Probing optical properties of nanomaterials
Light scattering due to angular reflectance is one of the most commonly measured and modeled optical phenomena, relevant in fields ranging from materials science and medical imaging to 3D computer graphics and animation. Analytical methods that make use of the phenomenon include remote sensing, target detection, and radiance signature evaluation for passive or active sensors. Modeling reflectance phenomena requires the formulation of a hyperspectral polarized BRDF (bidirectional reflectance distribution function) and an understanding of its importance in surface optical phenomena. The BRDF quantifies how light is angularly and spectrally reflected by a material illuminated from different directions (see Figure 1). It is a unique optical signature of a material and one of the crucial functions needed to model radiative transfer, laser imaging, medical imaging, and remote sensors.
We are interested in applying BRDFs to probe the optical properties of nanomaterials. These materials have attracted much attention in recent years because of their unique chemical, physical, and optical properties. This uniqueness results from the high surface-area-to-volume ratio (SA:V) of nanomaterials: the ratio of atoms at the surface to atoms within the volume. This is close to zero for bulk materials but increases significantly for nanomaterials, which are nearly all surface. A number of systems are used to measure the optical properties of these materials, including both near-field (e.g., scanning near-field optical microscopy, scanning tunneling microscopy) and far-field techniques (e.g., spectroscopy, imaging systems). Here we present an original analytical technique based on the study of hyperspectral polarized light scattering. It can be used to probe far-field optical properties of materials, especially nanomaterials that exhibit high SA:V.
At Onera: The French Aerospace Lab, the Optonics Department develops and manages optical systems from the source to post-processing, in a range of wavelengths from UV to IR. Our Melopee Lab (see Figure 2) is fully dedicated to characterizing materials and studying their optical properties. We have developed a new supercontinuum laser-based instrument to measure hyperspectral polarized optical signatures. It was initially designed to study dense media and multiple light scattering.1 This original, fast, in-line, non-contact, and highly resolved instrument provides specific angular, polarized (unpolarized or linearly polarized), and hyperspectral (from 480 to 2000nm) BRDFs resulting from complex surface light scattering.
Supercontinuum laser sources have been a growing interest for several applications, such as optical coherence tomography, frequency metrology, fluorescence lifetime imaging, optical communications, and gas sensing. Supercontinuum generation is a process where laser light (emitting at a fixed wavelength) is converted to light with a very broad spectral bandwidth (hyperspectral aspect). The spectral broadening is usually accomplished by propagating optical pulses through a strongly nonlinear device, such as an optical fiber. Recent advances2, 3 in nanostructured fiber optics and compact pulsed lasers have spurred the development of these hyperspectral coherent directional light sources, which achieve a broad white-light spectrum. Our instrument is based on this type of lighting source, which we combined with spectrophotometers. Previously, BRDFs were measured at different wavelengths using a number of single-wavelength sources, whereas our instrument allows simultaneous measurement over a range of wavelengths (hyperspectral BRDF measurements). In hyperspectral remote sensing, measurements are stored in a 3D hypercube that contains spatial and spectral information. We have extended this technique for hyperspectral polarized reflectance measurements. Hyperspectral calibration and corrections are known to be quite complex, involving a huge amount of data due to the great number of spectral bands. The calibration process we used (previously reported4) results in an averaged measurement error less than 2.5%.
Nanomaterials can be produced by a myriad of chemical and physical routes. Hyperspectral polarized BRDF provides a comprehensive analytical tool for non-destructive in-line investigations in nanomaterials production. Measurements have been carried out in suspension on nanoparticles made from a variety of materials, including silica, organic polymers such as latex, semiconductors including zinc oxide, and gold and silver nanoparticles. The volume fraction of nanomaterials in suspension ranges from 0.05 to 5%. Figure 3 shows results of our technique for monodisperse latex and polydisperse silicon dioxide nanoparticles.
To complement these measurements,5 we developed numerical tools based on stochastic or radiative transfer approaches. We simulated light scattering in various media—from dense suspensions to coatings—as detected by sensors taking into account the experimental protocol. Then, we compared these results with our own experimental data to determine the value of an objective function. This function is minimized by an iterative process, modifying numerical inputs such as the particle size distribution, the geometry, the concentration, and the optical index of scattering.
Our new instrument is dedicated to measuring the hyperspectral polarized angular signature of various materials, from liquid to solid samples. Measurements are carried out on bulk materials in addition to nanomaterials. We consider the hyperspectral polarized BRDF a powerful new way to study the optical properties of nanomaterials. In addition to finding new ways to optimize our calibration protocol, we are also introducing new physical models in our numerical models to reduce the error in determining a nanoparticle's optical properties. We are currently evaluating new materials to measure and simulate unexpected optical properties at the nanoscale.
Part of this work was supported by the Région Midi-Pyrénées. The authors wish to thank M.-L. de Solan from the Chemical Engineering Laboratory of Toulouse for helpful discussions and contributions.
Romain Ceolato graduated in physical engineering from the National Institute of Applied Sciences, Toulouse, France (2010). He subsequently worked in Masakazu Aono's group at the National Institute for Materials Science, Japan, on the fabrication of semiconductor nanolasers. He is currently a PhD candidate in applied optics. His research project is the optical diagnosis of dense scattering media for laser imaging systems and nanomaterials.
Nicolas Rivière is a research scientist in the Light Interaction with Matter, Imaging, and Detection Laser Systems Unit at Onera. He received his PhD in 2006 on polarized-BRDF data inversion to retrieve radiative parameters of dense media. He is involved in the development of direct codes and inverse techniques to determine the optical properties of scattering media (including radiative transfer modeling and simulation of speckle). He contributes to the development of new laser imaging systems and experimental setups dedicated to the optical diagnosis of scattering media.
Laurent Hespel received his PhD in 1999, submitting a thesis on experimental determination of radiative properties under high charge rate conditions. He subsequently joined Onera first as a postdoctoral fellow and then as a research scientist. He currently heads the Light Interaction with Matter, Imaging, and Detection Laser Systems Unit.
Béatrice Biscans obtained her degree in chemical engineering from the National Polytechnical Institute of Toulouse (1982) and was awarded her PhD in chemical and process engineering in 1985. She now works for CNRS, and since 1987 has been based at LGC. Her research focuses on solid and particulate processes, specifically crystallization and formulation processes, and nanoparticle production. She is vice president of the Working Party on Crystallization for the European Federation of Chemical Engineering, and also of the Working Party on Powders and Particles for the French Federation of Chemical Engineering. She has an official position in CNRS as the expert representative for chemical engineering in France.
