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In this article, based on the effective mass approximation and the density matrix formalism, we have performed a quantitative analysis of the quantum and dielectric confinement effect on the optical properties in a spherical core multishell quantum dot (MSQD) incorporated into six oxides (TiO2, ZrO2, HfO2, MgO, Si3N4, and SiO2). Formulas for the third-order nonlinear susceptibility χ(3), the optical absorption coefficients (OACs) and alterations in refractive index (RICs) are established using the density matrix formalism applied to a two-level system. Our findings indicated that the investigated coefficients exhibit significant sensitivity to variations in geometrical parameters, incident optical intensity, relaxation time, and the surrounding dielectric oxides. The dielectric discrepancy between the nanosystem and its new environment provides a manual-like guidance for the (MSQD) nanosystem which can be valuable in experimental applications. [ABSTRACT FROM AUTHOR]

In the present work, we explored the impact of doping concentration and applied electric field on both linear and nonlinear optical properties based upon the intersubband transitions of CdSe/MgSe single quantum wells (QWs) in the framework of the effective mass approximation (EMA) using the compact density matrix approach. The energy levels and their relative wave functions were obtained by solving the coupled Schrödinger–Poisson equations by the finite difference method (FDM) under the envelope wave function approximation. The third-order nonlinear optical susceptibility, the refractive index changes (RIC), and the absorption coefficients (AC) were investigated as a function of doping concentration. The numerical results revealed that an increase in the doping concentration ND results in a blue shift of the peak position of the linear and nonlinear optical properties, with a substantial enhancement of their magnitudes. The same behavior related to the third-order nonlinear susceptibility was noted when the considered structure was subjected to an external electric field. [ABSTRACT FROM AUTHOR]

Intersubband transitions in semiconductor heterostructures offer a way to achieve large and designable nonlinearities with dynamic modulation of intersubband energies through the Stark effect. One promising approach for incorporating these nonlinearities into free space optics is a nonlinear polaritonic metasurface, which derives resonant coupling between intersubband nonlinearities and optical modes in nanocavities. Recent work has shown efficient frequency mixing at low pumping intensities, with the ability to electrically tune the phase, amplitude, and spectral peak of it. However, the spectral tunability of intersubband nonlinearities is constrained by the static spectral response of nanocavities. To overcome this limitation, we present nonlinear polaritonic metasurfaces for a broadband giant nonlinear response. This is achieved by combining a Stark tunable nonlinear response from a quantum-engineered semi-conductor heterostructure with arrays of three nanocavities with different resonant wavelengths. We experimentally demonstrate broadband second harmonic generation (SHG) and a shift in the peak SHG efficiency within the range of 8.9–10.6 μm by applying bias voltage. This work will provide a promising route for achieving broadband and electrically tunable nonlinearities in metasurfaces.

In the present article, we derive optimal spatially varying control fields, which maximize the four-wave mixing efficiency in a four-subband semiconductor asymmetric double quantum well, following analogous works in atomic systems. The control fields coherently prepare the medium, where a weak probe pulse is propagated and eventually converted to a signal pulse at the output. The optimal fields, which maximize the conversion efficiency for a given propagation length, are obtained by applying optimal control theory to a simplified form of propagation equations but are tested with numerical simulations using the full set of Maxwell–Schrödinger equations, which accurately describe the propagation of light pulses in the medium. For short propagation distances, the proposed optimal scheme outperforms a simpler spatially changing control protocol that we recently studied, while for larger distances, the efficiency of both protocols approaches unity. The present work is expected to find application in frequency conversion between light beams, conversion between light beams carrying orbital angular momentum, and nonlinear optical amplification. [ABSTRACT FROM AUTHOR]

