This PDF file contains the front matter associated with SPIE Proceedings Volume 7214, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.

We demonstrate an experimental setup combining the near-field scanning optical microscope (NSOM) and timecorrelated single photon counting (TCSPC) system for high-resolution temporal and spatial spectroscopic measurements. In particular, a multiple-quantum-well (MQW) structure was excited with a tapered optical fiber with 100 nm opening aperture in near-field region and their temporal photoluminescence spectra were obtained by TCSPC system. We are able to measure fluorescence decay time of a GaAs/AlGaAs MQW structure with well width of 75Å in the near-field region at room temperature.

We have experimentally investigated injection currents generated by all-optical excitation of GaAs/AlGaAs quantum wells excited with 130 fs optical pulses. The currents have been detected via free-space THz experiments at room temperature. Our experiments prove that Coulomb effects strongly influence injection currents. This becomes most prominently visible when exciting light-hole exciton transitions. At this photon energy we observe a pronounced phase shift of the current transients which is due to oppositely oriented heavy-hole and light-hole type contributions. We are currently developing a microscopic theory based on a 14×14 k.p model in combination with the semiconductor Bloch equations to describe the observed features quantitatively. The combined theoretical and experimental approach will allow us to analyze the influence of the bandstructure and interaction effects on the injection current amplitude and current dynamics.

Nonstoichiometric ZnO with an excess of Zn atoms (ZnO:Zn) has a long history of use as a green/monochrome phosphor in electron-excited vacuum fluorescent and field emission displays. The advent of ultraviolet lasers and light emitting diodes presents the possibility of photoexciting the highly efficient, defect-related green emission in ZnO:Zn. Here we study experimentally the time-integrated quantum efficiency and the time-resolved photoluminescence decays of both near band edge and defect emissions in unannealed (ZnO) and annealed (ZnO:Zn) nanoparticles under femtosecond excitation. A comparison of results using one-photon excitation (excitation primarily near the particle's surface) versus two-photon excitation (uniform excitation throughout the particle's volume) elucidates how the quantum efficiencies depend on material properties, such as the spatial distributions of radiative and nonradiative defects, and on optical effects, such as reabsorption.

A series of experiments have been conducted that microscopically image thermal diffusion and surface acoustic phonon propagation within a single crystallite of a polycrystalline Si sample. The experimental approach employs ultrashort optical pulses to generate an electron-hole plasma and a second probe pulse is used to image the evolution of the plasma. By decomposing the signal into a component that varies with delay time and a steady state component that varies with pump modulation frequency, the respective influence of carrier recombination and thermal diffusion are identified. Additionally, the coherent surface acoustic phonon component to the signal is imaged using a Sagnac interferometer to monitor optical phase.

We describe experiments demonstrating the generation of ultrafast, high strain rate acoustic waves in a precompressed transparent medium at static pressure up to 24 GPa. We also observe shock waves in precompressed aluminum with transient pressures above 40 GPa under precompression. Using ultrafast interferometry, we determine parameters such as the shock pressure and acoustic wave velocity using multiple and single shot methods. These methods form the basis for material experiments under extreme conditions which are challenging to access using other techniques.

We will review recent progress on the generation and detection of picosecond shear acoustic waves. Examples will be shown in which the transverse isotropic symmetry of the sample structure is broken in order to permit shear wave generation through sudden laser heating. As an illustration of the technique, picosecond longitudinal and shear acoustic waves have been successfully employed to probe structural dynamics in nano-sized solids (gold nano-crystals assemblies) and nano-sized liquids (glycerol and water).

The optical properties of single-wall carbon nanotube sheets in the far-infrared (FIR) spectral range from few THz to several tens of THz have been investigated with terahertz spectroscopy both with static measurements elucidating the absorption mechanism in the FIR and with time-resolved experiments yielding information on the charge carrier dynamics after optical excitation of the nanotubes. We observe an overall depletion of the dominating broad absorption peak at around 4THz when the nanotubes are excited by a short visible laser pulse. This finding excludes particle-plasmon resonances as absorption mechanism and instead shows that interband transitions in tubes with an energy gap of ~10meV govern the far-infrared conductivity. A simple model based on an ensemble of two-level systems naturally explains the weak temperature dependence of the far-infrared conductivity by the tube-to-tube variation of the chemical potential. Furthermore, the time-resolved measurements do not show any evidence of a distinct free-carrier response which is attributed to the photogeneration of strongly bound excitons in the tubes with large energy gaps. The rapid decay of a featureless background with pronounced dichroism is associated with the increased absorption of spatially localized charge carriers before thermalization is completed.

