Large-Area Near-Infrared Emission Enhancement on Single Upconversion Nanoparticles by Metal Nanohole Array

Single lanthanide (Ln) ion doped upconversion nanoparticles (UCNPs) exhibit great potential for biomolecule sensing and counting. Plasmonic structures can improve the emission efficiency of single UCNPs by modulating the energy transferring process. Yet, achieving robust and large-area single UCNP emission modulation remains a challenge, which obstructs investigation and application of single UCNPs. Here, we present a strategy using metal nanohole arrays (NHAs) to achieve energy-transfer modulation on single UCNPs simultaneously within large-area plasmonic structures. By coupling surface plasmon polaritons (SPPs) with higher-intermediate state (1D2 → 3F3, 1D2 → 3H4) transitions, we achieved a remarkable up to 10-fold enhancement in 800 nm emission, surpassing the conventional approach of coupling SPPs with an intermediate ground state (3H4 → 3H6). We numerically simulate the electrical field distribution and reveal that luminescent enhancement is robust and insensitive to the exact location of particles. It is anticipated that the strategy provides a platform for widely exploring applications in single-particle quantitative biosensing.

Giant Photoluminescence Enhancement of Monolayer WSe2 Using a Plasmonic Nanocavity with On-Demand Resonance

Monolayer transition metal dichalcogenides (TMDs) are considered promising building blocks for next-generation photonic and optoelectronic devices, owing to their fascinating optical properties. However, their inherent weak light absorption and low quantum yield severely hinder their practical applications. Here, we report up to 18000-fold photoluminescence (PL) enhancement in a monolayer WSe2-coupled plasmonic nanocavity. A spectroscopy-assisted nanomanipulation technique enables the assembly of a nanocavity with customizable resonances to simultaneously enhance the excitation and emission processes. In particular, precise control over the magnetic cavity mode facilitates spectral and spatial overlap with the exciton, resulting in plasmon-exciton intermediate coupling that approaches the maximum emission rate in the hybrid system. Meanwhile, the cavity mode exhibits high radiation directivity, which overwhelmingly directs surface-normal PL emission and leads to a 17-fold increase in the collection efficiency. Our approach opens up a new avenue to enhance the PL intensity of monolayer TMDs, facilitating their implementation in highly efficient optoelectronic devices.

Bright Nonblinking Photoluminescence with Blinking Lifetime from a Nanocavity-Coupled Quantum Dot

Colloidal quantum dots (QDs) are excellent luminescent nanomaterials for many optoelectronic applications. However, photoluminescence blinking has limited their practical use. Coupling QDs to plasmonic nanostructures shows potential in suppressing blinking. However, the underlying mechanism remains unclear and debated, hampering the development of bright nonblinking dots. Here, by deterministically coupling a QD to a plasmonic nanocavity, we clarify the mechanism and demonstrate unprecedented single-QD brightness. In particular, we report for the first time that a blinking QD could obtain nonblinking photoluminescence with a blinking lifetime through coupling to the nanocavity. We show that the plasmon-enhanced radiative decay outcompetes the nonradiative Auger process, enabling similar quantum yields for charged and neutral excitons in the same dot. Meanwhile, we demonstrate a record photon detection rate of 17 MHz from a colloidal QD, indicating an experimental photon generation rate of more than 500 MHz. These findings pave the way for ultrabright nonblinking QDs, benefiting diverse QD-based applications.

The Nanoplasmonic Purcell Effect in Ultrafast and High-Light-Yield Perovskite Scintillators

The development of X-ray scintillators with ultrahigh light yields and ultrafast response times is a long sought-after goal. In this work, a fundamental mechanism that pushes the frontiers of ultrafast X-ray scintillator performance is theoretically predicted and experimentally demonstrated: the use of nanoscale-confined surface plasmon polariton modes to tailor the scintillator response time via the Purcell effect. By incorporating nanoplasmonic materials in scintillator devices, this work predicts over tenfold enhancement in decay rate and 38% reduction in time resolution even with only a simple planar design. The nanoplasmonic Purcell effect is experimentally demonstrated using perovskite scintillators, enhancing the light yield by over 120% to 88 ± 11 ph/keV, and the decay rate by over 60% to 2.0 ± 0.2 ns for the average decay time, and 0.7 ± 0.1 ns for the ultrafast decay component, in good agreement with the predictions of our theoretical framework. Proof-of-concept X-ray imaging experiments are performed using nanoplasmonic scintillators, demonstrating 182% enhancement in the modulation transfer function at four line pairs per millimeter spatial frequency. This work highlights the enormous potential of nanoplasmonics in optimizing ultrafast scintillator devices for applications including time-of-flight X-ray imaging and photon-counting computed tomography.

Ultrafast and Bright Quantum Emitters from the Cavity-Coupled Single Perovskite Nanocrystals

Perovskite nanocrystals (NCs) have attracted increasing interest in the realization of single-photon emitters owing to their ease of chemical synthesis, wide spectral tunability, fast recombination rate constant, scalability, and high quantum yield. However, the integration of a single perovskite NC into a photonic structure has yet to be accomplished. In this work, the integration of a highly stable individual zwitterionic ligand-based CsPbBr3 perovskite NC with a circular Bragg grating (CBG) is successfully demonstrated. The far-field radiation pattern of the NC inside the CBG exhibits high directionality toward a low azimuthal angle, which is consistent with the simulation results. A 5.4-fold enhancement in brightness is observed due to an increase in collection efficiency. Moreover, a 1.95-fold increase in the recombination rate constant is achieved. This study offers ultrafast (<100 ps) single-photon emission and an improved brightness of perovskite NCs, which are critical factors for practical quantum optical applications.

