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International audience; There is a constant need for direct counting of biotic nanoparticles such as viruses to unravel river functioning. We used, for the first time in freshwater, a new method based on interferometry differentiating viruses from other particles such as membrane vesicles. In the French Marne River, viruses represented between 42 and 72% of the particles. A spring monitoring in 2014 revealed their increase (2.1 × 107 to 2.1 × 108 mL−1) linked to an increase in algal biomass and diversity of bacterial plankton. Predicted virus size distributions were in agreement with transmission electron microscopy analysis suggesting a dominance of large viruses (≥60 nm).

Optical coherence tomography (OCT) has become a useful skin imaging tool in recent years. It can be used to image skin structure with great detail and deep below the surface. However, there is an unmet need for an inexpensive device that would be able to acquire high-quality en face images in short time. To this end, we report on a cost-effective full-field optical coherent tomography (FF-OCT) system comprised of a silicon camera with high full well capacity and a powerful near-infrared LED light source. The system, for example, is able to record 1.7 cm × 1.7 cm en face images in 0.12 sec with the spatial sampling rate of 2116 dpi and the sensitivity of 93 dB. We show that the FF-OCT system can be used for biometrics purposes by imaging internal fingerprints and sweat ducts with a good contrast and in short-enough time. Blood flow can also be sensed in dermis that, for example, can be used for liveness detection. The developed instrument could also be used in other en face deep-tissue imaging applications because of its high sensitivity and speed. One particular application is passive elastography that can map the stiffness of biological tissues. We will also present FF-OCT configurations that help improving the imaging depth through rejection of specular reflections and better use of a light budget.

Non-melanoma skin cancer is the most common cancer. On cosmetically sensitive areas, Mohs micrographic surgery is the standard of care; intra-operative margin assessment minimizes the surgical defect while helping to reduce the recurrence rate by a factor of 3 compared to surgical excision. The current Mohs technique relies on frozen section tissue slide preparation, which significantly lengthens operative time and requires on-site trained histotechnicians. Full-field micro-tomography is a novel optical imaging technique based on interferometry. It allows the extraction of a full-field optical coherence tomography (FFOCT) image, representative of the morphology of the tissue, and the dynamic cell information, representative of the intra-cellular metabolic activity. Both images are calculated at the micron-level in a few minutes and without tissue preparation. This multi-centric study aimed to evaluate these combined new imaging modalities for the analysis of skin cancer margins during Mohs surgery. Over 200 Mohs specimens were imaged in Drexel University College of Medicine, USA, and GHR Mulhouse Sud Alsace, France. An atlas was established of FFOCT images and corresponding histological slides to reveal FFOCT reading criteria of normal and cancerous structures. Dynamic cell information enhanced visualization of cancerous cells and surrounding immune cells, and yielded metabolic quantification of cancerous area versus benign areas. Finally, deep learning algorithms were developed for preliminary results for the automatic detection of cancerous tissue. Hybrid morphologic and metabolic micro-tomography techniques hold great potential for skin cancer margin assessment. They can potentially reduce recurrence rates and surgery times, optimize clinical workflow, and decrease healthcare costs.

Full Field OCT (FFOCT) is a shot noise limited interferometric microscopy technique that uses incoherent light and has proved to be an effective diagnostic tool in terms of sensitivity and specificity. We have used the FFOCT setup built by LLTech for the analysis of various cancerous tissues corresponding to the following organs: breast, skin, prostate, lungs, ENT, bladder, brain etc. The scores obtained were found in the range between 80 and 98%. To do better and to provide informations that the histology does not carry we have studied, using the same setup, the temporal dependence of our signals which we found to be related to the cellular metabolism. We have used the new high speed and high full well capacity of the Adimec camera to achieve a time analysis ranging between 2 and a few thousands of ms. We thus obtain a new contrast which constitutes a biomarker at the sub-cellular scale. We monitor the characteristic frequencies and amplitude of the signal and display them on the images of the tissues using a new processing code of the time series. This metabolic contrast also reveal the evolution of the activity of cancer cells under treatments such as chemotherapy. We will illustrate this new approach through examples of cancer tissues that are planned to be used as intraoperative tools.