We will report our study of photoinduced structural dynamics of nanomaterials of various shapes and sizes by a femtosecond laser heating pulse as detected by time-resolved electron diffraction or transient optical absorption. This work improves the understanding of nanoscale heat transfer and the ultrafast structural dynamics in nanomaterials such as thin films, spheres, prisms, discs, rods, pyramids and cubes. This work allows us to elucidate the roles of dynamic expansion/contraction and the more well-known static linear expansion. Both mechanisms play an important but different role in the observed structural changes at a different time scale. We will present comparative analysis of the experimental measurements for various samples as obtained by our own group and those reported by others.

We show that accumulation of charges at the metal edges via light-induced currents creates large horizontal electric field, which in effect attracts the incoming light. The enhanced field is fully propagating towards the far-field because no cut-off exists. With the amplitude enhancement in the range of 1,000, the intensity enhancement of 10⁶, and the nonlinear enhancement of 10¹², this structure can be an excellent launching pad for inducing broad-band nonlinearity, small signal detection in astronomy or biology, and for surface enhanced Raman scattering.

We demonstrate in this paper that laser ablation allows efficient analysis of organic and biological materials. Such analysis is based on laser-induced breakdown spectroscopy (LIBS) which consists in the detection of the optical emission from the plasma induced by a high intensity laser pulse focused on the sample surface. The optimization of the ablation regime in terms of laser parameters (pulse duration, wavelength, fluence) is important to generate a plasma suitable for the analysis. We first present the results of a study of laser ablation of organic samples with different laser parameters using time-resolved shadowgraph. We correlate the early stage expansion of the plasma to its optical emission properties, which allows us to choose suitable laser parameters for an efficient analysis of organic or biological samples by LIBS. As an illustration of the analytical ability of LIBS for biological materials, we show that the emission from CN molecules can be used to distinguish between biological and inorganic samples. Native CN molecular fragment directly ablated from a biological sample are identified using time-resolved LIBS. Those due to recombination with nitrogen contained in atmospheric air can be distinguished with their specific time evolution behavior.

Time shifting of optical pulses with duration in the range from 100 fs to a few ps represents one extreme of slow light, where THz bandwidth for the slow down or speed up is necessary. The physics of the time shifting of such very short pulses involves the gain saturation of the optical medium and is different from the slow-light mechanisms responsible for time shifting of pulses of narrower bandwidth. Experimental and theoretical results with semiconductor components are presented, emphasizing the physics as well as the limitations imposed by the dynamical processes.

Using terahertz time-domain spectroscopy we investigate how quantum, magnetic and electrostatic confinement alters the photoconductivity of nanostructured semiconductors. In 2.0 THz and 2.9THz GaAs/AlGaAs quantum cascade structures under a magnetic field we observe transitions from the 1s to 2p^- or 2p⁺ magneto-exciton states. The electron cyclotron resonance is prominent at high excitation fluence. Additionally, we report that the conductivity of photoexcited electrons in nanoporous InP honeycombs obeys the Drude model of free-carrier absorption, while the dark conductivity does not. This finding can be explained as a result of surface band bending spatially separating photoexcited electrons and holes, and also accounts for the long electron recombination lifetime (exceeding 100 ns) at low temperature.

We explore THz optical activity of ensembles of dense randomly oriented metallic helices by studying their time-domain response to THz electromagnetic pulse excitation. The interaction of the electromagnetic wave with this artificial chiral material mimics that interaction with highly concentrated optically active liquids. By dynamically accessing the THz electric field transmitted through the helical chiral media, optical activity signatures are correlated with the arrival time and polarization state of the detected THz electric field radiation. Our experiment show two distinct phases for optical rotation: one which is associated with scattering and the other is associated with phase accumulation.

Our simulations and experiments demonstrate a new physical mechanism for detecting acoustic waves of THz frequencies. We find that strain waves of THz frequencies can coherently generate radiation when they propagate past an interface between materials with different piezoelectric coefficients. By considering AlN/GaN heterostructures, we show that the radiation is of detectable amplitude and contains sufficient information to determine the time-dependence of the strain wave with potentially sub-picosecond, nearly atomic time and space resolution. This mechanism is distinct from optical approaches to strain wave measurement. We demonstrate this phenomenon within the context of high amplitude THz frequency strain waves that spontaneously form at the front of shock waves in GaN crystals. We also show how the mechanism can be utilized to determine the layer thicknesses in thin film GaN/AlN heterostructures.

