Strain-modulated anisotropic Andreev reflection in a graphene-based superconducting junction

We investigate the Andreev reflection across a uniaxial strained graphene-based superconducting junction. Compared with pristine graphene-based superconducting junction, three opposite properties are found. Firstly, in the regime of the interband conversion of electron-hole, the Andreev retro-reflection happens. Secondly, in the regime of the intraband conversion of electron-hole, the specular Andreev reflection happens. Thirdly, the perfect Andreev reflection, electron-hole conversion with unit efficiency, happens at a nonzero incident angle of electron. These three exotic properties arise from the strain-induced anisotropic band structure of graphene, which breaks up the original relation between the direction of velocity of particle and the direction of the corresponding wavevector. Our finding gives an insight into the understanding of Andreev reflection and provides an alternative method to modulate the Andreev reflection.     

Direct determination of reduced band gap and chemical potential in an electron-hole plasma in high-purity GaAs

We report the first transmission experiments with tunable i.r. Laser light through an electron-hole plasma in high purity GaAs. Negative absorption (gain) is observed at energies below the chemical potential, and positive absorption above. The experimentally determined energetic positions of the reduced band gap and chemical potential are lower than theoretically expected.     

Optical studies of fast plasma transport in Si

We present two new optical techniques for the investigation of the transport properties of ambipolar plasmas in semiconductors. The first method is a time-of-flight technique: Using surface doping with shallow impurities which provide a characteristic bound exciton emission we introduce optically active spatial markers into the thin Si wafers investigated. Carrier pairs are excited at the undoped surface of the wafer by short laser pulses. From time-resolved studies of the bound exciton emission we obtain average velocity values for the ambipolar transport through the sample. The most attractive feature of the time-of-flight method compared to other spatially resolved measurements is the combination of very high spatial resolution (submicron range) with a high sensitivity. The second method used to study the plasma transport is based on a time-resolved investigation of the Mott transition between the electron-hole plasma and free excitons in Si. Using well-established values for the Mott transition and the temporal evolution of the plasma and the free exciton emissions we obtain values for the velocity with which the plasma expands to the Mott density as functions of the excitation power and the temperature. The advantage of this method is the lack of any need for spatial resolution. Possible extensions of both methods are discussed.     

Optical gains associated with exciton collision processes and high density electron-hole plasma in CdSe

Optical gain spectra for CdSe are measured at 4.2 K changing excitation density. The gain in the 683–689 nm region is concluded to be due to exciton collision processes, in disagreement with the assignment to electron-hole liquid by other investigators. The gain due to high density electron-hole plasma, which has been found recently by the authors to be generated under very high excitation density, is observed in the 688–691 nm region.     

Electron-hole droplet formation in direct-gap semiconductors observed by mid-infrared pump-probe spectroscopy.

Mid-infrared pump-probe measurements with subpicosecond time resolution reveal the existence of a metastable condensed phase of the electron-hole ensemble in a direct-gap semiconductor CuCl. High-density electrons and holes are directly created in a low-temperature state by the resonant femtosecond excitation of excitons above the Mott transition density. Strong metallic reflection with a plasma frequency Planck's over 2pi(omega)p approximately 0.5 eV builds up within 0.3 ps. Within a few picoseconds, the mid-infrared reflection spectrum is transformed from metalliclike into colloidlike. The observed resonance feature at Planck's over 2pi(omega)p/sqrt[3] allows us to obtain the carrier density in the metastable electron-hole droplets of 2x10(20) cm(-3).     

Picosecond studies of highly excited CdS

Results on picosecond luminescence and excite-and-probe transmission as well as transient grating measurements for highly excited CdS measured at a bath temperature of 5 K will be presented. The luminescence and optical gain both due to electron-hole plasma and excitonic molecule recombination are observed. The electron-hole plasma decays very fast by bimolecular recombination of electrons and holes in the plasma and diffusion of the carrier toward the low density regions, and transforms into excitons and excitonic molecules within 100–200 ps. The possibility of electron-hole liquid formation is definitely excluded. The exciton and excitonic molecule decay rather slowly and govern the optical properties for times longer than 200 ps.     

