Time-resolved spectroscopy is performed in bulk samples of single wall carbon nanotubes. The carrier dynamics is monitored by probing the transient change of transmission of the sample in resonance with the interband transitions of the semiconducting tubes. Samples containing either isolated nanotubes or bundles of nanotubes are investigated. A strong increase of the relaxation time is observed for isolated nanotubes. We interpret this effect with a model based on a tunnel coupling between semiconducting and metallic nanotubes within a bundle. This model also accounts for the quenching of the luminescence in such samples.

Studies on the optical properties related to built-in internal electric field and carrier localization present in the GaN self-assembled quantum dots (QDs) are essential for the physical interest in atomic-like confined system and the visible and ultraviolet light emitting applications. We have systematically studied the optical properties of hexagonal GaN (h-GaN) and cubic GaN (c-GaN) self-assembled QDs by means of photoluminescence (PL), PL excitation (PLE), cathodoluminescence (CL), and time-resolved PL experiments. The GaN self-assembled QD samples were grown in Stranski-Krastanov mode by plasma-assisted molecular beam epitaxy. The substrates for the growth of h-GaN and c-GaN were 6H-SiC and 3C-SiC, respectively. With increasing temperature, the PL intensity of GaN quantum wells was dramatically decreased while that of GaN QDs was not changed much. From the wavelength-resolved CL images, strong carrier localization in the QD confinement was clearly observed. An apparent Stokes-like shift between PLE absorption edge and PL emission from the h-GaN QDs increases with increasing detection wavelength (so, with QD size), which is attributed to the separation of wavefunction overlap due to the built-in internal field present in the QDs. From the time-resolved PL experiments, we found that the measured lifetime of the h-GaN QDs emission increased with emission wavelength (i.e., with QD size), while that of the c-GaN QDs kept almost constant. It is concluded that the h-GaN QD emissions are strongly influenced by built-in internal electric field as well as carrier localization in the QDs.

Excitonic and spin excitations of single semiconductor quantum dots currently attract attention as possible candidates for solid state based implementations of quantum logic devices. Due to their rather short decoherence times in the picosecond to nanosecond range, such implementations rely on using ultrafast optical pulses to probe and control coherent polarizations. We combine ultrafast spectroscopy and near-field microscopy to probe the nonlinear optical response of a single quantum dot on a femtosecond time scale. Transient reflectivity spectra show pronounced oscillations around the quantum dot exciton line. These oscillations reflect phase-disturbing Coulomb interactions between the exitonic quantum dot polarization and continuum excitations. The results show that although semiconductor quantum dots resemble in many respects atomic systems, Coulomb many-body interactions can contribute significantly to their optical nonlinearities on ultrashort time scales.

The spectroscopic characteristics of GaSe nanoparticle aggregates are reported. We find that the lowest energy absorption band shifts slightly to the red and sharpens, while the next band shifts to the blue as the concentration is increased. The spectral changes are reversible and are interpreted in terms of the particles forming strongly interacting aggregates in high concentration, room temperature solutions. The absorption spectra can be semiquantitatively modeled in terms of the lowest two transitions observed in bulk GaSe, quantum confinement effects and dipolar coupling between excited state monomers. From these calculations, the lowest excited state interparticle coupling is estimated to be about -250 cm-1. Polydisperse samples have larger energy differences but comparable couplings between adjacent particles in the aggregates. As a result, the spectroscopic effects of aggregation are less pronounced in these samples. The nanoparticle aggregate spectra are reminiscent of J-aggregate spectra of organic dyes.

We use different theoretical approaches to demonstrate the qualitative and quantitative understanding of coherent electron tunneling transport through several organic molecules. Molecules that we consider are phenylenevinylene oligomer (OPV5), carotene, dithienylethene, and xyxyldithiol. The complex bandstructure technique is useful for molecules that have repeating units, and is used here to make estimates of the conductance and its dependence on molecular length for OPV5 and for carotene (in a charge state). For molecules of a general shape, such as that of the photoswitching dithienylethenes, we use Landauer theory to predict the I-V properties. The same analysis is used to study the conductance of xyxyldithiol molecules that are deformed by stretching as in AFM pulling experiments.

