Dylan Lu

Hyperlenses and metalenses for far-field super-resolution imaging

The resolution of conventional optical lens systems is always hampered by the diffraction limit. Recent developments in artificial metamaterials provide new avenues to build hyperlenses and metalenses that are able to image beyond the diffraction limit. Hyperlenses project super-resolution information to the far field through a magnification mechanism, whereas metalenses not only super-resolve subwavelength details but also enable optical Fourier transforms. Recently, there have been numerous designs for hyperlenses and metalenses, bringing fresh theoretical and experimental advances, though future directions and challenges remain to be overcome.

Wide field super-resolution surface imaging through plasmonic structured illumination microscopy.

We experimentally demonstrate a wide field surface plasmon (SP) assisted super-resolution imaging technique, plasmonic structured illumination microscopy (PSIM), by combining tunable SP interference (SPI) with structured illumination microscopy (SIM). By replacing the laser interference fringes in conventional SIM with SPI patterns, PSIM exhibits greatly enhanced resolving power thanks to the unique properties of SP waves. This PSIM technique is a wide field, surface super-resolution imaging technique with potential applications in the field of high-speed biomedical imaging.

Enhanced spontaneous emission inside hyperbolic metamaterials

Hyperbolic metamaterials can enhance spontaneous emission, but the radiation-matter coupling is not optimized if the light source is placed outside such media. We demonstrate a 3-fold improvement of the Purcell factor over its outer value and a significant enlargement in bandwidth by including the emitter within a Si/Ag periodic multilayer metamaterial. To extract the plasmonic modes of the structure into the far field we implement two types of 1D grating with triangular and rectangular profile, obtaining a 10-fold radiative enhancement at visible frequencies.

Ultralow Thermal Conductivity of Multilayers with Highly Dissimilar Debye Temperatures

Thermal transport in multilayers (MLs) has attracted significant interest and shows promising applications. Unlike their single-component counterparts, MLs exhibit a thermal conductivity that can be effectively engineered by both the number density of the layers and the interfacial thermal resistance between layers, with the latter being highly tunable via the contrast of acoustic properties of each layer. In this work, we experimentally demonstrated an ultralow thermal conductivity of 0.33 ± 0.04 W m(-1) K(-1) at room temperature in MLs made of Au and Si with a high interfacial density of ∼0.2 interface nm(-1). The measured thermal conductivity is significantly lower than the amorphous limit of either Si or Au and is also much lower than previously measured MLs with a similar interfacial density. With a Debye temperature ratio of ∼3.9 for Au and Si, the Au/Si MLs represent the highest mismatched system in inorganic MLs measured to date. In addition, we explore the prior theoretical prediction that full phonon dispersion could better model the interfacial thermal resistance involving materials with low Debye temperatures. Our results demonstrate that MLs with highly dissimilar Debye temperatures represent a rational approach to achieve ultralow thermal conductivity in inorganic materials and can also serve as a platform for investigating interfacial thermal transport.

Thermochromic halide perovskite solar cells

Smart photovoltaic windows represent a promising green technology featuring tunable transparency and electrical power generation under external stimuli to control the light transmission and manage the solar energy. Here, we demonstrate a thermochromic solar cell for smart photovoltaic window applications utilizing the structural phase transitions in inorganic halide perovskite caesium lead iodide/bromide. The solar cells undergo thermally-driven, moisture-mediated reversible transitions between a transparent non-perovskite phase (81.7% visible transparency) with low power output and a deeply coloured perovskite phase (35.4% visible transparency) with high power output. The inorganic perovskites exhibit tunable colours and transparencies, a peak device efficiency above 7%, and a phase transition temperature as low as 105 °C. We demonstrate excellent device stability over repeated phase transition cycles without colour fade or performance degradation. The photovoltaic windows showing both photoactivity and thermochromic features represent key stepping-stones for integration with buildings, automobiles, information displays, and potentially many other technologies.CsPbI3–xBrx solar cells, which undergo temperature- and moisture-driven reversible transitions between a non-perovskite transparent phase and a perovskite light-absorbing phase, are used as thermochromic photovoltaic devices integrated in windows.

Tunable surface plasmon polaritons in Ag composite films by adding dielectrics or semiconductors

We demonstrate that the surface plasmon polariton (SPP) properties of the silver composite films can be tuned by modest additions of silicon oxide or silicon. The dispersion relations deviate from that of pure silver films, and exhibit the capability to shift the surface plasmon frequency and provide larger SPP wave vectors at longer wavelengths. The effective permittivities are modeled phenomenologically by taking into account both filling ratios and size effects. These types of tunable composite films have various useful applications in areas, such as superlens imaging, SPP based sensing, enhanced photoluminescence, and SPP based photovoltatics.

