This conference presentation was prepared for SPIE Optics + Optoelectronics, 2023.

Inelastic electron-light scattering between electrons and optical modes renders ultrafast transmission electron microscopes an ideal platform for investigating optical properties on the nanoscale. Building on this technique, we demonstrate the spatial and spectral characterization of the intracavity field of a photonic chip-based, high-Q silicon nitride microresonator by means of free electron light interaction. By combining optical and electron spectroscopies, we probe the emergence of various nonlinear intracavity states, including dissipative Kerr solitons. This novel combination of nonlinear integrated photonics and electron microscopy promises new schemes in electron beam manipulation as well as electron-based probing of optical microresonator states.

Ultrafast control of light-matter interactions is fundamental in view of new technological frontiers, for instance in light-driven information processing and nanoscale photochemistry. In this framework, we explore metal-dielectric nanocavities to achieve all-optical modulation of the light reflectance at a specific wavelength. Without the need of driving higher order effects, our system is based on linear absorption, provides large relative modulation exceeding 100% and switching bandwidths of few hundred GHz at moderate excitation fluence. This archetypical system becomes even more interesting if the “gain medium” is an inorganic van der Waals bonded semiconductor, like a transition metal dichalcogenide (TMD). TMDs are subject of intense research due to their electronic and optical properties which are promising for next-generation optoelectronic devices. In this context, understanding the ultrafast carrier dynamics, as well as charge and energy transfer at the interface between metals and semiconductors is crucial and yet quite unexplored. By employing a pump-push-probe scheme, we experimentally study how thermally induced ultrafast charge carrier injection affects the exciton formation dynamics in bulk WS2, opening up excellent opportunities also in nano-chemistry. In fact, if an electronic transition strongly interacts with the light modes of a resonator, we can tailor the energetics and the morphology of a molecular state. By combining quantum mechanical modelling and pump-probe spectroscopy, we shed light on the ultrafast dynamics of a hybrid system composed of photo-switchable dye molecules coupled with optically anisotropic plasmonic nanoantennas, which allow us to selectively switch between two regimes where the light-matter interaction is either weak or strong. Our synergistic approach is instrumental to devise new strategies for tailoring electronic states by using plasmons for applications in polaritonic chemistry on femtosecond timescales.

Nonlinear light propagation inside the medium can be analyzed using the concept of nonlinear refractive index. Unique and popular nonlinear media with enhanced nonlinearity are photonic crystal fibers (PCFs). To numerically simulate nonlinear light propagation inside a PCF it is necessary to properly evaluate the fiber’s nonlinear refractive index at different wavelengths. This work presents a new method to measure nonlinear refractive index in optical fibers, which allowed to estimate the nonlinear refractive index of PCF over a broad range of wavelengths. This work has received funding from European Regional Development Fund (project No. 01.2.2-LMT-K-718-03-0004) under grant agreement with the Research Council of Lithuania (LMTLT).

This conference presentation was prepared for SPIE Optics + Optoelectronics, 2023.

Subnanosecond pulse duration optical parametric generator (OPG) based on fan-out grating design MgO:PPLN crystal is demonstrated. In a fan-out grating design crystal, the effective grating period and thus quasi-phase-matching (QPM) conditions change continuously throughout the width of the crystal. We show that the OPG, based on a fan-out periodically poled crystal and pumped by a micro-laser system, permits a compact and effective subnanosecond coherent light source that could be rapidly, widely, and continuously tunable in the near-infrared spectral region (1.4- 4.4 μm) just by laterally displacing the crystal with up to 50% conversion efficiency. Full characterization of the OPG is performed and the feasibility of broadband OPG output is evaluated with the addition of numerical calculations based on a nonlinear model. This work has received funding from European Regional Development Fund (project No. 01.2.2-LMT-K-718-03-0004) under grant agreement with the Research Council of Lithuania (LMTLT).

We demonstrate experimentally a nonlinear plasmonic metasurface that exhibits strongly asymmetric second-harmonic generation (SHG), depending on the illuminating direction: when the metasurface is illuminated from one side, it produces a significant SHG signal, whilst this nonlinear response is strongly reduced (approx. 10 dB), upon illumination from the opposite side. This surprising behavior stems from the bianisotropic response of the system, as confirmed by a homogenization analysis and the extraction of the effective susceptibility tensor. At first sight, it could be tempting to interpret this asymmetric response as a non-reciprocal phenomenon, but we will show that actually it is time-reversal asymmetric.

Flat optics has been recently unveiled as a powerful platform to perform data processing in real-time, and with small footprint [1, 2, 3]. So far, these explorations have been limited to linear optics, while arguably the most impactful operations stem from nonlinear processing of the incoming signals. In this context, here we add a new twist and depth to analog optical computing: we demonstrate that nonlinear phenomena combined with engineered nonlocality in flat-optics devices can be leveraged to synthesize Volterra kernels able to perform complex operations on incoming images in real-time. Metasurfaces have already introduced a paradigm shift for nonlinear optics enabling stronger nonlinearities in thin films and manipulation of the nonlinearly-generated wavefront [4, 5, 6]. In this framework, here we show that it is possible to exploit nonlocal nonlinearities as a powerful tool for analog computing with light waves. We show that using nonlinear nonlocality in flat optics we can realize analog image processing with previously not accessible functionalities. By exploring the simple scenario of a uniform χ2 thin sheet, we demonstrate edge detection operation with exciting potentials. In our proposed nonlinear flat-optics solution, the non-resonant nature of the nonlinear interaction involved in image processing allows edge detection over a broadband spectrum with ultra–high contrast and superior resilience to noise. Our results indicate that Volterra kernels of nonlinear nonlocal flat optics can open new opportunities in applications such as image processing, item recognition for computer vision, and high-contrast, high-resolution microscopy. References 1. Solli, D.R., Jalali, B., Nature Photonics 9(11), 704–706 (2015). 2. Silva, A., Monticone, F., Castaldi, G., Galdi, V., Alù, A., Engheta, N., Science 343(6167), 160–163 (2014). 3. Overvig, A., Alù, A., Laser & Photonics Reviews 16(8), 2100633 (2022). 4. Vabishchevich, P., Kivshar, Y., Photon. Res. 11, B50-B64 (2023) 5. Schlickriede, C., Waterman, N., Reineke, B., Georgi, P., Li, G., Zhang, S., Zentgraf, T., Advanced Materials 30(8), 1703843 (2018). 6. Gigli, C., Marino, G., Artioli, A., Rocco, D., De Angelis, C., Claudon, J., Gerard, J.-M., Leo, G., Optica 8(2), 269–276 (2021).

In this paper, the uncertainty on the phase noise measured for a low phase noise compact optical delay line optoelectronic oscillator is evaluated as ±2 dB at 2 σ.