The Theory Simulation Research Of The Terahertz Near-field Nanotip

With the development of terahertz source and the detection means, terahertz spectrscopy technology has made significant progresses in the semiconductor device testing, biological marker analysis, composite materials nondestructive testing and molecular imaging, etc. Terahertz wavelength is in 30 ? m- 3 mm range, due to the restriction of the diffraction limit, high spatial resolution terahertz spectroscopy imaging is a huge challenge. The near-field optical microscope based on the apex enhanced has been applied gradually in the visible and infrared frequency bands, and its imaging resolution can reach to nanoscale. Similar to visible light, infrared and Raman spectroscopies, whether can the terahertz spectroscopy also accomplished the high resolution imaging to break through the diffraction limit by local enhancement effects of the near-field nanotip? What is the difference between the imaging mechanism in the terahertz band and the traditional visible light and infrared bands?To cure the above problems, in this paper, the interaction between a gold nanotip and a silver substrate under the terahertz(THz) wave radiation is simulated by a finite difference time domain method and theoretically analyzed from the quasi-static charges model. Results from simulations and theoretical analysis show that with the increase in the distance between the nanotip and the substrate, the maximum electric field intensity on the substrate prominently decreases. With the increase in radius of the nanotip, diameter of the focused THz spot almost linearly increases and the intensity of THz spot also has a slight increase. These results are in agreement with the physical principles of the tip-enhanced near-field optical microscopy(TENOM) in the traditional infrared and Raman bands, which indicates the TENOM technique is also suitable for the THz frequency range. And factors that influence the enhancement effects are investigated and analyzed in detail. Simulation results show that the size of the nanotip apex, the apex-substrate distance, dielectric properties of the substrate and the detected sample, etc., have significant impacts on the electric field enhancement and spatial resolution of the terahertz near-field nanotip. In particular, because the resonance frequency of plasma materials is in the THz wave band unlike the visible and infrared bands, when the model applicates these materials, the researchers can get stronger local field enhancement signal. These results can be explained from the effective polarizability of the nanotip-sample/substrate system.To break through the restriction of optical diffraction limit to obtain a nanoscale detection resolution has become the inevitable development trend in the terahertz near-field microscopy. The nanotip plays an important role as the research core. The selection of its shape, material, structure and other factors and the study of its near-field enhancement mechanism will become the key to the terahertz near-field imaging with high sensitivity and spatial resolution.