The application of light for information processing opens up a multitude of possibilities. However, to be able to adequately use photons in circuits and sensors, materials need to have particular optical and mechanical properties. Researchers at the Karlsruhe Institute of Technology (KIT) have now for the first time used polycrystalline diamond to manufacture optical circuits.
Previously optical circuits have been manufactured using monocrystalline diamond substrates i.e., highly pure crystals with typically no more than one impurity atom to every one billion diamond atoms. Such circuits are bound to be small and their application to optical systems has required sophisticated fabrication methods.
Now, for the first time, the research group headed by Pernice used polycrystalline diamond for the fabrication of wafer-based optomechanical circuits. Even though its crystal structures are more irregular, polycrystalline diamond is robust and thus can be more easily processed. It is due to these specific properties that polycrystalline diamond can 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. As a matter of fact, monolithic optomechanical components could not have been manufactured without this new material.
Two parallel free-standing waveguides made of polycrystalline diamond serve as mechanical resonators. Optical fields (red/blue) are observed to propagate inside of them. (Graphic: KIT/CFN/Pernice)
Optomechanics combines integrated optics with mechanical elements e.g., with nanomechanical resonators in the case of the optomechanical circuit developed by Pernice and his group. These oscillatory systems react to a certain frequency. When that frequency occurs, the resonator is excited into vibration. “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,” explains 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.” This novel development has allowed the combining of once separate fields of research and has enabled the realization of highly efficient optomechanical circuits.
Integrated optics works in a similar manner to integrated electrical circuits. Whereas optical circuits transmit information via photons, conventional electronic circuits transfer data via electrons. Integrated optics aims to combine all components required for optical communication in an integrated optical circuit to avoid a detour via electrical signals. In both cases, the respective circuits are applied to slices less than one mm in thickness i.e., to the so-called wafers.
The polycrystalline diamond was manufactured in cooperation with the Fraunhofer Institute for Applied Solid State Physics and the company Diamond Materials in Freiburg, Germany. The prototypes manufactured within the Integrated Quantum Photonics-project at the DFG Center for Functional Nanostructures (CFN) in Karlsruhe open up new ways for entirely optically controlled platforms that are increasingly needed in fundamental research and advanced sensor technologies. These technologies include accelerometers that are integrated in various electronic devices such as airbag sensors or smartphone waterlevels.
Diamond optomechanical resonators.- (a) Schematic of coupled free-standing waveguides, which act as mechanical resonators. Propagating optical modes are overlaid in colour. (b) Fabrication routine used to prepare both photonic circuitry and mechanical elements on-chip
ABSTRACT – Diamond offers unique material advantages for the realization of micro- and nanomechanical resonators because of its high Young’s modulus, compatibility with harsh environments and superior thermal properties. At the same time, the wide electronic bandgap of 5.45 eV makes diamond a suitable material for integrated optics because of broadband transparency and the absence of free-carrier absorption commonly encountered in silicon photonics. Here we take advantage of both to engineer full-scale optomechanical circuits in diamond thin films. We show that polycrystalline diamond films fabricated by chemical vapour deposition provide a convenient wafer-scale substrate for the realization of high-quality nanophotonic devices. Using free-standing nanomechanical resonators embedded in on-chip Mach–Zehnder interferometers, we demonstrate efficient optomechanical transduction via gradient optical forces. Fabricated diamond resonators reproducibly show high mechanical quality factors up to 11,200. Our low cost, wideband, carrier-free photonic circuits hold promise for all-optical sensing and optomechanical signal processing at ultra-high frequencies.
Diamond-integrated nanophotonic circuits.