Modifying an Infrared Microscope To Characterize Propagating Surface Plasmon Polariton-Mediated Resonances

Wehave been developing techniques to apply propagating surface plasmon polariton (SPP) resonances to the spectroscopic study of individual, subwavelength-sized particles. SPPs, mixed states of light and conducting electrons at the surface of a patterned metal surface, are part of, that is, mediate, the extraordinary transmission/reflection resonances of patterned metal meshes. A fair body of work has been presented by us showing that plasmonic metal mesh is a very useful substrate for sensing resonance shifts and for infrared (IR) absorption studies for both benchtop 10 and microscope 13 studies. Dispersion studies 19 reveal much about the plasmonic nature of two-dimensional periodic mesh, which has been a subject of several reviews (to name only a few). Our plasmonic mesh is a freestanding nickel film with square holes (5 μm width), in a square lattice (L = 12.6 μm lattice parameter), with a thickness of ∼2 μm as shown in Figure 1a. Transmission spectra of the same piece of Ni mesh obtained with a Fourier transform infrared (FTIR) microscope as compared to a benchtop FTIR instrument are shown in Figure 1b. There are considerable differences: (i) the primary resonance at 752 cm 1 of the benchtop spectrum does not occur in the FTIR microscope spectrum, (ii) the FTIR microscope trace has two broad resonances at higher wavenumbers, and (iii) the FTIR microscope trace has much more transmission at the higher wavenumbers. The FTIR microscope spectrum of mesh has no predominant and assigned plasmonic resonance, which may be good for spectroscopy in an FTIR microscope, but is bad for sensing the shift of resonances. These differences can be understood by noting that the optical geometry in the sample region of a FTIR microscope system is significantly different from that of a typical benchtop FTIR system. Our IR microscope (Perkin-Elmer Spotlight 300) employs a pair of Cassegrain optics (reflective mirror combinations), which share a focal point at the sample. They only transmit rays from17 to 37 off of the optical axis; that is, a ring of light is focused onto and collected from the sample. In contrast, a desktop FTIR system has a beam that is perpendicular to the sample (an average angle of 0 relative to the optical axis with a Gaussian standard deviation of a few degrees about that average). A resonance at each angle in the large range of angles of an FTIR microscope will be dispersed differently, producing a smearing out of plasmonic resonances, instead of well-defined resonances. Also, the range of large off axis angles enables higher order diffraction spots to be collected, which would be lost in a zero order benchtopFTIR transmission spectrum, which manifests as greater transmission at higher wavenumbers. In this work, we describe how to add an aperture between the lower optics and the input of the sample, which greatly reduces the spread of angles narrowing transmission resonances. In a microscope, the mesh must be oriented perpendicular to the optical axis to keep the mesh in focus, so the natural coordinate for dispersion is rotation of the mesh within the focal plane about the microscope’s optical axis. A geometric arrangement between the aperture and mesh is described, which enables the production and identification of much sharper propagating SPP transmission resonances. This system has enabled a dispersion study with a Cassegrain microscope system by rotation of mesh rather than the tilting of mesh previously employed with a benchtop FTIR system.