  The development of compact and fieldable mid-infrared (mid-IR) spectroscopy
devices represents a critical challenge for distributed sensing with
applications from gas leak detection to environmental monitoring. Recent work
has focused on mid-IR photonic integrated circuit (PIC) sensing platforms and
waveguide-integrated mid-IR light sources and detectors based on semiconductors
such as PbTe, black phosphorus and tellurene. However, material bandgaps and
reliance on SiO$_2$ substrates limit operation to wavelengths
$\lambda\lesssim4\,\mu\textrm{m}$. Here we overcome these challenges with a
chalcogenide glass-on-CaF$_2$ PIC architecture incorporating split-gate
photothermoelectric graphene photodetectors. Our design extends operation to
$\lambda=5.2\,\mu\textrm{m}$ with a Johnson noise-limited noise-equivalent
power of $1.1\,\mathrm{nW}/\mathrm{Hz}^{1/2}$, no fall-off in photoresponse up
to $f = 1\,\mathrm{MHz}$, and a predicted 3-dB bandwidth of
$f_{3\textrm{dB}}>1\,\mathrm{GHz}$. This mid-IR PIC platform readily extends to
longer wavelengths and opens the door to applications from distributed gas
sensing and portable dual comb spectroscopy to weather-resilient free space
optical communications.
