Nano-imaging of waveguided infrared photoluminescence

Near-field photoluminescence (nano-PL) captures light-matter interactions that exist below the diffraction limit. An emitter placed in the vicinity of sub-wavelength modes, such as nano-gap gold plasmons, is to known to dramatically increase its spontaneous emission rate. However, such nano-gap Purcell enhancements are accompanied by high loss, predominate only in the visible range, and require complex lithography. The infrared range, so far unexplored with nano-PL, promises a variety of low loss (propagating) modes that confine light without additional patterning steps. Here, we develop a photoluminescence scanning near-field optical microscope (PL-SNOM) capable of imaging infrared waveguided photoluminescence in WSe2/2H-MoTe2/WSe2 heterostructures. The type-I band alignment ensures efficient electron-hole generation in the MoTe2 monolayer, and correspondingly low-absorption/high-index waveguided photoluminescence into the proximal bulk WSe2 slabs. Hyperspectral line profiles reveal bright sub-diffractional dispersing fringes near the WSe2 edge that agree with the expected waveguide dispersion. A novel model for the mode-dependent Purcell factor, informed by the measured dispersion, yields a maximum value of 1.5-2. While modest, this Purcell factor is predicted to dramatically increase as deep-infrared modes of stronger confinement are utilized.