Asymmetric Double Quantum Research Articles

Asymmetric light diffraction of two-dimensional electromagnetically induced grating with PT symmetry in asymmetric double quantum wells.

An asymmetric double semiconductor quantum well is proposed to realize two-dimensional parity-time (PT) symmetry and an electromagnetically induced grating. In such a nontrivial grating with PT symmetry, the incident probe photons can be diffracted to selected angles depending on the spatial relationship of the real and imaginary parts of the refractive index. Such results are due to the interference mechanism between the amplitude and phase of the grating and can be manipulated by the probe detuning, modulation amplitudes of the standing wave fields, and interaction length of the medium. Such a system may lead to new approaches of observing PT-symmetry-related phenomena and has potential applications in photoelectric devices requiring asymmetric light transport using semiconductor quantum wells.

Electron states and electron Raman scattering in an asymmetrical double quantum well: External electric field

In this paper studies an electron Raman scattering process for a semiconductor, coupled and asymmetrical double quantum well. Then, the presence of an electron in a single conduction band is considered. In addition, the system is subjected to an external electric field. To carry out this study, the net Raman gain and the differential cross section are calculated. The emission spectra are interpreted and discussed. For this, we obtain the exact solutions of the electron states considering the envelope function approximation and a single parabolic conduction band, which is split into a sub-bands system due to confinement. Furthermore, the effect of the electric field on the electron states and in the differential cross section is studied. To illustrate our findings, we have considered a system growing in a GaAs/AlxGa1−xAs matrix.

Four-wave mixing in an asymmetric double quantum dot molecule

The four-wave mixing (FWM) effect of a weak probe field, in an asymmetric semiconductor double quantum dot (QD) structure driven by a strong pump field is theoretically studied. Similarly to the case of examining several other nonlinear optical processes, the nonlinear differential equations of the density matrix elements are used, under the rotating wave approximation. By suitably tuning the intensity and the frequency of the pump field as well as by changing the value of the applied bias voltage, a procedure used to properly adjust the electron tunneling coupling, we control the FWM in the same way as several other nonlinear optical processes of the system. While in the weak electron tunneling regime, the impact of the pump field intensity on the FWM is proven to be of crucial importance, for even higher rates of the electron tunneling it is evident that the intensity of the pump field has only a slight impact on the form of the FWM spectrum. The number of the spectral peaks, depends on the relation between specific parameters of the system.

Erratum to: Nonlinear optical properties of asymmetric n-type double δ-doped GaAs quantum well under intense laser field

Erratum to: Nonlinear optical properties of asymmetric n-type double δ-doped GaAs quantum well under intense laser field

Tunneling-induced highly efficient four-wave mixing in asymmetric double quantum wells

The time-dependent analysis of four-wave mixing based on resonant tunneling is investigated in an asymmetric semiconductor double quantum well structure. We analytically demonstrate that a highly efficient mixing wave is found to be induced by resonant tunneling in such a quantum well scheme with low-light pump wave intensities. Especially for small propagation distances, a significant enhancement of four-wave mixing conversion efficiency can be achieved. Such an interesting scheme could be used to generate coherent short-wavelength radiation in solid-state materials.

Numerical simulation and experiment of high brightness tapered lasers

The optical properties of a tapered laser, which can be used to improve laser beam quality and generate a high brightness optical beam, are simulated and the tapered laser is fabricated. The beam quality, the index difference, and the index difference dependence on taper angle are studied. A tapered diode laser with a ridge waveguide is introduced and formed through etching the wafer with asymmetrical waveguide structure and double quantum wells. A high brightness power is achieved due to the combination of the ridge waveguide and the taper waveguide. In the experiment, the laser delivers 978 nm, 8 W CW with an 11.96 A current. The beam quality factor M2 in the vertical and lateral directions is measured at 1.42 and 2.21, respectively.

Improved projected Green's function approach to electron tunneling lifetime calculations in quantum wells

We demonstrate that the use of an optimal initial state of a wave function generated using Lanczos method allows the previously proposed projected Green's function approach to predict the tunneling lifetime of a particle crossing a double quantum well with a considerably improved accuracy. It is further revealed that the electron tunneling lifetime of such system depends linearly or quadratically on the barrier thickness or the well width. We also investigate the asymmetric double quantum well structure and find that the electron tunneling lifetime in it is strongly dependent on barrier thickness. Finally, we discuss the possible experimental designs to observe the above-mentioned tunneling features.

