This PDF file contains the front matter associated with SPIE Proceedings Volume 9746, including the Title Page, Copyright information, Table of Contents, Introduction (if any), and Conference Committee listing.

In this report, we present a novel approach to exploit the nonlinear response of terahertz (THz) field allowing the observation of ionization phenomenon in a single metal nano island. Because it is not easy to access such high power THz source to generate field over the threshold of the materials, fundamental studies on nonlinear terahertz waves and their applications in spectroscopy have been limited thus far. We are able to overcome this limitation through the use of a metallic nano island embedded in a slot antenna which strongly confines the terahertz electric field driving the system into a highly nonlinear regime. The structure, composed of a nano slot antenna and a nano island located at the center, highly confines THz electromagnetic field at the center of the structure, resulting in huge field enhancement by orders of magnitude at a specific frequency. Electrons on a metallic surface experience a ponderomotive force in a highly confined and enhanced THz electric field directed towards the weak field area by a field gradient. As a result, the accelerated electrons acquire enough energy to ionize ambient carbon atoms. It has to be stressed that it is the first time to observe the ionization of atoms induced by the enhanced terahertz radiation.

A microscopic theory is presented to describe high-harmonic generation in solids with the semiconductor-Bloch equations. The approach includes the relevant interband polarizations and intraband currents. The appearance of even harmonic orders is shown to require at least three electronic bands and a mutual interband coupling between them. In experimental and theoretical time-resolved studies, this also manifests as a unipolar emission signature of the high-harmonic radiation.

We present a theoretical analysis of Coulomb induced effects on intraband transitions between bound quantum dot and unbound continuum states of the host material. The intraband adsorption spectra show characteristic spectral signatures. In particular, the numerical results confirms the existence of bound excitons consisting of a localized carrier inside the quantum dot and a delocalized carrier of the continuum, which are spatially delocalized in the vicinity of the quantum dot.

A practical all-optical switch is necessary to alleviate electronic bottlenecks in fibre optic networks. Thus, a new alloptical switch is introduced here—exhibiting femtojoule switching energies and femtosecond switching times. The alloptical switches use 40 μm dielectric spheres to direct high-intensity photonic nanojets into peripheral coatings of semiconductor nanoparticles. Semiconductor nanoparticle coatings of Si, CdTe, InP, and CuO are studied and found to yield switching energies of approximately 1 pJ, 500 fJ, 400 fJ, and 300 fJ with switching times of 2 ps, 2.3 ps, 900 fs, and 350 fs, respectively.

We derive a theoretical framework for describing hybrid organic-inorganic systems consisting of an ordered organic molecular layer coupled to a semiconductor quantum well (e.g., ZnO). A Heisenberg equation of motion technique based on a density matrix formalism is applied to derive dynamical equations for the composite system on a mesoscopic scale. Our theoretical approach focuses on the inuence of nonradiative Förster excitation transfer across the hybrid interface on linear optical absorption spectra. Therefore, the dielectric screening is discussed at the interface of two materials with different dielectric constants. Moreover, the Förster transfer matrix element is calculated in the point-dipole approximation. For a consistent theoretical description of both constituents (i.e., the molecular layer and the semiconductor substrate), the problem is treated in momentum space. Solving the equations of motion for the microscopic polarizations in frequency space directly leads to an equation for the frequency-dependent linear absorption coefficient. Our theoretical approach forms the basis for studying parameter regimes and geometries with optimized excitation transfer efficiency across the semiconductor/ molecule interface.

