Robust lasing modes in coupled colloidal quantum dot microdisk pairs using a non-Hermitian exceptional point.

Evanescently coupled pairs of microdisk lasers have emerged as a useful platform for studying the non-Hermitian physics of exceptional points. It remains an open question how scalable and versatile such phenomena can be when carried over to other designs. Here we have studied the effect of gain/loss modulation in an evanescently coupled pair of microdisk optical resonators fabricated from solution-processed colloidal quantum dots. The emission spectra of these structures are sensitive to small imperfections, which cause frequency-splitting of the whispering gallery modes. Despite this inherent disorder, we found that when spatially modulating the optical pump to vary the gain differential between the coupled microdisks, the coupling drives the split parasitic intra-cavity modes into coalescence at an exceptional point of the resulting three-mode system. This unusual behavior is rationalized via a Hamiltonian that incorporates the intra-cavity coupling as well as the anisotropic inter-cavity coupling between modes in the microdisk pair.

Robust, Uniform, and Highly Emissive Quantum Dot-Polymer Films and Patterns Using Thiol-Ene Chemistry.

This work demonstrates a facile and versatile method for generating low scattering cross-linked quantum dot (QD)-polymer composite films and patterned highly emissive structures with ultrahigh QD loading, minimal phase separation, and tunable mechanical properties. Uniform QD-polymer films are fabricated using thiol-ene chemistry, in which cross-linked polymer networks are rapidly produced in ambient conditions via fast UV polymerization in bulk to suppress QD aggregation. UV-controlled thiol-ene chemistry limits phase separation through producing highly QD loaded cross-linked composites with loadings above majority of those reported in the literature (<1%) and approaching 30%. As the QD loading is increased, the thiol and ene conversion decreases, resulting in nanocomposites with widely variable and tailorable mechanical properties as a function of UV irradiation time with an elastic modulus decreasing to 1 GPa being characteristic of reinforced elastomeric materials, in contrast to usually observed stiff and brittle materials under these loading conditions. Furthermore, we demonstrate that the thiol-ene chemistry is compatible with soft-imprint lithography, making it possible to pattern highly loaded QD films while preserving the optical properties essential for high gain and low optical loss devices. The versatility of thiol-ene chemistry to produce high-dense QD-polymer films potentially makes it an important technique for polymer-based elastomeric optical metamaterials, where efficient light propagation is critical, like peculiar waveguides, sensors, and optical gain films.

Crafting Core/Graded Shell-Shell Quantum Dots with Suppressed Re-absorption and Tunable Stokes Shift as High Optical Gain Materials.

The key to utilizing quantum dots (QDs) as lasing media is to effectively reduce non-radiative processes, such as Auger recombination and surface trapping. A robust strategy to craft a set of CdSe/Cd(1-x)Zn(x)Se(1-y)S(y)/ZnS core/graded shell-shell QDs with suppressed re-absorption, reduced Auger recombination rate, and tunable Stokes shift is presented. In sharp contrast to conventional CdSe/ZnS QDs, which have a large energy level mismatch between CdSe and ZnS and thus show strong re-absorption and a constrained Stokes shift, the as-synthesized CdSe/Cd(1-x)Zn(x)Se(1-y)S(y)/ZnS QDs exhibited the suppressed re-absorption of CdSe core and tunable Stokes shift as a direct consequence of the delocalization of the electron wavefunction over the entire QD. Such Stokes shift-engineered QDs with suppressed re-absorption may represent an important class of building blocks for use in lasers, light emitting diodes, solar concentrators, and parity-time symmetry materials and devices.

Cellulose Nanocrystal Microcapsules as Tunable Cages for Nano- and Microparticles.

We demonstrate the fabrication of highly open spherical cages with large through pores using high aspect ratio cellulose nanocrystals with "haystack" shell morphology. In contrast to traditional ultrathin shell polymer microcapsules with random porous morphology and pore sizes below 10 nm with limited molecular permeability of individual macromolecules, the resilient cage-like microcapsules show a remarkable open network morphology that facilitates across-shell transport of large solid particles with a diameter from 30 to 100 nm. Moreover, the transport properties of solid nanoparticles through these shells can be pH-triggered without disassembly of these shells. Such behavior allows for the controlled loading and unloading of solid nanoparticles with much larger dimensions than molecular objects reported for conventional polymeric microcapsules.