引言#

三维微纳图案在光子学、电子学和生物医学工程等领域具有重要价值。传统的双光子吸收技术虽然能够制造高精度的三维图案，但通常需要昂贵的飞秒激光器以及具有大双光子吸收截面的特殊材料，限制了其广泛应用。相比之下，光子上转换技术能够将低能量的近红外光转换为高能量的紫外或可见光，且仅需使用价格亲民的连续波激光器，大幅降低了硬件成本并提升了能量转换效率。然而，以往基于上转换技术构建的三维结构大多是静态且不可逆的，缺乏动态可调控性。为了解决这一难题，研究团队设计了一套全新的多层近红外光响应纳米复合材料体系。通过结合计算机控制的三维样品台，该技术实现了对微结构的高精度直写、擦除与重构。如图1所示，该设计展示了上转换激光直写系统的物理过程以及不同光开关材料在多层膜结构中的多模态响应机制。

Fig.1 Upconversion direct laser writing using multilayers of NIR light-responsive nanocomposites. (a) Simulated intensity distribution of a 980 nm NIR laser beam at the focus. (b) Schematic illustration of the photophysical and photochemical processes of upconversion direct laser writing. At the laser focus, UCNPs absorb 980 nm light and emit UV light, triggering the photoisomerization of the photoswitches such as SP, DTE, and AZO. (c) Upconversion and UCNP-assisted photoisomerization. (d) Multilayers of NIR light-responsive nanocomposites for the fabrication of dynamic 3D patterns. The multilayers consist of a nanocomposite of DTE-UCNP, a nanocomposite of SP-UCNP, a nanocomposite of AZO-UCNP, and two glass spacers. A NIR laser is focused on the multilayers that are placed on a computer-controlled 3D sample stage. 3D patterns are prepared by moving the sample stage. (e) 3D patterns with different modes and functionalities. The patterns in the DTE-UCNP layer can be read out using color contrast and are erasable and rewritable under light irradiation; the patterns in the SP-UCNP layers can be read out using fluorescence signals and are self-erasable and photo-rewritable; the patterns in the AZO-UCNP layer can be read out using polarized light and are stable in ambient conditions.

主要实验及结论#

研究人员首先合成了平均粒径为47 nm的核壳结构UCNPs（NaYF4,Tm@NaYF4），并将其与螺吡喃分子均匀分散在聚PMMA基质中。如图2所示，透射电镜与扫描电镜图像证实了纳米颗粒在聚合物膜中的高度分散性。当聚焦的980 nm激光照射样品时，UCNPs吸收近红外光并发射出紫外光，诱导无色非荧光的螺吡喃分子异构化为具有紫色荧光的螺环开环体（MC）。实验表明，通过调控激光功率和照射时间，能够精准控制荧光斑点的尺寸，成功实现了最小3.2微米的加工分辨率。基于这一特性，研究人员在薄膜上成功写入了高分辨率的向日葵荧光图案。如图3所示，该图案不仅可以在530 nm可见光照射下快速擦除并在原位重写新图案，还具备在室温暗处40分钟内自动自擦除的动态特性，非常适合用于临时敏感信息的安全传输。

Fig.2 SP-UCNP nanocomposites and NIR light-induced SP-to-MC isomerization at the laser focus. Top-view (a) and cross-sectional (b) SEM images of a spin-coated film of an SP-UCNP nanocomposite. Scale bars: 1 µm. (c) Reversible photoisomerization of SP and MC. (d) Absorption and photoluminescence (PL) spectra of SP and MC (λex = 560 nm). (e) Absorption spectrum of SP and emission spectrum of UCNPs (λex = 980 nm, intensity = 2100 kW cm−2). (f) Emission spectra of UCNPs and SP-UCNP nanocomposites dispersed in chloroform (λex = 980 nm, intensity = 2100 kW cm−2). (g) Photograph of SP-UCNP nanocomposites dispersed in THF in a cuvette under the irradiation of focused 980 nm light (intensity at the focus = ∼500 kW cm−2). Photon upconversion generated a voxel at the focus. (h) Schematic illustration of upconversion laser direct writing of fluorescent MC spots in an SP-UCNP nanocomposite film. (i) Laser scanning confocal microscopy (LSCM) images of arrays of fluorescent MC spots prepared by upconversion laser direct writing of an SP-UCNP nanocomposite. The images were obtained by exciting the samples with 530 nm light and collecting signals from 600 to 700 nm. The laser power was 1–6 mW, and the irradiation time periods of the spots from left to right were from 0.2 to 5 s. Scale bar: 200 µm. (j) Spot diameters in the SP-UCNP nanocomposite film at different irradiation time periods and laser powers. The spot size was determined by the full width at half maximum of the intensity profiles from the LSCM images.

Fig.3 Writing, erasing, and rewriting of patterns in SP-UCNP nanocomposite films. (a) Schematic illustration of the writing, erasing, and rewriting processes. (b) Optical microscopy images of an SP-UCNP nanocomposite film before writing, after writing a sunflower pattern with focused NIR light (980 nm, 32 kW cm−2, 0.2 s for each spot), after erasing the pattern with visible light (530 nm, 42 mW cm−2, 30 s), and after rewriting a tulip pattern with focused NIR light. (c) LSCM images of a sunflower pattern that was kept in the dark for 0, 20, 40, and 60 min at room temperature. (d) LSCM images of an SP-UCNP nanocomposite film before writing, after writing a bus pattern, after self-erasing in the dark (1 h), and after rewriting an airplane pattern. Scale bars: 200 µm.