Nanostructuring Multilayer Hyperbolic Metamaterials for Ultrafast and Bright Green InGaN Quantum Wells

Semiconductor quantum well (QW) light-emitting diodes (LEDs) have limited temporal modulation bandwidth of a few hundred MHz due to the long carrier recombination lifetime. Material doping and structure engineering typically leads to incremental change in the carrier recombination rate, whereas the plasmonic-based Purcell effect enables dramatic improvement for modulation frequency beyond the GHz limit. By stacking Ag-Si multilayers, the resulting hyperbolic metamaterials (HMMs) have shown tunability in the plasmonic density of states for enhancing light emission at various wavelengths. Here, nanopatterned Ag-Si multilayer HMMs are utilized for enhancing spontaneous carrier recombination rates in InGaN/GaN QWs. An enhancement of close to 160-fold is achieved in the spontaneous recombination rate across a broadband of working wavelengths accompanied by over tenfold enhancement in the QW peak emission intensity, thanks to the outcoupling of dominating HMM modes. The integration of nanopatterned HMMs with InGaN QWs will lead to ultrafast and bright QW LEDs with a 3 dB modulation bandwidth beyond 100 GHz for applications in high-speed optoelectronic devices, optical wireless communications, and light-fidelity networks.

Bacteria photosensitized by intracellular gold nanoclusters for solar fuel production

The demand for renewable and sustainable fuel has prompted the rapid development of advanced nanotechnologies to effectively harness solar power. The construction of photosynthetic biohybrid systems (PBSs) aims to link preassembled biosynthetic pathways with inorganic light absorbers. This strategy inherits both the high light-harvesting efficiency of solid-state semiconductors and the superior catalytic performance of whole-cell microorganisms. Here, we introduce an intracellular, biocompatible light absorber, in the form of gold nanoclusters (AuNCs), to circumvent the sluggish kinetics of electron transfer for existing PBSs. Translocation of these AuNCs into non-photosynthetic bacteria enables photosynthesis of acetic acid from CO2. The AuNCs also serve as inhibitors of reactive oxygen species (ROS) to maintain high bacterium viability. With the dual advantages of light absorption and biocompatibility, this new generation of PBS can efficiently harvest sunlight and transfer photogenerated electrons to cellular metabolism, realizing CO2 fixation continuously over several days.A photosynthetic biohybrid system based on non-photosynthetic bacteria that incorporate gold nanoclusters achieves faster electron transfer and more durable solar CO2 fixation.

Giant Light-Emission Enhancement in Lead Halide Perovskites by Surface Oxygen Passivation

Surface condition plays an important role in the optical performance of semiconductor materials. As new types of semiconductors, the emerging metal-halide perovskites are promising for next-generation optoelectronic devices. We discover significantly improved light-emission efficiencies in lead halide perovskites due to surface oxygen passivation. The enhancement manifests close to 3 orders of magnitude as the perovskite dimensions decrease to the nanoscale, improving external quantum efficiencies from <0.02% to over 12%. Along with about a 4-fold increase in spontaneous carrier recombination lifetimes, we show that oxygen exposure enhances light emission by reducing the nonradiative recombination channel. Supported by X-ray surface characterization and theoretical modeling, we propose that excess lead atoms on the perovskite surface create deep-level trap states that can be passivated by oxygen adsorption.

Three-dimensional nanoscale imaging by plasmonic Brownian microscopy

Abstract Three-dimensional (3D) imaging at the nanoscale is a key to understanding of nanomaterials and complex systems. While scanning probe microscopy (SPM) has been the workhorse of nanoscale metrology, its slow scanning speed by a single probe tip can limit the application of SPM to wide-field imaging of 3D complex nanostructures. Both electron microscopy and optical tomography allow 3D imaging, but are limited to the use in vacuum environment due to electron scattering and to optical resolution in micron scales, respectively. Here we demonstrate plasmonic Brownian microscopy (PBM) as a way to improve the imaging speed of SPM. Unlike photonic force microscopy where a single trapped particle is used for a serial scanning, PBM utilizes a massive number of plasmonic nanoparticles (NPs) under Brownian diffusion in solution to scan in parallel around the unlabeled sample object. The motion of NPs under an evanescent field is three-dimensionally localized to reconstruct the super-resolution topology of 3D dielectric objects. Our method allows high throughput imaging of complex 3D structures over a large field of view, even with internal structures such as cavities that cannot be accessed by conventional mechanical tips in SPM.