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Andrea Alù

Publications and source records attributed to Andrea Alù.

At least 55 records · Page 3Linked to original sources
arXiv2024

Hyperbolic Shear Metasurfaces

Polar dielectrics with low crystal symmetry and sharp phonon resonances can support hyperbolic shear polaritons - highly confined surface modes with frequency-dependent optical axes and asymmetric dissipation features. So far, these modes have been observed only in bulk natural materials at mid-infrared frequencies, with properties limited by available crystal geometries and phonon resonance strength. Here we introduce hyperbolic shear metasurfaces: ultrathin engineered surfaces supporting hyperbolic surface modes with symmetry-tailored axial dispersion and loss redistribution that can maximally enhance light-matter interactions. By engineering effective shear phenomena in these engineered surfaces, we demonstrate geometry-controlled, ultra-confined, low-loss hyperbolic surface waves with broadband Purcell enhancements, applicable across a broad range of the electromagnetic spectrum.

physics.optics↗
arXiv2024

Intersubband polaritonic metasurfaces for high-contrast ultra-fast power limiting and optical switching

Nonlinear intersubband polaritonic metasurfaces support one of the strongest known ultrafast nonlinear responses in the mid-infrared frequency range across all condensed matter systems. Beyond harmonic generation and frequency mixing, these nonlinearities can be leveraged for ultrafast optical switching and power limiting, based on tailored transitions from strong to weak polaritonic coupling. Here, we demonstrate synergistic optimization of materials and photonic nanostructures to achieve large reflection contrast in ultrafast polaritonic metasurface limiters. The devices are based on optimized semiconductor heterostructure materials that minimize the intersubband transition linewidth and reduce absorption in optically saturated nanoresonators, achieving a record-high reflection contrast of 54% experimentally. We also discuss opportunities to further boost the metrics of performance of this class of ultrafast limiters, showing that reflection contrast as high as 94% may be realistically achieved using all-dielectric intersubband polaritonic metasurfaces.

physics.optics↗
arXiv2024

Temporal Signal Processing with Nonlocal Optical Metasurfaces

Nonlocal metasurfaces have recently enabled an ultra-compact, low-power and high-speed platform to perform analog image processing. While several computational tasks have been demonstrated based on this platform, most of the previous studies have focused only on spatial operations, such as spatial differentiation and edge detection. Here, we demonstrate that metasurfaces with temporal nonlocalities - that is, with a tailored dispersive response - can be used to implement time-domain signal processing in deeply subwavelength footprints. In particular, we show a passive metasurface performing first-order differentiation of an input signal with high-fidelity and high-efficiency. We also demonstrate that this approach is prone to scalability and cascaded computation. Our work paves the way to a new generation of ultra-compact, passive devices for all-optical computation, with applications in neural networks and neuromorphic computing.

physics.optics↗
arXiv2024

Space-Time Nonlocal Metasurfaces for Event-Based Image Processing

Analog computation with passive optical components can enhance processing speeds and reduce power consumption, recently attracting renewed interest thanks to the opportunities enabled by metasurfaces. Basic image processing tasks, such as spatial differentiation, have been recently demonstrated based on engineered nonlocalities in metasurfaces, but next-generation computational schemes require more advanced capabilities. Here, we tailor nonlocalities in space and time to design a metasurface that can perform mixed spatio-temporal differentiation of an input image, realizing event-based edge detection with a passive ultrathin silicon-based structured film compatible with standard fabrication techniques. The metasurface detects the object edges only when the object moves, and its design can be tailored to selectively enhance objects moving at desired speeds. Our results point towards fully-passive processing of spatio-temporal signals, for highly compact neuromorphic cameras.

physics.optics↗
arXiv2023

Extreme light confinement and control in low-symmetry phonon-polaritonic crystals

