Daisuke Fujisawa

Mushroom plasmonic metamaterial infrared absorbers

There has been a considerable amount of interest in the development of various types of electromagnetic wave absorbers for use in different wavelength ranges. In particular, infrared (IR) absorbers with wavelength selectivity can be applied to advanced uncooled IR sensors, which would be capable of identifying objects through their radiation spectrum. In the present study, mushroom plasmonic metamaterial absorbers (MPMAs) for the IR wavelength region were designed and fabricated. The MPMAs consist of a periodic array of thin metal micropatches connected to a thin metal plate with narrow silicon (Si) posts. A Si post height of 200 nm was achieved by isotropic XeF2 etching of a thin Si layer sandwiched between metal plates. This fabrication procedure is relatively simple and is consistent with complementary metal oxide semiconductor technology. The absorption spectra of the fabricated MPMAs were experimentally measured. In addition, theoretical calculations of their absorption properties were conducted usin...

Two-million-pixel SOI diode uncooled IRFPA with 15μm pixel pitch

We report the development of a 2-million-pixel, that is, a 2000 x 1000 array format, SOI diode uncooled IRFPA with 15 μm pixel pitch. The combination of the shrinkable 2-in-1 SOI diode pixel technology, which we proposed last year [1], and the uncooled IRFPA stitching technology has successfully achieved a 2-million-pixel array format. The chip size is 40.30 mm x 24.75 mm. Ten-series diodes are arranged in a 15 μm pixel. In spite of the increase to 2-million-pixels, a frame rate of 30 Hz, which is the same frame rate as our former generation (25 μm pixel pitch) VGA IRFPA, can be supported by the adoption of readout circuits with four outputs. NETDs are designed to be 60 mK (f/1.0, 15 Hz) and 84 mK (f/1.0, 30 Hz), respectively and a τth is designed to be 12 msec. We performed the fabrication of the 2-million-pixel SOI diode uncooled IRFPAs with 15 μm pixel pitch, and confirmed favorable diode pixel characteristics and IRFPA operation where the evaluated NETD and τth were 65 mK (f/1.0, 15 Hz) and 12 msec, respectively.

Theoretical investigation of all-metal-based mushroom plasmonic metamaterial absorbers at infrared wavelengths

Abstract. High-performance wavelength-selective infrared (IR) sensors require small pixel structures, a low-thermal mass, and operation in the middle-wavelength infrared (MWIR) and long-wavelength infrared (LWIR) regions for multicolor IR imaging. All-metal-based mushroom plasmonic metamaterial absorbers (MPMAs) were investigated theoretically and were designed to enhance the performance of wavelength-selective uncooled IR sensors. All components of the MPMAs are based on thin layers of metals such as Au without oxide insulators for increased absorption. The absorption properties of the MPMAs were investigated by rigorous coupled-wave analysis. Strong wavelength-selective absorption is realized over a wide range of MWIR and LWIR wavelengths by the plasmonic resonance of the micropatch and the narrow-gap resonance, without disturbance from the intrinsic absorption of oxide insulators. The absorption wavelength is defined mainly by the micropatch size and is longer than its period. The metal post width has less impact on the absorption properties and can maintain single-mode operation. Through-holes can be formed on the plate area to reduce the thermal mass. A small pixel size with reduced thermal mass and wideband single-mode operation can be realized using all-metal-based MPMAs.

Acoustic carrier transportation induced by surface acoustic waves in graphene in solution

The acoustic charge transportation induced by surface acoustic wave (SAW) propagation in graphene in solution was investigated. The sign of acoustic current (I A) was found to switch when crossing the Dirac point because the major carrier was transitioned from holes to electrons by the change in electrolyte-gate voltage. I A also exhibited a peak value under conditions of both hole and electron conduction. These results can be explained on the basis of a change in the type of major carrier in graphene, as well as a change in the carrier mobility of graphene.

Multi-color imaging with silicon-on-insulator diode uncooled infrared focal plane array using through-hole plasmonic metamaterial absorbers

This paper reports a silicon-on-insulator diode uncooled infrared focal plane array (IRFPA) with through-hole plasmonic metamaterial absorbers (TH-PLMAs) for multi-color imaging with a 320×240 array format. Through-holes formed on the PLMA can reduce the thermal mass while maintaining both the single-mode and high absorption due to plasmonic metamaterial structures, which results in fast response and high responsivity. The detection wavelength of the PLMA with through-holes can be controlled over a wide range of the IR spectrum by varying the size of the micropatches on the top layer.