ZnO-based heterostructures are up-and-coming candidates for terahertz (THz) optoelectronic devices, largely owing to their innate material attributes. The significant ZnO LO-phonon energy plays a pivotal role in mitigating thermally induced LO-phonon scattering, potentially significantly elevating the temperature performance of quantum cascade lasers (QCLs). In this work, we calculate the electronic structure and absorption of ZnO/ZnMgO multiple semiconductor quantum wells (MQWs) and the current density–voltage characteristics of nonpolar m-plane ZnO/ZnMgO double-barrier resonant tunnelling diodes (RTDs). Both MQWs and RTDs are considered here as two building blocks of a QCL. We show how the doping, Mg percentage and layer thickness affect the absorption of MQWs at room temperature. We confirm that in the high doping concentrations regime, a full quantum treatment that includes the depolarisation shift effect must be considered, as it shifts mid-infrared absorption peak energy for several tens of meV. Furthermore, we also focus on the performance of RTDs for various parameter changes and conclude that, to maximise the peak-to-valley ratio (PVR), the optimal doping density of the analysed ZnO/Zn88Mg12O double-barrier RTD should be approximately 10 18   c m − 3 , whilst the optimal barrier thickness should be 1.3 nm, with a Mg mole fraction of ~9%. [ABSTRACT FROM AUTHOR]

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.

Within the effective mass approximation framework, the electronic and optical properties of GaAs-based two-dimensional eccentric quantum rings subjected to a transverse magnetic field with an inclination θ-angle to the x-axis are investigated using a two-dimensional diagonalisation method. The results show that the electron energy spectra in concentric quantum rings reveal transparency to the action of magnetic field orientation, contrary to their optical properties, which turn out to be anisotropic with this later. In addition, the eccentricity preserves the absorption coefficient spectra for y-polarisation for all orientations of the magnetic field qualitatively, while those of x-polarisation are significantly modified by eccentricity and magnetic field. Specific peak transitions occurring at specific magnetic field values and θ-angle can be used as a signature of magnetic field orientation and eccentricity. [ABSTRACT FROM AUTHOR]

Electrically reconfigurable metasurfaces that overcome the static limitations in controlling the fundamental properties of scattered light are opening new avenues for functional flat optics. This work proposes and experimentally demonstrates electrically phase‐tunable mid‐infrared metasurfaces based on the polaritonic coupling of Stark‐tunable intersubband transitions in semiconductor heterostructures and electromagnetic modes in plasmonic nanoresonators. In the applied voltage range of −3 to +3 V, the local phase tuning of the light reflects from the metasurface, which enables the electrical control of the polarization state and wavefront of the reflected wave. Electrical beam polarization control, electrical beam diffraction control, and electrical beam steering are experimentally demonstrated as applications for local phase tunability. The proposed electrically tunable metasurfaces can easily tune the operating wavelength and function at relatively low voltages, which will enable various applications in the mid‐infrared region. [ABSTRACT FROM AUTHOR]

In this paper, we theoretically investigate intersubband absorption in La-doped BaSnO3/BaO perovskite-oxide double and triple symmetrical quantum wells. The goal is to assess the emerging possibilities offered by perovskite heterostructures for realization of oxide-based quantum well optoelectronic devices. Properties such as small electron effective mass, very high room-temperature mobility of BaSnO3, large conduction band offset between the two materials, are all promising for technological applications. Some of the possible applications include active n-channel materials in p-n junctions, field-effect transistors, ferroelectric field-effect transistors, sensitive UV photoconductors, and electron transport layers in perovskite solar cells. We are interested in tuning of the absorption spectra via quantum well width modulation, changing of doping density and variation of external electric field along the growth direction which is important for application in electro-optical light modulators. The electronic structure is calculated self-consistently by solving the Schrödinger–Poisson system of equations where the exchange correlation effects are also taken into the account. [ABSTRACT FROM AUTHOR]

