Intersubband Optical Transitions Research Articles

Magneto-polaron induced intersubband optical transition in a wide band gap II–VI semiconductor quantum dot

Effects of LO-phonon contribution on the electronic and the optical properties are investigated in a Cd0.8Zn0.2Se/ZnSe quantum dot in the presence of magnetic field strength. The magneto-polaron induced hydrogenic binding energy as a function of dot radius in the wide band gap quantum dot is calculated. The oscillator strength and the spontaneous lifetime are studied taking into account the spatial confinement, magnetic field strength and the phonon contribution. Numerical calculations are carried out using variational formulism within the single band effective mass approximation. The optical properties are computed with the compact density matrix method. The magneto-polaron induced optical gain as a function of photon energy is observed. The results show that the optical telecommunication wavelength in the fiber optic communications can be achieved using CdSe/ZnSe semiconductors and it can be tuned with the proper applications of external perturbations.

Intersubband optical transition energy in a CdTe/Zn0.2Cd0.8Te/ZnTe core/shell/shell spherical quantum dot with Smorodinsky–Winternitz confinement potential

Electronic and optical properties of a hydrogenic donor impurity in a CdTe/Zn0.2Cd0.8Te/ZnTe core/shell/shell quantum dot are discussed taking into consideration of geometrical confinement effect. The confining potentials on both the sides of the barrier are different and a two parametrical potential of Smorodinsky–Winternitz is considered in this problem. The dielectric mismatch is included in the Hamiltonian. The position dependent effective mass is applied. The electronic properties are studied using variational method and the optical properties are investigated using the density matrix approach. The intersubband optical absorption, the oscillator strength and the radiative life time between ground and the excited states are studied based on the wave functions and the confined energies with and without the impurity with various dot radii. The results show that the absorption wavelength in type-II core and shell semiconducting nanomaterials can be tuned over a wider range of wavelengths by altering their size and the composition.

Nonlinear Intersubband Transitions in Square and Graded Quantum Wells Modulated by Intense Laser Field

The intersubband optical absorption coefficients and the refractive index change depending on the intense laser field (ILF); both are calculated in a square quantum well (SQW) and a graded quantum well (GQW). Our results show that the position and the magnitude of the linear, nonlinear and total absorption coefficients and refractive index changes depend on the laser field parameter and the quantum well (QW) shape. By increasing the ILF value, we can obtain a red shift or a blue shift in the intersubband optical transitions as dependent on the shape of the QW. For the SQW, the intersubband absorption spectrum shows a blue shift up to the critical laser field value. This spectrum shows a red shift for ILF values larger than this certain value. For the GQW, the intersubband absorption spectrum shows a red shift by increasing the ILF. Thus the 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 the ILF value and the QW shape.

Mid-infrared optical transitions of a hydrogenic impurity in the conduction band of a Cd1 − xZnxSe/ZnSe parabolic quantum dot

Theoretical investigations on the absorption spectra for the intersubband optical transitions in a Cd1 − xZnxSe/ZnSe quantum dot are performed. Calculations on the electronic states are investigated using a variational scheme within the single band effective mass approximation and the optical gain is calculated using the compact density matrix approach with the iterative procedure. 2p-1s like transition energy and the spontaneous transition lifetime of the shallow donor hydrogenic impurity with the geometrical confinement and the Zn alloy content in a Cd1 − xZnxSe/ZnSe quantum dot are computed. The optical gain as a function of incident photon energy with the effects of spatial confinement and the Zn composition is found. The results show the dependence of the transition energy on the electron density in the Cd0.8Zn0.2Se/ZnSe quantum dot and the magnitude of the optical absorption coefficient enhances when the electron density increases. It is observed that a 1.55 μm wavelength is achieved for a 22 Å radius of a Cd1 − xZnxSe/ZnSe quantum dot. This band gap tunability brings out the possibility for near-infrared detectors, fiber optical communication systems and absorption modulators.

