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A simulation procedure for analyzing the electrical and optical characteristics of an AlGaN/GaN intersubbandquantum well middle-wavelength infrared photodetector is presented. The photoconductive gain spectrumwas simulated by coupling the drift-diffusion and capture-escape models in the active region of the devicestructure and by ignoring the contribution of radiative emission. It was shown that the photodetector at zero biasis sensitive over a spectral range from 4 to 6 μm, with the peak absorption occurring at 4.64 μm. The dependenceof the available photocurrent on both the wavelength and the angle of incidence of an unpolarized monochromaticbeam of light was also evaluated. An assessment of the dark current characteristics was estimated at varioustemperatures.

The theoretical investigation of the electronic and nonlinear optical properties in spheroid-shaped CdTe/ZnTe core/shell quantum dots (CSQDs) embedded into two commonly adopted dielectric matrices (PVA, SiO2) is done in the framework of the Effective Mass Approximation (EMA). The discrete intra-band confined energy levels and their matching wave functions were calculated by solving the stationary Schrödinger equation taking into account the Compact Density Matrix (CDM) approach. The effect of the dielectric mismatch between the system and the capped matrix has been studied and discussed. Our numerical results revealed that the third-order nonlinear (TON) optical susceptibility χ3 is strongly influenced by the geometrical parameters and the dielectric environment.

In this work, the electronic and optical properties of CdS/ZnS core–shell quantum dots (CSQDs) capped in different matrices were investigated theoretically. Through the effective mass approximation (EMA) and the density matrix approach (DMA), the quantized energy levels and their corresponding wave functions of the system were obtained by solving the Schrödinger equation in a spherical coordinates system. In addition, the effects of the incident optical intensity, the number of dots per unit volume, dielectric mismatch of the organic and inorganic matrix, and geometric parameters of the structure, such as the core/shell radius ratio for CdS/ZnS CSQDs on the optical properties, were evaluated and discussed. The results revealed that both the size and dielectric environments had a substantial effect on the optical features of these nanostructures. [ABSTRACT FROM AUTHOR]

In this study, we report on the ultra-short lifetime of excited intersubband electrons in a 38 Å wide AlGaN/GaN-based quantum well. The rapid decay of these charge carriers occurs due to a resonance between the relevant intersubband transition energy and the size of the GaN-based LO-phonon at 92 meV. Based on the experimentally observed Lorentz-shaped intersubband emission peak with a spectral width of roughly 6 meV (48 cm−1) respecting the Fourier transform limit, a very short lifetime, namely 111 fs, could be calculated. By comparing this lifetime to the existing literature data, our value confirms the potential high-speed capability of III-nitride-based optoelectronics. [ABSTRACT FROM AUTHOR]

In this work, we perform a theoretical analysis of the electronic and optical properties of CdS quantum dot (QD) encapsulated in ZnSe matrix. The intersubband energy levels and the matching wave functions are calculated using the effective mass approximation (E.M.A) and the compact density matrix approach. The effect of self-energy contribution due to the charging of CdS QD with ZnSe matrix has been taken into account from the beginning. We found that the linear and nonlinear optical properties are considerably impacted by the size parameter and the optical intensity I. The results obtained have essential consequences for the design of optoelectronic and photonic devices. [ABSTRACT FROM AUTHOR]

Using the matrix diagonalization method within the effective mass approximation and the rectangular potential profile, we investigate optical absorption coefficients (OAC) in the core-shell quantum antidot (QAD) Al0.3Ga0.7 As/GaAs/Al0.3Ga0.7 As in the presence of applied co-directed electrical and magnetic fields. The electric field effect on the electron wave functions, energy spectrum, oscillator strength of the intersubband electronic transitions is calculated for different size of QAD core and different value of the magnetic field induction. The total, linear and nonlinear optical absorption coefficients in QAD are obtained by considering 1 s-1p and 1p-1d electron quantum transitions as functions of the incident photon energy. [ABSTRACT FROM AUTHOR]

The electronic quasi-bound state in the continuum concept is explored in an InGaAs/InAlAs heterostructure to create a voltage-tunable dual-colour quantum Bragg mirror detector. This heterostructure is based on one main quantum well embedded between two different superlattices. By bandgap engineering, each superlattice gives rise to quasi-bound states in the continuum with a preferential direction for electron extraction. Due to these states, the photovoltaic photocurrent presents a dual-colour response, one in a positive direction at 340 meV (3.6 μm), and one in a negative direction at 430 meV (2.9 μm). The simultaneous dual-colour detection can be switched to a single-colour detection (340 meV or 430 meV) by applying a bias voltage. At 77 K, the specific detectivity for simultaneous dual-colour is 2.5·108 Jones, while the single-colour detectivities are 2.6·109 Jones at +2.0 V and 7.7·108 Jones at -1.6 V for 340 meV and 430 meV, respectively. [ABSTRACT FROM AUTHOR]

