We present a bias-free photoconductive terahertz generation scheme based on band-engineered photoconductive layers, enabling completely passive optical-to-terahertz conversion with zero dark current. By engineering the band structure of the photoconductive layers, a built-in electric field is induced, leading to efficient collection of almost all optically generated electrons. Additionally, a large area plasmonic nanoantenna array is utilized to enhance the optical absorption near each nanoantenna to reduce the average electron transit time to the radiating elements. Using this scheme, we demonstrate record-high optical-to-terahertz conversion efficiencies compared to previously demonstrated passive techniques utilizing nonlinear optical processes, spintronics, and photo-Dember effect.

The rapid acquisition of terahertz (THz) time-domain waveforms is a significant challenge in the study of fast and non-reproducible phenomena. To increase data acquisition rates, the THz waveform can be encoded on spectral components of individual near-infrared (NIR) ultrafast laser pulses. By using dispersive Fourier transform method, where spectral information are mapped in the time domain, we demonstrate time-resolved THz-spectroscopy at an unprecedented rate of 50 kHz. With this technique, we resolve sub-millisecond dynamics of carriers in silicon injected by successive resonant pulses as a saturation density is established.

We investigated THz phonon-polaritons by incorporating virtually free-standing lead halide perovskite into Fabry-Perot cavities. Rabi splitting occurred when the cavity resonance is in tune with the phonon resonance due to the strong coupling between the resonances. The Rabi splitting increased with the perovskite thickness, reaching 2.4 times of the strong coupling criterion; clear quantum beat oscillations were observed in the time domain. We also fabricated the flexible polariton devices using a plastic substrate for the development of tunable quantum states, controllable with mechanical bending processes.

Understanding and ultimately controlling the properties of matter, from molecular to quantum systems, requires imaging the elementary excitations on their natural time and length scales. To achieve this goal we developed scanning probe microscopy with ultrafast and shaped laser pulse excitation for multiscale spatio-temporal optical nano-imaging. In corresponding ultrafast movies we resolve the fundamental quantum dynamics from the few-femtosecond coherent to the thermal transport regime. I will discuss specific examples visualizing in space and time the nanoscale heterogeneity in competing structural and electronic dynamic processes that define the performance in perovskite photovoltaics or energy dissipation in 2D heterostructures. I will then extend the discussion to new forms of photon-matter hybrid states confining light on the nano- to atomic scale, with imaging in tip-enhanced strong coupling of single emitters.

I provide a novel concept of Tip-Enhanced Cavity-Spectroscopy (TECS) overcoming the limitations of previous approaches to induce, probe, and dynamically control ultra strong light-matter interactions. Furthermore, I provide several new directions of nano-spectroscopy and-imaging, which have not been thought in the near-field optics community before. First, we exploit extremely high tip-pressure (approximating GPa scale) to directly modify the lattice structure and electronic properties of materials. Second, we dynamically control the near-field polarization by adopting adaptive optics technique to near-field optics. Third, we develop conductive TECS to modify electrical properties of materials by directly flowing an electric current through the cavity junction.

Many molecular, quantum-dot, and optomechanical nanocavity-QED systems demonstrate strong nonlinear interactions between electrons, photons, and phonon (vibrational) modes. We show that such systems can be described by a universal model in the vicinity of the nonlinear resonance involving all three degrees of freedom. We solve for the ultrafast nonperturbative quantum dynamics in the strong coupling regime, taking into account quantization, dissipation, and fluctuations of all fields. We find analytic solutions for quantum states which demonstrate tripartite quantum entanglement. We show how the strong coupling at the nonlinear resonance modifies photon emission and vibrational spectra.

