Discovery of anomalous light transmission phenomena from the capped nanoaperture

Schools of Advanced Science and Engineering

Department of Chemistry and Biochemistry

Prof. Kohei Imura

〝Prof. Imura and collaborators discovered anomalous light transmission phenomena from the capped nanoaperture.”

When an aperture created in the opaque metal screen is illuminated, transmitted light through the aperture is observed at the backside of the screen.  Transmitted light intensity decreases when the aperture size decreases, and becomes extremely weak when the aperture size becomes smaller than the wavelength of light.  Let us consider here light transmission from the aperture when the aperture is capped with the other metal screen.  In common sense, no light is transmitted, and this is true when the aperture size is large.  Then, how about for the nano-aperture?  We studied the light transmission from the nanoaperture using near-field optical microscope.  In our experiment, we used an aperture near-field fiber probe and gold nanodisks as the nanometric aperture and the cap, respectively.  When the nanometric aperture is capped with a gold nanodisk, we found that the transmission light intensity is enhanced.  This is clear contrast to the large aperture case.  The enhancement is only observed in the vicinity of the disk.  The magnitude of enhancement depends strongly on the wavelength of light and reaches several times near the plasmon resonance of the nanodisk.  To understand the phenomena, we performed electromagnetic field calculations and found that the plasmon excited in the disk plays essential role.  The excited plasmon extracts near-field photons localized near the aperture and reradiates towards the far-field.  The plasmon works as antenna, and this is why the light transmission was enhanced when the aperture was capped with the gold nanodisk.  The antenna effect of plasmons has potentials in nanometric optical devices and the innovative use of light.

Reference: K. imura, K. Ueno, H. Misawa, H. Okamoto, Nano Lett. 11, 960-965 (2011).

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