Light Manipulates Photon Tunneling

Richard Gaughan

Through photon tunneling, quantum physics allows for the transmission of light through apertures that classical physics would declare too small. The utility of this phenomenon has been limited because it is dependent on little more than geometry: A given pinhole will allow tunneling, or it will not. But when the holes are filled with a highly nonlinear polymer, photon tunneling at one wavelength can be controlled with illumination at another.

Quantum physics allows the transmission of light through subwavelength apertures by photon tunneling. The intensity and polarization of tunneled light at one wavelength can be controlled by simultaneously illuminating an array of holes filled with a nonlinear polymer at another wavelength.

Researchers at the University of Maryland in College Park and at Queen's University of Belfast in the UK began an investigation into photon tunneling by thermally depositing a 0.5-µm layer of gold on a glass substrate. The deposition process produced a granular structure, with grain size in the range of 10 to 100 nm. At random intervals in the film, pinholes on the order of the grain size remained. The researchers covered the gold film with poly-3-butoxy-carbonyl-methyl-urethane (3BCMU), which is optically nonlinear. When they illuminated the surface of the film with 633-nm light from a HeNe laser, they observed some transmission through the grain-size pinholes.

To explain photon tunneling through subwavelength apertures, scientists have proposed that the incident radiation couples to surface plasmon states that propagate in the plane of the film, localizing in the pinholes. At the opposite end of the hole, the plasmon couples to a radiating state. If this model is correct, simultaneous illumination at another wavelength should affect coupling. And the researchers indeed found that when they shone control light of 488 nm from an argon-ion laser on the pinholes, the 3BCMU dramatically changed its dielectric constant at the 633-nm signal, the coupling changed, and the tunneling was reduced by 15 percent.

Igor I. Smolyaninov, a research scientist in the Maryland group, believes that a near-term application of the effect could be parallel all-optical signal and image processing. By projecting a reference image and a test image on an array of nanoholes, he suggested, the modulated portion of the transmitted one could indicate the degree of similarity or difference between the two.

But the physics of an ordered two-dimensional array of nanometer-scale pinholes is significantly different from that of a single hole or even of a one-dimensional array of slits. To investigate the properties of tunneling through an ordered array, Smolyaninov's team used focused-ion-beam milling to produce cylindrical holes through a 400-nm-thick gold film, which the researchers built on a thin film of chromium over a substrate of silicon nitride. Again, the pinholes were covered with 3BCMU.

The interactions were more complex in this experiment. Incident light again coupled to surface plasmons, but the periodic array established preferred directions. Besides modifying the intensity, the control beam modified the polarization of the transmitted light.

With the results verifying the surface plasmon model, Smolyaninov has concentrated on fleshing out the theory. "There is an apparent similarity between the nonlinear optical properties of narrow cylindrical nanochannels and a class of high-energy physics theories called Kaluza-Klein theories," he said. This deeper theoretical understanding is expected to open up new applications for gated photon tunneling.