In the present article, we derive optimal spatially varying control fields, which maximize the four-wave mixing efficiency in a four-subband semiconductor asymmetric double quantum well, following analogous works in atomic systems. The control fields coherently prepare the medium, where a weak probe pulse is propagated and eventually converted to a signal pulse at the output. The optimal fields, which maximize the conversion efficiency for a given propagation length, are obtained by applying optimal control theory to a simplified form of propagation equations but are tested with numerical simulations using the full set of Maxwell–Schrödinger equations, which accurately describe the propagation of light pulses in the medium. For short propagation distances, the proposed optimal scheme outperforms a simpler spatially changing control protocol that we recently studied, while for larger distances, the efficiency of both protocols approaches unity. The present work is expected to find application in frequency conversion between light beams, conversion between light beams carrying orbital angular momentum, and nonlinear optical amplification.

The main candidate to be a practical and low cost high power THz source is the intersubband-based quantum cascade laser, which can have a tremendous impact in many practical applications, including last mile and indoor telecommunication systems. In this review we discuss current challenges for THz intersubband device development from a microscopic point of view. Next summarize the search for new mechanisms and structure designs that can lead to intersubband gain without population inversion. This is a very important topic of current research, since is both an extremely elegant phenomenon from the basic physics of view and crucial for effective lasing in the THz range. The reason is that scattering phenomena can lead to level broadenings of the same order of magnitude of the lasing transitions, making population inversion by carrier injection in upper lasing subbands extremely difficult. Previous work in the literature is compared and contrasted with a new scheme that may lead to high temperature lasing by engineering the nonequilibrium population inversion with a combination of band structure and many body effects mediated by a k-space filter.

Abstract Electrically reconfigurable metasurfaces that overcome the static limitations in controlling the fundamental properties of scattered light are opening new avenues for functional flat optics. This work proposes and experimentally demonstrates electrically phase‐tunable mid‐infrared metasurfaces based on the polaritonic coupling of Stark‐tunable intersubband transitions in semiconductor heterostructures and electromagnetic modes in plasmonic nanoresonators. In the applied voltage range of −3 to +3 V, the local phase tuning of the light reflects from the metasurface, which enables the electrical control of the polarization state and wavefront of the reflected wave. Electrical beam polarization control, electrical beam diffraction control, and electrical beam steering are experimentally demonstrated as applications for local phase tunability. The proposed electrically tunable metasurfaces can easily tune the operating wavelength and function at relatively low voltages, which will enable various applications in the mid‐infrared region.

In this description, we have numerically studied and modulated the transition energy, the linear and third order nonlinear absorption coefficient as well as the refractive index changes under the effective mass approximation and the compact density matrix formalism in (CdS/ZnSe) core/shell prolate spheroid quantum dot (CSPSQD), incorporated in dielectric matrices as SiO2, PVC and PVA. Our results proved that the linear and nonlinear optical proprieties are sensitive by the geometry of the quantum dots (QDs) and the surrounding medium. The results shows, when the eccentricity increase, the transitions energy decrease considerably and the resonance peaks positions towards low energies. The suitable choice of the dielectric matrix and the geometrical size offers the possibility of tuning the optical properties of the QDs which are important of optoelectronic and photonic devices. [ABSTRACT FROM AUTHOR]

Polarizaton properties of ZnO well layers on BeMgZnO barrier layers grown in polar and semipolar orientations have been investigated. Cases of relaxed and strained barrier layers are considered. It is found that the polarizaton difference at the interfaces leads to a built-in electric field inside the well layer as much as 8 MV cmˆ(-1) in magnitude. Nonpolar ZnO/BeMgZnO quantum wells have been studied in terms of intersubband transitions. The calculations have covered Be and Mg concentrations up 0.18 and 0.5, respectively. It has been found that intersubband transition (ISBT) energies ranging from 50 to 700 meV are possible. The effect of barrier thickness on the ISBT energies has been studied. The results indicate insignificant changes in ISBT energies compared to the energies. [ABSTRACT FROM AUTHOR]

