Time-resolved photoinduced absorption spectroscopic study on trapped carriers at semiconductor–glass interfaces

0.4 Se_0.6 microcrystals was investigated by using time-resolved differential transmittance spectroscopy. The electron trapping at microcrystal–glass interfaces was found to occur within less than 1 ps after photoexcitation. At low excitation energy density, the excited electrons are trapped at point defects distributed over the nanocrystal interfaces. Such electrons give rise to long-lived photoinduced absorption with a lifetime of 3.2 ns. On the other hand, at high excitation energy density, transient absorption with a fast (60 ps) and simultaneously a slow decay component (3.2 ns) was observed. This short-lived photoinduced absorption is attributed to the electrons trapped at the shallow trap states of the semiconductor–glass interfaces. Received: 11 September 1996/Accepted: 9 December 1996     

Time-resolved 2-photon photoionization on metallic nanoparticles

Received: 16 October 1998 / Revised version: 11 December 1998     

Time-resolved thermoreflectivity of thin gold films and its dependence on film thickness

\valunit{10}{nm} to have been measured by femtosecond time-resolved pump-probe experiments. A conspicuous change of the relaxation behavior was found around for pump pulse fluences of . Thicker films show a nearly exponential decay of the transient linear reflectivity, which turns into a linear decay during the first for films with thicknesses of or less. This observation is evidence of a mean free path of about for hot electrons with temperatures around . Received: 17 December 1996     

Time-resolved spectroscopy of soantaneous luminescence of CdSSe1−x quantum dots

Picosecond time-resolved spectroscopy of the edge luminescence band of CdS _x Se_1–x quantum dots with crystallite diameters as small as a few nanometers under band-to-band excitation reveals strong enhancement of the radiative recombination rate compared to bulk CdS owing to quantum confinement. The splitting of the luminescence band into two lines originates from near-band-gap absorption. Analysis of the temperature as well as the spectral dependence of the decay time (leading to a red shift of the luminescence with increasing time) and of the total-light-decay law result in a new model for the dominant radiative recombination channel: donor-acceptor pair recombination instead of an excitonic mechanism as claimed in previous publications.Dedicated to H.-J. Queisser on the occasion of his 60th birthday     

Time-resolved two photon photoemission electron microscopy

Femtosecond, time-resolved two photon photoemission has been used to map the dynamics of photo-excited electrons at a structured metal/semiconductor surface. A photoemission microscope was employed as a spatially resolving electron detector. This novel setup has the potential to visualize variations of hot electron lifetimes in the femtosecond regime on heterogeneous sample surfaces and nanostructures. Received: 22 October 2001 / Revised version: 10 January 2002 / Published online: 7 February 2002     

Ultrafast laser spectroscopy of semiconductor nanocrystals

A large amount of work has been worldwide directed to understand the properties of semiconductor nanostructures. Ultrafast lasers with pulsewidths of a few femtoseconds allowed the investigation of the dynamics of elementary excitations in semiconductor structures on ultrashort time scales. Recent progress in technology made it possible to fabricate semiconductor nanocrystals (i.e. crystals of nanometer dimensions) of well-defined properties. The purpose of this paper is to review the understanding of carrier relaxation and recombination processes in semiconductor nanocrystals as studied by ultrafast laser spectroscopy. The up-to-date techniques of ultrafast laser spectroscopy as well as the fabrication of semiconductor nanocrystals are discussed in some detail.     

Infrared picosecond absorption spectroscopy of microcrystalline silicon: separation between carrier recombination in crystalline and amorphous fractions

We have studied ultra-fast carrier dynamics of photo-excited carriers in hydrogenated microcrystalline silicon prepared by a very high frequency glow-discharge technique. We report on direct observation of two types of dynamics using selective photo-excitation in picosecond pump and probe measurements. One type of the observed dynamics has been found to be independent of the sample preparation, while the other reflects the relative weights of crystalline and amorphous fractions. We propose a simple rate-equation model that describes the carrier dynamics in microcrystalline silicon in terms of the composition of those in Si microcrystallites and in the a-Si:H tissue which surrounds the microcrystallites. The model without any fitting parameters reproduces the experimental data very well when the dynamics are scaled with relative volume fractions as obtained from Raman spectra. Received: 23 November 2000 / Accepted: 17 March 2001 / Published online: 23 May 2001     

Ultrafast dynamics of carrier-induced absorption changes in highly-excited CdSe nanocrystals

We use femtosecond time-resolved transmission spectroscopy to study the density and time dependence of transient absorption changes of CdSe nanocrystals. Our data show pronounced absorption saturation up to complete bleaching of the lowest optical transition. At high carrier density the nonlinear spectra show several peaks that can be related to the two lowest quantized electron states. Thetime dependence of the carrier-induced absorption changes indicates an ultrafast relaxation process within the strongly broadened absorption lines.     

