Razorblade Technique Characterises Laser Irradiation for Nitrogen-Vacancy Centre Sensing.

Nitrogen-vacancy (NV) centres in diamond are rapidly becoming powerful tools for quantum sensing, but realising their full potential demands precise control over laser excitation. Alejandro Martínez-Méndez and Jesús Moreno-Meseguer, from the Universidad de Murcia, working with colleagues including Mariusz Mrózek et al. at the Jagiellonian University and Ulm University, present a comprehensive method for optimising laser performance in experiments using these single-photon emitters. The team meticulously characterised key laser parameters – including beam quality, spectral characteristics, and pulse timing – within a standard confocal microscope setup. This work establishes a straightforward protocol for fine-tuning laser excitation, significantly enhancing the performance and reliability of NV-centre based sensors and paving the way for more sensitive and accurate quantum measurements.

Nitrogen-vacancy (NV) centres in diamond are rapidly becoming a cornerstone of quantum sensing, offering remarkable potential for applications ranging from precision measurement of magnetic fields to nanoscale temperature mapping. These tiny defects within the diamond’s crystal structure possess unique quantum properties, behaving as miniature, solid-state spins that can be manipulated and read using light and microwaves. Realizing the full potential of NV centres, however, requires careful optimization of the laser light used to control and detect their quantum state. Researchers are employing a comprehensive methodology to optimise laser control within confocal microscopes, specifically for experiments utilising these NV centres. This work addresses the need for a robust and well-characterised optical setup to maximise sensor performance. The core of the approach lies in a detailed investigation of the laser beam itself, systematically assessing its quality to ensure it is tightly focused and delivers consistent intensity.

This is achieved using techniques to measure beam propagation and quantify its focus and stability, alongside examination of the laser’s spectral characteristics to confirm it matches the requirements for exciting the NV centre. Beyond basic characterisation, the methodology incorporates dynamic control over the laser light. Researchers investigate how the fluorescence emitted by the NV centre responds to varying laser power and polarisation, using a specialised plate to rotate the polarisation and fine-tune the interaction with the NV centre, optimising signal strength. They also modulate the laser’s temporal profile – how the power changes over time – using an acousto-optic modulator, allowing for rapid and accurate control of the excitation and readout pulses. Crucially, researchers confirm that the system is probing a single NV centre by measuring the statistical properties of the emitted light, analysing the timing of individual photons to verify that they originate from a single quantum emitter. Confocal microscopy, ideal for these experiments, achieves high resolution by focusing light to a tiny volume – approximately 0.1 cubic micrometres – and filtering out-of-focus light. This allows researchers to address and study individual NV centres within the diamond sample, crucial for sensitive measurements.

However, achieving this requires a laser beam with exceptional characteristics, including a Gaussian beam profile – a specific distribution of light intensity – for optimal focusing and resolution. A key metric for assessing beam quality is the M 2 factor, which compares the beam to an ideal Gaussian beam; lasers approaching this ideal deliver the tightest focus. Researchers discovered that excessive laser power can saturate the colour centres, diminishing the signal and introducing errors. By carefully controlling the laser intensity, they maintained a strong signal without compromising the NV centre’s quantum properties. Furthermore, they demonstrated the importance of polarization control, aligning the laser light to maximize fluorescence from the NV centre’s optical dipoles. The culmination of this optimization process is a significant improvement in the signal-to-noise ratio – a measure of the clarity of the signal received from the NV centre, allowing for more precise and reliable measurements. The optimised microscope successfully demonstrates the ability to drive spin transitions in a single NV centre, achieving a fluorescence contrast of approximately 28% in optically detected magnetic resonance experiments. While signal contrast is ultimately limited by the inherent electronic dynamics of the NV centre itself, and external factors like sample refractive index can complicate excitation, future work could focus on mitigating these influences to further improve signal quality and sensor performance.