Squeezing light through a slit
Researchers at the universities of Exeter (UK), Madrid and Zaragoza (Spain), and the FOM Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam (the Netherlands) have managed to triple the quantity of light transmitted through a slit. They designed the surroundings of the slit so that light could be actively guided over the surface of the material towards the slit, helped by light of a different colour. It is the first external switchable filter for which the transmission can be actively increased. This is important for ultrafast microscopy and spectroscopy of separate molecules. The results were published in Physical Review Letters on 28 March 2008.
Light that passes through an aperture or slit with a dimension smaller than its wavelength is attracting growing interest in physics, for example, for the spectroscopic observation of separate molecules. Influencing the transmission of light through these very small apertures is a considerable challenge. Of course the easiest way is to change the size of the slit. However, externally influencing the transmission of the light without having to change the size of the slit is better still, as this allows faster and more proactive control of the transmission. This is, for example, essential for the realisation of rapid, active optical components that allow dynamics beyond the diffraction limit to be observed.
In their publication, the researchers describe an ingenious approach for controlling the transmission of light through a sub-wavelength slit. They demonstrated their approach for long-waved terahertz (THz) light; this has a wavelength of 300 micrometres (i.e. 0.3 millimetres). They allowed the THz light to fall onto a slit 40 micrometres wide, made in the semiconducting material silicon. The trick was to make grooves in the silicon parallel to the slit (see Figure 1). THz light that falls onto the grooves can propagate itself across the silicon as surface waves. For ordinary silicon, the creation and propagation of surface waves is not efficient. However, rendering the silicon metallic, by bringing electrons into the conductance band, vastly increases the efficiency. This can be achieved by illuminating the silicon with a light pulse of a different wavelength (Figure 1). When the light pulse strikes the silicon, the THz light can propagate itself ten times further over the surface than before. THz light that would not initially fall onto the slit can then still reach the slit and creep through it (see Figure 2). The result is a tripling in the transmission.
This demonstrated switching of light with light in small structures is important for so-called near-field microscopy as well as for spectroscopy. The possibility of switching on very short timescales (picoseconds) makes this technique particularly attractive for ultrafast microscopy.
More information: Prof.dr. Mischa Bonn (AMOLF), phone +31 (0)20 608 12 34.
Reference:
Optical Control over Surface-Plasmon-Polariton-Assisted THz Transmission through a Slit Aperture, E. Hendry, F.J. García-Vidal, L. Martín-Moreno, J. Gómez Rivas, M. Bonn, A.P. Hibbins, and M.J. Lockyear, Physical Review Letters, 28 March 2008.