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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]