This PDF file contains the front matter associated with SPIE Proceedings Volume 11684

We have developed CUP to record 70 trillion fps. CUP can record the fastest phenomenon, namely, light propagation, and can be slowed down for slower phenomena. CUP can image in 2D non-repetitive events. CUP has a prominent advantage of measuring an x, y, t scene with a single exposure, thereby allowing observation of transient events occurring on a time scale down to 10s of fs. Further, CUP is receive-only—avoiding specialized active illumination required by other single-shot ultrafast imagers. CUP can be coupled with front optics ranging from microscopes to telescopes for widespread applications in both fundamental and applied sciences.

Skyrmions are topological defects in vector fields that exhibit a characteristic vector structure. When excited by electro-magnetic near fields on thin metal films, they are called plasmonic skyrmions. These fields exist at the sub–100-nm scale, oscillate with periods of a few femtoseconds, and thus are difficult to measure. So far, two-photon photoemission electron microscopy was able to image local plasmon fields with femtosecond time resolution. We now extend this technique to obtain time-resolved vector information that enables us to compose entire movies on a subfemtosecond time scale and a 10-nm spatial scale of the electric field vectors of surface plasmon polaritons (SPPs). We use this technique to image complete time sequences of propagating surface plasmons, demonstrating their spin-momentum locking, as well as plasmonic skyrmions on atomically flat single-crystalline gold films that have been patterned using gold ion beam lithography [1].

A single shot, complete spatiotemporal measurement of the complex electric field E(x,y,z,t) emitted by a high power (>0.1 TW) laser is demonstrated for the first time. We generate movies of the laser's electric field E(x,y,z,t) before and after the chirped pulse amplification chain and examine the temporal, spectral, and spatial field features.

On-demand engineering of condensed matter materials by external stimuli is a promising route for the dynamical control of physical and chemical properties. By applying a strong external perturbation, the driven system’s eigenstates are dependent on the crystal potential, and the periodicity of the external stimuli. In this contribution, I present two complementary case studies that facilitate the quantification of such light-engineered band structures using variants of photoelectron spectroscopy. I will exemplify the application of coherent two-dimensional photoelectron spectroscopy to resolve a dynamical band gap opening on the surface bands of Cu(111). Additionally, I will report on time-resolved momentum microscopy experiments on Au(111) that focus on the generation of light-engineered band structures throughout the full surface Brillouin zone.

We exploit the strong field enhancement offered by vertical gold nanocones resonating at 1 THz to induce THz field-driven electron emission. The nanocones are fabricated via an advanced 3D printing technique on a photopolymer and are successively gold coated. We demonstrate the clear advantage offered by nanocones featuring a monopolar resonance at THz frequencies with respect to traditional non-resonant tips via numerical modelling, THz far-field characterization, and the analysis of electron-induced argon gas fluorescence. Finally, we show that a further degree of optimization is enabled by tailoring the collective response of the nanocones when arranged in an array geometry.

We discuss our experiments that apply ultrafast electron diffraction (UED) to study structural dynamics of the phase transition in single crystal tantalum ditelluride, TaTe₂, a quasi-2D quantum material which exhibits a trimer superstructure at cryogenic temperatures. Intense near-infrared (NIR) pulses at 1030 nm are employed to quench the low temperature, atomically ordered state and the process is captured by ultrashort bunches of electrons as a function of pump-probe time delay. The diffraction signatures of the trimer superstructure recover on picosecond time scales. These measurements of TaTe₂ underscore moreover the applicability of the HiRES UED beamline at Lawrence Berkeley National Laboratory (LBNL) to probe ultrafast structural dynamics of complex materials.

Heterostructures of monolayers of transition-metal dichalcogenides are considered promising for future ultra-thin opto-electronic applications. They also host intriguing basic phenomena like moiré patterns and correlated electronic states. However, their absorption and emission spectra have so far been restricted to wavelengths shorter than 1000 nm, inaccessible to established technologies in the infra-red like silicon photonics. In this talk I will present the ongoing study of interlayer excitons in WSe2/MoS2 heterostructures. Owing to the large band offset between these materials, their heterostructure features an interlayer exciton emitting light around 1200 nm. This may couple photonic technologies in that range (close to the O-band) to the fundamental phenomena in these heterostructure. I will discuss the comprehensive exploration of the fundamental properties of these excitons such as: tunability, momentum assignment, polarization selection rules, lifetimes, moiré physics, etc.

