Active media

To make an oscillator from an amplifier requires, in the language of electronics, positive feedback. In lasers this is provided by the active medium being between two mirrors, both of them highly reflecting but one rather less so in order to allow some of the stimulated radiation to leak out and form the laser beam. The region bounded by the mirrors is called the laser cavity. Various mirror systems are used but that shown in Figure 9.1, consisting of... 

Photons of energy hcv are generated initially in the cavity through spontaneous emission. Those that strike the cavity mirrors at 90° are retained within the cavity causing the photon flux to reach a level which is sufflciently high to cause stimulated emission to occur, and the active medium is said to lase. 

In practice the laser can operate only when n, in Equation (9.2), takes values such that the corresponding resonant frequency v lies within the line width of the transition between the two energy levels involved. If the active medium is a gas this line width may be the Doppler line width (see Section 2.3.2). Figure 9.3 shows a case where there are twelve axial modes within the Doppler profile. The number of modes in the actual laser beam depends on how much radiation is allowed to leak out of the cavity. In the example in Figure 9.3 the output level has been adjusted so that the so-called threshold condition allows six axial modes in the beam. The gain, or the degree of amplification, achieved in the laser is a measure of the intensity. 

Excimer lasers employing NeF, ArF, KrF, XeF, ArCl, KrCl, XeCl, ArBr, KrBr, XeBr, Krl, and Xel as the active medium have been made. 

Laser action in some dye solutions was first discovered by Lankard and Sorokin in 1966. This led to the first laser which was continuously tunable over an appreciable wavelength range. Dye lasers are also unusual in that the active medium is a liquid. 

The state may decay by radiative (r) or non-radiative (nr) processes, labelled 5 and 7, respectively, in Figure 9.18. Process 5 is the fluorescence, which forms the laser radiation and the figure shows it terminating in a vibrationally excited level of Sq. The fact that it does so is vital to the dye being usable as an active medium and is a consequence of the Franck-Condon principle (see Section 7.2.5.3). 

Organic Dye Lasers. Organic dye lasers represent the only weU-developed laser type in which the active medium is a Hquid (39,40). The laser materials are dyestuffs, of which a common example is rhodamine 6G [989-38-8]. The dye is dissolved in very low concentration in a solvent such as methyl alcohol [67-56-17, CH OH. Only small amounts of dye are needed to produce a considerable effect on the optical properties of the solution. 

Free-Electron Lasers. The free-electron laser (EEL) directly converts the kinetic energy of a relativistic electron beam into light (45,46). Relativistic electron beams have velocities that approach the speed of light. The active medium is a beam of free electrons. The EEL, a specialized device having probably limited appHcations, is a novel type of laser with high tunabiHty and potentially high power and efficiency. 

The word laser is an acronym for light amplification by the stimulated emission of radiation. Lasers of all kinds consist of several basic components an active medium, an outside energy source, and an optical cavity with carefully designed mirrors on both ends. One of the mirrors is 100 percent reflective... 

Different lasers use different materials as the active medium. The medium can be either solid, liquid, or gas, and there are advantages for each in the amount of energy that can be stored, ease of handling and storage, secondary safety hazards, cooling properties, and physical characteristics of the laser output. 

The term solid-state laser refers to lasers that use solids as their active medium. However, two kinds of materials are required a host crystal and an impurity dopant. The dopant is selected for its ability to form a population inversion. The Nd YAG laser, for example, uses a small number of neodymium ions as a dopant in the solid YAG (yttrium-aluminum-gar-net) crystal. Solid-state lasers are pumped with an outside source such as a flash lamp, arc lamp, or another laser. This energy is then absorbed by the dopant, raising the atoms to an excited state. Solid-state lasers are sought after because the active medium is relatively easy to handle and store. Also, because the wavelength they produce is within the transmission range of glass, they can be used with fiber optics. 

