Femtosecond laser direct writing (FsLDW) technology has been widely used in high-quality micro–nano fabrication by scanning the ultrafast laser pulses with durations ranging from tens to hundreds of femtoseconds. In these applications, energy depositions induced by ultra-short pulse width occur at a time scale shorter than electron–phonon coupling processes in many materials. This feature can suppress the formation of heat-affected zone, resulting in the laser processing with high precision and resolution. In the FsLDW-based strategies, the subtractive and additive manufacturing are two of the mostly-adopted technologies. As a typical additive manufacturing technology, multi-photon lithography including the two-photon polymerization (2PP) is capable to form true 3D complex multi-element structures with sub-100-nm resolution. As an alternative approach to the conventional photolithographic patterning, the FLA process has also attracted considerable attention because it is a non-photolithographic, non-vacuum, on-demand, and cost-effective metal patterning fabrication route that can be applied to various substrates.

In order to fabricate advanced micro-nano devices with precise three-dimensional geometrical shape and complex functions, it is often required to combine different additive and subtractive manufacturing techniques. Although micro-/nano fabrication methods based on both 2PP and FLA are well established, considerable resistance still exists against their cooperation. The major reason lies in the incompatibility of material diversity and laser processing parameters. 2PP suffers from low material absorption cross-sections and a serial point-by-point writing strategy, resulting in low efficiency. FLA, on the other hand, is limited by its focal spot size, making large-area fabrication challenging without active focusing systems. Besides, hybrid manufacturing strategy in 3D direct laser writing usually requires switching between different equipment and/or adapting the laser processing setup. Therefore, the fabrication time consumption dramatically increases, resulting in existing works remaining at the laboratory research stage. Overcoming the inherent shortcomings of these two techniques will greatly facilitate the cost-effective and multi-scale fabrication of complex integrated devices. To address these challenges, Professor Chen Xie's team in Ultrafast Laser Laboratory (lead by Professor Minglie Hu) at Tianjin University, proposed a femtosecond adaptive optics-assisted hybrid additive-subtractive manufacturing technique. This technique successfully fabricated spoof surface plasmon polariton (SPP) waveguide devices exhibiting subwavelength confinement and localized field enhancement, with a more than 16-fold enhancement in fabrication efficiency. Relevant research results were recently published in Photonics Research, Volume 12, Issue 12, 2024. [Erse Jia, Chen Xie, Yue Yang, Xinyu Ma, Shixian Sun, Yanfeng Li, Xueqian Zhang, Minglie Hu, "Additive and subtractive hybrid manufacturing assisted by femtosecond adaptive optics," Photonics Res. 12, 2772 (2024)]

Figure 1 Design and matching manufacturing process of SPP waveguide devices. (a) and (b) Detailed geometric characteristics of SPP waveguide devices. (c) Manufacturing process flow of spoof SPP waveguide; the whole process is divided into two parts: additive manufacturing and subtractive manufacturing, with 2PP and FLA as the core technologies, respectively.

The process flow diagram of SPP waveguide devices based on the hybrid additive and subtractive manufacturing is shown in Fig. 1. This strategy utilizes spatial light modulation to generate Bessel beams. The Bessel beam features non-diffractive micrometer-scaled central lobes along a millimeter-scaled propagation distance outgoing the conventional Rayleigh range of Gaussian beams. During the additive manufacturing stage, the Bessel beam-assisted 2PP strategy enables the parallel direct writing of the SPP waveguide framework within 50 minutes. In the subtractive manufacturing stage, adaptive optics-assisted FLA is utilized to fabricate the excitation region of the millimeter-scale SPP waveguide device.

Collaborating with the researchers in Center for Terahertz Waves at Tianjin University, a fiber-scanning near-field terahertz microscopy system (SNTM) is used to test and validate the functionality of the on-chip terahertz waveguide devices fabricated with FsLDW. Experimental results confirmed the device exhibits transmission mode selectivity and strong confinement of the spatial sub-wavelength field. Compared to the commercial focused ion beam (FIB) methods, microscale defects induced in the FLA process have negligible impact on the SPP excitation. In addition, adaptive optics-assisted subtractive manufacturing dramatically reduces the fabrication time to 30 minutes (8 hours for FIB) with comparable device performance.

Professor Chen Xie, the corresponding author of this study, stated: "The adaptive optics have significantly empowered the laser-based additive and subtractive manufacturing respectively. In this hybrid manufacturing strategy, the diffraction-free nature of the Bessel beam is a key advantage. It significantly compensates the inherent inefficiencies of traditional 2PP technology and also spare the non-critical sample positioning, thus allowing fast patterning over large non-flat surfaces without the aid of any focusing feedback device."

In conclusion, femtosecond adaptive optics greatly facilitate the integration of additive manufacturing (2PP) and subtractive manufacturing (FLA). Subsequently, the team will continue to optimize the additive and subtractive hybrid manufacturing process, and this fabrication strategy will be applied in more fields such as complex terahertz on-chip integrated devices, flexible photonics and holographic metasurfaces.