arXiv subjects

Iñigo Liberal

Publications and source records attributed to Iñigo Liberal.

At least 19 recordsLinked to original sources

arXiv2026

Generalized Time-Varying Drude Model for Dispersive and Lossy Modulations

We develop a generalization of the time-varying Drude model, treating carrier density, effective mass, and collision rate as explicit functions of time. We derive expressions for polarization, susceptibility, displacement, and permittivity in different domains. Our analysis reveals that non-adiabatic modulations and time-dependent losses induce rich and distinct behaviors, leading to temporal blurring, selective gating and suppression, and low-frequency spectral reshaping. Besides underpinning and upgrading the current framework on photonics of time-varying media, this model may be useful in the design and fitting theoretical models with experimental realizations.

physics.optics↗

arXiv2025

Synthetic crystal rotation with spacetime metamaterials

The interaction of light with rotating bodies has been historically limited to rotation frequencies much smaller than optical frequencies. Here, we investigate synthetic crystal rotations, i.e., spatiotemporal modulations mimicking the rotation of an anisotropic crystal, which grant access to large rotation frequencies. Spatiotemporal modulations change the fundamental symmetries of the electromagnetic field, breaking temporal and rotation symmetries, but preserving a spatiotemporal rotation symmetry that enforces the conservation of a combination of energy and spin angular momentum (SAM). The scattering of optical pulses by synthetically rotating crystals results in spatiotemporal light with intra-pulse SAM changes. The frequency-domain response reveals sidebands with frequency/SAM locking, and negative frequency sideband transitions for large enough rotation frequencies. Our results highlight the qualitatively different light-matter interaction regimes accessed by synthetic rotations.

physics.optics↗

arXiv2025

Generalized Epsilon-Near-Zero Polaritons on Uniaxial Metasurfaces

Epsilon-near-zero (ENZ) thin films facilitate strong light-matter interactions with a widespread impact in nonlinear, quantum and thermal photonics. Here, we extend the scope of thin film ENZ modes by elucidating the generalized polaritonic modes emerging from anisotropy and near-zero permittivity. Through a theoretical investigation of generalized ENZ polaritons on silicon carbide (SiC) metasurfaces, we reveal a complex polaritonic landscape, consisting of a seven-region phase diagram. We show that the complex interplay between material properties and geometry changes the nature of the modes, and we clarify when ENZ modes with closed and open isofrequency curves (IFCs) can be observed, and how they coexist with surface phonon-polaritons and hyperbolic modes. The associated photonic topological transitions are accompanied by phase velocity sign reversals and induce dramatic modifications of near-field profiles and Purcell enhancement factors. Our work merges insights from anisotropic and ENZ effects, enabling enhanced control over polariton behavior with broad implications for subwavelength optics, material design, nonlinear effects, thermal emission, and quantum technologies.

physics.optics↗

arXiv2025

Mirrors without spatial boundaries

Mirrors are one of the most elementary and ubiquitous components of optical systems. They use a sharp refractive index contrast to provide the basic capability of reflecting light. Motivated by recent developments of photonic time-varying media, here we investigate the fundamental question on whether it is possible to have a mirror without any spatial boundary. In this vein, we first discuss how purely temporal mirrors are in general forbidden by the conservation of Minkowski momentum. However, we show that an exotic class of metamaterials, namely, temporal non-Foster left-handed media, exhibit anti-parallel Minkowski momentum and energy flow, thereby enabling mirrors without spatial boundaries. Upon this ground, we put forward some related photonic functionalities, including temporal cavities, pulse freezing, and frequency comb generators, which can be understood as the precursor of temporal lasers.

physics.optics↗

arXiv2024

Dispersion effects in thermal emission from temporal metamaterials: High-frequency cut-offs