The absence of strong losses in high-beta nanolasers makes the identification of the onset of lasing difficult to pinpoint, as the input-output characteristics can become almost thresholdless. The second-order photon correlation function g2(0) has become a valuable tool to assess the coherence properties of nanolasers, as its transition to a value of 1 clearly marks the laser threshold. Most measurements of the zero-delay-time autocorrelation function involve temporal averaging over g2(tau) due to the finite time resolution of the photon detectors. In the past, a generalized Siegert relation has been used to approximately obtain g2(tau). Using full quantum-optical two-time calculations, we address the question in how far it can be used in the partially coherent regime of conventional nanolasers that show a soft transition to lasing, and in few-emitter nanolasers that operate close to or in the regime of strong light-matter coupling.

Solid-state surface systems are particularly attractive because of their modified electronic, lattice and spin structures, resulting in strongly altered physical and chemical properties compared with the bulk. We have recently developed Ultrafast Low-Energy Electron Diffraction (ULEED) in a laser pump/electron-probe scheme to explore optically-induced structural dynamics at surfaces on their intrinsic time scales. This talk will introduce the basic principles of ULEED and discuss our recent advances regarding the coherent vibrational control over the phase transition in indium nanowires on the (111) surface of silicon by manipulating the vibrational amplitudes of key lattice modes. This mode-selective control of solids and surfaces could open new routes to switching chemical and physical functionalities, enabled by metastable and non-equilibrium states.

Understanding the mechanism of charge dynamics of photocatalytic materials is the key to design and optimize more efficient materials for many renewable energy applications such as solar water splitting, solar CO2 conversion, etc. In this study, the charge dynamics of CuO thin films is unraveled by the combination of ultrafast (picosecond) and nanosecond TAS techniques. To unravel the complicated charge dynamics of CuO, 3 different excitation wavelength are used which clarify the role of the defect states within the band gap. A compelling energy diagram with a rate-equation-based numerical model has proposed to successfully disentangle different transitions which captures both the spectral and time-dependent behaviors. This understanding can help us to better design CuO based photocatalysts.

Transition metal oxides such as TiO₂ and CeO₂ are wide bandgap semiconductors with photocatalytic properties used to drive electrical currents and enhance electrochemical reactions. The utility of wide bandgap semiconductors can be extended from the UV and into the visible by embedding metal nanoparticles into the semiconductor. Excitation of localized surface plasmon resonances in the metal nanoparticle generates free electrons that can be injected into the semiconductor and extend the photoactivity range using lower energy photons. In this work, ultrafast transient absorption spectroscopy is used to investigate the hot electron injection from Cu nanoparticles photodeposited onto a CeO₂ aerogel.

I will discuss how a combination of a femtosecond optical pulse and a strong lightwave can precisely excite quantum states as well as many-body dynamics that induces Harmonic Sideband (HSB) emission. By accurately clocking the optimal HSB emission conditions with a 0.7% precision of the driving field’s oscillation period, we detect that strong Coulombic interaction effects in a WSe2 monolayer shift the optimal timing at attosecond scales. I will summarize how this information yields direct attosecond clocking of many-body effects in diverse quantum systems.

Excitation of coherent high-frequency magnons (quanta of spin waves) is critical to the development of high-speed magnonic devices. Here we computationally demonstrate the excitation of coherent terahertz (THz) magnons in ferromagnetic and antiferromagnetic thin films by a photoinduced picosecond acoustic pulse. Analytical calculations are performed to reveal the magnon excitation mechanism. Through spin pumping and spin-charge conversion, these magnons can inject THz charge current into an adjacent heavy-metal film which in turn emits electromagnetic (EM) waves. Based on dynamical phase-field simulations, we show that the emitted EM wave retains the spectral information of all the magnon modes, providing a basis for detection via THz emission spectroscopy.

Quadratic magnetoresistive phenomena, such as the anisotropic and spin-Hall magnetoresistance, are powerful and versatile probes of magnetic order in ferro- and antiferromagnetic spintronic research. In this contribution, we study the ultrafast dynamics of these effects using the broadband time-domain terahertz (THz) spectroscopy and electrical detection, covering frequency responses ranging from DC to tens of THz. Owing to the wide spectral range, we can reveal the extrinsic (electron-scattering dependent) and a sizable intrinsic (scattering independent) contributions to the anisotropic magnetoresistance in common ferromagnets and identify specific ultrafast regimes of coupling of the spin accumulation to magnetic moments in spin-Hall magnetoresistive bilayers.

