Semiconductor Multiple Quantum Wells Research Articles

Optimization of slow light under EIT in semiconductor multiple quantum wells

Optimization of slow light under EIT in semiconductor multiple quantum wells

Broadband Photo-Excited Coherent Acoustic Frequency Combs and Mini-Brillouin-Zone Modes in a MQW-SESAM Structure

A multiple quantum-well semiconductor saturable absorber mirror (MQW-SESAM) structure has been investigated by femtosecond pump-probe laser spectroscopy at a central wavelength of around 1050 nm. Coherent acoustic phonons are generated and detected over a wide frequency range from ~15 GHz to ~800 GHz. In the optical absorption region, i.e., in the multiple quantum wells (In0.27Ga0.73As), acoustic frequency combs centered at ~365 GHz, with a comb spacing of ~33 GHz, are generated. Most importantly, in the transparent region, i.e., in the distributed Bragg reflector, which is formed by a non-doped long-period semiconductor GaAs/Al0.95Ga0.05As superlattice, the mini-Brillouin-zone center, as well as zone-edge acoustic modes, are observed. The mini-zone-center modes with a fundamental frequency of 32 GHz can be attributed to the spatial modulation of the pump optical interference field with a period very close to that of the distributed Bragg reflector, in combination with the periodic spatial modulation of the electrostriction coefficient in the distributed Bragg reflector. The excitation of mini-zone-edge modes is attributed to the stimulated subharmonic decay of the fundamental center modes. Their subsequent back-folding to the mini-Brillouin-zone center makes them Raman active for the probe light.

Numerical investigation of strain effects on properties of AlGaAs/InGaAs multiple quantum well solar cells.

The effects of strain on the electronics and optical properties of a p-i-n solar cell, composed of AlGaAs/InGaAs semiconductor multiple quantum wells, are investigated. The lattice mismatch between AlGaAs and InGaAs induces strain on the composite material; thus the solar cell structure is simulated first with strain and then without strain. Finally, the short-circuit current, fill factor, and cell efficiency for the two approaches are compared in order to determine the effects of the mentioned mismatch on the solar cell properties. In addition, the results are indicated for quantum wells, with different numbers and width. Moreover, the mole fraction of aluminum content of the material varies from 0.1 to 0.3. The simulations are carried out with the Silvaco-Atlas software package ver. 3.20.2.R. Our results show that the existence of strain leads to improvement of the solar cell operation.

Confinement effect on hole polarization in (Ga,Mn)As/AlAs diluted magnetic semiconductor multiple quantum wells

The influence of quantum confinement on the spin polarization of holes in ferromagnetic multiple quantum wells based on (Ga,Mn)As diluted magnetic semiconductor has been investigated. It is shown that the spin polarization of holes in the impurity band is more likely determined by the magnetic properties of GaMnAs rather than the quantum-confinement effect. The model of Mn acceptor in a QW, describing the polarization characteristics of photoluminescence in GaAs: Mn/AlAs QWs, has been developed. Experimental data and theoretical analysis show that the spin polarization of holes in (Ga, Mn)As/AlAs QWs can be explained within a model, which suggests that holes are localized in the impurity band.

Infrared supercontinuum generation in multiple quantum well nanostructures

The paper presents a theoretical study of broadband mid-infrared supercontinuum generation at low power in semiconductor multiple quantum wells (MQWs) facilitated by electromagnetically induced transparency. Pulses of 200 W peak power and 700 fs duration at 9.963 μm have been used to study the supercontinuum generation dynamics in a 1.374 μm long MQW system. The supercontinuum spectrum is 13.0 μm broad and asymmetric about the pump wavelength. Although the spectral broadening is dominated by self-phase modulation, four-wave mixing, modulation instability and soliton generation also contribute to the broadening.

