Squeezing light through a small slit

Researchers of the Universities of Exeter (Great Britain), Madrid and Zaragoza (Spain) and the FOM Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam have succeeded in tripling the transmission of light through a small slit.

This was achieved by designing the surroundings of the slit in such a way that light can be guided towards the slit - with the help of light of a different colour. It is the first externally switchable filter of which the transmission can be increased actively. This is important for ultrafast microscopy and single molecule spectroscopy. The results are published in Physical Review Letters on March 28.

Light that leaks through a hole or slit smaller than the wavelength of that light, is becoming increasingly interesting in physics, for instance for performing spectroscopic observations on isolated molecules. Influencing the transmission of light through very small apertures is a challenge. Obviously, it is possible to make a slit broader or narrower, but the possibility of controlling the transmission from the outside without having to change the size of the slit is highly desirable, since that allows for fast, active control over the transmission. This is essential for realizing fast, active optical components with which, for instance, dynamics can be followed beyond the diffraction limit.

Silicon
In their publication, the researchers describe an ingenious way of controlling the transmission of light through a sub-wavelength slit. They demonstrated their methodology for long-wavelength Terahertz (THz)-light; of which the wavelength is 300 micrometer (or 0,3 millimetres). The THz-light was incident on a 40-micrometer wide slit, structured in the semiconductor material silicon. The trick was to make grooves in the silicon that were parallel to the slit (see figure 1). THz-light that falls on the grooves can propagate on the surface of the silicon in the form of a surface wave. For ordinary silicon, however, the generation and propagation of surface waves occurs inefficiently. The efficiency can be increased significantly by making the silicon metallic by putting electrons in the conduction band. This is readily achieved by irradiating the silicon with an optical light pulse (figure 1). The light pulse excites the silicon and as a result the THz-light can propagate approximately ten times further over the surface than before. In this fashion, THz-light which does not directly impinge on the slit can reach the slit and thus contribute to transmission. (see figure 2). This resulted in a three-fold enhancement of the transmission.

The switching of light with light in small structures that was demonstrated here is important for so-called near-field microscopy and for spectroscopy. Mainly the possibility to switch on very short time scales (pico seconds) makes this a very appealing technique for ultra-fast transmission. (March 2008)

Results of the experiment. Above panel: THz-transmission spectra through the un-pumped (black line) and pumped structure (red line). The two lower panels show calculations that demonstrate how the transmission increases when the silicon structure is excited. The structure is grey, the THz-field strength leads from weak (blue) to strong (red). In the unexcited structure, the fields are weak, and only present near the slit. After excitation, it is clear that many grooves contribute to the enhanced transmission.