Electron-hole excitations, or excitons, play a key role in energy conversion processes and photophysics applications. The exciton transport and decay properties are strongly coupled to structural complexities. They are of particular interest upon layered heterostructures of transition metal dichalcogenides (TMDs), a structural composition that introduces non-trivial interlayer excitonic effects. In this talk, I will describe a computational approach to study the excitonic phenomena at TMD heterostructures, using ab initio many-body perturbation theory. I will discuss many-body effects on optical selection rules and exciton phenomena in and between layered transition metal dichalcogenides, where a mixed nature of electron-hole interactions control the optical transitions and the exciton fine structure. I will further present a new approach to study exciton decay processes upon such junctions from first principles.

Photonic chip frequency comb generation based on Kerr-mediated nonlinear optical processes in microresonators is promising for a number of applications in time and frequency metrology, spectroscopy, sensing, and communications. In this talk, I will discuss our work in developing soliton microresonator frequency combs whose spectral bandwidths exceed an octave, so that when self-referenced, they can be used as core components of optical frequency synthesizers and optical atomic clocks. I will present recent efforts to create even broader bandwidth microcombs and to access visible wavelengths, which is a particular challenge due to the large dispersion of the constituent materials. Finally, I will discuss some of the outstanding design and simulation challenges associated with the development of these microcombs.

Coherent Raman scattering (CRS) microscopy is a powerful third-order nonlinear optical technique for label-free chemical identification of molecules based on their intrinsic vibrational spectrum. We present several approaches to broadband stimulated Raman scattering (SRS) and broadband coherent anti-Stokes Raman scattering (CARS) spectroscopy and microscopy, based on: (1) convolutional neural networks to remove the unwanted non-resonant background from broadband CARS spectra. (2) A 32-channels lock-in amplifier for parallel detection of broadband SRS spectra. (3) Fourier-Transform SRS using a birefringent delay line. (4) Photonic time-stretch SRS, in which the broadband pulses are temporally stretched their spectra sampled at MHz repetition rates.

The electromagnetic field is quantized. This quantization gives rise to fluctuations in the interaction with matter, a prominent example being the photon recoil noise, often termed radiation pressure shot noise. A kinematic noise associated with the quantization of the light field that has remained unobserved to date is radiation torque shot noise. This torque noise arises from the quantization of photon spin and its observation requires an optomechanical system with a rotational degree of freedom. In this contribution, we show our progress torwards the observation of radiation torque shot noise using an optically levitated nanoparticle.

Characterization of the nonlinear refractive index and photothermal properties are essential for the application of new photonic materials. The advantages and limitations of the commonly used techniques are related to the material analyzed and the available optical source properties. Here we propose a novel modification of the I-scan experimental setup, the intensity scan technique, adding eclipse detection. The eclipse intensity scan, EI-scan, makes this technique more sensible, providing better signal noise than the current I-scan method. Compared to standard Z-scan procedures, I-scan techniques have better performance in analyzing samples with non-parallel surfaces and samples that degrade when interacting with high-intensity pulses. From the developed analytical model, a fit expression for the EI-scan is presented. We measured thermal effects produced by high repetition pulse rates and electronic effects produced by third-order optical nonlinear interaction in I-scan techniques. In both cases, this new method demonstrated the increase in sensitivity and the gain in signal-to-noise ratio.

