Nanophotonics:
enhanced optical transmission through nano-apertures

The optical transmission of an aperture punched in an opaque metal film is extremely small when the aperture diameter d is much smaller than the optical wavelength, d<<l.

The graph below shows the total transmitted light (when the incident light is focused onto a spot with diameter D=5l) as a function of d/l. The solid black line is the geometric limit, T=(d/D)2, appropriate for large d. The green dot-dashed line is Bethe's prediction for the case of an ideal metal, which follows T~(d/l)6. The graph shows that the throughput decreases precipitously as d/l goes into the sub-wavelength limit. For a real metal film with finite thickness the transmission follows that of a waveguide beyond cutoff, which drops even more rapidly (blue dashed line).


We have shown that the optical transmission can be enhanced considerably (red solid squares) when the metal surface around the aperture has a periodic surface corrugation. The peak wavelength depends on the lattice constant of the corrugation, and is thus tunable. The transmission can exceed the geometric limit (T>(d/D)2), even when d<<l.
Refs.: Ebbesen, Lezec, Ghaemi, Thio, Wolff, Nature 391, 667 (1998);
Thio, Pellerin, Linke, Ebbesen, Lezec, Optics Lett. 26, 1972 (2001).

Periodic arrays of apertures

A periodic aperture array constitutes an extreme case of surface corrugation. The optical transmission spectra have both high transmission peaks as well as deep transmission minima.

  

The transmission peaks are widely held to be cause by a resonance with surface plasmon polarition (SP) modes on the surface of the metal film, which cause the oscillating electric field to be strongly enhanced at the aperture entrances.

We have recently proposed a radically different model for the transmission enhancement. In the new model the incident light scatters off the subwavelength surface structure and generates an evanescent wave, labeled a composite diffracted evanescent wave (CDEW), which travels on the surface and interferes with the light that is directly impinging on the neighbouring holes. Transmission enhancement or suppression is obtained when the interference is constructive or destructive, respectively. The CDEW model thus naturally includes both the transmission maxima and minima; furthermore, it successfully explains certain aspects of the enhanced transmission that, in the framework of the SP model, have remained mysteries, including the occurrence of enhanced transmission in non-metallic systems (which do not support SPs), efficient beaming effects, and discrepancies between experiments and SP theory of the spectral positions as well as widths of the transmission maxima.

Ref: Lezec and Thio, Optics Express 12, 3629 (2004)
Details and FAQ (frequently asked questions) of CDEW model

 

Single apertures

Contrary to what is widely claimed in the literature, the transmission enhancement of aperture arrays is G<7 (when compared to a real single, isolated hole of the same dimensions). However, for a single hole surrounded by circular surface corrugation, the axial symmetry gives significantly higher enhancement: In an optimised device the transmission enhancement can be G>100. In this case also the peak wavelength is tunable by an appropriate choice of the periodicity.

    

A systematic study of the corrugation geometry shows that the optical geometry is a set of ring grooves, concentric around the central aperture, with a depth h=100nm, corresponding to about three times the skin depth of the metal, usually silver. The peak tranmission of such an optimised device as be as high as T/f~3: three times more light is coming through the aperture than is directly impinging on it. The enhanced transmission can therefore exceed the geometric limit by a factor of 3, even well into the subwavelength limit (see top figure of this page).

 

Applications

The wavelength at which the transmission maxima and minima occur can be tuned by varying the lattice constant of the surface structure, the refractive index of the dielectric medium adjacent to the metal, or the incident angle. Such a tunability, together with the high transmission even at subwavelength scale, make enhanced-transmission devices very attractive in a number of applications, including high-density optical data storage, near-field scanning optical microscopy (NSOM), optical switches and modulators, and high efficiency display devices.