Multilevel silicon diffractive optics for terahertz waves

Abstract

A multilevel microfabrication process has been developed to produce silicon Fresnel lenses for terahertz waves. A repeated binary fabrication process was used to create lenses with up to eight levels in complexity and these lenses have been compared to both less complex structures and refractive optic lenses. The microfabrication required deep reactive ion etching and multilevel resist processing using SU8 photoresist. At the design frequency of 1 THz an eight-level lens showed significant improvement in intensity at the focus and had much reduced energy lost into sidelobes compared with refractive lenses.

Review of Terahertz Tomography Techniques

“…Figure 10 is a schematic setup of Fresnel lens imaging. This distinctive property could be used to perform tomographic imaging of a target when used with broadband illumination [164,165]. The depth of focus of the THz wave mostly determines the resolution of this technique in the z dimension, so there is no restriction for narrowband imaging systems.…”

A Nipkow disk integrated with Fresnel lenses for terahertz single pixel imaging

“…The unmodified wafer has a thickness of 325 µm and a transmittance of 50%-60% at terahertz frequencies. A binary fabrication technique was used to develop the lenses on the silicon wafer [22][23][24]. Constrained by the size of the silicon wafer, 12 Fresnel lenses having a diameter of 10 mm were arranged spirally with an axial separation of 2 mm.…”

“…This leads the number of zones, n, to be 2 [23]. Since a binary photolithography fabrication technique was used to make the lenses, the number of photo masks, k, determines the number of phase steps, m (m = 2 k ).…”

“…Since a binary photolithography fabrication technique was used to make the lenses, the number of photo masks, k, determines the number of phase steps, m (m = 2 k ). Thus, the radius of each phase step, R ni , can be calculated from [23,24] (2 2 2)…”

“…The total etch depth, h, for silicon, with a refractive index of 3.42, is 49 µm [23]. Table 1 summarises all the design parameters for a binary diffractive lens with a diameter of 10 mm and focal length of 55 mm on a 4-inch double-side polished silicon wafer.…”