• Intersubband transition in elliptical cylindrical CdS/ZnS core/shell quantum dots. • The Photoionization cross-section is investigated with several capped matrices. • The binding energy affected by the electric field intensity, the degree of ellipticity and the immersed matrix. In this study, the effects of the electric field in different directions on hydrogenic donor impurity (HDI) confined to a CdS/ZnS elliptical core/shell cylindrical quantum dot (ECSCQD) immersed in a dielectric matrix (DM) were investigated. The binding energy (BE) and their corresponding photoionization cross-section (PICS) are computed by solving the Schrödinger equation in cylindrical coordinates in the framework of the effective mass approximation (EMA) using a variational and perturbation approach. The effects of different dielectric matrices, lateral and normal electric field (EF) force, geometric factors such as the size of ECSCQDs and ellipticity constants on BE and PICS were presented. It has been indicated that weak lateral EF strength compared to normal EF strength, dielectric matrix, and geometric factors have a significant effect on the BE and PICS. In addition, it was observed that dielectric matrices and different ellipticity constants were quite effective on the Stark shift (S.S.) in the hydrogenic donor impurity ground state energy due to the electric field. The results obtained from the calculations showed that the structure factors, different impurity positions, the external electric field applied to the structure, the elliptical cylinder shape and the dielectric matrix in which the structure is embedded are quite decisive in the magnitude of the PICS amplitudes and resonant peak values. [ABSTRACT FROM AUTHOR]

Metal-oxides hold promise as superior plasmonic materials in the mid-infrared compared to metals, although their integration over established material technologies still remains challenging. We demonstrate localized surface plasmons in self-assembled, hemispherical CdZnO metal-oxide nanoparticles on GaAs, as a route to enhance the absorption in mid-infrared photodetectors. In this system, two localized surface plasmon modes are identified at 5.3 and 2.7 μm, which yield an enhancement of the light intensity in the underlying GaAs. In the case of the long-wavelength mode the enhancement is as large as 100 near the interface, and persists at depths down to 50 nm. We show numerically that both modes can be coupled to infrared intersubband transitions in GaAs-based multiple quantum wells, yielding an absorbed power gain as high as 5.5, and allowing light absorption at normal incidence. Experimentally, we demonstrate this coupling in a nanoparticle/multiple quantum well structure, where under p-polarization the intersubband absorption is enhanced by a factor of 2.5 and is still observed under s-polarization, forbidden by the usual absorption selection rules. Thus, the integration of CdZnO on GaAs can help improve the figures of merit of quantum well infrared photodetectors, concept that can be extended to other midinfrared detector technologies.

In this study, we report on the ultra-short lifetime of excited intersubband electrons in a 38 Å wide AlGaN/GaN-based quantum well. The rapid decay of these charge carriers occurs due to a resonance between the relevant intersubband transition energy and the size of the GaN-based LO-phonon at 92 meV. Based on the experimentally observed Lorentz-shaped intersubband emission peak with a spectral width of roughly 6 meV (48 cm−1) respecting the Fourier transform limit, a very short lifetime, namely 111 fs, could be calculated. By comparing this lifetime to the existing literature data, our value confirms the potential high-speed capability of III-nitride-based optoelectronics.

The integration of quantum well infrared photodetectors with plasmonic cavities has allowed for demonstration of sensitive photodetectors in the mid-infrared up to room-temperature operating conditions. However, clear guidelines for optimizing device structure for these detectors have not been developed. Using simple stripe cavity detectors as a model system, we clarify the fundamental factors that improve photodetector performance. By etching semiconductor material between the stripes, the cavity resonance wavelength was expected to blue-shift, and the electric field was predicted to strongly increase, resulting in higher responsivity than unetched stripe detectors. Contrary to our predictions, etched stripe detectors showed lower responsivities, indicating surface effects at the sidewalls and reduced absorption. Nevertheless, etching led to higher detectivity due to significantly reduced detector dark current. These results suggest that etched structures are the superior photodetector design, and that appropriate sidewall surface treatments could further improve device performance. Finally, through polarization and incidence angle dependence measurements of the stripe detectors, we clarify how the design of previously demonstrated wired patch antennas led to improved device performance. These results are widely applicable for cavity designs over a broad range of wavelengths within the infrared, and can serve as a roadmap for improving next-generation infrared photodetectors.

Infrared phototransistors with very high performance show great potential in optoelectronic application, attributable to the intrinsic high transconductive gain of the transistor structure and therefore very large photo-responsivity. To distinguish multispectral information with phototransistors, however, the coincident spectral light has to been laterally dispersed and thereby coupled to the individual photo-gates sensing different target bands. Here, the dual-frequency plasmonic photo-coupler based on a single perforated metallic layer is numerically designed in the infrared region to improve photon absorption efficiency at two target wavelengths. The geometric parameters of the plasmonic grating composed of large and small cross-shaped holes are independently tuned to control the resonance frequencies under different incident (x- and y-) polarization conditions. While the dual-frequency resonant modes are induced by the hybridization between localized surface plasmons (LSPs) and propagating surface plasmons (PSPs), it is found that LSPs play the dominant role and can be effectively adjusted to achieve the target dual-frequency response. More importantly, the spectral response with low- (high-) frequency resonance corresponding to large (small) cross-hole region matches the spatially distributed photogates of the phototransistor structure and therefore realizes near-field enhancement and hence large photoresponsivity at target dual frequencies. In addition, the spectral response for x- and y-polarizations can be unified despite of the apparently broken symmetry between these two polarizations. Our work provides a general strategy to realize sensitive multiband-infrared phototransistors which require only state-of-the-art planar technology and are applicable for highly sensitive infrared photodetection. [ABSTRACT FROM AUTHOR]

