Winston Kong Chan

Field effect in epitaxial graphene on a silicon carbide substrate

The authors report a strong field effect observed at room temperature in epitaxially synthesized, as opposed to exfoliated, graphene. The graphene formed on the silicon face of a 4H silicon carbide substrate was photolithographically patterned into isolated active regions for the semimetal graphene-based transistors. Gold electrodes and a polymer dielectric were used in the top-gate transistors. The demonstration of a field effect mobility of 535cm2∕Vs was attributed to the transistor geometry that maximizes conductance modulation, although the mobility is lower than observed in exfoliated graphene possibly due to grain boundaries caused by the rough morphology of the substrate surface.

Room-temperature 2.5 μm InGaAsSb/AlGaAsSb diode lasers emitting 1 W continuous waves

We have characterized 2.5-μm-wavelength InGaAsSb/AlGaAsSb/GaSb two-quantum-well diode lasers that emit 1 W continuous waves from a 100-μm-wide aperture at a temperature of 12 °C. The threshold current density is 250 A/cm2, and the external quantum efficiency near threshold is 0.36. The wall–plug efficiency reaches a maximum of 12% at a current of 2 A. Operating in the pulsed-current mode, the devices output nearly 5 W at 20 °C. These lasers exhibit internal losses of about 4 cm−1 and differential series resistances of about 0.1 Ω. A broad-waveguide design lowers internal losses, and highly doped transition regions between the cladding layers and the GaSb reduces series resistance.

Single-lobe, surface-normal beam surface emission from second-order distributed feedback lasers with half-wave grating phase shift

Half-wave phase shifts were fabricated in the center of second-order GaAs gratings, for use in surface-emitting, horizontal-cavity, semiconductor diode lasers (λ=0.98 μm). Incorporating such gratings in diode lasers with distributed-feedback (DFB) active regions and distributed Bragg reflectors (DBRs) is found to provide surface-normal, single-lobe beam emission, as predicted by theory. InGaAs/AlGaAs/InGaP, two-quantum-well structures are employed. A 500-μm-long GaAs/Au second-order grating with half-wave phase shift represents the DFB region, which provides feedback and unidirectional light outcoupling. GaAs/SiO2/Au, 500-μm-long, second-order gratings are the DBR regions, on either side of the DFB region, which provide both frequency-selective feedback as well as unidirectional outcoupling. Lateral-mode control is achieved via a 2.5-μm-wide ridge waveguide. Surface emission is obtained through a 80-μm-wide window stripe in the metallization on the substrate n-side. Single-frequency lasing in an orthonorm...

Observation of low optical overlap mode propagation in nanoscale indium phosphide membrane waveguides

The authors have developed a nanoscale, rib-loaded waveguide that propagates a low optical overlap mode (LOOM) in which less than 1% of the modal field energy resides in the semiconductor material. Because of the small modal fill factor, the potential for extremely low waveguide propagation loss, on the order of 0.001dB or less, is predicted. Elevated membrane waveguides, 50nm thick with a 50nm thick rib, have been fabricated in InP using a multistep microelectromechanical release process. Both transverse electric and transverse magnetic LOOM propagations have been observed and measurements are compared to theoretical predictions.

Epitaxial InGaAsP/InP photodiode for registration of InP scintillation

Operation of semiconductor scintillators requires optically tight integration of the photoreceiver system on the surface of the scintillator slab. We have implemented an efficient and fast quaternary InGaAsP pin photodiode, epitaxially grown on the surface of an InP scintillator wafer and sensitive to InP luminescence. The diode is characterized by an extremely low room-temperature dark current, about 1 nA/cm 2 at the reverse bias of 2 V. The low leakage makes possible a sensitive readout circuitry even though the diode has a large area (1 � 1m m 2 ) and therefore large capacitance (50 pF). Results of electrical, optical and radiation testing of the diodes are presented. Detection of individual a-particles and g-photons is demonstrated.

Multi-frequency laser monolithically integrating InGaAsP gain elements with amorphous silicon AWG

We demonstrate a photonic integrated circuit using a novel monolithic integration platform combining InGaAsP gain elements and index matched amorphous silicon waveguide devices. The AWG based multi-frequency laser emits eight 100-GHz-spaced wavelengths near 1550 nm.

Amorphous Silicon Waveguide Components for Monolithic Integration with InGaAsP Gain Sections

Low loss, single mode rib waveguides, based on PECVD deposited multi-layer amorphous silicon are fabricated. These waveguide are refractive index and mode-matched to III/V laser waveguides. Methods for monolithic integration of these passive amorphous silicon waveguides with InGaAsP/InP gain sections are demonstrated. Results of a multi-wavelength laser based on an amorphous silicon arrayed waveguide grating integrated on a single chip with InGaAsP gain sections are presented.

Low optical overlap mode (LOOM) propagation in InP nanoscale membrane waveguides

A guided low optical overlap mode (LOOM) has a modal confinement factor below 1% and potential for extremely low loss. We demonstrate TM LOOM propagation in planar 50-nm-thick InP membrane waveguides and compare to theory.

Indium-Phosphide-Based Mutation-Designed Optical Waveguides for Application

Altering morphology at scales insensible to optical waves by nanofabrication mutates optical properties of semiconductors. Low-optical-overlap modes (LOOMs) are capable of ultra-low propagation loss, nonlinearity and dispersion in high power and signal processing applications.

High-performance surface-normal modulators based on stepped quantum wells

Surface-normal modulators based on stepped quantum wells at /spl lambda//spl sim/1.55 /spl mu/m are demonstrated. These devices have nearly two times better efficiency and 7 dB higher extinction ratio compared to devices with rectangular and coupled-quantum wells.