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Polaritons in polar biaxial crystals with extreme anisotropy offer a promising route to manipulate nanoscale light-matter interactions. The dynamical modulation of their dispersion is great significance for future integrated nano-optics but remains challenging. Here, we report a momentum-directed strategy, a coupling between the modes with extra momentum supported by the interface and in-plane hyperbolic polaritons, to tailor topological transitions of anisotropic polaritons in biaxial crystals. We experimentally demonstrate such tailored polaritons at the interface of heterostructures between graphene and {\alpha}-phase molybdenum trioxide ({\alpha}-MoO3). The interlayer coupling can be electrically modulated by changing the Fermi level in graphene, enabling a dynamic topological transition. More interestingly, we found that the topological transition occurs at a constant Fermi level when tuning the thickness of {\alpha}-MoO3. The momentum-directed strategy implemented by interface engineering offers new insights for optical topological transitions, which may shed new light for programmable polaritonics, energy transfer and neuromorphic photonics.

Advanced control over the excitation of ultraconfined polaritons—hybrid light and matter waves—empowers unique opportunities for many nanophotonic functionalities, e.g., on-chip circuits, quantum information processing, and controlling thermal radiation. Recent work has shown that highly asymmetric polaritons are directly governed by asymmetries in crystal structures. Here, we experimentally demonstrate extremely asymmetric and unidirectional phonon polariton (PhP) excitation via directly patterning high-symmetry orthorhombic van der Waals (vdW) crystal α-MoO 3 . This phenomenon results from symmetry breaking of momentum matching in polaritonic diffraction in vdW materials. We show that the propagation of PhPs can be versatile and robustly tailored via structural engineering, while PhPs in low-symmetry (e.g., monoclinic and triclinic) crystals are largely restricted by their naturally occurring permittivities. Our work synergizes grating diffraction phenomena with the extreme anisotropy of high-symmetry vdW materials, enabling unexpected control of infrared polaritons along different pathways and opening opportunities for applications ranging from on-chip photonics to directional heat dissipation.

7 pags., 4 figs., Negative reflection occurs when light is reflected toward the same side of the normal to the boundary from which it is incident. This exotic optical phenomenon is not only yet to be visualized in real space but also remains unexplored, both at the nanoscale and in natural media. Here, we directly visualize nanoscale-confined polaritons negatively reflecting on subwavelength mirrors fabricated in a low-loss van der Waals crystal. Our near-field nanoimaging results unveil an unconventional and broad tunability of both the polaritonic wavelength and direction of propagation upon negative reflection. On the basis of these findings, we introduce a device in nano-optics: a hyperbolic nanoresonator, in which hyperbolic polaritons with different momenta reflect back to a common point source, enhancing the intensity. These results pave way to realize nanophotonics in low-loss natural media, providing an efficient route to control nanolight, a key for future on-chip optical nanotechnologies., G.Á.-P. and J.T.-G. acknowledge support through the Severo Ochoa Program from the Government of the Principality of Asturias (grant numbers PA-20-PF-BP19-053 and PA-18-PF-BP17-126, respectively). Q.O. acknowledges support from the Australian Research Council (ARC; CE170100039 and DE220100154). This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). hBN crystal growth was supported by the National Science Foundation award number CMMI 1538127. V.G. acknowledges the Ministerio de Ciencia, Innovación y Universidades through the grant MELODIA (PGC2018-095777-B-C21). V.G. thanks the “ENSEMBLE 3–Centre of Excellence for nanophotonics, advanced materials and novel crystal growth-based technologies” project (GA no. MAB/2020/14) carried out within the International Research Agendas program of the Foundation for Polish Science cofinanced by the European Union under the European Regional Development Fund and the European Union’s Horizon 2020 research and innovation programme Teaming for Excellence (GA no. 857543) for support of this work. R.H. acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities (national project RTI2018-094830-B-100 and the project MDM-2016-0618 of the María de Maeztu Units of Excellence Program) and the Basque Government (grant no. IT1164-19). J.M.-S. acknowledges financial support from the Ramón y Cajal Program of the Government of Spain and FSE (RYC2018-026196-I) and the Spanish Ministry of Science and Innovation (State Plan for Scientific and Technical Research and Innovation grant number PID2019-110308GA-I00). A.Y.N. acknowledges the Spanish Ministry of Science and Innovation (grants MAT201788358-C3-3-R and PID2020-115221GB-C42) and the Basque Department of Education (grant PIBA-2020-1-0014). P.A.-G. acknowledges support from the European Research Council under starting grant no. 715496, 2DNANOPTICA and the Spanish Ministry of Science and Innovation (State Plan for Scientific and Technical Research and Innovation grant number PID2019-111156GB-I00). Q.B. acknowledges the support from Shenzhen Nanshan District Pilotage Team Program (LHTD20170006).

