Ultrafast relaxation dynamics of photoexcitations in semiconducting single walled carbon nanotubes (S-NTs) were investigated using polarized pump-probe photomodulation (with 150 fs time resolution) and cw polarized photoluminescence (PL). Both annealed and unannealed NT films and D₂O solutions of isolated NTs were investigated. Various transient photoinduced bleaching (PB) and photoinduced absorption (PA) bands, which show photoinduced dichroism, were observed in the ultrafast photomodulation spectra of all NT forms. Taking into account the PB spectral shift observed for NTs in solution, the PA and PB bands are seen to decay together by following a power law in time of the form (t)^-α, with α in the range of 0.7 to 1. The PL emission of S-NTs in D₂O solution shows a polarization degree that agrees with that of the transient photoinduced dichroism. We conclude that the primary photoexcitations in S-NTs are excitons that are confined along the NTs. From the average PL polarization degree and the transient polarization memory decay, we estimate the PL lifetime of isolated NTs in solution is of order 500ps. This relatively long PL lifetime is dominated by non-radiative decay processes, which when coupled with the tiny PL emission quantum efficiency indicates a very small radiative recombination rate, in good agreement with recent theories that include electron correlation.

Molecular dynamics simulations have been used to investigate the nature of heat pulse propagation through a Y-junction carbon nanotube consisting of a (14,0) trunk splitting into a pair of (7,0) branches. For comparison, these simulations were also carried out on straight (7,0) and (14,0) carbon nanotubes. Simulations of the Y-junction nanotube were run in three different configurations: with the heat pulse originating in the trunk, in one of the branches, or in both of the branches simultaneously. All of the simulations were run at 0K, and the length of the pulse was 1ps. Results have shown that the heat pulse excites a variety of traveling phonon modes. It has been found that the junction impedes the propagation of these modes. Furthermore, it has been observed that traveling modes originating in the trunk pass through the junction more easily than those originating in the branches. This provides preliminary evidence for anisotropic heat flow in Y-junction nanotube structures at low temperatures. Finally, it is possible for a single phonon mode passing through the junction to generate multiple phonon modes on the other side, all with velocities less than or equal to the original mode.

The excitation-fluence and magnetic-field dependence of terahertz (THz) radiation power from InAs is investigated. For magnetic-field dependence, two completely different behaviors were observed depending on the excitation fluence. At low excitation fluence, the enhanced THz-radiation mainly originates from the carrier acceleration by the surface electric field. The Lorentz force changes the direction of carrier acceleration toward surface parallel, and the THz-radiation power is enhanced regardless of magnetic field direction. In contrast, at high excitation fluence, the surface electric field is almost screened out and the diffusion process becomes significant. By applying a magnetic field, the dipole is rotated to the direction in which the THz-radiation is efficiently or inefficiently extracted from the surface, and the radiation power is either enhanced or reduced depending on the magnetic-field direction. Additionally, from the magnetic-field dependence up to 27 T, it is found that THz-radiation power saturates at approximately 3 T and also at 13 T, and that the THz-radiation power at 3 T is much higher than that at 13 T. For the generation of broadband THz-radiation, focus is made on n-type InAs irradiated by ultrafast optical pulses. From the magnetic-field dependence of THz-radiation power, using n-type InAs under magnetic field is found to be the practical method to generate broadband THz-radiation, and the origin of higher-frequency component is identified to the hybrid modes.

Terahertz electromagnetic pulse generation using photoconductive emitters, their characterization, real-time imaging, and spectroscopies using them are described. Intense THz pulses were generated using a large-aperture photoconductive antenna. Real-time imaging were performed for the pulse characterization and acquisition of 1-kHz high-speed movies of moving objects. Tunable THz pulse generation and ultrafast semiconductor spectroscopy using photoconductive antennas will also be described.