International audience

Full-Field Optical Coherence Tomography (FF-OCT) reveals submicrometric morphological details in retinal explants without the use of contrast agents. Dynamic FF-OCT (D-FF-OCT) takes advantage of the temporal evolution of the local FF-OCT signal to reveal a movement-dependent contrast inside tissues, mostly relying on cellular motility. Compared to regular FF-OCT images, the relative contrast from stationary structures such as nerve fibers is reduced, and contrast inside cells is enhanced, revealing many more cells, as well as the position of nuclei, and cell metabolism. We used a multimodal FF-OCT, D-FF-OCT and fluorescence microscope to compare and identify the structures observed in D-FF-OCT, which allowed us to reconstruct the full 3-D micrometric organization of corneal and retinal explants. In healthy explants, this multimodal association allows the label-free specific detection of all cell populations, except from the Mueller cells, and of several structural features such as nerve and collagen fibers, and pedicles and spherules. D-FF-OCT also accesses several functional contrasts (relying on metabolism, mechanical and electrical activity) that can be combined to monitor the tissue health over time. It is anticipated that such a combination of static and dynamic OCT information may be used in vivo in future for the early detection of ocular pathologies. To this end, we tried here to foster our understanding of the progression and occurrence of such diseases in animal models. We notably used this optical system to follow the evolution of stem cells injected in the cornea and to assess the concentration of macrophages in retinas with inflammation.

The hypersensitive response has been mostly studied by molecular and biochemical methods after sample destruction. The development of imaging techniques allows the monitoring of physiological changes before any signs of cell death. Here, we follow the early steps of a hypersensitive-like response induced by the bacterial elicitor harpin in Nicotiana sp. We describe cytological modifications after inoculation of the harpin protein, using confocal fluorescence microscopy (CFM) and optical coherence tomography (OCT), an interferometric-based microscopy. The changes detected by CFM occurred 5 h after harpin infiltration and corresponded to a redistribution of the chloroplasts from the upper to the inner regions of the palisade mesophyll cells which could be related to a perturbation in the microtubule network. Using OCT, we were able to detect a decrease in chloroplast backscattered signal as early as 30 min after harpin infiltration. A simple physical model, which accounted for the structure and distribution of thylakoid membranes, suggested that this loss of scattering could be associated with a modification in the refractive index of the thylakoid membranes. Our OCT observations were correlated with a decrease in photosynthesis, emphasizing changes in chloroplast structure as one of the earliest hallmarks of plant hypersensitive cell death.

New instrumentation has been developed that is dedicated to the measurement of surface morphology with high resolutions and short acquisition times. Sinusoidal phase-shifting interferometry is demonstrated to provide within an acquisition time of only a few milliseconds topographic images with ∼1 nm height precision, ∼1 µm lateral resolution, and a few tens of pm sensitivity. A white-light scanning microscope is also under development, using an in-house developed high speed, intelligent CCD camera, the first tests of which demonstrate the feasibility of providing topographic images of deep surface relief (several microns to several tens of microns) within less than 0.3 s. The useful lateral field of view can be extended by employing image "stitching" while maintaining a high lateral resolution. PACS. 07.60.Pb Conventional optical microscopes

Summary The establishment of Erwinia amylovora harpin-induced hypersensitive response (HR) in Nicotiana sylvestris was followed by infra-red thermography (IRT). Three to four hours after elicitation, the temperature decreased in the harpin-infiltrated zone associated to stomatal opening. The marked drop in temperature which reached 2°C and preceded necrosis symptoms for several hours, is thus likely caused by higher transpiration. Neither of these effects was observed in a respiratory mutant, affected in complex I structure and function and over-expressing alternative oxidase, indicating that they are directly or indirectly mediated by mitochondrial function. However, as the HR establishment was similar in both wild type and mutant, cell death was either uncorrelated with the observed epidermal changes or occurred by a different signalling pathway in the two genotypes. IRT revealed a novel aspect of plant–pathogen interactions and could be applied to screen for mutants affected in elicitor signalling and/or for respiratory mutants.

We use an imaging system taking advantage of a focused ultrasonic field to mark the photon trajectories in a biological sample by modulating the optical paths in the focal zone. We show the applicability of this setup to various geometrical configurations, such as forward and backward scattering. The modulated speckle detected on a CCD camera reflects, after parallel processing, the local optical properties of the tissue in the focal zone. We propose to use the backscattering geometry to estimate the local effective attenuation coefficient. Finally we compare images which have been obtained through several centimeters of biological tissue, both in forward and in backward geometry.

We describe a compensated common path lateral shearing interferometer that allows fast two-dimensional measurements of the optical thickness of transparent plates. The bench is able to provide accurate results even when heterogeneous residual birefringence is present in the sample. This bench brings several improvements to an optical system originally built by Mr G Nomarski.

