Three-Dimensionally Printed, Vertical Full-Color Display Pixels for Multiplexed Anticounterfeiting.

Information encryption strategies have become increasingly essential. Most of the fluorescent security patterns have been made with a lateral configuration of red, green, and blue subpixels, limiting the pixel density and security level. Here we report vertically stacked, luminescent heterojunction micropixels that construct high-resolution, multiplexed anticounterfeiting labels. This is enabled by meniscus-guided three-dimensional (3D) microprinting of red, green, and blue (RGB) dye-doped materials. High-precision vertical stacking of subpixel segments achieves full-color pixels without sacrificing lateral resolution, achieving a small pixel size of ∼µm and a high density of over 13,000 pixels per inch. Furthermore, a full-scale color synthesis for individual pixels is developed by modulating the lengths of the RGB subpixels. Taking advantage of these unique 3D structural designs, trichannel multiplexed anticounterfeiting Quick Response codes are successfully demonstrated. We expect that this work will advance data encryption technology while also providing a versatile manufacturing platform for diverse 3D display devices.

Two-Photon Lasing from Two-Dimensional Homologous Ruddlesden-Popper Perovskite with Giant Nonlinear Absorption and Natural Microcavities.

Two-dimensional Ruddlesden-Popper perovskites (RPPs) with multiple quantum well-like structures, strong excitonic quantum confinement, and high stability are promising optical gain media. However, the lasing from such material with a small number of inorganic well layers is difficult to achieve. Herein, we demonstrate the low-threshold upconversion lasing from the homologous RPP (PEA)2(MA)n-1PbnI3n+1 (n = 2 and 3) microflakes with wavelength varies from 598 to 637 nm under 800 nm laser excitation at low temperature (≤153 K). Using the micro Z-scan technique, we discovered that the RPP flakes have a giant two-photon absorption coefficient ß as high as 3.6 × 103 cm GW-1, resulting in the effective upconversion transition under two-photon excitation. Furthermore, the self-formation of Fabry-Pérot microcavities provides the support for lasing emission from the n ≥ 2 RPP flakes. Calculation results and microscopic transient absorption measurements reveal that low-threshold lasing is due to the high differential gain coefficient and the suppressed nonradiative Auger recombination rate inside the quantum confinement structures. These properties enable RPPs as potential gain media for developing upconversion microcavity lasers.

Nonlinear Optical Activities in Two-Dimensional Gallium Sulfide: A Comprehensive Study.

The nonlinear optical (NLO) properties of two-dimensional (2D) materials are fascinating for fundamental physics and optoelectronic device development. However, relatively few investigations have been conducted to establish the combined NLO activities of a 2D material. Herein, a study of numerous NLO properties of 2D gallium sulfide (GaS), including second-harmonic generation (SHG), two-photon excited fluorescence (TPEF), and NLO absorption are presented. The layer-dependent SHG response of 2D GaS identifies the noncentrosymmetric nature of the odd layers, and the second-order susceptibility (χ2) value of 47.98 pm/V (three-layers of GaS) indicates the superior efficiency of the SHG signal. In addition, structural deformation induces the symmetry breaking and facilitates the SHG in the bulk samples, whereas a possible efficient symmetry breaking in the liquid-phase exfoliated samples results in an enhancement of the SHG signal, providing prospective fields of investigation for researchers. The generation of TPEF from 800 to 860 nm depicts the two-photon absorption characteristics of 2D GaS material. Moreover, the saturable absorption characteristics of 2D GaS are realized from the largest nonlinear absorption coefficient (ß) of -9.3 × 103, -91.0 × 103, and -6.05 × 103 cm/GW and giant modulation depths (Ts) of 24.4%, 35.3%, and 29.1% at three different wavelengths of 800, 1066, and 1560 nm, respectively. Hence, such NLO activities indicate that 2D GaS material can facilitate in the technical advancements of future nonlinear optoelectronic devices.

