Tunable cyclotron resonance laser based on hot holes in germanium applied to FIR spectroscopy of GaAs/AlGaAs heterostructures

We have studied the stimulated far infrared (FIR) cyclotron resonance emission from the heavy holes in germanium. The emission spectrum consists of a single line which is linearly tunable by the magnetic field. We have measured the cyclotron resonance absorption of a GaAs/AlGaAs heterostructure using the stimulated cyclotron resonance emission as a light source for the first time. The photoconductive response of the GaAs/AlGaAs heterostructure is studied as a function of the magnetic field. We found that the photoconductive signal depends strongly on the filling factor.     

Magnetic quantum oscillation in a monolayer insulator

The Shubnikov-de Haas(SdH)oscillation,as evidenced by the oscillating resistivity as a function of magnetic field at sufficiently low temperature,is not uncommon in metals,semi-metals and narrow gap semiconductors at their conducting state.The origin of SdH oscillation is associated with Landau quantization,at which the cyclotron orbits of electrons are quantized.In other words,electrons can only occupy dis-crete energy levels,which are called the Landau levels.As the magnetic field increases,the split,quantized Landau levels move across the Fermi level and hence the material's trans-port properties start to oscillate periodically.     

Evolution of the photoresponse time of the GaAs/AlGaAs cyclotron resonance quantum Hall effect detector

The photoresponse time of the cyclotron resonance detector to terahertz radiation under integer quantum Hall effect conditions in the GaAs/AlGaAs heterostructure is shown to have a deep minimum at the Hall plateau center and two sharp maxima at the plateau edges. The minimum at the plateau center is associated with the fundamental property of vanishing of the random impurity potential screening under quantum Hall effect conditions. The decrease in the response time outside the plateau is related to the equilibrium population of the Landau level above (under) the Fermi level by electrons (holes), respectively, which increases the probability of photoexcited carrier recombination. It is shown that, under conditions of background radiation (300 K), relaxation times decrease by two orders of magnitude while retaining the characteristic magnetic field dependence.     

Magnetoabsorption in narrow-gap HgCdTe epitaxial layers in the terahertz range

The magnetoabsorption and photoconductivity spectra are investigated in the terahertz (THz) range at a temperature of T = 4.2 K for n-Hg_{1 ? x} Cd _x Te bulk epitaxial layers of various compositions (both semiconductor and semimetallic) grown by molecular-beam epitaxy. Within the framework of the Kane 8 · 8 model, the electron and hole Landau levels are calculated. It is shown that, in contrast to the results of previous investigations, all observed resonance lines are related to transitions between the Landau levels of free carriers (the cyclotron resonance in the conduction band and the transitions between heavy-hole and electron Landau levels), which is evidence of the high purity and structural perfection of the samples. The possibility of using zero-gap Hg_{1 ? x} Cd _x Te solid solutions as THz photodetectors tunable by magnetic field is shown.     

Hg1—xCdxTe表面电子的子能带结构

讨论了Hg_(1-x)Cd_xTe MIS结构N型反型层电子子能带结构的理论和实验研究结果。描述了采用电容-电压谱,回旋共振谱和磁导振荡谱定量地研究电子子能带结构的模型和方法。推导得到的子能带色散关系,朗道能级和有效g~*因子,与测得的子能带电子的回旋共振和自旋共振结果符合得很好,从而可以定量地研究由于表面电子的自旋轨道相互作用引起的零场分裂效应,朗道能级的移动、交叉,波函数的混合效应以及电致自旋分裂的色散关系。     

Far-infrared laser oscillation due to cyclotron emission in p-type germanium

Far-infrared laser oscillation due to cyclotron emission in the light hole band of p-type germanium was observed under crossed electric and magnetic fields. The wavelength is inversely proportional to magnetic fields with a cyclotron mass ofm _c =0.048m _c . Numerical calculations based on Luttinger Hamiltonian show that mixing of wavefunctions between the light and heavy hole bands causes population inversion betweenn=0 andn=1 light-hole Landau levels. It is also shown that non-equidistant energy spacing of light-hole Landau levels is essential to yield net amplification.     

Quantum cyclotron resonance in silicon

Quantum effects in the cyclotron absorption of thermally excited holes in the valence bands of Si are demonstrated for the first time. They become evident in the dependence of the cyclotron absorption spectra on uniaxial stress. The spectra were taken with fixed infrared excitation from an HCN laser (?ω=3.68 meV) and pulsed magnetic fields up to 35 Teslas. The uniaxial stress parallel to the magnetic field in <100> and <111> direction reached 10 kbars, the crystal temperature was set between 30 and 60 K. The interpretation of the measured spectra is possible using the stress-dependence of Landau level dispersion, which is calculated by numerical diagonalization of a Hamiltonian for the sixfold degenerate valence band maximum (split by spin-orbit coupling into J=3/2 and J=1/2 states). The small spin-orbit splitting in Si of 44 meV is responsible for an efficient coupling between light-hole and split-off-band and results, even in the high-stress-limit, in a remarkable non-parabolicity.     