Exciton-polaritons in halfspace

A rigorous method is presented describing the coupling between an exciton polariton in a halfspace semiconductor and the external driving field. The method is based on density matrix theory. It allows to consider realistic electron-hole interactions, spatial dispersion and extrinsic surface potentials. Without invoking additional boundary conditions or an artificial subdivision of the semiconductor it is shown that the influence of the surface can be isolated from the bulk behaviour. This is accomplished by a symmetric continuation of the restricted configuration space to bulk geometry inspired by the image source method in electrostatics. As a demonstration the solution is worked out for a simplified polariton model. The results are compared with other theories and with experimental reflection spectra.     

Exciton-contribution to the reflection spectrum at high excitation intensities in CdS

We report measurements of changes in reflection spectrum of CdS due to increasing the density of photoexcited carriers at temperatures above the critical temperature for electron-hole liquid formation. The contribution of the exciton resonance is seen to decrease and analysis of the lineshape indicates that this decrease is due to exciton-exciton collision and a change in exciton polarizability. These results are consistent with a transition from exciton to free electron-hole plasma (Mott transition) at a density of n ～ 2.5 x 10¹⁷ cm^-3.     

Transport of degenerate electron-hole plasmas in Si and Ge

At low crystal temperatures, pulsed-laser excitation of Si and Ge can produce a mobile electron-hole plasma with a Fermi energy much larger than k_BT. The motion of this degenerate plasma away from the excitation surface depends intimately on its interactions with high-frequency phonons. Momentum damping and phonon-wind forces are principal factors which determine the plasma motion on nanosecond and longer time scales. A variety of luminescence and heat-pulse experiments are reviewed here which characterize the transport behavior of photo-generated electron-hole plasma in these indirect-gap semiconductors.     

Nonequilibrium properties of electron-hole plasma in direct-gap semiconductors

The gain spectra of the electron-hole plasma recombination in CdS are investigated as a function of the excitation conditions and of the lattice temperature. From a lineshape analysis which includes such many-body effects as collision broadening, single-particle energy renormalization and excitonic enhancement, average plasma parameters are obtained. In contrast to the predictions of quasi-equilibrium theory, one finds that the electron-hole plasma does not reach a full thermal quasi-equilibrium in direct-gap materials because of the short lifetimes of the carriers. The nonequilibrium effects are shown to lead to the formation of electron-hole plasma density fluctuations. No well-defined coexistence region exists. The experimental results in the phase transition region can consistently be explained by theoretical treatments of this nonequilibrium phase transition.     

Stimulated emission and E-H plasma in gallium selenide

The stimulated emission from extremely high quality GaSe crystals is investigated at very high values of excitation intensity by means of a nitrogen laser. The rising of the laser action due to electron-hole plasma recombination is reported. The unsaturated optical gain spectrum confirms the stimulated effect either from excitonic interaction processes or from electron-hole plasma. The role of the surface quality of the samples in the competition among these two amplification mechanisms is particularly discussed.     

Optical nonlinearity of CdS

The discovery and investigation of optically nonlinear behavior of CdS is reviewed. The development is described from nonlinear anti-Stokes excitation via the characterization of inelastic light scattering processes and the identification of high-density phenomena like biexciton creation, excitonic collisions, and electron-hole plasma generation towards very recent time-resolved gain, light-induced grating, and in particular optical bistability experiments. CdS is thus shown to allow for an extremely wide variety of different optically nonlinear processes and may be regarded as a model material for this field. Possible applications show up for several of the described phenomena.     

Strongly confined semiconductor quantum dots: Pair excitations and optical properties

This paper reviews a microscopic model of basic electron-hole pair excitation processes in strongly confined semiconductor quantum dots (QD) and their influence on the optical QD properties. The effects of valence band mixing, Coulomb interaction, and surface polarization are taken into account. The exciton and biexciton wave functions and energies are obtained using a numerical diagonalization method. The computed optical spectra, such as absorption, gain, pump-probe, and two-photon absorption, agree well with experiments.     

Specular Andreev reflection in graphene

By combining the Dirac equation of relativistic quantum mechanics with the Bogoliubov-de Gennes equation of superconductivity we investigate the electron-hole conversion at a normal-metal-superconductor interface in graphene. We find that the Andreev reflection of Dirac fermions has several unusual features: (1) the electron and hole occupy different valleys of the band structure; (2) at normal incidence the electron-hole conversion happens with unit efficiency in spite of the large mismatch in Fermi wavelengths at the two sides of the interface; and, most fundamentally: (3) away from normal incidence the reflection angle may be the same as the angle of incidence (retroreflection) or it may be inverted (specular reflection). Specular Andreev reflection dominates in weakly doped graphene, when the Fermi wavelength in the normal region is large compared to the superconducting coherence length.     

