arXiv subjects

D. Faccio

Publications and source records attributed to D. Faccio.

At least 19 recordsLinked to original sources

arXiv2023

Amplification of electromagnetic waves by a rotating body

In 1971, Zel'dovich predicted the amplification of electromagnetic (EM) waves scattered by a rotating metallic cylinder, gaining mechanical rotational energy from the body. Since then, this phenomenon has been believed to be unobservable with electromagnetic fields due to technological difficulties in meeting the condition of amplification, that is, the cylinder must rotate faster than the frequency of the incoming radiation. Here, we show that this key piece of fundamental physics has been hiding in plain sight for the past 60 years in the physics of induction generators. We measure the amplification of an electromagnetic field, generated by a toroid LC-circuit, scattered by an aluminium cylinder spinning in the toroid gap. We show that when the Zel'dovich condition is met, the resistance induced by the cylinder becomes negative implying amplification of the incoming EM waves. These results reveal the connection between the concept of induction generators and the physics of this fundamental effect that was believed to be unobservable, and hence open new prospects towards testing the Zel'dovich mechanism in the quantum regime, as well as related quantum friction effects.

physics.class-ph↗

arXiv2022

Experimental observation of violent relaxation and the formation of out-of-equilibrium quasi-stationary states

Large scale structures in the Universe, ranging from globular clusters to entire galaxies, are the manifestation of relaxation to out-of-equilibrium states that are not described by standard statistical mechanics at equilibrium. Instead, they are formed through a process of a very different nature, i.e. violent relaxation. However, astrophysical time-scales are so large that it is not possible to directly observe these relaxation dynamics and therefore verify the details of the violent relaxation process. We develop a table-top experiment and model that allows us to directly observe effects such as mixing of phase space, and violent relaxation, leading to the formation of a table-top analogue of a galaxy. The experiment allows us to control a range of parameters, including the nonlocal (gravitational) interaction strength and quantum effects, thus providing an effective test-bed for gravitational models that cannot otherwise be directly studied in experimental settings.

physics.optics↗

arXiv2021

Super-Resolution Time-Resolved Imaging using Computational Sensor Fusion

Imaging across both the full transverse spatial and temporal dimensions of a scene with high precision in all three coordinates is key to applications ranging from LIDAR to fluorescence lifetime imaging. However, compromises that sacrifice, for example, spatial resolution at the expense of temporal resolution are often required, in particular when the full 3-dimensional data cube is required in short acquisition times. We introduce a sensor fusion approach that combines data having low-spatial resolution but high temporal precision gathered with a single-photon-avalanche-diode (SPAD) array with set of data that has high spatial but no temporal resolution, such as that acquired with a standard CMOS camera. Our method, based on blurring the image on the SPAD array and computational sensor fusion, reconstructs time-resolved images at significantly higher spatial resolution than the SPAD input, upsampling numerical data by a factor 12x12, and demonstrating up to 4x4 upsampling of experimental data. We demonstrate the technique for both LIDAR applications and FLIM of fluorescent cancer cells. This technique paves the way to high spatial resolution SPAD imaging or, equivalently, FLIM imaging with conventional microscopes at frame rates accelerated by more than an order of magnitude.

eess.IV↗

arXiv2020

Non-line-of-sight Imaging

Emerging single-photon-sensitive sensors combined with advanced inverse methods to process picosecond-accurate time-stamped photon counts have given rise to unprecedented imaging capabilities. Rather than imaging photons that travel along direct paths from a source to an object and back to the detector, non-line-of-sight (NLOS) imaging approaches analyse photons {scattered from multiple surfaces that travel} along indirect light paths to estimate 3D images of scenes outside the direct line of sight of a camera, hidden by a wall or other obstacles. Here we review recent advances in the field of NLOS imaging, discussing how to see around corners and future prospects for the field.

physics.optics↗

arXiv2020

Amplification of waves from a rotating body

In 1971 Zel'dovich predicted that quantum fluctuations and classical waves reflected from a rotating absorbing cylinder will gain energy and be amplified. This key conceptual step towards the understanding that black holes may also amplify quantum fluctuations, has not been verified experimentally due to the challenging experimental requirements on the cylinder rotation rate that must be larger than the incoming wave frequency. Here we experimentally demonstrate that these conditions can be satisfied with acoustic waves. We show that low-frequency acoustic modes with orbital angular momentum are transmitted through an absorbing rotating disk and amplified by up to 30% or more when the disk rotation rate satisfies the Zel'dovich condition. These experiments address an outstanding problem in fundamental physics and have implications for future research into the extraction of energy from rotating systems.

physics.class-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↗

arXiv2019

Non-line-of-sight 3D imaging with a single-pixel camera

Real time, high resolution 3D reconstruction of scenes hidden from the direct field of view is a challenging field of research with applications in real-life situations related e.g. to surveillance, self-driving cars and rescue missions. Most current techniques recover the 3D structure of a non-lineof-sight (NLOS) static scene by detecting the return signal from the hidden object on a scattering observation area. Here, we demonstrate the full colour retrieval of the 3D shape of a hidden scene by coupling back-projection imaging algorithms with the high-resolution time-of-flight information provided by a single-pixel camera. By using a high effciency Single-Photon Avalanche Diode (SPAD) detector, this technique provides the advantage of imaging with no mechanical scanning parts, with acquisition times down to sub-seconds.

