Efforts to create materials in which to burn holes at higher temperatures face an inherent contradiction: the need for inhomogeneous line broadening from host-guest interactions, vs. the desire to limit the homogeneous line broadening from thermal fluctuations of host-guest interactions (i.e. phonon broadening). A different approach to the question of how to burn holes at high temperatures was recently conceived and experimentally confirmed.1 This approach relies on using distributions of chromophore-doped optical cavities as the hole burning medium. The effect was recently observed in dye-doped micro-spheres. As a simple model illustrating the important effects characteristic to these Mie theory cavity resonances in spheres, the Fabry-Perot is treated analytically and numerically in this work.
Efforts to create materials in which to burn holes at higher temperatures face an inherent contradiction: the need for inhomogeneous line broadening from host-guest interactions, vs. the desire to limit the homogeneous line broadening from thermal fluctuations of host-guest interactions (i.e. phonon broadening). A different approach to the question of how to burn holes at high temperatures was recently conceived and experimentally confirmed.1 This approach relies on using distributions of chromophore-doped optical cavities as the hole burning medium. The effect was recently observed in dye-doped micro-spheres. As a simple model illustrating the important effects characteristic to these Mie theory cavity resonances in spheres, the Fabry-Perot is treated analytically and numerically in this work.
For next generation high density optical storage, extensive studies on persistent spectral hole burning (PSHB) have been performed, because they have a large potential for a high density frequency-domain optical data storage. PSHB has been observed at room temperature in Sm-doped crystals and glasses. Among them, glasses are considered to be more favorable than single crystals, due to their wide inhomogeneous line width and easy process for mass production. In the study, we developed a new glass mixture which shows room temperature PSHB with a inhomogeneous line width broadened up to 70 cm/sup -1/. The PSHB was characterized by investigating the time decay pattern of fluorescence and the excitation spectra.
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.
The study of optical relaxation processes in solids at low temperature has progressed tremendously in the last 20 years due to the development of coherent optical techniques. The latter would not have been possible without the advent of narrow-band tunable lasers, on the one hand, and lasers with very short pulses (picoseconds, femtosecoods) on the other hand. Such coherent techniques are necessary because spectral line shapes of solids doped with guest molecules are seldom determined by dynamical interactions but principally by strain or structural disorder. This gives rise to inhomogeneous broadening, rlOh, which in crystalline hosts at low temperature varies between ' 0.1 and 10 cm -I, whereas in glasses it may amount to � 100-500 em-I. One of the techniques to achieve narrowing of spectral lines is site selection spectroscopy (1-3). A sub-ensemble of molecules within thc inhomogeneously broadened absorption band is selectively excited by mcans of a narrow-band laser, and the fluorescence or phosphoresccnce signal is detected with a high-resolution monochromator and/or a Fabry Perot interferometer. Fluorescence line-narrowing ( I) is a special case of this technique (for a review see 3). The intrinsic or homogeneous spectral linewidth, r hom, which yields information on the relaxation processes of the excited state, is given by the effective optical dephasing time, T2:
Photon gated spectral hole-burning was first observed in a samarium doped crystal, BaFCl:Sm2+ [1]. This material was of interest to the developement of optical memories as the holes were found to be stable to room temperature thermal cycling. Holes could be burnt at liquid helium temperatures but as the temperature was raised the line quickly became homogeneously broadened. The addition of bromine to the melt succeeded in broadening the inhomogeneous linewidths to about 2 nm through substitutional disorder [2] and persistent holes were observed at 77 K [3]. Subsequently, a study of the parameters for spectral hole-burning in a BaFCl0.5Br0.5:Sm2+ crystalline powder was performed [4] and the highest temperature for which persistent hole-burning had been observed was raised to 133 K. Here we report spectral hole-burning at room temperature (figure 1) in a related compound, Sr0.5Mg0.5FCl0.5Br0.5:Sm2+.
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.
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.