Frequency-Tunable Terahertz Plasmonic Structure Based on the Solid Immersed Method for Sensing.

Toshio Sugaya,Yukio Kawano

doi:10.3390/s21041419

Abstract

Terahertz waves are located in the frequency band between radio waves and light, and they are being considered for various applications as a light source. Generally, the use of light requires focusing; however, when a terahertz wave is irradiated onto a small detector or a small measurement sample, its wavelength, which is much longer than that of visible light, causes problems. The diffraction limit may make it impossible to focus the terahertz light down to the desired range by using common lenses. The Bull’s Eye structure, which is a plasmonic structure, is a promising tool for focusing the terahertz light beyond the diffraction limit and into the sub-wavelength region. By utilizing the surface plasmon propagation, the electric field intensity and transmission coefficient can be enhanced. In this study, we improved the electric field intensity and light focusing in a small region by adapting the solid immersion method (SIM) from our previous study, which had a frequency-tunable nonconcentric Bull’s Eye structure. Through electromagnetic field analysis, the electric field intensity was confirmed to be approximately 20 times higher than that of the case without the SIM, and the transmission measurements confirmed that the transmission through an aperture had a gap of 1/20 that of the wavelength. This fabricated device can be used in imaging and sensing applications because of the close contact between the transmission aperture and the measurement sample.

Highlights

The terahertz (THz) frequency band has a substance-specific absorption spectrum for various chemicals, macromolecules and water vapor
When THz waves are applied in cancer cell detection, the size of a single cell is approximately 10 μm, which is very small when compared with the wavelength of THz waves, making it difficult to detect a single cell by focusing light with a common lens
The solid immersion method (SIM) utilizes the effect of the effective wavelength being shortened to 1/n by the refractive index n in the dielectric, and it is generally used in a lens [18,19]; its adaptation to the Bull’s Eye (BE) structures is being considered for application in thin-film sensing [20]

Summary

Introduction

The terahertz (THz) frequency band has a substance-specific absorption spectrum (fingerprint spectrum) for various chemicals, macromolecules and water vapor. Its energy is much lower than that of X-rays, and, it is being considered for non-destructive inspections, in the biotechnology and medical fields This is because of its ability to reduce the damage it can cause to the measurement sample [1,2,3]. The wavelengths of THz waves are approximately 3 mm (0.1 THz) to 30 μm (10 THz), which is tens or hundreds of times longer than those of visible light (400–800 nm). They can be problematic due to the limitation called the “diffraction limit”. We focused on one of the plasmonic structures, 4.0/)

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Discussion

Conclusion