Pico-second spectroscopy of highly excited CdSe and CdS—II high density electron-hole plasma

A new luminescence line characterized by a broad spectral width appears under intense pico-second pulse excitation at 4.2 K in both CdSe and CdS. With increasing excitation density the peak of the line shifts to lower energies and the spectral width becomes broader. Just opposite changes are observed with the lapse of time after excitation. It is concluded that the line is due to high density electron-hole plasma.     

Spectroscopy of pico-second light pulses with a television camera system

A television camera system using a high-sensitivity image orthicon is constructed for spectroscopic measurements in the pico-second region. It is possible to fix the spectrum of a relatively weak pico-second light pulse as a static spectrum on the screen of an oscilloscope. Time-resolved luminescence spectra from a heavily-excited GaAs crystal in the pico-second region are demonstrated as examples of measurements using this system.     

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.     

Time-dependent density-functional molecular-dynamics study of the isotope effect in chemicurrents

The energy dissipation into electron-hole pairs has been simulated ab initio within time-dependent density-functional theory for spin-unpolarized hydrogen atoms interacting with the Al on-top site at the Al(1 1 1) surface. The electron-hole pair excitation spectra are characterized by an approximately exponentially decaying tail of the electron energy distribution. It is shown that both the energy dissipated into electron-hole pairs and the excitation spectra, and hence the chemicurrent yield, show an isotope dependence identical to what expected from the linear friction ansatz and the forced oscillator model.     

Time-resolved spectra of spontaneous luminescence from high density electron-hole plasma in CdS

Pico-second time-resolved spectra of the spontaneous luminescence from high density electron-hole plasma in CdS are measured at 4.2 K suppressing the stimulation effect. It is found that after the pulse excitation hot carriers are cooled rapidly for the first 100 psec, and that therafter up to the 400 psec delay time the shape of spectra hardly changes but the spectral width depends on excitation density. Although it seems as if some kind of state like electron-hole liquid is formed, one cannot easily regard that state as the condensed electron-hole drop state.     

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.     

Correlated ejection of electron-hole drops from a continuously generated electron-hole liquid in Ge

The photocurrent power spectra were measured in a Ge photodiode as a function of the incident optical intensity at 1.8 K and 4.3 K. The observed spectra showed a peak at low frequencies superimposed on a continuum which has a cut-off at high frequency. The interpretation of the results implies that the electron-hole drops coming from a same region of the photoexcited liquid are ejected periodically.     

射频磁控溅射法制备高质量ZnO薄膜的激光特性研究

用射频磁控溅射方法在SiO2衬底上制备ZnO薄膜。在室温下观测到了A、B激子吸收以及在19K下发现的A、B、C激子的反射表明所制备的ZnO薄膜具有很好的纤锌矿结构。我们获得了来自于电子空穴等离子体的受激发射。进一步研究我们发现由大量窄峰所组成的激光发射，窄峰的间距都为0.5nm左右。根据理论计算，产生激光发射的自成腔的长度为31.5 μm。我们认为ZnO薄膜中产生激光发射的自成腔的形成与其六角型结构有重要关系。     

All-optical extinction-ratio enhancement of a 160 GHz pulse train by a saturable-absorber vertical microcavity

A vertical-access passive all-optical gate has been used to improve the extinction ratio of a 160 GHz pico-second pulse train at 1555 nm. An extinction ratio enhancement of 6 dB is observed within an 8 nm bandwidth. Such a device is a promising candidate for low-cost all optical reamplication and reshaping (2R) regeneration at 160 Gbits/s.     

Nonresonant two-photon generation of excitonic matter in a CuCl pellet

A pressed CuCl pellet is optically excited at 2 K using an excitation energy in the range from 1892 to 2843 meV, which is far below the bandgap. The steady-state population dynamics unambiguously indicates an unusual two-photon generation of ground-state excitons. At high-excitation levels, the observed spectra exhibit rich spectral features arising from electron-hole plasma and electron-hole droplets formation. This nonresonant two-photon excitation is presumably assisted by impurity bands due to grain boundaries and surfaces in this random semiconductor.     

Luminescence of a quasi-two-dimensional electron-hole liquid and excitonic molecules in Si/SiGe/Si heterostructures upon two-electron transitions

The low-temperature photoluminescence of Si/Si_0.91Ge_0.09/Si heterostructures in the near-infrared and visible spectral ranges is investigated. For the structure in which the barrier in the conduction band formed by the SiGe layer is transparent to electron tunneling, the broad luminescence line observed in the visible range is analyzed by comparing its shape with the numerical convolution of the spectrum of near-infrared recombination radiation originating from the electron-hole liquid. The comparison demonstrates that, at high excitation levels, the visible-range emission is caused by two-electron transitions in a quasi-two-dimensional spatially direct electron-hole liquid. Furthermore, the combined analysis of the photoluminescence spectra in the near-infrared and visible ranges yields the binding energy of a quasi-two-dimensional free biexciton in the SiGe layer of these heterostructures. In the structures with a wide SiGe layer that is not tunneling-transparent to electrons, a spatially indirect (dipolar) electron-hole liquid is observed.     