Strong Coupling Dynamics of a Quantum Emitter near a Topological Insulator Nanoparticle

We study the spontaneous emission dynamics of a quantum emitter near a topological insulator Bi2Se3 spherical nanoparticle. Using the electromagnetic Green's tensor method, we find exceptional Purcell factors of the quantum emitter up to 1010 at distances between the emitter and the nanoparticle as large as half the nanoparticle's radius in the terahertz regime. We study the spontaneous emission evolution of a quantum emitter for various transition frequencies in the terahertz and various vacuum decay rates. For short vacuum decay times, we observe non-Markovian spontaneous emission dynamics, which correspond perfectly to values of well-established measures of non-Markovianity and possibly indicate considerable dynamical quantum speedup. The dynamics turn progressively Markovian as the vacuum decay times increase, while in this regime, the non-Markovianity measures are nullified, and the quantum speedup vanishes. For the shortest vacuum decay times, we find that the population remains trapped in the emitter, which indicates that a hybrid bound state between the quantum emitter and the continuum of electromagnetic modes as affected by the nanoparticle has been formed. This work demonstrates that a Bi2Se3 spherical nanoparticle can be a nanoscale platform for strong light-matter coupling.

Controlling Valley-Specific Light Emission from Monolayer MoS2 with Achiral Dielectric Metasurfaces

Excitons in two-dimensional transition metal dichalcogenides have a valley degree of freedom that can be optically manipulated for quantum information processing. Here, we integrate MoS₂ monolayers with achiral silicon disk array metasurfaces to enhance and control valley-specific absorption and emission. Through the coupling to the metasurface electric and magnetic Mie modes, the intensity and lifetime of the emission of neutral excitons, trions, and defect bound excitons can be enhanced and shortened, respectively, while the spectral shape can be modified. Additionally, the degree of polarization (DOP) of exciton and trion emission from the valley can be symmetrically enhanced at 100 K. The DOP increase is attributed to both the metasurface-enhanced chiral absorption of light and the metasurface-enhanced exciton emission from the Purcell effect. Combining Si-compatible photonic design with large-scale 2D materials integration, our work makes an important step toward on-chip valleytronic applications approaching room-temperature operation.

Single-Photon Source in a Topological Cavity

The introduction of topological physics into the field of photonics has led to the development of photonic devices endowed with robustness against structural disorder. While a range of platforms have been successfully implemented demonstrating topological protection of light in the classical domain, the implementation of quantum light sources in photonic devices harnessing topologically nontrivial resonances is largely unexplored. Here, we demonstrate a single photon source based on a single semiconductor quantum dot coupled to a topologically nontrivial Su-Schrieffer-Heeger (SSH) cavity mode. We provide an in-depth study of Purcell enhancement for this topological quantum light source and demonstrate its emission of nonclassical light on demand. Our approach is a promising step toward the application of topological cavities in quantum photonics.

Enhanced Spontaneous Emission of CsPbI3 Perovskite Nanocrystals Using a Hyperbolic Metamaterial Modified by Dielectric Nanoantenna

In this work, we demonstrate, theoretically and experimentally, a hybrid dielectric-plasmonic multifunctional structure able to provide full control of the emission properties of CsPbI₃ perovskite nanocrystals (PNCs). The device consists of a hyperbolic metamaterial (HMM) composed of alternating thin metal (Ag) and dielectric (LiF) layers, covered by TiO₂ spherical MIE nanoresonators (i.e., the nanoantenna). An optimum HMM leads to a certain Purcell effect, i.e., an increase in the exciton radiative rate, but the emission intensity is reduced due to the presence of metal in the HMM. The incorporation of TiO₂ nanoresonators deposited on the top of the HMM is able to counteract such an undesirable intensity reduction by the coupling between the exciton and the MIE modes of the dielectric nanoantenna. More importantly, MIE nanoresonators result in a preferential light emission towards the normal direction to the HMM plane, increasing the collected signal by more than one order of magnitude together with a further increase in the Purcell factor. These results will be useful in quantum information applications involving single emitters based on PNCs together with a high exciton emission rate and intensity.

Resonance Photoluminescence Enhancement of Monolayer MoS2 via a Plasmonic Nanowire Dimer Optical Antenna

Two-dimensional transition-metal dichalcogenides (TMDs) such as monolayer MoS2 exhibit remarkable optical properties. However, the intrinsic absorption and emission rates of MoS2 are very low, thus severely hindering its application in electronics and photonics. Combining MoS2 with a plasmonic optical antenna is an alternative solution to enhance the emission rates of the 2D semiconductor, and this can drastically increase the photoresponsivity of the corresponding photodetector. Herein, we have constructed a plasmonic gap cavity of a nanowire dimer (NWD) system as an optical antenna to brighten the emission of MoS2 off the hot spot. Different from the conventional enhancement concept which occurred in the plasmonic hot spot, the light emission off the nanogap hot spot was thoroughly investigated. We demonstrate that this new plasmonic optical nanostructure leads to a strong enhancement due to the Purcell effect. The NWD optical antenna can trap light to the near field through a high-efficiency plasmonic gap mode (PGM); then the PL emission was enhanced drastically up to 14.5-fold due to the resonance of the plasmonic gap mode (PGM) in the NWD with the excitonic band of monolayer MoS2. Theoretical simulations reveal that this NWD can alter the efficiency of convergence and excitation, which was consistent with our experimental results. This study can provide a pathway toward enhancing and controlling PGM-enhanced light emission of TMD materials beyond the plasmonic hot spot.

