Achieving coherent perfect absorber or laser effects on the identical side of asymmetric metasurfaces

Coherent perfect absorber (CPA) and laser points are special spectral singularities where zero and infinitely large eigenvalues exist, respectively. The singularities have mainly been realized on the opposite surfaces (upper and lower sides) of the multilayer structures. Here, we demonstrate CPA and laser points separately on the identical side of a proposed bilayer metasurface. The structure is mainly composed of an asymmetric Si metagrating and a symmetric subwavelength grating. Two lightwaves from opposite angles above the metagrating give rise to a scattering matrix constructed by specular reflection (<i>r</i>₀) and retroreflection (<i>r</i>_±1) components, diverging from the commonly used reflections and transmissions. The conditions for CPA and the laser are derived from the scattering matrix. By fine-tuning the structural parameters, the CPA points are observed for both transverse-magnetic (TM) and transverse-electric (TE) incidences. Furthermore, upon the introduction of optical gain, laser points are achieved with the amplification factor of the reflection exceeding 10³ times. The laser point exhibits remarkable sensitivity to tiny perturbations of the environmental refractive index, with a figure of merit (FOM) reaching2.6×10⁵.