CVD process improves diamond quality for lasers

Dr. Jörg Schwartz, joerg.schwartz@photonics.com

Diamonds could become a laser’s best friend, thanks to new methods of creating man-made versions of the gem.

Researchers have proved the principle that diamonds offer high efficiency for lasing; however, demonstrating this has been an issue because it requires pure diamond crystals. “Using natural diamonds … is problematic – the quality is not consistent and, as everybody knows, they’re very expensive,” said associate professor Richard P. Mildren of Macquarie University.

The new synthetic diamonds not only cost much less but also are grown to researchers’ specifications using the process of chemical vapor deposition (CVD), a method that essentially creates a carbon crystal lattice by putting down atomic layers of carbon on a large, flat diamond crystal substrate. Today this allows creation of diamonds up to 8 mm in length, weighing a bit under a carat. Last year, when the researchers set a new record efficiency of 63.5 percent (see “Diamonds sparkle in Raman application,” Photonics Spectra, November 2009, p. 23), they used a 6.7-mm-long crystal and, going forward, “Diamonds larger than one centimeter are likely to be available very soon,” Mildren said. Another benefit of CVD is that it is compatible with photonic integration processes, so that diamond lasers or waveguides can be included in future photonic integration efforts.

Over the past few years, researchers from Macquarie University have made tremendous progress introducing diamond as a very attractive laser material. Presenting recent findings on this relatively young laser variant in an invited talk at this year’s CLEO/QELS conference, the scientists outlined the state of the art in Raman lasers based on undoped, single-crystal diamond, which typically uses an external cavity to generate nano- and picosecond pulses with high efficiency.

Richard Mildren and his team at Macquarie University have been pushing the limits of diamond-based Raman lasers for several years. Now these devices have become mature enough to compete with other lasers, mainly due to the availability of good-quality synthetic diamonds and a better understanding of the laser’s design. Courtesy of Macquarie University.

Aside from its outstanding efficiency – at least 40 percent higher than alternative Raman materials – diamond has two properties that make it attractive as a laser material. First, it offers very good heat conductivity; second, it is transparent over an extremely wide part of the optical spectrum. Localized heating is an unwanted side effect when building lasers with high power and/or when trying to make them small. Low absorption over a wide range is desired to give flexibility in terms of output wavelength.

For classical lasers, the choice of output wavelength is determined by the atomic or molecular energy levels and transitions available for lasing action. However, diamond lasers are Raman lasers: Unlike, say, diode lasers, they are optically rather than electrically pumped; the pump interacts with molecular or atomic vibrations in the material. These interactions make some of the pump photons lose or gain energy – i.e., the scattered light has a different wavelength. The laser action takes place with this secondary light, called Stokes or anti-Stokes, being amplified in an appropriate cavity while pumping energy into the system, resulting in coherent emission at the shifted wavelength.

Diamond not only transfers the pump energy more efficiently into the output but also enables a larger-than-usual shift, resulting in output light that is at a greater spectral distance from the pump. This means that, in combination with the wide window of transparency, diamond lasers can be made to lase at wavelengths between 225 nm in the ultraviolet and 100 µm in the far-infrared (with a gap between 3- and 6-µm wavelengths).