Engineering Plasmonic Environments for 2D Materials and 2D-Based Photodetectors

Two-dimensional layered materials are considered ideal platforms to study novel small-scale optoelectronic devices due to their unique electronic structures and fantastic physical properties. However, it is urgent to further improve the light–matter interaction in these materials because their light absorption efficiency is limited by the atomically thin thickness. One of the promising approaches is to engineer the plasmonic environment around 2D materials for modulating light–matter interaction in 2D materials. This method greatly benefits from the advances in the development of nanofabrication and out-plane van der Waals interaction of 2D materials. In this paper, we review a series of recent works on 2D materials integrated with plasmonic environments, including the plasmonic-enhanced photoluminescence quantum yield, strong coupling between plasmons and excitons, nonlinear optics in plasmonic nanocavities, manipulation of chiral optical signals in hybrid nanostructures, and the improvement of the performance of optoelectronic devices based on composite systems.

Fabry-Perot Mode-Limited High-Purcell-Enhanced Spontaneous Emission from In Situ Laser-Induced CsPbBr3 Quantum Dots in CsPb2Br5 Microcavities

The patterned metal halide perovskites exhibit novel photophysical properties and high performance in photonic applications. Here, we show that a UV continuous wave laser can induce in situ crystallization of individual and patterned CsPbBr₃ quantum dots (QDs) inside the CsPb₂Br₅ microplatelets. The microplatelet acts as a natural Fabry-Perot cavity and causes the high-Purcell-effect-enhanced (by 287 times) cavity mode spontaneous emission of the embedded CsPbBr₃ QDs. The luminescence exhibits a superlinear emission intensity-excitation intensity relation I(p) ∝ p^2.83, and the exponent is much bigger than that of the free-space exciton spontaneous emission, suggesting arising of stimulated emission at higher photon concentrations. These laser-driven crystallized and patterned cavity mode luminescent perovskite QDs in a waterproof wider-bandgap perovskite microcavity act as an ideal platform for studying the cavity quantum electrodynamics phenomena and for applications in information storage and encryption, anticounterfeiting, and low-threshold lasers.

Hybrid cube-in-cup nanoantenna: towards ordered photonics

The most significant goal of nanophotonics is the development of high-speed quantum emitting devices operating at ambient temperature. In this regard, plasmonic nanoparticles-on-mirror are potential candidates for designing high-speed photon sources. We introduce a novel hybrid nanoantenna (HNA) with CdSe/CdS colloidal quantum dots (QDs) based on a silver nanocube in a metal cup that presents a nanoparticle-in-cavity coupled with an emitters system. We use focused ion beam nanolithography to fabricate an ordered array of cups, which were then filled with colloidal nanoparticles using the most simple drop-casting and spin coating methods. The spectral and time-resolved studies of the samples with one or more nanocubes in the cup reveal a significant change in the radiation characteristics of QDs inside the nanoantenna. The Purcell effect causes an increase in the fluorescence decay rate (≥30) and an increase in the fluorescence intensity (≥3) of emitters in the HNA. Using the finite element method simulations, we have discovered that the proximity of the cups wall affects the oscillation modes of the gap plasmon, which, in turn, leads to changes in the electric field enhancement inside the nanoantenna gap. Additionally, substantial variations in the behavior of the gap plasmons at different polarizations of the exciting radiation have been revealed. The proposed nanoantenna can be useful in the development of plasmonic sensors, display pixels, and single-photon sources.

Purcell-like Enhancement of Electron-Phonon Interactions in Long-Period Superlattices: Linear-Temperature Resistivity and Cooling Power

In the Purcell effect, the efficiency of optical emitters is enhanced by reducing the optical mode volume. Here we predict an analogous enhancement for electron-phonon (el-ph) scattering, achieved by compressing the electronic Wannier orbitals. Reshaped Wannier orbitals are a prominent attribute of graphene moiré superlattices, where the orbital size is tunable by the twist angle. A reduction in the orbital size leads to an enhancement in the el-ph interaction strength, yielding the values considerably larger than those in pristine monolayer graphene. The enhanced coupling boosts the el-ph scattering rates, pushing them above the values expected for the flat-band-enhanced density of electronic states. The enhanced phonon emission and scattering rates are manifested through the observables such as the electron-lattice cooling and the linear-temperature (T) resistivity, both of which are directly tunable by the moiré twist angle.

Purcell-Enhanced Single Photon Source Based on a Deterministically Placed WSe2 Monolayer Quantum Dot in a Circular Bragg Grating Cavity

We demonstrate a deterministic Purcell-enhanced single photon source realized by integrating an atomically thin WSe2 layer with a circular Bragg grating cavity. The cavity significantly enhances the photoluminescence from the atomically thin layer and supports single photon generation with g(2)(0) < 0.25. We observe a consistent increase of the spontaneous emission rate for WSe2 emitters located in the center of the Bragg grating cavity. These WSe2 emitters are self-aligned and deterministically coupled to such a broadband cavity, configuring a new generation of deterministic single photon sources, characterized by their simple and low-cost production and intrinsic scalability.

