Beam, light

All the previous discussion in this chapter has been concerned with absorption or emission of a single photon. However, it is possible for an atom or molecule to absorb two or more photons simultaneously from a light beam to produce an excited state whose energy is the sum of the energies of the photons absorbed. This can happen even when there is no intemrediate stationary state of the system at the energy of one of the photons. The possibility was first demonstrated theoretically by Maria Goppert-Mayer in 1931 [29], but experimental observations had to await the development of the laser. Multiphoton spectroscopy is now a iisefiil technique [30, 31]. 

Optothemial spectroscopy is a bolonietric method that monitors the energy in a stream of molecules rather than in the light beam. A well collimated molecular beam is directed toward a liquid helium cooled bolometer. There will be energy... 

Figure Bl.18.8. Differential interferenee eontrast the light beam is split into two beams by a Wollaston prism. The two beams pass the objeet at elosely spaeed positions and give, after interferenee, a eontrast due to the phase differenee.

Figure Bl.18.11. Confocal scanning microscope in reflection the pinliole in front of the detector is in a conjugate position to the illumination pinliole. This arrangement allows the object to be optically sectioned. The lens is used to focus the light beam onto the sample and onto the pinliole. Thus, the resulting point spread fimctioii is sharpened and the resolution increased.

It is well known that a light beam carries momentum and tliat tire scattering of light by an object produces a force. This property of light was first demonstrated by Frisch [K)] tlirough tire observation of a very small transverse... 

Figure C 1.4.2. Spontaneous emission following absorjDtion occurs in random directions, but absorjDtion from a light beam occurs along only one direction.

Figure C 1.4.8. (a) An energy level diagram showing the shift of Zeeman levels as the atom moves away from the z = 0 axis. The atom encounters a restoring force in either direction from counteriDropagating light beams, (b) A typical optical arrangement for implementation of a magneto-optical trap.

The atom will therefore experience a net restoring force pushing it back to the origin. If the light beams are red detuned F, then the Doppler shift of the atomic motion will introduce a velocity-dependent tenn to the restoring force such that, for small displacements and velocities, the total restoring force can be expressed as the sum of a tenn linear in velocity and a tenn linear in displacement. 

Giordimaine J A 1962 Mixing of light beams in orystals Phys. Rev. Lett. 8 19... 

The observed rotation a of an optically pure substance depends on how many mol ecules the light beam encounters A filled polarimeter tube twice the length of another produces twice the observed rotation as does a solution twice as concentrated To account for the effects of path length and concentration chemists have defined the term specific rotation, given the symbol [a] Specific rotation is calculated from the observed rotation according to the expression... 

Specific Rotation. Optical rotation is caused by individual molecules of the optically active compound. The amount of rotation depends upon how many molecules the light beam encounters in passing through the tube. When allowances are made for the length of the tube that contains the sample and the sample concentration, it is found that the amount of rotation, as well as its direction, is a characteristic of each individual optically active compound. 

The small cross-sectional area covered by a laser light beam coupled with the energy density in the beam leads to power levels reaching from milliwatts to many hundreds of kilowatts per square meter. 

As with a light beam and glass lenses, an ion beam can be directed and focused using electric and magnetic fields, often called lenses by analogy with their optical counterparts. 

The subscript v is attached to both of these intensities as a reminder that the foregoing analysis is based on the assumption of vertical polarization for the incident light beam. The ratio of these intensities gives the fraction of light scattered per molecule by vertically polarized light ... 

The aluminium absorbs the radiation, heats up and transfers the heat to xenon gas contained in the cell. As the temperature of the gas varies the curvature of a flexible mirror of antimony-coated collodion, forming a part of the cell, changes. Reflection of a light beam from this mirror, which is on the outside of the Golay cell, indicates its curvature and therefore the intensity of radiation absorbed by the cell. 

The real utility of d comes in the analysis of thin films. Consider a substrate of refractive index supporting a thin film of thickness d and refractive index in contact with an internal reflection element (the prism) of refractive index as shown in Figure 24. In this case, d depends on the polarization of the incident light beam and is given by... 

The electromagnetic field of a light beam produces an electrical polari2ation vector in the material through which it passes. In ordinary optics, which may be termed linear optics, the polari2ation vector is proportional to the electric field vector E. However, the polari2ation can be expanded in an infinite series ... 

A wide variety of other nonlinear optical effects also have been demonstrated. According to equation 12, if two light beams having frequency CO and CO2 are combined in a material with a nonzero value of light waves of frequency + UJ2 and are produced. A combination of such effects, used... 

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