Takui Sakaguchi

Photonic bands in two-dimensional microplasma arrays. I. Theoretical derivation of band structures of electromagnetic waves

Two theoretical approaches appropriate for two-dimensional plasma photonic crystals reveal dispersions of propagating waves including photonic (electromagnetic) band gaps and multiflatbands. A modified plane-wave expansion method yields dispersions of collisional periodical plasmas, and the complex-value solution of a wave equation by a finite difference method enables us to obtain dispersions with structure effects in an individual microplasma. Periodical plasma arrays form band gaps as well as normal photonic crystals, and multiflatbands are present below the electron plasma frequency in the transverse electric field mode. Electron elastic collisions lower the top frequency of the multiflatbands but have little effect on band gap properties. The spatial gradient of the local dielectric constant resulting from an electron density profile widens the frequency region of the multiflatbands, as demonstrated by the change of surface wave distributions. Propagation properties described in dispersions including band gaps and flatbands agree with experimental observations of microplasma arrays.

Verification of a plasma photonic crystal for microwaves of millimeter wavelength range using two-dimensional array of columnar microplasmas

We experimentally verified that a microplasma assembly can create a functional dielectric layer for the propagation of electromagnetic waves as a “plasma photonic crystal.” A two-dimensional array in a square lattice was composed of columnar plasmas of about 2mm in diameter, and the transmitted microwaves at 70–75GHz showed a change of energy flow direction. This result is attributed to the fact that periodical structure is composed of individual plasma columns with a different dispersion than the ambient part and the experimental frequency range lies in the vicinity of the lowest band gap of the photonic crystal calculated theoretically.

Photonic bands in two-dimensional microplasma arrays. II. Band gaps observed in millimeter and subterahertz ranges

Plasma photonic band gaps have been observed in a two-dimensional microplasma array, and we have characterized their properties by both experimental and theoretical results. Microplasma columns ignited in helium near atmospheric pressure formed crystal-like structures in a square lattice with a lattice constant from 1.5to2.5mm. Microwaves in the millimeter range transmitting through the array region attenuated at frequencies of photonic band gap in the Γ‐X direction, as predicted by the modified plane-wave expansion method. Frequency dependence around the band gap was clarified in the numerical analysis of electromagnetic wave propagation and agreed with experimental results. Electron density in microplasmas was estimated to be 1×1013cm−3 from the attenuation rate at the band gap in the Γ‐X direction. Variation of the lattice constant induced frequency shift of the band gap in the millimeter and subterahertz regions, and so plasma photonic crystal can perform as a dynamically controllable band-stop filter.

Lasing band-edge identification for a surface-emitting photonic crystal laser

The possibility of single-mode oscillation over a large cavity area for photonic crystal lasers emitting at the photonic band edge has resulted in much interest in such materials for new forms of solid-state laser. In this paper, we measure the photonic bandstructure in our sample and identify the lasing band edge. By mapping out the bandstructure at the /spl Gamma/-point, we have observed fine structure at the band edge. The experimental results are in good agreement with the theoretically predicted bandstructure. Above threshold, we observe a lasing peak at 965 nm at one of the band edges. The far-field distribution of the laser is measured, showing an annular profile and azimuthal polarization. Calculations on the far-field distribution at the lasing band edge suggest the annular profile is due to an anti-symmetric resonant mode.

Interaction and control of millimetre-waves with microplasma arrays

Propagation of electromagnetic waves in several types of microplasmas has been examined experimentally in a frequency range 10?75?GHz. Firstly, the fundamental characteristics of the propagation were investigated using a planar geometry of microplasma assembly, and the electron density was derived by a comparison of the transmittance with the theoretical analyses using a Drude type model with collisional effects. Secondly, an extraordinary propagation phenomenon such as the focusing effect was observed in a two-dimensional periodical microplasma array. This kind of anomalous refraction cannot be interpreted only by predictions based on the dielectric property of bulk plasma, and it is suggested that a photonic-crystal-like periodical dielectric structure may play a significant role. Thirdly, it was demonstrated that the T-junction formed by a microplasma connected to a microstrip line can control the transmission of microwaves. An attenuation (or modulation) depth of about 35% was obtained with a series of two T-junctions connected to the strip line at the right-angled corners. All the above features come from (a) the relatively high electron density of the microplasmas near 1013?cm?3, (b) the complex dispersion relation with collisional effects and (c) the spatial arrangement with a characteristic scale of the same order of the wavelength of microwaves.

