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

Multiphoton lithography(MPL) offers the highest resolution 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 (CUC) 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 Ho ³⁺ /Yb ³⁺ doped upconversion nanoparticles (UCNPs) with triplet-triplet annihilation (TTA) upconversion to fabricate 3D polymer structures by low-cost continuous wave 980 nm laser at 10 ⁵ W/cm ² . This method overcomes the diffusivity caused by isotropy of UCNPs luminescence, and achieves two orders of magnitude improvement in resolution while maintaining the advantages of using near infrared(NIR) light. This new strategy provides a general way for designing four-photon and even six-photon MPL in week light.