The ultrafast monitoring of deoxyribonucleic acid (DNA) dynamic structural changes is an emerging and rapidly growing research topic in biotechnology. The existing optical spectroscopy used to identify different dynamical DNA structures lacks quick response while requiring large consumption of samples and bulky instrumental facilities. It is highly demanded to develop an ultrafast technique that monitors DNA structural changes with the external stimulus or cancer‐related disease scenarios. Here, we demonstrate a novel photonic integrated graphene‐optofluidic device to monitor DNA structural changes with the ultrafast response time. Our approach is featured with an effective and straightforward design of decoding the electronic structure change of graphene induced by its interactions with DNAs in different conformations using ultrafast nanosecond pulse laser and achieving refractive index sensitivity of ~3 × 10−5 RIU. This innovative technique for the first time allows us to perform ultrafast monitoring of the conformational changes of special DNA molecules structures, including G‐quadruplex formation by K+ ions and i‐motif formation by the low pH stimulus. The graphene‐optofluidic device as presented here provides a new class of label‐free, ultrafast, ultrasensitive, compact, and cost‐effective optical biosensors for medical and healthcare applications.

Twisted two-dimensional bilayer materials exhibit many exotic electronic phenomena. Manipulating the 'twist angle' between the two layers enables fine control of the electronic band structure, resulting in magic-angle flat-band superconductivity1,2, the formation of moire excitons3-8 and interlayer magnetism9. However, there are limited demonstrations of such concepts for photons. Here we show how analogous principles, combined with extreme anisotropy, enable control and manipulation of the photonic dispersion of phonon polaritons in van der Waals bilayers. We experimentally observe tunable topological transitions from open (hyperbolic) to closed (elliptical) dispersion contours in bilayers of α-phase molybdenum trioxide (α-MoO3), arising when the rotation between the layers is at a photonic magic twist angle. These transitions are induced by polariton hybridization and are controlled by a topological quantity. At the transitions the bilayer dispersion flattens, exhibiting low-loss tunable polariton canalization and diffractionless propagation with a resolution of less than λ0/40, where λ0 is the free-space wavelength. Our findings extend twistronics10 and moire physics to nanophotonics and polaritonics, with potential applications in nanoimaging, nanoscale light propagation, energy transfer and quantum physics.

The diffusion length of photogenerated carriers is a crucial parameter in semiconductors for optoelectronic applications. However, it is a challenging task to determine the diffusion length in layered nanoplatelets due to their anisotropic diffusion of photogenerated carriers and nanometer-thin thickness. Here, we demonstrate a novel method to determine the in-plane diffusion length of photogenerated carriers in layered nanoplatelets using local time-resolved photoluminescence. Also, the in-plane carrier diffusion length of 1.82 μm is obtained for an exfoliated (BA)

Broadband photodetectors are essential for various applications including spectroscopy, communication, imaging and sensing. Even though few materials show broadband photodetection, they are only capable of detecting light in limited spectral ranges from infrared (IR) to ultraviolet (UV) wavelengths. However, their functionalities at much shorter wavelengths such as ionizing radiation still present a challenge because of the limitations from the material properties. Here we demonstrate a room temperature, high performance broadband photodetector based on van der Waals (vdWs) magnetic material CrSiTe3 (CST) that can efficiently detect both soft X-rays and light in the near-infrared (NIR) and visible wavelength ranges. The CrSiTe3 based device can work with a high sensitivity (≈463 μC Gyair−1 cm−2) under synchrotron soft X-rays, which is more than twenty times more sensitive than the conventional α-Se device. In the NIR range, the device can achieve a maximum responsivity of around 206 A W−1 at 785 nm, an extremely high external quantum efficiency (EQE) of 32 600%, a detectivity of 1.25 × 1012 cm Hz1/2 W−1 and a relatively fast photoresponse time (rise time ∼43 ms under X-rays, 17 ms under a 785 nm laser). Together these results demonstrate a high-performance broadband photo and X-ray detector, offering intriguing opportunities for novel optoelectronic applications.

Back-contact electrodes for hybrid organic-inorganic perovskite solar cells (PSCs) eliminate the parasitic absorption losses caused by the transparent conductive electrodes that are inherent to conventional sandwich-architecture devices. However, the fabrication methods for these unconventional architectures rely heavily on expensive photolithography, which limits scalability. Herein, we present an alternative cost-effective microfabrication technique in which the conventional photolithography process is replaced by microsphere lithography in which a close-packed polystyrene microsphere monolayer acts as the patterning mask for the honeycomb-shaped electrodes. A comprehensive comparison between photolithography and microsphere lithography fabrication techniques was conducted. Using microsphere lithography, we achieve highly efficient devices having a stabilized power conversion efficiency (PCE) of 8.6%, twice the reported value using photolithography. Microsphere lithography also enabled the fabrication of the largest back-contact PSC to date, having an active area of 0.75 cm2 and a stabilized PCE of 2.44%., This work was financially supported by the Australian Government through the Australian Renewable Energy Agency (ARENA) the Australian Centre for Advanced Photovoltaics (ACAP) and the Australian Research Council (ARC, DE220100154). This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). The authors acknowledge use of facilities within the Monash Centre for Electron Microscopy (MCEM). The authors acknowledge use of facilities within the Flexible Electronics Laboratory (FEL) at the Commonwealth Scientific and Industrial Research Organisation (CSIRO), Clayton site. S.R.R. acknowledges the support from “la Caixa” Foundation (ID 100010434). Fellowship code LCF/BQ/PI20/11760024.

