Synthesis of Thermally Stable and Highly Luminescent Cs5 Cu3 Cl6 I2 Nanocrystals with Nonlinear Optical Response.

Low-dimensional Cu(I)-based metal halide materials are gaining attention due to their low toxicity, high stability and unique luminescence mechanism, which is mediated by self-trapped excitons (STEs). Among them, Cs5 Cu3 Cl6 I2 , which emits blue light, is a promising candidate for applications as a next-generation blue-emitting material. In this article, an optimized colloidal process to synthesize uniform Cs5 Cu3 Cl6 I2 nanocrystals (NCs) with a superior quantum yield (QY) is proposed. In addition, precise control of the synthesis parameters, enabling anisotropic growth and emission wavelength shifting is demonstrated. The synthesized Cs5 Cu3 Cl6 I2 NCs have an excellent photoluminescence (PL) retention rate, even at high temperature, and exhibit high stability over multiple heating-cooling cycles under ambient conditions. Moreover, under 850-nm femtosecond laser irradiation, the NCs exhibit three-photon absorption (3PA)-induced PL, highlighting the possibility of utilizing their nonlinear optical properties. Such thermally stable and highly luminescent Cs5 Cu3 Cl6 I2 NCs with nonlinear optical properties overcome the limitations of conventional blue-emitting nanomaterials. These findings provide insights into the mechanism of the colloidal synthesis of Cs5 Cu3 Cl6 I2 NCs and a foundation for further research.

Enhanced Multiphoton Processes in Perovskite Metasurfaces.

Multiphoton absorption and luminescence are fundamentally important nonlinear processes for utilizing efficient light-matter interaction. Resonant enhancement of nonlinear processes has been demonstrated for many nanostructures; however, it is believed that all higher-order processes are always much weaker than their corresponding linear processes. Here, we study multiphoton luminescence from structured surfaces and, combining multiple advantages of perovskites with the concept of metasurfaces, we demonstrate that the efficiency of nonlinear multiphoton processes can become comparable to the efficiency of the linear process. We reveal that the perovskite metasurface can enhance substantially two-photon stimulated emission with the threshold being comparable with that of the one-photon process. Our modeling of free-carrier dynamics and exciton recombination upon nonlinear photoexcitation uncovers that this effect can be attributed to the local field enhancement in structured media, a substantial increase of the mode overlap, and the selection rules of two-photon absorption in perovskites.

Multiphoton Bleaching of Red Fluorescent Proteins and the Ways to Reduce It.

Red fluorescent proteins and biosensors built upon them are potentially beneficial for two-photon laser microscopy (TPLM) because they can image deeper layers of tissue, compared to green fluorescent proteins. However, some publications report on their very fast photobleaching, especially upon excitation at 750-800 nm. Here we study the multiphoton bleaching properties of mCherry, mPlum, tdTomato, and jREX-GECO1, measuring power dependences of photobleaching rates K at different excitation wavelengths across the whole two-photon absorption spectrum. Although all these proteins contain the chromophore with the same chemical structure, the mechanisms of their multiphoton bleaching are different. The number of photons required to initiate a photochemical reaction varies, depending on wavelength and power, from 2 (all four proteins) to 3 (jREX-GECO1) to 4 (mCherry, mPlum, tdTomato), and even up to 8 (tdTomato). We found that at sufficiently low excitation power P, the rate K often follows a quadratic power dependence, that turns into higher order dependence (K~Pα with α > 2) when the power surpasses a particular threshold P*. An optimum intensity for TPLM is close to the P*, because it provides the highest signal-to-background ratio and any further reduction of laser intensity would not improve the fluorescence/bleaching rate ratio. Additionally, one should avoid using wavelengths shorter than a particular threshold to avoid fast bleaching due to multiphoton ionization.

Highly Efficient Multiphoton Absorption of Zinc-AIEgen Metal-Organic Frameworks.