Carrier transport effects and dynamics in multiple quantum well optical amplifiers

The gain recovery dynamics of multiple quantum well semiconductor optical amplifiers, following gain compression caused by ultrashort optical pulse excitation, have been studied for several devices of different structures. Fast, slow, and intermediate time constants are identified. The fast component (0.6 to 0.9 ps) corresponds to cooling of the dense, inverted electron-hole plasma. The slow component (150 to 300 ps) corresponds to replenishment of carriers from the external bias supply, with the dynamics dominated by spontaneous recombination (primarily Auger) of the electron-hole plasma. The intermediate time constant (2 to 14 ps) is caused by carrier capture by the quantum wells and is structure-dependent. For most of the devices, the capture process is dominated by diffusion-limited transport in the cladding/barrier region. The variation of carrier density and temperature also affects the refractive index profile of the devices and, hence, affects the waveguiding properties. Dynamical variation of the mode confinement factor is observed on the fast and slow timescales defined above.     

Influence of the electron-hole density profile on the reflectivity of laser irradiated silicon

We study the influence of the spatial extension of the electron-hole plasma created by a pump pulse on the reflectivity of a probe pulse. We show that the density deduced from reflectivity measurements is the surface density value with a very good accuracy, except very close to the plasma resonance. We also show that the resonance broadening due to the spatial inhomogeneity can be larger than the one due to free carriers absorption and has to be included in the usual experimental determination of the plasma relaxation time.     

基于毫米级单晶石墨烯的倍频器性能研究

石墨烯作为一种拥有高电子迁移率和高饱和速度的二维材料,在射频电子学领域具有很大的应用潜力,引起了人们广泛的研究兴趣.近些年随着化学气相沉积制备石墨烯技术的发展,高质量大尺寸的单晶石墨烯生长技术也愈加成熟.本文基于化学气相沉积生长的毫米级单晶石墨烯,在高介电常数介质上制备出高性能的石墨烯倍频器,并且对其倍频特性做了系统的研究.研究结果表明:在输入信号频率为1 GHz时,倍频增益可以达到-23.4 dB,频谱纯度可以达到94%.研究了不同漏极偏压以及输入信号功率下倍频增益的变化特性,随着漏极偏压以及输入信号功率的增加,倍频增益增加.对具有不同跨导和电子空穴电导对称性的器件的倍频增益和频谱纯度随输入信号频率f_(in)的变化关系进行了研究.结果表明,跨导对于倍频增益影响显著,在f_(in)=1 GHz时器件的频谱纯度差别不大,均大于90%,但是随着f_(in)增加至4 GHz,电子空穴电导对称性较差的器件频谱纯度下降至42%,电子空穴电导对称性较好的器件仍能保持85%的频谱纯度.这是电子空穴电导对称性和电子空穴响应速度共同作用的结果.本文的研究结果对于高性能石墨烯倍频器设计具有一定的指导意义.     

Lineshape theory of the plasmon-phonon sidebands of a semiconductor plasma gain spectrum

The gain spectrum of an electron-hole plasma in direct-gap semiconductors due to plasmon-phonon-assited radiative transitions is calculated for various plasma densities at zero temperature. The screened Coulomb interaction is treated within a damped plasmon-phonon-pole approximation. The validity of this approximation is checked in the simpler case without phonon participation by comparing it with the dynamical random-phase approximation (RPA). For CdS the calculated spectra are in good qualitative agreement with the spectra experimentally observed by Saito.     

Terahertz signatures of the exciton formation dynamics in non-resonantly excited semiconductors

A microscopic theory for the induced terahertz (THz) absorption of semiconductors is applied to study the time-dependent system response after non-resonant optical excitation. The formation of excitonic populations from an interacting electron-hole plasma is analyzed and the characteristic THz signatures are computed. Good qualitative agreement with recent experiments is obtained.     

Stimulated emission processes in highly excited CdS at 80 K

The stimulated emission from CdS at 80 K under high excitation density is studied by means of quasi-resonant dye laser pumping. The evidence of exciton-exciton (P line) and exciton-electron (E line) scattering and, at the highest excitation level, of electron-hole plasma (EHP) recombination are reported and discussed also by means of optical gain measurements.