Holographic camera for non-contact measurement of nanoscale surface heights

We report on the development of a holographic camera capable of measuring nanometer-scale surface features. The system is based on a modified off-axis Mach-Zehnder interferometer and was optimized to provide high-quality interference patterns. Fast imaging was implemented with a time-gated intensified CCD camera directly facing the surface of the object. By increasing the intensifier gain, holographic images with good contrast could be captured within 1 ns. We tested the ability of the camera to measure nanometer-scale height differences using a patterned USAF target. The depth resolution of the camera was estimated to be better than 10 nm. We also found that both the object-CCD distance and the angle between the object and reference beams had significant impact on the quality of the reconstructed surface profiles. Potential applications of the camera include measurement of tissue surface displacements for non-contact photoacoustic imaging.