Optoelectronic quantum telecommunications based on spins in semiconductors

The transmission of quantum information over long distances will allow new forms of data security, based on quantum cryptography. These new technologies rely for security on the quantum "uncertainty principle" and on the long distance transmission of "quantum entanglement." A new type of telecommunications device called the "quantum repeater" can allow the faithful transmission of quantum information over worldwide distances, in spite of the inevitably severe losses while propagating along optical fibers. In a quantum repeater, information is stored in the quantum state of a semiconductor electron spin, while complementary entangled information is transmitted as a photon down the optical fiber. This long-range entanglement permits the execution of the teleportation algorithm, which accurately transmits a quantum state over long distances. The quantum repeater is an excellent stepping stone to larger quantum information processors, since teleportation requires only three quantum logic gates. This paper reviews the experimental status of semiconductor quantum repeaters, including the spin resonance transistor logic gates, and the experimental detection of single photons in a manner that preserves their spin information.     

Optical effects based on intersub-band-transitions in quantum wells

Unipolar devices based on intersub-band-transition have been developed in the last decade [Appl. Phys. Lett. 65 (1994) 2901]. However, this technique will not be directed applicable for the longer wavelengths corresponding to Tetrahertz frequencies. In this work we analyze a THz device based on the intersub-band-transitions of an asymmetric double quantum well. We first study the intersub-band optical absorption in superlattice made of asymmetric double quantum wells tailored as a three level system. By applying an external electric field we obtain the Wannier-Stark ladder and can tune the transition energies between the sub-bands to reach the THz absorption between the two excited sub-bands. Although the dipole moments are big, the relative THz absorption to the other frequencies is small. However, reported lifetimes for this system encourage the possibility of getting population inversion, which is the main condition to design a laser. With this goal, it is presented a detailed study of the geometric design of the asymmetric double quantum well by performing an accurate calculation of the energies and wave functions, the dipole moments and the electron-LO-phonon interaction form factors, which are important ingredients of the scattering rates results. Furthermore, we analyze the role of electron-LO-phonon scattering in THz devices.     

Light-scattering determination of electron tunneling gaps in double quantum wells

Resonant inelastic light-scattering techniques have been used to measure directly the single-particle tunneling gap (Δ_SAS) in Al_xGa_1−xAs/GaAs double quantum wells. We have observed a systematic decrease in Δ_SAS with increasing height of the barrier in agreement with the Δ_SAS calculated self-consistently within a Hartree approximation.     

Temperature renormalization of the conduction electron g factor in silicon

The temperature dependence of the conduction electron g factor in silicon is studied theoretically and experimentally. The theory of renormalization of the electron energy in an external magnetic field due to interaction with lattice vibrations is developed. According to the results of the calculation, in second-order perturbation theory, the temperature renormalization of the electron g factor is determined mainly by the difference between the processes of intervalley scattering that occur with spin preservation for spin-up and spin-down electrons. Experimental investigations of n-Si samples by the electron spin resonance method demonstrated the almost linear decrease in the conduction electron g factor with an increase in the temperature in a wide range from 80 to 300 K. The results of the calculation are in good agreement with the experimental data.     

Optically pumped intersubband lasers based on quantum wells

A far infrared (FIR) laser based on intersubband transitions in quantum wells is proposed where a pumping laser is used to create population inversion in the structure. The goal is to develop a structure which operates essentially as a 4-level laser, to minimize bottlenecking of the lower laser state. Multiple quantum wells can be used in the active laser of these structures to enhance the laser gain and the minimum required reflectivity in the cavity structure. The possibility of using both conduction and valence band quantum-well structures are investigated. Our study shows that, due to high intersubband scattering rates in the valence band structure, the creation of population inversion is more difficult and requires a high pumping power density while in the conduction band structure, population inversion can be achieved by a moderate pumping power density. The maximum population inversion in the conduction band structure is estimated to be 2.1×10¹¹ cm², which requires a pumping power density 2 kW cm^-2 for a single quantum well. The threshold power as well as the minimum required reflectivity of the cavity structure for the conduction band scheme are estimated for different well numbers     

Mid-IR optical limiter based on type-II quantum wells

We show that strong optically induced intervalence band transitions in type-II InAs/GaSb/AlSb quantum wells lead to reverse saturable absorption, and propose to apply it to optical limiting in the mid-infrared spectral region. A salient feature of the proposed limiter is the flexibility of the design for different wavelengths and threshold powers. We develop a rigorous theoretical model of the proposed limiter, and use it to estimate the relationship between the key figures of merit: insertion loss, threshold, optical bandwidth and dynamic range. We investigate tradeoffs involving the dynamic range, optical bandwidth and thickness of the limiter, and show that one can attain a favorable combination of these.     

