P. M. Rentzepis

Three-Dimensional Optical Storage Memory

A novel three-dimensional (3-D) optical memory device is presented that allows fast random access of the information and extremely high bit densities. This device is based on two-photon writing, reading, and erasing of the information in a photochromic material embedded in a polymer matrix. Absorption and emission data show that two-photon writing and reading of information is feasible. The advantages and properties of such a 3-D optical memory are discussed.

TWO‐PHOTON EXCITATION OF FLUORESCENCE BY PICOSECOND LIGHT PULSES

Standing wave excitation of two‐photon fluorescence in solutions or organic molecules is reported. The observations allow the direct display and measurement of optical pulses as short as 1−2 × 10−2 sec.

Picosecond Kinetics of Events Leading to Reaction Center Bacteriochlorophyll Oxidation

A transient absorption spectrum has been measured in Rhodopseudomonas spheroides R26 reaction centers. Its salient features indicate that both the bacteriopheophytin and bacteriochlorophyll chromophores play a role in the excited state. Decay of this state yields a rise time for oxidation of the reaction center complex of about 150 picoseconds.

Two-Photon Three-Dimensional Optical Storage Memory

We describe the design and construction of ultrahigh capacity three-dimensional, 3D, optical storage devices that operate by two-photon absorption. The molecular systems and their properties that are used as two photon media for writing and one photon for accessing the stored information within the volume of the device are presented in some detail and the nonlinear two-photon absorption mechanism is briefly visited. The optical system and its components, which facilitated writing and reading, are also described and the bit density, bit error rate, store and access speeds, cycle times, and stability of the materials under various experimental conditions are also topics addressed in this review. The first ever storage of terabyte data in a removable storage disk is described in detail.

SOME APPROACHES TO VACUUM UV AND X‐RAY LASERS

The ejection of electrons from inner shells of atoms through photoionization is suggested as a straightforward means of creating population inversions at vacuum uv and x‐ray wavelengths. This can be accomplished by photon sources covering broad bands. Sodium vapor and solid copper are examined as possible candidates for laser action at 372 A and 1.54 A, respectively. A first‐rise traveling wave pump must be used; pumping powers required for superradiant operation run into several gigawatts, with rise times ranging from 10−9 sec in the near‐vacuum uv to 10−15 sec in the x‐ray region.

Picosecond spectroscopy using a picosecond continuum

Abstract This paper describes the generation of a continuum which has a time duration equal to or less than the picosecond pulse which generates it. The spectrum of the continuum is several thousand wavenumbers in width. Experimental methods are presented which enable one for the first time to conduct absorption spectroscopy measurements in the picosecond scale. As an example of this method we describe the absorption and emission picosecond kinetics of a bleachable dye 3,3 diethyloxodicarbocyanine (DODC).

Observations on the measurement of two-photon absorption cross-section

We have measured two-photon absorption (TPA) cross-sections of four organic molecules in solution. The data show that the nonlinear transmission method without consideration of other nonlinear effects results, under certain conditions, in erroneous values for the TPA cross-sections. We also find that the cross-sections measured by excited-state methods, namely two-photon induced fluorescence and a new excited-state method, which is based on transient spectroscopy following two-photon excitation, are in good agreement with the published data. Therefore, caution is warranted when using the transmission method.

Primary intermediates in the photochemical cycle of bacteriorhodopsin

Picosecond studies of the primary photochemical events in the light-adapted bacteriorhodopsin, bR570, indicate that the first metastable intermediate K610 is formed with a rise time of 11 ps. Difference spectra obtained at 50 ps after excitation show that K610 is the same species as that trapped in low temperature glasses. A precursor species (S) of the K610 intermediate has been observed which is red shifted with respect to K610 and is formed within the 6-ps time width of the excitation pulse. The formation of the precursor has no observable thermal dependence between 298 degrees and 1.8 degrees K. The formation of K610 has a very low thermal barrier and at very low temperatures, the rate of formation becomes practically temperature independent which is characteristic of a tunneling process. The rate of formation becomes practically temperature independent which is characteristic of a tunneling process. The rate of formation of K610 has a moderate deuterium isotope effect of kH/kD approximately 1.6 at 298 degrees K and 2.4 at 4 degrees K. The mechanism for formation of K610 is found to involve a rate-limiting proton transfer which occurs by tunneling at low temperatures.

Accessing 3D memory information by means of nonlinear absorption

A novel method for accessing information from two-photon 3D volume memory devices is presented. We have used the decrease in laser intensity induced by two-photon absorption as the means for reading 3D information. The ratio of the initial laser intensity to the intensity after two-photon absorption is increased drastically by passing both beams through a second and fourth harmonic crystal. This means eliminates the need for focusing and collecting lenses and the disadvantages associated with the fluorescence method of reading information.

Emission from the lowest singlet and triplet states of azulene

Abstract The emission from the first singlet state of azulene has been observed at ≈ 7000A. The phosphorescence was also measured at ≈ 10 000A at 77°K