Dispersion Control of Excitonic Thin Films for Tailored Superabsorption in the Visible Region

Abstract

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.

Semiconductor Thin Film Based Metasurfaces and Metamaterials for Photovoltaic and Photoelectrochemical Water Splitting Applications

“…Similar optical responses have been realized for another type of metals and semiconductors . This strong interference is not only probed in semiconductor based cavity designs but also excitonic thin films, such as dyes and organic molecules, and could also show light perfect absorption . The optical response of these organic materials ( n and k values) can be controlled with the change in the aggregate concentration.…”

Strong Nonlinear Optical Response in the Visible Spectral Range with Epsilon‐Near‐Zero Organic Thin Films

“…UV–vis normalized absorptance (= −log( R + T )) spectra of TDBC:PVA and HTJSq:PMMA at different ratio of dye compared to the polymer quantity are compared with the absorption of the dye in solution (Figure b,f). TDBC is found to show an intense J‐type transition in solution and in PVA blend . This transition is accompanied by broad shoulders in the blue side of the spectrum, which is indicative of a H‐like band in Davydov splitting .…”

“…As an example, a crystal‐like supramolecular assembly of dye aggregates such as cyanines can lead to a large‐scale coupling of excitonic transition dipoles and a delocalization of the photoexcited states. This effect was found to result in an ENZ response in the visible spectral range . However, both the role of the molecular aggregation and the enhancement of optical Kerr nonlinearity in organic ENZ materials have not been investigated yet.…”

Angle-dependent optical perfect absorption and enhanced photoluminescence in excitonic thin films

“…Figure 1(a) shows the dielectric constants of the excitonic film used in our measurement. These dielectric constants were extracted from the measured reflection-transmission spectra by fitting them to the Fresnel's equations [23,26]. The imaginary part of the dielectric constants (Im[ε]) has a sharp pole near 590 nm, and we have the optically metallic region (Re[ε] < 0) right below this excitonic pole wavelength.…”

“…Some organic molecules such as J-aggregates have large oscillator strength and can have very sharp excitonic absorption lines, which could lead to an optically metallic response (Re[ε] < 0) in the visible spectral range [19][20][21][22][23]. J-aggregates also support efficient exciton transfer across aggregated molecules, which could be useful for light detection or energy harvesting [24].…”