This PDF file contains the front matter associated with SPIE Proceedings Volume 6892, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing

We use the internal picosecond strain pulses to control the electron energy in a semiconductor quantum well. For generating the strain pulse a 100 nm thick metal transducer was hit by intense laser pulse and a strain pulse with duration ~10 ps and amplitude up to 0.1% was injected into a GaAs substrate. This strain pulse travels strongly directed through the crystal towards the quantum well generating at each momentary position a "nano-earthquake". When the quantum well is hit by this "earth quake", the exciton resonance is shifted on a value up to 10 meV on a ps time scale.

Time-resolved Raman spectroscopy on a subpicosecond time scale has been used to study the phonon dynamics of both the A₁(LO) and the E₁(LO) phonons in InN. From the temperature-dependence of their lifetimes, we demonstrate that both phonons decay primarily into a large wavevector TO phonon and a large wavevector TA/LA phonon consistent with the accepted phonon dispersion relationship for wurtzite InN. Their lifetimes have been found to decrease from 2.2 ps, at the low electron-hole pair density of 5×10¹⁷cm^-3 to 0.25 ps, at the highest density of 2×10¹⁹cm^-3. Our experimental findings demonstrate that carrier-density dependence of LO phonon lifetime is a universal phenomenon in polar semiconductors.

We present Raman-scattering results for PbTe quantum dots (QDs) in doped telluride glasses which clearly show the confinement effects on the phonon spectra as a function of the quantum-dot size.

We have observed that the temperature of the electrons drifting under a relatively-high electric field in an AlN/GaN-based high-electron-mobility transistor is significantly higher than the lattice temperature (i.e. the hot electrons are generated). These hot electrons are produced through the Fröhlich interaction between the drifting electrons and long-lived longitudinal-optical phonons. By fitting electric field vs. electron temperature deduced from the measurements of photoluminescence spectra to a theoretical model, we have deduced the longitudinal-optical-phonon emission time for each electron is to be on the order of 100 fs. This value is consistent with the value measured previously from Raman scattering.

A theory of electron relaxation for electron gases in semiconductor quantum well structures and at semiconductor surfaces is presented. The electron relaxation is described by quantum-kinetic equations. In the nonlinear optical response of a two dimensional electron gas in a GaN quantum well, polaronic signatures are clearly enhanced compared to the linear response, if the pump pulse is tuned to the polaron energy. For the phonon-induced electron relaxation at Si (001) surface structures, the interplay of bulk and surface states yields a complex temporal relaxation dynamics depending on the slab thickness.

High-quality (Q) factor photonic-crystal nanocavities are currently the focus of much interest because they can strongly confine photons in a tiny space. Nanocavities with ultrahigh Q factors of up to 2,500,000 and modal volumes of a cubic wavelength have been realized. If the Q factor could be dynamically controlled within the lifetime of a photon, significant advances would be expected in areas of physics and engineering such as the slowing and/or stopping of light and quantum-information processing. Here, we review the demonstration of dynamic control of the Q factor, by constructing a system composed of a nanocavity, a waveguide with nonlinear optical response and a photonic-crystal heterointerface mirror. The Q factor of the nanocavity was successfully changed from 3,800 to 22,000 within picoseconds.

Resonant-tunnelling diodes (RTDs) are used for studies of fundamental aspects of tunnelling and also for realization of oscillators at high frequencies, particularly in THz frequency range. Also, the RTDs can be considered as the building blocks of different electronic structures, including optical, e.g., quantum-cascade lasers. It is generally accepted that the inherent limitation of the operating frequency and the charge relaxation (response) time of RTD is determined by the quasi-bound-state lifetime. The simple picture is not generally correct. Here we show, first, that the Coulomb interaction between electrons can lead to large reduction/increase of the relaxation time. Second, we demonstrate that the operating frequencies of RTDs are limited neither by quasi-bound-state lifetime, nor by relaxation-time constants; particularly the differential conductance of RTDs can stay negative at the frequencies far beyond the limits imposed by the time constants. Here we provide the experimental evidences for both effects. We demonstrate negative differential conductance up to the frequency of 12 GHz in our RTDs with the inverse quasi-bound-state lifetime of around 1 GHz. Also the relaxation time in our RTDs was shown to be a factor of 2 shorter/longer (depending on the RTD operating point) than the quasi-bound-state lifetime. According to our theory, the effects are not limited to the low frequencies and the same effects should persist at higher frequencies also. Our results indicate not only that nowadays operating frequencies of RTDs could be increased, but the results also elucidate the fundamental limitations of the whole class of resonant-tunnelling structures: single-electron-transistor-like structures, multi-barrier structures, quantum-cascade lasers, etc.