Correlation of the Carrier Decay Time and Barrier Thickness for Asymmetric Cubic GaN/Al0.64Ga0.36N Double Quantum Wells

The carrier transfer via non‐resonant tunneling is of great significance for many devices like the quantum cascade laser. In this study time‐resolved photoluminescence is used for an investigation of this effect in asymmetric double quantum wells for low temperatures. The Al content in these asymmetric double quantum wells was determined by HRXRD to x = 0.64 ± 0.03. The growth of the asymmetric cubic GaN/AlxGa1−xN double quantum wells was performed by a radio‐frequency plasma‐assisted molecular beam epitaxy. As a substrate, 3C‐SiC (001) on top of Si (001) was used. Three samples with different barrier thickness d were analysed (1 nm, 3 nm, 15 nm). The two quantum wells are designed with the thicknesses 2.5 nm and 1.35 nm. Thus, three expected emission bands measured in luminescence can be resolved. The maximum intensities are 3.49 eV (wide well), 3.73 eV (narrow well) and 4.12 eV (AlGaN). A correlation between the carrier lifetimes of the quantum wells (QWs) and the barrier width is found. Exploiting rate equations, the intensity ratio of both QW emissions is calculated. The coupling of the two QWs starts below 3 nm barrier thickness, above this value there is no coupling.

Charge carrier dynamics of GaAs/AlGaAs asymmetric double quantum wells at room temperature studied by optical pump terahertz probe spectroscopy

Two asymmetric double quantum wells of different coupling strengths (barrier widths) were grown via molecular beam epitaxy, both samples allowing tunneling. Photoluminescence was measured at 10 and 300 K to provide evidence of tunneling, barrier dependence, and structural uniformity. Carrier dynamics at room temperature was investigated by optical pump terahertz probe (OPTP) spectroscopy. Carrier population decay rates were obtained and photoconductivity spectra were analyzed using the Drude model. This work demonstrates that carrier, and possibly tunneling dynamics in asymmetric double quantum well structures may be studied at room temperature through OPTP spectroscopy.

Spin and tunneling dynamics in an asymmetrical double quantum dot with spin-orbit coupling: Selective spin transport device

In this article, we study the spin and tunneling dynamics as a function of magnetic field in a one-dimensional GaAs double quantum dot with both the Dresselhaus and Rashba spin-orbit coupling. In particular, we consider different spatial widths for the spin-up and spin-down electronic states. We find that the spin dynamics is a superposition of slow as well as fast Rabi oscillations. It is found that the Rashba interaction strength as well as the external magnetic field strongly modifies the slow Rabi oscillations which is particularly useful for implementing solid state selective spin transport device.

Nonlinear optical properties of asymmetric n-type double δ-doped GaAs quantum well under intense laser field

The effect of non-resonant intense laser field on the intersubband-related optical absorption coefficient and refractive index change in the asymmetric n-type double δ-doped GaAs quantum well is theoretically investigated. The confined energy levels and corresponding wave functions of this structure are calculated by solving the Schrodinger equation in the laser-dressed confinement potential within the framework of effective mass approximation. The optical responses are reported as a function of the δ-doped impurities density and the applied non-resonant intense laser field. Additionally, the calculated results also reveal that the non-resonant intense laser field can be used as a way to control the electronic and optical properties of the low dimensional semiconductor nano-structures.

Effect of the magnetic field on the nonlinear optical rectification and second and third harmonic generation in double [formula omitted]-doped GaAs quantum wells

In this paper we are reporting the computation for the Nonlinear Optical Rectification (NOR) and the Second and Third Harmonic Generation (SHG and THG) related with electronic states of asymmetric double Si-δ-doped quantum well in a GaAs matrix when this is subjected to an in-plane (x-oriented) constant magnetic field effect. The work is performed in the effective mass and parabolic band approximations in order to compute the electronic structure for the system by a diagonalization procedure. The expressions for the nonlinear optical susceptibilities, χ0(2), χ2ω(2), and χ3ω(3), are those arising from the compact matrix density formulation and stand for the NOR, SHG, and THG, respectively. This asymmetric double δ-doped quantum well potential profile actually exhibits nonzero NOR, SHG, and THG responses which can be easily controlled by the in-plane (x-direction) externally applied magnetic field. In particular we find that for the chosen configuration the harmonic generation is in the far-infrared/THz region, thus and becoming suitable building blocks for photodetectors in this range of the electromagnetic spectra.

Comparison of symmetric and asymmetric double quantum well extended-cavity diode lasers for broadband passive mode-locking at 780 nm.

We present a compact, mode-locked diode laser system designed to emit a frequency comb in the wavelength range around 780 nm. We compare the mode-locking performance of symmetric and asymmetric double quantum well ridge-waveguide diode laser chips in an extended-cavity diode laser configuration. By reverse biasing a short section of the diode laser chip, passive mode-locking at 3.4 GHz is achieved. Employing an asymmetric double quantum well allows for generation of a mode-locked optical spectrum spanning more than 15 nm (full width at -20 dB) while the symmetric double quantum well device only provides a bandwidth of ∼2.7 nm (full width at -20 dB). Analysis of the RF noise characteristics of the pulse repetition rate shows an RF linewidth of about 7 kHz (full width at half-maximum) and of at most 530 Hz (full width at half-maximum) for the asymmetric and symmetric double quantum well devices, respectively. Investigation of the frequency noise power spectral density at the pulse repetition rate shows a white noise floor of approximately 2100 Hz2/Hz and of at most 170 Hz2/Hz for the diode laser employing the asymmetric and symmetric double quantum well structures, respectively. The pulse width is less than 10 ps for both devices.