In the present work, optical properties of tuned quantum dot/ring systems with considering the effect of Rashba spin–orbit interaction are theoretically investigated. To this end, we describe our system by using an appropriate model potential to obtain the energy levels and wave functions analytically. The system is excited by a monochromatic electromagnetic field and intersubband transitions for the electrons are considered. Then, analytical expressions for optical absorption coefficients and refractive index changes are used. The effect of quantum dot radius and the Rashba coupling constant on the optical properties have been investigated. The results show that: (i) The effect of Rashba spin–orbit interaction on the peak values of the refractive index changes is negligible. (ii) The total absorption coefficient is enhanced and shift toward higher energies with considering the effect of Rashba spin–orbit interaction. (iii) The impact rate of spin–orbit interaction on the optical properties relates to the quantum dot radius, depth of confining potential and the Rashba coupling constant. [ABSTRACT FROM AUTHOR]

We investigated theoretically a waveguided EAM (Electro-Absorption Modulator) based on ISBT (Intersubband Transitions) in an In(0.53)Ga(0.47)As/AlAs/AlAs(0.56) Sb(0.44) A-CDQWs (Asymmetric Coupled Double Quantum Wells) lattice-matched to InP at telecommunication wavelength ($\lambda =1.55~\mu \text{m}$) which offers ultrahigh-speed and moderate voltage swing. Likewise, the temperature dependency in the In(0.53)Ga(0.47)As/AlAs/AlAs(0.56) Sb(0.44) A-CDQWs was investigated at different temperatures from 300 K to 400 K and evidently the InP-based ISB (Intersubband) modulator offers better temperature stability (~ 0.05 nm/C) compared to the InP-based IB (Interband) modulator. The EAM investigated here is anticipated to have a RC-limited speed ($f_{3d\,B}$) of ~300 GHz with insertion loss of 5.1 dB, 10 dB extinction ratio and 5.18 dB/V modulation efficiency at a peak-to-peak voltage of 2.0 V which can support a data rate of up to 600 Gbps and beyond. [ABSTRACT FROM AUTHOR]

Intersubband transition was recently discovered in colloidal nanoplatelets, but the associated intersubband carrier relaxation dynamics remains poorly understood. In particular, it is crucial to selectively excite the intersubband transition and to follow the hot electron dynamics in the absence of valence-band holes. This is achieved herein by exciting the predoped electrons in CdSe/ZnS nanoplatelets using near-infrared femtosecond pulses and monitoring nonequilibrium electron dynamics using broad-band visible pulses. We find that the n = 2 electrons relax to the n = 1 subband and establish a Fermi-Dirac distribution within 200 fs, and finally reach an equilibrium with the lattice within a few ps. The cooling dynamics depend mainly on the excitation fluence but weakly on the doping density and the lattice temperature. These characteristics are well captured by our numerical simulation that explicitly accounts for the state occupation effect and optical phonon scattering.

In this work, we have modelled and computed the transport properties of the double-barrier InGaAs/GaAsSb structure from the resonant tunnelling point of view. Based on the classical Tsu-Esaki formula for the tunnelling current, we have calculated the current density-voltage characteristic of a Al-free type-II In 0.53 Ga 0.47 As/GaAs 0.51 Sb 0.49 structure at different cryogenic and elevated temperatures. The tunnelling coefficient has been calculated in the framework of effective mass approximation with nonparabolicity included, using the transfer matrix approach. A good qualitative agreement of the position of resonant current peaks with the existing experimental data was achieved. Our calculation shows a very high sensitivity of the tunnelling current peak on monolayer-scale layer structure fluctuation which strongly affects peak to valley ratio in the resonant tunnelling structure. [ABSTRACT FROM AUTHOR]

We compare magneto-resonance absorption FWHM (full-width at half-maximum) for the intrasubband transition (1 → 1) to that for the intersubband transition (1 → 2) in a square GaAs quantum well by using the projection operator method due to confined LO-phonon delineated by the model of Fuchs-Kliewer, Ridley, and Huang-Zhu. The numerical results show that the FWHM of the optically detected magnetophonon resonance (ODMPR) peak for the intrasubband/intersubband transition decreases with the rise of the width of well and increases with the growing temperature for all models of phonon. This is in accordance with the result has been investigated in the theory and experimental [1,2]. Furthermore, the FWHM of the above ODMPR peaks for the bulk phonon has a smaller and changes slower than it does for the confined phonon. The FWHM for the Huang-Zhu model is largest among three models of confined phonon (Fuchs-Kliewer, Ridley, and Huang-Zhu model), while it for Ridley model is smallest. This result is in accordance with the results have been investigated in the theory [3,4]. In addition, the FWHM of the ODMPR peak for the intrasubband transition has larger value than that for intersubband transition for both the bulk and confined phonon, this is in accordance with the result has been investigated in the theory and experimental [5]. [ABSTRACT FROM AUTHOR]

