Nonphotochemical hole burning of laser dyes, rare earth ions and photosynthetic pigments in polymer films

Abstract

The results of nonphotochemical hole burning (NPHB) and laser induced hole filling (LIHF) experiments are reported for several impurities intentionally doped into thin polymer films. The polymer films provide an amorphous environment for the dopants which can be easily and reproducibly prepared at room temperature, and they are convenient in handling since they are solids at ordinary temperatures. Thus, polymer films have several advantages over the various low temperature glasses which have been used previously for NPHB studies. The systems studied include the polymers poly(vinyl alcohol) and poly(acrylic acid) doped with various organic ionic dyes (cresyl violet, rhodamine 560 and 640, nile blue, and oxazine 720 and 725, all of which are used as the active gain medium in dye lasers) as well as the rare earth ions Pr('3+) and Nd('3+). Additionally, polystyrene was used as the amorphous host in NPHB and LIHF studies of chlorophyll a, chlorophyll b and self-aggregated dimers of chlorophyll a;The NPHB of the ionic dyes is efficient, and the spectra exhibit extensive intramolecular vibronic satellite holes which provide accurate S(,1) vibronic frequencies for the dopant. The homogeneous linewidths of the impurity S(,0) (--->) S(,1) transitions inferred from measured hole widths yields dephasing rates which are one to two orders of magnitude faster than observed in crystals and "hard" inorganic glasses (silica e.g.), and is taken as evidence for rapid pure dephasing processes occurring in the polymer systems even at 1.8 K. The LIHF of these systems is also efficient and shows a pronounced change in efficiency when the relative magnitudes of the laser frequencies for the first and second irradiations is reversed. Also, the efficiency of LIHF is insensitive to the amount of radiation absorbed by the dopant, and hence shows little change as the secondary burn frequency is moved relative to the primary frequency (except as noted above). This same LIHF behavior was observed in all the systems reported herein and may have profound implications as to the structure of glassy media;The chlorophyll systems also show efficient NPHB but no intramolecular vibronic holes are observed. These systems also show evidence of rapid pure dephasing processes even at 1.8 K. The results obtained for the monomer chlorophyll NPHB are used to help interpret the results obtained for the self-aggregated dimers which, in turn, were obtained to help in the interpretation of planned future experiments on chlorophylls in in vivo environments;Finally, a brief review of relevant NPHB research and the various theories used to describe the hole burning process is presented.