arXiv subjects

R. Sapienza

Publications and source records attributed to R. Sapienza.

9 recordsLinked to original sources

arXiv2026

Programmable Synthetic Motion at a Time-Varying Interface

Space-time metamaterials that exhibit synthetic motion promise arbitrary control over the momentum, frequency and energy of scattered light, but realising the required space-time modulation in a programmable way remains a challenge. Here we program synthetic motion using a single spatial light modulator in a 4f geometry which imprints a continuously tunable pulse-front tilt onto a high-intensity pump pulse, inducing reflectivity modulations at a sub-wavelength indium tin oxide thin film with synthetic velocities spanning the sub- and superluminal regimes. The angle-resolved spectrum of a scattered probe pulse reveals space-time diffraction patterns whose gradient and bandwidth vary continuously with synthetic velocity, in excellent agreement with theory. Splitting the shaped pump into two independently controlled pulses yields space-time double-slit diffraction with tunable fringe separation and frequency-momentum gradient. This programmable platform opens a path towards non-linear and periodic space-time trajectories for tabletop analogue studies of relativistic phenomena and space-time metasurfaces.

physics.optics↗

arXiv2024

Space-Time Optical Diffraction from Synthetic Motion

The interaction of light with objects and media moving at relativistic and superluminal speeds enables unconventional phenomena such as Fresnel drag, Hawking radiation, and light amplification. Synthetic motion, facilitated by modulated internal degrees of freedom, enables the study of relativistic phenomena unrestricted by the speed of light. In this study, we investigate synthetically moving apertures created by high-contrast reflectivity modulations, which are generated by ultrafast laser pulses on a subwavelength thin film of indium tin oxide. The space-time diffraction of a weaker probe beam reveals a complex, non-separable spatio-temporal transformation, where changes in the frequency of the wave are correlated to changes in its momentum. By using schemes of continuous or discrete modulation we demonstrate tunable frequency-momentum diffraction patterns with gradients that depend upon the relative velocity between the modulation and the probe wave. The diffraction patterns are matched by operator-based theory and the gradients are analytically predicted using a super-relativistic Doppler model, where the modulation is described as a superluminally moving scattering particle. Our experiments open a path towards mimicking relativistic mechanics and developing complex and programmable spatio-temporal transformations of light.

physics.optics↗

arXiv2023

Near-field imaging of optical nano-cavities in Hyperuniform disordered materials

Hyperuniform disordered photonic materials have recently been shown to display large, complete photonic band gaps and isotropic optical properties, and are emerging as strong candidates for a plethora of optoelectronic applications, making them competitive with many of their periodic and quasiperiodic counterparts. In this work, high quality factor optical cavities in hyperuniform disordered architectures are fabricated through semiconductor slabs and experimentally addressed by scanning near-field optical microscopy. The wide range of confined cavity modes that we detect arise from carefully designed local modifications of the dielectric structure. Previous works on hyperuniform disordered photonic systems have previously identified several Anderson localized states spectrally located at the PBG edges with relatively high quality factors. In this work, by engineering the structural parameters of the cavity, we achieve an experimental quality factor of order 6000 (higher than the one of the Anderson states) and we demonstrate that three types of localized modes of different nature coexist within a small area and in a relatively narrow spectral window of the disordered correlated system. Their compatibility with general boundary constraints, in contrast with ordered architectures that suffer strict layout constraints imposed by photonic crystals' axes orientation, makes optical cavities in disordered hyperuniform patterns a flexible optical insulator platform for planar optical circuits.

physics.optics↗

arXiv2019

Enhanced light-matter interaction in an atomically thin semiconductor coupled with dielectric nano-antennas

Unique structural and optical properties of atomically thin two-dimensional semiconducting transition metal dichalcogenides enable in principle their efficient coupling to photonic cavities with optical modes volumes close to or below the diffraction limit. Recently, it has become possible to make all-dielectric nano-cavities with reduced mode volumes and negligible non-radiative losses. Here, we realise low-loss high-refractive-index dielectric gallium phosphide (GaP) nano-antennas with small mode volumes coupled to atomic mono- and bilayers of WSe$_2$. We observe a photoluminescence enhancement exceeding 10$^4$ compared with WSe$_2$ placed on planar GaP, and trace its origin to a combination of enhancement of the spontaneous emission rate, favourable modification of the photoluminescence directionality and enhanced optical excitation efficiency. A further effect of the coupling is observed in the photoluminescence polarisation dependence and in the Raman scattering signal enhancement exceeding 10$^3$. Our findings reveal dielectric nano-antennas as a promising platform for engineering light-matter coupling in two-dimensional semiconductors.

physics.app-ph↗

arXiv2019

Negative refraction in time-varying, strongly-coupled plasmonic antenna-ENZ systems

Time-varying metasurfaces are emerging as a powerful instrument for the dynamical control of the electromagnetic properties of a propagating wave. Here we demonstrate an efficient time-varying metasurface based on plasmonic nano-antennas strongly coupled to an epsilon-near-zero (ENZ) deeply sub-wavelength film. The plasmonic resonance of the metal resonators strongly interacts with the optical ENZ modes, providing a Rabi level spitting of ~30%. Optical pumping at frequency ω induces a nonlinear polarisation oscillating at 2ω responsible for an efficient generation of a phase conjugate and a negative refracted beam with a conversion efficiency that is more than four orders of magnitude greater compared to the bare ENZ film. The introduction of a strongly coupled plasmonic system therefore provides a simple and effective route towards the implementation of ENZ physics at the nanoscale

physics.optics↗

arXiv2015

Nanophotonic boost of intermolecular energy transfer

We propose a scheme for efficient long-range energy transfer between two distant light emitters separated by more than one wavelength of light, i.e. much beyond the classical Forster radius. A hybrid nanoantenna-waveguide system mediates the transmission of energy, showing enhancements up to 10^8 as compared to vacuum. Our model shows how energy transfer in nanostructured media can be boosted, beyond the simple donor Purcell enhancement, and in particular for large donor-acceptor separations. The scheme we propose connects realistic emitters and could lead to practical on-chip implementations.

physics.optics↗

arXiv2011

Direct Determination of Diffusion Properties of Random Media from Speckle Contrast

We present a simple scheme to determine the diffusion properties of a thin slab of strongly scattering material by measuring the speckle contrast resulting from the transmission of a femtosecond pulse with controlled bandwidth. In contrast with previous methods, our scheme does not require time measurements nor interferometry. It is well adapted to the characterization of samples for pulse shaping, non-linear excitation through scattering media and biological imaging.

physics.optics↗

arXiv2010

Photonic Crystals with Controlled Disorder

Photonic crystals are extremely sensitive to structural disorder even to the point of completely losing their functionalities. While on one side this can be detrimental for applications in traditional optical devices, on the other side, it gives also rise to very interesting new physics and maybe even new applications. We propose a route to introduce disorder in photonic crystals in a controlled way, by creating a certain percentage of vacancies in the lattice. We show how the method works and what type of materials can be obtained this way. Also we characterize the resulting transport properties from various points of view, including measurements of the transport and scattering mean free path and the diffusion constant.

physics.optics↗

arXiv2008

Density of states governs light scattering in photonic crystals

We describe a smooth transition from (fully ordered) photonic crystal to (fully disordered) photonic glass that enables us to make an accurate measurement of the scattering mean free path in nanostructured media and, in turn, establishes the dominant role of the density of states. We have found one order of magnitude chromatic variation in the scattering mean free path in photonic crystals for just $\sim 3%$ shift around the band-gap ($\sim 27$ nm in wavelength).

physics.optics↗