Recent advances of femtosecond semiconductor physics at the limits of single electrons and photons down to sub-cycle time scales are presented. The first part deals with ultrafast measurements on single-electron systems: The transient quantum dynamics in a single CdSe/ZnSe quantum dot is investigated via femtosecond transmission spectroscopy. A two-color Er:fiber laser with excellent noise performance is key to these first resonant pump-probe measurements on a single-electron system. We have observed ultrafast bleaching of an electronic transition in a single quantum dot due to instantaneous Coulomb renormalization. Since we were also able to invert the two-level system, optical gain due to a single electron has been detected. By using π-pulses for probing, we could deterministically add or remove a single photon to or from a femtosecond light pulse, leading to non-classical states of the light field. In order to optimize electron-photon coupling, nanophotonic concepts like dielectric microresonators and metal optical antennas are explored. In the second part of the paper, we present multi-terahertz measurements on low-energy excitations in semiconductors. These studies lead towards a future time-domain quantum optics on a time scale of single cycles of light: Intense multiterahertz fields of order MV/cm are used to coherently promote optically dark and dense para excitons in Cu₂O from the 1s into the 2p state. The nonlinear field response of the intra-excitonic degrees of freedom is directly monitored in the time domain via ultrabroadband electro-optic sampling. The experimental results are analyzed with a microscopic many-body theory, identifying up to two internal Rabi cycles. Subsequently, intersubband cavity polaritons in a quantum well waveguide structure are optically generated within less than one cycle of light by a femtosecond near-infrared pulse. Mid-infrared probe transients trace the non-adiabatic switch-on of ultrastrong light-matter coupling and the conversion of bare photons into cavity polaritons directly in the time domain.

Understanding the nonlinear optical processes in semiconductor nanostructures leads to possible applications in areas including laser amplifiers, optical switches, and solar cells. Here we present a study of the frequency degenerate two-photon absorption (2PA) spectrum of a series of PbS and PbSe quantum dots (QDs). The influence of the quantum confinement is analyzed using a four-band model which considers the mixing of valence and conduction bands. In contrast to our observations of CdSe QDs, the present results point to an increase of the 2PA cross-section (normalized by the QD volume) as the quantum dot size is made smaller. This is explained by the symmetry between the valence and conduction bands which allows the density of states to remain high even for small QDs. A study of the ultrafast carrier dynamics of the PbS quantum dots is also presented. Through nondegenerate femtosecond pump-probe experiments we show evidence of multi-exciton generation with quantum yield (number of excitons generated per absorbed photon) up to 170% for excitation with hω> 3 E_g (where E_g is the bandgap energy).

We perform ultrafast pump-probe experiments on a 50 period Ge/SiGe multiple-quantum-well structure held at room temperature. Tunable 80 fs pulses emitted by an opto-parametric amplifier are used as a pump and a white-light supercontinuum generated directly from a 1 kHz Ti:sapphire regenerative amplifier system is used as a probe. The resulting spectro-temporal response shows three distinct temporal regimes. Coherent oscillations dominate at negative times yielding a well-defined time zero across the whole detected spectral range. Dynamics are observed within the direct conduction band valley during and shortly after the excitation while the electrons are also scattered towards the indirect minima. After several hundreds of fs to a few ps almost all electrons populate the L-valley states. These carriers decay out of the L-valleys on a timescale longer than several ns. During the first ps, carrier inversion is obtained for strong enough pumping due to faster intra-valley than intervalley scattering. The obtained gain values are similar in magnitude to those observed in typical III-V compound semiconductors.

Single particle transient absorption experiments have been used to study metallic and semiconducting nanowires. For the metal wires the major result is the observation of modulations in the transient absorption traces due to coherently excited breathing modes. The vibrational periods depend on the dimensions of the nanowire, and the decay times are sensitive to the environment. The nanowires in our experiments are spin coated from a polymer solution onto a glass substrate, and experience a range of different environments. This causes large variations in the quality factor of the breathing mode for different wires. Semiconducting nanowires of CdTe and CdSe were also examined. The CdTe wires show fast picosecond time scale dynamics, which are assigned to charge carrier trapping at surface states of the wires. In contrast, CdSe nanowires show no dynamics on the time scale of our measurements. For the CdTe nanowires the charge carrier trapping times vary from wire-to-wire, and also vary with position in a single wire. This is attributed to differences in surface chemistry. Overall these experiments illustrate the important of single particle techniques for studying nanomaterials, especially for elucidating how differences in local environment and structure affect dynamics.