Terahertz scanning near-field infrared microscopy (SNIM) below 1 THz is demonstrated. The near-field technique benefits from the broadband and highly brilliant coherent synchrotron radiation (CSR) from an electron storage ring and from a detection method based on locking on to the intrinsic time structure of the synchrotron radiation. The scanning microscope utilizes conical waveguides as near-field probes with apertures smaller than the wavelength. Different cone approaches have been investigated to obtain maximum transmittance. Together with a Martin-Puplett spectrometer the set-up enables spectroscopic mapping of the transmittance of samples well below the diffraction limit. Spatial resolution down to about λ/40 at 2 wavenumbers (0.06 THz) is derived from the transmittance spectra of the near-field probes. The potential of the technique is exemplified by imaging biological samples. Strongly absorbing living leaves have been imaged in transmittance with a spatial resolution of 130 μm at about 12 wavenumbers (0.36 THz). The THz near-field images reveal distinct structural differences of leaves from different plants investigated. The technique presented also allows spectral imaging of bulky organic tissues. Human teeth samples of various thicknesses have been imaged between 2 and 20 wavenumbers (between 0.06 and 0.6 THz). Regions of enamel and dentin within tooth samples are spatially and spectrally resolved, and buried caries lesions are imaged through both the outer enamel and into the underlying dentin.

The photoconductive (PC) antenna fabricated on arsenic-ion-implanted GaAs (GaAs:As⁺) and proton-bombarded InP (InP:H⁺) substrates are shown to have a useful detection bandwidths beyond 30 THz. This is comparable to that of the reference LT-GaAs PC antenna. The signal-to-noise ratio of these ion-implanted III-V PC antennas are, however, worse than that of the LT-GaAs devices because of the higher stray currents of the former under illumination. Ion-implanted III-V PC antennas are nevertheless attractive because the implanters are widely available, process parameters well-established and compatible with the IC industry. Implantations in selective areas are also straight forward. Our results suggest that ion-implanted III-V material can be a good choice as substrate or THz PC antennas, if the resistivity is increased by a proper annealing process and/or optimizing the implantation recipe.

We present a model of THz emission enhancement from femtosecond pulse excited n-GaAs and InAs surfaces with the application of a dc magnetic field. The far-field THz emission at different optical excitation densities, magnetic field strengths, and magnetic field orientations is determined. The model accurately describes the power dependence of THz emission from n-GaAs and InAs surfaces for magnetic field strengths up to ±10 T and ±6 T, respectively. THz emission saturation in both semiconductors for optical excitation densities from 40 nJ/cm² to 2.2 μJ/cm² are in accordance with previously reported experimental data. The model provides a useful tool for the description of ultra-fast processes in semiconductors.

Recent progress on ultrashort pulse lasers has made it possible to control the pulse timing and optical phases from different cavities very precisely. By applying detection and control techniques of the carrier-envelope phase to multicolor pulse sources with synchronized pulse timing, we obtained phase-coherent pulse-trains in different wavelengths. The pulse-trains have sufficient stability to achieve Fourier synthesis among optical fields. One of our test sources was a femtosecond optical parametric oscillator (OPO). Using a specially designed OPO whose signal and idler are sub-harmonics of the pump frequency, we obtained long-term stabilization of the optical phase among those three waves. Since the OPO also generates sum-frequencies of the three waves, six-color coherent pulse-trains from 425nm to 2550nm are available. Another test source was a passively synchronized mode-locked laser with two kinds of gain media, Ti:sapphire and Cr:forsterite. Although it is harder to reduce phase-noise between the different color pulses than in the OPO example, we can expect shorter pulse duration and higher average power from this type of coupled laser. These coherent multicolor pulse sources will be applied not only to shorter-pulse generation by field summation, but also to some applications that include competing non-linear processes among multicolor pulses.

A semiconductor-to-metal phase transition in vanadium dioxide results in a huge change of optical refractive index. Only a picojoule of the incident laser energy is required for this phase transition to happen, and the time scale of this transformation is very short -- semiconductor becomes metal within the first 100-fs. In this report we utilize this phase transformation to dramatically modify the shape of the incident laser pulse.

