A sole thin film of silver is a highly opaque material, but British researchers at University of Exeter have discovered that silver can be highly transparent when sandwiched between two transparent layers of zinc sulfide.

Light could sneak across narrow portions much like quantum particles can "tunnel" through impossibly high barriers. This experiment reached just 35 % of light transmission compared to the tunneling effect. The scientists think that building quantum barriers would permit designing loss-free tunneling for electrons in electronic circuits.

Light passing through a solid block will suffer total internal reflection if it strikes the surface at a very shallow angle, with no transmission into the air. Some of the electromagnetic waves, evanescent waves, stray across the boundary between the two materials, carrying no energy away.

If the lower intensity evanescent wave encounters another solid transparent block nearby, a light wave with reduced intensity appears in the next block. "Ordinarily, neither process allows complete transmission of light or particles, but a more complicated gap or barrier structure can lead to transmission with perfect efficiency," say Ian Hooper from the University of Exeter.

The scientists added layers of a different transparent material before and after the air gap. They proved that, at certain wavelengths, with certain materials, perfect transmission of light should occur. The trick is canceling light waves reflecting back and forth between the sandwiched solid layers.

The team wrapped the surface of a silica prism with a film of about 200 nanometers of zinc sulfide, a hard, transparent material. They then pressed two such prisms together, leaving a very thin air gap sandwiched between them because the surfaces were not perfectly smooth. "Laser light hitting one of the prisms at the optimal angle penetrated the three-layer sandwich of zinc sulfide/air/zinc sulfide with about 85% efficiency--not 100% because zinc sulfide absorbs a bit of light", says Hooper. The less perfect transmission is due to the zinc sulphide, which absorbs some light.

The scientists think that the light is transmitted because light reflected from the silica/zinc sulfide and the zinc sulfide/silver interfaces are 180 degrees out of phase with each other and of equal amplitude. "The reflection from the front interface combined with the multiple reflections from the subsequent interfaces also interfere such that they cancel, says Hooper. With no net reflection, and with a non-absorbing system, all the light must be transmitted."

After that, the physicists replaced the air gap with a 40 nm thick silver film. Silver was found to pass light with an efficiency as high as 35% at certain wavelengths. The analogous quantum effect should be easy to demonstrate by layering special designed semiconductors to create a barrier through which a current could tunnel with less loss, an useful electronic circuit component.

The work could help to improve the efficiency of organic light-emitting diodes, whose performance is limited by the light passing through a metal cathode.

Photo credit: I.R.Hooper/Univ. of Exeter