Phonons are considered as a most important origin of scattering and dissipation for electronic coherence in nanostructures. The generation of coherent acoustic phonons with femtosecond laser pulses opens the possibility to control phonon dynamics in amplitude and phase. We demonstrate a new experimental technique based on two synchronized femtosecond lasers with GHz repetition rate to study the dynamics of coherently generated acoustic phonons in semiconductor heterostructures with high sensitivity. High-speed synchronous optical sampling (ASOPS) enables to scan a time-delay of 1 ns with 100 fs time resolution with a frequency in the kHz range without a moving part in the set-up. We investigate the dynamics of coherent zone-folded acoustic phonons in semiconductor superlattices (GaAs/AlAs and GaSb/InAs) and of coherent vibration of metallic nanostructures of non-spherical shape using ASOPS.

We present our studies of femtosecond photoluminescence of colloidal solutions of CdSe and PbSe nanocrystals, using polychromatic fluorescence upconversion. Ultrafast relaxation processes are observed in both cases upon excitation at 400 nm and 800 nm respectively. Under moderately high excitation densities we studied the formation and dynamics of biexitons and triexcitons in CdSe nanocrystals. Contrary to earlier reports, all results could be understood without invoking the presence of charged particles. The dynamics of single excitons in PbSe nanocrystals is found to be similar to the case of CdSe, despite the high confinement in the former., The early-time spectra are characterized by emission from several low lying excited states. The kinetics point to a fast sequential cascade process between excited states, governed by energy gaps, and indicate the presence of additional dark states. The first direct measurement of the lowest excited states Huang-Rhys factors excludes that strong electron-phonon coupling mediates the intraband relaxation.

We report the application of high-intensity femtosecond laser pulses as a novel tool for manipulating biological specimens. When femtosecond laser pulses were focused to a near diffraction-limited focal spot, cellular material within the laser focal volume was surgically ablated. Several dissection cuts were made in the membrane of live mammalian cells, and membrane surgery was accomplished without inducing cell collapse or disassociation. By altering how the laser pulses were applied, focal adhesions joining live epithelial cells were surgically removed, resulting in single cell isolation. To further examine the versatility of this reported tool, cells were transiently permeabilized for introducing foreign material into the cytoplasm of live mammalian cells. Localizing focused femtosecond laser pulses on the biological membrane resulted in the formation of transient pores, which were harnessed as a pathway for the delivery of exogenous material. Individual mammalian cells were permeabilized in the presence of a hyperosmotic cryoprotective disaccharide. Material delivery was confirmed by measuring the volumetric response of cells permeabilized in 0.2, 0.3, 0.4 and 0.5 M cryoprotective sugar. The survival of permeabilized cells in increasing osmolarity of sugar was assessed using a membrane integrity assay. Further demonstrating the novelty of this reported tool, laser surgery of an aquatic embryo, the zebrafish (Danio rerio), was also performed. Utilizing the transient pores that were formed in the embryonic cells of the zebrafish embryo, an exogenous fluorescent probe FITC, Streptavidin-conjugated quantum dots or plasmid DNA (sCMV) encoding EGFP was introduced into the developing embryonic cells. To determine if the laser induced any short- or long-term effects on development, laser-manipulated embryos were reared to 2 and 7 days post-fertilization and compared to control embryos at the same developmental stages. Light microscopy and scanning electron microscopy were used to compare whole body mosaics of the developed larvae. Key developmental features that were compared included the olfactory pit, dorsal, ventral and pectoral fins, notochord, otic capsule and otic vesicle. No differences in the morphology and placement of the fore-, mid- and hindbrains were observed.

We present a novel theory of polaritons which contains the crucial tools to securely tackle the many-body physics of polaritons. Microcavity polaritons are central to exciting Bose-Einstein condensation and parametric scattering experiments in semiconductors. Besides Coulomb and Pauli scattering (dressed by the relevant exciton-Hopfield coefficients), it beautifully unravels a novel fundamental scattering between two polaritons which can be physically associated with a photon-assisted exchange process, without any Coulomb contribution. The non-trivial consequences of the polariton composite nature - here treated exactly - lead to results noticeably different from the ones of the conventional approaches in which polaritons are mapped into elementary bosons. As an example we compute the scattering rate of two pump polaritons in a typical stimulated scattering experiment. To evaluate the limits of bosonization techniques we also compare qualitatively and quantitatively the scattering rate obtained through the main bosonized approach used up to now in the field to the exact result, and show in particular that unphysical spurious terms appear with bosonization.