Hemispherical Cesium Lead Bromide Perovskite Single-Mode Microlasers with High-Quality Factors and Strong Purcell Enhancement

We realized a single-mode laser with an ultra-high quality factor in individual cesium lead bromide (CsPbBr₃) perovskite micro-hemispheres fabricated by chemical vapor deposition. A series of lasing property analysis based on cavity size was reported under this material system. Due to good optical confinement capability of the whispering gallery resonant cavity and high optical gain of CsPbBr₃ perovskite micro-hemispheres, single-mode lasing behavior was achieved with an ultra-high quality factor as large as 11,460 at room temperature. To study in detail the physical effects between lasing threshold and cavity, a set of cavity size dependence photoluminescence analyses were performed. We found that the lasing threshold increases while the cavity size decreases. Time-resolved PL analysis was conducted to confirm the relation between cavity size and lasing threshold. The larger cavity stands for longer PL lifetime and indicates easier-to-achieve carrier population inversion. Strong Purcell enhancement could be further investigated by the spontaneous emission coupling factor β and internal quantum efficiency as a function of cavity size. A high β-factor of 0.37 could be obtained from a 2.2 μm diameter hemisphere microcavity and a high Purcell factor of 14 in a 1.9 μm diameter hemisphere microcavity showing strong Purcell enhancement effect in our system.

Unprecedented Fluorophore Photostability Enabled by Low-Loss Organic Hyperbolic Materials

The dynamics of photons in fluorescent molecules plays a key role in fluorescence imaging, optical sensing, organic photovoltaics, and displays. Photobleaching is an irreversible photodegradation process of fluorophores, representing a fundamental limitation in relevant optical applications. Chemical reagents are used to suppress the photobleaching rate but with exceptionally high specificity for each type of fluorophore. Here, using organic hyperbolic materials (OHMs), an optical platform to achieve unprecedented fluorophore photostability without any chemical specificity is demonstrated. A more than 500-fold lengthening of the photobleaching lifetime and a 230-fold increase in the total emitted photon counts are observed simultaneously. These exceptional improvements solely come from the low-loss hyperbolic dispersion of OHM films and the large resultant Purcell effect in the visible spectral range. The demonstrated OHM platform may open up a new paradigm in nanophotonics and organic plasmonics for super-resolution imaging and the engineering of light-matter interactions at the nanoscale.

Frustules of Amphora sp. as a photonic crystal with photoluminescent CdS nanoparticles

Diatom frustules have species-specific patterns of pores, striae, pores, and nanopores, periodically arranged on its silica surface, as sets of cavities that modify the vacuum electromagnetic density of states. Therefore, frustules may be considered photonic crystals; the interaction with light-emitting sources inside the pores may potentially result in enhancement or inhibition of their spontaneous radiative emission rate and frequencies. In this work, we studied the photoluminescence of cadmium sulfide nanoparticles (CdS-NP) deposited inside frustule cavities that conveyed evidence of cavity-NP interaction. We synthesized CdS-NP, a semiconductor compound achieving quantum dots small enough to impose confinement effects to the electronic states. CdS-NP and their clusters were physiosorbed onto the surface, striae, and predominantly inside the pores of the cleansed frustules of Amphora sp. A broad peak with a maximum intensity at 437 nm (2.84 eV) was recorded after excitation with a 375 nm light source, showing a large blue shift and signal amplification of the CdS-NP photoluminescence when these were embedded inside the pores of the silica frustule. Using the Brus equation, we estimated a NP size of 4.1 ± 0.2 nm for the CdS-NP snuggly packed inside the smaller pores of the frustule, of 10 ± 0.7 nm in average diameter, The emission Purcell enhancement factor for an emitting atom in a cavity was calculated. The obtained Q factor (c. 5) was smaller than typical Q factors for designed semiconductor cavities of similar dimensions, an expected situation if it is assumed that the pores are open-ended cavities.

Metamaterial Enhancement of Metal-Halide Perovskite Luminescence

Metal-halide perovskites are rapidly emerging as solution-processable optical materials for light-emitting applications. Here, we adopt a plasmonic metamaterial approach to enhance photoluminescence emission and extraction of methylammonium lead iodide (MAPbI₃) thin films based on the Purcell effect. We show that hybridization of the active metal-halide film with resonant nanoscale sized slits carved into a gold film can yield more than 1 order of magnitude enhancement of luminescence intensity and nearly 3-fold reduction of luminescence lifetime corresponding to a Purcell enhancement factor of more than 300. These results show the effectiveness of resonant nanostructures in controlling metal-halide perovskite light emission properties over a tunable spectral range, a viable approach toward highly efficient perovskite light-emitting devices and single-photon emitters.

