Is the time reversed laser really a laser?

Question

I'm looking at the time reversed laser and I was having trouble understanding why we call this device a laser. To me this device is more like the absorbers found in FDTD codes, something like a CPML.

I am having trouble finding the analogs to the following processes in traditional lasers.

1. What is the mechanism for feedback. In a laser we have stimulated emission producing an exponential increase in output intensity, until the system saturates. In the reverse laser I only see an attenuation constant, whose constant value to me seems exactly like an absorber and not a laser.
2. Does this device have modes, does the feedback select an absorption width that narrows? The claims of an absorb band corresponding to the visible range (200nm-800nm) is a rather broad range that is characteristic of mostly everything?
3. What does population inversion and threshold mean for the reverse laser? If it is an absorber doesn't that mean that even a single photon knocks it out stability. In one paper the authors described a 2 state reverse laser system, we know that a 2 state laser is not possible....

Some images of time reversed lasers.

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Where is the lasing?

Answer 1

The energy it is absorbed is dissipated either as heat or as electron hole pairs but neglect to say how you can choose one outcome over the other, a potentially important point. Chong and co call their new devices coherent perfect absorbers or CPAs and a number of applications in communications spring to mind. They show that a single material can absorb perfectly at a number of different frequencies. However, they point out that if the material is bathed in broadband light, the effect averages out to the incoherent absorption level. That may place limits on how the idea can be applied to photovoltaics but these optics engineers are a clever bunch.