In this contribution, we present a novel type of sub-cycle field-resolved microscopy of Terahertz electric near-fields inside micro- and nanostructures. The “Quantum-probe Field Microscopy” (QFIM) scheme is based on fluorescence microscopy of semiconductor Quantum-dot luminescence and harnesses the Quantum-confined Stark effect for recording stroboscopic “movies” of ultrafast resonant and propagating THz-excitations. The scheme is compatible with strong local driving field strengths, sub-micrometer resolution and sub-cycle sampling of multi-THz waveforms. We discuss experimental implementations, recent results and future prospects of this versatile microscopy scheme.

Free electrons at relativistic velocities enable subwavelength spectroscopy of photons, plasmons, and excitons. The information transfer between these electronic and photonic systems points to their entanglement, standing in sharp contrast to the classical point-particle description of electron spectroscopy. Understanding the quantum electron-photon interaction and the quantum-to-classical transition would enable electron-based nanophononics to probe and drive novel phenomena. This talk discusses the electron-photon coupling phenomena of cathodoluminescence (CL), electron energy-loss spectroscopy (EELS), and photon-induced nearfield e-microscopy (PINEM). Starting from the electron-photon entanglement, I will show the emergence of the experimentally known EELS and PINEM spectra. I will present predictions of further properties of CL, such as its temporal coherence, and link them to the formation of attosecond-long electron pulses within the electron microscope.

We show theoretically and numerically that a time-reversed replica of the pump pulse envelope in a frequency converted signal can be achieved using a nonlinear time lens realized with an accelerating spatiotemporal Quasi-Phase-Matching (QPM) modulation. The nonlinear process depends on a suitable combination of input and output dispersion, before and after the nonlinear time lens, and on the acceleration rate of a quadratic spatiotemporal QPM modulation.

Magnetic field- and polarization-dependent measurements on bright and dark excitons in monolayer WSe2 combined with time-dependent density functional theory calculations reveal intriguing phenomena. Magnetic fields up to 25 T parallel to the WSe2 plane lead to a partial brightening of the energetically lower lying exciton, leading to an increase of the dephasing time. Using a broadband femtosecond pulse excitation, the bright and partially allowed excitonic state can be excited simultaneously, resulting in coherent quantum beating between these states. The magnetic fields perpendicular to the WSe2 plane energetically shift the bright and dark excitons relative to each other, resulting in the hybridization of the states at the K and K′ valleys. Our experimental results are well captured by time-dependent density functional theory calculations. These observations show that magnetic fields can be used to control the coherent dephasing and coupling of the optical excitations in atomically

Mixed dimensional heterostructures incorporating monolayers of transition metal dichalcogenides (TMDs) and non-van Der Waals nanomaterials provide interesting physics at low dimensions and beyond that of pristine TMDs. Of particular interest are light stimulated interfacial phenomena where excitons and/or charge carriers induced in either components diffuse at the interface of the heterostructure to provide enhanced optical activity. In this work we report ultrafast carrier dynamics in mixed 0D/2D PbS QD-monolayer MoS2 by transient absorption microscopy and time-resolved confocal luminesce. We show dependency of the rate for carrier transfer with PbS QD core size which we relate to changes in bandgap alignment at interface and resulting from changes in components band overlap and provide a mechanistic view of interfacial charge transfer via ultrafast transient absorption and emission microscopy.

In my talk, I will present the ultrafast formation dynamics of dark interlayer excitons in twisted WSe₂/MoS₂ heterostructures in space and time. First, I will report on the identification of a hallmark signature of the moiré superlattice that is imprinted onto the momentum-resolved interlayer exciton photoemission signal. With this data, we reconstruct the electronic part of the exciton wavefunction, and relate its extension to the moiré wavelength of the heterostructure. Second, I will show that interlayer excitons are effectively formed via exciton-phonon scattering, and subsequent interlayer tunneling at the interlayer hybridized ΣW valleys on the sub-50 fs timescale.