Polaritons, originating from the interactions between photons and material excitations, have attracted attention because of their strong field compression and deeply subdiffractional scales. For practical applications, it is crucial to manipulate polaritons efficiently, but doing so has remained challenging because of the relatively poor tunability of traditional polaritonic media. Fortunately, in the past decade, polaritons hosted by van der Waals (vdW) materials have allowed new opportunities to tackle this difficulty. We review the state of the art in the manipulation of polaritons at the extreme scale in vdW materials. Benefiting from the large and expanding catalogue of vdW materials and associated architectures and techniques, more accessible manipulation strategies are expected, not only offering control of light at the nanoscale with new degrees of freedom, but also offering insight into nanophotonics, meta-optics, topological physics and quantum materials., This work was supported by the Australian Research Council (ARC, CE170100039 and DE220100154) and Shenzhen Nanshan District Pilotage Team Program (LHTD20170006). P.L. acknowledges support from the National Natural Science Foundation of China (grant no. 62075070) and the Innovation Fund of WNLO. P.A.-G. acknowledges support from the European Research Council under starting grant no. 715496, 2DNANOPTICA and the Spanish Ministry of Science and Innovation (State Plan for Scientific and Technical Research and Innovation grant no. PID2019-111156GB-I00). J.D.C. acknowledges support from the Office of Naval Research, grant no. N000142212035.

Phonon-polaritons (PhPs) in layered crystals, including hexagonal boron nitride (hBN), have been investigated by combined scattering-type scanning near-field optical microscopy (s-SNOM) and Fourier transform infrared (FTIR) spectroscopy. Nevertheless, many of such s-SNOM-based FTIR spectra features remain unexplored, especially those originated from the impact of boundaries. Here we observe real-space PhP propagations in thin-layer hBN sheets either supported or suspended by s-SNOM imaging. Then with a high-power broadband IR laser source, we identify two major peaks and multiple auxiliary peaks in the near-field amplitude spectra, obtained using scattering-type near-field FTIR spectroscopy, from both supported and suspended hBN. The major PhP propagation interference peak moves toward the major in-plane phonon peak when the IR illumination moves away from the hBN edge. Specific differences between the auxiliary peaks in the near-field amplitude spectra from supported and suspended hBN sheets are investigated regarding different boundary conditions, associated with edges and substrate interfaces. The outcomes may be explored in heterostructures for advanced nanophotonic applications.

Metal halide perovskite nanostructures have emerged as low-dimensional semiconductors of great significance in many fields such as photovoltaics, photonics, and optoelectronics. Extensive efforts on the controlled synthesis of perovskite nanostructures have been made towards potential device applications. The engineering of their band structures holds great promise in the rational tuning of the electronic and optical properties of perovskite nanostructures, which is one of the keys to achieving efficient and multifunctional optoelectronic devices. In this article, we summarize recent advances in band structure engineering of perovskite nanostructures. A survey of bandgap engineering of nanostructured perovskites is firstly presented from the aspects of dimensionality tailoring, compositional substitution, phase segregation and transition, as well as strain and pressure stimuli. The strategies of electronic doping are then reviewed, including defect-induced self-doping, inorganic or organic molecules-based chemical doping, and modification by metal ions or nanostructures. Based on the bandgap engineering and electronic doping, discussions on engineering energy band alignments in perovskite nanostructures are provided for building high-performance perovskite p-n junctions and heterostructures. At last, we provide our perspectives in engineering band structures of perovskite nanostructures towards future low-energy optoelectronics technologies. Keywords: Band structure engineering, Perovskite nanostructures, Optoelectronic applications, Doping, Heterostructures

Lead halide perovskites are widely applied in not only photovoltaics but also on-chip light sources and photon detection. To promote the incorporation of perovskite into integrated devices, microscale color patterning flexibility is a very important step. Here, we demonstrate spatially resolved modulation of the fluorescence of nanoplatelets (NPs) by femtosecond direct laser writing (fs-DLW). As the perovskite NP for the fs-DLW pattern is specially designed with a gradual bromide-iodide composition along the depth, the replacement of iodide ions by bromide ions can be activated under a controlled laser pulse and fluorescence is thus modulated from red to green. The effect of processing depth and NP thickness on fluorescence modulation is systemically investigated. The as-grown thick NP (thickness ≈ 500 nm) mainly exhibits a 690 nm emission from the bottom iodine-rich phase. After halide substitution induced by fs-DLW, a new fluorescence peak appears in the wavelength range of 540-600 nm; the peak position and intensity are controlled by the DLW conditions. The fluorescent color is spatially modulated from red to green, enabling microscale-resolved multicolor emission. Compared with other currently available techniques, microscale color patterning via fs-DLW is a straightforward mask-free one-step operation, yielding high spatial resolution and enabling three-dimensional patterning by the multiple-photon method. We demonstrate that arbitrary patterns can be drawn on a wide range of perovskite NPs, implying the potential applications in microencryption, sensors, multicolor displays, lasers, and light-emitting devices.

