Wataru Kada

An ultra-thin diamond membrane as a transmission particle detector and vacuum window for external microbeams

Several applications of external microbeam techniques demand a very accurate and controlled dose delivery. To satisfy these requirements when post-sample ion detection is not feasible, we constructed a transmission single-ion detector based on an ultra-thin diamond membrane. The negligible intrinsic noise provides an excellent signal-to-noise ratio and enables a hit-detection efficiency of close to 100%, even for energetic protons, while the small thickness of the membrane limits beam spreading. Moreover, because of the superb mechanical stiffness of diamond, this membrane can simultaneously serve as a vacuum window and allow the extraction of an ion microbeam into the atmosphere.

DEVELOPMENT OF ANALYSIS SYSTEM OF MICRO-IBIL COMBINED WITH MICRO-PIXE

An ion beam induced luminescence analysis system using ion micro-beam (micro-IBIL) was newly developed on the microbeam system of the 3 MeV single-ended accelerator at ion-irradiation research facility TIARA, JAEA. The developed IBIL system consisted of an aspheric microlens, optical fibers, a monochromator and a photon-counting system to observe IBIL photons of specific wavelength with the resolution of 2 nm. A photomultiplier in the photon-counting system was cooled to around 0°C by a peltier device to reduce the background noises down to 10 cps and able to observe weak photon signals from specific chemical composites of the target. Experiments of micro-IBIL were performed using 3 MeV proton microbeam for several scintillators and particulate targets i.e. aerosol particles. The system had achieved chemical-imaging of aerosols by obtaining wavelength-dispersive micro-IBIL image at luminescence center of silicon dioxide.

DEVELOPMENT OF WAVELENGTH-DISPERSIVE MICRO-IBIL FOR THE CHEMICAL STRUCTURE ANALYSIS OF MICROMETER-SIZED PARTICLES

Ion-Beam-Induced Luminescence (IBIL) spectra and images for the chemical states and crystal structures of micrometer-sized mineral targets were obtained by wavelength-dispersive IBIL analysis. A wavelength resolution of approximately 2 nm was achieved with a remote-controlled compact monochromator installed on an IBIL analysis system using a 3 MeV H+ microbeam. Several particulate mineral targets and aerosol samples were prepared on a carbon-plate sample holder that was selected to reduce the background noise and achieve a high S/N ratio in both the IBIL and Particle-Induced X-ray Emission (PIXE) analyses. The chemical composition of small targets that could not be well determined using micro-PIXE analysis was successfully visualized by the proposed IBIL analysis.

EFFECTIVENESS OF A COMBINATION OF ML-EM AND STIM-CT IN PIXE-CT FOR BIOLOGICAL SPECIMEN

In this study, we applied ML-EM (maximum likelihood expectation maximization) iterative algorithm and STIM-CT to a three dimension imaging technique of PIXE -CT for cellular analysis using 3 MeV proton microbeam with a diameter of 1 μm at TIARA (Takasaki Ion Accelerators for Advanced Radiation Application). The algorithm had been applied to tomography such as SPECT (Single Photon Emission Computed Tomography). It is also possible and suitable to apply the algorithm to PIXE-CT because the algorithm can be used to project images taking into account errors due to rare events. Energy loss of incident particles and absorption of X-rays, however, affect quantitative values dramatically in the case of PIXE -CT. We estimated those effects from density distribution of major elements measured by STIM-CT in this work. In order to show the appropriateness of this method, X-ray yield and absorption were calculated using density distribution of two-dimensional Shepp phantom. The PIXE-CT experiments were carried out using a dried unicellulate as a test sample placed on an automatic rotation stage, which made a full turn by rotating 360° at each step of 18° in vacuum. The results of those simulation and experiments have proved that this method is effective.

