Laser focal point

Photoablation (diode-pumped Nd YV04 laser, X = 532 nm) was used to create a master on the PMMA layer coated on a Si wafer (see Figure 2.20a). The PMMA layer was doped with rhodamine B to facilitate the absorption of the laser radiation. The width of the ablated features depends on the diameter and the position of the laser focal point. The best aspect ratios were obtained with the laser beam focused 3 1 pm into the PMMA film. The ablated PMMA-Si master was used to cast a PDMS layer (see Figure 2.19b). The cast PDMS layer appeared to have smoother surfaces than the PMMA master [367]. 

The scattering cylinder of the focused laser beam must be aligned vertically to be parallel to the entrance slit of the spectrometer. The collection lens is used to image this cylinder onto the slit. The sample is then inserted at the laser focal point with its long axis parallel to the beam and a quick survey scan is taken until a Raman band is found. The sample, the excitation beam, and the collection lens are then carefully adjusted for maximum signal at this spectrometer setting. 

Many other types of atomization devices have been used in atomic spectroscopy. Gas discharges operated at reduced pressure have been investigated as sources of atomic emission and as ion sources for mass spectrometry. The glow discharge is generated between two planar electrodes in a cylindrical glass tube filled with gas to a pressure of a few torr. High-powered lasers have been employed to ablate samples and to cause laser-induced breakdown. In the latter technique, dielectric breakdown of a gas occurs at the laser focal point. 

A. Mizuno, M. Nishioka, Y. Ohno and L.-D. Dascalescu, Liquid microvortex generated around a laser focal point in an intense high-frequency electric field, IEEE Transactions on Industry Applications, 31(3), 464 68 (1995). 

Fig. 1.39 A schematic geometry of a laser heating arrangement. Representations of the horizontal and vertical temperature distributions are given, with typical dimensions of the sample and the YAG-laser focal point (reference 60).

The use of nanovials to confine the sample spots to a size approximately matching that of the laser focal point is also a major advantage in the automation process since searching for high intensity... 

Fig. 31 Two basic scanning modes for two-photon photopolymerization microfabrication. Conceptive illustration of how the two scan modes could be utilized for writing a character s . a raster scan and b vector scan. The solid and open circles denote exposed and unexposed dots, respectively, all scanned by the laser focal point. The dashed open circles in the right part means the dots that aren t scanned...

As was discussed in the previous part, the temperature elevation in the solutions can be ascribed to the absorption of the NIR light by the solvents. In order to quantitatively explain the temperature elevation coefficient, AT/AP, for other solvents, we proposed a simple model that can parametrize the temperature elevation. As easily predicted, the AT/AP value is closely related to the extinction coefficient of light absorption, a, and the thermal conductivity, X. Heat generated at the focal point ofthe NIR beam is proportional to the extinction coefficient, a, and the incident laser power, P, as represented by Eq. (8.5). 

As shown in Fig. 12 fluid flow can be determined by measuring the doppler shift in laser radiation scattered from particles in the moving fluid stream. No sensor is required in the moving stream. The laser radiation focal point can be moved across the flow tube to measure velocity profiles. Fluid linear flows from 0.01 to 5000 inches (0.03 centimeter to 127 mctersi per second hate been measured. Contaminants, such as smoke, may have to be added to gases to provide scattering centers for the laser beam. 

Let us now examine the emitted light rays in more detail and assume, in another example, that the traveler who is moving from left to right directs a laser in the direction Bfl of Fig. 12. The direction of the beam in relation to the stationary world will then be AG. Point G is found by drawing a line parallel to the line of travel (the x axis) from the point (H) on the sphere of observation to the corresponding point (G) on the ellipsoid of observation. As the point of observation is identical in space and time in the two systems, the center of the sphere should coincide with the focal point of the ellipsoid of observation [15]. 

This approach was employed, since spectra of single spots on the surface might have local variations in composition which would hinder a comparison of the two samples. The spot size at the focal point was approximately 150 microns in diameter. The instrument was operated in dual-cell mode, with ions detected in the analyzer side immediately after the laser was fired, in order to minimize undesirable ion-molecule reactions which might occur at the higher pressures in the source cell. Both positive and negative ion spectra were collected for each sample. 

The optical trapping method uses a highly focused laser beam to trap and manipulate particles of interest in a medium (illustrated in Figure 3). The laser is focused on a dielectric particle (e.g., a silica microscopic bead), the refractive index of which is higher than the suspension medium. This produces a light pressure (or gradient force), which moves the particle towards the focal point of the beam, that is, the beam waist (Lim et al., 2006). 

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