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Cristian Ciracì

Publications and source records attributed to Cristian Ciracì.

At least 19 recordsLinked to original sources
arXiv2026

Plasmonic-cavity Modulator for the Mid-IR with a Semi-transparent and Nonlinear Heavily-doped Semiconductor Mirror

We present a free-space plasmonic modulator based on a single heavily-doped semiconductor layer. We investigate its ability to modulate both the linear and nonlinear response at mid-infrared frequencies slightly below the plasma frequency of the semiconductor. We demonstrate electric control of the linear transmittance and reflectance, and of the efficiency of third-harmonic generation with a field-effect gate structure. We discuss further performance optimization of the device in terms of modulation speed and depth towards a fast modulator with very simple active material requirements. Our results establish a viable route toward practical plasmonic modulators and mixers operating in the mid-infrared atmospheric window available for free-space communications at wavelengths between 8 and 12 um.

physics.optics↗
arXiv2025

Modulating Low-Power Threshold Optical Bistability by Electrically Reconfigurable Free-Electron Kerr Nonlinearity

We propose a microscopic mechanism to electrically reconfigure the Kerr nonlinearity by modulating the concentration of free electrons in heavily doped semiconductors under a static bias. Our theory incorporates electrostatic and hydrodynamic frameworks to describe the electronic dynamics, demonstrating electrically tunable linear and nonlinear modulations. The power threshold of achieving optical bistability shows unprecedented tunability over two orders of magnitude, reaching values as low as 10 $μ$W through surface charge control. These findings offer new insights into understanding and actively controlling Kerr nonlinearities, paving the way for efficient refractive index engineering as well as the development of advanced linear and nonlinear electro-optical modulators.

physics.optics↗
arXiv2025

Roadmap on Nonlocality in Photonic Materials and Metamaterials

Photonic technologies continue to drive the quest for new optical materials with unprecedented responses. A major frontier in this field is the exploration of nonlocal (spatially dispersive) materials, going beyond the local, wavevector-independent assumption traditionally made in optical material modeling. On one end, the growing interest in plasmonic, polaritonic and quantum materials has revealed naturally occurring nonlocalities, emphasizing the need for more accurate models to predict and design their optical responses. This has major implications also for topological, nonreciprocal, and time-varying systems based on these material platforms. Beyond natural materials, artificially structured materials--metamaterials and metasurfaces--can provide even stronger and engineered nonlocal effects, emerging from long-range interactions or multipolar effects. This is a rapidly expanding area in the field of photonic metamaterials, with open frontiers yet to be explored. In the case of metasurfaces, in particular, nonlocality engineering has become a powerful tool for designing strongly wavevector-dependent responses, enabling enhanced wavefront control, spatial compression, multifunctional devices, and wave-based computing. Furthermore, nonlocality and related concepts play a critical role in defining the ultimate limits of what is possible in optics, photonics, and wave physics. This Roadmap aims to survey the most exciting developments in nonlocal photonic materials, highlight new opportunities and open challenges, and chart new pathways that will drive this emerging field forward--toward new scientific discoveries and technological advancements.

cond-mat.mes-hall↗
arXiv2025

Ultrahigh free-electron Kerr nonlinearity in all-semiconductor waveguides for all-optical nonlinear modulation of mid-infrared light

Nonlinear optical waveguides, particularly those harnessing the optical Kerr effect, are promising for advancing next-generation photonic technologies. Despite the Kerr effect`s ultrafast response, its inherently weak nonlinearity has hindered practical applications. Here, we explore free-electron-induced Kerr nonlinearities in all-semiconductor waveguides, revealing that longitudinal bulk plasmons (inherently nonlocal excitations) can generate exceptionally strong Kerr nonlinearities. We specifically develop a nonlinear eigenmode analysis integrated with semiclassical hydrodynamic theory to compute the linear and nonlinear optical responses originating from the quantum behavior of free electrons in heavily doped semiconductors. These waveguides achieve ultrahigh nonlinear coefficients exceeding 10$^7$ W$^{-1}$km$^{-1}$ and support long-propagating modes with propagation distances over 100 $μ$m. Additionally, we confirm the robustness of the nonlinear response under realistic conditions by considering viscoelastic and nonlinear damping mechanisms. Finally, we implement our all-semiconductor waveguides in a Mach-Zehnder interferometer, demonstrating efficient nonlinear modulation of the transmittance spectrum via the free-electron Kerr effect. This work evidences the transformative potential of free-electron nonlinearities in heavily doped semiconductors for photonic integrated circuits, paving the way for scalable on-chip nonlinear nanophotonic systems.