On the theory of relaxation of electrons excited by femtosecond laser pulses in thin metallic films

Received: 14 October 1998 / Revised version: 21 December 1998     

Time-resolved photo-induced nonlinear magneto-optical Kerr effect for the study of spin dynamics at a GaAs(001) surface

Received: 20 September 1998     

Time-resolved two-photon photoemission spectroscopy of HOPG and Ag nanoparticles on HOPG

Received: 6 November 1998     

Shock Induced Emission from Sapphire in High-Pressure Phase of Rh2O3 （Ⅱ） Structure

A distinct optical emission from the Rh203 （Ⅱ） structural sapphire is observed under shock compression of 125, 132, and 143 GPa. The emission intensity continuously increases with the thickness of shocked sapphire. The colour temperature is determined to be about 4000 K, which is obviously smaller than the reported value of the alpha phase alumina at the pressures below 80 GPa. The present results suggest that the structural transformation will cause an obvious change of optical property in sapphire.     

Ultrafast redistribution of vibrational excitation of CH-stretching modes probed via anti-Stokes Raman scattering

Resonant infrared femtosecond light pulses in the 3 μm region are used to excite specific CH-stretching modes of liquid 1,1,1-trichloroethane, cyclopentane, and cyclohexane. The ultrafast redistribution of vibrational excitation between different CH-stretching modes and the vibrational relaxation of these modes are monitored by spontaneous anti-Stokes Raman scattering of properly delayed and spectrally shaped probe pulses. The results are treated with a rate-equation system, yielding energy-redistribution and energy-relaxation times. Received: 16 December 1999 / Published online: 7 August 2000     

Doping-induced symmetry change of superconducting order

Superconductivity in doped cuprate compounds can be associated with different microscopic mechanisms: Coulomb, phonon- and magnon-mediated pairing. These mechanisms simultaneously contribute into superconducting order and their competition determines the symmetry of the order parameter. The magnon-mediated pairing is strongly dependent on doping, therefore, one can expect a change of the symmetry of the order parameter under variation of doping that can result in phase transitions inside the superconducting state.     

Inhomogeneous broadening of the dynamically split Kramers spectral line and up-conversion in the pair and quartet centers in CaF2:Nd

The site-selective and time-resolved fluorescence laser spectroscopy and kinetic measurements with high spectral and nanosecond temporal resolution was applied to analyze the high-energy wing of the M and N absorption bands of the ⁴I_9/2(1)→⁴G_5/2(1) crystal-field (CF) transition in a CaF₂:Nd³⁺ (0.6 wt%) crystal at 4.2 K. It was found that at helium temperatures the dynamically split spectral line assigned as the ⁴I_9/2(1)→⁴G_5/2(1) (CF) transition of coherently coupled Nd³⁺ ions in the pair M- and quartet N-centers of CaF₂:Nd³⁺ (0.6 wt%) is inhomogeneously broadened. It consists of the pair M- and quartet N-centers with at least 0.1 A variation of the positions of the fluorescence-excitation spectral lines registered at the ⁴F_3/2(1)→⁴I_9/2(1) CF transition. Small fluorescence-lifetimes variation of the ⁴F_3/2 and ⁴D_3/2 levels from the small variation of the distances R between Nd³⁺ ions in the pair is found. At least 2.7% variation of the value of the Nd-Nd distance R in the pair M-center was determined from the lifetime variation of the ⁴F_3/2 manifold with the assumption of a dipole-dipole interaction between the ions in the pair.The energy transfer up-conversion process responsible for the UV fluorescence observed when pumping the ⁴I_9/2(1)→⁴G_5/2(1) transition has been determined.     

Pair-density-wave pseudogap and superconducting states in cuprates

Bogoliubov quasiparticle interference and localized high-energy excitations observed in cuprates in nodal and antinodal regions of the momentum space, respectively, would lead to a conclusion that only the nodal region might give rise to superconductivity whereas the antinodal one might be associated with the pseudogap. We argue that both pseudogap and superconducting states arise exactly in the antinodal region with pronounced nesting feature of the Fermi contour as spatially inhomogeneous incoherent and coherent states of pairs with large momentum. The nodal region gives rise to conventional superconducting pairing with zero momentum which, together with the pairing with large momentum in the antinodal region, forms a biordered superconducting state in the whole of the Brillouin zone. This coherent state with complicated momentum dependence of the order parameter manifests itself as a pair-density wave that can exist without any driving insulating order. We believe that quasiparticle interference, other than observed in the nodal region, should be observable in the antinodal region as well.     

