Raman Line Broadening with Graphene Coated Broad-Band Nano-Antennas

Abstract: Raman spectroscopy is a well-established tool providing for molecular fingerprints.  Here we examine the Raman signal of graphene coated nano-antennas.  The antennas studied were in the shape of bow-tie and diamond.  Substantial signal amplification and line broadening have been observed signifying the nonlinear interaction between the antenna gain profile and the Raman line.

Summary: Surface enhanced Raman scattering (SERS) [1] is a major spectroscopic tool for an efficient molecular identification down to the level of a few molecules [2,3].  Driven by the notion of localized electric field intensities, research turned to aggregates of metal particles and then to the more structured nano-optical antennas [4-6].  Yet, most antennas used so far were narrow band; covering mostly the laser frequency.  Enhancement of the Raman signal has been further hindered by uncertainties in analyte concentration and location relative to the antenna.  Here we examined a new antenna type, the diamond-shape antenna (D-ant) [7] with a bandwidth capable of accommodating both the pump laser and scattered Raman frequencies.  The role of the antenna at resonance becomes two-fold: enhancing the absorption of the pump laser frequency and imposing unique conditions for plasmonic scattering of the Raman signal at the far-field.  By using graphene [8] as a test film we eliminated much of the uncertainty in molecule’s location and concentration, thus enabling better comparative studies. 

Fig. 1 is a scanning electron microscope (SEM) image of our graphene-coated antennas.  The metal layout included markers, which exhibited photoluminescence (PL) when illuminated by the 633 nm HeNe laser.  The PL signal of gold was small when illuminated with the 785 nm laser and thus, mostly the antennas glow was accentuated near resonance.  The background reflection has substantially elevated near resonance: this occurred because of antenna’s reflectivity and antenna’s directivity.

The Raman signal from a graphene coated diamond antenna exhibited a substantial enhancement and line broadening, associated which every Raman line.  The broadening becomes smaller as the Raman shift becomes larger.  It was as-if the enhancement and line broadening mapped the gain-profile of the diamond shaped antenna.

If we zoom at the 1300 cm^-1graphene line we may observe an enhancement factor of ca 70 and line broadening by a factor of almost 5.

In summary, diamond shape antennas exhibited substantial signal enhancement and line broadening compared to bow-tie antennas with similar dimensions.

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