We investigated the ultrafast terahertz response to the photoexcitation for vanadium dioxide single crystals and thin films using the optical-pump terahertz-probe technique at room temperature. The optical excitation at 800 nm induced an ultrafast decrease of the transmittance of the terahertz pulse within 0.7 ps, and then the transmittance decreases gradually up to 100 ps. The decrease of the transmittance is assigned to the appearance of the high electric conductivity due to metallic state. The conductivity increases more than ten times in the picoseconds time range after photoexcitation and it is concluded that the metallic electronic states appear. The rapid and gradual changes of the electric conductivity are very similar to the previous reports of the time resolved X-ray and electron diffractions. This fact indicates that the increase of the electric conductivity and the change of the lattice structure proceed in parallel. It is suggested that the photo-induced insulator-metal phase transition is of the Peierls type.

We address recent fiber-based femtosecond laser technology. Specifically, fiber-chirped pulse amplifier is discussed for the enabling the concept of real-world applications. We review recent selected material applications demonstrating advantages of ultrafast dynamics of highly repetitive pulse train in nanoparticle generation in pulsed-laser deposition and reliable Si wafer singulation.

Generation of free-carrier plasma and filamentation of the ultra-short laser pulse were investigated and modeled. Experimental results of filamentation are supported by numerical model which takes into account accumulation of refractive index modifications due to multi-pulse exposure. A contact acoustic monitoring technique was employed to perform spatially-resolved in situ detection of micro-plasma formation and filamentation of focused femtosecond laser pulses with critical and sub-critical powers in glass. The recorded acoustic signals reveal freecarrier generation mechanisms associated with the formation of plasma and filamentation of the propagating laser pulses. Optical opacity of the plasma region, which sets in at the irradiance of a few kJ/cm³ (close to the dielectric breakdown threshold) using pulse focusing optics with numerical aperture NA = 0.75, reveals its critical character, and allows the estimation of acoustic pressure in the ~GPa range. The pressure depended on the irradiance as P ~ I^0.59. In the case of loose focusing (NA = 0.035) filamentation of fs-pulses occurred at sub-critical plasma density with P ~ I. Detection and interpretation of these acoustic signatures thus enable real-time in situ monitoring of optical ionization, pulse filamentation in bulk dielectrics under the irradiation by femtosecond laser pulses.

We demonstrated sub-GHz operation of a single-photon-emitting diode at 1.55 μm using 80-ps-wide electrical pulses. A light-emitting diode (LED) with a quantum dot (QD) layer was fabricated into a nanoscale mesa structure with electrodes. The electroluminescence (EL) and radiative lifetime of a single exciton in the QD was directly determined to be 1.59 ns by time-resolved EL measurement. The single-exciton recombination time agrees with the radiative lifetime calculated with an eight-band kp model. The antibunching behavior of exciton radiative recombination in a currentinjected quantum dot was demonstrated at 1551.2 nm by Hanbury-Brown and Twiss-type photon correlation measurements. Device examination at a high drive rate by changing the delay time between two electrical pulses demonstrated that a QD LED can be used as source of sub-GHz single photons in the C-band triggered by current injection.

Ultrashort pulse lasers based on fiber optic architecture will play a dominant role in the spread of these lasers into research and industrial applications. The principle challenge is to generate adequate pulse energy from singlemode or quasi-singlemode amplifiers which have small cross-sectional area. We demonstrate a robust, all-fiber erbium amplifier system that produces >100 μJ per pulse with 701 fs pulsewidth and M² < 1.3. We will discuss the salient amplifier dynamics that influence the pulse generation, shaping, and propagation phenomena in state-of-the-art erbium fiber lasers. Furthermore, we show data relevant to applications and implementation of ultrashort pulse lasers.

Using a novel high-field THz source we performed various ultrafast experiments on n-type GaAs. Both nonlinear THz experiments driving resonantly the 1S-2P donor impurity transition and nonlinear transport experiments on free carriers in the conduction band of GaAs give new insights into the dynamics of localized and delocalized electrons surprisingly different from the well-known linear Drude theory.

