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September 29, 20224:00 pm – 5:00 pm (CDT)

Time-reversing a laser: What it means and what it’s good for


A. Douglas Stone (Yale University)


Alexey Belyanin



Mitchell Institute for Fundamental Physics & Astronomy

College Station, Texas 77843


Event Details

Over a decade ago an overlooked symmetry of Maxwell’s equations coupled to matter was discovered.  The linear Maxwell wave equation in the presence of a gain medium can describe a laser at threshold. This 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 wave equation at a real frequency; thus it acts as steady-state source of coherent radiation of negligible amplitude.  Time-reversing this unique behavior maps the system to another Maxwell solution, one which corresponds to time-reversing the outgoing lasing mode and replacing the replacing the spatial distribution of gain with equivalent absorption.  Because almost any system can lase with sufficient gain, this implies that under very general conditions any highly complex structure can be made to absorb perfectly a specific adapted input wavefront if absorption of the correct amount and spatial distribution is added, a phenomenon referred to as Coherent Perfect Absorption (CPA).  In the following years this effect has been demonstrated and extended in a wide variety of electromagnetic platforms, as well as in acoustic and other wave systems.  One dramatic discovery was that a cavity with balanced gain and loss can simultaneously lase in one mode and perfectly absorb a different one.  Another exciting generalization of CPA is a general theory of reflectionless excitation of arbitrary photonic structures.  Recently potentially important applications of the generalized theory of CPA have been demonstrated in microwave cavities to functions such as secure message transmission, analog computing, and signal routing.  I will present the basic concepts of CPA and discuss its generalizations in this talk, and present a few of these fascinating applications of the theory.

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