Ultrafast spectroscopic techniques have been crucial to gain fundamental insights into the topological properties of materials. We employed ultrafast time-resolved angle-resolved photoemission spectroscopy to probe and elucidate surface electronic structure and electron dynamics of Z2 topological insulators, Dirac-Weyl semimetals and exotic spin-orbit superconductors. In this talk, I plan to present a few recent examples including Weyl semimetal class TaIr(Te/Se)4; MoxW1−xTe2 materials and bulk-insulating topological insulator Bi2Te2Se where ultrafast techniques have been crucial to discover the full topological behavior.

We provide first-principles modelling of the ultrafast nonlinear optical response of low electron-density Drude materials (specifically, ITO) near their unique near-IR “epsilon-near-zero” point. We show that their 100’s-of-percent nonlinearity originates from stronger electron heating and faster cooling compared to noble metals, and unique dynamics of the chemical potential. The resulting drastic permittivity changes causes significant detunings from resonance, a rapid drop of absorptivity with increased illumination intensity and a sub-linear increase of phonon temperature with pump intensity, reaching the melting point at the experimentally-observed damage threshold. This furthers shows that the ITO nonlinearity is not electronic (“saturable”), but rather thermal.

The hybrid light-matter character of exciton-polaritons enables strikingly long-range energy transport in organic materials. We use femtosecond transient absorption microscopy to probe this behavior in the initial coherent regime, where photon and exciton wavefunctions are inextricably mixed. We achieve rapid polaritonic transport in highly ordered, pure organic semiconductor films without any external cavity. In a disordered system, confinement within a Fabry-Perot cavity provides enables comparable coherent transport effects. In both cases, the polaritons don’t travel alone: they are accompanied by intracavity dark states, which reduce the transport velocity, extend the lifetime, and provide a new mechanism for external control.

Light-harvesting technology converts photons into useful charge carriers that can be used to catalyze chemical reactions or be extracted for electricity. We have been examining the hot-carrier dynamics in type-II quantum well heterostructures finding metastable and protracted decay mechanisms in early delay times of transient absorption spectroscopy that may allow for hot carriers to be extracted in order to overcome the detailed-balance limit of single-junction heterostructures. Simultaneously, we have explored dielectric resonances in spherical and cubic nanoparticles that demonstrate coherent signatures in the negative delay time region of the transient absorption data and potentially explain improved photocatalysis for certain reactions excited with broadband light.

Layered organohalide perovskite films consist of quantum wells with concentration distributions tailored to enhance charge transport. Whereas cascaded energy and charge funneling behaviors have been detected with conventional optical spectroscopies, it is not clear that such dynamics contribute to the efficiencies of photovoltaic cells. Experimental methods based on a wide range of physical principles are used to determine carrier mobilities for light-harvesting materials in photovoltaic cells. For example, in a time-of-flight experiment, carrier transport is initiated by a single pulse of light, and the timescale of carrier transport across the active layer of a device is determined. With our newly developed multidimensional Time-Of-Flight (TOF) named nonlinear photocurrent spectroscopy (NLPC), transient populations of quantum wells with different sizes are established by tuning the wavelengths of the laser pulses into their respective electronic resonances. Multidimensional TOF data suggest that such light-harvesting processes do not assist long-range charge transport due to carrier trapping at interfaces between quantum wells and interstitial organic spacer molecules. Further, the measured instantaneous drift velocities show that trap-induced carrier deceleration is more evident with higher concentrations of organic spacer cations. Overall, our measurements indicate that the majority of photocurrent is produced when the thickest quantum wells absorb light and transport carriers without charge transfer transitions involving smaller quantum wells.

