This PDF file contains the front matter associated with SPIE Proceedings Volume 7937, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and the Conference Committee listing.

We review a comprehensive study of the ultrafast optoelectronic properties of a single self-assembled InGaAs/GaAs quantum dot. While manipulation of the artificial atom relies on two widely and independently tunable picoseconds pulse trains, sensitive readout is achieved via the ~pA photocurrent of the diode device. In particular, the absorption changes after occupation of an s-shell exciton reveal a biexciton absorption line as well as previously unobserved p-shell transitions in the presence of s-shell population. In addition, time-resolved data directly maps the picosecond tunneling times of electrons and holes out of the dot. Beyond these incoherent phenomena, we also realize coherent QD manipulations. Those comprise well-known excitonic Rabi-oscillations as well as single-pulse biexciton generation and conditional Rabi-oscillations of the exciton-biexciton transition after deterministic exciton preparation.

Semiconductor quantum posts (QPs) - nanowire-like InGaAs heterostructures in a GaAs matrix - resemble many properties of regular self-assembled quantum dots (QDs), to which they are closely related. Due to their increased size as compared to QDs, QPs have proven to be suitable for very low threshold interband lasers. However, their well controllable height makes them attractive for precise tuning of the interband energy spacing that in QDs can only be achieved via post-growth annealing. Specifically, the 1s - 2p transition energy is expected to drop below LO-phonon energies at post heights of more than 30 nm, making them attractive as frequency-agile structures at terahertz frequencies. In the work presented here we explore the capture dynamics of QP structures after photoexcitation into the GaAs matrix. While the combined electron-hole dynamics are studied using time-resolved photoluminescence spectroscopy, optical pump - THz probe experiments were performed in order to solely study the electron dynamics. The results of the THz experiment show that after ultrafast excitation, electrons relax within a few picoseconds into the quantum posts, which act as efficient traps. The saturation of the quantum post states, probed by photoluminescence, was reached at approximately ten times the quantum post density in the samples. Also studied was the presence of possible electronic resonances after direct photoexcitation into QPs where a broad absorption around 1.5 THz was observed.

We provide direct evidence that the macroscopic response of the gain dynamics in electrically-pumped In- GaAs/GaAs quantum dots is a superposition of intradot relaxation dynamics from microstates with multiple discrete carrier numbers. The gain recovery in the presence of an optical pre-pump fully depleting the ground-state gain is measured to be faster than without pre-pump. This effect, opposite to expectations from rate equations with mean-field carrier distributions, is due to a conditional gain recovery in which microstates with slow internal dynamics are suppressed by the pre-pump. The effect is evident at 15K and still observable at 300 K, beneficial for high-speed optical signal processing.

The incorporation of semiconductor quantum dots into different heterostructures for applications in nanoscale photodetection, lasing and amplification has been an active area of research in recent years. Here, we use ultrafast differential transmission spectroscopy to temporally and spectrally resolve density-and-temperature-dependent carrier dynamics in an InAs/InGaAs quantum dots-in-a-well (DWELL) heterostructure. In our experiments, electron-hole pairs are optically injected into the three dimensional GaAs barriers, after which we monitor carrier relaxation into the two dimensional InGaAs quantum wells and the zero dimensional InAs quantum dots by tuning the probe photon energy. We find that for low photoinjected carrier densities, carrier capture and relaxation are dominated by Auger carrier-carrier scattering at low temperatures, with thermal emission playing an increasing role with temperature. At low temperatures we also observe excitation-dependent shifts of the quantum dot energy levels. In contrast, high density measurements reveal an anomalous induced absorption at the quantum dot excited state that is correlated with quantum well population dynamics. Our experiments provide essential insight into carrier relaxation across multiple spatial dimensions and reveal unique Coulomb interaction-induced phenomena, with important implications for DWELL-based lasers and amplifiers.

The free-electron laser FELBE at the Helmholtz-Zentrum Dresden Rossendorf enables experiments with spectral, temporal, and, by means of near-field microscopy, also high spatial resolution. FELBE delivers picosecond IR and THz pulses in a wavelength range from 4 μm to 280 μm. Here we review the potential of the laser and focus on two highlight pump-probe experiments. In the first experiment, the relaxation dynamics in self assembled InGaAs quantum dots at energies below the Reststrahlen band is studied. Long intradot relaxation times (1.5 ns) are found for level separations of 14 meV (3.4 THz), decreasing very strongly to ~ 2 ps at 30 meV (7 THz). The results are in very good agreement with our microscopic theory of the carrier relaxation process, taking into account polaron decay via acoustic phonons. In the second experiment, the relaxation dynamics in graphene is investigated at photon energies E = 20 - 250 meV. For excitations below the energy of the optical phonon (G mode), the relaxation times are more than one order of magnitude longer as compared to the relaxation times observed for near infrared excitation.

