AbstractHere, we report template‐assisted assembly of emissive carbon quantum dot (CQD) microcrystals on organized cellulose nanocrystals templates at the liquid–air interface. This large‐scale assembly is facilitated by the complementary amphiphilic character of CQDs and cellulose nanocrystals in the organized nematic phase. The resulting large microcrystals up to 200 μm across show unusually high emission that is not observed for limited CQDs aggregates. The dense crystal packing of CQDs in the layered fashion suppresses local molecular rotations and vibrations, thus restricting the intermolecular energy transfer and corresponding quenching phenomena. The as‐prepared crystals are mechanically stable and can be exploited for recyclable catalysis, enabling applications beyond the individual nanoparticles or disordered aggregates. The ligand‐templated assembly can be used to diversify CQD crystal architectures to guide formation of fibers, microplates, and micro‐flowers.

Despite impressive advances in the synthesis of perovskite quantum dots (PQDs), the ability to craft PQDs of the same dimension yet different architectures (e.g., solid vs. hollow) remains a grand challenge. Moreover, precise control over the architectures and assemblies of PQDs renders new optical and optoelectronic properties. Herein, we report on a robust amphiphilic star-like block copolymer nanoreactor strategy to rapidly create monodisperse solid and hollow PQDs of the same external diameter in-situ. The dimension of PQDs can be readily regulated by utilizing star-like copolymers. Compared to solid PQDs, as the inner diameter of hollow PQDs increases, their photoluminescence progressively blue-shifts. Moreover, stripe patterns of PQDs can be conveniently formed via meniscus-assisted self-assembly (MASA) and subsequently anion-exchanged to yield multi-colored stripes with a heterostructured transition zone. Conceptually, an array of PQDs including all inorganic lead-free and organic-inorganic PQDs, can be easily accessed for applications in lasers, LEDs, and solar cells.

Since their inception, quantum dots have proven to be advantageous for light management applications due to their high brightness and well-controlled absorption, scattering, and emission properties. As quantum dots become commercially available at large scale, the need for robust, stable, and flexible optical components continues to drive the development of robust and flexible quantum dot composite materials. In this review, after a thorough introduction to quantum dots, discussion delves into methods for fabricating quantum dot loaded composite optical elements such as thin films, microfabricated patterns, and microstructures. The importance of surface chemistry and ligand engineering, host matrixes, wet processing, and unique patterning methodologies is presented by considering photostability, aggregation, and phase separation of quantum dots in corresponding composites. With regard to prospective optical applications of quantum dot materials, emphasis is placed on light emitting and guiding composite materials for lasing applications, specifically whispering gallery mode-based photonic microsystems. These developments will enable novel flexible, portable, and miniaturized optoelectronic devices such as light-emitting diodes, flexible pixelated displays, solar cells, large-area microwaveguides, omnidirectional micromirrors, optical metasurfaces, and directional microlasers.

Chiral fluorescent materials with fluorescent nanoparticles assembled into a chiral structure represent a grand challenge. Here, we report self -assembled emissive needle -like nanostructures through decorating cellulose nanocrystals (CNCs) with carbon quantum dots (CQDs). This assembly is facilitated by the heterogeneous amphiphilic interactions between natural and synthetic components. These emissive nanostructures can self organize into chiral nematic solid-state materials with enhanced mechanical performance. The chiral CQD/CNC films demonstrate an intense iridescent appearance superimposed with enhanced luminescence that is significantly higher than that for CQD films and other reported CQD/ CNC films. A characteristic fluorescent fingerprint signature is observed in the CQD/CNC film, proving the well-defined chiral organization of fluorescent nanostructures. The chiral organization of CQDs enables the solid CQD/CNC film to form a right-hand chiral fluorescence with an asymmetric factor of -0.2. Additionally, we developed chemical 2D printing and soft lithography patterning techniques to fabricate the freestanding chiral fluorescent patterns that combines mechanical intergrity and chiral nematic structure with light diffraction and emission.

Electrochromic polymer infused plasmonic nanohole array shows heterogeneous forward and backward optical scattering response upon polymer electrical permittivity modulation.

Whispering gallery mode resonators have been demonstrated to be a great way to achieve superior optical cavities with high quality factor and small mode volume. However due to the high sensitivity of these modes to the properties of the resonator boundary, they are susceptible to parasitic splitting of clockwise and counter-clockwise modes. In this work, we investigate the effect of implantation of an engineered notch into the boundary of a circular microdisk resonator fabricated from colloidal quantum dots, which are particularly sensitive to boundary defects. We observed a strong reduction of parasitic mode splitting with introduction of a large engineered notch, as well as enhanced directionality of laser emission. We further investigate the performance of these resonators in evanescently coupled pairs, where the modal interaction allows modulation of laser behavior through variation of the gain and loss induced by the optical pump. We show that two distinct cases of modal interaction can be achieved by adjusting the size of the engineered notch, providing a bridge between intra- and inter-disk modal interactions for laser spectral control.