Ultrafast terahertz spectroscopy is an ideal tool for observation of dynamics of charge, lattice and spin in solids on the most elementary timescale: in the regime ωτ ~ 1, where ω is the electromagnetic wave oscillation frequency, and τ is the characteristic timescale at which the fundamental phenomena in the three subsystems comprising the solid occur. In this paper two case studies will be discussed. (i) Ultrafast electron transport in graphene. We will show, that the free-carrier conductivity of graphene in arbitrary ultrafast, (sub-)picosecond electric fields is defined by the thermodynamic balance maintained within the electronic structure of graphene acting as thermalized electron gas. Within this simple thermodynamic picture, the electron gas quasi-instantaneously increases its temperature by absorbing the energy of driving ultrafast electric field, and at the same time cools down via a time-retarded, few picosecond-long process of phonon emission. The asymmetry in electron heating and cooling dynamics leads to heat accumulation in the electron population of graphene, concomitantly lowering the chemical potential for hotter electrons, and thereby reducing the intraband conductivity of graphene – an effect crucially important for understanding of ultrafast graphene transistors and photodetectors. (ii) We will also discuss the fundamental observation of spin-controlled electron conduction of Fermilevel electrons in ferromagnetic metals, and will directly quantify the Mott picture of conduction in ferromagnets - the effect directly employed in modern magnetic sensor technologies such as giant magnetoresistance.

This paper investigates the interaction of buckled Dirac materials (silicene and germanene) with ultrashort and ultrastrong optical pulses. Highly intensive few-cycle pulses strongly modify the electronic and optical properties of these two dimensional materials. Electron dynamics in such a short optical pulse is coherent and can be robustly controlled by altering the propagation direction, as well as the polarization angle of the pulse. The strong nonlinearity of the system for fields applied (~ V/Å) causes the violation of the charge (C) and parity (P) symmetries, effectively reducing the system’s symmetry from hexagonal to triangular. Such symmetry violations are related to the electron transfer between the sublattices caused by the normal field component and result in nonreciprocity, optical rectification and the appearance of a cross current.

Mode-locked fiber lasers are the most promising lasers for intracavity phase interferometry,¹ because they offer the possibility to have two orthogonally polarized pulses circulating independently in the cavity. The saturable absorbers based on polarization maintaining tapered fiber coated with carbon nanotubes are developed and analyzed for minimum coupling between the slow and fast axis of the fiber.

Quantum computing and ultrafast quantum electronics constitute pivotal technologies of the 21^st century and revolutionize the way we process information. Successful implementations require controlling superpositions of states and coherence in matter, and exploit nonlinear effects for elementary logic operations. In the THz frequency range between optics and electronics, solid state systems offer a rich spectrum of collective excitations such as excitons, phonons, magnons, or Landau electrons. Here, single-cycle THz transients of 8.7 kV/cm amplitude centered at 1 THz strongly excite inter-Landau-level transitions of magnetically biased GaAs quantum wells, facilitating coherent Landau ladder climbing by more than six rungs, population inversion, and coherent polarization control. Strong, highly nonlinear pump-probe and four- and six-wave mixing signals, entirely unexpected for this paragon of the harmonic oscillator, are revealed through two-time THz spectroscopy. In this scenario of nonperturbative polarization dynamics, our microscopic theory shows how the protective limits of Kohn’s theorem are ultimately surpassed by dynamically enhanced Coulomb interactions, opening the door to exploiting many-body dynamics for nonlinear quantum control.

In this proceedings article, we summarize our previous results on the novel applications using the coupled optical parametric oscillators (OPO’s). In a conventional OPO, a single pump wavelength is capable of generating a pair of the signal and idler beams by placing a bulk nonlinear crystal inside an OPO cavity. When a nonlinear crystal composite consisting of periodically-inverted KTiOPO₄ (KTP) plates bonded together by the adhesive-free-bonded (AFB) technique is used instead of the bulk nonlinear crystal, the optical parametric oscillation takes place at two sets of the new wavelengths for the signal and idler beams due to the phase shifts occurring at the interfaces of the adjacent domains making up the composite. These two sets of the signal and idler waves are effectively generated by the two OPO’s being coupled to each other. These signals and idlers exhibit ultrastability in terms of their frequency separation. We review the progress made by us on the applications being realized by using such coupled OPO’s such as THz generation and restoration of the blurred images after propagating through a distortion plate and a phase plate simulating atmospheric turbulence.

