Stefan Bernet

Storage of 2000 holograms in a photochemical hole-burning system

By use of a novel recording technique, 2000 high-resolution image holograms were recorded in a single spectral hole-burning sample at different frequencies and at different values of an applied electric field. We recorded each hologram by sweeping the frequency of a cw laser over a narrow interval while simultaneously changing the hologram phase by 2π. This swept recording technique produces holograms that have increased diffraction efficiency and that exhibit reduced cross talk with respect to conventional frequency-multiplexed holograms.

Spectral hole burning and molecular computing

The new concept of molecular computing based on spectral hole burning, the interaction of molecular energy levels with an externally applied electric field and the interferometric properties of holography, is presented. Data stored in the form of 2-D arrays are directly combined in parallel without the use of an external processor.

Holography in frequency selective media: hologram phase and causality

Abstract Diffraction properties of holograms stored in persistent spectral hole burning media are shown to be directly related to the causality principle and are studied experimentally by using a frequency-tunable arrow band laser. Amplification (erasing) of certain diffraction orders that are allowed (forbidden) by the causality principle is done by manipulating the spectral and phase properties of the holograms. The possibility to minimize cross-talk between holograms stored at different frequencies is also demonstrated.

Recording of 6000 holograms by use of spectral hole burning.

Experiments verifying a new method of storing spectral hole-burning holograms, which yields reduced cross talk as compared with standard spectral hole-burning holograms, have been conducted. Results demonstrating the reduced width of this type of hologram in both frequency and the applied electric-field dimension are presented. Analytic solutions for the spectral width and diffraction efficiency of these holograms are presented. Using this exposure technique, we have recorded 6000 holograms in a single spectral hole-burning sample.

From supramolecular photochemistry to the molecular computer

A molecule and its noncovalently bonded solvent shell can be termed “supermolecule”. It is assumed that its ground-state hypersurrace consists of many shallow local minima which have approximately the same energy and correspond to different arrangements of the solvent shell. At cryogenic temperatures each supermolecule sits in one of the different minima. Supermolecules might show different physical properties, specifically a spread in their absorption frequencies. This property forms the basis for spectral holeburning, a special type of photochemistry Spectral hole-burning not only allows high resolution spectroscopy of matrix isolated organic molecules, it also opens a wide field of technical applications, especially with respect to frequency selective information storage. Recently more than 2000 images were stored in a single polymer film at different frequencies of the visible spectrum. More general this type of wavelength-selective photochemistry allows the storage of all the properties associated with an optical wave field, such as frequency, polarization, direction of propagation, intensity, and, in conjunction with holographic methods, also the phase. One might say that supermolecular photochemistry freezes in all the properties of light. It is often claimed that the top end of the current generation of electronic computers will be replaced by optical computers having fast parallel computing capacities. A further development might be a molecular computer. The properiies of light are transferred to photochemical changes in the material and the stored patterns can then be operated on by logical operations at will. The interaction of the molecular energy levels with an external electric field provides dynamical responses. A functional model of a molecular processor which is based on spectral hole-burning and on the spectroscopic properties of a dye doped polymer film is described here. Hole-burning materials can be used for image recording as well as for parallel processing of stored information.

Spectral hole burning and holography. V. Asymmetric diffraction from thin holograms

Thin holograms with novel diffraction properties were created in spectral hole-burning materials. Simultaneously sweeping the frequency of the recording light and the phase of the holograms results in an asymmetric distribution of diffraction efficiency between positive and negative orders. Experiments were performed that demonstrate amplified or attenuated diffraction efficiency with respect to holograms recorded at a single frequency. The observed diffraction is well described by a simple model in which small grating amplitudes and weak burning are assumed.

Frequency and phase swept holograms in spectral hole-burning materials.

A new hologram type in spectral hole-burning systems is presented. During exposure, the frequency of narrow-band laser light is swept over a spectral range that corresponds to a few homogeneous linewidths of the spectrally selective recording material. Simultaneously the phase of the hologram is adjusted as a function of frequency-the phase sweep function. Because of the phase-reconstructing properties of holography, this recording technique programs the sample as a spectral amplitude and phase filter. We call this hologram type frequency and phase swept (FPS) holograms. Their properties and applications are summarized, and a straightforward theory is presented that describes all the diffraction phenomena observed to date. Thin FPS holograms show strongly asymmetric diffraction into conjugated diffraction orders, which is an unusual behavior for thin transmission holograms. Investigations demonstrate the advantages of FPS holograms with respect to conventional cw recording techniques in freq ncymultiplexed data storage. By choosing appropriate phase sweep functions, various features of holographic data storage can be optimized. Examples for cross-talk reduction, highest diffraction efficiency, and maximal readout stability are demonstrated. The properties of these FPS hologram types are deduced from theoretical considerations and confirmed by experiments.

Holography in frequency selective media II : controlling the diffraction efficiency

Abstract A novel hologram storage technique based on recently reported causality effects in spectral hole-burning media [1] is presented. Simultaneously sweeping the phase and frequency of CW recording light produces holograms with new diffraction properties. In particular the distribution of the light in the different diffraction orders becomes asymmetric and can be easily controlled by the direction of the phase shift. The line shape of phase and frequency swept holograms can be designed in a way which yields many advantages for optical data storage. Compared with normal holograms recorded at a single frequency the maximum diffraction efficiency has been increased by a factor of 5.

Power broadening of the spectral hole width in an optically thick sample

Under the assumption of negligible absorption, spectral holes are known to broaden in frequency proportional to the square root of the recording intensity. In this paper the standard expression for power broadening is compared with more general results for the case of non-zero absorption. Analytic results are derived for the upper and lower limits of the hole width, corresponding to the cases of low and high absorption, respectively. These results are compared with numerical solutions, and used to fit experimental data for an optically thick sample of chlorin in PVB, in a temperature range of 450 mK to 3 K. The temperature dependence and zero kelvin value of the dephasing time, T2, are evaluated.

Spectral holeburning and holography VI: Photon echoes from cw spectrally programmed holograms in a Pr3+:Y2SiO5 crystal

Abstract Spectral programming of stimulated photon echoes is presented. The preparation pulses — pulse #1 and #2 of the three pulses of the echo sequence—are substituted by writing a frequency dependent modulation into the inhomogenously broadened absorption line of a persistent spectral holeburning material. A holographic setup is used, and arbitrarily predefined echo signals can be generated. Linear response theory is applied to calculate the required modulation of both the intensity of the write beam and the holographic spatial phase as a function of optical frequency. Experimental results are shown using a Pr 3+ :Y 2 SiO 5 crystal at cryogenic temperatures.