arXiv subjects

Lotfi Berguiga

Publications and source records attributed to Lotfi Berguiga.

11 recordsLinked to original sources

arXiv2026

All-optical programming of polarization singularities in a photonic-crystal laser

Singular optics has emerged as an important research area with diverse applications, yet controlling optical singularities in nanophotonic emitters remains largely constrained by the fixed subwavelength geometry of optical resonators. Here, we circumvent this limitation and demonstrate all-optical programming of real-space polarization singularities in a photonic-crystal laser, while preserving a momentum-space vortex inherited from a symmetry-protected bound state in the continuum. The principle is to use a shaped optical pump to create a smooth mesoscopic potential, whose spatial variations are slow compared with the lattice period. This potential localizes a negative-mass Bloch band into trapped lasing states whose envelope functions, and therefore far-field singularity textures, are defined by the pump geometry. Using a honeycomb photonic crystal supporting a symmetry-protected bound state in the continuum, we achieve room-temperature telecom-band lasing with real-space polarization singularities pinned to the critical points of the envelope function, where its gradient vanishes, and reconfigurable in number and position by pump shaping, while the intrinsic momentum-space singularity at the $\Gamma$ point remains fixed. The experimental observations agree quantitatively with an analytical framework combining the Bloch mode of the photonic crystal with envelope-function theory, establishing optical envelope engineering as a route to programmable structured emission from active photonic lattices.

physics.optics↗

arXiv2025

A formalism for giant Goos-Hänchen shift in metasurface sensors with phase singularity

The Goos-Hänchen (GH) shift becomes giant in resonant photonic structures, making it promising for refractive index sensors with ultimate sensitivities. We provide here a complete formalism to analytically describe the GH shift and its associated sensitivity around the critical coupling regime in photonic structures. This analytical framework quantitatively connects physical parameters such as the quality factor, the angular dispersion, the beam size and the phase singularity to the GH shift. We numerically confirm this theory in two practical designs: a surface plasmon resonance sensor and a Bloch surface wave (BSW) metasurface sensor. Coupling our theory with numerical simulations, we design a BSW metasurface whose GH sensitivity ($10^{13} μm/RIU$) is more than 5 orders of magnitude higher than the current state-of-the art.We also reveal that the main practical limitation to reach ultimate GH sensitivities is the beam size. However, taking into account realistic beam sizes and introducing engineering dispersion for the metasurface, we calculate limits of detection for GH sensors as low as $10^{-13} RIU$ that still surpass current sensors. These results open the way for new sensing application needing high sensitivity and low limit of detection.

physics.optics↗

arXiv2025

Spatially-Controlled Planar Guided Crystallization of Low-Loss Phase Change Materials for Programmable Photonics

Photonic integrated devices are progressively evolving beyond passive components into fully programmable systems, notably driven by the progress in chalcogenide phase-change materials (PCMs) for non-volatile reconfigurable nanophotonics. However, the stochastic nature of their crystal grain formation results in strong spatial and temporal crystalline inhomogeneities. Here, we propose the concept of spatially-controlled planar guided crystallization, a novel method for programming the growth of optically homogeneous low-loss Sb2S3 PCM, leveraging the seeded directional and progressive crystallization within confined channels. This guided crystallization method is experimentally shown to circumvent the current limitations of conventional PCM-based nanophotonic devices, including a multilevel non-volatile optical phase-shifter exploiting a silicon nitride-based Mach-Zehnder interferometer, and a programmable metasurface with spectrally reconfigurable bound state in the continuum. Precisely controlling the growth of PCMs to ensure optically uniform crystalline properties across devices is the cornerstone for the industrial development of non-volatile reconfigurable photonic integrated circuits.

physics.optics↗

arXiv2025

Strong and Engineerable Optical Anisotropy in Easily Integrable Epitaxial SrO(SrTiO 3 ) N Ruddlesden--Popper Thin Layers

