Terahertz absorption by electrons and confined acoustic phonons in free-standing quantum wells

The acoustic phonon confinement in a free-standing quantum well (FSQW) results in an acoustic phonon energy quantization. Typical quantization energies are in the terahertz frequency range. Free electrons may absorb electromagnetic waves in this frequency range if they emit or absorb acoustic phonons. Therefore, the terahertz absorption reveals the characteristic features of the acoustic phonon spectrum in free-standing structures. We have calculated the absorption coefficient of an electromagnetic wave by free electrons in a FSQW in the terahertz frequency range. We took into account a time dependent electric field, an exact form of the acoustic phonon spectrum and eigenmodes, and electron interactions with confined acoustic phonons through the deformation potential. We demonstrate numerical results for GaAs FSQW of width 100 Å at low lattice temperatures in the frequency range 0.1-1 THz. The absorption coefficient exhibits several structures at frequencies corresponding to the lowest acoustic phonon modes. These features occur due to absorption of photons by electrons, which is accompanied by the emission of corresponding acoustic phonons.     

Effects of non-equilibrium polar optic phonons and the band non-parabolicity on small signal high-frequency hot electrons ac mobility in narrow gap semiconductors in high magnetic fields

The small signal high-frequency ac mobility of hot electrons in n-HgCdTe in the extreme quantum limit at low and high temperatures have been calculated considering the non-equilibrium phonon distribution as well as the thermal phonon distribution .The energy loss rate has been calculated considering only optical phonon scattering while the momentum loss rate has been calculated considering acoustic phonon scattering and piezoelectric scattering together with polar optical phonon scattering and separately considering only the polar optical scattering. The results have been discussed and compared. It has been observed that at 20 K, the normalized mobility considering all the three scattering mechanisms differs appreciably from that considering only the polar optical phonon scattering. However, at 77 K, there is no difference in the normalized mobility. This establishes the fact that at higher temperature, the effect of acoustic phonon scattering and piezoelectric coupling is negligible, compared to the polar optical phonon scattering. So the ac mobility considering only polar optical phonon scattering has been studied at 77 and 20 K. The ac mobility is found to remain constant up to 100 GHz and thereafter it started decreasing at higher frequencies at 77 K whereas the ac mobility reduces at much lower frequencies at lower temperature at lower field. The non-parabolicity of the band structure enhances the normalized mobility.     

Free carrier absorption in quantizing magnetic fields: Nonpolar optical phonon scattering

A quantum theory of free carrier absorption in nondegenerate semiconductors and in strong magnetic fields which was previously developed to treat the case when acoustic phonon scattering dominates the free carrier absorption process [1] is extended to treat the case when nonpolar optical scattering is important. When the electromagnetic radiation field is polarized parallel to the direction of the applied magnetic field, results are obtained which are similar to those when acoustic phonon scattering is dominant. The free carrier absorption is an oscillatory function of the magnetic field which on the average increases in magnitude with the magnetic field. However, more structure in the free carrier absorption occurs when nonpolar optical phonon scattering dominates. This is due to the fact that there are two periods in the oscillatory magnetic field dependence associated with the emission or the absorption of optical phonons during the intraband transitions. When the cyclotron frequency exceeds the sum of the photon and optical phonon frequencies, i.e. ω_c > θ + ω_o, the free carrier absorption is predicted to increase linearly with magnetic field when ?ω_c? k_BT. The magnetic field dependence of the free carrier absorption can be explained in terms of phonon-assisted transitions between the various Landau levels in a band involving the emission and absorption of optical phonons.     

Cyclotron Resonance of a Magnetopolaron in a Quantum Well at Finite Temperatures

Taking into account the interaction of an electron with both bulk longitudinal-optical (BO) and surface-optical (SO) phonons, the cyclotron resonance of a magnetopolaron in a quantum well at finite temperatures is investigated by using the generalized Larsen perturbationtheory method. It is shown that the absorption and emission resonances must be considered at the same time at finite temperatures. The results also show that the electron-SO phonon interaction plays an important role as well as the electron-BO phonon interaction, especially when the quantum well width is getting thinner. For an experimentally interesting GaAs/Ga_1-xAl_xAs sandwich structure, the splitting of the cyclotron resonance spectrum and the temperature dependence of two splitting cyclotron resonance mass of the magnetopolaron in the resonant magnetic field region have been studied.     