Dynamic analysis of optical soliton pair and four-wave mixing via Fano interference in multiple quantum wells

We perform a time-dependent analysis of the formation and stable propagation of an ultraslow optical soliton pair, and four-wave mixing (FWM) via tunable Fano interference in double-cascade type semiconductor multiple quantum wells (SMQWs). By using the probability amplitude method to describe the interaction of the system, we demonstrate that the electromagnetically induced transparency (EIT) can be controlled by Fano interference in the linear case and the strength of Fano interference has an important effect on the group velocity and amplitude of the soliton pair in the nonlinear case. Then, when the signal field is removed, the dynamic FWM process is analyzed in detail, and we find that the strength of Fano interference also has an important effect on the FWM’s efficiency: the maximum FWM efficiency is ~28% in appropriate conditions. The investigations are promising for practical applications in optical devices and optical information processing for solid systems.

All-optical switching based on the electromagnetically induced transparency effect of an active photonic crystal microcavity

We report a new all-optical switching in a two-dimensional photonic crystal microcavity made of semiconductor multiple quantum wells and realized based on the electromagnetically induced transparency effect with exciton and two-exciton energy levels. We use the quantum coherence effects to achieve small absorption of the probe field, and the absorption of the probe field can be adjusted by controlling the pump field and decay rate. We turn the control field into pulses of light field so that we can regulate the efficiency of the switch. Through selecting the appropriate control light field intensity, we can obtain a switching efficiency of 85% and a switching time is 10 ps. This result can be used for the development of new types of nanoelectronic devices for realizing the switching process.

Strain induced variations in band offsets and built-in electric fields in InGaN/GaN multiple quantum wells

The band structure, quantum confinement of charge carriers, and their localization affect the optoelectronic properties of compound semiconductor heterostructures and multiple quantum wells (MQWs). We present here the results of a systematic first-principles based density functional theory (DFT) investigation of the dependence of the valence band offsets and band bending in polar and non-polar strain-free and in-plane strained heteroepitaxial InxGa1-xN(InGaN)/GaN multilayers on the In composition and misfit strain. The results indicate that for non-polar m-plane configurations with [12¯10]InGaN//[12¯10]GaN and [0001]InGaN//[0001]GaN epitaxial alignments, the valence band offset changes linearly from 0 to 0.57 eV as the In composition is varied from 0 (GaN) to 1 (InN). These offsets are relatively insensitive to the misfit strain between InGaN and GaN. On the other hand, for polar c-plane strain-free heterostructures with [101¯0]InGaN//[101¯0]GaN and [12¯10]InGaN//[12¯10]GaN epitaxial alignments, the valence band offset increases nonlinearly from 0 eV (GaN) to 0.90 eV (InN). This is significantly reduced beyond x ≥ 0.5 by the effect of the equi-biaxial misfit strain. Thus, our results affirm that a combination of mechanical boundary conditions, epitaxial orientation, and variation in In concentration can be used as design parameters to rapidly tailor the band offsets in InGaN/GaN MQWs. Typically, calculations of the built-in electric field in complex semiconductor structures often must rely upon sequential optimization via repeated ab initio simulations. Here, we develop a formalism that augments such first-principles computations by including an electrostatic analysis (ESA) using Maxwell and Poisson's relations, thereby converting laborious DFT calculations into finite difference equations that can be rapidly solved. We use these tools to determine the bound sheet charges and built-in electric fields in polar epitaxial InGaN/GaN MQWs on c-plane GaN substrates for In compositions x = 0.125, 0.25,…, and 0.875. The results of the continuum level ESA are in excellent agreement with those from the atomistic level DFT computations, and are, therefore, extendable to such InGaN/GaN MQWs with an arbitrary In composition.

An optically detectable CO2 sensor utilizing polyethylenimine and starch functionalized InGaN/GaN multiple quantum wells

An optically detectable gas sensor based on the high surface sensitivity of functionalized polyethylenimine/starch In0.15Ga0.85N/GaN strained semiconductor multiple quantum wells (MQWs) has been developed. Due to the excellent piezoelectricity of the MQWs, the change of surface charges caused by chemical interaction can introduce a strain and induce an internal field. In turn, it tilts the energy levels of the MQWs and modifies the optical properties. Through the measurement of the changes in photoluminescence as well as Raman scattering spectra under different concentrations of carbon dioxide gas, we demonstrate the feasibility and high sensitivity of the sensors derived from our methodology.