Unlocking surface octahedral tilt in two-dimensional Ruddlesden-Popper perovskites.

Molecularly soft organic-inorganic hybrid perovskites are susceptible to dynamic instabilities of the lattice called octahedral tilt, which directly impacts their carrier transport and exciton-phonon coupling. Although the structural phase transitions associated with octahedral tilt has been extensively studied in 3D hybrid halide perovskites, its impact in hybrid 2D perovskites is not well understood. Here, we used scanning tunneling microscopy (STM) to directly visualize surface octahedral tilt in freshly exfoliated 2D Ruddlesden-Popper perovskites (RPPs) across the homologous series, whereby the steric hindrance imposed by long organic cations is unlocked by exfoliation. The experimentally determined octahedral tilts from n = 1 to n = 4 RPPs from STM images are found to agree very well with out-of-plane surface octahedral tilts predicted by density functional theory calculations. The surface-enhanced octahedral tilt is correlated to excitonic redshift observed in photoluminescence (PL), and it enhances inversion asymmetry normal to the direction of quantum well and promotes Rashba spin splitting for n > 1.

Robust and Flexible Random Lasers Using Perovskite Quantum Dots Coated Nickel Foam for Speckle-Free Laser Imaging.

The advantage of using flexible metallic structures as the substrate of flexible lasers over plastic materials is its strong mechanical strength and high thermal conductivity. Here, it is proposed to deposit CsPbBr3 perovskite quantum dots onto Ni porous foam for the realization of flexible lasers. Under two-photon 800 nm excitation at room temperature, incoherent random lasing emission is observed at ≈537 nm. By external deformation of the Ni porous foam, incoherent random lasing can be tuned to amplified spontaneous emission as well as the corresponding lasing threshold be controlled. More importantly, it is demonstrated that the laser is robust to intensive bending (>1000 bending cycles) with minimum effect on the lasing intensity. This flexible laser is also shown to be an ideal light source to produce a "speckle" free micro-image.

Ultraviolet C lasing at 263 nm from Ba2LaF7:Yb3+,Tm3+ upconversion nanocrystal microcavities.

It is a daunting challenge to realize ultraviolet C (UVC) lasing (i.e., has a wavelength range from 200 to 275 nm) from upconversion nanocrystals due to their low upconversion efficiency. Here, we fabricate Ba2LaF7:Yb3+(90mol%), Tm3+(5mol%) upconversion nanocrystals from amorphous borosilicate glass to support emission at ∼263nm under 980 nm ns laser excitation. The excitation threshold can be further reduced from ∼130 to ∼26.5mJ/cm2 by using a cylindrical microcavity. We also found that the growth of defect-free Ba2LaF7 nanocrystals with a high concentration of codoping Yb3+ and Tm3+ ions inside high optical damage threshold borosilicate glass is the key to achieving room-temperature UVC upconversion lasing under high-intensity excitation.

Ultrashort laser pulse doubling by metal-halide perovskite multiple quantum wells.

Multiple ultrashort laser pulses are widely used in optical spectroscopy, optoelectronic manipulation, optical imaging and optical signal processing etc. The laser pulse multiplication, so far, is solely realized by using the optical setups or devices to modify the output laser pulse from the optical gain medium. The employment of these external techniques is because the gain medium itself is incapable of modifying or multiplying the generated laser pulse. Herein, with single femtosecond laser pulse excitation, we achieve the double-pulsed stimulated emission with pulse duration of around 40 ps and pulse interval of around 70 ps from metal-halide perovskite multiple quantum wells. These unique stimulated emissions originate from one fast vertical and the other slow lateral high-efficiency carrier funneling from low-dimensional to high-dimensional quantum wells. Furthermore, such gain medium surprisingly possesses nearly Auger-free stimulated emission. These insights enable us a fresh approach to multiple the ultrashort laser pulse by gain medium.

Deep UV random lasing from NaGdF4:Yb3+,Tm3+ upconversion nanocrystals in amorphous borosilicate glass.

