Metallic nanostructures that exhibit plasmon resonances in the mid‐infrared range are of particular interest for a variety of optical processes where the infrared excitation and/or emission can be enhanced. This plasmon‐mediated enhancement can potentially be used toward highly sensitive detection of an analyte(s) by techniques such as surface‐enhanced infrared absorption (SEIRA). To maximize the SEIRA enhancement, it is necessary to prepare highly tuned plasmonic resonances over a defined spectral range that can span over several microns. Noteworthy, nanostructures with anisotropic shapes exhibit multiple resonances that can be exploited by controlling the polarization of the input light. This study demonstrates the role of polarization‐modulation infrared linear dichroism coupled to microscopy measurements (µPM‐IRLD) as a powerful means to explore the optical properties of anisotropic nanostructures. Quantitative µPM‐IRLD measurements are conducted on a series of dendritic fractals as model structures to explore the role of structural anisotropy on the resulting surface‐enhanced infrared absorption and sensing application. Once functionalized with an analyte, the µPM‐IRLD SEIRA results highlight that it is possible to selectively enhance further vibrational modes of analytes making use of the structural anisotropy of the metallic nanostructure.

The surface plasmon resonance (SPR) modes of the first three generations of a Sierpiński fractal triangle are investigated using electron energy loss spectroscopy (EELS) complemented with finite difference time domain simulations. The Sierpiński fractal geometry is created in a subtractive manner, by carving triangular apertures into the triangular prism of the previous fractal generation. The ability of the fractal antenna to efficiently utilize space in coupling to long wavelength excitations is confirmed on the single nanostructure level via redshifting of the primary dipole mode as the fractal generation is increased. Through application of the Babinet principle, it is demonstrated that this spectral shift is caused by coupling of two degenerate orthogonal dipolar modes of a single triangle with two degenerate orthogonal dipole modes of the triangular aperture occupying the center of the first generation fractal. It is also shown that the spectral position and strength of the dipole mode can be tuned by altering the size of the central aperture, and thus the capacitance of the equivalent circuit, and the width of the conductive channels joining different fractal building blocks, thereby altering the circuit inductance. Importantly, placing the aperture on a node of the SPR mode causes a shift in energy of this mode without changing the charge configuration; placing the aperture on an antinode of the SPR mode causes no shift in energy, but changes the field configuration, as revealed through EELS measurements. These fractal-specific properties provide new strategies to design, predict, and effectively exploit highly tunable SPR modes using simple building blocks.

An equilateral triangular prism is used as the fundamental building block to construct additive Sierpiński fractals, enabling new surface plasmon resonances (SPR) in the first three generations of Sierpiński triangles, as well as topological intermediaries between generations. The modes are characterized using electron energy loss spectroscopy accompanied by eigenmode calculations and optical finite-difference time-domain simulations. The complex fractal geometries present a predictable hierarchy of new resonances, each arising from the previous generational building blocks used to construct the fractal. Intermediate structures break the polarization degeneracy of the equilateral fractals while maintaining a rich multiband spectral response. Engineering defects in the narrow conductive channels of the fractal allows further manipulation of the SPR response, emphasizing higher order SPR modes over the lowest energy peak. The knowledge gained is used to develop guidelines for engineering the response of more complex fractal-based structures, including the spectral response and hotspot distribution.

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