Ryota Ito

Improvement of Phase-Shifting Properties in Coplanar Waveguide-Type Liquid Crystal Millimeter-Wave Phase Shifter by Introducing a Resonant Phenomenon

Nematic liquid crystal (LC) material is mounted on a coplanar waveguide (CPW) substrate, which is well known as an excellent transmission line for superhigh-frequency electromagnetic waves, to achieve an electronically tunable phase shifter. In our previous work, an upper electrode introduced for an LC drive significantly contributed to reducing the driving voltage with large phase-shifting properties. However, it also tended to increase transmission loss. Thus, it was clarified that it is not necessarily a desirable improvement if we evaluate the device performance by balancing the phase shift and loss. In this work, we propose a triangular pattern for the upper electrode to utilize a resonant phenomenon and to improve the phase-shifting properties. A periodic dip appears in the transmission spectra and some fluctuations appear in the phase properties near the dip frequency. It is found that the phase shift becomes very large at frequencies slightly lower than the dip frequency, although the frequency band in which the large phase shifts occur is limited.

Hole tunnelling properties in resonant tunnelling diodes with Si/strained Si0.8Ge0.2 heterostructures grown on Si(1 0 0) by low-temperature ultraclean LPCVD

Hole resonant tunnelling diodes (RTDs) with Si/strained Si1–xGex heterostructures epitaxially grown on Si(1 0 0) were fabricated, and sharp current peaks have been reproducibly observed. From the quantum well width dependence of the current–voltage characteristics, at the peak voltage, heavy holes in the accumulation region resonantly tunnel through the estimated resonant states of a light hole and a heavy hole in a Si1–xGex quantum well. The top contact and mesa area dependence of the peak current shows that the tunnel current only flows under a top contact electrode, i.e. the leakage current at the sidewall is negligibly small. The resonant tunnelling current density reaches as high as 3.6 kA cm−2 at 0.19 V with 2 nm thick barriers. Moreover, the introduction of higher Ge fraction effectively suppresses the increase of the valley current and enables negative differential conductance at higher operation temperature.

Discriminated Detection of Nuclear Tracks Recorded on Multilayers of Photographic Emulsions with Different Sensitivities by Color-Development Method

We have developed a new method for the detection of nuclear tracks recorded on nuclear plates composed of multiple photographic emulsion layers. Each layer of these plates contained a photographic coupler revealing a different color after color development. The developed silver grains left behind in each layer were colored with differently colored dye clouds when the bleaching process was omitted in the color-development process. The three-dimensional structure of a track was obtained from the color change of the track. The track indicated a different grain density in each layer of different sensitivity, thus expanding the dynamic range of such nuclear plates. Accurate discrimination of particles with different deposit energies by using emulsions with different properties in each layer is also expected.

Introduction of liquid crystal device into THz phase imaging

We investigate the terahertz (THz) phase imaging using liquid crystal (LC) device. Recently, there have been extensive efforts to measure the refractive indices and transmission losses of some LC materials in THz region. From these results, LC materials show relatively large refractive index anisotropy and they can have a potential application to some control devices similar to the display application. Furthermore, tunable LC THz phase shifter has been reported. In this study, an attempt was made to introduce the LC device into the THz phase imaging which was based on four-step phase-shifting algorithm. Four-step phase-shifting algorithm is one of the most effective methods of phase imaging and moving mirror is needed to introduce phase shift in standard measurements. In addition, we should prepare the two rays with common source, therefor experimental setup becomes complicated. While on the other hand, it is just needed to insert a LC phase shifter into the light path when we adopt LC device. Furthermore, low-voltage application is enough to introduce the phase-shifting, hence it is not necessary to prepare the moving mirror. In this work, we fabricate the electrically tunable LC phase shifter which has sandwich cell structure. The fundamental phase-shifting properties were measured by using an optically pumped gas laser system which can generate continuous wave (CW) THz waves. In addition, we investigated the phase shifting interferometry which was based on four-step phase-shifting algorithm by using LC phase shifter. We also estimate the birefringence of X-cut crystalline quartz at 2.5 THz.