In this paper we review our theoretical work on slow and fast light effects in quantum dot semiconductor optical amplifiers (QD SOAs), in particular we investigate the carrier dynamical contributions to the dynamic gain grating and cross gain modulation induced by unique ultrafast inter-subband carrier dynamics between discrete QD bound states. Our calculations predict that by increasing the injection current density, additional ultra-fast coherent gain contributions around 100GHz arise in contrast to the slow sub-gigahertz carrier density pulsation (CDP) effects. For potential applications in microwave photonics, especially targeting the millimeter wave range, we propose that quantum dot devices might be used to realize an optically fed microwave phase shifter in the frequency range of 100GHz.

Extraordinary optical transmission through subwavelength metallic hole-arrays has been an active research area since its first demonstration. The frequency selective resonance properties of subwavelength metallic hole arrays, generally known as surface plasmon polaritons, have potential use in functional plasmonic devices such as filters, modulators, switches, etc. Such plasmonic devices are also very promising for future terahertz applications. Ultrafast switching or modulation of the resonant behavior of the 2-D metallic arrays in terahertz frequencies is of particular interest for high speed communication and sensing applications. In this paper, we demonstrate ultrafast optical control of surface plasmon enhanced resonant terahertz transmission in two-dimensional subwavelength metallic hole arrays fabricated on gallium arsenide based substrates. Optically pumping the arrays creates a thin conductive layer in the substrate reducing the terahertz transmission amplitude of both the resonant mode and the direct transmission. Under low optical fluence, the terahertz transmission is more greatly affected by resonance damping than by propagation loss in the substrate. An ErAs:GaAs nanoisland superlattice substrate is shown to allow ultrafast control with a switching recovery time of ~10 ps. We also present resonant terahertz transmission in a hybrid plasmonic film comprised of an integrated array of subwavelength metallic islands and semiconductor hole arrays. Optically pumping the semiconductor hole arrays favors excitation of surface plasmon resonance. A large dynamic transition between a dipolar localized surface plasmon mode and a surface plasmon resonance near 0.8 THz is observed under near infrared optical excitation. The reversal in transmission amplitude from a stop-band to a pass-band and up to π/ 2 phase shift achieved in the hybrid plasmonic film make it promising in large dynamic phase modulation, optical changeover switching, and active terahertz plasmonics.

In this contribution we concentrate on two aspects of THz science related to surface and particle plasmons. Firstly, we report on the generation of THz pulses via irradiation of arrays of silver nanoparticles by femtosecond laser pulses. We propose that this effect arises from the emission of photoelectrons by multi-photon excitation and subsequent acceleration of these emitted electrons by ponderomotive forces associated with the optical fields of the plasmons in the metallic nanostructures. Secondly, we demonstrate that semiconductors supports strongly confined surface plasmons in the THz frequency range. We show that these SPs can be utilized to enhance the light-matter interaction with dielectric layers above the semiconductor surface, thereby allowing us to detect the presence of layers around one thousand times thinner than the free space wavelength of the THz light. We discuss the viability of using semiconductor SPs for the purposes of THz sensing and spectroscopy.

We report on a comprehensive investigation of radiation characteristics of THz emitters using structures based on the Sierpinski fractal. Self-similarity present in the geometrical properties of these antennas improves the coupling of emitted radiation to free space. Using THz-TDS it is shown that these novel antennas produce higher radiation power when compared to the bow-tie antenna. To the best of our knowledge this is the first time that Sierpinski fractal antennas have been used as emitters for generating THz radiation.

