Achromatization of conical diffraction: application to the generation of a polychromatic optical vortex.

Vortex beams are plagued by the intrinsic chromaticity of the physical phenomenon used to generate them. To the authors' best knowledge, attempts to generate them in a broad spectral range remain quite scarce and limited in their results. Crystal optics and especially conical diffraction (CD) (or refraction) intrinsically create achromatic vortices. The vortex is created by a wavelength-independent topological charge, embedded directly in the Fresnel equations. However, for biaxial crystals of low crystallographic symmetry, which includes all crystals used practically for CD, the dispersion of the binormal axis creates a chromaticity effect. In this Letter, we propose a new way to compensate this dispersion of the binormal axis of a biaxial crystal in order to generate white-light vortex beams by CD in a 250 nm spectral range, covering almost all the visible range. The advantages of the ability to use CD in a wide spectral range vastly exceed the sole generation of vortex beams.

Focusing by a high numerical aperture lens of distributions generated by conical diffraction.

This Letter examines the diffraction of a vortex beam by a high numerical aperture of topological charge ±1 generated by conical diffraction. Our research shows the behavior of the vortex beam is similar to the one already observed for Laguerre-Gauss and Bessel beams. We also highlight a similarity between the phase singularity created by a lens and the one created by conical diffraction through a thin crystal. More precisely, if the input beam is homogeneously polarized, we show the electric field in the image plane of a thin crystal caught between two orthogonal polarizers has the same expression as E(z), the component of the electric field along the propagation axis, in the focal plane of a lens.

Conical diffraction illumination opens the way for low phototoxicity super-resolution imaging.

We present a new technology for super-resolution fluorescence imaging, based on conical diffraction. Conical diffraction is a linear, singular phenomenon, taking place when a laser beam is diffracted through a biaxial crystal. We use conical diffraction in a thin biaxial crystal to generate illumination patterns that are more compact than the classical Gaussian beam, and use them to generate a super-resolution imaging modality. While there already exist several super-resolution modalities, our technology (biaxial super-resolution: BSR) is distinguished by the unique combination of several performance features. Using BSR super-resolution data are achieved using low light illumination significantly less than required for classical confocal imaging, which makes BSR ideal for live-cell, long-term time-lapse super-resolution imaging. Furthermore, no specific sample preparation is required, and any fluorophore can be used. Perhaps most exciting, improved resolution BSR-imaging resolution enhancement can be achieved with any type of objective no matter the magnification, numerical aperture, working distance, or the absence or presence of immersion medium. In this article, we present the first implementation of BSR modality on a commercial confocal microscope. We acquire and analyze validation data, showing high quality super-resolved images of biological objects, and demonstrate the wide applicability of the technology. We report live-cell super-resolution imaging over a long period, and show that the light dose required for super-resolution imaging is far below the threshold likely to generate phototoxicity.

Polarimetric imaging of uterine cervix: a case study.

We present a preliminary investigation of macroscopic polarimetric imaging of uterine cervix. Orthogonal state contrast (OSC) images of healthy and anomalous cervices have been taken in vivo at 550 nm. Four ex vivo cervix samples have been studied in full Muller polarimetry, at 550 nm and 700 nm, and characterized in detail by standard pathology. One sample was totally healthy, another one carried CIN lesions at very early stage (CIN1) in its visible exocervical region, while for the other two samples more advanced (CIN3) lesions were present, together with visible glandular epithelium (ectropion). Significant birefringence has been observed in the healthy regions of all six samples, both in vivo and ex vivo. Standard treatments of the Mueller images of the ex vivo samples allowed to quantify both retardation and depolarization. Retardation reached 60° in healthy regions, and disappeared in the anomalous regions of the other three ex vivo samples. The depolarization power was largest in healthy regions, and lower in CINs and ectropion. Possible origins of the observed effects are briefly discussed.

Multispectral Mueller polarimetric imaging detecting residual cancer and cancer regression after neoadjuvant treatment for colorectal carcinomas.

This work is devoted to a first exploration of Mueller polarimetric imaging for the detection of residual cancer after neoadjuvant treatment for the rectum. Three samples of colorectal carcinomas treated by radiochemotherapy together with one untreated sample are analyzed ex vivo before fixation in formalin by using a multispectral Mueller polarimetric imaging system operated from 500 to 700 nm. The Mueller images, analyzed using the Lu-Chipmann decomposition, show negligible diattenuation and retardation. The nonirradiated rectum exhibits a variation of depolarization with cancer evolution stage. At all wavelengths on irradiated samples, the contrast between the footprint of the initial tumor and surrounding healthy tissue is found to be much smaller for complete tumor regression than when a residual tumor is present, even at volume fractions of the order of 5%. This high sensitivity is attributed to the modification of stromal collagen induced by the cancer. The depolarization contrast between treated cancer and healthy tissue is found to increase monotonously with the volume fraction of residual cancer in the red part of the spectrum. Polarimetric imaging is a promising technique for detecting short-time small residual cancers, which is valuable information for pathological diagnosis and patient management by clinicians.

Ex vivo photometric and polarimetric multilayer characterization of human healthy colon by multispectral Mueller imaging.

Healthy human colon samples were analyzed ex vivo with a multispectral imaging Mueller polarimeter operating from 500 to 700 nm in a backscattering configuration with diffuse light illumination impinging on the innermost tissue layer, the mucosa. The intensity and polarimetric responses were taken on whole tissues first and after progressive exfoliation of the outer layers afterwards. Moreover, these measurements were carried out with two different substrates (one bright and the other dark) successively placed beneath each sample, allowing a reasonably accurate evaluation of the contributions to the overall backscattered light by the various layers. For the shorter investigated wavelengths (500 to 550 nm) the major contribution comes from mucosa and submucosa, while for the longer wavelengths (650 to 700 nm) muscular tissue and fat also contribute significantly. The depolarization has also been studied and is found to be stronger in the red part of the spectrum, mainly due to the highly depolarizing power of the muscular and fat layers.

Experimental validation of the symmetric decomposition of Mueller matrices.

We experimentally assess the validity of the symmetric decomposition of Mueller matrices [R. Ossikovski, J. Opt. Soc. Am A 26, 1109-1118 (2009)] into a sequence of five factors consisting of a diagonal depolarizer between two retarder and diattenuator pairs. The raw data were Mueller images of combinations of polarization components which were individually measured and then assembled in different combinations. The possibility to recover all the elements is discussed, including the experimentally relevant cases of "degenerate" depolarizers, with two equal eigenvalues, which were not explicitly considered in the general theory.

Experimental evidence for naturally occurring nondiagonal depolarizers.

We report on two Stokes nondiagonalizable Mueller matrices experimentally observed in a biological and in an organic sample. These matrices are examples of naturally occurring nondiagonal depolarizers whose unique property is to preserve the degree of polarization of all but one totally polarized light state. The description of the experimental matrices within the theory of Bragg scattering on cholesteric liquid crystals, as well as their interpretation in physical and structural terms, are likewise addressed.

Experimental implementation and properties of Stokes nondiagonalizable depolarizing Mueller matrices.

We report on the experimental realization of a family of depolarizing Mueller matrices that are Stokes nondiagonalizable. We likewise demonstrate a unique characteristic property of Stokes nondiagonalizable matrices consisting in the preservation of the degree of polarization of a single totally polarized input and illustrate it on our experimental example.