Diamond is considered by many to be the perfect material. Apart from its remarkable beauty when suitably cut, it is the hardest naturally occurring bulk material, has a record high thermal conductivity and offers excellent transparency from the ultraviolet to far infrared. However it is another characteristic which has quantum optics scientists excited about diamond. Analogous to semiconductors and conventional electronics, the key to making diamond functional is an impurity: a point defect called the nitrogen-vacancy (NV) center – ‘nature’s single photon source’.
The NV center, which is present in both naturally occurring and synthetically fabricated diamond, consists of a nitrogen with a neighboring empty site replacing carbon atoms in the diamond lattice. The optically active defect boasts long room temperature spin coherence time, making them attractive as quantum bits. Unlike classical computers which rely on digital 0s and 1s, quantum bits can be in 0 and 1 states simultaneously, enabling an exponential speed increase for certain calculations. Quantum computers are particularly useful for solving challenging multivariable problems such as nanoscale simulations in modern science or macroscale problems like predicting the world climate or fluctuations in the stock market. In addition, due to the magnetically sensitive ground state of NV centers, they can be used to measure weak magnetic fields with nanoscale resolution, which has triggered significant research into diamond-based optical magnetometers.
An integrated optics platform in diamond would be beneficial for magnetometry due to the enhanced interaction provided by waveguides, and quantum computing, in which NV centers could be optically linked together for long-range quantum entanglement, due to stability and integration provided by monolithic waveguides. However, it remains a challenge to fabricate optical waveguides in diamond, particularly in 3D architectures, due to its hardness and chemical inertness.
Figure 1. Femtosecond laser fabrication system for writing optical waveguides in diamond.
In an international collaboration between University of Calgary, Politecnico di Milano and the Institute for Photonics and Nanotechnologies (IFN) – CNR, we recently demonstrated the fabrication of 3D optical waveguides in bulk diamond using focused ultrashort laser pulses in a laboratory at CNST-IIT Milano (Figure 1). As confirmed by optically detected magnetic resonance, mRaman spectroscopy and photoluminescence measurements, we showed that the high repetition rate laser writing produced a waveguide with preserved crystallinity. Crucially, we found that the remarkable properties of the NV centers (Figure 2) were maintained, allowing photons to be efficiently carried between the defects, a crucial step in building a scalable quantum photonic platform.
Figure 2. Inset shows cross sectional microscope image of buried diamond waveguide, with the optical mode guided between two laser written modification tracks separated by 13 mm. The photoluminescence spectrum inside the waveguide is the same as the pristine diamond, demonstrating preserved nitrogen vacancy properties, crucial for applications in quantum computing and magnetometry.
The concentration of NV centers depends on the purity of the diamond, however the defects are randomly distributed throughout the volume. It is highly desirable to deterministically produce NVs on demand with submicron resolution, prealigned with existing photonic circuits. Recently, Chen et al. demonstrated that femtosecond laser static exposures produced vacancies in the bulk of diamond. After annealing at 1000°C, the laser formed vacancies diffused toward nitrogen impurities to produce on-demand and high quality single NVs .
We have taken these pioneering works of laser fabrication of optical waveguides[3, 5] and NVs a step further, by incorporating these important building blocks on the same integrated diamond chip, to enable the robust excitation and collection of light at NVs. Because a single laser microfabrication system is used, the alignment between NVs and waveguides is achieved with submicron resolution. Using confocal photoluminescence microscopy and wide-field EMCCD imaging, we demonstrated the coupling of single NVs using optical waveguides (Figure 3).
Optically addressed NV centers could open the door for more sophisticated quantum photonic networks in diamond. For example, in quantum grade diamond, the optically linked single NVs could be exploited for single photon sources or solid state qubits. In lower purity diamond, the laser writing of high density NV ensembles within waveguides could enable robust excitation and collection of the fluorescence signal for magnetometry.
Figure 3. Below: 532-nm wavelength excitation of single NV center using optical waveguide. Above: NV signature (650 nm – 800 nm) is captured from above using EMCCD imaging (shown) or confocal photoluminescence collection raster scan (not shown).
This work was funded by the FP7 DiamondFab CONCERT Japan project, DIAMANTE MIUR-SIR grant, and FemtoDiamante Cariplo ERC reinforcement grant.
Shane Eaton, Belén Sotillo, Roberta Ramponi
Istituto di Fotonica e Nanotecnologie
Consiglio Nazionale delle Ricerche (IFN-CNR)
Shane Eaton received his PhD at University Toronto in 2008. He is now a research associate with IFN. He is the winner of the prestigious SIR Italian project, to study laser microfabrication of quantum photonics in diamond. His h-index is 22 and he has over 50 papers.
Belén Sotillo received her PhD at the University of Madrid in 2014 with the highest distinction. She has been author or co-author of several papers published in international journals (h-index of 7). Currently she is a postdoctoral researcher with IFN characterizing the laser-material interaction in diamond.
Roberta Ramponi is the director of IFN-CNR and professor of physics at the Politecnico di Milano. She has been the president of the EOS and is now a member of the Board of the Stakeholders and the Executive Board of Photonics21. She has more than 130 journal papers.
Andrea Chiappini, Maurizio Ferrari
Istituto di Fotonica e Nanotecnologie
Consiglio Nazionale delle Ricerche (IFN-CNR)
Andrea Chiappini received his PhD in Physics from the University of Trento in 2006. Since August 2007, he has been the Principal Investigator on the research area “Sol-gel Photonics” at the Institute of Photonic and Nanotechnologies UOS Trento. He is coauthor of 50 papers and his h-index is 17.
Maurizio Ferrari received his PhD in Physics from the University of Trento in 1980. He is currently a Director of Research heading the IFN-CNR Trento unit. He is an SPIE Fellow, co-author of more than 400 publications, several book chapters, and is involved in numerous research projects concerning glass photonics.
JP Hadden, Paul E. Barclay
Institute for Quantum Science and Technology
University of Calgary
JP Hadden completed his PhD at the University of Bristol in 2013. His thesis focused on the use solid immersion lenses for enhanced photon collection efficiency from color centers in diamond. He joined Paul Barclay’s group in 2015 to investigate coupling between mechanical motion and colour centres in diamond.
Paul Barclay completed his PhD in Applied Physics at Caltech in 2007. In 2008 he joined HP Labs where he developed diamond nanophotonic devices. Since 2011, he has been a group leader at the University of Calgary and the National Institute for Nanotechnology, where he develops quantum and optomechanical nanophotonic devices.
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3. Sotillo, B., et al., Diamond photonics platform enabled by femtosecond laser writing. Scientific Reports, 2016. 6: p. 35566.
4. Chen, Y.-C., et al., Laser writing of coherent colour centres in diamond. Nature Photonics, 2016.
5. Courvoisier, A., M.J. Booth, and P.S. Salter, Inscription of 3D waveguides in diamond using an ultrafast laser. Applied Physics Letters, 2016. 109(3): p. 031109.
6. J. P. Hadden, V. Bharadwaj, B. Sotillo, S. Rampini, R. Osellame, T. T. Fernandez, A. Chiappini, C. Armellini, M. Ferrari, R. Ramponi, P. E. Barclay and S. M. Eaton, Waveguide-coupled single NV in diamond enabled by femtosecond laser writing (arXiv:1701.05885).