Solid state lasers

(a) Ruby. Ruby was the first material in which laser action was found. Ruby, a well-known gem, is a crystal consisting of aluminumoxide, A1₂O₃.

Fig. 1.1. (a) Maxwellian velocity distribution function f(v) of gas atoms which move at a velocity v in parallel to the laser axis. (b) The Doppler broadened emission line of gas atoms due to the Maxwellian velocity distribution according to fig. 1.1a.

Fig. 1.2. Energy level scheme of ruby: ( 1 ) ground state; (2) excited state from which the transition occurs; (3) pump levels.  

The lattice is doped with Cr³⁺ ions, i.e. triply ionized chromium, typically with a concentration of 0.05% (in weight). The chromium ions lend ruby its red color. Laser action takes place between levels of Cr³⁺ whose corresponding energy level scheme is shown in fig. 1.2. Thus basically we are dealing with the scheme, where the participating levels 2 and 3 are split. Optical pumping takes place into the levels denoted by ⁴F₂ and ⁴F₁ in fig. 1.2. From these levels the chromium ion relaxes into the level 2 which in fig. 1.2 is denoted by ²E In. fact this level is split into two further sublevels. The lower one of these two levels, which is denoted by E, serves as the initial state for the optical transition, i.e. for the laser transition into level 1 (which actually is fourfold degenerate). The optical transition denoted by R₁ takes place at 0.6943 pm. The lifetime of the upper laser level  is about 3 ×10^-3 s. The line width Δv strongly depends on temperature. At 300 K, Av = 2 × 10¹¹ Hz. Usually ruby is excited by intense flash lamps but other light sources are used also in order to generate cw emission.

(b) The neodymium glass laser. In this case glass serves as the basic substance which is doped with laser active neodymium ions ( Nd³⁺).  It is important to note that the lower laser level 1 is separated from the ground state 0 energetically so far that even at room temperature the occupation number of level 1 differs from that of the level 0 by a factor e^-10. Thus we may assume that the level 1 is initially practically unoccupied. The optical transition between levels 2 and 1 takes place at λ = 1.06 pm.

.Fig. 1.3. Compare text

(c) The neodymium YAG laser. In this case the neodymium ions are embedded in yttrium-aluminum garnets which consist of Y₃Al₅O₁₂. The level scheme corresponds to that of fig. 1.3. Laser action takes place at A = 1.0641 ym at room temperature

(d) Neodymium pentoxide crystals. Neodymium can be built in at regular lattice sites in the crystals mentioned above and can show laser action.

(e) As a last example of solid state lasers we mention calcium wolframate doped with ions of the rare earths.