Intersubband Electron Transitions Research Articles

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

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

Analytical Modeling for Photocurrent and Detectivity of TiO2/ZnS Core‐Shell Quantum Dot Photodetectors

AbstractIn this paper, a theoretical model has been presented to calculate the photocurrent and detectivity in a TiO2/ZnS core‐shell quantum dot (CSQD) photodetector. Analytical modeling permits to calculate the available photocurrent and corresponding detectivity from the CSQD structures. In this theoretical model, the subband energy levels of the CSQD are estimated using the 3D symmetric quantum box structures with a finite potential barrier. The finite band offset determines the number of available subband energy levels in the CSQD structures depending on the size of the core. Theoretical results show the variation of photocurrent and corresponding detectivity with different core sizes of the CSQD for a constant shell thickness. The calculations indicate that for electronic intersubband transitions detectivity is higher compared with the hole transitions.

Optical spectrum of n-type and p-type monolayer MoS2 in the presence of proximity-induced interactions

In this paper, we examined the effects of proximity-induced interactions such as Rashba spin-orbit coupling and effective Zeeman fields (EZFs) on the optical spectrum of n-type and p-type monolayer (ML)-MoS2. The optical conductivity is evaluated using the standard Kubo formula under random-phase approximation by including the effective electron–electron interaction. It has been found that there exist two absorption peaks in n-type ML-MoS2 and two knife shaped absorptions in p-type ML-MoS2, which are contributed by the inter-subband spin-flip electronic transitions within conduction and valence bands at valleys K and K′ with a lifted valley degeneracy. The optical absorptions in n-type and p-type ML-MoS2 occur in THz and infrared radiation regimes and the position, height, and shape of them can be effectively tuned by Rashba parameter, EZF parameters, and carrier density. The interesting theoretical predictions in this study would be helpful for the experimental observation of the optical absorption in infrared to THz bandwidths contributed by inter-subband spin-flip electronic transitions in a lifted valley degeneracy monolayer transition metal dichalcogenides system. The obtained results indicate that ML-MoS2 with the platform of proximity interactions make it a promising infrared and THz material for optics and optoelectronics.

Optical absorption in core-shell quantum antidot under applied co-directed electric and magnetic fields

Using the matrix diagonalization method within the effective mass approximation and the rectangular potential profile, we investigate optical absorption coefficients (OAC) in the core-shell quantum antidot (QAD) Al0.3Ga0.7 As/GaAs/Al0.3Ga0.7 As in the presence of applied co-directed electrical and magnetic fields. The electric field effect on the electron wave functions, energy spectrum, oscillator strength of the intersubband electronic transitions is calculated for different size of QAD core and different value of the magnetic field induction. The total, linear and nonlinear optical absorption coefficients in QAD are obtained by considering 1 s-1p and 1p-1d electron quantum transitions as functions of the incident photon energy.

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The spectral dependences of the absorption coefficient α in a submicron samples of the n-Bi2Te3 + CdBr2 topological insulator, which have optimal thermoelectric properties at high values of the Seebeck coefficient at temperatures below room temperature, are studied at temperatures of 205 and 300 K. The components of optical absorption spectra associated with interband and intersubband electron transitions are revealed and analyzed. It was found that in bismuth telluride, with decreasing temperature, the optical transition of electrons at the interband absortion threshold remain direct and allowed. The values of energy gaps between the absolute extrema of the conduction and valence bands, as well as between the main and additional subbands of conduction band, are estimated. It is shown that the rates of their change with temperature are opposite in sign. The energy band diagram of Bi2Te3 is constructed, which corresponds to experimental data.

