Chaotic lasers tamed
“Classical” laser light has become part of everyday life. There is a laser in every CD player, lecturers point to their slides with laser pointers and surgeons carry out medical operations with laser beams. Nevertheless there are numerous unusual kinds of laser light that are still largely unexplored, one of them being Diffusive Random Lasers (DRLs).
The quantum physicist Hakan Türeci from the Quantum Photonics Group of the ETH Zurich Institute for Quantum Electronics says “The basic idea behind DRLs is amazingly simple.” He and colleagues from Yale University and Vienna University of Technology describe new knowledge about the physics of DRLs in a paper published in “Science” on Friday 2 May 2008. They have also developed a new formula with which lasers can be re-computed from first principles. This means they have created a framework to understand the properties and development of such complex, exotic lasers.
Lasers without mirrors
A normal laser beam is generated in a cavity between two mirrors. The light flashes to and fro, passing through an amplifying medium on the way. An external “pump” supplies energy. One of the mirrors is semi-transparent and allows the laser beam to emerge. It is important that the light inside the cavity is not scattered, for example by impurities, as this would reduce the power of the laser beam. This kind of laser beam is directional and has a particular frequency, i.e. colour.
On the other hand the development of DRLs is still in its infancy, although the principle was already postulated by a Russian researcher in 1968. The advantage of a DRL: no expensive polished mirrors are needed to generate the laser light. The amplifier medium can be a dye solution containing nano-particles such as titanium dioxide. These particles are randomly distributed in the solution, which is excited by a light source and “pumped” with energy from outside.
The input light is scattered randomly on the nano-particles, bouncing from one particle to another and being amplified at the same time. This does not need a cavity like a conventional laser. With optimum pump power, i.e. the supply of external energy in the form of light or electric current, laser light finally emerges from the medium. However, the hot-spots with the maximum light intensity are unpredictable, but in most cases they are in a ring-shaped pattern at the edge of the amplifier medium. A random laser of this kind also does not have a sharply defined frequency. Countless frequencies that can mutually cancel each other out occur in a DRL system, i.e. they operate a kind of frequency Darwinism. In the end only the “strongest” frequencies remain. “However, the intensity of these winners is also unstable and fluctuates from one pulse to the next,” says Türeci.
The researchers have also designed a new mathematical model in their paper, an ab initio laser theory, a fundamental starting point that allows them to predict the power of lasers. The physicist explains: “We have a completely new formula with which we can calculate all the physical properties.” He says these can be used to develop novel laser methods, for example, and will become important in the future.
Laser ink for documents
According to the post-doc, DRL already has one interesting application, a kind of “laser ink”. This particle-enriched ink can be printed onto documents, for example. In this process the distribution of the nano-particles scattering the laser light is absolutely random and is individually different for each printed impression. To verify a document, it is irradiated with light and a detector is used to analyse the emerging laser light spectrum. This enables the authenticity of a document to be established definitively. This laser ink has already been patented in the USA.
DRL technology could also be used to reveal chemical impurities in water or for novel displays with extremely high switching speed and definition. Türeci also imagines that the technique could be used to detect injuries in human tissues. In this respect the tissue plays the part of the scattering medium, which would need to be dyed. Its properties – for example cellular composition and density – determine the spectral signature of the light formed by the tissue. If an organ contains a tumour, the light will be scattered differently compared to a healthy organ.
However, the researchers have not yet reached that point. More research on different DRLs is planned, for example to show how the intensity fluctuations of the frequencies that are generated can be brought under control.
Hakan E. Türeci, Li Ge, Stefan Rotter, A. Douglas Stone: Strong Interactions in Multimode Random Lasers, Science, 2 May 2008, Vol. 320. no. 5876, pp. 643 - 646 DOI: 10.1126/science.1155311