Intersubband Transitions Research Articles

Photodetectors performance of nanostructured materials in antimony III-V InGaAsSb/AlGaAsSb interband cascade structures

The aim of the study is to increase the performance of solar conversion by integrating nanostructured materials within conventional solar cells by employing multi-energy stages (i.e., interband and intersubband) in cascade systems to overcome the absorption length barrier and achieve large photo-generated carrier values. Specifically, InGaAsSb/Al GaAsSb MQWs are investigated with narrow well widths (2-20 nm) and transition energies of the structures are calculated for varied well/barrier widths and depths using numerical modelling. The results show that both interband and intersubband transitions are valuable for improving photovoltaic and photodetection applications, but their mechanisms and applications are distinct, so understanding and controlling these transitions within semiconductor materials is essential for optimising the performance of devices in both areas of optoelectronics, including strain as an important parameter of integrity. The data suggests that a single electron requires multiple photons to be transported, with the formation of a type-I broken-gap alignment between AlGa AsSb (barrier) and InGaAsSb (well). A wide range of wavelengths, from visible to medium infrared (MIR), will greatly enhance solar spectrum absorption. The long-term goal is to combine the thermal properties of antimonies (GaSbInSb) with a low band gap and the optical properties of arsenide (GaAsAlAs) with a large band gap.

Autler-Townes splitting and linear dichroism in colloidal CdSe nanoplatelets driven by near-infrared pulses.

Coherent interaction between light fields and matter has led to many exotic physical phenomena, one of which is the Autler-Townes splitting originating from the optical Stark effect of a three-level system. It has been well documented for atoms, molecules, and low-dimensional, epitaxial-grown semiconductors but rarely for solution-processed samples. Here, we report on Autler-Townes splitting observed in CdSe nanoplatelets. The strong intersubband transition of these nanoplatelets situated in the near-infrared allows us to coherently drive it with femtosecond near-infrared pulses, and meanwhile, their strong interband transition in the visible records the resultant spectral changes. The splitting is most prominent with cross-polarized, linear pump-probe pulses, consistent with the orthogonal polarizations of interband and intersubband transition dipoles. When the nanoplatelets are driven from tilted incident angles, we observe anomalous spectral lineshapes arising from a competition between interband and intersubband drivings, which are nevertheless quantitatively captured by our dressed-state model simulation.

Transmission coefficient and probability of intersubband transitions in [formula omitted] superlattices: Effect of non-parabolicity

Our study investigates optical transition energies in GaAs/AlGaAs quantum multiwell infrared photodetectors. These devices exploit transitions within subbands, necessitating accurate theoretical predictions to optimize their performance. We first determine the energetic positions of conduction minibands and investigate how band non-parabolicity influences the width of these minibands. Furthermore, we analyze the impact of non-parabolicity on transition probabilities within the quantum wells. Additionally, our study explores the transmission coefficient, which exhibits a decrease with increasing barrier height. This phenomenon affects electron transmission efficiency and is associated with quantum reflection processes. Higher barrier heights require greater energy for electrons to traverse, influencing device functionality. Understanding these factors is crucial for advancing the design and performance of GaAs/AlGaAs quantum multiwell photodetectors, offering insights into their operational limits and potential optimizations.

Hot Electron Cooling in n-Doped Colloidal Nanoplatelets Following Near-Infrared Intersubband Excitation.

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.

A Colloidal Route for Ubiquotous Mid-Wave Infrared Sensing

The rapid emergence of the Internet of Things, machine vision, and artificial intelligence have imposed critical requirements on the miniaturization, energy efficiency, and cost-effectiveness of infrared detectors. To meet these requirements, new sensor technologies that can reduce the fabrication cost associated with semiconductor epitaxy and remove the stringent requirement for cryogenic cooling are under active investigation. In the technologically important spectral region of mid-wavelength infrared (3–5 µm), colloidal quantum dots are currently at the forefront of this endeavor, with wafer-scale monolithic integration and Auger suppression being the key material capabilities to minimize the sensor's size, weight, power consumption, and cost (SWaP-C). Infrared sensors based on colloidal quantum dots have been studied for decades, and harnessing the interband transition in these quantum-confined nanostructures has been the mainstream theme in optoelectronic research. More unique photophysical properties can be harnessed by utilizing the transitions within the conduction levels or valence levels, known as intraband (intersubband) transitions. In this presentation, we discuss the progress, challenges, and opportunities of MWIR sensors fabricated from Ag2Se intraband colloidal quantum dots, a heavy metal-free colloidal nanomaterial that has merits for wide-scale adoption in consumer and industrial sectors.

