  Infrared spectroscopy currently requires the use of bulky, expensive, and/or
fragile spectrometers. For gas sensing, environmental monitoring, or other
applications in the field, an inexpensive, compact, robust on-chip spectrometer
is needed. One way to achieve this goal is through gradient permittivity
materials, in which the material permittivity changes as a function of position
in the plane. In this paper, we demonstrate the synthesis of infrared gradient
permittivity materials using shadow mask molecular beam epitaxy. The
permittivity of our material changes as a function of position in the lateral
direction, allowing us to confine varying wavelengths of infrared light at
varying horizontal locations. We see an electric field enhancement
corresponding to a wavenumber gradient of ~650 cm$^{-1}$ to 900 cm$^{-1}$ over
an in-plane gradient width of ~13 $\mu$ m on the flat mesa of our sample. In
addition, we see a wavenumber gradient of ~900 cm$^{-1}$ to 1250 cm$^{-1}$ over
an in-plane gradient width of ~13 $\mu$m on the slope of our sample. These two
different wavenumber gradient regions develop on two opposite sides of our
material. This demonstration of a scalable method of creating an in-plane
gradient permittivity material could be leveraged for the creation of a variety
of miniature infrared devices, such as an ultracompact spectrometer.