This review summarizes the recent progress in the study of ultrafast nonthermal effects of light on magnetic materials. Fundamental aspects of interaction between photons and spins, magneto-optical and opto-magnetic phenomena, microscopical mechanisms responsible for laser control of magnetism are discussed. Our recent experiments on laser excitation of magnetic resonances, quantum control of magnons, ultrafast phase transitions and femtosecond laser-induced switching of magnetization are reviewed.

We present the observation of ultrafast photo-enhancement of ferromagnetism in Mn-doped III-V semiconductor GaMnAs via photoexcited transient carriers. Our time-resolved magneto-optical Kerr spectroscopy reveals transient enhancement of the magnetization amplitude on a 100 ps time scale after initial sub-picosecond demagnetization. The dynamic enhancement of the magnetic ordering shows a maximum below the Curie temperature T_c and completely dominates the demagnetization at high temperatures approaching T_c. We attribute the observed ultrafast collective ordering to the transiently enhanced hole-Mn p-d exchange interaction, leading in particular to a correlation-induced maximum bellow T_c and transient increase of T_c. These results constitute the evidence for non-thermal, cooperative spin manipulation in (III,Mn)Vs on the ultrafast time scale.

We describe a new mechanism for ultrafast active control of plasmon propagation. By using time-domain terahertz spectroscopy, we demonstrate that electron spin state can influence plasmon propagation. Using a random spinplasmonic medium consisting of a dense ensemble of bimetallic ferromagnetic (F)/nonmagnetic (N) microparticles, plasmon propagation velocity, amplitude attenuation, phase retardation and magnetic field dependence are shown to be influenced by electron spin accumulation in the nonmagnetic layers. The observation of electron spin accumulation is attributed to the formation of a nonequilibrium spin-dependent potential barrier at the F/N interface that acts to resist the flow of a spin-polarized plasmon current. This phenomenon is similar to the electrically-driven spin accumulation phenomenon resulting from current transport between F/N layers. With this first demonstration of the merger between the plasmonics and spintronics fields, we envision the realization of a new class of ultrafast spinplasmonic devices having unique functionalities.

Owing to their reduced dimensionality, the behavior of quasi-one-dimensional systems is often strongly influenced by electron-electron interactions. We discuss some recent work on using theory and computation to understand and predict the electronic structure and the linear optical response of several one-dimensional (1D) nanostructures. The calculations are carried out employing a first-principles interacting-electron Green's function approach. It is shown that exciton states in the semiconducting carbon nanotubes have binding energies that are orders of magnitude larger than bulk semiconductors and hence they dominate the optical spectrum at all temperature, and that strongly bound excitons can exist even in metallic carbon nanotubes. In addition to the optically active (bright) exciton states, theory predicts a number of optically inactive or very weak oscillator strength (dark) exciton states. These findings demonstrate the importance of an exciton picture in interpreting optical experiments and in the possible applications of the carbon nanotubes. Our studies show that many-electron interaction (self-energy and excitonic) effects are equally dominant in the electronic structure and optical response of other potentially useful quasi-1D nanostructures such as the BN nanotubes, Si nanowires, and graphene nanoribbons.

We review our recent advances in four-wave mixing spectroscopy of individual semiconductor quantum dots using heterodyne spectral interferometry, a novel implementation of transient nonlinear spectroscopy allowing the study of the transient nonlinear polarization emitted from individual electronic transitions in both amplitude and phase. We present experiments on individual excitonic transitions localized in monolayer islands of a GaAs/AlAs quantum well. The detection of amplitude and phase allows the implementation of a twodimensional femtosecond spectroscopy, in which mutual coherent coupling of single quantum dot states can be observed and quantified. By combining two-dimensional femtosecond spectroscopy with four-wave mixing mapping in the real space we found coherent coupling between spatially separated excitons by up to ~ 0.8 μm.

We adopt a kinetic theory of polariton non-equilibrium Bose-Einstein condensation, to describe the formation of off-diagonal long-range order. The theory accounts properly for the dominant role of quantum fluctuations in the condensate. In realistic situations with optical excitation at high energy, it predicts a significant depletion of the condensate caused by long-wavelength fluctuations. As a consequence, the one-body density matrix in space displays a partially suppressed long-range order and a pronounced dependence on the finite size of the system.

