Abstract
Terahertz (THz) imaging techniques are attractive for a wide range of applications, such as non-destructive testing, biological sensing, and security imaging. We investigate practical issues in THz imaging systems based on a solid immersion lens (SIL). The system stability in terms of longitudinal misalignment of the SIL is experimentally verified by showing that the diffraction-limited sub-wavelength beam size (0.7 λ) is maintained as long as the SIL is axially located within the depth-of-focus (~13 λ) of the objective lens. The origin of the fringe patterns, which are undesirable but inevitable in THz imaging systems that use continuous waves, is analytically studied, and a method for minimizing the interference patterns is proposed. By combining two THz images obtained at different axial positions of the object and separated by λ/4, the interference patterns are significantly reduced, and the information hidden under the interference patterns is unveiled. The broad applicability of the proposed method is demonstrated by imaging objects with different surface profiles. Our work proves that the resolution of conventional THz imaging systems can easily be enhanced by simply inserting a SIL in front of the object with high tolerance in the longitudinal misalignment and provides a method enabling THz imaging for objects with different surface profiles.
Highlights
Terahertz (THz) imaging has proven its usefulness in various fields, such as nondestructive testing (NDT), biological sensing, and security imaging [1]
We investigated practical issues in THz imaging systems based on a solid immersion lens (SIL)
The results explicitly show that sub-wavelength resolution can be obtained by inserting a Si lens in front of the objects in a typical THz imaging system
Summary
Terahertz (THz) imaging has proven its usefulness in various fields, such as nondestructive testing (NDT), biological sensing, and security imaging [1]. The SIL-based imaging technique utilizes an additional high refractive index lens between the objective lens and the object to obtain more confined electromagnetic waves near the original focal plane formed by the objective lens [16] This technique has been applied for several applications, such as optical data storage [17] and super-resolution fluorescence microscopy under cryogenic conditions [18]. A high-resistivity float-zone (HRFZ) silicon (Si) lens is suitable for use as the SIL in a THz imaging system, as the HRFZ Si has high refractive index, low absorption coefficient, and low dispersion in the THz region [23] With this technique, spatial resolutions ranging from 0.15 to 0.35 λ have been reported [21,22]. The broad applicability of the proposed method is demonstrated by imaging objects having different surface profiles
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