High relaxation oscillation frequency operation of a QW-LD at thesecond quantized level

We describe a new approach for improving the relaxation oscillation frequency characteristics of quantum-well lasers at the second quantized level. Applying an antireflection (AR) and high-reflection (HR) coating to the laser facets suppresses lasing at the first quantized transition and promotes it at the second quantized transition. The relaxation oscillation frequency for the laser emitting at the second quantized transition is twice as high as that for the laser at the first quantized transition. The increase in the relaxation oscillation frequency of the laser operated at the second quantized transition is mainly due to the increase in the differential gain. Furthermore, the differential quantum efficiency increases to 0.5 in the AR/HR-coated laser     

Numerical analysis of the absorption and the refractive indexchange in arbitrary semiconductor quantum-well structures

A numerical method for the analysis of the absorption spectrum and the refractive index change due to an external electric field in quantum-well structures is presented. The finite-element method and the variational method are used to obtain the subband and the exciton energies in a quantum-well structure, respectively. The absorption spectrum due to the band-to-band and the excitonic transitions is then calculated, and the refractive index change is obtained using the Kramers-Kronig relations. This method is applicable to quantum-well structures with arbitrary potential profiles made of arbitrary semiconductors, because it is based on the finite-element method in which the general boundary condition for the heterointerface is employed. The validity of the method is confirmed by comparing the computed results with the measured ones     

Controlling chaotic behavior of heavy to light hole mixingtunneling by external electric fields

Oscillatory and chaotic motion of heavy to light hole mixing tunneling in asymmetric coupled quantum-well structures can be controlled by an external electric field. Chaotic behavior occurs if the heavy-hole state in the first well is aligned with the light-hole state in the second well under a significant in-plane vector k_∥. Oscillatory motion is recovered if the external electric field disrupts the alignment between the heavy-hole state in the first well and the light-hole state in the second well     

Quantum capture and escape in quantum-well lasers-implications ondirect modulation bandwidth limitations

Analytic expressions for carrier capture and escape currents into quantum wells are derived. The authors find that the escape rate can be as large as the capture rate under typical operating conditions in quantum-well lasers so that the damping and inertia of the relaxation oscillation are significantly increased in these lasers. Implications for the limitation of the modulation bandwidth of quantum-well lasers and its dependence on the quantum-well structure are discussed     

Generation of terahertz electromagnetic pulses from quantum-wellstructures

Terahertz generation from semiconductor quantum-well structures pumped by a femtosecond optical pulse is studied. We propose a three-level model for the electrons and holes in the quantum wells. We then solve the coupled optical Bloch equations directly using a Runge-Kutta method and calculate the terahertz radiation field. We study optical rectification and quantum beats caused by charge oscillations in 1) a coupled quantum well in which quantum beats occur between two electron states of the coupled system and 2) a single-quantum-well structure in which quantum beats occur between light-hole and heavy-hole excitons. Our theoretical results agree very well with the experimentally measured terahertz data     

Push-pull polarization conversion using novel asymmetric coupled quantum-well structures

A novel type of polarization conversion modulator, employing multiple repetitions of two novel InGaAlAs-InAlAs asymmetric coupled quantum-well structures, is proposed. The novelty of the proposed design is that, via the electrorefraction effect, it allows for a change in the TM mode's effective refractive index, which is opposite in sign to the change in the TE mode's effective refractive index. Numerical simulations based on the effective-mass envelope-function approximation show that the modulator provides not only an electrorefractive response similar to other quantum-well modulators but can also provide a low-chirp push-pull polarization modulation.     

Electric-field-induced refractive index variation in quantum-well structure

The refractive index variation in a quantum-well structure by an electric field is given theoretically. The calculated variation is ?1% for an applied field of 3.1×105 V/cm in a 300 ?-thick GaInAsP/InP single quantum well, which is about 39 times larger than the bulk value. A semiconductor quantum-well structure is found theoretically to be a new material with a larger electro-optic coefficient. Application to a new optical switching device is also suggested.     

Electrical and optical properties of near-surface AlGaAs/InGaAs/AlGaAs quantum wells with different quantum-well depths

A series of AlGaAs/InGaAs/AlGaAs quantum-well heterostructures with different quantum-well depths and approximately the same concentrations of two-dimensional electrons is grown by molecular-beam epitaxy. The built-in electric field in the grown samples is determined from the photoreflectance data and, on this basis, the energy-band structure in the quantum-well region is calculated. It is found that the highest mobility μ_e of two-dimensional electrons is attained in the sample with a barrier-layer thickness of L _b = 11 nm. Measurements of the photoluminescence spectra and the band-structure calculations demonstrate that, as the quantum well becomes closer to the surface, the doping profile broadens due to diffusion and segregation processes. The nonmonotonic dependence of μ_e on the distance between the surface and the quantum well is explained.     

