6 - Scopes of laser in spectroscopy

Abstract

Spectroscopy deals with the interaction of light radiation with matter, which provides information on the structure and properties of matter (solids, liquids, and gasses). If laser light is used in place of the light radiation, then the spectroscopy is known as laser spectroscopy. Laser spectroscopy has emerged as a tool in many scientific techniques like tracking air quality, process control, medical research, national security, agriculture, artwork authentication, and many more. This is due to the special characteristics of lasers as compared to ordinary light. Although the emission of laser radiation is governed by the same rules and principles as that of any other light sources, laser light is not like any other ordinary source of radiation found in nature. It is a much more powerful technological tool than light from ordinary sources. Its features like coherence, monochromaticity, and collimation (directionality or low-beam divergence) make it special. The laser beam emerging from the output mirror of the resonant cavity is highly parallel, and its divergence (the spread in a beam of light) is typically a few milliradians, that is, negligibly small. The photons emitted even at a slight angle with respect to the tube axis bounce back into the walls of the tube and do not contribute to the output beam (not 100% true due to diffraction). The laser cavity is resonant only for the frequencies ν=nc/2d, where d is the separation between the mirrors of the resonant cavity adjusted as an integral of half of the wavelength, limiting the wavelength range (production of laser of well-defined wavelength). The intensity of the laser, defined as the power emitted per unit area of the output mirror per unit solid angle, is extremely high compared with that of a conventional source. The conventional sources of radiation are incoherent in nature, which means that any two photons of the electromagnetic waves of the same wavelength are out of phase, while the laser is both temporally and spatially coherent, which means that the coherence of the laser medium exists for a relatively long time and over a relatively large distance. Laser, by virtue of its coherent nature, is used for local heating, as in metal cutting, metal welding, and for holography. The coherent nature of the laser is by virtue of the mechanism through which it is produced, that is, the process of stimulated emission where photons are essentially copied or exactly in phase. The production of laser in the same phase takes place as all emitted photons are at exactly the same wavelength due to the transition between two fixed energy levels (the amplification mechanism of the laser). The simplest explanation for these properties of the laser is in the mechanism of the laser itself. The process mainly includes the stimulated emission, which takes place in the amplifying medium contained by the laser. This is done with the application of a set of mirrors used for feeding the light back to the amplifying medium so that the developed beam is grown continuously. The key concept for the realization of the laser operation is the principle of coherence accompanying stimulated emission. This stimulated emission needs the process of population inversion, for which the lasing medium must have at least three energy levels.