Optical poling is a well-established technique for inducing χ^(2) nonlinearity, yet its impact on silica's molecular structure remains unexplored. Here, we report the first direct observation of molecular restructuring in large-core graded-index multimode fibers (MMFs) induced by optical poling, transforming the silica tetrahedral ring network. Through coherent light beating, this process converts large rings of more than four SiO_4 tetrahedra into smaller ones, altering both linear and nonlinear optical susceptibilities. Contrary to the assumption that poling efficiency stems solely from charge displacement, we show that structural modifications dominate, leading to record enhancements in third-order nonlinear processes, including geometric parametric instabilities (GPIs) and Kerr self-cleaning, despite a low modification of the Kerr coefficient. High-energy poling acts as an in situ annealing process, dynamically modulating the refractive index for unprecedented spatiotemporal light control. These findings provide fundamental insights into silica's molecular dynamics under intense optical fields and open avenues for ultra-efficient nonlinear optical devices, enabling next-generation fiber-based photonics for high-power lasers, broadband light generation, and all-optical signal processing.

We present a unified analytical framework for resonant optical tunnelling in planar three-layer photonic systems embedded in a transparent dielectric medium. Using a generalized Fresnel-coefficient approach, we derive compact expressions for the transmission and show that resonant tunnelling occurs in two fundamentally different regimes, determined by the nature of the waves supported in the core layer. When the core supports propagating harmonic waves, resonances obey the conventional Fabry–Perot phase condition. By contrast, when the core supports evanescent or damped waves, resonant tunnelling arises from an amplitude-matching condition governed by the magnitude of the composite reflection coefficient. These two regimes lead to qualitatively different transmission characteristics and distinct tunnelling behaviour. Transparent systems, including ideal metals, are analysed first in order to isolate the underlying physical mechanisms. Absorption is then incorporated, showing the transition from unitary resonant tunnelling to attenuated optical tunnelling in realistic plasmonic structures. Angular–spectral transmission maps illustrate the general features of each configuration and show that resonant tunnelling may occur even when the tunnelling layer is several wavelengths thick. The results provide a consistent physical interpretation of resonant tunnelling across dielectric and metal–dielectric multilayer systems.

Controlling complex light waves to achieve desired behaviours or characteristics on demand presents a significant challenge. This task becomes even more complicated when manipulating speckled light beams, owing to their inherently fuzzy intensity and phase structures. Here we demonstrate that a weak speckled second-harmonic signal in a multimode graded-index fibre can be manipulated via its conservative interaction with a high-power co-propagating fundamental pump wave. Specifically, the spatial quality of the signal can be either enhanced or degraded by varying the pump's power or its modal distribution. The underlying physical mechanism is the optically induced mode conversion that can be controlled by the pump beam shape. This phenomenon enables new possibilities to manipulate complex light in nonlinear multimode waveguides. A striking example of this novel light-by-light control is the experimentally observed enhancement or partial suppression of the visible Raman Stokes cascade regulated by the second harmonic beam, while modulated by the mode power distribution of the fundamental beam.

The traditional method for characterizing optical thin films consists in calculating the proportion of light that is reflected and transmitted by the film. In this work, we present an alternative procedure based on the analysis of electromagnetic fields and energy flow within the film. Using our approach, we provide a unified description of a plethora of phenomena in three-layered structures, including frustrated total internal reflection, dielectric waveguiding, or surface plasmon resonance. Our approach highlights the distinguishing features of evanescent and damped waves in opposition to travelling waves, particularly those concerning the propagation of energy. The theoretical framework proposed here contributes to explicitly visualize the intrinsic difference between propagation in thin films and thicker media, providing physical insight into energy transport through them.

Analytical modelling of light transmission through a metal-insulator-metal geometry embedded in a coupling glass surrounding medium is possible through an extended Fabry-Pérot formula. Two distinct coupled surface plasmon resonance branches are allowed inside such microcavity, where two thin metallic layers act as mirrors delimiting an inner dielectric material. In agreement with transfer-matrix method simulations, the resulting theoretical expressions predict a large and almost constant transmittance even for intracavity thicknesses greater than light’s penetration depth. Results at λ = 800 nm have been validated experimentally and show optical transmittance over 10% until nearly 3 μm. This high transmittance under such unexpected conditions, related to an anomalously high mirror reflection coefficient, sheds light on new possibilities for the design of optical devices. The experimental setup successfully used to corroborate the validity of the transmittance formula over different angular, spectral and geometrical conditions is also presented.

We discuss a numerical model and experimental results to study the effect of spatial beam self-cleaning in nonlinear graded-index fibers and how to control such process, including transient phenomena. We examine illustrative some cases of coherent combination of cleaned beams and supercontinuum generation.

