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Abstract Infrared (IR) gradient permittivity materials are the potential building blocks of miniature IR‐devices such as an on‐chip spectrometer. The manufacture of materials with permittivities that vary in the horizontal plane is demonstrated using shadow mask molecular beam epitaxy in Si:InAs films. However, to be useful, the permittivity gradient needs to be of high crystalline quality and its properties need to be tunable. In this paper, it is shown that it can control the permittivity gradient length and steepness by varying the shadow mask thickness. Samples grown with similar growth parameters and with 200 and 500 µm mask thicknesses show permittivity gradient widths of 18 and 39 µm on the flat mesa on one side and 11 and 23 µm on the film slope on the other side, respectively. The gradient steepnesses are 23.3 and 11.3 cm⁻¹/µm on the flat mesa and 21.8 and 9.1 cm⁻¹/µm on the film slope, for samples made with the 200 and 500 µm masks, respectively. This work clearly shows the ability to control the in‐plane permittivity gradient in Si:InAs films, setting the stage for the creation of miniature IR devices. 

Abstract Infrared spectroscopy currently requires the use of bulky, expensive, and/or fragile spectrometers. For gas sensing, environmental monitoring, or other applications, an inexpensive, compact, robust on‐chip spectrometer is needed. One way to achieve this is through gradient permittivity materials, in which the material permittivity changes as a function of position in the plane. Here, synthesis of infrared gradient permittivity materials is demonstrated using shadow mask molecular beam epitaxy. The permittivity of the material changes as a function of position in the lateral direction, confining varying wavelengths of infrared light at varying horizontal locations. An electric field enhancement corresponding to wavenumbers ranging from ≈650 to 900 cm⁻¹over an in‐plane width of ≈13 µm on the flat mesa of the sample is shown. An electric field enhancement corresponding to wavenumbers ranging from ≈900 to 1250 cm⁻¹over an in‐plane width of ≈13 µm on the slope of the sample is also shown. These two different regions of electric field enhancement develop on two opposite sides of the material. This demonstration of a scalable method of creating in‐plane gradient permittivity material can be leveraged for the creation of a variety of miniature infrared devices, such as an ultracompact spectrometer.