氮化物抛物量子阱中磁极化子的能量

利用改进的Lee-Low-Pines(LLP)方法和变分法研究了在外磁场作用下氮化物无限抛物量子阱中自由极化子的能级,得到了极化子基态能量随量子阱阱宽和外磁场变化的规律,对GaN/Al_0.3Ga_0.7N抛物量子阱进行了数值计算.结果表明:外磁场对极化子的能量有明显的影响,极化子基态能量随阱宽的增强而减小,随磁场的增强而增大,并且电子-声子相互作用对氮化物量子阱中极化子能量的贡献是很大的.     

Laser field induced interband absorption in a strained GaAs/GaAlAs double quantum well system

Effect of laser field intensity on exciton binding energies is investigated in a GaAs/ GaAlAs double quantum well system. Calculations have been carried out with the variational technique within the single band effective mass approximations using a two parametric trial wave function. The interband emission energy as a function of well width is calculated in the influence of laser field. The laser field induced photoionization cross-section for the exciton placed at the centre of the quantum well is computed as a function of normalized photon energy. The dependence of the photoionization cross-section on photon energy is carried out for the excitons. The resulting spectra are brought out for light polarized along and perpendicular to the growth direction. The intense laser field dependence of interband absorption coefficient is investigated. The results show that the exciton binding energy, interband emission energy, the photoionization cross-section and the interband absorption coefficient depend strongly on the well width and the laser field intensity. Our results are compared with the other existing literature available.     

Coherent acoustic phonon oscillations in semiconductor multiple quantum wells with piezoelectric fields

Large coherent acoustic phonon oscillations were demonstrated using InGaN/GaN multiple quantum wells with piezoelectric fields. With UV femtosecond pulse excitation, photogenerated carriers screened the piezoelectric field and initiated the displacive coherent phonon oscillations. The specific phonon frequency was selected by the coupling between the periodic carrier distribution and the corresponding acoustic phonon mode. The induced acoustic phonon oscillation resulted in piezoelectric field modulation and then caused absorption variation through the quantum confined Franz-Keldysh effect. The wave vector uncertainty due to the finite sample width was found to determine the observed dephasing time.     

Transient response of hot electrons in narrow-gap semiconductors at low temperatures in the presence of a longitudinal quantizing magnetic field

Transient response of hot electrons in narrow-gap semiconductors to a step electric field in the presence of a longitudinal quantizing magnetic field has been studied at low temperatures using displaced Maxwellian distribution. The energy and momentum balance equations are used assuming acoustic phonon scattering via deformation potential responsible for the energy relaxation and elastic acoustic phonon scattering together with ionized impurity scattering for momentum relaxation. The calculations for the variation of drift velocity and electron temperature as functions of time are made for n-Hg_0.8Cd_0.2 Te in the extreme quantum limit at 1.5 K and 4.2 K. The momentum and energy relaxation times are found to be of the same order of magnitudes as with the experimental values. The magnetic field and lattice temperature dependences of the relaxation rates have been investigated.One of the authors, Suchandra Bhaumik, acknowledges the Council of Scientific and Industrial Research (New Delhi) for financial support.     

Observation of coupled cyclotron-plasmonlo phonon modes in InSb for sample configurations ranging from Faraday to Voigt

In an external magnetic field the coupled plasmon-LO phonon modes interact with the single-particle cyclotron modes to produce, in general, three modes which are i.r. active for all sample configurations except the Faraday configuration. In normal-incidence experiments on a thin sample of n-InSb, we have observed the shift in position of the coupled-mode resonance absorption lines as the magnetic field is tilted in steps from the Faraday to the Voigt configuration. We also observed the magnetic field dependence of the intensity and width of the absorption lines which are related to the electron and LO phonon strengths of the modes. Calculations of transmittance and reflectance are in good qualitative agreement with the experimental observations.     

Application of a continued-fraction-based theory to magneto-optical intraband transition in GaAs and CdS

The quantum theory of intraband magneto-optical transition in semiconductors introduced previously in terms of continued-fraction-based power-series-expansion technique is reviewed in connection with examination of temperature- and dimensional-dependence of the width in GaAs and CdS in which piezoelectric scattering is dominant. With the same values of the piezoelectric coupling constant K and the expansion parameter the width in two-dimensions increases with temperature as in three-dimensions. Furthermore, the width becomes smaller uniformly as the dimension is reduced in the direction of static magnetic field in the quantum limit, which is physical. Therefore, the continued-fraction-based theory gives quite good interpretation of the acoustic phonon scattering in the quantum limit.     

