Strain-Relaxed Nanocrystalline Diamond Thin Films with Silicon Vacancy Centers Using Femtosecond Laser Irradiation for Photonic Applications

Abstract

Nanocrystalline diamond thin (thickness of 100–500 nm) films with ensembles of negatively charged light-emitting silicon vacancy (SiV) centers might find an application in the field of photonics or biosensing due to their scalable and low-price fabrication process in comparison with monocrystalline diamond. The potential application of diamond thin films is, however, limited due to the presence of radiative and nonradiative defects. The former type is a source of an unwanted background in the photoluminescence spectra, the latter lowers the quantum efficiency of light emission, and both types of defects cause the reabsorption of SiV emission during its propagation within the layer. Here we show that femtosecond (fs) laser pulses focused via a microscope objective on a nanocrystalline diamond thin film can be employed to reduce background photoluminescence intensity by a factor of 5 with respect to the luminescence of SiV centers. Raman spectra show that this decrease is mainly caused by laser ablation of the sp2-related carbon phase present in-between the grains. The reduction of the sp2 carbon phase also leads to a local decrease in the optical absorption followed by an almost twofold increase in the SiV emission peak intensity. Moreover, the fs-irradiation also entails the release of strain inside such a layer as manifested by the observed shift of the Raman diamond peak toward the value measured in the monocrystalline diamond and by a blue-shift of the zero-phonon-line peak position of the SiV centers. Such a finding might enable the improvement of the optical quality of the nanocrystalline diamond-based photonic nanostructures or even might be employed over a larger area by defocusing the irradiation laser in order to create large-scale strain-relaxed nanocrystalline diamond thin films.