Large Number Of Quantum Wells Research Articles

Mid-Infrared Intersubband Cavity Polaritons in Flexible Single Quantum Well

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.

Performance Improvement of Quantum Well Infrared Photodetectors Through Dark Current Reduction Factor

A device model for quantum well-infrared photodetectors (QWIPs) is studied. The developed model accounts for the self-consistent potential distribution. An analytical model for the calculation of dark current reduction factor (DCRF) and the signal to noise ratio (SNR) is presented using the Matlab environment. The aim is to reduce the dark current and improve the SNR by adjusting the device parameters such as spacing between the wells, bias voltage, and the operating temperature of these detectors. This model takes into account the donor concentration in the barriers and the concentration of electrons above the barriers that have little attention in the literature. In these devices, decreasing the amount of DCRF and increasing the SNR represents the main goal for the device improvements. From the proposed model, we concluded that the contact layer and space-charge slightly affect the dark current in QWIP with a large number of quantum wells (QWs). These effects are significant in the case of a moderate number of QWs.

Lateral carrier injection for the uniform pumping of several quantum wells in InGaN/GaN light-emitting diodes.

Most optoelectronic devices share the same basic epitaxial structure - a stack of quantum wells (QWs) sandwiched between p- and n-doped layers. In nitride semiconductors, where holes have 20-times lower mobility than electrons, the holes are able to populate only the topmost 1-2 QWs. The inability to distribute the holes in a large-enough number of QWs is a cause of high Auger recombination in nitride LEDs. Lateral carrier injection is an alternative design, in which the doped regions are situated at the sides of the QW stack and the carriers diffuse horizontally into the QWs. Given that the carriers are injected into all available QWs, it finally makes sense to grow structures with a large number of QWs. We report the results of our computer simulations, which explore the advantages of LCI-based LEDs in terms of energy efficiency.

High Efficiency Inverted GaAs and GaInP/GaAs Solar Cells With Strain‐Balanced GaInAs/GaAsP Quantum Wells

AbstractHigh‐efficiency solar cells are essential for high‐density terrestrial applications, as well as space and potentially vehicle applications. The optimum bandgap for the terrestrial spectrum lies beyond the absorption range of a traditional dual junction GaInP/GaAs cell, with the bottom GaAs cell having higher bandgap energy than necessary. Lower energy bandgaps can be achieved with multiple quantum wells (QWs), but such a pathway requires advanced management of the epitaxial growth conditions in order to be useful. Strain‐balanced GaAsP/GaInAs QWs are incorporated into a single junction GaAs solar cell and a dual junction GaInP/GaAs solar cell, leading to 27.2% efficiency in the single junction device and a one‐sun record 32.9% efficiency in the tandem device. Good carrier collection and low non‐radiative recombination are observed in the cells with up to 80 QWs. The GaAs cells employ a rear‐heterojunction architecture to boost the open‐circuit voltage to over 1.04 V in the quantum well device, despite the large number of QWs.

Quantum cascade laser with bound-to-quasi-continuum optical transitions at a temperature of up to 371 K

Based on a matched Ga0.47In0.53As/Al0.48In0.52As heteropair, we have developed a quantum cascade laser emitting at a wavelength of 7.4 μm. The chosen heterostructure with a relatively large number of quantum wells and barriers represents two mini-bands separated by a mini-gap with a localised doublet level near the upper mini-band, which provides a wide emission band (∼100 cm−1). In a pulse regime, the maximal laser operation temperature is 371 K. Such a high temperature is explained by two factors: a large energy of the transfer from the doublet to the upper mini-band and a large volt defect. The characteristic temperatures T0 are found, which are equal to 170 K for low (less than 300 K) temperatures and 270 K for the range of 300 − 370 K. In addition, optical cavity losses are determined to be 2.5 and 7.7 cm−1 at temperatures of 80 and 254 K, respectively. The pulse power is 0.3 W at 80 K and 0.05 W at 293 K.

Radiation-balanced tandem semiconductor/Yb3+:YLF lasers: feasibility study

A theoretical model of a tandem radiation-balanced laser is developed and applied to explore the feasibility of radiation-balanced operation. The external cavity tandem laser is composed of an optically pumped multiple quantum well InGaAs gain section that is monolithically integrated with a Yb:YLF cooling section. It is shown that performance depends critically on a number of parameters, the choice of the effective bandgap in the semiconductor being the most important one. It is established that radiation-balanced operation with efficiencies exceeding 10% can be attained provided that the active region contains a large number of quantum wells (N> -->10) and that an efficient multipass pump cavity can be implemented.

