Germanium-Vacancy Color Center in Diamond as a Temperature

Jan 22, 2018 - We present high-resolution, all-optical thermometry based on ensembles of germanium-vacancy (GeV) color center in diamond and implement...
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Germanium-Vacancy Color Center in Diamond as a Temperature Sensor Jing-Wei Fan, Ivan Cojocaru, Joe Becker, Ilya V Fedotov, Masfer Hassan A Alkahtani, Abdulrahman Alajlan, Sean Blakley, Mohammadreza Rezaee, Anna Lyamkina, Yuri N. Palyanov, Yuri M Borzdov, Ya-Ping Yang, Aleksei M Zheltikov, Philip R Hemmer, and Alexey V Akimov ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01465 • Publication Date (Web): 22 Jan 2018 Downloaded from http://pubs.acs.org on January 22, 2018

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Germanium-Vacancy Color Center in Diamond as a Temperature Sensor Jing-Wei Fan‡†◊, Ivan Cojocaru‡†, Joe Becker†, Ilya V Fedotov ┘ͳ, Masfer Hassan A Alkahtani₶ , Abdulrahman Alajlan†, Sean Blakley†, Mohammadreza Rezaee†, Anna Lyamkina┴ , Yuri N. Palyanov└╙, Yuri M. Borzdov└╙, Ya-Ping Yang◊, Aleksei Zheltikov† ┘ǂ, Philip Hemmer₶ , Alexey V Akimov†ǁ┘ † Texas A&M University, Department of Physics and Astronomy, 4242 TAMU, College Station, TX, USA ₶ Texas A&M University, Electrical & Computer Engineering Department, 3128 TAMU, College Station, TX, USA ◊ MOE Key Laboratory of Advanced Micro-Structured Materials, School of Physics, Science and Engineering, Tongji University, Shanghai 200092, China ǁ PN Lebedev Institute RAS, Leninsky Prospect 53, 119991, Moscow, Russia ┘Russian Quantum Center, 100A, Novaya Street, Skolkovo, 143025, Moscow Region, Russia └ Sobolev Institute of Geology and Mineralogy, Siberian Branch of the Russian Academy of Sciences, Koptyug Ave., 3, 630090, Novosibirsk, Russia ╙ Novosibirsk State University, 1, Pirogova str, 630090, Novosibirsk, Russia ┴ A.V. Rzhanov Institute of Semiconductor Physics, Siberian Branch of the Russian Academy of Sciences, Pr. Lavrent’eva 13, 630090, Novosibirsk, Russia

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 Physics Department, International Laser Center, M.V. Lomonosov Moscow State University, Moscow 119992, Russia ͳ Kazan Quantum Center, A.N. Tupolev Kazan National Research Technical University, 420126 Kazan, Russia ǂ Kurchatov Institute National Research Center, Moscow, Russia

KEYWORDS: thermometry, diamond, color center, all-optical high-resolution thermometry

ABSTRACT: We present high-resolution, all-optical thermometry based on ensembles of germanium-vacancy (GeV) color center in diamond and implement this method of thermometry in the fiber-optic format. Due to the unique properties of diamond, an all-optical approach using this method opens a way to produce back-action-free temperature measurements with resolution below 0.1 K in a wide range of temperatures.

Understanding the thermal properties of a living organism is a long-standing problem, since temperature is the most fundamental factor regulating all chemical reactions in vivo1–4. Living cells actively react to environmental changes in temperature and are likely to change their internal temperature during such processes as division, gene expression, enzyme reaction, and metabolism5,6. Therefore, real time monitoring of the thermal properties of cells and/or specific organelles opens the door to new possibilities for understanding intra-cell chemistry. Recently, high-resolution temperature measurement techniques made it possible to probe temperature fluctuations at the single-cell level, and even within the cell7–10. However, most biologically relevant temperature changes should be relatively small and transient, due to a cell’s strong interactions with its environment. Therefore, detecting this temperature change is quite challenging. ACS Paragon Plus Environment

