Abstract
Interferometric effects between two counter-propagating beams incident on an optical system can lead to a coherent modulation of the absorption of the total electromagnetic radiation with 100% efficiency even in deeply subwavelength structures. Coherent perfect absorption (CPA) rises from a resonant solution of the scattering matrix and often requires engineered optical properties. For instance, thin film CPA benefits from complex nanostructures with suitable resonance, albeit at a loss of operational bandwidth. In this work, we theoretically and experimentally demonstrate a broadband CPA based on light-with-light modulation in epsilon-near-zero (ENZ) subwavelength films. We show that unpatterned ENZ films with different thicknesses exhibit broadband CPA with a near-unity maximum value located at the ENZ wavelength. By using Kerr optical nonlinearities, we dynamically tune the visibility and peak wavelength of the total energy modulation. Our results based on homogeneous thick ENZ media open a route towards on-chip devices that require efficient light absorption and dynamical tunability.
Highlights
Coherent perfect absorption (CPA) was first proposed as a time-reversed version of a laser [1]
These results show that the system exhibits broadband coherent modulation of the energy with a maximum value close to one just below λENZ, independently of the thickness and of the single-pass absorption
We experimentally investigated the behaviour of CPA in ENZ films using aluminum-doped zinc oxide (AZO) films illuminated by two counter-propagating laser beams in a Sagnac-like interferometer configuration
Summary
Coherent perfect absorption (CPA) was first proposed as a time-reversed version of a laser [1]. While a resonant cavity is not required, single-pass absorption should be 50% to achieve perfect absorption [17,18] This is difficult to obtain in conventional dielectrics (too little losses) or metals (too high reflectivity). Applications of CPA in deeply subwavelength ENZ films could be found in photovoltaic energy conversion or devices such as bolometers which require large absorption with small masses Other applications, such as in nonlinear or quantum optics, may benefit from thicker films where the efficiency of the nonlinear process and the parametric gain generally scale with thickness. We study CPA in films of TCOs near their ENZ wavelength where the film’s refractive index exhibits large anomalous dispersion and a near-zero refractive index Such films can be treated as deeply subwavelength because the effective wavelength will increase drastically for wavelength approaching the ENZ wavelength. Our demonstration of broadband and tunable CPA in homogeneous ENZ films is relevant for practical nanoscale optical-switches and modulators where alternative nano-pattered metasurfaces would suffer from low switching efficiencies and detriments of nanofabrication processes
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