Background Reduction in Nano-Photoluminescence Measurements Through Tapping-Mode Demodulation

Nano-photoluminescence (nano-PL) measurements have proven in the past decade to be an incredibly powerful and direct way of probing the excited state physics beyond the diffraction limit. These measurements rely on confined plasmonic modes to boost the local Purcell enhancement at the apex of a tip, resulting in strongly enhanced spontaneous emission rate directly within the confined mode volume of the tip plasmon. Typically, nano-PL measurements are conducted by bringing an atomic force microscope (AFM) tip in contact and out of contact in quick succession. The PL spectrum collected when in contact and out of contact are subtracted from each other to obtain near-field contrast. However, the source of noise in nano-PL comes from emission of the sample. The focus of the excitation laser changes as the sample is retracted as part of the out-of-contact portion of the measurement, resulting in an inconsistent far-field subtraction. To this end, we have implemented a combination of tapping mode AFM and Fourier analysis to improve the far-field rejection of nano-PL measurements. By using tapping mode nano-PL to conduct measurements on various transition metal dichalcogenide (TMD) samples, we demonstrate that we can image features with better contrast than with the conventional nano-PL mode of operation.