Cold and Hot Spots: From Inhibition to Enhancement by Nanoscale Phase Tuning of Optical Nanoantennas.

Optical nanoantennas are well-known for the confinement of light into nanoscale hot spots, suitable for emission enhancement and sensing applications. Here, we show how control of the antenna dimensions allows tuning the local optical phase, hence turning a hot spot into a cold spot. We manipulate the local intensity exploiting the interference between driving and scattered field. Using single molecules as local detectors, we experimentally show the creation of subwavelength pockets with full suppression of the driving field. Remarkably, together with the cold excitation spots, we observe inhibition of emission by the phase-tuned nanoantenna. The fluorescence lifetime of a molecule scanned in such volumes becomes longer, showing slow down of spontaneous decay. In conclusion, the spatial phase of a nanoantenna is a powerful knob to tune between enhancement and inhibition in a 3-dimensional subwavelength volume.

Nanoscale Imaging and Control of Hexagonal Boron Nitride Single Photon Emitters by a Resonant Nanoantenna.

Defect centers in two-dimensional hexagonal boron nitride (hBN) are drawing attention as single-photon emitters with high photostability at room temperature. With their ultrahigh photon-stability, hBN single-photon emitters are promising for new applications in quantum technologies and for 2D-material based optoelectronics. Here, we control the emission rate of hBN-defects by coupling to resonant plasmonic nanocavities. By deterministic control of the antenna, we acquire high-resolution emission maps of the single hBN-defects. Using time-gating, we can discriminate the hBN-defect emission from the antenna luminescence. We observe sharp dips (40 nm fwhm) in emission, together with a reduction in luminescence lifetime. Comparing with finite-difference time-domain simulations, we conclude that both radiative and nonradiative rates are enhanced, effectively reducing the quantum efficiency. Also, the large refractive index of hBN largely screens off the local antenna field enhancement. Finally, based on the insight gained we propose a close-contact design for an order of magnitude brighter hBN single-photon emission.

Wavelength exponent based calibration for turbidity spectroscopy: Monitoring the particle size during emulsion polymerization reactions.

Particle size and particle size distribution (PSD) are important properties of polymer latexes because they strongly affect the film formation and the rheological properties of the latexes. Thus, monitoring the particle size is of paramount importance during the production of waterborne polymeric dispersions, for which online/inline measurements of the particle size are required. Herein, turbidity spectroscopy (TUS) is used to measure the particle size of nanoparticles in dispersed media. Calibration curves based on a modified wavelength exponent method are introduced and assessed to monitor the evolution of a broad range of particle sizes in seeded semibatch emulsion polymerizations of methyl methacrylate, butyl acrylate, methacrylic acid (MMA/BA/MAA 51/47/2) terpolymers. It is shown that the wavelength exponent based calibration curves can be successfully used to retrieve the particle size during the polymerization process.