A series of luminescent frameworks was synthesized from the selective combination of aggregation induced emission (AIE)-linker tetra-(4-carboxylphenyl)ethylene (H4 TCPE) and Zn2+ . Complex 1 was formed by the close packing of Zn-TCPE hinge, and isostructural complexes 2-5 were constructed by the linkage of Zn-TCPE layer and pillar ligands. These complexes exhibit highly efficient multiphoton excited photoluminescence (MEPL) and concomitant third-harmonic generation (THG). The multiphoton absorption (MPA) parameters of 1 are superior to other multiphoton emission materials including the perovskite nanocrystals. The incorporation of pillar linkers slows down the charge transfer between layers of Zn-TCPE, and the aromatic core of pillar linkers has a great influence on the MPA performance of the corresponding frameworks.

Frequency-Upconverted Stimulated Emission by Up to Six-Photon Excitation from Highly Extended Spiro-Fused Ladder-Type Oligo(p-phenylene)s.

Frequency-upconverted fluorescence and stimulated emission induced by multiphoton absorption (MPA) have attracted much interest. As compared with low-order MPA processes, the construction of high-order MPA processes is highly desirable and rather attractive, yet remains a formidable challenge due to its inherent low transition probability. We report the observation of the first experimental frequency-upconverted fluorescence and stimulated emission by simultaneous six-photon excitation in an organic molecular system. The well-designed organic conjugated system based on cross-shaped spiro-fused ladder-type oligo(p-phenylene)s (SpL-z, z=1-3) manifests reasonably high MPA cross-sections and brilliant luminescence emission simultaneously. The six-photon absorption cross-section of SpL-3 with an extended π-conjugation was evaluated as 8.67×10-169  cm12 s5 photon-5 . Exceptionally efficient 2- to 6-photon excited stimulated emission was achieved under near-infrared laser excitation.

Dynamic Supramolecular Ruthenium-Based Gels Responsive to Visible/NIR Light and Heat.

A simple supramolecular crosslinked gel is reported with a photosensitive ruthenium bipyridine complex functioning as a crosslinker and poly(4-vinylpyridine) (P4VP) as a macromolecular ligand. Irradiation of the organogels in H2 O/MeOH with visible and NIR light (in a multiphoton process) leads to cleavage of pyridine moieties from the ruthenium complex breaking the cross-links and causing degelation and hence solubilization of the P4VP chains. Real-time (RT) photorheology experiments of thin films showed a rapid degelation in several seconds, whereas larger bulk samples could also be photocleaved. Furthermore, the gels could be reformed or healed by simple heating of the system and restoration of the metal-ligand crosslinks. The relatively simple dynamic system with a high sensitivity towards light in the visible and NIR region make them interesting positive photoresists for nano/micropatterning applications, as was demonstrated by writing, erasing, and rewriting of the gels by single- and multiphoton lithography.

Evidence of Band-Edge Hole Levels Inversion in Spherical CuInS2 Quantum Dots.

CuInS2 (CIS) quantum dots (QDs) have emerged as one of the most promising candidates for application in a number of new technologies, mostly due to their heavy-metal-free composition and their unique optical properties. Among those, the large Stokes shift and the long-lived excited state are the most striking ones. Although these properties are important, the physical mechanism that originates them is still under debate. Here, we use two-photon absorption spectroscopy and ultrafast dynamics studies to investigate the physical origin of those phenomena. From the two-photon absorption spectroscopy, we observe yet another unique property of CIS QDs, a two-photon absorption transition below the one-photon absorption band edge, which has never been observed before for any other semiconductor nanostructure. This originates from the inversion of the 1S and 1P hole level order at the top of the valence band and results in a blue-shift of the experimentally measured one photon absorption edge by nearly 100 to 200 meV. However, this shift is not large enough to account for the Stokes shift observed, 200-500 meV. Consequently, despite the existence of the below band gap optical transition, photoluminescence in CIS QDs must originate from trap sites. These conclusions are reinforced by the multiexciton dynamics studies. From those, we demonstrate that biexciton Auger recombination behaves similarly to negative trion dynamics on these nanomaterials, which suggests that the trap state is an electron donating site.