The realization of lanthanide-doped upconversion nanocrystals embedded in a robust and transparent solid medium is highly desired to achieve deep UV (<300nm) lasing. Here, we fabricate NaGdF4:Yb3+,Tm3+ upconversion nanocrystals inside amorphous borosilicate glass to support ∼290nm random lasing emission under 980 nm excitation. We found that with careful control of the growth process, which is the key to achieve high-crystallinity nanocrystals, the nanocrystals can suppress defect-related quenching and enhance 1I6→3H6(6IJ→8S7/2) transition of Tm3+ (Gd3+) ions under five-photon absorption excitation so that high optical gain (>45cm-1) at ∼290nm can be obtained.

Atomic-Scale Insights into the Dynamics of Growth and Degradation of All-Inorganic Perovskite Nanocrystals.

An understanding of growth and degradation pathways is significant to solve the problem of the structural instability of all-inorganic perovskite nanocrystals (NCs). However, it is still a great challenge to directly record such dynamic processes with high spatial resolution owing to the existence of complex internal factors even using in situ transmission electron microscopy observation. Here, we employ a glassy matrix to produce CsPbBr3 NCs to ensure that the growth and degradation processes of CsPbBr3 NCs are recorded in the vacuum chamber, which could avoid the influence of the external factors, under electron beam (E-beam) irradiation. In addition, two stages of degradation pathways induced by the E-beam are observed sequentially: (1) a layer-by-layer decomposition and (2) instantaneous vanishing once the radius reaches the critical radius (∼2.3 nm). Indeed, we demonstrated that defects serve as a key flash point that could trigger the structural collapse of CsPbBr3 NCs. Our findings provide critical insights into the general instability issue of all-inorganic perovskite NCs in practical applications.

Enhanced laser speckle optical sensor for in situ strain sensing and structural health monitoring.

In this Letter, an enhanced laser speckle optical sensor (LSOS) for nondestructive, noncontact, and high-accuracy strain measurement has been developed. Subsystems of laser beam shaping and telecentric imaging were incorporated into the LSOS to achieve optimized speckle patterns, and a field-of-view (FOV) separation was introduced to extend sensor gauge length. Validation tests confirmed that the LSOS achieved consistent results with resistive strain gauges in laboratory conditions with maximum RMS error (RMSE) of $ 9.44\;\unicode{x00B5} \unicode{x03B5} $9.44µÎµ. Sensing practicality was demonstrated in field tests. The results showed that the LSOS is capable of achieving accurate strain measurements in an external environment with maximum RMSE of $ 13.34\;\unicode{x00B5}\unicode{x03B5} $13.34µÎµ.

Study of Crystallization and Coalescence of Nanocrystals in Amorphous Glass at High Temperature.

Amorphous fluoride glass is used as the reaction chamber (i.e., solid cell) to grow Ba2LaF7 (BLF) nanocrystals at elevation temperatures (i.e., 300-500 °C) so that in situ real-time crystallization and coalescence of BLF nanocrystals can be observed. Due to the inherent advantages of the liquid-like solid medium, high temporal and spatial resolution transmission electron microscopy images can be obtained. Hence, we reveal that the twinned and quadruplet BLF nanocrystals are formed at low temperature (≤430 °C) and the unification of two nanocrystals via the two pathways (i.e., migration with and without rotation) to a single defect-free BLF nanocrystal is favored at high temperature (≥470 °C).

Highly efficient and ultra-narrow bandwidth orange emissive carbon dots for microcavity lasers.

