Prakash Koonath

All optical switching and continuum generation in silicon waveguides

First demonstration of cross phase modulation based interferometric switch is presented in silicon on insulator waveguides. By using Mach-Zehnder interferometric configuration we experimentally demonstrate switching of CW signal ~25 nm away from the pump laser. We present the effect of free carrier accumulation on switching. Additionally, we theoretically analyze the transient effects and degradations due to free carrier absorption, free carrier refraction and two photon absorption effects. Results suggest that at low peak power levels the system is governed by Kerr nonlinearities. As the input power levels increase the free carrier effects becomes dominant. Effect of free carrier generation on continuum generation and power transfer also theoretically analyzed and spectral broadening factor for high input power levels is estimated.

Monolithic 3-D silicon photonics

A monolithic CMOS compatible process has been developed to realize vertically integrated devices in silicon. The method involves the implantation of an oxygen into a patterned silicon substrate to form buried guiding structures. These buried devices are separated from a surface silicon layer by an intervening layer of silicon dioxide formed through the implantation process. Photolithography and etching is used to define devices on the surface silicon layer. The method has been utilized to realize the vertically coupled microdisk resonators and a variety of microresonator-based integrated optical elements. A new method for extraction of the unloaded Q of a cavity from its measured spectrum is also described.

Multilayer 3-D photonics in silicon

Three-dimensionally (3-D) integrated photonic structures in multiple layers of silicon are reported. Implantation of oxygen ions into a silicon-on-insulator substrate with a patterned thermal oxide mask, followed by a high temperature anneal, creates photonic structures on 3-D integrated layers of silicon. This process is combined with epitaxial growth to achieve devices on three vertically integrated layers of silicon. As a demonstration vehicle, we report a multistage optical filter that comprises of coupled microdisks on two subsurface silicon layers with bus waveguides on the surface (3rd) layer. The optical filter shows extinction ratios in excess of 14 dB, with excess insertion loss of less than 1 dB.

Add-drop filters utilizing vertically coupled microdisk resonators in silicon

Add-drop filters, based on vertically coupled microdisk resonators, have been realized in silicon, using a modified separation by implantation of oxygen process. Buried rib waveguides in the bottom-layer silicon, of a two-layer structure, are coupled to microdisk resonators in the top-layer silicon through a silicon dioxide layer formed by oxygen implantation. The radii of the microdisk structures were varied suitably to obtain resonators with slightly shifted resonance wavelengths. The average adjacent channel crosstalk suppression of these filters exhibits an upper limit of 12.11dB and a lower limit of 6.2dB over the wavelength band under consideration.

Limiting nature of continuum generation in silicon

The generation of spectral continuum in silicon is studied experimentally and theoretically. The dynamics of the free carriers generated through two photon absorption (TPA) is found to limit the extent of the generated continuum.

Raman amplification and lasing in SiGe waveguides

We describe the first observation of spontaneous Raman emission, stimulated amplification, and lasing in a SiGe waveguide. A pulsed optical gain of 16dB and a lasing threshold of 25 W peak pulse power (20 mW average) is observed for a Si1-xGex waveguide with x=7.5%. At the same time, a 40 GHz frequency downshift is observed in the Raman spectrum compared to that of a silicon waveguide. The spectral shift can be attributed to the combination of composition- and strain-induced shift in the optical phonon frequency. The prospect of Germanium-Silicon-on-Oxide as a flexible Raman medium is discussed.

Vertically-coupled micro-resonators realized usingthree-dimensional sculpting in silicon

A modified separation by implantation of oxygen process has been developed to sculpt vertically coupled microdisk resonators in silicon. The approach involves the implantation of oxygen ions into a silicon substrate, patterned with thermal oxide, to define waveguides on the bottom silicon layer, and photolithography and reactive ion etching to define the microdisk resonators on the top silicon layer. The top and the bottom silicon layers are separated by the oxide layer that was formed after the oxygen implantation. Fabricated microdisk resonators show resonances with a Q value of 10 300 and a free spectral range of 5.4nm.

Sculpting of three-dimensional nano-optical structures in silicon

Separation by IMplantation of OXygen (SIMOX) based process has been developed to sculpt three-dimensionally integrated nano-optical waveguiding structures in silicon. An approach, based on the implantation of oxygen ions into a silicon substrate, patterned with thermal oxide, has been adopted to synthesize low loss buried rib waveguides in a single implantation step of thickness 286 nm and widths varying from 2 μm to 12 μm. These waveguides show propagation losses in the range of 3–4 dB/cm. The capability of the process to sculpt three-dimensional (3-D) structures has also been demonstrated by defining rib waveguides on the top silicon layer.

Subterranean silicon photonics: Demonstration of buried waveguide-coupled microresonators

Laterally-coupled silicon microresonators are fabricated beneath the surface of a silicon-on-insulator substrate using a modified separation by implantation of an oxygen technique. Implantation of oxygen ions into a substrate with patterned thermal oxide mask was utilized to realize buried waveguiding structures. Microdisk resonators in the buried silicon layer show loaded quality factors of 2000, with extinction ratios in excess of 20dB. The process also results in the formation of a silicon layer on the surface of the wafer that is suitable for the fabrication of electronic devices, thereby paving the way for three-dimensional monolithic integration of electronics and photonics in silicon.

Continuum generation and carving on a silicon chip

An attempt at creating a multi-wavelength silicon light source is reported. Self phase modulation is used to broaden the spectrum of an off-chip seed pulse. The spectrum is filtered into discrete channels using vertically-coupled microresonators.