Integrated Magnetometry Platform with Stackable Waveguide-Assisted Detection Channels for Sensing Arrays

The negatively charged nitrogen vacancy (N-$V^{-}$) center in diamond has shown great success in nanoscale, high-sensitivity magnetometry. Efficient fluorescence detection is crucial for improving the sensitivity. Furthermore, integrated devices enable practicable sensors. Here, we present an integrated architecture which allows us to create N-$V^{-}$ centers a few nanometers below the diamond surface, and at the same time covering the entire mode field of femtosecond-laser-written type-II waveguides. We experimentally verify the coupling efficiency, showcase the detection of magnetic resonance signals through the waveguides and perform proof-of-principle experiments in magnetic field and temperature sensing. The sensing task can be operated via the waveguide without direct light illumination through the sample, which is important for magnetometry in biological systems that are sensitive to light. In the future, our approach will enable the development of two-dimensional sensing arrays facilitating spatially and temporally correlated magnetometry.

Figure 1

(a) Sketch of the waveguide-assisted sensor array. Shallow-implanted N-$V^{-}$ centers act as local magnetometers on the diamond surface. The N-$V^{-}$ centers are optically accessed by means of laser-written type-II waveguides and low-numerical-aperture (low-NA) optics. The architecture can be extended to sensor arrays. The sketch incorporates microscope images of the waveguides in top and front view. (b) Sample fabrication process. First, the waveguide walls (black lines) are laser written from the top into the diamond. Second, nitrogen ions are shallow implanted into the front facet to form N-$V^{-}$ centers after annealing. (c) Level scheme and dynamics of the N-$V^{-}$ center as utilized to perform optically detected magnetic resonance (ODMR) spectroscopy. Dashed arrows depict phonon transitions and bold arrows photon transitions, respectively. The arrow width reflects the interaction strength. (d) Model for the splitting of the $m_s=\pm 1$ ground states with increasing magnetic field. The dashed lines indicate the magnetic field strengths that we apply in the experiments to perform ODMR scans.

Figure 2

(a) Confocal scan showing the fluorescence spots of the shallow-implanted N-$V^{-}$ centers on the front facet of the sample. The leftmost waveguide (marked with a green box) is studied in the following. (b) Mode shape of the waveguide (WG). The shape of the WG mode is revealed by laterally scanning a 738 nm laser while recording the collected counts in transmission. The two intersections at the side show the corresponding 2D Gaussian fit to the WG mode. The corresponding mode-polarization contrast is shown in the lower right corner. Note that the image is rotated by ${90}^{\circ }$ with respect to panel (a). (c) N-$V^{-}$ transmission through the waveguide. We excite the N-$V^{-}$ centers off-resonantly with 532 nm laser light from the front and collect the transmitted signal. Laterally scanning the excitation laser resembles the waveguide mode in transmission. The intersection of the dashed lines denotes the position which is at $61\mathrm{\%}$ of the maximum of the WG mode (green ellipse). (d) Experiment schematic. The sketch illustrates the two detection channels, reflection and transmission. (e) The PL spectra in reflection and transmission are compared. The exposure time of the transmission measurement is ten times larger than for the measurement in reflection. Both spectra are normalized to the ZPL. The inset shows the ZPL polarization measured in transmission through the waveguide, which resembles the waveguide mode polarization (marked as a green line). (f) We extract the wavelength-specific optical detection efficiency of the N-$V^{-}$ ensemble to the waveguide mode by comparing the transmitted PL spectrum with the fluorescence in reflection and by accounting for losses that occur until detection. The green curve is the inferred efficiency for a N-$V^{-}$ ensemble located at 61% of the optimal coupling corresponding to the green ellipse in (c). The black lines marks the $1/e^2$-profile of the Gaussian mode.

Figure 3

(a) Confocal scan of the backside of the sample with a low-NA objective and an enlarged view of the waveguide mode. (b) Comparison of the PL spectra in transmission (blue) and reflection (green) of the waveguide mode as well as the transmitted signal outside the waveguide mode which we assign to the background signal (red). The inset shows the experimental setup used for the measurement. (c) Comparison of the ODMR spectrum at zero magnetic field and with applied magnetic field detected in confocal configuration (blue, reflection) and in transmission through the waveguide (red, transmission). (d) ODMR scans with varying magnetic field strengths between 0 and 1.508 mT. The lowest plot is an enlarged view comparing the ODMR scans at 0.066 and 0.214 mT. We infer the magnetic field sensitivity from the accuracy of the resonance frequency extracted from the Lorentzian fits.

Figure 4

(a) Temperature $T$ dependence of the axial and transverse ZFS parameters $D$ and $E$. (b) ODMR scans at temperatures of 294 and 324 K are used to illustrate temperature sensitivity of the ZFS parameters.