Conference Agenda

Maxwell's equations are solved if and only if the amplitude and phase of the total electromagnetic fields are determined across all spatial points. Typically, the Stokes parameters can only capture the field's amplitude and polarization in the (far) radiation zone. Thus, relying solely on the measurement of the Stokes parameters proves insufficient to solve Maxwell's equations.

In this talk, I present a method that successfully solves Maxwell's equations for widely studied objects in Nanophotonics using only the Stokes parameters. This feature endows the Stokes parameters, primarily used to gain insight into the polarization state of electromagnetic radiation, with an even more fundamental role in electromagnetic scattering theory. Consequently, our findings, supported by analytical theory and exact numerical simulations, can find applications in all branches of Nanophotonics and Optics.

We show that certain desired behaviors of optical metasurfaces (e.g large amplitude of the transmission/reflection coefficients, variation of the phase between 0 and 2π) are linked to simple conditions on the positions of phase singularities in the complex-frequency plane. These positions are shown to be linked to certain symmetries of the metasurface. These findings provide a method for designing metasurfaces based on symmetry breaking.

The optimization of the quality factor (Q) of photonic resonators is of great importance for applications exploiting both ordered and disordered systems. Here we propose a gradient-based automated optimization approach to maximize the Q of optical resonances in ordered and disordered dielectric slabs which uses first-order non-hermitian perturbation theory. After benchmarking our method with an L3 photonic crystal cavity, we apply it to optimize a selected Anderson mode in a random design with initial Q-factor of 200, generating a new mode with Q = (10)^5.

We adopt a multi-objective optimization approach to design one-dimensional photonic crystals with large optical chirality enhancements. We show that this technique allows for a large design flexibility in terms of selected materials and operational wavelengths. Finally, we demonstrate that the designed platforms provide state of the art chirality enhancements above the two orders of magnitude over arbitrarily large areas and broad spectral ranges.

We theoretically show that under circularly polarized plane wave or vortex beam illumination, the second-harmonic circular dichroism is possible even if the nanostructure is achiral. The interplay of nanostructure's and crystalline lattice's symmetries leads to specific conditions for observation of the circular dichroism, which can be expressed by a short formula. This can be particularly important for chiral sensing enhancement with nanostructure, where it is important to separate the signal from the nanostructure itself.

In recent years a strong drive towards the miniaturization of nonlinear optics has been motivated by the functionalities it could empower in integrated devices. Among these, the upconversion of near-infrared photons to the visible and their manipulation is fundamental to downsize optical information. We realized a dual-beam scheme whereby a pulse at the telecom frequency ω (λ=1550 nm) is mixed with its frequency-doubled replica at 2ω. When the two pulses are superimposed on a nonlinear, all-dielectric metasurface two coherent frequency-tripling pathways are excited: third-harmonic generation (THG, ω+ω+ω) and sum-frequency generation (SFG, ω+2ω). Their coherent superposition at 3ω produces interference, which we enable by filtering the k-space with the metasurface diffraction. The emission routing among diffraction orders is sensitive to the relative phase between the two pumps. Therefore, by exploiting the phase difference as a control mechanism, the upconverted signal can be commuted between diffraction orders, with measured visibility >90%. The proposed approach can be envisioned as an all-optical method to route upconverted telecom photons.

Lattice resonances (LR), collective electromagnetic modes supported by periodic arrays of metallic nanostructures, produce very strong and spectrally narrow optical responses. Usually, they arise from the coherent interaction between the localized electric dipolar plasmonic modes of the individual constituents of the array. Unfortunately, a two-dimensional arrangement of electric dipoles cannot absorb more than half the incident power due to fundamental symmetry reasons, thus hindering the use of LRs for applications requiring an efficient absorption of light. In this communication, we report a novel approach to overcome this constraint, which is based on the use of an array made of a unit cell containing one metallic and one dielectric nanostructure. Using a rigorous coupled dipole model, we show that this system can support two independent LRs, one with magnetic and the other with electric dipolar character, whose properties are tuned independently by adjusting the periodicity of the array and the size of the nanostructures. Furthermore, we show that an appropriate choice of these parameters not only leads to perfect absorption but also to quality factors exceeding 10^3. This work provides a general framework to design and implement complex 2D arrays capable of sustaining LRs with perfect absorption.

Photonic crystal slabs (or patterned multilayer waveguides) are known to support truly guided modes with no losses, as well as quasi-guided modes that lie in the continuum of far-field radiation. In this contribution, we present a guided-mode expansion approach – and the corresponding free software named “legume” – that allows calculating a number of features of quasi-guided modes of photonic crystal slabs: (a) symmetry properties and the issue of polarization mixing in coupling to far-field radiation modes; (b) the occurence of bound states in a continuum, which have infinite Q-factor and give rise to topological singularities of the far-field polarization; (c) the description of active two-dimensional layers through a suitably formulated light-matter coupling Hamiltonian, allowing to describe the regime of strong coupling leading to photonic crystal polaritons. Comparison with rigorous coupled-wave analysis, and the insurgence of non-hermitian features in the optical properties, are also addressed.

