Nanodiamonds with photostable, sub-gigahertz linewidths quantum emitters

Single photon emitters with narrow linewidths are highly sought after for applications in quantum information processing and quantum communications. In this letter, we report on a bright, highly polarized near infrared single photon emitter embedded in diamond nanocrystals with a narrow, sub GHz optical linewidths at 10K. The observed zero phonon line at ~ 780 nm is optically stable under low power resonant excitation and blue shifts as the excitation power increases. Our results highlight the prospect for using new near infrared color centers in nanodiamonds for quantum applications.

In this work, we explore the optical properties and the linewidth of SPEs embedded in a chemically vapor deposition (CVD) grown nanodiamonds. At room temperature, this emitter have a linewidth of ~ 4 nm and a ZPL at ~ 780 nm, while at cryogenic temperatures, the linewidth reduces ~ 660 MHz, only slightly broader than the Fourier Transform (FT) limited linewidth. These properties of our emitter make them extremely promising for a variety of quantum photonic applications.
Furthermore, the sub GHz linewidths from defects in nanodiamonds, rather than in a bulk diamond crystal, offers interesting possibilities to realize hybrid quantum photonic networks [23,24].
The diamond nanocrystals were grown from detonation nanodiamond seeds (diameter 4-6 nm) that were spin coated on sapphire substrates. The use of sapphire substrates assists in minimizing incorporation of silicon atoms that would otherwise result in undesired silicon doping. The samples were then loaded into a microwave plasma chemical vapor deposition (MPCVD) system and the crystals were grown in mixed gases (hydrogen:methane = 100:1) with a microwave power of 900W, at 60 Torr of atmospheric pressure for 30 minutes. Under these conditions, nanodiamonds with diameters of ~0.3 -1 m were grown.
The optical properties of the emitters were studied using a home-built laser scanning confocal photoluminescence (PL) microscope equipped with a continuous wavelength tunable Titanium Sapphire laser (linewidths, 100 KHz) (figure 1a). The sapphire substrate with the grown diamond nanocrystals were mounted onto a cryostat equipped with a high-precision XYZ piezo scanning stage and were cooled to 10K using liquid helium. The excitation and collection were done via a high numerical aperture (NA = 0.9) objective mounted inside the cryostat, creating an excitation and collection spot size of ~ 430 nm. The signal collected from the emitters was analyzed with both a spectrometer equipped with a high-resolution silicon-based charge coupled device (CCD) camera and a Hanbury-Brown and Twiss (HBT) interferometer.
On average, two to three emitters were found in a 60 x 60 m 2 scan area. Bright spots that correlate  figure S1). For the remaining of the manuscript, we focus on a particular line at 780 nm. The line was selected arbitrary, with a goal to study emitters that emit further in the NIR. Consequently, figure 1c shows the narrow ZPL at 780 nm with a FWHM of ~ 0.16 nm that corresponds to our spectrometer resolution. Inset is the spectrum of the same emitter recorded over broader spectral range. The spectra were recorded at 10 K. From the spectrum, we deduced a Debye-Waller (DW) factor, = ⁄ = 0.87, a higher value than that of nitrogen-vacancy color center in diamonds (0.04) and comparable with the SiV and GeV defects).
To verify that the emitter is indeed a single photon emitter, a second-order autocorrelation function, 2 ( ), was recorded and is shown in figure 1d. The data is fitted by employing a standard three-level model for the color center: (2) where a is the bunching factor, while t1 and t2 are the lifetimes of the excited and metastable state, respectively.
The fit yields a value of 0.26 for g (2) (), indicating the quantum nature of the emission. The saturation curves of the SPE and its polarization properties are shown in the supporting information (figure s2).
To gain more information about the coherent properties of the SPEs, and unveil its natural linewidth, resonant exciton measurements were performed. While cross polarization schemes are often used for studying quantum dots [25], they were not practical in our measurement due to the high scattering from the diamond nanocrystals. To filter the exciting laser, we used a long-pass filter to collect only the emitter's phonon side band (PSB). suggesting that the optical linewidth is less prone to spectral diffusion [21,26,27], as will be discussed later.