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A number of different techniques for intra-cellular temperature mapping have been suggested in recent years. For example, fluorescent nanogel was used in combination with timeresolved photon counting, enabling temperature sensitivity of better than 0.5 degree inside the cell8. Local measurements with an ultrathin thermocouple were demonstrated to have a similar level of sensitivity9. Another interesting technique is based on quantum dots and the dependence of their photoluminescence spectra on temperature11. Probably the most precise measurement of temperature inside the cell was achieved using nitrogen-vacancy (NV) color center in diamond7,10. Here, by controllably heating a cell via laser illumination of a gold nanoparticle, researchers were able to achieve the temperature resolution well below 0.1 degree. In addition to providing superior sensitivity, this method has a number of advantages related to the intrinsic properties of diamond: it is chemically and physically inert, is not porous and has low toxicity, and thus is expected to have minimal effect on the cell (or organelle) functionality. In addition, the surface passivation chemistry of diamond nanoparticles is now relatively well developed12–16 enabling selective attachment of nanodiamonds to specific organelles of the cell, as required for in vivo measurement of their temperature response to various perturbations. While an NV-center-based sensor provides the best temperature sensitivity, it has a considerable limitation due to the fact that it requires the application of microwave radiation to the diamond. Since microwave radiation cannot be focused below a cell’s size, the cell must be exposed to its large dose. At the same time, microwave radiation can produce heating or otherwise influence cell chemistry17,18. A possible alternative for the NV center may be a siliconvacancy (SiV) center19 or the recently discovered germanium-vacancy (GeV) center, which has optical spectra dominated by a zero-phonon line, thus offering a possibility to measure temperature through detection of changes in the optical spectrum. The better-developed SiV color center also has a narrow spectral line, but a relatively low quantum efficiency. In contrast the GeV center has a nearly unitary quantum efficiency20 which minimizes possible heating effects due to the cell’s interaction with optical radiation. On another side the spectral line of the

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GeV center is outside of the near-infrared transparency windows in biological tissue which range approximately from 650 – 950 nm and 1000 – 1350 nm21,22 therefore we investigate a fiber based implementation of the sensor (see below).

RESULTS AND DISCUSSION The idea of GeV-based thermometry is based on optical measurements of the spectral shift of the zero-phonon line and its spectral width with the temperature changes. The GeV center has a similar level structure to the SiV center23–25. The temperature dependence of SiV energy levels was thoroughly analyzed by Jahnke et al26. The shift and broadening of the spectral lines of a SiV center are dominated by the second- and first-order processes of the corresponding electron-photon interaction in the excited state. The rate of electron-phonon interaction is much higher than the spontaneous decay rate of the exited state in a wide range of temperatures, leading to the strong temperature-dependent modification of the line width of transition and a shift of its position. Since two-phonon processes dominate over single-phonon processes, in the temperature ranges of interest, this interaction results in T 3 dependence of the zero-phonon line position. To verify that the physics of the temperature shift of the GeV color center spectral line is similar to that of the SiV, we first measured a temperature dependence of the GeV zero phonon line over a wide range of temperatures.

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Figure 1. A) Schematic of the experimental setup. Black outline presents cryostat volume. B) Level scheme of a GeV center. Color lines indicate allowed transitions. C) GeV center spectra at various temperatures. Solid lines correspond to fits, and dots to experimental data. Dashed vertical lines represent the “center of mass” of the zero-phonon line.

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Figure 2. Fitting details. A) Fit with 3 Lorentzian curves at room temperature. Dashed lines indicate individual Lorentzian curves, solid line stands for overall fit. B) Fit with 4 Lorentzian curves at low temperature. Inset demonstrates sharp features at the spectrum, which appear at low temperatures. These features do not affect fits of the zero phonon line position and width.

For our measurements, we used a home-built, 2-channel confocal microscope (see Figure 1A). One of the channels was connected to an avalanche photodiode, allowing the sample visualization and focusing; the other was connected to the spectrometer (see Supporting Information for more details).