Effect of graphene on plasmonic metasurfaces at infrared wavelengths

Significant enhancement of infrared transmittance by the presence of a graphene layer on a plasmonic metasurface (PLM) has been demonstrated. PLMs with different configurations were fabricated, and their transmittance with and without graphene was compared. Selective enhancement by graphene occurred at the plasmon resonance wavelength. The degree of enhancement was found to depend on the width of the gap between the periodic metal regions in the PLM. A maximum enhancement of ∼210% was achieved at a wavelength of 10 μm. The ability to achieve such a drastic increase in transmittance at the plasmon resonant wavelength is expected to lead to improvements in the performance of energy collecting devices and optical sensors.

Giant Dirac point shift of graphene phototransistors by doped silicon substrate current

Graphene is a promising new material for photodetectors due to its excellent optical properties and high-speed response. However, graphene-based phototransistors have low responsivity due to the weak light absorption of graphene. We have observed a giant Dirac point shift upon white light illumination in graphene-based phototransistors with n-doped Si substrates, but not those with p-doped substrates. The source-drain current and substrate current were investigated with and without illumination for both p-type and n-type Si substrates. The decay time of the drain-source current indicates that the Si substrate, SiO2 layer, and metal electrode comprise a metal-oxide-semiconductor (MOS) capacitor due to the presence of defects at the interface between the Si substrate and SiO2 layer. The difference in the diffusion time of the intrinsic major carriers (electrons) and the photogenerated electron-hole pairs to the depletion layer delays the application of the gate voltage to the graphene channel. Therefore, th...

Three-dimensional plasmonic metamaterial absorbers based on all-metal structures

Three-dimensional plasmonic metamaterial absorbers (3-D PMAs) based on all-metal structures were developed. 3-D PMAs consist of a periodic array of thin metal micropatches connected to a thin metal plate with narrow metal posts. The 3-D PMA consists of plasmonic metal (Au) based components. 3-D PMAs were fabricated by a two-step lift-off procedure with a carbon sacrifice layer and a narrow metal post with a height of 200 nm was achieved. Reflection spectroscopy measurements demonstrate that the wavelength-selective absorption was realized, and the absorption wavelength can be controlled by the micropatch size, regardless of the micropatch-array period, and can be longer than the micropatch array period. Wavelength selective absorption is possible due to the surface plasmonic resonant mode localized at the micropatches. The metal posts have negligible impact on the plasmonic resonance. 3-D PMAs based on all-metal structures can be applicable for a wide range of the middle- and long-wavelength IR region due to the lack of additional absorption by an insulator layer based on SiO2, SiN, or Al2O3, which are typically used in metal-insulatormetal absorbers. 3-D PMAs have a small thermal mass and an absorption wavelength beyond the period, which result in a fast response and small pixel size. The results obtained here should contribute to the high-performance wavelengthselective uncooled IR sensors and IR emitters.

Photocurrent enhancement of graphene phototransistors using p-n junction formed by conventional photolithography process

A p–n junction was developed in a graphene transistor by a simple photolithography process used in typical semiconductor processes. The p- and n-type regions were formed by coating photoresist on part of the graphene channel and immersion of the uncovered graphene region in alkali developer, respectively. A 3-fold enhancement of the photocurrent was observed at the maximum field effect mobility. It is therefore important to maximize the field effect mobility by doping to maximize the photocurrent. The results obtained here are an important step toward the production of high-sensitivity graphene-based phototransistors compatible with conventional industrial procedures.

Three-dimensional plasmonic metamaterial absorbers for high-performance wavelength selective uncooled infrared sensors

A three-dimensional plasmonic metamaterial absorber (3-D PMA) was theoretically investigated and designed for the performance enhancement of wavelength selective uncooled infrared (IR) sensors. All components of the 3-D PMA are based on thin layers of plasmonic metals such as Au. The post produces a narrow gap, such as a few hundred nanometers, between the micropatch and the metal plate. The absorption properties of the 3-D PMA were investigated by rigorous coupled-wave analysis. A strong wavelength selective absorption is realized by the plasmonic resonant mode of the micropatch and the narrow-gap resonant mode between the micropatch and the plate. The disturbance of the post for both resonance modes is negligible. The absorption wavelength is defined mainly by the size of the micropatch, regardless of the micropatch array period and is longer than the micropatch array period. The absorption mode can also be controlled by the shape of the micropatch. Through-holes can be formed on the plate area, where there is no gap resonance to the micropatch. The thickness of each component can be reduced considering the skin depth effect and there is no added absorption of materials such as SiO2. A small pixel size with reduced thermal mass can be realized using a 3-D PMA structure. The results obtained here will contribute to the development of high-performance uncooled IR sensors for multicolor imaging.