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.

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.

Label-free discrimination between protein families within the same biological environment remains a significant challenge for bioimaging. We present a novel method that addresses this issue by exploiting the non-resonant background signal in multiplex-coherent anti-Stokes Raman scattering (M-CARS) spectroscopy, a signal typically regarded as noise. Our approach leverages the contrast between resonant and non-resonant contributions in hyperspectral M-CARS data to enable the distinct detection and spatial mapping of individual protein families. As a proof of concept, we apply this method to distinguish actin and myosin filaments in muscle tissue. Myosin is first localized through its endogenous second harmonic generation (SHG) signal. Pixel-wise analysis of the ratio between resonant and non-resonant components then reveals the location of actin filaments, in agreement with SHG expectations. This work highlights the untapped potential of the silent spectral region, which carries valuable discriminative information. By leveraging the intrinsic differences in nonlinear optical responses between proteins, this strategy enables qualitative label-free imaging and enhances the analytical potential of M-CARS spectroscopy for multifactorial biological studies.

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.

Quantum technologies and applications is a recent, revolutionary and fast-growing domain. Single photon sources are crucial for quantum applications. Among the various mechanisms, GaAs-based semiconductor quantum dots (QD) are promising single-photon sources [1]. These particular QDs are excited by Ti:Sa oscillators delivering high repetition rate trains of picosecond transform-limited pulses at $(920\pm 20)$ nm with sub-nanojoule energy [2]. To increase compactness and portability, the development of custom-design excitation sources, based e.g. on fiber lasers, is in an active stage. Here, we propose a scheme based on second-harmonic generation (SHG) of pulses at 1840 nm. We followed a similar approach in a previous work [3], where we produced femtosecond pulses at 920 nm for two-photon microscopy from a soliton self-frequency shifted source at 1840 nm. However, for efficient generation of single photons in QDs, the pulses must be narrow linewidth (< 1 nm) corresponding to transform-limited duration longer than 1 ps. To this aim, we exploit SHG in a regime of large group-velocity mismatch (GVM) between the fundamental frequency (FF) and its second harmonic (SH). This way we achieve efficient conversion (~50%), while producing transform-limited SH pulses whose duration exceeds that of the fundamental radiation by a factor of 40.

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

We experimentally demonstrate a novel approach to control the brightness of a high energy beam by leveraging its interaction with a weaker beam that differs either in polarization or in wavelength. This allows for extending the spatial beam selfcleaning process in multimode fibers to multiple beams.

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.

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.

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.

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.

We discuss new ideas for developing fiber light sources for nonlinear imaging. We demonstrate high-power SC-based laser tunable within fingerprint region, as well as new methods of tuning FWM sidebands.

A coherent concatenation of multiple Townes solitons may lead to a stable infrared and visible broadband filament in ceramic YAG polycrystal. This self-trapped soli-ton train helps implement self-referenced multiplex coherent anti-Stokes Raman scattering imaging.

We experimentally study the spatial beam profile and the spectral broadening at the output of a multimode air-silica microstructure fiber taper, used along the direction of an increasing fiber diameter. By using a laser pump at 1064 nm emitting 60 ps Gaussian beam pulses, we observed a competition between Raman beam cleanup and Kerr beam self-cleaning: the multimode frequency conversion process permits to generate spectral sidebands with frequency detuning from the pump that are difficult to obtain in standard graded-index multimode fibers. The generated supercontinuum spans from 500 nm up to 2.5 µm.

We propose a novel approach using Multiplex-Coherent Anti-Stokes Raman Scattering (M-CARS) for la-bel-free discriminations in biomedical tissues. The strategy is based on the evaluation of the contrast be-tween resonant and nonresonant contributions in a M-CARS hyperspectral dataset, and tested to identify and differentiate thin actin filaments from thick myosin filaments in muscle tissue without any labeling. First step consists in ensuring knowledge of the spatial regions containing thick myosin filaments thanks to its endogenous second harmonic signal, deducing expected location for thin actin filaments between myosin filaments. The ratio of resonant and nonresonant contributions for each pixel of the hyperspectral image allows then to discriminate actin from myosin filaments, whose localization is in accordance with the SHG probing. This qualitative imaging represents a proof of principle for highlighting and discriminat-ing purposes in biological microscopy, thanks to the difference in the nonlinear properties of the related proteins. This paves the way for considering label-free imaging through a competition between two third-order nonlinear signatures.

We demonstrate that exploiting the cylindrical geometry of optical fibers allows for the ignition of helical-shaped plasma filaments.

Multimode fibres (MMFs) have recently demonstrated a great potential towards discovering numerous new and complex processes that have never been observed, so far [1]. Among others, Kerr self-cleaning has unveiled the ability of graded-index (GRIN) fibres to produce a quasi-single-mode beam, despite the incoherent multimode propagation involving a large number of modes [2]. A clean beam with its improved brightness can then be obtained at the output of the GRIN MMFs. This phenomenon is enabled by the combined effect of self-imaging and Kerr nonlinearity, followed by a nonlinear nonreciprocity of the mode coupling process. This high-intensity self-transformation can also be described by a thermodynamic approach, whenever a large number of modes is involved [3–4].