The dynamics of the bound and free excitons and exiton polaritons of the ZnO nanorods have been investigated by time resolved photoluminescence in the temperature range from 10 K to 300 K. The samples have been fabricated by catalyst-free metal organic chemical vapor deposition (MOCVD), and have a diameter 35 nm and lengths in the range of 150 nm to 1.1 μm. In the temperature range of 10 K to 50 K, the photoluminescence lifetime of the bound exciton increases as the temperature increases. Photoluminescence lifetime of the free excitons, however, decreases with the temperature. The low temperature (10 K) time resolved photoluminescence spectra reconstructed from the time profiles measured at different frequencies clearly show that the bound exciton decay faster than the free A exciton. This result may be due to the transition from the bound exciton to free exciton because of the local temperature increase. Free B exciton is dominant above 50 K, and forms exciton polariton at high temperatures. At low temperature, photoluminescence lifetimes of the free A and B excitons do not show a clear correlation with the length of the nanorods. At room temperature, however, the photoluminescence lifetime increases monotonically as the length of the nanorods increase in the range of 150 nm to 600 nm. Decrease of the radiative decay rate of the exciton polariton has been invoked to account for the results.

Ultrafast excitation of nonequilibrium quasi-free electrons by a femtosecond pulse and the first steps of their relaxation are studied in noble metal nanoparticles. Coherent electron-optical pulse coupling in or our of resonance with the surface plasmon resonance is shown to decay by single electron excitation (Landau damping) on a sub- 10 fs scale. This process is detected by monitoring transient depopulation of electron states well below the Fermi energy, in agreement with the free-electron optical absorption model. The subsequent loss of energy of the created nonequilibrium electron distribution show a particle size dependent slowing-down of the energy transfer to the lattice on a subpicosecond scale. These results are in quantitative agreement with numerical simulations of the athermal electron kinetics.

The optical properties and associated carrier dynamics of self-organized InP quantum dots embedded in a GaP matrix are presented and discussed, together with their growth and structural properties. InP deposited on GaP (001) using gas-source molecular-beam epitaxy forms Stranski-Krastanow quantum dots for an InP coverage greater than 1.8 monolayers. The size of dots is dependent on the growth conditions; supercritical InP deposition under a sufficiently high phoshine flux results in relatively small (≈20 nm) and dense (≈ 5 × 10⁹ dots/cm²) dots with intense optical emission. The photoluminescence from the quantum dots is observed up to room temperature at about 2 eV; photoluminescence from the strained two-dimensional InP wetting layer peaks at about 2.2 eV. Modeling based on the “model-solid theory” as well as time-resolved photoluminescence indicate that the band alignment for the InP wetting layer is indirect and probably type II; this emission results from spatially indirect recombination of electrons in the GaP X valley with holes in the InP and their phonon replicas. The band alignment of InP quantum dots, however, is type I. Whereas low-temperature time-resolved photoluminescence measurements indicate a rather long carrier lifetime of about 25 ns for the wetting layer, the carrier lifetime in the quantum dots is about 2 ns, typical for type-I quantum dots. Pressure-dependent photoluminescence measurements provide further evidence for a type-I band alignment for InP/GaP QDs at normal pressure, but indicate that they become type II under hydrostatic pressures of about 1.2 GPa and are consistent with an energy difference between the lowest InP and GaP states of about 31 meV. Exploiting the visible direct-bandgap transition in the GaP system could lead to an increased efficiency of light emission in GaP-based light emitters.

Surface plasmon excitation in metal nanoparticles has found great interest in the past. This collective oscillation of the conduction electrons can be stimulated with light and is characterized by resonances, whose positions depend on the material, the dimensions and the dielectric surrounding of the particles. Recently, the investigation of the ultrafast dephasing time T₂ and of the decay mechanisms of surface plasmon excitation have become of particular importance, an essential reason being that T₂ is proportional to the enhancement factor of the electric field in the vicinity of the nanoparticle surface. This enhancement plays a key role in a great variety of applications. The present paper presents an overview of our recent experiments on the ultrafast decay of surface plasmon excitation, in particular by using a technique that allows us to measure the homogeneous line widths of surface plasmon resonances in the presence of inhomogeneous broadening and thus determine T₂. The method is based on persistent spectral hole burning in the absorption profiles of supported metal nanoparticles by nanosecond laser pulses. The technique has been systematically applied to silver and, more recently, to gold nanoparticles on different substrates. Size and shape dependent dephasing times ranging from 2.6 to 15 fs have been extracted from the experimental results using a theoretical model. The values reflect the reduced dimensions of the nanoparticles and we conclude that additional damping mechanisms, in particular surface scattering and chemical interface damping, come into play.

Multiple quantum well (MQW) structure piezoelectric semiconductor can be treated as a piezoelectric transducer to generate nanometer wavelength and THz frequency acoustic waves. The generation mechanism of nano acoustic wave (NAW) in quantum wells induced by femtosecond optical pulses can be modeled by a macroscopic elastic continuum theory. The absorption of the MQW's modulated by NAW's through quantum confined Franz-Keldysh (QCFK) effect allows another femtosecond optical probe pulses to monitor the propagating NAW. Many applications in typical ultrasonics can be achieved by NAWs. The simultaneous waveform synthesis is demonstrated by an optical coherent control technique. The phase of the totally reflected NAW is studied. Acoustic coherent control can be achieved by designing the thickness of the cap layer on the MQW. We also demonstrated the feasibility to apply THz NAWs to acoustically control an electronic device with higher operation speed and spatial accuracy. Seismology, which is the first step toward ultrasonic imaging, was also demonstrated. The arrival time of the echo is obtained by processing the transient transmission changes of the probe. Ultrafast technique and nano technology are ready for nano ultrasonics.