Diagnostics of microdischarge-integrated plasma sources for display and materials processing

Two different types of microdischarge-integrated plasma sources have been operated at around the atmospheric pressure range. The discharge characteristics were diagnosed by optical emission spectroscopy (OES), laser absorption spectroscopy (LAS) and microwave transmission (MT) techniques. The dynamic spatiotemporal behaviour of excited atoms was analysed using OES and LAS and the temporal behaviour of the electron density was estimated using the MT method. In Ar and Xe/Ne gases, waveforms of the MT signal followed the current waveform in the rise period and lasted longer according to the recombination losses. However, in He the waveform followed the density of metastable atoms, reflecting the production of a large amount of electrons by the Penning ionization process with impurities. The estimated peak electron density in those plasma sources is of the order of 1012 cm−3, and the metastable atom density can reach 1013 cm−3. Thus, it is suggested that these sources can be potentially applied to convenient material processing tools of large area operated stably at atmospheric pressure.

Characteristics of metamaterials composed of microplasma arrays

Functional arrays of microplasmas were designed for the purpose of controlling electromagnetic waves in the range 10–100 GHz. A two-dimensional square-lattice array of microplasma columns was proved to have photonic-crystal-like properties showing the presence of photonic band gaps and flat bands. A one-dimensional linear array of cold cathode fluorescent lamps (CCFLs) embedded in between the truncated strip lines showed the resonant transmission characteristics due to the surface-wave modes corresponding to the flat-band frequencies. Another one-dimensional configuration of CCFLs vertically aligned along both sides of a coplanar strip line showed peculiar behavior at the lower and higher edges of the band gap. These unique wave-propagation properties are attributed to the periodic structures of which pitches are comparable to or smaller than the wavelengths of electromagnetic waves. Therefore, these artificial arrays are potential metamaterials, which can be used for plasma devices controlling electromagnetic waves.

Demonstration of 3 kV 4H-SiC reverse blocking MOSFET

The authors developed 3 kV 4H-SÍC reverse blocking (RB) metal-oxide-semiconductor field-effect transistors (MOSFETs) for the first time. To achieve reverse blocking capability, the n+-substrate layer was removed by polishing, and both a Schottky contact and edge-termination structure were introduced onto the wafer backside. Fabricated SiC RB MOSFETs exhibited good Schottky characteristics, and measured differential specific on-resistance was 20 mΩ·cm2. Both forward and reverse blocking voltages of RB MOSFETs are higher than 3 kV. On-state power loss of a developed RB MOSFET is 35% lower than that of anti-serially connected standard 3 kV SiC MOSFETs, demonstrating the advantage of the developed RB MOSFET as a high-voltage bi-directional switch.

High-Temperature Characteristics of 3-kV 4H-SiC Reverse Blocking MOSFET for High-Performance Bidirectional Switch

Novel 3-kV 4H-SiC reverse blocking (RB) metal–oxide–semiconductor field-effect transistors (MOSFETs) have been demonstrated for high-voltage bidirectional switching applications. To achieve RB capability, a series Schottky barrier diode structure was introduced onto the backside of the 4H-SiC MOSFET. The developed SiC RB MOSFET exhibits bidirectional blocking voltage over 3 kV and a differential specific on-resistance of 20

Plasma Photonic Crystals in Two‐Dimensional Arrays of Microplasmas

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