Emel Taşyürek

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.

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.

Scalable optical transmitter and receiver based on cascaded nanoresonator modulators and multiwavelength laser

Optical modulators are key components in optical data links and are, currently, the subject of much research. Here we combine a comb laser with a series of cascaded photonic crystal modulators. A silicon nitride waveguide is used as the bus providing a near ideal means of routing light around a photonic chip as well as low insertion losses from optical fibre. This is vertically coupled to the photonic crystal cavities, the large free spectral range of which provides very high scalability along with very low (~femtojoule) switching energies. Finally, we show a route to integrating optics with electronics at the frontend. Combined with the low silicon area consumed by vertically coupled photonic crystal cavities, this is very promising for future optical interconnects.

Simple electroabsorption model for silicongermanium/germanium quantum well devices

We present a simple model for the electroabsorption spectrum of germanium-based quantum wells, which can be used to optimize material design for modulators. The model gives very good agreement with our new experimental data.

Simple electroabsorption model for germanium quantum well devices

We present a simple electroabsorption model for germanium quantum wells to facilitate optical modulator design. We show this model is valid for a range of well sizes with an increased exciton-enhanced absorption for thinner wells.

Simple electroabsorption calculator for germanium quantum well devices

We present a simple quantum well electroabsorption calculator (SQWEAC) for the germanium material system to facilitate optoelectronic modulator design. We show SQWEAC is valid for a range of quantum well sizes and growth conditions.

Intervalley scattering and field screening in germanium/silicon-germanium quantum wells

The scattering time of electrons from the direct Γ to the indirect L valley in the conduction band of germanium/silicon-germanium quantum wells is measured to be ∼250 fs. Carrier field screening is also observed and modeled.

Ultrafast absorption recovery in germanium/silicon-germanium quantum wells

We show ultrafast absorption changes in germanium-based quantum wells, attributing these to exciton bleaching, ultrafast intervalley scattering, carrier escape and transport, and field screening, and compare these experimental results to modeling.