An ultra-compact angstrom-scale displacement sensor with large measurement range based on wavelength modulation.

Optical displacement metrology is important in nanotechnology and used to identify positions and displacements of nanodevices. Although several methods have been proposed, a sensor with ultracompact size, angstrom-scale resolution, and large measurement range is still lacking. We propose an optical displacement sensor with wavelength modulation that can demonstrate subwavelength footprint and angstrom-level resolution with large measurement range in this study. The proposed sensor consists of two optical slot antennas. Surface plasmon polaritons (SPPs) are launched at antennas and interfere when a tightly focused broadband light source illuminates the sensor. Spectrum of output SPPs presents a dip, which depends on the position of focal spot of incident light and is used to extract displacement. A maximum resolution of 0.734 nm was obtained. Furthermore, we used interference fringe of two broadband beams as light source and the measurement range of the sensor is not limited by the size of the tightly focused light source while maintaining high resolution. The method utilizes a new mechanism of wavelength modulation to overcome the trade-off between the high resolution and large measurement range, and achieve a variety of potential applications for nanometrology in the future.