具有分离门电抽运石墨烯中电子-空穴等离子体的冷却效应

本文研究了室温条件下具有分离门的电诱导石墨烯n-i-p结构中, 与电子和空穴注入有关的粒子数反转效应. 考虑n区横向电场的屏栅效应, 计算了电子-空穴的有效温度与门电压以及光声子的有效温度与门电压的关系, 结果表明注入可以导致n区中电子-空穴等离子体显著冷却, 直至低于晶格温度; 计算了电流-电压特性以及与频率有关的动态电导率, 在一定的电压下, 动态电导率在太赫兹频段可以为负值. 研究表明电子-空穴等离子体冷却能够加强负动态电导率效应, 提高实现太赫兹激射的可行性. 关键词： 石墨烯 n-i-p结构 有效温度 动态电导率     

Electronic structure of warm dense copper studied by ultrafast x-ray absorption spectroscopy

We use time-resolved x-ray absorption spectroscopy to investigate the unoccupied electronic density of states of warm dense copper that is produced isochorically through the absorption of an ultrafast optical pulse. The temperature of the superheated electron-hole plasma, which ranges from 4000 to 10?000 K, was determined by comparing the measured x-ray absorption spectrum with a simulation. The electronic structure of warm dense copper is adequately described with the high temperature electronic density of state calculated by the density functional theory. The dynamics of the electron temperature is consistent with a two-temperature model, while a temperature-dependent electron-phonon coupling parameter is necessary.     

Formation of excitons in semiconductor nanostructures in the presence of electron-hole plasma

We propose a mechanism of increase in the binding energy of an exciton in wide band-gap semiconductors in the presence of optically pumped electron-hole plasma. These excitons with relatively high binding energy (>150 meV) can exist at room temperature when the dielectric constant of semiconductor in the infrared region of spectrum approaches zero. Calculations for CdS show that the density of electron-hole plasma should be higher than 10¹⁹ cm^?3 for formation of such excitons. We show that there exist a considerable number of close-lying energy levels of excitons with high binding energy in the forbidden band of the semiconductor. We guess that these excitons participate in the process of laser generation in optically pumped semiconductor nanocrystals.     

Electron-hole plasma luminescence in GaP

We have studied the photoluminescence spectrum of the electron-hole gas phase in presence of electron-hole liquid in GaP. At low temperature, free excitons are clearly present. With increasing temperatures, i.e. density of the vapour phase, the exciton features evolve into a broad band shifted in energy, which we attribute to an electron-hole plasma. The results are discussed in view of an insulator-metal Mott transition.     

Large excitonic enhancement of optical refrigeration in semiconductors

We present a theoretical analysis for laser cooling of bulk GaAs based on a microscopic many-particle theory of absorption and luminescence of a partially ionized electron-hole plasma. Our cooling threshold analysis shows that, at low temperatures, the presence of the excitonic resonance in the luminescence is essential in competing against heating losses. The theory includes self-consistent energy renormalizations and line broadenings from both instantaneous mean-field and frequency-dependent carrier-carrier correlations, and it is applicable from the few-Kelvin regime to above room temperature.     

Electron-hole liquid in strained SiGe layers of silicon heterostructures

The electron-hole liquid has been found in strained SiGe thin films of Si/Si_1?x Ge_x/Si heterostructures. The density and binding energy of the electron-hole liquid have been determined. Owing to the presence of internal strains in the SiGe layer, the density and binding energy are significantly smaller than the respective quantities for the electron-hole liquid in a bulk single crystal of the solid solution of the same composition. The critical temperature of the transition from the exciton gas to the electron-hole liquid is estimated using the experimental data. The Mott transition (from the exciton gas to electron-hole plasma) occurs above the critical temperatures for high excitation intensities.     

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.     

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.     

Current relaxation processes in highly excited semiconductors

We compare the conductivity relaxation time due to electron-hole scattering to the one due to electron-phonon scattering. We first calculate the variations of both scatterings with the electron-hole plasma density from the dilute to the metal-like regime, using the appropriate screenings of the interaction potentials; we find that, unexpectedly, the electron-hole collisions dominate the electron-phonon scattering, not at very large density, but instead, at intermediate density, when the plasma becomes degenerate.     

