Efficient photon detection from color centers in a diamond optical waveguide

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Efficient photon detection from color centers in a diamond optical waveguide

D. Le Sage, L. M. Pham, N. Bar-Gill, C. Belthangady, M. D. Lukin, A. Yacoby, and R. L. Walsworth

Phys. Rev. B 85, 121202(R) – Published 23 March 2012

Abstract

A common limitation of experiments using color centers in diamond is the poor photon collection efficiency of microscope objectives due to refraction at the diamond interface. We present a simple and effective technique to detect a large fraction of photons emitted by color centers within a planar diamond sample by detecting light that is guided to the edges of the diamond via total internal reflection. We describe a prototype device using this “side-collection” technique, which provides a photon collection efficiency $\approx 47 %$ and a photon detection efficiency $\approx 39 %$ . We apply the enhanced signal-to-noise ratio gained from side collection to ac magnetometry using ensembles of nitrogen-vacancy (NV) color centers, and demonstrate an ac magnetic field sensitivity $\approx 100 pT / \sqrt{Hz}$ , limited by added noise in the prototype side-collection device. Technical optimization should allow significant further improvements in photon collection and detection efficiency as well as subpicotesla NV-diamond magnetic field sensitivity using the side-collection technique.

Received 24 December 2011

DOI:https://doi.org/10.1103/PhysRevB.85.121202

Authors & Affiliations

D. Le Sage^1,2, L. M. Pham³, N. Bar-Gill^1,2, C. Belthangady^1,2, M. D. Lukin², A. Yacoby², and R. L. Walsworth^1,2,*

¹Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138, USA
²Physics Department, Harvard University, Cambridge, Massachusetts 02138, USA
³School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA

^*rwalsworth@cfa.harvard.edu

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Issue

Vol. 85, Iss. 12 — 15 March 2012

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Images

Figure 1
(a) The NV color center consists of a substitutional nitrogen atom (N) adjacent to a vacancy (V) in the diamond lattice. (b) In the side-collection technique, a focused laser beam excites color centers in a specific volume within the diamond, and much of the resulting fluorescence is detected after it exits one or more sides of the diamond waveguide. In the demonstration experiments reported here, four photodetectors are arranged around the four primary sides of the diamond chip. (c) Ray diagrams illustrate refraction at the diamond surface and light guided by total internal reflection above the critical angle ( $θ_{c} = 24 . 6^{\circ}$ ). (d) Red-filtered photograph of NV fluorescence from the diamond chip used in the demonstrations reported here. Guiding of NV fluorescence light is evident as a bright glow around the diamond's perimeter, while a 532-nm laser beam passes through its center.Reuse & Permissions
Figure 2
(a) Prototype device to compare the photon collection and detection efficiency of the side-collection technique with that of a 0.4-NA microscope objective (Obj), using a high NV-density diamond chip, 532-nm excitation laser, dichroic mirror (DM), 650-nm long-pass filter (LP), and Si photodiodes (PD). (b) Photo of the side-collection prototype device (partially disassembled for viewing of the interior). The inset shows the locations of the LPs and PDs in relation to the diamond chip. (c) Side-collection and microscope objective optical signals were recorded using a charge-sensitive amplifier. (d) NV fluorescence (number of photons) detected by the two collection modalities during a 300-ns pulse of the excitation laser (532 nm), as a function of laser power, shows a 100-times larger side-collection signal.Reuse & Permissions
Figure 3
(a) Optical and microwave spin-echo pulse sequence used for NV-diamond ac magnetometry with the side-collection prototype device (Refs. 11 and 14). An optical pump pulse prepares the NV spins in the $| m_{s} = 0 〉$ state; a microwave spin-echo sequence allows the NV spins to probe an applied 20-kHz magnetic field; and an optical readout pulse allows measurement of the change in NV spin-dependent fluorescence compared to the initial state. (b) By varying $τ$ with $B_{ac} = 0$ , we measure the NV spin-echo decoherence curve shown, with a greatly enhanced signal-to-noise ratio (SNR) provided by the side-collection technique. (c) Varying $B_{ac}$ with $τ = 50 μ s$ produces the NV-diamond ac magnetometry curve shown. The zoomed-in region of the curve near $B_{ac} = 0$ depicts the mean and standard deviation of a large number of ac magnetometry measurements, each lasting $57 μ$ s. The $\pm 0.071 %$ uncertainty in the signal (vertical error bars) corresponds to a $\pm$ 13 nT uncertainty in $B_{ac}$ (horizontal error bars) for a single measurement, yielding 100 pT/ $\sqrt{Hz}$ sensitivity for a laser excitation volume $\sim 10^{- 4}$ mm $^{3}$ .Reuse & Permissions

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