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Research advances of high-resolution THz imaging based on terajet effect
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Abstract
Terahertz (THz) imaging technology has shown great advantages and potential applications in the fields of biomedicine, security, and aerospace, due to its low energy, high transmittance, wide bandwidth, and unique analysis abilities; while low spatial resolution restricts its further applications. Recently, a high-resolution, high-throughput, and broad-bandwidth THz imaging method has been proposed based on the terajet effect produced by dielectric structures with appropriate refractive index. The terajet beam can break through the restriction of the diffraction limit on the spatial resolution of the microscopic system without losing the energy and spectral bandwidth of the THz field. In this paper, firstly, a white-light nanoscopy based on photonic nanojet produced by microspheres is introduced, then the THz microscopy based on terajet effect produced by mesoscopic dielectric structures is reviewed. Finally, the prospect of THz high resolution imaging technology based on terajet effect is prospected.-
Keywords:
- terahertz imaging /
- terajet /
- spatial resolution
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References
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Overview
Overview: In the past decades, great advancements have been made to achieve super-resolution imaging, including near-field THz microscopy, metamaterial superlens, fluorescence microscopy and so on, pushing the resolution to ñm or nm scale. Unfortunately, applications of these methods have been limited in part due to their complication in access and operation, loss of energy and spectral bandwidth, difficulty in information extraction or limited choices of samples. Thus, it is highly desired to develop innovative super-resolution THz imaging modality that is easily accessible and low-cost. Fortunately, at visible frequencies, a unique easy-access super-resolution imaging where dielectric microsphere with appropriate refractive index has been presented and delivered a remarkable 50 nm resolution with white lights in 2010. Furthermore, super-resolution imaging was also presented with a large field-of-view using large polystyrene microspheres (above 30 μm). A strong continuing interest in the technique has led to numerous progress in visible light. In these works, the super-resolution capability of microspheres is determined by the "photonic nanojet" and coupling with evanescent waves. More recently, a straightforward THz imaging method based on terajet effect, analogous to microsphere optical nanoscope, is proposed and developed with spatial resolution beyond the diffraction limit by using either continuous or pulsed THz wave. The terajet beam can break through the restriction of the diffraction limit on the spatial resolution of the microscopic system without losing the energy and spectral bandwidth of the THz field, i.e., a high-resolution, high-throughput and broad-bandwidth THz imaging method. Also, with the extensively longer wavelength of the THz wave, the size of the dielectric spheres is much larger (on the order of millimeters), the spheres are easier to fabricate, simple to manipulate, and capable of handling energy and bandwidth losses. In addition, as unique spectroscopic technique, THz imaging reveals much richer subwavelength structural information, including frequency-dependent amplitude and phase, as well as time-dependent delay and thickness. In this review, firstly, a white-light nanoscopy based on photonic nanojet produced by microspheres is introduced, then the terahertz microscopy based on terajet effect produced by mesoscopic dielectric structures is reviewed. Finally, the prospect of terahertz high resolution imaging technology based on terajet effect is prospected.
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Figure 1.
(a) Experimental configuration of a white-light microsphere nanoscope; microsphere superlens imaging in transmission mode; (b) 360 nm wide lines spaced 130 nm apart; (c) A gold-coated fishnet AAO sample; and microsphere superlens imaging in reflection mode; (d) A commercial Blue-ray DVD disk; (e) A star structure made on GeSbTe thin film[27]
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Figure 2.
(a) Schematic diagram of photonic nanojet produced by microsphere[39]; (b) Comparison between single-layer homogeneous microspheres and double-layer heterogeneous microspheres[40]; (c) Microsphere optical nano-jet related to non-diffractive Bessel beams[41]
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Figure 3.
(a) Schematic diagram of terajet beam generated by dielectric cuboids with different refractive index[28]; (b) Dielectric cube coupled THz imaging system in reflection mode and its amplitude and phase images for aluminum plate samples[45]
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Figure 4.
(a) Schematic diagram of all-dielectric period terajet waveguide using an array of couple cuboids and its electric field distribution[46]; (b) Schematic diagram of the pupil-masked 3D dielectric cuboid and its electric field distribution[48]
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Figure 5.
Distribution of electric energy generated by different geometric structures. (a) Metal rod array filled rectangular condyle[49], (b) sphere[51], (c) triangular condyle[51], (d) trapezoidal condyle[51], and (e) fan-shaped condyle[51]
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Figure 6.
(a) Schematic diagram of a lens group setup; (b) Schematic diagram of THz transmission imaging system by using THz sheet; (c) Measured intensities of output beam from lens group system[53]
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Figure 7.
(a) Schematic diagram of dielectric sphere coupled THz microscopy; (b) Imaging resolution; (c) Contrast image with and without dielectric sphere[29]
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Figure 8.
(a) THz time-domain waveforms with and without dielectric sphere; (b) Frequency-domain signals; (c) Relative amplitude transmittance[54]
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Figure 9.
(a) The spatial energy distribution of 3 mm sphere in z-x plane at different frequencies; (b) Evolution of a jet-like energy distribution as the refractive index of 3 mm dielectric sphere increases from 1.4 to 2.0[29]
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Figure 10.
The spatial energy distribution of 3 mm sphere in x-z plane at (a) 0.5 THz, (b) 1.0 THz, (c) 1.5 THz, (d) 2.0 THz, (e) 2.5 THz, and (f) 3.0 THz[54]