Transient reflectivity pump-probe experiments are performed on a single gold particle to analyze the vibration of the particle as well as the propagation of the resulting GHz acoustic wave in the embedding medium. In a first part, the vibration of a single 430 nm diameter gold particle embedded in a silica matrix is investigated. A semi-analytical model is presented and demonstrates that the detection mechanism relies on an intrinsic common-path interferometer which directly images the particle interface displacement. The coherent phonon propagation inside the embedding medium is then studied in the case of a gold particle embedded in an agarose gel. A comparison between experimental results and calculations suggests a detection of the Brillouin scattering in agarose, so long as the Brillouin frequency at the considered probe wavelength matches the fundamental breathing mode frequency (or one of its harmonics) of the particle.

Nanosecond laser-induced damage (LID) in potassium dihydrogen phosphate (KH₂PO₄ or KDP) remains an issue for light-frequency converters in large-aperture lasers such as NIF (National Ignition Facility, in USA) and LMJ (Laser MegaJoule, in France). In the final optic assembly, converters are simultaneously illuminated by multiple wavelengths during the frequency conversion. In this configuration, the damage resistance of the KDP crystals becomes a crucial problem and has to be improved. In this study, we propose a refined investigation about the LID mechanisms involved in the case of a multiple wavelengths combination. Experiments based on an original pump-pump set-up have been carried out in the nanosecond regime on a KDP crystal. In particular, the impact of a simultaneous mixing of 355 nm and 1064 nm pulses has been experimentally studied and compared to a model based on heat transfer, the Mie theory and a Drude model. This study sheds light on the physical processes implied in the KDP laser damage. In particular, a three-photon ionization mechanism is shown to be responsible for laser damage in KDP.

We review our recent work in the field of organic spintronics, and show that the spin- and time-resolved two-photon photoemission (STR-2PPE) technique can be used to obtain quantitative information about the spin-dependent properties of hybrid organic-inorganic interfaces, as well as about the spin-dependent transport in organic semiconductors. In addition, we present STR-2PPE measurements performed on the Co-copper phthalocyanine (CuPc) system at different temperatures to investigate the microscopic processes repsonsible for the spin-polarization decay in organic semiconductors and at interfaces with such materials. We found no significant temperature dependence of the spin-injection efficiency across the Co-CuPc interface as well as of the spin-polarization decay length in CuPc.

Terahertz (THz) time domain spectroscopy (TDS) is widely used in a broad range of applications where knowledge of both the amplitude and phase of a THz wave can reveal useful information about a sample. However, a means of amplifying THz pulses which would be of great benefit for improving the applicability of TDS is lacking. While THz quantum cascade lasers (QCL) are promising devices for THz amplification, gain clamping limits the attainable amplification. Here we circumvent gain clamping and demonstrate amplification of THz pulses by ultrafast gain switching of a QCL via the use of an integrated Auston switch. This unclamps the gain by placing the laser in a non-equilibrium state that allows large amplification of the electromagnetic field within the cavity. This technique offers the potential to produce high field THz pulses that approach the QCL saturation field.

Quantum cascade lasers (QCLs) are unipolar devices composed of repeated stack of semiconductor multiple quantum well heterostructures utilizing intersubband transitions and resonant tunneling. In this paper, 120 fs Mid-IR pulses are used to investigate the nature of carrier transport through the quantum wells and barriers of a pulse biased, room temperature operating ultrastrong coupling design QCL. Despite the low average power of Mid-IR pulses, we managed to efficiently couple these pulses into the QCL waveguide so as to observe distinct phenomena by varying the pump and probe's power. Biased just below the threshold, we observed a strong gain depletion dip at t=0 which is mainly caused by the depletion of electrons from the upper lasing state mainly by stimulated emission. Ultrafast gain recovery within the first 200 fs was observed. This is mainly attributed to phonon scattering and electrons resonantly tunneling through a much thinner injector barrier, which overcomes the interface-roughness-induced detuning of resonant tunneling. Electron transport through the injector region contributes to a slower gain recovery lifetime of 2-3 ps.

Ultrashort laser pulses with a duration of 200 fs were obtained from a passively modelocked external cavity diode laser at 830 nm emission wavelength. By intracavity dispersion control the spectral bandwidth is increased and the emitted pulses are compressed externaly by a grating compressor. A tapered amplifier is used to achieve peak powers of up to 2.5 kW.

