1 Introduction

Additive manufacturing represents a highly advanced processing method for micro/nano functional structures and devices.^[1-4] 3D nanolithography based on two-photon polymerization (TPP) stands as a representative technology in this field, widely recognized for its true 3D manufacturing capability and high printing resolution beyond the optical diffraction limit,^[5-8] which enables the high-precision fabrication of metamaterials,^{[9, 10]} sensors,^{[11, 12]} microrobotics,^[13-15] and nanowrinkled structures.^[16] In addition to the ultimate goals of high resolution and high throughput, miniaturization and low cost represent the recent trend in the advancement of TPP technology,^{[17, 18]} which aims to reduce entry barriers and facilitate the widespread adoption of 3D nanolithography technology. This is especially critical for modern application fields such as in situ manipulation,^[19] lab-on-fiber,^[20] and aerospace, where there is a demand for compact, lightweight, and cost-effective optical systems.

Nevertheless, current TPP technologies are associated with intricate and expensive optical systems.^[21-28] For example, a standard single-focus TPP system typically comprises an objective lens, a beam expansion system, and other necessary optical components, while a multi-focus TPP system for increasing processing throughput requires additional functional elements to be added to the optical system, such as a diffractive beam splitter or a microlens array (MLA). Although there have been reports of simplified nanoprinters,^{[17, 29, 30]} challenges including optical path accumulation, size, and weight, still have to be resolved to facilitate the further integration of TPP systems. As a planar optical device, metasurface aligned with the development trend of pursuing compact and integrated optical systems,^[31] such as spectrometer,^[32] Hartmann–Shack wavefront sensor,^[33] two-photon microscopic imaging,^[34] chip-scale metalens microscope,^[35] orbital angular momentum demultiplexer, and decoder,^[36] laser direct writing,^[37] fluorescence imaging.^[38] Meanwhile, it shows the powerful modulation ability of various optical parameters (such as phase, amplitude, polarization, wavelength, etc.) by elaborately designing the parameters of meta-atoms, such as material, structure, arrangement, etc.^[39-47] However, research on integrating advanced optical element metasurfaces into TPP systems is presently limited. The application of metasurfaces into TPP may lead to be cost-effective and hold substantial development potential. It is expected to simplify optical setups while achieving richer and more flexible printing functionalities.

Here, we proposed an ultracompact metasurface-based TPP (M-TPP) technique compared with traditional optical setups. As an illustration, in this study, metasurface was employed to replace various light field modulator components in a multi-focus TPP system, including a beam splitter, a dispersion compensating system, a beam expanding system, and an objective lens, which realized seven-foci parallel processing with high uniformity. The designed metasurface was specifically a multi-focus metalens (MFM) with a high numerical aperture (NA). Several different arbitrary models from 2D to 3D were printed to demonstrate the practicability of M-TPP. In addition, an MLA device with consistent focusing and imaging quality was fabricated, showcasing the application prospects of M-TPP. The compact and cost-effective M-TPP technique is expected to significantly simplify the optical setup of micro/nano additive manufacturing systems, which is expected to lead to widespread applications such as imaging, optical storage, advanced sensing, and optical encryption.

2 Results

The design concept of M-TPP is illustrated in Figure 1. Compared to commonly used light field modulator devices (such as diffractive optical element (DOE), digital micro-mirror device, spatial light modulator, etc), metasurfaces offer significant advantages such as the miniaturization platform and versatile integration, which forms the basis for constructing simplified and multi-functional TPP systems. Take the traditional multi-focus TPP system based on a DOE as an example (Figure 1a), a beam splitter DOE, a dispersion compensating telescope (DCT), a 4f beam expanding system, and an objective lens are necessary. In this study, a metasurface element MFM was utilized to replace all these optical components above, which greatly simplified the complexity of the existing system and was simple to operate, as shown in Figure 1b. The dimension occupied by the replaced optical components and their required optical path length was approximately several centimeters transversely and tens of centimeters axially. In contrast, the dimension of the MFM was only ≈2.2 cm × 2.2 cm transversely and ≈0.17 cm axially. Meanwhile, the total weight of the replaced optical components was on the order of kilograms, while the weight of the MFM was only 0.00025 kg. It is worth noting that the calculations for the dimension and weight of the MFM here include the glass substrate. The actual dimension and weight of the meta-atoms responsible for modulation will be even lower. Figure 1c shows the printing process of an array of 3D micro/nanostructures using a MFM, increasing processing throughput by seven times compared to conventional single-focus manufacturing. And the morphological uniformity among structures was high. Furthermore, Figure 1d shows the comparison between the planar optical device metalens and the conventional objective lens. In comparison to the conventional objective lens, the MFM exhibits smaller size and weight, and the manufacturing cost is expected to be substantially lower, benefiting from advancements in deep-ultraviolet immersion lithography and wafer-scale nanoimprint lithography,^[48] as well as inverse design method.^[49]