Modeling techniques for quantum cascade lasers

Quantum cascade lasers are unipolar semiconductor lasers covering a wide range of the infrared and terahertz spectrum. Lasing action is achieved by using optical intersubband transitions between quantized states in specifically designed multiple-quantum-well heterostructures. A systematic improvement of quantum cascade lasers with respect to operating temperature, efficiency, and spectral range requires detailed modeling of the underlying physical processes in these structures. Moreover, the quantum cascade laser constitutes a versatile model device for the development and improvement of simulation techniques in nano- and optoelectronics. This review provides a comprehensive survey and discussion of the modeling techniques used for the simulation of quantum cascade lasers. The main focus is on the modeling of carrier transport in the nanostructured gain medium, while the simulation of the optical cavity is covered at a more basic level. Specifically, the transfer matrix and finite difference methods for solving the one-dimensional Schrödinger equation and Schrödinger-Poisson system are discussed, providing the quantized states in the multiple-quantum-well active region. The modeling of the optical cavity is covered with a focus on basic waveguide resonator structures. Furthermore, various carrier transport simulation methods are discussed, ranging from basic empirical approaches to advanced self-consistent techniques. The methods include empirical rate equation and related Maxwell-Bloch equation approaches, self-consistent rate equation and ensemble Monte Carlo methods, as well as quantum transport approaches, in particular the density matrix and non-equilibrium Green's function formalism. The derived scattering rates and self-energies are generally valid for n-type devices based on one-dimensional quantum confinement, such as quantum well structures.

Oscillator strengths of the intersubband transitions in a semiconductor spherical quantum dot with the donor impurity in the center

Oscillator strengths have been studied for a series of single-electron intersubband optical transitions nl → n′l′ (l ⩽ 4) in a spherical quantum dot (QD) with the donor impurity in the center (D0). Energy spectrum calculations were performed under effective mass approximation by assuming a spherically symmetric confining potential of finite depth. On the basis of the analytical solutions, the Schrödinger and Poisson equations' energies of states and corresponding wave functions were determined. The oscillator strength of each transition depends on the energy difference of two states and the dipole matrix element, i.e. wave functions of states. The effects of increasing the dot size in the presence of hydrogenic impurity on the oscillator strength of three series of transitions for a CdTe/ZnTe QD are presented in this paper.

Physical modeling and simulation of a high-performance charge sensitive infrared phototransistor

Charge sensitive infrared phototransistors are well known for their capability for response spectrum tuning and single photon detection. In this paper, we established a physical model for a charge sensitive infrared phototransistor operating at \(45\,\upmu \hbox {m}\) wavelength using Crosslight Apsys software. Several key physical mechanisms involved such as inter-subband optical transition and resonant tunneling of carriers were applied and fine tuned to obtain a better simulation result. The calculated absorption spectrum and the simulated data graphs demonstrate that this cell can be used for long wavelength detection with relatively high sensitivity.

Nonlinear intersubband transitions in a parabolic and an inverse parabolic quantum well under applied magnetic field

In this study, both the intersubband optical absorption coefficients and the refractive index changes as dependent on the magnetic field are calculated in a parabolic (PQW) and an inverse parabolic quantum well (IPQW) for different Al concentrations at the well center. Our results show that the position and the magnitude of the linear, nonlinear and total absorption coefficients and refractive index changes depend on the magnetic field strength and the Al concentration at the well center. By decreasing the Al concentration at the well center we can obtain a red shift and a blue shift in the intersubband optical transitions for PQWs and IPQWs, respectively. Thus, the 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 the applicable magnetic field value and the Al concentration at the well center.

Carrier Lifetime in Exfoliated Few-Layer Graphene Determined from Intersubband Optical Transitions

We report a femtosecond transient spectroscopy study in the near to middle infrared range, 0.8-0.35eV photon energy, on graphene and few layer graphene single flakes. The spectra show an evolving structure of photoinduced absorption bands superimposed on the bleaching caused by Pauli blocking of the interband optically coupled states. Supported by tight-binding model calculations, we assign the photoinduced absorption features to intersubband transitions as the number of layers is increased. Interestingly, the intersubband photoinduced resonances show a longer dynamics than the interband bleaching, because of their independence from the absolute energy of the carriers with respect to the Dirac point. The dynamic of these intersubband transitions reflects the lifetime of the hot carriers and provides an elegant method to access it in this important class of semimetals.

