1.1 Historical Background
1.1.1 Early developments
This section outlines the basic historical developments related to solid state laser materials and technology. It shows briefly how advances in basic science related to rare-earth-ion spectroscopy progressed, together with an understanding of physical processes in solids and solid state materials, which led to the first operation of the laser and subsequently to many technological advances.
In 1907, Becquerel made early observations of a rare earth's luminescence at low temperatures. During the years 1930â1940, an extensive theoretical analysis of the spectra of rare-earth ions was conducted, led by scientists such as Bethe, Kramers, Van Vleck, Racach, and others.
From 1961 through 1968, Dieke and others conducted research that led to the full assignment of the energy levels of trivalent rare-earth trichlorides. This whole assignment was accompanied by theoretical calculations of the transition strengths (mainly the electric dipole but also the magnetic dipole transitions) among various electronic levels in rare-earth ions.
Parallel to these developments, technology progressed toward the development of laser devices. Although in 1917 Einstein defined the concept of stimulated emission and formulated the relationship between induced and spontaneous transitions, Schawlow, Townes, Basov, and Prokhorov, who pioneered the theoretical predictions of laser action, made significant progress in the field in 1954. In 1960, Maiman operated the first ruby laser; this was soon followed by the operation of by Sorokin and Stevenson. In 1961, Snitzer operated the first Nd:glass laser. In the same year, Johnson et al. built and operated the first CW laser based on a laser. During the years 1964â1966, Johnson, Geusic, Van Uitert, and others conducted intensive research at Bell Telephone Laboratories toward the development of new inorganic hosts for laser materials based on various trivalent rare earths, mainly and . The main hosts investigated were various garnets, such as (YAG), (GGG), and (YGG).