Intersubband Absorption Coefficient Research Articles

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.

Temperature dependence analysis of intersubband absorption in asymmetric ZnO/MgZnO quantum wells: Potential for terahertz emission applications

In this paper, in ZnO/MgxZn1−xO quantum well structures, with 20% of Mg, we have calculated the electronic states of confined levels and the intersubband absorption coefficient with or without the effect of non-parabolicity. We use the finite difference approach to do our computations inside the envelope function formalism approximation. The results show that the wavelength range covered by the E12 transition (9–16 μm) is mid-infrared to far-infrared. According to our findings, ZnO/MgZnO quantum well structures, which have extremely similar lattice characteristics and the benefit of an unconstrained structure, have E23 transition energies that provide terahertz frequencies (1.1—20THz) that are utilized in airport security and explosives detection. This promotes intersubband photonic structure design and epitaxial development. Furthermore, we have examined the impact of temperature on the Fermi level and the load carrier population inside each level for these remarkable structures. In addition, each transition's oscillation force and intersubband absorption are considered.

Impurity photoionization cross-section and intersubband optical absorption coefficient in multilayer spherical quantum dots

Energy spectrum, wave functions and binding energies of the electron to the donor impurity ion located in the center of a multilayer spherical quantum dot (MSQD) consisting of a core and two spherical shells were studied within the effective mass approximation. Based on the exact wave functions of the electron expressed in terms of Coulomb functions of the first and second kind, the spectral dependences of the photoionization cross section of the impurity (PCS) and the intersubband optical absorption coefficient (OAC) for various geometric dimensions of the nanostructure were calculated. It is shown that the decrease in the width of the external potential well changes the localization of the electron in the nanosystem which significantly affects the binding energy of the electron with the impurity, photoionization cross section and interband absorption coefficient. The position of the PCS peak associated with the quantum transition of an electron from the ground state to the 1p0 state shifts to the region of higher energies, and its height decreases. At the same time, the height of PCS peaks associated with quantum transitions to higher excited states (2p0, 3p0) increases.The presence of impurities and changes in the MSQD size significantly affect the intersubband absorption coefficient. Decrease of the external potential well width in the absence of impurities leads to a monotonous increase in OAC through the first excited state, and in the presence of a central impurity, absorption through states with higher energy increases.

Donor impurity atom effect on the inter-subband absorption coefficient for symmetric double n-type δ-doped GaAs quantum well

In this work, the electronic structure and the inter-subband absorption coefficient (IAC) are theoretically studied for symmetric double n-type δ-doped GaAs quantum well considering the donor impurity atom effect. The electron states are determined by a diagonalization procedure, working within the effective mass and parabolic band approximations, and the effect of donor center is treated via the variational method. Meanwhile, linear and nonlinear contributions for the inter-subband absorption coefficient were evaluated from expressions usually derived within the perturbative solution of the Von Neumann equation for density matrix. We report the impurity binding energy by considering a donor atom located at the center of the system (at zi = 0). We found that the reported physical properties become more sensitive to the inter-well separation distance Lw than to the δ-doping density, N2d. In the former case the total optical absorption coefficient undergoes an important red-shift as well as a significant decrease in its magnitude. When N2d values increase, the binding energy exhibits a contrary effect, and the total optical absorption coefficient exhibits an small blue-shift with no significant changes in its magnitude. The presence of the donor impurity atom in the system represents one factor that can modify the location of inter-subband absorption coefficient, by inducing a blue-shift of the optical response. The resonant peak energies are within the range of several terahertz, showing potential device applications within this range of the electromagnetic spectrum.

GaAs quantum well in the non-parabolic case: the effect of hydrostatic pressure on the intersubband absorption coefficient and the refractive index

We investigated the effect of the hydrostatic pressure on the optoelectronic properties of a quantum well (QW) based on δ-doped GaAs sandwiched by Ga1-xAlxAs. We study the case of a non-parabolic conduction band where the aluminum content is set at 30%. We perform our calculations in the context of the approximation of the envelope function formalism using the finite difference method. Results show that the transition energies decrease with the increase of the hydrostatic pressure, which causes remarkable modifications on the optical properties of the QW nanostructure. The non-parabolicity effect is more important for small QW (Lw ≤ 5nm) and less marked in narrow and large QW. In addition, we study the absorption coefficient for 8 nm/4 nm/8 nm geometry. On the one hand, the pressure increase creates a displacement of the optical absorption coefficient towards low energies and a decrease of the absorption peak value. On the other hand, the refractive index moves towards higher energies. We show that in the presence of a hydrostatic pressure and following its effect on intersubband transitions, these optical properties also depend on the dopant concentration rate and the quantum well width. Our study finds interests for the nano-fabrication of quantum wells and in particular for those used in optical and electronic applications.