Metamaterial-Assisted Photobleaching Microscopy with Nanometer Scale Axial Resolution

The past two decades have witnessed a dramatic progress in the development of novel super-resolution fluorescence microscopy technologies. Here, we report a new fluorescence imaging method, called metamaterial-assisted photobleaching microscopy (MAPM), which possesses a nanometer-scale axial resolution and is suitable for broadband operation across the entire visible spectrum. The photobleaching kinetics of fluorophores can be greatly modified via a separation-dependent energy transfer process to a nearby metamaterial. The corresponding photobleaching rate is thus linked to the distance between the fluorophores and the metamaterial layer, leading to a reconstructed image with exceptionally high axial resolution. We apply the MAPM technology to image the HeLa cell membranes tagged with fluorescent proteins and demonstrate an axial resolution of ∼2.4 nm with multiple colors. MAPM utilizes a metamaterial-coated substrate to achieve super-resolution without altering anything else in a conventional microscope, representing a simple solution for fluorescence imaging at nanometer axial resolution.

Pulsed electron spin resonance spectroscopy in the Purcell regime

In EPR, spin relaxation is typically governed by interactions with the lattice or other spins. However, it has recently been shown that given a sufficiently strong spin-resonator coupling and high resonator quality factor, the spontaneous emission of microwave photons from the spins into the resonator can become the main relaxation mechanism, as predicted by Purcell. With increasing attention on the use of microresonators for EPR to achieve high spin-number sensitivity it is important to understand how this novel regime influences measured EPR signals, for example the amplitude and temporal shape of the spin-echo. We study this regime theoretically and experimentally, using donor spins in silicon, under different conditions of spin-linewidth and coupling homogeneity. When the spin-resonator coupling is distributed inhomogeneously, we find that the effective spin-echo relaxation time measured in a saturation recovery sequence strongly depends on the parameters for the detection echo. When the spin linewidth is larger than the resonator bandwidth, the different Fourier components of the spin echo relax with different characteristic times - due to the role of the resonator in driving relaxation - which results in the temporal shape of the echo becoming dependent on the repetition time of the experiment.

Study on Modulation Bandwidth and Light Extraction Efficiency of Flip-Chip Light-Emitting Diode with Photonic Crystals

In this study, the photonic crystal structure is employed to increase both the light extraction efficiency and the modulation bandwidth of flip-chip GaN-based light-emitting diodes (LEDs). The finite difference time domain method is utilized to investigate the influence of structure of photonic crystals on the Purcell factor and light extraction efficiency of flip-chip GaN-based LEDs. Simulation results show that the modulation bandwidth is estimated to be 202 MHz at current densities of 1000 A/cm². The experimental result of modulation bandwidth is in accord with the simulation. The optical f-3dB of the device achieves 212 MHz at current densities of 1000 A/cm² and up to 285 MHz at current densities of 2000 A/cm². This design of photonic crystal flip-chip LED has the potential for applications in high-frequency visible light communication.

Bacteriorhodopsin Enhances Efficiency of Perovskite Solar Cells

Recently, halide perovskites have upstaged decades of solar cell development by reaching power conversion efficiencies that surpass the performance of polycrystalline silicon. The efficiency improvement in the perovskite cells is related to repeated recycling between photons and electron-hole pairs, reduced recombination losses, and increased carrier lifetimes. Here, we demonstrate a novel approach toward augmenting the perovskite solar cell efficiency by invoking the Förster Resonance Energy Transfer (FRET) mechanism. FRET occurs in the near-field region as the bacteriorhodopsin (bR) protein, and perovskite has similar optical gaps. Titanium dioxide functionalized with the bR protein is shown to accelerate the electron injection from excitons produced in the perovskite layer. FRET predicts the strength of long-range excitonic transport between the perovskite and bR layers. Solar cells incorporating TiO₂/bR layers are found to exhibit much higher photovoltaic performance as compared to baseline cells without bR. These results open the opportunity to develop a new class of bioperovskite solar cells with improved performance and stability.

Improving Upconversion Efficiency Based on Cross-Patterned Upconversion Material Slot Waveguides on a Silicon Layer

Upconversion (UC) materials can be used to harvest near-infrared (NIR) light and convert it into visible light. Although this improves optical device operating spectral range and efficiency, e.g., solar cells, typical UC material conversion efficiency is too low for practical devices. We propose a cross-patterned slot waveguide constructed from UC material embedded in a high index semiconductor layer to improve UC. Since the slot waveguide mode is induced in the low index UC slot, NIR absorption (~970 nm) increased 25-fold compared with film structures. Furthermore, the spontaneous emission enhancement rate at 660 nm increased 9.6-fold compared to the reference film due to resonance excited in the UC slot (Purcell effect). Thus, the proposed UC slot array structure improved UC efficiency 240-fold considering absorption and emission enhancements. This double resonance UC improvement can be applied to practical optical devices.

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.

Light-Emitting Halide Perovskite Nanoantennas

Nanoantennas made of high-index dielectrics with low losses in visible and infrared frequency ranges have emerged as a novel platform for advanced nanophotonic devices. On the other hand, halide perovskites are known to possess high refractive index, and they support excitons at room temperature with high binding energies and quantum yield of luminescence that makes them very attractive for all-dielectric resonant nanophotonics. Here we employ halide perovskites to create light-emitting nanoantennas with enhanced photoluminescence due to the coupling of their excitons to dipolar and multipolar Mie resonances. We demonstrate that the halide perovskite nanoantennas can emit light in the range of 530-770 nm depending on their composition. We employ a simple technique based on laser ablation of thin films prepared by wet-chemistry methods as a novel cost-effective approach for the fabrication of resonant perovskite nanostructures.

