Inert light changes colour

Light does not change colour without any reason. The fact is that the colour of light, also expressed in wavelength, coheres with the energy of the light particles (photons) in a light beam: the longer the wavelength, the less energy. The law of conservation of energy does not allow the energy to change colour without any reason. Only at very particular circumstances change of colour does occur. If the intensity of light is increased immensely and the light is being transmitted through an appropriate material, then two photons are able to ‘melt’ into one photon with two-fold energy, so to speak. Thus, infrared light that cannot be observed with the naked eys, may change into blue light. In technical jargon this process is called a second-harmonic generation.

Unexpected effects with nanoscreen
Usually, light of a particular wavelength hardly passes a hole if its dimension is smaller than the wavelength. Still, a ‘nanoscreen’ is an appropriate material to get high intensities of light. By putting the holes in a recurrent pattern, they allow to pass a surprisingly large amount of light. The amount of light that passed through may even be increasing more than to be expected on the basis of the combined surface of the holes. The metal between the holes helps ‘to compress’ more light through the holes.

Two years ago Dutch and French researchers proved that they had been able to influence this 'exceptional’ transmission phenomenon by changing the shape of the holes, contrary to the then applicable knowledge**⁾. A nanoscreen that has rectangular holes sized 75 nanometres to 225 nanometres (one nanometre is 0.000001 millimetre) allowed to pass ten times as much light than a screen that had round holes and a diameter of 190 nanometres.

Inert light
It has now appeared that the researchers are able to generate very high intensities of light by transmitting light through a screen of rectangular nanoholes. These intensities are about one million times one thousand million as big as a heavy projector. The researchers have succeeded in changing the colour of light with this high intensity by compressing it through a nanoscreen: they changed the infrared light into a blue light. The intensity of light can be increased furthermore by elongating the rectangular holes again and again. This produced a correspondingly large amount of blue light.

It appears that there is a maximum in increasing the intensity of light by elongating the holes. The intensity decreases beyond specific proportions of length and width. Exactly this shape of the rectangles made the research team find their remarkable discovery: although the intensity of light did not increase, yet, ten times as much blue light was produced. Calculations show that it is hard for the light to pass the holes when they have the abovementioned ‘limiting properties’, in technical jargon this is called ‘cut-off’. If the propositions of length and width is exactly the same as the ‘cut-off’, the ligth crawls slowly through the holes. That makes it last longer before the light passes the holes. Thus, the photons get a better chance to blend to a new colour.

Change of colour for solar cells and biosensors
This observation opens up perspectives for the development of the next generation of solar cells and biosensors. A greater part of the light that the sun radiates to the earth cannot be used in a solar cell, in which the light is transformed into electric energy. The fact is that this light has a colour that a solar cell is not able to ‘see’: infrared. The photons do not have sufficient energy to generate electric power. When we are able to transform infrared light into photons that have more energy, the new photons will be able to generate useful electricity. By generating nanoscreens to the solar cell, it would be able to use a greater part of the solar spectrum.

Researchers often look for an optical fingerprint of the molecules in order to detect biomolecules. One way to acquire an optical fingerprint is to shoot at the molecules with a powerful laser. The molecular vibrations in the biomolecule are able to change the colour of the laser. This is a so-called nonlinear optical process that is also laborious, just as the earlier described second-harmonic generation, and that requires very high intensities of light. By putting the molecules in the holes of the nanoscreen, it must be possible to acquire a fingerprint of the biomolecule using a relatively cheap laser. This opens up perspectives to create a new generation of sensors that do not require a large laser infrastructure and that the general practitioner may perhaps use in his surgery.

*) The article is entitled Strong Modification of the Nonlinear Optical Response of Metallic Subwavelength Hole Arrays. The authors are Joris van Nieuwstadt, Marijn Sandtke, Rob Harmsen, Frans Segerink, Jord Prangsma,  Stefan Enoch and Kobus Kuipers. This research fits in the NWO-Vici-research by Kuipers, entitled Nonlinear Optics at the Nanoscale.

**) See also the newspaper report 'Doorlaten van licht door een zeef met nanometergaatjes hangt af van de vorm van de gaatjes', 4 May 2004
 
For more information, please contact professor dr. Kobus Kuipers, FOM Institute for Atomic and Molecular Physics, phone (020) 608 12 34 or Marijn Sandtke, FOM Institute for Atomic and Molecular Physics, phone (020) 608 12 34

website: www.amolf.nl