Controllable amplification, absorption, and dispersion in double‐cascade‐type four‐level system of multiple quantum wells

AbstractA double‐cascade‐type four‐level system of semiconductor multiple quantum wells (MQWs) was constructed with biexcitons and excitons. The nonlinear optical properties for amplification, absorption, and dispersion of 2‐weak fields in this scheme are investigated. It shows that the amplification, absorption, and dispersion responses of 2‐weak fields can be achieved by appropriately adjusting the relative phase, the probe detuning, and the two control Rabi frequencies. The investigation is much more practical than its atomic counterpart because of its flexible design and the widely adjustable parameters. It may provide a new possibility in technological applications for the light amplifier and optical switch working on quantum coherence effects in MQW solid‐state systems.

Electromagnetically induced transparency and theoretical slow light in semiconductor multiple quantum wells

We report an experimental demonstration of electromagnetically induced transparency in the transient optical response in a cascade-type three-level system of GaAs/AlGaAs multiple quantum wells, and analyze the cascade-type three-level schemes with density matrix and Maxwell equations to then obtain the phase shift, group velocity, and group-velocity dispersion. The calculated group velocity is ∼6.87×104 m/s and the corresponding pulse delay is ∼7 ps. Finally, we provide a convenient basis for investigating many-body effects in semiconductors.

Jumping magneto-electric states of electrons in semiconductor multiple quantum wells

Orbital and spin electron states in semiconductor multiple quantum wells in the presence of an external magnetic field transverse to the growth direction are considered. Rectangular wells of GaAs/GaAlAs and InAs/AlSb are taken as examples. It is shown that, in addition to magneto-electric states known from one-well systems, there appear magneto-electric states having a much stronger dependence of energies on a magnetic field and exhibiting an interesting anti-crossing behavior. The origin of these states is investigated and it is shown that the strong field dependence of the energies is related to an unusual ‘jumping’ behavior of their wavefunctions between quantum wells as the field increases. The ways of investigating the jumping states by means of interband magneto-luminescence transitions or intraband cyclotron-like transitions are considered and it is demonstrated that the jumping states can be observed. The spin g factors of electrons in the jumping states are calculated using the real values of the spin–orbit interaction and bands’ nonparabolicity for the semiconductors in question. It is demonstrated that the jumping states offer a wide variety of the spin g factors.

Effect of saturable index change on all-optical logic operations in a passive vertical cavity semiconductor saturable absorber

Semiconductor multiple quantum wells (QWs) have saturable absorption and saturable refractive index change properties. When it is embedded within a reflective Fabry-Perot cavity, the device shows forward and inverse saturation reflectivity for a radiation at cavity resonance wavelength with input intensity around the impedance matching point. Consideration of non-linear index change increases the extinction of forward saturation characteristics of the device. However its application in logic devices is very limited. Whereas, the inverse characteristics for which a number of logic devices can be proposed is severely affected in its extinction because of the consideration of non-linear index change. Here the authors optimise the parameters of vertical cavity semiconductor saturable absorber for the static and dynamic device performance of all-optical logic operations based on inverse saturation characteristics with both dispersive and absorptive saturation into consideration. The authors obtain an extinction of almost 77.5% with input intensity of the order of saturation intensity of a multiple QWs. The novelty of this work lies in the observation that the saturation of non-linear index of refraction has enormous effect on the characteristics of logic operations, which is not addressed in earlier reports. It is also observed that the detrimental effect of non-linear index saturation can be overcome by proper absorber parameters.

Cooperative recombination of electron-hole pairs in semiconductor quantum wells under quantizing magnetic fields

We present results of detailed investigations of light emission from semiconductor multiple quantum wells at low temperatures and high magnetic fields excited by intense femtosecond laser pulses. The intensity and linewidth as well as the directional and statistical properties of photoemission strongly depended on the magnetic field strength and pump laser fluence. We also investigated the effects of spot size, temperature, excitation geometry, and excitation pulse width on the emission properties. The results suggest that the initially incoherent photoexcited electron-hole pairs spontaneously form a macroscopic coherent state upon relaxation into the low-lying magnetoexcitonic states, followed by the emission of a superfluorescent burst of radiation. We have developed a theoretical model for superfluorescent emission from semiconductor quantum wells, which successfully explained the observed characteristics.

