Physically Grounded Monocular Depth via Nanophotonic Wavefront Prompting

Depth foundation models offer strong learned priors for 3D perception but lack physical depth cues, leading to ambiguities in metric scale. We introduce a birefringent metalens -- a planar nanophotonic lens composed of subwavelength pixels for wavefront shaping with a thickness of 700 nm and a diameter of 3 mm -- to physically prompt depth foundation models. In a single monocular shot, our metalens physically embeds depth information into two polarized optical wavefronts, which we decode through a lightweight prompting and fine-tuning framework that aligns depth foundation models with the optical signals. To scale the training data, we develop a light wave propagation simulator that synthesizes metalens responses from RGB-D datasets, incorporating key physical factors to minimize the sim-to-real gap. Simulated and physical experiments with our fabricated titanium-dioxide metalens demonstrate accurate and consistent metric depth over state-of-the-art monocular depth estimators. The research demonstrates that nanophotonic wavefront formation offers a promising bridge for grounding depth foundation models in physical depth sensing.