  Linear colour conversion is a process where an emitter absorbs a photon and
then emits another photon with either higher or lower energy, corresponding to
up- or down conversion, respectively. In this regard, the presence of a
volumetric cavity plays a crucial role in enhancing absorption and
photoluminescence (PL), as it allows for large volumes of interaction between
the exciting photons and the emissive materials, maximising the colour
conversion efficiency. Here, we present a dual resonance nanostructure made of
a titanium dioxide (TiO2) subwavelength grating to enhance the colour
down-conversion efficiency of green light at ~530 nm emitted by gradient
alloyed CdxZn1-xSeyS1-y colloidal quantum dots (QDs) when excited with a blue
light at ~460 nm. A large mode volume can be created within the QD layer by the
hybridisation of the grating resonances and waveguide modes. This allows
increasing mode overlap between the resonances and the QDs, resulting in large
absorption and tailored emission enhancements. Particularly, we achieved
polarized light emission with maximum photoluminescence enhancement of ~140
times at a specific angular direction, and a total enhancement of ~34 times
within 0.55 numerical aperture (NA) of the collecting objective. The
enhancement encompasses absorption enhancement, Purcell enhancement and
directionality enhancement (i.e., outcoupling). We achieved total absorption of
35% for green QDs with a remarkably thin colour conversion layer of ~ 400 nm
(inclusive of the TiO2 layer). This work provides a guideline for designing
large-volume cavities for practical application in absorption/fluorescence
enhancement, such as down colour conversion in microLED displays, detectors or
photovoltaics.
