Blocked Holes Can Enhance Light, Rather than Stop it

Physorg.com, Optics InfoBase: "Many optical systems today, such as those in sensing, nanolithography, and many others, are built on a general belief: An optically opaque metal film would block light transmission even if the film has small holes, as long as the holes are covered with opaque metals which geometrically block the light path through the holes. For example, light transmission from one side of a glass to the other side is assumed to be blocked, when an opaque metal film is coated on one surface of the glass, even if the surface unavoidably has tiny dusts. This is because the coated metal covers the dust completely, hence blocking the light geometric path through the dust. Here, we report our experimental and theoretical study that demonstrates otherwise: Not only the light can transmit, but also the transmission is greatly enhanced, which is much better than an open hole. Furthermore, we found the transmission can be tuned by the metal blocker’s geometry and by the gap between the blockers and the metal film."

These electron microscope images show an experiment in which Princeton Professor of Engineering Stephen Chou showed that blocking a hole in a thin metal film could cause more light to pass through the hole than leaving the hole unblocked. The top image shows an array of 60nm holes spaced 200nm apart with gold caps, each of which is 40 percent bigger than the hole on which it sits. The bottom image shows a cross-section view of one hole with the cap sitting on top of SiO2 pillar. The gold film in the experiment was 40nm thick. The hole covered with the cap surprisingly allows 70% more light to be transmitted through the film than a hole without the cap, Chou's research team found.

"We did not expect more light to get through," Chou said. "We expected the metal to block the light completely."

Chou said the metal disk acts as a sort of "antenna" that picks up and radiates electromagnetic waves. In this case, the metal disks pick up light from one side of the hole and radiate it to the opposite side. The waves travel along the surface of the metal and leap from the hole to the cap, or vice versa depending on which way the light is traveling. Chou's research group is continuing to investigate the effect and how it could be applied to enhance the performance of ultrasensitive detectors.

Comparison of transmittance measurements showing 70% transmission enhancement by the blocked hole array than the open hole array.

(a) Experimental transmittance spectra measured on a periodic gold hole array blocked by Au nanodisks and the same gold hole array after removal of the nanodisks. The hole array has a hole diameter of 70 nm and a gold thickness of 40 nm, the gold nanodisks have a diameter of 85 nm, and the SiO2 pillar height is 52 nm.

(b) Plot of transmission enhancement ratio calculated by dividing the optical transmission of blocked and open gold hole arrays. A maximum enhancement of 1.7x is observed at 680 nm.

Thanks to JM for sending me the link!