The latest breakthroughs in time-varying photonics are fueling novel thermal emission phenomena, for example, showing that the dynamic amplification of quantum vacuum fluctuations, induced by the time-modulation of material properties, enables a mechanism to surpass the black-body spectrum. So far, this issue has only been investigated under the assumption of non-dispersive time-modulations. In this work, we identify the existence of a non-physical diverging behavior in the time-modulated emission spectra at high frequencies, and prove that it is actually attributed to the simplistic assumption of a non-dispersive (temporally local) response of the time-modulation associated with memory-less systems. Accordingly, we upgrade the theoretical formalism by introducing a dispersive response function, showing that it leads to a high-frequency cut-off, thereby eliminating the divergence and hence allowing for the proper computation of the emission spectra of time-modulated materials.

physics.optics↗

arXiv2024

Spatiotemporal symmetries and energy-momentum conservation in uniform spacetime metamaterials

Spacetime metamaterials (ST-MMs) are opening new regimes of light-matter interactions based on the breaking of temporal and spatial symmetries, as well as intriguing concepts associated with synthetic motion. In this work, we investigate the continuous spatiotemporal translation symmetry of ST-MMs with uniform modulation velocity. Using Noether theorem, we demonstrate that such symmetry entails the conservation of the energy-momentum. We highlight how energy-momentum conservation imposes constraints on the range of allowed light-matter interactions within ST-MMs, as illustrated with examples of the collision of electromagnetic and modulation pulses. Furthermore, we discuss the similarities and differences between the conservation of energy-momentum and relativistic effects. We believe that our work provides a step forward in clarifying the fundamental theory underlying ST-MMs.

physics.optics↗

arXiv2024

Symmetries and conservation of spin angular momentum, helicity, and chirality in photonic time-varying media

Polarization-dependent dynamical properties of light as the spin angular momentum (SAM), helicity, and chirality are conserved quantities in free-space. Despite their similarities on account of their relationship with a circular state of polarization, SAM, helicity, and chirality emerge from distinct symmetries, which endows them with different physical meanings, properties, and practical applications. In this work, we investigate the behavior of such quantities in time-varying media (TVM), i.e., how a temporal modulation impacts their symmetries and conservation laws. Our results demonstrate that the SAM is conserved for any time modulation, helicity is only preserved in impedance-matched time modulations, while chirality is not conserved. In addition, the continuity equations highlight the dependence of the chirality with the energy content of the fields. These results provide additional insights into the similarities and differences between SAM, helicity, and chirality, as well as their physical meaning. Furthermore, our theoretical framework provides with a new perspective to analyze polarization-dependent light-matter interactions in TVM.

physics.optics↗

arXiv2024

Quantum and thermal noise in coupled non-Hermitian waveguide systems with different models of gain and loss

Non-Hermitian (NH) photonic systems leverage gain and loss to open new directions for nanophotonic technologies. However, the quantum and thermal noise intrinsically associated with gain/loss affects the eigenvalue/eigenvector structure of NH systems, as well as its practical noise performance. Here, we present a comparative analysis of the impact of different gain and loss mechanisms on the noise generated in gain-loss compensated NH waveguide systems. Our results highlight important differences in the eigenvalue/eigenvector structure, noise power, photon statistics and squeezing. At the same time, we identify some universal properties such as gain-loss compensation, broken to unbroken phase transitions, coalesce of pairs of eigenvectors, and linear scaling of the noise with the length of the waveguide. We believe that these results provide a more global understanding on the impact of the gain/loss mechanism on the noise generated in NH systems.

physics.optics↗

arXiv2024

Can thermal emission from time-varying media be described semiclassically?

