Brandon Redding

Free-carrier absorption modulation in silicon nanocrystal slot waveguides.

Free-carrier absorption (FCA) has proven to be an important obstacle in the development of a silicon-based laser; however, FCA may serve as a potential advantage in active silicon-based switches or modulators. In this work, we present FCA modulation in slot waveguides with silicon nanocrystals (Si-ncs) embedded in SiO(2) as the low-index slot material. Slot waveguides were fabricated with and without Si-ncs, and the presence of Si-ncs was shown to increase the pump-induced FCA loss in the waveguides by a factor of 4.5. We modeled the Si-nc material using a four-level rate equation analysis to estimate the excited population of Si-ncs, allowing us to extract a value of 2.6 × 10(-17) cm(2) for the FCA cross section of the Si-nc material.

Electromagnetic modeling of active silicon nanocrystal waveguides

In this paper we propose an electromagnetic analysis of active silicon nano-crystal (Si-nc) waveguide devices. To account for the nonlinearity in the active medium we introduce a four level rate equation model whose parameters are based on experimentally reported material properties. The electromagnetic polarization serves to couple the quantum mechanical and electromagnetic behavior within the ADE-FDTD scheme. The developed modeling tool is used to simulate waveguide amplifiers, enhanced spontaneous emission microcavities, and the temporal lasing dynamics of active Si-nc based devices.

Comparison of raised-microdisk whispering-gallery-mode characterization techniques

We compare the two prevailing raised-microdisk whispering-gallery-mode (WGM) characterization techniques, one based on coupling emission to a tapered fiber and the other based on collecting emission in the far field. We applied both techniques to study WGMs in Si nanocrystal raised microdisks and observed dramatically different behavior. We explain this difference in terms of the radiative bending loss on which the far-field collection technique relies and discuss the regimes of operation in which each technique is appropriate.

Mitigation of Si nanocrystal free carrier absorption loss at 1.5 μm in a concentric microdisk structure

We study the mitigation of Si nanocrystal (Si-nc) free carrier absorption (FCA) loss at telecom wavelengths in a concentric microdisk design. The concentric microdisk design relies on using the Si-nc emission as an optical pump for the surrounding Er-based lasing media without subjecting the lasing mode to the FCA loss present in the Si-ncs. We analyze the FCA loss as a function of overhang width in this design and show that for large enough overhang width the FCA loss is negligible. We also compute the FCA cross section from the FCA loss and number of excited Si-ncs, modeled by a four-level system, and show sigma(FCA)=1.08 +/- 2.3 x 10(-17) cm(2), which is in good agreement with reported cross sections for similar films.

Pulsed pumping of silicon nanocrystal light emitting devices

Typical silicon nanocrystal light emitting devices (LEDs) operate under direct current (DC) biasing conditions that require high electric fields or high current densities. The electroluminescence (EL) under these conditions relies on impact excitation that can be damaging to the material. In this work, we present bipolar injection into silicon nanocrystal LEDs using a pulsed pumping scheme. We measured the frequency dependence of the integrated and time-resolved EL of the LEDs. The frequency dependent behavior of the time-resolved characteristics is used to explain the integrated EL measurements. In addition, the light output of the device was measured under pulsed excitation and was found to increase by a factor of 18 as compared to the case of DC excitation.

Coupling Si nanocrystal microdisk emission to whispering-gallery modes in a concentric SiO2 ring.

We present a concentric microdisk design in which luminescence from an inner disk of Si nanocrystals (Si-ncs) contributes to resonant modes in an outer ring of SiO2. Photoluminescence from fabricated structures reveals the excitation of whispering-gallery modes (WGMs) with quality factors as high as 2850, limited by the spectral resolution of our spectrometer. Two-dimensional finite-difference time-domain simulations provide insight into the WGM properties and the role of disk and ring geometry. The presented concentric disk structure provides a means to use the efficient visible luminescence of Si-ncs as an optical pump for an extrinsic lasing material such as Er:SiO2.

Modeling of light amplification and enhanced spontaneous emission in silicon nanocrystals

Based on coupled quantum electrodynamics the light propagation in active silicon nanocrystals was reported in this paper. The classical electron oscillator model was employed to bridge the link between the rate equations of the four-level atomic system of the active medium and the electromagnetic interaction. With the assistance of auxiliary differential equations we numerically solved the system by using the Finite-difference Time-domain (FDTD) method. Both stimulated and spontaneous emissions were taken into account in the active medium system. Light amplification characteristics due to stimulated emission were investigated under various pumping rates. To enhance the spontaneous emission, microcavities based on one-dimensional photonic crystals were designed to maximize the nonlinear interaction between the active medium and electromagnetic waves. Preliminary experimental investigation of the cavity-enhanced luminescence was performed to demonstrate the validation of the proposed simulation scheme.

Fabrication and characterization of silicon/silicon dioxide super lattices for silicon based light emitting devices

Silicon based light emitting materials are of particular interest for integrating electric and photonic devices into an all-silicon platform. The progress of nano-scale fabrication has led to the ability to realize silicon emitters based on quantum confinement mechanisms. Quantum confinement in nano-structured silicon overcomes the indirect bandgap present in bulk silicon allowing for radiative emissions. Two common structures that utilize the quantum mechanisms leading to light emission in silicon are nanocrystals embedded in silicon dioxide and silicon/silicon dioxide super lattices. Nanocrystals employ quantum confinement in three dimensions while the super lattice structure induces two-dimensional confinement. Strong photoluminescence (PL) has been demonstrated in both structures, confirming the presence of quantum confinement effects. Our super lattice structures are grown using plasma enhanced chemical vapor deposition (PECVD) with alternating layers of silicon and silicon dioxide. We present here sub-10nm period superlattices confirmed via transmission electron microscopy and x-ray diffraction and reflectivity. We also present a new design for an electrically pumped device along with preliminary current-voltage characteristics.

Quantum Electrodynamic Modeling of Silicon-Based Active Devices

We propose a time-domain analysis of an active medium based on a coupled quantum mechanical and electromagnetic model to accurately simulate the dynamics of silicon-based photonic devices. To fully account for the nonlinearity of an active medium, the rate equations of a four-level atomic system are introduced into the electromagnetic polarization vector. With these auxiliary differential equations, we solve the time evolution of the electromagnetic waves and atomic population densities using the FDTD method. The developed simulation approach has been used to model light amplification and amplified spontaneous emission in silicon nanocrystals, as well as the lasing dynamics in a novel photonic crystal-based silicon microcavity.

Whispering gallery modes at 800 nm and 1550 nm in concentric Si-nc/Er:SiO2 microdisks

We examined a concentric Si nanocrystal (Si-nc) and Er doped SiO 2 (Er:SiO 2 ) microdisk structure supporting high-Q whispering gallery modes (WGMs) at both visible and telecom wavelengths. This structure provides a means to utilize Si-nc luminescence as an optical pump for an Er:SiO 2 cavity without subjecting a telecom-wavelength, Er:SiO 2 -based mode to loss mechanisms associated with the Si-nc material. After fabricating a concentric microdisk consisting of an inner Si-nc disk and an outer Er:SiO 2 ring, we characterize visible wavelength WGMs excited by the Si-nc photoluminescence and observed spectrometer limited quality factors as high as 10 3 . Telecom wavelength photoluminescence from the Er:SiO 2 ring was measured to have a quality factor as high as 10 4 in the erbium luminescence region using a passive pulled fiber setup.