Strained InGaAs-GaAs single-quantum-well lasers coupled to n-typeδ-doping-improved static and dynamic performance

A new concept for improving the performance of quantum-well (QW) lasers is reported. The enhancement, both in static and dynamic characteristics, was accomplished by the use of Te n-type δ-doping, coupled to a single strained InGaAs-GaAs QW. The internal parameters were investigated, and their enhancement origin is revealed. It is shown to be mainly a consequence of the higher carrier population in the QW and due to the strong coupling between the QW and the δ-doping well     

Eigenstates and absorption spectra of interdiffused AlGaAs-GaAsmultiple-quantum-well structures

We present a comprehensive analysis of the inter-well coupled eigenstates in interdiffused AlGaAs-GaAs multiple quantum-well structures. A full numerical calculation is considered without any approximation or presumption of the eigenstates. During the initial interdiffusion in the quantum-well structure with well and barrier thickness equaling 100 Å, the wavefunctions behave as in the case of a single quantum well with almost no well-to-well coupling of the states. However, as interdiffusion proceeds, the eigenstate in each subband forms minibands, as in the case of superlattice. Distortion of the coupled wavefunctions can also be observed as a consequence of the nonuniformity of the confined wells at the far sides of the multiquantum-well core. The polarized absorption coefficients, including valence band-mixing, are also calculated. Results show that the blue shift of the absorption edge is greater in the range of 10 Å⩽L _d⩽30 Å. The two-dimensional quantum-well properties are at its strongest in the beginning of interdiffusion. An estimation of the modulator performance also shows an improvement of the contrast ratio and lower absorption loss during the initial stage of interdiffusion. This predicts wavelength tuning range of almost 60 nm     

Inductively Coupled Argon Plasma-Enhanced Quantum-Well Intermixing: Cap Layer Effect and Plasma Process Influence

Comprehensive investigation on inductively coupled argon plasma-enhanced quantum-well intermixing is done on an InGaAs–InP quantum-well structure with p-/n- doped InP and InGaAs caps using polarized edge-emitting photoluminescence analysis technique. The derived diffusion lengths on group V and III sublattices show that the cap material plays an important role as it influences both the accumulation and diffusion of point defects during plasma process and annealing process, respectively. The large blue and red shift can be realized with p-InP and n-InGaAs caps, respectively, by controlling the diffusion length ratio for interdiffusion on two sublattices.      

Amplified spontaneous emission model for quantum-well distributedfeedback lasers

An amplified spontaneous emission model for quantum-well (QW) distributed feedback (DFB) lasers is presented, which takes into account local spontaneous emission, stimulated emission, and real refractive index change which are calculated from the Fermi-Dirac occupancy functions in a self-consistent manner. The local-normal-mode transfer-matrix method is used, which allows a coupling of the local DFB effect with the local QW spontaneous emission and gain. As an example, an analysis is given of a partly gain-coupled DFB laser with periodically etched QWs, which has a large discontinuity of spontaneous emission and gain in high- and low-corrugation regions. It is shown that the side-mode suppression improves with the increase of the number of etched QW's, due to the carrier-density-dependent gain-coupling     

Electrooptical Modulator at Telecommunication Wavelengths Based on GaN–AlN Coupled Quantum Wells

We report on the demonstration of an intersubband (ISB) coupled quantum-well modulator operating at room temperature and telecommunication wavelength using a GaN-AIN quantum-well structure. The optical modulation is shown to result from electron tunneling between the wells. Stark shifting of the ISB absorption is observed. The maximum modulation depth is 0.79% at lambda= 2.3 mum and 0.18% at lambda= 1.37 mum for a mesa device with only 151-nm interaction length. We show that by reducing the mesa size down to 15 times 15 mum², optical modulation bandwidth as large as 3 GHz can be obtained.     

