Ultrasonic modulation of persistent spectral holes in crystals

A variety of in homogeneously broadened optical and infrared transitions in solids display persistent spectral holes when irradiated with a narrowband laser. In the case of photochemical hole burning, absorption decreases at the laser frequency because the center undergoes an internal chemical change such as bond rearrangement or ionization. 1-3 Nonphotochemical hole burning is thought to occur via photophysical rearrangement of the impurity and/or its neighbors. Persistent spectral holes have been observed for electronic transitions of photostable molecules imbedded in organic glasses and crystals and for vibrational transitions of photostable molecules in an organic glass and in alkali halides. 7 We have investigated for the first time the influence of ultrasound on non photochemical persistent holes in alkali halides and find that ultrasonic waves modulate the hole depth. This mechanical modulation leads to a new optical method of monitoring ultrasound in crystals. In addition, prolonged exposure to ultrasonic radiation (in the absence of laser radiation) is found to permanently erase the persistent holes. The experiments were performed on KI crystals doped with 0.05% KRe04 and 2% NaI and on RbI crystals doped with 0.4% RbRe04' respectively. The crystals were mounted in the sample chamber of an optical cryostat and immersed in superfluid helium. Ultrasonic waves were generated and detected with a lO-MHzX-cut quartz transducer and commercial pulse-echo equipment. Because of the large difference in thermal contraction between quartz and alkali halides, most elastic bonds between them tend to break at low temperatures. 8 To counteract this problem, a special technique for bonding was developed. A drop of n-propyl alcohol was applied to the crystal surface and then a 20-f-l indium foil was stretched over the surface forcing a thin film of fluid between the crystal surface and foil. The quartz-transducer was then bonded onto the indium foil with nonaq stopcock grease. This method prevented fast evaporation of the organic liquid and, because of the low freezing point of n-propyl alcohol, most of the thermal contraction had taken place before the bond solidifiep. With this bond, good echoes were obtained as the sample was cooled from room temperature to liquid helium temperature. Holes were burned in the RbI crystal using the CO2 10P42 line (10.84 f-l) and in the KI sample with the N20 P19