Confining light to a random laser
Researchers at the University of Twente (UT) and the FOM-Institute for Atomic and Molecular Physics (AMOLF) in Amsterdam have discovered that a certain kind of laser, the random laser, can be very small. These small random lasers may possibly be used for authentication and encoding of documents, currencies and data. The researchers will have their findings published in the scientific journal Physical Review Letters.
It is impossible to imagine life today without the laser. The effects of a 'common' laser are based on confinement and gain of light. Gain of light may occur in certain atoms, molecules and crystals, by conversion energy from another source. Most lasers need mirrors that are exactly lined up, in order to confine light to a gain area.
Multiple scattering
The researchers at the University of Twente and AMOLF have been investigating a particular kind of laser that combine gain and scattering of light. Scattering plays a part in almost every discipline of science and technology. Materials like fog, milk and dye scatter light. Combining gain and scattering of light does not seem very logical, for scattering of light in lasers is usually being prevented. The fact is that mirrors are not able to confine scattered light. However, there is one kind of scattering - multiple scattering - that can be confined. The multiplication of light in a scattered material then follows a path of random 'drunken walks', which causes the light to remain in the same area for a long time. A strongly scattered medium that intensifies also the intensity of light, is called a random laser.
Two periods of time are important to a random laser: the length of stay of light and the gain length. The length of stay is the period of time that the light needs in order to cover a 'drunken walk'. The gain length is the period of time that the light has to remain in the medium to get a sufficient gain in order to create a laser out of the scattered medium. If the length of stay of a walk is longer than the gain length, then the effects of a laser come into being.
Spectral pure light
All over the world scientists have been experimenting on random lasers that combine gain and scattering. Unexpectedly, they have discovered that some of these lasers produce spectral pure light. Spectral pure light is light that is the size of one wavelength. The exact origine of this kind of light is controversial: does it arise from small areas or just from the laser as a whole?
Well, the researchers at the University of Twente and AMOLF have answered this question. They have created a very special random laser using a sponge of gallium phosphide. This is the strongest scattering non-absorbing material used for visible light. The spectrum of the light produced by this laser, is five times as pure as that of other disorderly lasers. The researchers have scrutinized the laser with a microscope area by area. They discovered that the light arises from very small areas two micrometres in diameter; this is an indication that strong scattering is able to confine light in an unexpectedly small scale.
Combining gain and scattering leads to valuable new sights into both phenomena. Random lasers will find possible applications in authorization and encoding of documents, currencies and data. The fact is that this kind of lasers is simply to be made, but not to be copied: every sponge has a unique structure and therefore, unique qualities being a laser. Moreover, it appeared that the random laser is a very small one. This is one step forward to the development of nanolasers, say, applicable to more and more reducing of integrated optical components.
For more information, please contact Karen van der Molen, University of Twente: email or Allard Mosk, University of Twente, phone +31 (0)53 489 53 92.
The article is entitled 'Spatial Extent of Random Laser Modes'. The authors are Karen van der Molen (UT), Willem Tjerkstra (UT), Allard Mosk (UT) and Ad Lagendijk (AMOLF).