  We show that fabrication imperfections in silicon nitride photonic crystal
waveguides can be used as a resource to efficiently confine light in the
Anderson-localised regime and add functionalities to photonic devices. Our
results prove that disorder-induced localisation of light can be utilised to
realise an alternative class of high-quality optical sensors operating at room
temperature. We measure wavelength shifts of optical resonances as large as
15.2 nm, more than 100 times the spectral linewidth of 0.15\,nm, for a
refractive index change of about 0.38. By studying the temperature dependence
of the optical properties of the system, we report wavelength shifts of up to
about 2 nm and increases of more than a factor 2 in the quality factor of the
cavity resonances, when going from room to cryogenic temperatures. Such a
device can allow simultaneous sensing of both local contaminants and
temperature variations, monitored by tens of optical resonances spontaneously
appearing along a single photonic crystal waveguide. Our findings demonstrate
the potential of Anderson-localised light in photonic crystals for scalable and
efficient optical sensors operating in the visible and near-infrared range of
wavelengths.