Metasurfaces control optical wavefronts via arrays of nanoscale resonators laid out across a surface. When combined with IIIâ€"V semiconductors with strong optical nonlinearities, a variety of nonlinear effects such as harmonic generation and all-optical modulation can be enabled and enhanced at the nanoscale. This review presents our research on engineering and boosting nonlinear effects in ultrafast and nonlinear semiconductor metasurfaces fabricated at the Center for Integrated Nanotechnologies. We cover our recent works on parametric generation of harmonic light via direct and cascaded processes in GaAs-metasurfaces using Mie-like optical resonances or symmetric-protected bound state in the continuum, and then describe the recent advances on harmonic generation in all-dielectric metasurfaces coupled to intersubband transitions in IIIâ€"V semiconductor heterostructures. The review concludes on the potential of metasurfaces to serve as the next platform for on-chip quantum light generation. [ABSTRACT FROM AUTHOR]

Terahertz quantum cascade laser sources with intra-cavity non-linear frequency mixing are the first room-temperature electrically pumped monolithic semiconductor sources that operate in the 1.2–5.9 THz spectral range. However, high performance in low-frequency range is difficult because converted terahertz waves suffer from significantly high absorption in waveguides. Here, we report a sub-terahertz electrically pumped monolithic semiconductor laser. This sub-terahertz source is based on a high-performance, long-wavelength (λ ≈ 13.7 μm) quantum cascade laser in which high-efficiency terahertz generation occurs. The device produces peak output power of 11 μW within the 615–788 GHz frequency range at room temperature. Additionally, a source emitting at 1.5 THz provides peak output power of 287 μW at 110 K. The generated terahertz radiation of

Terahertz (THz) quantum cascade lasers (QCLs) are electrically pumped and heterostructure based semiconductor laser sources with intersubband transitions of electrons in different layers of the quantum wells and barriers. The THz QCLs have high output power in THz region which make them important from application point of view. Recently intensive research has been carried out by researchers for obtaining efficient designs of THz sources. Most of the researchers have investigated the THz frequency range between 0.1 and 3 THz; however, the output power of the THz sources in the frequency range 3–5 THz is small because of transit time and resistance-capacitance effects. Nevertheless, the present review is focused for the development of efficient THz QCL sources in the frequency range from 3 to 5 THz where one of the major problem of thermal backfilling of the carriers has to be overcome by engineering the heterostructure. [ABSTRACT FROM AUTHOR]

GaN/AlN resonant tunneling diodes (RTDs) are studied to realize a high‐speed nonvolatile memory based on the intersubband transactions and electron accumulation in the quantum well, which has the potential to operate at picosecond time scales. The crystal quality of GaN/AlN RTDs is improved by changing the growth conditions and structure of the buffer layer. The surface roughness and dislocation density of the GaN/AlN RTDs are successfully suppressed, and clear ON/OFF switching due to intersubband transitions is observed by inputting pulse voltage sequences. However, the voltages for write and erase operations are changed by improving the crystal quality of GaN/AlN RTDs. The theoretical analysis of resonant levels in the GaN/AlN RTDs indicates that the voltages for write and erase operations are very sensitive to the well and barrier widths and the density of electrons accumulating in the quantum well. Based on the results, the design of GaN/AlN RTDs for higher‐performance nonvolatile memory operations is investigated. [ABSTRACT FROM AUTHOR]

