Nondiffusing localized excitons (XL) in two-dimensional semiconductors present a robust platform for mediating light-matter interactions, with potential applications in both photovoltaics and light-emitting devices. However, at room temperature, high thermal energy hinders XL formation, while excess charges diminish the quantum yield (QY) through nonradiative decay. Here, we present high-QY XL emission in ambient conditions by removing excess charges and inducing efficient exciton funneling into a Au nanohole. Specifically, by evaporating an H2O barrier between the n-type MoS2 and the Au substrate, we induce a grounding effect on electrons. Dominantly populating excitons are then funneled and bound to the nanohole through the strain-induced zero-dimensional quantum well effect. We confirm the exciton confinement efficiency of ~98% using a drift-diffusion model, enabling bright XL emission at the nanoscale. Using tip-induced gigapascal-scale pressure, we control XL dynamics and QY in a reversible manner. Our approach provides an innovative strategy for XL-based nanophotonic devices.

We combine modeling and experiments to investigate second- and third-harmonic generation (SHG/THG) in metal-indium tin oxide (ITO) metasurfaces. Linear optics at normal incidence show moderate field enhancement near the ITO epsilon-near-zero (ENZ) wavelength, steering the focus toward intrinsic, material driven nonlinear response rather than simple linear field boosting. Wavelength resolved SHG requires a Lorentz dispersive χ^2 for ITO to match spectra; a static χ^2 fails. Angle resolved SHG/THG cannot be reproduced with purely real coefficients; grouped contributions to χ^3 (and effective χ^2) must be complex. Using a hydrodynamic model for the metal and ITO with linear dispersion plus dispersive χ^2 and χ^3, we show that these complex phases arise from coherent interference of nonlinear sources in the metal, ITO, and interfaces, each weighted by distinct, complex local field and radiation factors. Experimentally, we fabricated split ring resonator metasurfaces on ITO films atop a metallic ground plate and measured linear reflectance and angle resolved SHG/THG in reflection geometry. The measurements quantitatively confirm the modeling: dispersive χ^2 is necessary to capture SHG spectra, and complex, interference induced effective coefficients are essential to reproduce angular SHG/THG patterns. Together, these results provide a unified, physically grounded interpretation of nonlinear emission from metal-oxide metasurfaces without relying on ENZ field enhancement.

Localization of exciton center-of-mass wavefunctions due to thermally-induced disorder in monolayer molybdenum diselenide (MoSe2) is observed. The magnetic-field and temperature dependent photoluminescence lineshapes provide evidence for exciton localization in encapsulated monolayers with low inhomogeneity. © 2024 The Author(s)

Photoluminescence (PL) emission in two-dimensional (2D) materials is of great interest for nanophotonics applications. While excitonic emission has been observed in numerous 2D materials, tunable multi-band luminescence is rare. Here, we present single-crystalline AgErP2Se6, a 2D material that exhibits bright, multi-band PL emission from Er3+ ions within the lattice. The emission bands cover a wide range (350- 1,550 nm), with ultra-narrow (as low as 0.5 nm at room temperature) emission peaks and room temperature lifetimes up to 4 ms. The intensities of the PL emission bands from the single crystals depend strongly on temperature and pressure, enabling sensing over a wide temperature and pressure range. Furthermore, the PL persists in exfoliated flakes down to at least 11 nm thick and demonstrates thickness-dependent Purcell enhancement. This work establishes 2D AgErP2Se6 as a multi-band luminescent emitter and sensor, poised to enable integration into a number of optoelectronic and nanophotonic applications.