Time-varying media, i.e., materials whose properties dynamically change in time, have opened new possibilities for thermal emission engineering by lifting the limitations imposed by energy conservation and reciprocity, and providing access to nonequilibrium dynamics. In addition, quantum effects, such as vacuum amplification and emission at zero temperature, have been predicted for time-varying media, reopening the debate on the quantum nature of thermal emission. Here, we derive a semiclassical theory to thermal emission from time-varying media based on fluctuational electrodynamics, and compare it to the quantum theory. Our results show that a quantum theory is needed to correctly capture the contribution from quantum vacuum amplifications effects, which can be relevant even at room temperature and mid-infrared frequencies. Finally, we propose corrections to the standard semiclassical theory that enable the prediction of thermal emission from time-varying media with classical tools.

physics.optics↗

arXiv2024

A Review on the Scientific and Technological Breakthroughs in Thermal Emission Engineering

The emission of thermal radiation is a physical process of fundamental and technological interest. From different approaches, thermal radiation can be regarded either as one of the basic mechanisms of heat transfer, as a fundamental quantum phenomenon of photon production, or as the propagation of electromagnetic waves. However, unlike light emanating from conventional photonic sources, such as lasers or antennas, thermal radiation is characterized for being broadband, omnidirectional, and unpolarized. Due to these features, ultimately tied to its inherently incoherent nature, taming the thermal radiation constitutes a challenging issue. Latest advances in the field of nanophotonics have led to a whole set of artificial platforms, ranging from spatially structured materials, and much more recently, time-modulated media, offering promising avenues for enhancing the control and manipulation of electromagnetic waves, from far to near-field regimes. Given the ongoing parallelism between the fields of nanophotonics and thermal emission, these recent developments have been harnessed to deal with radiative thermal processes, thereby conforming the current basis of thermal emission engineering. In this review, we survey some of the main breakthroughs carried out in this burgeoning research field, from fundamental aspects, theoretical limits, the emergence of new phenomena, practical applications, challenges, and future prospects.

physics.optics↗

arXiv2023

Quantum vacuum amplification in time-varying media with arbitrary temporal profiles

In this work we address quantum vacuum amplification effects in time-varying media with an arbitrary time-modulation profile. To this end, we propose a theoretical formalism based on the concept of conjugated harmonic oscillators, evaluating the impact on the transition time in temporal boundaries, shedding light into the practical requirements to observe quantum effects at them. In addition, we find nontrivial effects in pulsed-modulations, where the swiftest and strongest modulation does not lead to the highest photon production. Thus, our results provide key insights for the design of temporal modulation sequences to enhance quantum phenomena.

physics.optics↗

arXiv2023

Orthogonal thermal noise and transmission signals: A new coherent perfect absorption's feature

Coherent perfect absorption (CPA) is an interference process associated with the zeros of the scattering matrix that enables light-with-light interactions in linear systems, of interest for optical computing, data processing and sensing. However, the noise properties of CPA remain relatively unexplored. Here, we demonstrate that CPA thermal noise signals exhibit a unique property: they are orthogonal to the signals transmitted through the network. In turn, such property enables a variety of thermal noise management effects, such as the physical separability of thermal noise and transmitted signals, and "externally lossless" networks that internally host radiative heat transfer processes. We believe that our results provide a new perspective on the many CPA technologies currently under development.

physics.optics↗

arXiv2023

New horizons in near-zero refractive index photonics and hyperbolic metamaterials

The engineering of the spatial and temporal properties of both the electric permittivity and the refractive index of materials is at the core of photonics. When vanishing to zero, those two variables provide new knobs to control light-matter interactions. This perspective aims at providing an overview of the state of the art and the challenges in emerging research areas where the use of near-zero refractive index and hyperbolic metamaterials is pivotal, in particular light and thermal emission, nonlinear optics, sensing applications and time-varying photonics.

physics.optics↗

arXiv2023

Addressing the impact of surface roughness on epsilon-near-zero substrates

Epsilon-near-zero (ENZ) media have been very actively investigated due to their unconventional wave phenomena and strengthened nonlinear response. However, the technological impact of ENZ media will be determined by the quality of realistic ENZ materials, including material loss and surface roughness. Here, we provide a comprehensive experimental study of the impact of surface roughness on ENZ substrates. Using silicon carbide (SiC) substrates with artificially induced roughness, we analyze samples whose roughness ranges from a few to hundreds of nanometer size-scales. It is concluded that ENZ substrates with roughness in the few nanometer scale are negatively affected by coupling to longitudinal phonons and strong ENZ fields normal to the surface. On the other hand, when the roughness is in the hundreds of nanometer scale, the ENZ band is found to be more robust than dielectric and surface phonon polariton (SPhP) bands.