The propagation of picosecond duration heat pulses in single wall carbon nanotubes has been investigated using Molecular Dynamics simulations. It is found that the picosecond heat pulse in (10,0) and (5,5) induces several waves that propagate at different propagation speeds. The leading waves move at the speed of sound corresponds to LA phonons, followed by waves moving at TW phonon modes. The heat energy content in the waves corresponding to LA phonon modes in (10,0) zigzag nanotubes is significantly larger than in (5,5) armchair nanotubes.

The nonlinear optical response of carbon nanotubes (CNTs) to the interaction with intense ultrashort laser pulses was studied theoretically and experimentally. A full quantum-mechanical theory for harmonics generation from a single-walled CNT has been developed, using the quantum kinetic equations for π-electrons with both intraband and interband transitions taken into account. In the regime of strong driving fields, a non-perturbative approach with the numerical solution of the quantum kinetic equations in the time domain was used to calculate the density of the axial electric current in CNTs. The results of this theory are compared to experiments performed on samples of multi-walled CNTs, using pulses of 160 fs generated by a Cr:Forsterite laser, at a wavelength of 1250 nm. The experimental results show indeed an unusual nonperturbative behavior of the third-harmonic yield, for relatively low input laser fields of ~ 10¹⁰ W/cm², in very good agreement with the theoretical predictions. The interaction of CNTs with strong laser fields results not only in the generation of harmonics, but also in the generation of a broad spectral background. Generation of a continuous background in the vicinity of the third-harmonic of the laser field was also obtained from the quantum-mechanical calculations, however, with lower intensities than observed experimentally. Possible explanations for this discrepancy will be discussed.

Single-Wall Carbon Nanotubes (SWCNT) have stimulated extensive attention due to their extraordinary electronic properties. Unlike past studies of SWCNT where the tubes were agglomerated in bundles, or, suspended in a solution only recently, we were able to grow well-separated individual SWCNT in a controlled fashion. Moreover, unlike their counterparts, these SWCNT are strongly chiral and semiconductive. Ordered arrays of nano-size spheres (photonic crystals) attracted the attention of many researchers for their linear and nonlinear properties. For example, one is able to design and realize imaging elements thinner than the propagating wavelength or, use these highly dispersive structures to compress or, broaden ultra-short pulses. The optically confining environment of these three-dimensional, periodic structures is particularly attractive when it is imbedded with nonlinear material such as, SWCNT. In this talk I will review experimental results obtained for SWCNT. These tubes were encapsulated in polymers, suspended in solutions or, grown within the voids of photonic crystals. The experiments were conducted at the visible, near IR and THz frequency range using CW, nanosecond and femptosecond pulsed lasers. Overall, SWCNT exhibited a large nonlinear characteristic, which is associated with very short-lived photo-induced carriers.

We present a study of the optical properties of different GaN/AlGaN multiple quantum well heterostructure samples. In particular, a comparative study of room temperature intersubband electron scattering lifetimes in a variety of single and coupled quantum well samples is presented for different excitation powers and wavelengths. All samples were grown by molecular beam epitaxy on sapphire substrate. Typical quantum well widths of ~10 to 15 Å were used together with AlN bulk and superlattice barriers with an AlN-mole fraction of ~0.65 and ~0.9, respectively. In absorption measurements, the single quantum well samples show intersubband absorption peaks ranging from ~1.4 to ~1.7 μm wavelength. Two absorption peaks at ~1.75 and ~2.45 μm were measured for an asymmetric coupled double quantum well structure. The intersubband electron scattering lifetimes were determined with time resolved pump-probe measurements using pump and probe beams with a pulse width of ~130 fs and wavelengths of ~1.55 and ~1.7 μm, respectively. For the single quantum well samples, intersubband scattering lifetimes of ~260 to ~300 fs have been measured. For the upper excited state of the coupled double quantum well, a lifetime of ~230 fs has been found.

Ultrashort strain pulses are a promising tool for the analysis and manipulation of condensed matter, thin films, and nanostructures. We present a new and unconventional way to generate coherent longitudinal acoustic wavepackets of high amplitude in the THz frequency range using the nonlinear development of picosecond strain pulses in a crystal. Our work [PRL 89, 285504 (2002)] demonstrated breakup of an initial wavepacket into a train of ultrashort strain solitons, using position-dependent Brillouin scattering. We extend in this paper the interpretation of the Brillouin scattering data in terms of optical Bragg reflections off the moving soliton train, using the analogy with an N-slit diffraction grating. Finally, we show that these short pulses can excite an electronic two-level system at THz resonance frequency, allowing for coherent amplification and even the development of a phonon laser.

