# Transparent Perfect Mirror

## TL;DR

This paper introduces a non-Hermitian optical cavity that acts as a transparent perfect mirror, reflecting fully while remaining transparent, with potential applications in security and privacy.

## Contribution

It demonstrates a novel active optical cavity configuration that achieves perfect reflection and transparency simultaneously, a feat not possible with passive structures.

## Key findings

- Achieves spectrally flat unity-reflectivity at the threshold gain
- Eliminates cavity resonances to appear as a perfect mirror
- Confirmed in integrated waveguide and fiber systems

## Abstract

A mirror that reflects light fully and yet is transparent appears paradoxical. Current so-called transparent or "one-way" mirrors are not perfectly reflective and thus can be distinguished from a standard mirror. Constructing a transparent "perfect" mirror has profound implications for security, privacy, and camouflage. However, such a hypothetical device cannot be implemented in a passive structure. We demonstrate here a transparent perfect mirror in a non-Hermitian configuration: an active optical cavity where a certain prelasing gain extinguishes Poynting's vector at the device entrance. At this threshold, all vestiges of the cavity's structural resonances are eliminated and the device presents spectrally flat unity-reflectivity, thus, becoming indistinguishable from a perfect mirror when probed optically across the gain bandwidth. Nevertheless, the device is rendered transparent by virtue of persisting amplified transmission resonances. We confirm these predictions in two photonic realizations: a compact integrated active waveguide and a macroscopic all-optical-fiber system.