Laser science

Suckers for light

An optical device has been designed that performs a function exactly opposite to that of a laser. It perfectly absorbs incoming coherent radiation and turns it into thermal or electrical energy.

I enjoy being an experimental laser scientist. With lasers in the lab there is almost never any ambiguity — they either lase or they don't — and trying to set the pumping current by hand to exactly the required threshold for lasing is at once quite difficult and a great teaching moment. The strength of the phase transition occurring at the lasing threshold is palpable because some properties, such as the brightness or the linewidth of the emitted light, change, often by orders of magnitude, as the microcosm of light inside the laser crystallizes into one 'coherent' light beam. And we do understand this laser process quite well. Or do we? It is surprising that, after 50 years of lasers in the lab, we are still refining our models and finding simple yet novel questions to answer. A particularly intriguing one has been posed by Chong et al.1 in a paper published in Physical Review Letters.

The authors ask — and answer — the question of whether one can devise the exact opposite of the laser process and, in turn, design what they term a coherent perfect absorber (CPA). The device perfectly absorbs incoming coherent light (which is characterized by waves that have the same amplitude and frequency and are in step with one another) and turns it into some form of internal energy — heat or, preferably, useful electrical energy. The researchers' answer is that, in fact, such CPAs are possible and are also robust solutions: they are quite straightforward to realize once the conditions have been set up correctly.

Chong et al.1 arrive at both the proposal and the solution for the CPA from a semi-classical model of lasing, and by using the 'scattering matrix' approach for simple laser systems. The scattering matrix, or S-matrix, relates all incoming and outgoing electromagnetic waves of a specific optical system, and hence can be used to search for possible solutions that involve neither transmission nor scattering — that is, those that absorb all incoming light. For lasers, the S-matrix approach is made to work by considering the optical gain of the laser medium, which involves adding a negative imaginary component to its complex refractive index. Similarly for CPAs, dissipation must be added as a positive imaginary component to the refractive index of the material inside the CPA; that is, the material must display some absorption at the desired frequencies of light. Then, as the authors demonstrate, real solutions exist for CPAs for specific sets of real and imaginary components of the refractive index, and for certain light frequencies and geometrical dimensions of the absorber. Yet the absorption need not be strong or resonant — that is, matched to the frequencies of strongest absorption of the material, as determined by the material's quantum structure. Therefore, plain, general-use semiconductors such as the ubiquitous silicon would make for excellent CPAs.

The second element needed to design a CPA is an optical resonator — an arrangement of optical components that allows light waves to travel in a closed path and to interfere. To function, the CPA relies on the interplay between the absorbing material and light-wave interference provided by the resonator (Fig. 1, overleaf). For lasers, the higher the quality factor of the resonator — that is, the more light is reflected inside the resonator — the lower the overall gain that is needed to reach the lasing threshold. Similarly for CPAs, the higher the quality factor, the less absorbing the material needs to be for perfect absorption of the incoming light. Yet, even plain Fabry–Pérot-type resonators, such as those formed by two parallel reflective surfaces acting as mirrors, will work just fine. The resonator also selects the specific frequencies of light at which perfect absorption occurs.

Figure 1: The coherent perfect absorber.

Coherent light is incident on an absorbing material in a resonator formed by two parallel reflective surfaces or mirrors. The interplay of absorption and interference leads to perfect absorption of the incoming radiation and its conversion into other forms of energy1. The schematic of a laser would be entirely analogous, with only the arrows for light and energy reversed: energy pumped in would result in coherent light out.

The final step for perfect absorption is setting the phase of the incoming coherent light, which describes the positions of the light wave's peaks and troughs. A laser will naturally pick a frequency and phase for each beam it emits. But for the CPA, these phases must be properly chosen for either perfect absorption or, alternatively, increased scattering, depending on the phase relationships of the interfering waves inside the CPA. If a single incoming light beam is used, perfect absorption is achieved by tuning to the correct frequency. If two or more light beams are used, their relative phases can be set — and the ensuing mutual interference can be used — to switch between a perfectly absorbing and a minimally absorbing state.

Naturally, the development of strongly absorbing materials or devices has been, and will probably continue to be, a long-standing research field2,3,4,5. There are many modern-day applications that would benefit greatly from the strongest absorption possible. For example, such absorption would allow a more efficient conversion of solar energy into other forms of energy, and, in the field of optical communications, would permit the detection of that very last information-carrying photon. Many solutions for quasi-perfect absorption then rely on resonant absorption inside the material's quantum structure — which can be achieved by engineering the energy-band structure of the material — or make use of periodically arranged microstructures such as Bragg or photonic crystal structures.

Many of these solutions are constructed with incoherent and broadband light in mind, and several do not provide 'perfect' absorption. Chong and colleagues' work1 is different in that it starts from and mirrors the laser process, which produces coherent and narrow-band light, and is highly specialized for the perfect absorption of coherent light. Among the applications mentioned by the authors, such as generic detectors or switches, this latter aspect of selective absorption of coherent light is useful for a specific set of applications in laser spectroscopy in the mid- to far-infrared regions, where weak laser signals, such as those recovered from remote sensing, are easily overpowered by thermal background radiation. A detector that can efficiently detect, by means of perfect absorption, coherent light signals of specific frequencies against a noisy background would be highly beneficial. The next step in this field, then, is to turn Chong and colleagues' theoretical work into practical applications based on the CPA concept.


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Gmachl, C. Suckers for light. Nature 467, 37–39 (2010).

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