Uni-modal retroreflection in multi-modal elastic wave fields

•We present a novel principle of perfect retroreflection of elastic waves.•A uniquely engineered meta-corner reflector can realize the proposed principle.•Our findings are validated theoretically, numerically, and experimentally.•Its application in measuring a thickness of a thin film is presented. The phenomenon of retroreflection of light characterized by a single transversely polarized wave has been extensively explored. However, the phenomenon of retroreflection of a longitudinally polarized elastic wave in a solid medium is far behind the current theory because an incident longitudinal wave can be diffusively reflected into multiple wave modes, both longitudinal and transverse. Here, our experiments show that a uniquely engineered meta-corner reflector can perfectly retroreflect an incident longitudinal wave onto the same longitudinal wave without generating a transverse wave. The meta-corner reflector consisted of a novel sequential stack of a full longitudinal-to-transverse mode-converting layer and an angle-tuned corner reflector. We present a theoretical foundation for retroreflection and its critical application in measuring the thickness of a thin film coated on a substrate that is usually difficult to measure. [Display omitted]