It is necessary to use the information contained in the near field to get sub-wavelength details in optical imaging which are not revealed through the far-field image. We have designed and built various setups able to perform near-field measurements in the UV, visible and IR, both in transmission, reflection and dark field with a resolution of 10 nm, independent of the wavelength but related to the tip size. Images revealing local dielectric contrasts, small particle effects, as well as local field enhancements in random structures, are shown.

La localisation des excitations optiques dans des films semi-metalliques aleatoires est observee, a l'aide d'un microscope optique en champ proche, dans des domaines sub-longueur d'onde. Cet effet est attribue a la localisation d'Anderson des modes plasmons de surface dans ces films. Les fluctuations geantes du champ local sont tres superieures au champ applique et sont concentrees spatialement dans des domaines dont la taille typique est celle des grains metalliques (-30 nm). Le spectre optique de champ proche (700 nm< λ < 800 nm), constitue de pics de resonance, est detecte et observe experimentalement. Il depend fortement de la zone du film etudiee.

Localization of optical excitations within subwavelength areas of a random metal-dielectric film is observed using near-field scanning optical microscopy. This effect is attributed to Anderson localization of surface plasmon modes in a semicontinuous metal film. The localized modes are seen as giant fluctuations of local electric fields spatially concentrated in ``hot'' spots, where the fields are much greater than the applied field. The local near-field spectra consisting of strong resonance peaks are detected and shown to depend markedly on the sample site probed. The observed spectral peaks correspond to localized modes of random metal-dielectric films.

A new near-field optical microscope working in transmission is presented. Lateral optical resolution less than 10 nm is obtained with a pure metallic probe. Polarization images of a metallic step confirmed the good resolution by comparison with an analytical model. We also demonstrate the capability of the microscope to obtain images with polarization effects. The good resolution is used for the observation of small gold aggregates which confirm that this microscope is able to make spectroscopic measurements of the optical effect induced by a nanometric scale particle. The polarization sensibility allows us to measure near-field magneto-optical contrast on a multi-layer sample with magnetic domains. These results are promising for magneto-optical characterization with nanometre resolution.

The advantage of a high signal-to-input power and path-length ratio, while retaining very good flexibility of the operation, has made the recently developed gas-phase mirage-effect spectroscopy setup an excellent tool for the measurement of trace gases and organic volatiles up to ppb or lower concentration levels. The present study demonstrates an unusual application of this novel technique in the area of the nondestructive characterization and photodecomposition of polymers. Very small quantities of ethylene gas were found to be released when polyethylenes were exposed to a very low power mercury lamp (photoexposure). The extent (quantity) of the ethylene gas produced was found to be specific and characteristic of the type of polyethylenes (e.g., HDPE and LDPE). The source of ethylene was attributed to the rupture of the terminal monomer units of the branched chains. This study, apart from fundamental understanding, also demonstrated a direct industrial application in the fast and nondestructive characterization of various polyethylenes.

Our recent work on the Mirage-FTIR technique has led to the development of new applications, specially for nondestructive evaluation of high absorptivity (black) materials like carbon fibre and coating on carbon fibre mesh. Having succeeded in recording well resolved IR spectra of such materials, we report a preliminary study on nondestructive evaluation of the extent of thermo-oxidative conversion of precursor raw material (special acrylic fibre-SAF) into carbon fibres by vibrational spectroscopy. An attempt has been made to develop a semi-quantitative correlation between the performance characteristics (properties) of carbon fibres produced from different sources (manufacturers) and their IR spectral characteristics. In addition, the Mirage-FTIR technique has also been used as a surface characterization tool for the analysis of surface coatings.

A vibrating metallic conical probe has been used in near-field microscopic measurements to obtain optical resolutions much smaller than the wavelength. Numerical simulations of these complex problems have drawn most of the attention in the past. We propose here an analytical expression for the electric field intensity in the near region of a probe situated above the surface of a semi-infinite dielectric medium. It is given as a function of the observation point location, with the cone aperture, the dielectric medium permittivity, and the characteristics of the incident plane wave as parameters. © 1998 John Wiley & Sons, Inc. Microwave Opt Technol Lett 18: 120–124, 1998.

We demonstrate the unusual applications of the recently developed mirage-Fourier transform infrared (FTIR) spectroscopy for nondestructive characterization of polymer coatings, especially on absorbing substrates or very thin coatings, where conventional FTIR techniques tend to fail. The mirage-FTIR has been briefly described. A few specific examples have been given to demonstrate the potential of this technique as a tool for surface characterization.

