Fig. 1: Parallel photon avalanche in Ho3+ enables multicolor nanoscopy under 965 nm continuous-wave excitation.

Top: Schematic diagram of the parallel photon avalanche (PA) mechanism. Rare population on dual reservoir levels (⁵I₆ and ⁵I₇), originated from phonon-assisted GSA (multiphonon relaxation process is indicated by curved lines with an arrow), would increase exponentially at their excitation thresholds through the energy-looping process, which is composed of excited-state absorption (ESA) from reservoir level to emissive level (⁵F₅ and ⁵S₂/⁵F₄) as well as cross relaxation (CR) between emissive level and ground state (⁵I₈). The energy-looping process involved in green–blue (red) PA is indicated by clockwise circles with cyan (red) color. GSA, ground-state absorption. Solid line with an arrow, absorption or emission; dashed lines with an arrow linked by a thinner dashed line, cross relaxation. Middle–Bottom: Schematic sketch of multicolor sub-diffraction imaging enabled by PA emissions in red (R) and green–blue (GB) channels at different excitation power density (P_ex). I_em, emission intensity. For each imaging channel, the optimal imaging resolution of the designed nanoparticle is achieved at the P_ex, indicated by dotted lines, where the function of I_em to P_ex has the greatest slope. Diffraction limit is about 333 nm (full width at half maximum) when applying an objective lens with a numerical aperture of 1.45. White scale bar, 100 nm. Note that all the images are artificially drawn ones to help readers’ understanding