Subdiffraction, Luminescence-Depletion Imaging of Isolated, Giant, CdSe/CdS Nanocrystal Quantum Dots

Subdiffraction spatial resolution luminescence depletion imaging was performed with giant CdSe/14CdS nanocrystal quantum dots (g-NQDs) dispersed on a glass slide. Luminescence depletion imaging used a Gaussian shaped excitation laser pulse overlapped with a depletion pulse, shaped into a doughnut profile, with zero intensity in the center. Luminescence from a subdiffraction volume is collected from the central portion of the excitation spot, where no depletion takes place. Up to 92% depletion of the luminescence signal was achieved. An average full width at half-maximum of 40 ± 10 nm was measured in the lateral direction for isolated g-NQDs at an air interface using luminescence depletion imaging, whereas the average full width at half-maximum was 450 ± 90 nm using diffraction-limited, confocal luminescence imaging. Time-gating of the luminescence depletion data was required to achieve the stated spatial resolution. No observable photobleaching of the g-NQDs was present in the measurements, which allowed imaging with a dwell time of 250 ms per pixel to obtain images with a high signal-to-noise ratio. The mechanism for luminescence depletion is likely stimulated emission, stimulated absorption, or a combination of the two. The g-NQDs fulfill a need for versatile, photostable tags for subdiffraction imaging schemes where high laser powers or long exposure times are used

Far-field fluorescence nanoscopy of diamond color centers by ground state depletion

We report on two modalities of lens-based fluorescence microscopy with diffraction-unlimited resolution relying on the depletion of the fluorophore ground state. The first version utilizes a beam with a deep intensity minimum, such as a doughnut, for intense excitation followed by mathematical deconvolution, whereas in the second version, a regularly focused beam is added for generating the image directly. In agreement with theory, the subdiffraction resolution scales with the square root of the intensity depleting the ground state. Applied to the imaging of color centers in diamond our measurements evidence a resolving power down to ≈ 7.6 nm, corresponding to 1/70 of the wavelength of light employed. Our study underscores the key role of exploiting (molecular) states for overcoming the diffraction barrier in far-field optical microscopy