We investigate the origin of radiative recombination in (InGa)(AsN)/GaAs single quantum wells by means of continuous wave and time-resolved photoluminescence (PL) measurements. Samples with different indium and nitrogen concentration were investigated. An analysis of the whole set of data for different excitation densities and lattice temperatures, T, is reported. This analysis provides insights into radiative and non-radiative processes ruling the recombination dynamics and shows the predominant contribution of localized state emission at low T. The nature of these states is further studied by measuring the time necessary (rise time) for their population. We find that the PL rise time in (InGa)(AsN) is independent of temperature and detection energy, thus being not conclusive about the origin of the states involved in the emission processes. On the contrary, magneto-PL measurements show that the shift of the PL peak energy induced by a magnetic field, B, decreases sizably and changes its dependence on B from linear to quadratic when going from low to high temperature. This counterintuitive result shows that radiative recombination at low temperature (T<100 K) is not excitonic, contrary to previous assignments, and is due to loosely bound electron-hole pairs in which one carrier is localized by N-induced potential fluctuations and the other carrier is delocalized.

Excitonic optical properties of Ga-rich Al_xGa_1-xN ternary alloy epitaxial layers are reviewed on the basis of our recent experimental observations. Photoluminescence due to radiative recombination of biexcitons was clearly observed from the ternary alloys with different aluminum compositions (x=0.019 ~ 0.15). Recombination dynamics of excitons and biexcitons was studied by means of time-resolved photoluminescence spectroscopy. The effect of localization due to alloy disorder on biexcitons was also studied by means of photoluminescence excitation spectroscopy. A Stokes shift of biexcitons was defined experimentally on the basis of two-photon absorption of biexcitons in order to evaluate the degree of biexciton localization quantitatively. A binding energy of biexcitons was determined as a function of aluminum composition. The biexciton localization due to alloy disorder resulted in a strong enhancement of the biexciton binding energy.

Deep ultraviolet (UV) photoluminescence (PL) spectroscopy has been employed to study the optical properties and carrier dynamics in AlN and GaN epilayers at temperatures from 10 to 800 K. The parameters that describe the temperature variation of the energy bandgap (α and β, or a_B and θ) and linewidth broadening have been obtained and are compared with the previously reported values in AlN and GaN obtained by different measurement methods in narrower temperature ranges. Our experimental results demonstrate that the broader temperature range of measurements is necessary to obtain accurate values of these parameters, particularly for AlN. The phonon-carrier interactions were also investigated in both AlN and GaN epilayers. At low temperatures, the linewidth of PL emission lines increases with temperature due to the electron-acoustic phonon interaction. The electron-LO phonon interaction becomes important above 200 K and eventually dominant at high temperatures in both AlN and GaN. The temperature dependencies of the decay lifetimes were investigated up to 500 K, from which free excitons and free carriers interactions are discussed for AlN and GaN epilayers. The implications of our findings to the optoelectronic and electronic device applications at elevated temperatures are discussed.

Dye sensitized nanocrystalline semiconductor films are used as a photoactive part in dye-sensitized solar cells, which are recently attracting much interest both in basic and applied studies. Electron transfer reaction from a photoexcited dye molecule, which is chemically adsorbed on the surface of semiconductor, into the semiconductor conduction band is the primary step to generate photocurrent. Ultrafast pump-probe spectroscopy with a <100 fs time resolution and in a visible-to-IR wavelength range was used to elucidate the interfacial electron transfer mechanism in dye-sensitized nanocrystalline metal oxide films of ZnO, TiO₂, and others. We found two types of reaction paths; one is direct electron transfer from the excited molecule to the conduction band and the other is stepwise transfer through an intermediate, which was assigned to a charge transfer complex formed by the excited molecule and a surface state on the semiconductor. The order of the observed electron transfer rates for different semiconductors was qualitatively explained by the idea of the density of electron acceptor states; that is, the larger the density of states near the energy level of the excited molecules was, the faster the electron transfer took place.

Piezoelectric semiconductor with heterostructure can be treated as a piezoelectric transducer for the generation of acoustic waves with wavelength less than 10 nm (nano-acoustic waves) by optical technique. This optical piezoelectric transducer has also been utilized for the detection of the nano-acoustic waves (NAW). In this paper, we discuss the generation, detection, and propagation of nano-acoustic waves in piezoelectric semiconductors. We demonstrate that the acoustic frequency of the NAW can be tuned by an optical control technique. Besides, we have also developed an acoustic sensor with THz bandwidth which can be used to study NAW propagation control devices such as nano-phononic bandgap crystal. We demonstrated that the roughness of an interface can be evaluated by the NAW with a resolution less than 1 nm through the acoustic phasefront distortion effect. With the optical piezoelectric transducer, nano-ultrasonics, which is analogous to typical ultrasonics but on the nanometer scale, has been successfully developed.

