Interference and resonant cavity effects explain enhanced transmission through subwavelength apertures in thin metal films

Abstract

Transmission through an opaque Au film with a single subwavelength aperture centered in a smooth cavity between linear grating structures is studied experimentally and with a finite element model. The model is in good agreement with measured results and is used to investigate local field behavior. It shows that a surface plasmon polariton (SPP) is launched along the metal surface, while interference of the SPP with the incident light along with resonant cavity effects give rise to suppression and enhancement in transmission. Based on experimental and modeling results, peak location and structure of the enhancement/suppression bands are explained analytically, confirming the primary role of SPPs in enhanced transmission through small apertures in opaque metal films.

Experimental study of enhanced transmission through subwavelength linear apertures flanked by periodic corrugations

“…19,20 The model gives excellent agreement with transmission spectra, such as those in Figure 7, and allows us to examine the electromagnetic fields at the surface of the metal and near the aperture. Using this approach we have confirmed Lezec and Thio's conclusion that interference effects cause transmission enhancement and suppression.…”

“…We have developed analytical expressions based both on resonant cavity effects and interference between SPPs and incident and reflected light that does predict positions of maximum transmission. 19 The cavity resonances are given by where w c,ef f is the effective cavity width, m is an integer, and n SP is given in Equation 3. Values for w c,ef f are based on the effective mirroring locations of gratings which numerical simulations give on average as w c,ef f = w c + 50 nm.…”

“…For this case, the center of the first groove is considered the primary launching point of the SPP, 19 therefore w c,ef f = w c + 225 nm. Since this SPP launching site is protruding into the dielectric, φ = 0.…”

“…When an aperture is located at a position where power flow is in contact with the surface and flow is directed into the aperture, enhanced transmission occurs. 19 Similarly, suppressed transmission occurs if the aperture is located below a region where power flow is not in contact with the surface.…”

Nanometer scale light focusing with high cavity-enhanced output

“…3,4 However, the most widespread structure consists of exploiting the transmission through subwavelength apertures to obtain a localized and directional light beam. [5][6][7][8][9][10][11][12][13][14][15] In order to illustrate the major issues associated with nanoscale light focusing, let us consider a 30 nm wide slit perforated in a 60 nm thick silver film ͓n = 0.05+ 3.43i ͑Ref. 16͔͒, which is illuminated by a monochromatic ͑ = 532 nm͒ linearly polarized ͑perpendicular to the slit axis͒ normally incident plane wave.…”