为了获得热稳定性更高的动态图案，研究团队进一步制备了掺杂二芳基乙烯的DTE-UCNP纳米复合薄膜。如图4所示，开环态DTE在接收到上转换紫外光后转变为闭环态，产生显著的颜色变化。研究人员利用该材料成功写入了蝴蝶与向日葵等吸收光谱对比鲜明的色彩图案。与螺吡喃体系不同的是，DTE形成的图案在室温下表现出极高的热稳定性，放置30天后图案依然保持清晰无衰减。同时，该图案同样支持通过可见光照射进行无损擦除与多次重写，兼具长期保存与光控动态重构的双重优势。

Fig.4 Fabrication of thermally stable yet photochemically dynamic patterns using DTE-UCNP nanocomposites. (a) Photoisomerization of DTE. (b) UV–vis absorption spectra of DTE in the open and closed forms. (c) UV–vis absorption spectrum of o-DTE and emission spectrum of UCNPs (λex = 980 nm, intensity = 2100 kW cm−2). (d) Optical microscopy images of a DTE-UCNP nanocomposite film before writing patterns, after writing a butterfly pattern with focused NIR light (980 nm, 64 kW cm−2, 0.5 s for each spot), after erasing the pattern with visible light (530 nm, 42 mW cm−2, 30 s), and after rewriting a sunflower pattern with focused NIR light. (e) Optical microscopy images of a lotus pattern in a DTE-UCNP nanocomposite film, which was kept for 0, 10, 20, and 30 days at room temperature. Scale bars: 200 µm.

Fig.5 Polarized patterns in AZO-UCNP nanocomposite films fabricated by upconversion laser direct writing. (a) Photoisomerization of AZO. (b) Schematic illustration of the fabrication of an array of polarized spots via upconversion laser direct writing in an AZO-UCNP nanocomposite film. (c) POM image of spot arrays prepared at different irradiation times (2–20 s) and laser powers (10–70 mW). Scale bar: 100 µm; A: analyzer; P: polarizer. (d) Diameters of the spots in the AZO-UCNP nanocomposite film at different irradiation times and laser powers. (e) Polarized Raman spectra of an AZO-UCNP nanocomposite film after irradiation with UV light (365 nm, 67 mW cm−2, 10 min) and polarized blue light (470 nm, 29 mW cm−2, 30 min). The angles 0° and 90° indicate that the orientation of the AZO groups is parallel and perpendicular to the polarization direction of the Raman microspectrometer. (f,g) Polarized Raman spectra of an oriented AZO-UCNP nanocomposite film under the irradiation of focused NIR light from 0 to 20 s at 0° (f) and 90° (g). (h) POM images of a peace dove pattern on an AZO-UCNP film before and after being stored for 150 days in ambient laboratory conditions. Scale bar: 100 µm. (i) POM images showing an oriented AZO-UCNP nanocomposite film (1) before irradiation, (2) after focused NIR light irradiation, (3) after UV irradiation, (4) after polarized blue light irradiation, (5) after focused NIR light irradiation, and (6) after another UV erasing, polarized blue light-induced orientation, and NIR light rewriting process. Scale bars: 200 µm.

此外，研究团队还将偶氮苯聚合物与UCNPs结合，开发了具备偏振编码功能的AZO-UCNP复合材料。如图5所示，通过依次进行紫外光预照射、偏振蓝光定向以及聚焦近红外光局域打乱分子取向的三步法，研究人员实现了极高精度的偏振图案绘制，其最小像素斑点直径达到1.15微米。偏振拉曼光谱测定证实，近红外激光有效地降低了局域偶氮苯分子的有序度参数。所制备的和平鸽偏振图案在自然环境下保存150天仍完好无损，且同样具备多次循环擦写的能力，展示出出色的偏振多模态加密性能。

Fig.6 3D patterns with multimode, erasable, and rewritable features. (a) Schematic illustration of upconversion laser direct writing of 3D patterns on three SP-UCNP layers separated by two glass spacers. (b) Side view of 3D patterns in three SP-UCNP layers imaged by LSCM. The top views of the 3D patterns are in Figures S42 and S43. (c) LSCM images showing three SP-UCNP layers before writing 3D patterns, after writing 3D patterns, and after erasing and rewriting 3D patterns. (d) Schematic illustration of multimode 3D patterns on a DTE-UCNP layer, an SP-UCNP layer, and an AZO-UCNP layer separated by two glass spacers. (e) Optical microscopy images of a pattern in the DTE-UCNP layer in the initial state, after storage in the dark for 30 and 60 min, and after two erasing and rewriting processes. (f) LSCM images of a pattern in the SP-UCNP layer in the initial state, after being stored in the dark for 30 and 60 min, and after two self-erasing and rewriting processes. (g) LSCM images of the pattern in the AZO-UCNP layer kept for 0, 10, 20, 30, and 40 days in a laboratory. Scale bars: 200 µm.

总结与展望#

本研究成功开发了一种基于上转换激光直写技术与近红外光响应纳米复合材料的动态三维微纳加工新平台。该工作巧妙地将连续波980 nm激光器与光开关分子的可逆异构化相结合，不仅摆脱了对昂贵飞秒激光体系的依赖，还赋予了材料在色彩、荧光和偏振三个维度上的可逆擦写能力。该技术在实现低成本、高精度加工的同时，大幅提升了三维图案的动态可重构性。未来，这种多模态动态三维光刻技术有望在高密度光存储、动态信息加密、可重构光子学器件以及智能生物响应基质等多个前沿领域发挥重要作用。