二维极化子在磁场中的基态能量

谐振子算符的代数运算方法被用于研究磁场中同时与表面光学声子及表面声学声子相互作用的二维电子。得到二维极化子在强磁场中直至四级微扰的基态能量以及它在任意强经磁场中的二级微扰基态能量表达式。结果发现,对磁场中二维极化子基态能量的影响中,表面声学声子有着与表面光学声子同样的甚至更为突出的贡献,是不容忽视的。 关键词：     

Ultrafast optical probe of coherent acoustic phonons in Co_2MnAl Heusler film

In this work, pronounced oscillations in the time-resolved reflectivity of Heusler alloy Co_2MnAl films which are epitaxially grown on Ga As substrates are observed and investigated as a function of film thickness, probe wavelength,external magnetic field and temperature. Our results suggest that the oscillation response at 24.5 GHz results from the coherent phonon generation in Co_2MnAl film and can be explained by a propagating strain pulse model. From the probe wavelength dependent oscillation frequency, a sound velocity of(3.85±0.1)×10~3m/s at 800 nm for the epitaxial Co_2MnAl film is determined at room temperature. The detected coherent acoustic phonon generation in Co_2MnAl reported in this work provides a valuable reference for exploring the high-speed magnetization manipulation via magnetoelastic coupling for future spintronic devices based on Heusler alloy films.     

Effect of nonpolar optical phonon scattering on free-carrier absorption in heavily doped n-type germanium

The effect of nonpolar optical phonon scattering on the free-carrier absorption in n-type semiconductors such as germanium has been investigated quantum mechanically in quantizing magnetic fields. It is assumed that the energy band of electrons in semiconductors is nonparabolic and the dominant scattering mechanism for electrons in solids is that of nonpolar optical phonon scattering. When the radiation field is polarized parallel to the magnetic field, the absorption coefficient will be of complex value due to the interaction of the radiation field and the optical phonon field with electrons in semiconductors. Results show that real and imaginary parts of the absorption coefficient oscillate quite considerably with the magnetic field in the high fields for the heavily doped n-type Ge. Both real and imaginary parts of the absorption coefficient appear as positive and negative values when changing the magnetic field. In low magnetic fields, the imaginary part of the absorption coefficient disappears. However, if the density of electrons increases, the imaginary part of the absorption coefficient will increase with the magnetic field in low fields. Moreover, it is also shown that the amplitudes of oscillations for the real and imaginary parts of the absorption coefficient do not vary in a regular trend with the density of electrons.     

Polarization control of multi-photon absorption under intermediate femtosecond laser field

It has been shown that the femtosecond laser polarization modulation is a very simple and well-established method to control the multi-photon absorption process by the light–matter interaction. Previous studies mainly focused on the multiphoton absorption control in the weak field. In this paper, we further explore the polarization control behavior of multiphoton absorption process in the intermediate femtosecond laser field. In the weak femtosecond laser field, the secondorder perturbation theory can well describe the non-resonant two-photon absorption process. However, the higher order nonlinear effect(e.g., four-photon absorption) can occur in the intermediate femtosecond laser field, and thus it is necessary to establish new theoretical model to describe the multi-photon absorption process, which includes the two-photon and four-photon transitions. Here, we construct a fourth-order perturbation theory to study the polarization control behavior of this multi-photon absorption under the intermediate femtosecond laser field excitation, and our theoretical results show that the two-photon and four-photon excitation pathways can induce a coherent interference, while the coherent interference is constructive or destructive that depends on the femtosecond laser center frequency. Moreover, the two-photon and fourphoton transitions have the different polarization control efficiency, and the four-photon absorption can obtain the higher polarization control efficiency. Thus, the polarization control efficiency of the whole excitation process can be increased or decreased by properly designing the femtosecond laser field intensity and laser center frequency. These studies can provide a clear physical picture for understanding and controlling the multi-photon absorption process in the intermediate femtosecond laser field, and also can provide a theoretical guidance for the future experimental realization.     