In this study, stimulated emission (SE) and ultrafast carrier relaxation are explored in InGaN and AlGaN/GaN multiple quantum wells (MQWs). The SE threshold densities (I_th) in InGaN MQWs increase with increasing QW depth. By contrast, no significant variation is observed in AlGaN/GaN MQWs with varying barrier height and growth conditions (Ga-rich and N-rich). Wavelength non-degenerate time-resolved differential transmission (TRDT) measurements reveal that increased non-radiative recombination and fast capture of carriers to the localized states below the SE energy in deeper InGaN MQWs are responsible for the increased I_th. At high excitation densities SE is shown to remove carriers efficiently from the QWs with a time constant of a few picoseconds, causing carriers at higher energies to cascade down and refill these SE-emptied states. The strength and decay times of the SE feature, which are resolved from the spectrally integrated TRDT data, are seen to vary as a function of excitation energy and density. The fast, SE-accelerated decay in AlGaN MQWs occurs more than twice as fast as in InGaN MQWs for similar excitation densities. More importantly, recombination times are an order of magnitude faster in AlGaN MQWs than in InGaN MQWs.

The high instantaneous powers associated with femtosecond lasers can color many nominally transparent materials. Although the excitations responsible for this defect formation occur on subpicosecond time scales, subsequent interactions between the resulting electronic and lattice defects complicate the evolution of color center formation and decay. These interactions must be understood in order to account for the long term behavior of coloration. In this work, we probe the evolution of color centers produced by femtosecond laser radiation in soda lime glass and single crystal sodium chloride on time scales from microseconds to hundreds of seconds. By using an appropriately chosen probe laser focused through the femtosecond laser spot, we can follow the changes in coloration due to individual or multiple femtosecond pulses, and follow the evolution of that coloration for long times after femtosecond laser radiation is terminated. For the soda lime glass, the decay of color centers is well described in terms of bimolecular annihilation reactions between electron and hole centers. Similar processes appear to operate in single crystal sodium chloride. We report also fabrication of permanent periodic patterns in soda lime glass by two time coincident femtosecond laser pulses.

We discuss the generation mechanism of THz radiation and acoustic phonon pulse wave under ultra short pulse excitations in GaN-based light emitting diode (LED) structures containing InGaN/GaN multiple quantum wells. In order to understand the role of piezoelectricity in the THz radiation and acoustic phonon pulse wave generations, an external field was applied in these structures so that the piezoelectric field in the quantum wells was compensated under an external reverse bias. Coherent acoustic phonon pulse wave was found to be independent of the applied voltage, although the strain of the InGaN layers was crucial for the generation of the signals. The THz emission from these structures was found to increase with increasing reverse voltage and excitation energy, similar to the trend of the photocurrents in these structures. The bias and wavelength dependence of the THz generation suggests the carriers associated with the photocurrents are responsible for the THz radiation.

Si and Mg-doped AlN epilayers were grown by metal-organic chemical vapor deposition (MOCVD) on sapphire substrates. Deep ultraviolet (UV) picosecond time-resolved photoluminescence (PL) spectroscopy has been employed to study the optical transitions in the grown epilayers. The donor bound exciton (or I₂) transition was found to be the dominant recombination line in Si-doped AlN epilayers at 10 K and its emission intensity decreases with increasing Si dopant concentration. Doping induced band-gap renormalization effect has also been observed. Time-resolved PL results on Si-doped AlN revealed a linear decrease of PL decay lifetime with increasing Si dopant concentration, which was believed to be a direct consequence of the doping enhanced nonradiative recombination rates and corroborated the PL intensity results. For Mg-doped AlN epilayers, two emission lines at 4.70 and 5.54 eV have been observed at 10 K, which were assigned to donor-acceptor pair transitions involving Mg acceptor and two different donors (one deep and one shallow). From PL emission spectra and the temperature dependence of the PL emission intensity, a binding energy of 0.51 eV for Mg acceptor in AlN was determined. Together with previous experimental results, the Mg acceptor activation energy in AlGaN as a function of the Al content for the entire AlN composition range was obtained. The average hole effective mass in AlN was also deduced to be about 2.7 m₀ from the experimental value of Mg binding energy together with the effective mass theory. Although Mg acceptors are an effective mass state in ultra-large bandgap AlN, as a consequence of this large acceptor binding energy of 0.51 eV, only a very small fraction (about 10^-9) of Mg dopants can be activated at room temperature in Mg-doped AlN. Decay lifetimes of these emission lines are also measured as functions of emission energy, temperature, and excitation intensity. The implications of our finding on the applications of AlN epilayers for many novel devices will also be discussed.

