Mode evolution and nanofocusing of grating-coupled surface plasmon polaritons on metallic tip
1 Introduction
Plasmonic nanofoucsing techniques have received increasing attentions for concentrating light energy into a nanoscale spatial region1. In consequence, the electric field can be significantly enhanced with mode volume maintained in nanoscale. Such an excellent characteristic has been exploited in a wide range of applications in tip-enhanced Raman spectroscopy (TERS)2-4, surface enhanced Raman spectroscopy (SERS) 5-8, nonlinear optics9-13, and super-resolution imaging14, 15, etc. The key to achieving plasmonic nanofocusing is the design of metal nanostructures with excellent localized surface plasmon resonance (LSPR) effect, such as wedges16, tetrahedrons17, grooves18, nanoparticles19, etc. Besides, it is noteworthy that noble metallic tips can combine electric field enhancement and spatial confinement characteristics to generate nano-confined light source20, 21.
Conical metal tips have been commonly adopted for TERS in the configuration of scanning probe microscopy22. Conical metal tips are typically fabricated by electrochemically etching gold/silver wires23, and directly illuminated at the tip apex with a focused laser beam24. Localized surface plasmons (LSPs) are excited at the tip apex to enhance the near field as the nano-confined light source. Subject to the diffraction limit, however, the focus light spot under the far-field illumination is much larger than the size of the enhanced near field, and thus background signal excitation is inevitable, which in turn limits sensing performance in some circumstances22.
An effective way to avoid the background signal excitation is based on a conical metal tip with grating-assisted light coupling25-27, which has also been exploited in the fields of grating-assisted SPP interferences28, imaging29, lithography30, etc. In this scheme, a diffractive grating is fabricated on the metal tip body at a distance of tens of microns from the tip apex to achieve momentum matching between the far-field excitation light and the surface plasmonic polaritons (SPPs)31-33. The grating-coupled SPPs propagate along the metal body and are finally converted to LSPs confined at the tip apex. Although such a scheme has been applied in nano-Raman/nonlinear spectroscopy34, 35, the mechanism of plasmonic mode evolution and the resulting nanofocusing has not been clearly understood36-39.
In this paper, we present a detailed analysis on mode evolution of grating-coupled SPPs on the conical metal tip based on the guided-wave theory. The eigenvalue equations of guided SPP modes and mode fields of allowed eigenmodes are examined for cylindrical plasmonic guides. The propagation of the grating-coupled SPPs on the conical metal tip is numerical simulated by using the finite difference time domain (FDTD) solution software (Lumerical Inc.), revealing plasmonic mode evolution for realization of nanofocusing.
2 Results and discussion
As an excellent plasmonic material in the visible band, silver is selected for the metal tip model, as illustrated in
Fig. 1. Geometry of a conical silver tip with grating-assisted light coupling.The excitation light is focused onto the diffractive grating. R is the cross-sectional radius of the tip body gradually decreasing from 1 μm to 20 nm. The tip apex is modeled as a hemisphere with a radius of r .
Under the condition of
Fig. 2. (a ) Sketch map of SPPs excitation using a planar grating; (b ) Dispersion relationship of SPPs and the grating coupling; (c ) Reflection obtained from the field monitor located above the surface without grating; (d ) Re(H y ) distribution of the grating-coupled SPPs generation and propagation along the silver-air interface with excitation wavelength at λ =632.8 nm and θ =28.5°.
To understand the guided SPP modes in an adiabatic tapered plasmonic waveguide, the effective indices (
where
where
For a certain
According to Eqs. (1) and (2), we can calculate the effective indices of the cylindrical silver waveguide with a radius of
Fig. 3. Effective indices n eff in real part (a ) and imaginary part (b ) of guided SPP modes versus the radius of cylindrical silver guide R . (c ) Sketch map of a silver tip removing the diffracting grating. (d–j ) Transverse modes intensity distributions of guided modes for R =1 μm.
For the assembly of the diffraction grating and the metallic tip illustrated in
Fig. 4. (a ) Transverse mode intensity distributions of the grating-coupled SPPs at R =1 μm; (b–g ) Transverse mode intensity distributions of the hybrid mode at R =800, 750, 600, 350, 230, and 20 nm, respectively. (h ) Electric field intensity distribution at the tip apex. (i ) Transverse electric field intensity distribution at 1 nm below the tip apex.
In order to ensure that the TM01 mode can propagate to the tip apex with minimal non-ohmic loss, i.e. via radiation, the condition of the adiabatic parameter
Note that
To further increase the field enhancement factor at the silver tip apex, a silver substrate is placed 2 nm below the tip apex to construct the gap mode, as shown in
Fig. 6. (a ) Gap-mode configuration with the gap distance d =2 nm. (b ) Electric field intensity and polarization distributions in the x-z plane.
(c ) Electric field intensity distribution in the x-y plane at 1 nm below the tip apex. (d ) Comparison of electric field enhancement factor between the non-gap mode (Fig. 4(i) ) and gap mode located at 1 nm below the apex of the silver tip.
3 Conclusions
We present a detailed analysis on mode evolution of grating-coupled SPPs on a conical metal tip based on the guided-wave theory. The eigenvalue equation of HE
4 Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (NSFC) (61675169, 61377055 and 11634010), the National Key R & D Program of China (2017YFA0303800), and the Fundamental Research Funds for the Central Universities (3102017zy021, 3102017HQZZ 022).
5 Competing interests
The authors declare no competing financial interests.
6 Supplementary information
Section 1: Derivation of eigenvalue equation.
Section 2: Enhancement factor of the silver tip directly illuminated by far-filed excitation light.
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Article Outline
Fanfan Lu, Wending Zhang, Ligang Huang, Shuhai Liang, Dong Mao, Feng Gao, Ting Mei, Jianlin Zhao. Mode evolution and nanofocusing of grating-coupled surface plasmon polaritons on metallic tip[J]. Opto-Electronic Advances, 2018, 1(6): 180010.