A metallic hole array (MHA) supports the propagation of spoof surface plasmon polariton (SSPP) modes on its surface. We observe enhancement of dispersed waves at a specific angle when SSPP modes propagate. We propose a symmetrical estimation method for SSPP skin depth using waves dispersed from an MHA. The proposed method provides an experimental demonstration of the skin depth of the electric fields of SSPP modes. Using the dispersed waves emitted from the plane of the MHA, we are able to detect confined electric fields with decay lengths that resemble those theoretically predicted around a specific frequency of SSPP mode excitation.
In situ sensing with wireless digital-data transfer is a potential processing scheme that works very closely to the location of an event monitored by a sensor and converts the sensor’s raw output into digitized and informative small-volume bits, as suggested by recent proposals for edge computing and the Internet of Things (IoT). Colour perception may be a target of in situ sensor data acquisition; however, in contrast to from other sensing devices, colour sensors that detect visible light signals are usually located away from light-emitting sources, collecting light transmitting through the space and attenuating it in some manner. For example, in a vacuum chamber whose gas pressure is much less than the ambient atmosphere in which the sensors usually work, there are many veiled light sources, such as discharge plasma, for various industrial purposes including nanoscale manufacturing. In this study, we designed an in-vacuum colour sensor that can work with analogue-to-digital conversion and transfer data by wireless communication; this sensor is active in a low-pressure plasma chamber, detecting light signals and transferring them to a personal computer located outside the vacuum chamber. In addition to detecting lights with controlled spectra from outside successfully, we achieved complete operation of our in-vacuum active sensor for plasma emissions generated at 100 Pa. Comparing the signals with data from simultaneous monitoring by a monochromator, we established that the recorded signals arose from the plasma, confirming successful direct detection of low-pressure plasma emissions without any filtering effects between the sensor and the target object.
A novel 2D imaging method for permittivity imaging using a meta-structure with a functional scanning defect is proposed, working in the millimeter wave-range. The meta-structure we used here is composed of a perforated metal plate with subwavelength-holes and a needle-like conductor that can scan two-dimensionally just beneath the plate. The metal plate, which is referred to as a metal hole array (MHA) in this study, is known as a structure supporting propagation of spoof surface plasmon polaritons (SSPPs). High-frequency waves with frequencies higher than microwaves, including SSPPs, have the potential to detect signals from inner parts embedded beneath solid surfaces such as living cells or organs under the skin, without physical invasion, because of the larger skin depth penetration of millimeter wave-bands than optical wave-bands. Focused on activated SSPPs, the localized distortion of SSPP modes on an MHA is used in the proposed method to scan the electromagnetic properties of the MHA with a needle-like conductor (conductive probe), which is a kind of active defect-initiator. To show the validity of the proposed method, electromagnetic analyses of the localized distortions of wave fields were performed, and one- and two-dimensional imaging experiments were conducted with the aim of detecting both conductive and dielectric samples. The analytical results confirmed the localized distortion of the electric field distribution of SSPP modes and also indicated that the proposed method has scanning ability. In experimental studies, the detection of conductive and dielectric samples was successful, where the detected dielectrics contained pseudo-biological materials, with an accuracy on the order of millimeters. Finally, a biomedical diagnosis in the case of a rat lung is demonstrated by using the experimental system. These results indicate that the proposed method may be usable for non-invasive and low-risk biomedical diagnosis.
Go itami * & osamu Sakai A metal plate array (MpA) which is a structure complimentary to a metal hole array (MHA), supports spoof surface plasmon polaritons (SSpp) as well as an MHA does. Babinet's principle attributes the phenomenon of duality to transmission characteristics of the complimentary impedance surfaces because of the symmetry of the behaviors of electric and magnetic fields. However, it is also a fact that the complimentary structures do not follow this principle if they have wavelength-size thickness, because electromagnetic waves do not treat such thick structures as a boundary surface but as propagation spaces with the specific boundaries such as a waveguide which shows SSPP modes. If the thickness is so small that it is negligible, Babinet's principle is still valid, while it has been uncertain how the layer thickness works to break the principle as it is increased. The unconfirmed transformation is revealed analytically and experimentally with the use of MpAs and MHAs of varying thicknesses. The behavior of electromagnetic waves interacting with objects is broadly classified into two cases. For wavelengths significantly larger than the object, the wave's electric fields are treated as near fields and electromagnetic behavior is expressed as a scattering phenomenon. For wavelengths much smaller than the object the wave's electric fields are treated as far fields and the electromagnetic behavior is expressed as propagating waves, either transmitted or reflected. However, when the wavelength is of the same order as the object size, the scattering phenomena become more complex. In this case, if such objects are perforated periodically, they can be provided an interesting medium with properties, otherwise not attainable with naturally occurring materials, such as a negative refractive index (NRI) 1-6. In general, these artificial structures are called metamaterials. In 2000, Pendry showed theoretically the concept of a perfect lens with a negative refractive index (NRI) medium 1 , and in 2001, Shelby et al. verified the possibility of an NRI experimentally by using the two artificial resonators for negative permeability and permittivity media 2. Their studies demonstrated the feasibility of metamaterials, and accelerated the advances in the field in recent years 1-3, 7-12. In such studies, a split ring resonator (SRR) is often used as a negative permeability medium for NRI realization. And the complementary split ring resonator (CSRR) is also known as the resonator for negative permittivity realization, based on Babinet's principle 12-16. In these two resonators, the essential difference in their resonance is based on the following electromagnetic radiation mechanisms: SRR radiates the topologically overlaid waves from the infinitesimal electric dipoles, and CSRR radiates them from the infinitesimal magnetic dipoles. Based on this principle, the SRR and CSRR show band-stop and band-pass characteristics if they are used as planar spatial filters. A frequency selective surface (FSS) is also a spatial f...
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