Solid State lasers.

In a solid state laser, the material that participates in laser action is a solid (single crystal) or a glass. Ruby laser (Figure 31.5) consist of Al₂O₃ (alumina) doped with Cr³⁺ ions.

The ground state of Cr³⁺ is represented as ⁴A₂. The left superscript is 2S + 1 where S is the total spin of the system. There are three unpaired electrons in Cr³⁺ contributing to S = 3 x 1/2 = 3/2 . The right subscript indicates the value of the total angular momentum quantum number which is J = L+S, where L = total orbital angular momentum quantum number. The values of J vary from L + S to L - S. The Roman letter represents the value of L (S corresponds L = 0, P corresponds to L = 1 and so on) or the symmetry of the ground state. This ground state can be excited by a pump to one of the several excited states ⁴F₁ or ⁴F₂ . The exciting radiation need not be monochromatic as there are several excited states. These excited states quickly decay to a metastable ²E state. The ² E state gets populated in a short time span and can be stimulated to decay to the ground ⁴A₂ state by a stimulating photon of 694.3 nm wavelength.The energy level diagram is shown in Figure 31.5.

Figure 31.5 The energy level diagram for a ruby laser

Since this ground state ⁴A₂ is always significantly populated due to laser action, it is difficult to maintain a population inversion and the laser has to switched on and off intermittently. This is called a pulsed laser as against a continuous laser.