ODMR

Optically detected magnetic resonance (ODMR) is a spectroscopy technique in which magnetic resonance information is obtained from changes in an optical signal, most commonly the fluorescence emitted by a sample, rather than from direct microwave absorption detection. ODMR is widely used to study spin properties in solid-state systems and to develop quantum sensing technologies.

Principle and mechanism: In suitable solid-state spin systems, such as nitrogen-vacancy (NV) centers in diamond, optical pumping initializes electron spins into a defined quantum state with high polarization. Microwaves resonant with transitions between spin sublevels drive spin flips, altering the population distribution among states. Because the spin states influence the rate of radiative decay (fluorescence) through spin-dependent intersystem crossing, resonance conditions produce measurable changes in fluorescence intensity. By sweeping microwave frequency or applying a static magnetic field and scanning the signal, one detects resonance features that reflect the spin structure and environmental conditions.

Typical platforms and measurements: The prototypical ODMR system uses NV centers in diamond, but other defect centers in diamond or silicon carbide, and certain color centers in solids, can also support ODMR. ODMR can be performed on ensembles or single spins. The technique often operates at room temperature and can provide high sensitivity to external magnetic fields, temperature (via zero-field splitting), strain, and electric fields.

Instrumentation and setup: A light source or laser optically pumps the spins, while a detector records fluorescence. Microwave radiation is delivered to the sample through an antenna or waveguide. A static magnetic field is applied to define axes and enable Zeeman-split resonances. The experimental arrangement emphasizes stable optical and microwave control, noise suppression, and efficient signal collection.

Applications and advantages: ODMR enables nanoscale magnetic field imaging, high-sensitivity magnetometry, and quantum sensing in materials science, biology, and fundamental physics. Its strengths include operation at room temperature, potential for single-spin sensitivity, and compatibility with solid-state devices. Challenges include photoluminescence background, spectral diffusion, and the need for precise control of optical, microwave, and environmental conditions.