Last year researchers from Yale published a theoretical paper proposing how a time-reversed laser might be constructed. A conventional laser uses a gain medium in which electrons are pumped into higher energy states to emit coherent radiation. The proposed device is the time-reversed equivalent, and sees the gain medium replaced by an appropriate absorber. Now, less than 8 months later the team have constructed such a device, which they have reported in the journal Science [Wan et al., Science (2011) 331, 889].
In their original paper the team theoretically demonstrated that a block of silicon could act as a “coherent perfect absorber” (CPA) to light in the 500 – 900 nm wavelength range. Prof Hui Cao explained that the “CPA absorbs light by setting up an interferometric trap within a cavity that contains absorbing materials, so that all the light will be absorbed eventually. The perfect absorption condition does not require the material is strongly absorptive, in fact it can be realized with materials of low absorption coefficient.”
The absorption is a result of interference and dissipation. By destructively interfering two beams at the edges of a silicon cavity, the light from both beams can be indefinitely trapped inside. Then, due to dissipation, the energy of the light is lost as heat. However, as Prof A. Douglas Stone told Materials Today, there is the potential to extract electrical energy, “this is straightforward based on known photovoltaic technology. It probably cannot deliver a large amount of electrical power due to non-linear effects”.
As with a regular laser, the gain medium determines the resonant frequency. In this case strong absorption was possible in the range of 990 – 1010 nm, with the strongest absorption only occurring after further fine tuning. The finite linewidth of the incident laser light limited the amount of absorption that could be achieved with the anti-laser, although the absorption was still amazingly over 99 %.
The authors speculate that the device’s sensitivity to frequency, phase and amplitude could lead to potential applications in detection, modulation, interferometry and switching. The potential is therefore present for optical communications and “compact on-chip interferometers”. Stone and Cao described their strategy for developing the system, “We are trying to realize CPA in more complex systems where a complex input waveform is needed. We will also look into the practical applications of CPA to optical devices”.
Stewart Bland