  The propagation of waves in complex media can be harnessed either by taming
the incident wave-field impinging on the medium or by forcing waves along
desired paths through its careful design. These two alternative strategies have
given rise to fascinating concepts such as time reversal or negative
refraction. Here, we show how these two processes are intimately linked through
the negative reflection phenomenon. A negative reflecting mirror converts a
wave of positive phase velocity into its negative counterpart and vice versa.
In this article, we experimentally demonstrate this phenomenon with elastic
waves in a 2D billiard and in a disordered plate by means of laser
interferometry. Despite the complexity of such configurations, the negatively
reflected wave field focuses back towards the initial source location, thereby
mimicking a phase conjugation operation while being a fully passive process.
The super-focusing capability of negative reflection is also highlighted in a
monochromatic regime. The negative reflection phenomenon is not restricted to
guided elastic waves since it can occur in zero-gap systems such as photonic
crystals, chiral metamaterials or graphene. Negative reflection can thus become
a tool of choice for the control of waves in all fields of wave physics.