In the design of the MFM, phase retardations were achieved via polarization-independent SiN_x nanopillars based on transmission phase modulation, as shown in Figure 2a. The reason for the use of SiN_x was that it is almost transparent to laser at a wavelength of 780 nm and has a refractive index close to 2, much higher than normal glass materials. The finite-difference time-domain method was used to simulate and finally determine that the height (H) of the SiN_x nanopillars was 1150 nm, the period (P) was 520 nm, and the radius (R) was 95, 119, 138, 155, 173, 198, and 217 nm, respectively. Almost uniform seven-level phase modulation can be achieved under the premise of high amplitude transmission efficiency. Subsequently, the MFM was obtained by combining the established nanopillars library with the calculated phase distribution, which was fabricated through a combination of electron-beam lithography and inductively coupled plasma etching (see Note S5 and Figure S5, Supporting Information). Figure 2b–d presents the fabricated results of the MFM with a diameter of ≈1.04 mm, where the equivalent NA for each focus was 0.8 (see Note S1, Supporting Information).

We define the transmission efficiency as the ratio of the optical power at the back of the MFM substrate to the incident optical power ($$, see Figure S6 (Supporting Information), which was 80.13%. Meanwhile, the modulation efficiency was 73.28%, defined as the optical power at the back of the MFM substrate minus the zero-order optical power in the far field, divided by the incident optical power ($$, see Figure S6 (Supporting Information). Because in metalenses, not all transmitted light can be focused. Some incompletely modulated light may be diffracted to undesired angles, which was mainly zero-order light in the center. The high efficiencies ensured that the power at its seven foci exceeded the polymerization threshold. We utilized a microscopic system to characterize the focusing performance of the MFM (see Figure S7a, Supporting Information). Figure 2e shows measured focusing results for seven foci, we name them focus one to focus seven (F1–F7) for convenience. According to the focusing direction of different foci, 1D normalized intensity distribution was reasonably obtained (see Note S6 and Figure S8, Supporting Information) to calculate full width at half maximum (FWHM). Figure 2f displays normalized intensity profiles and 1D normalized intensity distributions corresponding to F1. The measured FWHM of seven foci were 500, 530, 529, 551, 534, 536, and 541 nm, respectively, with a uniformity of 95.15%, as shown in Figure 2g. The experimental focusing results for seven foci of the MFM are shown in Figure S9 (Supporting Information). In addition, the uniformity of normalized total intensity (see Note S7 and Table S1, Supporting Information) of each focus was 95.21%. The experimental results prove that the MFM has excellent focusing uniformity, suggesting that the MFM can maintain high parallel processing consistency (see Note S8 and Figure S10 in Supporting Information).

To investigate the ultimate processing capacity of M-TPP, a suspended nanowire was fabricated as shown in Figure 3a,b). The lateral and axial sizes of the thinnest linewidth were 100.8 and 159.7 nm ($$), respectively. It demonstrates that M-TPP can fabricate 3D micro/nanostructures with sub-diffraction-limited feature sizes. Subsequently, 2D and 3D micro/nanostructure arrays with exceptional uniformity were printed to showcase the arbitrary models manufacturing capability of M-TPP. As shown in Figure 3c, we printed the WNLO (Wuhan National Laboratory for Optoelectronics) logo array, which exhibited a favorable appearance. The SEM images demonstrate that M-TPP excels in 2D model forming. Besides, we printed an Arabic numerals array, and numerals 1–9 within the white dashed box in Figure 3d were printed using a single focus. This confirms the hexagonal topological splice capability of M-TPP. Movie S1 (Supporting Information) displayed its real-time printing process. Furthermore, this splicing method was not restricted to printing only nine models using a single focus, it can be extended to printing continuous structures and any quantity of models using a single focus (see Figure S11, Supporting Information). 2.5D models of the flower and the Great Wall of China were printed as shown in Figure 3f–i. The detailed grain structure of the petals in Figure 3g demonstrates the capability of M-TPP to print intricate models with fine details. 3D models of the bridge with cavity and microcantilever with overhanging part were printed as shown in Figure 3j–m, which demonstrates the 3D nanolithography capability of M-TPP.