Europium ion as a probe for binding sites to carrageenans

Carrageenans, sulfated polysaccharides extracted from red algae, present a coil-helix transition and helix aggregation dependence on the type and concentration of counterions. In this study, we focus attention on a mixed valence counterion system: Eu³⁺/Na⁺ or K⁺ with different gel-forming carrageenans: kappa, iota, and kappa-2. Results of stationary and time-dependent luminescence showed to be a suitable tool to probe ion binding to both the negatively charged sulfate group and the hydroxyl groups present in the biopolymer. For lower europium ion concentrations, a single longer decay emission lifetime was detected, which was attributed to the binding of europium ion to the carrageenan sulfate groups. An additional decay ascribed to europium binding to hydroxyl groups was observed above a threshold concentration, and this decay was dependent on the carrageenan charge density. Symmetry of the europium ion microenvironment was estimated by the ratio between the intensities of its emission bands, which has been shown to depend on the concentration of europium ions and on the specificity of the monovalent counterion bound to the carrageenan.     

Ultrafast all-optical polarization switching based on “virtual excitation” of InGaAs(P) MQWs

All-optical polarization switches based on near-resonant excitation have been demonstrated recently, which operate without significant real carriers excited in MQWs so as to avoid carriers accumulation. In this paper, we focus our investigation on the switch adopting InGaAsP MQWs because it could be compatible with the optical communication system. Our theoretical analysis is restricted to χ³ regime (i.e., the lowest-order nonlinear regime) and based on the dynamics-controlled truncation (DCT) scheme which provides a formalism for studying the coherent dynamics in weakly-nonlinear coherent optics of semiconductors. By using the theoretical model based on DCT theory, the switching action was simulated. With this theoretical model, we study the respective contributions of phase space filling and Hartree-Fock mean field as main terms of the optical Stark effect to the switching process, then exhibit the influence of delay time and control intensity for the switching response.     

Ultrafast motion of liquids C2H4Cl2 and C2H4Br2 studied with a femtosecond laser

Transient optical Kerr effect of liquids C₂H₄Cl₂ and C₂H₄Br₂ is investigated, for the first time to our knowledge, with a femtosecond (fs) probe laser delayed with respect to a coherent fs pump laser. Coherent coupling and electronic Kerr signals are observed around zero delay when pump and probe overlap. Persisting after the pump-probe overlap are Kerr signals arising from the torsional and other intramolecular vibrations of the trans and gauche conformations; Kerr signals arising from the intermolecular motion are also observed. Vibrational quantum interference is only observed in liquid C₂H₄Br₂ and the related beats data are fitted with the torsional vibrations, 91 cm⁻¹ (gauche) and 132 cm⁻¹ (trans), and the CCBr angle-bending vibrations, 231 cm⁻¹ (gauche) and 190 cm⁻¹ (trans), with dephasing times, 0.45 ps, 0.45 ps, 2 ps, and 1.5 ps, respectively. These vibrational frequencies agree with those obtained in the frequency-domain. That no vibrational mode is observed for C₂H₄Cl₂ might be attributed to ineffective Raman-pumping. Kerr signals observed after the pump-probe overlap are Fourier transformed to give the spectra of the intermolecular motion and the vibrational spectrum, which agrees with the one observed in the infrared absorption and/or Raman scattering heretofore.     

Direct nanosecond Nd→Ce nonradiative energy transfer in cerium trifluoride laser crystals

Using third harmonics of LiF:F₂⁺ tunable color center laser excitation and selective fluorescence detection the temperature and concentration dependencies of fluorescence decay curves of the high-lying manifold of the Nd³⁺ ion were measured in CeF₃ crystals. As a result the temperature dependence of energy transfer kinetics from the manifold of the Nd³⁺ donor ions to the manifold of the acceptor Ce³⁺ ions in the ordered practically 100% filled crystal lattice was determined for 13-. Based on the temperature dependence the mechanisms and the channels of the Nd→Ce nonradiative energy transfer have been recognized. The net growth of the resonance Nd→Ce energy transfer rate in the temperature range from 25 to is found to be almost 3 orders of magnitude from 9.0×10⁴ to .In a crystal a significant contribution of the Nd→Nd resonance energy transfer to the manifold quenching is found for 20- and its channel and mechanism are suggested.Discussion of the possibility of subpicosecond and picosecond nonradiative energy transfer in rare-earth doped laser crystals is provided.