Polaritons are a hybrid class of quasiparticles originating from the strong and resonant coupling between light and matter excitations. Recent years have witnessed a surge of interest in novel polariton types, arising from directional, long-lived material resonances, and leading to extreme optical anisotropy that enables novel regimes of nanoscale, highly confined light propagation. While such exotic propagation features may also be in principle achieved using carefully designed metamaterials, it has been recently realized that they can naturally emerge when coupling infrared light to directional lattice vibrations, i.e., phonons, in polar crystals. Interestingly, a reduction in crystal symmetry increases the directionality of optical phonons and the resulting anisotropy of the response, which in turn enables new polaritonic phenomena, such as hyperbolic polaritons with highly directional propagation, ghost polaritons with complex-valued wave vectors, and shear polaritons with strongly asymmetric propagation features. In this Review, we develop a critical overview of recent advances in the discovery of phonon polaritons in low-symmetry crystals, highlighting the role of broken symmetries in dictating the polariton response and associated nanoscale-light propagation features. We also discuss emerging opportunities for polaritons in lower-symmetry materials and metamaterials, with connections to topological physics and the possibility of leveraging anisotropic nonlinearities and optical pumping to further control their nanoscale response.

physics.optics↗
arXiv2023

Reconfigurable Image Processing Metasurfaces with Phase-Change Materials

Optical metasurfaces have been enabling reduced footprint and power consumption, as well as faster speeds, in the context of analog computing and image processing. While various image processing and optical computing functionalities have been recently demonstrated using metasurfaces, most of the considered devices are static and lack reconfigurability. Yet, the ability to dynamically reconfigure processing operations is key for metasurfaces to be able to compete with practical computing systems. Here, we demonstrate a passive edge-detection metasurface operating in the near-infrared regime whose image processing response can be drastically modified by temperature variations smaller than 10° C around a CMOS-compatible temperature of 65° C. Such reconfigurability is achieved by leveraging the insulator-to-metal phase transition of a thin buried layer of vanadium dioxide which, in turn, strongly alters the nonlocal response of the metasurface. Importantly, this reconfigurability is accompanied by performance metrics - such as high numerical aperture, high efficiency, isotropy, and polarization-independence - close to optimal, and it is combined with a simple geometry compatible with large-scale manufacturing. Our work paves the way to a new generation of ultra-compact, tunable, passive devices for all-optical computation, with potential applications in augmented reality, remote sensing and bio-medical imaging.

physics.optics↗
arXiv2023

Dispersion Engineered Metasurfaces for Broadband, High-NA, High-Efficiency, Dual-Polarization Analog Image Processing

Optical metasurfaces performing analog image processing - such as spatial differentiation and edge detection - hold the potential to reduce processing times and power consumption, while avoiding bulky 4F lens systems. However, current designs have been suffering from trade-offs between spatial resolution, throughput, polarization asymmetry, operational bandwidth, and isotropy. Here, we show that dispersion engineering provides an elegant way to design metasurfaces where all these critical metrics are simultaneously optimized. We experimentally demonstrate silicon metasurfaces performing isotropic and dual-polarization edge detection, with numerical apertures above 0.35 and spectral bandwidths of 35 nm around 1500 nm. Moreover, we introduce quantitative metrics to assess the efficiency of these devices. Thanks to the low loss nature and dual-polarization response, our metasurfaces feature large throughput efficiencies, approaching the theoretical maximum for a given NA. Our results pave the way for low-loss, high-efficiency and broadband optical computing and image processing with free-space metasurfaces.

physics.optics↗
arXiv2023

An Asymmetric Spoof-Fluid-Spoof Acoustic Waveguide and its Application as a CO$_2$ Sensor

We study pressure acoustic propagation in asymmetric spoof-fluid-spoof acoustic waveguides and its potential application in acoustic gas sensors. First, a stable and efficient analytical method is established for fast calculation of the dispersion curves based on spectral expansion and enforcement of continuity between segments at suitable collocation points. The analysis is validated by a commercial finite element software. The geometric design of the waveguide is then optimized for the emergence of a nearly-flat dispersion curve associated with vertical geometric asymmetry. The waveguide is fabricated using 3D printing technology and the measurement results corroborate the numerical simulations. Based on the nearly-flat dispersion curve supported by this waveguide, a CO$_2$ sensor is proposed allowing to relate the phase difference measured between two points in the waveguide to the composition of the gas in the waveguide. The proposed sensor is experimentally validated in a controlled environment and the measurement results match the computational predictions well. The sensor is robust with respect to noise and signal-recording duration due to fast phase measurements and shows high sensitivity to gas concentration due to reliance on the second, nearly-flat, dispersion curve. In addition, the sensor is label-free and low-cost, while exhibiting rapid response, low-maintenance requirements and potential for measurements in a wide range of CO$_2$ concentrations without saturation issues.

