This PDF file contains the front matter associated with SPIE Proceedings Volume 12419, including the Title Page, Copyright information, Table of Contents, and Conference Committee information.

Using a ZnSe large aperture photoconductive antenna with a specific electrode structure, we demonstrate the generation of intense, sub-cycle terahertz (THz) pulses with variable elliptical polarization and peak fields above 80 kV/cm. The electrode structure is composed of six units, where three units have interdigitated horizontal electrodes, and the other three units have an interdigitated structure with vertical electrodes. The units with horizontal and vertical electrodes are positioned alternatively onto the antenna, allowing the generation of two quasi-half-cycle THz pulses with orthogonal polarization. A time delay between the two THz pulses is introduced by a phase-delay mask covering only the parts of the antenna with horizontal electrodes. By changing the mask thicknesses, we can control on demand, the polarization state of the THz pulses from linear polarization to elliptical to circular polarization over a quarter of a THz cycle.

Understanding phase competition and phase separation in quantum materials requires access to the spatiotemporal dynamics of electronic ordering phenomena on a micro- to nanometer length- and femtosecond timescale. While time- and angle-resolved photoemission (trARPES) experiments provide sensitivity to the femtosecond dynamics of electronic ordering, they typically lack the required spatial resolution. Here, we demonstrate ultrafast dark-field photoemission microscopy (PEEM) using a momentum microscope, providing access to ultrafast electronic order on the microscale. We investigate the prototypical Charge-Density Wave (CDW) compound TbTe₃ in the vicinity of a buried crystal defect, demonstrating real- and reciprocal-space configurations combined with a pump-probe approach. We find CDW order to be suppressed in the region covered by the crystal defect, most likely due to locally imposed strain. Comparing the ultrafast dynamics in different areas of the sample reveals a substantially smaller response to optical excitation and faster relaxation of excited carriers in the defect area, which we attribute to enhanced particle-hole scattering and defect-induced relaxation channels.

The nature of the couplings within and between lattice and charge degrees of freedom is central to the physics of materials. These interactions are essential to phenomena as diverse as superconductivity, charge density waves and carrier mobility in semiconductors and metals. Despite their fundamental role, detailed momentum-dependent information on the strength of electron-phonon coupling (EPC) and phonon-phonon coupling (PPC) across the entire Brillouin zone has proved to be very difficult to obtain experimentally. An emerging pump-probe technique, ultrafast electron diffuse scattering (UEDS), provides such information from the perspective of the phonon system directly. The application of this technique to EPC related phenomena in a wide variety of layered materials has recently been demonstrated.

Tailored light excitations provide a powerful means of initiating novel transformations and manipulating the properties of quantum materials on-demand. This work explores advancements in ultrafast single-shot optical and THz spectroscopy and its applications in the study of nonlinear material dynamics. First, we demonstrate the development of a new single-shot THz spectrometer capable of collecting multidimensional THz spectra two orders of magnitude faster than conventional delay line techniques. Next, we apply single-shot spectroscopy to capture the real-time formation of a metastable hidden metallic state in 1T-TaS2.

Upon photo-excitation of a molecule it will break apart. We can see fragments following direct, conventional dissociation paths, as well as fragments deviating from this minimum energy path. The latter are called roaming fragments and explore the potential energy landscape in a statistical manner. Dissociating and roaming fragments are directly captured using Coulomb Explosion Imaging (CEI) and individual pathways are distinguished based on state-of-the-art theory analysis.

The efficiency of optoelectronic devices, such as photovoltaics and sensors, are limited by the speed and direction of exciton propagation in their constituent materials. Organic semiconductors represent one of the most promising candidates for next generation photovoltaics, yet demonstrate extremely slow exciton transport. These processes, and in particular the role of phonons, are poorly understood. In this work, we use a fully microscopic manyparticle theory to model exciton transport in organic semiconductors. We find that the exciton diffusion is anisotropic, and that this anisotropy increases with increasing temperature. We predict that the magnitude of the diffusion is highly temperature dependent, decreasing by a factor of 2 from 77 K to 300 K. Our results are in good agreement with previous experimental studies and open ways for the control of exciton propagation in organic semiconductors.

MXenes are a new class of intrinsically metallic 2D materials. Their wide range of optoelectronic properties they demonstrate as a function of their chemical composition suggest applications in electronic and photonic devices. In this work we present a comprehensive study of the optical properties of three members of the MXene family, Ti₃C₂T_z, Mo₂Ti₂C₃T_z, and Nb₂CT_z, using ultrafast transient optical absorption and THz spectroscopy. We find that those properties result from a complicated interaction between free carriers, interband transitions and localized surface plasmon resonances. Elucidating the nature of photoexcitation and dynamics of carriers in these emergent materials will lay the foundation for their potential for optoelectronic applications.

