by Simon Mitton

The last three papers on the grid for the Physics Top Ten feature advances in laser physics. The technical developments recorded in #8 and #9 are of strong interest for medical applications, particularly the design of laser tools for ophthalmology. Paper #10 on nanolasers is intriguing because it tells of a breakthrough in realizing ultracompact lasers with coherent light beams squeezed to about 1% of the diffraction limit.

Paper #8 reports a continuous wave diode-laser with emission at the 589 nm Na D₂ line. The crucial point concerns the device’s operating wavelength. Orange-yellow lasers have a multitude of potential applications in medicine, biology, display technology, and even the creation of laser guide stars for astronomical telescopes.

The first lasers operated via the stimulated emission from excited atoms, but modern solid-state lasers employ Raman scattering. The lasers in #8 and #9 comprise a diode laser to act as an optical pump, coupled to a cavity crystal doped with the rare earth neodymium, and this is the active medium. In the self-Raman set-up described in #8, the LVO₄ crystal was selected because it is more efficient than alternatives in which L is replaced by Y or Gd, and the LVO4 crystal has a higher damage threshold as well as being easy to process. A pulsed laser based on Nd₃:LuVO₄ was announced in 2009, but this generated 120 mW at 1179 nm. The scheme announced in #8 utilizes intracavity frequency doubling in the crystal borate LiB3O5 to halve the wavelength to that of the Na D2 line.

Putting all of this another way: the group led by Yanfei Lu at Changchun University of Science and Technology, China, has found a way of producing orange-yellow laser light in a compact set-up. A laser diode pumps a crystal of lutetium vanadate doped with neodymium, the output of which is frequency-doubled in the non-linear optical material lithium borate (LBO).

"Xiang Zhang (paper #10) and his team have opened up a new world in which coherent light replaces electronics, leading to processing at the speed of light."

The high level of citations to this paper is being driven by its application to ophthalmology, a discipline that pioneered the use of laser energy for patient treatment, and which still accounts for more laser operations than any other specialty. For treating retinal disorders, 589 nm light is producing the best clinical outcomes with the fewest side effects. Hemaglobin absorbs yellow light, so the surgeon can use this laser as a non-invasive tool for dealing with blood vessels in the eye. Continuous wave operation provides physicians a constant source of energy.

The same group is responsible for #9. The synthetic crystal material yttrium aluminium garnet (Y₃AI₅O₁₂, acronym YAG)) has been used in lasers for half a century. Green light (532 nm) laser pointers use Nd:YAG with a frequency doubler. Medical applications include ophthalmology (particularly photocoagulation), laser hair removal, and treatment of skin cancers. Paper #9 reports a new technical development, namely emission at 869 nm rather than a 946 nm emission.

An LBO frequency doubler of a slightly different composition to that used in #8 produced output beams at 435 nm. This experiment opens up a new way to reach deep wavelengths at the blue end of the optical spectrum. The Changchun group obtained good beam quality and high output power stability, both of which are essential for real world applications.

Finally, #10 is about making lasers that are much smaller than the wavelength of light. Why would anyone want to do that? The short answer is that in the emergent field of optoelectronics there is currently a mismatch between the length scales of electronics and optics. The smallest commercial transistor gate sizes are way below the diffraction limit for light. Laser physicists are working on the equivalent of putting the beam of a flashlight through a needle’s eye. The fundamental challenge is to develop ultracompact lasers that pump out coherent light on the nanometer scale.

In #10, nanoscience researchers at the University of California, Berkeley, announce a new milestone in laser physics by creating the world’s smallest laser. Light cannot be focused below half its wavelength, so smoke and mirrors are of no use. But by attaching photons to electrons in surface plasmons on a metal, it is possible to squeeze light. Xiang Zhang (pictured above) and his team have opened up a new world in which coherent light replaces electronics, leading to processing at the speed of light.

Dr. Simon Mitton is a Fellow of St. Edmund’s College, University of Cambridge, U.K.