This PDF file contains the front matter associated with SPIE Proceedings Volume 11999, including the Title Page, Copyright information, Table of Contents, and Conference Committee listings.

We study the spatio-temporal evolution of hot electrons generated in plasmonic nanostructures under resonant excitation with fs-laser pulses. A spatially inhomogeneous version of the Three-Temperature Model for hot-electrons dynamics, coupled to semiclassical calculations of third-order optical nonlinearity in gold, enabled us to engineer a transient symmetry breaking of the optical properties at the nanoscale. This effect is exploited to achieve all-optical control of light with unprecedented speed. For instance, a photoinduced broadband dichroism, fully reversible and transiently vanishing in less than 1 picoseconds (overcoming the speed bottleneck caused by slower, electron-phonon and phonon-phonon relaxation processes), has been experimentally demonstrated in plasmonic metasurfaces with nanocross metaatoms. Also, we designed a nonlinear plasmonic metagrating (based on cross-polarized gold nanostrip dimer metaatoms), where the nanoscale symmetry breaking enables ultrafast reconfiguration of diffraction orders via control laser pulses. The photoinduced power imbalance between symmetrical diffraction orders is calculated to exceed 20% under moderate (∼2 mJ/cm²) laser fluence, and returns to the balanced diffraction in about 2 ps. Our design has been developed for gold nanomaterials, but the concept of ultrafast all-optical symmetry breaking can be exploited beyond plasmonics (e.g. in semiconductor nanostructures), with potential impact on a broad range of applications in nanophotonics.

Plasmonics has been revolutionized by techniques that effectively couple light to plasmonic modes in nanomaterials. Although plasmonics was originally discovered using particle beams, ease of access to lasers and advances in coupling techniques such as nano-focusing have boosted optical plasmonics. In this work, we introduce plasmonic nano-focusing of particle beams as a means of accessing unprecedented PetaVolts per meter EM fields. PV/m fields are supported by electron bunches compressed to densities approaching that of the free electron Fermi gas in conducting materials. Strong focusing forces are sustained by a novel strongly electrostatic surface crunch-in plasmonic mode driven in a tube by charged particle bunches. Nano-focusing is made possible by this crunch-in mode when excited in tubes with tapered radius. Surface plasmons not only allow controlled focusing of the bunch but also efficient coupling by avoiding collisional disruption and losses. Plasmonic nano-focusing can bring forth ultra-solid particle beams of nanometric dimensions which stand to open unforeseen possibilities.

We investigate space-time surface plasmon polariton (ST-SPP) wave packet, a conceptual correspondence of a surface-electromagnetic wave to the space-time wave (ST wave) that is excited on a metal surface through a light-SPP coupling at a nano-scaled ridge. In a framework of the finite-difference time-domain (FDTD) method, a pulsed excitation light was constructed by using a couple of hundred plane waves with different frequencies whose intensity and incident angle were determined to satisfy the conditions for generating ST wave according to the dispersion relation of SPP on the metal surface. The ST-SPP WP launched from the nano-ridge exhibited propagation invariance and tunability of the group velocity.

We consider an electronic three-level system with two dipole-allowed transitions that are resonantly excited with two single-mode quantum fields, respectively. The interaction is described with a Jaynes-Cummings type model. In such a fully-quantized system, quantum correlations between initially independent quantum fields are found to arise. Their theoretical analysis is an important but challenging task since each field appears in a mixed state and the known criteria of entanglement are not suitable for such a multi-partite case. Here, we present a detailed insight into the formation of such correlations by using the cluster-expansion approach. With this approach, the hierarchy problem that arises due to the light-matter interaction can be truncated and analyzed by classifying many-body quantities systematically into clusters and omitting clusters above a predefined size. This leads to explicit expressions for the correlated part of high-order N-particle operators, which do not allow for further factorization. In our case, we consider N-particle operators that are composed of at least one bosonic operator of the respective fields, where the number of bosonic operators is limited by the chosen maximum cluster size. The obtained correlated parts are processed into a single measure for the correlation between the fields. We perform simulations based on the obtained equations for the expectation value of the correlated parts, which allow a deeper insight into the formation of quantum correlations and to study the contribution and behavior of different cluster sizes. Numerical results for the correlation between the two quantum fields are presented and discussed.

We demonstrate that the temporal profile of the transverse nanoseсond photovoltage pulse generated in the thin semiсonduсtor СuSe/t-Se nanoсomposite film under irradiation with elliptiсally polarized femtoseсond pulse is determined by the interplay of linear and сirсular photoсurrents. These photoсurrents have different durations indiсating the dependenсe of the relaxation time of the photogenerated сarriers on their spin. The interplay of photoсurrent results in the generation of either unipolar or bipolar pulse with temporal profile dependent on the handedness, shape, and orientation of the polarization ellipse of the laser beam relative to the plane of incidence.

Femtosecond coherence spectroscopy is a family of ultrafast techniques that utilize ultrafast laser pulses to prepare and monitor coherent states in resonant or non-resonant samples. Among coherence spectroscopies, 2D electronic spectroscopy (2DES) techniques have recently gained particular interest given their capability of following ultrafast processes in real-time. Indeed, 2DES is widely exploited nowadays to unveil subtle details of ultrafast relaxation dynamics, including energy and charge transport, in complex media such as biological and artificial light-harvesting complexes and solid-state materials. Particularly meaningful is the possibility of assessing coherent mechanisms active in the transport of excitation energy in these materials. With the development of promising new applications and rapidly evolving technical capabilities, the enormous potential of 2DES techniques to impact the field of nanosystems, quantum technologies, and quantum devices is here delineated. Two examples illustrate this aim: semiconductor quantum dots solid-state materials and colloidal suspensions of plexciton nanohybrids.