The nonlinear optical response of quantum well excitons is investigated experimentally using polarization resolved four wave mixing, optical-pump optical-probe, and optical-pump Terahertz-probe spectroscopy. The four-wave mixing data reveal clear signatures of coherent biexcitons which concur with straight-forward polarization selection rules at the Γ point. The type-I samples show the well-established time-domain beating signatures in the transients as well as the corresponding spectral signatures clearly. The latter are also present in type-II samples; however, the smaller exciton and biexciton binding energies in these structures infer longer beating times which, in turn, are accompanied by faster dephasing of the type-II exciton coherences. Furthermore, the THz absorption following spectrally narrow, picosecond excitation at energies in the vicinity of the 1s exciton resonance are discussed. Here, the optical signatures yield the well-established redshifts and blueshifts for the appropriate polarization geometries in type-I quantum well samples also termed “AC Stark Effect”. The THz probe reveals intriguing spectral features which can be ascribed to coherent negative absorption following an excitation into a virtual state for an excitation below the 1s exciton resonance. Furthermore, the scattering and ionization of excitons is discussed for several excitation geometries yielding control rules for elastic and inelastic quasiparticle collisions.

The nonlinear optical response of quantum well excitons excited by optical fields is analyzed by numerical solutions of the semiconductor Bloch equations. Differential absorption spectra are computed for resonant pumping at the exciton resonance and the dependence of the absorption changes on the polarization directions of the pump and probe pulses is investigated. Coherent biexcitonic many-body correlations are included in our approach up to third-order in the optical fields. Results are presented for spatially-direct type-I and spatiallyindirect type-II quantum well systems. Due to the spatial inhomogeneity, in type-II structures a finite coupling between excitons of opposite spins exists already on the Hartree-Fock level and contributes to the absorption changes for the case of opposite circularly polarized pump and probe pulses.

Indium Tin Oxide (ITO) is an example of a material with a greatly enhanced optical nonlinearity for wavelengths at which the dielectric permittivity is near zero. Its enormous nonlinearity may enable compact photonic devices. All-optical devices involve multiple beams, and it was found recently that two-beam interaction modifies the effective nonlinearity in ITO for co-polarized beams. In that work, results of a degenerate pump-probe experiment were compared to a numerical model of the hot electron nonlinearity in ITO. The numerical model successfully explained the polarization dependent differential transmission and reflection, including the dependence on the chirp of the pulses. Here, we consider a simpler analytical model for the two-beam interaction in the continuous wave limit. We directly test this analytical model using a nondegenerate pump-probe transmission experiment using long pulses and find reasonable agreement.

An experimental demonstration of a waveguide-integrated plasmonic slot photodetector based on MoTe2 with 30 GHz 3 dB roll-off bandwidth at telecom wavelength. To overcome the intrinsic low carrier mobility and weak light-matter interaction when applying two-dimensional material for optoelectronic devices, here we numerically and experimentally show a novel concept of the plasmonic slot structure to eliminate the transit time constant (τ=L^2/μV) from the device which is related to the material mobility. The nanometer-wide plasmonic slot offers a ‘squeezed’ mode that allows the 2d material can effectively absorb the light via band-to-band transitions with an overlap factor (Г) increasing more than 3 times compared to the bare waveguide structure. The ultra-narrow slot width reduces the carriers’ drift route to tens of nanometers and is only limited by the RC time constant. The MoTe₂ serves as the semiconducting light-absorbing material with its layer-dependent bandgap that encompasses the standard O-band wavelength for communications (1,260 nm -1,360 nm). The device's static performance under 1 V bias voltage also shows a high photoresponsivity of 0.8 A/W at 1310 nm with a low dark current of 90 nA. Furthermore, we study the slot width-dependent frequency response and static response to validate our concept, which shows that both the frequency and static response are inversely proportional to the slot width. The concept is not restricted to materials and the platform. This may pave the way for developing high-performance optoelectronic devices with materials that have unique optical and electric properties but suffer from low mobilities.^1–13 It has been shown that introducing plasmonic structures, cavities, resonators, or nanoparticles into waveguide-loaded systems can improve light-matter interactions.^14–17 Still, one of their biggest challenges today is the frequency response of TMDCbased devices for telecommunications or information processing.^18–30 For photodetectors using TMDCs as the lightabsorbing medium, this is particularly crucial.

