François Amblard

Speckle metrology harnesses the interferometric properties of disordered light to achieve remarkable sensitivities. Often relying on time-domain analysis, it is rate-limited by the acquisition of speckle images. In the present work instead, we use a frequency-domain approach which spans 8 to 10 frequency decades up to 100 MHz, and reveals minute changes of speckle decorrelation spectra. We built a 3D stochastic interferometer using a centimeter-sized quartz-powder cavity with arbitrary shape and high Lambertian reflectivity. Filled with a coherent monochromatic photon gas, it creates statistically isotropic and homogeneous 3D interference patterns, whose variations arise from cavity deformations or fluctuations of the dielectric tensor field inside. Speckle decorrelation depends neither on where the perturbation sits nor on where it is measured. With an average 62 m photon transit path and a finesse of 10500, cavity deformations are detected with a power noise floor of 4 × 10⁻³ pm², i.e., 2.7 pm at 1 kHz. We also demonstrate a 100-fold sensitivity gain compared to conventional light scattering techniques when probing thermal motions of single and multiply scattering colloids.

Skin pigmentation depends on cellular processes including biogenesis, transport, maturation, and transfer of melanosomes to keratinocytes. Howeve | POSTER

Dynamic light scattering techniques can give access to the motion spectrum of microscopic objects and are therefore routinely used for numerous industrial and research applications ranging from particle sizing to the characterization of the viscoelastic properties of materials. However, such measurements are impossible when samples do not scatter light enough, i.e., when light undergoes too few scattering events when passing through a sample, either due to excessively small scattering cross sections or due to low concentrations of scatterers. Here, we propose to amplify the light scattering efficiency by placing weakly scattering samples inside a Lambertian cavity with high-reflectance walls. When injected with laser light, the cavity produces a 3D isotropic and homogeneous light field, effectively elongates the photon scattering path length through the sample by 2-3 orders of magnitude, and leads to a dramatic increase in sensitivity. With a 10(4)-fold increase in sensitivity compared to classical techniques, we potentially expand the applications of light scattering to miniaturized microfluidics samples and to weakly scattering samples in general. We show that we can access the short-time dynamics of low-turbidity samples and demonstrate our sensitivity gain by measuring the diffusion coefficient and, therefore, the size of particles ranging from 5 nm to 20 mu m with volume fractions as low as 10(-9) in volumes as low as 100 mu L and in solvents with refractive index mismatches down to delta(n) asymptotic to 0.01. Beyond the realm of current applications of light scattering techniques, our cavity-amplified scattering spectroscopy method (CASS) and its high sensitivity represent a significant methodological step toward the study of short time dynamics problems such as the ballistic limit of Brownian motion, the internal dynamics of proteins, or the dielectric dynamics of liquids.

Optical interferometry has proven extremely powerful to investigate some of the most fundamental laws of physics, using light with very precisely controlled geometry. In the past few decades, various speckle metrology methods have emerged to harness the interferometric properties of strongly disordered light instead, using time-domain analysis of speckle patterns at a maximum rate limited by the frequency of image acquisition. The present work is based on using a centimeter-sized quartz-powder cavity with an arbitrary shape that is engineered with very high Lambertian reflectivity. When filled with a coherent monochromatic photon gas, a statistically isotropic and homogeneous 3D speckle interference pattern is obtained. A single-mode fiber is then used in combination with photon number autocorrelation analysis to detect minute changes of the speckle decorrelation spectrum, caused either by cavity deformations or fluctuations of the dielectric tensor field inside. The decorrelation spectrum is acquired over 8 to 10 frequency decades below 100 MHz, with a sensitivity that is only limited by the intrinsic photon statistics and the extrinsic instrumental noises. With typically 1700 reflections and an average photon transit path length of 62m, our 3D stochastic interferometer yields a typical finesse of 10500, and cavity deformations are detected in an ergodic fashion over a six-decade dynamic range with a power noise floor of 4x{10}^{-3} pm^2 which corresponds to 2.7 pm at 1 kHz. The cavity also operates as a speckle fluctuation amplifier that reveals decorrelation spectra due to picometric thermal motions of colloids in both single and multiple scattering regimes with a typical 100-fold sensitivity gain compared to conventional light scattering techniques.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Skin pigmentation is dependent on cellular processes including melanosome biogenesis, transport, maturation and transfer to keratinocytes. However, how the cells finely control these processes in space and time to ensure proper pigmentation remains unclear. Here, we show that a component of the cytoplasmic dynein complex, Dynlt3, is required for efficient melanosome transport, acidity and transfer. In Mus musculus melanocytes with decreased levels of Dynlt3, pigmented melanosomes undergo a more directional motion, leading to their peripheral location in the cell. Stage IV melanosomes are more acidic, but still heavily pigmented, resulting in a less efficient melanosome transfer. Finally, the level of Dynlt3 is dependent on β-catenin activity, revealing a function of the Wnt/β-catenin signalling pathway during melanocyte and skin pigmentation, by coupling the transport, positioning and acidity of melanosomes required for their transfer.

Blackbody radiation is a fundamental phenomenon in nature, and its explanation by Planck marks a cornerstone in the history of Physics. In this theoretical work, we show that the spectral radiance given by Planck’s law is strongly superlinear with temperature, with an arbitrarily large local exponent for decreasing wavelengths. From that scaling analysis, we propose a new concept of super-resolved detection and imaging: if a focused beam of energy is scanned over an object that absorbs and linearly converts that energy into heat, a highly nonlinear thermal radiation response is generated, and its point spread function can be made arbitrarily smaller than the excitation beam focus. Based on a few practical scenarios, we propose to extend the notion of super-resolution beyond its current niche in microscopy to various kinds of excitation beams, a wide range of spatial scales, and a broader diversity of target objects.

A new kind of interferometry is presented, based on the maximal disorder of a single-frequency laser light coherently resonating inside a closed cavity with diffuse reflective walls. After thousands of Lambertian reflections, the optical field at any point inside the cavity is the coherent superposition of a huge number of modes that individually and independently obey a single probability distribution, which is invariant by rotations and translations regardless of the cavity geometry. The resulting interference speckle field has the same statistical properties and exhibits extreme sensitivity to phase fluctuations. From speckle intensity fluctuations, we can measure the various causes that perturb light propagation, be it deformations of the cavity, dynamic scattering of the propagation medium, or fluctuations of the input light phase. Preliminary experiments indicate that strains down to 10^-10 can be detected, that correspond to picometer cavity deformations. This concept of ”scrambled” interferometer opens new possibilities in the field of accelerometry or vibrometry, as well as for the enhancement of dynamic or multiple light scattering techniques.