Silicon optoelectronics devices have been well explored in the near-IR regime with emphasis on telecom applications. In the mid-IR regime, group IV optoelectronic devices (silicon and/or germanium based) could one day serve as waveguides, nonlinear media for χ⁽²⁾ and χ⁽³⁾ wave mixing, and highly adaptable platforms for low cost, lab-on-chip chemical and biological sensors. However, nonlinear optical absorption in these materials limit potential applications. In this report, we observe dramatic decreases in transmission in silicon and germanium at middle-infrared wavelengths when utilizing intense (~ 10 GW/cm²) 100 fs pulses. We suggest potential mechanisms to explain the observed nonlinear effects and describe future experiments to decouple high order multiphoton absorption, electron-hole pair generation and light-dopant interactions which might contribute to observed effects.

The semiconductor Bloch equations provide a very versatile and microscopic approach to compute and analyze optical and electronic properties of semiconductors. Here, we focus on high harmonic generation arising from the driving of crystalline systems with very strong optical and Terahertz pulses. Implementing a proper gauge allows us to solve the semiconductor Bloch equations in the length gauge. The length gauge turns out to be advantageous since it converges for a smaller number of bands than the velocity gauge and, in addition, enables a unique distinction between inter- and intraband contributions. Besides odd harmonics polarized parallel to the incoming field our approach also describes even harmonics which originate from the Berry curvature and are polarized perpendicular to the incident field. Next, we demonstrate that the electron and hole collision/recombination dynamics is mainly responsible for the anisotropy of the interband high harmonic generation. Our findings connect the electron/hole backward scattering to van Hove singularities and the forward scattering with critical lines in the band structure and we show that this dynamics can be controlled by properly designed two-color fields. Furthermore, we consider excitonic effects within a two-band model and show that they can strongly enhance the high harmonic emission intensity for suitably chosen incident pulses. When an odd-order harmonic corresponds to the energy of the 1s exciton this harmonic is several orders of magnitude larger than the emission from non-interacting electrons and holes.

High-order harmonics can be efficiently generated by high power mid-infrared ultrashort laser excitation of semiconductor materials. Interaction of an intense femtosecond pulse with finite structures involves a complex interplay of linear and nonlinear propagation effects and electron-hole carrier dynamics, which can be self-consistently analyzed numerically by a coupled Maxwell-Semiconductor Bloch model. In the current work, such an approach based on a three-band model for gallium arsenide [111] is applied to elucidate the influence of multiple reflections and transmissions from a finite slab on the high-order harmonic emission. Reflected and transmitted spectra including even and odd harmonics are theoretically analyzed as a function of the slab thickness and the field amplitude. Spatial distributions of laser-induced carriers are shown to be strongly inhomogeneous and thickness-dependent. The developed approach opens new frontiers for exploring ultrashort laser interaction regimes with nanostructures of arbitrary geometry.

Ultrafast non-thermal control of quantum materials has gained growing interest over decades. Contrary to the conventional knowledge that the photoexcitation causes heating of materials and destroys the low temperature ordered phases, recent developments of ultrafast light sources have shown the possibility of creating symmetry-broken ordered phases before the system reaches thermal equilibrium state. As a new route for such a light-induced phase transition, we have investigated the effect of strong excitation of amplitude mode in a charge density wave (CDW) phase in a layered transition-metal dichalcogenide compound, 3R-Ta_1+xSe₂¹. A soft phonon mode associated with the CDW phase transition, namely the amplitude mode, is identified at 2.3 THz at the lowest temperature through the optical pump and optical probe experiments. When this amplitude mode is coherently driven by an intense THz pulse through the two-photon excitation process, a dynamical suppression of the CDW order is manifested by the mode softening of the CDW amplitude mode with intense THz excitation. Furthermore, a gap opening is observed in the THz-frequency optical conductivity spectrum, indicating that an insulating-like metastable state is induced by the amplitude mode excitation. The formation dynamics of the gap synchronizes with the oscillation of CDW amplitude mode, which indicates the intimate interplay between the order parameters of the equilibrium CDW and the induced metastable hidden state. In this presentation, we overview the above results which have been recently published in Ref.1.

Semiconductor photocathodes with gradient-doping structures have attracted lots of interest in recent years because of their improved performances, such as higher quantum efficiency and longer diffusion length, over uniform-doped devices. It has been suggested that such improvement is due to the built-in electric field generated by the gradient of the doping concentration in the active layer. Under this built-in field, photoelectrons migrate toward the device surface via both diffusion and directional drift. While some past reports have studied and compared the photoelectron behaviors in uniform- and gradient-doped GaAs photocathodes, most of them are based on steady-state measurement and analysis. There has been little prior work focusing on dynamic responses. In this presentation, we report a comparative study of the ultrafast response of a uniform-doped and a gradient-doped GaAs photocathode, both theoretically and experimentally. We first develop a generalized diffusion-drift model, which adds a built-in electric field to a carrier diffusion model to incorporate the carrier drift. Then the theoretical model is used to predict the ultrafast transient behaviors of photoelectrons in both uniform- and gradient-doped photocathodes. Finally, the transient reflectivity of the photocathode devices is experimentally measured using pump-probe reflectometry (PPR), and the results are compared to the theoretical predictions. These comparisons indicate that the theoretical model is able to offer an appropriate physical picture of carrier transportation inside GaAs photocathodes of different doping profiles. It also enables the evaluation of device parameters such as diffusion coefficient and carrier decay time via PPR measurement.

We spontaneously generated idler and signal beams with four-wave mixing process. Next, we measured with a time-lens their internal structure and found that the statistics of the different peaks and the separation between the peaks follow stochastic process. This is essential first step before checking the correlation between the beams and the entanglement of the generated photons.