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Linear colour conversion is a process where an emitter absorbs a photon andthen emits another photon with either higher or lower energy, corresponding toup- or down conversion, respectively. In this regard, the presence of avolumetric cavity plays a crucial role in enhancing absorption andphotoluminescence (PL), as it allows for large volumes of interaction betweenthe exciting photons and the emissive materials, maximising the colourconversion efficiency. Here, we present a dual resonance nanostructure made ofa titanium dioxide (TiO2) subwavelength grating to enhance the colourdown-conversion efficiency of green light at ~530 nm emitted by gradientalloyed CdxZn1-xSeyS1-y colloidal quantum dots (QDs) when excited with a bluelight at ~460 nm. A large mode volume can be created within the QD layer by thehybridisation of the grating resonances and waveguide modes. This allowsincreasing mode overlap between the resonances and the QDs, resulting in largeabsorption and tailored emission enhancements. Particularly, we achievedpolarized light emission with maximum photoluminescence enhancement of ~140times at a specific angular direction, and a total enhancement of ~34 timeswithin 0.55 numerical aperture (NA) of the collecting objective. Theenhancement encompasses absorption enhancement, Purcell enhancement anddirectionality enhancement (i.e., outcoupling). We achieved total absorption of35% for green QDs with a remarkably thin colour conversion layer of ~ 400 nm(inclusive of the TiO2 layer). This work provides a guideline for designinglarge-volume cavities for practical application in absorption/fluorescenceenhancement, such as down colour conversion in microLED displays, detectors orphotovoltaics.

Dual-resonance nanostructures for colour down-conversion of colloidal quantum emitters