Interfaces between metals and semiconducting materials can inevitably influence the magnetotransport properties, which are crucial for technological applications ranging from magnetic sensing to storage devices. By taking advantage of this, a metallic graphene foam is integrated with semiconducting copper-based metal sulfide nanocrystals, i.e., Cu2ZnSnS4 (copper-zinc-tin-sulfur) without direct chemical bonding and structural damage, which creates numerous nanoboundaries that can be basically used to tune the magnetotransport properties. Herein, the magnetoresistance of a graphene foam is enhanced from nearly 90 to 130% at room temperature and under the application of 5 T magnetic field strength due to the addition of Cu2ZnSnS4 nanocrystals in high densities. We believe that the enhancement of magnetoresistance in hybrid graphene foam/Cu2ZnSnS4 nanocrystals is due to the evolution of the mobility fluctuation mechanism, triggered by the formation of nanoboundaries. Incorporating Cu2ZnSnS4 nanocrystals into a graphene foam not only provides an effective way to further enhance the magnitude of magnetoresistance but also opens a suitable window to achieve efficient and highly functional magnetic sensors with a large, linear, and controllable response.

Effective assembly of perovskite quantum dots is a huge challenge to develop high-performance optoelectronic devices with large scale integration functionality. Here, we demonstrate a facile way to fabricate one-dimensional arrays consisting of aligned perovskite quantum dots and their remarkable application as high-performance photodetectors. All inorganic perovskite CsPbX3 (X = Cl, Br, I) quantum dots, synthesized by a hot injection method, are precisely assembled into the one-dimensional arrays using a liquid bridge induced assembly strategy. Systematic studies have shown that photodetectors based on such one-dimensional CsPbI3 quantum dot arrays have excellent performance in terms of high responsivity and high detection sensitivity. It is suggested that the availability of more oxygen-rich hole trapping states on the surface of the quantum dots play an important role in the photocurrent generation. This work presents a significant step forward towards scalable self-assembly of perovskite quantum dots into one-dimensional arrays, indicating an enormous potential in manufacturing of high-performance integrated optoelectronic sensor circuitry.

Phonon polaritons-light coupled to lattice vibrations-in polar van der Waals crystals offer unprecedented opportunities for controlling light at the nanoscale due to their anisotropic and ultralow-loss propagation. While their analog plasmon polaritons-light coupled to electron oscillations-have long been studied and exhibit interesting reflections at geometrical edges and electronic boundaries, whether phonon polaritons can be reflected by such barriers has been elusive. Here, the effective and tunable reflection of phonon polaritons at embedded interfaces formed in hydrogen-intercalated α-MoO3 flakes is elaborated upon. Without breaking geometrical continuity, such intercalation interfaces can reflect phonon polaritons with low losses, yielding the distinct phase changes of -0.8π and -0.3π associated with polariton propagation, high efficiency of 50%, and potential electrical tunability. The results point to a new approach to construct on-demand polariton reflectors, phase modulators, and retarders, which may be transplanted into building future polaritonic circuits using van der Waals crystals.

Surface phonon polaritons (SPhPs) in polar dielectrics offer new opportunities for infrared nanophotonics. However, bulk SPhPs inherently propagate isotropically with limited photon confinement, and how to collectively realize ultralarge confinement, in-plane hyperbolicity, and unidirectional propagation remains elusive. Here, we report an approach to solve the aforementioned issues of bulk SPhPs in one go by constructing a heterostructural interface between biaxial van der Waals material (e.g., α-MoO

Surface phonon polaritons (SPhPs) in polar dielectrics offer new opportunities for infrared nanophotonics due to sub-diffraction confinement with low optical losses. Though the polaritonic field confinement can be significantly improved by modifying the dielectric environment, it is challenging to break the fundamental limits in photon confinement and propagation behavior of SPhP modes. In particular, as SPhPs inherently propagate isotropically in these bulk polar dielectrics, how to collectively realize ultra-large field confinement, in-plane hyperbolicity and unidirectional propagation remains elusive. Here, we report an approach to solve the aforementioned issues of bulk polar dielectric's SPhPs at one go by constructing a heterostructural interface between biaxial van der Waals material (e.g., MoO3) and bulk polar dielectric (e.g., SiC, AlN, and GaN). Due to anisotropy-oriented mode couplings at the interface, the hybridized SPhPs with a large confinement factor (>100) show in-plane hyperbolicity that has been switched to the orthogonal direction as compared to that in natural MoO3. More interestingly, this proof of concept allows steerable, angle-dependent and unidirectional polariton excitation by suspending MoO3 on patterned SiC air cavities. Our finding exemplifies a generalizable framework to manipulate the flow of nano-light and engineer unusual polaritonic responses in many other hybrid systems consisting of van der Waals materials and bulk polar dielectrics.