Formation of the electron-hole droplet cloud in germanium

We describe luminescence imaging experiments that probe the nature of the phonon wind which transports electron-hole droplets in Ge. The participation of non-equilibrium phonons emanating near the excitation region is supported by: (a) sharp features in the droplet spatial distribution (b) a dependence of the average cloud density on the excitation photon energy, and (c) a rapid initial buildup of the cloud.     

Plasma dynamics in GaAs under strong picosecond surface excitation

We have investigated the time-resolved luminescence of GaAs in air at room temperature, under strong picosecond surface excitation. The energy density was in the range 4–40 $\text{mJ}\text{cm}{}^2$ and the excitation wavelength λ_ex ? 0.53 μ. The peak temperature reached by the generated electron-hole plasma at the end of the excitation pulse amounts to T_M ? 720 K at an energy density of about 10 $\text{mJ}\text{cm}{}^2$. Further increase of the power induces surface damage on the sample. The plasma relaxes its kinetic energy at a slow rate of the order of 10¹⁰$\text{eV}\text{s}$, supporting our previous calculations which concluded that electron-phonon interactions are strongly screened by intravalley free-carrier collisions at high plasma density, so that the plasma cooling slows down.     

Luminescence of high density electron-hole plasma in CdSe at elevated temperature

Luminescence of high density electron-hole plasma in CdSe is observed in the 77–300 K temperature range by picosecond pulse excitation. With increasing temperature from 77 K the stimulated emission band is replaced by the spontaneous emission band. Temperature changes of spectral features of these two bands and also their time dependence after pulse excitation are consistent with the theoretical consideration.     

Formation of a high T(c) electron-hole liquid in diamond

We report on the observation of electron-hole ( e-h) liquid (EHL) in diamond by time-resolved luminescence measurements under an intense femtosecond photoexcitation above the band gap. The EHL luminescence band is observed below the e-h plasma band, showing a finite rise time of several tens of picoseconds. The rise time, which corresponds to the nucleation and the growth of the e-h droplets, plummets on approaching the EHL critical temperature. Time-resolved spectral shape analysis reveals a very high carrier density of 1x10(20) cm(-3) and very high critical temperature of T(c) = 165 K of EHL.     

Light-controlled microstrip line coupler

Conclusions We have analyzed coupling properties of the coupled microstrip lines whose substrate contain electron-hole plasma by means of the spectral domain method. Scattering parameters in each ports have been evaluated numerically as a function of plasma density.Experiments were carried out using high resistivity silicon and LEDs. The agreement of the theoretical and experimental results was satisfactory. The plasma density observed in the experiments is one-third as large as the theoretically estimated value.     

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.     

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.     

BCS wave-function approach to the BEC-BCS crossover of exciton-polariton condensates

The crossover between low and high density regimes of exciton-polariton condensates is examined using a BCS wave-function approach. Our approach is an extension of the BEC-BCS crossover theory for excitons, but includes a cavity photon field. The approach can describe both the low density limit, where the system can be described as a Bose-Einstein condensate (BEC) of exciton-polaritons, and the high density limit, where the system enters a photon-dominated regime. In contrast to the exciton BEC-BCS crossover where the system approaches an electron-hole plasma, the polariton high density limit has strongly correlated electron-hole pairs. At intermediate densities, there is a regime with BCS-like properties, with a peak at nonzero momentum of the singlet pair function. We calculate the expected photoluminescence and give several experimental signatures of the crossover.     

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.     

Cooling of hot electron-hole plasmas in the presence of screened electron-phonon interactions

We have computed for several semiconductors, (GaAs, CdSe, CdS), the energy loss-rate in hot electron-hole plasmas, at high density, when occurs the screening of the electron-phonon interactions. We show that the long range interactions (piezoelectrical and polar) are quickly reduced when the plasma density is raised, and that the energy loss-rate diminishes therefore significantly (one order of magnitude).     

Picosecond buildup and relaxation of intense stimulated emission in GaAs

In support of the idea developed previously based on circumstantial evidence, we have found that stimulated emission emerges in GaAs and its intensity increases with a picosecond delay relative to the front of powerful picosecond optical pumping that produced a dense electron-hole plasma. The emission intensity relaxes with decreasing pumping with a characteristic time of ～10 ps. We have derived the dependences of the delay time, the relaxation time, and the duration of the picosecond emission pulse on its photon energy. The estimates based on the fact that the relaxation of emission is determined by electron-hole plasma cooling correspond to the measured relaxation time.