We report electrically tunable nonlinear optical metasurfaces for Second Harmonic Generation (SHG) based on coupling of intersubband transitions with plasmonic nanocavity. In the structure, conversion efficiency, spectral bandwidth, and local nonlinear phase of SHG can be controlled by the nonlinear response of the constituent meta-atoms. Experimentally, we demonstrate over 2900% of SHG intensity modulation depth, SHG beam diffraction, and SHG beam steering by applying electrical bias voltages. In addition, we propose a new meta-atom structure with full 2π phase tunability, which can be used to construct electrically tunable nonlinear flat optics.

Electro-optic modulators are critical building blocks for many signal processing systems which adhere to requirements given by both electrical and optical constraints. We discuss the fundamental speed limitations of recently developed charge and field driven nanophotonic electro-optic devices enabling the next generation of electro-optic modulators featuring a significantly improved device performance regarding modulation efficiency (VπL ⪅1 Vcm), device footprint (⪅ 1 mm2) and bandwidth (⪆ 100 GHz). We show that the practical limits of the operation speed is for both, ferroelectric Pockels modulators and the proposed transparent conducting oxides (ITO) modulators, determined by the frequency response of the corresponding electronic circuit driving the modulator.

Plasmonic colors have gained significant interest for flat panel displays due to their broad color gamut and high subwavelength resolution. The reversible metal deposition, having tunable nanostructures, along with their localized surface plasmon resonance (LSPR), is considered as a promising strategy for dynamic color displays. Herein, a demonstration of the manipulation of plasmonic silver adatoms through reversible metal deposition is presented for dynamic light modulation. The voltage-activated reversible silver nanoparticles deposition enables a wide range of dynamic plasmonic color change of 100 nm, and also facilitates a size and shape control of the grown silver nanoparticles. The silver nanoparticles interact with visible light through LSPR, the size and shape of the particles affect their optical properties. Our findings provide a favorable and novel platform for low energy-consumption tunable photonic and nanoplasmonic devices, as well as provide a simple and reliable process for rapid, scalable, and green preparation of tunable plasmonic Ag nanoparticles.

Anomalous currents refer to electronic currents that flow perpendicularly to the direction of the accelerating electric field. Such anomalous currents can be generated when Terahertz fields are applied after an optical interband excitation of GaAs quantum wells. The underlying processes are investigated by numerical solutions of the semiconductor Bloch equations in the length gauge. Excitonic effects are included by treating the manybody Coulomb interaction in time-dependent Hartree-Fock approximation and additionally also carrier-phonon scattering processes are considered. The band structure and matrix elements are obtained from a 14-band k · p model within the envelope function approximation. The random phase factors of the matrix elements that appear due to the separate numerical diagonalization at each k-point are treated by applying a smooth gauge transformation. We present the macroscopic Berry curvature and anomalous current transients with and without excitonic effects. It is demonstrated that the resonant optical excitation of excitonic resonances can significantly enhance the Berry curvature and the anomalous currents.

The nonlinear optical response of an ensemble of semiconductor quantum dots is analyzed by wave-mixing processes, where we focus on four-wave mixing with two incident pulses. Wave-mixing experiments are often described with semiclassical models, where the light is modeled classically and the material quantum mechanically. Here, however, we use a fully quantized model, where the light is given by a quantum state of light. Quantum light involves more degrees of freedom than classical light as e.g., its photon statistics and quantum correlations, which is a promising resource for quantum devices, such as quantum memories. The light-matter interaction is treated with a Jaynes-Cummings type model and the quantum field is given by a single mode since the quantum dots are embedded in a microcavity. We present numerical simulations of the four-wave-mixing response of a homogeneous system for pulse sequences and find a significant dependence of the result on the photon statistics of the incident pulses. The model constitutes a problem with a large state space which arises from the frequency distribution of the transition energies of the inhomogeneously broadened quantum dot ensemble that is coupled with a quantum light mode. Here we approximate the dynamics by summing over individual quantum dot-microcavity systems. Photon echoes arising from the excitation with different quantum states of light are simulated and compared.

