Strong light absorption in ultrathin films has been of great interest for both fundamental studies and device applications. Here we demonstrate and analyze controllable superabsorption in excitonic thin films in the visible region. By adjusting the concentration of J-aggregate dyes, we control the dispersion of excitonic films (from optically metallic to nonmetallic ones) and show that this leads to drastic changes in the optical response of organic thin films. We find that planar excitonic films can have various optical features in the visible region, for example, surface polaritons, epsilon-near-pole, asymmetric Fabry−Perot type resonances, and so on. We leverage these diverse features to study perfect absorption in planar films without additional structural patterning. We also demonstrate that strong light absorption can even occur away from an excitonic absorption peak (i.e., maximum optical loss position) due to cavity-like resonances in the high dielectric constant region. Our work demonstrates that there are unique opportunities for dispersion control in the visible region with easy-to-handle organic molecules, and this can be useful for novel nano-optical studies or energy conversion devices. Collaborative synergy between molecular photonics and nanoscale optics has been demonstrated throughout this work.
We propose and analyze a scheme for active switching and spectral tuning of mid-infrared Fano resonances. We consider dielectric resonators made of semiconductor cylinder arrays and block pairs, and theoretically investigate their optical response change due to carrier generation. Owing to sharp optical resonances in these structures and large dielectric constant variations with carrier densities, the significant spectral tuning of Fano resonances is achievable. Furthermore, selective optical pumping in coupled semiconductor structures can even enable dynamic switching of Fano resonances. This leads to a drastic change in the scattering spectra as well as in the near-field intensity. We also observe a stark difference between Fano resonances in cylinder arrays and block pairs. To understand this unusual behavior, we adopt the two coupled oscillator model, and extract the relevant Fano resonance parameters that explain this difference. Our findings and in-depth analyses can be useful for molecular sensors and switching devices in the technologically important mid-infrared spectral region.
Transformation Optics (TO) enables arbitrary control of electromagnetic waves via spatiallytailored effective material parameters. TO is revolutionizing our understanding of how to control the flow of light. It not only revolutionizes the fundamental physics of light-matter interactions, but also implements totally new optical functions. For instance, it enables highly unusual optical functions that may otherwise be almost impossible or difficult to obtain, such as invisibility cloaking, illusion optics, optical black holes, lossless waveguide bends, etc. The basic idea is that a mathematical transformation of space can be represented by a change in the material effective parameters. TO provides an elegant way to control the flow of light, but it usually requires extreme material parameters that can be too complicated to realize or can be implemented only through resonant metallic metamaterials. However, meaningful advances have been made recently to resolve these difficulties. In this review article, we introduce the concepts of TO and explore recent developments. There has been a plethora of new ideas in this field, but large opportunities for both fundamental and applied research are still waiting for us.
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