Abstract
The terahertz (THz, 0.1–10 THz) region has been attracting tremendous research interest owing to its potential in practical applications such as biomedical, material inspection, and nondestructive imaging. Those applications require enhancing the spatial resolution at a specific frequency of interest. A variety of resolution-enhancement techniques have been proposed, such as near-field scanning probes, surface plasmons, and aspheric lenses. Here, we demonstrate for the first time that a mesoscale dielectric cube can be exploited as a novel resolution enhancer by simply placing it at the focused imaging point of a continuous wave THz imaging system. The operating principle of this enhancer is based on the generation—by the dielectric cuboid—of the so-called terajet, a photonic jet in the THz region. A subwavelength hotspot is obtained by placing a Teflon cube, with a 1.46 refractive index, at the imaging point of the imaging system, regardless of the numerical aperture (NA). The generated terajet at 125 GHz is experimentally characterized, using our unique THz-wave visualization system. The full width at half maximum (FWHM) of the hotspot obtained by placing the enhancer at the focal point of a mirror with a measured NA of 0.55 is approximately 0.55λ, which is even better than the FWHM obtained by a conventional focusing device with the ideal maximum numerical aperture (NA = 1) in air. Nondestructive subwavelength-resolution imaging demonstrations of a Suica integrated circuit card, which is used as a common fare card for trains in Japan, and an aluminum plate with 0.63λ trenches are presented. The amplitude and phase images obtained with the enhancer at 125 GHz can clearly resolve both the air-trenches on the aluminum plate and the card’s inner electronic circuitry, whereas the images obtained without the enhancer are blurred because of insufficient resolution. An increase of the image contrast by a factor of 4.4 was also obtained using the enhancer.
Highlights
Associating this result with the result previously obtained with terajets generated under planar incidence (FWHM = 0.6λ),[28] an numerical aperture (NA) = 0 focusing device, indicates that mesoscale dielectric cubes can enhance the spatial resolution to the subwavelength region by being placed at the focused imaging points of continuous wave (CW)-THz imaging systems with different values of the NA
To demonstrate the subwavelength-resolution imaging capability obtained by placing the enhancer at the focused imaging point of the THz imaging system at 125 GHz, two demonstration samples were used: an aluminum plate with 1.5 mm (0.63λ) trenches and a 1.5 mm (0.63λ) diameter hole (Fig. 6(a)) and a Suica IC card, a common fare card used for trains in Japan (Fig. 6(b))
For the first time, that a mesoscale dielectric cube can be used as a novel resolution enhancer by placing it at the focused imaging point of a CW-THz imaging system, regardless of the value of the NA
Summary
The terahertz (THz, 0.1–10 THz) region has attracted significant research attention, owing to the unique features of the electromagnetic waves in this frequency band, which show great promise in a lot of practical applications.[1,2,3,4] One of those unique features is the spectral fingerprint that many materials possess in these frequencies,[5,6] which can be exploited for nondestructive imaging applications such as biomedical, materials inspection, and security.[7,8,9] high spatial resolution imaging at a specific frequency of interest is a highly desirable capability in THz imaging applications.[10,11]. NA = 0.55 using our unique THz-wave visualization system based on a self-heterodyne nonpolarimetric electro-optic (EO) detection technique.[31,32,33] An FWHM of approximately 0.55λ (1.32 mm) is obtained, which is even superior to the FWHM obtained by conventional focusing devices with the ideal maximum NA = 1 in air (0.67λ). This result, when associated with the result previously reported in Ref. 28, indicates that this enhancer can enhance the spatial resolution into the subwavelength region by being placed at the focal point of the focusing device, regardless of its NA value.
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