We investigate the effects of direct deposition of different dielectric materials (AlOx, SiOx, SiNx) onto atomically thin TMDC WS2 on its optical response using atomic layer deposition (ALD), electron beam evaporation (EBE), plasma-enhanced chemical vapor deposition (PECVD), and magnetron sputtering. The photoluminescence measurements reveal quenching of the excitonic emission after all deposition processes, which is linked to the increased level of charge doping and associated rise of the trion emission and/or the localized (bound) exciton emission. Furthermore, Raman spectroscopy allows us to clearly correlate the observed changes in excitonic emission with the increased levels of lattice disorder and defects. In particular, we show that the different doping levels in a monolayer WS2 capped by a dielectric material are strongly related to the defects in the WS2 crystal introduced by all capping methods, except for ALD. The strong charge doping in the ALD-capped sample seems to be caused by other factors, such as deviations in the dielectric layer stoichiometry or chemical reactions on the monolayer surface, which makes ALD distinct from all other techniques. Overall, the EBE process results in the lowest level of doping and defect densities and in the largest spectral weight of the exciton emission in the PL. Sputtering is revealed as the most aggressive dielectric capping method for WS2, fully quenching its optical response. Our results demonstrate and quantify the effects of direct deposition of dielectric materials onto monolayer WS2, which can provide valuable guidance for the efforts to integrate monolayer TMDCs into functional optoelectronic devices.

As the performance of organic–inorganic halide perovskite solar cells approaches their practical limits, the use of back-contact architectures, which eliminate parasitic light absorption, provides an effective route toward higher device efficiencies. However, a poor understanding of the underlying device physics has limited further performance improvements. Here a mesoporous charge-transporting layer is introduced into quasi-interdigitated back-contact perovskite devices and the charge extraction behavior with an increased interfacial contact area is studied. The results show that the incorporation of a thin mesoporous titanium dioxide layer significantly shortens the charge-transfer lifetime and results in more efficient and balanced charge extraction dynamics. A high short-circuit current density of 21.3 mA cm–2 is achieved using a polycrystalline perovskite layer on a mesoscopic quasi-interdigitated back-contact electrode, a record for this type of device architecture., The authors are grateful for the financial support by the Australian Centre for Advanced Photovoltaics (ACAP), the Australian Renewable Energy Agency (ARENA), and the Australian Research Council (ARC) ARC Centre of Excellence in Exciton Science (ACEx: CE170100026). This work was performed in part at the Melbourne Centre for Nanofabrication (MCN) in the Victorian Node of the Australian National Fabrication Facility (ANFF). Q.O. acknowledges the support from the ARC Centre of Excellence in Future Low-Energy Electronics Technologies (FLEET).

Highly confined and low-loss polaritons are known to propagate isotropically over graphene and hexagonal boron nitride in the plane, leaving limited degrees of freedom in manipulating light at the nanoscale. The emerging family of biaxial van der Waals materials, such as α-MoO3 and V2O5, support exotic polariton propagation, as their auxiliary optical axis is in the plane. Here, exploiting this strong in-plane anisotropy, we report edge-tailored hyperbolic polaritons in patterned α-MoO3 nanocavities via real-space nanoimaging. We find that the angle between the edge orientation and the crystallographic direction significantly affects the optical response, and can serve as a key tuning parameter in tailoring the polaritonic patterns. By shaping α-MoO3 nanocavities with different geometries, we observe edge-oriented and steerable hyperbolic polaritons as well as forbidden zones where the polaritons detour. The lifetime and figure of merit of the hyperbolic polaritons can be regulated by the edge aspect ratio of nanocavity., The possibility to manipulate the propagation of polaritons is limited by the isotropy of 2D materials like graphene and hexagonal boron nitride. Here, the authors exploit the anisotropy of α-MoO3 and study edge tailored hyperbolic polariton manipulation in α-MoO3 nanocavities via real space nanoimaging.

Owing to its good air stability and high refractive index, two-dimensional (2D) noble metal dichalcogenide shows intriguing potential for versatile flat optics applications. However, light field manipulation at the atomic scale is conventionally considered unattainable because the small thickness and intrinsic losses of 2D materials completely suppress both resonances and phase accumulation effects. Here, we demonstrate that losses of structured atomically thick PtSe2 films integrated on top of a uniform substrate can be utilized to create the spots of critical coupling, enabling singular phase behaviors with a remarkable π phase jump. This finding enables the experimental demonstration of atomically thick binary meta-optics that allows an angle-robust and high unit thickness diffraction efficiency of 0.96%/nm in visible frequencies (given its thickness of merely 4.3 nm). Our results unlock the potential of a new class of 2D flat optics for light field manipulation at an atomic thickness.

