Intersubband Optical Transitions Research Articles

Intersubband optical gain spectra in compressively strained InGaAs/GaInP quantum well

In this paper, the theoretical analyses of the optical intersubband transitions in strained InGaAs/GaInP quantum well are investigated. A simplified method of the treatment of the compressive strain effects incorporation for the wavefunction and thus the intersubband dipole matrix is discussed. The effects of Gallium mole fraction, quantum well width and compressive strained on the band properties, intersubband transitions and optical absorption/gain spectra are shown to be pronounced. Thus this present investigation could become a good guild for tuning the optoelectronic properties of many quantum devices utilizing intersubband electronics in their various technological applications.

Interband second-order nonlinear optical susceptibility of asymmetric coupled quantum wells

Asymmetric molecular bonds possess a microscopic second-order nonlinear optical polarizability p(2). Crystals built from them possess a macroscopic second-order nonlinear optical susceptibility, χ(2), if their structure lacks centrosymmetry. χ(2) can be enhanced by introducing additional asymmetry at the meta-structural level. Here, we use a dipole matrix formalism to calculate χ(2) of asymmetric GaAs/AlGaAs coupled quantum well structures at telecommunication frequencies, for which interband (rather than previously considered intersubband) optical transitions govern optical nonlinearities. Using unit cell and envelope wavefunctions and considering all possible transitions between two bound electron and two bound hole states, we predict tenfold enhancement in χ(2) in previously underexplored ranges of quantum well asymmetry and coupling barrier thickness. This work paves the way toward enhanced, tailorable second-order optical nonlinearities for semiconductor digital alloy and superlattice structures.

Controllable terahertz intersubband absorptions in ZnO/(Sb,N) co-doped ZnO quantum wells: First-principles study

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/MgxZn1–xO, ZnO/CdxZn1–xO 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.

Magnetic-field-induced cavity protection for intersubband polaritons

We analyse the effect of a strong perpendicular magnetic field on an intersubband transition in a disordered doped quantum well strongly coupled to an optical cavity. The magnetic field changes the lineshape of the intersubband optical transition due to the interface roughness of the quantum well from a Lorentzian to a Gaussian one. In this regime, a novel form of magnetic-field-induced cavity protection sets in, which strongly reduces the polariton linewidth to the cavity contribution only. Implications of our results for fundamental studies of nonlinear polariton dynamics and for technological applications to polariton lasers are finally highlighted.

Structure dependent third order nonlinear susceptibility in the presence of impurity and magnetic field in CdS/ZnS core/shell quantum dot

Third order nonlinear optical susceptibility due to intersubband optical transition of an electron confined in a CdS/ZnS core-shell quantum dot is investigated for various sizes of core and shell radii in the presence of magnetic field. It is computed by employing the variational approach within the effective mass approximation and the two level density matrix method is applied to obtain the transition energy between two levels. The dependence on third order susceptibility of the incident light is investigated. The results show that the third order nonlinear optical susceptibility is blueshifted with the reduction of inner core radius for a constant outer radius and the same trend is observed with the decreasing outer shell radius. The resonant wavelength and the peak of third order susceptibility will be managed by the control over the structure of core-shell materials and the external perturbation.

Current induced drag of photons in GaAs/AlGaAs quantum wells

The effect of current drag of photons is experimentally investigated in the mid-infrared spectral range corresponded to the intersubband optical electron transitions in GaAs/AlGaAs quantum wells at room temperature for the first time. The optical electron transitions between the first and second electron subbands in quantum wells are revealed in light absorption spectra in the mid-infrared range. The dependences of the change in the refractive index on the lateral current in quantum wells are obtained for different light polarizations. A substantial spectral dependence of the effect is observed for p-polarized radiation.

The drag of photons by electric current in quantum wells

The flow of electric current in quantum well breaks the space inversion symmetry, which leads to the dependence of the radiation transmission on the relative orientation of current and photon wave vector, this phenomenon can be named current drag of photons. We have developed a microscopic theory of such an effect for intersubband transitions in quantum wells taking into account both depolarization and exchange-correlation effects. It is shown that the effect of the current drag of photons originates from the asymmetry of intersubband optical transitions due to the redistribution of electrons in momentum space. We show that the presence of dc electric current leads to the shift of intersubband resonance position and affects both transmission coefficient and absorbance in quantum wells.