Vertical-Cavity Surface-Emitting Lasers (VCSELs) have become the preferred option for energy-efficient, high-speed optical interconnects in data centers and supercomputers due to their cost-effectiveness and reliability. However, current VCSELs have limitations in modulation speeds, with a roll-off of 20 GHz. In this work, we propose a novel hexagonal transverse-coupled-cavity design that adiabatically couples through a central cavity. Through this design, we have successfully developed a prototype VCSEL with a 3-dB roll-off modulation bandwidth of 45 GHz, an improvement of five times that of a standard VCSEL on the same epiwafer. This design utilizes the Vernier effect to widen the laser's aperture, extending the dynamic roll-off point and increasing power output. With the increased modulation speed and output power, this new VCSEL design offers new possibilities for data communication, sensing, automotive, and photonic AI systems.

We apply a spatially multiplexed low-noise solid-state dual-comb oscillator delivering two pulse trains at slightly different repetition rates for picosecond ultrasonics measurements. This novel laser delivers more than 2 W of average power per pulse train with 200fs pulses. We characterize the laser performance demonstrating its ultra-low relative timing jitter and relative intensity noise. By employing a high-sensitivity detection scheme we achieve <5e-5 pump-probe detection sensitivity in a single 12-ns-long trace with 250-fs delay resolution and an acquisition time of only 2 ms. We explain how to predict the measurement performance and optimize the data acquisition scheme accordingly.

Vortices provide the elementary quanta of rotation in superfluids. Here we exploit the natural assets of open-dissipative exciton-polariton fluids to set into ultrafast spiraling motion a doubly-charged vortex, which also results in the time-varying orbital angular momentum of emitted light. The created topology comprises all the possible polariton pseudospins filling the associated Hilbert space of states, with each of them appearing twice in real space where the Bloch sphere is mapped. The intrinsic quantity characterizing the topological link between the two spaces— the Berry curvature—is reshaping in time, but always keeps a space integral of twice the solid angle.

High-Harmonic Generation (HHG) in solids has attracted great attention in recent years as it represents a promising tool to probe the structure, topology, and ultrafast dynamics of condensed matter systems. We report on our recent theoretical findings on the microscopic generation mechanisms behind HHG. We show that the interband HHG mechanism pictured by the three-step model involving electron-hole-pair creation, propagation and recollision should be greatly expanded to encompass general cases: often the electron-hole pairs created away from the minimal band gap can dominate the HHG emission, and the electron-hole pair can undergo imperfect recolliions, i.e. recollisions where the electron and hole do not exactly overlap spatially. We further characterize the anomalous HHG mechanism in solids, i.e. the perpendicular-polarized harmonics generated by the contribution of the Berry curvatures and the anomalous velocities. We find that the anomalous harmonics will in general be competing with the interband harmonics, and state general situations where one dominates over the other. This resolves a recent debate on the origin of the perpendicular-polarized harmonics.

Recently an all-optical Quasi Phase Matching (QPM) scheme for High Harmonic Generation (HHG) was introduced in which the pump beam was constructed in the form of an intensity grating whose periodicity could be easily tuned to allow selective enhancement of specific harmonic orders. This was performed using a two-component fundamental beam, superposing a Bessel beam and a Gaussian beam. Here we extend this scheme by constructing the pump as a travelling grating with both controlled periodicity and velocity - utilizing a spatiotemporal modulation, thus allowing both selective harmonic enhancement and fine-tuning of its frequency. Therefore, this scheme can be extremely useful for spectroscopic applications.

This conference poster presentation was prepared for the Photonics West OPTO 2023 Symposium.

Progress towards broadband active mode-locked operation of terahertz metasurface Quantum-Cascade (QC) Vertical External Cavity Surface Emitting Lasers (VECSELs) is presented. First we show that broadband gain over a 1.1 THz bandwidth (centered at 3.5 THz) can be obtained from coupled-resonator QC amplifying metasurfaces. Second we show multi-mode operation can be encouraged by using a specially designed output coupler to compensate for the frequency dependent metasurface gain. Third, we show that injecting a strong RF tone onto the laser bias current at a frequency near the cavity round trip frequency induces multimode lasing over a 290 GHz bandwidth and stabilizes the round-trip frequency.