Ultrafast laser frequency microcombs provide equidistant coherent frequency markers over a broad spectrum, enabling new frontiers in chip-scale frequency metrology, laser spectroscopy, dense communications, precision metrology. Measuring and understanding the fundamental noise parameters in these high-clock-rate frequency microcombs are essential to advance the underlying physics and the precision microwave-optical clockwork. In this talk we describe the noise characteristics and timing jitter in adiabatic laser frequency microcombs. We compare and contrast the fundamental noise and fluctuation parameters for a series of laser frequency microcomb states, from multiple soliton to soliton crystals and single-soliton regimes. Each of the noise families and their noise coupling mechanisms are examined with our theoretical models. This aids the understanding of frequency, intensity and phase noise characteristics of frequency microcombs towards the precision limits.

In quantum materials, exotic quantum states can emerge as a result of strong many-body interaction that are of charge, magnetic, orbital and structural origins. The delicate balance among these interacting degrees of freedom engenders not only a ground state, but also many other competing metastable states with distinct macroscopic properties. Despite static tuning methods, the rapidly developing ultrafast science has now made it possible to dynamically control quantum materials at an unprecedented level, that is, the direct manipulation of elementary excitations at their fundamental time and energy scales. Here, we show examples on how ultrafast laser excitation induces a novel insulator-metal transition in manganite thin film (La1-xCaxMnO3), evidenced by combined methods of ultrafast terahertz spectroscopy, scanning near-field microscopy and X-ray scattering. We acknowledge support from Hong Kong Research Grants Council (Project NO. ECS26302219).

Recent breakthroughs in electrical detection and manipulation of antiferromagnets have opened a new avenue in the research of non-volatile spintronic devices. Antiparallel spin sublattices in antiferromagnets lead to the insensitivity to magnetic field perturbations, multi-level stability and ultra-fast spin dynamics. However, these features also make the characterization of antiferromagnetic materials, in particular of thin metallic films suitable for spintronics, a major challenge [1]. In this contribution we show how thin films of compensated antiferromagnetic metal CuMnAs, where non-volatile antiferromagnetic memory functionality has been demonstrated [5], can be studied by pump-probe experiments [3-5].

As of late, research related to plasmonic-electrochromic (“plasmochromic”) devices and nanostructures has gained significant interest from a multidisciplinary field of researchers. The dynamic optical properties of electrochromic materials in combination with the enhanced light-matter interaction of plasmonic nanostructures and metal films, makes this new class of devices contenders in the fields of color printing, light, and resonance modulation. While conventionally used in electrochromic smart windows, plasmochromic devices use the individual parts of the refractive index. The most important electrochromic material is tungsten oxide (WO3), which exhibits a high change in the refractive index () and extinction (Delta k=0.5) during reversible ion intercalation. Here, plasmochromic resonance modulation is used to create a dynamic reflective display with a wavelength modulation of over 64 nm in the visible range. The results are verified via FDTD analysis, which projects a maximum wavelength shift of over 100 nm.

Nanoscale photothermal effects enable important applications in cancer therapy, imaging and catalysis. These effects also induce substantial changes in the optical response experienced by the probing light, thus suggesting their application in all-optical modulation. Here, we demonstrate the ability of graphene, thin metal films, and graphene/metal hybrid systems to undergo photothermal optical modulation with depths as large as >70% over a wide spectral range extending from the visible to the terahertz frequency domains.

Controlling the permittivity of materials enables control over the amplitude, phase and polarization of light interacting with them. Tailorable and tunable transparent conducting oxides have applications in optical switching, beam steering, imaging, sensing, and spectroscopy. In this work, we experimentally demonstrate wide tailoring and tuning of the optical properties of oxides to achieve fast switching with large modulation depths. In cadmium oxide, the permittivity and the epsilon-near-zero points can be tailored via yttrium doping to achieve large, ENZ-enhanced mid-IR reflectance modulation. In zinc oxide, the permittivity is tuned by interband pumping, achieving large reflectance modulation in the telecom regime. With aluminum-doped zinc oxide, we demonstrate tailorable Berreman-type absorbers that can achieve ultrafast switching in the telecom frequencies. Our work will pave the way to practical optical switching spanning the telecom to the mid-infrared wavelength regimes.

