Stephanie A. Claussen

Ge/SiGe Quantum Well Waveguide Modulator Monolithically Integrated With SOI Waveguides

We present a Ge/SiGe quantum well QCSE waveguide modulator that is monolithically integrated with SOI waveguides. The integrated device shows 3.2 dB contrast ratio with 1 V swing at 7.0 Gbps.

Measurement and modeling of ultrafast carrier dynamics and transport in germanium/silicon-germanium quantum wells

We measure the intervalley scattering time of electrons in the conduction band of Ge quantum wells from the direct Γ valley to the indirect L valley to be ~185 fs using a pump-probe setup at 1570 nm. We relate this to the width of the exciton peak seen in the absorption spectra of this material, and show that these quantum wells could be used as a fast saturable absorber with a saturation fluence between 0.11 and 0.27 pJ/μm. We also observe field screening by photogenerated carriers in the material on longer timescales. We model this field screening by incorporating carrier escape from the quantum wells, drift across the intrinsic region, and recovery of the applied voltage through diffusive conduction.

Ge/SiGe asymmetric Fabry-Perot quantum well electroabsorption modulators

We demonstrate vertical-incidence electroabsorption modulators for free-space optical interconnects. The devices operate via the quantum-confined Stark effect in Ge/SiGe quantum wells grown on silicon substrates by reduced pressure chemical vapor deposition. The strong electroabsorption contrast enables use of a moderate-Q asymmetric Fabry-Perot resonant cavity, formed using a film transfer process, which allows for operation over a wide optical bandwidth without thermal tuning. Extinction ratios of 3.4 dB and 2.5 dB are obtained for 3 V and 1.5 V drive swings, respectively, with insertion loss less than 4.5 dB. For 60 ?m diameter devices, large signal modulation is demonstrated at 2 Gbps, and a 3 dB modulation bandwidth of 3.5 GHz is observed. These devices show promise for high-speed, low-energy operation given further miniaturization.

A Ge/SiGe quantum well waveguide modulator monolithically integrated with SOI waveguides

We report a Ge/SiGe quantum well waveguide electroabsorption modulator that is monolithically integrated with silicon-on-insulator waveguides. The active quantum well section is selectively grown on a silicon-on-insulator substrate and has a footprint of 8 . The integrated device demonstrates more than 3.2-dB contrast ratio with 1-V direct voltage swing at 3.5 GHz. We also show the potential of this device to operate in the telecommunication C-band at room temperature.

Femtosecond carrier dynamics in Ge/SiGe quantum wells

We resolve photoinduced changes in carrier populations of Ge SiGe quantum wells using femtosecond pump-probe spectroscopy. Absorption transients 400 fs indicate rapid GammararrL intervalley scattering that may explain exciton linewidth and suggest saturable absorber applications.

Surface-Normal Ge/SiGe Asymmetric Fabry–Perot Optical Modulators Fabricated on Silicon Substrates

We demonstrate the first vertical-incidence Ge/SiGe quantum well reflection modulators fabricated entirely on standard silicon substrates. These modulators could help enable massively parallel, free-space optical interconnects to silicon chips. An asymmetric Fabry-Perot resonant cavity is formed around the quantum well region by alkaline etching the backside of the Si substrate to leave suspended SiGe membranes, upon which high-index-contrast Bragg mirrors are deposited. Electroabsorption and electrorefraction both contribute to the reflectance modulation. The devices exhibit greater than 10 dB extinction ratio with low insertion loss of 1.3 dB. High-speed modulation with a 3 dB bandwidth of 4 GHz is demonstrated. The moderate-Q cavity (Q ~ 600) yields an operating bandwidth of more than 1 nm and permits operation without active thermal stabilization.

Si-Ge surface-normal asymmetric Fabry-Perot quantum-confined stark effect electroabsorption modulator

The strong electroabsorption modulation possible in Ge/SiGe quantum wells promises efficient, CMOS-compatible integrated optical modulators. Using an asymmetric Fabry-Perot design, we demonstrate the first surface-normal semiconductor modulator structure grown on silicon.

Selective area growth of germanium and germanium/silicon-germanium quantum wells in silicon waveguides for on-chip optical interconnect applications

We propose a robust fabrication process for growing Ge and Ge-based heterostructures in growth windows with Si sidewalls which can be applied to growth in thick Si optical waveguides. Sidewall growth is eliminated by the presence of a dielectric spacer layer which covers the sidewalls. We demonstrate the effectiveness of this process by selective-area growth of Ge and Ge/SiGe quantum wells, and show an improved performance and increased process reliability over previous work.

Ge quantum well resonator modulators

The strong electroabsorption modulation possible in Ge/SiGe quantum wells promises efficient, CMOS-compatible integrated optical modulators. We demonstrate surface-normal asymmetric Fabry-Perot and microdisk resonator modulators employing Ge quantum wells grown on silicon.

Spot size effects in asymmetric fabry-perot electroabsorption modulators

Simulations of an asymmetric Fabry-Perot modulator reveal degradation of the contrast ratio as the incident spot size is decreased. The minimum practical spot size imposes constraints on the modulator dimensions and hence device capacitance.