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     

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     

Spectroscopic study of Ga-doped Ge under uniaxial pressure

To identify the optical transitions responsible for the excitation of long-wavelength stimulated emission in uniaxially compressed Ga-doped Ge, the optical absorption and photoconductivity spectra of the material were investigated at a wide range of pressures in directions [111] and [001]. The dependence of the valence band splitting between the light-and heavy-hole subbands in Ge as a function of the applied pressure was found. As determined from this dependence, the deformation potential constants for the valence band appeared to be less than the presently accepted values. It is shown that, as pressure increases, some of the excited states of the Ga impurity levels reach the light-hole band, enter it, and remain close to its edge (the resonant states). It is possible that a population inversion of these resonant states gives rise to the excitation of stimulated emission at a photon energy of about 10 meV. No specific features confirming the existence of resonant impurity states near the edge of the heavy-hole band were found in the spectra.     

The small-signal response of 1.5 μm multiple-quantum-well lasersin a two-band-model approximation

The variation of the small-signal response of 1.5 μm unstrained multiple quantum-well lasers with the number of wells (N_W ) is studied theoretically in a two-band-model (TB) approximation. The quasi-Fermi energies, together with gain and spontaneous emission rate spectra, are formulated analytically assuming a finite-well model and flatband conditions, including the contributions from carriers in both the wells and the barriers. The gain spectrum shows two major peaks located at the lowest heavy-hole and light-hole transitions. Therefore, the lasers under investigation are treated as three-level systems. The optical confinement factors are evaluated numerically by the matrix transfer method. The traditional rate equations are reformulated and solved for the frequency and damping rate of the relaxation oscillations in terms of an equivalent circuit     

三维半导体GaAs量子阱微腔中的腔极化激元

半导体微腔中腔模和激子模耦合形成腔极化激元,三维微腔中由于横向限定腔模和激子模形成离散化的本征模式.本文计算了远离截止近似下,三维半导体微腔中空腔腔模的能量与微腔半径的关系;及腔模和激子模耦合后,三维半导体柱型微腔中具有相同角量子数和径向量子数的两个低阶腔模、重空穴激子模、轻空穴激子模耦合形成的腔极化激元能量随微腔半径变化的情况.结果表明随着微腔半径的减小腔模能量蓝移,腔模与相应的重空穴激子模、轻空穴激子模耦合形成的腔极化激元的三支随着微腔半径的减小存在明显的反交叉行为.随着微腔半径的变化,极化激元的三支所体现的模     

Si-based quantum staircase terahertz lasers

Design results are presented for electrically pumped quantum staircase intersubband p-i-p SiGe/Si strain-balanced superlattice lasers to be operated at 77 K or higher. The wavelength of laser emission will be in the THz range. Two approaches of quantum staircase lasers will be presented, one utilizes the inverted light-hole effective mass, while the other inverted heavy-hole mass. Optical gain on the order of a few 100 cm⁻¹ can be achieved for both laser designs.     

Envelope function calculations of linear and nonlinear opticalgains in a strained-layer quantum-well laser

The linear and nonlinear optical gain of strained-layer InGaAs-AlGaAs quantum well (QW) lasers are studied theoretically, with band mixing effects taken into account. Effects of the biaxial compressive strain of the InGaAs-AlGaAs QW on the band structure are investigated by solving for the Pikus-Bir Hamiltonian. The biaxial compressive strain separates the HH and the LH subbands by pulling down the HH subbands and pushing the LH subbands away from the valence band edge. Since the C-HH transition is dominated by the TE polarization, it is expected that the TE mode gain would be substantially larger than the TM mode gain. The gain and the gain-suppression coefficient are calculated from the complex optical susceptibility obtained by the density matrix formalism. Optical output power is calculated by solving the rate equations for the stationary states with nonlinear gain suppression. The calculated L-I characteristics shows reasonable agreement with the experimental data     

GaAs／AlGaAs非对称耦合双阱光荧光的温度依赖关系

报道了ＧａＡｓ／ＡｌＧａＡｓ非对称耦合双量子阱ｐｉｎ结构在不同温度下的光荧光谱，观察到宽阱与窄阱重空穴激子峰荧光强度随温度上升而较快下降的不同变化关系，结果表明窄阱电子的热发射是导致窄阱光荧光强度随温度上升而较快下降的主要原因。同时观测到宽阱轻空穴激子峰强度特环的温度依赖关系，并分析了其物理机制。     

On the optimization of InGaAs-InAlAs quantum-well structures forelectroabsorption modulators

Several InGaAs-InAlAs multiple quantum-well structures grown by metalorganic vapor phase epitaxy (MOVPE), with various Ga content and quantum-well width, have been investigated for electroabsorption modulators (EAM's). The light-hole heavy-hole splitting, the chirp parameter, the insertion loss and the figure of merit ΓΔα/F of the different InGaAs-InAlAs structures have been evaluated with photocurrent, photoluminescence, absorption and X-ray measurements. It was then possible to experimentally study the influence of different parameters of the multiple quantum-well structures on the device performance. The use of tensile strained barriers are believed to be responsible for the improvement in the figure of merit. Structures with unresolved light-hole and heavy-hole transitions, with negligible chirp, with adequate insertion loss and with extremely high values for ΓΔα/F have been obtained, however, not simultaneously     