We examine ultrafast photoconductivity in functionalized pentacene single crystals using optical-pump terahertz-probe techniques. The 0.5 ps rise time observed in the photoconductive transients, which is limited by the response time of the terahertz pulse setup, suggests that mobile charge carriers are a primary photoexcitation. The peak of the photoconductive signal increases as the temperature decreases due to higher carrier mobilities at lower temperatures. A lower limit for the carrier mobility of 1.6 cm²/Vs at 10 K and 0.2 cm²/Vs at room temperature is obtained. We further show that the absorption edge near the pump excitation wavelength of 800 nm remains temperature independent, and is therefore not a contributing factor in our observation of larger transient signals at lower temperatures. After an initial fast decay, a power-law decay is observed in the tail of the transient response from 2 to 600 ps. The dependence of the photoconductive response on the pump fluence and the electric field amplitude of the terahertz pulse are examined. Finally, we show some preliminary results of transient photocurrent measurements on contact-biased samples using a fast oscilloscope with a system rise time of about 50 ps.

We report the study of far-infrared dielectric properties of a polar crystal using the THz time domain spectroscopy. We measured the power and phase transmission spectra by coherent THz radiation. The anisotropy of dielectric properties has been investigated simply by rotating the polarization plane of an incident beam. The frequency dependence of complex dielectric constant has been determined without the Kramers-Kronig analysis. The complex dispersion relation of phonon polariton frequency and damping has been also determined. These far-infrared properties have been consistently reproduced by the damped harmonic oscillator model related to the lowest frequency polar mode.

THz time-domain spectroscopy (THz TDS) with ultrafast photo-excitation is applied to probe the complex conductivity of the charge carriers in sapphire over the temperature range of 40 - 350 K. A comparison of the measured complex conductivity to the Drude model yields the carrier scattering rate and density. The dependence of the carrier scattering rate on temperature and sample purity is used to identify the scattering mechanisms in sapphire. In the higher temperature range, scattering is determined by intrinsic phonon processes, but impurity scattering becomes dominant at low temperatures in typical optical-grade samples. In high-purity samples, however, impurity scattering remains negligible down to 40 K, and carrier mobilities exceeding 10,000 cm²/Vs can be achieved.

Structural dynamics of 300-ps laser irradiated semiconductor is studied by means of picosecond time-resolved X-ray diffraction. Picosecond pulsed X rays are generated by focusing intense femtosecond laser beams onto metal target. Time-resolved X-ray diffraction is performed by a laser pump and X-ray probe technique. Lattice expansion due to acoustic phonon generation and propagation is observed in a silicon crystal in a single laser shot experiment at laser energy density of 1.0 J/cm². On the other hand, in a multiple laser shot experiment, lattice compression due to laser shock compression is observed at 1.4 J/cm².

We have generated pulsed beams of longitudinal and transverse polarized acoustic phonons by ultra fast optical excitation of gallium arsenide/aluminium arsenide superlattice structures. The phonons propagated ballistically over macroscopic (~ mm) distances at low temperatures and were detected using superconducting bolometers. We used superlattice phonon filters and the frequency-dependent phonon scattering in gallium arsenide to analyse the phonon spectrum. The phonons were found to be monochromatic, with a centre frequency given by υ = c_s/d_SL, where c_s is the phonon speed and d_SL is the superlattice period, and having a spectral line width (full width at half maximum) of less than 50 GHz. We measured a mean free path of 0.8 mm for both the longitudinal and transverse modes, consistent with point defect scattering in the GaAs substrate. Such phonons, with frequencies in the THz range, have potential applications in a number of areas, e.g. acoustic microscopy of solid-state nanostructures.

Due to its very short carrier lifetime and its absorption window in the 1550nm spectral range, ion-irradiated InGaAs is a material of choice for opto-electronic telecommunication systems. Ion irradiated-InGaAs is a well adapted material for realizing fast saturable absorbers and fast photoconductive antennas. However, to our knowledge, no detailed experimental study has been reported on the thermal stability of ion-irradiated InGaAs. Post-irradiation annealing of such a material is required to enhance opto-electrical response, and the thermal stability of irradiated devices. Moreover, the study of annealing kinetics provides useful information about the nature of defects and their initial distribution. The carrier lifetime, the mobility and the residual carrier concentration versus anneal in heavy(Au⁺) and light(H⁺) ion-irradiated InGaAs samples have been measured. The defect annealing kinetics observed in proton-irradiated samples is described well by a Frenkel pair recombination model, thereby indicating the dominance of isolated point defects. In contrast, the model is not adapted to describe the thermal behavior of Au⁺-irradiation-induced defects that are clusters of point defects as observed via Transmission Electronic Microscopy. A higher thermal stability for the components based on Au⁺-irradiated InGaAs than on H⁺-irradiated ones is then expected.