(Invited) Conduction Band Structure of Colloidal Quantum Wells

Similar to epitaxial quantum wells, colloidal quantum wells display electron intersubband transitions when photoexcited or electrochemically charged. Compared to epitaxial quantum wells, the strong confinement effect of thin, monodisperse colloidal quantum wells results in narrow intersubband transitions which reach high energies spanning the near-infrared spectral range. The composition of surface ligand chemistry subtly tunes the strain of the colloidal quantum wells to generate intersubband transitions over nearly continuously tunable energies. The divergent behavior of the band gap energy and intersubband transition energy implicates a substantial role of biaxial (in-plane) strain on the valence band structure. Multiple pump transient spectroscopy was furthermore employed to examine the lifetime of intersubband transitions, which is critical to their implementation in quantum cascade lasers or infrared detectors. In all cases, including core/shell samples, the lifetime of intersubband transitions is limited by surface or LO phonons to 1 ps or less, much shorter than comparable intraband transitions in quantum dots.

Recent Advances of Efficient Design of Terahertz Quantum-Cascade Lasers

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.

Band Structure and Intersubband Transitions of Three-Layer Semiconductor Nanoplatelets.

This paper presents the first general theory of electronic band structure and intersubband transitions in three-layer semiconductor nanoplatelets. We find a dispersion relation and wave functions of the confined electrons and use them to analyze the band structure of core/shell nanoplatelets with equal thicknesses of the shell layers. It is shown that the energies of electrons localized inside the shell layers can be degenerate for certain electron wave vectors and certain core and shell thicknesses. We also show that the energies of intersubband transitions can be nonmonotonic functions of the core and shell thicknesses, exhibiting pronounced local minima and maxima which can be observed in the infrared absorption spectra. Our results will prove useful for the design of photonic devices based on multilayered semiconductor nanoplatelets operating at infrared frequencies.

Theoretical simulation of quantum cascade lasers based on InGaAs/AlInAs and InGaAs/AlAsSb quantum wells

Quantum cascade lasers based on semiconductor InGaAs/AlInAs and InGaAs/AlAsSb quantum wells exhibit electronic intersubband transitions within conduction bands and attract much attention due to their operation at room-temperature short wavelengths. In this work we performed a theoretical simulation of such quantum cascade lasers and studied the influence of their construction and application conditions on optical behaviour. The intersubband absorption processes in these quanta well structures were studied. Electronic properties and the conduction band edge profiles were simulated as well as the probability densities of the Wannier-Stark states were determined. The simulation results showed that with the rise of the temperature the threshold current density increases which also leads to a decrease in the optical gain. An increase of the applied electric field is accompanied by the optical gain rising and decrease of the threshold current density that results in a blue shift of laser frequency.

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.

Near-infrared optical absorption in GaN/AlN quantum wells grown by molecular-beam epitaxy

Optical absorption of s- and p-polarized light was studied in GaN/AlN quantum wells in the near-infrared spectral range. An absorption peak associated with intersubband electron transitions in quantum wells was observed near a wavelength of 1.55 μm. Optoelectronic devices based on these structures can be used in fiber-optic telecommunication technologies.

Influence of electric field on direct and indirect exciton in a concentrically coupled quantum ring heterostructure embedded in SiO2 matrix

Abstract Excitonic states of Concentric Double Quantum Ring (CDQR) heterostructures embedded in SiO2 matrix under the influence of electric field has been calculated theoretically using variational technique. CDQR has been confined in a quantum well of suitable width which serve as the height of the quantum ring along which the electric field is applied. Central barrier width, outer ring width and carrier location has been tuned to understand the localization of both direct and indirect excitons. Effect of ring dimension and electric field on the energy states of Inner Ring Exciton (IRE) and Outer Ring Exciton (ORE) has been determined. Tilting of conduction and valence bands due to the application of electric field invokes a triangular confinement to the carriers and the same has been discussed. Diamagnetic susceptibility which is merely an indicator of electron hole separation and intersubband transition energy has also been calculated for both symmetric and asymmetric quantum ring. Height of the QR has also been changed to study its effect on the diamagnetic susceptibility of an exciton.