Ultrashort laser-excited semiconductor nanostructures, supporting individual Mie or collective resonances, can serve as efficient miniaturized sources for low- and high-order harmonic generation. Upon laser excitation, multiple nonlinearities come into interplay on subwavelength spatial and ultrafast temporal scales, including surface and bulk effects, contributions from bound electrons and photo-excited carriers. In turn, transient optical properties affect the structure and the amplitude of the transmitted laser pulse. Computational approaches, coupling ultrashort pulse propagation with semiconductor nonlinear optical response, compatible with the considered spatial and temporal scales, are urgently needed to provide new strategies for efficient light modulation and manipulation, for instance, in order to enhance the nonlinear conversion efficiency. Nonlinear dynamics in ultrashort laser-excited nanostructures will be discussed from the perspective of classical perturbative, semi-classical, and microscopic non-perturbative models based on semiconductor Bloch equations, considering electronic multi-band structure of the material and involved intra- and inter-band transitions. As an example, an enhanced harmonic generation will be shown from a single nanoparticle or periodic array of nanoparticles, supporting Mie and collective lattice resonances, and a subwavelength resonator supporting quasi-bound states in the continuum. Ultrafast processes involved in nonlinear pulse propagation such as spectrum broadening and plasma blue-shift, frequency mixing and saturation in the harmonic yield, as well as the restrictions due to carrier absorption and heating of the sample, will be discussed within the framework of a classical model. Perspectives of applying self-consistent nonlinear Maxwell-based approaches to large-scale problems in nonlinear meta-photonics as well as their current limitations will be finally outlined.

We propose the generation of 3D linear light bullets propagating in free space using a single passive optical surface. The device is a single-layer photonic crystal slab. It can automatically transform an incident conventional Gaussian pulse into a light bullet in the reflection. Our approach also provides simultaneous control of various properties including group velocity, spin, and orbital angular momentum. Our results may advance practical applications of light bullet.

Discovered in 2013, 2D niobium carbide (Nb₂C), a member of the MXene family, has been shown to have many extraordinary properties, such as high photothermal conversion efficiency, strong electron-phonon interactions, strong optical absorption in the near-infrared, and even saturable optical absorption. These unique properties of Nb₂C render this MXene potentially useful for a variety of applications, including photonic and optoelectronic devices and even photothermal cancer therapy. Here, we employ both terahertz time-domain spectroscopy (TDS) and time-resolved terahertz spectroscopy (TRTS) to investigate intrinsic and photoinduced conductivity and dynamics of optically injected charge carriers with 1.55 eV excitations in order to understand the photoinduced processes taking place in Nb₂C. We find that the photoinduced conductivity in this MXene shows an initial rapid decay over a picosecond time scale, followed by a much longer-lived component that lasts for nanoseconds. We also observe that the long-range conductivity is strongly limited by the nanoflake boundaries.

Measuring the material’s third-order nonlinear optical refraction (NLR) and absorption (NLA) spectra over a broad wavelength range remains challenging and time consuming. Nevertheless, it is crucial to know these spectra to establish the material’s potential for photonic applications, the best spectral range for a specific application, etc. GaP is an important nonlinear material which its NLR and NLA still not fully determined over a broad wavelength range for all-optical switching (AOS) applications, for instance. Here, we have determined the figure-of-merit (FOM) spectrum of a GaP crystal (100-cut) for ultrafast AOS applications based on its nonlinear properties. Using ultrafast laser pulses from an optical parametric amplifier, we performed openaperture Z-scan measurements associate to the nonlinear ellipse rotation (NER) effect to determine simultaneously the NLR and NLA spectra. In addition, besides its wavelength dependence, we measured the FOM as a function of the crystal orientation. We observed that the crystal orientation is very critical to attains high or low FOM values.

Germanium sulfide (GeS) is a 2D semiconductor with high carrier mobility, a moderate band gap of about 1.6 eV, and highly anisotropic optical properties. In-plane anisotropy and a large in-plane spontaneous electric polarization in GeS monolayers have been predicted to result in significant second order nonlinear effects in response to above-the-gap excitation with photon energy < 2.5 eV¹. We have further confirmed it experimentally by demonstrating surface shift current generation in GeS using THz emission spectroscopy with 3.1 eV excitation.³ Here, we use time-resolved THz spectroscopy to investigate the dynamics and lifetimes of photoexcited carriers in GeS single crystals and nanoribbons in response to excitations with energies ranging from 1.5 eV, resonant with the bulk gap, to 3.1 eV. We find that resulting dynamics vary considerably. Lower energy (1.5 eV) excitation injects carriers directly into three low-lying valleys in the conduction band. Those carriers have long, which photoconductivity persisting for over 500 ps, as it can be seen in Fig. 1(a). On the other hand, injecting carriers high into the conduction band results in THz emission due to the shift current as well as into transient photoconductivity that recovers over <100 ps. Pronounced changes in the transient photoconductivity in response to optical excitation with photon energy across the visible-NIR range open intriguing possibilities for applications in ultrafast spectrally-sensitive photodetectors and solar energy conversion.