Laser Materials

the substances used as active mediums in lasers. The first laser, in which a ruby crystal (Al₂O₃ — Cr³⁺) was the medium, was developed in 1960. A mixture of neon and helium gases (1960), silicate glass with a trace of Nd³⁺ ions (1961), crystals of the semiconductor compound GaAs (1962), solutions of neodymium in the inorganic liquid SeOCl₂, and solutions of organic dyes (1966) were used later. By 1973 about 200 different laser materials, encompassing substances in all states of aggregation—solid, liquid, gaseous, and plasma—were known.

Laser materials must satisfy a number of requirements: they must have a set of energy levels that make possible efficient absorption of the energy supplied from without and its conversion into electromagnetic radiation with the lowest possible losses; they must have high optical uniformity, to eliminate light losses to scattering, as well as high thermal conductivity and a low coefficient of thermal expansion; they must be resistant to various physicochemical effects and drops in temperature and humidity; and they must retain their composition and properties in operation. In addition, solid laser materials must have high strength and must withstand the mechanical processing (cutting, grinding, and polishing) necessary to produce active elements from them.

Ionic crystals with impurities. Ionic crystals with impurities are the most representative group of laser materials. Crystals of inorganic compounds of fluorides (such as CaF₂, LaF₃, and L1YF₄) and oxides (such as AI₂O₃) or complex compounds [such as CaWO₄, Y₃A1₅O₁₂, and Ca₅(PO₄)₃F] contain in their crystal lattice the ions of active impurities: the rare earths (Sm²⁺, Dy²⁺, Tu²⁺, Pr³⁺, Nd³⁺, Er³⁺, HO³⁺, and Tu³⁺) and the transition elements (Cr³⁺, Ni²⁺, Co²⁺, and V²⁺) or U³⁺ ions. The concentration of active impurities in the crystals ranges from 0.05 to several percent by mass. Generation is excited by optical pumping and the energy is usually absorbed directly by the impurity ions. These laser materials are distinguished by a high concentration of active particles (10¹⁹− 10²¹ ions per cu cm), a narrow generation line (0.001–0.1 nanometers [nm]), low angular divergence of the generated radiation, and the ability to function in both the pulsed and continuous modes. Their shortcomings are the low efficiency of conversion of electric power into the energy of laser radiation in the pumping lamp-crystal system (1–5 percent) and the difficulty of producing laser rods of large size with the necessary optical uniformity.

Laser crystals with impurities are grown mainly by directed crystallization of a melt in crystallization units that provide high stability of the melt temperature and of the rate of growth of the crystal. The content of impurities in the raw materials used to grow crystals must not exceed 0.01 percent by mass and, in the case of the most dangerous impurities, must not exceed 0.0001 percent. Cylindrical rods up to 250 mm long and 2–20 mm in diameter are cut from the crystals grown. The end faces of the rods are ground and then polished. The rods generally are produced with flat end faces that are parallel to each other with a precision of 3–5″ and strictly perpendicular to geometrical axes; in some cases end faces of a spherical or other configuration are used. The chemical composition and physical properties of the most important laser materials based on impurity crystals are given in Table 1.

Glasses. In contrast to crystals, laser glasses have a disordered internal structure. In addition to such glass-forming components as SiO₂, B₂O₃, P₂O₅, and BeF₂, they contain Na₂O, K₂O, Li₂O, MgO, CaO, BaO, A1₂O₃, La₂O₃, and Sb₂O₃. The active impurity is most often Nd³⁺ ions, but Gd³⁺, Er³⁺,

Table 1. Composition and physical properties of laser materials based on crystals with Impurities
Crystal	Active impurity		(Density (kg/m3)	Index of refraction	Melting point °K)	Hardness (on Mohs’ scale)	Main wave-lengths of generation (μ)
	Substance	Content (% by mass)
AI2O3.........	Cr3+	0.03–0.7	3,980	1.764	2303	9	0.6943 R1 line 0.6929 R2 line
Y3AI5O12.......	Nd3+	0.5–2.5	4,560	1.8347	2203 ± 20	8.5	1.0641 at 300°K
CaWCO4........	Nd3+	0.5–3.0	6,066	1.926	1843	4.5–5.0	1.058 at 300°K
CaF2 .........	Dy2+	0.02–0.06	3,180	1.4335	1639	4	2.36 at 77°K
LaF3 .........	Nd3+	0.5–2.0	—	—	1766		1.0633 at 295°K

Ho³⁺, and Yb³⁺ also are used. The concentration of Nd³⁺ in the glass may be as high as 6 percent by mass. An advantage of glasses as laser materials, in addition to the high concentration of active particles, is the possibility of producing large active elements (up to 1.8 m long and 70 mm in diameter) and of virtually any shape, with very high optical uniformity. Their shortcomings include a wide generation line (3–10 nm) and low thermal conductivity, which impedes rapid dissipation of heat during high-power optical pumping. The chemical composition and physical properties of laser glasses are given in Table 2.

