Reflected light exhibits complete phase reversal in traditional metallic mirrors, causing a standing wave with reduced electric field intensity directly above the reflective surface. This diminished field intensity leads to poor absorption near the interface. Metamaterial mirrors are increasingly being explored as a solution to this problem because they allow the phase of the reflected light to be tuned and can be used to eliminate phase reversal upon reflection. Here, we explore the incorporation of quantum dot (QD) films on dielectric metasurfaces, as the tunable QD band-gap makes them well-suited to take advantage of the wavelength-dependent electric field enhancements that metasurfaces exhibit. We are particularly interested in QDs with core/shell heterostructures. Thick QD shells help passivate the core surface and preserve their optical properties, but also lead to a lower optical density when in the solid-state, something that metasurfaces can help mitigate.
To avoid parasitic Joule losses exhibited by metal structures, we explored metasurfaces based on high-contrast dielectric materials. Using FDTD simulations and optical models measured via ellipsometry, we designed TiO2 structures for phase tuning. Our set-up consists of an aluminum back reflector, a 20nm Al2O3 spacer layer, and a metasurface patterned using TiO2 nanodisks. It was found that when using CdSe/CdS quantum dots, a nanodisk 80nm high and 175nm in diameter gave the desired 180° phase shift for 405nm incident light. For 445nm light, only a 65nm diameter was required for the desired phase shift. In both cases, simulations showed much higher electric field intensity and absorption in the quantum dot layer, compared to the case with no TiO2 structures or with non-optimized TiO2 diameters.
Experimentally, we patterned TiO2 structures via both e-beam lithography and colloidal methods. For e-beam patterning, a top-down approach is used. PMMA is patterned on top of TiO2 and a thin layer of chromium is deposited to serve as the TiO2 mask. After lift-off, a dry-etching process is used to create TiO2 pillars. Colloidally, TiO2 nanospheres are synthesized through a sol-gel process. By varying the water content in our reaction mixtures, we have tuned the targeted diameter from 160 - 200nm with a size distribution of 11%. For the absorbing layer, CdSe/CdS QDs were synthesized using a non-hot injection method. Neat, thin-films of these QDs were spin-cast on top of our metasurface using a solid-state ligand exchange process. Once the structures are created, a customized optical setup was used to map the film’s optical characteristics over hundreds of square microns with nanometer control of the sample position, allowing us to track how different optical parameters change in the presence of the metasurface mirror and as a function of the local environment. The optical response of the colloidally-synthesized metasurface will be compared to the response of the pattern made via e-beam lithography.