High-power pulsed laser

Optical second-harmonic generation (SHG) has recently emerged as a powerful surface probe [95, 96]. Second harmonic generation has long been used to produce frequency doublers from noncentrosymmetric crystals. As a surface probe, SHG can be caused by the break in symmetry at the interface between two centrosymmetric media. A high-powered pulsed laser is focused at an angle of incidence from 30 to 70° onto the sample at a power density of 10 to 10 W/cm. The harmonic is observed in reflection or transmission at twice the incident frequency with a photomultiplier tube. 

SHG Optical second-harmonic generation [95, 96] A high-powered pulsed laser generates frequency-doubled response due to the asymmetry of the interface Adsorption and surface coverage rapid surface changes... 

The transversely excited atmospheric-pressure (TEA) laser, inherently a pulsed device rather than a continuous laser, is another common variety of carbon dioxide laser (33,34). Carbon dioxide—TEA lasers are an important class of high-power pulsed lasers. Pulse durations are in the submicrosecond regime peak powers exceed 10 MW. 

Laser removal of tattoos and of colored birthmarks has been widely studied. A high power pulsed laser at a wavelength absorbed by the pigment is used to vaporize the pigment and to bleach the colored area. Ruby, Nd YAG, and dye lasers are favored for this purpose. 

High power pulsed lasers are used to produce plasmas and thus to sample and excite the surfaces of soHds. Improvements in minimum detectable limits and decreases in background radiation and in interelement interference effects result from the use of two lasers (99) (see Surface and interface analysis). 

A survey of the application of high-power pulsed lasers to experiments on a nanosecond time scale has been given by Bradley 2 ). 

High-power pulsed lasers offer the possibility of studying nonlinear phenomena such as stimulated Raman scattering, the inverse Raman effect and the hyper-Raman effect. These investigations have contributed much to our knowledge of the solid-state and liquid stucture of matter and its higher order constants. 

The effect that the light itself induces as it propagates through the medium determines the different types of nonlinear processes and optical phenomena. These phenomena are usually only observed at very high light intensities and such nonlinearity requires the use of high-power pulsed lasers [20]. 

Optical multipass cells have been used for the enhancement of CW Raman scattering(4) however, these cells are typically not well-suited for use with high power, pulsed lasers. A new multipass cell for use with a pulsed Nd YAG laser is proposed whereby the 1.06 micron laser output is admitted into a multipass cell cavity where it is partially converted to 532nm with a Brewster s angle cut second harmonic generating crystal The 532nm pulse is trapped in the mirrored cavity while the 1.06 micron pulse is dumped. This multipass cell concept has been demonstrated with the experimental set-up shown in figure 1. 

The laser ablation system consists of a high-power pulsed laser, optics to focus the laser at or near the surface of the sample, and an ablation cell. Small ablated particles are swept out of the ablation cell and carried into the ICP in a flowing gas. Often a microscope lens and video camera are positioned to allow the operator to view the sample surface before and after ablation. A high-quality microscope and precise positioning of the sample relative to the laser beam are essential for good spatially resolved sampling. 

Our current theoretical model constitutes a significant improvement upon our obsolete scheme [19] which has been stringently verified recently (to the accuracy of about 2 MHz) for the deuterium IS — 2S transition [14] driven by a high-power pulsed laser [22,23] with an rms chirp of 2.3 MHz (see Figs. 2 and 4). These improvements have resulted eventually in by a factor 10 higher overall accuracy (now, < 100kHz) as well as in a wider scope of physical pro-... 

Molecular beam sources for use at high collisional energies have also employed the methods of charge exchange [41], sputtering [42] and rotor beams [43]. Pulsed metal beams have been generated by the novel method of the evaporation of a metal film by a high power pulsed laser [44],... 

It is difficult to span the intervening energy gap between photo- and radiation chemistry, however, high powered pulsed lasers, utilising multiphoton absorption by the medium, do much to remedy this situation. For the most part, the work described falls into two categories, data with steady state irradiation i.e. light sources and Co-y rays, and pulsed experiments as with lasers and pulsed electron accelerators such as Van de Graaffs and Linacs. 

Crystals of BBO and LBO are extensively used for generating visible and UV light from high-power pulsed lasers. The general utility of the materials derives from their high optical damage thresholds and transparency ranges that extend to 190 nm for BBO and 170 mn for LBO. 

Access to central facilities. National laboratories now provide important new tools for in situ electrochemical characterization, including facilities for synchrotron radiation, soft neutrons, high-power pulsed laser light, and supercomputers. These provide investigators with new capabilities but demand a new mode of operation. Experiments must be prepared and rehearsed and then transported to the central facility for an intensive, scheduled experimental run. The complexity of the apparatus may require collaboration with others more familiar with the equipment. The central laboratories are essential for many of the research opportunities identified herein and must be funded at levels appropriate to the anticipated new users. 

PLD is a thin film deposition technique akin to physical vapour deposition (PVD) whereby a high-power pulsed laser beam is focused inside a vacuum chamber to strike a target of the material to be deposited (Figure 5.47). 

An alternate approach used by the Miller group is to pulse amplify a CW dye laser. This approach degrades the laser resolution from about 1 MHz to about 200 MHz but results in a high-power pulsed laser beam with excellent mode quality. This high peak power and high spectral resolution results in excellent spectra with an improved S/N ratio. The only serious drawback of this scheme is the increased complexity and cost of the laser systems. 

Figure 17.1.21 Experimental apparatus for SHG experiments. A small portion of the beam from a high-power pulsed laser is sent to a reference channel, after frequency doubling, to provide a signal to normalize for fluctuations in the laser intensity. The main beam is linearly polarized and filtered before impinging on the sample. The resulting beam at 2

The CARS measurement system consists of two high powered pulsed lasers, optics and detection equipment, and specialized software for a system dedicated to the acquisition and analysis of CARS spectra for the time resolved point measurements of temperature in a large scale flame. 

Studies of vibrational relaxation using lasers have expanded dramatically in the past fourteen years due largely to the increased availability of versatile, high-power, pulsed-laser devices. Increased demand for energy-transfer parameters has resulted from attempts to develop new lasers and to push existing systems to their practical limits. Thus the development of lasers and the study of energy-transfer phenomena have been mutually synergistic processes. 

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