Spin dynamics in the III-V dilute magnetic semiconductor GaMnAs is investigated by photo-induced demagnetization. Experimental results obtained from two different time-dependent characterization techniques - "two color-probe" magneto-optical Kerr effect (TR-MOKE) and mid-infrared differential transmittance -- are compared. Upon photo-excitation with a 100 fs, 3.1 eV light pulse, a long demagnetization time in the hundreds of picoseconds timescale is found by TR-MOKE, indicating a spin-dependent band structure in this material. In mid-infrared measurements, a positive increase of the differential transmittance is observed in the same time interval when the sample is cooled below its Currie temperature. It is shown that this mid-infrared absorption feature is directly related to ferromagnetism in this material. The magnetism-related component of the broad DC mid-infrared absorption peak characteristic of this p-type material could be observed with this time-resolved measurement. Experimental results were simulated with a model describing the interaction between three thermal reservoirs (hole, spin and lattice) and taking thermal diffusion into account.

We present a scheme to control the spin-polarization of photoelectrons/photoions using short laser pulses. It is based on the pump-probe method. By exciting fine structure manifolds of a system by the short pump pulse with a sufficient bandwidth, a superposition of fine structure states is created. After the pump pulse, the coherently superposed excited state evolves in time under the field-free condition with a period determined by the inverse of energy difference between them. In terms of the spin state, this suggests that the different spin state evolves differently in time, leading to the time-varying spin-polarization. Therefore, varying the time delay between the pump and probe pulses leads to the control of spin states upon photoionization. Specific theoretical results are presented for two-valence-electron atoms, in particular for Mg, which demonstrate that, under certain conditions, not only the degree of spin-polarization but also its polarity can be manipulated through time delay. Furthermore, we propose a new technique to accelerate the coupling time by the introduction of a dressing laser with a few ns duration. Since the underline physics is rather general and transparent, the presented scheme may be potentially applied to nanostructures such as quantum wells and quantum dots.

Both ultrafast thermal and photocurable nanoimprint lithography (NIL) are studied and high fidelity transfers of nanopatterns from molds to resists have been achieved. In ultrafast thermal NIL, we use a single excimer laser pulse to melt a NIL resist polymer and imprint it using a fused silica mold. The entire imprint process, from melting the polymer to completion of the imprint, takes less than 200 ns. This technique, termed laser assisted nanoimprint lithography (LAN), has patterned nanostructures in various polymer films with high fidelity over the entire mold area. In LAN, the short laser pulse is absorbed primarily by the resist and the laser energy is minute, hence substrate heating and distortion are negligible. In ultrafast photocurable NIL, a flash lamp (pulse width 94 μs) is used to crosslink photo curable resists over a 4 in. wafer with high uniformity by a single pulse. The significant reduction of the heating of the substrate and mold will greatly benefit overlay alignment.

This paper reports on progress in the VUV spectroscopy of surfaces and nanostructures, as facilitated by two experimental innovations. In Scanning Probe Energy Loss Spectroscopy (SPELS) electrons backscattered from the tipsurface junction are detected when an STM tip is operated in field emission mode. Energy loss features in the range 1300 eV have been observed, corresponding to optical absorption spectra from the IR through to the VUV and X-ray regions. Recent data suggests in addition that SPELS can record local secondary electron emission spectra. SPELS offers spatial resolution on the 10 nm scale. High Harmonic Generation (HHG) of VUV light provides a complementary probe of the excited states of surfaces and nanostructures, offering a temporal resolution of order 100 fs or below. The utility of the High Harmonic source is demonstrated by an investigation of the visible fluorescence from thin films of passivated CdSe nanocrystals, as a function of excitation energy, decay time and temperature, which thus emerge as new scintillators for VUV applications.

