arXiv subjects

Agostino Di Francescantonio

Publications and source records attributed to Agostino Di Francescantonio.

5 recordsLinked to original sources

arXiv2026

Nonlinear Self-Action across Temporal Regimes in Resonant Dielectric Metasurfaces

Quasi-bound states in the continuum (qBICs) enable exceptional field confinement, strongly reducing the pump intensity threshold for nonlinear light-matter interaction in dielectric metasurfaces. As a result, nonlinear self-action effects, often elusive in bulk nonlinear media, emerge at moderate excitation intensities. Here, nonlinear self-action in resonantly enhanced third-harmonic (TH) generation from a dielectric metasurface supporting a qBIC resonance is investigated across distinct temporal excitation regimes. These regimes establish different coupling conditions between the excitation and the resonant mode, causing the same nonlinear self-action to emerge through complementary intensity-dependent signatures. Under spectrally narrow picosecond excitation, resonance-enhanced TH generation shows pronounced deviations from cubic scaling at high intensities. In contrast, broadband femtosecond excitation transiently drives the resonance, encoding the nonlinear response in the spectral reshaping and broadening of the TH signal. Simulations reproduce both regimes: continuous-wave modeling captures picosecond power scaling and the role of higher-order nonlinear susceptibilities, while time-domain simulations resolve femtosecond dynamics. These results clarify how pulse duration, bandwidth, and resonant coupling determine the observable signatures of nonlinear self-action in resonant dielectric metasurfaces, linking field confinement to conversion efficiency, power-law scaling, and ultrafast spectral dynamics.

physics.optics↗

arXiv2026

Electro-optical modulation of light polarization in a nonlocal lithium niobate metasurface

We report the experimental realization of a LiNbO3 metasurface for electro-optic modulation of light polarization in the telecommunication band. High-Q quasi-bound states in the continuum are emploied to enhance the modulation of amplitude and phase of an impinging beam by a driving electric field, leading to efficient polarization rotation and conversion. We quantified modulation effects under a CMOS-compatible bias at 1 MHz frequency, achieving a variation of 5% in the Stokes parameters and a variation of the polarization ellipse angles of about 3° for the transmitted light. These results demonstrate that dynamic polarization and phase modulation can be attained in a compact platform, highlighting the potential of high-Q resonant LiNbO3 metasurfaces for enhanced light-matter interaction in subwavelength electro-optic devices.

physics.optics↗

arXiv2024

All-optical polarization control and routing by nonlinear interferometry at the nanoscale

Optical metasurfaces are rapidly establishing as key-enabling platforms for nanophotonics applications. Along with the ability of taming light in subwavelength thicknesses, they can feature multiple functionalities in one device. The generation and control of light polarization by metasurfaces already provided a route towards ultracompact polarimetry devices in the linear regime. If translated to the nonlinear optical regime it may become a key-enabling tool in nonlinear imaging, optical holography and sensing. Here, we report the experimental ultrafast all-optical polarization modulation of upconverted light by all-dielectric metasurfaces via nonlinear interferometry. By controlling the relative phase between a pump beam at $ω$ and its frequency-double replica at 2$ω$, we can set the phase relation between two frequency-degenerate upconverted processes at 3$ω$ $-$ Sum-Frequency Generation (SFG) and Third-Harmonic Generation (THG) $-$ stemming from an AlGaAs metasurface. By exploiting the opposite parity of the two nonlinear process and tuning their relative powers, we can achieve modulation of the polarization states of the light emitted at 3$ω$ between linear and circular states with a degree of circular polarization (DOCP) up to 83$\%$. In particular, circularly polarized light features opposite handedness symmetrically located in the Fourier space, at coincidence with the first diffraction orders of the metasurface. Moreover, by toggling the phase delay by $π$, the handedness can be fully switched between the diffraction. Our work adds an additional layer of modulation in polarization beyond intensity to all-optical routing with precise phase control. The capability to route circular polarization states in the k-space holds significant potential for chiral sensing and advanced imaging techniques.

physics.optics↗

arXiv2024

Efficient GHz electro-optical modulation with a nonlocal lithium niobate metasurface in the linear and nonlinear regime

Electro-optical modulation is widely employed for optical signal processing and in laser technology. To date, it is efficiently realized in integrated photonic systems as well as in bulk optics devices. Yet, the achievement of modulators exploiting Pockels effect in flat optics, essential to scale down the electric radiation-optical control in free space, currently lag behind bulk and on-chip integrated platforms in terms efficiency and speed. We bridge this gap realizing a metasurface based on lithium niobate (LiNbO3) on insulator that leverages on resonances with quality-factor as high as 8e3 to achieve fast electrical modulation of both linear and nonlinear optical properties. LiNbO3, well known for its high nonlinear susceptibility and wide transparency window across the infrared and visible spectrum, is employed to realize an asymmetric, one-dimensional array of nanowires, exhibiting resonances with linewidth < 0.2 nm. By applying a CMOS-compatible electrical bias, the metasurface imparts a relative reflectivity modulation around 0.1, with a modulation efficiency, defined as relative modulation per applied Volt, larger than 0.01 V^-1 on a bandwidth of about 1 GHz. We also demonstrated more than one order of magnitude intensity modulation of the second harmonic seeded by a continuous-wave laser, with a modulation efficiency of about 0.12 V^-1. This dual modulation capability, rooted in the interplay between optical resonances and electric field manipulation, holds significant potential for cutting-edge applications in high-speed photonics, nonlinear optics, and reconfigurable communication systems. Our findings highlight the transformative potential of LiNbO3-based metasurfaces for integration into next-generation optical technologies that demand rapid, efficient electrical control of light.

physics.optics↗

arXiv2023

All-optical free-space routing of upconverted light by metasurfaces via nonlinear interferometry

All-optical modulation yields the promise of high-speed information processing. In this frame, metasurfaces are rapidly gaining traction as ultrathin multifunctional platforms for light management. Among the featured functionalities, they enable light wavefront manipulation and, more recently, demonstrated the ability to perform light-by-light manipulation through nonlinear optical processes. Here, by employing a nonlinear periodic metasurface, we demonstrate all-optical routing of telecom photons upconverted to the visible range. This is achieved via the interference between two frequency-degenerate upconversion processes, namely third-harmonic and sum-frequency generation, stemming from the interaction of a pump pulse with its frequency-doubled replica. By tuning the relative phase and polarization between these two pump beams, and concurrently engineering the nonlinear emission of the individual elements of the metasurfaces (meta-atoms) along with its pitch, we route the upconverted signal among the diffraction orders of the metasurface with a modulation efficiency up to 90%. Thanks to the phase control and the ultrafast dynamics of the underlying nonlinear processes, free-space all-optical routing could be potentially performed at rates close to the employed optical frequencies divided by the quality factor of the optical resonances at play. Our approach adds a further twist to optical interferometry, which is a key-enabling technique in a wide range of applications, such as homodyne detection, radar interferometry, LiDAR technology, gravitational waves detection, and molecular photometry. In particular, the nonlinear character of light upconversion combined with phase sensitivity is extremely appealing for enhanced imaging and biosensing.

physics.optics↗