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.

Abstract As photonic systems grow more complex, it becomes increasingly difficult to capture their behaviour within the conventional four dimensions of space and time, particularly for systems operating at the nanometer scale, where strong confinement effects, near-field interactions, and subwavelength structuring introduce additional layers of complexity. The concept of 5D photonics reflects this shift by incorporating additional physical, material, computational, adaptive, and quantum degrees of freedom as active components in design, control, and function. Rather than defining a single extra coordinate, higher-dimensional photonics is about operating photonic systems within expanded, dynamically accessible state spaces where multiple dimensions can interact and evolve together. This roadmap brings together perspectives ranging from modeling and design concepts to experimental platforms, materials, components, and system-level implementations. It covers a wide spectrum of synthetic and structured dimensions, nonlinear and strong-field regimes, adaptive and reconfigurable architectures, cyber-physical and engineering approaches, as well as inherently high-dimensional quantum and excitonic systems. Across all these areas, higher-dimensional thinking emerges not as an abstract construct but as a practical tool for enabling new functionalities, overcoming conventional design limitations, and bridging physical systems with digital and AI-driven layers. By framing these diverse developments within a shared higher-dimensional perspective, the roadmap aims to provide orientation in a rapidly expanding field, reveal conceptual connections between traditionally separate areas of photonics, and highlight common challenges and opportunities. In doing so, it positions higher-dimensional photonics as a central paradigm for developing future photonic technologies that are increasingly adaptive, intelligent, and integrated across physical and virtual domains.

We investigated the efficiency limitations of second-harmonic generation (SHG) in spliced optically poled Corning HI980 fiber segments. Although theory predicts quadratic growth with segment number, experiments show nearly linear efficiency scaling. Using a continuous wave model (CW), we demonstrate this subquadratic behavior primarily stems from random longitudinal shifts between quasi-phase-matching (QPM) regions in spliced segments. Further investigations through coupled generalized nonlinear Schrodinger equations (coupled GNLSEs) confirm fundamental frequency (FF) power depletion through Raman scattering and spectral broadening during propagation. Our numerical simulations successfully reproduced experimental spectral measurements, validating the model's accuracy. For the first time, we report the effective quadratic nonlinear coefficient induced by optical poling in this fiber d(eff) = 9 x 10-(4) pm/

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.

Machine learning is bringing revolutionary approaches into many fields of physics. Among those, photonics enables fast and scalable information processing. Photonics platforms further possess rich nonlinear dynamics that drive fundamental interest but also prove powerful for applications in computation, imaging, frequency conversion, source development and advanced signal processing. However, incoherent processes of nonlinear optics are hardly exploited in practice as the control of noise-driven dynamics remains challenging. Here, we exploit deep learning strategies and demonstrate that coherent optical seeding can effectively shape incoherent spectral broadening. We focus on the intricate interplay between weak coherent pulses and broadband noise, competing during nonlinear fibre propagation within an amplification process known as modulation instability. We demonstrate artificial neural networks' capability to efficiently predict these complex incoherent dynamics, both numerically and experimentally. Our results show that input seed properties can be inferred from the incoherent output signal. Furthermore, our approach enables reliable prediction of output spectral fluctuations, paving the way to tailoring complex photonic signals with specific correlation features.

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.

The propagation of light in disordered media is a rich and emerging topic of research [1], recently reinvigorated by the discovery of the regime of branched flow at light wavelengths [2]. The interest in this topic can be twofold: for fundamental physics, owing to the ubiquitous nature of branching for waves, and for conceiving new technologies for sensing applications. From the seminal works of Refs. [2], [3], light branching is a particular regime that appears before the better-known diffusion regime: to observe branching the refractive index spatial variation should occur over a scale much longer that the wavelength while the refractive index variation should be weak enough to induce small scattering. Such phenomenon in optics has been observed first in a thin soap membrane either with coherent [2] and incoherent light [3]. A recent numerical analysis shows the occurrence of light branching in thick media or multimode slab waveguides [4] and how wavefront shaping can be used to steer and control the branches. More recently, electrical tunability of light branches in liquid crystals has been also demonstrated experimentally [5].

Seeded optical poling is a promising and simple method to write an effective quadratic coefficient in silica optical fibers doped with germanium. We considered commercial solid core fibers since these fibers could allow for a wide adoption of optical poling. For each fiber, we optimized the poling process and analyzed the second harmonic conversion efficiencies in relation with the fiber chemical composition as obtained resorting to energy-dispersive X-ray spectroscopy. Our comprehensive investigation shows that segments of fibers spanning 20 cm, featuring a germanium content of around 6%, can effectively convert 30 mW of 1064 nm light with efficiencies hovering around 1%.

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 experimentally reveal a so-far unnoticed high-power limit to spatial beam self-cleaning in graded-index nonlinear multimode optical fibers. As the optical pulse power is progressively increased, we observed that the initial Kerr-induced beam clean-up and associated improvement of the spatial beam quality are eventually spoiled. Based on a holographic mode decomposition of the output field, we show that this beam degradation can be described in a thermodynamic approach to wave propagation as a manifestation of "high-temperature" thermalization, which depletes the fundamental mode in favor of a highly multimodal power distribution.

We review recent works in optical signal shaping and advanced characterization techniques within the framework of nonlinear fiber propagation. Specifically, we focus on the development of characterization methods based on the dispersive Fourier transform to monitor incoherent spectral broadening processes with enhanced resolution and sensitivity. In this framework, we also discuss recent studies of modulation instability in a noise-driven regime. Paired with suitable optical monitoring techniques, we show that controlled coherent optical seeding can be leveraged via several machine learning approaches to tailor and optimize incoherent spectral broadening dynamics.

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.

We numerically study the nonlinear propagation in multimode GRIN fibers. By exploiting the time-reversal symmetry, we identify the conditions for robust on-demand modal distributions. We extend our study to the reversal of spatial beam self-cleaning.