Selective Enrichment of Fluorescent Nanodiamonds by Stimulated Recoil Forces

Detonation nanodiamonds containing silicon-vacancy (SiV) centers (SiV-DNDs) exhibit spectrally sharp optical transitions and are promising nanoscale emitters. After purification and oxidative postprocessing, SiV-DNDs are obtained with a mean particle diameter of ∼10 nm, a size scale at which single-color-center occupancy per particle may be expected. Yet, practical applications require selective enrichment from mixtures that also contain undoped nanodiamonds. Conventional separation methods lack sufficient selectivity, and resonant absorption-based optical sorting is fundamentally constrained by excited-state saturation, rendering it ineffective in the single-color-center regime. Building on our recent theoretical predictions that stimulated emission can generate a dissipative optical force beyond this limit, we demonstrate that the stimulated recoil force (SRF) provides a scalable mechanism for emission-line-selective manipulation of nanodiamonds in liquid. Using a glass capillary with counter-propagating pump beams and a manipulation beam resonant with SiV emission, we observe millimeter-scale downstream depletion and upstream enrichment of SiV-DNDs, while a spectrally distinct, off-resonant fluorescent nanodiamond population remains unchanged. The magnitude and spatial extent of the transport show that SRF overcomes Brownian diffusion and enables long-range, species-selective transport under realistic conditions. To identify the physical mechanism, we perform complementary glass-cell experiments under a well-defined focusing geometry and compare the observed enrichment with optical-force calculations based on density-matrix dynamics and Brownian-dynamics simulations. Qualitative agreement supports SRF as the dominant dissipative contribution responsible for the transport. These results demonstrate practical, emission-energy-selective optical sorting of fluorescent nanodiamonds and define design principles for extending this approach to capillaries, microfluidic systems, and other fluorescent nanomaterials.