Masaaki Shibata

Photorefractive properties of undoped, Cr-doped, and Cu-doped potassium sodium strontium barium niobate crystals

We determine several photorefractive properties of undoped, Cr-doped, and Cu-doped (KyNa1–y)2z, (SrxBa1–x)1–z Nb2O6 (KNSBN) crystals by means of two-beam coupling measurements. An elemental analysis of the crystals to determine their compositions and impurities is also performed. It is found that two-beam coupling gain of the Cr-doped KNSBN at 514.5 nm is enhanced by ~10 times that of the undoped KNSBN, while its response speed remains substantially unchanged. Longer-wavelength photorefractivity is also observed in the Cr-doped KNSBN. Quasi-periodic oscillations of the probe-beam intensity in the two-beam coupling experiment are observed in the Cu-doped KNSBN.

A passive micro gas regulator for hydrogen flow control

This paper presents the design, fabrication process, experimental characterizations and simulations of a novel passive micro gas regulator. The device is to be used in a miniature fuel cell for portable electronic applications in order to regulate the hydrogen flow feeding the fuel cell from a high-pressure tank. Its structure and working principle are similar to some macroscopic devices. Using MEMS technologies, e.g., deep-RIE etching and multiple wafer bonding, the dimensions of the device have been made less than 8 × 8 × 1 mm3. Moreover, as it is passive, it consumes no power from the fuel cell. Thanks to these two features, it is well suited for portable applications. To our knowledge, this is the first passive micro gas regulator, except those actuated by hydrogels. The experimental characterization shows that the opening and closing are controlled by the released gas pressure. Up to an input pressure of 8 atm, the device has been successfully driven and the leakage has been measured to be below 0.1 sccm nitrogen. Analytical models for the moving part and for the flow rate show a good agreement when compared to the experimental data.

A low melting point alloy as a functional material for a one-shot micro-valve

A one-shot valve made using MEMS (micro-electro-mechanical systems) technologies has been designed, fabricated and characterized. The operation of the device is temperature and pressure dependent. It consists in a channel obstructed by a membrane coated with a low melting point alloy. The valve is normally closed and opens if the ambient temperature is higher than the melting temperature of the alloy and if the pressure difference across the channel is such as to fracture the membrane. The working principle of the device has been demonstrated and several characteristics have been measured.

A micro-machined safety valve for power applications with improved sealing

A novel micro-safety valve for portable hydrogen fuel cells has been designed, fabricated and characterized. This device is intended to prevent over-pressure on the hydrogen side of the fuel cell. A careful study of its sealing properties has been conducted under conditions at which it is nearly open. The leakage rate depending on the size of the sealing surface and on the material of the gasket (fluorocarbon and parylene films) has been investigated using hydrogen gas. The experimental results show that small sealing surfaces lead to small leakage rates and that the parylene films are easily damaged, leading to important leakage rates. Moreover, it is shown that the resistance to flow of the device is diminished by reducing the width of the valve nozzle.

Near-infrared photorefractivity in Cr-doped potassium sodium strontium barium niobate single crystal

We report on the photorefractive properties of a Cr-doped (K1−xNax)2A−2 (SryBa1−y)2−ANb10O30 (x=0.586, y=0.659, A=1.12) single crystal in the near-infrared spectrum. The sample exhibits photorefractivity for wavelengths up to at least 840 nm where the steady-state two-beam coupling gain is found to be larger than 2 cm−1. Photorefractive gain and decay rate are measured as a function of wavelength, grating spacing and intensity. The wavelength dependence of gain fluctuations in two-beam coupling are also measured.

Pulsewidth dependence of time‐resolved two‐photon absorption with picosecond pump–probe excitation

Pulsewidth dependence of time‐resolved two‐photon absorption with picosecond pump–probe excitation is investigated. It is shown theoretically that the time‐resolved probe transmittance change by two‐photon absorption almost follows a temporal autocorrelation of the pump and probe pulses. Time‐resolved two‐photon absorption measurements of 1.064 μm, 29 ps pulses with semi‐insulating undoped GaAs are also performed. The comparison of the experimental results with second harmonic generation autocorrelation measurements confirms the theory.

Fluidic self-alignment applied to a micro-fluidic system

A technique for the self-alignment of components is applied to the assembly of a passive micro-valve which is to be used for safety purposes in a miniature fuel cell. The technique uses the capillary forces originating in the interactions of HMDS (Hexadimethyldisilazane) and the faces of the components to achieve their passive alignment. HMDS is chosen because it is a volatile, low surface tension and inert liquid. The originality of this method is that the liquid area is defined by the edges of the components. Compared to a traditional pick-and-place technique, it leads to a leakage rate more than five times smaller.

Effects of free‐carrier absorption on time‐resolved pump‐probe two‐photon absorption in semi‐insulating semiconductors

Free‐carrier absorption induced by picosecond pump excitation is investigated for the time‐resolved pump‐probe nonlinear transmission process in semi‐insulating semiconductors. Time‐resolved nonlinear absorption measurements of 1.064 μm, 29 ps pulses with semi‐insulating undoped GaAs and Ga‐doped CdTe are performed. Because of very small linear absorption at 1.064 μm for Ga‐doped CdTe, the effect of the nonlinear free‐carrier absorption is found to be noticeable. A theoretical analysis is also made to understand the role of the free‐carrier absorption and the linear absorption saturation at large positive time delays of the probe. From the fitting procedure by the theory free‐carrier absorption coefficients as well as two‐photon absorption coefficients for the two crystals are obtained.

Thermal modification of the rigidity of micro-structures by the phase transition of a fusible alloy

An original micro-valve has been designed, modeled, fabricated and characterized. Its novelty resides in the thermal modification of the rigidity of its movable element by the phase transition of a fusible alloy integrated by self-assembly onto a silicon membrane. It is applied as a purge and safety valve for hydrogen micro fuel cells. The experimental results demonstrate the modification of the rigidity of the mechanical structure by melting and freezing one of its constituents, a property favored at a small size. This principle allows one to latch a mechanical element in the open and closed positions, as well as for ambient temperature sensing. Its potential applications include other devices such as electrical relays and optical switches, and can be integrated with active materials.