Terahertz quantum cascade laser (QCL) sources based on intra-cavity difference frequency generation are currently the only electrically pumped monolithic semiconductor light sources operating at room temperature in the 1–6-THz spectral range. Relying on the active regions with the giant second-order nonlinear susceptibility and the Cherenkov phase-matching scheme, these devices demonstrated drastic improvements in performance in the past several years and can now produce narrow-linewidth single-mode terahertz emission that is tunable from 1 to 6 THz with power output sufficient for imaging and spectroscopic applications. This paper reviews the progress of this technology. Recent efforts in wave function engineering using a new active region design based on a dual-upper-state concept led to a significant enhancement of the optical nonlinearity of the active region for efficient terahertz generation. The transfer of Cherenkov devices from their native semi-insulating InP substrates to high-resistivity silicon substrates resulted in a dramatic improvement in the outcoupling efficiency of terahertz radiation. Cherenkov terahertz QCL sources based on the dual-upper-state design have also been shown to exhibit ultra-broadband comb-like terahertz emission spectra with more than one octave of terahertz frequency span. The broadband terahertz QCL sources operating in continuous-wave mode produces the narrow inter-mode beat-note linewidth of 287 Hz, which indicates frequency comb operation of mid-infrared pumps and thus supports potential terahertz comb operation. Finally, we report the high-quality terahertz imaging obtained by a THz imaging system using terahertz QCL sources based on intra-cavity difference frequency generation.

Intersubband transitions (ISBTs) in conduction‐band quantum wells (QWs) have attracted tremendous attention for their high technological potential, ranging from quantum cascade lasers, quantum well infrared photodetectors, to various nonlinear optical elements. One of the main characteristics of using the ISBTs is their polarization selection rule, which forbids a normal‐incidence geometry. Here, it is shown that the ISBT selection rule is not strict on optical nonlinearities in metallic QWs (MQWs). The nonlinear process of second harmonic generation nearly follows the selection rule, while the optical Kerr nonlinear process severely deviates from it. The anomalous optical selection rules result from the non‐negligible ultrafast electron–electron scattering in these plasmonic systems, and a coupled mode theory is provided to get a physical grasp of the problem. The flexible selection rule in MQWs could bring drastic improvements in efficiency and diversity of ISBT‐based devices. [ABSTRACT FROM AUTHOR]

Metasurfaces that generate nonlinear optical responses provide new degrees of freedom for applications such as nonlinear holography, wave mixing, and new frequencies generation. To extend the utility of flat nonlinear optics, metasurfaces providing both giant nonlinear response for efficient frequency mixing in subwavelength films and a single‐beam output with a local wavefront control need to be developed. Metasurfaces with giant nonlinear response have recently been realized using the concept of intersubband polaritons. Separately, nonlinear metasurfaces providing a single‐beam nonlinear output with a local wavefront control have been studied. However, a metasurface platform that possesses both of these properties has yet to be demonstrated. Herein, the first such platform is presented and its operation for three‐ and four‐wave mixing processes is demonstrated. This approach is based on using Pancharatnam–Berry phase control of local nonlinear response and on employing meta‐atoms with specific symmetries to enable generation of only a single nonlinear beam. Experimentally, 400 nm‐thick metasurfaces with a wavefront‐controlled single‐beam output are demonstrated for second‐ and third‐harmonic generation at pump wavelength of approximately 10 µm. Power conversion efficiencies of 7.6 × 10−4% and 3.6 × 10−4% are obtained for the two nonlinear processes, respectively, at peak pumping intensity of only 80 kW cm−2. [ABSTRACT FROM AUTHOR]

