Plasmon-Assisted Two-Slit Transmission: Young’s Experiment Revisited

We present an experimental and theoretical study of the optical transmission of a thin metal screen perforated by two subwavelength slits, separated by many optical wavelengths. The total intensity of the far-field double-slit pattern is shown to be reduced or enhanced as a function of the wavelength of the incident light beam. This modulation is attributed to an interference phenomenon at each of the slits, instead of at the detector. The interference arises as a consequence of the excitation of surface plasmons propagating from one slit to the other.

Figure 1

The frame at the top shows the Young-type interference pattern behind the screen, as recorded with a charge coupled device camera. The other frames display experimental transmission spectra for a TM-polarized input beam (polarization perpendicular to the long axis of the 200 nm wide slits), recorded by integrating over the interference pattern. The value of the slit separation $d$ is indicated in each of the frames. In the frame at the bottom ($d=24.5\mu \mathrm{m}$) the results for TE-polarized incident light (open squares) are included; the scale at the right applies to this choice of polarization.

Figure 2

Two interfering paths leading to light emission from the slit at the left. A similar set of paths gives rise to emission from the slit on the right. The dashed line indicates the propagating surface plasmon.

Figure 3

The calculated angular-integrated transmission coefficient of a double slit in a 200 nm thick gold film as a function of the wavelength of the incident light. The slits are 200 nm wide and separated by $25.0\mu \mathrm{m}$. The full line displays the results for TM polarization, while the dotted line (magnified 10 times) shows the results for the case of TE polarization. The transmission coefficient is normalized to the area of the slits.

Figure 4

Intensity distribution in the immediate vicinity of the double-slit system for TM-polarized incident radiation when the transmission is maximum (top frame, slit separation equal to $5{\lambda }_{\mathrm{s}\mathrm{p}}/2$), and minimum (bottom frame, slit separation equal to $4{\lambda }_{\mathrm{s}\mathrm{p}}/2$). The field is incident from below. All lengths are in nm.