physics.app-ph↗
arXiv2023

Polarization Imaging and Edge Detection with Image-Processing Metasurfaces

Optical metasurfaces have been recently explored as ultrathin analog image differentiators. By tailoring the momentum transfer function, they can perform efficient Fourier-filtering - and thus potentially any linear mathematical operation - on an input image, replacing bulky 4f systems. While this approach has been investigated in different platforms, and several techniques have been explored to achieve the required angular response, little effort has been devoted so far to tailor and control also the polarization response of an image-processing metasurface. Here, we show that edge-detection metasurfaces can be designed with tailored polarization responses while simultaneously preserving an isotropic response. In particular, we demonstrate single-layer silicon metasurfaces yielding efficient Laplacian operation on a 2D image with either large polarization asymmetry, or nearly polarization-independent response. In the former case, we show that a strongly asymmetric polarization response can be used to unlock more sophisticated on-the-fly image processing functionalities, such as dynamically tunable direction-dependent edge detection. In parallel, metasurfaces with dual-polarized response are shown to enable efficient operation for unpolarized or arbitrarily polarized images, ensuring high efficiency. For both devices, we demonstrate edge detection within relatively large numerical apertures, with excellent isotropy and intensity throughput. Our study paves the way for the broad use of optical metasurfaces for sophisticated, massively parallel analog image processing with zero energy requirements.

physics.optics↗
arXiv2023

Spatio-Temporal Coupled Mode Theory for Nonlocal Metasurfaces

Diffractive nonlocal metasurfaces have recently opened a broad range of exciting developments in nanophotonics research and applications, leveraging spatially extended (yet locally patterned) resonant modes to control light with new degrees of freedom. While conventional grating responses are elegantly captured by temporal coupled mode theory (TCMT), TCMT is not well equipped to capture the more sophisticated responses observed in the nascent field of nonlocal metasurfaces. Here, we introduce spatio-temporal coupled mode theory (STCMT), capable of elegantly capturing the key features of the resonant response of wavefront-shaping nonlocal metasurfaces. This framework can quantitatively guide nonlocal metasurface design, and is compatible with local metasurface frameworks, making it a powerful tool to rationally design and optimize a broad class of ultrathin optical components. We validate this STCMT framework against full-wave simulations of various nonlocal metasurfaces, demonstrating that this tool offers a powerful semi-analytical framework to understand and model the physics and functionality of these devices, without the need for computationally intense full-wave simulations. We also discuss how this model may shed physical insights into nonlocal phenomena in photonics and into the functionality of the resulting devices. As a relevant example, we showcase STCMT's flexibility by applying it to study and rapidly prototype nonlocal metasurfaces that spatially shape thermal emission.

physics.optics↗
arXiv2023

Ghost line waves

Time-harmonic electromagnetic plane waves in anisotropic media can exhibit complex-valued wavevectors (with nonzero real and imaginary parts) even in the absence of material dissipation. These peculiar modes, usually referred to as "ghost waves," hybridize the typical traits of conventional propagating and evanescent waves, displaying both phase accumulation and purely reactive exponential decay away from the direction of power flow. Their existence has been predicted in several scenarios, and has been recently observed experimentally in the form of surface phonon polaritons with complex-valued out-of-plane wavevectors propagating at the interface between air and a natural uniaxial crystal with slanted optical axis. Here, we demonstrate that ghost waves can arise also in lower-dimensional flat-optics scenarios, which are becoming increasingly relevant in the context of metasurfaces and in the field of polaritonics. Specifically, we show that planar junctions between isotropic and anisotropic metasurfaces can support "ghost line waves" that propagate unattenuated along the line interface, exhibiting phase oscillations combined with evanescent decay both in the plane of the metasurface (away from the interface) and out-of-plane n the surrounding medium. Our theoretical results, validated by finite-element numerical simulations, demonstrate a novel form of polaritonic waves with highly confined features, which may provide new opportunities for the control of light at the nanoscale, and may find potential applications in a variety of scenarios, including integrated waveguides, nonlinear optics, optical sensing and sub-diffraction imaging.