Optical anisotropy is a key property for numerous photonic devices. However, bulk anisotropic materials suitable for such applications remain relatively scarse and are often challenging to synthesize as thin films. Additionally, the optical losses as well as the complex structuration of anisotropic metamaterials hinder their integrability in photonic devices. Based on ellipsometry measurements coupled with reflectance, it is demonstrated here that Ruddlesden-Popper (RP) SrO(SrTiO 3 ) N phases (STO-RP N ), epitaxial thin films composed of a SrTiO 3 lattice periodically interrupted by one SrO atomic plane every N unit cells, exhibit pronounced dichroism and birefringence over a broad spectral range. Notably, this anisotropy is tunable by adjusting the RP order N. In contrast to most other anisotropic materials reported in the literature, STO-RP N thin layers can be fabricated using industry-standard growth processes. As it can be epitaxially grown on Si and GaAs using SrTiO 3 templates, the work paves the way for their compact integration on these photonic platforms.

cond-mat.mtrl-sci↗

arXiv2024

Reversible single-pulse laser-induced phase change of Sb$_2$S$_3$ thin films: multi-physics modeling and experimental demonstrations

Phase change materials (PCMs) have gained a tremendous interest as a means to actively tune nanophotonic devices through the large optical modulation produced by their amorphous to crystalline reversible transition. Recently, materials such as Sb$_2$S$_3$ emerged as particularly promising low loss PCMs, with both large refractive index modulations and transparency in the visible and NIR. Controlling the local and reversible phase transition in this material is of major importance for future applications, and an appealing method to do so is to exploit pulsed lasers. Yet, the physics and limits involved in the optical switching of Sb$_2$S$_3$ are not yet well understood. Here, we investigate the reversible laser-induced phase transition of Sb$_2$S$_3$, focusing specifically on the mechanisms that drive the optically induced amorphization, with multi-physics considerations including the optical and thermal properties of the PCM and its environment. We theoretically and experimentally determine the laser energy threshold for reversibly changing the phase of the PCM, not only between fully amorphous and crystalline states but also between partially recrystallized states. We then reveal the non-negligible impact of the material's polycrystallinity and anisotropy on the power thresholds for optical switching. Finally, we address the challenges related to laser amorphization of thick Sb$_2$S$_3$ layers, as well as strategies to overcome them. These results enable a qualitative and quantitative understanding of the physics behind the optically-induced reversible change of phase in Sb$_2$S$_3$ layers.

physics.app-ph↗

arXiv2024

Super Bound States in the Continuum on Photonic Flatbands: Concept, Experimental Realization, and Optical Trapping Demonstration

In this work, we theoretically propose and experimentally demonstrate the formation of a super bound state in a continuum (BIC) on a photonic crystal flat band. This unique state simultaneously exhibits an enhanced quality factor and near-zero group velocity across an extended region of the Brillouin zone. It is achieved at the topological transition when a symmetry-protected BIC pinned at $k=0$ merges with two Friedrich-Wintgen quasi-BICs, which arise from destructive interference between lossy photonic modes of opposite symmetries. As a proof-of-concept, we employ the super flat BIC to demonstrate three-dimensional optical trapping of individual particles. Our findings present a novel approach to engineering both the real and imaginary components of photonic states on a subwavelength scale for innovative optoelectronic devices.

physics.optics↗

arXiv2023

A simple method for programming and analyzing multilevel crystallization states in phase-change materials thin film

We propose and demonstrate a simple method to accurately monitor and program arbitrary states of partial crystallization in phase-change materials (PCMs). The method relies both on the optical absorption in PCMs as well as on the physics of crystallization kinetics. Instead of raising temperature incrementally to increase the fraction of crystallized material, we leverage the time evolution of crystallization at constant temperatures and couple this to a real-time optical monitoring to precisely control the change of phase. We experimentally demonstrate this scheme by encoding a dozen of distinct states of crystallization in two different PCMs: GST and Sb2S3. We further exploit this time-crystallization for the in-situ analysis of phase change mechanisms and demonstrate that the physics of crystallization in Sb2S3 is fully described by the so-called Johnson-Mehl-Avrami-Kolmogorov formalism. The presented method not only paves the way towards real-time and model-free programming of non-volatile reconfigurable photonic integrated devices, but also provides crucial insights into the physics of crystallization in PCMs.

physics.app-ph↗

arXiv2021

Ultimate Phase Sensitivity in Surface Plasmon Resonance Sensors by Tuning Critical Coupling with Phase Change Materials