We report both temperature and excitation density dependent, four wave mixing measurements on Si-D stretch vibrations in deuterated amorphous silicon thin films. Utilising the infrared output of a free electron laser (FEL), we have made transient grating measurements of the temperature dependent anharmonic decay rate of Si-D stretch vibrations in deuterated amorphous silicon. Unlike Si-H vibrations, it is round that the excited deuterium mode relaxed with a single exponential decay rate into collective modes of the host, bypassing the local bending modes. Vibrational photon echo measurements suggest that phase coherence is lost via elastic phonon scattering with excitation (but not temperature) dependent contributions from non-equilibrium phonons. The degradation of p-i-n solar cells with identical intrinsic absorber layers (to those used for the time domain experiments) under prolonged light soaking treatments show that α-Si:D has a superior resistance to light induced defect creation.

Two-dimensional (2D) Fourier-transform optical spectroscopy is demonstrated on GaAs quantum wells. This technique represents a highly enhanced version of transient four-wave mixing (FWM). The 2D spectra are generated by measuring four-wave mixing signal fields as a function of the time after the third pulse and the time delay between the first two excitation pulses. Signal fields are measured, including phase information, by spectral interferometry. Active stabilization of the interferometer allows us to measure the phase of the emitted signal as a function of the phase between the first two excitation pulses. This enables the implementation of the Fourier transform analysis. Our spectra show the light-hole and heavy-hole exciton transitions on the main diagonal as well as coupling between those two levels as off-diagonal peaks.

Coherent phonons of semiconductor-metal interfaces are impulsively generated and detected with time-resolved second-harmonic generation. Coherent longitudinal optical phonons are launched in the near surface depletion regions of GaP/Au and GaAsP/Au Schottky photodiodes. Photoexcited electrons ballistically transport from the metal layer into the semiconductor region rapidly screen the near surface depletion field and launches these coherent LO phonons. The dephasing of these coherent LO phonons is mainly due to the anharmonic decay at zero or reverse bias. Their dephasing times decrease significantly as a forward bias is applied to the heterointerface. This effect is attributed to the strong carrier-phonon scatterings induced by the electrical driven electrons flowing across the heterointerface. Meanwhile, coherent longitudinal acoustic wave packet is observed in the GaP/Au heterointerface via transient thermal absorption in the gold thin layer. This acoustic wave packet propagates into the GaP bulk with the sound velocity ~5.8×10⁵ cm/sec of GaP LA phonon.

We demonstrate that longitudinal-optical (LO) phonons efficiently pump electrons from the quasi-X states to the quasi-Γ states in short-period type-II GaAs/AlAs superlattices. At a very low temperature, the phonon-assisted electron up-transfer can occur if the energy difference between the lowest quasi-Γ states and quasi-X states is equal to or less than the LO phonon energies. As the temperature increases, however, kinetic energies of the electrons can facilitate the electron up-transfer. As a result, we have observed peculiar behaviors on these superlattices. First, photoluminescence intensity for the quasi-direct transition drastically increases as the temperature or pump power increases. Second, the dependence of the integrated photoluminescence intensity on the pump power exhibits a square power law.

Quantum well intersubband transitions display some of the most interesting many-body effects including various collective excitations, such as intersubband plasmon and Fermi-edge singularity (repellon). To describe these various excitations and to study the effects of scatterings on the intersubband lineshape, we have performed a systematic microscopic theoretical investigation of intersubband transitions. The theory leads to a set of intersubband semiconductor Bloch equations (ISBEs) at the first order of Coulomb interaction. The extension to include the second order Coulomb interaction and LO-phonon interaction leads to optical dephasing or linewidth broadening. Using this theory, we have studied systematically the interplay of collective excitations in quantum well intersubband transitions. Our results show that such interplay leads to dramatic changes in spectral features, such as absorption peak positions and lineshape, compared with a free-particle theory. We will also show that the typical usage of the dephasing rate approximation is generally invalid for intersubband transitions. There is a strong cancellation effects between the in- (off-diagonal) and out-(diagonal) scattering terms at the second order and the out-scattering alone overestimates the linewidth significantly. Such a cancellation is much stronger for intersubband transitions than for interband case, because of the much smaller inhomogeneous broadening in the intersubband case. We also show that there is a cancellation of electron-electron and electron-phonon scatterings in their contributions to the linewidth. Finally we will show that our theory agrees very well with recent infrared absorption experiments.