A high-speed intersubband modulator based on quantum interferencein double quantum wells

Calculations on a modulator based on quantum interference in AlGaAs/GaAs asymmetric double quantum wells (QWs) are performed. The modulation of the absorption is based on the anti-crossing behavior of the two lowest states in the coupled wells. At anti-crossing, the oscillator strengths of the transitions from these two lowest states to a higher state are changed in opposite directions. The width of the barrier between the wells should be thick enough to allow a large change in oscillator strength with applied field, yet thin enough so that the absorption peaks of the transitions are resolved. The QWs are designed so that one absorption peak has only a small energy shift for the transition used for modulation while the absorption varies rapidly with the applied voltage. A complete structure including a surface plasmon waveguide is proposed enabling calculations of modal absorption. Parameters important for the performance of the modulator are then determined. An extinction ratio of 10 dB at a wavelength of 8.4 μm is predicted for a device length of 18 μm and a peak-to-peak voltage of 0.9 V. The resistance-capacitance-limited 3-dB bandwidth is 130 GHz. The predicted performance compares very favorably with present interband modulators based on the quantum-confined Stark effect     

Spatial reproduction and multiplication effects arising from the interference of electron waves in 2D semiconductor nanostructures based on rectangular quantum wells

The effects of spatial reproduction and multiplication of the probability current density that arise in a system composed of narrow and wide rectangular quantum wells (QWs) joined in the direction of propagation of the electron wave are investigated theoretically. It is found that the transverse distribution existing at the input of the wide QW is reproduced with a certain accuracy in its repetitive cross sections and that the initial peak splits into several identical peaks of lower intensity within each interval between these cross sections (the multiplication effect). It is shown that these effects arise from the interference of electron waves in the case when the waves propagate simultaneously in several quantum-dimensional subbands of the wide QW.     

基于量子阱带间跃迁的红外探测器原型器件

近期,实验发现PN结中局域载流子具有极高提取效率,并导致吸收系数的大幅度增加。本文报道基于上述现象的新型量子阱带间跃迁红外探测器原型器件的性能。利用含有InGaAs/GaAs多量子阱的PIN二极管,在无表面减反射膜的实验条件下,利用仅100nm的有效吸收厚度,实现了31%的外量子效率。基于该数值推算得到,量子阱的光吸收系数达3.7×104 cm-1,该数值高于传统透射实验测量结果一个数量级。上述实验结果指出,利用量子阱带间跃迁工作机制,有望实现新颖的器件结构设计和提高现有器件性能。     

Current-injection efficiency in semiconductor lasers with a waveguide based on quantum wells

A dynamic distributed diffusion-drift model of laser heterostructures, which takes into account carrier capture by quantum wells, is developed. The leakage currents in the lasing mode are calculated for different laser structures without wide-gap emitters: InGaAs/GaAs (lasing wavelength λ = 0.98 μm), InGaAsP/InP (λ = 1.3 μm), and InGaAs/InP (λ = 1.55 μm). It is shown that consideration of the finite carrier-capture time is of major importance for calculating structures with deep quantum wells. The ratio of the leakage currents to the total current in the structures with deep quantum wells (InGaAsP/InP and InGaAs/InP) increases with an increase in the injection current and may reach a few percent when the lasing threshold is multiply exceeded.     

Electron mobility and electron scattering by polar optical phonons in heterostructure quantum wells

Basic features of confined electron scattering in quantum wells (QWs) by confined polar optical (PO) phonons are analyzed. The dependence of electron mobility in Al_0.25Ga_0.75As/GaAs/Al_0.25Ga_0.75As QWs on the well width is calculated. It is demonstrated that increases and decreases in electron mobility (relative to the bulk value) as a function of the QW width occur due to resonance intersubband scattering. The dependence of electron mobility limited by PO phonon scattering on the electron density n _s in the QW is calculated. It is shown that anomalous behavior of electrical conductivity, which in certain cases decreases with increasing electron density, can take place in Al_0.25Ga_0.75As/GaAs/Al_0.25Ga_0.75As QWs for n _s >10¹⁶ m^?2.     

