Laser beam focusing

The depth profiling of the Cu-Ag-Si samples was performed with the laser beam focused on to the sample surface to the spot of approximately 30-40 pm. To obtain the best depth resolution the laser fluence was maintained near the 1 J cm level, close to the threshold of the LIBS detection scheme. The intensity profiles of Cu and Ag emission lines are shown in Fig. 4.43. The individual layers of Cu and Ag were defi-... 

Fig. 5.5. Schematic view of the deflection sensing system as used in the NanoScope III AFM (Digital Instruments, Santa Barbara, CA, USA). The deflection ofthe cantilever is amplified by a laser beam focused on the rear ofthe cantilever and reflected towards a split photodiode detector.

Movements of single myosin molecules along an actin filament can be measured by means of an optical trap consisting of laser beams focused on polystyrene beads attached to die ends of actin molecules. (Adapted from Finer et at., 1994. Nature 368 113- 119. See also Block, 1995. Nature 378 132 133.)... 

Since the demonstration by Schumacher et al ) of the use of alkali metal vapor inclusion into a supersonic beam to produce clusters, there have been a number of attempts to generalize the approach. It has recently been recognized that instead of high temperature ovens, with their concommitant set of complex experimental problems, an intense pulsed laser beam focused on a target could be effectively used to produce metal atoms in the throat of a supersonic expansion valve. ) If these atoms are injected into a high pressure inert gas, such as helium, nucleation to produce clusters occurs. This development has as its most important result that clusters of virtually any material now can be produced and studied with relative ease. 

The radiation pressure exerted by light is very weak. A bright laser beam of several milliwatts of power can exert only a few piconewtons (pN) of force. However, a force of 10 pN is enough to pull a cell of E. coli through water ten times faster than it can swim.213 In about 1986, it was found that a laser beam focused down to a spot of - one K ( 1 pm for an infrared laser) can trap and hold in its focus a retractile bead of 1 pm diameter. This "optical tweezers" has become an important experimental tool with many uses.213 214 For example, see Fig. 19-19. Not only are optical tweezers of utility in studying biological motors but also mechanical properties of all sorts of macromolecules can be examined. For example, DNA can be stretched and its extensibility measured.215 Actin filaments have even been tied into knots 216... 

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]. 

New developments in spectroscopic techniques have made it possible to obtain Raman spectra and more recently also FTIR spectra of oligomer crystals. The crystal area used for FTIR studies with all reflecting Cassegrain type microscopes is about 30 x 30 um, while the laser beam focused through the microscope covers about 5 j,m in diameter. Several examples of IR and Raman crystal studies, some of them correlating with X-ray crystal diffraction studies, are listed in Sec. 4.7.1.3.5. 

In the case of a bull s eye structure, the relative electromagnetic enhancement is about 100 (computed for an incoming plane wave and consistent with experiment (72)), but the structure radius and thus the laser beam focus spot are about 4 times the diffraction limit (minimum focus size). If we compare the relative intensity enhancement to the case of a diffraction-limited spot, the intensity with the bull s eye is increased by 100x(l/4) = 6.25, which is about the enhancement with a single (bare) aperture with a tightly focused beam. Of course, this argument has to be supplemented with the influence the emission rate and the directivity of the fluorescence beam, which contribute to the overall enhanced fluorescence. However,... 

Fig. 9.6. Schematic diagram of the experimental system for laser ablation-assisted radio-frequency atomization excitation. (I) Sample holder, (2) tantalum lid, (3) graphite cup, (4) graphite disc, (5) rf power source, (6) different types of sample holder for sintered ceramics, (7) NdiYAG laser, (8) laser beam-focusing lens, (9) spectrometer, (10) quartz window for laser irradiation, (11) central electrode, (12) discharge chamber, (13) quartz window for optical observation. (Reproduced with permission of the Royal Society of Chemistry.)...

One of the recent techniques most commonly used to measure diffusion in 2-dimensions is fluorescence recovery after photobleaching (FRAP). This method uses a laser beam focused through a fluorescence microscope to follow the diffusion of fluorescent molecules in a plane perpendicular to the laser beam. The fluorescence intensity from a laser spot of known diameter, typically a few microns, is measured. The laser intensity is then increased by approximately 1000 times. This irreversibly photobleaches any fluorophore in the spot. The intensity is then decreased again and the recovery in fluorescence intensity measured as unbleached molecules diffuse into the spot. The time function of fluorescence intensity is then analysed to give surface self-diffusion coefficient (Clark et al. 1990a, b, Wilde Clark 1993, Ladhaetal. 1994). 

Observing single molecules. A major advance in the study of molecular motors has been the development of ways to observe and study single macromolecules. The methods make use of optical traps (optical "tweezers") that can hold a very small ( 1 pm diameter) polystyrene or silica bead near the waist of a laser beam focused through a microscope objective. ° ... 

In contrast to the setup shown in Figure 5, our experiment uses two coaxial laser beams focused onto the sample C by a lens L (Figure 7). The dye laser beam (power 1 to 10 mW) creates the thermal lens in the sample, whereas the helium-neon laser is used only for monitoring development of the thermal lens. To avoid a thermal lens being induced by the helium-neon laser, a neutral density filter F2 reduces the power of its beam to 6 to 7 /xW in the sample. In front of the detector D, an interference filter F, blocks the beam of the dye laser, and a pinhole P is placed such that only light near the optical axis reaches the detector. To monitor the wavelength of excitation Ao, part of the dye laser beam is deflected by a glass plate Gj onto a... 

Scanning tunneling microscopy (STM) and atomic force microscopy (AFM) are scanning probe techniques, in which sharp solid probe scans over the specimen surface and provides topographic information. STM provides information at the atomic level and can identify the arrangement of atoms on sample surface, hi this technique, a sharp tungsten tip is scanned very close to the sample surfece and tunneling current is measured. The current is inversely proportional to the tip-sample separation. AFM makes nse of a sihcon nitride probe mounted on a flexible cantilever. The movement of the cantilever is monitored by a laser beam focused on the surface as the probe scans the sample surface, attractive and repulsive forces between the probe and the surface are measured. 

A laser beam focused into the diffusion layer in combination with a diode array detector may even be used to directly image the concentration profile inside the diffusion layer, as described by Posdorfer et al. [76,77]. 

The primary limitations on laser energy and power are the restrictions on the size of the amplifier and the damage thresholds when the beam arrives at the laser mirrors and windows. A 1-W continuous carbon dioxide laser beam, focused on a fire brick, will cause the brick to glow white hot and begin to disintegrate. A carbon dioxide laser has been made to continuously operate at a level 1,000,000 times that powerful Similarly, a pulsed laser of 0.0005 J, when focused into the eye, can cause severe retinal damage. A ptrlsed neody mittm laser has been made to produces a bilhon times that energy I... 

In principle, Raman spectroscopy is a microtechnique [161) since, for a given light flux of a laser source, the flux of Raman radiation is inversely proportional to the diameter of the laser-beam focus at the sample, i.e., an optimized Raman sample is a microsample. However, Raman microspectroscopy able to obtain spatially resolved vibrational spectra to ca. 1 pm spatial resolution and using a conventional optical microscope system has only recently been more widely appreciated. For Raman microspectroscopy both conventional [162] and FT-Raman spectrometers [ 163], [ 164] are employed, the latter being coupled by near-infrared fiber optics to the microscope. 

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