The cyclotron resonance in Heterostructures with the InSb/AlInSb quantum wells

The absorption of two-dimensional electrons in InSb-based quantum wells in the quantized magnetic fields in the terahertz spectral region are studied. A p-Ge-based cyclotron laser was used as the radiation source. The effective mass of carriers at the Fermi level equal to 0.0219m ₀ (m ₀ is the mass of a free electron) is determined from the cyclotron resonance spectra. It is shown that the electron spectrum is described by the Kane model in a wide range of magnetic fields. An anomalously pronounced splitting of the cyclotron resonance line not associated with the nonparabolicity of the conduction band of InAs is observed in low magnetic fields, which can be attributed to the effect of the spin-orbit interaction.     

On the electrically detected cyclotron resonance of holes in silicon nanostructures

The cyclotron resonance in semiconductor nanostructures is electrically detected for the first time without an external cavity, a source, and a detector of microwave radiation. An ultranarrow p-Si quantum well on an n-Si (100) surface confined by superconducting heavily boron-doped δ-shaped barriers is used as the object of investigation and provides microwave generation within the framework of the nonstationary Josephson effect. The cyclotron resonance is detected upon the presence of a microcavity, which is incorporated into the quantum-well plane, by measuring the longitudinal magnetoresistance under conditions of stabilization of the source-drain current. The cyclotron-resonance spectra and their angular dependences measured in a low magnetic field identify small values of the effective mass of light and heavy holes in various 2D subbands due to the presence of edge channels with a high mobility of carriers.     

Far-infrared radiation induced photovoltage of inversion electrons on GaAs and Si

We report a novel far-infrared radiation induced photoresponse of quasi two-dimensional space-charge layers on GaAs and Si in quantizing magnetic fields. In gated AlAs---GaAs heterojunctions and in metal-oxide-silicon field-effect transistors significant changes of the gate potential are observed around even Landau level filling factors, whenever radiation of a far-infrared molecular laser is absorbed at cyclotron resonance by inversion electrons. The photosignal can be understood as reflecting the difference in the inversion electron chemical potentials at two different electron temperatures. On GaAs with far-infrared laser intensities in the regime below 1mW/cm² electron temperatures are obtained that exceed the lattice temperature by up to 10K, whereas on Si at comparable conditions changes in the electron temperature are about 2 orders of magnitude smaller.     

THz Gyrotron and BWO Designed for Operation in DNP-NMR Spectrometer Magnet

Dynamic nuclear polarization (DNP) in high-field nuclear magnetic resonance (NMR) spectroscopy requires medium-power terahertz radiation, which nowadays can be provided basically by gyrotrons with superconducting magnets. As the electron cyclotron frequency is very close to the frequency of electron paramagnetic resonance for the same magnetic field, under certain conditions the gyrotron can be installed inside the same solenoid used for NMR spectrometer. This eliminates the need for an additional superconducting magnet, results in a shorter terahertz transmission line, and can make DNP systems practical. In addition to an extremely low-voltage gyrotron (“gyrotrino”), we analyze also advantages of strong magnetic field for a slow-wave electron device as an alternative terahertz source.     

Experimental-theoretical Landau energy level data and its use in making p-Ge laser transition assignments

Landau energy levels are determined from cyclotron resonance absorption data and compared to theoretical calculations. Energy level diagrams are presented to graphically display the nonequal energy level spacings and identify absorption transitions. This information is then used to make transition assignments responsible for the p-Ge laser signals seen by V. N. Shastin et al. and C. Kremser et al. This work suggests that the p-Ge laser yields signals via the following transitions: (a) the expected cyclotron transitions for Δn=1, (b) harmonic cyclotron transitions for Δn=2, 3, (c) light-to-heavy hole transitions and (d) light hole to acceptor level transitions. The material of this paper should be of aid in the study of the pumping-oscillation cycle of the p-Ge laser.     

