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 λ) and a pattern period of 0.41 λ 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.

Gradient refractive index (GRIN) metamaterial lenses can achieve multi-band fusion infrared imaging and has the characteristics of integration, light weight, and achromaticity, compared with existing curved refractive lenses group. Constructing a three-dimensional (3D) GRIN lens for dual-band (3.0–5.0 and 7.5–9.2 µm) mid-infrared imaging is challenging due to the difficulty of fabricating the desired 3D GRIN materials with continuously changing linewidths. Here, we present a 3D self-focusing GRIN lens with a cylindrical symmetry configuration in the mid-infrared band. Such a 3D GRIN lens is designed with gradient woodpile polymer metamaterials based on effective medium theory and fabricated with high fidelity by grayscale two-photon lithography. Simulated and experimental results simultaneously exhibit a 3D GRIN lens possessing dual-band, achromatic, near-diffraction-limit focusing on the wavelengths of 4.5 and 7.5 µm. The protocol for developing the 3D GRIN lens with dual-band fusion imaging would prompt potential applications in integrated light-coupled devices and lightweight infrared imaging devices.

Asymmetric transmission (AT) materials allow light to pass through differently depending on its direction, which is important for optical devices like isolators, encryption, and solar cells. Here, we propose and experimentally demonstrate an approach to achieve AT at optical frequencies by using three-dimensional gradient spiral photonic crystals (PCs). By analyzing the iso frequency surface's response of spiral photonic crystals, we predict the AT of light at certain incident angles. We fabricate dielectric spiral PCs with different gradients of 1%, 11%, and 28% in the z-direction using the femtosecond laser direct writing technique. The predicted AT of light and the dependence of AT on the structural gradient amplitude due to the complex dispersion relations in PCs have been revealed through experimental measurements. These results demonstrate the potential of gradient 3D PCs in the development of diverse (AT) optical devices.

Digital micromirror device-based maskless projection lithography (DMPL) has emerged as a transformative nanopatterning platform, yet faces fundamental challenges in resolving high-density nanostructures due to optical diffraction constraints. The filtering characteristics of the pupil function and the phase modulation caused by defocus lead to pattern distortion and loss of high-frequency information, making it difficult to ensure consistency and distinguishability in dense pattern lithography. Here, we introduce multiple exposures (ME) to DMPL, developing a multiple-exposure without alignment solution to solve the problem of indistinguishable dense pattern exposure. At the same time, we also propose a novel method to maximize the spacing uniformity of the layout decomposition scheme based on the gradient descent algorithm. We further improved the depth of focus by maximizing the uniformity of spacing, achieving decoherence of sub-layouts within the laser field, thereby ensuring the printability of dense patterns. Simulations and experimental results verify that this approach significantly expands the process window and improves lithographic resolution. For one-dimensional dense lines, the minimum resolvable period decreases from 378 nm (half pitch ~ 0.5λ/NA) to 223 nm (half pitch ~ 0.3λ/NA). Furthermore, the metal layer layout of the integrated circuit chip has a minimum gap of 1 DMD pixel (75.6 nm in image plane) is achieved, demonstrating the efficacy of the proposed methods in addressing the challenges posed by dense layouts in DMPL applications. This work provides a paradigm-shifting solution to the persistent “density-fidelity trade-off” in digital nanofabrication, charting new pathways toward next-generation functional meta-devices.

The generation of supercontinuum spectra on integrated nonlinear optical platforms opens up new possibilities for applications in spectroscopy, metrology, signal processing, optical communications, and more. As an extremely promising low-loss nonlinear integrated photonics platform, there is an urgent need to achieve efficient and broadband supercontinuum generation (SCG) on thin-film lithium niobate (TFLN). However, most advancements in this field have not fully exploited the potential of χ(2) nonlinearity, leading to constraints in both spectral width and flatness. In this work, we introduce broadband quasi-phase matching into TFLN, effectively leveraging the synergistic effects of χ(2) and χ(3) nonlinearities. By combining cascaded χ(2) frequency upconversion, self-phase modulation (SPM), and dispersive wave (DW) generation, we have achieved, for the first time, a broadband, flat, three-octave-spanning SCG covering the ultraviolet–visible–NIR region (320–2500 nm, −30 dB) with microjoule-level energy in z-cut quasi-phase matching TFLN. The ultraviolet–visible spectral region consists of second–fourth harmonics, while the NIR region is composed of the spectrally broadened pump wave by SPM and DW generated by soliton fission, with a conversion efficiency of up to 31% from the pump to the ultraviolet–visible region. We conducted a comprehensive study on the critical roles of χ(2) and χ(3) nonlinearities in SCG and provided a systematic, in-depth analysis of the spectral evolution process. The experimental results show excellent agreement with numerical simulations. Our work paves the way for SCG across the full transparency window of LN.