The Design, Fabrication And Experimental Charaterization Of The Ultrathin Terahetz Long-focal-depth Lens

Terahertz waves have many unique properties, such as coherence, low photon energy, fingerprint spectrum, etc. In recent years, with the rapid development of terahertz sources and detecting techniques, the terahertz wave attracts intensive research interest and is now becoming a very important subject. On the other hand, long-focal-depth (LFD) lenses have wide applications in optical coupling, optical imaging, optical interconnections and optical communications, because they have a long axial focal region. In this thesis, we design an LFD lens in the terahertz waveband, so as to realize its focusing function in a long axial range.In traditional optics, the phase change is realized through optical path accumulation, therefore, the thickness of diffractive optical elements is generally comparable to the incident wavelength. Since the terahertz wavelength is hundreds of micrometers, terahertz optical elements are usually too large for system integration and miniaturization. Quite recently, scientists have discovered a new phase modulation mechanism based on metallic surface plasmonic resonance. Phase changes universally exist in metallic-dielectric stratified structures, electromagnetic resonant cavities, metallic nanoparticle clusters, and plasmonic antennas. Especially, through studying the interacting behavior between the incident light and the subwavelength metallic structures, the generalized laws for reflection and refraction are formulated. Experimental results have validated the theoretical laws. Because the subwavelength metal surface brings about an abrupt phase change, it does not rely on the element thickness. Normally, the element thickness is about100nanometers. Therefore, this novel surface is called as a metasurface. Compared with diffractive optical elements, the element thickness based on metasurfaces is about only1/1000.By using the abrupt phase modulation mechanism, we design the ultrathin LFD lens based on metasurfaces. Main research contents and achievements are listed as follows:(1) Phase distribution design of the LFD lens. By using the Yang-Gu algorithm, the phase distribution of the LFD lens is designed. Theoretical simulations reveal that the designed LFD lens has an axial focal depth of8.40mm. In comparison, the conventional lens with the same diameter and numerical aperture has a focal depth of2.41mm.(2) Designs of the metasurface antennas. By using the commercial software ’Concerto’ based on the finite-element method, we have designed eight antenna units. These eight antennas modulate the incident light with almost the same amplitude, while their phases range from0to2π for every π/4. They act as eight quantization levels in diffractive optics. Through quantizing the obtained phases in step (1) into eight levels and arranging the appropriate antenna on the corresponding pixel, the LFD lens is finally formed.(3) Experimental fabrication of the ultrathin LFD terahertz lens. Firstly, on a500nm silicon substrate, the designed antenna patterns are obtained through ultraviolet photolithography. Secondly, a50to100nm gold film is deposited above by using the electron beam evaporation technology. Finally, by using the lift-off technology, the photoresist is removed and the LFD lens is fabricated. With the optical microscope, it is found that the ultrathin LFD terahertz lens has small fabricating errors and a good surface uniformity.(4) Experimental characterization of the fabricated ultrathin LFD terahertz lens. By using the terahertz quasi-near-field focal-plane imaging system, focal properties of the fabricated LFD lens are characterized. Firstly, axial intensity measurements reveal that the experimental focal depth is8.96mm, which basically agrees with the theoretically predicted value. Secondly, the intensity distributions on several transverse planes within the LFD region is measured, experimental results demonstrate that the LFD lens possesses high lateral focusing resolution within the LFD region. Thirdly, the fabricated LFD lens holds an LFD property with a bandwidth of200GHz.