一、导读

金属谐振器允许操纵亚波长体积的电磁场，以实现强光物质相互作用，支撑新兴的平面光学器件。设计具有优化有效模体积(V_eff)的超材料以激励和最大化地利用高密度光子是传感应用中非常理想的。

2024年7月，福建省太赫兹功能器件与智能传感重点实验室在国际知名光学期刊《Optics and Laser Technology》（中科院2区TOP，IF:4.6）发表了题为“Femtosecond laser direct writing wedge metallic microcavities for terahertz sensing”的文章。论文提出了一种具有低V_eff的领结型太赫兹金属孔径超材料结构，用于增强生物传感。实验室2021级博士研究生林廷玲为本论文的第一作者，2022级博士研究生曾秋铭为本论文的共同第一作者，实验室主任钟舜聪教授和实验室黄异老师是本文的共同通讯作者。

二、内容简介

本研究提出的楔形金属微腔是通过飞秒激光直写加工而成。重要的是，该器件利用飞秒激光制造工艺的固有特性，通过定制脉冲能量实现低至3 μm 的楔形腔，其传感性能优于传统的梯形腔。

Figure 1.Wedge metallic microcavities for THz sensing enhancement. (a) Schematic of the bowtie-type metamaterial sensor structure and the geometrical parameters of the unit cell. Microscopic images of fabricated sample, which show the top and bottom cell views, respectively. (b) Cross-sectional view of the |E| profiles of the wedge (left) and trapezoidal (right) cavity structure. (c) |E| on the bottom surface (z= 0) atx= 0 (left) andy= 0 (right). (d) Normalized sensitivity S_nand V^N_effof the structure with varied gap ratio (g_b/g_t). Nonlinear fits to the data points were performed, with all R²> 0.99.

楔形金属微腔的器件加工结果表征显示出与模拟结果高度吻合，证明了更低的飞秒激光脉冲能量可以实现更小微腔的加工，其具有更高的电场增强。

Figure 2.Design optimization of the wedge metallic microcavities. (a) Sketch of the ultrafast femtosecond laser machining process. (b) Microscopic images of bowtie-type metamaterials with different wedge gap sizes realized by adjusting the laser energy. (c) Distribution of |E| for differentg_batz= 0. (d) |E| on the bottom surface atx= 0 andy= 0. (e) Experimental and simulated transmittance spectra for differentg_b. Q factor are presented for each resonance.

通过搭载液体样品池来利用液体石蜡表征了楔形金属微腔的折射率传感能力，其中3 μm楔形腔实现了最高0.654 RIU^-1的归一化折射率灵敏度以及4.643的品质因数，实验证实了更小的楔形腔具有更优的传感性能。

Figure 3.Refractive index sensing using bowtie-type THz metamaterials with different sizes. (a) Schematic diagram of metamaterials sensing based on a liquid sample cell. (b) Experimental transmittance spectra in response to liquid paraffin, where the thickness of the spacer is 10 μm. (c) Simulated transmittance spectra corresponding to (b), using the same refractive index of analyte (n = 1.49). (d) Extracted FOM from experimental and simulated data as well as V^N_effof the resonator as a function of parameterg_b. The FOM increases with decreasingg_b, demonstrating the capability of reducing the wedge gap to improve the sensing performance.

为了验证所设计的优化楔形金属微腔具有的生物分子传感能力，以 L-脯氨酸作为检测目标，采用滴干检测的方法，最终实现了最低浓度为0.01 mg/mL的脯氨酸分子的检测。

Figure 4.Optimized bowtie-type metamaterial for biomolecular sensing.(a) Schematic of the THz metamaterial biosensor for L-proline detection. (b) Measured transmittance spectra of the biosensor for different L-proline concentrations. (c) Extracted the frequency shifts of resonance. (d) Calculated refractive indices at different concentrations of L-proline and a fitting curve with R²= 0.98.

三、总结

论文展示了将飞秒激光加工特性与亚波长金属孔径结构相结合的力量。楔形微腔允许更强的限制电磁辐射，克服了飞秒激光脉冲直接烧蚀制造中逐渐变细的缺点，使金属孔径超材料成为实现极端光学操纵的可靠和有效平台。通过调整激光脉冲能量以优化穿透深度，实现了最小间隙为3 μm的楔形微腔，使V^N_eff达到 9.8 × 10⁻⁷可增强液体传感与具有较大间隙的配置相比。因此，具有增强光-物质相互作用的优化楔形微腔被证明可产生 0.675 RIU⁻¹的超高归一化灵敏度以及检测低至0.87 nmol的L-脯氨酸的能力。结果强调了具有优化楔形腔的金属孔径超材料在与加工精度相匹配的飞秒激光加工相结合时的重要应用前景，特别是对于技术相关的太赫兹光谱区域。

原文链接：https://doi.org/10.1016/j.optlastec.2024.111434