We propose optical discs that store data holographically in three dimensions by combining multiplexing techniques and multilayer storage. Microholograms can be stored overlapping in the same volume by using angle multiplexing, wavelength multiplexing or the combination of both. Storage of multiple information bits in one single position on the disc increases the data rates and the storage capacity can reach more than 100 Gbytes /1/. In contrast to previous holographic storage systems, the storage media are made of cheap and mass-produceable photopolymer layers instead of expensive crystals. Furthermore, the microholographic storage method can be downward compatible with today's Compact-Disk (CD)- and Digital-Versatile-Disk (DVD)-systems.

We report on the holographic recording and read-out of microscopic Bragg-reflectors in DuPont Omnidex HRF-800 photopolymer films with 20 micrometer thickness. A Helium-Neon laser operating at (lambda) equals 632.8 nm was employed as light source. Lenses from Digital Versatile Disk (DVD) pick- ups were used to realize a beam waist with submicron diameter. The local distribution of the diffraction efficiency was investigated by applying a confocal scanning microscope setup. Holograms with a 1/e2-radius of 1.8 micrometer and 12 micrometer depth (FWHM) have been recorded. Due to shrinkage, the maximum diffraction efficiency was 0.4%, which can be improved by adjusting the read-out wavelength.

We propose a concept of holographic data storage which promises storage capacities of more than 100-Gbyte on a digital versatile disk (DVD)-sized disk. The information is stored bitwise in form of microscopic reflection holograms. High storage densities can be achieved by combining multiplexing methods and multilayer storage. A theoretical model for microscopic reflection holograms generated by focused Gaussian beams is proposed, Experimental results are presented for the recording and characterization of microholograms in DuPont's HRF-800 photopolymers. The local distribution of the diffraction efficiency was investigated by applying a confocal scanning microscope setup. Single-color holograms with a radius of 1.8 and 12 mu m depth have been recorded. We observe a blue shift in the spectral response of the microholograms of less than 2% due to shrinkage of the polymer, In the case of threefold wavelength multiplexing, all wavelengths are clearly resolved in the spectral response having spectral width Delta lambda of less than 10 nm (FWHM), Baking the holograms for 1 h at 120 degrees C nearly doubled the diffraction efficiency while the spectral response of the microholograms broadened by a factor two. We showed that 80-ps pulses are sufficient for holographic recording in the photopolymers. An optimized pre-illumination allows a significant increase in diffraction efficiency.

An optical disc system has been proposed that stores data holographically in three dimensions as microholograms combining different multiplexing techniques. Such microholograms can be stored overlapping in the same volume by using angle multiplexing, wavelength multiplexing or the combination of both. Storage of several information bits in one single position on the disc increases the data rate so that the total storage capacity could reach more than 100 GBytes on a DVD sized disc. In contrast to photorefractive memory systems, the storage media are made of cheap and mass- producible photopolymer layers instead of expensive crystals. Furthermore, the microholographic storage method can be downward compatible with today's Compact-Disk (CD)- and Digital-Versatile-Disk (DVD)-systems. In this paper we report on wavelength multiplexing of microscopic reflection holograms and on thermal processing of the photopolymer.

A new storage technique based on microscopic reflection holograms is presented. This technique has the potential to provide storage capacities of up to 100 Gbyte on a CD-like disk. The influence of wavelength- and angle multiplexing on the storage density is investigated and an estimation of the total storage capacity as a function of the focus radius is given. The major advantage of our holographic technique over other holographic storage approaches is the opportunity to achieve compatibility to existing CD and DVD systems.

The energy relaxation of excitons in the organic semiconductor tetracene (film thickness 160 nm) is studied by photoluminescence from 15 to 300 K using time-correlated single-photon counting. The well-known relaxation of triplet excitons with time constant ∼7 ns is accompanied by a faster relaxation process with time constants ranging from 260 ps to 1.2 ns, depending on temperature and wavelength.

Ternary II-VI semiconductors, like CdSxSe1-x or ZnxCd1-xS, were prepared by thermal evaporation of binary semiconductors from a double source. The influence of substrate temperature during thin film deposition and of thermal annealing in argon atmosphere on absorption edge steepness is studied. A high absorption edge steepness is advantageous to achieve large resonant optical nonlinearities, like absorption bistability and electronic nonlinearities. The position of the absorption edge can be tuned by choosing a proper film composition. Thermally induced optical bistability is demonstrated. Electronic nonlinearities are investigated in ns- and ps-pump-probe experiments. Fast bleaching with < 60 ps relaxation time is observed at the absorption edge. The maximum bleaching coefficient for CdS0.69Se0.31 films is 1.4 cm/kW at (lambda) equals 587 nm for pumping with (lambda) equals 377 nm. Annealing the films showed no effect on the relaxation time, but the bleaching coefficient was increased by a factor of five.

Nonlinear optical absorption of evaporated, polycrystalline CdSSe thin films is studied experimentally. A cw Rh6G-dye laser and a frequency doubled alexandrite laser with 5 ns pulse duration are used for excitation. Thermal and electronic nonlinearities are studied under steady state conditions. The thermal effect is used to realize absorption bistability and to determine thermal relaxation times. Absorption bleaching is observed with ns excitation and the maximum bleaching coefficient is 1.4 cm/kW. The low quantum efficiency of < 3 x 10(-2) for fluorescence indicates that the charge carrier lifetime is dominated by nonradiative processes. The relaxation time for bleaching determined by a ps-pump-probe technique is about 50 ps.