  The ability to perform broadband optical spectroscopy with
sub-diffraction-limit resolution is highly sought-after for a wide range of
critical applications. However, sophisticated tip-enhanced techniques are
currently required to achieve this goal. We bypass this challenge by
demonstrating an extremely broadband photodetector based on a two-dimensional
(2D) van der Waals heterostructure that is sensitive to light across over a
decade in energy from the mid-infrared (MIR) to deep-ultraviolet (DUV) at room
temperature. The devices feature high detectivity (> 10^9 cm Hz^1/2 W^-1)
together with high bandwidth (2.1 MHz). The active area can be further
miniaturized to submicron dimensions, far below the diffraction limit for the
longest detectable wavelength of 4.1 um, enabling such devices for facile
measurements of local optical properties on atomic-layer-thickness samples
placed in close proximity. This work can lead to the development of low-cost
and high-throughput photosensors for hyperspectral imaging at the nanoscale.