The operation of the Hot Electron Light Emitting and Lasing in Semiconductor Heterostructure -- Vertical Cavity Surface Emitting Laser (HELLISH-VCSEL) devices is based on hot carrier transport parallel to the layers of Ga_1-xAl_xAs p-n junction. It is therefore a field - effect device and the light emission from the device is independent of the polarity of the applied voltage. In this study, we present the temperature dependence of the operational characteristics of the device. Experimental studies comprising of the measurements of the I-V characteristics, electroluminescence, reflectivity, and temperature dependent light-applied electric field (L-F) characteristics are conducted to find the optimum operating temperature of the device.

The formation of helical nanowires -- nanosprings -- of boron carbide have been observed and a growth mechanism, based on the work of adhesion of the metal catalyst and the tip of the nanowire, developed. The model demonstrates that the asymmetry necessary for helical growth is introduced when the following conditions are met: (1) The radius of the droplet is larger than the radius of the nanowire, and (2) The center of mass of the metal droplet is displaced laterally from the central axis of the nanowire. Furthermore, this model indicates that only amorphous nanowires will exhibit this unique form of growth and that in monocrystalline nanowires it is the crystal structure that inhibits helical growth. High-resolution transmission electron microscopy and electron diffraction has been used to compare the structure of both amorphous and crystalline nanowires.

Representative examples of applications research based on Free-Electron Lasers are reviewed. Research highlights include: observation of absolute negative conductance in semiconductor superlattices using a terahertz Free-Electron Laser at the University of California, Santa Barbara; infrared photon echoes as a technique in nonlinear spectroscopy to investigate vibrational dynamics in liquids and glasses using an infrared Free-Electron Laser at Stanford University; attributing the 20.1 μm stellar spectral feature to titanium carbide clusters using an infrared Free-Electron Laser in The Netherlands; human laser neurosurgery and ophthalmic laser surgery using an infrared Free-Electron Laser at Vanderbilt University; imaging of nanoscale island dynamics during thin film growth using the ultraviolet Free-Electron Laser at Duke University; and nuclear resonant fluorescence measurements for parity assignments in ¹³⁸Ba using the high intensity gamma ray source at Duke University.

Third-order optical nonlinearity in optical fibers has many attractive applications to all-optical signal processing that will be employed in future large-capacity photonic networks. After reviewing the third-order nonlinear optical property of optical fibers, we describe our recent experimental results on all-optical signal processing functions such as wideband wavelength conversion, ultrafast gate switching, and ultrafast pulse reshaping. These functions are based on self-phase modulation (SPM), cross-phase modulation (XPM), and four-wave mixing (FWM) in nonlinear fibers.

Ultrafast (picosecond range) switching of a GaAs-based BJT (bipolar junction transistor) in the avalanche mode has recently been demonstrated experimentally. It was found to be caused by the formation and spread of ultra-high amplitude multiple Gunn domains, which cause extremely powerful avalanching in the volume of the switching filaments. Unavoidable parasitic impedance of an external circuit limits the rate of avalanche carrier generation in the channels, however, which slows down the switching and increases the residual voltage across the switch. We present here the results of simulations which show that the switching transient can be significantly accelerated and the residual voltage reduced due to the supporting of a higher current density in the channels by the charge stored in the barrier capacitance of the non-switched part of the structure. The corresponding circuital currents are confined in low-inductance loops inside the structure and are not critically affected by the parameters of the external circuit. This provides very fast and effective reduction in the collector voltage, provided the parameters of the semiconductor layers and the geometry of the device are selected properly. Particularly significant in this process is the effect of circuital current saturation in the lightly doped collector region of the non-switched part of the transistor. The results of the simulations with the barrier capacitance included in the model are in excellent agreement with the experimental data.