Nonlinear Intersubband Optical Absorption Coefficient and Refractive Index Changes in Asymmetric Parabolic Double Quantum Wells

The change in optical absorption coefficients and refractive index in GaAs-AlGaAs asymmetric parabolic double quantum wells (DQWs) with applied electric field are studied in detail. Analytical expressions for the linear, nonlinear and total intersubband absorption coefficient and refractive index changes are obtained by using compact density matrix approach. It is found that the magnitude of the nonlinear part of the change in refractive index and absorption coefficients are larger than the linear part in the given frequency region. The value of the total change in refractive index is negative and the asymmetric DQWs becomes left handed media. This property is of great importance for metamaterials science and can contribute substantially to the present search for simple and inexpensive left handed media. Moreover the negative value of the total change in absorption coefficient may be used for developing optical device such as maser at different frequency regime.

Intersubband transition in lattice-matched BGaN/AlN quantum well structures with high absorption coefficients.

Intersubband absorption properties of lattice-matched BGaN/AlN quantum well (QW) structures grown on AlN substrate are theoretically investigated using an effective mass theory considering the nonparabolicity of the conduction band. These results are compared with those of GaN/AlN QW structures. The intersubband absorption coefficient of the BGaN/AlN QW structure is shown to be enhanced significantly, compared to that of the conventional GaN/AlN QW structure. This can be explained by the fact that the BGaN/AlN QW structure exhibits larger intersuband dipole moment and quasi-Fermi-level separation than the GaN/AlN QW structure, due to the increase in the carrier confinement by a larger internal field. We expect that the BGaN/AlN QW structure with a high absorption coefficient can be used for telecommunication applications at 1.55 µm under the lattice-matched condition, instead of the conventional GaN/AlN QW structure with the large strain.

Intersubband absorption coefficients of GaN/AlN and strain-compensated InGaN/InAlN quantum well structures

Intersubband (ISB) transition absorption properties of strain-compensated InGaN/InAlN quantum well (QW) structures grown on GaN substrate were investigated using an effective mass theory. The interband dipole moment of the strain-compensated InGaN/InAlN QW structure is shown to be comparable to that of the conventional GaN/AlN QW structure. However, the strain-compensated InGaN/InAlN QW structure shows much larger intersubband absorption coefficient than the conventional GaN/AlN QW structure. This is mainly due to the fact that the former shows much larger quasi-Fermi-level separation than the latter because the electron effective mass is reduced with the inclusion of Indium. As a result, the strain-compensated QW structures could be used as ISB photodetectors for telecommunication operating at 1.55 μm with a higher absorption coefficient and a reduced strain, compared to conventional GaN/AlN QW structures.

On intersubband absorption of radiation in delta-doped QWs

The results of calculation of intersubband absorption coefficients for either center-, or edge-delta-doped with Phosphorus 10nm and 20nm-wide Si0.8Ge0.2/Si/Si0.8Ge0.2 quantum wells are presented. It is shown, that the absorption for delta-doped structures differs substantially from that of a pure rectangular or uniformly doped ones. There are two main features for delta-doped quantum wells. The first one is the blue-shift for optical transitions between first and others (more pronounced), and second and others (less pronounced) space quantized energy levels. The second one is that edge doping changes the symmetry of the quantum well and forbidden optical transitions for the rectangular structure become now allowed. The influences of temperature, quantum well width, and impurity concentration on the optical absorption are studied. It is shown that the most dramatic changes in comparison with rectangular quantum wells are for wider investigated edge-doped structures with bigger number of ionized impurities.

Nonequilibrium Green's function formulation of intersubband absorption for nonparabolic single-band effective mass Hamiltonian

The formulas are derived that enable calculations of intersubband absorption coefficient within nonequilibrium Green's function method applied to a single-band effective-mass Hamiltonian with the energy dependent effective mass. The derivation provides also the formulas for the virtual valence band components of the two-band Green's functions which can be used for more exact estimation of the density of states and electrons and more reliable treatment of electronic transport in unipolar n-type heterostructure semiconductor devices.