Features of absorption of a weak electromagnetic wave in nanowires

Interband and intersubband optical transitions in quantum wires are studied theoretically with allowance for scattering of carriers both by long-wavelength acoustic fluctuations and by a rough surface. The frequency dependence of the coefficient of light absorption is shown to be described by a bell-shaped curve, the half-width of which depends on the temperature and radius of the nanowire. The theoretical results are compared with experimental data on optical properties of Bi nanowires. In particular, it is shown that scattering of carriers by a rough surface at low temperatures must be taken into account for a satisfactory agreement between the theory and experiment.

Linear and nonlinear intersubband optical absorption coefficient and refractive index change in n-type δ-doped GaAs structure

In the effective mass approximation, we have theoretically investigated the subband structure of single Si δ-doped GaAs by solving the Schrödinger and Poisson equations self-consistently. Both the linear and nonlinear intersubband optical absorption coefficients and the refractive index changes are calculated as dependent on the doping concentration and thickness. Our results show that the position and the magnitude of the linear and total absorption coefficients and refractive index changes depend on the doping concentration and thickness. The shape of potential profile and the subband energy differences are changed as dependent on the donor concentration and thickness. By considering the variation of the energy difference we can obtain a blue shift or a red shift in the intersubband optical transitions. The resonant peak of total absorption coefficient can be bleached at sufficiently high incident optical intensities. Such a dependence of the exciting optical intensity on the doping concentration and thickness can be very useful for several potential device applications.

Nano-Device Modeling for Charge-Sensitive Infrared Photodetector Used in Very Long Wavelength and THz

This paper discusses modeling for the design of charge-sensitive nano-device for 15μm wavelength using Crosslight Apsys software. First, a double quantum well heterostructure was established using Graphic User Interface. Second, a computer simulation program based on physical mechanisms involved such as intersubband optical transition was compiled and executed. The scan conditions and iterative algorithm were also carefully set up. Finally, band diagrams and I-V characteristics were obtained and were in good agreements with the measured result.

Design of Terahertz Quantum Dot Cascade Laser Using Raman Amplification Process

As we know, quantum cascade lasers (QCLs) are currently the most advanced electrically pumped semiconductor lasers, which emit radiation due to intersubband optical transitions in semiconductor superlattices. Also, quantum cascade laser is one of the best alternatives for reaching the terahertz frequency by semiconductor structures. In this paper, in order to engineer the QCL, effects of Raman inversion as an optical nonlinearity process in active region (QD Hetero structures) of the THz laser has been investigated by using Launda-type three-level system. This would allow suppression of thermally activated optical phonons scattering processes. At the first, the three level QD is designed, where we have considered three Al0.48In0.52As/Ga0.47In0.47As QDs in z direction with the layer sequence of 4/1.5/4/3.5/4 in nanometers. Then, by rate equations in steady-state condition, the laser gain has been calculated.

Linear and nonlinear optical absorption in different graded quantum wells modulated by intense laser field

The laser field dependence of the linear and nonlinear intersubband optical absorption in different graded quantum wells (GQWs) is investigated in the effective mass approximation. Results obtained show that the position and the magnitude of the linear and total absorption coefficients depend on the laser parameter and the shape of GQW. The resonant peak of total absorption coefficient can be bleached at sufficiently high incident optical intensities. Such a dependence of the exciting optical intensity on the external field strengths in different GQWs can be very useful for several potential device applications. It should point out that by applying the laser field we can obtain a blue shift or a red shift in the intersubband optical transitions.

Effect of electric field on the oscillator strength and cross-section for intersubband transition in a semiconductor quantum ring

In this paper, we study the oscillator strength and cross-section for intersubband optical transition in an n-type semiconductor quantum ring of cylindrical symmetry in the presence of an electric field perpendicular to the plane of the ring. The analysis is done considering Kane-type band non-parabolicity of the semiconductor and assuming that the polarization of the incident radiation is along the axis of the ring. The results show that the oscillator strength decreases and the transition energy increases with the electric field. The assumption of a parabolic band leads to an overestimation of the oscillator strength. The effects of the electric field, band non-parabolicity and relaxation time on absorption cross-section for intersubband transition in a semiconductor quantum ring are also shown.