ZnO-based heterostructures have emerged as promising materials for infrared and terahertz (THz) optoelectronic technologies capable of operating at room temperatures. This is attributed to the intrinsically high longitudinal-optical (LO) phonon energy which suppresses the phonon scattering to boost the temperature performance. Numerous high-performance THz sources rely on single doping (e.g., ZnO/Mg x Zn 1–x O, ZnO/Cd x Zn 1–x O hybrids). Herein, the combined theoretical approaches based on density functional theory (DFT) and the numerical matrix method were employed to unravel the electronic and optical properties of a quantum well made from ZnO/(Sb,N) co-doped ZnO heterostructures. The findings reveal that (Sb,N) co-doped relatively requires the lower formation energy than the single doping counterparts. Both un-doped and (Sb,N) co-doped ZnO are characterized as semiconductors of which the energy gap of the latter is relatively lower. The combination of these materials consequently creates type-I band lineups with the valence and conduction band offsets of 1.03 eV and 0.61 eV, respectively. Intriguingly, a confined electron in the resultant quantum well derived from the conduction band offset exhibits the intersubband optical transitions in the energy range of 6.0–10.0 THz which is the challenging regime in THz technology. Moreover, the optical absorptions can be effectively modulated by the width of the quantum well and the external electric fields. Specifically, the increases in the well width and field strength result in the redshift and blueshift in the absorption spectrum, respectively. Hence, ZnO/(Sb,N) co-doped ZnO/ZnO heterostructure is conclusively a potential candidate for THz optoelectronic devices. [Display omitted] • The electronic and optical properties of ZnO/(Sb,N) co-doped ZnO heterostructure are presented. • The heterostructure adopts type-I band lineups with the valence (conduction) band offset of 1.03 (0.61) eV. • A quantum well from the heterostructure exhibits intersubband transitions in the challenging range of 6.0–10.0 THz. • Optical absorptions can be effectively tuned by the width of the quantum well and the external electric fields. [ABSTRACT FROM AUTHOR]

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., Competing Interests: The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (© 2024 The Author.)

In this study, we propose a theoretical simulation of a Ge 0.9 Sn 0.1 rectangular SQW with GeSn parabolically graded barriers (PGBs) in the terahertz (THz) region. The discrete intra-band confined energy levels and their matching wave functions were calculated by solving the stationary Schrödinger equation by the finite difference method taking into account the Compact Density Matrix approach under the framework of both the effective mass and the envelope wave function approximations. In this work, we studied the effect of the quantum square-well width on the intersubband transition and oscillator strength in a GeSn-strained-based Barrier-Well-Barrier GeSn/(Ge,α-Sn) PGBs to obtain the optimum quantum confinement of electrons. The electronic states and their wave functions in the conduction band were computed by solving the Schrödinger equation without and under the effect of an applied external electric field at room temperature. We then investigated the effect of the electric field on the optical absorption coefficient (OAC). Our numerical results show that for external fields (>15 kV/cm), an intersubband transitions (ISBTs) frequency band of 2–14 THz (8–58 meV) was obtained for the specific optimized parameters. These results should be beneficial to the design of devices based on GeSn QWs with PGB structures operating in the THz frequency range. • Effect of the compositions and thicknesses on the GeSn parabolically graded barriers. • The Strak effect on OAC was investigated. • GeSn parabolic quantum wells design is a good candidate for optoelectronic devices in the THz frequency range. [ABSTRACT FROM AUTHOR]

n-type Ge/SiGe asymmetric coupled quantum wells represent the building block of a variety of nanoscale quantum devices, including recently proposed designs for a silicon-based THz quantum cascade laser. In this paper, we combine structural and spectroscopic experiments on 20-module superstructures, each featuring two Ge wells coupled through a Ge-rich SiGe tunnel barrier, as a function of the geometry parameters of the design and the P dopant concentration. Through a comparison of THz spectroscopic data with numerical calculations of intersubband optical absorption resonances, we demonstrated that it is possible to tune, by design, the energy and the spatial overlap of quantum confined subbands in the conduction band of the heterostructures. The high structural/interface quality of the samples and the control achieved on subband hybridization are promising starting points towards a working electrically pumped light-emitting device.

