Bite Sized Science: Set Lasers to Reverse!
By Keir Liddle
The science of lasers has just taken a large step backwards.
Lasers generally work by creating coherent beams of identical light particles, or photons. The most basic types consist of an inert gas placed between two mirrors. When photons pass into the laser cavity, they excite the electrons of the gas into a semi-stable quantum state. Passing photons can cause the electrons to drop to a lower energy state, releasing still more, identical photons. The photons multiply as they bounce between the mirrors and interact with other electrons, until the laser creates a glittering beam of light.
Researchers led by physicists Hui Cao and Douglas Stone at Yale University in New Haven, Connecticut, wondered what would happen if you attempted to run the above process in reverse.
In principle, such a thing should be possible. The rules of quantum mechanics mean that the process is the same going forwards or backwards, and that it should be possible to make lasers absorb photons. However, in practice, the experiment required a high level of precision. Cao and her group used a 110-micrometre silicon wafer on an optical bench, along with a tunable infrared laser. They split the laser beam into two and shine it into both sides of the silicon slice. The front and back of the slice act as mirrors and the silicon in-between takes the place of the gas found in a common laser. Tuning the incoming beam’s frequency and other properties causes the photons to become trapped between the surfaces of the silicon. As the photons bounce back and forth, the silicon absorbs them until all the photons are sucked up by the device and converted into heat.
This device itself may not be particularly useful, but the principle could lead to far more sensitive sensors.