Fig. 1
3D sub-diffraction resolution achieved by SEE microscopy. a Sub-diffraction resolution can be achieved in a conventional confocal microscope when using super-linear emitters, in our case upconversion nanoparticles (UCNPs of NaYF4: 20% Yb, 8% Tm). b The emission from the UCNPs at λ = 455 nm, exhibits a strong super-linear dependence on the excitation power, with a maximum of slope s = 4.1 in the low-power range. The open square symbols represent the peak power densities at which the measurements in panel (d-e) were obtained. c Experimentally, the diffraction limit of the setup is determined from the PSF of the excitation beam, obtained by scanning a gold particle through the beam and recording the scattering intensity at each point. The diffraction-limited lateral (x direction) resolution is 408 nm and the axial (z direction) resolution is 977 nm as shown in the corresponding cross sections. In contrast, when using non-linear emitters (d-e), the size of the PSF, representing the resolution, changes depending on the excitation power. d In the low-power super-linear range we achieve sub-diffraction imaging (uSEE microscopy), with lateral resolution 216 nm (λ/4.5 compared with the diffraction limit of λ/2.4) and axial resolution 542 nm (λ/1.8 compared with the diffraction limit of λ). e In the high-power saturation regime the resolution is worse than the diffraction limit. The dashed black box in (d) indicates the span of the experimentally measured area. The area outside of the black box contains a flat background equal to the average background in the measured area. This facilitates the visual comparison of panels (c–e)