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The zero-phonon line of the GeV center consists of 4 components, as shown in Figure 1B. At room temperature, splitting between these 4 lines is completely covered by temperaturedependent broadening and, therefore, the band could be considered a single peak (see Figure 1C). Besides the zero-phonon line, the spectrum of the GeV center has a considerable fraction of emission to the phononic sideband. To take into account the presence of this sideband, we performed a fitting of the spectrum with 3 Lorentzian curves, as shown in Figure 2A. As noted, the theoretic model26 predicts a cubic temperature dependence for the zero-phonon line width when temperatures are sufficiently high. Indeed, Figure 3B shows a dependence that is close to cubic, in agreement with the model used (see Supplementary information). As temperature decreases, line broadening also decreases, and at about 150 K, the zerophonon line splits into two peaks (Figure 3B & Figure 1C), corresponding to transitions A & B and C & D (Figure 1B). Starting from this point, fitting the spectrum with 3 Lorentzians becomes invalid. At temperatures lower than 150 K, 4 components/curves were used to account for the splitting of the zero-phonon line (see Figure 2B). To correlate the spectral position of the single line at higher temperatures with the doublet structure after splitting, one could calculate the “center of mass” of the doublet, as shown in Figure 1C. It is clear from Figure 3A that the “center of mass” of the doublet smoothly continues the dependence of the spectral shift of the single line on temperature, but deviates from the expected T 3 dependence. This is due to the fact that after the splitting, the relative intensities of the 2 lines in the doublet change with temperature (see Figure 1C). This change is rather easy to understand: the mixing of excited states by interaction with phonons leads to the rapid thermalization of the exited stated population. Therefore, the ratio of spectral line intensities should be proportional to the Boltzmann factor e −∆e

kT

where ∆ e is the energy splitting in the excited state and T is the

temperature. In this case, to calculate the “center of mass” of the doublet, one needs to use the Boltzmann factor to properly weight the spectral lines (see Figure 3A). One can see that this correction predicts the “center of mass” position reasonably well below 150 K. Unfortunately, at

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temperatures right above that of spectral line splitting, it is not practical to correct weights of the unresolved lines using the Boltzmann factor. Nonetheless, the Boltzmann factor is already affecting the observed line position, causing a slight deviation from the expected T 3 dependence. At lower temperatures, the two peaks continue to shift apart (see Figure 3A) and change their relative amplitude, slowly converting to a single line due to the Boltzmann factor in the low temperature limit. In principle, one could expect further splitting of the remaining line into another doublet due to the ground state splitting, but due to the limitation of our spectrometer, this splitting was not observed. To estimate an absolute uncertainty of our temperature measurements for randomly taken GeV color centers, we compared several bulk samples with different concentrations of GeV centers and different methods of center creation. As one can see from Figure 3B, for all 4 samples, the temperature dependence is the same. Nevertheless, there is a small offset of the line position at room temperature, which is most likely due to the different local strain, which leads to a considerable uncertainty in the absolute temperature measurement. This uncertainty could be estimated as 0.5 K when one averages all the data taken. The relative temperature measurement precision will depend on the range in which the thermometer is used since a response curve has a cubic dependence. In the most interesting range around room temperature ( T = 300 K ) the temperature response of both spectral line position and width is almost linear with a coefficient of k p for position and k w

3a pT 2 = 6.8(4) GHz/K (or k p −1 = 121(7) K/nm )

3awT 2 = 11.4(7) GHz/K for width. Coefficient a p for the cubic dependence

has a standard deviation of about 5% for different samples, which will translate into a relative temperature measurement uncertainty of the same 5%. An example of temperature measurement near room temperature was performed for sample #5 and is presented in Figure 4A . Here, the integration time was chosen to be one minute to resolve temperature changes below 0.1 K (see below). It should be noted that the spectral width dependence has a similar, though positive slope ACS Paragon Plus Environment

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and in principle could be used for temperature measurement (see Figure 4B), except with reduced precision.

Figure 3. A) Temperature dependence of the zero-phonon line position for sample #1. B) Temperature dependence of the zero-phonon line position for various samples fitted with

aT 3 + b dependence. Index S stands for the sample, and p for position on the sample. C) Width ACS Paragon Plus Environment

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of the zero phonon line versus temperature fitted with aT α + b dependence where α is used as a fit parameter. High power corresponds to 12 mW; low power corresponds to 1.4 mW of excitation power.

The minimum detectable change in temperature depends on the integration time and the collected count rate (or number of color centers used). Figure 4C demonstrates the Allan deviation measured on sample #5 having a high signal rate of 7 ⋅106 counts / sec corresponding to

about 600 color centers in the focal spot. The deviation shows a square-root dependence, matching shot noise predictions with a minimal detected temperature change of 300 mK