Hybridization of excitons with photons to form hybrid quasiparticles-exciton-polaritons (EPs)-has been widely investigated in a range of semiconductor material systems coupled to photonic cavities. Self-hybridization occurs when the semiconductor itself can serve as the photonic cavity medium, resulting in strongly coupled EPs with Rabi splitting energies ((h) over bar Omega) of >200 meV at room temperature, which were recently observed in layered two-dimensional excitonic materials. Here we report an extreme version of this phenomenon-an ultrastrong EP coupling-in a nascent, two-dimensional excitonic system, namely, the metal-organic chalcogenolate compound called mithrene. The resulting self-hybridized EPs in mithrene crystals placed on Au substrates show Rabi splitting in the ultrastrong-coupling range ((h) over bar Omega > 600 meV) due to the strong oscillator strength of the excitons concurrent with the large refractive indices of mithrene. We further show that bright EP emission occurs at room temperature as well as EP dispersions at low temperatures. Importantly, we find lower EP emission linewidth narrowing to similar to 1 nm when mithrene crystals are placed in closed Fabry-Perot cavities. Our results suggest that metal-organic chalcogenolate materials are ideal for polaritonics in the deep green-blue part of the spectrum in which strong excitonic materials with large optical constants are particularly scarce.

We combine modeling and experiments to investigate second- and third-harmonic generation (SHG/THG) in metal-indium-tin oxide (ITO) metasurfaces. Linear optics at normal incidence shows moderate field enhancement near the ITO epsilon-near-zero (ENZ) wavelength, steering the focus toward intrinsic, material-driven nonlinear response rather than simple linear field boosting. Wavelength-resolved SHG requires a Lorentz-dispersive chi(2) for ITO to match spectra; a static chi(2) fails. Angle-resolved SHG/THG cannot be reproduced with purely real coefficients; grouped contributions to chi(3) (and the effective chi(2)) must be complex. Using a hydrodynamic model for the metal and ITO with linear dispersion plus dispersive chi(2) and chi(3), we show that these complex phases arise from coherent interference of nonlinear sources in the metal, ITO, and interfaces, each weighted by distinct, complex local fields, and radiation factors. Experimentally, we fabricated split-ring resonator metasurfaces on ITO films atop a metallic ground plate and measured linear reflectance and angle-resolved SHG/THG in reflection geometry. The measurements quantitatively confirm the modeling: dispersive chi(2) is necessary to capture SHG spectra, and complex interference-induced effective coefficients are essential to reproduce angular SHG/THG patterns. Together, these results provide a unified, physically grounded interpretation of nonlinear emission from metal-oxide metasurfaces without relying on ENZ field enhancement. This interference-based picture enables inverse design of metal-ITO ENZ metasurfaces with tailored SHG/THG angular patterns for compact frequency converters, directional nonlinear emitters, and on-chip spectroscopy.

Chalcogenides and oxy-chalcogenides, including complex chalcogenides and transition metal dichalcogenides, are emerging semiconductors with direct or indirect band gaps within the visible spectrum. These materials are being explored for various photonic and electronic applications such as photodetectors, photovoltaics, and phase-change electronics. Understanding the fundamental properties of these materials is crucial for optimizing their functionalities. Therefore, the availability of large, high-quality single crystals of chalcogenides and oxy-chalcogenides is essential for a better comprehension of their structure and properties. In this study, we present a novel crystal growth method that utilizes the exchange reaction between BaS and ZrCl4/HfCl4. By carefully controlling the stoichiometric ratio of the binary sulfide to the chloride, we can grow single crystals of several materials, such as ZrS2, HfS2, BaZrS3, and ZrOS. This method results in large single crystals with a short reaction time of 24 to 48 h. High-resolution thin-film X-ray diffraction and single-crystal X-ray diffraction confirm the quality of the crystals produced through this exchange reaction. Importantly, we report optical properties, especially static and time-resolved photoluminescence, by studying their emission behavior and carrier dynamics to evaluate their suitability for optoelectronic applications. The chloride exchange reaction method paves the way for the synthesis of single crystals of chalcogenides and oxy-chalcogenide systems with a short reaction time but with low mosaicity and shall be an alternative single-crystal growth technique for materials that are challenging to grow, especially optical and optoelectronic materials.

