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Femtosecond laser writing as an enabling tool for diamond photonics and NV centers.

Eaton S.E., Ramponi R.
  Mercoledì 13/09   09:00 - 13:00   Aula A107   II - Fisica della materia
Diamond is an extremely attractive material for the realization of photonics-integrated platforms for quantum applications, due to its wide-range transparency, chemical inertness and hardness. Even more important, intrinsic defects such as nitrogen vacancy (NV) centers allow additional functionalities. Diamond's NV centers, which are present in both naturally occurring and synthetically fabricated diamond, consist 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. 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, due to its hardness and chemical inertness. We recently demonstrated the fabrication of 3D optical waveguides in bulk diamond using focused ultrashort laser pulses. As confirmed by optically detected magnetic resonance and $\mu Raman$ spectroscopy, we showed that the high-repetition-rate laser writing produced a waveguide with preserved crystallinity. 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 exposures produced vacancies in bulk diamond. After annealing at 1000 ${}^{\circ} C$, 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 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. Using wide-field Electron multiplying CCD (EMCCD) imaging, we demonstrated the coupling of single NVs using optical waveguides. Optically addressed NV centers could pave the way 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. In this work, the laser writing technique will be illustrated and its potential in diamond and other transparent materials will be discussed, and the results achieved in diamond will be shown.