“Fata Morgana” or “Mirage” phenomena have long been captivated as optical illusions, which actually relies on gradient‐density air or vapor. Man‐made optical illusions have witnessed significant ...progress by resorting to artificially structured metamaterials. Nevertheless, two long‐standing challenges remain formidable: first, exotic parameters (negative or less than unity) become inevitable; second, the signature of original object is altered to that of a virtual counterpart. It is thus not able to address the holy grail of illusion per se, since a single virtual object still exposes the location. In this study, those problems are successfully addressed in a particular setup—illusion thermotics, which identically mimics the exterior thermal behavior of an equivalent reference and splits the interior original heat source into many virtual signatures. A general paradigm to design thermal illusion metadevices is proposed to manipulate thermal conduction, and empower robust simultaneous functions of moving, shaping, rotating, and splitting heat sources of arbitrary cross sections. The temperature profile inside the thermal metadevice can mislead the awareness of the real location, shape, size, and number of the actual heat sources. The present concept may trigger unprecedented development in other physical fields to realize multiple functionalized illusions in optics, electromagnetics, etc.
Existing optical/thermal illusions fail to address the holy grail of illusion per se, since a single virtual object still exposes the location. A solution to this problem is found in illusion thermotics, which identically mimics the exterior thermal behavior of an equivalent reference and splits the interior original heat source into many virtual signatures to enhance the deceptiveness unprecedentedly.
The geometry and dimension of a gold nanorod (GNR) are optimally designed to enhance the fluorescence intensity of a lanthanide-doped upconversion nanocrystal placed in close proximity to the GNR. A ...systematic study of the electromagnetic interaction between the upconversion emitter of three energy levels and the GNR shows that the enhancement effect arising from localized electric field-induced absorption can be balanced by the negative effect of electronic transition from an intermediate state to the ground state of the emitter. The dependence of fluorescence enhancement on the emitter-GNR separation is investigated, and the results demonstrate a maximum enhancement factor of 120 folds and 160 folds at emission wavelengths 650 and 540 nm, respectively. This is achieved at the emitter-GNR separation ranging from 5 to 15 nm, depending on the initial quantum efficiency of the emitter. The modified upconversion luminescence behavior by adjusting the aspect ratio of the GNR and the relative position of the emitter indicates the dominate role of excitation process in the total fluorescence enhancement. These findings are of great importance for rationally designing composite nanostructures of metal nanoparticles and upconversion nanocrystals with maximized plasmonic enhancement for bioimaging and sensing applications.
Chirality is a universal geometric property in both micro‐ and macroworlds. Recently, optical chiral effects have drawn increased attention due to their great potential in fundamental studies and ...practical applications. Significantly, the optical chiral response of artificial structures can be enhanced by orders of magnitude compared to that of their naturally occurring counterparts. These man‐made structures generally exhibit two types of optical chirality: extrinsic chirality and intrinsic chirality. The former relies on external illumination conditions, while the latter arises from the geometric characteristics of 3D objects. Herein, this review mainly focuses on the intrinsic chirality of artificial structures and discusses the existing realizations based on their design principles. In particular, an overview is given of the recent demonstrations of nonlinear optical effects in chiral structures and active chiral structures. Lastly, some promising prospects for future studies in the field are outlined.
3D metaphotonic structures with intrinsic chirality support both optical rotation and circular dichroism, which are orders of magnitude higher than that of their naturally occurring counterparts. They have drawn increased attention due to their potential in fundamental studies in physics, chemistry and biology, and in practical applications, such as negative refractive index media, molecule sensors, and modulators.
