  The large transparency window of silicon, covering the 1.1 um 8 um wavelength
range, makes it a promising platform for the implementation of
photothermal-based absorption spectrometers. These devices indirectly sense
absorption in the mid-infrared (MIR) by using near-infrared (NIR) wavelengths,
thereby enabling the realization of MIR absorption spectrometers without the
need for MIR photodetectors. Nevertheless, due to their comparatively large
index contrast and cross-sections, MIR Si strip waveguides are multi-mode at
NIR wavelengths, hindering device implementation. Here we present, for the
first time, an integrated Bragg grating waveguide filter for hybrid near- and
mid-infrared operation. Specifically, the filter is implemented in a
single-etch suspended silicon corrugated waveguide with an effectively
single-mode operation in NIR region for a waveguide cross-section as large as
0.5 um x 1.1 um. At the same time, the waveguide supports single-mode
propagation in MIR region. We demonstrate a long-period waveguide Bragg grating
yielding a sharp third-order Bragg resonance for the fundamental waveguide mode
and radiating the higher order modes. We experimentally demonstrate a notch
filter with a 4nm bandwidth and 40 dB extinction ratio with a
temperature-dependent Bragg wavelength shift of 70pm/K.
