Dendritic Plasmonics for Mid-Infrared Spectroscopy

Abstract

Metallic nanostructures that exhibit tailored optical resonances spanning from the near to midinfrared spectral range are of particular interest for spectroscopic and optical measurements in these spectral domains that can benefit from localized surface-enhancement effects. Plasmon resonances shifted in the near or mid-infrared range could be used to further enhance the excitation and/or the emission of an optical process. Surface-enhanced infrared absorption (SEIRA) is one of such processes and can particularly benefit from plasmon-enhanced local fields yielding an increase in sensitivity towards the detection of an analyte. Herein, we have fabricated a series of gold dendritic nanostructures, prepared by electron-beam lithography, that exhibit plasmon resonances spanning the near and mid-infrared spectral regions. We explore the influence of the number of branches of the dendritic structures, as well as the length of each generation together with the overall effect of the shape and symmetry on the resulting optical

Exploiting Anisotropy of Plasmonic Nanostructures with Polarization Modulation Infrared Linear Dichroism Microscopy (µPM‐IRLD)

“…At the HE overlap, the enhancement of the EM field occurs only in the outermost structures ( Figure A,B), whereas the LE overlap incorporated the structures from both the first and second‐order generations (Figure C,D). This distribution has been previously related to the hybridization of the LSPRs associated with the incorporation of more structures with increasing generations …”

“…Altering the dimensions of the fractal nanostructures is a practical way to tune the spectral position of their resonances, as was shown in previous studies where a 1 nm increase in the side length yields a red spectral shift of 6 to 7 nm . To demonstrate that this size dependence continues with increasing side length, a narrow range of sizes (350–400 nm) were prepared such that the two LSPRs of the three‐branched second‐order generation were still located within the spectral range of interest.…”

“…Inspired by the Cayley Tree fractal, we have previously explored a more general version of radial fractals, classified as dendritic fractals . For such structures, with each additional generation, a new lower energy (LE) (lower wavenumber) resonance is introduced.…”

“…For such structures, with each additional generation, a new lower energy (LE) (lower wavenumber) resonance is introduced. The spectral position of the resonance can then be tuned by altering the size, shape, and configuration of the fractal . Since studies involving SEIRA emphasize the detection of small molecules, polymers, proteins, and lipids, the spectral regions between 1000–2000, and 2800–3200 cm −1 are of the greatest interest.…”

“…Once fabricated, a “duck foot”‐like structure is observed. This effect can be minimized by truncating (removing branches) from the outer generation, and/or altering the dimensions of the branches (i.e., increasing the length of the nanorods). With a particular interest in working with the intrinsic fractal, we have decided to not explore truncated fractals in this study.…”