Performance Prediction of a Quantum Well Infrared Photodetector Using GeSn/SiGeSn Structure

QWIP using group IV elements have created more interest among researchers for its potential application in optical communication as well as in optical interconnects. This paper presents modeling and theoretical analysis of Sn-based tensile strained type I direct band gap QWIP in which the active region has a multiple quantum well structure formed with Ge0.92Sn0.08 quantum wells separated by Si0.11Ge0.7Sn0.19 barriers. The structure reported by V. Ryzhii et al. has been reproduced and the parameters like responsivity, power density and the dark current density have been analytically calculated. A comprehensive comparison of responsivity and power density of this proposed structure with the existing QWIP structure made of GaAs/InGaAs is reported here. An improvement in the field of responsivity is observed with the proposed model. The reduction of threshold power density corresponds to an effective operation of the QWIPs in incident infrared radiation. The intersubband electron transition and tunneling electron injection effects are considered here to predict a better performance of the proposed structure operated in infrared region.

Polarized near-infrared intersubband absorptions in CdSe colloidal quantum wells

Colloidal quantum wells are two-dimensional materials grown with atomically-precise thickness that dictates their electronic structure. Although intersubband absorption in epitaxial quantum wells is well-known, analogous observations in non-epitaxial two-dimensional materials are sparse. Here we show that CdSe nanoplatelet quantum wells have narrow (30–200 meV), polarized intersubband absorption features when photoexcited or under applied bias, which can be tuned by thickness across the near-infrared (NIR) spectral window (900–1600 nm) inclusive of important telecommunications wavelengths. By examination of the optical absorption and polarization-resolved measurements, the NIR absorptions are assigned to electron intersubband transitions. Under photoexcitation, the intersubband features display hot carrier and Auger recombination effects similar to excitonic absorptions. Sequenced two-color photoexcitation permits the sub-picosecond modulation of the carrier temperature in such colloidal quantum wells. This work suggests that colloidal quantum wells may be promising building blocks for NIR technologies.

Observation of Intersubband Polaritons in a Single Nanoantenna Using Nano-FTIR Spectroscopy.

Strong coupling of an intersubband (ISB) electron transition in quantum wells to a subwavelength plasmonic nanoantenna can give rise to intriguing quantum phenomena, such as ISB polariton condensation, and enable practical devices including low threshold lasers. However, experimental observation of ISB polaritons in an isolated subwavelength system has not yet been reported. Here, we use scanning probe near-field microscopy and Fourier-transform infrared (FTIR) spectroscopy to detect formation of ISB polariton states in a single nanoantenna. We excite the nanoantenna by a broadband IR pulse and spectrally analyze evanescent fields on the nanoantenna surface. We observe the distinctive splitting of the nanoantenna resonance peak into two polariton modes and two π-phase steps corresponding to each of the modes. We map ISB polariton dispersion using a set of nanoantennae of different sizes. This nano-FTIR spectroscopy approach opens doors for investigations of ISB polariton physics in the single subwavelength nanoantenna regime.

Optical absorption via intersubband transition of electrons in GaAs/AlxGa1−xAs multi-quantum wells in an electric field * * Project supported by the National Natural Science Foundation of China (No. 61764012).

Based on the effective mass approximation, the Schrödinger equation and Poisson equation in GaAs/ AlxGa1−xAs multi-quantum wells (MQWs) are self-consistently solved to obtain the wave functions and energy levels of electrons in the conduction band for the ground first excited state by considering a lateral electric field (LEF). Then, the effects of size, ternary mixed crystal, doping concentration, and temperature on linear and nonlinear intersubband optical absorption coefficients (IOACs), and refractive index changes (RICs) due to the transition between ground states and the first excited states of electrons are discussed based on Fermi’s golden rule. The results show that, under a fixed LEF, with increase of Al composition and doping concentration, the IOACs produce a red shift. With increases of both widths of the wells and barriers IOACs appear as blue shifts and their amplitudes increase, but the barrier width change is much more important to affect nonlinear IOACs, whereas increasing the temperature results in a blue shift first and then a red shift of IOACs. When the other parameters are fixed but there is an increase in the LEF, IOACs occur with a blue shift, and the RICs have similar properties.