Measurements of exciton absorption with pico-second light pulses produced by a self-phase modulation technique

Time-resolved absorption spectra of the A exciton in CdSe are measured by using pico-second white-light pulses produced by a self-phase modulation technique. The absorption peak of the n = 1 A exciton shifts to the high-energy side due to the high-density effect. The magnitude of the shift reaches a maximum at 20 psec after the pulse excitation, which is considered to indicate that the time constant for the formation of the n = 1 excitons by band-to-band excitation is about 20 psec.     

Evidence of the exciton-plasma transition in the emission spectra of GaSe

Summary We present luminescence spectra of gallium selenide at 2 K in which a slow and continuous evolution from excitonic recombination to an electron-hole plasma emission is observed when the excitation intensity is increased. We find a small red-shift and a broadening of the direct exciton emission line which is followed by its disappearing. We explain these results with a model which takes into account the electron-hole correlation. Part of this work was carried out during a stage at the Institut de Physique Appliquée of the Ecole Polytechnique Fédérale de Lausanne (Switzerland).     

磁控溅射制备ZnO薄膜的受激发射特性的研究

用射频磁控反应溅射法在二氧化硅衬底上制备ZnO薄膜。得到了在不同温度下ZnO薄膜的吸收与光致发光。观测到了纵光学波 (LO)声子吸收峰与自由激子吸收峰 ;室温 (30 0K)下 ,PL谱中仅有自由激子发光峰。这些结果证实了ZnO薄膜具有较高的质量。探讨了变温ZnO薄膜的发光特性。研究了ZnO薄膜的受激发射特性。     

Reflectance anisotropy spectra of the diamond (100)-(2x1) surface: evidence of strongly bound surface state excitons

We compare the results of ab initio calculations with measured reflection anisotropy spectra and show that strongly bound surface-state excitons occur on the clean diamond (100) surface. These excitons are found to have a binding energy close to 1 eV, the strongest ever observed at a semiconductor surface. Important electron-hole interaction effects on the line shape of the optical transitions above the surface-state gap are also found.     

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.     

Optical properties of highly excited direct gap semiconductors

The electronic excitations in direct gap semiconductors interact strongly with the photon field. We discuss both the experimental and the theoretical aspects of the optical properties of these materials under strong optical excitation. We distinguish between intermediate excitation levels at which the electronic excitations form a dense system of excitons and excitonic molecules and very high excitation levels at which a degenerate electron-hole plasma occurs. The optical spectra of dense excitonic systems, which are mainly observed in copper halides and II–VI compounds, are shown to be determined mainly by the interaction processes between excitonic molecules, polaritons and free carriers. The optical properties of the electron-hole plasma, which has been observed in II–VI and especially in III–V compounds, can be understood only by taking into account many-body effects, such as dynamical screening of the Coulomb interactions, plasmon-assisted transitions and excitonic enhancement.     

Luminescence from silicon-surface metallized by extremely high excitation

Time resolved spectra of photoluminescence in Si at 4.2K and 20K under high excitation levels by a Q-switched laser confirm that a new component exists at the higher energy side of the Haynes line. This component is ascribed to an electron-hole liquid phase extending uniformly over the crystal surface and having larger carrier concentration compared to the equilibrium electron-hole drop.     

Electron-hole coupled plasma mode in the magnetoreflection of graphite under high magnetic fields

A new structure which cannot be explained by any transitions between Landau levels was found in the far-infrared magnetoreflection spectra in graphite under high magnetic fields up to 45 T. The photon energy at which this structure is observed tends to zero at zero field, and it increases superlinearly with increasing magnetic field. The origin of this mode is interpreted in terms of the electron-hole coupled plasma which has never been observed in other substances before.     

Self-consistent Green's function theory for interacting electrons in a random potential

A new set of self-consistent equations is proposed for the calculation of the disorder and Coulomb contributions to the electron self-energy and the electron-hole interaction in a many-electron system in the presence of a site-diagonal random potential. The treatment of the disorder is along the lines of the coherent-potential approximation. An explicit expression for the Coulomb self-energy is derived within the time-dependent screened Hartree-Fock approximation. Some possible applications of the formalism to the formation of a Coulomb gap in a doped semiconductor and to electron-hole interaction effects in the optical spectra of alloy semiconductor systems are discussed.Dedicated to B. Mühlschlegel on the occasion of his 60th birthday