Abstract: Single rare-earth ions in solids show great potential for quantum applications, including single-photon emission, quantum computing, and high-precision sensing. However, the linewidths of single rare-earth ions are often broadened due to perturbations associated with the detection methods. Identifying the dominant broadening sources is key to reduce the linewidths for practical applications. We report a spectral broadening study on a single Er3+ ion in a Si nanotransistor. The single ion spectra display a Lorentzian lineshape at all light intensities considered. The linewidth remains nearly constant at 32±2 MHz in the low-intensity regime and shows a monotonic increase with the intensity in the high-intensity regime. The power broadening does not persist over the microsecond time scales considered after resonant excitation. Nor does it depend on the resonant excitation intensity or the Zeeman shift. These observations and temperature-dependent measurements suggest that charge fluctuations are likely to be a dominant broadening source. Laser heating may also contribute to the power broadening. Charge suppression in the Er3+-doped region and coupling Er3+ ions to a confined optical mode could be implemented to reduce the spectral linewidth and to enhance the sensing precision of single Er3+ ions in Si.

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