The Optics Laboratory
Group of Hans Hallen, Physics Department, North Carolina State University
Gradient field Raman effects are also observed in other cases when a metal is in close proximity to the measured molecules. This is more common than might be naively expected, since small metal particles are often used to locally enhance fields for spectroscopic measurements. The nano-bowtie antenna is also described here: nano-bowtie antenna.
First, an introduction to the antenna. Our aim is a deep UV nano-antenna that can be used for plasmon excitation light focussing and DUV resonance Raman. More detail is here ,  . It extends the wavelength range dramatically compared to earlier work, as seen in Fig. 1.
Figure 1. An extrapolation from the literature [Crozier, K. B., Sundaramurthy, A., Kino, S., and Quate, C. F., J. Applied Physics 94, 4632 (2003).], we show that our antenna resonance is consistent with the simple model used to determine the resonance of a metal triangle. Our antenna has two triangles to form a bowtie, but the same plasmon resonance holds for each.
It is interesting to compare three spectra: the far-field off resonance, the far field with excitation tuned to an absorption (resonance Raman), and the near-field (bowtie) plasmon enhanced resonance Raman. Figure 2 shows that the resonance Raman has many more excited vibration lines than the nonresonant case, and that different vibrations are excited. This is explained by the fact that vibrations which are strongly affected by the bond breaking of the absorption will be enhanced, and that several of these phonon types may be created at once (overtones) or mixtures with these phonons (combinations) are often created, as discussed on the resonance Raman page. The addition of the plasmonic nano-enhancement does not dramatically change the observed overtone and combination bands compared to the just-resonance-Raman case. New, unexpected vibration lines, which are large enough that should have been observed if they were present, are found, however, as shown in Fig. 2 (b).
Figure 2. (a) Raman spectra of various types are compared. The upper plot contains a non-resonant Raman spectrum and a resonance Raman spectrum taken without any nearby metal. The lower plot contains a resonance Raman spectra in the presence of the metal nano-antenna. The plots are shifted so that the horizontal energy axes align, and the green dotted lines relate stronger Raman features in the plots. The label colors are black: observed in both resonant Raman experiments (with and without metal), blue: likely too weak to be observed without metal enhancement, and red: infrared vibrations, the GFR effect, so only in the metal nearby, bowtie spectrum. (b) The bowtie spectrum repeated with zoom-ins to show the presence of the infrared peaks. The same color scheme is used.
To give further evidence that these bands observed in the bowtie, near-metal spectrum but not in the far-field spectrum are gradient-field Raman, GFR, related, we must show that infrared spectra are strong at these energies (wavenumber shifts). This is done in Fig. 3. It is interesting that we easily observe the low energy combination mode line in the far-infrared specteal region. The consistency between the strong infrared lines from the literature and our 'new' peaks is good evidence that the gradient field effect is present here, as expected.
Figure 3. The resonance Raman peak with nearby metal (nano-antenna) shows GFR effects, as indicated by comparison to the infrared spectrum [NIST WebBook entry for benzene,
More info is in the papers.
NC State University | Physics | Optics Home
Copyright ©2000-2017, Hallen Laboratory, NCSU, Raleigh, NC. www.physics.ncsu.edu/optics
Comments or questions? Hans_Hallen@ncsu.edu