Recently, two rich and exciting research fields, layered two-dimensional (2D) materials and metamaterials, have started overlapping. Metamaterials are artificial, engineered materials with broad metaphotonic prospects such as negative refraction, perfect lensing, subwavelength imaging, and cloaking. The possibility of achieving metaphotonic properties using metamaterials based on layered 2D materials has been extensively exploited. Because they are highly tunable and adjustable with the ease of micro- and nanofabrication, 2D materials exhibit diverse optical properties such as natural negative refraction, natural anisotropic behavior, and even hyperbolic dispersion. A combination of 2D materials with conventional metamaterials promises a variety of prospective applications. In this review, we illustrate how the concept of metamaterials and their associated metaphotonic capabilities are naturally born in 2D materials. The multifunctionality of 2D materials may enable the manufacture of novel optical devices that work in a broad frequency range, from visible to terahertz, with particularly low loss, high speed, gated tunability, and miniaturized sizes. This new area of research links the fields of photonics, optoelectronics, and plasmonics with that of metamaterials and may provide insights to future innovations for 2D-material-inspired metaphotonic devices.

Twisted two-dimensional bilayer materials exhibit many exotic electronic phenomena. Manipulating the 'twist angle' between the two layers enables fine control of the electronic band structure, resulting in magic-angle flat-band superconductivity

We demonstrate a nonlinear microscopy-based non-invasive technique for characterisation of PbI2 nanosheets. By using the polarisation, wavelength and thickness dependences of the harmonic emissions we can precisely determine their thickness, strain and crystalline orientation. 160.4236 160.4330 Lead iodide (PbI2) is a layered material with unique optical and electrical properties, including direct bandgap in the bulk and a layered crystalline structure, consisting of close-packed Pb atoms sandwiched between two layers of Iodine atoms [1]. Compared to the widely-studied TMDCs, 2D PbI2 is a halide semiconductors with a larger visible bandgap (Eg ∼ 2.4 eV) which endows its distinct optical properties. Despite being extensively studied in its bulk form and being used as a precursor for perovskite materials, the recently developed PbI2 nanosheets have shown a great promise for high-performance optoelectronic devices, such as flexible photodetectors [2] and nanolasers [3]. However, such novel applications of PbI2 nanosheets require careful characterization of their crystalline structure, thickness, strain and nonlinearity. Here we deploy a nonlinear optical microscopy as a non-invasive technique to investigate PbI2 nanosheets and fully determine their crystalline properties. To achieve such complete characterisation we analyse the polarisation, thickness and strain dependence of the second harmonic generation (SHG) and third harmonic generation (THG) from solution-grown PbI2 nanosheets. Our experimental results are also compared with numerical simulations of the conversion efficiency of the nonlinear harmonic generation with different thickness and wavelength, revealing the influence of the layered crystalline structure. Our measurements allow, for the first time, to precisely determine their thickness, crystalline orientation and strain with a non-invasive optical technique.

Lead iodide (PbI2) is a van der Waals layered semiconductor with a direct bandgap in its bulk form and a hexagonal layered crystalline structure. The recently developed PbI2 nanosheets have shown great promise for high-performance optoelectronic devices, including nanolasers and photodetectors. However, despite being widely used as a precursor for perovskite materials, the optical properties of PbI2 nanomaterials remain largely unexplored. Here, we determine the nonlinear optical properties of PbI2 nanosheets by utilising nonlinear microscopy as a non-invasive optical technique. We demonstrate the nonlinearity enhancement dependent on excitonic resonances, crystalline orientation, thickness, and influence of the substrate. Our results allow for estimating the second- and third-order nonlinear susceptibilities of the nanosheets, opening new opportunities for the use of PbI2 nanosheets as nonlinear and quantum light sources.

Flexible white organic light-emitting diodes (FWOLEDs) have garnered progressively increasing attention in recent years due to their unique capacity to be integrated with bendable or curvilinear optoelectronics. However, highly efficient FWOLEDs are still challenging to make for application in general solid-state lighting because of its inherent optical confinement and imperfect device architectures. Here, in-depth investigation is performed on designing an alternative route for wide-viewing angle optical power enhancement of FWOLEDs based on a simple synergetic electrode architecture. The key feature is the electrode architecture that takes a full advantage of the synergetic collaboration between the tunable density of built-in random inverted micro-cone arrays (IMCAs) and embedded silver nanowires controlling both optical outcoupling enhancement and electrical shorts suppression. The maximum power efficiency (PE) and external quantum efficiency (EQE) reach 58.7 lm W−1 and 28.9% (@ 1000 cd m−2), respectively, which are 70% and 80% enhancement compared with a similarly structured device fabricated on traditional indium-tin-oxide (ITO) glass. Furthermore, the proposed method is directly compatible with inexpensive and scalable roll-to-roll manufacture technology, offering additional appealing features for next-generation higher-valued-added functional solid-state lighting apparatus.

