Hybrid-lens for Terahertz Subwavelength Focusing beyond the Diffraction Limit
The relatively low energy of the terahertz light (THz, 1 THz = 4.14 meV) promises the non-invasive optical imaging of the biomedical objects. However, the THz light inherently suffers the optical diffraction limit and the subwavelength microstructures cannot be resolved. We propose the planar hybrid-lens comprising the multiple slits and the two-dimensional rod array for the subwavelength focusing of the THz light beyond the diffraction limit. The hybrid-lens is investigated by the finite difference time domain simulation (FDTD). The spectra of the rod array obtained by FDTD are compared with the spectra calculated by the analytical model. The subwavelength focal spot about 20 µm (λ/7.5) at the frequency 2 THz is demonstrated by FDTD. The array of the subwavelength focal spots on the opposite side of the multiple slits are generated by the interferences of the Floquet-Bloch waves which can be guided through the rod array. The hybrid-lens can function as the subwavelength focusing and the substrate simultaneously which may serve as the new tool in the high-speed imaging and sensing of the subwavelength biomedical objects by using the THz light.