Heterostructures between 2D and 3D electron systems remain critically important in developing novel and efficient optoelectronic and electronic devices. In this study, a vertical heterojunction between monolayer MoS2 and bulk InSe was developed. This heterojunction exhibits a type-I band alignment that facilitates rapid energy transfer from the wide bandgap MoS2 to the narrow bandgap InSe resulting in quenching of the MoS2 photoluminescence (PL) emission and enhancement of the A exciton emission in InSe. Temperature-dependent PL measurements of MoS2 on SiO2, MoS2 on InSe, and bare InSe revealed the critical role of defect trapping and electron-phonon coupling in the optical response of MoS2 on InSe. These results demonstrate that heterostructures combining monolayer MoS2 on bulk InSe, showing marked improvement relative to bare InSe, would be advantageous when incorporated into optoelectronic devices such as photodetectors, light emitters, and color converters and highlights the benefit of creating van der Waals (vdW) heterostructures with tailored properties.

Monolayer transition-metal dichalcogenides (TMDCs) host tightly bound excitons with unique valley pseudospin properties, establishing them as an emerging material platform for nanophotonics and quantum technologies. Exciton-exciton interactions modify light-matter coupling and significantly affect the formation of exciton complexes. Here, we employ a top-down nanofabrication technique to manipulate interexcitonic interactions in WS2 monolayers through lateral confinement. By restricting the motion of excitons in confined two-dimensional (2D) spaces, interexcitonic interactions are significantly modified, resulting in strong biexcitonic emission in nanodots smaller than 100 nm that is obscured in pristine monolayers. Moreover, we demonstrate selective optical excitation of valley pseudospins for excitonic quasiparticles in confined monolayers. Our work highlights the role of spatial confinement in excitonic behavior in 2D systems and provides new insights into the development of future photonic and valleytronic devices with low-dimensional platforms.

Localized emission in atomically thin semiconductors has sparked significant interest as single-photon sources. Despite comprehensive studies into the correlation between localized strain and exciton emission, the impacts of charge transfer on nanobubble emission remains elusive. Here, we report the observation of core/shell-like localized emission from monolayer WSe2 nanobubbles at room temperature through near-field studies. By altering the electronic junction between monolayer WSe2 and the Au substrate, one can effectively adjust the semiconductor to metal junction from a Schottky to an Ohmic junction. Through concurrent analysis of topography, potential, tip-enhanced photoluminescence, and a piezo response force microscope, we attribute the core/shell-like emissions to strong piezoelectric potential aided by induced polarity at the WSe2-Au Schottky interface which results in spatial confinement of the excitons. Our findings present a new approach for manipulating charge confinement and engineering localized emission within atomically thin semiconductor nanobubbles. These insights hold implications for advancing the nano and quantum photonics with low-dimensional semiconductors.

Quantum emitters are essential components of quantum photonic circuitry envisioned beyond the current optoelectronic state-of-the-art. Two dimensional materials are attractive hosts for such emitters. However, the high single photon purity required is rarely realized due to the presence of spectrally degenerate classical light originating from defects. Here, we show that design of a van der Waals heterostructure effectively eliminates this spurious light, resulting in purities suitable for a variety of quantum technological applications. Single photon purity from emitters in monolayer WSe2 increases from 60% to 92% by incorporating this monolayer in a simple graphite/WSe2 heterostructure. Fast interlayer charge transfer quenches a broad photoluminescence background by preventing radiative recombination through long-lived defect bound exciton states. This approach is generally applicable to other 2D emitter materials, circumvents issues of material quality, and offers a path forward to achieve the ultrahigh single photon purities ultimately required for photon-based quantum technologies.