Abstract In this work, we theoretically investigate the structural dependence of intersubband absorption of InGaAsSb/GaAs single quantum well structures. We begin by analyzing the impact of In and Sb incorporation on the critical thickness, conduction band offset and band gap energy. The first two electron energy levels E 12 and the corresponding wavelength were made for the In x Ga 1− x As 1- y Sb y /GaAs system and are analyzed in detail by solving the Schrödinger equation. Also, we have investigated effects of composition and well width on the intersubband absorption. In addition, the wavelength and absorption coefficient of the ISBT can be adjusted and optimized by changing the composition and the width of the SQW. Finally, strain effects on intersubband absorption and on the peak response wavelength have also been systematically studied. Our study shows that InGaAsSb/GaAs SQW will play a key role in research of electronics and photonic devices in the future. Highlights • Studied the effects of structural parameters on the Intersubband transition of InGaAsSb/GaAs QWs. • Absorption coefficient dependency on the InGaAsSb structural parameters has been investigated. • The effects of these parameters are numerically studied. • Effect of the strain on peak wavelength and on the maximum absorption has been verified. • InGaAsSb/GaAs is a good candidate as a novel material system for optoelectronic intersubband devices. [ABSTRACT FROM AUTHOR]

• Intersubband transition in CdTe/ZnTe core/shell prolate spheroidal QDs. • Nonlinear third-order optical susceptibility is investigated with eccentricity e 1. • Nonlinear third-order optical susceptibility is shifted with surrounding polymer matrix. The theoretical investigation of the electronic and nonlinear optical properties in spheroid-shaped CdTe/ZnTe core/shell quantum dots (CSQDs) embedded into two commonly adopted dielectric matrices (PVA, SiO 2) is done in the framework of the Effective Mass Approximation (EMA). The discrete intra-band confined energy levels and their matching wave functions were calculated by solving the stationary Schrödinger equation taking into account the Compact Density Matrix (CDM) approach. The effect of the dielectric mismatch between the system and the capped matrix has been studied and discussed. Our numerical results revealed that the third-order nonlinear (TON) optical susceptibility χ 3 is strongly influenced by the geometrical parameters and the dielectric environment. [ABSTRACT FROM AUTHOR]

An asymmetric quantum well (AQW) system that is pumped by two infrared beams is designed to generate terahertz (THz) waves. The refractive index and phase mismatch associated with the intersubband transition of the AQW structure are calculated and analyzed for both present and absent conduction band nonparabolicity. The calculated results reveal that, for increasing wavelengths, the refractive index of the AQW for the short-wavelength pump beam varies more than 0.83 and undergoes a 0.204  μ m redshift, when the conduction band nonparabolicity is considered. The variation of the refractive index of the AQW with the long-wavelength pump beam, changes from 0.225 to 0.316 after considering the conduction-band nonparabolicity. In addition, no redshift is observed. Whether the refractive index of one pump beam with its specific wavelength increases is determined mainly by the linear terms. However, for increasing the other pump wavelengths, the refractive index of one pump beam mainly depends on the nonlinear terms. Subband energy-levels and dipole transition matrix elements show noticeable changes due to conduction-band nonparabolicity, which change the refractive index. Phase matching can be achieved by adjusting the wavelength of the two pump beams. However, both phase mismatch and coverage increase when the conduction band nonparabolicity is considered. [ABSTRACT FROM AUTHOR]

The four-wave mixing (FWM) effect of a weak probe field, in an asymmetric semiconductor double quantum dot (QD) structure driven by a strong pump field is theoretically studied. Similarly to the case of examining several other nonlinear optical processes, the nonlinear differential equations of the density matrix elements are used, under the rotating wave approximation. By suitably tuning the intensity and the frequency of the pump field as well as by changing the value of the applied bias voltage, a procedure used to properly adjust the electron tunneling coupling, we control the FWM in the same way as several other nonlinear optical processes of the system. While in the weak electron tunneling regime, the impact of the pump field intensity on the FWM is proven to be of crucial importance, for even higher rates of the electron tunneling it is evident that the intensity of the pump field has only a slight impact on the form of the FWM spectrum. The number of the spectral peaks, depends on the relation between specific parameters of the system. [ABSTRACT FROM AUTHOR]

In this paper, we explore THz range optical intersubband transition energies in a donor doped quantum well of a GaAs/AlGaAs system as a function of the insertion position of an AlAs monolayer in the GaAs quantum well. In simulated models, the optical transition energies between electron subband levels 1 and 2 were higher in the doped structure than in the undoped structure. This may be because the envelope wave function of the second electron subband strongly overlapped the envelope wave function of the first electron subband and influenced the optical intersubband transition between the two levels in the THz range. At different levels of bias voltage at the Schottky barrier on the donor doped structure, the electric field in the growth direction of the structure linearly increased the further away the AlAs monolayer was placed from the reference position. We also simulated the optical transition energies between acceptor energy levels of the acceptor doped structure as a function of the insertion position of the AlAs monolayer. The acceptor doped structure induced THz range emission whereas the undoped structure induced mid-IR emission. [ABSTRACT FROM AUTHOR]

