| Summary: | We dismantle the previously held misconception that it is impossible for bulk
rectification mechanisms to induce a net DC electric current when the frequency
of the impinging radiation lies within the optical gap of a metal in the limit
of small carrier relaxation rates. We argue that generically such in-gap
rectification mechanisms are irreversible and accompanied by a continuous
exchange of energy with a heat bath and must also be necessarily accompanied by
a small but finite absorption of radiation in order to guarantee the positivity
of the net entropy production and abide by the second law of thermodynamics. We
show, however, that the intra-band non-linear Hall effect arising from the
Berry curvature is a special kind of in-gap rectification mechanism that
behaves as a ``photo-voltaic demon'', namely it can operate as an ideal
reversible and dissipationless conveyor of energy between the radiation and an
external circuit. Its reversible nature allows for an interesting mode of
operation as an amplifier of circularly polarized light, whose efficiency can
approach 100%, and which could be technologically promising especially in the
infrared frequency range.
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