Heavy ion implantation into InP and In_0.53Ga_0.47As and rapid thermal annealing has been applied to produce materials with high resistivity, good mobility and ultrashort carrier lifetime, as required for ultrafast optoelectronic applications. Two implantation methods have been analyzed: Fe⁺ implantation into semi-insulating InP and InGaAs, and P⁺ implantation into p-doped InP and InGaAs. Both approaches allow production of layers with high sheet resistance, up to 10⁶ Ω/square for the P⁺-implanted compounds. Electron mobility in the high resistivity layers is of the order of 10² cm²V^-1s^-1. Carrier lifetimes, measured by the time-resolved photoluminescence and reflectivity, can be tuned from ~100 femtoseconds to tens of picoseconds by choosing implantation and annealing conditions. Measurements of carrier dynamics have shown that carrier traps act as efficient recombination centers, at least for the case of InP. The dependencies of electrical and ultrafast optical properties on the implantation dose and annealing temperature are determined by the interplay between shallow P and As antisite-related donors, deep Fe-related acceptors and defect complexes.

Quantum interference of single and two photon absorption pathways connecting valence and conduction band states in a semiconductor allow one to generate spin currents with or without charge currents. The underlying principles for these generation processes are outlines. We offer experimental demonstration of pure spin currents in GaAs using two color beams configured collinearly to produce spatially homogeneous currents, or non-collinearly to produce spin current gratings.

We report on fabrication and high-frequency performance of our photodetectors and photomixers based on freestanding low-temperature-grown GaAs (LT-GaAs). In our experiments, the LT-GaAs/AlAs bilayers were grown on 2-inch diameter, semi-insulating GaAs wafers by a molecular beam epitaxy. Next, the bilayer was patterned to form 10×10 μm² to 150×150 μm² structures using photolithography and ion beam etching. The AlAs layer was then selectively etched in diluted HF solution, and the LT-GaAs device was lifted from its substrate and transferred on top of a variety of substrates including Si, MgO/YBaCuO, Al₂O₃, and a plastic foil. Following the transfer, metallic coplanar transmission lines were fabricated on top of the LT-GaAs structure, forming a metal-semiconductor-metal photodetectors or photomixer structures. Our freestanding devices exhibited above 200 V breakdown voltages and dark currents at 100 V below 3×10^-7 A. Device photoresponse was measured using an electro-optic sampling technique with 100-fs-wide laser pulses at wavelengths of 810 nm and 405 nm as the excitation source. For 810-nm excitation, we measured 0.55 ps-wide electrical transients with voltage amplitudes of up to 1.3 V. The signal amplitude was a linear function of the applied voltage bias, as well as a linear function of the laser excitation power, below well-defined saturation thresholds. Output power from the freestanding photomixers was measured with two-beam laser illumination experimental setup. Reported fabrication technique is suitable for the LT-GaAs integration with a range of semiconducting, superconducting, and organic materials for high-frequency hybrid optoelectronic applications.

We present an experimental study of electron transport in electrically driven quantum cascade laser structures. Ultrafast quantum transport from the injector into the upper laser subband is investigated by mid-infrared pump-probe experiments directly monitoring the femtosecond saturation and subsequent recovery of electrically induced optical gain. For low current densities, low lattice temperatures, and low pump pulse intensities the charge transport is dominantly coherent, i.e., we observe pronounced gain oscillations upon excitation giving evidence for a coherent electron motion between the injector and the upper laser subband. Increasing either the current density, the lattice temperature, or the pump pulse intensity the gain recovery shows an additional slow incoherent component which essentially follows the pump-initiated heating and subsequent cooling of the carrier gas in the injector.

Non-linear carrier-photon dynamics are studied for optically pumped InAs quantum dot (QD) laser structures, using excitation into the GaAs barrier by two degenerate pump and probe laser pulses. The non-linear emission from QDs excited by the pump pulse is further amplified by the probe excitation. By varying the delay between the two pulses a very fast decay of the QD excited state emission is measured. Notably slower dynamics for the QD ground state are observed, governed by state filling phenomena, which result in gain saturation.

Characteristics of the absorption recovery and the saturation of intersubband transition in GaN/AlN quantum wells are investigated for the purpose of applying these quantum wells to optical switches operating at a higher bit rate than 1 Tb/s. The pump-probe measurement verifies the absorption recovery time to be 150 fs at a wavelength of 4.5 μm. Dependence on the absorption on the input light intensity is examined at a wavelength of 1.48 μm for an optical pulse with a width of 130 fs. The characterization is performed with the Lorentzian fit of the absorption spectrum on the assumption of a two-level system. The result indicates that the recovery time is much less than 1 ps and the absorption saturation intensity is of the order of pJ/μm². A ridge waveguide was fabricated and the onset of the intersubband absorption was confirmed. Finally, the switching performance is studied by means of the finite-difference time-domain (FDTD) simulation combined with three-level rate equations. Ridge waveguide structures with 3-QWs in the mid-layer are examined. Control and signal pulses are assumed to be the Gaussian pulses with a width of 250 fs. The results show that an extinction ratio of larger than 10 is achievable with an input control pulse energy of less than 1 pJ.

