37 research outputs found
Imaging light transport at the femtosecond scale
Paper, milk, clouds and white paint share a common property: they are opaque disordered media through which light scatters randomly rather than propagating in a straight path. For very thick and turbid media, indeed, light eventually propagates in a âdiffusiveâ way, i.e. similarly to how tea infuses through hot water. Frequently though, a material is neither perfectly opaque nor transparent and the simple diffusion model does not hold. In this work, we developed a novel optical-gating setup that allowed us to observe light transport in scattering media with sub-ps time resolution. An array of unexplored aspects of light propagation emerged from this spatio-temporal description, unveiling transport regimes that were previously inaccessibile due to the extreme time scales involved and the lack of analytical models
Imaging light transport at the femtosecond scale
Paper, milk, clouds and white paint share a common property: they are opaque disordered media through which light scatters randomly rather than propagating in a straight path. For very thick and turbid media, indeed, light eventually propagates in a âdiffusiveâ way, i.e. similarly to how tea infuses through hot water. Frequently though, a material is neither perfectly opaque nor transparent and the simple diffusion model does not hold. In this work, we developed a novel optical-gating setup that allowed us to observe light transport in scattering media with sub-ps time resolution. An array of unexplored aspects of light propagation emerged from this spatio-temporal description, unveiling transport regimes that were previously inaccessibile due to the extreme time scales involved and the lack of analytical models
Optimized White Reflectance in Photonic Network Structures
Three-dimensional disordered networks are receiving increasing attention as
versatile architectures for highly scattering materials. However, due to their
complex morphology, little is still known about the interplay between their
structural and optical properties. Here, we describe a simple algorithm that
allows to generate photonic network structures inspired by that of the
Cyphochilus beetle, famous for the bright white reflectance of its thin
cuticular scales. The model allows to vary the degree of structural anisotropy
and filling fraction of the network independently, revealing the key
contribution of these two parameters to the overall scattering efficiency.
Rigorous numerical simulations show that the obtained structures can exceed the
broadband reflectance of the beetle while using less material, providing new
insights for the design of advanced scattering materials.Comment: 10 pages, 3 figures. peer reviewed version, published in final form
at https://doi.org/10.1002/adom.20190004
Experimental imaging and Monte Carlo modeling of ultrafast pulse propagation in thin scattering slabs
Significance: Most radiative transport problems in turbid media are typically associated with mm or cm scales, leading to typical time scales in the range of hundreds of ps or more. In certain cases, however, much thinner layers can also be relevant, which can dramatically alter the overall transport properties of a scattering medium. Studying scattering in these thin layers requires ultrafast detection techniques and adaptations to the common Monte Carlo (MC) approach.Aim: We aim to discuss a few relevant aspects for the simulation of light transport in thin scattering membranes, and compare the obtained numerical results with experimental measurements based on an all-optical gating technique.Approach: A thin membrane with controlled scattering properties based on polymer-dispersed TiO2 nanoparticles is fabricated for experimental validation. Transmittance measurements are compared against a custom open-source MC implementation including specific pulse profiles for tightly focused femtosecond laser pulses.Results: Experimental transmittance data of ultrafast pulses through a thin scattering sample are compared with MC simulations in the spatiotemporal domain to retrieve its scattering properties. The results show good agreement also at short distances and time scales.Conclusions: When simulating light transport in scattering membranes with thicknesses in the orders of tens of micrometer, care has to be taken when describing the temporal, spatial, and divergence profiles of the source term, as well as the possible truncation of step length distributions, which could be introduced by simple strategies for the generation of random exponentially distributed variables. (C) The Authors. Published by SPIE under a Creative Commons Attribution 4.0 International License
Diffusion of light in semitransparent media
Light diffusion is usually associated with thick, opaque media. Indeed,
multiple scattering is necessary for the onset of the diffusive regime and such
condition is generally not met in almost transparent media. Nonetheless, at
long enough times, transport in an infinite thin slab is still determined by a
multiple scattering process whose complete characterization is lacking. In this
paper we show that, after a short transient, the mean square width of the
transmitted intensity still exhibits a simple linear increase with time as
predicted by diffusion theory, even at optical thickness as low as one.
Interestingly, such linear growth is predicted not to depend neither on the
slab thickness nor on its refractive index contrast, yet the accuracy of this
simple approximation in the ballistic-to-diffusive regime hasn't been
investigated so far. By means of Monte Carlo simulations, we find clear
evidence that boundary conditions play an active role in redefining the very
asymptotic value of the diffusion coefficient by directly modifying the
statistical distributions underlying light transport in such media. In this
respect, we demonstrate the need to distinguish between a set of intrinsic and
effective transport parameters, whose relation and interplay with boundary
conditions remains to be fully understood
Deducing effective light transport parameters in optically thin systems
We present an extensive Monte Carlo study on light transport in optically
thin slabs, addressing both axial and transverse propagation. We completely
characterize the so-called ballistic-to-diffusive transition, notably in terms
of the spatial variance of the transmitted/reflected profile. We test the
validity of the prediction cast by diffusion theory, that the spatial variance
should grow independently of absorption and, to a first approximation, of the
sample thickness and refractive index contrast. Based on a large set of
simulated data, we build a freely available look-up table routine allowing
reliable and precise determination of the microscopic transport parameters
starting from robust observables which are independent of absolute intensity
measurements. We also present the Monte Carlo software package that was
developed for the purpose of this study
Role of packing density and spatial correlations in strongly scattering 3D systems
Discrete random media have been investigated extensively over the past century due to their ability to scatter light. Even so, the link between the three-dimensional (3D) spatial distribution of the scattering elements and the resulting opacity is still lively debated to date due to different experimental conditions, range of parameters explored, or sample formulations. On the other hand, a unified numerical survey with controlled parameters has been impractical up to date due to the sheer computational power required to address samples with representative size. In this work, we exploit a graphics processing unit implementation of the T-matrix method to investigate the complete range of particle volume concentration and packing-induced spatial correlations, allowing us to reveal and elucidate a twofold role played by spatial correlations in either enhancing or suppressing opacity. By applying these findings to the illustrative case of white paint, we determine the optimal combination of density and spatial correlations corresponding to the highest opacity. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreemen
Biomimetic Polymer Film with Brilliant Brightness Using a OneâStep Water VaporâInduced Phase Separation Method
The scales of the white Cyphochilus beetles are endowed with unusual whiteness arising from the exceptional scattering efficiency of their disordered ultrastructure optimized through millions of years of evolution. Here, a simple, oneâstep method based on water vaporâinduced phase separation is developed to prepare thin polystyrene films with similar microstructure and comparable optical performance. A typical biomimetic 3.5 ”m PS film exhibits a diffuse reflectance of 61% at 500 nm wavelength, which translates into a transport mean free path below 1 ”m. A complete optical characterization through Monte Carlo simulations reveals how such a scattering performance arises from the scattering coefficient and scattering anisotropy, whose interplay provides insight into the morphological properties of the material. The potential of brightâwhite coatings as smart sensors or wearable devices is highlighted using a treated 3.5 ”m film as a realâtime sensor for human exhalation
Remote Control of Liquid Crystal Elastomer Random Laser
We demonstrate random lasing in a composite system fabricated from elastomeric liquid crystals and TiO2 nanoparticles. By moving this random laser near the focal plane of the pump beam, we achieve control of its emission properties