<title>Two-dimensional array of optical interference filters produced by lithographic alterations of the index of refraction</title>

Abstract

We describe a new concept for producing, on a single substrate, a two-dimensional array of optical interference filters where the pass-band of each element can be independently specified. The interference filter is formed by optically contacting two dielectric mirrors so that the top quarter-wave films of the two mirrors form a Fabry-Perot cavity having a half-wave thickness. In the new device, we propose to etch an array of sub-wavelength patterns into the top surface of one of the mirrors before forming the cavity. The patterns must have a pitch shorter than the operational wavelength in order to eliminate diffraction. By changing the index of refraction of the half-wave layer, or the optical thickness of the cavity, the patterning is used to shift the pass-band and form an array of interference filters. One approach to producing the array is to change the fill factor of the pattern. Once the filter array is produced it may be mated to a two-dimensional detector array to form a miniature spectrophotometer.KEYWORDS: sub-wavelength structures, interference filter, spectrophotometer, CURRENT TECHNOLOGY FOR FILTER ARRAYSOptical interference filters appear in a number of forms. The class of filters with the highest transmission and narrowest linewidths are Fabry-Perot filters. These filters are formed by sandwiching a half-wave (or some multiple thereof) spacer layer between two mirrors. The lowest absorptive losses are found in mirrors made from multilayer dielectric stacks that are usually quarter-wave in thickness. The pass-band of these filters (i.e. the center wavelength, b)is determined by the optical thickness of the spacer layer. The bandwidth is determined by both the optical thickness of the spacer layer and the reflectivities of the two mirrors. The most common Fabry-Perot filters commercially available have pass-bands that are spatially uniform with variations as small as 0.1%. Filters with spatially varying pass-bands are also commercially available and are manufactured by proportionally varying the thickness of all the layers uniformly across the surface of the filters. These filters, called wedge filters, are either circular (circular variable filter -CVF) or linear (linear variable filter -LVF). In the former, the thicknesses of the multilayer films vary linearly with the angular position on a circular substrate; in the latter, they vary linearly with position across a rectangular substrate.Although wedge filters are widely used, they have several limitations. First, it is not possible to have both large dispersion (>> 10 d m m ) and retain other desirable characteristics. The dispersion is proportional to the wedge angle; however, a large wedge angle results in poor transmission characteristics (due to n o n -n o d reflections fiom the angled mirror facet), and an increase in spectral bandwidth (due to wavefront inhomogenity across the angled facets). Owing to these limitations, wedge filters are large (e.g. typical LVFs in the 300 nm -700 nm spectral range are 60 mm in length) and have dis...

Wide bandwidth and high resolution planar filter array based on DBR-metasurface-DBR structures

“…Walls et al have demonstrated FP filter arrays using metallic mirrors and effective index medium created by subwavelength patterning [21]. Filter arrays composed of dielectric mirrors and 1D subwavelength gratings have arXiv:1604.03167v1 [physics.optics] 11 Apr 2016 also been proposed [22], but, to the best our knowledge, have not been experimentally demonstrated. Furthermore, compared with 1D subwavelength gratings, the dielectric metasurfaces provide more control over the phase shifts and are polarization insensitive [16,18,23].…”

Mono-Material Multilayer Interference Optical Filter with Sub-Wavelength Structure for Infrared and Terahertz Optics

“…Altering the refractive index in a portion of the filter enables us to obtain a monolithic filter array. 18,19) A birefringent wave plate or polarizer can also be fabricated. A more complete review of the applications of the SWS can be found in Kikuta et al 12) As the first demonstration of a mono-material multilayer interference filter with SWS, we have fabricated one-layer silicon SWS on the surface of a silicon wafer, which is supposed to work as an antireflection (AR) layer (sample 1), and have compared its optical property with theoretical calculations.…”