This study reports a facile method for the assembly of large, array style, coupled dye‐doped microsphere resonators by template‐assisted, in which an aqueous suspension of colloidal microspheres assemble on a patterned template. By exploiting the high resolution of 3D (two‐photon) lithography derived templates, closely packed large arrays, hundreds to thousands of dimers with controlled gap spacing, only limited by the size of the substrate can be achieved. Dye‐doped emissive microspheres with Q ‐factors >2.5 × 10 2 can be achieved and trapped into predetermined cavity positions, thereby controlling the distance between adjacent microspheres. This design allows to scale down dimer spacing from usual 400 nm for traditional photolithography to very small spacing of 50 nm. It is found that exciting individual microspheres in the ensemble shows intense optical cavity modes, whereas closely coupled pairs show controlled mode splitting. Coupling between photoluminescent microspheres is strongly influenced by the gap distance, with strong coupling, equating to normal mode splitting, arising as the gap distance is reduced below traditional sub‐micrometer scale. The coupled dimer assemblies are promising candidates for advancing the development of large‐area coupled nanophotonic structures, beyond the spatial resolution‐limited photolithographical derived arrays.

We report the multibody coupling behavior of a vertical hybrid plasmonic-photonic cavity whose output mode can be selected by different lateral geometrical configurations of plasmonic nanostructures for a novel compact optical modulation strategy. The hybrid cavity has a Fabry-Perot-based configuration with a SiO2/Si dielectric interface at one end and plasmonic individual or coupled Au nanospheres (AuNSs) on the other end. The AuNS acts as an optical antenna that outcouples the standing wave inside the cavity to the far-field. The behavior of this hybrid antenna can be altered by the lateral near-field coupling of two AuNSs, allowing different output mode to be amplified. Upon assembly, the plasmonic peaks of the AuNSs are discretized by the photonic cavity, forming hybrid modes that are distinctively different from the original Fabry-Perot modes. A different primary mode can be selected according to the spectral envelope imposed by the plasmonic oscillation from respective AuNS nanostructures. Optical responses from monomer and dimer are recorded at single-particle resolution, and their interactions with a photonic cavity are analyzed using subnanometer grid finite difference time domain (FDTD) simulations to reveal the underlying multibody coupling mechanism. We also demonstrate the potential for the hybrid microcavities to incorporate gain media for potential applications in photonic circuits and near-field spectroscopy.

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.

We have fabricated microdisk lasers of colloidal quantum dots in circular and elliptical forms with different aspect ratios. By characterizing the laser emission spectrum under optical pumping and through mode simulation calculations, we demonstrate that the elliptical resonators can display two sets of whispering-gallery modes that interact to varying degrees depending on the aspect ratio. This causes significant mode splitting in the emission spectra when the mode interaction is at maximum. We also performed angular-dependent laser emission measurements for characterizing the emission pattern of the microlasers. We found that the emission pattern becomes less isotropic and more directional as the boundary deviates further from circular symmetry. In addition, we demonstrate that the emission directional properties of these microlasers can be further tailored by coupling pairs of elliptical microcavities together in different manners, where the long or short elliptical axis interacts most strongly. (C) 2018 Society of Photo-Optical Instrumentation Engineers (SPIE)

Herein, a novel orthogonal lithography process is reported to pattern all‐inorganic perovskite CsPbX 3 (X = Cl, Br, I) quantum dot (QD) arrays which cannot be patterned with traditional approaches. This approach involves a combination of fluorinated polymer and solvent to resolve issues of polar–nonpolar solvent constraints thus enabling the fabrication of complex patterns with high optical gain and multicolor emission. This approach is utilized to fabricate high‐resolution large‐area arrays of microdisk lasers and multicolor (binary and ternary emission) pixels. The optical cavity modes of CsPbBr 3 QD microdisk lasers are readily controlled by tuning the disk size, where the mode spacing decreases while the number of modes increases with increasing disk diameter. Finally, the versatility of this approach for the integration of environmentally sensitive QDs with different emission signatures and composition on the same chip, while achieving high‐density, high‐resolution large‐area QD arrays with multicolor pixels, is demonstrated.

We report on the intriguing phenomenon of the evolution of photoluminescence from CdSe core, CdSe/ZnS core/shell, and CdSe/Cd1–xZnxSe1–ySy core/graded shell green-emitting quantum dots (QDs). The thickness and composition profile of QDs was found to control how the optical characteristics evolve under different light exposure conditions. Unexpectedly, changes of emission intensity (decreasing and increasing) and spectral band position were observed, which can be reversible or irreversible, depending on the QD architecture and exposure conditions. It is revealed that competition between the reversible and irreversible optical changes led to unique decay-to-recovery behavior for the QD emission, metastable bright states of QDs that can be activated and deactivated numerous times, and decoupled optical changes (reversible intensity changes vs irreversible spectral shifts). We suggest that the distinct dynamic response of each QD architecture arises from how the core and shell interact with each other and the...