The last decade has witnessed the remarkable research progress of lanthanide‐doped upconversion nanocrystals (UCNCs) at the forefront of promising applications. However, the future development and ...application of UCNCs are constrained greatly by their underlying shortcomings such as significant nonradiative processes, low quantum efficiency, and single emission colors. Here a hybrid plasmonic upconversion nanostructure consisting of a GNR@SiO2 coupled with NaGdF4:Yb3+,Nd3+@NaGdF4:Yb3+,Er3+@NaGdF4 core–shell–shell UCNCs is rationally designed and fabricated, which exhibits strongly enhanced UC fluorescence (up to 20 folds) and flexibly tunable UC colors. The experimental findings show that controlling the SiO2 spacer thickness enables readily manipulating the intensity ratio of the Er3+ red, green, and blue emissions, thereby allowing us to achieve the emission color tuning from pale yellow to green upon excitation at 808 nm. Electrodynamic simulations reveal that the tunable UC colors are due to the interplay of plasmon‐mediated simultaneous excitation and emission enhancements in the Er3+ green emission yet only excitation enhancement in the blue and red emissions. The results not only provide an upfront experimental design for constructing hybrid plasmonic UC nanostructures with high efficiency and color tunability, but also deepen the understanding of the interaction mechanism between the Er3+ emissions and plasmon resonances in such complex hybrid nanostructure.
The plasmon‐mediated selective excitation and emission enhancement on the blue, green, and red emissions of Er3+ in a rationally designed hybrid plasmonic upconversion (UC) nanostructure enables achieving enhanced UC luminescence and tunable UC color.
The use of rare-earth (RE) (e.g., Eu2+/Ce3+) ions as single luminescent centers in phosphors with tailorable emission properties has been extensively studied for their potential use in white LEDs. ...However, significant limitations remain, in particular, for red-emitting phosphors due to the inherently broad excitation bands which result from the underlying d–f transitions and span large parts of the visible spectral region. Guided by density functional theory calculations on the ligand structure of the non-RE Bi3+ ion, we report here on an alternative class of phosphors, (Y,Sc)(Nb,V)O4:Bi3+, which exhibit homogeneous Bi3+ luminescence. In these materials, adjustment of the cation fractions enables dedicated tailoring of the excitation scheme within the spectral range of ∼340–420 nm and, in the meanwhile, allows for tunable emission spanning from about 450 nm (blue) to 647 nm (orange-red). The practical absence of any overlap between the emission and excitation spectra addresses the issues of emission color purity and visible reabsorption. Tailoring through band-gap modulation is achieved by single or parallel substitution of Nb by V and Y by Sc. Such topochemical design of the ligand configuration enables modulation of the electronic band gap and thus provides a new path toward tunable phosphors, exemplarily based on Bi3+ single doping.
Improving the ferromagnetism property of graphene materials is particularly important for a wide range of applications such as spintronics, magnetic memory, and other electromagnetic devices. ...Pyrrolic nitrogen (N) doping is an effective means to enhance the ferromagnetism of graphene materials. Here we report the synthesis of N-doped graphene with 6.02at.% doping concentration through a high-throughput hydrothermal method. X-ray photoelectron spectroscopy reveals that the pyrrolic N bonding configuration dominates over the other bonding types observed in our samples, which is in good agreement with the Raman spectroscopy and first-principle calculations. The vibrating sample magnetometer and SQUID are employed to further analyze the magnetic properties of the pyrrolic N-doped graphene. At room temperature, the sample exhibits significant ferromagnetism with a high saturation magnetic moment (1.4×10−2emu/g) among graphene materials and a narrow coercivity (181.4Oe). Our results have not only extended the synthesis method of N-doped graphene materials but also deepened the fundamental understanding of the N doping behaviors in enhancing their magnetism.
Plasmon‐induced hot carriers have vast potential for light‐triggered high‐efficiency carrier generation and extraction, which can overcome the optical band gap limit of conventional ...semiconductor‐based optoelectronic devices. Here, it is demonstrated that Au/TiO2 dumbbell nanostructures assembled on a thin Au film serve as an efficient optical absorber and a hot‐carrier generator in the visible region. Upon excitation of localized surface plasmons in such coupled particle‐on‐film nanocavities, the energetic conduction electrons in Au can be injected over the Au/TiO2 Schottky barrier and migrated to TiO2, participating in the chemical reaction occurring at the TiO2 surface. Compared with the same dumbbell nanostructures on an indium tin oxide (ITO) film, such nanocavities exhibit remarkable enhancement in both photocurrent amplitude and reaction rate that arise from increased light absorption and near‐field amplification in the presence of the Au film. The incident‐wavelength‐dependent photocurrent and reaction rate measurements jointly reveal that Au‐film‐mediated near‐field localization facilitates more efficient electron–hole separation and transport in the dumbbells and also promotes strong d‐band optical transitions in the Au film for generation of extra hot electrons. Such nanocavities provide a new plasmonic platform for effective photoexcitation and extraction of hot carriers and also better understanding of their fundamental science and technological implications in solar energy harvesting.
