Femtosecond mid-infrared spectroscopy of low-energy excitations in solids

Recent progress in the generation and spectroscopic application of femtosecond pulses in the wavelength range between 3 and 25 m has led to new insight into low-energy excitations of solids, in particular semiconductors, semiconductor nanostructures, and high-T_C superconductors. The ultrafast nonequilibrium dynamics of such excitations has been observed in real-time and the underlying microscopic interactions have been analyzed. This article gives a brief overview of this exciting field of ultrafast science with the main emphasis on pulse generation and applications in semiconductor research where femtosecond intersubband Rabi flopping in quantum wells and quantum coherent electron transport in quantum cascade devices have been observed. PACS 78.67.De; 73.21.Fg; 07.57.Hm; 42.65.Re     

Theoretical description of ultrafast phenomena in clusters

A theoretical study of different ultrafast nonequilibrium processes taking place during and after ultrashort excitation of clusters is presented. We discuss similarities and differences for several processes involving nonequilibrium ultrafast motion of atoms and electrons. We study ultrashort relaxation of clusters in response to excitations produced by femtosecond laser pulses of different intensities. We show how different relaxation processes, such as bond breaking, melting, fragmentation, emission of atoms, or Coulomb explosion, can be induced, depending on the laser intensity and laser pulse duration. We also discuss processes involving nonequilibrium electron dynamics, such as intraband Auger decay in clusters and ultrafast electronic motion during collisions between clusters and surfaces. We show that this electron dynamics leads to Stückelberg-like oscillations of measurable quantities, such as the electron emission yield. Received: 4 April 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Applications of time-resolved terahertz spectroscopy in ultrafast carrier dynamics

Three time-resolved terahertz (THz) spectroscopy methods (optical-pump/THz-probe spectroscopy,THzpump/THz-probe spectroscopy,and THz-pump/optical-probe spectroscopy) are reviewed.These are used to characterize ultrafast dynamics in photo-or THz-excited semiconductors,superconductors,nanomaterials,and other materials.In particular,the optical-pump/THz-probe spectroscopy is utilized to investigate carrier dynamics and the related intervalley scattering phenomena in semiconductors.The recent development of intense pulsed THz sources is expected to affect the research in nonlinear THz responses of various materials.     

Recombination in narrow-gap semiconductors

This review is intended to be an introduction to the recombination mechanisms important for narrow-gap semiconductors. The study of recombination mechanisms gives information on the various interactions the free carriers happen to meet in crystals. On the other hand recombination mechanisms are limiting the performance of all the modern solid state electronic devices. It was recently observed that the dominant recombination processes in narrow-gap semiconductors are different from those important for the classical semiconductors. Due to the narrow band gap the intrinsic carrier density is rather large and in addition the band gap energy is of the same order of magnitude as the energy of elementary excitations as LO-phonons or plasmons, and typical cyclotron-resonance energies. Accordingly processes as the Auger effect or recombination by emitting single phonons or single plasmons become highly probable. These mechanisms have been observed recently and also a new magnetic quantum effect was discovered. The new processes have proved to be important for devices like infrared detectors and infrared solid state lasers, but revealed also some fundamental properties of semiconductors.     

Kinematical and Dynamical Inelastic Light Scattering in Conducting Crystals

Fundamentals of the kinematical quantum theory of inelastic light scattering from electronic excitations in conducting crystals are reviewed. Keeping both the →Δ and →p·→A terms in the photon-electron interaction the general expression for the differential scattering cross-section is obtained. Emphasizing opacity effects and anisotropies arising from the Doppler shift the scattering kinematics is analyzed. The scattering from free-carrier density fluctuations is considered briefly. The basic concepts of a recently established phenomenological theory of dynamical light diffraction in opaque crystals are discussed, and some aspects of a predicted phonon induced anomalous transmission of light below the plasma edge in semiconductors are studied.     

Advances in Mössbauer Emission Spectroscopy

After effects following nuclear transformation have been extensively studied in a large variety of matrices by Mössbauer Emission Spectroscopy (MES). The branching ratios of transient atomic charge states and energetically unstable states of the environment, excitations of electronic ligand field states and populations out of equilibrium within the electronic ground state manifold have been studied. Recent developments and also new insights and understandings of different aspects of the aftereffects of radioactive decay of⁵⁷Co in semiconductors and molecular crystals are reviewed. A detailed picture of the decay process within the electronic states of the nucleogenic Fe³⁺ species could be obtained from emission spectroscopy of sources in applied magnetic fields.     

