利用準波導模態之多波長可調共振應用於中紅外表面增強紅外吸收

Abstract

本論文旨在提出以鍺銻碲化物(Ge₂Sb₂Te₅, GST)為可調介電層之金屬–絕緣體–金屬(Metal-Insulator-Metal, MIM)不對稱光柵結構，研究其中紅外波段(3–20 μm)的多模態共振調控與多峰可調應用。首先，利用能我折疊機制設計可激發高Q值準波導模態(Quasi-Guided Mode, QGM)之結構，並透過特徵頻率分析驗證其物理本質。藉由數值模擬系統性探討光柵線寬、間隙、GST 厚度與入射角等參數對間隙模態(Gap-Cavity, GC)、域性性表面電子共共振(Localized Surface Plasmon, LSP)、波導模態共振 (Guided Mode Resonance, GMR)與 QGM 等模態的影響，並比較對稱與不對稱結構於橫向電場(Transverse Electric, TE)與橫向磁場(Transverse Magnetic, TM)極化下的差異。實驗方面，本研究採用單次黃光微影製程完成樣品製作，並透過熱處理調控 GST 相變化，成功驅動多組共振峰產生紅移與分裂，展現出具結構與極化可調性的多峰光學響應。在分共檢測應用中，QGM 可與聚甲基丙烯酸甲酯 (Poly(Methyl Methacrylate), PMMA)的碳氧雙鍵(C=O)吸收峰有效耦合，顯示高靈敏表面增強紅外吸收(Surface-enhanced infrared absorption, SEIRA)能力，而 S1813 光阻因吸收峰分散未能顯現顯著增強，反映出多峰分共檢測的挑戰性。綜合而言，本研究提出之結構具備多模態、窄頻與多峰可調特性，並能以簡化製程實現大面積製作，對於未來可調紅外熱輻射與高效分共感測元件的發展具有重要參考價值。

關鍵字：中紅外、可調式熱輻射、不對稱光柵、GST相變化、SEIRA

An asymmetric metal–insulator–metal (MIM) grating incorporating a tunable Ge₂Sb₂Te₅ (GST) phase-change layer is proposed to realize multi-modal resonances and multi-peak tunability across the mid-infrared (3–20 µm), with applications in molecular sensing. Band folding of the dispersion produces high-Q quasi-guided modes (QGMs), whose physical origin is confirmed by eigenfrequency analysis. Numerical simulations systematically investigate the effects of grating linewidth, gap width, GST thickness, and incident angle on the gap-cavity (GC), localized surface plasmon (LSP), guided-mode resonance (GMR), and QGM responses under both TE and TM polarizations. We fabricate the structures in a single photolithography step. Subsequent thermal annealing drives GST to an intermediate crystalline state and induces red-shifts and splitting of multiple resonances, yielding tunable multipeak optical responses. In sensing experiments, QGM resonances couple to the C=O vibration of PMMA and provide high surface-enhanced infrared absorption (SEIRA) sensitivity. In contrast, S1813 photoresist shows little enhancement because its absorption is broad and dispersed. This result underscores the difficulty of analytes with multipeak vibrational spectra. Overall, the proposed design combines multi-modal, narrowband, and multi-peak tunable characteristics with simplified, large-area fabrication, providing a promising platform for reconfigurable mid-infrared emitters and high-performance SEIRA sensors.

Keywords: Thermal emitter, mid-infrared, phase-change material, asymmetric grating, SEIRA