Laser-beam deflection

The laser-beam deflection technique was first employed to determine the vibrational relaxation times of hydrogen [9] and deuterium [10]. The 

A laser beam which has been reduced approximately six-fold in diameter is passed through the shock tube perpendicular to the direction of shock front movement and focused onto the apex of a right angle prism. The deflection of the beam is directly proportional to the density gradient and is measured by the difference in light intensity falling upon two photomultiplier tubes. 

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Binnig et al. [48] invented the atomic force microscope in 1985. Their original model of the AFM consisted of a diamond shard attached to a strip of gold foil. The diamond tip contacted the surface directly, with the inter-atomic van der Waals forces providing the interaction mechanism. Detection of the cantilever s vertical movement was done with a second tip—an STM placed above the cantilever. Today, most AFMs use a laser beam deflection system, introduced by Meyer and Amer [49], where a laser is reflected from the back of the reflective AFM lever and onto a position-sensitive detector. 

The cantilever bending-technique requires a sensitive displacement detection such as a capacitance probe (Klokholm 1976, 1977), optical interferometry (Sontag and Tam 1986), a tunnelling tip (Wandass et al. 1988) or angular detection (e.g. laser beam deflection, Son-tag and Tam 1986 Trippel 1977 Tam and Schroeder 1988 Betz 1997 Sander et al. 1998). 

Fig. 7. (a) Laser beam deflection detection systems as often used in ambient AFMs. 

Bending beam method — The principles of the bending beam ( bending cantilever , laser beam deflection wafer curvature ) method were first stated by Stoney [i], who derived an equation relating the stress in the film to the radius of curvature of the beam. The bending beam method can be effectively used in electrochemical experiments, since the changes of the surface stress... 

Figure 3.15 Shock-tube arrangement of Kiefer and Lutz [129], utilizing laser-beam deflection, for study of vibrational relaxation in D2. Tx and T2 are collimating telescopes, Wx and W2 are windows (P) designates pressure transducers for measurement of shock velocity.

After these initial and promising lateral-force results, the friction force microscopy (FFM) was introduced. The FFM is a modified SFM with a four-quadrant photodiode, based on the laser beam deflection technique (Meyer Amer 1988) (Fig. 2.17). The beam is emitted by a low-voltage laser diode and reflected from the rear side of the cantilever to the four-quadrant photodiode. With this detection scheme, normal and torsional forces can be measured simultaneously. The torsional forces correspond to the lateral forces measured with the instrument of Mate et al. (1987). In 1993, Ovemey introduced the threefold measurement of topography, friction and elasticity on a polymer sample using an ITM. With this latest achievement, a wide spectram of tribological information was opened up, limited only by the lattice parameters of the sample. 

We have developed a laser beam deflection technique which, although similar to the method employed by Amer s group, differs in several important respects. U4-15) In particular, our method employs highly focused heating and probe laser beams, both incident normal to the sample surface, and the experiments are performed at high modulation frequencies of up to 10 MHz. 

Figure 1. Schematic depiction of laser beam deflection technique used for the thin-film thickness measurement experiments.

Figure 5. Relative amplitude at 1 MHz of laser beam deflection signal as a function of Al film thickness for a series of Al-on-Si and Al-on-Si02-on-Si films. Circles are experimental data, and curves are from the extended Opsal-Rosencwaig model.

Fig. 14. Rate coefficients derived from laser-beam deflection technique [40].

The most precise work to date is that obtained by the laser-beam deflection technique over the temperature range 3500—8000° K. The precision of the data is displayed in Fig. 14. Rate coefficients for hydrogen dissociation for several colliding partners including the hydrogen atom were calculated as... 

Shock tube studies of the decomposition of Oj have revealed the presence of an incubation period which precedes the observation of a steady rate of dissociation [11, 13, 58]. The length of the incubation period at a particular temperature was related to the vibrational relaxation time for oxygen at that temperature using previously measured relaxation data [59, 60]. The decomposition has been observed in a variety of inert diluents (He, Ar, Kr and Xe) in a wide range of oxygen concentrations (1—50%) over an extensive temperature range (2850—8500°K) by several different analytical methods including ARAS [13] and the laser-beam deflection technique [11]. 

The precision of the laser-beam deflection data for various amounts of oxygen diluted in krypton is comparable with that of the results displayed in Fig. 14. The variation of the rate coefficients over the 6000° K temperature interval is linear using the strjct Arrhenius equation... 

The complexity introduced by including reactions (15)—(19) precludes the analysis of the entire density gradient oscillogram as was done for homonuclear dissociations. The determination of atomic efficiencies for heteronuclear decompositions is not possible using the laser-beam deflection technique. 

Instrumentation. A cantilever with a sharp tip interacting with the surface under investigation is used. The actual bending of the cantilever is measured with a laser beam deflected from a mirror-like surface spot on the back of the cantilever towards a position-sensitive photodetector. The measured signal is used to control the piezo actuators. A constant force mode in which the cantilever-surface distance is kept at a preset interaction force and a constant height mode of scanning operation are possible. The principle of operation is schematically outlined in Fig. 7.9. 

Laser beam deflection offers a convenient and sensitive method of measuring cantilever deflection. In the non-contact mode, the AFM derives topographic images from measurements of attractive forces the tip does not touch the sample. On the other hand, in the contact mode, repulsion forces between the tip and the sample produce topographic images. 

ACOUSTO-OPTICS deals with the interaction between sound and laser. It can result in laser beam deflection, laser intensity modulation, and phase modulation and frequency shifting of laser. Devices and systems based on acoustooptic interaction have been playing a major role in various types of optical information processing as well as optical display systems. In Section II, we cover and review some basics of acousto-optic interactions and discuss a plane-wave scattering model for the sound-light interaction. In Section III, we discuss how intensity and frequency modulation of a laser beam can be accomplished by the acoustooptic effect. Applications such as laser beam deflection and laser TV display will then be discussed. In Section IV, we present modem applications of acousto-optics... 

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