Permanent and transient hole-burning experiments carried out on various types of amorphous systems at temperatures between 0.3 and 20 K are presented. Optical dephasing on organic molecules in glasses and polymers measured on different time scales are discussed in relation to spectral diffusion processes and compared to the literature. The pure dephasing time appears to be proportional to the excited state lifetime of the guest. Organic molecules adsorbed on the surface of porous silica glass show that their homogeneous linewidth is an order of magnitude larger than in polymers and glasses in the bulk, but follows the same temperature dependence. Hole-burning results on bacteriochlorophyll in various disordered hosts are compared to those on a isolated light-harvesting photosynthetic pigment-protein complex (B800–850). From the holewidth in the latter the energy transfer rate between aggregated bacteriochlorophyll pigments is determined and compared to time-resolved spectroscopy results in the literature.
Permanent and transient hole-burning experiments carried out on various types of amorphous systems at temperatures between 0.3 and 20 K are presented. Optical dephasing on organic molecules in glasses and polymers measured on different time scales are discussed in relation to spectral diffusion processes and compared to the literature. The pure dephasing time appears to be proportional to the excited state lifetime of the guest. Organic molecules adsorbed on the surface of porous silica glass show that their homogeneous linewidth is an order of magnitude larger than in polymers and glasses in the bulk, but follows the same temperature dependence. Hole-burning results on bacteriochlorophyll in various disordered hosts are compared to those on a isolated light-harvesting photosynthetic pigment-protein complex (B800–850). From the holewidth in the latter the energy transfer rate between aggregated bacteriochlorophyll pigments is determined and compared to time-resolved spectroscopy results in the literature.
First preliminary persistent spectral hole-burning (PSHB) investigations of dye-molecules adsorbed onto the disordered surfaces of metal oxide powders or porous glasses1,2,3 indicated that these systems seem to behave quite similar to doped glasses. The optical absorption is strongly inhomogeneously broadened due to the disorder of the surface and at 1.5 K holewidths in the 1 cm1 range - even an order of magnitude broader than in glasses - have been reported. In the following we will give a short summary of our latest more detailed investigations of surface adsorbed dye-molecules which on the one hand reaffirm that these systems indeed behave in many respects as 3-D disordered systems but on the other hand point to some remarkable specific features of adsorbed dye-molecules.
Picosecond photon echo experiments on pentacene in polymethylmethacrylate (PMMA) show that the homogeneous width exhibits a T1.3 temperature dependence. This behavior is ascribed to coupling of the pentacene transition to randomly distributed low-frequency excitations in the amorphous host. A calculation of the resulting homogeneous width is presented which uses optical Redfield theory as a starting point. From the results of this calculation we conclude that the coupling between the pentacene molecule and the host low frequency excitations is of static dipolar nature and that the density of states of these excitations varies as ω0.3. Results of nonphotochemical hole-burning experiments on the same system are also reported. Comparison of these results with the ones obtained by the photon echo method indicates, that in the hole-burning experiments, the hole width and its temperature-induced broadening are dominated by spectral diffusion.
Picosecond photon echo (PE) experiments are used to examine the optical dephasing and intermolecular interactions of organic chromophores in a variety of glassy organic and polymer hosts. The inherent disorder in these systems leads to dynamics which span times from picoseconds to hours and longer. The temperature dependent optical dephasing as measured by the PE follows a power law characteristic of tunnelling twolevel systems (TLS) at low temperatures (1 °K to 4 °K) and is exponentially activated at higher temperatures due to dephasing by additional modes. The nature of these modes will be addressed. These data and those from other line narrowing experiments such as hole burning allow the study both of fast dephasing processes and spectral diffusion due to slow fluctuations in the host. The observed lineshapes and holewidths require dipolar coupling between the chromophore and TLS and offer information on the distribution of relaxation rates. Furthermore, techniques sensitive to different timescales are used to examine host dynamics by removing the guest-host coupling from consideration. The data are discussed in conjunction with recently developed theoretical treatments of dephasing in disordered systems.
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.
An overview of recent results concerning molecular doped solids is presented. The hole burning performances and the photophysics of polymer and xerogel-based systems are compared. Three areas are considered in more detail: the problem of electron-phonon coupling, the one of high-temperature persistent spectral hole burning materials, and the search for new frequency-selective photosensitive systems for fast optical pulse processing at 800 nm. This comparison demonstrates that molecular doped polymers and molecular doped xerogels have similar hole burning behavior but mechanisms in these systems can be different. However, in respect of the areas considered, inorganic hosts based on silicate xerogels or porous glasses have shown the best results. Moreover, all the possibilities of hybrid xerogels created by combining inorganic and organic capabilities or by grafting organic species to the host have not yet been revealed.
The persistent spectral hole-burning (PSHB) phenomenon has been known since 1974. It is still an important research area for the study of the intimacy of complex molecular systems in the solid state, revealing high-resolution spectra, photophysics, photochemistry, and dynamics of molecular doped amorphous media, organic as well as inorganic. From another point of view, PSHB allows the engraving of any spectral structures in the inhomogeneous absorption band profile of molecular doped amorphous hosts or ion doped crystals cooled down to liquid helium temperatures. Therefore, a PSHB material is programmable in the spectral domain and consequently it can be transformed in an optical processor capable of achieving user-defined optical functions. Some aspects of both fields are illustrated in the present paper. Concerning the search for efficient PSHB materials, the hole-burning performances and the photophysics of polymer and xerogel based systems are compared. The problem of high-temperature persistent spectral hole-burning materials and the search for new frequency selective photosensitive systems for fast optical pulse processing at 800nm are considered. Regarding the points treated, inorganic hosts based on silicate xerogels or porous glasses have shown the best results. Moreover, by combining inorganic and organic capabilities or by grafting organic species to the host, hybrid xerogels have not yet revealed all possibilities. Also, the interest of two-photon materials for engraving spectral features with near-infrared or infrared light is developed. As an introduction to possible applications of PSHB material, the basics of spectro-temporal holography are remembered and a demonstrative experiment using a naphthalocyanine-doped polymer film is described, proving that the temporal aberration free recompression of ultrashort light pulses is feasible, therefore opening a way for applications in ultrashort light pulse shaping. Aspects for a comparison between cw hole-burning and femtosecond spectro-temporal experiments are considered, and prospects for coherent control of light fields or photochemical processes are also evoked.