Nonlinear opto-magnetic response of magnetic nanoparticles: Opto-magnetic particle spectroscopy

Opto-magnetic response of magnetic nanoparticles (MNPs) has been investigated as a means for rapid and sensitive biomolecule detection. However, current studies primarily focus on the linear opto-magnetic response of MNPs. In this study, we explore the nonlinear opto-magnetic response of MNPs induced in a sufficiently strong alternating-current (AC) magnetic field (≥3 mT) and under laser light, referred to as opto-magnetic particle spectroscopy (optoMPS). The underlying mechanism of the optoMPS signal is attributed to physical rotation of MNPs in an AC magnetic field and under laser light. Fokker-Planck equation is numerically solved to calculate the optoMPS signal while an optoMPS system is built for measurements. Experimental results show that the harmonic vs excitation frequency curve shifts to lower frequencies as the harmonic index increases. In addition, the optoMPS signal of MNP samples with different iron concentrations is measured to investigate the signal-to-noise ratio in AC magnetic fields with amplitudes ranging from 3 to 9 mT. Notably, the 2nd and 4th harmonics in an AC magnetic field with 9 mT allow for a limit-of-detection of 25 ng in terms of iron, paving the way for highly sensitive detection of MNPs with the optoMPS signal.