Intersubband optoelectronic devices rely on transitions between quantum-confined electron levels in semiconductor heterostructures, which enables infrared (IR) photodetection in the 1-30 μm wavelength window with picosecond response times. Incorporating nanowires as active media could enable an independent control over the electrical cross-section of the device and the optical absorption cross-section. Furthermore, the three-dimensional carrier confinement in nanowire heterostructures opens new possibilities to tune the carrier relaxation time. However, the generation of structural defects and the surface sensitivity of GaAs nanowires have so far hindered the fabrication of nanowire intersubband devices. Here, we report the first demonstration of intersubband photodetection in a nanowire, using GaN nanowires containing a GaN/AlN superlattice absorbing at 1.55 μm. The combination of spectral photocurrent measurements with 8-band k·p calculations of the electronic structure supports the interpretation of the result as intersubband photodetection in these extremely short-period superlattices. We observe a linear dependence of the photocurrent with the incident illumination power, which confirms the insensitivity of the intersubband process to surface states and highlights how architectures featuring large surface-to-volume ratios are suitable as intersubband photodetectors. Our analysis of the photocurrent characteristics points out routes for an improvement of the device performance. This first nanowire based intersubband photodetector represents a technological breakthrough that paves the way to a powerful device platform with potential for ultrafast, ultrasensitive photodetectors and highly efficient quantum cascade emitters with improved thermal stability. [ABSTRACT FROM AUTHOR]

The hot-electron energy-loss rate via the acoustic-phonons in an embedded semiconductor quantum wire of circular cross section in the presence of external electric field has been investigated using deformation potential theory. Dimensional confinement effect on modifying acoustic-phonon modes are taken into account. The energy-loss rate as a function of electric field strength, electron density and electron temperature is obtained. Our calculations show that the electric field applied perpendicularly to the wire axis can be used as an important tool for the control of the energy-loss processes in nanowires. [ABSTRACT FROM AUTHOR]

The linear intersubband optical absorption in the polarization semiparabolic quantum wells (SPQWs) are investigated for typical AlN/Al x Ga 1 − x N/AlN. First, the one-dimensional Poisson and Schrödinger equations have been solved by the variational method within a finite potential barrier model and a bent band figured by all confinement sources (realistic model). Then, the intersubband optical absorption between the lowest two subbands has been theoretically studied under an uniform external electric field. Computed results including the effective confining potential profile, the wave function and the distribution of electron gas in quantum wells, intersubband optical absorption coefficient have been discussed. Our calculation shows that the positive interface polarization charges affect on the distribution of the two-dimensional electron gas (2DEG) in SPQWs so it has great influences on the total optical absorption coefficients significantly. [ABSTRACT FROM AUTHOR]

A cascade infrared upconverter (CIUP) avoids mechanical bonding between photodetectors and read-out integrated circuits, being a promising candidate for very large-format infrared imaging. This letter demonstrates a CIUP with an Al0.52In0.48P hole-blocking layer whose valence band discontinuity with GaAs is large, which will tightly confine injected holes into the emission region. The device shows a peak response wavelength at 4.9~\mu \textm and a peak emission wavelength at 870 nm. At 1.4 V bias and 78 K, shot noise limited detectivity of the upconversion system is 1.17\times 10^9 Jones, about one order of magnitude higher than the previously reported CIUP with an AlAs hole-blocking layer. [ABSTRACT FROM PUBLISHER]

The propagation of an initially chirped incident few-cycle pulse through an ensemble of quantum wells is numerically investigated. It is found that the distribution characteristic of the transmitted spectrum sensitively depends on the incident laser parameters, especially its positive or negative chirp property. As for the incident pulse with a positive initial chirp, beyond the obvious spectral blue-shift, the transmitted spectral distribution is discrete. In contrast, as for a negative initial chirp, the spectral distribution is continuous instead. In addition, the insensitivity of chirp-dependent spectral distribution to medium symmetry character is also tested and the intensity of high-frequency spectral components enhances obviously due to the nonlinear propagation effects. [ABSTRACT FROM AUTHOR]

Semiconductor-based up-conversion infrared (IR) photodetectors have the advantages of pixelless imaging and being free from thermal mismatch between photodetector and read-out integrated circuit. Profited by cascade carrier transportation, cascade IR up-converters (CIUPs) provide a fresh idea to ease the contradiction between dark current and responsivity. For GaAs-based mid-wave IR CIUP, a strained InGaAs/AlGaAs material system is an essential configuration, in spite of the limitation of lattice-mismatched epitaxy. An InGaAs/InAlAs material system lattice-matched to InP substrate is a promising alternative to the strained InGaAs/AlGaAs structures. In this letter, an InP-based mid-wave IR CIUP is demonstrated with a 4.7- \mu \textm peak response wavelength and 1.19- \mu \textm peak emission wavelength. For the up-conversion system, the dark-current-limited detectivity reaches 1.1\times 10^11 Jones at 78 K and 1.3 V CIUP bias, and the background-limited infrared performance condition is achieved at 107 K with the detectivity of 1.2\times 10^10 Jones. [ABSTRACT FROM PUBLISHER]

