Wide-Dynamic-Range Cantilever Magnetometry Using a Fiber-Optic Interferometer and its Application to High-frequency Electron Spin Resonance Spectroscopy

We present a method of broadening the dynamic range of optical interferometric detection of cantilever displacement. The key idea of this system is to use a wavelength-tunable laser source. The wavelength is subject to proportional-integral control, which is used to keep the cavity detuning constant during a measurement. Under this control, the change in wavelength is proportional to the cantilever displacement. Using this technique, we can measure large displacements ($>1\ \mathrm{\mu m}$) without degradation of sensitivity. We apply this technique to high-frequency electron spin resonance spectroscopy and succeed in removing an irregular background signal that arises from the constantly varying sensitivity of the interferometer.