The excited state lifetime of the emitter was measured using a pulsed laser to find out whether the emitter's linewidth is Fourier Transformed (FT) limited. The results are shown in Figure 2b. Using a single exponential fit, the lifetime of the excited state of the emitter was determined to be τ1 = 1.1 ns. By applying the expression: γ = 1 2 1 ⁄ where γ and τ1 are the FT limited linewidth (natural linewidth) and the excited state's lifetime of the emitter, respectively, we estimated a value for γ of 145 MHz. This means that the emitter's linewidth is ~ 4.5 times broader than the expected value.
The linewidth broadening can arise from numerous factors including ultrafast spectral diffusion due to interaction of the strong emitter dipole with fluctuating electric field from surrounding defects (inhomogeneous broadening), or alternatively, a homogeneous broadening due to phonon coupling. In addition, spectral diffusion often results in intensity fluctuations, associated with slow frequency jumps, which were not observed in our case. Combined with a more favorable Lorentzian fit, we therefore conclude that the line broadening is predominantly due to phonon interactions. Similar behavior was also reported for the SiV [15,28] and the GeV [17] in diamond.
Despite the line broadening, achieving a sub GHz linewidths from single emitters in nanodiamonds, particularly at the NIR spectral range is valuable, as it opens excellent pathways to couple these emitters to high quality optical resonators and photonic cavities [23,24].
Next, we measured the emitter's linewidth off resonantly as a function of laser power to determine the photostability of the emitter under increasing excitation power. Figure 3 (a, b) show the optical stability of the emitters under a low 300 W and high 3 mW excitation laser power, respectively.
For these measurements, a total of 200 PL spectra were acquired at intervals of 200 ms. At low power (300 W), almost no spectral diffusion or blinking to a different frequency was witnessed (within the spectrometer resolution). On the other hand, at high excitation power of ~ 3 mW the spectral fluctuation of the ZPL were noticeable. Spectral jumps as large as ~1.5 nm away from the ZPL position were observed, several orders of magnitude higher than the measured FWHM of ~ 660 MHz. This suggests that the emitter is highly susceptible to the strength of the laser electromagnetic field, and it may possess a linear permanent dipole behavior that may result in spectral diffusion under increased excitation power. Additionally, an increased pumping intensity may result in a photo-ionization, similarly to what occurs with the NV center [29]. Photoionization will result in a frequency drift, and can be evidenced as blinking. Note, however, that the spectral jumps occurred on second time scale, which means techniques such as dynamic stabilization using applied electric fields can be employed to stabilized the emitter under high excitation power. [30] In addition to the power induced blinking, the ZPL exhibits broadening as a function of excitation power. Figure 3c shows several spectra from the same SPE under increased excitation power which reveals power broadening and a blue shift in ZPL spectral positions. Figure 3d presents a plot of the FWHM values as a function of power, showing a good fit with a logarithmic function. These measurements are in agreement with previous optical studies on carbon nanotubes, suggesting that the broadening arises from an increase of the local temperature induced by an increase in power of the excitation laser. [31,32] Finally, we discuss the potential origin of the emitters. Amongst color centers in diamond, only SiV, GeV and an unknown NIR defect [21] exhibited narrow lines amenable to resonant excitation.
Our emitters cannot be attributed to the SiV, since we did not observe splitting into 4 spectral lines at cryogenic temperatures -a typical signature of a single SiV defect. The emitter's ZPL at ~ 780 nm is also far from the standard SiV emission centered at 738 nm. [14] The defects can be attributed to Cr related defects, as the sample was grown on sapphire, in a similar procedure as was reported in previous works [33], although previous measurements from these emitters revealed a broader GHz lines [21]. Another viable explanation is the recently discovered narrowband emitters that appear at secondary nucleation cites and extended defects in CVD grown nanodiamonds. [34] As seen from SEM image in figure S1a, some nanodiamonds possess such defects, and therefore give rise to the narrowband PL lines, as indeed observed in our experiments. While we cannot conclusively identify the origin of the emitters, or its crystallographic structure, the scalable production of these emitters from a standard growth procedure enables unique opportunities to explore quantum optics experiments and future applications with single emitters in nanodiamonds.
In     3mW (b). All the spectra shown were normalized. c) Power-induced linewidth broadening measurements for the same emitter with laser power increasing from 10 W to 3mW. The open markers and solid lines are experimental and fit data, respectively. The red semitransparent arrow serves as a guide-to-theeye for the shift in the spectra. d) Lorentzian fitted FWHM of the linewidths of the emitter from (c). The spectral broadening is evident. The data is fit with a log function.