Quantitative phase imaging of adherent mammalian cells: a comparative study.

The Quantitative phase imaging methods have several advantages when it comes to monitoring cultures of adherent mammalian cells. Because of low photo-toxicity and no need for staining, we can follow cells in a minimally invasive way over a long period of time. The ability to measure the optical path difference in a quantitative manner allows the measurement of the cell dry mass, an important metric for studying the growth kinetics of mammalian cells. Here we present and compare cell measurements obtained with three different techniques: digital holographic microscopy, lens-free microscopy and quadriwave lateral sheering interferometry. We report a linear relationship between optical volume density values measured with these different techniques and estimate the precisions of this measurement for the different individual instruments.

Wavefront sensing applied to determine the temperature dependence of the refractive index of liquids.

A method based on wavefront sensing is described to determine the temperature dependence of the refractive index of liquids. The technique only implies measuring the wavefront of a light beam passing through a micro-vessel containing the liquid. Here, this vessel is a crater made by CO2 laser processing in a fused silica plate. From the wavefront analysis, the optical path that is related to the refractive index of the liquid can be determined. This measurement can be done at different temperatures to obtain the temperature dependence of the refractive index. This method is applied to three liquids: water, ethanol, and cyclohexane at λ=630 nm. The results show a linear dependence in the range of 17°C-50°C and give coefficients dndT that are in good agreement with values from the literature.

Image-based adaptive optics for in vivo imaging in the hippocampus.

Adaptive optics is a promising technique for the improvement of microscopy in tissues. A large palette of indirect and direct wavefront sensing methods has been proposed for in vivo imaging in experimental animal models. Application of most of these methods to complex samples suffers from either intrinsic and/or practical difficulties. Here we show a theoretically optimized wavefront correction method for inhomogeneously labeled biological samples. We demonstrate its performance at a depth of 200 µm in brain tissue within a sparsely labeled region such as the pyramidal cell layer of the hippocampus, with cells expressing GCamP6. This method is designed to be sample-independent thanks to an automatic axial locking on objects of interest through the use of an image-based metric that we designed. Using this method, we show an increase of in vivo imaging quality in the hippocampus.

Tomographic diffractive microscopy with a wavefront sensor.

Tomographic diffractive microscopy is a recent imaging technique that reconstructs quantitatively the three-dimensional permittivity map of a sample with a resolution better than that of conventional wide-field microscopy. Its main drawbacks lie in the complexity of the setup and in the slowness of the image recording as both the amplitude and the phase of the field scattered by the sample need to be measured for hundreds of successive illumination angles. In this Letter, we show that, using a wavefront sensor, tomographic diffractive microscopy can be implemented easily on a conventional microscope. Moreover, the number of illuminations can be dramatically decreased if a constrained reconstruction algorithm is used to recover the sample map of permittivity.

Image Upconversion from the Visible to the UV Domain: Application to Dynamic UV Microstereolithography.

We present what to our knowledge is a new application of optical frequency upconversion of images in quadratic materials to dynamic UV microstereolithography. A 150 x 150 point visible image transmitted by a liquid-crystal display was upconverted in a lithium triborate crystal, and the UV image was successfully used to polymerize a commercial stereolithographic resin.

Fluorescence of Ta(2)O(5) thin films doped by kilo-electron-volt Er implantation: application to microcavities.

Luminescent layers are prepared by the implantation of kilo-electron-volt Er ions into tantalum pentoxide (Ta(2)O(5)) thin films made by ion plating. The implantation fluences range from 3.3 × 10(14) to 2 × 10(15) ions/cm(2), and the energies range from 190 to 380 keV. Refractive index, extinction coefficient, and losses on guided propagation are investigated. We show that these Er-implanted layers present an absorption as low as that of the nonimplanted films. When optically pumped with an Ar(+) laser (λ = 0.488 µm) beam, implanted films show peaked fluorescence spectra centered near 1.53 and 0.532 µm. We show that the fluorescence intensity is correlated with the intensity of the pump beam in the region where Er ions are implanted. Radiation patterns of Er ions located inside a single layer or inside a Ta(2)O(5)/SiO(2) dielectric stack made by ion plating are also investigated. We show that, in any case, spontaneous emission of Er ions can be spatially controlled.

Light-induced refractive-index modifications in dielectric thin films: experimental determination of relaxation time and amplitude.

A two-beam setup based on the totally reflecting prism coupler is shown to be a powerful means of characterizing light-induced refractive-index modifications in dielectric thin films. Rise and relaxation times and amplitudes of thin-film refractive-index variations can be measured. Some developments of the electromagnetic theory of prism coupling are presented for Gaussian incident beams. Measurements made on a single Ta(2)O(5) layer deposited on a silica glass are presented. Relaxation times of a few milliseconds reveal the thermal origin of the phenomena. The thermal nonlinear coefficient of this Ta(2)O(5) layer is nearly 10(-15) m(2)/W.

Nonlinear totally reflecting prism coupler: thermomechanic effects and intensity-dependent refractive index of thin films.

Starting with an accurate linear electromagnetic theory of a totally reflecting prism coupled to a dielectric waveguide, we implement a numerical technique to take into account optogeometric perturbations in stratified media. We calculate both the reflected fields in intensity on the prism base (near field) and in infinity (far field) for an incident Gaussian beam. The study of the variations of the intensity in the reflected beam (near and far fields) versus light power shows thermoinduced dilation of the prism and an intensity-dependent refractive index of thin films composed of tantalium pentoxyde and titanium dioxide.