Optical absorption properties in multi-layer InGaN/GaN core-shell quantum dots: The influences of ternary mixed crystal effect and hydrogen-like impurity

Based on the effective mass approximation and the compact density matrix theory, the electronic states and optical absorption properties of intersubband transitions between the conduction-band energy levels in zinc blende InGaN/GaN/InGaN/GaN core-shell quantum dots (CSQDs) are investigated by using the finite difference method. The effects of hydrogen-like impurity and In component on the wave functions, transition energies, and optical absorption properties are discussed in detail. The results show that hydrogen-like impurity can cause the peak positions of optical absorption coefficients (OACs) and refractive index changes (RICs) to move to high energy area, and the peak values of OACs rise. Increasing In component of InGaN crystal in the core region of a CSQD has the same effect. But an opposite tendency will be found when increasing In component in the shell-well of a CSQD, i.e. optical absorption peaks move to low energy area and their values decrease. Comparing to the shell-well, the core region has more powerful confinement effect on electrons. So the better absorption properties are more likely to be obtained for this type of multi-layer InGaN/GaN CSQDs when In component in the core is larger than that in the shell-well.

Effects of hydrostatic pressure, aluminium composition, and external magnetic field on intersubband and intrasubband transitions characteristics in symmetric potential quantum wells

Aluminium composition (λ), magnetic field (B), and hydrostatic pressure (P) influences on magneto-optical (MO) properties of intra- and inter-subband transitions in GaAs/AlλGa1−λAs symmetric QWs for four models of confined optic phonons are studied and compared. The findings indicate: the MO characteristics of intra- and inter-subband transitions are significantly influenced by B, λ, and P for phonon absorption and emission. The e–p interaction intensity decreases as P increases, whereas it increases as λ increases. The influences of λ and P on phonon absorption are more significant than those on emission. E–p interaction intensity dependence on λ and P for intra-subband transitions is stronger than that for inter-subband ones. The e–p interaction in QWs is well described by Knipp–Reinecke and Huang–Zhu models. To reduce the influence of P on MO properties, we can increase T and λ or decrease B. At T≈600 K, the influence of P is insignificant and can be neglected.

Enhanced terahertz ISBT in GaAsBi/AlGaAs quantum well: impact of doping modulation

One of the pivotal characteristics of quantum wells (QW) designed for applications in filters or modulators is its absorption coefficient. This study investigates the combined influence of doping and active layer thickness on the absorption coefficient in GaAsBi/AlGaAs quantum wells. The self-consistent coupling between Schrödinger and Poisson equations was employed for this purpose. A comprehensive analysis of conduction band structure, energy levels, potentials, and densities is presented. This analysis discerns the effects of doping on intersubband transitions (ISBTs) in the terahertz (THz) frequency domain. An introduction of a donor impurity into the system greatly influenced the intersubband absorption coefficient, increasing its intensity and causing the optical response to shift toward the lowest frequency. The obtained resonant peak energies, situated within the terahertz spectrum, hint at promising applications for GaAsBi-based devices within this frequency band.

Electrically tunable third-harmonic generation using intersubband polaritonic metasurfaces

Nonlinear intersubband polaritonic metasurfaces, which integrate giant nonlinear responses derived from intersubband transitions of multiple quantum wells (MQWs) with plasmonic nanoresonators, not only facilitate efficient frequency conversion at pump intensities on the order of few tens of kW cm-2 but also enable electrical modulation of nonlinear responses at the individual meta-atom level and dynamic beam manipulation. The electrical modulation characteristics of the magnitude and phase of the nonlinear optical response are realized through Stark tuning of the resonant intersubband nonlinearity. In this study, we report, for the first time, experimental implementations of electrical modulation characteristics of mid-infrared third-harmonic generation (THG) using an intersubband polaritonic metasurface based on MQW with electrically tunable third-order nonlinear response. Experimentally, we achieved a 450% modulation depth of the THG signal, 86% suppression of zero-order THG diffraction tuning based on local phase tuning exceeding 180 degrees, and THG beam steering using phase gradients. Our work proposes a new route for electrically tunable flat nonlinear optical elements with versatile functionalities.

Electric field-induced electronic structure and intersubband transitions of a hydrogen molecular ion in a gaussian-type quantum ring

In this work, the influence of an external electric field on the electronic structure and intersubband transitions of a singly ionized double-donor system in a GaAs quantum ring defined by Gaussian-type potentials is investigated theoretically. Within the framework of the effective mass approach, the two-dimensional diagonalization method is used for the solution of the Schrödinger equation to obtain the eigen energies and corresponding wave functions. Numerical results reveal that the electronic energy spectrum and the linear optical absorption coefficient of the ring are remarkably affected by the strength of the lateral electric field, internuclear distance and parameters defining the confinement potential. Also, it has been shown that beyond the anti-crossing point, the wave functions exchange their symmetries without mixing, which is a characteristic feature of energy-level anti-crossing.