Ultrashort electrical pulses were used to characterize the magnetoplasmon resonance of a two-dimensional electron gas formed in an AlGaAs/GaAs heterostructures at frequencies up to 400 gigahertz. This was accomplished by incorporating the sample into a guided wave probe operating in a dilution refrigerator. A bath temperature of 50mK was recorded during measurements, demonstrating the feasibility of this approach in other situations requiring high magnetic field and mK temperatures.

Field emission, quantum tunneling from the clean surface of a nanoscale conductor tip in vacuum, is an extremely fast process, where the instantaneous value of the current responds to the intense applied electric field with a delay of less than 2 fs. The cause for this intrinsic delay is shown to be the traversal time for quantum tunneling. Because the tip is much smaller than optical wavelengths, quasistatic conditions require that the potential of the tip must follow the instantaneous electric field in the imposed optical radiation. There is a resonance that is caused by virtual photon processes in which the electrons absorb single quanta from the radiation field while they are tunneling, to be promoted to energies where the wave function is reinforced by reflections at the classical turning points. Numerical solutions of the time-dependent Schrödinger equation show that the transient response to pulsed radiation consists of beating of the radiation with this resonance, and is intensified by the resonance. Experiments show that when a field emission tube is used as a two-terminal device, by placing the load in the external bias circuit, the response to a pulsed laser is delayed by a time constant equal to the product of the load resistance and the electrode capacitance, typically 10-100 μs. Thus, other means for coupling are recommended, including propagation as surface waves on an extended tip and radiation from antennas formed on the tip, and these methods have been tested with microwave prototypes. Ultimately miniature multifunction devices could be built to implement this new technology because nanoscale field emission tubes are now available, and field emitter arrays with 10¹⁰ tips/cm² are used in flat panel displays.

Superfast high current switching of a GaAs-based JBT in the avalanche mode has been achieved experimentally for the first time. A very fast reduction in the voltage across the transistor was observed (~ 200-300 ps) and the amplitude of the current pulses ranged from 2 to 130 A depending on the load resistance. It was observed experimentally that the switching occurs in a number of synchronized current channels with a characteristic diameter of <~10 microns. A 1D simulation code was developed and the switching transient for a single channel was simulated, with the external circuit incorporated into the simulations. Photon-assisted carrier transport and negative differential electron mobility were taken into account in the theoretical model. The former does not play an appreciable role in the 1D switching transient, although the latter determines superfast switching at extreme current densities (> 1 MA/cm²). Superfast switching occurs due to the appearance of a number of Gunn domains at any instant (up to ~ 20 domains across a collector region ~30 microns in thickness). These domains of huge amplitude (up to ~700 kV/cm) are moving towards the cathode and give rise to extremely high ionization rates across the volume of the channel in the n0 collector region. The simulations provide a fairly reliable interpretation of the experimentally observed switching time, which is shorter than that in Si avalanche transistors by a factor of ~15. The new device is fairly attractive, e.g. for feeding pulsed laser diodes when the current rise time should be shorter than the lasing delay.

We present the design and analysis of an optical receiver front-end configuration that uses a single heterojunction bipolar transistor for both photodetection and amplification purpose. The performance characteristics an InP/InGaAs HBT receiver operating in 1.55 μm wavelength region have been studied on the basis of our model. Theoretical results indicate a high transimpedance gain (~54 dBΩ), a large bandwidth (~29.5 GHz) and a reasonably high sensitivity (-24 dBm at 10 Gb/s) for the receiver configuration. Use of a single HBT in the front-end would greatly simplify the fabrication of optoelectronic integrated circuit (OEIC) receiver in the monolithic form.

Field-induced electron transport in an In_xGa_1-xN (x≅0.4) sample grown on GaN has been studied by subpicosecond Raman spectroscopy. Non-equilibrium electron distribution and electron drift velocity due to the presence of piezoelectric and spontaneous fields in the In_xGa_1-xN layer have been directly measured. The experimental results are compared with ensemble Monte Carlo calculations and reasonable agreements are obtained.

We report experimental results on simultaneous measurement of electron as well as hole transient transport in an Al_0.3Ga_0.7As-based p-i-n semiconductor nanostructure by using picosecond/subpicosecond Raman spectroscopy. Electron and hole velocity overshoots are directly observed. These experimental results are discussed and explained.