Hz

leading to uncertainty of less than 0.1 K in 10 seconds integration time (see Supplementary Information). Note that the Allan variance has not yet “bottomed-out” even for 30 sec of averaging time. To avoid uncertainty in absolute temperature measurements, one could pre-calibrate the sensor. In case of nanocrystals, when a nanodiamond is randomly selected and cannot be fully calibrated, a single-point calibration technique could be used. In this technique, the position of the spectral line of GeV center is first measured for a known temperature, and then a relative change of temperature is measured by change in the position of the GeV line. If calibrated by 1 point, the uncertainty of the absolute temperature measurement will be limited by several percent due to lack of precise knowledge of the slope of the response curve. At low temperatures, the main limitation is the resolution of our spectrometer which is needed to accurately measure the positions of individual spectral lines. As mentioned above the temperature dependence is no longer cubic below 150 K, but relying on the intensity ratios of the splitted spectral lines A and C (see Figure 1B) accurate measurements can be made down to temperatures of around 20 K. At temperatures between 20 and 4 K only line C survives, but this line in its turn splits into lines C and D whose relative amplitude is again driven by the Boltzmann factor. While we did not measure the upper temperature limit of our sensor, we estimate that at high temperatures the ACS Paragon Plus Environment

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gradual disappearance of the zero phonon line will greatly reduce precision beyond about 860 K, in analogy to the maximum temperature of observation for SiV27.

Figure 4. A) High-resolution temperature dependence of the GeV center zero-phonon line position with 1 minute integration time at each point. Red solid line indicates linear fit of the

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dependence. The fitted slope of the curve is indicated in the legend. B) Same for the GeV center zero-phonon line width. C) Allan deviation for sample #5. The slope indicates a square-root dependence of temperature measurement resolution. Temperature sensitivity of 0.3 K / Hz is achieved in these experiments. All the graphs are taken with a 7 ⋅106 counts/sec count rate.

For in-vivo biological applications, GeV color center based thermometer could be used if one integrates it with an optical fiber10. A fiber based sensor allows for a full calibration of the sensor before the measurement and therefore enables absolute temperature measurements with sensitivity only limited by the signal-to-noise ratio and the integration time. To demonstrate GeV-center-based thermometry in the fiber format, we crushed sample #4 into microdiamonds. One of the microdiamonds was then attached to the tip of a multimode optical fiber using a technique described in detail elsewhere28,29. GeV centers were excited by a second-harmonic, 532-nm output of a continuous-wave Nd: YAG laser (Figure 5A). With the microdiamond properly coupled to an optical fiber, both SiV and GeV color centers are found to be efficiently excited by the laser radiation that is delivered through the fiber. With a proper design of the fiber probe28,29, this fluorescence response can be efficiently collected by the same fiber, which then delivers it to a detector. The fiber-coupled fluorescence spectra are dominated by well-resolved, intense, narrowband zero-phonon lines (Figure 5B). As the thermostat temperature was gradually varied in a precisely controlled fashion, the zero-phonon lines displayed spectral shifts and broadening (Figure 5C), closely following all the above-described tendencies. The temperature dependencies and sensitivities also closely matched the above results, thus demonstrating a robust operation of the sensor in a format compatible with in-vivo biological applications.

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Figure 5. A) Experimental setup for all-optical temperature measurements using split-vacancy centers integrated with a fiber. B) Fluorescence spectra from a diamond with split-vacancy centers collected through an optical fiber. C) Fluorescence spectra from GeV centers measured for four different temperatures (as specified in the figure).

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In conclusion, we proved that the GeV spectral line depends on temperature via coupling of excited states to lattice phonons. Based on this mechanism/dependence, an all-optical, highresolution thermometry technique was proposed that covers a wide temperature range from 150 K to 400 K with potential extension to the 4 – 860 K range. Temperature measurements from low to room temperature were demonstrated. The precision of the thermometer was shown to be as high as 0.3K / Hz for about 600 color centers. Finally, a fiber optic realization of sensor was demonstrated in a biologically relevant range of temperatures to establish the potential of this temperature sensor for a variety of biological applications, including in vivo measurements. ASSOCIATED CONTENT

Supporting Information. The following files are available: PDF, with details of experimental setup, samples used and fitting procedure AUTHOR INFORMATION Corresponding Author

Alexey Akimov, [email protected] Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. ACKNOWLEDGMENT We thank Denis Sukachev, Alp Sipahigil, Christian Nguyen and Rufin Evans for fruitful discussions and samples #1 and #2.

