# Sensitive detection of level anti-crossing spectra of nitrogen-vacancy centers in diamond

## TL;DR

This study investigates the magnetic field dependence of photoluminescence in diamond NV centers, revealing new level anti-crossing lines and demonstrating the impact of modulation frequency on detection sensitivity, supported by a theoretical spin dynamics model.

## Contribution

The paper introduces a sensitive lock-in detection method for LAC lines in NV centers and provides a theoretical model explaining the frequency-dependent effects observed.

## Key findings

- LAC lines are strongly dependent on modulation frequency, increasing in amplitude at lower frequencies.
- New LAC lines were detected using lock-in detection techniques.
- Good agreement between experimental results and the theoretical spin dynamics model.

## Abstract

We report a study of the magnetic field dependence of photoluminescence of NV$^-$ centers (negatively charged nitrogen-vacancy centers) in diamond single crystals. In such a magnetic field dependence characteristic sharp features are observed, which are coming from Level Anti-Crossings (LACs) in a coupled electron-nuclear spin system. For sensitive detection of such LAC-lines we use lock-in detection to measure the photoluminescence intensity. This experimental technique allows us to obtain new LAC lines. Additionally, a remarkably strong dependence of the LAC-lines on the modulation frequency is found. Specifically, upon decrease of the modulation frequency from 12 kHz to 17 Hz the amplitude of the LAC-lines increases by approximately two orders of magnitude. To take a quantitative account for such effects, we present a theoretical model, which describes the spin dynamics in a coupled electron-nuclear spin system under the action of an oscillating external magnetic field. Good agreement between experiments and theory allows us to conclude that the observed effects are originating from coherent spin polarization exchange in a coupled spin system comprising the spin-polarized NV$^-$ center. Our results are of great practical importance allowing one to optimize the experimental conditions for probing LAC-derived lines in diamond crystals comprising NV$^-$ centers and for indirect detection and identification of other paramagnetic defect centers.