A novel method for the synthesis of highly monodispersed hydrophillic InP-ZnS nanocrystals and their use as luminescence probes for live cell imaging is reported. Hydrophobic InP-ZnS nanocrystals are prepared by a new method that yields high-quality, luminescent core-shell nanocrystals within 6-8 h of total reaction time. Then by carefully manipulating the surface of these passivated nanocrystals, aqueous dispersions of folate-conjugated nanocrystals (folate-QDs) with high photostability are prepared. By use of confocal microscopy, we demonstrate the receptor-mediated delivery of folic acid conjugated quantum dots into folate-receptor-positive cell lines such as KB cells. These folate-QDs tend to accumulate in multi-vescicular bodies of KB cells after 6 h of incubation. Receptor-mediated delivery was confirmed by comparison with the uptake of these particles in folate-receptor-negative cell lines such as A549. Efficient two-photon excitation of these particles and two-photon imaging using these particles are also demonstrated. The use of these InP-ZnS nanoparticles and their efficient two-photon excitation can be potentially useful for deep tissue imaging for future in vivo studies.

Emerging quantum technologies for cryptography, computing and metrology exploit quantum mechanical effects for enhanced information processing and nanoscale sensing. Though different platform systems are currently being explored, light-based quantum technologies using single-photon emitters as the basic building block are among the frontrunners 1 . Several strategies have been used to realize deterministic single photon sources in the solid state 2 , including quantum dots 3 , single molecules 4 , and point defects in wide bandgap materials such as diamond and silicon carbide 5-9 . Single photon emitters in novel van der Waals materials have garnered recent attention due to their potential for integration with waveguides, microcavities, and other passive components typical in photonic devices. Example 2D systems hosting quantum emitters include WSe 2 and MoS 2 as well as other transition metal dichalcogenides (TMDs) 10-14 .Here we focus on hexagonal boron nitride (hBN), a wide bandgap semiconductor where defect emission has been shown to be tunable and robust at room temperature 15-22 and above 23 . We use confocal microscopy to investigate the photoluminescence (PL) of point defects within thin hBN flakes deposited on a lithographically patterned SiO 2 substrate. Due to Van der Waals forces the flake conforms to the surface topography thus accumulating significant local strain near protruding features. Using large structured arrays of different sizes and geometries we find nearly perfect correspondence between the strained areas of the flake and defect emission. Our modeling supports the notion of defect activation via charge trapping in deformation potential wells. The physics at play has some similarities with that governing the dynamics of excitons in WSe 2 monolayers subjected to comparable geometries, as reported recently 24,25 . Unlike TMDs, however, the wide bandgap of hBN can accommodate large potential modulations, sufficient to stabilize the defect charge at room temperature. In particular, we calculate deformation potential wells as deep as 500 meV confined to regions of tensile and compressive strain in the hBN flake that correlate well with the spatial localization of the emitters.For the first set of experiments we use an array of 155-nm-high nanopillars with diameters ranging from 200 to 700 nm fabricated via electron beam lithography over a large-area silica substrate (Figure 1a); the sample is a commercial, 20-nmthick flake of hBN grown via chemical vapor deposition (CVD). We follow a wet transfer protocol 26 to drape the flake on the patterned silica substrate (see Methods). This technique takes advantage of the Van der Waals forces to make the flake conform to the surface topography. As an illustration, Figure 1b shows an atomic force microscopy (AFM) image from an hBN fragment where the 20-nm-thick flake folds on itself: We identify single-and double-layer sections near the left and right areas, respectively; bare pillars -visible on the lower, right corner of the image -provide a direct view...

Dissipative and dispersive optomechanical couplings are experimentally observed in a photonic crystal split-beam nanocavity optimized for detecting nanoscale sources of torque. Dissipative coupling of up to approximately $500$ MHz/nm and dispersive coupling of $2$ GHz/nm enable measurements of sub-pg torsional and cantilever-like mechanical resonances with a thermally-limited torque detection sensitivity of 1.2$\times 10^{-20} \text{N} \, \text{m}/\sqrt{\text{Hz}}$ in ambient conditions and 1.3$\times 10^{-21} \text{N} \, \text{m}/\sqrt{\text{Hz}}$ in low vacuum. Interference between optomechanical coupling mechanisms is observed to enhance detection sensitivity and generate a mechanical-mode-dependent optomechanical wavelength response.Comment: 11 pages, 6 figure

Fluorescent defects recently observed under ambient conditions in hexagonal boron nitride (h-BN) promise to open novel opportunities for the implementation of on-chip photonic devices that rely on identical photons from single emitters. Here we report on the room-temperature photoluminescence dynamics of individual emitters in multilayer h-BN flakes exposed to blue laser light. Comparison of optical spectra recorded at successive times reveals considerable spectral diffusion, possibly the result of slowly fluctuating, trapped-carrier-induced Stark shifts. Large spectral jumpsreaching up to 100 nmfollowed by bleaching are observed in most cases upon prolonged exposure to blue light, an indication of one-directional photochemical changes possibly taking place on the flake surface. Remarkably, only a fraction of the observed emitters also fluoresce on green illumination, suggesting a more complex optical excitation dynamics than previously anticipated and raising questions on the physical nature of the crystal defect at play.