We have realized a tandem solar cell design that combines a pin-junction with a photovoltaic intersubband absorber. This concept allows harvesting light in the visible range and the near- and mid-infrared at the same time, and theoretically, energy conversion efficiencies beyond the Shockley-Queisser-limit could be achieved. A test structure was grown, and the operation of this concept could be confirmed, in principal with an optical two-beam experiment. The basic characteristics of the device can be explained with an equivalent circuit design that consists of three individual cells, and we find an obvious analogy to the concept of the intermediate band solar cell with noteworthy advantages at some points. Our results show, that for a working device it is crucial to adjust the properties of the photovoltaic intersubband absorber for optimal charge separating performance at the working point of the solar cell. Copyright © 2015 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

Strong and ultrastrong coupling between intersubband transitions in quantum wells and cavity photons have been realized in mid-infrared and terahertz spectral regions. However, most previous works employed a large number of quantum wells on rigid substrates to achieve coupling strengths reaching the strong or ultrastrong coupling regime. In this work, we experimentally demonstrate ultrastrong coupling between the intersubband transition in a single quantum well and the resonant mode of photonic nanocavity at room temperature. We also observe strong coupling between the nanocavity resonance and the second-order intersubband transition in a single quantum well. Furthermore, we implement for the first time such intersubband cavity polariton systems on soft and flexible substrates and demonstrate that bending of the single quantum well does not significantly affect the characteristics of the cavity polaritons. This work paves the way to broaden the range of potential applications of intersubband cavity polaritons including soft and wearable photonics.

The coherent light–matter interaction in a 4-level cascade-type subband system of an asymmetric GaAs/AlGaAs quantum well structure is studied in pump-probe transmission experiments with picosecond (ps) time resolution. Coupling two excited subbands by an intense mid-infrared laser pulse at low sample temperatures is found to result in a substantially increased transparency of the fundamental e 1 –e 2 transition. We find a reduction of the absorption coefficient by ~80%, which is one of the most pronounced electromagnetically induced transparency in solid state systems observed so far. [ABSTRACT FROM AUTHOR]

In this article, a large Kerr nonlinearity induced by THz driven to intersubband of a GaN/AlGaN asymmetric quantum well nanostructure is investigated. To obtain the Kerr nonlinearity of the nanostructure, potential profiles, energy eigenvalues and wavefunctions, and overlap integrals are obtained by solving the Schrödinger and Poisson equations self-consistently. Knowing energy levels and related wavefunctions, one can investigate the nonlinear response of the structure by solving the density matrix equations of motion in the rotating wave approximation. The effect of the THz field, the control field, and the effect of the THz field detuning on the Kerr nonlinearity and the nonlinear absorption are observed. Our obtained results show that these parameters variations can cause a large Kerr nonlinearity and nonlinear absorption in GaN/AlGaN asymmetric quantum well nanostructure. [ABSTRACT FROM PUBLISHER]

We studied in detail the voltage dependence of two-step photocurrent generation through a two-step process of absorbing sub-band gap photons of different photon energies in a GaAs/AlGaAs quantum dot Intermediate Band Solar Cell. Our experiments revealed that two-step photocurrent generation is largely dependent on voltage, and exhibits a maximum at −0.3 V. A notable feature is a monotonic decrease in two-step photocurrent in the forward bias region, where the operating point of the solar cell lies. Using a model of rate equations, we extracted the voltage dependence of the individual escape and recombination rates, and found that the decrease in two-step photocurrent in the forward bias region is related to a monotonic increase in recombination rate in the quantum dots with increasing bias. [ABSTRACT FROM AUTHOR]

In this work, the effects of wetting layer (WL) thickness on the electronic and transition properties of strain-free pyramid-shaped GaAs/Al 0.3 Ga 0.7 As quantum dots (QDs) coupled to a WL, which are relaxed from piezoelectric effect, are investigated. It is shown that the WL, which is usually ignored in the literature, severely influences properties, such as transition dipole moment and oscillator strength of QD/WL coupled structures. Two in-plane-polarized transitions of P-to-S and WL-to-P transitions were investigated. Although, the oscillator strength of P-to-S transition decreases with increasing the WL thickness, it still dominates the oscillator strength of WL-to-P transition. Also, an increasing trend for WL-to-P transition is shown with increasing the WL thickness. The results, which were compared with experimental data Wang et al. (2009), are of importance in QD-based devices such as lasers and photo-detectors, as well as harmonic generation. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical analysis for the characteristics of quantum well infrared phototransistors (QWIPTs). A mathematical model describing this device is introduced under nonuniformity distribution of quantum wells (QWs). MATLAB environment is used to devise this model. Furthermore, block diagram models through the VisSim environment were used to describe the device characteristics. The developed models are used to investigate the behavior of the device with different values of performance parameters such as bias voltage, spacing between QWs, and temperature. These parameters are tuned to enhance the performance of these quantum phototransistors through the presented modeling. Moreover, the resultant performance characteristics and comparison between both QWIPTs and quantum wire infrared phototransistors are investigated. Also, the obtained results are validated against experimental published work and full agreements are obtained. [ABSTRACT FROM AUTHOR]