physics.optics↗

arXiv2023

A tutorial on the conservation of momentum in photonic time-varying media

Time-varying media break temporal symmetries while preserving spatial symmetries intact. Thus, it represents an excellent conceptual framework to investigate the fundamental implications of Noether's theorem for the electromagnetic field. At the same time, addressing momentum conservation in time-varying media sheds light on the Abraham-Minkowski debate, where two opposing forms of the electromagnetic field momentum are defended. Here, we present a tutorial review on the conservation of momentum in time-varying media. We demonstrate that the Minkowski momentum is a conserved quantity with three independent approaches of increasing complexity: (i) via the application of the boundary conditions for Maxwell equations at a temporal boundary, (ii) testing for constants of motion and deriving conservation laws, and (iii) applying temporal and spatial translations within the framework of the Lagrangian theory of the electromagnetic field. Each approach provides a different and complementary insight into the problem.

physics.optics↗

arXiv2022

Incandescent temporal metamaterials

Regarded as a promising alternative to spatially shaping matter, time-varying media can be seized to control and manipulate wave phenomena, including thermal radiation. Here, based upon the framework of macroscopic quantum electrodynamics, we elaborate a comprehensive quantum theoretical formulation that lies the basis for investigating thermal emission effects in time-modulated media. Our theory unveils new physics brought about by time-varying media: nontrivial correlations between thermal fluctuating currents at different frequencies and positions, thermal radiation overcoming the black-body spectrum, and quantum vacuum amplification effects at finite temperature. We illustrate how these features lead to striking phenomena and novel thermal emitters, specifically, showing that the time-modulation releases strong field fluctuations confined within epsilon-near-zero (ENZ) bodies, and that, in turn, it enables a narrowband (partially coherent) emission spanning the whole range of wavevectors, from near to far-field regimes.

physics.optics↗

arXiv2022

Suppressed-scattering windows for radiative cooling applications

The scattering of light by resonant nanoparticles is a key process for enhancing the solar reflectance in daylight radiative cooling. Here, we investigate the impact of material dispersion on the scattering performance of popular nanoparticles for radiative cooling applications. We show that, due to material dispersion, nanoparticles with a qualitatively similar response at visible frequencies exhibit fundamentally different scattering properties at infrared frequencies. It is found that dispersive nanoparticles exhibit suppressed-scattering windows, allowing for selective thermal emission within an highly reflective sample. The existence of suppressed-scattering windows solely depends on material dispersion, and they appear pinned to the same wavelength even in random composite materials and periodic metasurfaces. Finally, we investigate calcium-silicate-hydrate (CSH), the main phase of concrete, as an example of a dispersive host, illustrating that the co-design of nanoparticles and host allows for tuning of the suppressed-scattering windows. Our results indicate that controlled nanoporosities would enable concrete with daylight passive radiative cooling capabilities.

physics.app-ph↗

arXiv2022

Spectrally stable thermal emitters enabled by material-based high-impedance surfaces

Radiative thermal engineering with subwavelength metallic bodies is a key element for heat and energy management applications, communication and sensing. Here, we numerically and experimentally demonstrate metallic thermal emitters with narrowband but extremely stable emission spectra, whose resonant frequency does not shift with changes on the nanofilm thickness, the angle of observation and/or polarization. Our devices are based on epsilon-near-zero (ENZ) substrates acting as material-based high-impedance substrates. They do not require from complex nanofabrication processes, thus being compatible with large-area and low-cost applications.

physics.optics↗