Blue-Shifting Intramolecular Charge Transfer Emission by Nonlocal Effect of Hyperbolic Metamaterials

Metallic nanostructures permit controlling various photophysical processes by coupling photons with plasmonic oscillation of electrons confined in the tailored nanostructures. One example is hyperbolic metamaterial (HMM) leading to an enhanced spontaneous emission rate of emitters located nearby. Noting that emission in organic molecules is from either π-π* or intramolecular charge-transfer (ICT) states, we address here how HMM modifies ICT emission spectral features by comparing them with a spectral shift dependent on the local polarity of the medium. The 7.0 nm blue shift is observed in ICT emission from 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dispersed into a polymer matrix prepared on HMM multilayered structure, while no spectral shift is observed in π-π* emission from perylene diimide. In the frame of the Lippert-Mataga formalism, the blue shift is explained by the HMM nonlocal effects resulting from 8% decrease in refractive index and 18% reduction in dielectric permittivity. This phenomenon was also shown in a hemicurcuminoid borondifluoride dye yielding 15.0 nm blue shift. Such a capability of spectral shift control in films by HMM structure opens new prospects for engineering organic light-emitting devices.

Making two-photon processes dominate one-photon processes using mid-IR phonon polaritons

The recent discovery of nanoscale-confined phonon polaritons in polar dielectric materials has generated vigorous interest because it provides a path to low-loss nanoscale photonics at technologically important mid-IR and terahertz frequencies. In this work, we show that these polar dielectrics can be used to develop a bright and efficient spontaneous emitter of photon pairs. The two-photon emission can completely dominate the total emission for realistic electronic systems, even when competing single-photon emission channels exist. We believe this work acts as a starting point for the development of sources of entangled nano-confined photons at frequency ranges where photon sources are generally considered lacking. Additionally, we believe that these results add a dimension to the great promise of phonon polaritonics.

Phonon polaritons are guided hybrid modes of photons and optical phonons that can propagate on the surface of a polar dielectric. In this work, we show that the precise combination of confinement and bandwidth offered by phonon polaritons allows for the ability to create highly efficient sources of polariton pairs in the mid-IR/terahertz frequency ranges. Specifically, these polar dielectrics can cause emitters to preferentially decay by the emission of pairs of phonon polaritons, instead of the previously dominant single-photon emission. We show that such two-photon emission processes can occur on nanosecond time scales and can be nearly 2 orders of magnitude faster than competing single-photon transitions, as opposed to being as much as 8–10 orders of magnitude slower in free space. These results are robust to the choice of polar dielectric, allowing potentially versatile implementation in a host of materials such as hexagonal boron nitride, silicon carbide, and others. Our results suggest a design strategy for quantum light sources in the mid-IR/terahertz: ones that prefer to emit a relatively broad spectrum of photon pairs, potentially allowing for new sources of both single and multiple photons.

Sculpting Fano Resonances To Control Photonic-Plasmonic Hybridization

Hybrid photonic-plasmonic systems have tremendous potential as versatile platforms for the study and control of nanoscale light-matter interactions since their respective components have either high-quality factors or low mode volumes. Individual metallic nanoparticles deposited on optical microresonators provide an excellent example where ultrahigh-quality optical whispering-gallery modes can be combined with nanoscopic plasmonic mode volumes to maximize the system's photonic performance. Such optimization, however, is difficult in practice because of the inability to easily measure and tune critical system parameters. In this Letter, we present a general and practical method to determine the coupling strength and tailor the degree of hybridization in composite optical microresonator-plasmonic nanoparticle systems based on experimentally measured absorption spectra. Specifically, we use thermal annealing to control the detuning between a metal nanoparticle's localized surface plasmon resonance and the whispering-gallery modes of an optical microresonator cavity. We demonstrate the ability to sculpt Fano resonance lineshapes in the absorption spectrum and infer system parameters critical to elucidating the underlying photonic-plasmonic hybridization. We show that including decoherence processes is necessary to capture the evolution of the lineshapes. As a result, thermal annealing allows us to directly tune the degree of hybridization and various hybrid mode quantities such as the quality factor and mode volume and ultimately maximize the Purcell factor to be 10⁴.

Tuning Excitonic Properties of Monolayer MoS2 with Microsphere Cavity by High-Throughput Chemical Vapor Deposition Method

Tuning the optical properties of 2D direct bandgap semiconductors is crucial for applications in photonic light source, optical communication, and sensing. In this work, the excitonic properties of molybdenum disulphide (MoS₂ ) are successfully tuned by directly depositing it onto silica microsphere resonators using chemical vapor deposition method. Multiple whispering gallery mode (WGM) peaks in the emission wavelength range of ≈650-750 nm are observed under continuous wave excitation at room temperature. Time-resolved photoluminescence (TRPL) and femtosecond transient absorption (TA) spectroscopy are conducted to study light-matter interaction dynamics of the MoS₂ microcavities. TRPL study suggests radiative recombination rate of carrier-phonon scattering and interband transition processes in MoS₂ is enhanced by a factor of ≈1.65 due to Purcell effect in microcavities. TA spectroscopy study shows modulation of the interband transition process mainly occurs at PB-A band with an estimated F ≈ 1.60. Furthermore, refractive index sensing utilizing WGM peaks of MoS₂ is established with sensitivity up to ≈150 nm per refractive index unit. The present work provides a large-scale and straightforward method for coupling atomically thin 2D gain media with cavities for high-performance optoelectronic devices and sensors.