The strong optical nonlinearity of silicon and tight optical field confinement in silicon waveguides, accompanied by silicon's unique material properties such as high optical damage threshold and thermal conductivity, enable compact nonlinear photonic devices to be fabricated in silicon using cost effective CMOS compatible fabrication technology. By integrating a p-i-n diode into the silicon waveguide, the nonlinear optical loss due to two photon absorption induced free carrier absorption in silicon waveguides can be dramatically reduced, and efficient nonlinear optical devices can be realized on silicon chips for high speed optical communications. In this paper, we report recent development of silicon p-i-n waveguide based nonlinear photonic chips for wavelength conversion and dispersion compensation applications. Wavelength conversion efficiency of -8.5 dB can be achieved in an 8-cm long p-i-n silicon waveguide by four-wave mixing in continuous-wave operation, and chromatic dispersion compensation by mid-span spectral inversion is demonstrated experimentally using silicon spectral inverter at the mid-span of a fiber optical link, achieving transmission of optical data at 40 Gb/s over 320 km of standard fiber with negligible power penalty. The unique advantages of using silicon over previously proposed nonlinear optical media for dispersion compensation are discussed.

We perform degenerate four-wave-mixing (FWM) studies of GaN excitons especially for an understanding of the strain-fields in the heteroepitaxial films. The shifts of exciton energies and their beating oscillation variations highlight the biaxial strain, allowing for a precise determination of the strain parameters. The uniaxial strain field can be characterized by the polarization dependence of FWM, which shows distinct polarizations and energy variations depending on the sample and its position. The minimum changes of the polarized FWM intensity and exchange energy splittings correspond to a uniaxial strain of 5.0 × 10^-5, which currently gives a lower resolution limit of this technique and is comparable with that of conventional X-ray diffraction. In the time-evolutions, we investigate the strain effects on the phase of the quantum beats (QBs), giving insight into the excitons interactions. By using time-resolved FWM, difference between two-types of exciton transitions is identified. In addition, coherent manipulations of QBs are successfully realized in the FWM with a Michelson interferometer.

Time-resolved photoluminescence (TRPL) across the full spectrum of a 240 layer ZnTe/ZnSe super-lattice structure has been performed using a femtosecond Ti-Sapphire laser and a streak camera system for detection. A significant change of the PL lifetime is observed across the emission spectrum decreasing smoothly from ~ 100 ns at 2.35 eV, to less than a few nanoseconds at 2.8 eV. The increase of the PL lifetime in the low energy region of the emission spectrum is strong evidence to support the formation of type-II quantum dots (QDs) from excitons bound to clusters of Te-atoms. In such QDs the confined holes and the free electrons are spatially separated, thus increasing the radiative lifetime. This result is consistent with recent photoluminescence (PL) measurements in which evidence of the combined contribution of excitons bound isoelectronically to Te-atoms, and type-II QDs was observed.

By using a frequency-controlled narrow band THz source, a Fourier Transform Infrared (FTIR) spectroscopy system, and a frequency-controlled terahertz (THz) emitter, for the first time, we studied the THz photon absorption related to the THz confined acoustic vibrations in semiconductor nanocrystals. Through a specific charge separation in the CdSe/CdTe type-II nanocrystals and a piezoelectric coupling in the wurtzite CdSe nanocrystals, the THz photons can be resonantly coupled with (l=1) dipolar modes and the (l=0) breathing modes, respectively. Our results provide new mechanisms for low dimensional systems to convert a THz photon into a phonon of the same frequency.

The second-order processes of optical rectification and photoconduction are well known and widely used to produce ultrafast electromagnetic pulses in the terahertz frequency domain. We present a new form of rectification relying on the excitation of surface plasmons (SPs) in metallic nanostructures. Multiphoton ionization and ponderomotive acceleration of electrons in the enhanced evanescent field of the SPs, results in a femtosecond current surge and emission of terahertz electromagnetic radiation. Using gold, this rectification process is third or higher order in the incident field.

The terahertz response of a two-dimensional electron gas (2DEG) is investigated theoretically. The developed microscopic model shows that the terahertz absorption sensitively detects Coulomb-induced many-body correlations within the entire 2DEG system. In particular, the resulting response follows from a nontrivial competition between the ponderomotive and the Coulomb-correlation contributions. The result is in good agreement with recent experiments while the response cannot be explained with a simple Drude-model analysis.

We propose various designs of porous polymer fibers for guiding terahertz radiation. Numerical simulations are presented for three fiber geometries: a Bragg fiber consisting of periodic multilayers of ferroelectric polyvinylidene fluoride (PVDF) and polycarbonate (PC), a sub-wavelength waveguide containing multiple sub-wavelength holes, as well as a cobweb-like porous Bragg fiber consisting of solid film layers suspended by a network of bridges. Various properties of these fibers are presented. Emphasis is put on the optimization of the geometries to increase the fraction of power guided in the air, thereby alleviating the effects of material absorption. Losses of about 10 dB/m, 7.8 dB/m, and 1.7 dB/m at 1 THz are respectively predicted for these three structures.