In this paper we describe an interferometric polarization microscope. By scanning the sample, we reach the thermoelastic deformation at the sample surface with high spatial resolution. The thermoelastic deformation is produced by the absorption of a modulated laser beam, focused by the microscope. The thermoelastic images obtained reveal the granular structure of the polycrystalline material with a new kind of contrast which cannot be seen by other optical or thermal techniques. The experimental set-up, very compact, has a sensitivity in the picometric range. We also present a model to calculate the thermoelastic deformation using a finite-element analysis that takes into account the orientation of the cubic axes in polycrystalline materials and that allows a first interpretation of images obtained with the microscope. Nous d?crivons dans cet article un microscope interf?rentiel ? polarisation. Par balayage de l'?chantillon, nous atteignons la d?formation thermo?lastique de la surface de l'?chantillon. Cette d?formation thermo?lastique est produite par l'absorption d'un flux laser modul? et focalis? par le microscope. Les images thermo?lastiques obtenues r?v?lent la structure granulaire du mat?riau polycristallin sous forme de contrastes qui ne peuvent ?tre obtenus par des mesures optiques ou thermiques. Ce montage, tr?s compact, poss?de une sensibilit? qui se situe en-de?? de la gamme du picom?tre. Nous pr?sentons ?galement un mod?le de calcul des d?formations thermo?lastiques par ?l?ments finis qui tient compte de l'orientation des grains dans les mat?riaux m?talliques ? structure cubique et qui permet un d?but d'interpr?tation des images obtenues par le microscope.

Carbon fiber, being black (high absorptivity) material, is difficult to analyze by the conventional infrared (IR) techniques. Probably due to this fact, most of the studies reported in literature have been restricted to the thermal degradation of polyacrylonitrile-based fibers only. In the present work, we have used a recently developed mirage-FTIR technique for recording the IR spectra of carbon fibre at different stages of production, right from the precursor to final product. A study of microstructural/chemical changes occuring not only provides the information on structure but also demonstrates an unusual and highly technical industrial application. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 63: 1785–1791, 1997

Mass transfers in solution from or to an electrode surface occur at a micronic scale in many processes such as corrosion, electroless plating or electrocrystallization. Direct knowledge of local information is important for a better understanding of these processes. In this paper, we propose the use of an interferometric microscope to measure at a micronic scale concentration gradients which are directly related to a 3D mass transfer. After validation the technique with the Fe 2+ Fe 3+ electrochemical system, we have detected the local concentration gradient of the counter-ions exchanged between a ruthenium oxide film and its surrounding electrolyte with a spatial resolution of 5 micrometres. These preliminary results allow us to conclude that this method can be very fruitful for local mass transfer studies.

In spectroscopic ellipsometry (SE) measurements of thin films with inhomogeneous optical thickness, the spot size of the probe beam on the sample surface is a highly critical parameter in the treatment of the experimental data. SE measurements were performed on transparent samples prepared by various techniques: plasma enhanced chemical vapor deposition of diamond film, spin-coated PMMA films and thermally grown SiO 2 film. The first two deposition techniques can produce films with inhomogeneous optical thickness. By comparing SE measurements using different set-up configurations, we demonstrate that a combination of lenses and small diaphragms is most suitable for minimizing the effects related to the optical thickness distribution and light beam divergence. In this way mistakes are avoided in extracting physical parameters from SE optical modeling of these films.

We present a quantitative and comparative study of three different photothermal techniques based on the signal produced on vertical/slanted thermal barriers under a variety of barrier/sample conditions. Models based on integral methods are developed to calculate the amplitude and phase of the sample surface temperature and mirage deflection in surrounding air. The geometries studied include: vertical, tilted, buried, and finite size barriers separating identical or different media. The models incorporate the probe and pump beam sizes, the thermal resistance of the barrier and the optothermal characteristics of the sample. Experimental measurements are performed on a variety of fabricated barriers with three modulated photothermal techniques: the thermoreflectance, infrared radiometry, and mirage detection. We discuss in a comparative way the limits, drawbacks and the applicability of each technique. Model fits to the experimental results allow characterization of the thermal barrier (spatial localization, geometry, orientation, and size) and provide an accurate determination of its thermal resistance.

Wavefront control is crucial to the Virgo experiment. We have developed a profilometer sensitive to the sample local slope, which measures mirror profiles with nanometre sensitivity over a large frequency range, without any need for a reference surface. In order to reduce measurement errors due to atmospheric turbulence and mechanical defects, a compensation system is included as part of the bench.