Effect of radial electrostatic field on optical absorption in semiconductor nanotube

In the effective-mass approximation the single-electron states in a wide-band semiconductor nanocylindrical heterolayer in the presence of strong lateral-radial electrical field are considered. The explicit forms of the energy spectrum and envelope wave functions of single-particle states for charge carriers are obtained using variation approach. Corresponding absorption characteristics of interband and inter-subband optical transitions in the layer in the presence of strong radial field are calculated. The radial field leads to the explicit dependence of absorption intensity on the values of effective masses of charge carriers. It is shown that absorption's threshold frequencies depend explicitly on geometrical sizes of sample and intensity of external field.

The role of intersubband optical transitions on the electrical properties of InGaAs/GaAs quantum dot solar cells

ABSTRACTWe report on an experimental study of the intersubband optical response of an In0.5Ga0.5As/GaAs quantum dot solar cell (QDSC). By calculating the quantum dot absorption cross section, the strength of the intersubband optical transitions is gauged, and their importance and influence on the electrical properties of the solar cell can be compared with those of other physical processes such as thermal intersubband and optical interband transitions. The temperature‐dependent photocurrent and dark current characteristics of the QDSC have also been analyzed in detail, leading to an understanding of the specific effects reducing the open‐circuit voltage. The deviation of QDSCs from idealized models is discussed, and some conditions required for an improved open‐circuit voltage are suggested. Copyright © 2012 John Wiley & Sons, Ltd.

Intersubband transitions and refractive index changes in coupled double quantum well with different well shapes

In this study, both the linear intersubband transitions and the refractive index changes in coupled double quantum well (DQW) with different well shapes for different electric fields are theoretically calculated within framework of the effective mass approximation. Results obtained show that intersubband transitions and the energy levels in coupled DQW can importantly be modified and controlled by the electric field strength and direction. By considering the variation of the energy differences, it should point out that by varying electric field we can obtain a blue or red shift in the intersubband optical transitions. The modulation of the absorption coefficients and the refractive index changes which can be suitable for good performance optical modulators and various infrared optical device applications can be easy obtained by tuning applied electric field strength and direction.

Coherent population trapping in intersubband photocurrent spectra

We present results from numerical simulations of the photocurrent generated by intersubband optical transitions in a double quantum well coupled with a continuum of extended states. The photocurrent spectra are obtained directly from the time-dependent Schr\"odinger equation for the coherent regime without any adjusting parameters in the calculations other than the ones that define the physical system, also in a nonperturbative way and without basis-set expansions or truncations. A realistic representation of a three-level system in the Lambda (``$\ensuremath{\Lambda}$'') configuration is investigated when two bound states in each quantum well are coupled by exciting fields via an excited quasibound state. Resonance between the exciting fields and the quantum states leads to coherent effects such as Rabi-dressed states, electromagnetically induced transparency, and population trapping, which are investigated in terms of the photocurrent spectral changes; that is, the coherent optical dynamics can be seen from the photocurrent signal. An excitation scheme involving two-photon absorption was proposed to produce the population-trapping effects using only one exciting field.

Quantum Dots-Based All-Optical-Readout Middle-Far-Infrared Imaging

A quantum dot (QD) middle-far-infrared (MFIR) detection and imaging approach based on all-optical-readout (AORO) is proposed. Using the intersubband (ISB) optical transition to change the interband (IB) absorption, it converts the ISB MFIR signal into the IB near-infrared signal. It might overcome the problems in the electrical readout infrared detection and imaging, such as dark current, low-temperature operation, low resolution, and vulnerability to the high radiation intensity. Based on the In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.5</sub> As/GaAs lens-shaped QD structure, our calculation shows the potential of this AORO approach for the MFIR detection and high-resolution imaging, the potential for room-temperature operation, and the ability for frequency selectivity and spectrum scanning.