Laser glasses are founded in platinum or ceramic crucibles. Platinum, which enters the glass from the crucible, decreases the radiation power of the laser, since it creates sites of mechanical failure in the working element. The initial components of the charge for glass founding must not contain more than 0.01–0.001 percent impurities by mass. Impurities of Fe²⁺, Sm³⁺, Pr³⁺, Dy³⁺, Co, Ni, and Cu are particularly harmful for neodymium glasses.

Semiconductors. Semiconductor laser materials are crystal compounds of the types A^IIB^VI (ZnS, ZnSe, CdS, CdSe, CdTe, PbS, PbSe, and PbTe) and A^IIIB^V (GaPAs, GaAs, GaSb, InAs, and InSb), as well as crystals of tellurium. Semiconductor crystals are grown from a melt or the gas phase. Crystals for injection lasers, which are excited by passing an electric current through the working element, have a p-n junction. The thickness of the p-n junction is 0.1 microns (μ). Radiation arises in a layer of the p-n junction, but the radiating layer is thicker than the p-n junction (~2μ). The working elements of injection lasers made from semiconductor crystals are rectangular plates measuring 1 × 1 × 0.2 mm. The p-n junctions in GaAs crystals have the best energy parameters. The advantages of semiconductor laser materials with a p-n junction are high efficiency (up to 50 percent), small size of the working elements, and high radiation power per square centimeter of radiating surface area. Their shortcomings are technological difficulties in producing uniform, high-quality p-n junctions, the broad radiation line (~ 10 nm at room temperature), and large angular divergence of the radiation (l°-2°). Crystals of pure compounds without the introduction of any impurities are used in semiconductor lasers with electron excitation or optical pumping.

Gases. The distinctive features of gas laser materials are precise correspondence of the pattern of the energy levels of the gas to the levels of its individual atoms or molecules, high optical uniformity (the light beam passing through the gaseous medium is virtually not scattered), very low angular divergence, and narrow generation lines. Their shortcoming is the low concentration of working particles (only 10¹⁴-10¹⁷ particles per cu cm). In gas-discharge lasers, where excitation is accomplished by developing an electrical discharge in the gas, the pressure ranges from hundredths of an atmosphere, or 10³ newtons per sq m (N/m²), to several atmospheres, or (1–9) × 10⁵ N/m². Atoms of a gas (neon or xenon), positively charged ions (Ne²⁺, Ne³+, Ar²⁺, or Kr²⁺) or molecules (N₂, CO₂, H₂O, or HCN) are the working particles. In some cases another gas is added to the main working gas to improve its operation. For example, neon atoms are the active radiating particles in a helium-neon laser. A helium impurity improves the conditions for exciting neon atoms through the resonant transfer of energy to their upper working levels. The gas CF₃I is used at a pressure of 6.7 kN/m², or 50 millimeters of mercury (mm Hg), in lasers excited as a result of photodissociation. Vapors of the alkali metal cesium are used in gas lasers excited by an external light source.

Liquids. Liquid laser materials are comparable to gas lasers in optical uniformity and have a high density of active particles. In addition, the liquid can circulate in the laser resonator, providing efficient dissipation of the liberated heat. Their shortcoming is low resistance to laser radiation and the effect of the powerful radiation of optical pumping. In inorganic liquids the active impurity (Nd³⁺ ions) is dissolved to a concentration of several percent by mass in oxychlorides of selenium (SeOCl₂) or phosphorus (POC1₃), which contain chlorides of certain metals. The width of the generation line does not exceed a few tenths of a nanometer. Liquid laser materials based on organic dyes are molecular solutions of rhodamines, pyronine, Trypaflavine, 3-aminophthalamide, and other substances in ethyl alcohol, glycerol, water, and solutions of sulfuric acid. Generation is excited by the radiation of ruby or neodymium glass lasers or by the light of pulsed gas-discharge lamps. Because of the broad radiation spectrum of solutions of organic dyes, smooth tuning of the wavelength of the laser radiation within the radiation band is possible.