Y. Arakawa

Multidimensional quantum well laser and temperature dependence of its threshold current

A new type of semiconductor laser is studied, in which injected carriers in the active region are quantum mechanically confined in two or three dimensions (2D or 3D). Effects of such confinements on the lasing characteristics are analyzed. Most important, the threshold current of such laser is predicted to be far less temperature sensitive than that of conventional lasers, reflecting the reduced dimensionality of electronic state. In the case of 3D‐QW laser, the temperature dependence is virtually eliminated. An experiment on 2D quantum well lasers is performed by placing a conventional laser in a strong magnetic field (30 T) and has demonstrated the predicted increase of T0 value from 144 to 313 °C.

Structural and optical properties of type II GaSb/GaAs self-assembled quantum dots grown by molecular beam epitaxy

We report structural and optical properties of GaSb/GaAs self-assembled quantum dots (QDs) grown by molecular beam epitaxy. The QDs, with nanometer-scale dimensions, were characterized by atomic force microscopy. Furthermore, in photoluminescence (PL) measurements the feature from the QDs was observed at ∼1.1 eV, clearly separated from that of the wetting layer at ∼1.3 eV. With increasing excitation power, the peak from the QDs displayed a large shift towards higher energy. In addition, the temperature dependence of PL yielded a large thermal activation energy, 130 meV, confirming the strong localization of excitons in the QDs.

Artificial control of optical gain polarization by stacking quantum dot layers

Polarization insensitivity of InAs∕GaAs quantum dot (QD) optical amplifier has been demonstrated by controlling the dot shape. The height of the QD has been controlled by stacking closely InAs islands to form a columnar QD. Room-temperature polarized amplified spontaneous emission from the columnar QDs has been investigated by using variable stripe-length method. With increasing the aspect ratio, transverse-magnetic-mode-dominant optical gain has been achieved. We obtained almost polarization insensitive optical gain for QDs with seven stacking layers.

Nonlinear gain effects due to carrier heating and spectral holeburning in strained-quantum-well lasers

The authors present numerical results for the nonlinear gain effects due to carrier heating and spectral holeburning in 50 AA strained In/sub x/Ga/sub 1-x/As/Al/sub 0.3/Ga/sub 0.7/As quantum-well lasers. Calculations are performed on the basis of a 4*4 matrix system consisting of the usual Kohn-Luttinger Hamiltonian and a strain Hamiltonian for the valence band structure. In addition, the authors perform a small-signal analysis based on four dynamic equations for the photon density, carrier density, and two supplementary equations for the electron and hole energy densities to obtain information about nonlinear gain coefficients. The results indicate that the nonlinear gain is enhanced with the strain mainly due to the rapid increase of the carrier heating effect as the carrier density at the lasing threshold decreases, and that carrier heating is about five times as important compared to spectral holeburning.<<ETX>>

Control of optical polarization anisotropy in edge emitting luminescence of InAs/GaAs self-assembled quantum dots

We have studied the polarization properties of cleaved-edge photoluminescence (PL) from InAs/GaAs self-assembled quantum dots. Transverse-electric (TE) and transverse-magnetic (TM) mode PL intensities have been analyzed for the dots having 8 nm InxGa1−xAs capping layer with indium (In) composition of x=0 and 0.13. Polarization results show a dramatic change with the capping layer In compositions; TE-mode dominant PL is observed for dots with x=0, on the other hand, TM-mode dominant PL for dots with x=0.13. This polarization change has been attributed to the dot shape change using transmission electron microscopy images. These results suggest that the optical polarization anisotropy of the quantum dots can be controlled by manipulating the capping layer In composition.

Electric-field control of tunneling magnetoresistance effect in a Ni∕InAs∕Ni quantum-dot spin valve

The authors demonstrate an electric-field control of tunneling magnetoresistance (TMR) effect in a semiconductor quantum-dot spin-valve device. By using ferromagnetic Ni nanogap electrodes, they observe the Coulomb blockade oscillations at a small bias voltage. In the vicinity of the Coulomb blockade peak, the TMR effect is significantly modulated and even its sign is switched by changing the gate voltage, where the sign of the TMR value changes at the resonant condition.

Conductance of single thiolated poly(GC)-poly(GC) DNA molecules

We use ultrahigh vacuum scanning tunneling microscopy∕spectroscopy (UHV-STM∕STS) to investigate the electronic properties of single thiolated 12-base-pair poly(GC)-poly(GC) DNA molecules on a Au(111) surface at room temperature. Reproducible current-voltage curves of the DNA are obtained at variable sample-tip separations. The normalized conductance, which can be interpreted as the density of states, shows a well-defined wide band gap. UHV-STM∕STS opens up a novel technique to probe the electronic properties of biomolecules on surfaces at the atomic level.

Quantum-Dot Semiconductor Optical Amplifiers With Polarization-Independent Gains in 1.5-

We have demonstrated a polarization-independent gain in semiconductor optical amplifiers that have columnar quantum dots surrounded by strained side barriers in 1.5-mum wavelength bands. We obtained a polarization-dependent gain of 0.5 dB with a gain of 10 dB and a saturation output power of 18 dBm at a wavelength of 1.55 mum.

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The growth of InAs on GaAs(001) is of great interest primarily due to the self-assembly of arrays of quantum dots (QDs) with excellent opto-electronic properties. However, a basic understanding of their spontaneous formation is lacking. Advanced experimental methods are required to probe these nanostructures dynamically in order to elucidate their growth mechanism. Scanning tunneling microscopy (STM) has been successfully applied to many GaAs-based materials grown by molecular beam epitaxy (MBE). Typical STM-MBE experiments involve quenching the sample and transferring it to a remote STM chamber under arsenic-free ultra-high vacuum. In the case of GaAs-based materials grown at substrate temperatures of 400-600^oC, operating the STM at room temperature ensures that the surface is essentially static on the time scale of STM imaging. To attempt dynamic experiments requires a system in which STM and MBE are incorporated into one unit in order to scan in situ during growth. Here, we discuss in situ STM results from just such a system, covering both QDs and the dynamics of the wetting layer.

m Wavelength Bands

We experimentally study the transport features of electrons in a spin-diode structure consisting of a single semiconductor quantum dot (QD) weakly coupled to one nonmagnetic and one ferromagnetic (FM) lead, in which the QD has an artificial atomic nature. A Coulomb stability diamond shows asymmetric features with respect to the polarity of the bias voltage. For the regime of two-electron tunneling, we find anomalous suppression of the current for both forward and reverse bias. We discuss possible mechanisms of the anomalous current suppression in terms of spin blockade via the QD-FM interface at the ground state of a two-electron QD.