Antenna and Plasmonic Properties of Scanning Probe Tips at Optical and Terahertz Regimes

Electric field enhancement (γ) at the end of a sharp tip is calculated by finiteelement method (FEM). The metal tip is modeled as a 3-D gold hemi-ellipsoid illuminated by two different excitation frequencies in the optical and Terahertz (THz) regimes with the wavelengths of 630 nm and 0.3 mm, respectively. The dependence of field enhancement on apex radius, tip geometry, radiation wavelengths, tip and sample materials at optical and THz regimes is investigated. First, Maxwell’s equations are solved in the frequency domain and the reliability of FEM method is validated by comparing the results with analytical solutions and the finite element time domain method. Our calculation shows the competition between antenna and plasmon resonances and associated dephasing effects for the field enhancement at optical and THz regimes. When the size of the metal tip is larger than approximately 0.4λ (λ is the radiation wavelength), the field enhancement is less than 10 for λ=630 due to the dephasing effects whereas at λ=0.3 mm (THz radiation) the antenna effect is dominant leading to an extremely high field enhancement.