Two-photon absorption (TPA), a technique that is widely used in optical nanoprinting, cannot achieve high speed and high resolution at the same time. High light intensity boosts printing speed in TPA, but degrades resolution and risks damaging materials. Low light intensity preserves resolution, but significantly slows the printing process. To encompass both high speed and high resolution in TPA, researchers at Jinan University, in collaboration with the Chinese Academy of Sciences, developed few-photon two-photon absorption (fpTPA). The technique, for implementing TPA under ultralow photon irradiance using a single, tightly focused femtosecond laser pulse, enabled the researchers to achieve efficient two-photon absorption even under ultralow photon exposure. Obtaining precise control of the number of photons within the femtosecond laser pulses is central to the success of the technique. By improving the speed and resolution of nanoprinting, the technique supports improved results in support of optical, microelectronic, and biomedical applications. Additionally, its compatibility with standard digital optical projection systems makes it a cost-effective and scalable solution for next-generation nanomanufacturing. The technique is based on the principles of wave-particle duality and the spatiotemporal uncertainty of photons that is inherent to femtosecond laser pulses. It leverages the wave-particle duality of light, coordinating femtosecond pulses to ensure that two photons are absorbed by a molecule in rapid succession, even at very low photon fluxes. Time-dependent quantum mechanism of two-photon absorption. Courtesy of Nature Communications (2025). DOI: 10.1038/s41467-025-57390-9. The researchers also created a space-time model that describes the quantum-mechanical interaction pathways under such sparse photon conditions. The spatiotemporal model illustrates the precise, time-dependent mechanism of TPA under ultralow irradiance with a focused femtosecond laser pulse. Through simulations using the model, the researchers determined that the probability of TPA depends strongly on the lifetime of the molecule’s virtual state under few-photon irradiation. Unlike traditional TPA, which relies on robust, continuous photon fields, fpTPA enables highly localized, two-photon events within nm-scale regions, breaking through the diffraction limit dictated by classical optics. To validate the fpTPA concept and spatiotemporal model, the researchers integrated fpTPA with two-photon digital optical projection lithography. They achieved feature sizes as small as 26 nm, equivalent to 1/20 of the operating wavelength. The two-photon digital optical projection lithography system improved throughput by five orders of magnitude, enabling large-scale, high-resolution structures to be printed rapidly — essentially ending the trade-off between resolution and efficiency in TPL. Additionally, the researchers developed the in-situ digital multiple exposures (iDME) method, enabling fine, dense, complex patterning with two-photon digital optical projection lithography. By sequentially exposing multiple digital mask patterns, they fabricated dense nanostructures with a periodicity of just 210 nm (around 0.41 × the wavelength) — well below traditional optical limits — without sacrificing structural integrity. The researchers used the technique to fabricate micro- and nanostructures including optical waveguides, microring resonators, and complex biomicrofluidic channels. The variety of structures created by the team showed that fpTPA could be used broadly across the optical communications and biomedical engineering sectors. “This technology fundamentally changes how we think about two-photon processes,” professor Yuanyuan Zhao said. “It opens up a practical and efficient route to achieve ultrahigh-resolution fabrication without compromising throughput, paving the way for new advances in microelectronics, photonics, and biomedicine.” The research was published in Nature Communications (www.doi.org/10.1038/s41467-025-57390-9).