We report the fabrication and validation of an experimental system that exploits plasmonic Au nanoparticles to generate heat on a nanometric lateral scale, and measures the local temperature increase in the surroundings of these nanoparticles by means of carefully placed monolayers of transition metal dichalcogenide (TMDC).

Photonic bound states in the continuum (BICs) have enabled a new class of spectrally selective metasurfaces supporting ultrasharp resonances, enabling breakthroughs in higher-harmonic generation, strong light-matter coupling, biodetection, and lasing. However, many implementations still face constraints related to large metasurface footprints, fabrication limits requiring constant resonator heights throughout the structure, or limited numbers of resonances in a given metasurface area. In this talk, I will present some of our recent concepts for obtaining additional nanophotonic functionalities in BIC-driven systems, including the arrangement of resonators in radial configurations for polarization invariance and reduced footprints, height-driven BICs for obtaining maximally chiral light-matter interactions, and active resonance control by incorporating an electro-optically active polymer. Finally, I will show how BIC metasurfaces with continuously varying structural parameters can be leveraged to spatially encode spectral and molecular coupling information simultaneously, enabling new perspectives for biochemical spectroscopy.

Exceptional points (EPs) attract lots of attention due to the richness of the phenomenology associated to their presence in the complex eigenspectrum of coupled non-Hermitian systems. Here we provide both a coupled mode theory analysis and an experimental investigation of two nanolasers interacting through a channel-mediated coupling. We demonstrate the transition from Parity-Time (PT) symmetric to PT-broken regime using a thermo-optic control over the laser frequency detuning. This regime is associated with a significant reduction of the laser threshold as well as we a enhanced sensitivity to external perturbations in the vicinity of the EP.

Our investigation focused on fluorescence enhancement mechanisms using metal nanoantennas with Alexa Fluor-647. By exploiting numerical modelling tools, we fabricated non-interacting Au nanodisc arrays on glass substrates, achieving a maximum fluorescence enhancement factor of 180 at an optimal spacer thickness of approximately 10 nm. Comparative analysis with bare glass substrates revealed significant improvements in excitation and emission dynamics, attributed to nanoscale field confinement and the Purcell effect. Time- and space-resolved photoluminescence measurements unveiled a distance-dependent interaction between the fluorophore and localized plasmons, modulated by thin polyelectrolyte monolayers, with prevalent radiative processes in samples exhibiting maximum signal.

The manipulation of the the field from quantum emitters directly at the source level is becoming an increasingly viable option for obtaining single photons with specific polarizations or phase profiles. In this context, the combination of metasurfaces with surface propagating modes like Bloch Surface Waves have emerged as an ideal candidate. In this work we present a radially distributed metasurface-like grating designed to produce vortex beams with arbitrary spin and orbital angular momentum. This functionality is realized through its synergy with a dipole-like source, coupled with a TM polarized Bloch Surface Wave.

Two dimensional (2D) semiconductors are exceptional materials for exploring light-matter interactions at the nanoscale. Here I will discuss the integration of monolayer semiconductors and hybrid nanophotonic platforms based on Mie-resonant nanostructures, achieving enhanced light-matter interaction up to the strong coupling regime and generation of strain-induced single photon sources.

Monolayer transition metal dichalcogenides (TMDs) like WS2 exhibit strong exciton resonances in the visible spectral range that govern their optical response. The excitonic light-matter interaction in these 2D quantum materials is inherently strong and highly tunable, which can be leveraged to realize mutable flat optical elements as well as novel spin-valley coupled information carriers.

Here, I will showcase experimental realizations of coherent coupling to hybrid light-matter quasiparticles known as 2D exciton polaritons (2DEPs) in nanopatterned monolayers of WS2, allowing for enhanced and tunable photonic functionality given directly by the geometry of the monolayer itself. Using guided mode resonances in sub-wavelength gratings structured in mm-sized continuous WS2 monolayers, we can realize dynamic control of light scattered coherently off the hybrid light-matter state via electrical and/or thermal tuning. Further utilization of photonic metasurface concepts allows for angle-dependent amplitude switching of grating diffraction orders via perturbative approaches, leading to expected modulation depths exceeding 13dB stemming from an atomically thin optical element. This opens a path to full active control over the complex optical response in tailored atomically thin metasurfaces via exciton resonance tuning.

