Cascade upconversion: a strategy enabling four-photon lithography in weak light intensity

Multiphoton lithography offers the minimum feature size available in submicron scale true 3D printing, but excessive femtosecond laser intensity prevents it from leading to higher photon counts. To circumvent away this effect, we present a cascade upconversion strategy, which is a combination of two efficient two-photon upconversion processes to achieve four-photon photopolymerization. In order to demonstrate the advantages and feasibility of this approach, we combine excited state absorption upconversion using high concentration Ho³⁺/Yb³⁺ doped upconversion nanoparticles with triplet-triplet annihilation upconversion to fabricate 3D polymer structures by low-cost continuous wave 980 nm laser at 10⁵W/cm². This method overcomes the diffusivity caused by isotropy of nanoparticles luminescence, and achieves two orders of magnitude reduction in feature size while maintaining the advantages of true 3D printing (fast, freedom, high quality) and using near infrared light. This new strategy provides a general way for designing four-photon and even six-photon multiphoton lithography in weak light intensity.