The ongoing quest for semiconductor lasers with low threshold has led to the development of new materials (quantum wells, wires, and dots) and new optical resonators (microdisks and photonic bandgap crystals). In a novel approach to "thresholdless" lasers, we have developed a new growth technique for self-assembled deep-centers in the technologically important semiconductor gallium-arsenide (GaAs). We recently demonstrated the first GaAs deep-center laser. These lasers, which intentionally utilize GaAs deep-center transitions, exhibit a threshold current density of less than 70mA/cm2 in continuous-wave mode at room temperature at the important 1.54 μm fiber-optic wavelength in a single-pass geometry. We studied fast carrier dynamics in the new GaAs deep-center laser. The low threshold current was a consequence of fast subpicosecond capture of free holes onto deep-centers. This fast capture of free holes onto deep-centers allowed fast depopulation of electrons out of the lower energy level of the optical transition. We demonstrated laser action at many wavelengths between 1.2 μm and 1.6 μm, including fiber-optic wavelengths. A significance is that it has been a long-sought goal to tune the stimulated-emission from the same semiconductor over a wide wavelength range. A semiconductor source of tunable coherent radiation would have many applications, e.g., fiber-optics, spectroscopy, lab-on-a-chip, chemical species identification.

A microscopic theory for the terahertz response of a semiconductor quantum well under coherent conditions is presented. It is shown that excitonic effects influence the intersubband absorption under certain conditions. For high-quality samples, one should be able to resolve both band-to-band and excitonic intersubband transitions in an terahertz absorption measurement. Due to the competition of intersubband transitions and classical field-induced carrier accelerations, an unexpected Fano feature is observed in the terahertz spectra. This result is in excellent agreement with recent measurements.

We review our recent efforts on power scaling of THz pulses generated by several nonlinear crystals. By using a single GaP crystal and stacking three GaP plates, we have significantly increased the output peak power to as high as 722 W and 2.36 kW, respectively. On the other hand, by using CO₂ laser pulses, we have obtained the average output power of 260 μW. We have also used these laser pulses to scale up the output power for the THz pulses to 29.8 μW by stacking GaAs wafers. Indeed, by stacking up to ten wafers, we have increased the output power by a factor of 160. Finally, by using ultrafast laser pulses, we have achieved record-high output powers for the THz pulses generated from multi-period periodically-poled LiNbO₃ crystals based on a backward configuration. The highest output power obtained by us so far is 10.7 μW.

We report ultrafast carrier dynamics in hybrid CdSe nanorod/poly(3-hexythiophene) (P3HT) bulk heterojunction films measured by time-resolved terahertz spectroscopy, and compare to the well studied P3HT/phenyl-C61-butyric acid methyl ester (PCBM) blend. Both films show an improved peak photoconductivity compared to P3HT alone, consistent with efficient charge transfer. The photoconductivity dynamics show fast, picosecond trapping or recombination in the hybrid blend while the all-organic film shows no such loss of mobile charge over ns time scales. The ac conductivity for all samples is well described by a Kramers-Kronig compatible Jonscher-type power law with exponent between 0.5 and 1 suggesting that interchain hopping in the polymer or between nanorods occurs at frequencies higher than 3 THz immediately after photoexcitation.

Femtosecond pump-probe methods are useful tools for investigating transient electronic and vibrational states of conducting materials and molecular photochemistry. Ultraviolet and visible excitation pulses (<150 fs, <20 μJ, 400-800 nm) with time-delayed broadband terahertz (~500 GHz to 3 THz) probing pulses (Time-Resolved Terahertz Spectroscopy; TRTS) are used to measure linear spectroscopic transmission changes resulting from exciton and free carrier population in organic semiconducting thin films. Picosecond timescale exciton geminate recombination and longer-time free-carrier conduction in semiconductor polymers and nanolayered donor-acceptor films are discussed. Systems investigated with terahertz probe pulses include thiophene-based polymers (P3HT, PBTTT) studied as drop and spin-cast films on transparent quartz substrates. The relative conductivity of these films increases with increasing P3HT polymer molecular weight, structural regularity, and the fused rings in PBTTT further increases conduction. Recent studies of composite and nanolayered films (by vapor deposition) containing alternating Zn-phthalocyanine (ZnPc) and buckminsterfullerene (C₆₀) also yield high conduction that scales linearly with the number of interfaces and total film thickness. We find evidence for a short-lived charge transfer state of C₆₀ that decays within several picoseconds of excitation. In contrast, both composite and multilayered films exhibit long-lived THz dynamics that depends on the composition and structure of the films. The optimum composition for charge transfer within composite films is observed for a ~1:1 blend of ZnPc with C₆₀ and a 4:1 blend of P3HT with Phenyl C₆₁ Butyric Acid Methyl Ester (PCBM) while an increase in charge photo-generation with decreasing layer thickness (2 nm) exhibits the strongest THz signal. These findings parallel results for FET polymer transistor devices pointing to the advantage of optically measuring material properties before device test.