Tunable Spectrum Selectivity for Multiphoton Absorption with Enhanced Visible Light Trapping in ZnO Nanorods.

Observation of visible light trapping in zinc oxide (ZnO) nanorods (NRs) correlated to the optical and photoelectrochemical properties is reported. In this study, ZnO NR diameter and c-axis length respond primarily at two different regions, UV and visible light, respectively. ZnO NR diameter exhibits UV absorption where large ZnO NR diameter area increases light absorption ability leading to high efficient electron-hole pair separation. On the other hand, ZnO NR c-axis length has a dominant effect in visible light resulting from a multiphoton absorption mechanism due to light reflection and trapping behavior in the free space between adjacent ZnO NRs. Furthermore, oxygen vacancies and defects in ZnO NRs are associated with the broad visible emission band of different energy levels also highlighting the possibility of the multiphoton absorption mechanism. It is demonstrated that the minimum average of ZnO NR c-axis length must satisfy the linear regression model of Z p,min = 6.31d to initiate the multiphoton absorption mechanism under visible light. This work indicates the broadening of absorption spectrum from UV to visible light region by incorporating a controllable diameter and c-axis length on vertically aligned ZnO NRs, which is important in optimizing the design and functionality of electronic devices based on light absorption mechanism.

Highly Efficient Multiphoton-Pumped Frequency-Upconversion Stimulated Blue Emission with Ultralow Threshold from Highly Extended Ladder-Type Oligo(p-phenylene)s.

A series of highly extended π-conjugated ladder-type oligo(p-phenylene)s containing up to 10 phenyl rings with (L)-Ph(n)-NPh (n=7-10) or without diphenylamino endcaps (L)-Ph(n) (n=7 and 8) were synthesized and investigated for their multiphoton absorption properties for frequency upconverted blue ASE/lasing. Extremely large two-photon absorption (2PA) cross-sections and highly efficient 2PA ASE/lasing with ultralow threshold were achieved. (L)-Ph(10)-NPh exhibits the highest intrinsic 2PA cross-section of 3643 GM for a blue emissive organic fluorophore reported so far. The record-high 2PA pumped ASE/lasing efficiency of 2.06 % was obtained by un-endcapped oligomer, (L)-Ph(8) rather than that with larger σ2 , suggesting that a molecule with larger σ2 is not guaranteed to exhibit higher η2 . All of these oligomers exhibit exceptionally ultralow 2PA pumped ASE/lasing thresholds, among which the lowest 2PA pumped threshold of circa 0.26 µJ was achieved by (L)-Ph(10)-NPh.

Harnessing Metal-Organic Frameworks for NIR-II Light-Driven Multiphoton Photocatalytic Water Splitting in Hydrogen Therapy.

The construction of near-infrared (NIR) light-activated hydrogen-producing materials that enable the controlled generation and high-concentration release of hydrogen molecules in deep tumor tissues and enhance the effects of hydrogen therapy holds significant scientific importance. To address the key technical challenge of low-efficiency oxidation-reduction reactions for narrow-bandgap photocatalytic materials, this work proposes an innovative approach for the controllable fabrication of multiphoton photocatalytic materials to overcome the limitations imposed by traditional near-infrared photocatalysts with "narrow-bandgap" constraints. Herein, an NIR-responsive multiphoton photocatalyst, ZrTc-Co, is developed by utilizing a post-synthetic coordination modification strategy to introduce hydrogenation active site CoII into a multiphoton responsive MOF (ZrTc). The results reveal that with the introduction of the CoII site, electron-hole recombination can be efficiently suppressed, thus promoting the efficiency of hydrogen evolution reaction. In addition, the integration of CoII can effectively enhance charge transfer and improve static hyperpolarizability, which endows ZrTc-Co with excellent multiphoton absorption. Moreover, hyaluronic acid modification endows ZrTc-Co with cancer cell-specific targeting characteristics, laying the foundation for tumor-specific elimination. Collectively, the proposed findings present a strategy for constructing NIR-II light-mediated hydrogen therapeutic agents for deep tumor elimination.