Ultrafast 3D nonlinear multiphoton imaging holds great promise for the visualization of complex biological structures. However, its practical implementation remains constrained, primarily due to the limited longitudinal scanning speed achievable with tightly focused laser pulses within the sample. In this work, we propose a 3D nonlinear imaging concept that harnesses a spatiotemporal Kerr self-focusing process for depth-resolved imaging without mechanical scanning. We exploit Townes solitons to generate ultrafast nonlinear flying focus pulses that are well-suited for nonlinear fluorescence imaging. A key feature of this approach is the power-dependent longitudinal displacement of the self-focusing point within the sample. This displacement, combined with the power-limiting effects from conical-wave emissions at multiple wavelengths, enables precise control and calibration of the beam's focal position in three dimensions. Consequently, by implementing a temporal pulse encoding, either by splitting and delaying an initial pump pulse or by introducing an inter-pulse amplitude modulation, we can induce multiple self-focusing events at different time bins. This allows us to probe multiple axial planes in a single laser shot. As a result, a full 3D image can be reconstructed from a single 2D transverse scan, significantly accelerating imaging speed. Additionally, the stable nonlinear propagation of these filaments leads to collinear supercontinuum generation in the form of self-guided light beams. Overall, our method presents a pathway for ultrafast, high-resolution 3D nonlinear imaging. (c) 2025 Optica Publishing Group under the terms of the Optica

Frustrated total internal reflection (FTIR) is analyzed from a novel perspective. Unlike similar works, the angle of incidence is used here as the experimental variable instead of the film thickness through which light tunnels. This method makes it possible to visualize not only the phenomenon of FTIR but also the resonance processes that occur for angles of incidence below the critical angle. An affordable straightforward experiment appropriate for undergraduates is presented. The experiment involves measuring the reflection and transmission of light through a pair of prisms separated by an air or water layer, and the results are in fair agreement with theory.

We experimentally demonstrate that Townes solitons generated in a Nd:YAG crystal enable the formation of ultrafast nonlinear flying focus pulses that can be exploited for nonlinear fluorescence imaging. The power dependent longitudinal shift of the self-focusing point along the crystal, accompanied by a power-limiting effect caused by conical-wave emission at different wavelengths, permits a precise spatial calibration of the beam profile. Consequently, by splitting and delaying an initial pump pulse or, equivalently, by introducing an inter-pulse amplitude modulation to implement a temporal pulse encoding, one can generate different self-focusing points at different time bins for the same pump pulse. This permits to analyze a biological sample at different depths in a single laser shot. Such a method avoids repeating multiple transverse scans while imaging at different depths. The nonlinear propagation of such stable filaments also leads to collinear supercontinuum generation in the form of self-guided polychromatic filaments. Our method paves the way for ultrafast 3D nonlinear multiphoton imaging.

In this work, we study the contributions that different molecular blocks have in the wavelength-dependence of the refractive index in ionic liquids. The ionic liquids chosen for this work are combinations of the bis(trifluoromethylsulfonyl)imide anion with cations based on four different heterocycles with different extents of charge delocalization. The analysis is performed in terms of the experimental electronic polarizability, which is obtained by combining measurements of refractive index curves and densities via the Lorentz-Lorenz equation. Exploiting the additivity of electronic polarizability in ionic liquids, the contribution of the anion and the heterocycles of the cations is separated from that of the alkyl chains. Our results show important differences in these contributions, revealing a key influence of the charge delocalization in the cationic rings on the behavior of the refractive index dispersion. The understanding of how different parts of ionic liquids affect their refractive index dependence on wavelength would allow to gain precise control of this magnitude, enabling the development of customized optical materials for diverse applications in photonics and sensing technologies.

A symmetrical structure consisting of a low refractive index dielectric layer between two metallic films, i.e. an optical cavity, surrounded by a semi-infinite dielectric medium of higher refractive index, forms an optical system capable of supporting both volume and surface resonances. The latter are associated with synchronized collective electronic oscillations in the inner surfaces of the two thin metallic films, called coupled surface plasmons. These oscillations are generated by an evanescent wave in the cavity and therefore the thickness of the cavity is limited to the micron range for visible radiation. Under suitable incident conditions, light propagating in the microcavity will resonate with these plasmonic oscillations and can be strongly transmitted into the surrounding medium. In this work, we establish a simple model of the transmission characteristics of the cavity and define resonance conditions that allow high transmittance even for inner dielectric layer thicknesses of various wavelengths. This phenomenon is an enhanced version of the optical process of frustrated total reflection between dielectrics analogous to quantum tunnelling effect. In the present situation, the phenomenon is more striking because it takes place in a system with two absorbing metal films which, under resonant conditions, favour a superior transmission.