Lead halide perovskites are widely applied in not only photovoltaics, but also on-chip light source, nanolaser, and photon detection. In order to promote the incorporation of perovskite into integrated devices, microscale color patterning flexibility is a very important step. Femtosecond (fs) laser fabrication has shown significant advantages of high spatial resolution, low surround damage, and high processing efficiency over the other laser fabrication. Compared to the state-of-art techniques, the straightforward fs-direct laser writing (fs-DLW) also has advantages of mask-free, simple one step, and contactless. Here, a specially designed formamidinium lead mixed-halide nanoplatelet (FAPb(BrxI1-x)3 NP) with gradient bandgap is fabricated by chemical vapor deposition method. Then, spatially resolved modulation of the fluorescence by fs-DLW is demonstrated on the as-grown NP. The fluorescence color is modulated from red to green under a controlled laser pulse, due to the replacement of iodide ions by bromide ions. Specifically, the as-grown NP (thickness≈800 nm) is with a gradual bromide-iodide composition along the depth, mainly exhibits an emission of 710-nm from the bottom iodine rich phase. After halide substitution induced by fs-DLW, new fluorescence peaks appear in the wavelength range of 540 to 700 nm, which is controlled by the fs-DLW conditions. The fluorescent color is spatially modulated from red to green, enabling microscale resolved multicolor emission, implying the potential applications in micro-encryption, sensors, multicolor displays, lasers, and light-emitting devices.

Halide perovskites are an extremely promising material platform for solar cells and photonic devices. The role of surface carrier recombination-well known to detrimentally affect the performance of devices-is still not well understood for thin samples where the thickness is comparable to or less than the carrier diffusion length. Here, using time-resolved microspectroscopy along with modeling, we investigate charge-carrier recombination dynamics in halide perovskite CH3NH3PbI3 nanoplatelets with thicknesses from ∼20 to 200 nm, ranging from much lesser than to comparable to the carrier diffusion length. We show that surface recombination plays a stronger role in thin perovskite nanoplatelets, significantly decreasing photoluminescence (PL) efficiency, PL decay lifetime, and photostability. Interestingly, we find that both thick and thin nanoplatelets exhibit a similar increase in PL efficiency with increasing excitation fluence, well described by our excitation saturation model. We also find that the excited carrier distribution along the depth impacts the surface recombination. Using the diffusion-surface recombination model, we determine the surface recombination velocity. This work provides a comprehensive understanding of the role of surface recombination and charge-carrier dynamics in thin perovskite platelets and reveals valuable insights useful for applications in photovoltaics and photonics.

Hybrid organic–inorganic metal halide perovskites have been considered as promising materials for boosting the performance of photovoltaics and optoelectronics. Reduced-dimensional condiments and tunable properties render two-dimensional (2D) perovskite as novel building blocks for constructing micro-/nanoscale devices in high-performance optoelectronic applications. However, the stability is still one major obstacle for long-term practical use. Herein, we provide microscale insights into the degradation kinetics of 2D CH3NH3PbI3 (MAPbI3) perovskite and CH3NH3PbI3/graphene heterostructures. It is found that the degradation is mainly caused by cation evaporation, which consequently affects the microstructure, light–matter interaction, and the photoluminescence quantum yield efficiency of the 2D perovskite. Interestingly, the encapsulation of perovskite by monolayer graphene can largely preserve the structure of the perovskite nanosheet and maintain reasonable optical properties upon exposure to high temper...

Two novel D–A bipolar blue phosphorescent host materials based on phenothiazine-5,5-dioxide: 3-(9H-carbazol-9-yl)-10-ethyl-10H-phenothiazine-5,5-dioxide (CEPDO) and 10-butyl-3-(9H-carbazol-9-yl)-10H-phenothiazine-5,5-dioxide (CBPDO) were synthesized and characterized. The photophysical, electrochemical and thermal properties were systematically investigated. CEPDO and CBPDO not only have a high triplet energy but also show a bipolar behavior. Moreover, their fluorescence emission peaks are in the blue fluorescence region at 408 nm and the fluorescence quantum efficiency (Φ) of CEPDO and CBPDO were 62.5% and 59.7%, respectively. Both CEPDO and CBPDO showed very high thermal stability with decomposition temperatures (Td) of 409 and 396 °C as well as suitable HOMO and LUMO energy levels. This preferable performance suggests that CEPDO and CBPDO are alternative bipolar host materials for the PhOLEDs.

An intermediate connector of Cs 2 CO 3 /Al/MoO 3 used in tandem organic light-emitting diodes (OLEDs) was investigated in this work. Here, an ultrathin Cs 2 CO 3 /Al was used as an electron-injection layer (EIL) from MoO 3 to the adjacent electron transporting layer (ETL). To verify the function of this intermediate connector, the device performances were evaluated through current density-voltage-luminance characteristics, current density-efficiency curves, and EL spectra. Additionally, the effect of photon energy on carriers behavior in the Cs 2 CO 3 /Al/MoO 3 connector is also estimated. The electrical properties and EL spectra of tandem OLEDs show that the Cs 2 CO 3 /Al/MoO 3 can function well for charge generation and transport, and the current density-voltage curves of Cs 2 CO 3 /Al/MoO 3 based special multilayer device shows the photovoltaic effect as a photovoltaic cell.

