Diamond nanophotonics is a rapidly evolving platform in which non-classical light—emitted by defect centers in diamond—can be generated, manipulated, and detected in a single monolithic device (e.g., for quantum information processing applications).1–3 Indeed, novel diamond fabrication techniques make it possible to engineer unique nanostructures in which diamond's extraordinary material properties (e.g., high refractive index, wide band gap, and large optical transmission window) can be exploited.4, 5 The relatively large Kerr non-linearity6 of diamond also makes it an attractive platform for on-chip nonlinear optics at visible and IR wavelengths.7 This nonlinearity could be used for frequency conversion of photons generated by color centers in diamond (i.e., from their typical visible wavelengths to telecom wavelengths).8 In turn, this would enable transmission of quantum information and distribution of quantum entanglement9, 10 over long distances. Such integrated diamond–quantum photonics platforms would benefit from the use (and realization) of high-performance single-photon detectors that have broadband photon sensitivity and are integrated on the same diamond chip.
A scanning electron microscope image of several freestanding diamond waveguides (with triangular cross sections) is shown in Figure 2(a). We etched these waveguides from single-crystal diamond with the use of our angled-etching fabrication method.4 The waveguides are supported periodically by thin support structures underneath the waveguide that are created by slightly increasing the width of the waveguide at the support locations. This allows long segments of the waveguide to remain freestanding (while not perturbing the waveguide mode).19 In addition, single meander SNSPDs—see Figure 2(b)—are located on both ends of the waveguide. The SNSPDs are then connected to titanium/gold contact pads for electrical readout.
Finite-difference time-domain simulations of the diamond waveguide SNSPD device are shown in Figure 3. The normalized field distribution of the optical mode in the diamond waveguide is shown in Figure 3(a), which illustrates the capacity for single-mode waveguide operation in the triangular cross section diamond waveguide. In addition, the absorption characteristics of the device—Figure 3(b)—indicate that more than 99% of the optical power has been absorbed by the SNSPD after a propagation distance of 15μm.
The photon-counting performance of an SNSPD on one of the freestanding diamond waveguides (at 4.2K)—when illuminated with vertically incident 705nm photons—is depicted by the blue curve in Figure 4, and the red curve indicates the dark count response of the detector. The temperature (4.2K) and superconductor thickness (10.5nm) of the device limit the SNSPD from reaching a fully saturated photon count rate. However, we do observe a wide photon-counting operational range (i.e., the region where the device count rate begins to level off and approach an ideal saturated regime) that is still far from the detector's intrinsic dark counts.
In summary, we have developed a platform with which SNSPDs can be fabricated on freestanding waveguides that are etched from single-crystal diamond (which can host quantum emitters with good spectral properties).20 We have also characterized the photon-counting performance of our fabricated detectors. With our approach it is possible to achieve monolithic and scalable integration of diamond quantum optical circuits that are based on defect color centers. In the next stages of our work, we plan to improve the filtering of the pump beam (i.e., that is used to excite the color centers) so that the SNSPDs are no longer saturated by pump photons.
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