The use of ultrashort optical and X-ray pulses offers new opportunities to study fundamental interactions in materials exhibiting unconventional quantum states, such as stripes, charge density waves and high-temperature superconductivity. To understand the microscopic interdependence between these states a probe capable of discerning their interaction on the natural length and time scales is necessary. In this talk, I will present ultrafast resonant soft x-ray scattering results to track the transient evolution of nanoscale charge density wave correlations in the high temperature superconductor, YBa2Cu3O6+x. Ultrashort infrared pulses produce a non-thermal quench of the superconducting state while X-ray pulses detect the reaction of charge density waves. We observe a picosecond response, characterized by a large enhancement of spatial coherence of charge density waves, nearly doubling their correlation length, and a smaller increase of their amplitude. This ultrafast snapshot directly reveals the interaction between these quantum states on their natural timescales. It demonstrates that their competition manifests inhomogeneously, as disruption of nanoscale spatial coherence, indicating the role of superconductivity in stabilizing topological defects within charge density waves domains.

Following the ultrafast optical excitation of an inhomogeneously broadened ensemble, the macroscopic optical polarization decays rapidly due to dephasing. This destructive interference is, however, reversible in photon echo experiments. Here, we propose a concept in which a control pulse slows down either the dephasing or the rephasing of the exciton ensemble during its presence. We analyze and visualize this optical freezing process by showing and discussing results for different single and multiple sequences of control pulses using a simple model of inhomogeneously broadened two-level systems. This idea has been realized in experiments performed on self-assembled (In,Ga)As quantum dots where it was possible to retard or advance the photon echo emission time by several picoseconds. The measurements are in very good agreement with numerical simulations for a more realistic model which, in particular, takes the spatial shape of the laser pulses into account.

Attosecond soft X-ray pulses across the oxygen edge at 543 eV are a powerful means for X-ray absorption spectroscopy. I will show how the combination of attosecond temporal resolution with the soft X-ray’s element and state specificity provides unprecedented insight into the electronic and nuclear dynamics in real time. First experiments will be presented which resolve the dynamics in a quantum material in real time. These results provide comprehensive insight into the dynamics in condensed matter, with the future possibility to address fundamental and long-standing questions such as phase transitions and superconductivity

This work presents a novel approach to investigate coherent dynamics in condensed matter materials by means of ultrafast noncollinear multidimensional spectroscopy in the mid infrared with high spatio-temporal resolution. High-order nonlinear wave-mixing processes with background-free and field resolved detection allow to analyze the shape and time evolution of signals in time or frequency domain, demonstrated with bulk indium antimonide, thin-film graphite, and a high-temperature cuprate superconductor. The experiments show high potential to deepen our understanding of (strong) electronic correlations and quasi-particle dynamics in such materials that lead to exciting phenomena from the complex refractive index to high-temperature superconductivity.

Photoexcitation by an intense laser pulse can alter the free energy landscape of a solid so that it can access states of matter that do not normally exist in thermal equilibrium. Discoveries of such light-induced states are often found in materials that host phase competition, where one state of matter lies in close proximity to another and photoexcitation provides sufficient energy to overcome the barrier between the neighboring phases. Here, we study the rare-earth tritelluride family (RTe₃): It possesses two nearly-equivalent and competing charge density waves (CDWs) in equilibrium, out of which one dominates over the other due to a small lattice anisotropy. When both CDWs are present, such as in ErTe₃, an optical pulse transiently weakens both orders. If only the dominant CDW exists, such as in LaTe₃, this order is suppressed by photoexcitation while the subdominant density wave emerges. The light-induced subdominant CDW is distinct from its equilibrium counterpart. Nonetheless, this light-induced CDW relaxes at the same time when the original CDW is re-established, indicating a strong phase competition between the two in the out-of-equilibrium regime. Our results provide a framework for understanding the interplay between competing orders and for unleashing novel states of matter that are "trapped" under equilibrium conditions.

