IDART: Interferometric Dual-AC Resonance Tracking for Nano-Electromechanical Mapping.

Piezoresponse force microscopy (PFM) has established itself as a successful and reliable imaging and spectroscopic tool for measuring a wide variety of nanoscale electromechanical functionalities. Quantitative imaging of nanoscale electromechanical phenomena requires high sensitivity while avoiding large bias artifacts. Conventional PFM often relies on high voltages to overcome optical detection noise, increasing the likelihood of non-ideal effects including electrostatic crosstalk, Joule heating, and tip-induced switching. To reduce the required biases, and with them the likelihood and severity of these effects, we introduce interferometrically detected dual AC resonance tracking (iDART), which combines the femtometer-scale displacement sensitivity of quadrature phase differential interferometry with contact resonance amplification. iDART achieves at least a tenfold improvement in signal-to-noise ratio over state-of-the-art PFM approaches, including single-frequency interferometric PFM and conventional resonance-enhanced PFM using optical beam detection. We demonstrate this improvement on PZT and a variety of hafnium-based ferroelectric thin films. Switching spectroscopy shows similar gains, revealing reliable hysteresis loops at small biases and mitigating the nonlinearities and device failures that can occur at higher excitation amplitudes. These results establish iDART as a sensitive, low-bias approach for nanoscale electromechanical imaging and spectroscopy of weak piezoelectric systems, extending functional imaging to 2D ferroelectrics, beyond-CMOS technologies, and biomaterials.