Theoretical study of acousto-optical coherence tomography using random phase jumps on ultrasound and light.

Acousto-optical coherence tomography (AOCT) is a variant of acousto-optic imaging (also called ultrasonic modulation imaging) that makes it possible to get the z resolution with acoustic and optic continuous wave beams. We describe here theoretically the AOCT effect, and we show that the acousto-optic "tagged photons" remain coherent if they are generated within a specific z region of the sample. We quantify the z selectivity for both the "tagged photon" field and for the Lesaffre et al. [Opt. Express 17, 18211 (2009)] photorefractive signal.

Exploiting the time-reversal operator for adaptive optics, selective focusing, and scattering pattern analysis.

We report on the experimental measurement of the backscattering matrix of a weakly scattering medium in optics, composed of a few dispersed gold nanobeads. The decomposition of the time-reversal operator is applied to this matrix and we demonstrate selective and efficient focusing on individual scatterers, even through an aberrating layer. Moreover, we show that this approach provides the decomposition of the scattering pattern of a single nanoparticle. These results open important perspectives for optical imaging, characterization, and selective excitation of nanoparticles.

Measuring the transmission matrix in optics: an approach to the study and control of light propagation in disordered media.

We introduce a method to experimentally measure the monochromatic transmission matrix of a complex medium in optics. This method is based on a spatial phase modulator together with a full-field interferometric measurement on a camera. We determine the transmission matrix of a thick random scattering sample. We show that this matrix exhibits statistical properties in good agreement with random matrix theory and allows light focusing and imaging through the random medium. This method might give important insight into the mesoscopic properties of a complex medium.

Defocus test and defocus correction in full-field optical coherence tomography.

We report experimental evidence and correction of defocus in full-field optical coherence tomography of biological samples owing 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.

Detection of the tagged or untagged photons in acousto-optic imaging of thick highly scattering media by photorefractive adaptive holography.

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 the art and understanding on the problem of breast imaging with PR detection of the acousto-optic signal.

High-speed wave-mixing laser Doppler imaging in vivo.

An interferometric method for parallel optical spectroscopy in the kilohertz range is reported, as well as its experimental validation in the context of high-speed laser Doppler imaging in vivo. The interferometric approach enables imaging in the low light conditions of a 2 kHz frame rate recording with a complementary metal-oxide semiconductor camera. Observation of mice craniums with near-infrared (lambda=785 nm) laser light in reflection configuration is reported. Doppler spectral images allegedly sensitive to blood flow are sequentially measured at several optical frequency detunings, to shift the spectral range of analysis in the radio-frequency spectrum.

Analytical method for localizing a fluorescent inclusion in a turbid medium.

We describe a novel method for localizing a fluorescent inclusion in a homogeneous turbid medium through the use of time-resolved techniques. Based on the calculation of the mean time of the fluorescence curves, the method does not require a priori knowledge of either the fluorescence lifetime or the mean time of the instrument response function since it adopts a differential processing approach. Theoretical expressions were validated and experiments for assessing the accuracy of localization were carried out on liquid optical phantoms with a small fluorescent inclusion. The illumination and detection optical fibers were immersed in the medium to achieve infinite medium geometry as required by the model used. The experimental setup consisted of a time-correlated single-photon counting system. Submillimeter accuracy was achieved for the localization of the inclusion.

In situ monitoring of the photorefractive response time in a self-adaptive wavefront holography setup developed for acousto-optic imaging.

The measurement of optical contrasts within thick biological tissues can be performed with the hybrid technique of acousto-optic imaging, but it has been shown that an acquisition rate in the 1-10kHz range is required for a good efficiency. This comes from the interferometric nature of the signal, blurred by speckle decorrelation in a time t(c), due to a decrease of the speckle pattern contrast at the exit of the sample. An holographic setup that associates a fast and large area single photodetector and a photorefractive crystal, can measure in real-time the acousto-optic signal: this is the so-called self-adaptive wavefront holography technique. Nevertheless, it is essential to size the photorefractive response time ( t(PR)) of the crystal with t(c) in order to optimize the signal-to-noise ratio of the measurement. This time mainly depends on the overall light intensity within the crystal. We have developed an original in situ method to determine t(PR) with the combination of acoustic pulses and a frequency de-tuning of the reference beam. We can measure precisely this time but also monitor it according to a theoretical model that we have previously described. We are able to adapt the response time of the setup to the decorrelation time of the medium under study.

Full-field optical sectioning and three-dimensional localization of fluorescent particles using focal plane modulation.

We present a technique for imaging fluorescent particles based on the axial modulation of the objective's focal plane position. This technique provides full-field optical sectioning and can be used to localize the fluorophores in three dimensions. We describe the technique and apply it to image 200 nm diameter fluorescent beads immobilized in a gel. We show that full-field optical sectioning is obtained and that the beads are localized with a precision of 10 nm in the transverse plane and 14 nm in the axial direction.

Stroboscopic ultrahigh-resolution full-field optical coherence tomography.

We present a new technique that produces en face tomographic images with a 10-micros acquisition time per image. The setup consists of an interference microscope with stroboscopic illumination provided by a xenon arc flash lamp (10-micros flashes at 15 Hz). The tomographic images are obtained from two phase-opposed interferometric images recorded simultaneously by two synchronized CCD cameras. Transverse resolution better than 1.0 microm is achieved by use of high-numerical-aperture microscope objectives. The short coherence length of the source yields an axial resolution of 0.9 microm. 3 x 3 pixel binning leads to a detection sensitivity of 71 dB. Our system is suitable for various applications, particularly in biology for in vivo cellular-level imaging.

