Abstract
Exciton-polaritons, a hybrid state of strongly coupled light and matter, present an exciting avenue for potential applications in chemistry, optics, and solid-state physics. Polariton quasi-particles provide new allowed quantum states and additional degrees of freedom to participate in cavity dynamics enabling new chemical reactions in inorganic and organic systems. Applications in light emitting diodes, low-threshold lasing, photonic switching as well as the potential to study Bose-Einstein condensation at high temperatures show the merit of continued research.
Colloidal cadmium selenide semiconducting nanocrystals (NCs) known as nanoplatelets (NPLs) (a two-dimensional variant of the quantum dot) are increasingly used in optical devices due to their bright and tunable emission characteristics. However, to use these colloidal particles in practical photonic devices, the direct patterning of micron-scale features remains a grand-challenge.
Optimal conditions for polariton creation using NPLs inside Fabry-Perot (FP) cavities fabricated in the URnano cleanroom as well as the ligand-stripping procedure for colloidal nanocrystal solutions are explored. The ability to selectively pattern nanoplatelet films and optical cavities enables new modalities for lateral electric field confinement and fabricating metastructures.
Introduction
Exciton-polaritons show promise in the utilization and understanding of new chemical reactions as well as the creation of opto-electronic devices. Polaritons arise from the strong coupling of light and matter inside regions of localized resonant electric field. Plasmonic resonators and Fabry-Perot (FP) cavities provide ways to confine the electric field of certain resonant wavelengths into an area with excitons to observe strong coupling. Semiconducting nanocrystals (NCs) with allowed electronic transitions at the appropriate frequencies are placed in the regions of strong electric field to enable coupling.
Nanoplatelets are 2-dimensional CdSe nanocrystals with their electric dipole oriented perpendicular to the face of the platelet. The differences in geometry between other materials such as quantum dots allows for different exciton mode volume, exciton region thickness, oscillator strength, dipole direction, and different cavity dynamics.
Strong coupling of exciton-polaritons and cavity mode-splitting can be described by
Cavity Fabrication and NPL Deposition
The confined feature view shows an example of a laterally confined mode-volume used in this project to observe polaritons with significant Rabi splitting.
Protocol:
- P20 (HMDS) primer is spin coated* onto a clean half-cavity substrate
- S-1805 UV photoresist is spin coated* to achieve film thickness of 500 nm.
- Substrate is baked at 115°C for 60 S
- 405 nm UV laser-writer exposes arbitrary pattern at 270 mJ cm-2
- Development (1) using MF-319 removes exposed photoresist under light agitation for 60 s before being submersed in DI-H2O stop bath
- A ~150 nm Colloidal NPL film is drop casted onto the remaining resist
- Development (2) is carried out using acetone Leaving NPL pattern
- The top half of the Fabry-Perot cavity is deposited in the PVD
*Spin coating cycle is 3 steps with an acceleration and deceleration rate of 12,000 RPM/s:
500 RPM | 5 s
6500 RPM | 45 s
500 RPM | 5 s
Experimental Setup
Example of DBR-metal FP cavities without lateral confinement and polariton observation using angle resolved white light reflectance and photoluminescense obtained from the above setup diagram.
Results
An array of NPL patterns illustrating achievable feature size.
White light reflectance demonstrates strong coupling in the patterned NPL cavities. Large Rabi splittings are observed in a) and b). In a) strong coupling to the NPLs’ light hole exciton transition is also observed indicating strong mode confinement.
Summary and Future Directions
Fabricating NPL microstructures allows us to study cavity geometries that enable a defined mode volume with increased electric field confinement which will be modeled in Lumerical in the future.
Creating chiral NPL meta-structures opens up the possibility to study chiral strong light-matter coupling.
Rabi splitting of the NPL light hole excitonic transition as seen with this cavity geometry poses new questions about the underlying physics of hybridized polariton states.