Polycrystalline Diamond Used to Fabricate Optical Circuits

Optical circuits manufactured from polycrystalline diamond create new possibilities for all-optical sensor technology platforms.

Diamond is optically transparent to lightwaves over a wide range of wavelengths, including the visible spectrum between 400 and 750 nm. Because of this feature, it can be used specifically in optomechanical circuits for applications in sensor technology and fluorescence imaging or for novel optical biological measuring methods. Whereas the high refractive index of diamond and the absence of absorption allow an efficient photon transport, its high modulus of elasticity makes it a robust material that adapts to rough surfaces and releases heat rapidly.

“Diamond has several properties that allow us to manufacture all components of a ready-to-use optomechanical circuit monolithically,” said Karlsruhe Institute of Technology research group leader Wolfram Pernice. “The elements thus manufactured — that is, the resonators, circuits and the wafer — are attractive because of their high quality.”

To adequately use photons in circuits and sensors, materials need to have particular optical and mechanical properties. Until recently, optical circuits were manufactured using monocrystalline diamond substrates — highly pure crystals with no more than one impurity atom to every 1 billion diamond atoms. Such circuits are bound to be small, and their application to optical systems has required sophisticated fabrication methods.

Two parallel free-standing waveguides made of polycrystalline diamond serve as mechanical resonators in a KIT experiment. Optical fields (red/blue) are observed to propagate inside them. Courtesy of KIT/CFN/Pernice.

Now, for the first time, researchers at KIT have used polycrystalline diamond to fabricate wafer-based optomechanical circuits. These oscillatory systems react to a certain frequency that excites the resonator into vibration.

Although its crystal structures are more irregular, polycrystalline diamond is robust and thus can be easily processed. These specific properties make it possible for polycrystalline diamonds to be used on much larger areas than monocrystalline material. Polycrystalline diamond conducts photons almost as efficiently as the monocrystalline substrate and is suitable for industrial use.

“Nanomechanical resonators are among today's most sensitive sensors and are used in various precision measurements. It is extremely difficult, however, to address such smallest components through conventional measuring methods,” said Patrik Rath, first author of the study. “In our study, we have made use of the fact that today, nanophotonic components can be manufactured in the same sizes as nanoscale mechanical resonators. When the resonator responds, corresponding optical signals are transferred directly to the circuit.”

The polycrystalline diamond was manufactured in cooperation with the Fraunhofer Institute for Applied Solid State Physics and Diamond Materials of Freiburg, Germany.

Results were published in Nature Communications (doi: 10.1038/ncomms2710).

For more information, visit: www.kit.edu