Parallel accelerated electron paramagnetic resonance spectroscopy using diamond sensors

Abstract

The nitrogen-vacancy (NV) center can serve as a magnetic sensor for electron paramagnetic resonance (EPR) measurements. Benefiting from its atomic size, the diamond chip can integrate a tremendous amount of NV centers to improve the magnetic-field sensitivity. However, EPR spectroscopy using NV ensembles is less efficient due to inhomogeneities in both sensors and targets. Spectral line broadening induced by ensemble averaging is even detrimental to spectroscopy. Here we show a kind of cross-relaxation EPR spectroscopy at zero field, where the sensor is tuned by an amplitude-modulated control field to match the target. The modulation makes detection robust to the sensor's inhomogeneity, while zero-field EPR is naturally robust to the target's inhomogeneity. We demonstrate an efficient EPR measurement on an ensemble of roughly 30000 NV centers. Our method shows the ability to not only acquire unambiguous EPR spectra of free radicals, but also monitor their spectroscopic dynamics in real time.

Figures (4)

• Figure 1: The parallel acceleration detection scheme. (a) An illustrative diagram of NV center (blue arrow) detecting electron spins (red arrow). Upper panel: The structure of NV center and nitroxide radical spin. Lower panel: Spectroscopic measurements necessitate the application of external fields, such as optical pulses, bias magnetic field, and control fields. In widefield setup, NV centers measure an ensemble of target radicals on surface in nano-meter scale distances. (b) Schematic diagram of cross-relaxation detection. Depending on whether the energy levels of the sensor and target spins match, the relaxation of the sensor NV will be accelerated or remain unchanged. (c) Schematic diagram of cross-relaxation based EPR spectroscopy. Left Panel: By applying an amplitude-modulated microwave sequence, all sensor spins are synchronized to have the same energy levels determined by the modulation frequency $f$. Right Panel: Different colored energy levels indicate different orientations of the targets, corresponding to the 'emission' of different colors of light. By conducting experiments at zero field, the energy levels of all target spins are equivalent regardless of orientations. The modulation frequency is then swept to obtain spectra.
• Figure 2: Experimental demonstration of parallel measurements. (a) Experimental setup of the widefield microscopy. (Abbreviations: APD: Avalanche photodiodes, LP: Long pass filter, DM: Dichroic mirror, Obj.: Microscope objective). (b) Rabi oscillation of NV ensembles. The points are experimental results, while the line is a cosine damping fit. (c) Relaxation measurements. The inset gives the pulse sequence, where the polarization time and readout window are $1.04$ and $0.52\ms$, respectively. The points are experimental results, while the lines are exponential fits giving $T_1=3.7\pm0.1ms$, $T_1^{'}=1.3\pm0.5ms$. The driving strength and modulation frequency are $35$ and $50\MHz$, respectively. (d) NV-EPR measurement of $^{14}$N-TEMPO methacrylate (inset: chemical structure). The points are experimental results, while the line is a Gaussian fit. The spectrum in the upper panel is a prediction according to the conventional EPR measurements. Other measurement parameters are: Evolution time $0.9m\s$, polarization time $0.41\ms$, readout time $0.2\ms$, $\kappa = 0.57$, and the laser power density $101W\per\cm\squared$. (e) NV-EPR measurement of $\text{4-Oxo-TEMPO-d}_{16}$, $^{15}$N (inset: chemical structure). The points are experimental results, while the line is a 3-peak Gaussian fit. The spectrum in the upper panel is a prediction according to the conventional EPR measurements. Other measurement parameters are: Evolution time $1m\s$, polarization time $0.76\ms$, readout time $0.25\ms$, $\kappa = 0.71$, and the laser power density is $81W\per\cm\squared$.
• Figure 3: Dynamics of nitroxide EPR spectra (a) Real-time monitoring of the spectral lines, showing a decaying trend. The points are experimental results, while the lines are 3-peak Gaussian fits. (Evolution time $1.2m\s$, polarization time $1.01\ms$, readout time $0.5\ms$, $\kappa = 0.71$, and the laser power density is $37W\per\cm\squared$). (b) Schematic diagram of the possible laser-quenching process. Gray points indicate loss of spin signal. (c) Dependence of the fitted peak area on total illumination time. Gray line is a further exponential fit ($y = A\exp(-t/t_d)+y_0$) to the fitted data (characteristic time $t_d = 18\pm1h$). (d) Dependence of the fitted linewidth of peak 3 on total illumination time. The error bars are the fitting errors. The hatched area represents the upper and lower limits of the fit to avoid divergence. The gray dashed line is a guide to the eye.
• Figure 4: Dependence of quenching process on laser power density. (a) Sequence of quenching experiment under high-power laser. The power of the laser quenching block($\sigma$) applied between each sequential measurement is fixed. (b) Quenching of EPR signal of $^{14}$N-TEMPO methacrylate under different laser power density. Data points are fitted peak areas of the sequential EPR spectra, while the lines are exponential fits, where the characteristic time is defined as the nitroxide lifetime. (c) Dependence of nitroxide lifetime on laser power density. Points are the mean lifetime of repeated quenching measurements with error bars indicating standard error of the mean. Line is a fit with formula of $\log{t_d} = a\log{\sigma} + b$, which gives $a=-1.0(2),b=14(1)$, indicating a linear dependence of the quenching rate ($1/t_d$) on laser power density.