In situ observation of plasmon tuning in a single gold nanoparticle during controlled melting.

In this Letter we study the relations among shape, symmetry, and plasmon resonance shift in a single gold nanoparticle during laser melting. A beam of an argon ion laser is focused on a selected particle, while its optical and shape properties can be observed with the help of a combined dark-field/photoluminescence microscope and an atomic force microscope, respectively. Starting from a spherical shape, radiation pressure forms the melting gold particle into an upright standing rod on a glass substrate, showing a characteristic dipole scattering pattern. A red-shift of the photoluminescence signal and the scattering spectrum is observed. The melting process can be controlled allowing the formation of different particle heights and plasmon resonance shifts. In situ tuning of the plasmon resonance of individual particles is possible with this reversible melting process.

Plasmon-enhanced upconversion in single NaYF4:Yb3+/Er3+ codoped nanocrystals.

In this Letter we report the plasmon-enhanced upconversion in single NaYF(4) nanocrystals codoped with Yb(3+)/Er(3+). Single nanocrystals and gold nanospheres are investigated and assembled in a combined confocal and atomic force microscope setup. The nanocrystals show strong upconversion emission in the green and red under excitation with a continuous wave laser in the near-infrared at 973 nm. By the use of the atomic force microscope, we couple single nanocrystals with gold spheres (30 and 60 nm in diameter) to obtain enhanced upconversion emission. An overall enhancement factor of 3.8 is reached. A comparison of time-resolved measurements on the bare nanocrystal and the coupled nanocrystal-gold sphere systems unveil that faster excitation as well as faster emission occurs in the nanocrystals.

Nanoassembled plasmonic-photonic hybrid cavity for tailored light-matter coupling.

We propose and demonstrate a hybrid cavity system in which metal nanoparticles are evanescently coupled to a dielectric photonic crystal cavity using a nanoassembly method. While the metal constituents lead to strongly localized fields, optical feedback is provided by the surrounding photonic crystal structure. The combined effect of plasmonic field enhancement and high quality factor (Q approximately 900) opens new routes for the control of light-matter interaction at the nanoscale.

Plasmon-enhanced single photon emission from a nanoassembled metal-diamond hybrid structure at room temperature.

In this Letter we present the controlled coupling of a single nitrogen vacancy center to a plasmonic structure. With the help of an atomic force microscope, a single nanodiamond containing a single nitrogen vacancy center and two gold nanospheres are assembled step-by-step. We show that both the excitation rate and the radiative decay rate of the color center are enhanced by about 1 order of magnitude, while the single photon character of the emission is maintained. Hot spots between diamond and gold nanoparticles provide an efficient near-field coupling, despite the mismatch in size and shape. Our approach provides hybrid systems as important building blocks for novel nanophotonic light sources in advanced plasmonic devices stable even at room temperature.

Observation of size dependence in multicolor upconversion in single Yb3+, Er3+ Codoped NaYF4 nanocrystals.

In this Letter we report on the investigation of the upconversion emission of single NaYF(4) nanocrystals codoped with Yb(3+) and Er(3+). Single nanocrystals on a coverslip are excited with continuous wave laser light at 973 nm in a confocal setup and the upconversion fluorescence is analyzed with a spectrometer. With the help of an atomic force microscope the size of the nanocrystals is simultaneously determined. A strong size-dependence of the spectral properties of the upconversion signal of individual nanocrystals is observed. We attribute this to a differing number of available phonons in the individual crystals for multiphonon relaxation processes, depending on their size. We believe that this result provides a new strategy in the synthesis of upconversion nanoparticles with different spectral properties by changing only their size as it is well-known from the case of semiconductor quantum dots.

One-by-one coupling of single defect centers in nanodiamonds to high-Q modes of an optical microresonator.

In this letter, we present the on-demand coupling of single NV(-) defect centers in nanodiamonds to a polystyrene microspherical resonator. From an ensemble on a coverslip, we select single nanodiamonds containing a single defect proven by a pronounced antibunching dip. With the help of a scanning near-field probe, we can attach these nanodiamonds to a microsphere resonator one-by-one. A clearly modulated fluorescence spectrum demonstrates coupling of the single defect centers to high-Q whispering-gallery modes. Our experiments establish a toolbox to assemble complex systems consisting of single quantum emitters and (coupled) microresonators.