The influence of water on biomolecular interfaces and functionality has been in the focus of hydration studies. Improved experimental and computational probes gave insight to this question from different perspectives. The aspect of collective water network dynamics has been experimentally accessed by terahertz (THz) spectroscopy, which is sensitive to even small solute-induced rearrangements of the water network in the biomolecular surroundings. THz hydration studies uncovered that the dynamical hydration shell of saccharides consists of several hundred water molecules and up to thousand water molecules for proteins. Mutations at the protein surface and inside the core perturb the dynamical hydration, whereas it is noticeable that native wild-type proteins most significantly affect hydration dynamics. Kinetic THz absorption (KITA) studies of protein folding recently revealed that solvent dynamics are coupled to secondary structure formation of the protein. The solvent water network is dynamically rearranged in milliseconds before the protein folds to its native state within the following seconds. THz spectroscopy gives experimental evidence that collective long-range dynamics are a key factor of biomolecular hydration.

We demonstrate the potential utilization of a Schottky barrier in the plasmonic regime at terahertz (THz) frequencies. Experimental evidence of local THz plasmonic field enhancement via radiation from the space-charge distribution at a Schottky interface is shown. A 12% increase in the plasmonic-mediated transmission of THz radiation through random, dense ensembles of Cu particles is observed when a Cu_xO/Au structure is introduced to the surface of the particles. The THz electric field induces oscillations of the local charge density at the Schottky interface leading to emission of high frequency radiation as the charges settle back into equilibrium. The non-linear THz response of the Schottky interface introduces the physical groundwork for the implementation of plasmonic circuits that have operational frequencies exceeding the limits of traditional semiconductor electronics.

An all-optical magnetization switching protocol is presented where the magnetization associated with a single Mn atom embedded in a single CdTe quantum dot is controlled on a picosecond time scale. Even though there is no direct optical coupling to the Mn spin, the control may be achieved by the optical excitation and manipulation of spin-polarized carriers that couple to the Mn spin via the exchange interaction. It is shown that the Mn spin can be selectively driven into each of its spin eigenstates. By suitably chosen pulse sequences also well defined superposition states can be prepared. The Mn spin dynamics is directly reflected in pump probe type signals.

A microscopic theory that describes injection currents in GaAs quantum wells is presented. 14 × 14 band k.p theory is used to compute the band structure including anisotropy and spin-orbit interaction. Transient injection currents are obtained via numerical solutions of the semiconductor Bloch equations. Depending on the growth direction of the considered quantum well system and the propagation and polarization directions of the incident light beam, it is possible to generate charge and/or spin photocurrents on ultrashort time scales. The dependence of the photocurrents on the excitation conditions is computed and discussed.

We review recent experiments on spin excitation and manipulation in the ferromagnetic semiconductor GaMnAs. Spin dynamics in GaMnAs have been studied by two complementary approaches - by frequency-domain techniques, such as Brillouin light scattering (BLS) and ferromagnetic resonance (FMR); and by optical real-time techniques, such as ultrafast pump-probe magneto-optical spectroscopy. Using BLS and FMR, magnon frequencies (or resonance fields), were investigated as a function of Mn concentration, temperature and direction of magnetization, leading to information on magnetic anisotropy. Time-resolved magneto-optical Kerr effect, on the other hand, was used to study photo-induced femtosecond magnetization rotation, ultrafast optical demagnetization, and collective magnetization precession. Optically-induced transient changes in magnetization of GaMnAs produced by femtosecond laser pulses are analyzed and discussed in terms of the Landau-Lifshitz-Gilbert model. Finally, for completeness, we also discuss carrier-mediated nonthermal and thermal (lattice-heating) contributions to spin dynamics.

