Naoshi Uesugi

Time- and frequency-domain hybrid optical memory: 1.6-kbit data storage in Eu3+:Y2SiO5

We propose and demonstrate a novel type of frequency-selective optical memory that writes and reads the data in both the time domain and the frequency domain. Temporal 16-bit data were stored by accumulated-photon-echo bit-by-bit storage at 103 frequency addresses within the inhomogeneous line of the (7)F(0)-(5)D(0) transition of Eu(3+):Y(2)SiO(5), which yields a total memory capacity of 1.6 kbits in a single spot of 240-microm diameter. The keys to the success of this experiment are this materials long dephasing time and lack of spectral diffusion.

Ultralong optical dephasing time in Eu 3+ :Y 2 SiO 5

We report what is to our knowledge the first measurement of linear and nonlinear spectroscopic properties for the (7)F(0)-(5)D(0) transition of Eu(3+):Y(2)SiO(5). Two clearly resolved lines at 579.879 and 580.049 nm, stemming from different sites, show dissimilar photoluminescence and hole spectra. In addition, these two sites have different inhomogeneous and homogeneous linewidths, which suggests that the local-field effect is smaller for one site. Specifically, the less affected site exhibits the longest dephasing time (822 micros) of any solid, which corresponds to a homogeneous linewidth of 387 Hz, and this linewidth is found to persist for hours without apparent spectral diffusion.

Efficient second‐harmonic generation in three‐dimensional LiNbO3 optical waveguide

Highly efficient second‐harmonic generation using a titanium in‐diffused LiNbO3 three‐dimensional optical waveguide was performed with the cw Nd : YAG laser at 1.064‐μm wavelength. The phase‐matching condition to convert the TE00 fundamental frequency to the TM00 or TM20 second harmonic was demonstrated by adjusting the crystal temperature. The conversion efficiency is 1.5×10−4 at 2‐mW fundamental input power under the phase‐matching condition. By using a low‐loss LiNbO3 optical waveguide, equipped with highly reflective cavity mirrors on the input and output surfaces, parametric oscillation is expected.

Size dependence of optical nonlinearity of CdSSe microcrystallites doped in glass

This paper investigates the size dependence of the effective nonlinear cross section σeff and the carrier recombination time on the size of CdS0.12Se0.88 microcrystallites with average radii of 10, 30, 50, and 100 A by degenerate four‐wave mixing (DFWM) experiments. The decay curves of the DFWM signal as a function of the probe delay time have fast and slow components. As the microcrystallite size decreases, the fast component becomes dominant and a carrier recombination time of 2 ps was observed in the 10‐A microcrystallites. The diffraction efficiency of DFWM is almost the same for all microcrystallites, that is, σeff has a weak size dependence. Smaller microcrystallites show smaller magnitudes of the third‐order nonlinear susceptibility ‖χ(3)‖, which is calculated from the measured effective nonlinear cross section and the carrier recombination time.

X-ray generation enhancement from a laser-produced plasma with a porous silicon target

X-ray generation enhancement from a laser-produced plasma with a porous Si target is reported. For a porous surface formed on a Si wafer, the self-reflectivity of a femtosecond pulse becomes considerably small. The observed energy penetration depth is 25–30 μm, which is much larger than the skin depth of solid density matter. Using a porous Si target, the threshold of the pre-pulse intensity required for soft x-ray emission enhancement can be reduced. It also contributes to enhance the pre-pulse effect, and soft x-ray generation enhancement ranging from 1.6 to 6.5 times is observed depending on the pre-pulse intensity.

Optical loss increase of phosphor‐doped silica fiber at high temperature in the long wavelength region

The wavelength‐dependent loss increase in the long wavelength region is revealed for phosphor‐doped silica fiber at a high temperature around 200 °C. The loss increases around 1.3 μm and becomes larger for longer wavelength regions. The loss increase is suppressed by reducing P2O5 dopant concentration. The overtone absorption of Ge–OH at 1.41 μm as well as that of Si–OH at 1.39 μm also increases markedly. The possible origin of the loss increase is discussed.

Temporal evolution of soft x-ray pulse emitted from aluminum plasma produced by a pair of Ti : sapphire laser pulses

The temporal and spectral evolution of soft x‐ray pulses (40–100 A) emitted from Al plasma produced by a pair of femtosecond Ti:sapphire laser pulses at normal incidence was studied. Both the soft x‐ray emission and the pulse duration increased with increase in the scale length of the preformed plasma. Prepulse enhanced soft x‐ray emission about 100 times with a pulse duration of 100–130 ps. A spectrally resolved time history revealed that the emission at shorter wavelengths started and decayed relatively more quickly, and emissions at longer wavelengths built up and decayed more slowly.

Loss increase for optical fibers exposed to hydrogen atmosphere

Loss spectrum changes for optical fibers exposed to a hydrogen atmosphere in the 15-200\deg C temperature range are measured. Loss increase due to molecular hydrogen dissolved into fibers is investigated from the loss peak at 1.24 μm, and that due to hydroxyl group formation from the loss peak at 1.41 μm. The loss increase due to molecular hydrogen is fully explained by physical solubility theory and diffusion equation. The empirical formula for time, temperature, and hydrogen-pressure dependences of the loss increase due to hydroxyl group formation is evaluated from the experimental results. The loss increase at 1.3- and 1.5-μm wavelength band at room temperature are estimated.

Time-resolved soft x-ray absorption spectroscopy of silicon using femtosecond laser plasma x rays

We measured time-resolved soft x-ray absorption of photoexcited silicon by means of pump-probe spectroscopy, using a picosecond soft x-ray pulse from femtosecond laser-produced plasma as a probe. We observed a 5% increase in the absorption caused by 1010 W/cm2 intensity laser pulse irradiation near the LII,III edge at 100 eV. The change was observed only when the laser and the soft x-ray pulses overlapped on the sample both in time and space.

Microcrystallite size dependence of absorption and photoluminescence spectra in CdSxSe1−x‐doped glass

Absorption and photoluminescence peak shifts due to the quantum size effect are observed in CdSxSe1−x microcrystallites with average radii ranging from several angstroms to 100 A. For microcrystallite radii between 15 and 100 A, the observed peak shifts can be described using an effective mass of 0.46m0 (m0 is the free‐electron mass), which is 4.6 times as large as the reduced mass in CdS0.12Se0.88. When the radius is reduced to less than 15 A, the effective mass, which is estimated from the experimental results, increases. The discrepancy between the theoretical prediction and the obtained results is discussed.