Tunable Bragg polaritons and nonlinear emission from a hybrid metal-unfolded ZnSe-based microcavity

Strong light-matter interaction in Bragg structures possesses several advantages over conventional microcavity system. These structures provide an opportunity to incorporate a large number of quantum wells without increasing the mode volume. Further, it is expected that the strong coupling could occur over the entire thickness of the Bragg structure, and the system offers an improved overlap between exciton wave function and light mode. However, advanced experiments in Bragg structures require a precise control and manipulation of quantum states of Bragg polaritons. Here, we propose and experimentally demonstrate novel methods for the modulation of Bragg polariton eigenstates. The modulation will be shown to even exceed 10 meV if the thickness of the top layer of the ZnSe-based Bragg structure is changed or if a thin silver layer is deposited on top of the structure. The Q value of the Bragg mode will be enhanced by a factor of 2.3 for a 30 nm silver layer. In addition, we report on the observation of nonlinear emission of the lower Bragg polariton mode in the hybrid structure being achieved when excitation dependent measurements are performed. Our results open the door to create a confined Bragg polariton system similar to conventional microcavities.

Correlation between the structural and cathodoluminescence properties in InGaN/GaN multiple quantum wells with large number of quantum wells

Cathodoluminescence (CL) characteristics on 30-period InGaN/GaN multiple quantum well (MQW) solar cell structures are investigated, revealing the relationship between optical and structural properties of the MQW structures with a large number of quantum wells. In the bottom MQW layers, a blueshift of CL peak along the growth direction is found and attributed to the decrease of indium content due to the compositional pulling effect. An obvious split of emission peak and a redshift of the main emission energy are found in the top MQW layers when the MQW grows above the critical layer thickness. They are attributed to the segregation of In-rich InGaN clusters rather than the increase of indium content in quantum well layer. The MQW structure is identified to consist of two regions: a strained one in the bottom, where the indium content is gradually decreased, and a partly relaxed one in the top with segregated In-rich InGaN clusters.

AlGaN-Based Lateral Current Injection Laser Diodes Using Regrown Ohmic Contacts

A lateral current injection (LCI) design for AlGaN deep-ultraviolet edge-emitting laser diodes (LDs) on AlN substrates is presented. Two-dimensional optoelectronic simulation predicts lasing at a wavelength of 290 nm. Unlike vertical current injection designs, LCI designs can take advantage of narrow-bandgap p-type contact layers with minimal impact on the optical confinement factor through a partial decoupling of the problems of optical confinement and electrical injection. With polarization-charge-matched quantum well barriers, a large number of quantum wells can be used in LCI designs, which not only enhances the optical confinement factor but also distributes joule heating over a large volume. This is the first theoretical investigation of an LCI LD design in the III-V nitride material system.

Guiding effect of quantum wells in semiconductor lasers

The guiding effect of InGaAs quantum wells in GaAs- and InP-based semiconductor lasers has been studied theoretically and experimentally. The results demonstrate that such waveguides can be effectively used in laser structures with a large refractive index difference between the quantum well material and semiconductor matrix and a large number of quantum wells (e.g. in InP-based structures).

Modelling of a diode laser with a resonant grating of quantum wells and an external mirror

A three-dimensional numerical model of a diode laser with a resonant grating of quantum wells (QWs) and an external mirror is developed and used to calculate diode laser pulses that are long compared to the time of reaching a stationary regime and are short enough to neglect heating of the medium. The consistent solutions of the Helmholtz field equation and the system of diffusion equations for inversion in each QW are found. A source of charge carriers can be both an electron beam and a pump laser beam. The calculations yielded the longitudinal and radial profiles of the generatedfield, as well as its wavelength and power. The effective threshold pump current is determined. In the created iteration algorithm, the calculation time linearly increases with the number of QWs, which allows one to find the characteristics of lasers with a large number of QWs. The output powers and beam divergence angles of a cylindrical laser are calculated for different cavity lengths and pump spot radii. After calculating the fundamental mode characteristics, high-order modes were additionally calculated on the background of the frozen carrier distributions in the QW grating. It is shown that all the competing modes remain below the excitation threshold for the pump powers used in the experiment. The calculated and experimental data for the case of pumping by a nanosecond electron beam are qualitatively compared.

