Plasmonic nanostructures are widely utilized in surface-enhanced Raman spectroscopy (SERS) from ultraviolet to near-infrared applications. Periodic nanoplasmonic systems such as plasmonic gratings are of great interest as SERS-active substrates due to their strong polarization dependence and ease of fabrication. In this work, we modelled a silver grating that manifests a subradiant plasmonic resonance as a dip in its reflectivity with significant near-field enhancement only for transverse-magnetic (TM) polarization of light. We investigated the role of its fill factor, commonly defined as a ratio between the width of the grating groove and the grating period, on the SERS enhancement. We designed multiple gratings having different fill factors using finite-difference time-domain (FDTD) simulations to incorporate different degrees of spectral detunings in their reflection dips from our Raman excitation (488 nm). Our numerical studies suggested that by tuning the spectral position of the optical resonance of the grating, via modifying their fill factor, we could optimize the achievable SERS enhancement. Moreover, by changing the polarization of the excitation light from transverse-magnetic to transverse-electric, we can disable the optical resonance of the gratings resulting in negligible SERS performance. To verify this, we fabricated and optically characterized the modelled gratings and ensured the presence of the desired detunings in their optical responses. Our Raman analysis on riboflavin confirmed that the higher overlap between the grating resonance and the intended Raman excitation yields stronger Raman enhancement only for TM polarized light. Our findings provide insight on the development of fabrication-friendly plasmonic gratings for optimal intensification of the Raman signal with an extra degree of control through the polarization of the excitation light. This feature enables studying Raman signal of exactly the same molecules with and without electromagnetic SERS enhancements, just by changing the polarization of the excitation, and thereby permits detailed studies on the selection rules and the chemical enhancements possibly involved in SERS.

中文翻译：

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适用于制造的偏振敏感等离子体激元光栅，可实现最佳的表面增强拉曼光谱

等离子纳米结构广泛用于从紫外线到近红外应用的表面增强拉曼光谱（SERS）中。周期性的纳米等离子体系统，如等离子体激元光栅，由于其强烈的偏振依赖性和易于制造，作为SERS活性衬底引起了人们的极大兴趣。在这项工作中，我们建模了一个银光栅，该银光栅表现出亚辐射等离子体共振，这是其反射率的下降，并且仅对光的横向磁（TM）偏振具有明显的近场增强。我们研究了其填充因子（通常定义为光栅凹槽宽度与光栅周期之比）对SERS增强的作用。我们使用有限差分时域（FDTD）模拟设计了具有不同填充因子的多个光栅，以将不同程度的光谱失谐纳入我们来自拉曼激发（488 nm）的反射波中。我们的数值研究表明，通过调整光栅的光学共振光谱位置，通过修改其填充系数，我们可以优化可实现的SERS增强。此外，通过将激发光的偏振从横磁改变为横电，我们可以禁用光栅的光学共振，从而导致SERS性能可忽略不计。为了验证这一点，我们制造并光学表征了建模光栅，并确保了其光学响应中存在所需的失谐。我们对核黄素的拉曼分析证实，光栅共振和预期的拉曼激发之间的较高重叠仅对TM偏振光产生更强的拉曼增强。我们的发现为制造友好的等离激元光栅的发展提供了见识，这些激子可通过激发光的偏振来以最佳程度的控制来优化拉曼信号的增强。该特征使得仅通过改变激发的极化即可研究具有和不具有电磁SERS增强的完全相同分子的拉曼信号，从而允许对选择规则和SERS可能涉及的化学增强进行详细研究。我们的发现为制造友好的等离激元光栅的发展提供了见识，该光栅可通过激发光的偏振来以额外程度的控制来优化拉曼信号的增强。该特征使得仅通过改变激发的极化就可以研究具有和不具有电磁SERS增强的完全相同的分子的拉曼信号，从而可以对SERS可能涉及的选择规则和化学增强进行详细研究。我们的发现为制造友好的等离激元光栅的发展提供了见识，该光栅可通过激发光的偏振来以额外程度的控制来优化拉曼信号的增强。该特征使得仅通过改变激发的极化即可研究具有和不具有电磁SERS增强的完全相同分子的拉曼信号，从而允许对选择规则和SERS可能涉及的化学增强进行详细研究。