We analyze four-wave-mixing experiments with three incident laser pulses performed on a semiconductor quantum well embedded in a microcavity. The coupling of the intracavity field and the exciton transition leads to exciton polaritons. The many-body hierarchy problem that arises due to the Coulomb interaction is treated by the dynamics-controlled truncation scheme, which leads to a set of Bloch equations that contain optical nonlinearities including biexcitonic many-body correlations and contributions beyond the coherent limit, which have not been thoroughly explored for a microcavity yet. A numerical solution of these Bloch equations is performed by projecting onto the 1s-exciton and biexciton states. We present the two-dimensional Fourier transform of the four-wave-mixing signal for different polarization directions of the incident pulses, which allows us to investigate the absorption and emission of the system and the couplings among the different resonances from the lower polariton, the upper polariton, and the biexciton. The numerical results are compared with measurements, in which a GaAs quantum well sample enclosed in distributed Bragg reflectors is investigated for four different polarization configurations, and we find a good agreement.

Plasmonic gap governs much of the electromagnetic response of metamaterials. Meanwhile, nano and subnanometric gap control achieved by exceptional advancement of nanotechnology has paved the way for quantum plasmonics. However, practical applications have been hindered by difficulties of active nano-control over a broad spectral range. We report on mechanically nano-controllable plasmonic metamaterials fabricated on flexible substrate with a broad spectral response from the visible to the terahertz waves. By closing and opening the metallic nanogap via macroscopic control, we observed both classical and quantum plasmonic responses. Using our devices functioning between the two extreme regimes of classical gaps and full-contact mode, we achieve unprecedented performances of light modulation in a broad spectral range.

Over the last decade, the development of high-power ultrafast laser systems led to the emergence of intense pioseconds terahertz (THz) pulses, which provide a new tool for studying fundamental aspects of light-matter interactions by driving out-of-equilibrium electrons, phonons or magnons at ultrafast time scale. Thanks to spectral weight in the THz frequency range, it is possible to directly couple light to infrared-active optical phonon mode in solid and it has been widely demonstrated and studied in various materials. However, only sparse and incomplete reports are available on THz-induced coherent acoustics phonons and none of them clearly demonstrate the origin of coherent acoustics phonons generation. Here, we report on the generation of coherent acoustic phonons in materials with terahertz ultrashort pulses. This is demonstrated in metals and topological insulators by exciting acoustic eigenmode in nanometric sized thin films.

We generated THz phonon-polaritons in lead halide perovskite films coated on metamaterials consisting of split-ring resonator arrays. Rabi splitting occurred when the metamaterial resonance is in tune with the phonon resonance, due to the strong coupling between the resonances. By varying the metamaterial resonance, we obtained the dispersion curves with a clear anti-crossing behavior, which is consistent with the simulation results. By monitoring the Rabi splitting as a function of the annealing time, we investigated the correlation between the interaction potential and crystallized fraction, in which we found a unique power-law scaling in association with the crystal growth dimensions.

Femtosecond mode-locked laser frequency combs have served as the cornerstone in precision spectroscopy, all-optical atomic clocks, and measurements of ultrafast dynamics. Recently frequency microcombs based on nonlinear microresonators have been examined – affording remarkable precision approaching that of laser frequency combs, and now on a solid-state chip-scale platform and from a fundamentally different physical origin. Here we unravel the transitional dynamics of frequency microcombs from chaotic background routes to femtosecond mode-locking in real-time, enabled by our ultrafast temporal magnifier metrology and enlarged stability of dispersion-managed dissipative solitons. Through our dispersion-managed oscillator, we report a stability zone more than an order-of-magnitude larger than prior static homogeneous counterparts, providing a novel platform for understanding ultrafast dissipative dynamics and offering a new path towards high-power frequency microcombs.

Autonomous vehicles and robots, key components of the 4th industrial revolution, must be able to observe their 3D environment in realtime. Currently, the 3D video camera using time-of-flight imaging is the most viable solution however raster speeds are currently limited by the speed of mechanical scanners or by the wavelength tuning speed of pulsed lasers. By adapting techniques from ultrafast time-stretch imaging, a new Lidar platform scans orders of magnitude faster than today’s commercial line-scanning pulsed-Lidar systems.

