[1] D K Gramotnev, S I Bozhevolnyi. Nanofocusing of electromagnetic radiation. Nat Photonics, 2013, 8: 13-22.

[2] R M Stöckle, Y D Suh, V Deckert, R Zenobi. Nanoscale chemical analysis by tip-enhanced Raman spectroscopy. Chem Phys Lett, 2000, 318: 131-136.

[3] S Jiang, Y Zhang, R Zhang, C R Hu, M H Liao, et al.. Distinguishing adjacent molecules on a surface using plasmon-enhanced Raman scattering. Nat Nanotechnol, 2015, 10: 865-869.

[4] J H Zhong, X Jin, L Y Meng, X Wang, H S Su, et al.. Probing the electronic and catalytic properties of a bimetallic surface with 3 nm resolution. Nat Nanotechnol, 2017, 12: 132-136.

[5] J F Li, Y F Huang, Y Ding, Z L Yang, S B Li, et al.. Shell-isolated nanoparticle-enhanced Raman spectroscopy. Nature, 2010, 464: 392-395.

[6] W D Zhang, C Li, K Gao, F F Lu, M Liu, et al.. Surface-enhanced Raman spectroscopy with Au-nanoparticle substrate fabricated by using femtosecond pulse. Nanotechnology, 2018, 29: 205301.

[7] H Wei, F Hao, Y Z Huang, W Z Wang, P Nordlander, et al.. Polarization dependence of surface-enhanced Raman scattering in gold nanoparticle-nanowire systems. Nano Lett, 2008, 8: 2497-2502.

[8] K C Xu, Z Y Wang, C F Tan, N Kang, L W Chen, et al.. Uniaxially stretched flexible surface Plasmon resonance film for versatile surface enhanced Raman scattering diagnostics. ACS Appl Mater Interfaces, 2017, 9: 26341-26349.

[9] C C Neacsu, G A Reider, M B Raschke. Second-harmonic generation from nanoscopic metal tips: symmetry selection rules for single asymmetric nanostructures. Phys Rev B, 2005, 71: 201402.

[10] M Kauranen, A V Zayats. Nonlinear plasmonics. Nat Photonics, 2012, 6: 737-748.

[11] Y J Jin, L W Chen, M X Wu, X Z Lu, R Zhou, et al.. Enhanced saturable absorption of the graphene oxide film via photonic nanojets. Opt Mater Express, 2016, 6: 1114-1121.

[12] L W Chen, X R Zheng, Z R Du, B H Jia, M Gu, et al.. A frozen matrix hybrid optical nonlinear system enhanced by a particle lens. Nanoscale, 2015, 7: 14982-14988.

[13] Z R Du, L W Chen, T S Kao, M X Wu, M H Hong. Improved optical limiting performance of laser-ablation-generated metal nanoparticles due to silica-microsphere-induced local field enhancement. Beilstein J Nanotechnol, 2015, 6: 1199-1204.

[14] C Chen, N Hayazawa, S Kawata. A 1.7 nm resolution chemical analysis of carbon nanotubes by tip-enhanced Raman imaging in the ambient. Nat Commun, 2014, 5: 3312.

[15] R Zhang, Y Zhang, Z C Dong, S Jiang, C Zhang, et al.. Chemical mapping of a single molecule by plasmon-enhanced Raman scattering. Nature, 2013, 498: 82-86.

[16] K V Nerkararyan. Superfocusing of a surface polariton in a wedge-like structure. Phys Lett A, 1997, 237: 103-105.

[17] N C Lindquist, P Nagpal, A Lesuffleur, D J Norris, S H Oh. Three-dimensional plasmonic nanofocusing. Nano Lett, 2010, 10: 1369-1373.

[18] V S Volkov, S I Bozhevolnyi, S G Rodrigo, L Martín-Moreno, F J García-Vidal, et al.. Nanofocusing with channel plasmon polaritons. Nano Lett, 2009, 9: 1278-1282.

[19] A I Fernández-Domínguez, S A Maier, J B Pendry. Collection and concentration of light by touching spheres: a transformation optics approach. Phys Rev Lett, 2010, 105: 266807.

[20] E Verhagen, A Polman, L K Kuipers. Nanofocusing in laterally tapered plasmonic waveguides. Opt Express, 2008, 16: 45-57.

[21] B N Tugchin, N Janunts, A E Klein, M Steinert, S Fasold, et al.. Plasmonic tip based on excitation of radially polarized conical surface plasmon polariton for detecting longitudinal and transversal fields. ACS Photonics, 2015, 2: 1468-1475.

[22] J Stadler, T Schmid, R Zenobi. Developments in and practical guidelines for tip-enhanced Raman spectroscopy. Nanoscale, 2012, 4: 1856-1870.

[23] T X Huang, S C Huang, M H Li, Z C Zeng, X Wang, et al.. Tip-enhanced Raman spectroscopy: tip-related issues. Anal Bioanal Chem, 2015, 407: 8177-8195.

[24] P Verma. Tip-enhanced Raman spectroscopy: technique and recent advances. Chem Rev, 2017, 117: 6447-6466.