Photon-assisted tunneling in coupled double quantum wells

Using different techniques to individually contact two closely spaced electron gases, we study the tunneling characteristics between weakly coupled GaAs quantum wells, with and without resonant far-infrared excitation. We find that for barriers as thick as 300 Å, the alignment between the subbands in the wells can be observed as an increased tunneling conductivity. To study photon-assisted tunneling in our samples, we use the cyclotron resonance as a strong, tunable electronic excitation in the far-infrared. When the Landau-level spacing [hstrok]ω_c corresponds to the laser energy [hstrok]ω_L, the carriers are effectively pumped to higher Landau levels, which leads to a reduced resistance across the tunnel barrier. This photo-conductive signal is “doubly resonant” in that it is at maximum when ω_c coincides with ω_L, and at the same time the subbands of the two wells are aligned.     

Mechanism of terahertz photoconductivity in semimetallic HgTe/CdHgTe quantum wells

Terahertz photoconductivity in magnetic fields in semimetallic HgTe/CdHgTe quantum wells has been studied. The main contribution to photoconductivity comes from a signal that appears as a result of electron-gas heating. It is shown that, with the cyclotron resonance conditions satisfied, the photoconductivity signal is composed of cyclotron-resonance and bolometric components. However, in this case too, the bolometric contribution predominates.     

Design of the Upgraded TJ-II Quasi-optical Transmission Line

TJ-II plasma start-up and heating are made by electron cyclotron resonance waves at the second harmonic of the electron cyclotron frequency. Two quasi-optical transmission lines transmit the microwave power of the gyrotrons to the vacuum vessel. The first line launches the microwave power under fixed injection geometry, i.e. there is no possibility to change the launching angle and the wave polarization. The second line has a moveable mirror installed inside the TJ-II vessel. To get high absorption efficiency and a narrow energy deposition profile the internal mirror focuses the wave beam at plasma center. To get more flexibility in the experiments on heating and current drive the first transmission line needs to be upgraded. The design is presented in this paper. The new launching antenna includes an internal mirror to focus the beam and to change the injection angle. Both launchers are then symmetrical. A polarizer consisting of two corrugated mirrors is used to get any wave polarization. Two mirrors with an array of coupling holes and calorimetric measurements of the energy absorbed in the barrier window allow the estimation of the microwave power launched into the TJ-II.     

Splitting and shift of hot magnetophonon resonance peaks in a two-dimensional electron gas

The splitting of the magnetophonon resonance peaks in a two-dimensional electron system is investigated as function of the electric field (or average electron velocity) for different values of the broadening of the Landau levels. We found that for small broadening the maxima in the magnetophonon oscillations are split into two peaks. A new physical interpretation is presented for this splitting which is based on the separate contributions of LO-phonon absorption and emission processes. A shift of the resonance maxima is found when the broadening of the Landau levels is large. A new explanation is given for the apparent temperature and electron density dependence of the optical phonon frequency in heterostructures as determined from magnetophonon resonance experiments. A mechanism is proposed which is able to produce the observed shift in the resonant position and which is consistent with an interaction with the optical phonon mode of the bulk material.     

束缚磁极化子的有效质量与共振频率的关系

采用改进的线性组合算符法和幺正变换方法,研究极性晶体膜中束缚磁极化子的有效质量与振动频率λ的变化关系.得出束缚磁极化子的有效质量均由两部分组成:第一部分是由于电子-体LO声子相互作用所引起的;第二部分则是电子-SO声子相互作用引起的.后者又包含两部分,分别是电子与极性膜中两支表面声子相互作用的贡献.而且当λ＜20×10...     

极性晶体中磁极化子声子色散效应

利用线性组合算符和幺正变换相结合的方法,研究了声子色散对极性晶体中磁极化子性质的影响．计及纵光学（LO）声子色散,在抛物近似下导出了极性晶体中磁极化子基态能量、自陷能和基态Landau能随声子色散系数、回旋共振频率和电子-纵光学声子耦合强度的变化关系．数值计算结果表明基态能量随声子色散系数、电子-纵光学声子耦合强度的增大而减小．自陷能随声子色散系数和电子-纵光学声子耦合强度的增大而增大．基态Landau能随电子-纵光学声子耦合强度的增大先增大到最大值后又减小,随声子色散系数的增大而增大．     

磁场和库仑场对量子点中强耦合束缚极化子性质的影响

采用线性组合算符和幺正变换方法研究了磁场和库仑场对半导体量子点中强耦合极化子性质的影响.导出了强耦合束缚磁极化子的振动频率和基态能量与量子点的有效受限长度、库仑束缚势、磁场的回旋共振频率和电子-声子耦合强度的变化关系.数值计算结果表明:强耦合束缚磁极化子的振动频率和基态能量随量子点的有效受限长度的减小而迅速增大,随磁场的回旋共振频率的增加而增大.基态能量随电子-声子耦合强度和库仑束缚势的增加而减少.