High-temperature excitons and enhanced electroabsorption in InGaAs/InAlAs multiple quantum wells

Sharp heavy-hole and light-hole excitons are clearly observed for the first time in InGaAs/InAlAs multiple-quantum-well (MQW) structures at temperatures ranging from ?190°C to 70°C. The halfwidth of the heavy-hole exciton line is as narrow as 6.2 meV at room temperature. InGaAs/InAlAs MQWs are prepared in a PIN doped configuration by molecular beam epitaxy. An enhanced electroabsorption effect is also clearly observed in long-wavelength-region MQWs.     

Analysis of GaInP/AlGaInP compressive strainedmultiple-quantum-well laser

We have analyzed GaInP/AlGaInP compressive strained MQW lasers, with theoretical calculation and experimental results. Our calculations of TE polarized gain, where the valence subband mixing and the heterobarrier leakage current are taken into account, are in good agreement with the experimental results. When a compressive strain of up to 0.5% is induced in the quantum wells, the density of states near the valence band edge is decreased, due to the reduction of heavy-hole and light-hole subband mixing. At the threshold condition, the compressive strain reduces not only the radiative recombination current, but also the hetero-barrier leakage current. Therefore, the threshold current is reduced, and its temperature dependence is found to be small, In the analysis, we also show that when larger compressive strain of more than 0.5% is induced in the 40-Å-thick quantum wells, the threshold characteristics are degraded     

Bipolar quantum-transport modeling of carrier injection into aSCH-quantum-well laser

A quantum mechanical model of carrier transport in separate confinement heterostructure (SCH) quantum-well lasers based on the Wigner function is presented for the first time. In the simulation, the three quantum Liouville equations with respect to the Wigner functions defined for electron, heavy-hole, and light-hole are solved simultaneously with the Poisson's equation to consider self-consistency in potential, As a simulation model, InGaAsP-InGaAs SCH quantum-well lasers are considered. The carrier injection into single and double quantum-well lasers is simulated. It is demonstrated that the amount of electrons and holes injected into the single quantum-well active layer is not equal in general if the quantum transport is considered. In the double quantum-well structure, the bottleneck phenomenon of heavy-hole injection into the second well and the quite different shape of heavy-hole density profiles in the two wells are simulated     

Optical properties of strained layer (111)B Al0.15Ga0.85As-In0.04Ga0.96as quantum well heterostructures

Data presented here demonstrate that strained-layer (111)B Al_0.15Ga_0.85As-In_0.04Ga_0.96As quantum wells exhibit unique optical properties when compared to otherwise identical (100) oriented strained layers. Photoluminescence measurements identify a strain-induced electric field of order 6.7 Vμm^-1 within the (111)B quantum well that is not present for the (100) case. Photoluminescence excitation spectroscopy measurements show that the heavy-hole to light-hole energy band splitting is approximately 7 meV larger for the (111)B structure than for the (100) structure. Howard Hughes Doctoral Fellow IBM Graduate Fellow     

Tensile-strained GaInAsP-InP quantum-well lasers emitting at 1.3μm

Tensile-strained GaInAsP-InP quantum-well (QW) lasers emitting at 1.3 μm are investigated. Low-pressure metalorganic chemical vapor deposition (LP-MOCVD) is used for crystal growth. High-resolution X-ray diffraction shows good agreement with theoretical simulation, photoluminescence spectra have good energy separation between light-hole and heavy-hole bands due to biaxial tension. The lowest threshold current density for infinite cavity length J_th/N_w ^∞ of 100 A/cm² is obtained for the device with -1.15% strain and N_w=3. The amount of strain which gives the lowest J_th/N_w^∞ experimentally clarified is around -1.2%. Threshold current of a buried-heterostructure (BH) laser is reduced to be as low as 1.0 mA. Enhanced differential gain of 7.1×10^-16 cm² is also confirmed by measurements of relative intensity noise. Much improved threshold characteristic with the feasibility of submilliamp threshold current can be achievable by optimizing the BH structure. The tensile-strained QW laser emitting at 1.3 μm with very low power consumption is attractive for the light source of fiber in the loop system and optical interconnection applications     

Theoretical and experimental study of optical gain and linewidth enhancement factor of type-I quantum-cascade lasers

A theoretical and experimental study of the optical gain and the linewidth enhancement factor (LEF) of a type-I quantum-cascade (QC) laser is reported. QC lasers have a symmetrical gain spectrum because the optical transition occurs between conduction subbands. According to the Kramers-Kronig relation, a zero LEF is predicted at the gain peak, but there has been no experimental observation of a zero LEF. There are other mechanisms that affect the LEF such as device self-heating, and the refractive index change due to other transition states not involved in lasing action. In this paper, the effects of these mechanisms on the LEF of a type-I QC laser are investigated theoretically and experimentally. The optical gain spectrum and the LEF are measured using the Hakki-Paoli method. Device self-heating on the wavelength shift in the Fabry-Perot modes is isolated by measuring the shift of the lasing wavelength above the threshold current. The band structure of a QC laser is calculated by solving the Schro/spl uml/dinger-Poisson equation self-consistently. We use the Gaussian lineshape function for gain change and the confluent hypergeometric function of the first kind for refractive index change, which satisfies the Kramers-Kronig relation. The refractive index change caused by various transition states is calculated by the theoretical model of a type-I QC laser. The calculated LEF shows good agreement with the experimental measurement.     