Abstract Using the genuine superlattice eigenvalues and eigenfunctions, and their parity symmetries, new transition selection rules are derived, and a novel approach for the explicit calculation of the optical spectra of periodic structures presented. The optical response and infrared spectra published in the last 40 years for (In, Ga)N and (Al, Ga)As superlattices, with different number of unit cells n (varying from ∼ 10 to ∼ 400) and barrier and valley widths in the range of 2.5 nm to 40 nm, and different properties have been revisited. Among the various examples discussed here, we study the narrow peaks clustered in groups reported in high resolution spectra of blue-emitting devices, that could not be explained until now. These properties are faithfully reproduced and fully understood. We show that these effects are related to and depend on the surface energy levels and their tuneable detachment. We shown also that the optical transitions experimentally observed, but forbidden by the current optical physics, are really allowed transitions. Since the number of energy eigenvalues E μ , ν r , v and eigenfunctions Ψ μ ′ , ν ′ r , v (z) is now much greater, the number of allowed matrix-elements can be extremely large (of the order of n 2 n c n v ∕ 2 , with n c and n v the number of subbands in the conduction and valence bands). We show that this number can be reduced by using the eigenfunctions' symmetries, which are behind the parity symmetry selection rules and the leading order selection rules. These rules reduce the number of matrix-elements evaluations from ∼ n 2 n c n v ∕ 2 to ∼ n n c n v ∕ 2 , i.e., depending on the superlattice, from about 1000 to 100. We discuss also a third rule, that collects the contributions of the surface and edge states, reducing the number of transitions even more to N s ≤ n c n v. With these rules, the main peaks are conserved and their number practically matches that of the real spectrum. Excellent agreements are found with experimental results. Highlights • Truly quantum calculation of optical properties in superlattices. • An approach that goes beyond the abilities of the current approaches in the literature. • Accurate calculations based on bona fide superlattice eigenvalues and eigenfunctions. • Accurate calculation of optical-transition matrix elements and new selection rules. • Optical spectra of AlGaAs and InGaN superlattices agree with experimental results. • Optical spectra features of blue diodes faithfully reproduced and fully explained. • Unexplained lines in high-resolution spectra assigned to surface and edge states. [ABSTRACT FROM AUTHOR]

The intersubband absorption linewidth dependence on well width in GaAs quantum well is calculated. Three mechanisms of scattering have been discussed there: carriers scattering by optical (LO), acoustic (LA) phonons and ionized impurity centers (ION). The method which used for calculations is similar to a well-known method of calculating transport mobility. The estimation for absorption coefficient is proposed, based on two-dimensional dynamical conductivity expression. The LO phonon emission process is activated starting from some quantum well (QW) width so it has its impact on absorption linewidth. [ABSTRACT FROM AUTHOR]

Nonpolar AlGaN/GaN heterostructures have the potential to supplant polar heterostructures in infrared optoelectronic devices due to their theoretical advantages stemming from the absence of built‐in polarization fields along nonpolar directions of the nitride wurtzite lattice. However, development of nonpolar m‐plane infrared devices in a broad spectral range has been hampered, so far, by challenges to grow homogeneous high Al‐composition AlGaN on m‐plane GaN. AlxGa1‐xN layers with 0.6< x <0.8 are found to be kinetically unstable under metal‐rich growth conditions by plasma‐assisted molecular‐beam epitaxy. After reviewing recent progress in the field, this paper focuses on the effect of the structure of m‐plane AlxGa1−xN/GaN (x <0.6) superlattices on near‐infrared intersubband absorption. Even at these intermediate Al‐compositions, the effective growth rate of AlGaN is drastically reduced, and the AlGaN‐GaN interface roughness is unexpectedly high. Consequently, accurate determination of layer thicknesses and alloy composition necessitates structural characterization by a combination of scanning transmission electron microscopy and high‐resolution X‐ray diffraction. The energy and linewidth of near‐infrared intersubband transitions are also significantly affected by this unusual growth behavior. The experimental results for intersubband absorption of m‐ and c‐plane superlattices are compared to each other and with numerical calculations, and the main reasons for discrepancies are discussed. [ABSTRACT FROM AUTHOR]

We report on the systematic study of two main scattering mechanisms on intersubband transitions, namely ionized impurity scattering and interface roughness scattering. The former mechanism has been investigated as a function of the dopants position within a multiple GaAs/AlGaAs quantum well structure and compared to the transition of an undoped sample. The study on the latter scattering mechanism has been conducted using the growth interruption technique. We report an improvement of the intersubband (ISB) transition linewidth up to 11% by interrupting growth at GaAs-on-AlGaAs interfaces. As a result, the lifetime of intersubband polaritons could be improved up to 9%. This leads to a reduction of 17% of the theoretical threshold intensity for polaritonic coherent emission. This work brings a useful contribution towards the realization of polariton-based devices.