Plasmonic sensing is an established technology for real-time biomedical diagnostics and air-quality monitoring. While intensity and wavelength tracking are the most commonly used interrogation methods for Surface Plasmon Resonance (SPR), several works indicate the potential superiority of phase interrogation in detection sensitivity. Here, we theoretically and numerically establish the link between ultra-high sensitivities in phase interrogation SPR sensors and the critical coupling condition. However, reaching this condition requires a technically infeasible angstrom-level precision in the metal layer thickness. We propose a robust solution to overcome this limitation by coupling the SPR with a phase-change material (PCM) thin film. By exploiting the multilevel reconfigurable phase states of PCM, we theoretically demonstrate ultra-high phase sensitivities with a limit of detection as low as $10^{-10}$ refractive index unit (RIU). Such a PCM-assisted SPR sensor platform paves the way for unprecedented sensitivity sensors for the detection of trace amounts of low molecular weight species in biomedical sensing and environmental monitoring.

physics.optics↗

arXiv2020

Reconfigurable Flat Optics with Programmable Reflection Amplitude Using Lithography-Free Phase-Change Materials Ultra Thin Films

We experimentally demonstrate a very large dynamic optical reflection modulation from a simple unpatterned layered stack of phase-change materials ultrathin films. Specifically, we theoretically and experimentally demonstrate that properly designed deeply subwavelength GeSbTe (GST) films on a metallic mirror produce a dynamic modulation of light in the near-infrared from very strong reflection (R>80%) to perfect absorption (A > 99,97%) by simply switching the crystalline state of the phase-change material. While the amplitude of modulation can lead to an optical contrast up to 10^6, we can also actively "write" intermediate levels of reflection in between extreme values, corresponding to partial crystallization of the GST layer. We further explore several layered system designs and provide guidelines to tailor the wavelength efficiency range, the angle of operation and the degree of crystallization leading to perfect absorption.

physics.optics↗

arXiv2019

Unified Weak-to-Strong Coupling Transitions and Radiation Interference Induced by Vertical-Symmetry Breaking in Photonic Crystal Slabs

Vertical-symmetry breaking provides a versatile means of coupling leaky photonic-crystal resonances that are otherwise protected by opposite out-of-plane parity. Here, we develop a unified two-mode framework for the weak-to-strong coupling transition induced by vertical-symmetry breaking in photonic crystal slabs. The symmetry-breaking perturbation simultaneously generates a near-field coherent coupling and modifies the overlap of the radiation channels of the two parent resonances. Their interplay drives the transition from frequency crossings to avoided crossings through an exceptional point (EP), while also governing radiative linewidth exchange through Friedrich-Wintgen interference. Using a radiation-vector description resolved into the upper and lower half-spaces, we show that total radiation cancellation and one-sided radiation cancellation correspond, respectively, to quasi-bound states in the continuum (quasi-BIC) and quasi-unidirectional guided resonances (quasi-UGR). This framework distinguishes the spectral condition for EP formation from the far-field conditions controlling linewidth suppression and directional emission. We validate this picture numerically and experimentally in square-lattice photonic crystal slabs. Tuning the superstrate index drives a weak-to-strong coupling transition through an EP, while partial etching yields a broad off-$\Gamma$ quasi-BIC regime with strongly asymmetric top and bottom radiation. We further apply the same radiation-vector framework to a laterally shifted bilayer grating, where quasi-BICs and quasi-UGR emerge along a continuous symmetry-breaking pathway. These results establish vertical-symmetry breaking as a general route for controlling hybridization, radiation interference, and directional leakage in photonic crystal slabs.

physics.optics↗

arXiv2018

Tamm plasmon Photonic Crystals : from Bandgap Engineering to Defect Cavity

We report for the first time the bandgap engineering of Tamm plasmon photonic crystals - Tamm plasmon structures of which the metalic layer is periodically patterned into lattice of subwavelength period. By adopting a double period design, we evidenced experimentally a complete photonic bandgap up to $150\,nm$ in the telecom range. Moreover, such design offers a great flexibility to tailor on-demand, and independently, the band-gap size from $30\,nm$ to $150\,nm$ and its spectral position within $50\,nm$. Finally, by implementing a defect cavity within the Tamm plasmon photonic crystal, an ultimate cavity of $1.6μm$ supporting a single highly confined Tamm mode is experimentally demonstrated. All experimental results are in perfect agreement with numerical calculations. Our results suggests the possibility to engineer novel band dispersion with surface modes of hybrid metalic/dielectric structures, thus open the way to Tamm plasmon towards applications in topological photonics, metamaterials and parity symmetry physics.

physics.optics↗