We discuss high speed switching of lasing circular polarizations in VCSELs by optical spin injection. We conducted polarization- and time-resolved measurements of two consecutive lasing outputs from a (110)-InGaAs/GaAs VCSEL at 77 K with different time delays between the two optical excitations for alternately up- and down-spin electrons. 1-GHz switching of lasing circular polarizations has been demonstrated with taking advantage of the long electron spin relaxation time τ_s in (110)-QWs. Rate equation analysis closely reproduced the measured results and showed that shortening the carrier lifetime τ_c while preserving the long τ_s is a straightforward solution for faster switching since the residual unpolarized electrons limit the switching speed. Thus, we dry-etched the (110)-QWs into micro-posts to introduce the surface non-radiative recombination using ECR-RIE, and investigated the τ_c and τ_s. Spin-polarized carriers were optically excited in square posts with different sizes from 0.5 μm to 30 μm, and the time evolutions of two orthogonal circular polarization components of photoluminescence were measured by a streak camera. The long τ _s (~1.3 ns) in the (110)-QW wafer is found to be preserved even when the sidewall boundaries with fast surface recombination are introduced and the τ_c is drastically shortened. The same rate equation analysis indicated that spin-controlled VCSELs with such (110)-QW micro-posts will exhibit faster switching thanks to the shortened τ_c and preserved long τ_s. In particular, 20-GHz switching is expected with 0.5-μm posts, although the threshold pulse energy per unit area becomes 2.9 times larger than that for 1-GHz switching without post structure.

We experimentally and theoretically investigate injection currents generated by femtosecond single-color circularly-polarized laser pulses in (110)-oriented GaAs quantum wells. The current measurements are performed by detecting the emitted Terahertz radiation at room temperature. The microscopic theory is based on a 14 x 14 k • p band-structure calculation in combination with the multi-subband semiconductor Bloch equations. For symmetric GaAs quantum wells grown in (110) direction, an oscillatory dependence of the injection currents on the exciting photon energy is obtained. The results of the microscopic theory are in good agreement with the measurements.

Using our novel high field terahertz source we performed various nonlinear experiments on semiconductor nanostructures. In one-dimensional nonlinear propagation experiments on n-type GaAs we observed ballistic high-field transport and THz-induced interband tunneling of electrons. Two-dimensional THz correlation spectroscopy performed on intersubband transitions of two coupled quantum wells shows distinct polaronic features of the intersubband transitions.

We report the carrier density dependence of carrier dynamics of Mg-doped InN (InN:Mg) films. Recently, we have demonstrated a significant enhancement of terahertz emission from InN:Mg, which is due to the temporal evolution of drift and diffusion currents depending on the background carrier density. We studied the details of carrier dynamics of InN:Mg which is crucial for the clarification of the terahertz emission mechanism by performing the time-resolved optical reflectivity measurement on InN:Mg films grown with different Mg-doping levels. Experimental analysis demonstrates that the initial sharp drop and recovery of reflectivity response of InN:Mg films are dominated by photocarrier-dependent bandgap renormalization and band filling processes, whereas the slow decay time constant (τ₂) of reflectivity of InN:Mg has the strong dependence on the background carrier density. As the carrier density decreases from that of undoped InN, τ₂ of InN:Mg continuously increases and reaches the maximum value at a critical value of ~1x10¹⁸ cm^-3. Interestingly, the strongest terahertz radiation was observed at this carrier density and it keeps decreasing with the increase of carrier density. Intense terahertz radiation corresponds to the fast and large spatial separation of charged carrier density through diffusion and drift. Large spatial separation results in the longer decay time for charged carriers to reach equilibrium after strong emission of terahertz waves, and it explains the similar carrier density dependence of terahertz emission and τ₂.

Ultrafast hot carrier dynamics in Indium nitride (InN) epitaxial films were investigated by femtosecond time-resolved pump-probe reflection measurements. Carrier density and carrier energy dependence of the hot carrier dynamics in InN were studied by varying the pump laser power and wavelength, respectively. Experimental results showed that the hot carrier relaxation can be fitted by a biexponential relaxation process. The fast relaxation rate increased with increasing carrier density (N), which was measured as N^0.5. The fast relaxation rate also increased with increasing carrier energy (E), which was measured as E^0.53. These observations revealed that electron-electron scattering plays an important role in the fast relaxation process. The slow relaxation process was found to be dominated by Auger scattering and the slow relaxation rate was independent of the carrier energy. The defect-related trapping time in InN was estimated to be ~515 ps.

