Wavelength Tunable Infrared Perfect Absorption in Plasmonic Nanocrystal Monolayers

The ability to efficiently absorb light in ultrathin subwavelength) layers is essential for modern electro-optic devices, including detectors, sensors, and nonlinear modulators. Tailoring these ultrathin films’ spectral, spatial, and polarimetric properties is highly desirable for many, if not all, of the above applications. Doing so, however, often requires costly lithographic techniques or exotic materials, limiting scalability. Here we propose, demonstrate, and analyze a mid-infrared absorber architecture leveraging monolayer films of nano-plasmonic colloidal tin-doped indium oxide nanocrystals (ITO NCs). We fabricate a series of ITO NC monolayer films using the liquid-air interface method; by synthetically varying the Sn dopant concentration in the NCs, we achieve spectrally selective perfect absorption tunable between wavelengths of two and five micrometers. We achieve monolayer thickness-controlled coupling strength tuning by varying NC size, allowing access to different coupling regimes. Furthermore, we synthesize a bilayer film that enables broadband absorption covering the entire mid-wave IR region (λ = 3−5 μm). With perfect absorption in monolayer films only hundredths of a wavelength in thickness, we demonstrate a scalable platform enabling strong light-matter interaction, with potential applications for molecular detection and ultrafast nonlinear optical applications.