The bistability characteristics of GaN/AlN resonant tunneling diodes (RTDs) grown on a sapphire substrate were investigated. The RTDs exhibit bistability characteristics with high and low resistivity switched by varying the polarity of the bias voltage. Negative differential resistance is realized in the current–voltage $(I-V)$ characteristics after low resistivity is achieved by application of a negative bias. The calculated $I-V$ characteristics based on self-consistent methods indicate that the bistability characteristics are caused by the accumulation of electrons in the quantum well due to intersubband transitions. Also, the bistability characteristics of GaN/AlN RTDs were discussed toward the application to the ultrafast nonvolatile memory. [ABSTRACT FROM PUBLISHER]

Highlights: [•] We model the electrical and optical characteristics of (CdS/ZnSe)/BeTe QWIP near 300K. [•] This system is used for the first time in such application. [•] The simulation takes into account three mechanisms of carrier transport: G–R, Diff and TAT processes. [•] The investigations have been performed over a wide range of temperatures and bias voltages. [•] For 1.55μm cutoff wavelength, D * ∼5.7×1010 cmHz1/2/W has been achieved in X BeTe minimum. [ABSTRACT FROM AUTHOR]

Abstract: Efficient design of optoelectronic devices based on electron intersubband transitions depends critically on the knowledge of the intersubband relaxation times which in turn, depends on electron scattering with LO and acoustic phonons. In this article the intersubband scattering time associated with electron–acoustic-phonon interaction has been discussed in terms of phonon mode quantization and phonon confinement with describing the acoustic phonon dispersion relation in detail by introducing the cut-off frequency for each mode. It has been shown that the quantization of acoustic phonon modes lead to an enhancement in electron–phonon scattering time in AlGaAs quantum well structures. Based on the presented model, a new tailoring method has presented to adjust the electron–phonon scattering time in intersubband-transition-based structures while keeping the electronic properties unaltered. Also, we illustrated that for a quantum well with subband energy separation of ∼30meV, the intersubband scattering time with acoustic-phonon-assisted transitions could be tailored from ∼120ps to increased value of ∼400ps or reduced value of ∼45ps by inserting a 1nm-thickacoustically soft or hard layers, respectively, while keeping the same the initial energy separation. [Copyright &y& Elsevier]

In this paper, we report on the design and characterization of a quantum well based infrared photodetectors covering simultaneously infrared radiation within mid- and long-infrared spectral regions. The proposed infrared photodetectors rely on intersubband transitions in asymmetric ZnSe/CdS double quantum wells. The three-energy-level and the wavelengths of the intersubband transitions in the asymmetric double quantum wells are obtained by solving the Schrödinger and Poisson equations self consistently, the influence of the right well width on the absorption coefficient is studied. The peak positions of intersubband absorption coefficients in the structure are found at 3.31, 4.4 and 13.5µm for a 1nm right well width while the absorption peak positions are located at 3.33, 6.43 and 6.95µm for a 1.4nm right well thickness. Then, the electro-optic performances of the infrared photodetector are evaluated; the dark current dependence with the applied voltage and temperature is discussed. This work demonstrates the possibility of detection of widely separated wavelength bands using intersubband transitions in quantum wells with a low dark current. [ABSTRACT FROM AUTHOR]

The concept of strong light-matter coupling has been demonstrated in semiconductor structures, and it is poised to revolutionize the design and implementation of components, including solid state lasers and detectors. We demonstrate an original nanospectroscopy technique that permits the study of the light-matter interaction in single subwavelength-sized nanocavities where far-field spectroscopy is not possible using conventional techniques. We inserted a thin (∼150 nm) polymer layer with negligible absorption in the mid-infrared range (5 μm < λ < 12 μm) inside a metal-insulator-metal resonant cavity, where a photonic mode and the intersubband transition of a semiconductor quantum well are strongly coupled. The intersubband transition peaks at λ = 8.3 μm, and the nanocavity is overall 270 nm thick. Acting as a nonperturbative transducer, the polymer layer introduces only a limited alteration of the optical response while allowing to reveal the optical power absorbed inside the concealed cavity. Spectroscopy of the cavity losses is enabled by the polymer thermal expansion due to heat dissipation in the active part of the cavity, and performed using atomic force microscopy (AFM). This innovative approach allows the typical anticrossing characteristic of the polaritonic dispersion to be identified in the cavity loss spectra at the single nanoresonator level. Results also suggest that near-field coupling of the external drive field to the top metal patch mediated by a metal-coated AFM probe tip is possible, and it enables the near-field mapping of the cavity mode symmetry including in the presence of a strong light-matter interaction.