Nonlinear intersubband transitions in different shaped quantum wells under intense laser field

In this study, the intersubband optical absorption coefficients and the refractive index changes are calculated in a square quantum well (SQW), a parabolic quantum well (PQW), a graded quantum well (GQW) and a V-shaped quantum well (VQW), without and with the intense laser field (ILF). Our results show that the position and the magnitude of the linear, nonlinear and total absorption coefficients and refractive index changes depend on ILF parameter and the quantum well (QW) shape. While for SQW the intersubband absorption spectrum shows blue shift up to the critical ILF value, this spectrum shows red shift for greater ILF values than this certain value. Whereas, for PQW, GQW and VQW the absorption spectrum shows always red shift by increasing ILF. Thus, the intersubband absorption coefficients and the refractive index changes which can be suitable for great performance optical modulators and multiple infrared optical device applications can be easily obtained by tuning ILF value and QW shape.

The linear and nonlinear optical properties of an off-center hydrogenic donor impurity in nanowire superlattices: Comparison between arrays of spherical and cylindrical quantum dots

Nanowire superlattices (NWSLs) are objects with a wide range of potential applications in nanoscale electronics and optics. In this paper, we consider an off-center hydrogenic donor impurity in two different NWSL with cylindrical cross-section which involved an array of spherical or cylindrical quantum dots (QDs) along their axis and called Sph and Cyl NWSLs, respectively. The electronic eigenstates and energy eigenvalues of NWSLs are determined by applying finite difference method. Additionally, optical properties such as linear, nonlinear and total intersubband absorption coefficients (ACs) and refractive index (RI) changes are investigated by means of compact density matrix approach. The results show that (I) optical ACs and RI changes of a Sph NWSL are smaller than those of Cyl NWSL. (II) The amplitudes of ACs and RI changes are strongly affected by impurity position and show an oscillatory behavior as the impurity shifts away from the center. (III) Resonance condition in optical spectrum can be adjusted by shape of QDs and impurity position and both blue and red shifts are appeared in ACs as the impurity shifts away from the center of NWSLs. (IV) Total optical absorption saturation intensity can be controlled by the shape of QDs or the impurity position. Moreover, optical incident intensities correspond to the saturation in optical spectrum in Cyl NWSL are smaller than their values for Sph NWSL and have a minimum values as the impurity is located at the center of barriers for both Sph and Cyl NWSLs. (V) By increasing the relaxation time the amplitude of the total AC will increase and for large values of relaxation time a saturation appears in optical spectrum which strongly depends on the Al concentration, impurity position and type of NWSL.

The linear and nonlinear optical properties of GaAs/GaAlAs nanowire superlattices

In the present work, we intend to study the effects of aluminum (Al) concentration, hydrogenic donor impurity and shape of quantum dots (QDs) on optical properties of nanowire superlattices (NWSLs). We consider two different NWSL which involved an array of spherical or cylindrical QDs along their axis and called Sph and Cyl NWSLs, respectively. In this regard, the energy eigenvalues and corresponding wave functions are calculated using finite difference approximation within the effective mass framework. Also, the linear, third order nonlinear and total intersubband absorption coefficients (ACs) and refractive index (RI) changes are investigated by means of the compact density matrix approach. The results show that (I) ACs and RI changes of Cyl NWSL are larger than those of Sph NWSL. (II) Presence of impurity decreases ACs and RI changes and shifts optical spectrum towards higher energies (blue shift) for both Sph and Cyl NWSLs. (III) In absence of hydrogenic donor impurity, by increasing the volume of QDs a red shift appears in optical spectrum of Cyl NWSL whereas for Sph NWSL first red shift and then blue shift appear. In the presence of hydrogenic impurity an opposite behavior is observed, i.e., optical spectrum of Cyl NWSL shifts towards higher energies with an augment in volume of QDs while for Sph NWSL first a blue shift and then a red shift appear in optical spectrum. (IV) Through increasing the Al concentration, ACs and RI changes decrease and shift towards higher energies for both Sph and Cyl NWSLs. This blue shift is more enhanced in the case of Sph NWSL. (V) By increasing incident optical intensity the magnitude of total AC decreases and absorption saturation occurs for large values of incident intensity. We find that the critical incident intensity; which corresponds to the saturation, is increased by decreasing aluminum concentration and also is smaller for Cyl NWSL.

Intersubband optical absorption coefficients and refractive index changes in spherical hydrogenic antidots

In this work, we have studied the optical absorption coefficients and refractive index changes in spherical quantum antidots with hydrogenic donor impurity at the center. For this purpose, the energy spectrum and wavefunctions are first determined, analytically. Then, we have used analytical expressions for the intersubband absorption coefficients and refractive index changes obtained by the compact density matrix formalism. The results show that i—total absorption coeficient increase with increasing size of antidot, ii—the refractive index changes decrease with decreasing antidot size.

