Advanced Optical Materials最新文献

Organic luminescent radicals have emerged as promising open-shell emitters for advanced optoelectronic and sensing applications. Among them, the carbazole-substituted tris(2,4,6-trichlorophenyl)methyl radical (Cz-TTM) exhibits remarkable photostability and emission efficiency compared with the parent TTM radical. Nevertheless, Cz-TTM can be visibly degraded by indoor light irradiation and its photodegradation mechanism is poorly understood. Here, the study systematically investigates the photochemical behavior of Cz-TTM in solutions, together with the influence of impurities on the photophysical properties of the compound. It is found that emissive and non-emissive impurities are introduced depending on the purification methods and the light irradiation conditions. Impurities cause underestimation of photoluminescence quantum yield through competitive absorption. Photodegradation under continuous irradiation undergoes a multi-step reaction via metastable intermediates, which react with dissolved oxygen upon local excitation of the TTM radical moiety. Quantum-chemical calculations indicate a ring-closure followed by dechlorination within the TTM moiety as the key degradation pathway. The superior photostability of Cz-TTM to TTM originates from its stabilized first excited doublet energy level, which increases the activation energy for ring-closure reaction in Cz-TTM. These combined experimental and theoretical insights reveal the fundamental factors governing the photostability of TTM-type luminescent radicals and offer design principles for next-generation organic luminescent materials.

3R-MoS₂, a MoS₂ polytype with broken inversion symmetry, enables unique light-matter interactions and is promising for linear and nonlinear integrated photonics beyond the monolayer limit. Yet, systematic studies of its thickness-dependent reflectivity and its impact on harmonic generation are still lacking. While AFM can offer atomic-scale resolution, measuring 3R-MoS₂ on non-solid substrates like PDMS remains challenging. To address this, a fast, non-destructive optical method is introduced to determine the thickness of 3R-MoS₂ flakes from reflectivity measurements with a mean bias of less than 2 nm in the 3–200 nm range. Nonlinear characterization further reveals distinct thickness-dependent maxima in second- and third-harmonic generation (SHG/THG), with the first clear peak at ≈200 nm. These maxima arise from Fabry–Pérot-type phase matching conditions mediated by the film thickness and can further be shaped by absorption. This work thus provides both a practical thickness metrology and new insights for exploiting thickness-dependent 3R-MoS₂ nonlinearities in scalable photonic technologies.

When exciting cathodoluminescence from a material using an electron beam, the light emission is due to electronic transitions within the material. The fundamental transition is related to the bandgap, while other emissions can be related to material defects and/or impurities. Since the bandgap of materials is temperature- and strain-dependent, cathodoluminescence emission can, in principle, be dynamically tuned. Herein, a proof-of-concept of a thermally tunable cathodoluminescence light source is demonstrated using a triode electron gun and a wide-bandgap z-cut ZnO crystal as anode. At room temperature (295 K), an ultraviolet (UV) emission peak corresponding to the ZnO bandgap is identified (398 nm), and a wide emission in the visible range (centered at ≈510 nm) is also detected (recombination of electron-hole pairs in singly occupied oxygen vacancies). Additionally, the cathodoluminescence emission spectrum is measured as a function of the ZnO anode temperature (295–373 K), resulting in a shift of the UV emission peak from 398 to ≈409 nm (3.115–3.032 eV), which translates to a temperature dependency of −1.06 ± 0.06 meV K⁻¹. The results establish the feasibility of a compact, wavelength-tunable cathodoluminescence-based UV source, with future potential for integration into field-deployable systems for gas sensing and lab-on-chip spectroscopy applications.

Periodically structured materials offer an attractive means to generate color by selective reflection but suffer from angle-dependence and generally require a black background. The former may be solved by molding the material into spheres but then an index-matching binder is often required to avoid Mie scattering that otherwise dwarfs the structural color. However, this leads to blueshifted reflections that reduce color purity. Here it is shown that shells of polymerized cholesteric liquid crystal infused with dye (dCSRs) solve these problems while adding circular polarization contrast. The dye absorbs Mie-scattered light, rendering binder and black background unnecessary. Moreover, refraction at the air–dCSR interface shrinks the bandwidth of selective reflections, turning dCSRs into discrete pixels of structural color, easily tunable across the visible spectrum. By mixing red, green and blue dCSRs, non-spectral colors can be generated, allowing dCSRs to truly compete with traditional dyes. By arranging dCSRs onto a background with identical color, fixing and protecting them with a matte clearcoat, graphical information like text or QR-code-like patterns are encoded such that the information is camouflaged to the human eye. Only through a circular polarizer is the pattern revealed, with extraordinary contrast, of great use for robotics and Augmented Reality.