We report on the coupling between the coherent GaAs-like longitudinal optical (LO) phonons and the excitonic quantum beats in GaAs/AlAs multiple quantum wells with different splitting energies of the heavy-hole (HH) and light-hole (LH) excitons. The time-domain signal in each GaAs/AlAs multiple quantum well observed by using a reflection-type pump-probe technique shows a strong coherent oscillation with the fast dephasing time less than 0.5 ps and a weak coherent oscillation with the long dephasing time over 4 ps: The former and later oscillations correspond to the excitonic quantum beat and the coherent GaAs-like LO phonon, respectively. When the splitting energy between the HH and LH excitons becomes close to the energy of the GaAs-like LO phonon, the amplitude of the coherent GaAs-like LO phonon is resonantly enhanced, and the pump-energy dependence of the amplitude of the GaAs-like LO phonon shows the similar profile to that of the excitonic quantum beat. These results indicate that in the multiple quantum wells with the splitting energy almost equal to the LO phonon energy, the excitonic quantum beat acts as a driving force for the coherent GaAs-like LO phonon due to the coupling between the GaAs-like LO phonon and the excitonic quantum beat. The dependence of the frequency of the excitonic quantum beat on the splitting energy exhibits an anticrossing behavior in the vicinity of the resonance energy that the splitting energy becomes equal to the LO phonon energy, which demonstrates the existence of the coupled mode between the GaAs-like LO phonon and the excitonic quantum beat.

Picosecond acoustic phonon pulses are generated with ultrashort laser pulses in a sample containing three GaAs-Al_0.3Ga_0.7As quantum wells of different thickness. The pump photon energy is tuned through the hh1-e1 transitions of each well (1.44 - 1.64 eV) and the probe photon energy is chosen to allow detection of the phonon pulses at the sample surface (3.06 eV). Transient optical reflectance and phase changes are recorded as a function of the delay time between the pump and probe light pulses using an interferometric technique. The transition between the valence and conduction sublevels of the wells is observed to strongly influence the pump-photon-energy dependence of the acoustic phonon pulse generation. The data are analyzed with a model that relates the carrier wavefunctions in the quantum wells to the acoustic strain through the deformation potential, and the acoustic strain to the transient optical reflectance and phase.

We have investigated the ultrafast relaxation dynamics of intersubband transition (ISBT) in GaN/AlN, using a two-color pump-probe technique, in a wide energy range around the optical communication wavelength. We suggest that the origin of the signal depends on the relation between the pump and probe pulse energies. We have observed an ultrafast induced absorption signal and a slow negative component which are due to the absorption of electrons during intra-subband scattering and a carrier cooling process with a hot-phonon effect, respectively. Moreover, we clarify the origin of the inhomogeneous broadening width of the ISBT and of the intrinsic absorption width of ISBT from the detailed analyses of the result. We have reproduced the relaxation dynamics by a numerical calculation to confirm this interpretation of ISBT relaxation dynamics.

We studied the influence of the populations of neutral and positively charged excitons (trions) on optical absorption of modulation p-doped CdTe-based quantum wells. The density of 2D hole gas in the quantum well was controlled by an additional cw illumination in the range from 10¹⁰ cm^-2 to 10¹¹ cm^-2. Time-resolved absorption was measured following a picosecond, circularly polarized, resonant pump pulse, which created significant exciton population. A spectrally broad femtosecond probe pulse was used to detect the absorption over the excitonic region, including exciton, trion and biexciton transition energies. Besides, we used a small magnetic field (below 1T) to create a steady-state spin polarization of the hole gas. By exploiting polarization-dependent selection rules, we were able to identify exciton, trion and biexciton absorption lines without ambiguity. We studied the evolution of these absorption lines under influence of photo-created populations of excitons and trions. The results are interpreted in terms of spin-dependent exciton-exciton and exciton-carrier interaction, the latter being dominant, in contrast with results obtained on GaAs-based quantum wells. We propose a new explanation of the oscillator strength stealing phenomena observed in doped quantum wells, based on the screening of neutral excitons by charge carriers. We have also found that binding holes into charged excitons excludes them from the interaction with the rest of the system, so that oscillator strength stealing is partially blocked. Experimental evidence is presented for creation of a transient spin polarization in the system by a circularly polarized pump pulse.

We study the charge dynamics in a double quantum well and in ballistic mesoscopic rings driven by half-cycle pulses. It is shown that such pulses can be utilized to localize, within femtoseconds, and control, for picoseconds, the electronic motion in a Ga_1-xAl_xAs based double quantum well. To identify the pulse parameters that appropriate for an efficient control process we developed a simplified analytical model and corroborated the results by performing full numerical calculations. We also show that when a thin ballistic mesoscopic ring is subjected to a linearly polarized HCP a post-pulse (and therefore field-free) polarization is induced in the ring. The non-equilibrium post-pulse polarization oscillates in the ring as long as the coherence is preserved and decays on a time scale determined by the relaxation time.