Using a microscopic theory that combines k.p band structure calculations with multisubband semiconductor Bloch equations we are capable of computing coherent optically-induced rectification, injection, and shift currents in semiconductors and semiconductor nanostructures. A 14-band k.p theory has been employed to obtain electron states in non-centrosymmetric semiconductor systems. Numerical solutions of the multisubband Bloch equations provide a detailed and transparent description of the dynamics of the material excitations in terms of interband and intersubband polarizations/coherences and occupations. Our approach allows us to calculate and analyze photocurrents in the time and the frequency domains for bulk as well as quantum well and quantum wire systems with various growth directions. As examples, we present theoretical results on the rectification and shift currents in bulk GaAs and GaAs-based quantum wells. Moreover, we compare our results with experiments on shift currents. In the experiments the terahertz radiation emitted from the transient currents is detected via electro-optic sampling. This comparison is important from two perspectives. First, it helps to validate the theoretical model. Second, it allows us to investigate the microscopic origins of interesting features observed in the experiments.

Miniaturized plasmonic and photonic integrated circuits are generally considered as the core of future generations of optoelectronic devices, due to their potential to bridge the size-compatibility gap between photonics and electronics. However, as the nanoscale is approached in increasingly small plasmonic and photonic systems, experimentally observing their behavior involves ever more stringent requirements in terms of both temporal and spatial resolution. This talk focuses on the use of time-resolved Photon-Induced Near-Field Electron Microscopy (PINEM) to study the excitation, propagation, (self-)interference and dynamics of surface plasmon polaritons (SPPs) in various plasmonic nanostructures with both nanometer and ultrafast resolution in a transmission electron microscope. Using this field-ofview technique, we directly show how photo-excited plasmonic interference patterns are controlled through the combination of excitation polarization and nanostructure geometry. Moreover, we capture the propagation of the photoinduced self-interfering plasmonic wave, clearly demonstrating the effects of axial confinement in nanostructured plasmonic thin film stacks.

In this work, we present the design of integrable magnetoplasmonic isolators and modulators, based on a longrange magnetoplasmonic waveguide structure. With the addition of magnetized cerium-substituted yttrium iron garnet waveguides and planar samarium-cobalt biasing magnets to a Mach-Zehnder interferometer (MZI), we show that an efficient isolator architecture can be implemented with insertion loss of 2.51 dB and an isolation of 22.82 dB within a small footprint of 6:4 x 10^-3 mm². Additionally, employing bismuth-substituted yttrium iron garnet in a MZI and transient magnetic fields from nearby transmission lines, we propose a high-speed electrical-to-optical clock multiplier. Such a device exhibits a modulation depth of 16.26 dB, and an output modulation frequency of 279.9 MHz. Thus, input clock signals can be multiplied by factors of 2:1 x 10³. These devices are envisioned as fundamental constituents of future integrated nanoplasmonic circuits.

The potential of terabit-per-second fibre optics can be unlocked with emerging all-optical networks and processors employing all-optical switching. To be effective, all-optical switching must support operations with femtojoule switching energies and femtosecond switching times. With this in mind, this work studies geometrical and material characteristics for all-optical switching and develops a new all-optical switching architecture. A nanojet focal geometry is applied, in the form of dielectric spheres, to direct high-intensity photonic nanojets into peripheral semiconductors. Theoretical and experimental analyses demonstrate photonic nanojets, enabling femtojoule switching energies through localized photoinjection, and semiconductor nanoparticles, enabling femtosecond switching times through localized recombination.

The Ultrastrong light-matter coupling regime is attained when the coupling rate Ω is comparable or larger than the resonance frequency of the light ω_LC and electronic transition frequency of the matter part ω_c. This regime has attained much attention due to the peculiar properties of the coupled state. Here, we present a physical system showing a high normalized coupling rate not far from unity. The LC-mode of a THz-metasurface unitcell at ω_LC = 2π x 0:25 THz is coupled to the collective cyclotron transition of 4 two dimensional electron gases (2DEGs) resulting in a normalized coupling rate of Ω/ω_c = 0:9, Ω being the vacuum Rabi frequency. The high coupling rate is enabled by the small mode volume provided by the LC-mode of the split ring resonator geometry which was employed. Equally important is the large collective transition dipole element of the cyclotron transition.