This study reports a facile on‐chip fabrication of CdSe/Cd1−xZnxSe1−ySy quantum dot microdisk lasers and their large‐area arrays via a pattern‐assisted layer‐by‐layer assembly process. This approach combines the versatility of colloidal semiconducting nanoparticles (bright emission, solubility, and high stability) with the spatial precision of optical lithography to create robust large‐area optical lasing arrays (up to a few thousand disks). Specifically, microdisk lasers with high quality factors (within 1000–2000) are fabricated with predefined size and shape (as controlled by master templates) with high consistency and throughput, essentially providing a new approach to fabricate difficult‐to‐control on‐chip optical cavities in a low‐cost and effective manner. Notably, the number of longitudinal cavity modes in the microdisk laser can be precisely controlled by varying the microdisks' diameter, allowing for either near‐single mode or multimode operation while preserving high quality factors. Furthermore, the cross‐linking of quantum dots imparts high chemical resistance and mechanical robustness that helps retain the structural integrity under harsh processing conditions (such as sonication or direct exposure to various solvents). As such, these quantum dot microdisk laser arrays are promising candidates for advancing the development of large‐area, low‐cost on‐chip photonic structures with controlled lasing modes.

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.

In 1998, Carl M. Bender made the interesting observation that non‐Hermitian Hamiltonians may still possess real eigenvalues if they obey parity‐time (PT) symmetry [1]. More recently, it was demonstrated that optical systems provide fertile experimental test‐grounds for these abstract principles, as the complex PT‐ symmetric potential could be constructed by spatial balancing of loss and gain [2]. In addition to providing model systems for non‐Hermitian quantum mechanics, PT‐ symmetric optical structures are capable of producing novel, counter‐intuitve optical behavior including non‐reciprocal propagation, loss‐induced transparency, and gain‐induced suppression of lasing. After introducing these concepts, I will describe our work in bringing the versatility of solution‐processed semiconductors to study PT‐symmetric optical phenomenon, focusing on collaborative work with chemists and materials scientists at Georgia Tech University to develop, characterize and manipulate optical gain in colloidal quantum dot films. After achieving optimal material properties and device structures for laser action, we fabricated microdisk laser cavities that exhibit whispering gallery mode emission upon optical pumping. These resonators then serve as molecular units in near‐field coupled structures where we measure the microdisk(s) emission pattern while modulating the spatial and intensity profile of the pump beam to create different conditions of gain/loss in the structures. In this way we explore the exotic effects arising from their mutual interaction.

We present novel photopatterning approaches based on near-field and far field interference lithography techniques that yield highly uniform high-resolution large area face-profile and edge-profile photopatterns. The near-field interference methods utilize a phase-shift mask while the far-field method uses two-beam interference. These interference-based techniques yield photopatterns with minimum feature sizes near 500 nm, which matches the current resolution of photopatterning. Furthermore, these interference techniques drastically increase the patternable area (up to cm(2)) and the throughput (increases of up to 3 orders of magnitude) while maintaining pattern uniformity. Furthermore, these strategies use easy-to-handle reusable photomasks or no masks at all and address the major constraints associated with obtaining high resolution without compromising throughput that have often limited the applicability of traditional photopatterning. Finally, all approaches can be applied multiple times on the same film area to yield ultradense multilevel intensity contrast photopatterns that are very difficult to obtain using traditional strategies. These interference-based exposure techniques represent a paradigm shift in the field of photopatterning and will be valuable for applications that require uniform high-resolution patterns over large areas, such as photosensors, anticounterfeiting labels, and virtual displays.

Microdisk lasers of quantum dots were fabricated with both unintentional defects from fabrication, and intentional defects engineered with controlled size. The effects of the defects on the spectral and directional emissive properties were studied.

Positive and negative photoluminescent photopattern contrasts arising from intrinsic modification of quantum dot (QD) emission (decay or recovery) upon exposure to light are reported. The ability to fabricate a variety of photo­pattern types using a single type of quantum dot is due to a two‐step decay‐to‐recovery evolution upon light exposure. It is shown that high‐contrast photopatterns spanning mm2 areas can be fabricated within seconds with a facile one‐step process, representing a drastic reduction in the time required to develop an emissive pattern in a QD‐polymer film (from hours to seconds). Furthermore, the controlled light exposure allows for a programmed transformation of the emissive pattern contrast, with a reversal of the bright/dark regions of the QD‐polymer photopattern demonstrated. Finally, it is shown that the photopatterns can be stored over a period of time and then “recharged” using simple light exposure to partially recover the intensity and contrast of aged photopatterns. The outlined patterning strategies open up new pathways for facile, one‐step parallel fabrication of anti‐counterfeiting emitting labels and light sensors, as well as for gain‐loss parity‐time systems where an emission contrast is required but where physical patterning may not be appropriate.