The interplay between disorder and Coulomb interactions ubiquitously affects the properties of condensed matter systems. We examine its role in the nonlinear optical response of semiconductor quantum wells. In particular, we investigate the coherent coupling strength between exciton resonances that are spectrally split by interface fluctuations. Previous studies yielded conflicting results. In light of rising interest in semiconductor devices that rely on spatial and/or temporal coherence, we revisit this problem by applying a newly developed spectroscopy method: electronic two-dimensional Fourier transform spectroscopy (2DFTS). 2DFTS is a powerful technique for revealing the presence of coupling and for distinguishing the (coherent or incoherent) nature of such coupling, especially in complex systems with several spectrally overlapping resonances. Even the most basic information about such complex systems, including the homogeneous and inhomogeneous linewidths of various resonances, cannot be extracted reliably using conventional spectroscopic tools. In these new 2DFTS measurements, we did not observe any clear cross peaks corresponding to coherent couplings between either heavy-hole or light-hole excitons. These measurements allow us to place a quantitative upper bound on the possible coupling strength in this prototypical system. A modified mean-field theory reveals a simple yet important relation that determines how the coherent coupling strength depends on the disorder correlation length and Coulomb interaction length.

Nanoscale laser processing of wide-bandgap materials with temporally shaped femtosecond laser pulses is investigated experimentally. Femtosecond pulse shaping in frequency domain is introduced and applied to two classes of shaped pulses relevant to laser nano structuring. The first class, characterized by a symmetric temporal pulse envelope but asymmetric instantaneous frequency allows us to examine the influence of the sweep of the photon energy. In contrast, asymmetric temporal pulse envelopes with a constant instantaneous frequency serve as a prototype for pulses with time-dependent energy flow but constant photon energy. In our experiment, we use a modified microscope set up to irradiate the surface of a fused silica sample with a single shaped pulse resulting in ablation structures. The topology of the laser generated structures is measured by Atomic Force Microscopy (AFM). Structure parameters are investigated as a function of the pulse energy and the modulation parameters. We find different thresholds for surface material modification with respect to an asymmetric pulse and its time reversed counterpart. However, we do not observe pronounced differences between up- and down-chirped radiation in the measured structure diameters and thresholds.

Using two-color time-resolved Faraday rotation and ellipticity, we demonstrate ultrafast optical control of electron spins in GaAs quantum wells and InAs quantum dots. In quantum wells, a magnetic-field induced electron spin polarization is manipulated by off-resonant pulses. By measuring the amplitude and phase of the spin polarization as a function of pulse detuning, we observe the two competing optical processes: real excitation, which generates a spin polarization through excitation of electron-hole pairs; and virtual excitation, which can manipulate a spin polarization through a stimulated Raman process without exciting electron-hole pairs. In InAs quantum dots, the spin coherence time is much longer, so that the effect of many repetitions of the pump pulses is important. Through real excitation, the pulse train efficiently polarizes electron spins that precess at multiples of the laser repetition frequency, leading to a "mode-locking" phenomenon. Through virtual excitation, the spins can be partially rotated toward the magnetic field direction, leading to a sensitive dependence of the spin orientation on the precession frequency and detuning. The electron spin dynamics strongly influence the nuclear spin dynamics as well, leading to directional control of the nuclear polarization distribution.

We provide a simple analytical model for the modification of optical properties of active molecules and other objects when they are placed in the vicinity of metal nanoparticles of sub-wavelength dimensions. Specifically, we study the enhancement of optical absorption and emission by these objects. The theory takes into account the radiative decay of the surface plasmon mode supported by the metal nanospheres - a basic phenomenon that has been ignored in electro-static treatment. In addition, the theory adequately treats the quenching effect of high order surface plasmon modes on luminescence efficiency. Using the examples of Ag and Au nanospheres embedded in GaN dielectric, we show that enhancement for each case depends strongly on the nanoparticle size and molecule-nanoparticle separation, enabling optimization for each combination of absorption cross section and original radiative efficiency. The enhancement effect is most significant for relatively weak and diluted absorbers and rather inefficient emitters that are placed at some optimal distances from the metal nanoparticles.

Several silicon-based plasmonic waveguides are proposed for long propagation and ultrafast all-optical modulation and switching applications. Above-bandgap femtosecond pump pulses are used to generate free carriers in ion-implanted silicon, resulting in ultrafast nonlinear phase and amplitude modulation. It is demonstrated that by carefully designing a 5-layer device from silver, ion-implanted silicon and air, it is possible to achieve long propagation distances (~100μm), or switching times of 5ps and an on-off contrast of 35dB.