We report on very long electron spin lifetimes in cubic GaN measured by time-resolved Kerr-rotation-spectroscopy. The spin coherence times with and without external magnetic field exceed 500 ps at room temperature, despite a high n-type doping level of more than 10¹⁹ cm^-3 in the bulk sample under investigation. Our findings are therefore highly relevant for spin optoelectronics in the blue wavelength regime. The spin lifetimes are found to be almost temperature independent in accord with a prediction for degenerate electron gases of Dyakonov and Perel from 1972. These results are discussed also in comparison to wurtzite GaN, which shows much shorter spin lifetimes and a dependence of spin lifetimes on the spin orientation.

The relaxation dynamics of photo-excited carriers of indium nitride (InN) films and nanocolumns were examined using degenerate pump-probe measurements at room temperature. We measured two InN films and nanocolumns with different background carrier densities, and performed numerical calculations incorporating band-filling and bandgap-renormalization effects, as well as LO phonon scattering. We found that the intrinsic relaxation properties of InN can be understood by considering the density of states and electron occupation number of the conduction band. It was also revealed that the decay dynamics of InN are not affected by the carrier recombination time under the appropriate conditions. In addition, we examined the differences in carrier relaxation properties between films and nanocolumns.

Time integrated and time resolved microphotoluminescence studies have been performed on In_xGa_1-xN quantum disks embedded in GaN nanocolumns. The results are analysed in context of current theories regarding an inhomogeneous strain distribution in the disk, which is theorised to generate lateral charge separation in the disks by strain induced band bending, an inhomogeneous polarization field distribution, and Fermi surface pinning. It is concluded that no lateral separation of carriers occurs in the quantum discs under investigation.

Ultrafast x-ray diffraction (UXRD) has become more and more prevalent in various scientific disciplines that are interested in directly observing atomic motion in real time. The timescale, amplitude and phase of collective atomic motion can be determined with high accuracy, even when the induced amplitude is smaller than thermal fluctuations. The structural rearrangements induced by an ultrafast stimulus (charge carriers excitation or heat deposition by a laser pulse) can be recorded in real time. Here we report on a new laser-driven plasma-x-ray source (PXS) and discuss different applications which will be addressed in UXRD experiments.

Heat dissipation from a nanoscale hot-spot is expected to be non-diffusive when a hot-spot is smaller than the phonon mean free path of the substrate. Our technique of observing diffraction of coherent soft x-ray pulses allows for very high resolution (~pm) of thermally-induced surface distortion, as well as femtosecond time resolution of dynamics. We successfully model our experimental results with a diffusive transport model that is modified to include an additional boundary resistance. These results confirm the importance of considering ballistic transport away from a nanoscale heat source, and identify a means of correctly accounting for this ballistic transport.

The amplification of surface plasmon polaritons in planar metallic waveguides via propagation through an optically pumped dipolar gain medium incorporated into one of the claddings is discussed theoretically and experimentally. Physically realisable arrangements based on the single-interface and on the thin metal film are described. Experimental results are given, demonstrating amplification of the long-range surface plasmon-polariton along a thin metal stripe at near infrared wavelengths using a dye gain medium. Low amplified spontaneous emission noise into this mode is simultaneously observed.

The energy states in semiconductor quantum dots are discrete as in atoms, and quantum states can be coherently controlled with resonant laser pulses. Long coherence times allow the observation of Rabi-flopping of a single dipole transition in a solid state device, for which occupancy of the upper state depends sensitively on the dipole moment and the excitation laser power. We report on the robust preparation of a quantum state using an optical technique that exploits rapid adiabatic passage from the ground to an excited state through excitation with laser pulses whose frequency is swept through the resonance.

We present a microscopic description of the photon statistics and phonon signatures in the quantum light emission from semiconductor quantum dots. Using higher-order Born approximation, time-resolved optical emission and scattering spectra are calculated in the weak coupling regime. In the strong coupling regime, a mathematical induction method is presented to calculate the longitudinal optical (LO) phonon-assisted quantum dot cavity-QED. The phonon interaction at room temperature has a strong impact on the quantum correlation of the cavity field and enhances the non-classicality of the cavity field, initially prepared in a thermal state.

The two-photon excited fluorescence spectra from CdSe quantum dots on SiN film with two-dimensional photonic crystal are studied. The fluorescence and decay process from quantum dots in solvent are compared. Two-photon excited fluorescence is enhanced by photonic crystal in vertical direction, and the blue shift of the spectrum is found on photonic crystal. The mechanism of the enhancement and the blue shift of spectrum are explained.

