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Advancement and application of terahertz pulsed focal-plane imaging technique
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Abstract
As an important composition of terahertz (THz) technology, THz pulsed focal-plane imaging has been paid widely attention since it was invented. Until now, researchers have introduced all kinds of methods to enhance the performance of this imaging technique. Simultaneously, this imaging technique has been tried to apply into various industrial and fundamental research fields. In this paper, recent technique improvements and application researches for THz pulsed focal-plane imaging are reviewed, including the spatial resolution enhancement, signal-to-noise ratio improvement, information acquiring ability as well as applications of this imaging technique in spectroscopic identification inspections, function demonstrations of meta-surface devices, measurements of THz special beams, observations of THz surface electromagnetic waves, and so on. The aim of this paper is to push the technique innovation and application exploration of THz pulsed focal-plane imaging.-
Keywords:
- terahertz /
- focal-plane imaging /
- resolution /
- meta-surface
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References
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Overview
Overview: As a class of novel far-infrared testing technology, terahertz (THz) imaging has been rapidly developed for recent decades due to characteristics of the THz radiation, such as low photon energy, broad bandwidth, and high transmission to non-polar materials. Notably, the THz pulsed focal-plane imaging technique has become an important composition in all kinds of THz imaging methods because of its obvious measurement advantages. When the THz pulsed focal-plane imaging is employed, two-dimensional THz information of a substance can be accurately acquired in a single measurement and the raster scan process in traditional THz imaging is effectively avoided, which leads to the reduction of the experimental time as well as the enhancements of the measurement stability and sampling ratio. In this review, the technique innovations and application explorations of THz pulsed focal-plane imaging are introduced. This THz imaging technique was firstly proposed in 1996 and various means have been applied to improve its performance. With the development of the imaging technique, the super-thin sensor crystal and the quasi-near-field detection are introduced to improve the imaging spatial resolution; the dynamics subtraction and the balanced electro-optic detection are applied to enhance the signal-to-noise ratio of the imaging system. In addition, this imaging system can individually measure different THz polarization components (Ex, Ey, and Ez) by varying the polarization of the probe beam and using the sensor crystals with different crystalline orientations. Currently, it can be said that almost all of THz wave-front information can be obtained by using this imaging technique. With the maturation of the imaging technique, it has been applied into various industrial and fundamental research fields. Utilizing the spectroscopic measurement ability of the imaging system, identification of different chemical and biological samples can be achieved. Utilizing the vectorial measurement ability of the imaging system, the function of THz meta-surface devices, characterizations of THz special beams, and observations of THz surface electromagnetic waves have been demonstrated. Besides, this imaging technique has been also applied to measure transmission modes of THz waveguides, inspections to concealed objects, and so on. Of course, there is still much room for the future improvement of this imaging technique, such as the further enhancement of the signal-to-noise ratio, the enlargement of the imaging region, and the simplification of the optical configuration. Nevertheless, it can be expected that the imaging technique will show its tremendous application potentials in the future.
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Figure 1.
Comparison of THz pulsed focal-plane imaging and THz raster scan imaging[12-13]. (a) Scheme of a THz raster scan imaging system and (b) THz images of a leaf by THz raster scan imaging; (c) Scheme of a THz pulsed focal-plane imaging system and (d) THz intensity distribution obtained by THz pulsed focal-plane imaging
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Figure 2.
THz pulsed focal-line imaging[16]. (a) Scheme of a THz pulsed focal-line imaging system; (b) Photo of a metal hole array and its THz images at 0.407 THz, 0.815 THz, 1.600 THz
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Figure 3.
Improvement of a THz pulsed focal-plane imaging system[17-19]. (a) Improved THz pulsed focal-plane imaging system; (b) Quasi-near-field detection technique; (c) Balanced electro-optic detection technique; (d) Polarization detection technique
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Figure 4.
THz pulsed focal-plane microscopy[21]. (a) Scheme of a THz pulsed focal-plane microscopy system; (b) Visible and THz images of a double slit metallic mask; (c) THz intensity profile curve extracted along the dashed line in (b)
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Figure 5.
Vectorial measurement of a THz field by THz pulsed focal-plane imaging[22]. (a) THz pulsed focal-plane imaging system in which a < 100> ZnTe is selected as the sensor crystal; (b) Evolutions of the Ez amplitude and phase for a converging linearly polarized THz field and (c) evolution of the Ez complex field for a focused left circularly polarized THz field
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Figure 6.
Spectroscopic identification by THz pulsed focal-plane imaging[24-25]. (a) Inspection of different chemicals by reflective THz pulsed focal-plane imaging; (b) Visible and THz images of 2, 4-DNT, theopylline, RDX, glutamic acid, and glass samples; (c) Inspection to biological tissues by THz pulsed focal-line imaging; (d) Visible and THz images of a human tooth slice
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Figure 7.
Function characterization of metasurface elements by THz pulsed focal-plane imaging[29-32]. (a) Imaging function characterization of a THz metasurface lens; (b) Polarization-dependent response of a THz metasurface photonic Hall element; (c) Function characterization of a THz metasurface ring Airy beam modulator; (d) Function characterization of a wavelength de-multiplexing THz metasurface hologram
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Figure 8.
Vectorial characterization of THz special beams by THz pulsed focal-plane imaging[38-39, 41]. (a) Vectorial characterization of a THz vortex beam; (b) Vectorial characterization of a THz Bessel beam; (c) Vectorial characterization of a THz bottle beam
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Figure 9.
Characterization of THz surface waves by THz pulsed focal-plane imaging[47-48]. (a) Characterization of a THz surface converging beam; (b) Characterization of a THz surface bottle beam