Halide perovskites have emerged as promising candidates for high-performance solar cells. This study investigates the temperature-dependent optoelectronic properties of mixed-cation mixed-halide perovskite solar cells using electroluminescence (EL) and photoluminescence (PL) hyperspectral imaging, along with current–voltage analysis. Luminescence images, which are converted to EL and PL external radiative efficiency (ERE) maps, reveal significant changes in the optoelectronic behavior of these devices at low temperatures. Specifically, it is found that a substantial source of heterogeneity in the low-temperature EL ERE maps below 240 K is related to local charge injection and extraction bottlenecks, whereas PL ERE maps show suppressed nonradiative recombination and significant improvements in efficiency throughout the investigated temperature range. The spatial distribution of ERE and its variation with applied current are analyzed, offering insights into charge-carrier dynamics and defect behavior. These results reveal that while the perovskite layer exhibits enhanced ERE at low temperatures, charge injection barriers at the interfaces of the perovskite solar cells can suppress EL and degrade the fill factor below 240 K. These findings reveal that a deeper understanding of the performance of perovskite solar cells under low-temperature conditions is an essential step toward their potential application in space power systems and advanced semiconductor devices.

The selective growth, unambiguous identification, and optical characterization of 2H and 3R polytypes in bulk single crystals of the van der Waals polar insulator α-In₂Se₃ are reported. High-quality single crystals are obtained by optimizing chemical vapor transport (2H) and horizontal Bridgman (3R) methods, and their phases are reliably and efficiently verified by X-ray Laue back-reflection. Optical transmission spectra show the 2H phase has a slightly smaller band gap and steeper absorption edge than the 3R phase, consistent with first-principles predictions of their electronic structures. Furthermore, absolute reflectance measurements unveil rich peak-valley structures reflecting distinct van Hove singularities in their joint density of states, providing a physical characteristic that more clearly distinguishes the polytypes than the band-gap feature. The good agreement with DFT simulations confirms that absolute reflectivity provides crucial insights into fine electronic structures beyond the band-gap region. The results highlight pathways to further explore polytype-dependent functional properties of α-In₂Se_3, such as nonlinear optical responses.

Photon upconversion has potential applications in light–emitting diodes, photocatalysis, bio-imaging, microscopy, 3D printing, and photovoltaics. Bulk lead-halide perovskites have emerged as promising sensitisers for solid-state photon upconversion via triplet–triplet annihilation due to their excellent optoelectronic properties. In this system, a perovskite-sensitiser absorbs photons and subsequently generates triplet excitons in an adjacent emitter material, where triplet–triplet annihilation can occur, allowing for the emission of higher energy photons. However, a major loss pathway in perovskite-sensitised upconversion is the back-transfer of singlet excitons from the emitter to the sensitiser via Förster Resonance Energy Transfer. In this investigation, a 2D perovskite spacer layer is introduced between the bulk perovskite-sensitiser and rubrene emitter to mitigate back-transfer of singlet excitons. This modification reveals the inherent balance between efficient triplet exciton transfer across the interface with a potential barrier vs the mitigation of near-field back-transfer by increasing the distance between the sensitiser and singlet excitons in the emitter. Notably, the introduction of this spacer layer enhances the relative upconversion efficiency at lower excitation power densities while also sustaining performance over extended timescales. This work represents significant progress toward the practical applications of perovskite-sensitised photon upconversion.

Plasmonic nanostructures photoexcited with ultrashort light pulses exhibit a strong nonlinear optical response driven by nonequilibrium ‘hot’ carriers. Studying the spectro-temporal evolution of such nonlinearities to extract information on hot electron dynamics has attracted significant interest, given the unparalleled opportunities unlocked by these high-energy carriers in fields ranging from photocatalysis to optical communications. However, in typical samples of size-dispersed nanoparticles, effects such as inhomogeneous broadening and pump-pulse-induced selectivity can distort the system response, hindering accurate characterizations. This study dissects the ultrafast response of polydisperse gold nanorods employing two-dimensional electronic spectroscopy (2DES), a powerful technique offering a unique combination of temporal and spectral resolution. The ultrabroadband pulses cover both the transverse and longitudinal nanorod resonances, enabling an accurate analysis of their distinct behavior. By complementing experiments with a quantitative model of hot-carrier-mediated nonlinearities that incorporates sample polydispersity, the broadband excitation, and the nanorods’ resonant absorption, the work provides a comprehensive understanding of the underlying mechanisms and identifies fingerprints of electron–electron scattering in the 2DES maps. Performed on a simple yet prototypical system, this analysis advances the study of plasmonic hot carriers and supports further applications of 2DES to explore ultrafast mechanisms in more advanced hybrid plasmon-based systems, e.g. strongly-coupled complexes.

PtNi Nonlinear Photonics

PtNi octahedral alloy nanoparticles, oil-phase synthesized, depict an alloyed platform for nonlinear photonics. Near-infrared multiwavelength Q-switched operation is realized in 1-, 1.5-, and 2-µm bands with PtNi saturable absorbers. First principles calculations identify surface ferromagnetic Ni as the major absorption center, enabling the enhanced nonlinear response and broadband ultrafast modulation. More details can be found in the Research Article by Bo Fu, Haichang Lu, Wei Zhou, and co-workers (DOI: 10.1002/adom.202502331).

Organic Nanoparticles for Photocatalytic Hydrogen Evaluation

In the Research Article (DOI: 10.1002/adom.202500678), Sae Byeok Jo and co-workers reveal the photophysical governance of “dark” excited states in organic heterojunction nanoparticles in water, enabling highly efficient broadband photocatalytic hydrogen evolutions. By inducing new transition routes that extend charge generation timescale, chronic limitations from the predominance of short-lived singlets are alleviated.