The temperature distribution throughout arrays of illuminated metal nanoparticles is investigated numerically and experimentally. The two cases of continuous and femtosecond-pulsed illumination are addressed. In the case of continuous illumination, two distinct regimes are evidenced: a temperature confinement regime, where the temperature increase remains confined at the vicinity of each nanosource of heat, and a temperature delocalization regime, where the temperature is uniform throughout the whole nanoparticle assembly despite the heat sources' nanometric size. We show that the occurrence of one regime or another simply depends on the geometry of the nanoparticle distribution. In particular, we derived (i) simple expressions of dimensionless parameters aimed at predicting the degree of temperature confinement and (ii) analytical expressions aimed at estimating the actual temperature increase at the center of an assembly of nanoparticles under illumination, preventing heavy numerical simulations. All these theoretical results are supported by experimental measurements of the temperature distribution on regular arrays of gold nanoparticles under illumination. In the case of femtosecond-pulsed illumination, we explain the two conditions that must be fulfilled to observe a further enhanced temperature spatial confinement.

In recent decades, optogenetics has been transforming neuroscience research, enabling neuroscientists to drive and read neural circuits. The recent development in illumination approaches combined with two-photon (2P) excitation, either sequential or parallel, has opened the route for brain circuit manipulation with single-cell resolution and millisecond temporal precision. Yet, the high excitation power required for multi-target photostimulation, especially under 2P illumination, raises questions about the induced local heating inside samples. Here, we present and experimentally validate a theoretical model that makes it possible to simulate 3D light propagation and heat diffusion in optically scattering samples at high spatial and temporal resolution under the illumination configurations most commonly used to perform 2P optogenetics: single- and multi-spot holographic illumination and spiral laser scanning. By investigating the effects of photostimulation repetition rate, spot spacing, and illumination dependence of heat diffusion, we found conditions that make it possible to design a multi-target 2P optogenetics experiment with minimal sample heating.

We demonstrate stimulated Raman microscopy with broadband pump and Stokes pulses, using spectral focusing to attain spectral resolution and to rapidly acquire spectra within a spectral window determined by the bandwidth of the pulses. As the Stokes pulse, we use the redshifted soliton generated in a photonic crystal fiber, which allows for simple shifting of the accessible spectral window.

Optical metasurfaces have shown to be a powerful approach to planar optical elements, enabling an unprecedented control over light phase and amplitude. At that stage, where wide variety of static functionalities have been accomplished, most efforts are being directed towards achieving reconfigurable optical elements. Here, we present our approach to an electrically controlled varifocal metalens operating in the visible frequency range. It relies on dynamically controlling the refractive index environment of a silicon metalens by means of an electric resistor embedded into a thermo-optical polymer. We demonstrate precise and continuous tuneability of the focal length and achieve focal length variation larger than the Rayleigh length for voltage as small as 12 volts. The system time-response is of the order of 100 ms, with the potential to be reduced with further integration. Finally, the imaging capability of our varifocal metalens is successfully validated in an optical microscopy setting. Compared to conventional bulky reconfigurable lenses, the presented technology is a lightweight and compact solution, offering new opportunities for miniaturized smart imaging devices.

We propose a three-color, double-modulation scheme for the background-free detection of stimulated Raman scattering (SRS). We call the scheme stimulated Raman gain and opposite loss detection (SRGOLD). It exploits the symmetric nature of potential parasitic signals (cross phase modulation, two-photon absorption, and thermal effects) to the end of suppressing them. Conversely, the antisymmetric nature of SRS provides for a twofold increase in the magnitude of the SRS signal. We experimentally demonstrate SRGOLD spectroscopy and microscopy on test samples as well as on mice skin samples.

The advent of spatial control over the phase and amplitude of light waves has profoundly transformed photonics, enabling major advances from imaging and information technology to biomedical optics. Here, we propose a novel approach to deterministic phase-front shaping through a planar thermo-optical module using designed microheaters to locally shape the refractive index distribution. When combined with a genetic algorithm optimisation, this SmartLens can produce free-form optical wavefront modifications. Individually or in arrays, it can generate complex functions based on either pure, or a combination of, Zernike polynomials, including lenses or aberration correctors of electrically-tuneable magnitude. This simple and compact concept complements the existing optical shaping toolbox by offering low chromatic aberrations, polarisation-insensitive and transmission-mode components which can be readily integrated into existing optical systems.

We propose and implement a broadband, compact, and low-cost wavefront sensing scheme by simply placing a thin diffuser in the close vicinity of a camera. The local wavefront gradient is determined from the local translation of the speckle pattern. The translation vector map is computed thanks to a fast diffeomorphic image registration algorithm and integrated to reconstruct the wavefront profile. The simple translation of speckle grains under local wavefront tip/tilt is ensured by the so-called "memory effect" of the diffuser. Quantitative wavefront measurements are experimentally demonstrated both for the few first Zernike polynomials and for phase-imaging applications requiring high resolution. We finally provided a theoretical description of the resolution limit that is supported experimentally.

Coherent Raman scattering processes such as coherent anti-Stokes Raman scattering and stimulated Raman scattering are described in a tutorial way keeping simple physical pictures and simple derivations. The simplicity of the presentation keeps however most of the key features of these coherent and resonant processes and their intimate relation with spontaneous Raman scattering. This tutorial provides a digest of introduction to the fundamental physics at work, and it does not focus on the numerous technological implementations; rather, it provides the concepts and the physical tools to understand the extensive literature in this field. The presentation is made simple enough for under-graduate students, graduate students, and newcomers with various scientific backgrounds.