Spectral and Temporal Dynamics of Far Infrared Generated Phonons in Lanthanum Fluoride
Abstract
This thesis is concerned with the far infrared (FIR) generation of nonequilibrium acoustic phonons in LaF_3:Pr^{3+ } (0.5%), and the subsequent spectral and temporal dynamics that occur as the system relaxes toward equilibrium. Monochromatic, high occupation number ( ~1) populations of acoustic phonons are generated at frequencies throughout the acoustic spectrum via FIR pumping. This direct generation of phonons at the frequency of the FIR photons, a process forbidden in a perfect lattice, is attributed to defect-induced one -phonon absorption (DIOPA). This mechanism is discussed, as are other possible impurity induced effects. The impurities are not expected to significantly perturb the lattice modes, nor introduce any resonant or local phonon modes. Phonons are detected by optically pumping the anti-Stokes absorption sideband of the ^3 P_0<=vel of the Pr ^{3+} dopant ion. This technique provides temporal and spectral resolutions of ~eq 10 ns and ~eq 0.25 cm^{-1}, respectively. Primary consideration is given to three-phonon anharmonic processes, phonon breakup and confluence, which occur on a time scale that is short compared with transport time out of the interaction volume. The observed anharmonic decay times of initially monochromatic populations vary from ~eq 10 ns to ~eq 150 ns with a frequency dependence that is in good agreement with the theoretical nu^ {-5} dependence predicted by the isotropic dispersionless model. The temporal evolution of the phonon spectral distribution is observed, from the monochromatic generation through the establishment of a quasi-equilibrium. Broadband spectral features attributed anharmonic decay products and phonon backfeeding have been identified. Also, the appearance of a narrowband phonon distribution at twice the FIR frequency is attributed to phonon upconversion i.e., the confluence of two FIR generated phonons to create one phonon at twice the energy. The observed spectral dynamics have been modeled with the rate equations for a set of modified Einstein modes. The results indicate that phonon confluence processes play an important role in the dynamics following monochromatic generation, and suggest that momentum conservation is at least somewhat relaxed in the anharmonic decay process.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- January 1990
- Bibcode:
- 1990PhDT........81T
- Keywords:
-
- SPECTRAL DYNAMICS;
- LANTHANUM FLUORIDE;
- Physics: Condensed Matter