Strong coupling in the interaction of free electrons with photons will allow for the exploration of various new effects. Here, we demonstrate CW-driven inelastic electron-photon scattering at a fiber-integrated high-Q Si3N4 microresonator, enabled by resonant field enhancement and electron-light phase matching. Employing energy-filtered imaging and laser detuning-dependent measurements, we characterise the electron’s interaction with the whispering gallery mode spatially and spectrally. Finally, we discuss prospects of electron-driven photon generation in the resonator. This combination of electron microscopy and integrated photonics opens up new paths for optical electron beam modulation, electron probing of nonlinear optical effects and free-electron cavity quantum optics.

We discuss theoretical and experimental results on the ultrafast detection of quantum emitters. We employ nano-antennas, specifically gold nanocones, to accelerate the spontaneous emission rate of silicon-vacancy color centers in diamond by orders of magnitude to generate single photons with picoseconds timescales. Moreover, we implement an optical Kerr shutter under tight focusing to be able to detect such ultrafast single photons with time resolutions down to a few hundreds of femtoseconds.

Semiconductor quantum dots (QDs) may be applied to solid-state quantum emitters of interest for quantum light sources or nodes in distributed quantum networks. For such emitters, resonant optical driving results in the highest degree of photon indistinguishability, but leads to the need to reject scattered light from the laser used to drive the emitter. In this work, we apply femtosecond pulse shaping techniques to the development of quantum state inversion strategies for QD emitters that optimize fidelity and source brightness. The control protocols we have developed would be applicable to a wide range of solid-state QE systems.

We propose a scheme for the generation of highly indistinguishable single photons using semiconductor quantum dots and demonstrate its performance and potential. The scheme is based on the resonant two-photon excitation of the biexciton followed by stimulation of the biexciton to selectively prepare an exciton. Quantum-optical simulations and experiments are in good agreement and show that the scheme provides significant advantages over previously demonstrated excitation methods. Specifically, the scheme allows for ultra-low multi-photon error rates, high indistinguishability, high brightness and programmable linear polarization.

Our ability to structure magnetic fields in space and time is limited by the electrical conductors and superconductors used to carry currents in electromagnets. In this work, we explore the possibility to transfer spatial and temporal structure from femtosecond laser pulses to currents in solids and gases via coherent control. In particular, we demonstrate how structured light can be used to sculpt and reconfigure complex current arrangements including ring currents in solids and gases. I will discuss our experimental efforts to use these currents for reconfigurable optoelectronic circuitry, to control THz bandwidth magnetic fields, and to serve as active metasurfaces for generating THz impulses with exotic spatio-temporal structure.

The measurement and control of nanoscale spin dynamics is essential for the understanding and development of quantum materials. We present a new instrument along with precise spectroscopic measurements of ultrafast magnetic phenomena using extreme ultraviolet high harmonic sources. Our unique instrument allows us to optically control and probe spin dynamics of complex magnetic materials with element specificity on few-femtosecond timescales on up.

Band bending at semiconductor surfaces induced by e.g. chemical doping can create metallic surfaces with properties not found in the bulk, such as high electron mobility, magnetism or superconductivity. Optical generation of such metallic surfaces on ultrafast timescales would be appealing for high-speed electronics. We demonstrate the ultrafast generation of a metal at the surface of ZnO upon photoexcitation. Compared to known ultrafast photoinduced semiconductor-to-metal transitions that occur in the bulk of inorganic semiconductors, the metallization of the ZnO surface is launched by 3–4 orders of magnitude lower photon fluxes. Using time- and angle-resolved photoelectron spectroscopy, we show that the phase transition is caused by photoinduced downward surface band bending due to photodepletion of donor-type deep surface defects. These findings present a general route for controlling surface-confined metallicity on ultrafast timescales.

In this study, we propose the concept of generating transient nonlinearity via nonlinear carrier lifetime variation based on Auger recombination in silicon nanostructures. The nonlinear Auger lifetime variation creates a common crossing point for all pump-probe transient traces at different pump fluences, presenting a fluence-independent property. Furthermore, we observe that sub-linear and super-linear responses exist before and after the crossing point, revealing an unconventional temporal tunability of Auger-induced transient nonlinearity. Leveraging the combination of a laser scanning microscope and pump-probe technique, these temporally transient nonlinear behaviors are applicable to spatial resolution enhancement beyond the diffraction limit.

