Progress toward high-$Q$ perfect absorption: A Fano antilaser

Here we propose a route to the high-$Q$ perfect absorption of light by introducing the concept of a Fano antilaser. Based on the drastic spectral variation of the optical phase in a Fano-resonant system, a spectral singularity for scatter-free perfect absorption can be achieved with an order of magnitude smaller material loss. By applying temporal coupled mode theory to a Fano-resonant waveguide platform, we reveal that the required material loss and following absorption $Q$ factor are ultimately determined by the degree of Fano spectral asymmetry. The feasibility of the Fano antilaser is confirmed using a photonic crystal platform, to demonstrate spatiospectrally selective heating. Our results utilizing the phase-dependent control of device bandwidths derive a counterintuitive realization of high-$Q$ perfect conversion of light into internal energy, and thus pave the way for a new regime of absorption-based devices, including switches, sensors, thermal imaging, and optothermal emitters.

Figure 1

Fano antilaser system based on a stub-waveguide structure. (a) Schematic of Fano antilaser compared to Fano laser dynamics (orange for gain and blue for lossy stub, gray for feeding waveguides, and green for the junction). (b) CMT model of the structure in (a). Transmission ($T=|S_{-2}/S_{+1}{|}^2$, blue solid) and reflection ($R=|S_{-1}/S_{+1}{|}^2$, red solid) spectra for (c) non-Fano $({\omega }_A={\omega }_0)$ and (d) Fano $({\omega }_A\ne {\omega }_0)$ cases, both with a single port excitation $(S_{+1}=1,S_{+2}=0)$. Arrows in (c) and (d) denote the stub resonance $\left({\omega }_0\right)$, and dashed lines denote junction resonator responses without a stub $({\tau }_s\to \infty )$. The phase information for the non-Fano (c) and Fano (d) cases are shown in (e) and (f), respectively. Dotted lines denote the phases [purple for the stub phase $-2n{k}_{0}d$ and green for the junction phase arg $\left(a\right)]$ before the connection between the stub and the junction, and the solid green line shows the phase arg $\left(a\right)$ after the coupling. The stub length $d=(7/4){\lambda }_0/\mathrm{Re}\left[n\right]$ where $n=1.5$. ${\omega }_A=1.3{\omega }_0$ for the Fano case. $Q_w={\omega }_A{\tau }_w/2=6$ and $Q_s={\omega }_A{\tau }_s/2=6$.

Figure 2

Absorption spectra of a non-Fano antilaser $({\omega }_A={\omega }_0)$ with [(a)–(e)] a single port incidence and [(f)–(j)] coherent incidences, for [(c),(h)] $Q_s=4$, [(d),(i)] $Q_s=8$, and [(e),(j)] $Q_s=12$. (a),(f) Schematics and (b),(g) energy level diagrams are shown for each case. The absorption spectra are obtained as a function of $\mathrm{Im}\left[n\right]/\mathrm{Re}\left[n\right]$. $d=(7/4)\left({\lambda }_0\right)/\mathrm{Re}\left[n\right]$, and $Q_w=6$.

Figure 3

(a)–(e) Absorption spectra of Fano antilasers $({\omega }_A=1.3{\omega }_0)$ with coherent incidences, for (c) $Q_s=4$, (d) $Q_s=8$, and (e) $Q_s=12$. (a) Schematics and (b) energy level diagram are also shown. (f)–(h) show $D\left(\omega \right)$ for (f) the non-Fano $({\omega }_A={\omega }_0)$ and Fano cases of (g) ${\omega }_A=1.3{\omega }_0$ and (h) ${\omega }_A=1.5{\omega }_0$. $Q_s=12$ for (f)–(h). (i) Spectral variations of $D\left(\omega \right)$ for (f)–(h) at the perfect absorbing point of $\mathrm{Im}\left[n\right]/\mathrm{Re}\left[n\right]$. Variations of (j) $Q_{\mathrm{abs}}$ and (k) required loss $\left(\mathrm{Im}\right[n]/\mathrm{Re}[n\left]\right)$ for different degrees of Fano asymmetry are shown as functions of $Q_s$. Arrows in (i)–(k) present changes per the increase of Fano asymmetry. All other parameters are equal to those in Fig. 2.

Figure 4

(a) Fano antilaser in PC realization $(d_A=5,d_s=15)$. (b) Transmission spectra for a single port incidence. (c)–(e) Absorption spectra of PC Fano antilasers $(d_A=5)$ with (c) $d_s=5$, (d) $d_s=9$, and (e) $d_s=15$. $\mathrm{Re}\left[n\right]=3.5$ for dielectric rods embedded in air. All the results are obtained using comsol.

Figure 5

(a)–(c) Field distributions and (d)–(f) heat generation maps around the perfect absorption [(b),(e)], for selective thermal emitters $(d_A=5,d_s=15)$. (g) Switching as a function of $\mathrm{Im}\left[n\right]$ for different degrees of Fano spectral asymmetry. Solid (or dotted) lines denote the absorbed power (or the scattering for a single port). All other parameters are equal to those in Fig. 4.