[25] C Ropers, C C Neacsu, T Elsaesser, M Albrecht, M B Raschke, et al.. Grating-coupling of surface plasmons onto metallic tips: a nanoconfined light source. Nano Lett, 2007, 7: 2784-2788.

[26] C C Neacsu, S Berweger, R L Olmon, L V Saraf, C Ropers, et al.. Near-field localization in plasmonic superfocusing: a nanoemitter on a tip. Nano Lett, 2010, 10: 592-596.

[27] S Berweger, J M Atkin, R L Olmon, M B Raschke. Light on the tip of a needle: plasmonic nanofocusing for spectroscopy on the nanoscale. J Phys Chem Lett, 2012, 3: 945-952.

[28] T Xu, C T Wang, C L Du, X G Luo. Plasmonic beam deflector. Opt Express, 2008, 16: 4753-4759.

[29] T Xu, C L Du, C T Wang, X G Luo. Subwavelength imaging by metallic slab lens with nanoslits. Appl Phys Lett, 2007, 91: 201501.

[30] X G Luo, T Ishihara. Surface plasmon resonant interference nanolithography technique. Appl Phys Lett, 2004, 84: 4780.

[31] D Sadiq, J Shirdel, J S Lee, E Selishcheva, N Park, et al.. Adiabatic nanofocusing scattering-type optical nanoscopy of individual gold nanoparticles. Nano Lett, 2011, 11: 1609-1613.

[32] M Müller, V Kravtsov, A Paarmann, M B Raschke, R Ernstorfer. Nanofocused Plasmon-driven sub-10 fs electron point source. ACS Photonics, 2016, 3: 611-619.

[33] S Schmidt, B Piglosiewicz, D Sadiq, J Shirdel, J S Lee, et al.. Adiabatic nanofocusing on ultrasmooth single-crystalline gold tapers creates a 10-nm-sized light source with few-cycle time resolution. ACS Nano, 2012, 6: 6040-6048.

[34] S Berweger, J M Atkin, R L Olmon, M B Raschke. Adiabatic Tip-Plasmon focusing for Nano-Raman spectroscopy. J Phys Chem Lett, 2010, 1: 3427-3432.

[35] V Kravtsov, J M Atkin, M B Raschke. Group delay and dispersion in adiabatic plasmonic nanofocusing. Opt Lett, 2013, 38: 1322-1324.

[36] M Esmann, S F Becker, B B da Cunha, J H Brauer, R Vogelgesang, et al.. k-space imaging of the eigenmodes of sharp gold tapers for scanning near-field optical microscopy. Beilstein J Nanotechnol, 2013, 4: 603-610.

[37] J Mihaljevic, C Hafner, A J Meixner. Grating enhanced apertureless near-field optical microscopy. Opt Express, 2015, 23: 18401-18414.

[38] J S Lee, S Han, J Shirdel, S Koo, D Sadiq, et al.. Superfocusing of electric or magnetic fields using conical metal tips: effect of mode symmetry on the plasmon excitation method. Opt Express, 2011, 19: 12342-12347.

[39] P Andrey. Nanofocusing of surface Plasmons at the apex of metallic probe tips. J Nanoelectron Optoe, 2010, 5: 310-314.

[40] P B Johnson, R W Christy. Optical constants of the noble metals. Phys Rev B, 1972, 6: 4370-4379.

[41] PalikE DHandbook of Optical Constants of Solids (Academic, San Diego, America, 1998)

[42] M I Stockman. Nanofocusing of optical energy in tapered plasmonic waveguides. Phys Rev Lett, 2004, 93: 137404.

[43] Z Y Fang, C F Lin, R M Ma, S Huang, X Zhu. Planar plasmonic focusing and optical transport using CdS nanoribbon. ACS Nano, 2010, 4: 75-82.

[44] Z Y Fang, L R Fan, C F Lin, D Zhang, A J Meixner, et al.. Plasmonic coupling of bow tie antennas with Ag nanowire. Nano Lett, 2011, 11: 1676-1680.

[45] V S Gurevich, M N Libenson. Surface polaritons propagation along micropipettes. Ultramicroscopy, 1995, 57: 277-281.

[46] A J Babadjanyan, N L Margaryan, K V Nerkararyan. Superfocusing of surface polaritons in the conical structure. J Appl Phys, 2000, 87: 3785.

[47] W D Zhang, L G Huang, K Y Wei, P Li, B Q Jiang, et al.. Cylindrical vector beam generation in fiber with mode selectivity and wavelength tunability over broadband by acoustic flexural wave. Opt Express, 2016, 24: 10376-10384.

[48] L Novotny, C Hafner. Light propagation in a cylindrical waveguide with a complex, metallic, dielectric function. Phys Rev E, 1994, 50: 4094-4106.

[49] D K Gramotnev, M W Vogel, M I Stockman. Optimized nonadiabatic nanofocusing of plasmons by tapered metal rods. J Appl Phys, 2008, 104: 034311.

[50] N A Issa, R Guckenberger. Optical nanofocusing on tapered metallic waveguides. Plasmonics, 2007, 2: 31-37.