Identifying Defect Origins of Single-Photon Emitters Using Infrared Nano-Optic Probes

Abstract

Abstract Body: Point defects in wide bandgap semiconductors can provide robust, on-demand single photons that can be used in quantum technologies. Despite the appeal of defect-based single-photon emitters (SPEs), sub-diffractional length-scales of defects make traditional optical characterization techniques insensitive to local variations in the material response. Traditionally, photoluminescence (PL) spectroscopy is used to measure the emission spectrum and even map local intensities, however, spatial resolution is typically restricted to sizes on the order of a micron. However, advances in nano-optic probe technologies such as scattering-type scanning near-field optical microscopy (s-SNOM), can provide optical information about a material with nanoscale spatial resolution (100μm2) and imaging at only a single incident frequency, therefore another imaging technique must be used to navigate the sample of interest and to identify spectroscopic resonances of interest. Here, PL and nano-FTIR spectroscopies offer a highly complementary toolset with s-SNOM. Using PL intensity mapping in a scanning confocal microscope, ensembles of nano diamonds (d=134nm) with nitrogen-vacancy (NV) defects can be identified by high-intensity regions (Fig. 1a) and verified from the corresponding collected emission spectrum (Fig. 1b). FTIR spectroscopic measurements of similar diamond samples with very high NV densities have been found to correlate with additional vibrational absorption bands in the 1100-1400cm-1 range (Fig. 1c) with two prominent peaks2, indicative of vibrational modes due to the modified crystal bonds making up the NV defect. By mapping spatial regions featuring nano diamond NVs via s-SNOM, we hypothesize that one can map out the local distribution of the defect vibrational states. This is presented through s-SNOM phase maps recorded within a region of high PL intensity using a probing frequency (1177cm-1) within the absorption band of the NVs (Fig. 1d). Regions of high phase contrast correspond to high local absorption and three regions, circled in white, display magnitudes of high contrast at this frequency. However, the phase contrast is diminished when probing the sample with a different frequency (1450cm-1), outside of the NV absorption band (Fig. 1e). Regions of high phase contrast present in both images (1d,e) are attributed to fabrication residue, polymethyl methacrylate, which absorbs at both probed frequencies. The s-SNOM and PL maps suggest that the highlighted features are nano diamonds containing photoluminescent NVs but is still inconclusive. However, nano-FTIR provides direct measurements of IR reflection and absorption spectra from samples without diffraction limitations3 using s-SNOM principles. Employing it to measure the spectra of the high contrast regions will enable a direct comparison to its bulk counterpart, conclusively determining the nature of the additional vibrational modes. Correlating PL with s-SNOM imaging has been demonstrated on extended defects, but not so with point defects4. Diamond NVs are appropriate for developing this technique because of their high brightness at room temperature5. This technique could provide a non-destructive method of investigating defect-based SPEs in materials such as hexagonal boron nitride and gallium nitride, whose defect structures have not been fully characterized but are yet still under intense investigation.