Long-distance channeling of cold atoms exiting a 2D magneto-optical trap by a Laguerre-Gaussian laser beam.

Using a blue-detuned laser, shaped into a nearly Laguerre-Gaussian (LG) donut mode, we channel atoms exiting a two-dimensional magneto-optical trap (2D-MOT) over a 30 cm distance. Compared to a freely propagating beam, the atomic flux (∼10(10) at/s) is conserved whereas the divergence is reduced from 40 to 3 mrad. So, 30 cm far the 2D-MOT exit, the atomic beam has a 1 mm diameter and the atomic density is increased by a factor of ∼200. The LG-channeled-2D-MOT has been studied versus the order of the LG mode (from 2 to 10) and versus the laser-atom frequency detuning (from 2 to 120 GHz).

Negative experimental evidence for magneto-orbital dichroism.

A light beam can carry both spin angular momentum (SAM) and orbital angular momentum (OAM). SAM is commonly evidenced by circular dichroism (CD) experiments i. e. differential absorption of left and right-handed circularly polarized light. Recent experiments, supported by theoretical work, indicate that the corresponding effect with OAM instead of SAM is not observed in chiral matter. Isotropic materials can show CD when subjected to a magnetic field (MCD). We report a set of experiments, under well defined conditions, searching for magnetic orbital dichroism (MOD), differential absorption of light as a function of the sign of its OAM. We experimentally demonstrate that this effect, if any, is smaller than a few 10(−4) of MCD for the Nd:YAG 4/9/2 →4 F5/2 transition. This transition is essentially of electric dipole nature. We give an intuitive argument suggesting that the lowest order of light matter interaction leading to MOD is the electric quadrupole term.

Droplet digital PCR improves absolute quantification of viable lactic acid bacteria in faecal samples.

Analysing correlations between the observed health effects of ingested probiotics and their survival in digestive tract allows adapting their preparations for food. Tracking ingested probiotic in faecal samples requires accurate and specific tools to quantify live vs dead cells at strain level. Traditional culture-based methods are simpler to use but they do not allow quantifying viable but non-cultivable (VBNC) cells and they are poorly discriminant below the species level. We have set up a viable PCR (vPCR) assay combining propidium monoazide (PMA) treatment and either real time quantitative PCR (qPCR) or droplet digital PCR (ddPCR) to quantify a Lactobacillus rhamnosus and two Lactobacillus paracasei subsp. paracasei strains in piglet faeces. Adjustments of the PMA treatment conditions and reduction of the faecal sample size were necessary to obtain accurate discrimination between dead and live cells. The study also revealed differences of PMA efficiency among the two L. paracasei strains. Both PCR methods were able to specifically quantify each strain and provided comparable total bacterial counts. However, quantification of lower numbers of viable cells was best achieved with ddPCR, which was characterized by a reduced lower limit of quantification (improvement of up to 1.76 log10 compared to qPCR). All three strains were able to survive in the piglets' gut with viability losses between 0.78 and 1.59 log10/g faeces. This study shows the applicability of PMA-ddPCR to specific quantification of small numbers of viable bacterial cells in the presence of an important background of unwanted microorganisms, and without the need to set up standard curves. It also illustrates the need to adapt PMA protocols according to the final matrix and target strain, even for closely related strains. The PMA-ddPCR approach provides a new tool to quantify bacterial survival in faecal samples from a preclinical and clinical trial.

Real time control of the spatial selectivity during laser treatment of a biological surface.

We propose a new irradiation technique to improve the spatial selectivity and the dose control during a laser treatment of a biological surface. The technique is based on the use of a spatial light modulator to project a rectangular laser image of 8 mm x 5 mm. The spatial distribution of the irradiation is controlled in real time by image analysis applied to the acquired image of the surface to be treated. The aim is to discriminate between the target areas to be irradiated and the surrounding parts to be preserved. The resolution is about 12 microm and is limited by the resolution of the camera and the spatial light modulator. For treatments of a large surface we use an intensity correlator to measure the displacement and to avoid an unwanted second irradiation on a target.

Spatially selective laser irradiation method controlled by real-time image analysis: optical aspects.

We present a spatially selective irradiation method for laser treatment of biological surfaces. The purpose is to irradiate only the pathological targets and to preserve the healthy surrounding parts. We are interested here in the optical arrangement of the device and in the evaluation of its physical limitations before it is used for medical purposes. The method uses real-time image analysis applied to the video imagery of the surface to be treated. The result of the image analysis generates the control signal to a spatial light modulator, allowing us to project an adequate repartition of laser irradiance on the surface to be treated. Experimental results demonstrate a spatial selectivity of approximately 10 microm for a 6.7 mm x 5 mm field. The optical system has no moving parts (unlike galvanometric scanners) and is able to irradiate multiple targets simultaneously with different doses.