M. Tsuchiya

Formation of GaAs ridge quantum wire structures by molecular beam epitaxy on patterned substrates

A ridge quantum wire structure has been successfully fabricated on a patterned (001) GaAs substrate by first growing a (111)B facet structure with a very sharp ridge and then depositing a thin GaAs quantum well on its top. Electron microscope study has shown that a GaAs wire with the effective lateral width of 17–18 nm is formed at the ridge top. Photoluminescence and cathodoluminescence measurements indicate that one of the luminescence lines comes from the wire region at the ridge and its blue shift (∼60 meV) agrees with the quantum confined energy calculated for the observed wire structure.

Dependence of resonant tunneling current on well widths in AlAs/GaAs/AlAs double barrier diode structures

The dependence of current components on GaAs well widths is studied in AlAs/GaAs/AlAs double barrier diode structures having AlAs barriers of 8 atomic layers. It is shown for the first time that the density JRT of resonant current varies from 8×102 to 1.6×104 Au2009cm−2 by the choice of the well width from 9 to 5 nm in accordance with theoretical calculations. Furthermore, one of these diodes shows excellent current‐voltage characteristics at room temperature with a peak to valley ratio of 3, the highest value ever reported.

Nonlinear photodetection scheme and its system applications to fiber-optic millimeter-wave wireless down-links

We report on our study on the nonlinear photodetection (NL-PD) scheme, which we have proposed and demonstrated as an extremely simple configuration for the optoelectronic millimeter-wave (MM-wave) mixing. The topics described in this paper are as follows: (1) advantageous optical MM-wave link architectures employing the NL-PD techniques; (2) operation principle of NL-PD; (3) detailed characterization of optoelectronic MM-wave mixer properties of a waveguide p-i-n photodiode; (4) experimental demonstration of NL-PD performance in analog and digital fiber-optic MM-wave transmission over a 30 km single-mode fiber; and (5) proposal of its possible applications to fiber-optic MM-wave wireless down-links, such as passive antenna base stations and coax-wireless-hybrid fiber-optic CATV systems.

Dependence of resonant tunneling current on Al mole fractions in AlxGa1−xAs‐GaAs‐AlxGa1−xAs double barrier structures

Electron transport in AlxGa1−xAs‐GaAs‐AlxGa1−xAs double barrier tunneling structures is studied as a function of Al content x in the barrier for the case where the thickness of the barrier is 11 atomic layers and that of the GaAs well layer is 7 nm. It is shown that the density JRT of resonant tunneling current varies systematically from 7.0×102 to 3.5×104 Au2009cm−2 as x is varied from 1.0 to 0.43. These data are found to be quantitatively explained by the theoretical calculation, in which the band discontinuity at Γ valley is taken as the barrier height. This result indicates that the electron tunneling through the double barrier structure is dominated by the barrier height of Γ minimum even when the subsidiary minima have lower energies (0.45≤x≤1.0).

Chirp and stability of mode-locked semiconductor lasers

A numerical study of mode-locked semiconductor lasers is presented with special attention to the chirp characteristics and to dispersion-related criteria for stable pulse-train emission. The dependence of the pulse chirp upon the refractive-index change, both with carrier density and carrier temperature changes, is discussed. The experimental observation of blue-chirped pulses for passive mode-locking in contrast to red-chirped pulses for active mode-locking is found to be due to the different contributions of gain and absorber media to the refractive-index change. In addition, it is revealed that the boundary of the stable operation regime is critically influenced by the spectral characteristics of laser and external cavity. Design considerations toward the achievement of high pulse energy, narrow spectral bandwidth, and linear chirp are given.

Gigantic negative transconductance and mobility modulation in a double‐quantum‐well structure via gate‐controlled resonant coupling

Transport of two‐dimensional electrons in a novel double‐quantum‐well (DQW) field‐effect transistor was systematically studied with emphasis on the effect of resonant interaction. By introducing ionized impurities appropriately into one of the QWs, wave‐function‐dependent scattering process was sensitively controlled by the gate voltage Vg. A prominent valley structure was observed in the channel conductance −Vg characteristics at resonance with the peak‐to‐valley ratio of 3 at 4.2 K. This nonlinear characteristic is caused by the deformation of electron wave functions in the DQW and is found to be well explained by the theoretical calculation. The DQW structure can be utilized for both negative transconductance and velocity modulation devices.

MODULATION OF ONE-DIMENSIONAL ELECTRON DENSITY IN N-ALGAAS/GAAS EDGE QUANTUM WIRE TRANSISTOR

An array of AlGaAs/GaAs edge quantum wires (EQWIs) with an effective width of 80 nm was successfully prepared on a (111)B microfacet structure on a patterned substrate by molecular beam epitaxy. By forming a gate electrode on the wires, field effect transistor action has been successfully demonstrated. The conductance of the wire measured in magnetic fields has exhibited a clear Shubnikov–de Haas (SdH) oscillation, and its Landau plot shows a characteristic nonlinearity caused by the magnetic depopulation of one‐dimensional (1D) subbands. It has been found that as the gate voltage decreases, the SdH peaks shift systematically toward lower magnetic fields, indicating a successful modulation of 1D electron density in the EQWI.

Energy levels and electron wave functions in semiconductor quantum wells having superlattice alloylike material (0.9 nm GaAs/0.9 nm AlGaAs) as barrier layers

Energy levels and wave functions of carriers are studied both experimentally and theoretically in 4 nm GaAs quantum wells (QW’s), for the case when barriers are formed with alternating layers of 0.9 nm GaAs/0.9 nm AlxGa1−xAs (x=0.39). The photoluminescence spectra of the QW’s are studied at 77 K and are found nearly equivalent to that of conventional QW’s having alloy barriers with Al content of 0.26, which is much higher than the averaged alloy composition (∼0.2). The modified Kronig–Penney analysis is found effective in predicting the observed energy and has clarified a feature of enhanced penetration of wave function into the novel barrier layer.

Resonant tunneling in quantum heterostructures: electron transport, dynamics, and device applications

The current understanding of the resonant tunneling process of electrons in double-barrier (DB) diodes and in coupled quantum well structures is described. The authors examine the validity of the simple Fabry-Perot model in describing the electron transport in actual DB diodes. They then describe a picosecond laser study to clarify the dynamics of resonant tunneling, including the intrinsic time delay associated with the multiple reflection of electron waves. Lastly, they discuss both the current state and prospects of device applications for high-speed electronics and optoelectronics. >

Angle-independent beam steering using a liquid crystal grating with multi-resistive electrodes

Abstract We report on the design and experimental evaluation of a liquid crystal beam deflector generating a blazed refractive index profile. By using multi-resistive electrodes, the wavefront of monochromatic light passing through the active areas of the array becomes spatially modulated in a continuous and linear manner. The special features of such a beam deflector are reduced number of electrodes and constant intensity of the deflected beam over the whole beam deflection range. Owing to the careful design and optimized fabrication process of the electrodes, the angular independence of steering efficiency has been successfully confirmed for the first time in a beam deflection experiment. The optical throughput of our beam deflector was enhanced by a novel Talbot array illuminator.