Enhanced THz Field Confinement and Broadband Concentration using a Split Tapered Plasmonic Aperture and its Application to Near Field Imaging

Shuchang Liu Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah, USA Oleg Mitrofanov Department of Electronic and Electrical Engineering, University College London, London, UK Ajay Nahata Department of Electrical and Computer Engineering, University of Utah, Salt Lake City, Utah, USA

Tapered structures have long been used in near-field optical microscopy and allow for concentration of the incident radiation. Among the various embodiments of the tapered structures, tapered apertures (TAs) can, in principle, more tightly confine radiation based on the geometry of the output surface. However, a simple TA suffers from a cutoff frequency associated with the aperture diameter.

In the first step, we inserted a gap in a TA that was fabricated in a large metal block, which was equivalent to the superposition of a TA and a parallel plate waveguide. Although the TA allowed for transmission below the cutoff frequency, radiation leaked from the TA into the gap. As a result, the concentration and confinement capabilities of the structure were limited.

In order to improve the properties of the structure, we describe the fabrication and characterization of a metallic conically TA with a gap, in which the wall thickness is small. Such a structure should allow for high field concentration without an apparent cutoff frequency and, correspondingly, tight confinement of the transmitted terahertz (THz) radiation, making it well-suited for THz near-field imaging. The final device [Figs. 1(a) and 1(b)] consisted of a conically TA with a circular output aperture of a diameter D2 = 200 μm, an aperture length of d = 3.0 mm and a taper full angle α = 30°, which we have previously found to be the optimal taper angle [1]. Here, the thickness of the side walls is ~ 60 um. Using a THz time-domain spectroscopy system [Fig. 1(c)], we characterized the spectral and spatial properties and field mapping of the structure, shown in Figs. 1(d) and 1(e) respectively, and clearly demonstrate high field concentration (~7 times in amplitude), tight radiation confinement (within the aperture area) and broadband operation (beyond the cutoff frequency) as a near-field probe. We describe the near-field imaging capabilities using such structures.

Fig.1

Fig. 1 (a) Schematic diagram of the thin-wall split tapered aperture. (b) Microscope photo of the structure. (c) The near-field setup. (d) Transmission spectrum of the structure compared with input and transmission without the structure. (e) Field mapping at the output surface of the structure with a gap g = ~10 um.

References

[1] S. Liu, et al., Opt. Express 21, 12363 (2013).

ajay.nahata@utah.edu









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