Laser and Spectroscopic Properties of Chromium - Forsterite

This thesis describes the discovery of a new tunable solid state laser, chromium-doped forsterite (Cr:Mg _2SiO_4), its spectroscopic, quantum electronic and laser characteristics. The main goal of spectroscopic investigations was to identify optically active ions in chromium-doped forsterite. The spectroscopic studies performed on Cr:Mg _2SiO_4 crystals grown under different conditions include: measurements of the polarized absorption and fluorescence spectra, polarized excitation spectra, temperature and wavelength dependence of the fluorescence lifetime, relative quantum efficiency, and theoretical calculations using Tanabe-Sugano formalism. The conclusions based on these measurements and calculations are as follows. (1) There are at least three inequitably optically active centers in chromium-doped forsterite. (2) Trivalent chromium (Cr^{3+}) substitutes for divalent magnesium (Mg^ {2+}) in two distinct octahedrally coordinated sites, one with mirror (C_{rm s}) and the other with inversion (C _{rm i}) symmetry. (3) Tetravalent chromium (Cr^{4+}) substitutes for tetrahedrally coordinated silicon (Si^ {4+}). (4) Absorption and emission spectra show contributions from all three centers. (5) Center responsible for laser operation in the near-infrared spectral region is tetrahedral Cr^{4+} ion. Laser properties of chromium-doped forsterite for different modes of operation were investigated. Pulsed laser operation was obtained for 532-nm, 578-nm, 629-nm, and 1064-nm pumping, with repetition rate ranging from 10 Hz to 6.5 kHz. Tuning range covers the 1167-1345 nm wavelength region. Highest slope efficiency for pulsed operation obtained so far is ~23%. Continuous -wave operation was demonstrated by pumping the crystal with the 1064-nm radiation from a cw Nd:YAG laser. Highest slope efficiency for continuous-wave operation is ~38%. Limiting slope efficiency that may be obtained if all passive losses are eliminated is estimated to be >65%. Effective gain cross section was measured using the pump-and-probe technique. The peak gain cross section is estimated to be 1.9 times 10^{-19}cm ^2 at 1215 nm which is comparable to the peak value of ~2.1 times 10^{-19} cm ^2 predicted by fluorescence linewidth and lifetime measurements. The results of the effective gain measurements indicate that excited-state absorption is not a major loss mechanism in chromium-doped forsterite laser for wavelengths shorter than 1.3 mu m. There is an indication of the onset of weak excited-state absorption in the wavelength region beyond 1.3 mum. Further research and development of chromium-doped forsterite laser is suggested. The research to be performed is devoted to investigation of fundamental processes in this crystal such as nonradiative transition and phonon dynamics. The development aspect of this project points to various modes of operation, such as flashlamp-pumped operation, diode pumping, active and passive mode locking and Q-switching and building different configurations of oscillators and amplifiers for applications. The development of new lasers based on tetravalent chromium and other transition metal ions with identical electronic configuration in various host crystals is proposed.