Background impurities in a delta-doped QW. Part II: Edge doping

This is the second part of our study of the background impurity influence on the intersubband energy structure of a single SiGe/Si/SiGe quantum well with the impurity delta layer within the well. By the background impurity we mean sparse shallow donor doping throughout the infinitely wide barriers. In this part we consider a situation where the delta layer is positioned near the edge of the well and the structure symmetry is broken. We explain in detail the necessary modifications of our self-consistent method that includes calculation of impurity binding energy. The results particularly show that the mentioned asymmetry combined with the background impurity in the barriers provides new features to the effect of tuning the intersubband optical transitions by the ionization grade of the impurity in delta-layer that provides new technological possibilities.

Tunable intersubband absorption spectrum between Landau levels in GaAs/AlGaAs quantum well

Intersubband optical absorption spectrum was studied in quantum well structures in quantizing magnetic field tilted with respect to the structure layers. Violation of the selection rule forbidding optical intersubband transitions in structures made of quantum wells with an asymmetric potential is proposed. The importance of asymmetric structure design to achieve considerable values of optical absorption is demonstrated.

Theoretical investigation of optical intersubband transitions and infrared photodetection in β-(AlxGa1 − x)2O3/Ga2O3 quantum well structures

We provide theoretical consideration of intersubband transitions designed in the ultra-wide bandgap aluminum gallium oxide [(AlxGa1 − x)2O3]/gallium oxide (Ga2O3) quantum well system. Conventional material systems have matured into successful intersubband device applications such as large-area quantum well infrared photodetector (QWIP) focal plane arrays for reproducible imaging systems but are fundamentally limited via maximum conduction band offsets to mid- and long-wavelength infrared applications. Short- and near-infrared devices are technologically important to optical communication systems and biomedical imaging applications but are difficult to realize in intersubband designs for this reason. In this work, we use a first-principles approach to estimate the expansive design space of monoclinic β-(AlxGa1 − x)2O3/Ga2O3 material system, which reaches from short-wavelength infrared (1–3 μm) to far infrared (>30 μm) transition wavelengths. We estimate the performance metrics of two QWIPs operating in the long- and short-wavelength regimes, including an estimation of high room temperature detectivity (∼1011 Jones) at the optical communication wavelength λp = 1.55 μm. Our findings demonstrate the potential of the rapidly maturing (AlxGa1 − x)2O3/Ga2O3 material system to open the door for intersubband device applications.

Terahertz generation in quantum-cascade lasers through down-conversion optical nonlinearity: Design and modeling

In this paper, we propose an electrically driven optically excited laser structure for terahertz (THz) generation in quantum cascade lasers (QCLs) through frequency down-conversion optical nonlinearity. In our distinct design, both the pump and THz radiations are generated simultaneously by optical and electronic intersubband transitions (ISTs) between the quantized states within the conduction band of the same active region of a QCL. The modeling of laser bandstructure is achieved using a self-consistent solution of Schrödinger–Poisson equations, and the detailed analysis of electronic transport of the proposed structure is performed using the density matrix approach and energy-density balance equation that self-consistently consider the optical, electrical and thermal interactions in the structure. Additionally, the waveguide loss for the THz optical mode is considered using the bulk Drude model. Using a stationary solution of the density matrix equations, an expression for the THz intensity gain is derived as a function of pump field intensity, carrier distributions and quantum coherence contributions. Using the presented model, we analyze the steady-state and transient characteristics of the device for two different heatsink temperatures, 220 and 300 K. Simulation results show that the proposed structure has a potential to generate room-temperature THz radiation in QCLs.

Nonparabolicity of size-quantized subbands of bilayer semiconductor quantum wells with heterojunction.