We discuss nanoscale metal-dielectric-semiconductor resonant gain geometries to create a new type of light emitters focusing on three key aspects: second order intensity correlation characterizations, direct modulation and coupled nanolasers dynamics.

The advent of dispersion-engineered and highly nonlinear nanophotonics is expected to open up an all-optical path towards the strong-interaction regime of quantum optics by combining high transverse field confinement with ultra-short-pulse operation. Obtaining a full understanding of photon dynamics in such broadband devices, however, poses major challenges in the modeling and simulation of multimode non-Gaussian quantum physics, highlighting the need for sophisticated reduced models that facilitate efficient numerical study while providing useful physical insight. In this manuscript, we review our recent efforts in modeling broadband optical systems at varying levels of abstraction and generality, ranging from multimode extensions of quantum input-output theory for sync-pumped oscillators to the development of numerical methods based on a field-theoretic description of nonlinear waveguides. We expect our work not only to guide ongoing theoretical and experimental efforts towards next-generation quantum devices but also to uncover essential physics of broadband quantum photonics.

The broad applications of ultrawide-band signals and terahertz waves in quantum measurements, imaging and sensing techniques, advanced biological treatments5, and very-high-data-rate communications6 have drawn extensive attention to ultrafast electronics. In such applications, high-speed operation of electronic switches is challenging, especially when high-amplitude output signals are required. For instance, although field-effect and bipolar junction devices have good controllability and robust performance, their relatively large output capacitance with respect to their ON-state current substantially limits their switching speed. In this talk, we present a novel on-chip, all-electronic device based on a nanoscale plasma (nanoplasma) that enables picosecond switching of electric signals with a wide range of power levels. The very high electric field in the small volume of the nanoplasma leads to ultrafast electron transfer, resulting in extremely short time responses. We achieved an ultrafast switching speed, higher than 10 volts per picosecond, which is about two orders of magnitude larger than that of field-effect transistors and more than ten times faster than that of conventional electronic switches. We measured extremely short rise times down to five picoseconds, which were limited by the employed measurement set-up. By integrating these devices with dipole antennas, high-power terahertz signals with a power–frequency trade-off of 600 milliwatts terahertz squared were emitted, much greater than that achieved by the state of the art in compact solid-state electronics. The ease of integration and the compactness of the nanoplasma switches could enable their implementation in several fields, such as imaging, sensing, communications and biomedical applications.

We report the spatiotemporal photoconductivity imaging on monolayer semiconductors and perovskite solar cell thin films by laser-illuminated microwave impedance microscopy (iMIM). With a spatial resolution of sub-100 nm and a temporal resolution of sub-10 ns, we are able to quantitatively measure the diffusion length and lifetime of photo-generated free carriers in these materials. The results are in good agreement with the diffusion equation and Einstein relation. Our work reveals the intrinsic time and length scales of electrical response to photo-excitation in optoelectronic van der Waals systems and photovoltaic materials.

The role of quantum memory in ultrafast, nonperturbative lightwave electronics is discussed with several examples involving one- and two-dimensional quantum materials. Since interactions are strong in these materials, scattering emerges often on sub-100fs timescales. Nevertheless, quantum memory has extremely important implications for lightwave electronics – several examples will be given to demonstrate how omitting quantum memory eliminates many key effects of measured in lightwave electronics. These insights have direct implications to all aspects of lightwave electronics, including multi-photon absorption, optical switching, and high-harmonic generation.

We consider intra-band electron oscillations driven by intense few-cycle laser pulses carrying as many as 15 cycles and show feasibility of generation of femtosecond photocurrent pulses in semiconductors without external bias. The mechanism of this phenomenon is attributed to generation of non-zero net momentum within each oscillation cycle due to sub-cycle violation of symmetry of momentum departures. Combined with laser-induced increase of free-electron population, it induces photocurrent pulses. Reported analytical quantum-mechanical model delivers scaling of peak photocurrent with material and laser parameters. Based on it, we discuss the type of semiconductor nanostructures most favorable for detection of that phenomenon.