Multiphoton upconversion in rare earth doped nanocrystals for sub-diffractive microscopy

Summary form only given. Overcoming the spatial resolution limitations in optical microscopy due to the diffraction of light has become a major challenge in modern microscopy. To overcome this limit, we demonstrate here the upconversion process in rare-earth-doped fluoride nanoparticles (RE:NPs) that may serve as novel sources for fluorescent microscopy. RE:NPs have been reported to serve as efficient biolabels for cellular and small animal imaging [1]. While almost all work devoted to RE doped nanocrystals in biological applications had investigated NaYF₄, here we performed experiments with LaF₃ and KY₃F₁₀, other possible materials to host upconversion. Specifically, (2at.%) Er³⁺, (2at.%) Yb³⁺: KY₃F₁₀, (1at.%) Tm³⁺, (6at.%) Yb³⁺: LaF₃ and (0.5at.%) Tm³⁺, (12at.%) Yb³⁺: KY₃F₁₀ 20-50 nm-sized crystals had been characterized using AFM, STEM, X-ray diffraction and dynamic light scattering techniques and investigated by luminescence spectroscopy and nonlinear optical microscopy [2]. The latter was done on NP´s dispersed in PEG and spun-coated on a glass cover slip. One of the fluorescence images produced with upconversion process in these nanoparticles is presented in Fig. 1. The increase of the photon process order and the corresponding decrease in the full width of half-maximum (FWHM) of intensity point spread function (IPSF) value can be clearly observed. As the result of the photon process order, N, increase, the instrument´s optical resolution becomes: Δx, ΔY ≅ 0.61λ/NA√N (1) where λ is the excitation wavelength and NA is the objective´s numerical aperture.Whereas the resolution of a mul tipho ton microscopy does not exceed λ 3 in the lateral directions [3], in our study we demonstrate that the FWHMIPsF can be down-scaled up to 297nm for a two-- hoton process, 239nm for a three-photon process, and /90nm for a four-photon process, corresponding respectively to λ 3.29 , λ 4.29 and λ 5.15 improvement in the resolution. Our experimental results demonstrate several advantages of RE:NPs nanoparticles over conventional bio and medical imaging probes: low backgrounds noise due to near infrared excitation, low power (μW) excitation, and absence of photo-blinking and pho tobleaching. This technique together with the detection of the two-, three-, and four-photon processes at single particle level open new opportunities in single-molecule tracking applications in biology as well as novel low-cost high-resolution optical microscopy systems.