physics.optics↗
arXiv2023

Twist-Induced Hyperbolic Shear Metasurfaces

Following the discovery of moiré-driven superconductivity in twisted graphene multilayers, twistronics has spurred a surge of interest in tailored broken symmetries through angular rotations, enabling new properties from electronics to photonics and phononics. Analogously, in monoclinic polar crystals a nontrivial angle between non-degenerate dipolar phonon resonances can naturally emerge due to asymmetries in their crystal lattice, and its variations are associated with intriguing polaritonic phenomena, including axial dispersion, i.e., a rotation of the optical axis with frequency, and microscopic shear effects that result in asymmetric loss distributions. So far these phenomena were restricted to specific mid-infrared frequencies, difficult to access with conventional lasers, and fundamentally limited by the degree of asymmetry and the strength of light-matter interactions available in natural crystals. Here, we leverage twistronics to demonstrate giant axial dispersion and loss asymmetry of hyperbolic waves in elastic metasurfaces, by tailoring the angle between coupled pairs of anisotropic metasurfaces. We show extreme control over elastic wave dispersion via the twist angle, and leverage the resulting phenomena to demonstrate reflection-free negative refraction, as well as the application of axial dispersion to achieve diffraction-free non-destructive testing, whereby the angular direction of a hyperbolic probe wave is encoded into its frequency. Our work welds the concepts of twistronics, non-Hermiticity and extreme anisotropy, demonstrating the powerful opportunities enabled by metasurfaces for tunable, highly directional surface acoustic wave propagation, of great interest for applications ranging from seismic mitigation to on-chip phononics and wireless communications, paving the way towards their translation into emerging photonic and polaritonic metasurface technologies.

cond-mat.mes-hall↗
arXiv2023

Interaction-driven transport of dark excitons in 2D semiconductors with phonon-mediated optical readout

The growing field of quantum information technology requires propagation of information over long distances with efficient readout mechanisms. Excitonic quantum fluids have emerged as a powerful platform for this task due to their straightforward electro-optical conversion. In two-dimensional transition metal dichalcogenides, the coupling between spin and valley provides exciting opportunities for harnessing, manipulating and storing bits of information. However, the large inhomogeneity of single layers cannot be overcome by the properties of bright excitons, hindering spin-valley transport. Nonetheless, the rich band structure supports dark excitonic states with strong binding energy and longer lifetime, ideally suited for long-range transport. Here we show that dark excitons can diffuse over several micrometers and prove that this repulsion-driven propagation is robust across non-uniform samples. The long-range propagation of dark states with an optical readout mediated by chiral phonons provides a new concept of excitonic devices for applications in both classical and quantum information technology.

cond-mat.mes-hall↗
arXiv2023

Loss compensation and super-resolution with excitations at complex frequencies

Abbe diffraction limit fundamentally bounds the resolution of conventional optical imaging and spectroscopic systems. Along the years, several schemes have been introduced to overcome this limit, each offering opportunities and trade-offs. Metamaterials have been proposed as a promising platform in this context, in principle enabling superlenses capable of drastic resolution enhancements. However, their performance is severely hindered by material loss and nonlocal phenomena, which become increasingly more detrimental as we attempt to image more subwavelength details. Active metamaterials have been explored to overcome these challenges, however material gain introduces other obstacles, e.g., instabilities, nonlinearities and noise. Here, we demonstrate that the temporal excitation of passive superlenses using signals oscillating at complex frequencies compensates material loss, leading to resolution enhancement. Our results demonstrate that virtual gain stemming from tailored forms of excitation can tackle the impact of loss in superlenses. More broadly, our work opens promising avenues for loss compensation in metamaterials, with a broad range of applications extendable to optics and nanophotonic systems.

physics.class-ph↗
arXiv2023

Lower Bounds to the Q factor of Electrically Small Resonators through Quasistatic Modal Expansion