Enhanced Telecom Emission from Single Group-IV Quantum Dots by Precise CMOS-Compatible Positioning in Photonic Crystal Cavities

Efficient coupling to integrated high-quality-factor cavities is crucial for the employment of germanium quantum dot (QD) emitters in future monolithic silicon-based optoelectronic platforms. We report on strongly enhanced emission from single Ge QDs into L3 photonic crystal resonator (PCR) modes based on precise positioning of these dots at the maximum of the respective mode field energy density. Perfect site control of Ge QDs grown on prepatterned silicon-on-insulator substrates was exploited to fabricate in one processing run almost 300 PCRs containing single QDs in systematically varying positions within the cavities. Extensive photoluminescence studies on this cavity chip enable a direct evaluation of the position-dependent coupling efficiency between single dots and selected cavity modes. The experimental results demonstrate the great potential of the approach allowing CMOS-compatible parallel fabrication of arrays of spatially matched dot/cavity systems for group-IV-based data transfer or quantum optical systems in the telecom regime.

Bridging the Gap between Dielectric Nanophotonics and the Visible Regime with Effectively Lossless Gallium Phosphide Antennas

We present all-dielectric gallium phosphide (GaP) nanoantennas as an efficient nanophotonic platform for surface-enhanced second harmonic generation (SHG) and fluorescence (SEF), showing negligible losses in the visible range. Employing single GaP nanodisks, we observe an increase of more than 3 orders of magnitude in the SHG conversion signal in comparison with the bulk. This constitutes an SHG efficiency as large as 0.0002%, which is to the best of our knowledge the highest yet achieved value for a single nano-object in the optical region. Furthermore, we show that GaP dimers with 35 nm gap can enhance up to 3600 times the fluorescence emission of dyes located in the gap of the nanoantenna. This is accomplished by a fluorescence lifetime reduction of at least 22 times, accompanied by a high-intensity field confinement in the gap region. These results open new avenues for low-loss nanophotonics in the optical regime.

Surface Lattice Resonances for Enhanced and Directional Electroluminescence at High Current Densities

Hybrid photonic-plasmonic modes in periodic arrays of metallic nanostructures offer a promising trade-off between high-quality cavities and subdiffraction mode confinement. However, their application in electrically driven light-emitting devices is hindered by their sensitivity to the surrounding environment and to charge injecting metallic electrodes in particular. Here, we demonstrate that the planar structure of light-emitting field-effect transistor (LEFET) ensures undisturbed operation of the characteristic modes. We incorporate a square array of gold nanodisks into the charge transporting and emissive layer of a polymer LEFET in order to tailor directionality and emission efficiency via the Purcell effect and variation of the fractional local density of states in particular. Angle- and polarization-resolved spectra confirm that the enhanced electroluminescence correlates with the dispersion curves of the surface lattice resonances supported by these structures. These LEFETs reach current densities on the order of 10 kA/cm², which may pave the way toward practical optoelectronic devices with tailored emission patterns and potentially electrically pumped plasmonic lasers.

Lithographically Defined, Room Temperature Low Threshold Subwavelength Red-Emitting Hybrid Plasmonic Lasers

Hybrid plasmonic lasers provide deep subwavelength optical confinement, strongly enhanced light-matter interaction and together with nanoscale footprint promise new applications in optical communication, biosensing, and photolithography. The subwavelength hybrid plasmonic lasers reported so far often use bottom-up grown nanowires, nanorods, and nanosquares, making it difficult to integrate these devices into industry-relevant high density plasmonic circuits. Here, we report the first experimental demonstration of AlGaInP based, red-emitting hybrid plasmonic lasers at room temperature using lithography based fabrication processes. Resonant cavities with deep subwavelength 2D and 3D mode confinement of λ²/56 and λ³/199, respectively, are demonstrated. A range of cavity geometries (waveguides, rings, squares, and disks) show very low lasing thresholds of 0.6-1.8 mJ/cm² with wide gain bandwidth (610 nm-685 nm), which are attributed to the heterogeneous geometry of the gain material, the optimized etching technique, and the strong overlap of the gain material with the plasmonic modes. Most importantly, we establish the connection between mode confinements and enhanced absorption and stimulated emission, which plays critical roles in maintaining low lasing thresholds at extremely small hybrid plasmonic cavities. Our results pave the way for the further integration of dense arrays of hybrid plasmonic lasers with optical and electronic technology platforms.

Optical Gain in MoS2 via Coupling with Nanostructured Substrate: Fabry-Perot Interference and Plasmonic Excitation

Despite the direct band gap of monolayer transition metal dichalcogenides (TMDs), their optical gain remains limited because of the poor light absorption in atomically thin, layered materials. Most approaches to improve the optical gain of TMDs mainly involve modulation of the active materials or multilayer stacking. Here, we report a method to enhance the optical absorption and emission in MoS2 simply through the design of a nanostructured substrate. The substrate consisted of a dielectric nanofilm spacer (TiO2) and metal film. The overall photoluminescence intensity from monolayer MoS2 on the nanostructured substrate was engineered based on the TiO2 thickness and amplified by Fabry-Perot interference. In addition, the neutral exciton emission was selectively amplified by plasmonic excitations from the local field originating from the surface roughness of the metal film with spacer thicknesses of less than 10 nm. We further demonstrate that the quality factor of the device can also be engineered by selecting a spacer material with a different refractive index.