Luminescent materials with high efficiency, narrow emission bandwidth and long emission wavelength have attracted extensive attention in recent years. However, for novel luminescent carbon dots, it is still a major challenge to obtain these properties simultaneously. Here, this type of carbon dot was proposed using 1,4-diaminonaphthalene as the initial source. The carbon dots demonstrate strong orange emission with the highest quantum yield of 82% and an extremely narrow emission bandwidth of 30 nm. It is found that the orange emission of carbon dots is attributed to the high defect amounts including nitrogen and oxygen doping. The high carboxyl group content leads to a high efficiency and the uniform size distribution results in a narrow bandwidth. The carbon dots are used as the gain medium of a whispering gallery mode microcavity laser. A low excitation threshold of 12 kW cm-2 and a high quality factor of ∼3600 can be obtained from the microcavity lasers. This work has provided a didactic example to develop high-quality long emission-wavelength carbon dots with strong emission and an ultra-narrow emission bandwidth, which makes it possible to expand the application of original and high-performance lasers or other optical devices.

Electrochemically assisted flexible lanthanide upconversion luminescence sensing of heavy metal contamination with high sensitivity and selectivity.

Heavy metal contamination in water can pose lethal threats to public health; therefore it is highly desired to develop a rapid and sensitive sensor for monitoring water quality. Owing to their superior optical features, upconversion nanoparticles (UCNPs) are widely explored to detect metal ions based on resonance energy transfer to dye quenchers. However, these schemes heavily rely on the optical properties of the molecules, which limits the flexibility of the probe design. Herein, a flexible carbon fiber cloth/UCNP composite probe was fabricated for sensing copper(ii) (Cu2+) ions and an electrochemical (E-chem) technique was implemented for the first time to enhance its sensing performance. By applying 0.3 V on the composite probe, Cu2+ ions can be effectively accumulated through oxidation, yielding a remarkable improvement in the selectivity and sensitivity. A more outstanding detection limit of the sensor was achieved at 82 ppb under the E-chem assistance, with 300-fold enhancement compared to the detection without the E-chem effect. This sensing approach can be an alternative to molecular quenchers and open up new possibilities for simple, rapid and portable sensing of metal ions.

Influence of Plasmonic Effect on the Upconversion Emission Characteristics of NaYF4 Hexagonal Microrods.

The influence of plasmonic effect on the upconversion emission characteristics of Yb3+-Er3+-Tm3+ tridoped ß-NaYF4 hexagonal microrods is studied. Upconversion spontaneous emission can be improved by 10 times if the microrod is deposited on an Ag-coated substrate. The enhancement is also dependent on the emission wavelength and the polarization of the excitation source. Furthermore, upconversion lasing is supported by the geometry of the microrods via the formation of whispering gallery modes. The corresponding excitation threshold can also be reduced by 50% through the influence of plasmonic effect, the coupling between the whispering gallery modes and the surface plasmonic resonance modes.

Multiphoton Upconversion Emission from Diamond Single Crystals.

Room-temperature upconversion emission up to eight-photon absorption is demonstrated from diamond single crystals under femtosecond laser excitation for the first time. The low concentration of defects and impurities is attributed to the support of free excitons emission at 235 nm. Nonlinear optical properties are also investigated by using an open-aperture Z-scan technique. The corresponding three-, five-, and eight-photon absorption coefficients of the diamonds are found to be 1.8 × 10-2 cm3/GW2, 5 × 10-9 cm7/GW4, and 1.6 × 10-19 cm13/GW7, respectively. Considering its high hardness and high thermal conductivity, diamonds are a versatile nonlinear optical material suitable for high-power deep ultraviolet applications under multiphoton excitation.

Frequency upconverted amplified spontaneous emission and lasing from inorganic perovskite under simultaneous six-photon absorption.

Multiphoton pumped stimulated emission requires simultaneous absorption of photons for the creation of population inversion to sustain optical amplification. Recently, stimulated emission by simultaneous absorption of up to five photons has been realized. To achieve more diverse nonlinear optical applications, it is desired to have more photons involved in the upconversion process. Here, we demonstrate unambiguously frequency upconverted amplified spontaneous emission and lasing via simultaneous six-photon absorption from inorganic perovskite. Our finding allows the utilization of inorganic perovskite as the novel alternative for higher-order multiphoton fluorescent applications.