Using a vibrating opaque metallic tip, which periodically and locally modifies the electromagnetic field distribution of a diffraction spot focused onto the sample surface through a microscope objective lens, we have observed a clear improvement of the optical resolution compared to the diffraction limit both with topographical and with purely optical features. This procedure simultaneously generates a reflection mode near field optical signal and a “tapping mode” Atomic Force Microscope (AFM) signal, and thus enables control of the effects of the local topography. The study of several samples has revealed a 50–100 nm optical resolution and the independence of the AFM and the near field optical signals.

Intracellular motion can be quantitatively monitored in tissues using coherence-gated microscopic techniques. With full-field optical coherence tomography (FFOCT), the use of high numerical aperture microscope objectives provides a high resolution mapping of intracellular dynamics that are probed with subwavelength sensitivity. In the upper temporal bandwidth that we have used (1-6 Hz) the main contribution to the dynamic signal arises from the overall dynamical, optically heterogeneous cytoplasm. We propose a method to specifically study the impact of actomyosin contractility on the intracellular dynamic signal by performing high throughput, comparative measurements of multicellular aggregates with and without blebbistatin action, a selective inhibitor of class-II myosins that disrupts actomyosin contractile activity. Our results indicate a significant increase in the fraction of the signal that decorrelates within 1 second after inhibition of contractility. This observation mitigates the anticipated importance of actomyosin contractile forces to directly move organelles, but highlights their role in hindering organelle transport via their stiffening effect of the viscoelastic cytoplasm.

The mechanical, thermal and optical properties of thin coatings have been considerably improved these last years. For instance optical coatings for laser mirrors with sub ppm losses are now available. Moreover thermal diffusivity varying over more than 4 orders of magnitude can be found when going from polymers coatings to diamond ones. Reliable sensitive techniques are necessary to characterize them. First, we present two optical techniques: ellipsometry and polarization microscopy which lead to determination of optical (refractive index) and geometrical (thickness, homogeneity) parameters. Second, we describe a few optically based photothermal techniques in which heating and detection are contactless and non-destructive: photothermal microscopy, mirage detection, time resolved ellipsometry, etc. These techniques allow local (down to micron scale) to global determination of the optical properties (absorption with a few ppb sensitivity) as well as thermal properties (diffusivity, effusivity) and reveal inhomogeneities, adhesion as well as anisotropies.

We present here an alternative experimental approach to achieve a local selection of a field distribution smaller than the wavelength. The sample surface is irradiated by a strongly converging beam issued from a microscope objective. A tungsten tip is used to perturb locally and periodically the electromagnetic field in the diffraction limited spot. The vibration amplitude is kept at a constant value when scanning the sample. We expect this to discard the topographical response and to retain mainly the optical response of the sample. We can use our microscope as an ordinary scanning metallographic one by monitoring the DC level of the reflected light and compare this signal with the AC signal induced by the tip oscillation on the same detector. Preliminary results obtained show a clear improvement of the resolution compared to the diffraction limited one.

InGaAsP/InP buried heterostructure lasers were investigated using reflectance microscopy. Measurements were performed for both biased (current modulation) and non-biased (modulated pump beam) device. Maps of the facets of non-degraded and degraded lasers are presented and discussed.

We have studied heat diffusion from rough surfaces in different materials exposed to a constant heating and we observed anomalously slow temperature rises over about 3 time decades which indicate a hyperdiffusion phenomenon.

This demonstration of mirage effect under a microscope provides a new possibility to measure the thermal diffusivity of a transparent sample as small as a few microns, with micronic spatial resolution.

We experimentally measure the monochromatic transmission matrix (TM) of an optical multiple scattering medium using a spatial light modulator together with a phase-shifting interferometry measurement method. The TM contains all information needed to shape the scattered output field at will or to detect an image through the medium. We confront theory and experiment for these applications and we study the effect of noise on the reconstruction method. We also extracted from the TM informations about the statistical properties of the medium and the light transport whitin it. In particular, we are able to isolate the contributions of the Memory Effect (ME) and measure its attenuation length.

Usually sample surface structures generate large flunctuations of the photothermal response due to local variations of optical, thermal and geometrical properties which mask the crack signal when using single detection ir radiometry. Demonstrated experimentally and theoretically is the ability of a differential stimulated ir detection scheme to obtain a better contrast in crack detection on real samples. A finite element simulation confirms the magnitude and shape of the signals for vertical and slanted cracks.