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This research was supported in part by the Russian Foundation for Basic Research (16-0200843, 17-52-53092, and 17-00-00212), Russian Science Foundation (project no. 17-12-01533, assembling fiber based thermometer), Welch Foundation (Grant No. A-1801), the Government of Russian Federation (project no. 14.Z50.31.0040, Feb. 17, 2017), and ONR (Award No. 00014-16-1-2578). National Natural Science Foundation of China (Grants No. 11474221, and No. 11574229), the Joint Fund of the National Natural Science Foundation of China (Grant No. U1330203), International Exchange Program for Graduate Students, Tongji University and the Ministry of Education and Science of the Russian Federation in the framework of increase Competitiveness Program of NUST ‘‘MISIS”, implemented by a governmental decree dated 16th of March 2013, No 211. HPHT synthesis and characterization of Ge-doped diamond were accomplished with support from the Russian Science Foundation (Grant No. 14-27-00054). Agreement No. 14.W03.31.0028 with host organization ZPTI of FRC KazanSC of RAS ABBREVIATIONS NV nitrogen-vacancy, SiV silicon-vacancy, GeV germanium-vacancy. REFERENCES

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Lagomarsino, S.; Gorelli, F.; Santoro, M.; Fabbri, N.; Hajeb, A.; Sciortino, S.; Palla, L.; Czelusniak, C.; Massi, M.; Taccetti, F.; Giuntini, L.; Gelli, N.; Fedyanin, D. Y.; Cataliotti, F. S.; Toninelli, C.; Agio, M. Robust Luminescence of the Silicon-Vacancy Center in Diamond at High Temperatures. AIP Adv. 2015, 5 (12), 127117.

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Fedotov, I. V.; Doronina-Amitonova, L. V.; Voronin, a. a.; Levchenko, a. O.; Zibrov, S. A.; Sidorov-Biryukov, D. a.; Fedotov, a. B.; Velichansky, V. L.; Zheltikov, a. M. Electron Spin Manipulation and Readout through an Optical Fiber. Sci. Rep. 2014, 4, 5362.

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Fedotov, I. V.; Doronina-Amitonova, L. V.; Sidorov-Biryukov, D. A.; Safronov, N. A.; Blakley, S.; Levchenko, A. O.; Zibrov, S. A.; Fedotov, A. B.; Kilin, S. Y.; Scully, M. O.; Velichansky, V. L.; Zheltikov, A. M. Fiber-Optic Magnetic-Field Imaging. Opt. Lett. 2014, 39 (24), 6954.

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Figure 1. A) Schematic of the experimental setup. Black outline presents cryostat volume. B) Level scheme of a GeV center. Color lines indicate allowed transitions. C) GeV center spectra at various temperatures. Solid lines correspond to fits, and dots to experimental data. Dashed vertical lines represent the “center of mass” of the zero-phonon line. 108x157mm (300 x 300 DPI)

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Figure 2. Fitting details. A) Fit with 3 Lorentzian curves at room temperature. Dashed lines indicate individual Lorentzian curves, solid line stands for overall fit. B) Fit with 4 Lorentzian curves at low temperature. Inset demonstrates sharp features at the spectrum, which appear at low temperatures. These features do not affect fits of the zero phonon line position and width. 107x145mm (300 x 300 DPI)

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Figure 3. A) Temperature dependence of the zero-phonon line position for sample #1. B) Temperature dependence of the zero-phonon line position for various samples fitted with a·T3+b dependence. Index S stands for the sample, and p for position on the sample. C) Width of the zero phonon line versus temperature fitted with Tα+b dependence where α is used as a fit parameter. High power corresponds to 12 mW; low power corresponds to 1.4 mW of excitation power. 96x202mm (300 x 300 DPI)

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Figure 4. A) High-resolution temperature dependence of the GeV center zero-phonon line position with 1 minute integration time at each point. Red solid line indicates linear fit of the dependence. The fitted slope of the curve is indicated in the legend. B) Same for the GeV center zero-phonon line width. C) Allan deviation for sample #5. The slope indicates a square-root dependence of temperature measurement resolution. Temperature sensitivity of 0.3·K/√Hz is achieved in these experiments. All the graphs are taken with a 7·106 count rate. 96x207mm (300 x 300 DPI)

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Figure 5. A) Experimental setup for all-optical temperature measurements using split-vacancy centers integrated with a fiber. B) Fluorescence spectra from a diamond with split-vacancy centers collected through an optical fiber. C) Fluorescence spectra from GeV centers measured for four different temperatures (as specified in the figure). 98x192mm (300 x 300 DPI)

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