Interference in Optical Cavity Demonstrated To Be Equivalent To Time Reversal In Lasers

                                  Diagram of how time-reversed laser would work

For graduate physics students it's no secret that in an optical cavity, light passes through the material many times. Also, it's known that under the right conditions, nearly all the light is eventually absorbed, even by a weakly absorbing material. Such a system is an example of what is known as a coherent perfect absorber, which achieves its performance with the help of interference effects.

Interestingly, the conditions for perfect absorption in such an optical cavity are the same as those for a laser that runs in reverse. Hence, the laser’s gain medium becomes an absorbing medium, and it stimulated emission becomes absorption-  effectively a state of time reversal. However, the laser - or perfect absorber cavity - generally does its job only for a specific spatial mode and direction of propagation. 

Now Ori Katz of the Hebrew University of Jerusalem and his colleagues have demonstrated a simple design for a coherent perfect absorber (shown above) that overcomes those limitations.  Katz and his team from Yeshiva University realized that to absorb various modes of light simultaneously, they could borrow ideas from a type of laser that emits multiple modes simultaneously: the degenerate-cavity laser. In a conventional coherent perfect absorber, light traveling normal to the cavity’s two mirrors bounces back and forth along the same path. Light at any other angle instead ricochets and eventually leaves the cavity.

The degenerate cavity (shown at top, Physics Today) includes the addition of two lenses placed such that light traveling along any path retraces its steps after each round trip. Because light propagating in any given direction travels in a closed loop, different incoming angles and spatial modes are simultaneously trapped in the cavity. Ultimately, light then ends up where it entered the cavity, so it can also destructively interfere with any light that would otherwise be reflected.  This occurs at the cavity’s entrance mirror, so the light ends up in the cavity and bounces back and forth until it’s absorbed.

That a laser can be operated in reverse to achieve a coherent perfect absorber (CPA) has been known for some time.  Although appealing from a conceptual basis, such devices are limited to a single, appropriately shaped wavefront or wave mode.  But given Ori Katz's team and their colleagues have since demonstrated how a CPA can be designed to overcome those limitations we have a new perspective.  In particular, time-reversing a degenerate cavity laser is equivalent to a unique perfectly absorbing cavity that self-images any incident light field onto itself. 

A. Douglas Stone of Yale University and colleagues first introduced the theory behind coherent perfect absorbers in 2010. Consider just a laser hitting a slab of material leading to a fraction of the light reflected, another fraction transmitted, and a final fraction absorbed. If two antiparallel beams then entered opposite sides of the material, they could interfere such that the transmitted and reflected light in each direction cancel.   

What was once commonplace in optical physics has now been revived and advanced in the novel experiments of Katz et al, which hopefully will make its way into undergrad physics textbooks soon.