Terahertz confocal microscopy based on Fibonacci super-oscillatory lens

Terahertz (THz) waves are electromagnetic waves between microwave and infrared with low energy, water absorption, high penetration, and other important characteristics. The frequency range of THz waves is between 0.1 ~ 10 THz. With the development of THz sources and detectors, the imaging technology in the THz band has also been greatly developed. THz confocal imaging technology is performed by scanning the sample layer-by-layer. In which, the combination of pinhole and point light source can filter out the stray beam from the defocusing plane and improve the axial resolution. The superoscillation is a phenomenon in which the band-limited function can oscillate faster than the highest Fourier component. A super-oscillating lens based on such method can generate a focal spot beyond the diffraction limit at any position through complex interference, which is called terahertz super-oscillating lens (TSOL). In this paper, we present a phase-type Fibonacci TSOL, which can obtain a short focal depth and narrow spot diameter. The Fibonacci lens is a diffractive lens that produces multiple foci along the axial coordinate. Its focal depth and focal spot at around 0.3 THz are investigated by simulation and experiments. A THz confocal system based on Fibonacci TSOL is built, and the confocal imaging of 3D printed samples is studied. The lateral and axial resolution of the system is higher than the traditional THz confocal microscope using a single lens.