physics.optics↗

arXiv2018

Metasurface imaging with entangled photons

Plasmonics and metamaterials have recently been shown to allow the control and interaction with non-classical states of light, a rather counterintuitive finding given the high losses typically encountered in these systems. Here, we demonstrate a range of functionalities that are allowed with correlated and entangled photons that are used to illuminate multiple, overlaid patterns on plasmonic metasurfaces. Correlated photons allow to nonlocally determine the pattern that is imaged or, alternatively to un-scramble an image that is otherwise blurred. Entangled photons allow a more important functionality whereby the images imprinted on the metasurface are individually visible only when illuminated with one of the entangled photons. Correlated single photon imaging of functional metasurfaces could therefore promise advances towards the use of nanostructured subwavelength thin devices in quantum information protocols.

quant-ph↗

arXiv2018

Spontaneous photon production in time-dependent epsilon-near-zero materials

Quantum field theory predicts that a spatially homogeneous but temporally varying medium will excite photon pairs out of the vacuum state. However, this important theoretical prediction lacks experimental verification due to the difficulty in attaining the required non-adiabatic and large amplitude changes in the medium. Recent work has shown that in epsilon-near-zero (ENZ) materials it is possible to optically induce changes of the refractive index of the order of unity, in femtosecond timescales. By studying the quantum field theory of a spatially homogeneous, time-varying ENZ medium, we theoretically predict photon pair production that is up to several orders of magnitude larger than in non-ENZ time-varying materials. We also find that whilst in standard materials the emission spectrum depends on the time scale of the perturbation, in ENZ materials the emission is always peaked at the ENZ wavelength. These studies pave the way to technologically feasible observation of photon pair emission from a time-varying background with implications for quantum field theories beyond condensed matter systems and with potential applications as a new source of entangled light.

quant-ph↗

arXiv2017

Controlling hybrid nonlinearities in transparent conducting oxides via two-colour excitation

Nanophotonics and metamaterials have revolutionised the way we think about optical space (epsilon, mu), enabling us to engineer the refractive index almost at will, to confine light to the smallest of the volumes, and to manipulate optical signals with extremely small footprints and energy requirements. Significant efforts are now devoted to finding suitable materials and strategies for the dynamic control of the optical properties. Transparent conductive oxides exhibit large ultrafast nonlinearities under both interband and intraband excitations. Here, we show that combining these two effects in aluminium-doped zinc oxide via a two colour laser field discloses new material functionalities. Owing to the independence of the two nonlinearities the ultrafast temporal dynamics of the material permittivity can be designed by acting on the amplitude and delay of the two fields. We demonstrate the potential applications of this novel degree of freedom by dynamically addressing the modulation bandwidth and optical spectral tuning of a probe optical pulse.

physics.optics↗

arXiv2017

Phase-Insensitive Scattering of Terahertz Radiation

The nonlinear interaction between Near-Infrared (NIR) and Terahertz pulses is principally investigated as a means for the detection of radiation in the hardly accessible THz spectral region. Most studies have targeted second-order nonlinear processes, given their higher efficiencies, and only a limited number have addressed third-order nonlinear interactions, mainly investigating four-wave mixing in air for broadband THz detection. We have studied the nonlinear interaction between THz and NIR pulses in solid-state media (specifically diamond), and we show how the former can be frequency-shifted up to UV frequencies by the scattering from the nonlinear polarisation induced by the latter. Such UV emission differs from the well-known electric-field-induced second harmonic (EFISH) one, as it is generated via a phase-insensitive scattering, rather than a sum- or difference-frequency four-wave-mixing process.

physics.optics↗

arXiv2016

Synthetic magnetism for photon fluids

We develop a theory of artificial gauge fields in photon fluids for the cases of both second-order and third-order optical nonlinearities. This applies to weak excitations in the presence of pump fields carrying orbital angular momentum, and is thus a type of Bogoliubov theory. The resulting artificial gauge fields experienced by the weak excitations are an interesting generalization of previous cases and reflect the PT-symmetry properties of the underlying non-Hermitian Hamiltonian. We illustrate the observable consequences of the resulting synthetic magnetic fields for examples involving both second-order and third-order nonlinearities.

cond-mat.quant-gas↗

arXiv2016

Enhanced nonlinear refractive index in epsilon-near-zero materials

New propagation regimes for light arise from the ability to tune the dielectric permittivity to extremely low values. Here we demonstrate a universal approach based on the low linear permittivity values attained in the epsilon-near-zero (ENZ) regime for enhancing the nonlinear refractive index, which enables remarkable light-induced changes of the material properties. Experiments performed on Al-doped ZnO (AZO) thin films show a six-fold increase of the Kerr nonlinear refractive index ($n_2$) at the ENZ wavelength, located in the 1300 nm region. This in turn leads to ultrafast light-induced refractive index changes of the order of unity, thus representing a new paradigm for nonlinear optics.