physics.optics↗
arXiv2024

Low-power threshold optical bistability enabled by hydrodynamic Kerr nonlinearity of free-carriers in heavily doped semiconductors

We demonstrate nanoscale optical bistability at an exceptionally low power threshold of 1 mW by leveraging Kerr-type hydrodynamic nonlinearities due to the heavily doped semiconductor's free carriers. This high nonlinearity is enabled by a strong coupling between metallic nanopatch surface plasmons and longitudinal bulk plasmons (LBP) that arise in heavily doped semiconductors due to the nonlocality. Through the coupling, an efficient, near-unity conversion of far-field energy into LBP states could be achieved. These findings offer a viable approach to experimentally probe LBPs and lay the groundwork for developing efficient and ultrafast all-optical nonlinear devices.

physics.optics↗
arXiv2024

Origin of optical nonlinearity in plasmonic semiconductor nanostructures

The development of nanoscale nonlinear elements in photonic integrated circuits is hindered by the physical limits to the nonlinear optical response of dielectrics, which requires that the interacting waves propagate in transparent volumes for distances much longer than their wavelength. Here we present experimental evidence that optical nonlinearities in doped semiconductors are due to free-electron and their efficiency could exceed by several orders of magnitude that of conventional dielectric nonlinearities. Our experimental findings are supported by comprehensive computational results based on the hydrodynamic modeling, which naturally includes nonlocal effects, of the free-electron dynamics in heavily doped semiconductors. By studying third-harmonic generation from plasmonic nanoantenna arrays made out of heavily n-doped InGaAs with increasing levels of free-carrier density, we discriminate between hydrodynamic and dielectric nonlinearities. As a result, the value of maximum nonlinear efficiency as well as its spectral location can now be controlled by tuning the doping level. Having employed the common material platform InGaAs/InP that supports integrated waveguides, our findings pave the way for future exploitation of plasmonic nonlinearities in all-semiconductor photonic integrated circuits.

physics.optics↗
arXiv2022

Nanogap-Enhanced Infrared Spectroscopy with Template-Stripped Wafer-Scale Arrays of Buried Plasmonic Cavities

We have combined atomic layer lithography and template stripping to produce a new class of substrates for surface-enhanced infrared absorption (SEIRA) spectroscopy. Our structure consists of a buried and U-shaped metal-insulator-metal waveguide whose folded vertical arms efficiently couple normally incident light. The insulator is formed by atomic layer deposition (ALD) of Al2O3 and precisely defines the gap size. The buried nanocavities are protected from contamination by a silicon template until ready for use and exposed by template stripping on demand. The exposed nanocavity generates strong infrared resonances, tightly confines infrared radiation into a gap that is as small as 3 nm (λ/3300), and creates a dense array of millimeter-long hotspots. After partial removal of the insulators, the gaps are backfilled with benzenethiol molecules, generating distinct Fano resonances due to strong coupling with gap plasmons, and a SEIRA enhancement factor of 10^5 is observed for a 3 nm gap. Because of the wafer-scale manufacturability, single-digit-nanometer control of the gap size via ALD, and long-term storage enabled by template stripping, our buried plasmonic nanocavity substrates will benefit broad applications in sensing and spectroscopy.

physics.optics↗
arXiv2022

Second-harmonic generation from singular metasurfaces

We present a theoretical study of second-harmonic generation from a singular metasurface. The singular metasurfaces strongly interact with the incident light, where the large field enhancement forms an intense surface polarization that generates the second-harmonic field. By using transformation optics, the calculation of nonlinear optical response is converted from the metasurface frame to that of a simple slab geometry, largely reducing the complexity of the problem. In addition, the singular metasurface exhibits a weak dependence on the incident angle of light, which can be potentially used as an all-angle device for harmonic generations. Finally, we study the symmetry dependence of second-harmonic generation in the far field for the singular metasurface and show how to enhance the conversion efficiency under normal incidence by breaking the surface inversion symmetry.

physics.optics↗
arXiv2022

Second-harmonic generation in plasmonic waveguides with nonlocal response and electron spill-out