Light and matter can be unified under the strong coupling regime, creating superpositions of both, called dressed states or polaritons. After initially being demonstrated in bulk semiconductors and atomic systems ,strong coupling phenomena have been realized in solid state optical microcavities. They form an essential ingredient in the exciting physics spanning from many-body quantum coherence phenomena, like Bose-Einstein condensation and superfluidity, to cavity quantum electrodynamics (cQED). A widely used approach within cQED is the Jaynes-Cummings (JC) model that describes the interaction of a single fermionic two-level system with a single bosonic photon mode. For a photon number larger than one, known as quantum strong coupling (QSC), a significant anharmonicity is predicted for the ladder-like spectrum of dressed states. For optical transitions in semiconductor nanostructures, first signatures of the quantum strong coupling were recently published. In our latest report we applied advanced coherent nonlinear spectroscopy to explore a strongly coupled exciton-cavity system. Specifically, we measured and simulated its four-wave mixing (FWM) response, granting direct access to the first two rungs of the JC ladder. This paper summarizes the main results of Ref. 15 and adds FWM experiments obtained on a micropillar cavity in which a doublet of quantum dot (QD) excitons interacts with the cavity mode in the limit of weak to strong coupling.

Recently a remarkable phenomenon in ultrafast laser processing of transparent materials has been reported manifesting itself as a change in material modification by reversing the writing direction. It has been experimentally demonstrated that the pulse front tilt is responsible for the occurrence of directional dependence. Additionally, an anisotropic cavitation was observed in the vicinity of the focus at high fluences. The bubbles, formed in the bulk of the glass, can be trapped and manipulated in the plane perpendicular to the light propagation direction by controlling the laser writing direction relative to the tilt of the pulse front. Another intriguing effect recently discovered occurs when the direction of the femtosecond laser beam is reversed from +Z to - Z directions, the structures written in a lithium niobate crystal are mirror images when translating the beam along the +Y and -Y directions. In contrast to glass, the directional dependence of writing in lithium niobate depends on the orientation of the crystal with respect to the direction of the beam movement and the light propagation direction. A theoretical model was created to demonstrate how in the lithium niobate, the nonreciprocal photosensitivity manifests itself as a changing the sign of the light-induced current when the light propagation direction is reversed. Therefore, in a non-centrosymmetric medium, modification of the material can be different when light propagates in opposite directions.

Efficient soft x-ray lasing was achieved by using plasma waveguide to confine the pump beam. With a 9-mm-long pure krypton plasma waveguide prepared by using the axicon-ignitor-heater scheme, lasing at 32.8 nm is enhanced by 400 folds. An output level of 8×10¹⁰ photon/shot is reached at an energy conversion efficiency of 2×10^-6. Seeding the laser with high-harmonic generation yields small divergence, high coherence, and controlled polarization. Application in digital holographic microscopy was demonstrated.

In two-dimensional electron gas (2DEG), spatial gradient of effective magnetic field due to spin orbit interaction yields spin dependent force. By taking this advantage, Stern-Gerlach spin filter in 2DEG has been proposed for generating spin polarized currents without any external magnetic fields and ferromagnetic materials [Phys. Rev. B 72, 041308(R) (2005)]. In order to demonstrate the spin filtering effect, detection of spin polarized electrons becomes crucial importance. Here, we propose an electrical detection of spin filtering by introducing an in-plane magnetic field in mesoscopic Stern-Gerlach spin filter. In-plane magnetic field induces spin polarized electrons due to Zeeman splitting, generating the imbalance between up-spin and down-spin currents after the spin separation. Calculated spin separation angle becomes 20º based on experimentally accessible parameters. Time evolution of wave packet shows the spin separation as well as the charge imbalance under the in-plane magnetic field. By fabricating Y-branch shaped narrow wire structure with two split gate electrodes at the junction, spin filtering effect can be detected as the magnitude difference of each branch currents. Gate bias dependence of each branch current is measured in B_ex= ±15 T at T=4.2 K.

