ИСТИНА

Nanodiamonds with color centers present a valuable object for studies in quantum optics, bioimaging and nanosensing [1]. To efficiently handle the fluorescence of the color centers, hosting nanodiamonds were proposed to be placed into properly designed resonators. In this way, the emission can be enhanced and directed into a desired direction. However, the fact that the diamond nanoparticle itself can be resonant, is usually omitted from consideration. High-index nanoparticles exhibit a set of optical resonances, first of which is excited, when the particle size is approximately equal to the wavelength in the particle material [2]. As they reveal themselves in the scattering spectra, these resonances are also referred to as Mie resonances. The high refractive index of diamond does allow for the excitation of Mie resonances in subwavelength diamond particles, although no evidence of this had been shown until our recent work [3]. In this contribution, we report an experimental study of intrinsic optical resonances of nanodiamonds. First, we employ single-particle dark-field scattering spectroscopy to reveal the fundamental optical resonances of diamond nanoparticles. Second, we study the lifetime of the color centers in nanodiamonds at the excitation of the Mie resonances. The particles for this study were grown on a sapphire substrate by chemical vapor deposition and characterized by scanning electron microscopy. The size of the studied diamonds varies from about 300 to 400 nm. In this size range, the magnetic dipole and quadrupole resonances are shown to be the most dominant in the scattering spectra. The scattering resonances are shown to shift to the red side of the spectrum with increasing particle size. In the case of an anisotropic particle, the scattering spectrum depends on the polarization of the input light. The results of the scattering spectroscopy are verified by numerical and analytical calculations. To characterize the influence of the excited resonances on the lifetime of color centers, we first calculate the Purcell factor for a dipole source in a free-standing diamond nanosphere. At the excitation of the magnetic quadrupole resonance, the Purcell factor is shown to be enhanced by almost two orders of magnitude compare to a non-resonant nanoparticle. Finally, by measuring the saturation intensity for color centers in nanodiamonds, we show experimentally that the lifetime varies from one particle to another by almost an order of magnitude. The obtained results demonstrate that the efficiency of diamond-based sensors and single-photon sources can be significantly improved by choosing nanoparticles of a proper size. [1] Aharonovich I., et al. Nature Photonics 5(7), 397–405 (2011). [2] Kuznetsov A.I., et al. Science 354(6314), aag2472 (2016). [3] Shilkin D.A., et al. ACS Photonics 4(5), 1153–1158 (2017).