Fig. 1. Bowtie apertured (a) and gaped (b) antennas. (c) Induced surface charges and electric dipole when incident electric field polarizes along the tips. Figure reproduced with permission from: (a-b) ref.⁴, Copyright 2006 American Chemical Society.

Fig. 2. Near-field imaging and trapping using (apertured) bowties. (a) Transmission through the subwavelength apertures. Bowtie apertures show much enhanced transmission. Left: different apertures. Right: far-field transmission measurements. (b) Bowtie apertures fabricated on the SNOM probe. (c) Line profiles of SNOM images using bowtie (solid line) and square (dashed line) aperture probes. (d) 5-nm-gap bowtie apertures. (e) Optical potentials U and the corresponding optical forces F along the x-axis. Figure reproduced with permission from: (a–c) ref.³, Copyright 2007 AIP Publishing; (d–e) ref.²⁷, Copyright 2018 the author(s), under a Creative Commons Attribution 4.0 International License.

Fig. 3. Nonlinear response in bowties. (a) Three-dimensional (3D) gold bowties array. (b) Nonlinear emission spectrum from a single bowtie (inset). (c) Spectrum of generated high harmonics from 2D bowties array (inset). (d) Experimental (TPPL, circles) and theoretical (field enhancement, squares) results versus bowtie gap size. Inset shows a bowtie with a 22 nm gap. Figure reproduced with permission from: (a) ref.³², American Chemical Society; (b) ref.³⁵, under a Creative Commons Attribution 3.0 License; (c) ref.¹⁵, American Physical Society.

Fig. 4. Nanolithography using bowties. (a) Bowtie apertures with a 30 nm gap. (b) AFM image of 40 nm × 50 nm lithography hole. (c–d) AFM image (c) and cross section (d) along the nanoantenna axis of bowties exposed at 25 μW laser power. Feature size of ~30 nm for each of the resist pillars was measured. (e) SEM image of the fabricated circular contact probe. (f) AFM image of a 22-nm half pitch resolution line array pattern. (g–h) Bowtie nanolithography combined with metal-insulator-metal (g) and hyperbolic metamaterials (h). Figure reproduced with permission from: (a, b) ref.⁴, Copyright 2016 American Chemical Society; (c, d) ref.¹⁶, American Chemical Society; (e, f) ref.³⁷, Copyright 2012 John Wiley and Sons; (g) ref.³⁸, Copyright 2019 Optical Society of America; (h) ref.⁴¹, IOP Publishing.

Fig. 5. Bowtie based nanosources. (a) Schematic of 3D bowtie plasmonic lasers. (b) Evolution of lasing spectra from 3D Au bowties under pump polarization parallel to the tip axis. Inset shows emission intensity versus pump pulse energy density plotted on a semilogarithmic scale. (c) Directional SP out-coupling emission. (d) Bowties with a small gap. (e) Simulated near-field patterns of one of resonant modes in bowties. (f) Thermal emission spectrum under different bowtie gap sizes. Figure reproduced with permission from: (a–c) ref.¹⁷, Copyright 2012 American Chemical Society; (d–f) ref.⁴⁴, Copyright 2017 the author(s), under the ACS AuthorChoice via CC-BY-NC-ND Usage Agreement.

Fig. 6. 2D materials photodetectors based on bowties. (a) Schematic and (b) SEM image of the plasmonically enhanced graphene photodetector. (c) The magnitude of in-plane electric fields. Strong plasmonic field enhancements in the gap were observed. (d) Eye diagram of 100 Gbit/s OOK optical signals. (e) Optical and (f) SEM image of bowtie gap antennas for high responsivity detectors. (g) Optical and (h) SEM image of bowtie aperture antennas for high polarization. Figure reproduced with permission from: (a–d) ref.⁴⁵, Copyright 2019 The Author(s), under the ACS AuthorChoice Usage Agreement; (e–h) ref.¹⁸, Copyright 2018 American Chemical Society.

Fig. 7. Surface plasmons mediated via the IET process. (a) Schematic of the Al-AlO_x-Au tunnel junction. (b) Energy level diagram of the IET process. (c) Bias dependent emission spectra. Emitted photons with cut-off frequencies can be seen. Figure reproduced with permission from ref.⁴⁹, under a Creative Commons Attribution 4.0 International License.

Fig. 8. Light emission from bowtie antenna based tunnel junctions. (a) Lateral tunnel junctions made by bowtie gap-antennas. Inset shows that the bowtie antennas are connected before the electro-migration process. (b) Time evolution of normalized conductance (G/G₀). (c) Light emission spectra. Left column: images captured under different biases. Right column: spectral evolution under different bias. (d) Enhanced LDOS in the order of 10⁵. Solid line: bowties case. Dashed lines: nanowire case. (e) Wavelength dependent normalized LDOS (red) and radiation efficiency (black). They both contribute to the ultimate emission spectrum. Figure reproduced with permission from: (a–d) ref.¹⁹, Copyright 2019 American Chemical Society; (e) ref.⁶⁴, Copyright 2020 Optical Society of America.

Table 1. Comparison on the performance of tunnel junctions