We present an investigation of electron-spin dynamics in p-doped bulk GaAs due to the electron-hole exchange interaction, aka the Bir-Aronov-Pikus mechanism. We discuss under which conditions a spin relaxation times for this mechanism is, in principle, accessible to experimental techniques, in particular to 2-photon photoemission, but also Faraday/Kerr effect measurements. We give numerical results for the spin relaxation time for a range of p-doping densities and temperatures. We then go beyond the relaxation time approximation and calculate numerically the spin-dependent electron dynamics by including the spin-flip electron-hole exchange scattering and spin-conserving carrier Coulomb scattering at the level of Boltzmann scattering integrals. We show that the electronic dynamics deviates from the simple spin-relaxation dynamics for electrons excited at high energies where the thermalization does not take place faster than the spin relaxation time. We also present a derivation of the influence of screening on the electron-hole exchange scattering and conclude that it can be neglected for the case of GaAs, but may become important for narrow-gap semiconductors.

Electronic structure and dynamics are captured by optical 2D-Fourier-transform (2DFT) spectroscopy, which tracks the phase of the nonlinear signal during two time delays of a multi-pulse excitation sequence. These Fourier-transformed spectra separate and isolate overlapping and competing contributions to the coherent response. We have developed an ultra-stable platform consisting of nested interferometers with active phase control, allowing for exploration of single- and two-quantum coherences. Phase-resolved spectra are retrieved by all-optical determination of experimental phase ambiguities. GaAs quantum wells show suppression of many-body interactions in cross-linear polarized 2DFT spectra and many-body two-quantum coherences. Potassium vapor also shows unexpected two-quantum coherences.

Semiconducting single-walled carbon nanotubes (SWNTs) are one of the most intriguing nanomaterials due to their large aspect ratios, size tunable properties, and dominant many body interactions. While the dynamics of exciton population relaxation have been well characterized, optical dephasing processes have only been examined indirectly through steady-state measurements such as single-molecule spectroscopy that can yield highly variable estimates of the homogeneous linewidth. To bring clarity to these conflicting estimates, a time-domain measurement of exciton dephasing at an ensemble level is necessary. Using two-pulse photon echo (2PE) spectroscopy, comparatively long dephasing times approaching 200 fs are extracted for the (6,5) tube species at room temperature. In this contribution, we extend our previous study of 2PE and pump-probe spectroscopy to low temperatures to investigate inelastic exciton-exciton scattering. In contrast to the population kinetics observed upon excitation of the second transition-allowed excitonic state (E₂₂), our one-color pump-probe data instead shows faster relaxation upon cooling to 60 K when the lowest transition-allowed state (E₁₁) is directly excited for the (6,5) tube species. Analysis of the kinetics obtained suggests that the observed acceleration of kinetic decay at low temperature originates from an increasing rate of exciton-exciton annihilation. In order to directly probe exciton-exciton scattering processes, femtosecond 2PE signal is measured as a function of excitation fluence and temperature. Consistent with the observed enhancement of exciton-exciton scattering and annihilation at low temperatures, the dephasing rates show a correlated trend with the temperature dependence of the population lifetimes extracted from one-color pump-probe measurements.

Multidimensional analysis of coherent signals is commonly used in nuclear magnetic resonance to study correlations among spins. These techniques were recently extended to the femtosecond regime and applied to chemical, biological and semiconductor systems. In this work, we apply a two-dimensional correlation spectroscopy technique which employs double-quantum-coherence to investigate many-body effects in a semiconductor quantum well. The signal is detected along the direction k₁+ k₂- k₃, where k₁, k₂ and k₃ are the pulse wave vectors in chronological order. We show that this signal is particularly sensitive to many-body correlations which are missed by time-dependent Hartree-Fock approximation. The correlation energy of two-exciton can be probed with a very high resolution arising from a twodimensional correlation spectrum, where two-exciton couplings spread the cross peaks along both axes of the 2D spectrum to create a characteristic highly resolved pattern. This level of detail is not available from conventional onedimensional four-wave mixing or other two-dimensional correlation spectroscopy signals such as the photo echo (-k₁+ k₂+ k₃).

We describe the conditions of shaping regular trains of optical dissipative solitary pulses, excited by multi-pulse sequences of periodic modulating signals, in the actively mode-locked semiconductor laser heterostructure with an external long-haul single-mode silicon fiber exhibiting square-law dispersion, cubic Kerr nonlinearity, and linear optical losses. The presented model for the analysis includes three principal contributions associated with the modulated gain, optical losses, as well as linear and nonlinear phase shifts. In fact, the trains of optical dissipative solitary pulses appear within simultaneous presenting and a balance of mutually compensating interactions between the second-order dispersion and cubic-law Kerr nonlinearity as well as between active medium gain and linear optical losses in the combined cavity. Within such a model, a contribution of the nonlinear Ginzburg-Landau operator to shaping the parameters of optical dissipative solitary pulses is described via exploiting an approximate variational procedure involving the technique of trial functions. Finally, the results of the illustrating proof-of-principle experiments are briefly presented and discussed in terms of optical dissipative solitary pulses.