This paper presents a theory of size quantization and intersubband optical transitions in bilayer semiconductor quantum wells with asymmetric profile. We show that, in contrast to single-layer quantum wells, the size-quantized subbands of bilayer quantum wells are nonparabolic and characterized by effective masses that depend on the electron wave number and the subband number. It is found that the effective masses are related to the localization of the electron wave function in the layers of the quantum well and can be controlled by varying the chemical composition or geometric parameters of the structure. We also derive an analytical expression for the probability of optical transitions between the subbands of the bilayer quantum well. Our results are useful for the development of laser systems and photodetectors based on colloidal nanoplates and epitaxial layers of semiconductor materials with heterojunctions.

Ultra-thin van der Waals crystals as semiconductor quantum wells

Control over the quantization of electrons in quantum wells is at the heart of the functioning of modern advanced electronics; high electron mobility transistors, semiconductor and Capasso terahertz lasers, and many others. However, this avenue has not been explored in the case of 2D materials. Here we apply this concept to van der Waals heterostructures using the thickness of exfoliated crystals to control the quantum well dimensions in few-layer semiconductor InSe. This approach realizes precise control over the energy of the subbands and their uniformity guarantees extremely high quality electronic transport in these systems. Using tunnelling and light emitting devices, we reveal the full subband structure by studying resonance features in the tunnelling current, photoabsorption and light emission spectra. In the future, these systems could enable development of elementary blocks for atomically thin infrared and THz light sources based on intersubband optical transitions in few-layer van der Waals materials.

Effect of the hydrostatic pressure and shell’s Al composition in the intraband absorption coefficient for core/shell spherical GaAs/Al[formula omitted]Ga[formula omitted]As quantum dots

In this paper we theoretically investigate the role of hydrostatic pressure by analyzing its influence on potential barrier’s height in GaAs/AlxGa1−xAs core/shell spherical quantum dots. The values of hydrostatic pressure considered here are always below the Γ−X crossover. In addition, we take into account the barrier shell’s size effects and the barrier’s aluminum concentration, looking for a description of the features of the intraband optical absorption coefficient in the system. The electronic structure is calculated within the effective mass approximation. From the numerical point of view the hybrid matrix method was implemented to avoid numerical instability issues that appears in the conventional transfer matrix method. The main intersubband optical transition is considered to take place between the 1s and 1p computed electronic states. The results show that the absorption coefficient undergoes first a red-shift and later a more pronounced blue-shift, depending on the AlxGa1−xAs barrier width (wb1). The absorption coefficient experiences a blue-shift as the barrier’s aluminum concentration increases, and it is non monotonically red-shifted as the hydrostatic pressure augments, due to the barrier’s height pressure dependency. For the chosen system parameters, the absorption coefficient resonant peak lies within the range of 20 to 30 meV, that corresponds to the THz frequency region. Accordingly, this system can be proposed as a building block for photodetectors in the THz electromagnetic spectrum region.

Study on Inter Band and Inter Sub-Band Optical Transitions With Varying InAs/InGaAs Sub-Monolayer Quantum Dot Heterostructure Stacks Grown by Molecular Beam Epitaxy

Multiple stacking of sub-monolayer (SML) quantum dot (QD) heterostructure exhibits high optical quality and is seen in devices like lasers diodes, photodetectors, etc. In this study, we have investigated the optical and material characterization of InAs/InGaAs SML quantum dot (QD) heterostructure with multiple stacking layers (nSML) on GaAs substrates. The experimentally calculated PL emission energies were found to be 1.19, 1.13, 1.11 and 1.12 eV for 4, 6, 8 and 10 QD stacks at 19 K, excitation power of 1.1 kW/cm2 (25 mW) respectively. A feature of increased strain with increasing nSML was verified experimentally by high-resolution X-ray diffraction (HRXRD) and Raman measurements as well. The experimental PL peak energy data were then validated with nextnano++ simulations based on Schrodinger – Poisson device solver. The hydrostatic and biaxial strain components were computed to correlate the experimental and simulation data. Hence with these enunciated understandings, we conclude that an ideal choice on the number of SML stacks that can be grown on a GaAs substrate was found to be 6 stacks, helpful to realize and fabricate QD based infrared photodetectors (QDIPs) devices in long wavelength regime.

