The nitrogen-vacancy (NV) centre in diamond is a promising candidate for quantum computing applications and magnetic sensing applications, because it is an atomic-scale defect with stable coherence time (T2) and reliable accessibility at room temperature. We demonstrated a method for improving the NV spin properties (the full width half maximum (FWHM) value of the magnetic resonance spectrum and T2) through a near-field (NF) etching method under ambient conditions. The NF etching method, based on a He-Cd ultraviolet laser (325 nm), which is longer than the absorption edge of the oxygen molecule, enabled selective removal of defects on the nanodiamond surface. We observed a decrease in the FWHM value close to 15% and an increase in T2 close to 25%. Since our technique can be easily reproduced, a wide range of NV centre applications could be improved, especially magnetic sensing applications. Our results are especially attractive, because they have been obtained under ambient conditions and only require a light source with wavelength slightly above the O2 absorption edge.

By using gold (Au) nanoparticles (NPs) as an optical near-field source under far-field illumination in combination with a silver (Ag) ion solution containing a photoinitiator, we coated Ag on Au NPs using a near-field (NF)-assisted process. We evaluated the change in the size of the NPs using transmission electron microscopy. Evaluation of the synthesized Ag volume over illumination power confirmed the squared power dependence of the NP volume with illumination using 808 nm light, i.e., a wavelength longer than the absorption edge wavelength of the photoinitiator molecules. The rate of volume increase was much lower than the plasmonic field enhancement effect. Therefore, the squared power dependency of the volume increase using a wavelength longer than the absorption edge wavelength originated from NF-assisted second-harmonic generation and the resulting excitation.

Surface flattening techniques are extremely important for the development of future electrical and/or optical devices because carrier-scattering losses due to surface roughness severely limit the performance of nanoscale devices. To address the problem, we have developed a near-field etching technique that provides selective etching of surface protrusions, resulting in an atomically flat surface. To achieve finer control, we examine the importance of the wavelength of the near-field etching laser. Using light sources at wavelengths of 325 and 405 nm, which are beyond the absorption edge of the photoresist (310 nm), we compare the resulting cross-sectional etching volumes. The volumes were larger when 325 nm light was employed, i.e., closer to the absorption edge. Although 405 nm light did not cause structural change in the photoresist, a higher reduction of the surface roughness was observed as compared to the 325 nm light. These results indicate that even wavelengths above 325 nm can cause surface roughness improvements without notably changing the structure of the photoresist.

The luminescence of a nitrogen-vacancy (NV) center in a nanodiamond (ND) is of great interest because of its features, especially in the field of nanophotonics. When an NV center in an ND is located in the vicinity of the surface, the emission is often disturbed by any surface defects, resulting in non-radiative recombination. In this work, we performed dressed photon-phonon (DPP) etching of the NDs, and found that the size of the NDs decreased, while the cathodoluminescence (CL) intensity increased. We assume that this increase in the CL intensity originates from the removal of the surface protrusions and/or defects by DPP etching.