All-optical signal amplification in multiple-quantum-well resonant photonic bandgap structures

Using a microscopic many-particle theory, we predict all-optical amplification of low-intensity light pulses in resonant photonic bandgap structures realized by Bragg-spaced semiconductor multiple quantum wells. The discussed scheme is based on mirrorless parametric amplification.

Photoluminescence studies from micropillars fabricated on IV–VI multiple quantum-well semiconductor structure

Micropillars were fabricated on multiple quantum-well structure of PbSe/PbSrSe grown on top of BaF 2 substrate in molecular beam epitaxy (MBE). The photoluminescence spectra from the pillar structure, having a diameter of 5 μm and inter-pillar distance of 8 μm, was studied at various temperature starting from 77 to 300 K. There had been an approximately consistent red-shift of photoluminescence peak of ∼3.62 cm −1 for unit K change in ambient temperature. The prominent and repeatable emission from the micropillars at various temperatures signified high crystalline nature of the fabricated micro-objects. This type of micropillar structure is one of the magnificent contenders of future opto-electronic micro-features.

Strong optical emissions from two-dimensional metal photonic crystals with semiconductor multiple quantum wells

We investigated two-dimensional (2D) metal photonic crystals (PhCs) with semiconductor multiple quantum wells as an active material. The 2D metal PhCs were fabricated using only a lift-off process. The lift-off process leads to a simpler fabrication process than the one for conventional PhCs and avoids any process damage induced by dry etching. The 2D metal PhCs exhibited clear photonic band effects: (1) photoluminescence (PL) peak positions changed depending on the lattice constant and (2) the PL intensity from 2D metal PhCs was higher than that without 2D metal PhCs. These effects are suitable for fabricating light-emitting diodes with a high external quantum efficiency. PL intensity depending on PL excitation power revealed slightly improved luminescence efficiency than the sample without 2D metal PhCs.

All-Optical Processing for Pulse Position Coded Header in Packet Switched Optical Networks Using Vertical Cavity Semiconductor Gates

In this paper, we demonstrate all-optical header extraction and seed-pulse extraction for routing and synchronization of optical packets with pulse-position-coded headers. Our approach exploits self-induced effects in a passive, low-power, polarization-independent, compact vertical cavity semiconductor gate (VCSG). The VCSG operation is based on saturable absorption in semiconductor multiple quantum wells placed within an asymmetric Fabry-Perot cavity. Experiments were performed for both amplitude-modulated and differential-phase-shift-keying-modulated payload data. The payload repetition rate was 12.5 Gbit/s. The packet-bit rate was not limited by the gate recovery time. The operation of the VCSG was also investigated numerically.

Room temperature electron spin relaxation in GaInNAs multiple quantum wells at 1.3μm

The authors report a direct measurement of electron spin relaxation in GaInNAs semiconductor multiple quantum wells at room temperature. Multiple quantum wells of widths 5.8, 7, and 8nm exhibiting excitonic absorption around 1.3μm have been studied. Spin relaxation times were found to increase with well width in the range of 77–133ps. The spin relaxation time dependence on first electron confinement energy suggests the Elliot-Yafet mechanism [A. Tackeuchi et al., Physica B 272, 318 (1999)] as the dominant relaxation process.

Ultrafast all-optical pattern matching using differential spin excitation and its application to bypass/drop self-routing for asynchronous optical packets

This paper presents a novel scheme for ultrafast all-optical pattern matching using the differential spin excitation in semiconductor multiple quantum wells (MQWs). In a demonstration of an all-optical pattern matching between two 100-Gb/s 16-bit optical packets, the contrast ratio of the photodiode (PD) output from the pattern matcher, between the pattern matched and the pattern-unmatched cases, was more than four for packets with a 2-dB power fluctuation. As an application of the pattern matcher to optical-packet-switched ring networks, bypass/drop self-routing is demonstrated for asynchronous 100-Gb/s 32-bit optical packets with 8-bit labels. In the experiment, a label of an incoming packet was compared to a local address (LA) given to a node in the optical domain. By changing the pattern of the LA packet instead of that of the incoming packet, the pattern matching was carried out for packets with various kinds of patterns. The contrast ratio of the PD output was more than six for all patterns.