Pablo O. Vaccaro

Strain-driven self-positioning of micromachined structures

We introduce a method to make self-positioned micromachined structures by using the strain in a pair of lattice-mismatched epitaxial layers. This method allows the fabrication of simple and robust hinges for movable parts, and it can be applied to any pair of lattice-mismatched epitaxial layers, in semiconductors or metals. As an application example, a standing mirror was fabricated. A multilayer structure including an AlGaAs/GaAs dielectric mirror and an InGaAs strained layer was grown by molecular-beam epitaxy on a GaAs substrate. After releasing the multilayer structure from the substrate by selective etching, it moved to its final position powered by the strain release in the InGaAs layer.

Plasma mechanism of terahertz photomixing in high-electron mobility transistor under interband photoexcitation

We show that modulated near-infrared radiation can generate terahertz plasma oscillations in the channel of a high-electron mobility transistor. This effect is associated with a temporarily periodic injection of the electrons photoexcited by modulated near-infrared radiation into the transistor channel. The excitation of the plasma oscillations has the resonant character. It results in the pertinent excitation of the electric current in the external circuit that can be used for generation of terahertz electromagnetic radiation.

An InGaAs-GaAs vertical-cavity surface-emitting laser grown on GaAs(311)A substrate having low threshold and stable polarization

We have fabricated an InGaAs-GaAs vertical-cavity surface-emitting laser (VCSEL) grown on a GaAs(311)A substrate by molecular beam epitaxy (MBE) and demonstrated continuous wave operation at room temperature. A threshold current density of 80 A/cm/sup 2//well and very stable polarization characteristics up to 2.7 times the threshold were achieved. The polarization state with the highest optical intensity was oriented along the [2~33] direction, which is the crystallographic axis exhibiting the maximum gain. An extinction ratio of more than 12.7 dB was obtained between two orthogonal polarization states. The high and anisotropic gain of a (311)A-oriented VCSEL will drastically improve the device performance by optimization and process engineering.

Optical actuation of micromirrors fabricated by the micro-origami technique

Micromirrors were fabricated by the micro-origami technique. This technique allows the fabrication of simple and robust hinges for movable parts, and it can be applied to any pair of lattice mismatched epitaxial layers, in semiconductors or metals. A multilayer structure, including AlGaAs/GaAs component layers and an InGaAs strained layer, was grown by molecular beam epitaxy on a GaAs substrate. After definition of the hinge and mirror’s shape by photolithography, the micromirrors were released from the substrate by selective etching. They moved to their final position powered by the strain release in the InGaAs layer. Optical actuation was achieved by irradiation with the 488 nm line of an argon laser, and the mirror’s position was measured by sensing the reflection of a He–Ne laser. Continuous wave irradiation with a power density of 450 mW/mm2 produced an angular deflection of the mirror of around 0.5°. The frequency response of the mirrors shows a resonance at 25 kHz.

A light-emitting device using a lateral junction grown by molecular beam epitaxy on GaAs (311)A-oriented substrates

A lateral p–n junction allows direct injection of electrons and holes in the active layer of devices such as laser diodes and can reduce carrier relaxation time and increase modulation bandwidth. Light-emitting diodes were made on patterned GaAs (311)A-oriented substrates by using a lateral p–n junction formed in GaAs–silicon-doped epilayers grown by molecular beam epitaxy. Good electroluminescence at room temperature was obtained for both GaAs single layers and GaAs/AlGaAs multiple quantum well structures.

Morphological dependence of lasing modes in two-dimensional quasi-stadium laser diodes

Three types of quasi-stadium laser diodes, which have confocal, concentric, and geometrically unstable end mirrors, were fabricated by a reactive-ion-etching technique. It is shown that the experimental results on the lasing characteristics of these lasers excellently correspond to the theoretical results by the extended Fox–Li mode calculation method. The morphological effects on lasing modes are discussed from the viewpoint of ray-wave correspondence.

Quantum-well microtube constructed from a freestanding thin quantum-well layer

We fabricated and experimentally investigated a nanostructure known as a quantum-well (QW) microtube, which is a fine tube with a micron- or nanometer-order diameter fabricated by rolling a semiconductor GaAs QW. Although the wall thickness is only 40 nm, the system retains the quantum properties of a QW, and photoluminescence from the QW subband can be clearly observed. Even though the QW width is sufficiently small to make the QW subband type-II band-aligned, a type-II to type-I transition caused by uniaxial strain in the microtube allows for optical emission.

Ring and axis mode lasing in quasi-stadium laser diodes with concentric end mirrors

We fabricated quasi-stadium laser diodes whose resonators consist of two concentric curved end mirrors and two straight sidewall mirrors. We observed two lasing modes that correspond to different beam propagations along the cavity axis and along a ring trajectory, and different far-field patterns with wide angular separation. The modes can be selected by control of an electrode pattern. We also show that the far-field patterns numerically obtained by the extended Fox-Li mode calculation method are in good agreement with the experimental results.

Photoluminescence of GaAs/AlGaAs micro-tubes containing uniaxially strained quantum wells

Abstract We fabricated micro-tubes containing two GaAs/AlGaAs quantum wells (QWs) in a section of the tube layer, and studied the optical properties of the embedded QWs. A multilayer structure composed of GaAs and AlGaAs layers and a lattice-mismatched InGaAs layer was epitaxially grown on a GaAs substrate by MBE. This multilayer structure rolled up by the built-in strain when it was freed from the substrate. By measuring the photoluminescence peak shift of the QWs caused by the uniaxial strain, we were able to determine the radial profile of the strain within the micro-tube wall.

Growth and characterization of vertical-cavity surface-emitting lasers grown on (311)A-oriented GaAs substrates by molecular beam epitaxy

We have fabricated InGaAs/GaAs vertical-cavity surface-emitting lasers (VCSELs) grown on GaAs(311)A substrates by molecular beam epitaxy. High-quality crystals for VCSEL structures on (311)A having flat and smooth heterointerfaces were obtained by optimizing the growth conditions. It was experimentally demonstrated that the optical anisotropic gain of (311)-oriented strained-layer quantum wells is effective for polarization control. A [*BAR*2*BAR*33] polarization mode is stable against the injection currents of 3 times the threshold. In addition, the threshold current density of 80 A/(cm2well) is achieved for a device with a laser aperture as large as 25 µ m square, which is comparable to or even lower than that of (100)-oriented VCSELs.