Population Control of Upconversion Energy Transfer for Stimulation Emission Depletion Nanoscopy

Upconverting stimulated emission depletion microscopy (U-STED) is emerging as an effective approach for super-resolution imaging due to its significantly low depletion power and its ability to surpass the limitations of the square-root law and achieve higher resolution. Though the compelling performance, a trade-off between the spatial resolution and imaging quality in U-STED has been recognized in restricting the usability due to the low excitation power drove high depletion efficiency. Moreover, it is a burden to search for the right power relying on trial and error as the underpinning mechanism is unknown. Here, a method is proposed that can easily predict the ideal excitation power for high depletion efficiency with the assistance of the non-saturate excitation based on the dynamic cross-relaxation (CR) energy transfer of upconversion nanoparticles. This allows the authors to employ the rate equation model to simulate the populations of each relevant energy state of lanthanides and predict the ideal excitation power for high depletion efficiency. The authors demonstrate that the resolution of STED with the assistance of nonsaturated confocal super-resolution results can easily achieve the highest resolution of sub-40 nm, 1/24^th of the excitation wavelengths. The finding on the CR effect provides opportunities for population control in realizing low-power high-resolution nanoscopy.