Three-Dimensional Proton Beam Writing of Optically Active Coherent Vacancy Spins in Silicon Carbide

Constructing quantum devices comprises various challenging tasks, especially when concerning their nanoscale geometry. For quantum color centers, the traditional approach is to fabricate the device structure after the nondeterministic placement of the centers. Reversing this approach, we present the controlled generation of quantum centers in silicon carbide (SiC) by focused proton beam in a noncomplex manner without need for pre- or postirradiation treatment. The generation depth and resolution can be predicted by matching the proton energy to the materials stopping power, and the amount of quantum centers at one specific sample volume is tunable from ensembles of millions to discernible single photon emitters. We identify the generated centers as silicon vacancies through their characteristic magnetic resonance signatures and demonstrate that they possess a long spin-echo coherence time of 42 ± 20 μs at room temperature. Our approach hence enables the fabrication of quantum hybrid nanodevices based on SiC platform, where spin centers are integrated into p-i-n diodes, photonic cavities, and mechanical resonators.

Single photon sources in 4H-SiC metal-oxide-semiconductor field-effect transistors

We present single photon sources (SPSs) embedded in 4H-SiC metal-oxide-semiconductor field-effect transistors (MOSFETs). They are formed in the SiC/SiO2 interface regions of wet-oxidation C-face 4H-SiC MOSFETs and were not found in other C-face and Si-face MOSFETs. Their bright room-temperature photoluminescence (PL) was observed in the range from 550 to 750 nm and revealed variable multi-peak structures as well as variable peak shifts. We characterized a wide variety of their PL spectra as the inevitable variation of local atomic structures at the interface. Their polarization dependence indicates that they are formed at the SiC side of the interface. We also demonstrate that it is possible to switch on/off the SPSs by a bias voltage of the MOSFET.

Effect of a radical exposure nitridation surface on the charge stability of shallow nitrogen-vacancy centers in diamond

A nitridation process of a diamond surface with nitrogen radical exposure far from the radio-frequency plasma for the stabilization of a negatively charged nitrogen-vacancy (NV−) centers near the surface is presented. At a nitrogen coverage of as high as 0.9 monolayers, high average Rabi contrasts of 0.40 ± 0.06 and 0.46 ± 0.03 have been obtained for single NV− centers formed by shallow nitrogen implantation with acceleration voltages of 1 and 2 keV, respectively. This indicates that nitrogen termination by a radical exposure process produces an electric charge state suitable for single NV− centers near the surface compared with the states obtained for alternatively terminated surfaces.

Demonstration of Thermo-Optic Switch Consisting of Mach-Zehnder Polymer Waveguide Drawn Using Focused Proton Beam

We demonstrated a PMMA-based thermo-optic switch consisting of a Mach-Zehnder (MZ) type waveguide drawn by proton beam writing (PBW) and working at λ = 1.55 μm. The MZ waveguide was drawn by symmetrically coupling two Y junctions with a core width of 8 μm and a branching angle of 2°. A Ti thin-film heater and Al electrodes were formed on the surface of the MZ waveguide using conventional photolithography and wet-etching processes. An ON/OFF ratio of 9.0 dB and a switching power of 43.9 mW were obtained from the sample. The switching power is lower than for conventional commercial silica-based switches.

Fabrication of Mach-Zehnder Polymer Waveguides by a Direct-Drawing Technique Using a Focused Proton Beam

Proton beam writing (PBW) has recently attracted much attention as a next-generation microfabrication technology. This is a direct-drawing technique and does not need any masks to transfer micropatterns to sample surfaces. In our previous work, we demonstrated the first single-mode straight-line and Y-junction PMMA-based waveguides fabricated using PBW and working at λ = 1.55 μm. In this work, we fabricated the first PMMA-based Mach-Zehnder waveguides for the wavelength utilizing PBW in order to construct thermo-optic switches.

Fabrication of polymer optical waveguides for the 1.5-μm band using focused proton beam

Proton beam writing (PBW) has attracted much attention recently as a next-generation micro-fabrication technology. It is a direct-drawing technique and does not need any masks to transfer micro-patterns to sample surfaces. In addition, the refractive index of a poly (methyl methacrylate) (PMMA) can be increased by proton-beam irradiation. In this study, we fabricated the first 1.5-μm-band single-mode, straight-line waveguides and Y-junction waveguides consisting of PMMA layers using the PBW technique.