The dynamics of Förster resonance energy transfer (FRET) in mixed-size water-soluble CdTe quantum dots (QDs) are studied by using photoluminescence (PL) and time-resolved PL spectroscopy. When donor concentration is increased, an enhancement of both the FRET and quantum efficiency in the mixed-size CdTe QDs films can be observed. Increasing donor concentration significantly quenches the emission intensity and lifetime in donor QDs and enhances that in acceptor QDs. However, as D/A ratios exceed 6, the emission intensity and the lifetime of acceptor QDs start to decline, reflecting a decreasing in both quantum and FRET efficiency due to a markedly declining availability of acceptor QDs.

GaAs/AlAs coupled multilayer cavity structures on high-index substrates have been proposed as novel terahertz emission devices. Two cavity modes with an optical frequency difference in the terahertz region are realized when two cavity layers are coupled by an intermediate distributed Bragg reflector multilayer. Optical responses to ultrashort laser pulses have been simulated using the transfer matrix method. Interference between the enhanced light fields of the cavity modes was demonstrated when they were simultaneously excited by 100 fs Gaussian pulses. Extremely strong sum-frequency generation was experimentally observed in the (113)B coupled multilayer cavity. We also found that the polarization control by wafer-bonding might be one of the best ways to generate terahertz difference-frequency signal of two modes.

Fiber-optic Cherenkov radiation has emerged as a wavelength conversion technique to achieve isolated spectrum in the visible wavelength range. Most published results have reinforced the impression that CR forms a narrowband spectrum with poor efficiency. We both theoretically and experimentally investigate fiber-optic Cherenkov radiation excited by few-cycle pulses. We introduce the coherence length to quantify the Cherenkov-radiation bandwidth and its dependence on propagation distance. Detailed numerical simulations verified by experimental results reveal three unique features that are absent when pumped with often-used, long pulses; that is, continuum generation (may span one octave in connection with the pump spectrum), high conversion efficiency (up to 40%), and broad bandwidth (70 nm experimentally obtained) for the isolated Cherenkov radiation spectrum. These merits allow achieving broadband visible-wavelength spectra from low-energy ultrafast sources which opens up new applications.

We summarized our recent studies on the photocarrier recombination dynamics of SrTiO₃ by means of time-resolved photoluminescence (PL) spectroscopy and transient absorption (TA) spectroscopy. Novel room-temperature blue PL was observed in strongly photoexcited SrTiO₃ and electron-doped SrTiO₃. We revealed that both PL and TA dynamics are determined by a simple carrier recombination model and nonradiative three-particle Auger process determines the carrier recombination dynamics under high-density photoexcitation. We discussed the temperature-dependent PL spectrum and dynamics of strongly photoexcited SrTiO₃.

We perform two-colour pump probe experiments on metals and superconductors using synchronized 10-fs pulses generated by optical parametric amplifiers, tunable from the visible to the near-infrared, mapping with unprecedented detail the energy equilibration dynamics of the free electron gas. In gold films we observe dramatic changes of the differential reflectivity spectrum on the 100-fs timescale, corresponding to the establishment of the thermal electron distribution, with dynamics dictated by excess energy. In high-Tc cuprate superconductors we observe fast electron relaxation, attributed to a strong electron-phonon coupling which may play a role in the superconductivity mechanism.

We evaluate a density matrix theory for the description of ultrafast relaxation processes in low-dimensional carbon nanostructures. The theory is based on Bloch equations describing the temporal dynamics of charge carrier population and transition probabilities. In combination with tight-binding wavefunctions, the approach allows the microscopic calculation of linear and nonlinear optical properties of graphene and carbon nanotubes with arbitrary chirality. This way, we have access to time- and momentum-resolved relaxation dynamics of non-equilibrium charge carriers. We study absorption spectra in graphene and carbon nanotubes illustrating the importance of excitonic effects in these structures including the formation of exciton-phonon induced side-bands in carbon nanotubes. Furthermore, we illustrate the relaxation of optically excited charge carriers toward equilibrium via electron-phonon and electron-electron scattering. We observe an ultrafast thermalization of excited carriers within the first hundred femtoseconds followed by a cooling of the electronic system on the picosecond time scale. Moreover, we investigate phonon-induced intersubband relaxation between the two energetically lowest transitions in nanotubes leading to a better understanding of photoluminescence excitation (PLE) experiments.