Enlightened by holography metasurfaces for controlling the propagation of light, speckle image holography nanostructures were introduced into both fluorescent and phosphorescent OLEDs in order to achieve high efficiency and engineered emission profile. The resulting current efficiency and external quantum efficiency of the nanostructured fluorescent green emission OLEDs were enhanced by up to 99% and 104%, respectively. This enhancement feature is comparable to that of phosphorescent red- and blue-emission OLEDs. Furthermore, the enhancement ratios were retained approximately as a constant when the viewing angle varied from −60° to 60° for all three color devices. Experimental and theoretical results directly prove that the light steering is independent of the emission materials, the wavelength, radiation angle, and the polarization of the emitted light. We anticipate this paradigm may offer new opportunities for high-end optoelectronic designs and applications.

Phonon polaritons (PhPs) have attracted significant interest in the nano-optics communities because of their nanoscale confinement and long lifetimes. Although PhP modification by changing the local dielectric environment has been reported, controlled manipulation of PhPs by direct modification of the polaritonic material itself has remained elusive. Here, chemical switching of PhPs in α-MoO3 is achieved by engineering the α-MoO3 crystal through hydrogen intercalation. The intercalation process is non-volatile and recoverable, allowing reversible switching of PhPs while maintaining the long lifetimes. Precise control of the intercalation parameters enables analysis of the intermediate states, in which the needle-like hydrogenated nanostructures functioning as in-plane antennas effectively reflect and launch PhPs and form well-aligned cavities. We further achieve spatially controlled switching of PhPs in selective regions, leading to in-plane heterostructures with various geometries. The intercalation strategy introduced here opens a relatively non-destructive avenue connecting infrared nanophotonics, reconfigurable flat metasurfaces and van der Waals crystals., Phonon polaritons hold promises for nanophotonic applications but external control of phonon polaritons remains challenging. Here, the authors achieve reversible and non-volatile switching of phonon polariton by modifying crystal structure and lattice vibrations via hydrogenation.

Lead iodide (PbI 2 ) is a layered material with unique optical and electrical properties, including direct bandgap in the bulk and a layered crystalline structure, consisting of close-packed Pb atoms sandwiched between two layers of Iodine atoms [1]. Compared to the widely-studied TMDCs, 2D PbI2 is a new type of halide semiconductors with a relatively larger visible bandgap (E g ∼ 2.4 eV) which endows its distinct optical properties. Despite being studied for decades and being used as a precursor for perovskite materials, the recently developed PbI 2 nanosheets have shown a great promise for high-performance optoelectronic devices, such as flexible photodetectors [2] and nanolasers [3]. Such 2D nanosheets also show a great potential for low-dimensional nonlinear optical devices, However, their nonlinear properties are still unexplored, while novel applications of PbI2 nanosheets require careful characterization of their crystalline structure, thickness and nonlinearity. Here we demonstrate the nonlinear microscopy of PbI2 nanosheets using the polarization and thickness dependence of the second harmonic generation (SHG) and third harmonic generation (THG) from solution-grown nanosheets. Our measurements allow to precisely determine their thickness and crystalline orientation with a non-invasive optical technique.

The insufficient electron injection constitutes the major obstacle to achieving high-performance inverted organic light-emitting diodes (OLEDs). Here, a facile electron-injection architecture featuring a silver nanoparticle (AgNPs) interlayer-modified sol-gel-derived transparent zinc oxide (ZnO) ultrathin film is proposed and demonstrated. The optimized external quantum efficiencies of the developed inverted fluorescent and phosphorescent OLEDs capitalized on our proposed electron-injection structure reached 4.0 and 21.2% at a current density of 20 mA cm-2 and increased by a factor of 1.90 and 2.86 relative to a reference device without the AgNP interlayer, while simultaneously reducing the operational voltage and substantially ameliorating the device efficiency. Detailed analyses reveal that the local surface plasmon resonance emanated from AgNPs plays three meaningful roles simultaneously: suppressing the surface plasmon polariton mode loss, aiding in energy-level alignments, and inducing and reinforcing the local exciton-plasmon coupling electric field. Among these interesting and multifunctional roles, the enhanced local exciton-plasmon coupling electric field dominates the electron injection enhancement and substantial increases the device efficiency. Additionally, the light-scattering effect also helps in recovering the trapped light energy flux and thus improves the device efficiency. The proposed approach and findings provide an alternative path to fabricate high-performance inverted OLEDs and other related organic electronic or optoelectronic devices.

This thesis focuses on advanced 2D perovskite nanomaterials by engineering band structures for optoelectronic device applications, including the realization of 2D hybrid perovskite p-n junctions with strong charge depletion, surface functionalization of 2D perovskites, and mixed-halide perovskite nanoplatelets with a gradient bandgap. We demonstrate that the engineered band structures of 2D perovskites benefit the performance improvement of the perovskite nano-devices, such as photodiodes, photodetectors and X-ray detectors.