Tip‐Enhanced Imaging and Control of Infrared Strong Light‐Matter Interaction

AbstractOptical antenna resonators enable control of light‐matter interactions on the nano‐scale via electron–photon hybrid states in strong coupling. Specifically, mid‐infrared (MIR) nano‐antennas coupled to saturable intersubband transitions in multi‐quantum‐well (MQW) semiconductor heterostructures allow for the coupling strength to be tuned through antenna resonance and field intensity. Here, tip‐enhanced nano‐scale variation of antenna‐MQW coupling across the antenna is demonstrated, with a spatially‐dependent coupling strength varying from 73 (strong coupling) to 24 (weak coupling). This behavior is modeled based on the spatially dependent local constructive and destructive interference between tip and antenna fields. Using a quantum‐mechanical density‐matrix model of the MQW system with its designed values of transition dipole moment, doping density, and population decay time, the picosecond IR pulse coupling to intersubband transitions and the associated tip induced strong‐field saturation effects are described. These results present a new regime of nonlinear IR light‐matter control based on the dynamic manipulation of quantum hybrid states on the nanoscale and in the infrared, with a perspective regarding extension to molecular vibrations.

Intersubband polaritonic metasurfaces for high-contrast ultra-fast power limiting and optical switching

Nonlinear intersubband polaritonic metasurfaces support one of the strongest known ultrafast nonlinear responses in the mid-infrared frequency range across all condensed matter systems. Beyond harmonic generation and frequency mixing, these nonlinearities can be leveraged for ultrafast optical switching and power limiting, based on tailored transitions from strong to weak polaritonic coupling. Here, we demonstrate synergistic optimization of materials and photonic nanostructures to achieve large reflection contrast in ultrafast polaritonic metasurface limiters. The devices are based on optimized semiconductor heterostructure materials that minimize the intersubband transition linewidth and reduce absorption in optically-saturated nanoresonators, achieving a record-high reflection contrast of 54% experimentally. We also discuss opportunities to further boost the metrics of performance of this class of ultrafast limiters, showing that reflection contrast as high as 94% may be realistically achieved using all-dielectric intersubband polaritonic metasurfaces.

The effect of hydrostatic pressure, temperature and impurity factor on linear and nonlinear optical properties of InxGa1-xAs tuned quantum dot with modified Gaussian potential under the action of a vertical magnetic field

This study presents a detailed theoretical analysis of the effects of hydrostatic pressure, temperature, impurity factors, and external electric fields on the optical properties of InxGa1-xAs tuned quantum dot. A uniform magnetic field, directed perpendicular to the quantum dot plane, is applied. The system is excited using a monochromatic electromagnetic field, considering electron intersubband transitions. Energy levels and wave functions are determined using the parabolic band approximation and effective mass approximation. The linear and nonlinear optical absorption coefficients and refractive index changes are computed using the compact-density matrix approach and the iterative method. Numerical results indicate that the peaks of the linear and nonlinear optical absorption coefficients and refractive index changes shift towards lower energies (red shift) and higher energies (blue shift) under varying hydrostatic pressure, temperature, and impurity factor strength. Additionally, the peak values of the absorption coefficients and refractive index changes increase or decrease based on these parameter variations. These results have significant implications for the design and optimization of quantum dot-based optoelectronic devices.

Giant optical second- and third-order nonlinearities at a telecom wavelength.

A material platform that excels in both optical second- and third-order nonlinearities at a telecom wavelength is theoretically and experimentally demonstrated. In this TiN-based coupled metallic quantum well structure, electronic subbands are engineered to support doubly resonant inter-subband transitions for an exceptionally high second-order nonlinearity and provide single-photon transitions for a remarkable third-order nonlinearity within the 1400-1600 nm bandwidth. The second-order susceptibility χ(2) reaches 2840 pm/V at 1440 nm, while the Kerr coefficient n2 arrives at 2.8 × 10-10 cm2/W at 1460 nm. The achievement of simultaneous strong second- and third-order nonlinearities in one material at a telecom wavelength creates opportunities for multi-functional advanced applications in the field of nonlinear optics.

