Two-photon absorption under few-photon irradiation for optical nanoprinting

Two-photon absorption (TPA) has been widely applied for three-dimensional imaging and nanoprinting; however, the efficiency of TPA imaging and nanoprinting using laser scanning techniques is limited by its trade-off to reach high resolution. Here, we unveil a concept, few-photon irradiated TPA, supported by a spatiotemporal model based on the principle of wave-particle duality of light. This model describes the precise time-dependent mechanism of TPA under ultralow photon irradiance with a single tightly focused femtosecond laser pulse. We demonstrate that a feature size of 26 nm (1/20 lambda) and a pattern period of 0.41 lambda with a laser wavelength of 517 nm can be achieved by performing digital optical projection nanolithography under few-photon irradiation using the in-situ multiple exposure technique, improving printing efficiency by 5 orders of magnitude. We show deeper insights into the TPA mechanism and encourage the exploration of potential applications for TPA in nanoprinting and nanoimaging.