Nonlinear optical properties of an off-center donor in a quantum dot under applied magnetic field

The nonlinear optical properties of an off-center hydrogenic donor in a two-dimensional quantum dot under applied magnetic field are investigated in detail by using the matrix diagonalization method. Based on the computed energies and wave functions, the linear, third-order and total optical absorption coefficients as well as the refractive index changes have been examined between the ground state (L=0) and the first excited state (L=1). The results show that the ion position, the applied magnetic field, the confinement frequency, and the incident optical intensity have an important influence on the nonlinear optical properties of off-center donors.     

确定磁性体在绝热磁化过程中达到最高温度的方法

在磁性体磁化过程中, 决定其能够达到的最高温度, 对磁热材料的优化选取是重要的. 本文以钆镓石榴石(Gd₃Ga₅O₁₂) 为例, 根据高磁场下趋近饱和定律的思想, 给出了低温、超强磁场下, Gd₃Ga₅O₁₂晶体等效磁化率的定量形式. 在外磁场从0---40 T范围内, 计算了该晶体的磁熵变、声子熵变以及磁性体温度随外磁场的变化, 结果均与实验值符合较好. 利用声子熵变与饱和磁熵变曲线交点的唯一性, 给出了在磁性体磁化过程中, 确定其温度达到最大值的方法, 预言了Gd₃Ga₅O₁₂晶体在绝热磁化过程中达到的最高温度为64.7K. 该方法还可以对所加外磁场大小进行预言或估计.     

The theory of the cyclotron resonance line half-width in limited-size systems

Many-phonon optical transitions between Landau levels and size quantization levels in a longitudinal magnetic field are investigated in solitary quantum wells. The developed theory makes it possible to describe the intensity of the cyclotron resonance line as well as the temperature and field dependences of its half-width. The theoretical results are compared with experimental data. It is shown that when the interaction between electrons and optical phonons is taken into account, phonon satellites may appear as a result of an electron transition between the size quantization levels and magnetic levels.     

Free-carrier absorption in quantum well structures for acoustic phonon scattering

Free-carrier absorption has been studied for quantum well structures fabricated from III-V semiconducting materials where the acoustic phonon scattering is important. The energy band of carriers is assumed to be nonparabolic. We discuss the effect of acoustic phonon scattering on the free-carrier absorption for both deformation-potential coupling and piezoelectric coupling. It is found that the free-carrier absorption coefficient depends upon the polarization of the electromagnetic radiation relative to the layer plane or quantum well, the photon frequency, and the temperature. When the deformation-potential coupling is dominant, the free-carrier absorption coefficient increases with increasing temperature for photons polarized in the layer plane or perpendicular to the layer plane. However, when the piezoelectric coupling is dominant, the free-carrier absorption coefficient increases with increasing temperature for photons polarized in the layer plane, but for photons polarized perpendicularly to the layer plane, the free-carrier absorption coefficient decreases with increasing temperature. Moreover, at high temperatures such as T = 300 K, the free-carrier absorption coefficient oscillates with the film thickness in a small quantum well region and then decreases monotonically with increasing the film thickness. This is different from the result for three-dimensional semiconducting solids.     

Magnetoelectronic transport of the two-dimensional electron gas in CdSe single quantum wells

Hall mobility and magnetoresistance coefficient for the two-dimensional (2D) electron transport parallel to the heterojunction interfaces in a single quantum well of CdSe are calculated with a numerical iterative technique in the framework of Fermi-Dirac statistics. Lattice scatterings due to polar-mode longitudinal optic (LO) phonons, and acoustic phonons via deformation potential and piezoelectric couplings, are considered together with background and remote ionized impurity interactions. The parallel mode of piezoelectric scattering is found to contribute more than the perpendicular mode. We observe that the Hall mobility decreases with increasing temperature but increases with increasing channel width. The magnetoresistance coefficient is found to decrease with increasing temperature and increase with increasing magnetic field in the classical region.      

Anomalous dephasing of two-dimensional electrons in an AlGaAs/GaAs parabolic quantum well structure

Magnetoconductivity measurements are performed on a parabolic quantum well structure. The weak localization effect is observed at a low magnetic field for both single-subband and double-subband occupation regimes. Applying weak-localization theory, we have extracted the dephasing rate. The extracted dephasing rate increases with increasing conductivity in the small-energy-transfer regime and shows a similar trend as the electron density is increased in the large-energy-transfer regime. This is in conflict with Fermi-liquid theory, and cannot be attributed to electron–phonon scattering.