Plasmonic waveguides provide an integrated platform to develop efficient nanoscale ultrafast photonic devices. Theoretical models that describe nonlinear optical phenomena in plasmonic waveguides, usually, only incorporate bulk nonlinearities, while nonlinearities that arise from metallic constituents remained unexplored. In this work, we present a method that enables a generalized treatment of the nonlinearities present in plasmonic waveguides and use it to calculate second-harmonic generation from free electrons through a hydrodynamic nonlocal description. As a general application of our method we also consider nonlinearities arising from the quantum hydrodynamic theory with electron spill-out. Our results may find applicability in design and analysis of integrated photonic platforms for nonlinear optics incorporating wide variety of nonlinear materials such as heavily doped semiconductors for mid-infrared applications.

physics.optics↗
arXiv2022

Fluorescence quenching in plasmonic dimers due to electron tunneling

Plasmonic nanoparticles provide an ideal environment for the enhancement of fluorescent emission. On the one hand, they locally amplify the electromagnetic fields, increasing the emitter excitation rate, and on the other hand, they provide a high local density of states that accelerates spontaneous emission. However, when the emitter is placed in close proximity to a single metal nanoparticle, the number of nonradiative states increases dramatically, causing the fluorescence to quench. It has been predicted theoretically that, through a judicious placing of the emitter, fluorescence in plasmonic nanocavities can be increased at monotonically. In this article, we show that such monotonic increase is due to the use of local response approximation in the description of the plasmonic response of metal nanoparticles. We demonstrate that taking into account the electron tunneling and the nonlocality of the surrounding system via the quantum hydrodynamic theory results eventually in a quenching of fluorescence enhancement also when the emitter is placed in a nanocavity, as opposed to local response and Thomas-Fermi hydrodynamic theory results. This outcome marks the importance of considering the quantum effects, in particular, the electron tunneling to correctly describe the emission effects in plasmonic systems at nanoscale.

physics.optics↗
arXiv2021

Enhancing second-harmonic generation with electron spill-out at metallic surfaces

Second-order nonlinear optical processes do not manifest in the bulk of centrosymmetric materials, but may occur in the angstroms-thick layer at surfaces. At such length-scales, quantum mechanical effects come into play which could be crucial for an accurate description of plasmonic systems. In this article, we develop a theoretical model based on the quantum hydrodynamic description to study free-electron nonlinear dynamics in plasmonic systems. Our model predicts strong resonances induced by the spill-out of electron density at the metal surface. We show that these resonances can boost second-harmonic generation efficiency up to four orders of magnitude and can be arbitrarily tuned by controlling the electron spill-out at the metal surface with the aid of thin dielectric layers. These results offer a possibility to artificially increase nonlinear susceptibilities by engineering optical properties at the quantum level.

physics.optics↗
arXiv2021

Laplacian-Level Quantum Hydrodynamic Theory for Plasmonics

An accurate description of the optical response of subwavelength metallic particles and nanogap structures is a key problem of plasmonics. Quantum hydrodynamic theory (QHT) has emerged as a powerful method to calculate the optical response of metallic nanoparticles (NPs) since it takes into account nonlocality and spill-out effects. Nevertheless, the absorption spectra of metallic NPs obtained with conventional QHT, i.e., incorporating Thomas-Fermi (TF) and von Weizsäcker (vW) kinetic energy (KE) contributions, can be affected by several spurious resonances at energies higher than the main localized surface plasmon (LSP). These peaks are not present in reference time-dependent density-functional-theory (TD-DFT) spectra, where, instead, only a broad shoulder exists. Moreover, we show here that these peaks incorrectly reduce the LSP peak intensity and have a strong dependence on the simulation domain size so that a proper calculation of QHT absorption spectra can be problematic. In this article, we introduce a more general QHT method accounting for KE contributions depending on the Laplacian of the electronic density ($q$), thus, beyond the gradient-only dependence of the TFvW functional. We show that employing a KE functional with a term proportional to $q^2$ results in an absorption spectrum free of spurious peaks, with LSP resonance of correct intensity and numerically stable Bennett state. Finally, we present a novel Laplacian-level KE energy functional that is very accurate for the description of the optical properties of NPs with different sizes as well as for dimers.

cond-mat.mes-hall↗
arXiv2020

Free electron nonlinearities in heavily doped semiconductors plasmonics

Heavily doped semiconductors have emerged as tunable low-loss plasmonic materials at mid-infrared frequencies. In this article we investigate nonlinear optical phenomena associated with high concentration of free electrons. We use a hydrodynamic description to study free electron dynamics in heavily doped semiconductors up to third-order terms, which are usually negligible for noble metals. We find that cascaded third-harmonic generation due to second-harmonic signals can be as strong as direct third-harmonic generation contributions even when the second-harmonic generation efficiency is zero. Moreover, we show that when coupled with plasmonic enhancement free electron nonlinearities could be up to two orders of magnitude larger than conventional semiconductor nonlinearities. Our study might open a new route for nonlinear optical integrated devices at mid-infrared frequencies.