We printed an MLA device with a hexagonal topology arrangement to verify the practical application potential of M-TPP. MLAs were widely used in optical sensors,^[59] integral imaging,^[60] and other fields due to their high integration and consistency. For analytical convenience, we selected the printing of a single microlens using a single focus as an example to fabricate an MLA. Figure 4a shows SEM images of the fabricated MLA. IP-S was used as the material of the MLA, which has a high proximity effect, low shrinkage, and is almost transparent to visible light. The illustrations show that the fabricated microlens had smooth surfaces and stable 3D structures. The measured diameters of each microlens are 58.68, 58.29, 58.34, 58.28, 58.30, 58.37, and 58.31 µm (the uniformity is 99.66%), closely matching the design diameter of 56 µm. We increased the laser power to ensure sufficient polymerization, resulting in the diameter measurement values being slightly larger than the design value, but this hardly affects the optical performance of the MLA. Figure 4b presents the focusing performance results of the MLA (the measurement system is shown in Figure S7b (Supporting Information), where a uniform focus array can be clearly seen. Subsequently, Figure 4c,d) shows the imaging photographs of the letter “H” by the fabricated MLA, achieving clear imaging functions. Figure S18 (Supporting Information) presents lateral 1D normalized intensity distributions corresponding to lenses 1–7, respectively. FWHM was 1.554, 1.575, 1.629, 1.606, 1.587, 1.58, and 1.609 µm, respectively, with a uniformity of 97.64%. Figure S19 (Supporting Information) presents axial 1D normalized intensity distributions corresponding to lenses 1–7, respectively. Focal length was 78, 78, 82, 82, 80, 82, and 78 µm, respectively, with a uniformity of 97.5%. The design focal length of the plano-convex lens is calculated to be ≈69 µm using the formula $$, where the radius of curvature (RC) is 35 µm and the refractive index of IP-S (TPP solid) is 1.507 @ 780 nm. There are two main reasons why the measured value is slightly greater than 69 µm. On one hand, there are no specific parameter requirements, only to verify the optical performance of the MLA. Therefore, we did not optimize the surface shape in the fabrication process, resulting in a larger radius of curvature. On the other hand, measurement errors also contributed to this discrepancy. In short, the focusing and imaging results demonstrate a consistent optical quality of the fabricated MLA due to the excellent performance of M-TPP.

The above results prove that replacing a high-end objective lens with a metalens brings integration and functionality enhancement, but it also leads to some performance degradation. First, an objective lens can compensate for spherical aberration by rotating a single correction collar, while the MFM lacks flexibility for adjustment according to actual printing conditions. Second, an objective lens is typically designed with broadband achromatic capability, making it compatible with nonlinear optical characterization techniques such as coherent anti-stokes Raman scattering, enabling in situ monitoring of the printing process. However, the MFM only has a 10 nm bandwidth centered at 780 nm. It is challenging to integrate it with advanced nonlinear optical characterization techniques. Third, an objective lens can achieve diffraction-limited focusing under light incident at different angles, thereby increasing printing throughput by using galvanometers. However, the MFM is currently designed only for normal incidence conditions, which makes it limited to the scanning mode based on a piezo stage. In the future, the performance of MFM can be further improved through tunable metasurface methods, advanced metalens chromatic aberration correction techniques, and optimization of the electromagnetic response characteristics of meta-atoms at different incident angles. This will advance the widespread application of metasurface platforms in TPP technology.

3 Conclusion

In conclusion, we have proposed and demonstrated a highly integrated and functional M-TPP technique. The metasurface provided a promising platform for the development of laser precision engineering which is not limited to the proposed MFM in this work. It is expected to realize almost any desired light field modulation based on this platform, and the processing mode can be expanded from the scanning method to the plane projection method, and even the holographic projection method. Meanwhile, M-TPP can flexibly switch functions according to demands. If the functions of traditional systems need to be switched between different printing modes (such as multi-focus printing, projection printing, photoinhibition printing, etc.), it is often necessary to make numerous changes and recalibrations to the optical setup, which is both troublesome and time-consuming. In contrast, on the metasurface platform, it is easy to choose suitable printing functions by simply replacing metasurface elements with different functions, akin to the way commercial microscopes interchange objective lenses. It is believed that a highly integrated and versatile nanoprinter will be established based on the flat optical devices of metasurfaces, which can promote the widespread adoption of laser micro/nano 3D printing technology in scientific research and industry.

4 Experimental Section

Conflict of Interest

X.W., X.F., Y.L., K.X., F.C., X.Y., H.G., and W.X. are inventors on a patent application related to this work filed by Huazhong University of Science and Technology (no. CN116430678A, filed 23 March 2023). The authors declare that they have no other competing interests.