Telecommunications and polarimetry both require the active control of the polarization of light. Currently, this is done by combining intrinsically anisotropic materials with tunable isotropic materials into heterostructures using complicated fabrication techniques owing to the lack of scalable materials that possess both properties. Tunable birefringent and dichromic materials are scarce and rarely available in high-quality thin films over wafer scales. Here we report semiconducting, highly aligned, single-walled carbon nanotubes (SWCNTs) over 4 '' wafers with normalized birefringence and dichroism values of 0.09 and 0.58, respectively. The real and imaginary parts of the refractive index of these SWCNT films are tuned by up to 5.9% and 14.3% in the infrared at 2,200 nm and 1,660 nm, respectively, using electrostatic doping. Our results suggest that aligned SWCNTs are among the most anisotropic and tunable optical materials known and open new avenues for their application in integrated photonics and telecommunications. Using electrostatic doping, the real and imaginary parts of the refractive index along the extraordinary axis of semiconducting, highly aligned, single-walled carbon nanotubes over 4 '' wafers can be tuned by up to 5.9% and 14.3% in the infrared at 2,200 nm and 1,660 nm, respectively.

Two-dimensional (2D) semiconductors are promising candidates for optoelectronic application and quantum information processes due to their inherent out-of-plane 2D confinement. In addition, they offer the possibility of achieving low-dimensional in-plane exciton confinement, similar to zero-dimensional quantum dots, with intriguing optical and electronic properties via strain or composition engineering. However, realizing such laterally confined 2D monolayers and systematically controlling size-dependent optical properties remain significant challenges. Here, we report the observation of lateral confinement of excitons in epitaxially grown in-plane MoSe2 quantum dots (~15-60 nm wide) inside a continuous matrix of WSe2 monolayer film via a sequential epitaxial growth process. Various optical spectroscopy techniques reveal the size-dependent exciton confinement in the MoSe2 monolayer quantum dots with exciton blue shift (12-40 meV) at a low temperature as compared to continuous monolayer MoSe2. Finally, single-photon emission was also observed from the smallest dots at 1.6 K. Our study opens the door to compositionally engineered, tunable, in-plane quantum light sources in 2D semiconductors.

Rare earth dopants are one of the most extensively studied optical emission centers for a broad range of applications such as laser optoelectronics, sensing, lighting, and quantum information technologies due to their narrow optical linewidth and exceptional coherence properties. Epitaxial doped oxide thin films can serve as a promising and controlled host to investigate rare-earth dopants suitable for scalable quantum memories, on-chip lasers and amplifiers. Here, we report high-quality epitaxial thin films of Tm-doped CaZrO$_3$ grown by pulsed laser deposition for infrared optoelectronic and quantum memory applications. We perform extensive structural and chemical characterization to probe the crystallinity of the films and the doping behavior. Low temperature photoluminescence measurements show sharp radiative transitions in the short-wave infrared range of 1.75 - 2 \mu m.

Excitons, bound electron-hole pairs, in Two-Dimensional Hybrid Organic Inorganic Perovskites (2D HOIPs) are capable of forming hybrid light-matter states known as exciton-polaritons (E-Ps) when the excitonic medium is confined in an optical cavity. In the case of 2D HOIPs, they can self-hybridize into E-Ps at specific thicknesses of the HOIP crystals that form a resonant optical cavity with the excitons. However, the fundamental properties of these self-hybridized E-Ps in 2D HOIPs, including their role in ultrafast energy and/or charge transfer at interfaces, remain unclear. Here, we demonstrate that > 0.5 um thick 2D HOIP crystals on Au substrates are capable of supporting multiple-orders of self-hybridized E-P modes. These E-Ps have high Q factors (> 100) and modulate the optical dispersion for the crystal to enhance sub-gap absorption and emission. Through varying excitation energy and ultrafast measurements, we also confirm energy transfer from higher energy upper E-Ps to lower energy, lower E-Ps. Finally, we also demonstrate that E-Ps are capable of charge transport and transfer at interfaces. Our findings provide new insights into charge and energy transfer in E-Ps opening new opportunities towards their manipulation for polaritonic devices.