Direct Identification of Surface Defects and Their Influence on the Optical Characteristics of Upconversion Nanoparticles.

Core-shell structure is an obvious concept to suppress surface-related deactivations in lanthanide-doped upconversion nanoparticles (UCNPs). However, no direct observation of atomic-scale surface restoration, which can improve the upconversion photoluminescence, has been reported. Here, we use aberration-corrected high-angle annular dark field scanning transmission electron microscopy to study the surface condition of KLu2F7:Yb3+,Er3+ bare core UCNPs. Due to the very thin and uniform thickness of the UCNPs, we observe unambiguously that the recovery from surface defects enhances upconversion photoluminescence. Furthermore, the realization of dominant green lasing emission under pulsed laser excitation confirms the high crystallinity of the UCNPs.

Broadband Ce(III)-Sensitized Quantum Cutting in Core-Shell Nanoparticles: Mechanistic Investigation and Photovoltaic Application.

Quantum cutting in lanthanide-doped luminescent materials is promising for applications such as solar cells, mercury-free lamps, and plasma panel displays because of the ability to emit multiple photons for each absorbed higher-energy photon. Herein, a broadband Ce3+-sensitized quantum cutting process in Nd3+ ions is reported though gadolinium sublattice-mediated energy migration in a NaGdF4:Ce@NaGdF4:Nd@NaYF4 nanostructure. The Nd3+ ions show downconversion of one ultraviolet photon through two successive energy transitions, resulting in one visible photon and one near-infrared (NIR) photon. A class of NaGdF4:Ce@NaGdF4:Nd/Yb@NaYF4 nanoparticles is further developed to expand the spectrum of quantum cutting in the NIR. When the quantum cutting nanoparticles are incorporated into a hybrid crystalline silicon (c-Si) solar cell, a 1.2-fold increase in short-circuit current and a 1.4-fold increase in power conversion efficiency is demonstrated under short-wavelength ultraviolet irradiation. These insights should enhance our ability to control and utilize spectral downconversion with lanthanide ions.

Energy Migration Upconversion in Ce(III)-Doped Heterogeneous Core-Shell-Shell Nanoparticles.

One major challenge in upconversion research is to develop new materials and structures to expand the emission spectrum. Herein, a heterogeneous core-shell-shell nanostructure of NaYbF4 :Gd/Tm@NaGdF4 @CaF2 :Ce is developed to realize efficient photon upconversion in Ce3+ ions through a Gd-mediated energy migration process. The design takes advantage of CaF2 host that reduces the 4f-5d excitation frequency of Ce3+ to match the emission line of Gd3+ . Meanwhile, CaF2 is isostructural with NaGdF4 and can form a continuous crystalline lattice with the core layer. As a result, effective Yb3+ → Tm3+ → Gd3+ → Ce3+ energy transfer can be established in a single nanoparticle. This effect enables efficient ultraviolet emission of Ce3+ following near infrared excitation into the core layer. The Ce3+ upconversion emission achieved in the core-shell-shell nanoparticles features broad bandwidth and long lifetime, which offers exciting opportunities of realizing tunable lasing emissions in the ultraviolet spectral region.

Realization of multiphoton lasing from carbon nanodot microcavities.

The use of organosilane chains to link carbon nanodots (CDs) through organosilane surface functional groups is proposed to improve the efficiency of multiphoton absorption. As a result, a large absorption coefficient of 1.16 × 10-6 cm5 per GW3 is obtained and four-photon luminescence under 1900 nm excitation is observed from the CDs at room temperature. Furthermore, a CD laser, which demonstrates random lasing under three-photon (i.e. 1400 nm) excitation, can be realized by sandwiching a CD film between a quartz substrate and a dielectric mirror. The formation of strongly confined microcavities, which arise from the non-uniform distribution of refractive indices inside the CD film, is attributed to the realization of lasing emission.