Waveguide effect of GaAsSb quantum wells in a laser structure based on GaAs

The waveguide effect of GaAsSb quantum wells in a semiconductor-laser structure based on GaAs is studied theoretically and experimentally. It is shown that quantum wells themselves can be used as waveguide layers in the laser structure. As the excitation-power density attains a value of 2 kW/cm² at liquid-nitrogen temperature, superluminescence at the wavelength corresponding to the optical transition in bulk GaAs (at 835 nm) is observed.     

立方晶体非线性折射系数的各向异性

本文分析了立方晶体三次非线性极化强度P~((3))的各向异性,导出了非线性折射系数各向异性的一般表达式,讨论了强光束在立方晶体中的自感生偏振变化规律.     

A dual-color injection laser based on intra- and inter-band carriertransitions in semiconductor quantum wells or quantum dots

A new type of semiconductor injection laser capable of simultaneously generating radiation in the mid-infrared (MIR) (λ~10 μm) and near-infrared (NIR) (λ~0.9 μm) spectral regions is proposed. The MIR emission is a result of intersubband (intraband) electron transitions within a three-level conduction band in a quantum well or a quantum dot. The NIR emission, on the other hand, is due to conventional interband recombination of injected electrons and holes into the conduction and valence bands, respectively. The conditions for population inversion in the intersubband emission process are determined by an appropriately engineered energy structure for a three-level system in the conduction band of a quantum well or dot structure: for the quantum-well-based system, the structure has an asymmetric funnel shape to provide long electron-phonon lifetime at the third (top) energy level. Under high carrier injection, NIR interband emission depopulates the conduction ground level of the quantum well, thereby stabilizing the electron concentration at this level-a necessary condition fur the operation of the MIR laser. This paper discusses the calculation of the population inversion conditions, the requisite gain, and threshold current for MIR laser operation. We also present a preliminary design of the laser structure with a composite waveguide that accommodates both mid- and NIR stimulated emission     

Inversion of the electron population in subbands of dimensional quantization with longitudinal transport in tunnel-coupled quantum wells

The scheme of a laser which can operate in the far-infrared range (λ ∼ 150 μm) is suggested. In order to attain the inversion of the subband population it was suggested that electron transport in three tunnel-coupled quantum wells in a strong electric field, which lies in the plane of quantum wells, be used. An important specific feature of the structure suggested is the presence of a single rough heterointerface. The electron trans-port was simulated by the Monte Carlo method for the Al_xGa_1−x As/GaAs (x=0.2–0.3) heterostructure. The simulation demonstrated that the population inversion in the first and second subbands of dimensional quantization is realized in the field above 1.2 kV/cm at T=4.2 and 77 K. __________ Translated from Fizika i Tekhnika Poluprovodnikov, Vol. 36, No. 6, 2002, pp. 724–729. Original Russian Text Copyright ? 2002 by Aleshkin, Dubinov.     

Comparison of GaInNAs laser diodes based on two to five quantum wells

We have investigated GaInNAs-GaAsN multiquantum-well (MQW) lasers with two-QW (DQW), three-QW (TQW), and five-QW (5QW) active regions and emission in the 1.3-/spl mu/m range. A solid-source molecular beam epitaxy system has been used to grow the structures. Operation of a GaInNAs 5QW laser is reported. Low threshold currents of 22 (DQW) to 52 mA (5QWs) and external efficiencies of 0.25 W/A (DQWs) to 0.16 W/A (5QWs) per facet are realized under CW operation. T/sub 0/-values of 121 K are obtained.     

Ballistic electron transport in semiconductors

The possibility of ballistic transport in semiconductors is discussed and criteria are given for the experimental verification of ballistic behavior. The potential of purely ballistic motion, velocity overshoot, and inhomogeneities of the free carrier concentration in the space charge limited regime for device performance is assessed.     

Refractive index modulation based on excitonic effects inGaInAs-InP coupled asymmetric quantum wells

The effect of excitons in GaInAs-InP coupled asymmetric quantum wells on the refractive index modulation, is analyzed numerically using a model based on the effective mass approximation. It is shown that two coupled quantum wells brought in resonance by an applied electric field will, due to the reduction in the exciton oscillator strengths, have a modulation of the refractive index which is more than one order of magnitude larger than in a similar quantum well structure based on the quantum confined Stark effect, but with no coupling between the quantum wells. Calculations show that combining this strong electrorefractive effect with self-photo-induced modulation in a biased-pin-diode modulator configuration, results in an optical nonlinearity with a figure of merit of 20 cm³/J at a wavelength of 1.55 μm. This value is large compared to optical nonlinearities originating from band edge resonance effects in III-V semiconductor materials