Douglas Stone - "Time-reversing a Laser: What it means and Why it's Important"

Douglas Stone

Yale University, Dept of Applied Physics

Abstract­­

Over a decade ago an overlooked symmetry of Maxwell’s equations coupled to matter was recognized, a relationship between a laser at threshold and a perfectly absorbing resonator.  The threshold condition for lasing is the point at which gain balances loss, and the system self-organizes to oscillate coherently at a specific frequency in the highest Q electromagnetic mode.  At this special point the system supports a purely outgoing solution of the Maxwell wave equation at a real frequency but with negligible amplitude, heralding the turn-on of a steady-state source of coherent radiation.  Time-reversing this threshold lasing equation maps the laser system to another physical realizable electromagnetic system, one in which the time-reflected lasing mode is incident on an identical resonator, except that absorption loss replaces gain.  This mapping implies that under very general conditions, any complex structure can be made to absorb perfectly at a specific frequency, if a specific adapted input wavefront is imposed and the loss is appropriately tuned, a phenomenon now known as Coherent Perfect Absorption (CPA).  In the following years this effect has been demonstrated in a wide variety of electromagnetic platforms, as well as in acoustic and other wave systems.  More recently CPA has been understood to be one limiting case of a completely general theory of reflectionless scattering of linear waves. Just as every scattering structure has a complex spectrum of resonances, when excited at short enough wavelengths, one can show that every such structure has a complex spectrum of Reflectionless Scattering Modes (RSMs), distinct from the resonances, which can be tuned to enable perfect impedance-matching.  I will review a few dramatic experimental and technological applications of CPA and RSM.  One novel proposal applies these theories to the quantum scattering of atomic condensates to detect bound states in the continuum. 

Leuchs-Russell Auditorium, A.1.500, Staudtstr. 2
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