The common liquid lasers utilize a flowing dye as the active medium and are pumped by a flash lamp or another laser. These are typically more complex systems requiring more maintenance. They can he operated as either CW (continuous wave) or pulsed. One advantage liquid lasers have is they can be tuned for different wavelengths over a 100-nm range. 

Over the next few years, Townes, with the help of graduate students Herbert Zeiger and Janies Gordon, calculated the precise size of the necessaiy cavity and built a device that used ammonia as an active medium. Success came in 1954 with the completion of the first maser, an acronym for microwave amplification by stimulated emission of radiation. 

Schawlow continued working on his laser at Bell Labs. He had rejected ruby as an active medium because he felt it would not reach population inversion. By pumping the ruby with the light from a photographer s flash lamp, however, Maiman succeeded, created the world s first laser in June 1960. 

If the optically active medium is not transparent at the wavelength of the incident radiation, the transmitted intensity may be further reduced by an absorptive contribution to the index of refraction. Because of preferential absorption of either the left or the right circularly polarized component, the emerging beam would no longer be the sum of equal amplitudes and trace out an ellipse with ellipticity tp = (kt — kr). Practical details of the measurement and chemical applications of optical activity are discussed by Charney[34],... 

Figure 12.10c shows a contour plot of the nanolaser index profile superimposed on a cross-section of the modal field intensity profile in the center of the active medium. As shown in the figure, the modal profile of the nanocavity is confined almost completely in the 300-nm wide central pillar with a modal volume of 0.213 (1/n)3 (0.024 pm3) only 1.75 times the theoretically possible limit of a cubic half... 

Since all the physical properties of two given enantiomers are the same in the absence of a chiral, or optically active, medium, their chromatographic resolution needs a different approach from the relatively simple separation of geometrical isomers, stereoisomers or positional isomers. Two methods are used. The older technique of indirect resolution, requires conversion of the enantiomers to diastereoisomers using a suitable chiral reagent, followed by separation of the diastereoisomers on a non-chiral GC or LC stationary phase. This technique has now been largely superseded by direct resolution, using either a chiral mobile phase (in LC) or a chiral stationary phase. A variety of types of chiral stationary phase have been developed for use in GC, LC and SFC(21 23). 

An active medium, consisting of a collection of atoms, molecnles, or ions in a gaseous, liquid, or solid state, which generates and amplifies light by means of appropriate transitions between its quantum energy levels. 

Both the active medium and the resonator determine the light frequencies generated. 

Let us assume an active medium that responds to the energy-level diagram of Figure 2.6(a). It consists into four energy levels E, with respective population densities M (i = 0,..., 3). Let us also assume that laser action can take place due to the stimulated emission process E2 Ei. When a monochromatic electromagnetic wave with frequency v, such as (E2 — E )lh = v, travels in the z direction through the medium, the intensity of the beam at a depth z into the crystal is given by... 

However, a series of factors introduce losses in the system namely, the reflectivities of the mirrors (RiandR2) on the figure, which reflect only a fraction, Ri and R2, of the intensity. Additional losses can be produced by absorption in the windows of the cell that contains the active medium (if this is the case), diffraction by apertures, and scattering due to particles or imperfect surfaces. All of those losses can be included in a loss factor per trip, expressed as e. Thus, considering both amplification and intensity decrease per round trip, the intensity after a single round trip through a resonator of length d is... 

In a laser system, the wave is initiated by spontaneous emission from the excited state atoms in the active medium. The spontaneously emitted photons traveling parallel to the resonator axis are able to create new photons by stimulated emission. Above the threshold they induce a photon avalanche, which grows until the depletion of the population inversion compensates the repopulation due to pumping. 

The characteristics of the active medium determine those of the laser action. According to Equation (2.7), the gain G(v) is directly related with the particular characteristics of the quantum energy levels of the active medium via the transition cross section a. As follows from Sections 1.3 and 1.4 (see also Exercise 5.4) ... 

The active medium also determines the pumping scheme. Commonly, two types of operational schemes are used to describe laser operation /oMr-/cvc/ and three-level laser systems ... 

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