The problem of finding the optimal current distribution supported by small radiators yielding the minimum quality (Q) factor is a fundamental problem in electromagnetism. Q factor bounds constrain the maximum operational bandwidth of devices including antennas, metamaterials, and nanoresonators, and have been featured in seminal papers in the past decades. Here, we determine the lower bounds of Q factors of small-size plasmonic and high-permittivity dielectric resonators, which are characterized by quasi-electrostatic and quasi-magnetostatic natural modes, respectively. We expand the induced current density field in the resonator in terms of these modes, leading to closed-form analytical expressions for the electric and magnetic polarizability tensors, whose largest eigenvalue is directly linked to the minimum Q factor. Our results allow also to determine in closed form the corresponding optimal current density field. In particular, when the resonator exhibits two orthogonal reflection symmetries the minimum Q factor can be simply obtained from the Q factors of the single current modes with non-vanishing dipole moments aligned along the major axis of the resonator. Overall, our results open exciting opportunities in the context of nano-optics and metamaterials, facilitating the analysis and design of optimally shaped resonators for enhanced and tailored light-matter interactions.

physics.optics↗
arXiv2023

The bulk van der Waals layered magnet CrSBr is a quasi-1D material

Correlated quantum phenomena in one-dimensional (1D) systems that exhibit competing electronic and magnetic order are of strong interest for studying fundamental interactions and excitations, such as Tomonaga-Luttinger liquids and topological orders and defects with properties completely different from the quasiparticles expected in their higher-dimensional counterparts. However, clean 1D electronic systems are difficult to realize experimentally, particularly magnetically ordered systems. Here, we show that the van der Waals layered magnetic semiconductor CrSBr behaves like a quasi-1D material embedded in a magnetically ordered environment. The strong 1D electronic character originates from the Cr-S chains and the combination of weak interlayer hybridization and anisotropy in effective mass and dielectric screening with an effective electron mass ratio of $m^e_X/m^e_Y \sim 50$. This extreme anisotropy experimentally manifests in strong electron-phonon and exciton-phonon interactions, a Peierls-like structural instability and a Fano resonance from a van Hove singularity of similar strength of metallic carbon nanotubes. Moreover, due to the reduced dimensionality and interlayer coupling, CrSBr hosts spectrally narrow (1 meV) excitons of high binding energy and oscillator strength that inherit the 1D character. Overall, CrSBr is best understood as a stack of weakly hybridized monolayers and appears to be an experimentally attractive candidate for the study of exotic exciton and 1D correlated many-body physics in the presence of magnetic order.

cond-mat.mes-hall↗
arXiv2023

Demonstration of a Polarization-Agnostic Geometric Phase in Nonlocal Metasurfaces

Symmetry-driven phenomena arising in nonlocal metasurfaces supporting quasi-bound states in the continuum (q-BICs) have been opening new avenues to tailor enhanced light-matter interactions via perturbative design principles. Geometric phase concepts - observed in many physical systems - are particularly useful in nonlocal metasurfaces, as they enable to locally pattern the q-BIC scattering rate and phase across the metasurface aperture without affecting the delocalized nature of the q-BIC resonance. However, this control typically comes with stringent limitations in terms of efficiency and/or of polarization operation. Here, we unveil a new form of geometric phase control, accumulated along a continuous contour of geometric perturbations that parametrically encircle a singularity associated with a bound state. This response is obtained regardless of the chosen polarization state, which can be rationally specified by a judicious choice of the perturbation geometry. Our findings extend geometric phase concepts to arbitrary polarizations, including linear and elliptical states, with near-unity scattering efficiency. This polarization-agnostic geometric phase offers new opportunities for wavefront manipulation, multifunctional operation and light emission, with applications in augmented reality, secure communications, wireless systems, imaging, lasing, nonlinear and quantum optics.

physics.optics↗
arXiv2023

Stationary Charge Radiation in Anisotropic Photonic Time Crystals

Time metamaterials exhibit a great potential for wave manipulation, drawing increasing attention in recent years. Here, we explore the exotic wave dynamics in an anisotropic photonic time crystal (APTC), formed by an anisotropic medium whose optical properties are uniformly and periodically changed in time. Based on a temporal transfer matrix formalism, we show that a stationary charge embedded in an APTC can emit radiation, in contrast to the case of an isotropic photonic time crystal, and its distribution in momentum space is controlled by the APTC band structure. Our approach extends the functionalities of time metamaterials, offering new opportunities for simultaneous radiation generation and control, with implications for both classical and quantum applications.

physics.optics↗