Controllable coherent perfect absorption

There has been recently a great deal of interest in critical coupling (CC) in micro and nano structures. Critical coupling of incident electromagnetic radiation to a given micro– nano– structure refers to the case where the entire incident energy is absorbed in the structure, leading to null scattering. Initial research on critical coupling involved the coupled resonator optical waveguides (CROWs) proposed by Yariv. A realization of such system was the coupled fiber–microsphere or the fiber–disc system. The purpose here was to slow down light in order to stop and store it eventually. Later studies were directed to planar structures with a very thin layer of absorbing material on a distributed Bragg reflector (DBR) substrate. A spacer layer between the absorbing layer and the DBR controlled the amplitude and phase of the waves reflected from the different interfaces. For light at normal incidence from the top (on absorbing layer side), the DBR ensured null transmission for waves with frequency in the rejection band of the structure. The spacer layer thickness was controlled such that all the reflected waves coming from different interfaces interfered destructively in the medium of incidence, leading to null reflection. Thus, the structure neither transmitted nor reflected, implying the critical coupling of the incident light to the structure. The entire incident light was “perfectly” absorbed by the only few–nanometer thick lossy layer. Physically this amounts to having a purely imaginary component of the Poynting vector normal to the surface.

In a recent paper with collaborators in India and the United States, we have shown that the concept of coherent perfect absorption (CPA) can be extended to versatile geometries using a heterogeneous metal–dielectric composite layer as absorber. This approach provides great flexibility with respect to the material sample, illumination configuration, and frequency range where CPA can be realized. In a heterogeneous composite medium it is possible to tune the localized plasmon resonance and the resulting absorption and dispersion, by simply varying the inclusions volume fraction. In this publication, we have presented a detailed study of CPA in a slab of composite medium with illumination by coherent waves at identical angle of incidence from both sides. Identical angle of incidence ensures the interference of the transmitted light, say, incident from the left of the medium, with the reflected one incident from right. Perfect destructive interference corresponds to the case when the amplitude of both waves are the same and their phase difference is π. If both incident waves are identical, this leads to perfect cancellation on both sides and CPA. We present results which are indicative of a critical minimum of absorption below which CPA is not possible. This clearly demonstrates the necessity of absorption for CPA. We further demonstrate the controllability of the frequency over a broad range (from visible to near-IR) at which null-scattering can take place. We also show that anti-lasing can be realized at two distinct frequencies even in presence of dispersion. Note that in the experimental demonstration of anti-laser dispersion was ignored assuming the material (Si wafer) to be with uniform complex refractive index over a small range of frequencies in the near-IR domain. Such a flexibility in the design and prospects, as offered by our scheme, may lead to an easier realization of anti-lasers over a broader frequency domain.

The previous figure shows (a) the absolute values of reflected (dashed line) and transmitted (solid line) amplitudes |r f | and |tb| (arrow shows the point where conditions for CPA are satisfied), (b) the phase difference Δφ between the forward reflected and backward transmitted plane waves and (c) the log10|r f +tb|2 as a function of the wavelength λ for a film with a metal fraction fm=0.004.

Check the corresponding publication: PDF External link: doi: 10.1364/OE.20.001330