Electron paramagnetic resonance (EPR) spectroscopy is the method of choice to investigate and quantify paramagnetic species in many scientific fields, including materials science and the life sciences. Common EPR spectrometers use electromagnets and microwave (MW) resonators, limiting their application to dedicated lab environments. Here, we present an improved design of a miniaturized EPR spectrometer implemented on a silicon microchip (EPR-on-a-chip, EPRoC). In place of a microwave resonator, EPRoC uses an array of injection-locked voltage-controlled oscillators (VCOs), each incorporating a 200 μm diameter coil, as a combined microwave source and detector. The individual miniaturized VCO elements provide an excellent spin sensitivity reported to be about 4 × 109spins/√Hz, which is extended by the array over a larger area for improved concentration sensitivity. A striking advantage of this design is the possibility to sweep the MW frequency instead of the magnetic field, which allows the use of smaller, permanent magnets instead of the bulky and powerhungry electromagnets required for field-swept EPR. Here, we report rapid scan EPR (RS-EPRoC) experiments performed by sweeping the frequency of the EPRoC VCO array. RS-EPRoC spectra demonstrate an improved SNR by approximately two orders of magnitude for similar signal acquisition times compared to continuous wave (CW-EPRoC) methods, which may improve the absolute spin and concentration sensitivity of EPR-on-a-Chip at 14 GHz to about 6 × 107 spins/√Hz and 3.6 nM⁄√Hz, respectively.