Doping effects on intersubband and interband optical transitions inGaSb-InAs superlattices

Normal incidence intersubband and interband absorptions of a novel type II GaSb-InAs superlattices can be obtained by utilizing the various doped-type cap and buffer layers. Moreover, the types and intensities of the absorptions could also be modulated by changing doping concentration. The intersubband transition can occur due to the strong mixing of the heavy-hole band and the light-hole band for InAs n-type cap and buffer layers. But the interband transition is a result of coupling between the wave-functions of the first conduction subband and the first heavy-hole subband for GaSb p-type cap and buffer layers. Both the intensities of intersubband can be modulated by changing doping concentration, and the corresponding wavelengths are in the ranges of 3-5 μm and 8-14 μm, respectively. Hence, it shows the potential application as an infrared photodetector     

Enhanced optical polarization anisotropy in quantum wells under anisotropic tensile strain

Anisotropic in-plane strain in quantum wells leads to an optical polarization anisotropy that can be exploited in optoelectronic devices such as modulators. A theoretical model shows that the behavior of the polarization anisotropy with increasing strain anisotropy is radically different for quantum wells under anisotropic tensile and compressive strains of equal magnitude. This strikingly different behavior arises from the different valence-subband mixing that occurs in the cases of anisotropic tensile and compressive strain. Specifically, the mixing of the first heavy- and light-hole subbands that occurs only under anisotropic tensile strain is central to the polarization anisotropy.     

Modeling of modulation-doped multiple-quantum-well structures inapplied electric fields using the transfer-matrix technique

Modulation-doped and intrinsic multiple-quantum-well (MQW) structures under applied electric fields are investigated using the transfer-matrix technique (TMT). A method for locating quasi-eigenvalue energies is introduced and compared to traditional techniques on the basis of the occupation probability and transmission coefficient. Electron and heavy-hole energy quasi-eigenvalues and wave functions are calculated for modulation-doped and intrinsic quantum wells. The upper subbands of the two cases are found to vary significantly from one another in the presence of applied electric fields. The TMT is demonstrated to be a versatile method for modeling MQW structures under linear and nonlinear electric fields     

Optical Gain of V—groove Zn1—xCdxSe／ZnSe Quantum Wires

The subband structures, distributions of electron and hole wave functions, state density, optical gain spectra, and transparency carrier density of the V-groove Zn 1-x Cd x Se/ZnSe quantum wires are investigated theoretically using four band effective-mass Hamiltonian, which takes into account the effects of the valence band anisotropy and the band mixing. The biaxial strain effect for quantum wires is included in the calculation. The compressive strain in the Zn 1-x Cd x Se wire region increases the energy separation between the uppermost subbands. The optical gain with xy -polarized light is enhanced, while optical gain with z -polarized light is strongly decreased. The xy -polarized optical gain spectrum has a peak at around 2.541 eV, with the transparency carrier density of 0.75×10 18 cm -3 . The calculated results also show that the strain tends to increase the quantum confinement and enhance the anisotropy of the optical transitions.     

Uniaxial strain effect on the electronic and optical properties ofwurtzite GaN-AlGaN quantum-well lasers

The valence subband structures of uniaxial-strained wurtzite (WZ) GaN-AlGaN quantum wells (QW's) are calculated using multiband effective-mass theory. The optical gain is investigated using a numerical approach in which we account for the subband structure modification and mixing due to the anisotropic strain in the QW plane. We show that the mixing of the HH and LH bases in the uniaxial-strained (0001) GaN-AlGaN QW decouples |X〉 and |Y〉 at the Γ point, giving two topmost subbands, Y1 and X1, which can be more widely separated than the HH1 and LH1 subbands in the biaxial-strained (0001) GaN-AlGaN QW. We resolve the states of the subband dispersion in terms of the |X〉, |Y〉, and |Z〉 bases, and show the compositional variation as a function of the in-plane wavevector. Under uniaxial strain, it is possible to exploit the existence of the preferred symmetry at the valence band maximum and the reduced band-edge density-of-states due to the anisotropic in-plane energy dispersion to achieve lower transparency carrier and current densities and higher differential gain in comparison with a pseudomorphic biaxial-strained QW. We show that, for a QW laser structure with the optical cavity along the x axis, uniaxial compressive strain in the y direction shows greater improvement than the uniaxial tensile strain in the x direction of the same magnitude. Thus, a suitable uniaxial strain could be used to improve the threshold performance of WZ GaN-based QW lasers