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Molecular crystals and liquid crystals science technology. Section A, Molecular crystals and liquid crystals/Molecular crystals and liquid crystals science and technology. Section A, Molecular crystals and liquid crystals

Hole-Burning Memories: A Frequency Selective Mesoscopic Approach to Molecular Storage

D. Haarer

Published Oct 1, 1993• 2 citations

Abstract

Due to the long wavelength of visible light, most straight forward optical methods, like absorption or emission spectroscopy, are not well suited for spectroscopy on a “molecular level”. The hole-burning method is a selective spectroscopy scheme based on energy selection. This method yields, at low temperatures, very high spectral resolution and, hence, can be used to investigate molecular environments and molecular potentials. With the simultaneous application of external fields the hole-burning technique is a true two-dimensional spectroscopy method. Hole-burning can also be used for enhancing the schemes of presently used optical memories by many orders of magnitude. In this context new developments based on the use of semiconductor laser diodes are reported. The question of the requirement of low temperatures is also critically reviewed and, finally, a comparison between mesoscopic storage schemes and molecular schemes is given.

DOI: 10.1080/10587259308055202

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Laser Hole Burning Spectroscopy: A High Resolution Probe of Molecular Environments.

R. M. Macfarlane, Robert M. Shelby

Laser induced hole burning in optical absorption bands of low temperature solids has been investigated with emphasis on mechanisms for population hole burning, the interactions with the environment which limit the hole widths and the use of hole burning for ultra high resolution spectroscopy. Hole burning was observed in a wide range of solids including organic and organometallic molecules in crystals and rare earth ions in crystals and glasses. The mechanisms for hole burning also varied widely and a new mechanism involving optically induced nuclear spin flips of near neighbor nuclei was discovered. Examples of hole burning in new classes of materials are presented here for the first time, namely inorganic glasses (silicate glasses containing Nd(3+), Pr(3+) or Eu(3+)) and undoped stoichiometric compounds (e.g., EuP5O14). Hole recovery was used to determine nuclear spin lattice relaxation rates in praseodymium doped crystals. For allowed optical transitions (e.g., singlet states of zinc porphyrin, dibromoindigo and color centers in alkali halides) the hole widths at 2K are limited by population decay to approximately 50 MHz. For longer lived optical levels in crystalline materials, e.g., those containing rare earth ions, the hole widths are narrower (less than 10 MHz) and are limited by dynamical interactions with surrounding nuclei. In amorphous materials such as europium doped silicate glass, hole widths are much broader, being limited by low frequency tunneling motions. Hole burning measurements were complemented by time domain coherent transient (photon echo, and optical free decay) measurements of homogeneous linewidths which enabled us to measure optical resonances as narrow as 760 Hz in Eu(3+):Y2O3.

Final Report•Jun 1, 1981•0 citations

Due to the long wavelength of visible light, most straight forward optical methods, like absorption or emission spectroscopy, are not well suited for spectroscopy on a “molecular level”. The hole-burning method is a selective spectroscopy scheme based on energy selection. This method yields, at low temperatures, very high spectral resolution and, hence, can be used to investigate molecular environments and molecular potentials. With the simultaneous application of external fields the hole-burning technique is a true two-dimensional spectroscopy method. Hole-burning can also be used for enhancing the schemes of presently used optical memories by many orders of magnitude. In this context new developments based on the use of semiconductor laser diodes are reported. The question of the requirement of low temperatures is also critically reviewed and, finally, a comparison between mesoscopic storage schemes and molecular schemes is given.

Recent developments in the new spectroscopic technique concerning photochemical hole burning (PHB) and its possible application to the wavelength-multiplexed high density optical memory systems are reviewed. The basic concept of PHB is the site-selective optical bleaching usually observed in the solid systems doped with organic dye molecules at liquid helium temperature region. The molecular conditions and the fundamental mechanisms for the appearance of the holes are discussed in relation to the ordinary photochromic molecules. The review also includes discussions on the present research activities on PHB materials for future optical storage systems.

Zero-phonon lines in impurity optical spectra and their inhomogeneous broadening are discussed. Photoburning of persistent spectral holes is not only an effective method to eliminate the inhomogeneous broadening and perform high resolution spectroscopy, but it serves also as a powerful tool to control by means of illumination the absorption coefficient and refractive index of the matter. It opens new horizons for frequency-selective optical memories, data storage and data processing. Hole burning by picosecond pulses and the time-and space-domain holography based on it are discussed and the results of the experiments described.

One very exciting application of laser spectroscopy would be to use the high-resolution capabilities of today’s lasers and the phenomenon of persistent spectral hole-burning to form a frequency domain optical storage system.1 In such a system, bits of information would be stored by locally modifying (in the frequency domain) the shape of an inhomogeneously broadened absorption line of an absorber in a solid at low temperatures. Because typical homogeneous widths are 1000X (or more) narrower than typical inhomogeneous widths, as many as 1000 bits could be written in one diffraction-limited 1-μm spot In a sample, yielding the impressive storage density of 1011 bits/cm2.

The use of persistent spectral hole burning in optical data storage allows an increase of the storage density up to a factor of 1000 compared to the common spatial matrix of optical bits.1 The information is stored by burning at low temperature a sequence of narrow spectral holes in the inhomogeneously broadened optical transition of optical centers, e.g. dye molecules embedded in an amorphous polymer, with a tunable narrow-bandwidth laser. One bit of information is encoded by the presence or absence of a hole at a given location in frequency space. The information is read by observing the absorption or the fluorescence of the storage material using a laser with tunable frequency. One of the problems of spectral hole burning data storage arises from the fact that for addressing the memory locations the laser frequency has to be changed over a large frequency range with high accuracy and speed.

Laser induced hole burning in optical absorption bands of low temperature solids has been investigated with emphasis on mechanisms for population hole burning, the interactions with the environment which limit the hole widths and the use of hole burning for ultra high resolution spectroscopy. Hole burning was observed in a wide range of solids including organic and organometallic molecules in crystals and rare earth ions in crystals and glasses. The mechanisms for hole burning also varied widely and a new mechanism involving optically induced nuclear spin flips of near neighbor nuclei was discovered. Examples of hole burning in new classes of materials are presented here for the first time, namely inorganic glasses (silicate glasses containing Nd(3+), Pr(3+) or Eu(3+)) and undoped stoichiometric compounds (e.g., EuP5O14). Hole recovery was used to determine nuclear spin lattice relaxation rates in praseodymium doped crystals. For allowed optical transitions (e.g., singlet states of zinc porphyrin, dibromoindigo and color centers in alkali halides) the hole widths at 2K are limited by population decay to approximately 50 MHz. For longer lived optical levels in crystalline materials, e.g., those containing rare earth ions, the hole widths are narrower (less than 10 MHz) and are limited by dynamical interactions with surrounding nuclei. In amorphous materials such as europium doped silicate glass, hole widths are much broader, being limited by low frequency tunneling motions. Hole burning measurements were complemented by time domain coherent transient (photon echo, and optical free decay) measurements of homogeneous linewidths which enabled us to measure optical resonances as narrow as 760 Hz in Eu(3+):Y2O3.