We analyze the output power versus temperature characteristics of two GaAs/Al0.15Ga0.85As terahertz quantum cascade lasers (THz-QCLs) with maximum operating temperature Tmax=200 and 177 K as well as of one GaAs/Al0.30Ga0.70As THz-QCL with Tmax=150 K and identify the thermally-activated leakage paths responsible for the laser performance degradation as the temperature increases. We identify the specific carrier leakage path active in each THz-QCL structure and are able to reconstruct the output power versus temperature profile over the entire laser operation range. We find that using high barriers in the active region design virtually eliminates carrier leakage from the upper laser level into the continuum, opening a non-radiative scattering path from the upper into the lower laser level parallel to standard electron-LO-phonon emission. This effect, together with the reduced leakage from the lower laser level into the continuum in the high-barrier device, significantly contributes to the Tmax decrease from 177 to 150 K. We further show how electron leakage from the lower laser level into the continuum is enhanced in a GaAs/Al0.15Ga0.85As design with thin barriers, significantly improving the laser performance ( Tmax=200 K). Finally, we propose future design strategies for highly temperature-insensitive THz-QCLs. Our approach offers a straightforward method to analyze and troubleshoot thermally-activated carrier leakage dynamics in THz-QCLs. [ABSTRACT FROM PUBLISHER]

This paper assesses the difficulties associated with Al incorporation in nonpolar m-plane GaN/AlGaN multi-quantum-wells grown on free-standing m-plane GaN. Structures with average Al mole fraction below 6% show atomically flat surfaces and no extended structural defects, in spite of alloy fluctuations up to 30% of the average concentration. Increasing the average Al composition of the alloy above 23% induces anisotropic degradation of the surface morphology, with appearance of elongated features which increase the surface roughness, along with formation of stacking faults, dislocations, and nm-sized Al-rich clusters. The effect of all these structural features on the MQWs optical performance is discussed. [ABSTRACT FROM AUTHOR]

We investigate the impact of interface roughness (IFR) scattering on the performance of a series of the state-of-the-art GaAs/AlxGa1–xAs terahertz quantum cascade lasers (THz-QCLs) through a calculation of the induced inhomogeneous broadening and intersubband scattering rates. Our analysis includes two GaAs/Al0.15Ga0.85As THz-QCL devices with measured maximum operating temperatures at T \mathrm {max} =177 K and 175 K, two GaAs/Al0.30Ga0.70As devices with T \mathrm {max} = 150 K and 89 K, a GaAs/AlAs-Al0.15Ga0.85As device with T \mathrm {max} = 181 K, and a GaAs/AlAs device that did not lase. The investigated QCL wafers were grown at the same solid-source molecular beam epitaxy facility and at fixed parameters, so that we expect a constant interface roughness quality for all devices. We find negligible impact of IFR scattering on the performance of devices that use $x =0.15$ barriers as well as for devices that use x =0.30$ barriers with wide > 40 monolayers (ML) wells to support upper and lower laser levels. Fixing the barrier height to x =0.30$ , we calculate a drastic increase of the IFR-induced linewidth broadening from ~0.66 meV to ~2 meV when the quantum wells thicknesses reduce from ~40 ML to ~30 ML and relate this effect to the observed reduction of from 150 to 89 K. Furthermore, we calculate a large (~2 meV) IFR linewidth broadening and short (~1 ps) IFR intersubband scattering times for the device with pure AlAs layers and relate the consequent reduction of optical gain to the nonlasing of this device. [ABSTRACT FROM AUTHOR]