We present measurements of microphotoluminescence decay dynamics for single InGaN quantum dots. The recombination is shown to be characterized by a single exponential decay, in contrast to the non-exponential recombination dynamics seen in the two-dimensional wetting layer. The lifetimes of single dots in the temperature range 4 K to 60 K decrease with increasing temperature. Microphotoluminescence measurements of exciton complexes in single MOVPE-grown InGaN quantum dots are also reported. We find the exciton-biexciton and exciton-charged exciton splitting energies to be 25 meV and 10 meV to the higher-energy side of the exciton ground state, respectively. Assignments of the ground state exciton, biexciton and charged exciton are supported by theoretical calculations. These measurements have been extended to investigate the time-resolved dynamics of biexciton transitions in the quantum dots. The measurements yield a radiative recombination lifetime of 1.0 ns for the exciton and 1.4 ns for the biexciton. The data can be fitted to a coupled differential equation rate equation model, confirming that the exciton state is refilled as biexcitons undergo radiative decay.

Time-resolved photoluminescence decay measurements have been performed on samples with varying sized self-assembled InAs/GaAs quantum dot ensembles, formed by substrate mis-orientation alone, but otherwise under identical growth conditions. Ground-state radiative recombination lifetimes from 0.8 to 5.3 ns in the incident energy density range of 0.79 pJcm^-2 - 40 nJcm^-2 at a temperature of 77 K were obtained. It was found that a reduction of the quantum dot size led to a corresponding reduction of the radiative lifetime. The evident bi-exponential decay was obtained for the ground state emission of the quantum dot array, with the slower second component attributed to a carrier re-capturing and indirect radiative recombination processes. Also experimental evidence of the effect of the AlGaAs barrier in InAs QDs emitting in the wavelength range 1200-1300nm is presented. Time-resolved photoluminescence measurements have been performed on samples with different compositions of Al in the barrier. A full discussion of the lifetimes of these near infra-red emitting dots will be presented.

Far-field optical detection of a single metal nanoparticle using a space modulation technique is modeled and the results compared to experimental data in the case of gold nanospheres. The measured size and wavelength dependence of the absolute absorption cross-section of single nanospheres deposited on a transparent substrate is discussed and compared to theoretical predictions.

We report on the impulsive generation of optical and acoustic phonons in CdTe_0.68Se_0.32 nanocrystallites embedded in glass, at room temperature. Using ultrafast laser pulses in a pump-probe configuration, we were able to generate coherent vibrations. The energy of our laser was tuned to the absorption edge of the nanocrystals so as to resonantly excite the quantum dots. We identified two longitudinal optical phonons, an optical mode of mixed longitudinal-transverse nature and a longitudinal-like acoustic mode. The frequency, amplitude, decay and phase as a function of excitation energy were determined for the optical modes. These results clearly identify impulsive stimulated Raman scattering as the underlying mechanism of the coherent field generation. The acoustic oscillations are associated with the lowest confined acoustic mode with pseudo angular momentum l=0. We find that the frequency of this mode increases as the laser central energy increases. Since the energy of the exciton at the fundamental gap depends strongly on the particle size, such a behavior is attributed to resonant size-selective excitation of the nanocrystallites. In contrast, spontaneous Raman measurements obtained from the same sample do not show size selectivity and, in addition, the resonant spectra show l=1 and l=2 modes, which are not seen in the pump-probe data. Possible explanations and comparison with other reports are discussed.

In a context where ultrafast lasers have become ideal tools for material probing and processing we present various concepts for process control and optimization. Temporal tailoring of ultrashort laser pulses enables synergies between radiation and material and, therefore, new opportunities for optimal processing of materials. The concept of optimizing laser interactions is based on the possibility to adjust energy delivery so that control of laser-induced processes can be achieved and particular states of matter can be accessed. We present recent results related to the implementation of adaptive feedback loops based on temporal shaping of ultrafast laser pulses to control laser-induced phenomena for practical applications. The chosen example indicates the possibility to manipulate the kinetic properties of ions emitted from ultrafast laser irradiated semiconducting samples, using excitation sequences synchronized with the phase-transformation characteristic times. Versatile sub-keV ion beams are obtained exploiting transitions to supercritical fluid states with minimal energetic expenses, while achieving very efficient energy coupling and thermodynamic paths towards highly volatile states. Temporally selective irradiation can thus open up efficient thermodynamic paths towards critical points, delivering at the same time an extended degree of control in material processing.

