Detuning-insensitive wide-field imaging of vector microwave fields with diamond sensors

TL;DR

Addresses the problem of robust, high-resolution vector microwave-field imaging with NV-diamond sensors in the presence of detuning fluctuations. Introduces a detuning-insensitive method based on MW-power–induced ODMR linewidth broadening measured via amplitude-modulated microwaves and lock-in detection, enabling vector reconstruction with four NV orientations. Demonstrates 800 nm wide-field imaging and a single-axis sensitivity of about $600\, \mathrm{nT}/\sqrt{\mathrm{Hz}}$, achieving detailed near-field maps around an Ω-shaped antenna and four-direction vector components. The approach offers fast, large-dynamic-range microwave microscopy suitable for circuit diagnostics, magnon studies, and dielectric mapping, with potential for portable, robust sensing using diamond sensors.

Abstract

Nitrogen vacancy (NV) centers in diamond have precipitated profound advances in microwave detection, manifesting themselves both in spatial resolution and sensitivity. However, typical methods based on Rabi oscillations are subject to detunings due to thermal and magnetic fluctuations and/or gradients, which introduce systematic errors and render the measurements susceptible to environmental perturbations. Here, we propose and demonstrate a novel approach for determining both the magnitude and direction of microwaves, by exploiting the spectral line broadening effect in the optically detected magnetic resonance of NV centers. This method eliminates the requirement of aligning the MW frequency to the spin transitions and is therefore immune to variations and inhomogeneities of the magnetic field and temperature, providing an optimal tool for fast imaging applications. With this method, we achieved wide-field imaging of near field microwaves generated with a microscale $\rmΩ$-pattern antenna with a resolution of 800\,nm. Combining with the vector detection using multi-axis NVs, a full reconstruction of the vector microwave fields is obtained. Besides, our scheme also exhibits excellent linearity over a broad range of MW amplitudes, and the scale is theoretically calculated to be more than four orders. Our results augment the applicability of diamond-based microwave devices in applications under complex scenarios, especially where large dynamic range, fast test speed, and high spatial resolution are demanded.

Figures (4)

• Figure 1: (a) The energy levels of the electron spin of the NV center, and their interaction with the MW field. (b) The sensing scheme of MW detection. The laser remains continuously activated while the MW signal is square-wave amplitude-modulated with a depth of 80 %. (c) The black curve represents the ODMR spectrum obtained under lower MW power, while the red corresponds to full MW power. The subtracted signal is shown by the blue curve. (d) The theoretical dependence of the linewidth of the subtracted spectrum on the MW magnitude.
• Figure 2: (a) Schematic diagram of the vector microwave field imaging setup. The optical system comprises a laser confocal scanning system and a wide-field imaging system. In the wide-field system, a lens is placed in front of the beam splitter to ensure uniform illumination of the spot on the sample in Köhler illumination mode, and the spot covers the "$\Omega$"-shaped antenna area. (b) Confocal microscopy image. The dark fluorescent region corresponds to the antenna. The white dot indicates the interrogated region. The coordinate system is defined as indicated: the x- and y-axes lie parallel to the diamond surface, whereas the z-axis is oriented perpendicularly to surface. (c) After applying a biased magnetic field, the AM ODMR spectrum exhibit splittings at each peak under modulated MW irradiation.
• Figure 3: (a-b) The Rabi oscillations and AM ODMR spectral lines of NV centers with spatial orientations closest to the direction of the external magnetic field under varying MW power levels. The MW power input to the omega antenna, corresponding to the data lines from top to bottom, is 15.8 mW, 25.1 mW, 39.8 mW, 63.1 mW, 100 mW, and 158.5 mW, respectively. (c) The data and fitting curve of the left peak in the AM ODMR shown in Fig. \ref{['fig:Experimental_setup']} c. (d) The calibrated magnetic component of the MW field detected by NVs as a function of the applied MW powers. The data is fitted with a stringent linear function.
• Figure 4: (a-d) The distribution of the microwave field generated by the omega antenna was examined in the x, y, and z directions, along with the spatial distribution of the total intensity.