Dramatically Enhancing Multiphoton Harvesting Metal-Organic Frameworks for NIR-II Photocatalysis through Functional Regulation of Octupolar Molecules.

The development of effective multiphoton absorption (MPA) materials for near-infrared (NIR) light-driven photocatalysis holds great significance. In this study, we incorporated two multibranched cyclometallated iridium(III) modules with varying degrees of conjugation onto MPA-inert metal-organic frameworks (MOFs) to active MPA performance. Subsequently, the MOFs were further modified with Co(II) and hyaluronic acid (HA) to fabricate MINCH and MISCH, respectively. By introducing octupolar molecules and expanding the conjugation, MISCH exhibited a larger MPA cross section for efficient NIR light absorption and improved carrier transfer, leading to outstanding NIR light-driven multiphoton photocatalytic hydrogen production. Moreover, the HA modification enabled MISCH to achieve specific multiphoton photocatalytic hydrogen therapy for cancer cells. This study provides valuable insights into constructing highly active MPA materials for NIR light-driven photocatalysis, presenting a potential platform for hydrogen therapy in tumor treatment.

Manipulating Charge Distribution of Graphitic Carbon Nitride for Boosting NIR-II Light-Activated Reactive Oxygen Species Generation.

Structure engineering is of great importance to enhance the carrier separation efficiency of multiphoton absorption (MPA) materials for near-infrared (NIR) light-driven reactive oxygen species (ROS) generation. In this study, the MPA-responsive potassium/cyano group-functionalized graphitic carbon nitride was investigated, demonstrating charge redistribution and improved carrier separation efficiency by density functional theory calculations and experimental results. With various types of boosted ROS generation under UV-vis or NIR-II light irradiation, the potassium/cyano group-functionalized graphitic carbon nitride could achieve efficient multiphoton photodynamic therapy after reducing the particle size. This study developed a simple strategy to manipulate charge distribution for booting NIR light-activated ROS generation in efficient multiphoton photodynamic therapy.

Deep subsurface cavities in skin utilizing mechanical optical clearing and femtosecond laser ablation.

BACKGROUND AND OBJECTIVES: High precision subsurface ablation can be produced in transparent materials using femtosecond laser pulses and multiphoton absorption. Light scattering limits application of the same technique to most biological tissues. Previously, subsurface ablation was demonstrated at superficial depths (50-250 µm) in highly scattering tissues including murine skin and human sclera. We report application of mechanical optical clearing to produce deeper subsurface femtosecond ablation in rodent skin. Ability to target deeper structures in skin using subsurface ablation may allow novel clinical applications for dermatological laser surgery. STUDY DESIGN/MATERIALS AND METHODS: Operation of a prototype tissue optical clearing device (TOCD) was verified with white light photography in ex vivo rodent skin. A focused femtosecond beam transmitted through the TOCD and was scanned across rodent skin to produce subsurface ablation at increasing focal depths. Histological sections with H&E staining of the laser irradiated rodent skin were examined for subsurface ablation features following laser irradiation. RESULTS: Subsurface cavities were observed as deep as 1.7 mm below the skin surface in histological tissue sections. Diameter of subsurface cavities varied from tens of microns to over 100 µm. Subsurface cavities produced by scanning the focused femtosecond beam were contiguous and formed a continuous cut. Mechanical disruption of the overlying tissues was not observed. CONCLUSIONS: Mechanical optical clearing can be applied directly to in situ rodent skin and produces an optical clearing effect. High precision subsurface ablation can be produced at positions substantially deeper than previously demonstrated. Future studies may be targeted in in vivo human skin to investigate potential clinical applications of subsurface femtosecond ablation using mechanical optical clearing.