Time- and spatial-resolved circular-polarized photoluminescence measurement was performed on an n-type modulation-doped (Cd,Mn)Te quantum well. The spatial extent of PL in the right circular polarization (RCP) got broad with an increase in magnetic field, although, for the left circular polarization (LCP), the extent showed a tendency to contract with the magnetic field. This difference between RCP and LCP is considered to be resulting from the difficulty difference in the formation of a negatively-charged exciton X^-. We successfully observed the time-development of an electric-field-induced drift of X^-. The drift of X^- was found to be promoted by the magnetic fields. This is considered to originate in the suppression of an exciton-magnetic-polaron formation or/and the decrease of a simple spin scattering under the magnetic fields. The scattering time of X^- was found to be about the same value as that of the two-dimensional conduction electron. This is the same result as the case of an n-type modulation-doped GaAs quantum well which had been already reported by other group (See References 4 and 5).

Velocity overshoot phenomenon for electrons as well as holes in a GaAs-based p-i-n nanostructure have been studied by using transient picosecond Raman spectroscopy. Under the picosecond laser excitation, we have found that the extent of velocity overshoot for electrons is comparable to holes. These experimental results have been explained in terms of various carrier scattering processes.

Non-equilibrium longitudinal optical phonons in a high quality, single crystal wurtzite structure InN sample have been studied by picosecond Raman spectroscopy. Our experimental results demonstrate that the bandgap of InN cannot be around 1.89 eV; but are consistent with a bandgap of around 0.8 eV. In addition, they disprove the idea that 0.8 eV-luminescence observed recently in InN is due to deep level radiative emission in InN.

The results of studying the steady states for bright and dark dissipative optical solitons, appearing in single-mode semiconductor laser structure multilayered in a direction of passing the waves, are presented. These types of solitons occur due to reshaping the incoming optical pulses via the passive mode-locking process in traveling-wave regime. The relations between the pulse parameters and the structures’ properties are chosen in such a way that the mode-locking process is incoherent in behavior that leads to the phase decay of the incoming pulses. The analysis demonstrates that both bright asymmetrical and dark (or flashing) dissipative optical solitons can be supported by such structure with a weakly illocal slowly saturable gain and a fast-relaxing saturable absorption.

We have directly determined the spectral shape of the complex conductivities of Bloch oscillating electrons by using time-domain terahertz (THz)-electrooptic sampling technique and presented an experimental evidence for a dispersive Bloch gain in superlattices. This unique dispersive gain without population inversion arises from a non-classical nature of Bloch oscillations; that is, the phase of the Bloch oscillation (BO) is shifted by π/2 from that of the semi-classical charged harmonic oscillation when driven by the same ac field. By increasing the bias electric field, the gain bandwidth reached ~3 THz in our particular sample. It was also found that the dominant dephasing mechanism of the BOs is identified to be the interface roughness scattering (alloy disorder scattering) below (above) the critical bias electric field.

The Gd(0001) surface is investigated by femtosecond pump-probe experiments using laser pulses at 740-860 nm wave length. By non-linear optical second harmonic generation a coherent phonon-magnon mode at a frequency of 3 THz is observed which is excited through the exchange-split surface state. In parallel, electron-electron and electron-phonon interaction and their magnetic counterparts lead to incoherent dynamics of the electron, lattice, and spin subsystems. Variation of the optical wave length shows that for longer wave lengths up to 860 nm the coherent mode is excited predominantly while for shorter ones (less than or equal to 740 nm) incoherent contributions are favored. This presents a strong indication that depopulation of the occupied surface state component drives the coherent excitation. We find identical time scales for damping of the coherent mode and for electron-lattice equilibration which identifies electron-phonon scattering as an important relaxation channel for the coherent contribution. Increasing the temperature results in faster damping indicating that scattering of coherent phonons with thermal ones is an active relaxation channel as well.