physics.optics↗

arXiv2016

Optically induced metal-to-dielectric transition in Epsilon-Near-Zero metamaterials

Epsilon-Near-Zero materials exhibit a transition in the real part of the dielectric permittivity from positive to negative value as a function of wavelength. Here we study metal-dielectric layered metamaterials in the homogenised regime (each layer has strongly subwavelength thickness) with zero real part of the permittivity in the near-infrared region. By optically pumping the metamaterial we experimentally show that close to the Epsilon-Equal-to-Zero (EEZ) wavelength the permittivity exhibits a marked transition from metallic (negative permittivity) to dielectric (positive permittivity) as a function of the optical power. Remarkably, this transition is linear as a function of pump power and occurs on time scales of the order of the 100 fs pump pulse that need not be tuned to a specific wavelength. The linearity of the permittivity increase allows us to express the response of the metamaterial in terms of a standard third order optical nonlinearity: this shows a clear inversion of the roles of the real and imaginary parts in crossing the EEZ wavelength, further supporting an optically induced change in the physical behaviour of the metamaterial.

physics.optics↗

arXiv2015

Terawatt-level few-cycle mid-IR pulses through nonlinear self-compression in bulk

The physics of strong-field applications requires driver laser pulses that are both energetic and extremely short. Whereas optical amplifiers, laser and parametric, boost the energy, their gain bandwidth restricts the attainable pulse duration, requiring additional nonlinear spectral broadening to enable few or even single cycle compression and a corresponding peak power increase. In the mid-IR wavelength range that is critically important for scaling the ponderomotive energy in strong-field interactions, we demonstrate a remarkably simple energy-efficient and scalable soliton-like pulse compression in a mm-long YAG crystal with no additional dispersion management. Sub-three-cycle pulses with >0.65 TW peak power are compressed and extracted before the onset of modulation instability and multiple filamentation as a result of a favorable interplay between strong anomalous dispersion and optical nonlinearity around the wavelength of 3900 nm. As a striking manifestation of the increased peak power, we show the evidence of mid-infrared pulse filamentation in atmospheric air.

physics.optics↗

arXiv2014

Experimental quantum cosmology in time-dependent optical media

It is possible to construct artificial spacetime geometries for light by using intense laser pulses that modify the spatiotemporal properties of an optical medium. Here we theoretically investigate experimental possibilities for studying spacetime metrics of the form $\textrm{d}s^2=c^2\textrm{d}t^2-η(t)^2\textrm{d}x^2$. By tailoring the laser pulse shape and medium properties, it is possible to create a refractive index variation $n=n(t)$ that can be identified with $η(t)$. Starting from a perturbative solution to a generalised Hopfield model for the medium described by an $n=n(t)$ we provide estimates for the number of photons generated by the time-dependent spacetime. The simplest example is that of a uniformly varying $η(t)$ that therefore describes the Robertson-Walker metric, i.e. a cosmological expansion. The number of photon pairs generated in experimentally feasible conditions appears to be extremely small. However, large photon production can be obtained by periodically modulating the medium and thus resorting to a resonant enhancement similar to that observed in the dynamical Casimir effect. Curiously, the spacetime metric in this case closely resembles that of a gravitational wave. Motivated by this analogy we show that a periodic gravitational wave can indeed act as an amplifier for photons. The emission for an actual gravitational wave will be very weak but should be readily observable in the laboratory analogue.

gr-qc↗

arXiv2014

On the nature of spatiotemporal light bullets in bulk Kerr media

We present a detailed experimental investigation, which uncovers the nature of light bullets generated from self-focusing in a bulk dielectric medium with Kerr nonlinearity in the anomalous group velocity dispersion regime. By high dynamic range measurements of three-dimensional intensity profiles, we demonstrate that the light bullets consist of a sharply localized high-intensity core, which carries the self-compressed pulse and contains approximately 25% of the total energy, and a ring-shaped spatiotemporal periphery. Sub-diffractive propagation along with dispersive broadening of the light bullets in free space after they exit the nonlinear medium indicate a strong space-time coupling within the bullet. This finding is confirmed by measurements of spatiotemporal energy density flux that exhibits the same features as stationary, polychromatic Bessel beam, thus highlighting the physical nature of the light bullets.

physics.optics↗

arXiv2013

Blackbody emission from light interacting with an effective moving dispersive medium

Intense laser pulses excite a nonlinear polarisation response that may create an effective flowing medium and, under appropriate conditions, a blocking horizon for light. Here we analyse in detail the interaction of light with such laser-induced flowing media, fully accounting for the medium dispersion properties. An analytical model based on a first Born-approximation is found to be in excellent agreement with numerical simulations based on Maxwell's equations and shows that when a blocking horizon is formed, the stimulated medium scatters light with a blackbody emission spectrum. Based on these results, diamond is proposed as a promising candidate medium for future studies of Hawking emission from artificial, dispersive horizons.

physics.optics↗