Koichi Sakita

Analysis of Weighting Function in Transient Spectroscopy for Precise Measurement of Deep States

The weighting function in transient deep-level spectroscopy has been analyzed for precise measurement of deep states. The relation among weighting functions of different measurement modes is discussed. It is shown that unless the deep energy level is a well-defined discrete one, reconstruction of the density-of-states from the observed quantity is not straightforward and the weighting function greatly depends upon the energy dependence of attempt-to-escape frequency or capture cross section of the states. Approximate and exact methods to deduce the density-of-states are presented emphasizing that the neglect of energy dependence of the attempt-to-escape frequency brings a large error in the density-of-states estimation.

Rotationally symmetric wavefront coding for extended depth of focus with annular phase mask

Wavefront coding (WFC) with a cubic phase mask (CPM) for extended depth of focus (EDOF) has been studied since 1995. However, the asymmetric surface shape of a CPM has several disadvantages, such as necessity of a square aperture, directional artifacts in images, rotational adjustment during assembly, defocus-dependent lateral image displacement, and difficulty in fabrication. To overcome these disadvantages, we propose an annular phase mask (APM) with a rotationally symmetric form. An APM adds the phase of the parabolic profile in plural annuluses and phase steps between annuluses to the penetrating light. We confirmed an EDOF effect comparable to that of a CPM in our APM design by simulation. The depth of focus of a WFC system with the APM was estimated to be about 6.8 times larger than that of a conventional system. The effect observed in the experiments using a fabricated APM is consistent with the simulation result.

Visible–infrared achromatic imaging by wavefront coding with wide-angle automobile camera

We perform an experiment of achromatic imaging with wavefront coding (WFC) using a wide-angle automobile lens. Our original annular phase mask for WFC was inserted to the lens, for which the difference between the focal positions at 400 nm and at 950 nm is 0.10 mm. We acquired images of objects using a WFC camera with this lens under the conditions of visible and infrared light. As a result, the effect of the removal of the chromatic aberration of the WFC system was successfully determined. Moreover, we fabricated a demonstration set assuming the use of a night vision camera in an automobile and showed the effect of the WFC system.