Spectral Hole-Burning and Related Spectroscopies: Science and Applications
Iron-free Cytochrome C and Myoglobin in Buffer Glass as Weakly-coupling Mesoscopic Molecular System: Holeburning, Absorption and Fluorescence Spectra and their Temperature Properties
Persistent spectral hole burning is a useful tool to detect zero-phonon line and phonon sideband of organic dye molecules doped in glasses and polymers, which are usually covered under inhomogeneous broadening. These observation bring us information on the electron-phonon interaction such as Debye-Waller factor a in these molecular systems. From the viewpoint of the issue of materials designing for wavelength-multiplexed optical memory and also with naive interest to obtain efficient sharp holes, we have paid much attention to the molecular systems in the category of rather weaker inter/intra-molecular coupling.
Persistent spectral hole burning is a useful tool to detect zero-phonon line and phonon sideband of organic dye molecules doped in glasses and polymers, which are usually covered under inhomogeneous broadening. These observation bring us information on the electron-phonon interaction such as Debye-Waller factor a in these molecular systems. From the viewpoint of the issue of materials designing for wavelength-multiplexed optical memory and also with naive interest to obtain efficient sharp holes, we have paid much attention to the molecular systems in the category of rather weaker inter/intra-molecular coupling.
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.
Persistent spectral hole burning (PSHB) has attracted much attention due not only to its usefulness in site-selective spectroscopy but also to its applicability to wavelength multiplexed optical memory in the future.1) Though the application is still far from realization, potentially 103 times higher storage density would be attainable. Technological breakthrough is largely concerned with the issue of optimizing the storage materials. For the formation of persistent holes, the existence of both photoreactive centers and a solid matrix is essential, hence the comprehension of intramolecular and intermolecular electron-phonon interaction is of great importance. We have already proposed one working hypothesis, which, derived mainly from the experiments related to the intermolecular interactions, emphasizes the possibility of controlling the mesoscopic structure of the materials.2-4)
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.
So far, persistent spectral hole-burning (PSHB) phenomenon has been observed in a number of materials. They are molecules in glasses, crystals and polymers, color centers in alkali halide crystals, and rare earth and transition metal ions in glasses and crystals. They are molecular or ionic guests embedded in glass, crystal or polymer hosts. Basic requirements for the PSHB are summarized as follows: (1) The optical absorption of guests embedded in hosts must be broadened inhomogeneously. (2) There must exist more than one ground state configurations of the total system consisting of guests and hosts. (3) The relaxation among the ground states must be slower than the decay rate of any excited state. In fact, this set of requirements is satisfied in many kinds of molecular or ionic guests in hosts, such as glasses, crystals and polymers, at low temperatures. We can naturally understand that guests consisting of small number of atoms are greatly influenced by the surrounding hosts and that the energy levels of the guests are inhomogeneously broadened.
At very low temperatures, the spread in the different environments of matrix embedded dye molecules results in an inhomogeneously broadened S1 ← S0 band. Spectroscopic techniques based on the appearence of optical zero phonon lines, such as fluorescence line narrowing, spectral hole-burning, and single molecule spectroscopy provide an experimental resolution limited by the homogeneous linewidth. From the investigation of the homogeneous line width as a function of different parameters (time, temperature) information on the guest host interactions responsible for the observed linewidth is obtained. Spectrally narrow features are extremely sensitive probes for external perturbations, such as electric fields, magnetic fields, or hydrostatic pressure.
Recently, the interests in optical memory based on persistent spectral hole burning (PSHB) are increased because of its possibility of the application to high density optical memory. PSHB was observed for an organic dye doped in polymer and rear earth or transition metal doped in inorganic crystal or glasses up to now. Since PSHB was observed by two Russian groups (Gorokhovskii et al. and Kharlamov et al.) for free base phthalocyanine in a n-octane Shpol'skii matrix1 and for perylene and 9-aminoacridine molecules in glassy ethanol matrix2, PSHB materials are studied primarily for the polymer doped with an organic dye. But such materials can make hole only at very low temperature. Room temperature PSHB phenomena were observed on Sm2+ doped fluoride crystals3-5 and glasses6 recently, however these materials' Гih(inhomogeneous line width)/Гh(homogeneous line width)'s which are the parameters of data multiplicity in PSHB optical memory are an order of unity. For the application of PSHB materials to optical memory, high operating temperature, high Гih/Гh, and rapid reaction rate are wanted. Glass has a superiority on the view point of optical memory application because of its broad inhomogeneous line width and productivity. Our group has discovered room temperature PSHB for Sm2+ in borate glass systems whose Гih/Гh is 24.7 PSHB for rear earth metal in glass matrix has not been studied seriously, yet. It is thought that the study of the relationship between the optical hole and glass structure is necessary.