An attractive but challenging technology for high efficiency solar energy conversion is the intermediate band solar cell (IBSC), whose theoretical efficiency limit is 63%, yet which has so far failed to yield high efficiencies in practice. The most advanced IBSC technology is that based on quantum dots (QDs): the QD-IBSC. In this paper, k·p calculations of photon absorption in the QDs are combined with a multi-level detailed balance model. The model has been used to reproduce the measured quantum efficiency of a real QD-IBSC and its temperature dependence. This allows the analysis of individual sub-bandgap transition currents, which has as yet not been possible experimentally, yielding a deeper understanding of the failure of current QD-IBSCs. Based on the agreement with experimental data, the model is believed to be realistic enough to evaluate future QD-IBSC proposals. [ABSTRACT FROM AUTHOR]

Abstract: The electric field dependence of the linear and nonlinear intersubband optical absorption in double semi-graded quantum wells (DSGQWs) is investigated in the effective mass approximation. In our calculations, the position and the magnitude of the linear and total absorption coefficients depend on the electric field strength and barrier width. The resonant peak of total absorption coefficient can be bleached at sufficiently high incident optical intensities. The modulation of the absorption coefficients and the refractive index changes which can be suitable for good performance optical modulators and various infrared optical device applications can be easily obtained by tuning the electric field strength and barrier width. [Copyright &y& Elsevier]

Highlights: [•] An infrared detector based on (CdS)/ZnSe)/BeTe quantum wells is modeled. [•] The intersubband absorption, the dark current and the quantum efficiency are reported. [•] The low dark current at 300K reveals the good performance of the QWIP. [•] The detectivity curves confirm its good quality at 3.3μm wavelength. [ABSTRACT FROM AUTHOR]

Highlights: [•] Subband energies in normal/inverted CSQD computed by finite difference technique. [•] Hamiltonian matrices for cubic & spherical dots derived from Schrödinger equation. [•] Spherical CSQD exhibits higher transition energy than similar cubic CSQD. [•] Transition energy varies in opposite direction for inverted cubic & spherical dots. [•] Wide range of tuning of intersubband transition by tailoring core/shell dimensions. [Copyright &y& Elsevier]

Highlights: [•] Finite difference method is demonstrated effectively to investigate the 3D DWELL structure. [•] The finite scheme incorporates the jump condition automatically. [•] The energy band spectrum of isolated and array-based DWELL structures are shown. [ABSTRACT FROM AUTHOR]

This letter demonstrates bandgap control of ion-induced intermixing on InGaAs/AlAsSb coupled double quantum wells (CDQWs). With phosphorus ion implantation and different rapid thermal annealing (RTA) processes, the blueshift of the bandgap profile can be tailored. The influence of quantum well intermixing on the cross-phase modulation (XPM), caused by the intersubband transition in the CDQWs, is also experimentally investigated. Higher RTA temperature or longer annealing time could cause a larger bandgap blueshift that decreases the XPM efficiency. After annealing, the implanted CDQWs have a relatively low transmission loss, whereas the non-implanted sample can maintain a high XPM efficiency. This result can be used for monolithic integration of different bandgap modules with spatially selective mannerisms, and the implications are vast for optical communication applications. [ABSTRACT FROM PUBLISHER]

The effects of polarization and related structural parameters on the intersubband transitions of AlGaN/GaN multi-quantum wells (MQWs) have been investigated by solving the Schrodinger and the Poisson equations self-consistently. The results show that the intersubband absorption coefficient increases with increasing polarization while the transition wavelength decreases, which is not identical to the case of the interband transitions. Moreover, it suggests that the well width has a greater effect on the intersubband transitions than the barrier thickness, and the intersubband transition wavelength of the structure when doped in the barrier is shorter than that when doped in the well. It is found that the influences of the structural parameters differ for different electron subbands. The mechanisms responsible for these effects have been investigated in detail. [ABSTRACT FROM AUTHOR]

Abstract: A detailed simulation on the intersubband absorption for 1–2, 2–3, and 1–3 optical transitions in ZnO/Mg x Zn1− x O quantum wells is presented. The quantum-confined Stark effect induced by the internal polarization field on the absorption process is effectively controlled through an external electric field. It is easy to obtain the structural optimization of light absorption in different terahertz ranges in our numerical analysis. The absorption wavelengths corresponding to almost all of the transitions increase as the applied field varies from −500kV/cm to 500kV/cm. The small absorption coefficient corresponding to the 1–3 optical transition is increased by enhancing the structural asymmetry of quantum-well potential. The present work can be extended to the structural design and simulation of new optoelectronic devices based on other low-dimensional structures such as quantum wires and quantum dots. [Copyright &y& Elsevier]

Abstract: The laser field dependence of the linear and nonlinear intersubband optical absorption in different graded quantum wells (GQWs) is investigated in the effective mass approximation. Results obtained show that the position and the magnitude of the linear and total absorption coefficients depend on the laser parameter and the shape of GQW. The resonant peak of total absorption coefficient can be bleached at sufficiently high incident optical intensities. Such a dependence of the exciting optical intensity on the external field strengths in different GQWs can be very useful for several potential device applications. It should point out that by applying the laser field we can obtain a blue shift or a red shift in the intersubband optical transitions. [Copyright &y& Elsevier]