Fabry-Perot tuning of the band-gap polarity in plasmonic crystals

We report experimental observation of Fabry-Perot resonances and Fabry-Perot-induced band-structure flipping in quasi-one-dimensional plasmonic crystals. Angle-resolved transmission spectra of nanoslit arrays in metal films demonstrate band-gap formation resulting from surface plasmon polariton couplings mediated via nanometer-sized waveguide channels. Tuning the waveguide dielectric function and thickness allows for a pronounced resonant enhancement of the coupling strength and band-gap energy and may even induce changes in sign of the coupling—i.e., an effective band-gap flipping. Our results indicate an interesting route towards band-gap engineering in plasmonic crystals.

Figure 1

(Color online) (a) SEM image of the slit sample with $d=537\mathrm{nm}$. (b) Sketch of the microscopic origin of Fabry-Perot (FP) resonances in transmission. (c) Angle-dependent linear transmission spectra in air. (d) Transmission spectrum at $\theta =0°$ for air (black line) and water [red (dark gray) line] as dielectric. The arrows indicate the FP resonances. (e) Experimental values of the FP resonance wavelength versus $n_d$ and linear extrapolation.

Figure 2

(Color online) (a) Angle-dependent transmission spectra in air [Fig. 1c], in a narrow range around the $\mathrm{SM}[\pm 1]$ crossing. The spectrum is dominated by the pronounced FP resonance around $760\mathrm{nm}$. The band gap at the $\mathrm{SM}[\pm 1]$ crossing has negative polarity. (b) Contour plot when the top metal surface is immersed in water. The FP resonance is shifted towards $900\mathrm{nm}$ and the $\mathrm{DM}[\pm 1]$ crossing has positive polarity.

Figure 3

(Color online) Angle-dependent transmission spectra evidencing polarity flipping of the $\mathrm{SM}[\pm 1]$ band gap. (a) Transmission spectra around the $\mathrm{SM}[\pm 1]$ crossing for $n_d=1$ (air) and incidence angles between $\theta =0°$ and 3°. (b) Transmission spectra for $n_d=1.512$ (index matching oil). (c) Contour plot around the $\mathrm{SM}[\pm 1]$ crossing, showing a negative polarity band gap in air. (d) Same as (c) except that an index matching oil $n_d=1.512$ has been added. The band-gap polarity has changed sign.

Figure 4

Measured transmission spectra and plasmonic band gaps of the $\mathrm{SM}[\pm 1]$ crossing for a $d=537\mathrm{nm}$ immersed in four different dielectric media (a) in air $(n_d=1.0)$, (b) in water $(n_d=1.33)$, and in two different immersion oils (c) $n_d=1.512$ and (d) $n_d=1.65$.

Figure 5

(Color online) Angle-dependent transmission spectra through nanoslit arrays in a $h=75\mathrm{nm}$ thick gold film, calculated using the dynamic diffraction model (Refs. 27, 28). The calculations are performed for an array period of $d=537\mathrm{nm}$ and different refractive indices $n_d$ of the embedding dielectric: (a) $n_d=1.0$, (b) $n_d=1.33$, (c) $n_d=1.0$ ($\mathrm{SM}[\pm 1]$ crossing), and (d) $n_d=1.512$. In all cases, a slit width of $a=10\mathrm{nm}$ is assumed to approximately match the experimental FP resonances.