Background impurities and a delta-doped QW. Part I: Center doping

The influence of shallow background donor impurities on the energy characteristics of the SiGe/Si/SiGe quantum well structure with centered delta-doping is studied numerically. The description of the self-consistent method includes the calculation of the donors impurity binding energy in the delta-layer. The delta-layer impurity binding energy as well as the energy differences between the first quantized electron subbands in the well demonstrate a significant dependence on the characteristics of the background doping. Therefore, the background doping cannot be neglected when studying phenomena like intersubband optical transitions.

Linear and Nonlinear Intersubband Optical Properties of Direct Band Gap GeSn Quantum Dots.

Intersubband optical transitions, refractive index changes, and absorption coefficients are numerically driven for direct bandgap strained GeSn/Ge quantum dots. The linear, third-order nonlinear and total, absorption coefficients and refractive index changes are evaluated over useful dot sizes’ range ensuring p-like Γ-electron energy state to be lower than s-like L-electron energy state. The results show strong dependence of the total absorption coefficient and refractive index changes on the quantum dot sizes. The third order nonlinear contribution is found to be sensitive to the incident light intensity affecting both total absorption coefficient and refractive index changes, especially for larger dot sizes.

Anisotropic Quantum Well Electro-Optics in Few-Layer Black Phosphorus.

The incorporation of electrically tunable materials into photonic structures such as waveguides and metasurfaces enables dynamic, electrical control of light propagation at the nanoscale. Few-layer black phosphorus is a promising material for these applications due to its in-plane anisotropic, quantum well band structure, with a direct band gap that can be tuned from 0.3 to 2 eV with a number of layers and subbands that manifest as additional optical transitions across a wide range of energies. In this Letter, we report an experimental investigation of three different, anisotropic electro-optic mechanisms that allow electrical control of the complex refractive index in few-layer black phosphorus from the mid-infrared to the visible: Pauli-blocking of intersubband optical transitions (the Burstein-Moss effect); the quantum-confined Stark effect; and the modification of quantum well selection rules by a symmetry-breaking, applied electric field. These effects generate near-unity tuning of the BP oscillator strength for some material thicknesses and photon energies, along a single in-plane crystal axis, transforming absorption from highly anisotropic to nearly isotropic. Lastly, the anisotropy of these electro-optical phenomena results in dynamic control of linear dichroism and birefringence, a promising concept for active control of the complex polarization state of light, or propagation direction of surface waves.

Position-dependent mass effects on the optical responses of the quantum well with Tietz–Hua potential

In the present study we have investigated the bound energy levels and intersubband optical transitions in the Tietz–Hua quantum well for a spatially varying effective mass distribution. The energy levels and eigenfunctions are obtained by solving the effective mass equation through a variational approach. The obtained numerical calculations show that the spatially varying effective mass has a significant effect on the subband energies and transitions between the subbands in the Tietz–Hua quantum well. We conclude that, the energy levels and hence energy differences between the subband states decrease with increasing the stability and uniformity of the effective mass and eventually converge to the results of the constant effective mass. Furthermore, it was found that the size of the Tietz–Hua quantum well has an important effect on optical responses based on the intersubband transitions and additionally, the asymmetrical form of the Tietz–Hua quantum well is more predominant in stable and uniform effective mass case.

‘Forbidden’ intersubband optical transitions in quantum well structures in a tilted magnetic field

In this paper, we report on the behavior of intersubband optical absorption and emission spectrum in quantum well structures with an asymmetrical potential profile in a quantizing magnetic field tilted with respect to the layers in the considered structure. The analysis of the effect of both the magnetic field value and orientation and potential profile of a quantum well structure on the intensity of the ‘forbidden’ ( ) intersubband optical transitions is presented. We also present the approximate analytical expression for the estimation of the contribution to the absorption/emission coefficient of intersubband transitions with different