Optical bistability in mesoporous silicon microcavity resonators

Abstract

Porous silicon (PSi) has been shown to be a material with varieties of nonlinear optical properties. These properties have been discussed in terms of its nature of photoluminescence mechanism such as quantum confinement, surface states and the role of impurities. PSi has also been shown to be a useful material for optical devices, and have potential applications in fields such as bio-sensing and efficient lighting. In the following thesis, I present my theoretical and experimental results in studying a nonlinear optical property, known as optical bistability, in mesoporous silicon microcavity devices. The results show significant optical hysteresis in the transmission and reflection properties of mesoporous silicon microcavities when illuminated with a 150 nanosecond pulsed laser at 532 nm. The optical hysteresis is shown to be transient in nature and the properties are strongly dependent on the porosity of the cavity layer. The onset and damage threshold intensity are also shown to be porosity dependent. Modeling suggests that the observed effects are due to changes in the nonlinear refractive index and the transient lifetime increases with increasing porosity. The role of surface states was also investigated on influencing the bistable process by passivating the internal porous surface with hydrosilylation chemistry. The role of the number of periods in the cavity structure is also studied. The cavity is doped with quantum dot to improve its switching intensity. The optical bistable property of the cavity was also investigated by using a 532 nm continuous wave (CW) laser with maximum power of 10 mW, modulated by an optical chopper. The only sample that yields significant hysteresis is the sample doped with quantum dot, with millisecond respone time. The results show that the sample doped with quantum dot shows both electronic and thermal process in forming optical hysteresis. Finally, I develop a theoretical model to study the transient hysteresis shape. The model suggests that the power and pulse width of the excitation laser source strongly influence the shaping of the bistable curves. The nature of the nonlinear refractive index is also quantified in terms of the power and the width of the excitation pulse. The third order nonlinear coefficient is calculated to be in the order of 10-10 esu while the passivated samples yield results in the order of 10-9 esu, which agree with previous findings in literature.

Overview of Nonlinear Optics

“…When the nonlinearity is saturating, it can be shown that if ∆n is the maximum refractive index change, then the condition for bistability is ∆nk o L > αL + (1-R eff ), where α is the (unsaturable) loss per unit length inside the cavity and R eff is the average mirror reflectivity (Garmire, 1989). Semiconducting quantum wells have been shown to exhibit optical bistability when placed in a Fabry-Perot (Vivero, 2010), as does porous silicon (Pham, 2011).…”

Magnesiothermically Formed Porous Silicon Thin Films on Silicon‐on‐Insulator Optical Microresonators for High‐Sensitivity Detection

“…[19][20][21] The use of applied currents with HF-bearing solutions makes integration with silicon-oninsulator platforms diffi cult. [21][22][23][24][25][26][27][28][29][30] To the best of our knowledge, the largest Q-factor is ∼3380 with a free-standing resonator [ 21 ] and ∼1530 for a multilayer resonator. [21][22][23][24][25][26][27][28][29][30] To the best of our knowledge, the largest Q-factor is ∼3380 with a free-standing resonator [ 21 ] and ∼1530 for a multilayer resonator.…”