Broadband Tunable, Polarization-Selective and Directional Emission of (6,5) Carbon Nanotubes Coupled to Plasmonic Crystals

We demonstrate broadband tunability of light emission from dense (6,5) single-walled carbon nanotube thin films via efficient coupling to periodic arrays of gold nanodisks that support surface lattice resonances (SLRs). We thus eliminate the need to select single-walled carbon nanotubes (SWNTs) with different chiralities to obtain narrow linewidth emission at specific near-infrared wavelengths. Emission from these hybrid films is spectrally narrow (20-40 meV) yet broadly tunable (∼1000-1500 nm) and highly directional (divergence <1.5°). In addition, SLR scattering renders the emission highly polarized, even though the SWNTs are randomly distributed. Numerical simulations are applied to correlate the increased local electric fields around the nanodisks with the observed enhancement of directional emission. The ability to control the emission properties of a single type of near-infrared emitting SWNTs over a wide range of wavelengths will enable application of carbon nanotubes in multifunctional photonic devices.

Exciton-Plasmon Coupling and Electromagnetically Induced Transparency in Monolayer Semiconductors Hybridized with Ag Nanoparticles

Exciton-plasmon coupling in hybrids of a monolayer transition metal dichalcogenide and Ag nanoparticles is investigated in the weak and strong coupling regimes. In the weak coupling regime, both absorption enhancement and the Purcell effect collectively modify the photoluminescence properties of the semiconductor. In the strong coupling regime, electromagnetically induced transparency dips are displayed, evidencing coherent energy exchange between excitons and plasmons.

On-Demand Coupling of Electrically Generated Excitons with Surface Plasmons via Voltage-Controlled Emission Zone Position

The ability to confine and manipulate light below the diffraction limit is a major goal of future multifunctional optoelectronic/plasmonic systems. Here, we demonstrate the design and realization of a tunable and localized electrical source of excitons coupled to surface plasmons based on a polymer light-emitting field-effect transistor (LEFET). Gold nanorods that are integrated into the channel support localized surface plasmons and serve as nanoantennas for enhanced electroluminescence. By precise spatial control of the near-infrared emission zone in the LEFET via the applied voltages the near-field coupling between electrically generated excitons and the nanorods can be turned on or off as visualized by a change of electroluminescence intensity. Numerical calculations and spectroscopic measurements corroborate significant local electroluminescence enhancement due to the high local density of photonic states in the vicinity of the gold nanorods. Importantly, the integration of plasmonic nanostructures hardly influences the electrical performance of the LEFETs, thus, highlighting their mutual compatibility in novel active plasmonic devices.

Self-aligned deterministic coupling of single quantum emitter to nanofocused plasmonic modes

The quantum plasmonics field has emerged and been growing increasingly, including study of single emitter-light coupling using plasmonic system and scalable quantum plasmonic circuit. This offers opportunity for the quantum control of light with compact device footprint. However, coupling of a single emitter to highly localized plasmonic mode with nanoscale precision remains an important challenge. Today, the spatial overlap between metallic structure and single emitter mostly relies either on chance or on advanced nanopositioning control. Here, we demonstrate deterministic coupling between three-dimensionally nanofocused plasmonic modes and single quantum dots (QDs) without any positioning for single QDs. By depositing a thin silver layer on a site-controlled pyramid QD wafer, three-dimensional plasmonic nanofocusing on each QD at the pyramid apex is geometrically achieved through the silver-coated pyramid facets. Enhancement of the QD spontaneous emission rate as high as 22 ± 16 is measured for all processed QDs emitting over ∼150-meV spectral range. This approach could apply to high fabrication yield on-chip devices for wide application fields, e.g., high-efficiency light-emitting devices and quantum information processing.

Two-dimensional metal-chalcogenide films in tunable optical microcavities

Integration of quasi-two-dimensional (2D) films of metal-chalcogenides in optical microcavities permits new photonic applications of these materials. Here we present tunable microcavities with monolayer MoS2 or few monolayer GaSe films. We observe significant modification of spectral and temporal properties of photoluminescence (PL): PL is emitted in spectrally narrow and wavelength-tunable cavity modes with quality factors up to 7400; a 10-fold PL lifetime shortening is achieved, a consequence of Purcell enhancement of the spontaneous emission rate.

Inhibition and enhancement of the spontaneous emission of quantum dots in micropillar cavities with radial-distributed Bragg reflectors

We present a micropillar cavity where nondesired radial emission is inhibited. The photonic confinement in such a structure is improved by implementation of an additional concentric radial-distributed Bragg reflector. Such a reflector increases the reflectivity in all directions perpendicular to the micropillar axis from a typical value of 15-31% to above 98%. An inhibition of the spontaneous emission of off-resonant excitonic states of quantum dots embedded in the microcavity is revealed by time-resolved experiments. It proves a decreased density of photonic states related to unwanted radial leakage of photons out of the micropillar. For on-resonance conditions, we find that the dot emission rate is increased, evidencing the Purcell enhancement of spontaneous emission. The proposed design can increase the efficiency of single-photon sources and bring to micropillar cavities the functionalities based on lengthened decay times.