A multi-color CdS/ZnSe quantum well photodetector for mid- and long-wavelength infrared detection

In this paper, we report on the design and characterization of a quantum well based infrared photodetectors covering simultaneously infrared radiation within mid- and long-infrared spectral regions. The proposed infrared photodetectors rely on intersubband transitions in asymmetric ZnSe/CdS double quantum wells. The three-energy-level and the wavelengths of the intersubband transitions in the asymmetric double quantum wells are obtained by solving the Schrödinger and Poisson equations self consistently, the influence of the right well width on the absorption coefficient is studied. The peak positions of intersubband absorption coefficients in the structure are found at 3.31, 4.4 and 13.5µm for a 1nm right well width while the absorption peak positions are located at 3.33, 6.43 and 6.95µm for a 1.4nm right well thickness. Then, the electro-optic performances of the infrared photodetector are evaluated; the dark current dependence with the applied voltage and temperature is discussed. This work demonstrates the possibility of detection of widely separated wavelength bands using intersubband transitions in quantum wells with a low dark current.

Effects of polarization on intersubband transitions of AlxGa1−xN/GaN multi-quantum wells

The effects of polarization and related structural parameters on the intersubband transitions of AlGaN/GaN multi-quantum wells (MQWs) have been investigated by solving the Schrödinger and the Poisson equations self-consistently. The results show that the intersubband absorption coefficient increases with increasing polarization while the transition wavelength decreases, which is not identical to the case of the interband transitions. Moreover, it suggests that the well width has a greater effect on the intersubband transitions than the barrier thickness, and the intersubband transition wavelength of the structure when doped in the barrier is shorter than that when doped in the well. It is found that the influences of the structural parameters differ for different electron subbands. The mechanisms responsible for these effects have been investigated in detail.

Pressure effect on optical properties of modified Gaussian quantum dots

In the present work, we intend to study the pressure effect on optical properties of spherical quantum dots by using the modified Gaussian potential. In this regard, the linear, nonlinear and total intersubband absorption coefficients and refractive index changes are investigated for different hydrostatic pressures. According to the results obtained from the present work, it is deduced that: (i) the linear, nonlinear and total refractive index changes decrease and shift towards higher energies when the pressure increases and (ii) the linear, nonlinear and total absorption coefficients increase and shift towards higher energies by increasing the pressure.

Effect of position-dependent effective mass on linear and nonlinear optical properties of a cubic quantum dot

In this paper, we first obtain an analytic relation for studying the position-dependent effective mass in a GaAs/Al x Ga 1− x As cubic quantum dot. Then, the effect of position-dependent effective mass on the intersubband optical absorption coefficient and the refractive index change in the quantum dot are studied. Our numerical calculations are performed using both a constant effective mass and the position-dependent effective mass. We calculate the linear, nonlinear and total intersubband absorption coefficient and refractive index change as a function of the incident optical intensity and structural parameters such as dot length. The results obtained from the present work show that spatially varying electron effective mass plays an important role in the intersubband optical absorption coefficient and refractive index change in a cubic quantum dot.

Intersubband optical absorption coefficients and refractive index changes in modulation-doped asymmetric double quantum well

The linear and the third-order nonlinear optical absorption coefficients and refractive index changes in a modulation-doped asymmetric double quantum well are studied theoretically. The electron energy levels and the envelope wave functions in this structure are calculated by the Schrödinger and Poisson equations self-consistently in the effective mass approximation. The analytical expressions of optical properties are obtained by using the compact density-matrix approach. In this regard, the linear, nonlinear and total intersubband absorption coefficients and refractive index changes are investigated as a function of right-well width ( L w 2 ) of asymmetric double quantum well. Our results show that the total absorption coefficients and refractive index changes shift toward higher energies as the right-well width decreases. In addition, the total optical absorption coefficients and refractive index changes is strongly dependent on the incident optical intensity.

Effects of an Intense Laser Field and Hydrostatic Pressure on the Intersubband Transitions and Binding Energy of Shallow Donor Impurities in a Quantum Well

We have calculated the intersubband transitions and the ground-state binding energies of a hydrogenic donor impurity in a quantum well in the presence of a high-frequency laser field and hydrostatic pressure. The calculations are performed within the effective mass approximation, using a variational method. We conclude that the laser field amplitude and the hydrostatic pressure provide an important effect on the electronic and optical properties of the quantum wells. According to the results obtained from the present work, it is deduced that (i) the binding energies of donor impurity decrease as the laser field increase, (ii) the binding energies of donor impurity increase as the hydrostatic pressure increase, (iii) the intersubband absorption coefficients shift toward lower energies as the hydrostatic pressure increases, (iv) the magnitude of absorption coefficients decrease and also shift toward higher energies as the laser field increase. It is hopeful that the obtained results will provide important improvements in device applications.