Varying the effective refractive index to measure optical transport in random media

Nonlinear optical memory effect

Light propagating through random media produces characteristic speckle patterns, directly related to the large multitude of scattering events. These complex dynamics remarkably display robustness to perturbation of the incoming light parameters, maintaining correlation in the scattered wavefront. This behavior is known as the optical memory effect. Here we unveil the properties of the nonlinear optical memory effect, which occurs when an optothermal nonlinearity perturbs the random material. The effect is characterized through a series of pump and probe experiments in silica aerogel, in the visible range. This additional degree of freedom further generalizes the memory effect, opening the road to applications based on the nonlinear response of random media.

Dispersion relation for electromagnetic propagation in stochastic dielectric and magnetic helical photonic crystals

We theoretically study the dispersion relation for axially propagating electromagnetic waves throughout a one-dimensional helical structure whose pitch and dielectric and magnetic properties are spatial random functions with specific statistical characteristics. In the system of coordinates rotating with the helix, by using a matrix formalism, we write the set of differential equations that governs the expected value of the electromagnetic field amplitudes and we obtain the corresponding dispersion relation. We show that the dispersion relation depends strongly on the noise intensity introduced in the system and the autocorrelation length. When the autocorrelation length increases at fixed fluctuation and when the fluctuation augments at fixed autocorrelation length, the band gap widens and the attenuation coefficient of electromagnetic waves propagating in the random medium gets larger. By virtue of the degeneracy in the imaginary part of the eigenvalues associated with the propagating modes, the random medium acts as a filter for circularly polarized electromagnetic waves, in which only the propagating backward circularly polarized wave can propagate with no attenuation. Our results are valid for any kind of dielectric and magnetic structures which possess a helical-like symmetry such as cholesteric and chiral smectic-C liquid crystals, structurally chiral materials, and stressed cholesteric elastomers.

Deterministic control of broadband light through a multiply scattering medium via the multispectral transmission matrix

We present a method to measure the spectrally-resolved transmission matrix of a multiply scattering medium, thus allowing for the deterministic spatiospectral control of a broadband light source by means of wavefront shaping. As a demonstration, we show how the medium can be used to selectively focus one or many spectral components of a femtosecond pulse, and how it can be turned into a controllable dispersive optical element to spatially separate different spectral components to arbitrary positions.

Rayleigh scattering of surface plasmons by sub-wavelength holes

We study the scattering of surface plasmons from sub-wavelength holes and find that it exhibits a stronger wavelength dependence than the traditional λ(-4) scaling found for Rayleigh scattering of light from small particles. This experimental observation is consistent with recent theoretical work and linked to the two-dimensional nature of the surface plasmon and the wavelength dependence of its spatial extent in the third dimension. The scattering cross sections are obtained with a frequency-correlation technique, which compares intensity speckle patterns observed behind various random structures of holes and recorded at different wavelengths. This powerful technique even allows us to distinguish between scattering of surface plasmons into photons and scattering into other surface plasmons.

Empirical model of total internal reflection from highly turbid media

We demonstrate, to the best of our knowledge, a first accurate empirical model for reflectance measurements from highly turbid media over the full range of incident angles, i.e., for reflectivity values going from unity in the total internal reflection regime to nearly zero when almost all the light is transmitted. Evidence that our model is accurate is provided by extraction of the particle size, followed by independent verification with dynamic light scattering. Our methodology is in direct contrast with the prevalent approach in turbid media of focusing on only the critical angle region, which is just a small subset of the entire reflectance data.

Observation of anisotropic diffusion of light in compacted granular porous materials

By analyzing spatio-temporal characteristics of short optical pulses diffusively transmitted through compacted granular materials, we reveal that powder compaction can give rise to strongly anisotropic light diffusion. Our disclosure represents a revision of the understanding of optics of powder compacts. Routes to material characterization and investigation of compression-induced structural anisotropy are opened, and the falsification of isotropic models have implications for quantitative spectroscopy of powder compacts (e.g., pharmaceutical tablets).

Disordered, strongly scattering porous materials as miniature multipass gas cells

We investigate the interaction of light and gas in strongly scattering nano- and macroporous media. Manufacturing and structural characterization of ZrO(2), Al(2)O(3) and TiO(2) ceramics with different pore sizes, measurements of optical properties using photon time-of-flight spectroscopy, and high-resolution laser spectroscopy of O(2) at 760 nm are reported. We show that extreme light scattering can be utilized to realize miniature spectroscopic gas cells. Path length enhancement factors up to 750 are reached (5.4 m path through gas for light transmitted through a 7 mm ZrO(2) with 49% porosity and 115 nm pores).

Direct determination of diffusion properties of random media from speckle contrast

We present a simple scheme to determine the diffusion properties of a thin slab of strongly scattering material by measuring the speckle contrast resulting from the transmission of a femtosecond pulse with controlled bandwidth. In contrast with previous methods, our scheme does not require time measurements nor interferometry. It is well adapted to the characterization of samples for pulse shaping, nonlinear excitation through scattering media, and biological imaging.

Ultrafast dephasing of light in strongly scattering GaP nanowires

We demonstrate ultrafast dephasing in the random transport of light through a layer consisting of strongly scattering GaP nanowires. Dephasing results in a nonlinear intensity modulation of individual pseudomodes which is 100 times larger than that of bulk GaP. Different contributions to the nonlinear response are separated by using total transmission, white-light frequency correlation, and statistical pseudomode analysis. A dephasing time of 1.2±0.2  ps is found. Quantitative agreement is obtained with numerical model calculations which include photoinduced absorption and deformation of individual scatterers. Nonlinear dephasing of photonic eigenmodes opens up avenues for ultrafast control of random lasers, nanophotonic switches, and photon localization.