We study excitation energy and charge transfer in small aggregates of chirality enriched carbon nanotubes by transient absorption spectroscopy. Ground state photobleaching is used to monitor exciton population dynamics with sub-10 fs time resolution. Upon resonant excitation of the first exciton transition in (6,5) tubes, we find evidence for energy transfer to (7,5) tubes within our time resolution (< 10 fs). After pumping at high pump energies, free charge carriers are produced via exciton scattering into the underlying continuum bands. We obtain clearly distinguished photoinduced features in the visible spectral range, that allow for real-time tracing of charge carrier dynamics in carbon nanotubes on a sub-picosecond time scale.

A novel metrological approach bridges a dynamic holography and photoelectrical phenomena in semiconductors for monitoring the temporal and spatial non-equilibrium carrier dynamics. Light interference pattern of two coherent picosecond pulses was used to inject spatially modulated carrier pattern, modulate temporally the complex refractive index of a semiconductor, and thus create a light-induced transient diffraction grating (LITG). Recording of a thin grating at interband carrier generation with subsequent probing of spatial and temporal carrier dynamics by a delayed probe beam allowed investigation of various recombination mechanisms, covering linear, surface-limited, and nonlinear (bimolecular and Auger). Decay of LITG at its various spacings provided either the bipolar carrier mobility or minority one in heavily doped layers, diffusivity of degenerate plasma, as well revealed impact of carrier localization and band gap renormalization on carrier transport. Diffraction on thick Bragg gratings, recorded via deep impurity-assisted carrier generation revealed simultaneous index modulation by free-carriers, space-charge electric field, and recharged deep traps, thus enabling access to photoelectric parameters of the compensating centers. Grating decay in multiple quantum well structures (MQWS) provided carrier and spin relaxation rates, electron mobility, in-plane and cross-well transport. Spatial and temporal carrier dynamics in a wide excitation and temperature range is reviewed in a variety of III-nitride compounds (GaN, InGaN, AlGaN), GaAs, CdTe, InP, SiC, diamond films, and MQWS.

In this experimental work, we optically excite the eigenmodes of an elliptical resonator in a semiconductor micro-cavity. Using a pulsed excitation, we create a superposition of eigenmodes, and image the time evolution of the coherent emission pattern. Oscillations between vortex and anti-vortex states are observed, and remarkably well described within the Poincaré sphere representation for an eigenmode containing an orbital angular momentum. A semiconductor quantum well is embedded in the microcavity structure. The system is operated in the strong light matter coupling regime, where the eigenmodes are hybrid half-photonic half-excitonic quasiparticles called exciton polaritons.

Volume holographic gratings (VHG) are of wide interest in many applications because of their properties of high diffraction efficiency, excellent wavelength selectivity and angular selectivity. Recently, the potential application of VHG for manipulating laser pulses to achieve functional waveforms with large bandwidth that can be used for a variety of applications has attracted a significant amount of attention because of the flexibility and the possibility of using VHG to implement dynamic processing. However, these studies have dealt with the properties due to a single VHG rather than with a class of novel diffraction elements - multiple-layer volume holographic gratings (MVHG). In this paper, we extend the coupled wave theory of multi-layer gratings to study the Bragg diffraction properties of ULP, and present a systematically theoretical analysis on the spectrum distribution of the diffracted intensities, the diffraction bandwidth, and the total diffraction efficiency of a system of MVHG. The analysis and observations of this paper will be valuable for the accurate analysis of the interaction of ultrashort optical pulses and a variety of periodic structures, facilitating the design and the investigation of novel optical devices based on multiple layers of VHG.

We propose a simple method to create a local magnetic field minima for the magnetic trapping and confining of excitons. We observe an enhanced spatially resolved photoluminescence of the optically active heavy-hole excitons concentrated at the confining region in a multiple quantum wells system. We draw the attention to consider the proposed trapping mechanism as an approach to reach the Bose-Einstein condensate limit of excitons.

We report on the observation of an inhomogeneous spin-dependent spatial distribution of heavy-hole excitons generated by a localized inhomogeneous magnetic weak field. An exciton energy splitting is observed between the spin-up and spin-down states with an energy gap as a function of the magnetic field.