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Mechanical decoupling of quantum emitters in hexagonal boron nitride from low-energy phonon modes

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Science Advances  30 Sep 2020:
Vol. 6, no. 40, eaba6038
DOI: 10.1126/sciadv.aba6038
  • Fig. 1 Characterization and spectral properties.

    (A) Illustrative drawing of quantum emitters in hBN. (B) PL spectrum of emitter A. Emitter A yields an intense ZPL at 651 nm with low background. The inset shows the ZPL in high resolution, thereby revealing two distinct peaks that we identify as independent ZPLs. Since one peak is clearly dominating, the latter one can be treated as background. arb. units, arbitrary units. (C) PL spectrum of emitter B. The PL spectrum of emitter B is dominated by a ZPL at 673 nm. The ZPL in high resolution (see inset) shows a dominant ZPL together with a second, less intense peak compared to emitter A. (D) Correlation function of emitter A. The correlation function of emitter A with ZPL at 651 nm at 5 and 295 K is plotted here (blue dots), together with a fit to the data (red curve), revealing single quantum emitter characteristics with a dip at 0-ns delay below 0.5 at both temperatures. The gray lines mark the threshold at 0.5 for single-photon emitters. (E) Inhomogeneous linewidth of emitter A. Here, we illustrate 10 subsequent scans over the ZPL resonance. The total inhomogeneous linewidth with a Lorentzian fit to the data is shown in the top panel. Below, we plot the single scans in color scale. The red dots and error bars denote the line positions and linewidths resulting from Lorentzian fits, respectively. (F) Lifetime limited homogeneous lines at 5 K and room temperature. Scanning the excitation laser wavelength over the ZPL resonance reveals lifetime limited linewidths when fitting a Lorentzian (red curve) to the measurement data (blue dots) at both, cryogenic temperatures (left) and room temperature (right).

  • Fig. 2 Polarization-dependent PL and phonon mode decomposition.

    (A) PL spectrum. Here, we display the full spectrum (blue curve) of emitter A. For better visibility, the gray background illustrates the magnified PSB. For comparison, we plot the one-phonon band, derived from PSB decomposition in red. (B) Comparison of the one-phonon band and the phonon band structure. Each band in the band structure is labeled by its symmetry at the Γ-point (48). The green highlighted peaks in the one-phonon band (plotted in red at the left) are coincident with bands leveling off at high symmetry points in the band structure. The gray bars indicate peaks that are not coincident with any features in the band structure. The phonon mode marks label L as in-plane longitudinal, T as in-plane transverse and Z as out-of-plane combined with A for acoustic or O for optical. (C) The B1g/A2u modes at the A-point. Here, we illustrate the atom motion resulting in the B1g/A2u bands at the A-point. (D) The A2u (top) and B1g (bottom) modes at the Γ-point. This out-of-plane mode is sketched here with arrows denoting atom motion. (E) Polarization-dependent PL. The image shows a polarization-dependent PL spectroscopy with the logarithmic color scale at the ZPL at 651 nm being cut off for better total visibility. (F) Polarization-dependent PL in high resolution. Here, we plot polarization-dependent PL of the PSB close to the ZPL with high-resolution spectra. For better visibility, the color scale is logarithmic. (G) Absorption and emission polarization (532 nm). The red data points, together with the red fit, denote the polarization of the 532-nm laser light absorbed for PL spectroscopy. The polarization of light emitted from the emitter is plotted with blue dots and a blue curve marking the measurement data and the fit, respectively. Both measurements reveal a clearly distinct orientation of the 532-nm absorption and the emission dipole.

  • Fig. 3 Coupling to excited state PSB.

    (A) PLE measurement scheme for excitation via PSB. The blue curve shows the spectrum of emitter A, with the magnified PSB in red. The PLE excitation wavelengths are shaded in red. For detection, we integrate over ZPL or PSB as intensity measure (gray shaded areas). (B) Polarization of first phonon mode. We excite emitter A 6.8 meV detuned from resonance. We fit the results with a sine (red), yielding a polarization angle of 23°. For comparison, we plot the ZPL emission polarization in blue. (C) Polarization-dependent PLE at the excitation PSB. In the right panel, blue and red dots represent results for excitation with parallel and perpendicular polarized light. Circles and triangles denote two different measurement runs, using the optical mode for detection. Squares extend the measurement to higher energy differences with data extracted from the ZPL integral. The gray shaded spectrum is a mirrored PL spectrum for comparison. In the left panel, we illustrate the extracted gap width with rescaled intensity. (D) PLE at the excitation PSB with emitter B. We plot results extracted from the ZPL integral for parallel (blue) and perpendicular polarization (red). The dark gray shaded curve is the PL spectrum mirrored at the ZPL. The magnified PSB is plotted in lighter gray.

  • Fig. 4 Temperature-dependent gap size.

    (A) Gap size analysis procedure. For evaluation of the gap size, we fit a Gaussian (dashed red curve) to the ZPL in the PL spectrum (blue curve) and to the PSB. The inset shows the fitted PSB (green curve) with the PL spectrum (blue dots). (B) Gap size distribution. We determine the gap size of multiple emitters at 5 K and display the abundance distribution, peaking at 2 THz. (C) Evolution of the gap size. Here, we measured the gap size (blue points) and the half width of the ZPL (red squares) for different temperatures. The data here stem from emitter A, which we used for resonant PLE and PLE at the excitation PSB. For extrapolation, we assumed proportionality of both measurement values to the Boltzmann factor. The fitted curves are plotted in blue and red for the according parameters, respectively. (D) Evolution of the gap size. As in (C), we depict the gap data for emitter B using the same labeling.

  • Fig. 5 AFM characterization.

    Here, we present the key results of an AFM characterization of flakes hosting quantum emitters with and without gap between ZPL and PSB. The left plot (A) depicts the tilt angle of host flakes versus the gap size of the emitters. The dashed black line marks 12° tilt, thereby highlighting the minimum tilt for emitters with gap. In (B), we illustrate the flake that provides the data of the green triangle in (A). The white dashed line in the three-dimensional (3D) AFM height profile marks the segment we use for measuring the tilt of the flake. Below, we plot this 2D height profile (C), together with a straight dashed line illustrating the tilt angle determination. In the same manner, we exemplary picture one flake hosting an emitter with gap in the right panel (D). The 2D height profile (E) reveals the strong tilt of the host flake resulting in the red squared data point in (A).

Supplementary Materials

  • Supplementary Materials

    Mechanical decoupling of quantum emitters in hexagonal boron nitride from low-energy phonon modes

    Michael Hoese, Prithvi Reddy, Andreas Dietrich, Michael K. Koch, Konstantin G. Fehler, Marcus W. Doherty, Alexander Kubanek

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