The local extraction of electrons from metal nanotips is an essential component of both scanning tunneling microscopes and transmission or scanning electron microscopes based on field emission cathodes. Laser-induced electron emission from sharp tip structures is a prerequisite for equipping such methods with ultrafast temporal resolution. In this paper, recent experiments on femtosecond electron emission from sharp gold tips are discussed. Based on far-field and near-field characterization, confined multiphoton electron emission from the apex is demonstrated. The effective nonlinearity can be tuned by the application of an additional static bias voltage.

Multiple-quantum two-dimensional Fourier transform optical (2D FTOPT) spectroscopy was developed and conducted on GaAs quantum wells. Spatiotemporal femtosecond pulse shaping was used to control the optical phases and time delays of ultrashort pulses in multiple non-collinear beams so that fully coherent four-wave and higher-order mixing measurements could be conducted without delay stages, multiple interferometers, or any active phase control. Coherences of biexcitons, unbound but correlated exciton pairs, and excitons undergoing rephasing were observed directly.

We report the development of an experimental technique to measure the dynamics of electrical currents on the femtosecond timescale. The technique combines methods of coherent control with time- and angle-resolved photoelectron spectroscopy. Direct snapshots of the momentum distribution of the excited electrons as function of time are then determined by photoelectron spectroscopy. In this way we gain information on the generation and decay of ultrashort current pulses in unprecedented detail. In particular, this technique allows the observation of elastic electron scattering in terms of an incoherent population dynamics in momentum space. We have applied this optical current generation and detection scheme to electrons in so-called image-potential states which represent a prototype of two-dimensional electronic surface states. Electrons in these states are bound perpendicular to the metal surface by the Coulombic image potential whereas they can move almost freely parallel to the surface. For the (n=1) image-potential state of Cu(100) we find a decay time of 10 fs due to electron scattering with steps and surface defects.

A theoretical proposal is presented for the generation of mode-locked light-bullets in planar waveguide arrays, extending the concept of time-domain mode-locking in waveguide arrays to spatial (transverse) mode-locking in slab waveguides. The model presented yields three-dimensional localized states that act as global attractors to the waveguide array system. Single pulse stationary and time-periodic solutions as well as the transition to multi-pulse solutions as a function of gain are observed to be stabilized in such a system.

We present the possibility of light beam propagation control in a Kerr nonlinear magneto-optic medium through the efficient management of linear and circular birefringences. We show numerically that the joint birefringences, achieved through the combined use of the Cotton-Mouton and Faraday effects, can effectively accelerate, postpone or even arrest the nonlinear collapse (for a fixed value of the optical power). We also present the experimental observation of collapse tuning in a bulk Yttrium Iron Garnet (YIG) crystal placed in an external magnetic field. The obtained results offer new possibilities towards the use of magneto-optic effects for controlling various nonlinear phenomena.

A semiconductor quantum dot (QD) containing a single Mn atom is a promising system from the point of view of future information processing and storage devices. An efficient optical read-out of the single Mn spin state in a CdTe/ZnTe quantum dot, as well as studies of dynamics of this state, were recently reported by L. Besombes and co-workers. However, to construct the building blocks of future memory devices basing on single magnetic atoms the ability to control a single spin is still needed. This work is focused on the advancement in writing and storing of information on the Mn spin state. We demonstrate optical writing of information on the spin state of a single Mn ion embedded in a CdTe QD and we test the storage time in the range of a few tenths of a millisecond. A spin-conserving excitation transfer between two coupled QDs is used as a tool for optical manipulation of the Mn spin. Excitons resonantly created in a dot without magnetic atom by circularly polarized light tunnel to the dot with the Mn ion in a few picoseconds. Then they act on the Mn ion via the sp-d exchange interaction and orient its spin. The orientation is much more efficient in presence of a magnetic field of about 1T, due to suppression of fast spin relaxation channels. Dynamics of the Mn spin under polarized excitation as well as the information storage time on the Mn spin was measured in a time-resolved experiment, in which the intensity and polarization of excitation were modulated. Observed dynamics can be described with a simple rate equation model. The storage time was enhanced by the magnetic field and reached about half a millisecond at 1T.