We report a new concept in hardware acceleration of AI that exploits femtosecond pulses for both data acquisition and computing. Data is first modulated onto the spectrum of a supercontinuum laser. Nonlinear optical propagation then projects the data into an intermediate space in which data classification accuracy is enhanced. This nonlinear optical kernel operation improves the linear classification results similar to a traditional numerical kernel (such as the radial-basis-function) but with orders of magnitude lower latency. The performance is data-dependent due to the limited degrees of freedom in the optical part of the system.

We review the use of machine learning techniques in ultrafast photonics applications with emphasis on fiber-optics systems. In particular, we discuss how neural networks can be used to extract quantitative time-domain information in the development of nonlinear instabilities from spectral intensity measurements. We also show how neural networks can be efficiently applied to predict nonlinear dynamics in optical fibres for a wide range of scenarios, from pulse compression to ultra-broadband supercontinuum generation in both single and multimode fibers.

This Conference Presentation, “All-dielectric nanostructures for nonlinear optical applications” was recorded for the Ultrafast Phenomena and Nanophotonics conference at SPIE Photonics West On Demand 2022.

This conference presentation, “Multi-band photonic sources based on phase-matched second-order nonlinear interactions” was presented at the Ultrafast Phenomena and Nanophotonics XXVI conference at SPIE Photonics West 2022.

Bismuth oxyiodide (BiOI) is a promising material for photocatalysis with intriguing optical and structural properties. We demonstrate that excitation by a femtosecond laser pulse creates coherent phonons inducing a time-variant oscillating modulation of the optical density. The two underlying frequencies originate from vibrations along the stacking direction of oppositely charged layers in BiOI. This is consistent with a subpicosecond charge separation driven by a built-in dipolar field. This partially screens the field, launching coherent phonons. Our results demonstrate the presence of an electric field and show its role in efficient charge separation that is crucial for photocatalytic applications of BiOI.

This talk will show our recent theoretical and computational studies of new exciton physics in monolayer transition metal dichalcogenides. By developing a first-principle method based on many-body perturbation theories, we find that the photoelectrons from excitons hold unique energy dispersions and spectra weights, which unveil the fundamental physical properties of the excitons. The theoretical findings agree well with the experimentally measured pump-probe photoemission spectra of excitons in monolayer WSe2 (Science 370, 1199 (2020) and Science Advances 7, eabg0192 (2021)). We then demonstrate a valley- and spin-selective excitonic energy relaxation pathway, which leads to novel ultrafast dynamics in monolayer transition metal dichalcogenides. We further connect our theoretical discoveries to experimental results and explore their potential applications.

In this work, we take advantage of the strong temperature modulation of the graphene conductivity to propose an all-optical technique of excitation and manipulation of plasmons in graphene and thin metallic films, via the spatial patterning of the temperature of electrons in a graphene film, which can diffract a probe beam and directly excite plasmons. Additionally, we demonstrate the ability of graphene, thin metals films, and graphene-metal hybrid systems to undergo photothermal optical modulation with depth as large as >70% over a wide spectral range extending from the visible to the terahertz spectral domains. We envision the use of ultrafast pump laser pulses to raise the electron temperature of graphene during a picosecond timescale in which its mid-infrared plasmon resonances undergo dramatic shifts and broadenings, while visible and near-infrared plasmons in neighbouring metal films are severely attenuated by the presence of hot graphene electrons.

Increased modulation speed in quantum-dot lasers is possible by means of a tunnel-injection design. The concept was introduced to improve the dynamical properties of semiconductor lasers by avoiding the problem of hot carrier injection, which increases the gain nonlinearity thereby limiting the modulation capabilities. Cold carriers are efficiently provided via an injector quantum well that is tunnel coupled to excited QD states. We study the ultrafast carrier population dynamics in tunnel-injection lasers by comparing LO-phonon-assisted tunneling processes and Coulomb-scattering-assisted processes.

We explore the role that the electron wave function plays in cathodoluminescence (CL) emission when an external laser pulse is synchronized with the electron probe at the sample. We show that the the far-field emission is composed by coherent and incoherent contributions where the latter can only be modified by changing the electron density profile. In particular, shaped electrons lead to a partial suppression of the CL signal while its complete cancellation can be only achieved in the point-particle limit. We believe that our results open new routes toward coherent control of optical excitations at the atomic scale as well as toward a new method of studying ultrafast phenomena with a time resolution only limited by the width of the spectral window in the CL measurement.