Excellent Multiphoton Excitation Fluorescence with Large Multiphoton Absorption Cross Sections of Arginine-Modified Gold Nanoclusters for Bioimaging.

Fluorescent gold nanoclusters (Au NCs) with excellent one-photon and multiphoton properties have been demonstrated as promising candidates in many application fields. However, small multiphoton absorption (MPA) cross sections and weak multiphoton excitation (MPE) fluorescence impede their practical applications under near-infrared (NIR) excitation for biological imaging. Here, we report the regulated one-photon and multiphoton properties and mechanisms of arginine-stabilized 6-aza-2-thiothymine Au NCs (Arg/ATT-Au NCs) and the applications for MPE fluorescence imaging. The introduction of arginine into the capping layer of ATT-Au NCs significantly modifies the electronic structure, the absorption cross sections, and the relaxation dynamics of the lowest excited state, drastically reducing the nonradiative relaxation, suppressing the blinking, and greatly enhancing the fluorescence. Besides the improved one-photon properties, Arg/ATT-Au NCs demonstrate remarkable MPE fluorescence with a large MPA cross section. The two-photon (λex = 850 nm), three-photon (λex = 1400 nm), and four-photon (λex = 1700 nm) absorption cross sections have been determined to be 6.1 × 10-47 cm4 s1 photon-1, 1.5 × 10-78 cm6 s2 photon-2, and 5.5 × 10-108 cm8 s3 photon-3, respectively, much higher than those of conventional organic compounds and previously reported Au NCs. Moreover, Arg/ATT-Au NCs have been successfully applied in two-photon and three-photon excitation fluorescence imaging of living cells with NIR excitation. The manifold advantages of small size, high quantum yield, suppressed blinking, good photostability and cytocompatibility, large MPA cross sections, and excellent MPE fluorescence imaging performances make fluorescent Arg/ATT-Au NCs a great candidate of imaging probes with vis-NIR excitation.

Multiphoton Absorption Simulation of Sapphire Substrate under the Action of Femtosecond Laser for Larger Density of Pattern-Related Process Windows.

It is essential to develop pattern-related process windows on substrate surface for reducing the dislocation density of wide bandgap semiconductor film growth. For extremely high instantaneous intensity and excellent photon absorption rate, femtosecond lasers are currently being increasingly adopted. However, the mechanism of the femtosecond laser developing pattern-related process windows on the substrate remains to be further revealed. In this paper, a model is established based on the Fokker-Planck equation and the two-temperature model (TTM) equation to simulate the ablation of a sapphire substrate under the action of a femtosecond laser. The transient nonlinear evolutions such as free electron density, absorption coefficient, and electron-lattice temperature are obtained. This paper focuses on simulating the multiphoton absorption of sapphire under femtosecond lasers of different wavelengths. The results show that within the range of 400 to 1030 nm, when the wavelength is large, the number of multiphoton required for ionization is larger, and wider and shallower ablation pits can be obtained. When the wavelength is smaller, the number of multiphoton is smaller, narrower and deeper ablation pits can be obtained. Under the simulation conditions presented in this paper, the minimum ablation pit depth can reach 0.11 µm and the minimum radius can reach 0.6 µm. In the range of 400 to 1030 nm, selecting a laser with a shorter wavelength can achieve pattern-related process windows with a smaller diameter, which is beneficial to increase the density of pattern-related process windows on the substrate surface. The simulation is consistent with existing theories and experimental results, and further reveals the transient nonlinear mechanism of the femtosecond laser developing the pattern-related process windows on the sapphire substrate.

Clusters formation and fragmentation of nitromethane at 266 nm.

We carry out experiments on the fragmentation of nitromethane by multiphoton absorption at the wavelength 266 nm. This was conducted in a reflectron (Jordan), modified in the laboratory. Due to the large number of fragments, special care has been taken into the calibration of the system, in the simultaneity between the laser pulse and the sample, and the associated electronics to ensure that produced fragment spectra arise from the interaction laser-sample. We emphasize the next aspects of the method:•Simple design for introducing a gas sample at laser interaction region to facilitate the cluster formation•Astonishing number of fragments produced by multiphoton absorption.