Unraveling the solid-liquid-vapor phase transition dynamics at the atomic level with ultrafast x-ray absorption near-edge spectroscopy

X-ray absorption near-edge spectroscopy (XANES) is a powerful probe of electronic and atomic structures in various media, ranging from molecules to condensed matter. We show how ultrafast time resolution opens new possibilities to investigate highly nonequilibrium states of matter including phase transitions. Based on a tabletop laser-plasma ultrafast x-ray source, we have performed a time-resolved (～3 ps) XANES experiment that reveals the evolution of an aluminum foil at the atomic level, when undergoing ultrafast laser heating and ablation. X-ray absorption spectra highlight an ultrafast transition from the crystalline solid to the disordered liquid followed by a progressive transition of the delocalized valence electronic structure (metal) down to localized atomic orbitals (nonmetal-vapor), as the average distance between atoms increases.     

Characterization of the electronic excitations in Alq3 using electron energy-loss spectroscopy

We have studied the electronic excitations of tris-(8-hydroxyquinoline)-aluminium(III) (Alq₃) thin films using electron energy-loss spectroscopy in transmission. This allowed us to derive an effective dielectric function of such films in a large energy range. Moreover, an analysis of the momentum dependence of the lowest lying excitation allowed us to gain information on its localization. We show that this excitation does not disperse, i.e., is localized in the condensed phase. In contrast to many other molecular organic semiconductors, its spectral weight does not follow the behavior of a simple dipole-allowed electronic transition.     

Raman spectroscopy for impurity characterization in III–V semiconductors

Raman spectroscopy is discussed as a tool for the quantitative assessment of impurities and defects in III-V semiconductors. After a brief discussion of the effect of defects on Raman scattering by intrinsic phonon modes emphasis is laid on the characterization of individual impurities either via scattering by impurity-induced local vibrational modes or via scattering by internal electronic excitations.     

光散射谱作为研究半导体材料的探针

本文讨论了半导体材料激发的光散射谱,主要包括传导电子和空穴的单粒子跃迁;具有各种施主态和受主态的束缚电子和空穴的跃迁;杂质和缺陷的局域化振动和电子振动跃迁以及诸如等离子体子散射的集体激发。     

Theory of Raman scattering of light on spin-flip and electronic excitation in Europium chalcogenides

The inelastic scattering of light in magnetic semiconductors from the family of the Europium chalcogenides is discussed in relation with spin-orbit coupling and d-f exchange interaction. It is shown that the Raman processes connected with spin-flip and other electronic excitations can occur in the energy range of 0.1-0.5 eV. In this case and for the typical values of d-f exchange interaction and spin-orbit coupling parameter lying between 0.05 and 0.1 eV, the values of the differentional cross-sections are between 10^-9 - 10^-11 cm²sr^-1 sec. The selection rules for the polarization of the incident and scattered photons as well as the Raman tensors for the four allowed transitions are derived. The possibility of applying spin-flip Raman processes in magnetic semiconductors in the tuning of electromagnetic radiation in Raman lasers is analyzed briefly.     

The dispersion of excitons,polaritons and biexcitons in direct-gap semiconductors

Excitons are the energetically lowest excitations of the electronic system in an ideal semiconductor at zero temperature. If the excitons couple to the electromagnetic field, a mixed state is formed, the quanta of which are called excitonic polaritons. Associates of two excitons, so-called biexcitons, have been observed in many semiconductors. Excitons are known for about forty years. During the first three decades, they have been investigated mainly by the classical spectroscopic methods, i.e., reflection, transmission and luminescence spectroscopy. In the last decade, several new techniques have been developed, which allow for a direct spectroscopy in momentum space. In this contribution, we review these novel techniques, both linear and nonlinear ones, and present results obtained for excitons, polaritons and biexcitons. The review is restricted to semiconductors which have their conduction band minimum and their valence band maximum at the same point of the Brillouin zone (direct-gap materials) and which have a band to band transition which is dipole allowed.     

Light scattering as a form of modulation spectroscopy

In the conventional modulation spectroscopy the optical constants of a solid are modulated by means of an external parameter (stress, electric field, etc). As a result of the presence of elemental excitations in the solid (usually phonons) a spontaneous modulation of the optical constants takes place at the frequency of the excitations. This modulation can be observed in light scattering experiments (Raman and Brillouin). Such measurements yield information about the frequency and density of states of the excitations (conventional Raman and Brillouin measurements), about their interaction with electronic states, and about the electronic transitions themselves (resonant scattering experiments). The theory of resonant Raman and Brillouin scattering in semiconductors will be discussed in connection with that of other modulation experiments (piezoreflectance, stress induced birefringence). Another closely related phenomenon, that of spatial-dispersion-induced birefringence will also be presented.     