Integrating coherent light sources at the nanoscale with spasers is one of the most promising applications of plasmonics. A spaser is a nano-plasmonic counterpart of a laser, with photons replaced by surface plasmon polaritons and the resonant cavity replaced by a nanoparticle supporting localized plasmonic modes. Despite the large body of experimental and theoretical studies, the understanding of the fundamental properties of the spaser emission is still challenging. In this work, we investigated the ultrafast dynamics of the emission from a core-shell spaser by developing a rigorous first-principle numerical model. Our results show that the spaser is a highly nonlinear system with many interacting degrees of freedom, whose emission sustain a rich manifold of different spatial phases. In the regime of strong interaction we observed that the spaser emission manifests an irreversible ergodic evolution, where energy is equally shared among all the available degrees of freedom. Under this condition, the spaser generates ultrafast vortex lasing modes that are spinning on the femtosecond scale, acquiring the character of a nanoparticle with an effective spin. Interestingly, the spin orientation is defined by spontaneous symmetry breaking induced by quantum noise, which is a fundamental component of our ab-initio model. This opens up interesting possibilities of achieving unidirectional emission from a perfectly spherical nanoparticle, stimulating a broad range of applications for nano-plasmonic lasers as unidirectional couplers, random information sources and novel form of photonics neural-networks.

Multidimensional coherent spectroscopy maps the detuning dependence of the upper (UP) and lower (LP) excitonpolariton branches¹ in a wedged microcavity with a single InGaAs quantum well at 5 K. Features on the diagonal correspond to intra-action coherences of the UP and LP branches. Off-diagonal peaks are interaction coherences between the UP and LP branches. With increasing detuning (Δ), all peaks move to higher energy, the exciton-like (E_EX) and cavity-like (E_γ) modes swap position and have maximum intensity near the anti-crossing at Δ=0. An isolated biexciton (B) is only seen at Δ<0, separated by a binding energy of approximately 2 meV. For Δ>0, the spectral weight of the off-diagonal features swap, as the LP and B come into resonance. This indicates that the off-diagonal features are sensitive to the interactions including two-quantum contributions and that a situation similar to a Feshbach resonance exists.² Polarization of two-quantum contributions show spin sensitive two-polariton and new biexciton correlations. The latter likely influence the Feshbach resonance between biexcitons and two-polariton states. The two-quantum signatures also demonstate that biexcitons perturb the light-matter coupling in the microcavity to reduce the mixed two-polariton contributions. Detuning dependence of zero-quantum contributions show Raman-like coherences that are enhanced near zero detuning. Asymmetry of the Raman coherences are indicative of many-body interactions, which also grow stronger as the light-matter interactions are enhanced near zero deuning.

Typically, to calculate the two-particle Coulomb interaction between nanostructures, a six dimensional spatial integral need to be evaluated. For increasing size or complexity of the system, the calculation of the Coulomb coupling elements presents a significant limiting factor for simulations. The number of integrals in real space can be reduced by using a Green's function representation of the solution of a generalized Poisson equation. Without the restriction to specific symmetries, this efficient numerical method allows the inclusion of an arbitrary dielectric function. The Coulomb interaction between two colloidal quantum dots (QDs) is calculated without specifying the Green's function to an explicit analytic form. Nevertheless, the monopole-monopole interaction and the Förster induced excitation transfer are calculated separately. The Coulomb coupling between semiconductor QDs depends on the center-to-center distance between the nanostructures as well as on their relative dipole orientation to each other. To identify the effects of the spatially dependent Coulomb coupling on single excitons and biexcitons, a multidimensional coherent spectroscopy is used. The characteristic two dimensional optical signatures of different spatial arranged colloidal QDs are calculated with respect to the arrangement dependent Coulomb coupling between the nanostructures.

Ultrafast demagnetization after femtosecond laser excitation of thin ferromagnetic films has been shown to occur due to a combination of spin-flip scattering in the film and spin transport to a conducting substrate or adjacent layer. Here we demonstrate that the inherent depth sensitivity of the transversal magneto-optical Kerr effect can be employed to derive conclusions on a transient spatial profile in the magnetization in the direction normal to the sample surface. This magnetization profile is qualitatively different for demagnetization caused by spin flips and spin transport. With the help of simulations based on simple phenomenological models we show that spin transport to the substrate in Co/Cu(001) films dominates the demagnetization before the thermalization of the electronic system, i.e. at times < 100 fs, while after approximately 200 fs mainly spin-flip scattering determines the magnetization profile, in agreement with our earlier findings employing the longitudinal magneto-optical Kerr effect.