Spectroscopic ellipsometry is a widely used technique for the determination of the optical and geometrical characteristics of complex multilayered structures. We show in this communication that photostimulated ellipsometry is able to account for the time (or frequency) dependence of heat and carrier diffusion processes in such structures. The results of various experiments on SIMOX structures are reported.

This contribution presents a new kind of spectroscopic ellipsometry under modulated or pulsed external excitation when the sample physical properties (such as the refractive indices or thicknesses, e.g. in a layered structure) can be changed by the absorption of the incident energy. These photoinduced variations are detected through the ellipsometric signals. The experimental procedure used is first to fix the azimuth of polarizer, next successively to set the azimuth of analyser at four positions with an interval of 45°, then to measure the static signal and the photoinduced signal and finally to determine the ellipsometric parameters tan ψ and cos Δ, and their variations d(tan ψ) and d(cos Δ). So that the d(tan ψ) and d(cos Δ) can be recorded as a function of time or frequency, the time resolution is as fast as a few nanoseconds (limited by our laser pulse duration). Minimum variations in the ellipsometric parameters d(tan ψ) and d(cos Δ) of the order of 10−5 under modulated excitation, and 10−6 under pulsed excitation have been measured. This new technique has been applied to bulk and implanted semiconductors; dynamic indices and various heat diffusion processes can be observed from the stimulated spectroscopic ellipsometry.

We report the optical conductivity of YBa 2 Cu 3 O 7 and Bi 2 Sr 2 CaCu 2 O 8+δ thin films at room temperature. The optical conductivity in the 500–7000 cm −1 frequency range is analysed in terms of a frequency dependent relaxation time τ and effective mass of the normal carriers. The relaxation rate 1/τ exhibits the same linear dependence on frequency whatever the sample. The assumption that 1/τ also depends linearly on temperature accounts satisfactorily for the temperature variation (90–300K) of the transmission in the 1000–7000 cm −1 range. These results are consistent with the marginal Fermi liquid model.

Light-CT offers valuable diagnostics of skin pathologies. We show sections down to sub-nuclei sizes and images of patients’ skins exhibiting Werner syndrome. Light-CT merits are compared to other OCT approaches, confocal and optical coherence microscopy.

We have built a visible and near ir Fourier transform dichrometer able to perform spectra with a 8 μm spatial resolution. Circular dichroism (CD) measurements in the transmission on two superconducting thin films of YBa 2 Cu 3 O 7 and Bi 2 Sr 2 Ca 2 Cu 3 O 10 have been performed in the spectral range 13000–21000 cm -1 . In the first film below the transition temperature, a CD signal (absorbance unit) up to few times 10 -4 has clearly appeared far above the limit of sensitivity of our detection ( βA =2 x 10 -5 ). This value of β A corresponds to a rotation angle of a linearly polarized was βo =10 -5 rad. In the normal state no detectable CD signal has been observed. All the signals with the second film were found to be below our detection limit at any temperature.

International audience; We propose an original adaptive wavefront holographic setup based on the photorefractive effect (PR), to make real-time measurements of acousto-optic signals in thick scattering media, with a high flux collection at high rates for breast tumor detection. We describe here our present state of art and understanding on the problem of breast imaging with PR detection of the acousto-optic signal.

We report experimental evidence and correction of defocus in full-field OCT of biological samples due to mismatch of the refractive index of biological tissues and water. Via a metric based on the image quality, we demonstrate that we are able to compensate this index-induced defocus and to recover a sharp image in depth., 7 pages, 3 figures, minor changes, 1 figure added

International audience; The authors present a method of focusing high intensity ultrasound by time-reversing the photoacoustic response of an optically selective target in a nonselective background. The target’s photoacoustic response was isolated from the background by subtracting the photoacoustic waveforms obtained at different optical wavelengths and convolved with a continuous signal. It was found that the focus produced was comparable in quality to that obtained by delay-law beam-forming. The method holds the promise of allowing precise targeting of high intensity focused ultrasound on nonechogenic targets, in moving environments, independently of the presence of aberrating layers.

Les experiences de PDS (spectroscopie de deflection photothermique) en fonction de l'epaisseur de la couche (0,5-10 μm) confirment l'existence d'une absorption de surface et/ou interfaciale. Les spectres d'absorption derives des methodes du photocourant constant (CPM) obtenus pour ces memes echantillons presentent un effet surfacial ou interfacial tres faible voir inexistant. Presentation d'une methode capable de differencier l'absorption du volume de celle de la surface ou de l'interface. Cette methode est basee sur les experiences PDS a haute frequence de modulation