Yoshitate Takakura

Properties of a three-dimensional photonic jet

By focusing light with a sphere several wavelengths in diameter, we can obtain a photonic nanojet [Opt. Express 13, 526 (2005)]: if light is focused on the surface of the sphere, the width of the beam stays smaller than the wavelength along a distance of propagation of approximately two wavelengths and reaches a high intensity. We use the rigorous Mie theory to analyze the basic properties of the photonic jet in the general three-dimensional polarized case. This fast algorithm allows us to determine the influence of the radius and the refractive index of the sphere on the photonic jet. The polarization response is also studied. We observe that high-intensity concentrations and subwavelength focusing are two different effects. Their basic properties are analyzed, and explanations are proposed.

Target detection with a liquid crystal-based passive Stokes polarimeter

We present an imaging system that measures the polarimetric state of the light coming from each point of a scene. This system, which determines the four components of the Stokes vector at each spatial location, is based on a liquid-crystal polarization modulator, which makes it possible to acquire four-dimensional Stokes parameter images at a standard video rate. We show that using such polarimetric images instead of simple intensity images can improve target detection and segmentation performance.

Clustering of Mueller matrix images for skeletonized structure detection

This paper extends and refines previous work on clustering of polarization-encoded images. The polarization-encoded images used in this work are considered as multidimensional parametric images where a clustering scheme based on Markovian Bayesian inference is applied. Hidden Markov Chains Model (HMCM) and Hidden Hierarchical Markovian Model (HHMM) show to handle effectively Mueller images and give very good results for biological tissues (vegetal leaves). Pretreatments attempting to reduce the image dimensionality based on the Principal Component Analysis (PCA) turns out to be useless for Mueller matrix images.

Clustering of polarization-encoded images

Polarization-encoded imaging consists of the distributed measurements of polarization parameters for each pixel of an image. We address clustering of multidimensional polarization-encoded images. The spatial coherence of polarization information is considered. Two methods of analysis are proposed: polarization contrast enhancement and a more-sophisticated image-processing algorithm based on a Markovian model. The proposed algorithms are applied and validated with two different Mueller images acquired by a fully polarimetric imaging system.

Error analysis for rotating active Stokes-Mueller imaging polarimeters

Retrieval of Mueller matrices from intensity measurements is a noise-sensitive process. In addition, the choice of the method used for extracting Mueller matrix elements greatly influences the precision of the final results. Among available procedures, three have been tested and their robustness analyzed by adding Gaussian noise to computer synthesized data and have been verified by comparing experimental data. As expected, the methods based on classical matrix inversion reach their noise reduction limit even if more data are considered. In contrast, the discrete Fourier transform approach features a remarkable stability. The identified reason is that the retrieval process corresponds to filtering with a periodic kernel.

Using polarimetric imaging for material classification

In this paper we are mainly interested in the classification of images into different clusters, each corresponding to an object material in the imaged scene. For that purpose, the use of a polarimetric-based imaging system is proposed. This system has been developed in order to enlighten the huge potential of the information contained in polarization encoded images. This paper reports the experimental apparatus and its application to the classification process.

Estimation of physically realizable Mueller matrices from experiments using global constrained optimization

One can explicitly retrieve physically realizable Mueller matrices from quantified intensity data even in the presence of noise. This is done by integrating the physical realizability criterion obtained by Givens and Kostinski, [J. Mod. Opt. 40, 471 (1993)], as an active constraint in a global optimization process. Among different global optimization techniques, two of them have been tested and their robustness analyzed: a deterministic approach based on sequential quadratic programming and a stochastic approach based on constrained simulated annealing algorithms are implemented for this purpose. We illustrate the validity of both methods on experimental data and on the inadmissible Mueller matrix given by Howell, [Appl. Opt. 18, No. 6, 808-812 (1979)]. In comparison, the constrained simulated annealing method produced higher accuracy with similar computing time.

Improving Segmentation Maps using Polarization Imaging

Within the frame of polarimetric imagery, segmentation of 4 times 4 Mueller images consists in isolating objects that have different polarizing properties. Such objects are either partial polarizers, rotators or phasors. This means that there are 3 main polarization classes to consider. The difficulty in polarimetric segmentation comes from the fact that the relations between each of the mentioned class and the 4 times 4 elements of a Mueller matrix are not completely identified. Rather than dealing with unidentified quantities, Mueller images are transformed into intensity images so that robust classical segmentation procedures such as hidden Markov chain (HMC) can be applied. Such transformation is possible because it is the reversion procedure of the Mueller matrices retrieval procedure. Also, it is worth mentioning that the noise in the intensity images can be inferred so that the approach is mathematically rigorous. When applied to simulated or recorded images, it appears that the method outperforms approaches based on direct segmentation of Mueller images.

Very high angular selectivity system for measuring backscatter from rough surfaces

After recalling rough surfaces modelling and theoretical study of light scattering from such surfaces, we present in this paper an experimental set-up to measure backscattered light with a very high angular selectivity. The device is then tested for various samples such as white diffusers, ground metallic surfaces and eventually silver mirrors. When the surface is diffusive, very accurate results can be easily obtained. For surfaces with a specular peak, stray scattering from the surrounding can make the measurement very difficult. However, even in that extreme case, we have been able to investigate into the inverse problem thanks to the angular selectivity and have found quite a reasonable value for the correlation length.

A model to describe light scattering from material made of sensitized proteins

The interest for organic molecules in photonics and their use in the conception of new photonic devices, makes necessary a better understanding of their interactions with light. In particular, the modelling of these interactions is complex for biological molecules like proteins or polypeptides. The recent work of a team in our laboratory, using sensitized proteins to carry out holographic memories, has shown this interest of biological materials and the necessity to model their interaction with light. We present a theoretical electromagnetic model for such molecules. The real structure of the molecule is replaced by a theoretical aggregate of sub-wavelength spheres. A T-matrix algorithm is used to calculated the field scattered by these aggregates. The principle of this algorithm is explained. Their advantages and limitations are compared with other rigorous numerical methods used to study electromagnetism problems. The reasons, why we can model a molecule with a theoretical aggregate of spheres, are explained. The choices of the optical parameters of these aggregates are discussed. Some electromagnetic simulations of simple cases are presented in order to illustrate these choices. The experimental validation has to be done.