Study of non-linear optical properties of center and edge delta-doped multiple quantum wells

In the present paper, the linear and non-linear optical properties of GaAs/AlGaAs multiple quantum well with on-center and on-edge delta doping are studied within the effective mass approximation. The delta potential is analytically modeled within each quantum well potential to obtain the energy levels and the corresponding wave functions using the robust finite difference method. The linear and non-linear optical absorption coefficients and changes in the refractive index are studied in the presence of a static magnetic and a periodic laser field using the density matrix approach. The obtained results show that the position of resonances and the amplitude of the optical absorption coefficients and the refractive index changes can be modified by varying the magnetic field and strength and position of doping potential. Lastly, an increase of the optical intensity appreciably changes the total absorption coefficient, as well as the total refractive index changes. Obtained results are important for the design of various electronic components such as high-power FETs and infrared photonic devices based on the intersubband transition of electrons in δ-doped multiple quantum wells.

Optical properties in GaAs/AlGaAs semiparabolic quantum wells by the finite difference method: Combined effects of electric field and magnetic field

In the present work, the optical properties of GaAs/AlGaAs semiparabolic quantum wells (QWs) are studied under the effect of applied electric field and magnetic field by using the compact-density-matrix method. The energy eigenvalues and their corresponding eigenfunctions of the system are calculated by using the differential method. Simultaneously, the nonlinear optical rectification (OR) and optical absorption coefficients (OACs) are investigated, which are modulated by the applied electric field and magnetic field. It is found that the position and the magnitude of the resonant peaks of the nonlinear OR and OACs can depend strongly on the applied electric field, magnetic field and confined potential frequencies. This gives a new way to control the device applications based on the intersubband transitions of electrons in this system.

Intersubband optical absorption of electrons in double parabolic quantum wells of AlxGa1−xAs/AlyGa1−yAs**Project supported by the National Natural Science Foundation of China (Grant No. 61274098).

Some realizable structures of double parabolic quantum wells (DPQWs) consisting of AlxGa1−xAs/AlyGa1−yAs are constructed to discuss theoretically the optical absorption due to the intersubband transition of electrons for both symmetric and asymmetric cases with three energy levels of conduction bands. The electronic states in these structures are obtained using a finite element difference method. Based on a compact density matrix approach, the optical absorption induced by intersubband transition of electrons at room temperature is discussed. The results reveal that the peak positions and heights of intersubband optical absorption coefficients (IOACs) of DPQWs are sensitive to the barrier thickness, depending on Al component. Furthermore, external electric fields result in the decrease of peak, and play an important role in the blue shifts of absorption spectra due to electrons excited from ground state to the first and second excited states. It is found that the peaks of IOACs are smaller in asymmetric DPQWs than in symmetric ones. The results also indicate that the adjustable extent of incident photon energy for DPQW is larger than for a square one of a similar size. Our results are helpful in experiments and device fabrication.

Tunable resonant radiation force exerted on semiconductor quantum well nanostructures: Nonlocal effects**Project supported by the National Natural Science Foundation of China (Grant No. 11474106), the Natural Science Foundation of Guangdong Province, China (Grant No. 2016A030313439), and the Science and Technology Program of Guangzhou City, China (Grant No. 201707010403)

Resonant radiation force exerted on a semiconductor quantum well nanostructure (QWNS) from intersubband transition of electrons is investigated by taking the nonlocal coupling between the polarizability of electrons and applied optical fields into account for two kinds of polarized states. The numerical results show the spatial nonlocality of optical response can induce the spectral peak position of the exerted force to have a blueshift, which is sensitively dependent on the polarized state and the QWNS width. It is also demonstrated that resonant radiation force is controllable by the polarization and incident directions of applied light waves. This work provides effective methods for controlling optical force and manipulating nano-objects, and observing radiation forces in experiment. This nonlocal interaction mechanism can also be used to probe and predominate internal quantum properties of nanostructures, and to manipulate collective behavior of nano-objects.