physics.optics↗
arXiv2020

Impact of surface roughness in nanogap plasmonic systems

Recent results have shown unprecedented control over separation distances between two metallic elements hundreds of nanometers in size, underlying the effects of free-electron nonlocal response also at mid-infrared wavelengths. Most of metallic systems however, still suffer from some degree of inhomogeneity due to fabrication-induced surface roughness. Nanoscale roughness in such systems might hinder the understanding of the role of microscopic interactions. Here we investigate the effect of surface roughness in coaxial nanoapertures resonating at mid-infrared frequencies. We show that although random roughness shifts the resonances in an unpredictable way, the impact of nonlocal effects can still be clearly observed. Roughness-induced perturbation on the peak resonance of the system shows a strong correlation with the effective gap size of the individual samples. Fluctuations due to fabrication imperfections then can be suppressed by performing measurements on structure ensembles in which averaging over a large number of samples provides a precise measure of the ideal system's optical properties.

physics.optics↗
arXiv2018

Nanowire-Intensified MEF in Hybrid Polymer-Plasmonic Electrospun Filaments

Hybrid polymer-plasmonic nanostructures might combine high enhancement of localized fields from metal nanoparticles with light confinement and long-range transport in subwavelength dielectric structures. Here we report on the complex behavior of fluorophores coupling to Au nanoparticles within polymer nanowires, which features localized metal-enhanced fluorescence (MEF) with unique characteristics compared to conventional structures. The intensification effect when the particle is placed in the organic filaments is remarkably higher with respect to thin films of comparable thickness, thus highlighting a specific, nanowire-related enhancement of MEF effects. A dependence on the confinement volume in the dielectric nanowire is also evidenced, with MEF significantly increasing upon reducing the wire diameter. These findings are rationalized by finite element simulations, predicting a position-dependent enhancement of the quantum yield of fluorophores embedded in the fibers. Calculation of the ensemble-averaged fluorescence enhancement unveils the possibility of strongly enhancing the overall emission intensity for structures with size twice the diameter of the embedded metal particles. These new, hybrid fluorescent systems with localized enhanced emission, as well as the general Nanowire-Intensified MEF effect associated to them, are highly relevant for developing nanoscale light-emitting devices with high efficiency and inter-coupled through nanofiber networks, highly sensitive optical sensors, and novel laser architectures.

cond-mat.mtrl-sci↗
arXiv2016

Current-dependent exchange-correlation potential for non-local absorption in quantum hydrodynamic theory

The quantum hydrodynamic theory is a promising method for describing microscopic details of macroscopic systems. The hydrodynamic equation can be directly obtained from a single particle Kohn-Sham equation that includes the contribution of an external vector potential. This derivation allows to straightforwardly incorporate in the hydrodynamic equation an exchange-correlation viscoelastic term, so that broadening of collective excitation can be taken into account, as well as a correction to the plasmon dispersion. The result is an accurate self-consistent and computationally efficient hydrodynamic description of the free electron gas. A very accurate agreement with full quantum calculations is shown.

cond-mat.mes-hall↗
arXiv2016

Quantum Hydrodynamic Theory for Plasmonics: Impact of the Electron Density Tail

Multiscale plasmonic systems e.g. extended metallic nanostructures with sub-nanometer inter-distances) play a key role in the development of next-generation nano-photonic devices. An accurate modeling of the optical interactions in these systems requires an accurate description of both quantum effects and far-field properties. Classical electromagnetism can only describe the latter, while Time-Dependent Density Functional Theory (TD-DFT) can provide a full first-principles quantum treatment. However, TD-DFT becomes computationally prohibitive for sizes that exceed few nanometers, which are irrelevant for practical applications. In this article, we introduce a method based on the quantum hydrodynamic theory (QHT), which includes non-local contributions of the kinetic energy and the correct asymptotic description of the electron density. We show that our QHT method can predict both plasmon energy and spill-out effects in metal nanoparticles in excellent agreement with TD-DFT predictions, thus allowing a reliable and efficient calculations of both quantum and far-field properties in multiscale plasmonic systems.

cond-mat.mes-hall↗