Amplified stimulated emission in upconversion nanoparticles for super-resolution nanoscopy

Area of Science:

• Nanotechnology
• Materials Science
• Optics

Background:

• Lanthanide-doped materials are crucial for laser applications due to their metastable energy levels.
• Upconversion nanoparticles (UCNPs) offer tunable luminescence and single-nanoparticle sensitivity for advanced imaging.
• Thulium-doped UCNPs are particularly interesting for their potential in optical applications.

Purpose of the Study:

• To investigate the potential of high-concentration thulium-doped UCNPs for super-resolution microscopy.
• To explore the mechanism of population inversion and amplified stimulated emission in these UCNPs.
• To demonstrate the application of these UCNPs in low-power stimulated emission depletion (STED) microscopy.

Main Methods:

• Fabrication of lanthanide-doped upconversion nanoparticles with controlled thulium ion concentration.
• Excitation of UCNPs at 980 nm to induce population inversion on the 3H4 level via cross-relaxation.
• Application of 808 nm laser to trigger amplified stimulated emission and optical inhibition of upconversion.
• Implementation of low-power STED microscopy using engineered UCNPs for super-resolution imaging.

Main Results:

• High Tm3+ doping concentration in UCNPs leads to intense cross-relaxation and a photon-avalanche effect.
• Population inversion on the 3H4 level is readily established within single nanoparticles.
• Optical inhibition of blue luminescence upconversion pathway is achieved.
• Super-resolution STED microscopy with 28 nm resolution (1/36th wavelength) is demonstrated.

Conclusions:

• Engineered thulium-doped UCNPs can achieve population inversion and amplified stimulated emission at low pump power.
• These UCNPs offer significantly lower saturation intensity compared to conventional fluorescent probes for STED microscopy.
• This work presents a novel approach to overcome resolution limits in STED microscopy, enabling nanoscopy with engineered nanocrystals.