超快速激光光谱学技术在晶态半导体低维材料中的应用

本文结合作者近年来的工作，评述了用超快速激光光谱学技术对晶态半导体低维材料研究的进展，分别讨论了用超快光谱研究晶态半导体低维材料中载流子各种非平衡过程的优势和原理，提出了需要进一步探讨的方面。     

Kinetics of ultrafast migration-accelerated quenching in nanoparticles

It has been analytically shown that the kinetics of ultrafast migration-accelerated quenching of luminescence in nanoparticles for the case of the short-pulse-induced excitation of donors has a complex multistage character. This conclusion has been confirmed by Monte Carlo computer simulation. An order stage (exponential in time) has been observed at small times. However, in contrast to a bulk crystal, quenching in nanoparticles is exponential only to a depth determined by the average number of donors in nanoparticles. Then, differences in the quenching rates of excitations in individual nanoparticles lead to the onset of a disordered stage of ultrafast quenching, which satisfies a t ^3/S time law for the S multipole interaction type. The quenching rate of excitations at the disordered stage depends on the average number of donors in nanoparticles. In this property, ultrafast quenching in nanoparticles qualitatively differs from that in a bulk crystal: the ultrafast quenching rate in the bulk crystal is independent of the donor subsystem parameters as a whole and the donor concentration in particular. The time of the transition between stages has been determined.     

First principles study of ceramic materials (IVB group carbides) under ultrafast laser irradiation

Group IVB carbides have been applied in extreme aerospace environments as hard ceramic coatings; ZrC is being considered as a replacement for SiC in nuclear reactors. Therefore, a thorough understanding of the laser irradiation response of group IVB carbides is of clear significance. However, the existing knowledge on the fundamental properties of IVB group carbides is limited and insufficient with regard to both irradiated and non-irradiated characteristics. We investigate the effect of ultrafast laser irradiation on the lattice stability of ceramic materials(IVB group carbides) using the density functional perturbation theory(DFPT). The calculated phonon frequencies of TiC and ZrC at the ground state are in good agreement with previous calculations and experimental values. The phonon frequencies of IVB group carbides are positive, even though the electronic temperature reached 5 eV. Thus, IVB group carbides are more stable under ultrafast laser irradiation, which has greater benefits in nuclear and aeronautical applications compared to metals(W,Na), semimetals(Bi),and semiconductors(Si,SiC). The thermodynamic properties of ZrC are calculated as functions of their lattice temperature at different electronic temperatures. The elastic shear constants of IVB group carbides satisfy the Born stability criteria at Te = 5 eV. In addition, a comparison of the predicted melting temperatures of IVB group carbides, reveal that HfC is better suited for extreme high-temperature environments.     

The 2644-Å band system of fluorobenzene. The hot-band spectrum

The 2644-Å band system of fluorobenzene and two deuterated isotopes has been photographed and a vibrational analysis proposed. The present paper is limited to the hot-band spectrum. Several in-plane fundamental vibrations in the ground electronic state can be identified, and these agree quite well with values obtained from infrared spectroscopy. The major sequence bands have also been identified. Anharmonic coupling between vibrations 11 and 16a plays a major role in these isotopes, and this is described in detail.     

Elementary relaxation processes investigated by femtosecond photoelectron spectroscopy of two‐dimensional materials

Elementary scattering processes in solid matter occur on ultrafast timescales and photoelectron spectroscopy in the time domain represents an excellent tool for their analysis. Conventional photoemission accesses binding energies of electronic states and their momentum dispersion. The use of femtosecond laser pulses in pump‐probe experiments allows obtaining direct insights to the energy and momentum dependence of ultrafast dynamics. This article introduces the elementary interaction processes and emphasizes recent work performed in this rapidly developing field. Decay processes in the low excitation limit are addressed, where electrons decay according to their interaction with carriers in equilibrium. Here, hot electron relaxation in epitaxial metallic film is reviewed. In the limit of an intense optical excitation, scattering of the excited electrons among each other establishes a non‐equilibrium state. Results on charge‐density wave materials and the effect of coherent nuclear motion on the electronic structure, which can break low symmetry ground states, are discussed. Figure reprinted with permission from [71].     

Optical and luminescent VUV spectroscopy of La2Be2O5 crystals

Electronic excitations and the processes of their radiative relaxation are studied in pure and Ce³⁺ ion-doped crystals of lanthanum beryllate excited by synchrotron radiation in the x-ray and VUV ranges by methods of optical and luminescent vacuum ultraviolet time-resolved spectroscopy. Manifestations of excitons of the valence band are absent in the reflection spectra. However, a fast (τ=1.7 ns) and a slow (microsecond range) channel corresponding to two possible types of self-trapped excitons (STE) are found in radiative relaxation of intrinsic electronic excitations at T=10 K. The slow channel corresponds to emission of STE formed through recombination, the fast channel corresponds to emission of relaxed metastable excitons from the STE state. In the energy region higher than 14 eV (E>2E _g), the effect of multiplication of electronic excitations due to generation of secondary electron-hole pairs resulting from inelastic scattering of both hot photoelectrons and hot photoholes is exhibited.