facing great challenges. In recent decades, photonic devices have attracted considerable attention thanks to its stability, high speed and low energy consumption compared to the traditional electronic devices. In order to maintain Moore’s law, it is necessary to switch from the development of electronics device to photonics device. Nowadays, silicon photonics gradually becomes one of the most important research fields and realize large-scale commercial application, owing to little material cost with large element reserves, perfect compatibility with microelectronic processing technology, and good characteristics of hybrid optoelectronic integration. Limited by the indirect band gap structure, bulk silicon can hardly be used for light emission or absorption devices. However, when the size of silicon shrinks to several nanometers, its light absorption/emission no longer needs to follow the law of conservation of momentum and quantum effect will appear, we can take these advantages to design devices. At present, most studies on nano-scale silicon are about its linear optical properties, but seldom on the nonlinear optics of hydrogenated nanocrystalline silicon. Such property is quite important in application, fabricating multiple photonic devices if we take advantage of it, or leading to unexpected consequences otherwise.The primary achievement of this thesis is as follows:(1) a series of hydrogenated nanocrystalline silicon(nc-Si:H) thin film samples with different bandgap were fabricated by plasma enhanced chemical vapor deposition(PECVD) under different H dilution, radio frequency(RF) power, operating temperature, etc.;(2) we studied the nonlinear absorption of nc-Si:H by open aperture(OA) Z-scan technique;(3) the nonlinear refraction of nc-Si:H by close aperture(CA) Z-scan technique;(4) realized micrometer scale laser-induced crystallization by femtosecond laser and studied the unique nonlinear absorption property afterwards by OA Z-scan.The mixed-phase material nc-Si:H consists of nanometer grains embedded in disordered amorphous silicon matrix. As it can be fabricated by PECVD in large scale and little cost, it could be integrated into most silicon based optoelectronics devices. Former researches on nc-Si:H mainly focus on its quantum confinement, electrical and linear optical properties, while, yet little on nonlinear optical properties especially on photonic applications, such as all-optical switching. In this thesis, we studied the nonlinear optical properties of nc-Si:H with Z-scan method in detail, found the nonlinear absorption could be modified from saturable absorption to reverse saturable absorption sensitively by adjusting the incident wavelength, incident intensity and the bandgap of our sample. Z-scan results have turned out to be well-described by the modified third-order nonlinear absorption equation, and we think the band tail of our samples appears to play a crucial role in these nonlinear behaviors. On the other hand, since silicon has the significant carrier dispersion effect, electrons jumping into band tail and conduction band state after absorbing photons would significantly affect refraction index. By means of CA Z-scan, we demonstrated a unique feature of controllable nonlinear refraction characteristics between saturable and Kerr nonlinear refraction with the incident photon energy. We numerically evaluated the proportion of these two mechanisms in different wavelengths by a modified nonlinear refraction equation. As the nonlinear properties of nc-Si:H can be tuned by the incident wavelength, incident intensity and the bandgap, it shows great potential on photonic devices.During the OA Z-scan experiment, we found that when the incident intensity is beyond a threshold, there will be an irreversible mutation on the result of OA Z-scan of our sample, with absorption increasing more than 700%, and keeping still regardless of the change of incident wavelength and intensity. We did detailed physical and optical analysis on the mutated sample and found it was crystallized on micrometer scale by femtosecond laser. We found such interesting nonlinear absorption behavior can be well described by the gradient absorption model we proposed afterwards. Both the theoretical and experimental part of this result can be instrumental in designing novel low-intensity nonlinear optical devices. We also demonstrated that this absorption engineering technique can be utilized to build an optical limiter, and tested the performance of the optical limiter on high power laser.