Advances in interference filter technology permit currently using selected parts of the N2 and O2 pure rotational Raman spectra with very low temperature sensitivity, while rejecting sufficiently the elastic return. The Raman technique to retrieve the aerosol extinction coefficient can then be used with higher signal-to-noise ratios, because of a higher (about 8 times) effective differential backscatter cross-section as compared to the cross-section of the N2 vibro-rotational spectra. The design and results of pure rotational Raman channels at 354 nm and 530 nm allowing daytime aerosol extinction measurements implemented at the EARLINET/ACTRIS Barcelona lidar station are presented and discussed.
Manipulation and trapping of particles have taken a huge relevance in recent years thanks to many applications with revolutionary contributions to diverse fields. Several experiments have demonstrated that thermal effects can improve the current micromanipulation techniques such as DNA manipulation or assembly of colloidal crystals. In this work, we present the effect of laser-induced thermal effects, such as convection currents and thermophoresis, on the trap stiffness (spring constant) constant of an optical trap of 3-micrometer particles suspended in water. These effects are a consequence of light absorption in a thin layer of hydrogenated amorphous silicon (a-Si:H) deposited at the bottom of the chamber which generates a thermal gradient. Since these effects (and its correspondent forces) are symmetric around the beam focus, trapped particles, experience an increment in the trapping force. Around the beam focus, the drag force associated with convective currents is directed upwards and are compensated by optical scattering force. Depending on the laser power, the trap stiffness increases significantly, so a trapped particle can be dragged along the cell (by displacing the sample and leaving the beam fixed) at velocities around 90 μm/s without escaping the trap, whereas in the absence of the a-Si:H film, the escape velocity of the particle in the trap drops to velocities around 30 μm/s. This presents a simple, yet effective, option for optical manipulation at low powers (<5 mW) and its possible applications in the manipulation of a variety of biological micro samples.
In this work, we compare two techniques to make point-diffraction interferometers (PDI): microlithography and the mercury drop method to know with which of these the best results can be obtained. For the comparison, we used the wavefront generated by a commercial reference surface of λ/20 analyzing the interference pattern generated by the PDIs, we obtained information from the wavefront generated by the pinhole. Several PDIs were created and analyzed to have a statistical error information of both techniques.
In this work we demonstrate the increasing of the trap stiffness (spring constant) constant of an optical trap of particles suspended in water by laser-induced convection currents. These currents are the result of thermal gradients created by a light absorption in a thin layer of hydrogenated amorphous silicon (a:Si-H) deposited at the bottom of cell. Since convection currents (and therefore drag forces) are symmetric around the beam focus particles trapped by the beam are further contained. Around the focus the drag force is directed upwards and partially compensated by radiation pressure depending on the laser power increasing the stiffness of the optical trapping increases significatively so a particle trapped could dragged (by moving the translation stage leaving the beam fixed) at velocities as high as 90μm/s without escaping the trap, whereas with no a:Si-H film, the particle escapes from the trap at lower velocities (30μm/s).
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