We review a recently introduced simple model that describes magnetization dynamics in a system consisting of localized and itinerant carriers which are coupled by an antiferromagnetic exchange interaction. By also introducing a Stoner-like mean-field splitting in the itinerant carrier system, the model is intended to capture some important aspects of the electronic structure and the magnetization dynamics of ferrimagnets that involve itinerant carriers, such as GdFe(Co). In the framework of this model one can describe microscopically scattering dynamics due to the exchange interaction as well as coupling to phonons. We review some details of this model and show how it explains heat-induced magnetic switching in two-sublattice ferrimagnets. Further, we discuss the inuflence of angular momentum exchange with the environment, and present results for the magnetization dynamics that result from excitation above the compensation temperature.

The combined applications of vertical and lateral electric fields give rise to a novel degree of freedom in controlling transverse acoustic (TA) and longitudinal acoustic (LA) phonons. The coherent acoustic pulses were generated in GaN-based quantum wells (QWs) along polar c-axis under the ultrafast optical screening of the field-dependent strains and measured via the dynamic interference between reflected optical beams off the surface and the acoustic pulses. We electrically turned on the otherwise forbidden TA mode in the laterally biased region. The frequencies of the modes could be also modified via the field-induced changes in the propagation velocities. Finally, we experimentally investigated a novel method of controlling the phase of LA mode, generated from oppositely strained quantum wells and barriers under the vertical electric field.

We present the design of an ultrafast conical lens based nanoplasmonic electron gun. Through excitation with a radially polarized laser pulse, and a combination of magnetostatic and spatial filtering, high energy electron packets with attosecond durations can be achieved.

Transition metal dichalcogenides (TMDs) have garnered considerable interest in recent years owing to their layer thickness-dependent optoelectronic properties. In monolayer TMDs, the large carrier effective masses, strong quantum confinement, and reduced dielectric screening lead to pronounced exciton resonances with remarkably large binding energies and coupled spin and valley degrees of freedom (valley excitons). Coherent control of valley excitons for atomically thin optoelectronics and valleytronics requires understanding and quantifying sources of exciton decoherence. In this work, we reveal how exciton-exciton and exciton-phonon scattering influence the coherent quantum dynamics of valley excitons in monolayer TMDs, specifically tungsten diselenide (WSe₂), using two-dimensional coherent spectroscopy. Excitation-density and temperature dependent measurements of the homogeneous linewidth (inversely proportional to the optical coherence time) reveal that exciton-exciton and exciton-phonon interactions are significantly stronger compared to quasi-2D quantum wells and 3D bulk materials. The residual homogeneous linewidth extrapolated to zero excitation density and temperature is ~1:6 meV (equivalent to a coherence time of 0.4 ps), which is limited only by the population recombination lifetime in this sample.

The results of the practical implementation of the concept of field-emission current source with high average current density of 0.1-0.3 A-cm^-2 are shown. The durability of cathode samples at a level of 6000 hours is achieved under conditions of technical vacuum. A phenomenological model is suggested that describes the tunneling of both equilibrium and nonequilibrium electrons in a vacuum from the zone of concentration of electrostatic field. Conditions are discussed as the resulting increase in the emission current due to the connection mechanism of the photoelectric effect is thermodynamically favorable, that is not accompanied by an undesirable increase in the temperature of the local emission zone. It is shown that to ensure stability and durability of the cathode is also important to limit the concentration of equilibrium carriers using composite structures «DLC film on Mo substrate." This helps to reduce the criticality of the CVC. A possible alternative is to use a restrictive resistance in the cathode. However, this increases the heat losses and thus decreases assembly efficiency. The results of experimental studies of the structure showing the saturation of photoemission current component with an increase in operating voltage. This fact suggests the existence of an effective mechanism for control of emission at constant operating voltage. This is fundamentally important for the stabilization of field emission cathode, providing a reliability and durability. The single-photon processes and the small thickness DLC films (15-20 nm) provide high-speed process of control.