Fast and pure phase-shifting off-axis holographic microscopy with a digital micromirror device.

We present a phase-shifting digital holographic microscopy technique, where a digital micromirror device enables to perform a precise phase-only shift of the reference wave. By coupling the beam into a monomode fiber, we obtain a laser mode with a constant phase shift, equally acting on all pixels of the hologram. This method has the advantage of being relatively simple and compatible with high frame rate cameras, which makes it of great interest for the observation of fast phenomena. We demonstrate the validity of the technique in an off-axis configuration by imaging living paramecia caudata.

4D compressive sensing holographic imaging of small moving objects with multiple illuminations.

In previous work [Opt. Lett.44, 2827 (2019)OPLEDP0146-959210.1364/OL.44.002827], we presented a method based on digital holography and orthogonal matching pursuit, which is able to determine the 3D positions of small objects moving within a larger motionless object. Indeed, if the scattering density is sparse in direct 3D space, compressive sensing algorithms can be used. The method was validated by imaging red blood cell trajectories in the trunk vascular system of a zebrafish (Danio rerio) larva. We give here further details on the reconstruction technique and present a more robust version of the algorithm based on multiple illuminations.

Blood flow imaging in zebrafish by laser doppler digital holography.

Microvessel blood flow imaging techniques are widely used in biomedical research and clinical diagnostics where many diseases have a vascular etiology or involvement. For testing purposes, zebrafish embryo provides an ideal animal model to achieve high-resolution imaging of superficial and deeply localized vessels. Moreover, the study of the formation of a closed circulatory system in vertebrates is a topic of recent interest in biophysics. However, most of the existing techniques are invasive due to the use of a contrast agent for imaging purposes. Recent developments in Digital Holography and Laser Doppler Holography techniques can be considered to alleviate this issue. Laser Doppler holography and transmission microscopy can be coupled to analyze blood flow in fish embryos by adapting a laser Doppler holographic setup to a standard bio-microscope: the two beams of the holographic interferometer (illumination of the object and reference), whose frequency offset is controlled, were addressed to the microscope by optical fibers. Multimodal acquisition and analysis of the data is made by acting on the frequency offset of the two beams, and on the location of the Fourier space filtered zone. In this work, we show that it is possible to select the signal of moving scatterers, and to image Red Blood Cells (RBCs) and blood vessels. Individual RBCs are imaged, and movies showing the RBC motion are obtained. Microsc. Res. Tech. 81:153-161, 2018. © 2016 Wiley Periodicals, Inc.

4D holographic microscopy of zebrafish larvae microcirculation.

An original technique that combines digital holography, dual illumination of the sample and cleaning algorithm 3D reconstruction is proposed. It uses a standard transmission microscopy setup coupled with a digital holography detection. The technique is 4D, since it allows to determine, at each time step, the 3D locations (x,y,z) of many moving objects that scatter the dual illumination beam. The technique has been validated by imaging the microcirculation of blood in a fish larvae sample (the moving objects are thus red blood cells RBCs). Videos showing in 4D the moving RBCs superimposed with the perfused blood vessels are obtained.

MHz Ultrasound Induced Roughness of Fluid Interfaces.

The interface between two fluids is never flat at the nanoscale, and this is important for transport across interfaces. In the absence of any external field, the surface roughness is due to thermally excited capillary waves possessing subnanometric amplitudes in the case of simple liquids. Here, we investigate the effect of ultrasound on the surface roughness of liquid-gas and liquid-liquid interfaces. Megahertz (MHz) frequency ultrasound was applied normal to the interface at relatively low ultrasonic pressures (<0.6 MPa), and the amplitudes of surface fluctuations have been measured by light reflectivity and ellipsometry. We found a dramatic enhancement of surface roughness, roughly linear with intensity, with vertical displacements of the interface as high as 50-100 nm. As a consequence, the effective contact area between two fluids can be increased by ultrasound. This result has a clear impact for enhancing interface based processes such as mass or heat transfer.

Translational viscous drags of an ellipsoid straddling an interface between two fluids.

We study the dynamics of individual polystyrene ellipsoids of different aspect ratios trapped at the air-water interface. Using particle tracking and in situ vertical scanning interferometry techniques we are able to measure translational drags and the protrusion in air of the ellipsoids. We report that translational drags on the ellipsoid are unexpectedly enhanced: despite the fact that a noticeable part of the ellipsoid is in air, drags are found larger than the bulk one in water.

Heterodyne holography with full control of both the signal and reference arms.

Heterodyne holography is a variant of phase-shifting holography in which the reference and signal arms are controlled by acousto-optic modulators. In this review paper, we will briefly describe the method and its properties, and we will illustrate its advantages in experimental applications.

Brownian diffusion of a partially wetted colloid.

The dynamics of colloidal particles at interfaces between two fluids plays a central role in microrheology, encapsulation, emulsification, biofilm formation, water remediation and the interface-driven assembly of materials. Common intuition corroborated by hydrodynamic theories suggests that such dynamics is governed by a viscous force lower than that observed in the more viscous fluid. Here, we show experimentally that a particle straddling an air/water interface feels a large viscous drag that is unexpectedly larger than that measured in the bulk. We suggest that such a result arises from thermally activated fluctuations of the interface at the solid/air/liquid triple line and their coupling to the particle drag through the fluctuation-dissipation theorem. Our findings should inform approaches for improved control of the kinetically driven assembly of anisotropic particles with a large triple-line-length/particle-size ratio, and help to understand the formation and structure of such arrested materials.

Holographic microscopy reconstruction in both object and image half-spaces with an undistorted three-dimensional grid.

