Light lasers

SIAM Scanning interferometric apertureless microscopy [103b] Laser light is reflected off the substrate, and scattering between an AFM tip and sample is processed interferometrically Diffraction Surface structure... 

In the previous section we discussed light and matter at equilibrium in a two-level quantum system. For the remainder of this section we will be interested in light and matter which are not at equilibrium. In particular, laser light is completely different from the thennal radiation described at the end of the previous section. In the first place, only one, or a small number of states of the field are occupied, in contrast with the Planck distribution of occupation numbers in thennal radiation. Second, the field state can have a precise phase-, in thennal radiation this phase is assumed to be random. If multiple field states are occupied in a laser they can have a precise phase relationship, something which is achieved in lasers by a teclmique called mode-locking Multiple frequencies with a precise phase relation give rise to laser pulses in time. Nanosecond experiments... 

Cohen-Tannoud]i C 1991 Atomic motion in laser light Fundamental Systems in Quantum Optics ed J Dalibard et al (Oxford Elsevier)... 

Surface photochemistry can drive a surface chemical reaction in the presence of laser irradiation that would not otherwise occur. The types of excitations that initiate surface photochemistry can be roughly divided into those that occur due to direct excitations of the adsorbates and those that are mediated by the substrate. In a direct excitation, the adsorbed molecules are excited by the laser light, and will directly convert into products, much as they would in the gas phase. In substrate-mediated processes, however, the laser light acts to excite electrons from the substrate, which are often referred to as hot electrons . These hot electrons then interact with the adsorbates to initiate a chemical reaction. 

PES of neutral molecules to give positive ions is a much older field [ ]. The infomiation is valuable to chemists because it tells one about unoccupied orbitals m the neutral that may become occupied in chemical reactions. Since UV light is needed to ionize neutrals, UV lamps and syncln-otron radiation have been used as well as UV laser light. With suitable electron-energy resolution, vibrational states of the positive ions can be... 

Ulness D J, Stimson M J, Kirkwood J C and Albrecht A C 1997 Interferometric downconversion of high frequency molecular vibrations with time-frequency-resolved coherent Raman scattering using quasi-cw noisy laser light C-H stretching modes of chloroform and benzene J. Rhys. Chem. A 101 4587-91... 

Ulness D J and Albrecht A C 1996 Four-wave mixing in a Bloch two-level system with incoherent laser light having a Lorentzian spectral density analytic solution and a diagrammatic approach Rhys. Rev. A 53 1081-95... 

Chu B 1991 Laser Light Scattering, Basic Principies and Practice 2nd edn (New York Academic) (See also the first edition (published in 1974) that contains more mathematical derivations.)... 

Figure B2.5.11. Schematic set-up of laser-flash photolysis for detecting reaction products with uncertainty-limited energy and time resolution. The excitation CO2 laser pulse LP (broken line) enters the cell from the left, the tunable cw laser beam CW-L (frill line) from the right. A filter cell FZ protects the detector D, which detennines the time-dependent absorbance, from scattered CO2 laser light. The pyroelectric detector PY measures the energy of the CO2 laser pulse and the photon drag detector PD its temporal profile. A complete description can be found in [109].

Dalibard J and Cohen-Tannoudji C 1985 Dressed-atom approach to atomic motion in laser light the dipole force revisited J.Opt.Soc.Am. B 21707-20... 

Figure C 1.5.6. Single Ag nanoparticles imaged with evanescent-wave excitation. (A) Unfiltered photograph showing scattered laser light (514.5 nm) from Ag particles immobilized on a polylysine-coated surface. (B) Bandpass filtered (540-580 nm) photograph taken from a blank Ag colloid sample incubated witli 1 mM NaCl and...

The acronym LASER (Light Amplification via tire Stimulated Emission of Radiation) defines the process of amplification. For all intents and purjDoses tliis metliod was elegantly outlined by Einstein in 1917 [H] wherein he derived a treatment of the dynamic equilibrium of a material in a electromagnetic field absorbing and emitting photons. Key here is tire insight tliat, in addition to absorjDtion and spontaneous emission processes, in an excited system one can stimulate tire emission of a photon by interaction witli tire electromagnetic field. It is tliis stimulated emission process which lays tire conceptual foundation of tire laser. 

The importance of laser light, in brief, is tliat its base characteristics, coherence, spectral and polarization purity, and high brilliance allow us to manipulate its properties. Gain switching [i, 10] and mode locking [16] are prime examples of our ability to very specifically control tire laser output. It is easy to see why lasers are tire ideal sources for optoelectronic applications. 

In experimental measurements, sueh sharp 5-funetion peaks are, of eourse, not observed. Even when very narrow band width laser light sourees are used (i.e., for whieh g(co) is an extremely narrowly peaked funetion), speetral lines are found to possess finite widths. Let us now diseuss several sourees of line broadening, some of whieh will relate to deviations from the "unhindered" rotational motion model introdueed above. 

To understand the production of laser light, it is necessary to consider the interaction of light with matter. Quanta of light (photons) of wavelength X have energy E given by Equation 18.1, in which h is Planck s constant (6.63 x 10 J-sec) and c is the velocity of light (3 x 10 m-sec-h-... 

The timing of the emission is clearly dependent on the system in use. For example, if pumping is relatively slow and stimulated emission is fast, then the emergent beam of laser light will appear as a short pulse (subsequent lasing must await sufficient population inversion). This behavior is... 

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