Examples of plasmonic excitations in atomically thin metal films and wires are presented. The low-energy electron energy loss spectroscopy with high momentum and energy resolutions allows us to determine the strongly dispersing low-energy collective excitation from mid- to near-infrared frequency range with momentum range up to ~0.1 Å^-1. The dispersion relation is far apart from the light line and strongly reflects the shape and size effects in nanometer to Ångstrom scale. The two-dimensional type plasmon is observed in metallic atom sheets and the one-dimensional type plasmons are also measured from some metallic atom chains. From the direct measurement of their plasmonic band dispersion, we are able to detect the carrier doping effect, electronic correlation effect, and Rashba-spin-orbit splitting effect in these ultimately tiny systems.

Phase-locked electromagnetic transients in the terahertz (THz) spectral domain have become a unique contact-free probe of the femtosecond dynamics of low-energy excitations in semiconductors. Access to their nonlinear response, however, has been limited by a shortage of sufficiently intense THz emitters. Here we introduce a novel high-field source for THz transients featuring peak amplitudes of up to 108 MV/cm. This facility allows us to explore the non-perturbative response of semiconductors to intense fields tailored with sub-cycle precision. In a first experiment intense transients drive Rabi-oscillations between excitonic states in Cu₂O, implying exciting perspectives for future THz quantum optics. At electric fields beyond 10 MV/cm, we observe the breakdown of the power expansion of the nonlinear polarization in bulk semiconductors. Furthermore, we employ the intense magnetic field components of our transients to coherently control spin waves in antiferromagnetically ordered solids. Finally, intersubband cavity polaritons in semiconductor microcavities are exploited to push light-matter coupling to an unprecedented ultrastrong and sub-cycle regime.

Our recent development of a simple and reliable method to generate terahertz (THz) pulses with high intensities (equivalent to high electric field amplitudes) has paved the way for nonlinear optics in the THz regime. We present experiments on bulk n-type GaAs which give new insights into phenomena like Bloch oscillations and Zener tunneling in bulk semiconductors. Another novel development in this field is two-dimensional THz spectroscopy. Here, we present first experiments on intersubband transitions in GaAs/AlGaAs quantum wells.

Indium nitride (InN) is a recently discovered source of broadband terahertz (THz) frequency radiation. Emission of THz-radiation occurs upon irradiation of InN with femtosecond (fs) near-infrared laser (nir) pulses. As a narrow band gap semiconductor, InN is an exciting material for future time-domain THz-spectroscopy and THz-imaging systems powered by femtosecond fiber lasers operating at communication wavelengths (1550nm) and directly diode-laser pumped femtosecond solid state lasers emitting in the 1000-1600nm wavelengths range. Advantages of InN as THz-emitter are strong intrinsic electric fields, potentially low intrinsic carrier concentrations and a very low probability of intervalley scattering of photocarriers. Recent results on the impact of n- and p-type dopants on THz-radiation emission from InN thin films are discussed. Emission of THz-radiation from silicon (Si) -doped and native n-type InN increases with mobility as expected for transient photocurrents as primary mechanism of terahertz radiation emission. Doping of InN with magnesium (Mg) enhances the emission of THz-radiation compared to doping of InN with Si. This is experimental evidence for Mg acting as an electrically active acceptor in InN. THz- radiation emission from InN:Si is weaker than emission of THz-radiation from native n-type InN because of increased electron concentrations due to Si being an electrically active donor in InN.

We investigate ultrafast carrier dynamics in photoexcited InGaN/GaN multiple quantum wells by time-resolved terahertz spectroscopy. The initially very strong built-in piezoelectric field is screened upon photoexcitation by the polarized carriers, and is gradually restored as the carriers recombine. The conductivity related to the presence of photoexcited carriers, sensed by the THz probe pulses, shows a non-exponential, slowing-down decay with time, which is explained by the gradual restoration of the built-in field in the QWs and consequent quenching of recombination. Screening and restoration of the built-in field are confirmed by the photoluminescence measurements.

The application of THz plasmonics in imaging dielectric objects embedded in the metallic media is presented. Signatures of the embedded object was detected when the time domain information of the transmitted pulse was analyzed by THz time domain spectroscopy. The resolution of the acquired images was enhanced by using a super-resolution image processing technique. It is further shown that the images acquired from the pulse arrival time and phase magnitude reveal more details of the embedded object compared to the pulse power information.