Comparison of asymmetric double parabolic-inversed parabolic quantum wells for linear optical (1–2) transition

In present study, for asymmetric double parabolic quantum well (ADPQW) and asymmetric double inverse parabolic quantum well (ADIPQW) the linear optical absorption (OA) coefficient, the linear refractive index change (RIC), resonant peaks of the linear OA and the optical rectification (OR) coefficients are examined as dependent on the structural parameters (barrier width (b) and Al concentration at the well center (σ)). The results display that the variation of all OA and OR coefficients and RIC depend on b and σ-parameter. I have also shown that the σ-parameter has a significant effect on the electronic and optical properties of ADPQW and ADIPQW, and that the energy levels and the dipole moment matrix elements vary depending on the shape of the limiting potential. The intersubband absorption spectrum in the ADPQW shows the blue shift by increasing the b and σ values. Whereas the absorption spectrum for ADIPQW shows blue shift for σ=1 with increasing b-values, this spectrum shows red shifts for σ=5. The electro-optic properties of these structures promise many potential applications in high speed spatial light modulators and switches. So, it can be adjusted RIC and the resonant peak size of the linear OA and OR coefficient by changing Al density at the well center in addition to the barrier dimensions.

Enhancement of multisubband electron mobility in square-parabolic asymmetric double quantum well structure

We show that higher multisubband electron mobility μ can be achieved in AlxGa1-xAs based hybrid type square-parabolic asymmetric double quantum well (HDQW) compared to that of the symmetric square double quantum well (SDQW) and parabolic double quantum well (PDQW) structures. We show that μ(HDQW) > μ(PDQW) > μ(SDQW). This trend in the enhancement of μ is mostly governed by the change in the ionized impurity scattering potential. The mobility of the HDQW can be enhanced by increasing the well width and also the curvature of the parabolic potential. In addition, μ(HDQW) becomes larger when the parabolic well lies towards the substrate side compared to that of the surface side due to the reduced effect of the interface roughness scattering. The mobility also increases with increase in the surface electron density Ns. The enhancement of μ in HDQW is because of the reduction of the intersubband scattering rate matrix elements due to the asymmetric distribution of the subband wave functions. Our results can be utilized to develop asymmetric double quantum well field effect transistor.

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.

Effects of electromagnetic fields on the nonlinear optical properties of asymmetric double quantum well under intense laser field

In this study, the effects of electric and magnetic fields on the optical rectification and second and third harmonic generation in asymmetric double quantum well under the intense non-resonant laser field is theoretically investigated. We calculate the optical rectification and second and third harmonic generation within the compact density-matrix approach. The theoretical findings show that the influence of electric, magnetic, and intense laser fields leads to significant changes in the coefficients of nonlinear optical rectification, second and third harmonic generation.

Electromagnetically induced transparency in an asymmetric double quantum well under non-resonant, intense laser fields

Electromagnetically induced transparency in an asymmetric double quantum well subjected to a non-resonant, intense laser field is theoretically investigated. We found that the energy levels configuration could be switched between a Λ-type and a ladder-type scheme by varying the non-resonant radiation intensity. This effect is due to the laser-induced electron tunneling between the wells and it allows a substantial flexibility in the manipulation of the optical properties. The dependence of the susceptibilities on the control field Rabi frequency, intensity of the nonresonant laser, and the control field detuning for both configurations are discussed and compared.

Electronic States and Optical Transitions in an Asymmetric Quantum Dot Molecule

In the framework of adiabatic approximation the electronic states and direct interband absorption of light in the asymmetric double quantum dot molecule (QDM) having a shape of Cassini lemniscate revolution are discussed. Analytical expressions for the wave functions and energy spectrum of the electron in the QDM are derived. Nonmonotonic split and step-like behavior of the energy spectrum is revealed due to the possibility of the electron tunneling between quantum dots (QDs) in the molecule. The corresponding selection rules of quantum transitions for the direct interband absorption of light are obtained. The absorption edge characteristics depending on the QDM geometrical sizes are also revealed.

Nonlinear optical rectification, the second and third harmonic generation in asymmetric double quantum well under the intense laser field

Abstract . In this study, the effects of a non-resonant intense laser field and the Al-concentration on the optical rectification, second and third harmonic generation in asymmetric double quantum well are investigated within the compact density matrix approach theoretically. The results show that the intense laser field and the Al-concentration lead to significant changes in the coefficients of nonlinear optical rectification, second and third harmonic generations. Keywords: Asymmetric double quantum well, Nonlinear optical rectification, Second and third harmonic generations, Intense laser field Ozet. Bu calismada, asimetrik cift kuantum kuyusunun dogrusal olmayan optik duzeltme, ikinci ve ucuncu harmonikleri uzerine Al-konsantrasyonu ve rezonans olmayan yogun lazer alaninin etkisi kompakt yogunluk matris yaklasimi kullanilarak teorik olarak incelenmistir. Sonuclar, yogun lazer alani ve Al-konsantrasyonun dogrusal olmayan optik duzeltme, ikinci ve ucuncu harmonikler uzerine onemli degismelere neden oldugunu gostermektedir.