A coherent concatenation of multiple solitary waves may lead to a stable infrared and visible broadband filament in a ceramic YAG polycrystal. This self-trapped soliton train is leveraged to implement self-referenced multiplex coherent anti-Stokes Raman scattering (SR-M-CARS) imaging. Simulations and experiments illustrating the filamentation process and the concatenation of focusing-defocusing cycles in ceramic and crystal YAG are presented. In addition, our simulations and experiments further examine the dependence of the filamentation onset location and supercontinuum (SC) generation upon peak input power. Understanding this dependence is key for the implementation of viable CARS imaging techniques, due to the comparatively exceptional ability of YAG to generate supercontinuum that can enable higher-sensitivity imaging without delay lines.

This work models light transmission through metal-dielectric-metal microcavities supporting Coupled Surface Plasmons (CSP). An extended Fabry-Perot formula reveals two plasmonic resonances that merge beyond a critical cavity thickness. Remarkably, transmittance at these resonances remains high and nearly constant for thicknesses exceeding the light's penetration depth. Results at a 1 mu m wavelength show over 10% transmittance up to 3.5 mu m, offering new insights for photonic device design

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Optical poling of multimode graded-index fibers (GRIN) has emerged as a promising technique for creating periodic inscriptions of the second-order nonlinear optical susceptibility χ(2), enabling the generation of a second harmonic in silica fibers. In this work, we investigate the generation of multiple spectral peaks using a continuous broadband source in the infrared domain, generated in the same fiber by a femtosecond laser with a central wavelength different from the one used for the poling process. Building upon theoretical foundations, this work contributes at explaining how second and third-order nonlinear processes participate to the broad generation of the second harmonic in GRIN fibers.

We investigate an original approach for the generation of unequally spaced frequency combs using (2) –(3) nonlinearities in multimode graded-index (MM-GRIN) fiber. In a preliminary step, the MM-GRIN fiber (50 µm of core diameter and 125 μm of cladding diameter) is optically poled with a Nd:YAG sub-nanosecond microchip laser at 1064 nm. As a results, a double periodical inscription of a complex second order non-linearity χ(2) grating was led. The resulting χ(2) inscription allows the generation of second harmonic wave (SH) from a supercontinuum obtained in the infrared domain under the Raman and soliton propagation actions. We then detect the generation of various irregularly spaced spectral peaks surrounding the original SH (532 nm) at the fiber output allowing harmonic generation on more than 100 nm in the visible domain.

In the last decade, the intrinsic tunability of ionic liquids has attracted interest well beyond the disciplines that inaugurated the research on these materials at the beginning of the 1990s. One of these emerging fields is photonics, where the possibility of designing materials with task-specific optical responses is promising for a large number of potential applications. Among the optical properties of ionic liquids, refractive index has been the subject of a thorough investigation because of its relevance in the performance of most optical devices. In this chapter, we address the recent experimental and computational advances on the characterization of the refractive index of ionic liquids and the relations of this magnitude with their structure. Furthermore, we provide an important review of works introducing ionic liquids as essential optical materials in diverse applications.

Coupled surface plasmons arise in the surfaces of a dielectric layer between two metallic media when a dim wave propagating in the dielectric generates resonant free charge oscillations at the interfaces. Here, we consider surface plasmon resonance in a Fabry-Perot type cavity with plane metallic mirrors and an inner dielectric medium, optically less dense than the outer surrounding dielectric medium. The experimentally observed transmission as a function of both the angle of incidence of light and the wavelength is well modelled by an elementary transmittance function from which resonance conditions are obtained both in the Fabry-Perot and Surface-Plasmon regime.

Multiplex Coherent Anti-Stokes Raman Scattering (M-CARS) is an innovative nonlinear spectroscopic approach designed to characterize the vibrational modes of molecular structures. Coherent Raman scattering has been used for the characterization of biomedical targets for about 20 years and the multiplex aspect was introduced 10 years ago thanks to the use of a supercontinuum laser system. For each of these systems, the combination of a pump and a probe wave, driven by an external delay line, is however required to produce the vibrations. In the present work, we propose a new M-CARS system, free of the external delay line. A few-mode microstructured fiber enables merging both wave-packets (pump and supercontinuum) within a single waveguide. We showcase the capability of this system in generating hyperspectral images of biochemically active compounds. Curcumin I, the principal yellow compound isolated from Curcuma longa (Turmeric), is distinguishable by its multiple functional groups that display a nonlinear spectroscopic behavior.

We present experimental evidence (supported by numerical simulations) of the Goos-Hanchen effect and a nonlinear reflection process, after filament onset in Yttrium Aluminium Garnet. We then examine the dependence of these phenomena on input power.