Absorbing physics reverses laser

Physicists at Yale University have created the first functioning reverse laser, suggesting that the technology could be used in next-generation optical communications and computing.

In a paper entitled Time-Reversed Lasing and Interferometric Control of Absorption (PDF), Wenjie Wan, Yidong Chong, Li Ge, Heeso Noh, A Douglas Stone, and Hui Cao describe the device, dubbed a coherent perfect absorber (CPA), as producing the "time-reversed counterpart to laser emission". The researchers said in the paper — published in the journal Science on Friday — that the device could have potential practical applications in integrated optics.

The researchers' CPA is a silicon slice, around 110μm thick, which completely absorbs photons in a coherent stream — this is the precise reverse of what lasers do, which is to produce coherent beams of photons. The team shone a tunable infrared laser into the silicon from both sides, and with the right tuning the silicon absorbed all the photons and turned them into heat.

"The properties of CPAs point to a new method for controlling absorption through coherent illumination," the team wrote, adding that this means it can be used to control and measure the wavelengths of light. The CPA works through a consequence of the underlying symmetry in quantum physics: a process that occurs over time can be reversed as if time itself was reversed, by changing the direction of momentum and the polarity of fields within the constituents of that process.

As the team points out, the CPA relies on very specific tuning — it can perfectly absorb incoming laser beams, but only when those beams have the right relative phase and amplitude. Its nature means it could be used as a modulator, laser detector or phase-controlled optical switch.

"Coherent perfect absorbers may enable novel functionalities in silicon-integrated photonic circuits of the type envisioned for next-generation optical communications and computing applications as well as for coherent laser spectroscopy," the team wrote.