Emission linewidth broadening in quantum cascade lasers induced by multiple intersubband transitions

Emission linewidth broadening in quantum cascade lasers induced by multiple intersubband transitions

Enhanced response over wavelength range of 7–12 µm for quantum wells in asymmetric micro-pillars

Efficient coupling in broad wavelength range is desirable for wide-spectrum infrared light detection, yet this is a challenge for intersubband transition in semiconductor quantum wells (QWs). High-Q cavities mostly intensify the absorption at peak wavelengths but with shrinking bandwidth. Here, we propose a novel approach to expand the operating spectral range of the Quantum Well Infrared Photodetectors (QWIPs). By processing the QWs into asymmetric micro-pillar array structure, the device demonstrates a substantial enhancement in spectral response across the wavelength from 7.1 µm to 12.3 µm with guided mode resonance (GMR) effects. The blackbody responsivity is then increased by 3 times compared to that of the 45° polished edge-coupled counterpart. Meanwhile, the dark current density remains unchanged after the deep etching process, which will benefit the electrical performance of the detector with reduced volume duty ratio. In contrast to the symmetric micro-pillar array that contains simple resonance mode, the detectivity of QWIP in asymmetric pillar structure is found to be improved by 2-4 times within the range of 9.5 µm to 15 µm.

Conduction-band engineering of polar nitride semiconductors with wurtzite ScAlN for near-infrared photonic devices

Wurtzite ScxAl1−xN/GaN (x = 0.13–0.18) multi-quantum wells grown by molecular beam epitaxy on c-plane GaN are found to exhibit remarkably strong and narrow near-infrared intersubband absorption in the technologically important 1.8–2.4 μm range. Band structure simulations reveal that, for GaN wells wider than 3 nm, the quantized energies are set by the steep triangular profile of the conduction band caused by intrinsic polarization fields. As a result, the intersubband transition energies provide unique and direct access to essential ScAlN polarization parameters. Measured infrared absorption indicates that the spontaneous polarization difference of the presumed lattice-matched Sc0.18Al0.82N/GaN heterostructure is smaller than the theoretically calculated value. The intersubband transition energies are relatively insensitive to the barrier alloy composition indicating negligible variation of the net polarization field in the probed 0.13–0.18 Sc composition range.

Nonvolatile memory operations using intersubband transitions in GaN/AlN resonant tunneling diodes grown on Si(111) substrates

Nonvolatile memory using intersubband transitions and quantum-well electron accumulation in GaN/AlN resonant tunneling diodes (RTDs) is a promising candidate for high-speed nonvolatile memory operating on a picosecond timescale. This memory has been fabricated on sapphire(0001) substrates to date because of the high affinity between the nitride materials and the substrate. However, the fabrication of this memory on Si(111) substrates is attractive to realize hybrid integration with Si devices and nonvolatile memory and three-dimensional integration such as chip-on-wafer and wafer-on-wafer. In this study, GaN/AlN RTDs are fabricated on a Si(111) substrate using metal-organic vapor phase epitaxy. The large strain caused by the differences in the thermal expansion coefficients and lattice constants between the Si(111) substrate and nitride materials are suppressed by a growth technique based on the insertion of low-temperature-grown AlGaN and thin AlN layers. The GaN/AlN RTDs fabricated on Si(111) substrates show clear GaN/AlN heterointerfaces and a high ON/OFF ratio of >220, which are equivalent to those for devices fabricated on sapphire(0001) substrates. However, the nonvolatile memory characteristics fluctuate by repeated write/erase memory operations. Evaluation of the ON/OFF switching time and endurance characteristics indicates that the instability of the nonvolatile memory characteristics is caused by electron leakage through deep levels in the quantum-well structure. Possible methods for suppressing this are discussed with an aim of realizing high-speed and high-endurance nonvolatile memory.

Giant Tunability of Intersubband Transitions and Quantum Hall Quartets in Few-Layer InSe Quantum Wells.

A two-dimensional (2D) quantum electron system is characterized by quantized energy levels, or subbands, in the out-of-plane direction. Populating higher subbands and controlling the intersubband transitions have wide technological applications such as optical modulators and quantum cascade lasers. In conventional materials, however, the tunability of intersubband spacing is limited. Here we demonstrate electrostatic population and characterization of the second subband in few-layer InSe quantum wells, with giant tunability of its energy, population, and spin-orbit coupling strength, via the control of not only layer thickness but also the out-of-plane displacement field. A modulation of as much as 350% or over 250 meV is achievable, underscoring the promise of InSe for tunable infrared and THz sources, detectors, and modulators.

Optical rectification and second harmonic generation in CdSe/MgSe asymmetric double quantum wells

In this study, we report theoretically the effect of well, barrier widths and polarization on optical properties of intersubband transitions (ISBT) in CdSe/MgSe asymmetric quantum wells (ADQWs). Eigenenergies and their corresponding wave functions have been calculated by solving numerically the Schrödinger equation. The second harmonic generation and the optical rectification including intersubband transition energies have been discussed. Obtained results revealed that intersubband transition depends strongly on the quantum wells and the barrier widths as well as the stark effect. With appropriate intensity, optical rectification can reach great magnitude. We hope that the numerical results of our research are valuable theoretically and experimentally to our scientific community in nonlinear optics.