We independently determine the subpicosecond cooling rates for holes and electrons in CdSe quantum dots using time-resolved luminescence and time-resolved TeraHertz spectroscopy. The rate of hole cooling, following photoexcitation of the quantum dots, depends critically on the electron excess energy. This constitutes a direct proof of electron-to-hole energy transfer, the hypothesis behind the Auger cooling mechanism proposed in quantum dots, which is found to occur on a 1 ± 0.15 ps time scale. This is only marginally slower than the timescale of intraband relaxation of electrons in single crystal CdSe. In bulk CdSe, sequential multi-phonon emission allows for cooling rates of (0.4 ps)^-1 at room temperature.

Carrier-envelope phase-stabilized laser pulses brought significant advances in investigating laser-solid interactions, as well, with the potential of revealing carrier dynamics in solids on unprecedented time-scales. More specifically, multi-photon induced photoemission from metals proved to be sensitive to the waveform of few-cycle pulses, however, underlying mechanisms are not fully understood. Combining surface plasmonic effects with photoemission demonstrates a potentially more promising approach to investigate laser-surface interactions induced by few-cycle pulses. Numerical results from a simple model on this phenomenon are presented. Related to this, previously unaddressed carrier-envelope phase phenomena in the vicinity of the focus are also considered.

The temporal relaxation of optically excited electrons at the In-rich reconstructed InP(100) surface was studied using time-resolved two-photon-photoemission spectroscopy (TR-2PPE). High-energy carriers were generated at laser pump energies chosen to populate hot electron bulk states or the well known C₂ surface state via resonant direct optical excitation. The different relaxation pathways arising from these population schemes involve Γ-L-Γ intervalley scattering and the transient occupation of an additional surface state, C₁. The dynamics of these processes were recorded with a novel experimental setup using two ultra-low power 150 KHz repetition rate sub-20 fs NOPAs enabling two-colour pump-probe experiments in the linear regime. These experiments provide useful information in understanding the dynamics of devices on the basis of this semiconductor medium such as solar cells and high-speed switching circuits.

Two-dimensional Fourier transform spectroscopy, an extension of four-wave mixing (FWM), is able to resolve numerous aspects of many-body effects and higher-order Coulomb interactions that contribute to the ultrafast dynamics of quantum wells and bulk GaAs resonances. Coherent oscillations between heavy-hole and light-hole excitons - or quantum beats - can be unfolded from the exciton populations by Fourier transforming FWM data with respect to two time-axes. Excitation conditions such as pulse ordering, polarization, tuning and pulse energy can isolate Feynman pathways and highlight the coherent many-body correlations, including those from biexcitons. In addition, the bulk exciton and continuum states are studied more carefully for their dynamics.

We study Landau damping of a coherent, solid-state plasma by means of ultrafast THz spectroscopy. The onset of this phenomenon occurs when momentum and energy conservation are satisfied for single-particle excitations; this diminishes collective mode behavior. A series of InSb-based bulk heterostructures have varying amounts of spatial confinement, allowing direct access to a different range of wave vectors in the electron-electron interaction. Sufficient confinement leads to disappearance of the plasmon quasi-particle via Landau (collisionless) damping. The experimental results are quantitatively reproduced by model calculations.

Laser ablation (LA) is a thin film fabrication technique which has generated a lot of interest in the past few years as one of the simplest and most versatile methods for the deposition of a wide variety of materials. With the rapid development experienced in the generation of ultra short laser pulses, new possibilities were opened for the laser ablation technique, using femtosecond lasers as ablation source. It is commonly believed that when the temporal length of the laser pulse became shorter than the several picoseconds required to couple the electronic energy to the lattice of the material, thermal effects could not play a significant role. Since the pulse width is too short for thermal effects to take place, with each laser pulse a few atom layers of material are direct vaporized away from the target surface and a better control in the quantum dots (QDs) fabrication could be achieved. In this work we report the fabrication of PbTe QDs by femtosecond laser ablation of a PbTe target in argon atmosphere. Experiments were carried out using a typical LA configuration comprising a deposition chamber and an ultra short pulsed laser (100 fs; 30 mJ) at a central wavelength of 800 nm. PbTe was chosen because its QDs absorption band can be controlled by its size to fall in the spectral window of interest for optical communications (1.3-1.5 μm). This, together with the QD high optical nonlinearity, makes this material an excellent candidate for development of photonic devices. It was investigated the influence of the number of laser pulses in the formation of the nanoparticles. The structural parameters and the surface density of the nanoparticles were studied by high resolution transmission electron microscopy (HRTEM).