We report the first observation of coherent plasmon emission of THz radiation from arrays of semiconductor nanowires. The THz signal strength from InAs nanowires is comparable to a planar substrate, indicating the nanowires are highly efficient emitters. This is explained by the preferential orientation of plasma motion to the wire surface, which overcomes radiation trapping by total-internal reflection. Using a bulk Drude model, we identify the average donor density and mobility in the nanowires in a non-contact manner. Contact IV transconductance measurements provide order of magnitude agreement with values obtained from the THz spectra.

The fascinating properties of plasmonic structures have had significant impact on the development of next generation ultracompact photonic and optoelectronic components. We study two-dimensional plasmonic structures functioning at terahertz frequencies. Resonant terahertz response due to surface plasmons and dipole localized surface plasmons were investigated by the state-of-the-art terahertz time domain spectroscopy (THz-TDS) using both transmission and reflection configurations. Extraordinary terahertz transmission was demonstrated through the subwavelength metallic hole arrays made from good conducting metals as well as poor metals. Metallic arrays made from Pb, generally a poor metal, and having optically thin thicknesses less than one-third of a skin depth also contributed in enhanced THz transmission. A direct transition of a surface plasmon resonance from a photonic crystal minimum was observed in a photo-doped semiconductor array. Electrical controls of the surface plasmon resonances by hybridization of the Schottky diode between the metallic grating and the semiconductor substrate are investigated as a function of the applied reverse bias. In addition, we have demonstrated photo-induced creation and annihilation of surface plasmons with appropriate semiconductors at room temperature. According to the Fano model, the transmission properties are characterized by two essential contributions: resonant excitation of surface plasmons and nonresonant direct transmission. Such plasmonic structures may find fascinating applications in terahertz imaging, biomedical sensing, subwavelength terahertz spectroscopy, tunable filters, and integrated terahertz devices.

We report a femtosecond response in photoinduced magnetization rotation in the ferromagnetic semiconductor GaMnAs, which allows for detection of a four-state magnetic memory at the femtosecond time scale. The temporal profile of this cooperative magnetization rotation exhibits a discontinuity that reveals two distinct temporal regimes, marked by the transition from a highly non-equilibrium, carrier-mediated regime within the first 200 fs, to a thermal, lattice-heating picosecond regime.

We have examined ultrafast carrier dynamics and light amplification in ZnO nanowires following subpicosecond excitation at room temperature. We performed time- and wavelength-resolved pump-probe transmission and gain measurements on a 'forest' of 100- to 500-nm thick and 20-μm long nanowires, epitaxially grown on a sapphire wafer. Measurements were done using 267-nm pump pulses for direct, but inhomogeneous excitation, and 800-nm pulses to achieve homogeneous excitation via three-photon absorption. At the highest fluences, both for 267-nm and 800-nm pump pulses, a degenerate electron-hole plasma (EHP) is generated with carrier densities of 10²⁵ m^-3 or higher. We observed strong amplification of the probe, accompanied by a rapid decay (~ 1.5 ps) of the charge carriers. Below ~ 10²⁵ m^-3, the EHP becomes non-degenerate and the decay much slower. A dip in the pump-probe signal was observed, caused by ionization of probe exciton-polaritons by the pump. This effect allows for a measurement of the exciton-polariton dispersion relation and enhanced light-matter interaction in ZnO nanowires.

The pump-probe technique was used to study the reflectance of CdTe based quantum wells. The energy of a pump beam was scanned across excitonic resonances. The observed dynamics for both the energetic position and oscillator strength of excitonic lines is caused by the changes of the population of charge carriers as well as by the spin dependent interactions. The polarization resolved resonance technique was applied to the system of single CdTe - based quantum dots. Observation of linear to circular polarization conversion for pair of the adjacent, anisotropic dots allowed determining the time of excitation transfer between them.

Ultrafast time-resolved pump-probe measurements are used to study low energy excitations and dynamics of electronic transport in various semiconductor nanostructures. In quantum cascade lasers, we observe ultrafast gain recovery dynamics due to electronic transport in the structures. In particular, the nature of electronic transport had been addressed by using ultrafast optical techniques. Sub-picosecond resonant tunneling injection from the quantum cascade laser injector ground state into the upper lasing state was found to be incoherent due to strong dephasing in the active subband. We also observed the strong coupling of the electronic transport to the intra-cavity photon density, which we term "photon-driven transport". Note that this invited paper reviews the details of our recent observations (H. Choi et al., Phys. Rev. Lett., 100, 167401, 2008 and H. Choi, et al., Appl. Phys. Lett. 92, 122114 (2008)).