A Au/TiO2‐dumbbell‐on‐Au‐film‐nanocavity system is presented herein. This system features a dual hot electron injection channel recognized as an efficient localized plasmon resonance mechanism and a d‐band optical transition mechanism.
Metal-film-coupled nanoparticles with subnanometer particle–film gaps possess an ultrasmall mode volume, responsible for a variety of intriguing phenomena in plasmonic nanophotonics. Due to the large ...radiative loss associated with dipolar coupling, however, the plasmonic-film-coupled nanocavities usually feature a low-quality factor, setting an ultimate limit of the increased light–matter interaction strength. Here, we demonstrate a plasmonic nanocavity composed of a metal-film-coupled nanoparticle dimer, exhibiting a significantly improved quality factor. Compared to a silica-supported dimer, the spectral line width of the nanocavity plasmon resonance is reduced by a factor of ∼4.6 and is even smaller than its monomer counterpart (∼30% reduction). Comprehensive theoretical analyses reveal that this pronounced resonance narrowing effect can be attributed to intense film-mediated plasmon hybridization between the bonding dipolar and quadrupolar gap modes in the dimer. More importantly, the invoking of the dark quadrupole resonance leads to a giant photoluminescence intensity enhancement (∼200 times) and dramatic emission line-width narrowing (∼4.6 times), compared to the silica-supported dimer. The similar spectral characteristics of the measured plasmonic scattering and photoluminescence emission indicate that the radiative decay of the coupled plasmons in the nanocavity is the origin of the observed photoluminescence, consistent with a proposed phenomenological model. Numerical calculations show that the intensity enhancement is mainly contributed by the dimer–film gap rather than the interparticle gap. These findings not only shed more light on the hybridized interaction between plasmon modes but also deepen the understanding of photoluminescence emission in coupled plasmonic nanostructures.
Hierarchical porous plasmonic metamaterials consisting of periodic nanoholes with tunable diameter and uniformly distributed mesopores over the bulk are developed as a new class of 3D ...surface‐enhanced Raman spectroscopy (SERS) substrates. This multiscale architecture not only facilitates efficient cascaded electromagnetic enhancement but also provides an enormous number of Raman‐active binding sites, exhibiting excellent reproducibility and ultrasensitive detection of aromatic molecules down to 10−13 M.
The ability to harvest thermal energy and manipulate heat fluxes has recently attracted a great deal of research interest because this is critical to achieve efficient solar-to-thermal energy ...conversion in the technology of concentrated solar thermal collectors. Thermal metamaterials with engineered thermal conduction are often utilized to control the diffusive heat flow in ways otherwise not possible with naturally occurring materials. In this work, we adopt the transformation thermodynamics approach to design an annular fan-shaped thermal metamaterial which is capable of guiding heat fluxes and concentrating thermal energy to the central region of the metamaterial device without disturbing the temperature profile outside the structure--a fascinating and unique feature impossibly achieved with homogeneous materials. In experiment, this rationally-designed metamaterial structure demonstrates extreme heat flux compression from both line-shaped and point thermal sources with measured concentration efficiency up to 83.1%, providing the first experimental realization of our recent theoretical prediction (T. Han et al., Energy Environ. Sci., 2013, 6, 3537-3541). These unprecedented results may open up new possibilities for engineering thermal materials with desired properties that can be used for dramatically enhancing the efficiency of the existing solar thermal collectors.
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