Negative Index Makes a Perfect Time-Domain Lens, Generating Slow Playback of Ultrafast Events

Abstract

We explore the effects of incorporating negative index materials into the physics of time-varying media and find that changing the refractive index from positive to negative creates a perfect time-reversed wave: a perfect time-domain lens. Unlike other mechanisms of phase conjugation, the perfect time-domain lens time-reverses both the propagating waves and the evanescent part of the spectrum. Moreover, we find that the time-reversed wave can be slowed down or accelerated, depending on the refractive index ratio. We show that this effect remains strong even when the refractive index varies arbitrarily slow, in sharp contradistinction to time-reflection which necessitates large index changes at sub-cycle rates. This is the first avenue found to yield significant negative-frequency waves using a temporal interface without the need for sub-cycle modulation or impedance matching. The effect can be used to record extreme ultrafast information and subsequently play it backwards at a slow rate, and vice-versa.