Observation of interlayer, charge transfer (CT) excitons in van der Waals heterostructures (vdWHs) based on 2D-2D systems has been well investigated. While conceptually interesting, these charge transfer excitons are highly delocalized and spatially localizing them requires twisting layers at very specific angles. This issue of localizing the CT excitons can be overcome via making nanoplate-2D material heterostructures (N2DHs) where one of the components is a spatially quantum confined medium. Here, we demonstrate the formation of CT excitons in a mixed dimensional system comprising MoSe2 and WSe2 monolayers and CdSe/CdS-based core/shell nanoplates (NPLs). Spectral signatures of CT excitons in our N2DHs were resolved locally at the 2D/single-NPL heterointerface using tip-enhanced photoluminescence (TEPL) at room temperature. By varying both the 2D material and the shell thickness of the NPLs and applying an out-of-plane electric field, the exciton resonance energy was tuned by up to 100 meV. Our finding is a significant step toward the realization of highly tunable N2DH-based next-generation photonic devices.

Indium selenide (InSe) multilayers have attracted much interest recently due to their electronic and optical properties, partially dependent on the existence of an indirect-to-direct bandgap transition that is correlated to the multilayer thickness. In this work, we investigate stacks of van der Waals-bonded multilayer InSe, ordered similarly to the gamma phase of bulk InSe. We analyze the indirect-to-direct bandgap transition using first-principles methods, identifying the structural changes in the multilayer structures that cause the electronic modifications that result in this transition. We highlight differences between InSe and transition-metal dichalcogenides. Our calculations confirm the thickness dependence of the crossover between the indirect and direct bandgaps in multilayer InSe and emphasize changes in the structure and orbital nature of the valence and conduction bands. We compare the optical spectra predictions performed at a high level of theory to our thorough experimental photoluminescence characterization of our high-quality gamma phase bulk InSe. These predictions are correlated to electronic transitions and elucidate the relative contributions of in-plane and out-of-plane dipoles. The insights gained from our study could contribute to the design of multicomponent heterostructures with InSe for future electronic and electro-optical devices.

Hybridization of excitons with photons to form hybrid quasiparticles, exciton-polaritons (EPs), has been widely investigated in a range of semiconductor material systems coupled to photonic cavities. Self-hybridization occurs when the semiconductor itself can serve as the photonic cavity medium resulting in strongly-coupled EPs with Rabi splitting energies > 200 meV at room temperatures which recently were observed in layered two-dimensional (2D) excitonic materials. Here, we report an extreme version of this phenomenon, an ultrastrong EP coupling, in a nascent, 2D excitonic system, the metal organic chalcogenate (MOCHA) compound named mithrene. The resulting self-hybridized EPs in mithrene crystals placed on Au substrates show Rabi Splitting in the ultrastrong coupling range (> 600 meV) due to the strong oscillator strength of the excitons concurrent with the large refractive indices of mithrene. We further show bright EP emission at room temperature as well as EP dispersions at low-temperatures. Importantly, we find lower EP emission linewidth narrowing to ~1 nm when mithrene crystals are placed in closed Fabry-Perot cavities. Our results suggest that MOCHA materials are ideal for polaritonics in the deep green-blue part of the spectrum where strong excitonic materials with large optical constants are notably scarce.

Interlayer excitons (IXs) in two-dimensional (2D) heterostructures provide an exciting avenue for exploring optoelectronic and valleytronic phenomena. Presently, valleytronic research is limited to transition metal dichalcogenide (TMD) based 2D heterostructure samples, which require strict lattice (mis) match and interlayer twist angle requirements. Here, we explore a 2D heterostructure system with experimental observation of spin-valley layer coupling to realize helicity-resolved IXs, without the requirement of a specific geometric arrangement, i.e., twist angle or specific thermal annealing treatment of the samples in 2D Ruddlesden-Popper (2DRP) halide perovskite/2D TMD heterostructures. Using first-principle calculations, time-resolved and circularly polarized luminescence measurements, we demonstrate that Rashba spin-splitting in 2D perovskites and strongly coupled spin-valley physics in monolayer TMDs render spin-valley-dependent optical selection rules to the IXs. Consequently, a robust valley polarization of ∼14% with a long exciton lifetime of ∼22 ns is obtained in type-II band aligned 2DRP/TMD heterostructure at ∼1.54 eV measured at 80 K. Our work expands the scope for studying spin-valley physics in heterostructures of disparate classes of 2D semiconductors.