Anomalous characteristics of lasers with a large number of quantum wells

Semiconductor lasers with an active region containing six quantum wells are investigated experimentally. The temperature dependences of working characteristics (threshold current density, external differential quantum efficiency, and directional pattern) are analyzed. Anomalous behavior of the temperature dependence of the threshold current and external differential quantum efficiency, associated with a negative characteristic temperature and a decrease in the quantum efficiency of radiation upon a decrease in temperature, is detected. A narrowing of the directional patterns in the plane perpendicular to the p-n junction upon an increase in temperature is revealed.

Design optimization of InGaAsP–InGaAlAs 1.55 µm strain-compensated MQW lasers for direct modulation applications

In this paper, a simulation study of InGaAsP(well)/InGaAlAs(barrier) 1.55 µm strain-compensated multi-quantum well (MQW) lasers is presented. Due to a large conduction band discontinuity in this material system, a higher material gain and differential gain can be obtained from such a quantum well (QW) as compared to a traditional InGaAsP/InGaAsP quantum well. The deeper electron well should also improve elevated temperature operating characteristics and reduce the electron spillover from QWs. For MQWs, a uniform vertical distribution of holes is achieved due to a reduced effective hole confinement energy by optimizing the bandgap and the strain in the barriers. A large number of quantum wells can be uniformly pumped, reducing the carrier density in each individual well. A uniform and low carrier density in all the wells help reduce the total Auger recombination current. High p-doping in the active region is shown to enhance the carrier and gain non-uniformity in the MQWs. A simulated high modulation bandwidth has been demonstrated, promising directly modulated lasers as a low-cost source for short to medium distance (1–10 km) high speed optical links.

Infrared quantum well intersubband modulator

A modulator based on intersubband transition in asymmetrical quantum wells having a large linear Stark shift is demonstrated. Using a high doping density and a large number of quantum wells, a strong intersubband absorption (∼85%) is achieved. With an appropriate asymmetrical quantum well design, a large linear Stark shift is obtained.

Polariton dispersion of periodic quantum well structures

We studied the polariton dispersion relations of a periodic quantum-well structure with a period in the vicinity of half the exciton resonance wavelength, i.e., the Bragg structure. We classified polariton modes using an approximation of a large number of quantum wells. The polariton effective masses are found to be very small and equal to 10−3−10−4 of the free-electron mass.

Coupling of infrared radiation to intersubband transitions in multiple quantum wells: The effective-medium approach

The coupling of infrared radiation to intersubband excitations in multiple-quantum-well structures is discussed theoretically, employing the effective-medium approach. In contrast with previous papers, nonlocal effects in the intersubband optical response of quasi-two-dimensional electrons are taken into account. The transmission and total internal reflection geometries are considered. Special attention is paid to effects induced by (i) multiple reflections in the multiple-quantum-well structure itself, and (ii) the standing-wave effect connected with background phase-matched interferences. The results obtained indicate that the standing-wave effect plays an important role only in the case of thin structures. It can be taken into account by an appropriate modification of Beer's law. Multiple reflections substantially modify the intersubband absorption line shape of structures with a large number of quantum wells and near grazing incidence. A correct description of this modification must take into account even small differences between the dielectric constants of the well and barrier materials.

Waveguide properties of gallium, aluminum, and indium nitride heterostructures

The waveguide properties of double heterostructures constructed from nitrides of group-III elements are investigated. It is established that a maximum 8–10% AlN is sufficient (for limiting the optical mode inside the active region) in the solid solution of the wide-gap layer. The influence of light absorption in the wide-gap layers on the threshold current density of heterostructure lasers is taken into account. This form of light absorption is probably a decisive factor for heterostructures utilizing III-nitrides. A threshold current density at levels of the order of 10 kA/cm2 is expected for lasers utilizing such structures. Because of strong light absorption in the waveguide layer, the threshold current density for lasers with solitary quantum wells cannot be expected to be any lower than that of conventional double-heterostructure lasers. The threshold current density can be lowered to a few kA/cm2 in the case of lasers with a large number of quantum wells.

Electric-field effects on finite-length superlattices.

In this paper, we study the Wannier-Stark ladder by carrying out numerical calculations on a multiple-quantum-well structure under an applied electric field. The variation in the Wannier-Stark-ladder energies and the degree of localization of the corresponding wave function are examined over a wide range of values of the applied electric field. Our results show that the Wannier-Stark ladder does exist for a finite, but periodic system that consists of a large number of quantum wells having a multiple-miniband structure.