The study of Dirac plasmon polaritons (DPPs) in two-dimensional materials has raised considerable interest in the last years for the development of tunable optical devices, plasmonic sensors, ultrafast absorbers, modulators, and switches. In particular, topological insulators (TIs) represent an ideal material platform by virtue of the plasmon polaritons sustained by the Dirac carriers in their surface states. However, tracking DPP propagation at terahertz (THz) frequencies, with wavelength much smaller than that of the free-space photons, represents a challenging task. Herein, we trace the propagation of DPPs in TI-based coupled antennas. We show how Bi2Se3 rectangular nano-antennas effectively confine DPPs propagation to one dimension, enhancing their visibility despite intrinsic attenuation. Furthermore, plasmon dispersion and loss properties of coupled antenna resonators, patterned at varying lengths and distances are experimentally determined using holographic near-field nano-imaging at different THz frequencies. Our study evidences modifications on the DPP wavelength along the single nano-antenna ascribable to the cross-talk between neighbouring elements. The results provide insights into DPPs characteristics, paving the way for the design of novel topological devices and metasurfaces by leveraging their directional propagation capabilities.

Van der Waals (vdW) materials are ideal for nanoscale light-matter interactions. Here, we use quasi-bound states in the continuum (qBIC) to achieve high Q factor optical resonances in hBN and TMDC metasurfaces. In hBN metasurfaces, we achieve spectral tuning across the visible spectrum, enhance non-linear optical processes, and coupling of optically active defects. In WS2 metasurfaces, we observe strong anti-crossing between qBIC resonances and excitons, with Rabi splitting up to 116 meV. These results demonstrate the potential of vdW materials combined with qBIC for advanced nanophotonic platforms and room-temperature polaritonic devices.

In this work, we discuss the challenges associated with measuring and interpreting the vibrational properties of nanomaterials at mid- and far-infrared frequencies, where vibrational bands are often broad and overlapping. This issue is compounded by the complex interaction between infrared light and particulate samples, which depends on packing density and particle connectivity. Preliminary results concerning the far-infrared optical properties of Fe 3 O 4 nanoparticles have been obtained using the two most reliable methods (specular reflectance and attenuated total reflectance). These results are compared to one another and to their Raman counterparts. Finally, the influences of particle size and composition on the vibrational spectra are qualitatively discussed.

A variety of applications requires light and infrared radiation to be shaped and controlled actively. In our laboratory we are working on shaping light using silicon photonics on a chip and in free space using metasurfaces. Key to these applications are materials that can be tuned or switched optically, electrically or thermally. In this presentation I will give an overview of cutting edge developments in shaping of light using phase change materials. I will also address efforts at modelling these effects using new techniques from the toolbox of deep learning neural networks.

We show that, by exploiting the optical memory effect, it is possible to track a moving object through a strongly scattering layer, despite its image being obscured to us.

Here we propose to exploit the natural instability of thin solid films, i.e. solid state dewetting, to form regular patterns of monocrystalline atomically smooth Si, Si1-xGex and Ge nanostructures that cannot be realized with conventional methods. Additionally, the solid-state dewetting dynamics is guided by prepatterning the sample by a combination of electron-beam lithography and reactive-ion etching, obtaining precise control over number, size, shape, and relative position of the final structures. Methods and structures will be optimized towards their exploitation mainly in photonic devices application (e.g. anti-reflection coatings, colour-filters, random lasers, quantum emitters and photonic sensors).

Spin-orbit coupling in nanoscale optical fields induces the formation of a spin momentum component transverse to the orbital momentum. Herein, we firstly explore the manipulation of dynamics by the spin-orbit effect on gold monomers, observing how the self-induced spin from localized resonance results in trajectory shifts controlled by the incident polarization. Secondly, we discuss the spin-orbit behavior in systems of gold dimers that due to field hybridization effects within the gap exhibit changes in the nontrivial spin momentum that lead to the formation of a vortex and anti-vortex spin angular momentum (SAM) pair on the opposite surfaces of the nanoparticles. The results could offer advantages for biological and aerospace fields, leading to the development of new systems and applications in the field of spin-optics.

Photonic band gap crystals are being pursued for their ability to control emission and vacuum fluctuations radically. Here, we examine the spontaneous emission of lead sulfide quantum dot nanocrystals within 3D photonic band gap crystals. These crystals, formed by etching deep pores in a silicon bar from two directions, possess a diamond-like inverse-woodpile structure, with quantum dots infiltrated into the pores using a toluene suspension. At frequencies just above the band gap, we observe >18x more intensity from quantum dots inside the crystal than the same number of dots outside, indicative of strongly enhanced emission. Time-correlated single photon counting shows fast non-exponential decay, reflecting the varied local densities of states of different quantum dots within the nanopores. We employ a log-normal model to interpret properties of the time-resolved decay at different emission frequencies and observe that the most frequently occurring decay rate is up to a factor 10x greater in the photonic crystal than outside. Current efforts focus on enhancing signal-to-background and signal-to-noise ratio, essential for probing inhibited decay within the band gap, and further studies on 3D band gap cavity superlattices and quasicrystals.