We propose a holographic microscopy reconstruction method that propagates the hologram in the object half-space in the vicinity of the object. The calibration yields reconstructions with an undistorted reconstruction grid, i.e., with orthogonal x, y, and z axes and constant pixel pitch. The method is validated with a USAF target imaged by a ×60 microscope objective (MO), whose holograms are recorded and reconstructed for different USAF locations along the longitudinal axis: -75 to +75 µm. Since the reconstruction numerical phase mask, the reference phase curvature, and the MO form an afocal device, the reconstruction can be interpreted as occurring equivalently in the object or image half-space.

Stochastic 3D optical mapping by holographic localization of Brownian scatterers.

We present a wide-field microscopy technique for the 3D mapping of optical intensity distributions using Brownian gold nanopar-ticles as local probes, which are localized by off-axis holography. Fast computation methods allow us to localize hundreds of particles per minute with accuracies as good as 3 × 3 × 10nm³ for immobilized particles. Factors limiting this accuracy are discussed and the possibilities of the technique are illustrated through the 3D optical mapping of an evanescent and a propagative wave. Our results pave the way for a new stochastic imaging technique, well adapted to subwavelength optical characterization in water-based systems.

Laser Doppler holographic microscopy in transmission: application to fish embryo imaging.

We have extended Laser Doppler holographic microscopy to transmission geometry. The technique is validated with living fish embryos imaged by a modified upright bio-microcope. By varying the frequency of the holographic reference beam, and the combination of frames used to calculate the hologram, multimodal imaging has been performed. Doppler images of the blood vessels for different Doppler shifts, images where the flow direction is coded in RGB colors or movies showing blood cells individual motion have been obtained as well. The ability to select the Fourier space zone that is used to calculate the signal, makes the method quantitative.

Capillary force on a micrometric sphere trapped at a fluid interface exhibiting arbitrary curvature gradients.

We report theoretical predictions and measurements of the capillary force acting on a spherical colloid smaller than the capillary length that is placed on a curved fluid interface of arbitrary shape. By coupling direct imaging and interferometry, we are able to measure the in situ colloid contact angle and to correlate its position with respect to the interface curvature. Extremely tiny capillary forces down to femtonewtons can be measured with this method. Measurements agree well with a theory relating the capillary force to the gradient of Gaussian curvature and to the mean curvature of the interface prior to colloidal deposition. Numerical calculations corroborate these results.

Experimental study of z resolution in 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 ultrasound modulated optical tomography) that makes possible to get resolution along the ultrasound propagation axis z. We present here AOCT experimental results, and we study how the z resolution depends on time step between phase jumps T(φ), or on the correlation length Δz. By working at low resolution, we perform a quantitative comparison of the z measurements with the theoretical point spread function. We also present images recorded with different z resolution, and we qualitatively show how the image quality varies with T(φ), or Δz.

Phase-resolved heterodyne holographic vibrometry with a strobe local oscillator.

We report a demonstration of phase-resolved vibrometry, in which out-of-plane sinusoidal motion is assessed by heterodyne holography. In heterodyne holography, the beam in the reference channel is an optical local oscillator (LO). It is frequency-shifted with respect to the illumination beam to enable frequency conversion within the sensor bandwidth. The proposed scheme introduces a strobe LO, where the reference beam is frequency-shifted and modulated in amplitude, to alleviate the issue of phase retrieval. The strobe LO is both tuned around the first optical modulation sideband at the vibration frequency, and modulated in amplitude to freeze selected mechanical vibration states sequentially. The phase map of the vibration can then be derived from the demodulation of successive vibration states.

Acousto-optical coherence tomography with a digital holographic detection scheme.

Acousto-optical coherence tomography (AOCT) consists in using random phase jumps on ultrasound and light to achieve a millimeter resolution when imaging thick scattering media. We combined this technique with heterodyne off-axis digital holography. Two-dimensional images of absorbing objects embedded in scattering phantoms are obtained with a good signal-to-noise ratio. We study the impact of the phase modulation characteristics on the amplitude of the acousto-optic signal and on the contrast and apparent size of the absorbing inclusion.

Video-rate laser Doppler vibrometry by heterodyne holography.

We report a demonstration video-rate heterodyne holography in off-axis configuration. Reconstruction and display of a one megapixel hologram is achieved at 24 frames per second, with a graphics processing unit. Our claims are validated with real-time screening of steady-state vibration amplitudes in a wide-field, noncontact vibrometry experiment.

Parallel heterodyne detection of dynamic light-scattering spectra from gold nanoparticles diffusing in viscous fluids.

We developed a microscope intended to probe, using a parallel heterodyne receiver, the fluctuation spectrum of light quasi-elastically scattered by gold nanoparticles diffusing in viscous fluids. The cutoff frequencies of the recorded spectra scale up linearly with those expected from single-scattering formalism in a wide range of dynamic viscosities (1 to 15 times water viscosity at room temperature). Our scheme enables ensemble-averaged optical fluctuations measurements over multispeckle recordings in low light, at temporal frequencies up to 10 kHz, with a 12 Hz framerate array detector.

Heterodyne holographic microscopy of gold particles.

We report experimental results on heterodyne holographic microscopy of subwavelength-size gold particles. The apparatus uses continuous green-laser illumination of the metal beads in a total internal reflection configuration for dark-field operation. Detection of the scattered light at the illumination wavelength on a charge-coupled-device array detector enables 3D localization of brownian particles in water.

Spatiotemporal heterodyne detection.

We describe a scheme in which a camera is turned into an efficient tunable frequency filter of a few-Hertz bandwidth in an off-axis, heterodyne optical mixing configuration, enabling one to perform parallel, high-resolution coherent spectral imaging. This approach is made possible through the combination of a spatial and temporal modulation of the signal to reject noise contributions. Experimental data obtained with dynamically scattered light by a suspension of particles in Brownian motion is interpreted.