In this paper we present our recent developments in terahertz (THz) metamaterials and devices. Planar THz metamaterials and their complementary structures fabricated on suitable substrates have shown electric resonant response, which causes the band-pass or band-stop property in THz transmission and reflection. The operational frequency can be further tuned up to 20% upon photoexcitation of an integrated semiconductor region in the split-ring resonators as the metamaterial elements. On the other hand, the use of semiconductors as metamaterial substrates enables dynamical control of metamaterial resonances through photoexcitation, and reducing the substrate carrier lifetime further enables an ultrafast switching recovery. The metamaterial resonances can also be actively controlled by application of a voltage bias when they are fabricated on semiconductor substrates with appropriate doping concentration and thickness. Using this electrically driven approach, THz modulation depth up to 80% and modulation speed of 2 MHz at room temperature have been demonstrated, which suggests practical THz applications.

The optical emission and gain properties of Ga(AsSb) quantum-islands are investigate. These islands form during growth in a self-organized process in a series of Ga(AsSb)/GaAs/(AlGa)As heterostructures, resulting in an additional in-plane hole confinement of several hundreds of meV. The shape of the in-plane confinement potential is nearly parabolic and thus yields almost equidistant hole energy levels. Transmission electron microscopy reveals that the quantum islands are 100nm in diameter and exhibit an in-plane variation of the Sb concentration of more than 30 %. Up to seven bound hole states are observed in the photoluminescence spectra. Time-resolved photoluminescence data are shown as function of excitation density, lattice temperature, and excitation photon energy and reveal fast carrier capture into and relaxation within the quantum islands. Furthermore, the optical gain is measured using the variable stripe-length method and the advantages of such structures as active laser material are discussed.

Due to the recent progress of material science, quasi-one-dimensional (1D) materials provide an opportunity for investigating the influence of topology and dimensionality on their optical and electrical properties. In this study, we report the phase transition properties of such quasi-1D compounds by utilizing an ultrafast optical spectroscopy. Photoexcited nonequilibrium carrier dynamics yield characteristic features around the phase transition temperatures. We also discuss the toplogical effects on the phase coherence of correlated carriers by using both polarization and excitation energy dependences of the transient signals and their spatial characteristics.

We review our recent results on ultrafast dynamics of photogenerated electrons in Si and at Si(001)-(2x1) surfaces studied by femtosecond time-resolved two-photon photoemission spectroscopy. The photoemissin from the conduction band minimum (CBM) in Si, emitted via an inverse LEED state promoted by the surface photoeffect, provides a powerful tool to study the hot-electron dynamics in the bulk conduction band of Si. The relaxation in the X valley has been characterized with a fast formation of quasi-equilibrated hot electron system near the CBM and the energy relaxation process specified with the time constant of 240 fs (at 296 K), which is not dependent on the electron excess energy initially given to the electrons. The bulk conduction electrons are transferred into the surface un-occupied state on the Si(001)-(2x1) surface with respective contributions; the hot electrons are less effective in the transition. Ultrafast density loss process of conduction electrons is induced in 1 ps of excitation near the surface, which is specific to the relaxing electrons with higher energy and higher T*. The dynamical electron-hole recombination mechanism via a surface deep localized state has been proposed.

In this work we investigate the miniband relaxation dynamics of electrons in doped GaAs/AlGaAs superlattices by twocolor infrared pump-probe experiments using a free electron laser synchronized to a table top broadband IR source. In contrast to single color experiments, by this technique we are able to separate the different contributions from inter- and intraminiband relaxation to the transient behavior after an ultrafast excitation. In particular, the intraminiband relaxation is studied for different miniband widths, below and above the optical phonon energy of GaAs. For minibands wider than this critical value we find fast relaxation, nearly constant for different excitation intensities whereas for narrow minibands, a strong temperature and intensity dependence of the relaxation is found. The results are in good agreement with previously published Monte Carlo simulations.

The two dimensional Photonic crystal fiber (PCF) with a triangular lattice cross-section pattern of circular air hole is investigated for four and five layer by the use of finite difference time domain (FDTD) method to investigate the single mode property and the effect on it by increasing the number of layer as well as by varying the air hole diameter.