Heterodyne detection of multiply scattered monochromatic light with a multipixel detector.

A new technique is presented for measuring the spectral broadening of light that has been multiply scattered from scatterers in motion. In our method the scattered light is detected by a heterodyne receiver that uses a CCD as a multipixel detector. We obtain the frequency spectrum of the scattered light by sweeping the heterodyne local oscillator frequency. Our detection scheme combines a high optical etendue (product of the surface by the detection solid angle) with an optimal detection of the scattered photons (shot noise). Using this technique, we measure, in vivo, the frequency spectrum of the light scattered through the breast of a female volunteer.

Pulsed acousto-optic imaging in dynamic scattering media with heterodyne parallel speckle detection.

We present a new detection scheme for acousto-optic tomography based on pulsed-wave ultrasound and illumination combined with heterodyne parallel speckle detection. This setup can perform tomographies inside several-centimeter-thick scattering samples. Test experiments confirm the suitability of this method for performing tomographies inside various types of optically scattering media, including liquids.

Three-dimensional cellular-level imaging using full-field optical coherence tomography.

An ultrahigh-resolution full-field optical coherence tomography (OCT) system has been developed for cellular-level imaging of biological media. The system is based on a Linnik interference microscope illuminated with a tungsten halogen lamp, associated with a high-resolution CCD camera. En face tomographic images are produced in real time, with the best spatial resolution ever achieved in OCT (0.7 microm x 0.9 microm, axial x transverse). A shot-noise limited detection sensitivity of 80 dB can be reached with an acquisition time per image of 1 s. Images of animal ophthalmic biopsies and vegetal tissues are shown.

Photorefractive detection of tagged photons in ultrasound modulated optical tomography of thick biological tissues.

We present a new and simple method to obtain ultrasound modulated optical tomography images in thick biological tissues with the use of a photorefractive crystal. The technique offers the advantage of spatially adapting the output speckle wavefront by analysing the signal diffracted by the interference pattern between this output field and a reference beam, recorded inside the photorefractive crystal. Averaging out due to random phases of the speckle grains vanishes, and we can use a fast single photodetector to measure the ultrasound modulated optical contrast. This technique offers a promising way to make direct measurements within the decorrelation time scale of living tissues.

Thermal-light full-field optical coherence tomography.

We have built a high-resolution optical coherence tomography (OCT) system, based on a Linnik-type interference microscope, illuminated by a white-light thermal lamp. The extremely short coherence length of the illumination source and the large aperture of the objectives permit resolution close to 1 microm in three dimensions. A parallel detection scheme with a CCD camera provides cross-section (x-y) image acquisition without scanning at a rate of up to 50 Hz. To our knowledge, our system has the highest resolution demonstrated to date for OCT imaging. With identical resolution in three dimensions, realistic volume rendering of structures inside biological tissues is possible.

Imagery of local defects in multilayer components by short coherence length interferometry.

We propose to image local defects inside multidielectric optical components by using a special configuration Linnik interference microscope, along with a CCD camera and a dedicated detection, to extract the amplitude scattered by the defects in the interference image. The use of a short coherence length source allows one to obtain interference only from a thin slice (~1microm) within the observed object. The object is tilted to permit the use of a dark-field configuration to enhance the defect contrast. We describe the experimental setup and the detection scheme. Images that exhibit local point defects on the interfaces of various multilayer optical components (laser mirrors) are presented.

Infra-red thermography revealed a role for mitochondria in pre-symptomatic cooling during harpin-induced hypersensitive response.

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 degrees 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.

Sinusoidally phase-modulated interference microscope for high-speed high-resolution topographic imagery.

We describe an interference microscope that produces topographic images with a minimum acquisition time of 20 ms. The system is based on phase-shifting interferometry with sinusoidal phase modulation induced by the oscillation of an interferometric objective (Michelson or Mirau). A CCD camera captures four images per oscillation period to produce a phase map in real time. The system is installed on a commercial microscope.

Phase measurements with wide-aperture interferometers.

An interferogram produced by wide-aperture interferometers is studied both theoretically and experimentally. The fringe spacing is shown to increase nonlinearly with the numerical aperture and the fringe envelope to become narrower as the numerical aperture is increased. Phase measurements with wide-aperture interferometers therefore require calibration, and the phase can be measured only over a limited range. A calibration is given for accurate phase measurements, and the range over which the phase can be measured is calculated. Experimental measurements are presented and compared with theory.

Apertureless scanning near-field magneto-optical microscopy of magnetic multilayers.

We imaged magnetic domains in Pt/Co/Pt multilayers using an apertureless scanning near-field optical microscope operating in reflection mode. As the magneto-optical effects are weak for this kind of structure, a polarization modulation technique with a photoelastic modulator was used to reveal the contrast between magnetic domains. In the case of a Pt/Co/Pt trilayer structure, a strong improvement in lateral resolution is observed compared with far-field magneto-optical images and good sensitivity is achieved. In the case of a Pt/[Co/Pt]Pt multilayer structure, stripe domains of 200 nm width could be resolved, in good agreement with images obtained by magnetic force microscopy on the same structure.