D. Basu

Electrically injected InAs∕GaAs quantum dot spin laser operating at 200K

A spin-polarized vertical cavity surface emitting laser, with InAs∕GaAs self-organized quantum dots as the active gain media, has been fabricated and characterized. Electron spin injection is achieved via a MnAs∕GaAs Schottky tunnel contact. The laser is operated at 200K and, at this temperature, the degree of circular polarization in the output is 8% and the maximum threshold current reduction is 14%. These effects are not observed in identical control devices with nonmagnetic contacts.

High performance tunnel injection quantum dot comb laser

A high-speed multiwavelength quantum dot comb laser, grown by molecular beam epitaxy, is demonstrated. The device is characterized with a 75.9 nm (full width at half maximum) and a 91.4 nm (Δ−15 dB) wide lasing spectrum. There are 105 and 185 simultaneously emitted longitudinal modes with a maximum channel intensity nonuniformity of less than 3 dB in the spectral range of 1231–1252 nm and 1274–1311 nm, respectively, for a laser with 1040 μm cavity length. The channel spacing can be tuned with cavity length and remains invariant in the temperature range of 300–323 K. The small signal modulation bandwidth is 7.5 GHz.A high-speed multiwavelength quantum dot comb laser, grown by molecular beam epitaxy, is demonstrated. The device is characterized with a 75.9 nm (full width at half maximum) and a 91.4 nm (Δ−15 dB) wide lasing spectrum. There are 105 and 185 simultaneously emitted longitudinal modes with a maximum channel intensity nonuniformity of less than 3 dB in the spectral range of 1231–1252 nm and 1274–1311 nm, respectively, for a laser with 1040 μm cavity length. The channel spacing can be tuned with cavity length and remains invariant in the temperature range of 300–323 K. The small signal modulation bandwidth is 7.5 GHz.

Quantum dot polarized light sources

The design, operation and performance of quantum dot spin-polarized vertical cavity surface emitting lasers (VCSELs) and single-photon sources are described and discussed. The effects of spin-induced gain anisotropy on output polarization and threshold current reduction have been studied along with the high-frequency response in a spin-polarized VCSEL. While the output circular polarization in a VCSEL follows the out-of-plane magnetization characteristics of the ferromagnetic spin injector, the output polarization of the spin-polarized single-photon source shows a switching behavior which is explained by invoking the exciton fine structure in the quantum dots and the effects of electron–hole exchange splitting due to in-plane quantum dot rotational asymmetry.

An electrically injected quantum dot spin polarized single photon source

The characteristics of an electrically injected spin polarized single photon source have been investigated. The GaAs-based microcavity diode consists of a single InAs/GaAs self-organized quantum dot as the single photon source and a MnAs/Al0.1Ga0.9As Schottky tunnel barrier for the ferromagnetic contact to inject spin polarized electrons. The measured output circular polarization of the biexciton emission at λ∼1130 nm exhibits a switching behavior as a function of magnetic field, in the Faraday geometry, the value remaining near-zero for B<1 T and ∼6%–8% for B≥1 T. The linear polarization shows a complementary trend. The results are explained in the framework of the exciton fine structure in the quantum dot and the effects of electron-hole exchange splitting, due to in-plane quantum dot anisotropy, and Zeeman splitting on the spin eigenstates and their coupling to the photon field.

Electric field control of magnetoresistance in a lateral InAs quantum well spin valve

The control of magnetoresistance of a lateral spin valve with bias applied to a gate placed outside the channel region is demonstrated. The spin valve channel consists of an InAs/In0.53Ga0.47As/In0.52Al0.48As two-dimensional electron gas lattice matched to (001) InP. The polarizer and analyzer contacts are made with 35 nm type B MnAs/In0.52Al0.48As Schottky tunnel barriers. The magnetoresistance changes from 0.14% to 4% at 10 K in a device in which the spin transport is in the direction of magnetization of the polarizer and analyzer contacts. The effect is absent in a GaAs channel spin valve and other control devices indicating that the change in magnetoresistance is due to Rashba spin-orbit coupling.

A monolithically integrated magneto-optoelectronic circuit

The monolithic integration of a spin valve, an amplifier, and a light emitting diode to form a magneto-optoelectronic integrated circuit on GaAs is demonstrated. The circuit converts the spin polarization information in the channel of the spin valve to an amplified change in light intensity with a gain of 20. The monolithic circuit therefore operates as a magnetoelectronic switch which modulates the light intensity of the light emitting diode.

Gate control and amplification of magnetoresistance in a three-terminal device

Gate control and amplification of magnetoresistance are demonstrated at room temperature in a fully epitaxial three-terminal GaAs-based device. In addition to the two ferromagnetic spin injector and detector electrodes of a MnAs/AlAs/GaAs:Mn/AlAs/MnAs vertical spin valve, a third non-magnetic gate electrode (Ti/Au) is placed directly on top of the heavily p-doped GaAs channel layer. The magnetoresistance of the device can be amplified to reach values as high as 500% at room temperature with the application of a bias to the gate terminal, which modulates the spin selectivity of the tunnel barriers. The experimental results are modeled by solving spin drift-diffusion and tunneling equations self consistently.

Two-dimensional spin diffusion in multiterminal lateral spin valves

The effects of two-dimensional spin diffusion on spin extraction in lateral semiconductor spin valves have been investigated experimentally and theoretically. A ferromagnetic collector terminal of variable size is placed between the ferromagnetic electron spin injector and detector of a conventional lateral spin valve for spin extraction. It is observed that transverse spin diffusion beneath the collector terminal plays an important role along with the conventional longitudinal spin diffusion in describing the overall transport of spin carriers. Two-dimensional spin diffusion reduces the perturbation of the channel electrochemical potentials and improves spin extraction.

Magnetoresistance of lateral semiconductor spin valves

The magnetoresistance of two terminal lateral semiconductor spin valves with respect to varying mesa size is studied. It is shown theoretically that extended regions outside the spin-current path can act as an additional source of spin-relaxation, decreasing the magnetoresistance response. From a simplified expression of magnetoresistance derived from spin-diffusion equations, we show that it is important to etch away these extended regions for devices with channel lengths much smaller than the spin-diffusion length in order to achieve maximum magnetoresistance. Preliminary experimental data on a two terminal local spin valve are in good agreement with the theory established in this article.

Epitaxial growth and characterization of MnAs on InP and In0.53Ga0.47As

The heteroepitaxial growth of type-B ferromagnetic MnAs on InP and lattice matched In0.53Ga0.47As has been investigated for the first time. In situ reflection high energy electron diffraction during molecular beam epitaxy and atomic force microscopy are used to study the reconstruction and morphology, respectively, of the MnAs surface. The in-plane magnetic properties of the film are studied by magneto-optic Kerr effect measurements. The Curie temperature is estimated to be 315 K. The coercivity of 35 nm films measured at room temperature and 10 K are 860 Oe and 1410 Oe, respectively. The measured in-plane magnetocrystalline anisotropy constants Ku1 and Ku2 for the film are 2.747 × 106 and 7.086 × 106 erg cm−3, respectively. The magnetization and hysteresis in the out-of-plane direction are characterized by a saturation magnetic field of 1.2 T and coercivity of 1600 Oe at 10 K.