Gengxu Chen

Ultrasensitive Polarized Up-Conversion of Tm3+–Yb3+ Doped β-NaYF4 Single Nanorod

Up-conversion luminescence in rare earth ions (REs) doped nanoparticles has attracted considerable research attention for the promising applications in solid-state lasers, three-dimensional displays, solar cells, biological imaging, and so forth. However, there have been no reports on REs doped nanoparticles to investigate their polarized energy transfer up-conversion, especially for single particle. Herein, the polarized energy transfer up-conversion from REs doped fluoride nanorods is demonstrated in a single particle spectroscopy mode for the first time. Unique luminescent phenomena, for example, sharp energy level split and singlet-to-triplet transitions at room temperature, multiple discrete luminescence intensity periodic variation with polarization direction, are observed upon excitation with 980 nm linearly polarized laser. Furthermore, nanorods with the controllable aspect ratio and symmetry are fabricated for analysis of the mechanism of polarization anisotropy. The comparative experiments suggest that intraions transition properties and crystal local symmetry dominate the polarization anisotropy, which is also confirmed by density functional theory calculations. Taking advantage of the REs based up-conversion, potential application in polarized microscopic multi-information transportation is suggested for the polarization anisotropy from REs doped fluoride single nanorod or nanorod array.

Intense multiphoton upconversion of Yb3+–Tm3+ doped β-NaYF4 individual nanocrystals by saturation excitation

Saturation excitation of individual nanocrystals represents a powerful strategy to explore unique optical characteristics of luminescent activators. Here we report the intense multiphoton upconversion of β-NaYF4: Yb3+–Tm3+ individual nanocrystals benefiting from the perfect ladder-type electron configuration of Tm3+ under saturation illumination excitation. This leads to a luminescence switching between the emissions from 2-photon upconversion and 4-photon upconversion, which exhibits significantly enhanced luminescence intensity by up to a factor of 70. Such intense multiphoton upconversion may have potential application in super-resolution imaging.

Enhanced Photoluminescence of Single-Photon Emitters in Nanodiamonds on a Gold Film

Engineering the photo-emission characteristics of single-photon emitters is crucial for the growing field of quantum technologies. We show that the photoluminescence of a single nitrogen-vacancy color center in nanodiamond could be significantly enhanced when attached to a gold film of 50 nm with preserving photophysical properties as a single photon source. The nitrogen-vacancy center on the gold film showed a decreased excited-state lifetime and a modified saturation characteristic. The photoluminescence enhancement was due to the enhanced electric field by the surface plasmon and the energy transfer from the excited state of nitrogen-vacancy centers to the surface plasmon.

Polarization properties of surface plasmon enhanced photoluminescence from a single Ag nanowire.

Metallic nanowires are of great research interest due to their applications in surface plasmon polariton coupling of light. The efficiency is much dependent on the polarization of the light due to the phase matching requirement in the light-surface plasmon polariton coupling. By scanning confocal microscope, the photoluminescence from a single Ag nanowire was demonstrated strongly dependent on the excitation laser polarization, showing good consistency with the theoretical simulation. Meanwhile strong avalanche photoluminescence from a single Ag nanowire was observed when the excitation laser was polarized along the long axis of the Ag nanowire. The photoluminescence emission exhibited a polarization-sensitive spatial distribution. This may stimulate promising applications in designing polarization-controllable nanoscale plasmonic devices.

Magnetic-based Fano resonance of hybrid silicon-gold nanocavities in the near-infrared region

Direct interference between the orthogonal electric and magnetic modes in a hybrid silicon-gold nanocavity is demonstrated to induce a pronounced asymmetric magnetic-based Fano resonance in the total scattering spectrum at near-infrared frequencies. Differing from the previously reported magnetic-based Fano resonances in metal nanoparticle clusters, the narrow discrete mode provided by the silicon magnetic dipole resonance can be directly excited by external illumination, and greatly enhanced electric and magnetic fields are simultaneously obtained at the Fano dip.

Fabrication of nitrogen vacancy color centers by femtosecond pulse laser illumination

We report on a novel method to fabricate single, multiple and large-area high-density ensembles of nitrogen vacancy (NV) color centers in synthetic type Ib bulk diamond by femtosecond laser illumination. Electron beams generated in propagation of intense infrared laser pulses in air sputtered on a diamond sample under high temperature aroused by the laser illumination, creating NV color centers. Typical photoluminescence (PL) spectra of NV centers could be observed on the illuminated spots. Photon streams from individual photoluminescent points exhibited anti-bunching effect by the second-order correlation measurement, evidencing single and multiple photon-emitters around the laser illuminated spots.

Photoluminescence Enhancement Dependent on the Orientations of Single NV Centers in Nanodiamonds on a Gold Film

We studied the orientations of single Nitrogen-Vacancy (NV) color centers in nanodiamonds on a gold film by defocused imaging with a scanning confocal microscopy system and an electron-multiplying charge-coupled device. It was demonstrated that the perpendicular oriented dipole of single NV color center showed shorter fluorescence lifetime and higher fluorescence intensity than any other oriented ones on account of higher coupling efficiency of local electronic field induced by the surface plasmon resonance on the gold film for a vertical transition dipole.

Morphology and polarization-dependent second harmonic generation in single hexagonal sodium niobate micro/nano-crystals

We synthesized NaNbO3 micro/nano-crystals with six morphologies and three different crystal structures using the hydrothermal method. Each single hexagonal NaNbO3 micro/nano-crystal (samples No. 1–3) exhibited a strong second-order nonlinear response. By calculating the average intensity and measuring the polarization response of each of these hexagonal crystals (samples No. 1–3), it was demonstrated that the same crystal structure displays a similar optical second harmonic generation (SHG) response. Furthermore, SHG was shown to be independent of the morphology and identical for similar crystals, which is expected for a volume effect with a second-order susceptibility tensor depending on the properties of the material. The optical SHG properties of these NaNbO3 micro/nano-crystals together with the previous related research suggest that these non-toxic crystals may find promising applications in biological imaging and related fields.

Tip-Enhanced Second Harmonic Generation From a Single NaNbO 3 Nanocrystal

We demonstrated the enhanced second harmonic generation (SHG) from a single sodium niobate (NaNbO3) nanocrystal by approaching a metal tip on top of it. By scanning a laser-focused NaNbO3 nanocrystal with a gold-coated tip, the mapping of the enhanced SHG from the nanocrystal was obtained. We observed that the SHG of the single NaNbO3 nanocrystal was strongly enhanced with the gold-coated tip on the top of it. And the enhancement factor showed strong dependence on the excitation polarization and the excitation power. An over 12-fold tip enhancement was observed when the excitation power was 12 mW.

Fluorescence Polarization Switching from a Single Silicon Vacancy Colour Centre in Diamond.

Single-photon emitters with stable and uniform photoluminescence properties are important for quantum technology. However, in many cases, colour centres in diamond exhibit spectral diffusion and photoluminescence intensity fluctuation. It is therefore essential to investigate the dynamics of colour centres at the single defect level in order to enable the on-demand manipulation and improved applications in quantum technology. Here we report the polarization switching, intensity jumps and spectral shifting observed on a negatively charged single silicon-vacancy colour centre in diamond. The observed phenomena elucidate the single emitter dynamics induced by photoionization of nearby electron donors in the diamond.