Akinori Harada

BULK PERIODICALLY POLED MGO-LINBO3 BY CORONA DISCHARGE METHOD

We propose and demonstrate a novel corona discharge method for the fabrication of bulk periodically poled MgO‐LiNbO3 substrates. The bulk periodic domain inversion has been successfully formed with a first‐order modulation period of 5.2 μm for a wavelength of 980 nm over a 6.5 mm interaction length in a Z‐cut 0.4‐mm‐thick MgO‐LiNbO3 substrate for the first time. It was found that MgO‐LiNbO3 crystal is capable of forming domain inversion under a low applied voltage of no more than 6 kV/mm, which is lower than the case of LiNbO3 and LiTaO3. A 490 nm blue light power of 6.7 mW was achieved in a single pass through periodically poled MgO‐LiNbO3, by using a high‐power monolithically integrated flared master oscillator power amplifier of 790 mW as a fundamental source.

Generation of blue coherent light from a continuous‐wave semiconductor laser using an organic crystal‐cored fiber

Efficient frequency doubling of a continuous‐wave semiconductor laser was attained by using a nonlinear optical fiber with a single crystal core of an organic 3,5‐dimethyl‐1‐ (4‐nitrophenyl)pyrazole. The second‐harmonic power generated with a 15 mm length of the fiber reached 0.16 mW from the incident 16.6‐mW fundamental laser power at 884 nm. Nonlinear optical properties of the crystal and a fabrication technique of the fiber are also reported.

Intracavity frequency doubling of a diode-pumped 946-nm Nd:YAG laser with bulk periodically poled MgO LiNbO(3).

We report what we believe is the first cw quasi-phase-matched intracavity frequency doubling of a diode-pumped 946-nm Nd:YAG laser by use of bulk periodically poled MgO-LiNbO(3). A maximum second-harmonic power of 5.9 mW was obtained at a laser diode power of 500 mW. The blue beam profile showed a TEM(00) single spatial mode of low ellipticity (1:1) because there was no walk-off problem. Bulk periodic domain inversion was accomplished by a corona discharge method with a first-order modulation period of 4.75 microm for a wavelength of 946 nm over a 2-mm interaction length in Z-cut 0.4-mm-thick MgO-LiNbO(3).

Temperature Characteristics of 1.5-µm-Band MgO Doped LiNbO3 Quasi-Phase Matched Wavelength Converters

Temperature characteristics of MgO doped LiNbO3 quasi-phase matched (QPM) wavelength converters are studied based on different frequency generation (DFG) for the first time. The temperature sensitivity of the QPM wavelength is found to be about 0.113 nm/°C for the device with a period of 18.7 µm, while the wavelength conversion efficiency at a pump power of -0.5 dBm is found to be -26 dB. These values are compatible with the best values for non-doped LiNbO3 QPM-DFG wavelength converters reported to date. It is shown that the converted wavelength can be tuned by changing device operation temperature and pump wavelength, indicating the capability of variable-in variable-out wavelength conversions required in optical cross connect networks.

Efficient 1.5-µm-Band MgO-Doped LiNbO3 Quasi-Phase-Matched Wavelength Converters

1.5-µm-band wavelength converters based on cascaded χ(2) in periodically poled MgO:LiNbO3 (PPMGLN) waveguides are fabricated and studied for the first time. A conversion efficiency of -26 dB is obtained with the pump power of only 16 mW, which is compatible with the highest reported value of the PPLN wavelength converters based on nondoped substrates. The 3 dB conversion bandwidth is found to be as large as 68 nm for a 45-mm-long device.

Enhanced stability of the HfO2 electrolyte and reduced working voltage of a CB-RAM by an ionic liquid

Supplying a trace volume of ionic liquid, [bmim][Tf2N], which contains 5000 ppm of H2O, on the HfO2 film in the conducting-bridge random access memory composed of Cu/HfO2/Pt allows improved efficiency of the memory properties: reduction of operating voltage and prevention of destruction of the electrolyte.

All-Optical Variable-In Variable-Out Wavelength Conversions by Using MgO:LiNbO3 Quasiphase Matched Wavelength Converters

A novel method of all-optical variable-in variable-out wavelength conversions is, for the first time, proposed and demonstrated by using MgO:LiNbO3 quasiphase matched (QPM) waveguides. It is shown that input signal in a wavelength range of 60 nm can be converted to any output signal in a wavelength range of 30 nm with almost the same efficiency (-26 dB with a pump power of -0.5 dBm) by changing device temperature and pump wavelength.

Improved performance of a conducting-bridge random access memory using ionic liquids

The conducting-bridge random access memory (CB-RAM) is a promising candidate for the next-generation memory. In this study, the addition effect of 18 types of ionic liquids (ILs) on the performances of the Cu/HfO2/Pt type CB-RAM has been investigated; the switching voltage was lowered by supplying a trace amount of the IL to the HfO2 layer of the Cu/HfO2/Pt cell. It was revealed that the ionic conductivity (σi) of the IL and the coordinating ability (β) of the anion in the IL towards the copper (Cu) ion are key competing factors that determine the voltage required for the formation of the Cu filament (Vset); the increase in σi and the decrease in β are effective in enhancing the ionization and diffusion of the Cu atoms, resulting in the reduction of Vset. Therefore, among the tested ILs, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([bmim][Tf2N]), 1-ethyl-3-methylimidazoliumbis(trifluoromethanesulfonyl)amide ([emim][Tf2N]) and 1-butyl-3-methylimidazolium bis(fluorosulfonyl)amide ([bmim][FSA]), which have high σi and low β values were found to be the best additives for the Cu/HfO2/Pt cell. On the other hand, reset was revealed to occur due to the rupture of the Cu filament by the thermal diffusion of the Cu atoms by Joule heat. Reducing Vset and making a thin filament are effective in reducing the reset voltages. We further traced the origin of the improved dispersion of the switching voltages by the addition of the Cu(Tf2N)2 doped [bmim][Tf2N] to the HfO2 film, which we recently reported. A complex-impedance measurement suggested that the addition of Cu doped IL involves two competing factors: the decline of the Cu ion diffusion and enhancement of the Cu ionization. The balance of these two factors determines the important parameters such as Vset and its dispersion. The performance of the CB-RAM can be optimized by the adjustment of the IL, considering the balance of the competing factors revealed by the present study.

Comparisons of Piezoelectricities of Lead Zirconate Titanate for Various Phases by a First-Principles Method

We calculated and compared the piezoelectric coefficients and Born effective charges of various PZT crystals, which have different B-site orderings and crystal symmetries by a first-principles method. The piezoelectric coefficient of rhombohedral structures is smaller than that of tetragonal structures because of the weakness of the hybridization change between the oxygen and B-site orbitals. We discuss this reason using a simple analytic model. We also performed the first-principles calculation of the piezoelectric coefficient with emphasis on its dependencies on the k-point number and calculation scheme (GGA or LDA), and found that calculated piezoelectric coefficients are very sensitive to these factors. This is due to the fact that the piezoelectric coefficient strongly depends on the small changes of the internal displacement gradient.

Improvement of switching endurance of conducting-bridge random access memory by addition of metal-ion-containing ionic liquid

A remarkable improvement of cycling endurance was achieved by the addition of a trace amount of an ionic liquid ([bmim][Tf2N]) containing Cu(Tf2N)2 to the HfO2 layer of a conducting-bridge random access memory (CBRAM) with a Cu/HfO2/Pt structure. The improvement was brought about by the significant improvement of the dispersion of a set voltage and the suppression of the generation and migration of oxygen vacancies owing to the smooth diffusion of Cu ions in ionic liquids.