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

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

[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, 10, 865-869(2015).

[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, 12, 132-136(2017).

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

[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, 29, 205301(2018).

[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, 8, 2497-2502(2008).

[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, 9, 26341-26349(2017).

[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, 71, 201402(2005).

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

[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, 6, 1114-1121(2016).

[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, 7, 14982-14988(2015).

[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, 6, 1199-1204(2015).

[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, 5, 3312(2014).

[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, 498, 82-86(2013).

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

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

[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, 9, 1278-1282(2009).

[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, 105, 266807(2010).

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

[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, 2, 1468-1475(2015).

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

[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, 407, 8177-8195(2015).

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

[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, 7, 2784-2788(2007).

[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, 10, 592-596(2010).

[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, 3, 945-952(2012).

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

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

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

[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, 11, 1609-1613(2011).

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

[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, 6, 6040-6048(2012).

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

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

[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, 4, 603-610(2013).

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

[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, 19, 12342-12347(2011).

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

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

[41] [41] PalikE DHandbook of Optical Constants of Solids (Academic, San Diego, America, 1998)Palik E D. Handbook 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, 93, 137404(2004).

[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, 4, 75-82(2010).

[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, 11, 1676-1680(2011).

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

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

[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, 24, 10376-10384(2016).

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

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

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