A. F. J. Levi

Whispering-gallery mode microdisk lasers

A new microlaser design based on the high‐reflectivity whispering‐gallery modes around the edge of a thin semiconductor microdisk is described and initial experimental results are presented. Optical confinement within the thin disk plane results in a microresonator with potential for single‐mode, ultralow threshold lasers. Initial experiments use selective etching techniques in the InP/InGaAsP system to achieve 3–10 μm diameter disks as thin as 500 A suspended in air or SiO2 on an InP pedestal. Optically pumped InGaAs quantum wells provide sufficient gain when cooled with liquid nitrogen to obtain single‐mode lasing at 1.3 and 1.5 μm wavelengths with threshold pump powers below 100 μW.

Graphene-Silicon Schottky Diodes

We have fabricated graphene-silicon Schottky diodes by depositing mechanically exfoliated graphene on top of silicon substrates. The resulting current-voltage characteristics exhibit rectifying diode behavior with a barrier energy of 0.41 eV on n-type silicon and 0.45 eV on p-type silicon at the room temperature. The I-V characteristics measured at 100, 300, and 400 K indicate that temperature strongly influences the ideality factor of graphene-silicon Schottky diodes. The ideality factor, however, does not depend strongly on the number of graphene layers. The optical transparency of the thin graphene layer allows the underlying silicon substrate to absorb incident laser light and generate a photocurrent. Spatially resolved photocurrent measurements reveal the importance of inhomogeneity and series resistance in the devices.

Threshold characteristics of semiconductor microdisk lasers

This letter describes the threshold characteristics of InGaAs/InGaAsP microdisk lasers with optical emission near a wavelength λ=1.52 μm. More than 5% of the total spontaneous emission feeds into the lasing mode as the microdisk diameters reach 2 μm.

Transistor action in Si/CoSi2/Si heterostructures

We report transistor action in a Si/CoSi2/Si structure. The thin silicide layer (<100 A), which acts as the base, is a single‐crystal metal, essentially continuous and locally exhibiting atomically perfect interfaces with Si. The transistor action is manifested by a common base current gain α as high as 0.6 and a voltage gain greater than 10.

Directional light coupling from microdisk lasers

We describe methods for directional coupling of light output from whispering‐gallery mode microdisk lasers. Patterned asymmetries in the shape of microdisk resonators provide control of both direction and intensity of light output without dramatically increasing laser thresholds.

Control of a natural permeable CoSi2 base transistor

We report results on the fabrication of a natural permeable base transistor in a Si/CoSi2/Si heterostructure using molecular beam epitaxy (MBE). No photolithography is required, the transistor action being controlled by MBE growth conditions through the density and size of natural openings in the silicide base. The electrical characteristics of devices processed from such heterostructures are intimately related to the presence of these openings. Common base current gains in the range 0.01–0.95 have been observed and correlated with the size and density of the openings.

Room‐temperature operation of hot‐electron transistors

We demonstrate the first room‐temperature operation of a double heterojunction unipolar hot‐electron transistor. Our test structure has a current gain greater than 10 and a measured current drive capability in excess of 1200 A cm−2. The device uses an indirect, wide‐band‐gap AlSb0.92As0.08 emitter and the transistor base is a 100‐A‐wide InAs layer.

Controllable reduction of critical currents in YBa2Cu3O7−δ films

The critical currents in high quality thin films of the high Tc superconductor, YBa2Cu3O7−δ, can be controllably reduced by orders of magnitude using ion irradiation. This reduction in critical current occurs without substantial decrease in Tc or increase in room‐temperature resistivity. Using this technique, we have fabricated weak links that exhibit an ac Josephson effect at 77 K.The critical currents in high quality thin films of the high Tc superconductor, YBa2Cu3O7−δ, can be controllably reduced by orders of magnitude using ion irradiation. This reduction in critical current occurs without substantial decrease in Tc or increase in room‐temperature resistivity. Using this technique, we have fabricated weak links that exhibit an ac Josephson effect at 77 K.

High-Q microphotonic electro-optic modulator

Abstract A microphotonic mm-wave modulator using simultaneous RF and optical resonance in an electro-optic medium is presented. Theory and simulation of modulator operation is discussed, and experimental results demonstrating modulation using simultaneous resonance in the mm-wave range are reported.

Room‐temperature lasing action in In0.51Ga0.49P/In0.2Ga0.8As microcylinder laser diodes

We report room‐temperature operation of electrically pumped whispering‐gallery mode In0.51Ga0.49P/In0.2Ga0.8As microcylinder laser diodes with emission at wavelength λ=1.0 μm and threshold current Ith=5 mA. Because the lasing modes do not overlap the diode’s central region, carrier density is not efficiently pinned by above‐threshold stimulated emission.