  Efficient electrical generation of mid-infrared light is challenging because
of the dearth of materials with natural dipole-active electronic transitions in
this spectral region. One approach to solve this problem is through
quantum-engineering of the electron dispersion to create artificial
transitions, as in quantum cascade devices. In this work we propose an
alternative method to generate mid-infrared light, utilizing the coupling
between longitudinal and transverse degrees of freedom due to the nonlocal
optical response of nanoscopic polar dielectric crystals. Polar crystals
support sub-diffraction photonic modes in the mid-infrared. They also support
longitudinal phonons, which couple efficiently with electrical currents through
the Fr\"ohlich interaction. As we have shown in previous theoretical and
experimental works, these two degrees of freedom can hybridize forming
longitudinal-transverse polaritons. Here we theoretically demonstrate that
longitudinal-transverse polaritons can be efficiently generated by electrical
currents, leading to resonant narrowband photonic emission. This approach can
therefore be utilised to electrically generate far-field mid-infrared photons
in the absence of dipole-active electronic transitions, potentially
underpinning a novel generation of mid-infrared optoelectronic devices.