Broad-band wavelength tunable picosecond pulses from CW passivelymode-locked two-section multiple quantum-well lasers

Wavelength tunable CW (continuous-wave) passive mode-locking of a two-section quantum-well laser coupled to an external cavity is demonstrated. A tuning range of 26 nm is achieved with typical autocorrelation full widths at half maximum of 4.5 ps. The pulses are not transform limited, having a typical time-bandwidth product of 2.5     

Design of a Grating-Assisted Lateral Directional Coupler by Impurity-Induced Quantum-Well Intermixing of InGaAs/GaAs

An asymmetric waveguide grating-assisted coupler, which is suitable for a coarse wavelength division multiplexing system, based on a quantum-well intermixing process in InGaAs/GaAs, has been designed. The proposed device processing is similar to that used in nominal microelectronics processing. The device has been modeled from the first principle of refractive- index change due to F-implantation and anneal. The coupler has been analyzed by quasi-static effective index as well as finite difference methods in conjunction with an orthonormal-mode beam propagation method. A power-coupling ratio of ~90% at a wavelength of 1.51 mum with a coupled wavelength bandwidth of ~22 nm has been obtained. The coupler lengths are within a few millimeters, and the device is suitable for multiwavelength integration.     

Determination of Piezoelectric Fields Across InGaN/GaN Quantum Wells by Means of Electron Holography

Electron holography was used to determine the piezoelectric fields across an InGaN/GaN quantum-well structure. Holograms were taken with the sample intentionally tilted such that adjacent layers overlapped. The phase changes in the overlapping regions were analyzed to determine the piezoelectric fields in each well. It was shown that the piezoelectric field was strongest at the central part of the quantum-well structure. The field strength averaged over eight InGaN wells was ~2.2 MV/cm.     

Electroluminescence properties of a whispering-gallery-mode laser with coupled disk cavities

Whispering-gallery-mode (WGM) lasers (emission wavelength λ = 2.1–2.4 μm) with coupled disk cavities connected by a bridge are developed on the basis of a GaInAsSb/GaAlAsSb quantum-well nano-heterostructure and their emission spectra and optical far-field patterns are studied. Almost single-mode lasing in one spatial mode is observed in a wide range of supply currents. The emission wavelength grows with increasing current due to heating of the laser, and this phenomenon can be used in diode-laser spectroscopy. An assumption is made that the flux of generated emission can pass in lasers of this kind from one cavity to the other and back along the cavity-connecting bridge. It is found that strong narrowing of the laser emission pattern occurs with increasing current near the plane separating the coupled cavities.     

Quantum-well SEED optical oscillators

Oscillation at 110 MHz in a GaAs-GaAlAs quantum-well SEED (self-electrooptic effect device) optical oscillator is considered. Optimization of device length and optical pump wavelength for high-frequency oscillation is discussed. Frequency tuning is obtained by adjusting the oscillator bias voltage or optical pump power, and the oscillator can be injection locked to modulated optical signals. Frequency fluctuations caused by perturbative Gaussian noise and 1/f frequency noise are observed; the 1/f noise in an 8.5-MHz oscillator limited the minimum frequency variance to 230 Hz²     

High-power and low-divergence 980-nm InGaAs-GaAsP-AlGaAs strain-compensated quantum-well diode laser grown by MOCVD

High-power InGaAs-GaAsP-AlGaAs strain-compensated separate-confinement heterostructure double quantum-well lasers emitting at 980-nm wavelength have been grown by low-pressure metal-organic epitaxial chemical vapor deposition. Fabricated with a ridge waveguide, the lasers achieved an output power of 386 mW in the fundamental lateral mode without any kink being observed. By optimizing the laser structure parameters, a very low transverse beam divergence of 22.1/spl deg/ and a high slope efficiency of up to 0.89 mW/mA were obtained. The narrow transverse far field enables an output power of over 290 mW to be coupled into a single-mode fiber with a high coupling efficiency of 83.2% at 430 mA. Life testing at various powers shows excellent long-term reliability after 3500 h.     

I-V characteristics by radial tunneling in double-barrier tunnelingdiodes with cylindrical barriers

Radial tunneling transport in a double-barrier tunneling diode (DBTD) with cylindrical barriers has been theoretically studied. Calculations use a generalization of the transfer matrix method. Fine oscillation is demonstrated in transmission spectra and I-V characteristics. It is attributed to the coherence of traveling waves associated with cylindrical geometry. Various Al_xGa_1-x As-GaAs DBTD's with cylindrical barriers have been examined at absolute zero and room temperatures. Results are compared with DBTD's with planar geometry. It is recognized that different bias direction induces asymmetric I-V characteristics. Devices that operate on this inborn asymmetry are proposed. The device characteristics could be less sensitive to temperature than that of planar structures due to the fundamental operation principle. The results are useful to understand charge transport by the fast tunneling in quantum-well and superlattice structures with different geometries