Ryuzi Yano

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.

Nonlinear laser spectroscopy of eu3+: Y2sio5 and its application to time-domain optical memory

Linear- and nonlinear-optical properties of Eu3+:Y2SiO5 have been investigated. All the energy levels of the7F0,1,2,… ground-state manifold and the 5D0,1,2 excited-state manifold were identified, and they were found to be doubly structured because of the two inequivalent optical sites. Two absorption lines of the 7F0–5D0 transition were studied in detail by using nonlinear spectroscopic techniques, photon-echo and stimulated-photon-echo methods. It was found by the photon-echo measurement that the optical dephasing time for one site is extremely long and is excitation-intensity dependent. With the smallest possible intensity we observed the longest ever dephasing time in a solid, 822 μs, or a 387-Hz homogeneous linewidth. From the stimulated-photon-echo measurement, the linewidth was found to be constant over a wide range of time scales, implying the absence of spectral diffusion in this crystal. These quite advantageous properties of this crystal were used for the demonstration of two novel types of time-domain optical data readout: time- and frequency-domain hybrid optical memory and spectrally programmed coherent transients.

Spectrally programmed stimulated photon echo.

A stimulated photon echo has been observed from the artificially modulated inhomogeneous distribution of a medium. A cryogenic Eu (3+):YAlO(3) sample with a quasi-persistent hole lifetime is exposed to a frequency-scanned and intensity-modulated cw laser for multiple hole burning. A coherent burst equivalent to the stimulated photon echo is obtained by triggering this sample by a pulsed laser. We demonstrate further that any desired transient pulse shape can, in principle, be obtained by appropriate frequency-domain programming, which opens the way to a novel type of optical memory.

Terahertz wave detection performance of photoconductive antennas: Role of antenna structure and gate pulse intensity

We studied the receiver performance of two photoconductive antennas (bow tie and dipole antennas) fabricated on the same low-temperature-grown GaAs substrate to clarify the effect of the antenna structure and gate pulse intensity on terahertz wave detection. We observed the gate pulse intensity dependence of the temporal profiles of the terahertz waves or terahertz spectra. For both antennas, the sensitivity in the low-frequency regime (<0.5THz) was enhanced compared to that in the high-frequency regime for large gate pulse intensities. This is because the carrier trap time increased due to the saturation of the GaAs defect levels. We also observed that the peak-to-peak amplitude of the terahertz wave detected by one antenna was not always larger than that detected by the other antenna, and the peak-to-peak amplitude of the bow tie antenna was larger (smaller) than that of the dipole antenna when the gate pulse intensity was high (low). This was explained by the gate pulse intensity dependence of the freq...

Incoherent light gated optical Kerr shutter for fluorescence lifetime measurement

Abstract A new method of measuring fluorescence lifetime which is much shorter than the pulse width was developed using an optical Kerr shutter and incoherent light. While a sample was excited by second harmonic light of the output pulse of a Q-switched YAG laser, an optical Kerr shutter was gated with the fundamental light of the same laser pulse. The transmitted intensity of the fluorescence through the Kerr shutter was measured by changing the delay time between the second harmonic and the fundamental light. A resolution time of 30 ps was obtained by using a Q-switched YAG laser pulse of 7 ns duration.

Pump–probe spectroscopy of low-temperature grown GaAs for carrier lifetime estimation: arsenic pressure dependence of carrier lifetime during MBE crystal growth

Abstract We have performed reflection-type degenerate pump–probe spectroscopy to measure the carrier lifetime of GaAs grown by molecular beam epitaxy (MBE) at 250°C under four different effective arsenic pressures. The temporal behavior of the reflected probe pulse intensity strongly depends on both the wavelength and the intensity of the exciting laser pulse. The overall feature can be interpreted by the carrier-density- and wavelength-dependent refractive index change caused by the bandgap shrinkage, band filling, and plasma effect. We also found that the carrier lifetime depends on the effective arsenic pressure during the MBE growth, which is considered to determine the density of the defects in the GaAs. A carrier lifetime of 1.9 ps was obtained when GaAs was grown under the highest effective arsenic pressure at 250°C and subsequently annealed at 650°C.

Thermal conductivity measurement of water-ethanol solutions by the laser-induced transient grating method

Abstract The laser-induced transient grating (LITG) method has been applied to the measurement of thermal conductivity of water-ethanol solutions. A frequency-doubled Q -switched YAG laser is used for generating LITGs and a He-Ne laser is used for monitoring their decay. Taking advantage of the novel features of this method, we obtained accurate and reliable values of the thermal conductivity of water-ethanol solutions at various concentrations of ethanol at 15° C. A comparison of our data with Losenickys theory of thermal conductivity of binary solutions suggests that the thermal conductivity of water-ethanol solutions is enhanced by the effect of the hydrogen bonding.

Systematic pump-probe terahertz wave emission spectroscopy of a photoconductive antenna fabricated on low-temperature grown GaAs

We performed pump-probe terahertz (THz) wave emission spectroscopy for a photoconductive antenna fabricated on low-temperature grown GaAs. When the pump pulse intensity was increased, the THz wave generated by the probe pulse showed an increase of the spectral amplitude in the low-frequency regime (<0.5THz) and a decrease of the peak amplitude. The increase of the spectral amplitude and the decrease of the peak amplitude are related to the carrier density generated by the pump pulse, and are explained by the increase of the carrier trap time due to the saturation of the defect levels and by the carrier-carrier scattering process between carriers in the conduction band, respectively.

Arsenic pressure dependence of carrier lifetime and annealing dynamics for low-temperature grown GaAs studied by pump–probe spectroscopy

Reflection-type degenerate pump-probe spectroscopy was performed for low-temperature grown (LT-) GaAs to study the effects of arsenic pressure during crystal growth and annealing on carrier lifetime and to ascertain the annealing dynamics. It was found that a sample grown under a high arsenic pressure has a shorter carrier lifetime for both as-grown and anneal conditions. It was also found that the carrier decay times of samples changed drastically when the annealing temperature was above 550 °C. We determined the annealing dynamics of LT-GaAs based on a model in which AsGa antisite defects trap photoexcited carriers. An Arrhenius plot of the carrier decay rate vs. annealing temperature in the high temperature regime gave an energy EPA that was different from the true activation energy. The annealing time dependences of EPA obtained by the two diffusion models (self diffusion and VGa vacancy assisted diffusion of defects) were compared with EPAs of our data and other works, which proved that the annealing...