Halide Perovskites for Nonlinear Optics.

Halide perovskites provide an ideal platform for engineering highly promising semiconductor materials for a wide range of applications in optoelectronic devices, such as photovoltaics, light-emitting diodes, photodetectors, and lasers. More recently, increasing research efforts have been directed toward the nonlinear optical properties of halide perovskites because of their unique chemical and electronic properties, which are of crucial importance for advancing their applications in next-generation photonic devices. Here, the current state of the art in the field of nonlinear optics (NLO) in halide perovskite materials is reviewed. Halide perovskites are categorized into hybrid organic/inorganic and pure inorganic ones, and their second-, third-, and higher-order NLO properties are summarized. The performance of halide perovskite materials in NLO devices such as upconversion lasers and ultrafast laser modulators is analyzed. Several potential perspectives and research directions of these promising materials for nonlinear optics are presented.

Plasmon-Enhanced Resonant Photoemission Using Atomically Thick Dielectric Coatings.

By proposing an atomically thick dielectric coating on a metal nanoemitter, we theoretically show that the optical field tunneling of ultrafast-laser-induced photoemission can occur at an ultralow incident field strength of 0.03 V/nm. This coating strongly confines plasmonic fields and provides secondary field enhancement beyond the geometrical plasmon field enhancement effect, which can substantially reduce the barrier and enable more efficient photoemission. We numerically demonstrate that a 1 nm thick layer of SiO2 around a Au-nanopyramid will enhance the resonant photoemission current density by 2 orders of magnitude, where the transition from multiphoton absorption to optical field tunneling is accessed at an incident laser intensity at least 10 times lower than that of the bare nanoemitter. The effects of the coating properties such as refractive index, thickness, and geometrical settings are studied, and tunable photoemission is numerically demonstrated by using different ultrafast lasers. Our approach can also directly be extended to nonmetal emitters, to-for example-2D material coatings, and to plasmon-induced hot carrier generation.

Maskless 3D Ablation of Precise Microhole Structures in Plastics Using Femtosecond Laser Pulses.

Femtosecond laser ablation is a robust tool for the fabrication of microhole structures. This technique has several advantages compared to other microfabrication strategies for reliably preparing microhole structures of high quality and low cost. However, few studies have explored the use of femtosecond laser ablation in plastic materials because of the lack of controllability over the fabrication process in plastics. In particular, the depth profile of microhole structures prepared by conventional laser ablation techniques in plastics cannot be precisely and reproducibly controlled. In this paper, a novel three-dimensional femtosecond laser ablation technique was developed for the rapid fabrication of precise microhole structures in multiple plastics in air. Using a three-step fabrication scheme, microholes demonstrated extremely clean and sharp geometric features. This new technique also enables the precise creation of arbitrary-shaped microwell structures in plastic substrates through a rapid single-step ablation process, without the need for any masks. As a proof of concept for practical applications, precise microhole structures prepared by this novel femtosecond laser ablation technique were exploited for robust resistive-pulse sensing of microparticles.

Precise Spatially Selective Photothermolysis Using Modulated Femtosecond Lasers and Real-time Multimodal Microscopy Monitoring.

The successful application of lasers in the treatment of skin diseases and cosmetic surgery is largely based on the principle of conventional selective photothermolysis which relies strongly on the difference in the absorption between the therapeutic target and its surroundings. However, when the differentiation in absorption is not sufficient, collateral damage would occur due to indiscriminate and nonspecific tissue heating. To deal with such cases, we introduce a novel spatially selective photothermolysis method based on multiphoton absorption in which the radiant energy of a tightly focused near-infrared femtosecond laser beam can be directed spatially by aiming the laser focal point to the target of interest. We construct a multimodal optical microscope to perform and monitor the spatially selective photothermolysis. We demonstrate that precise alteration of the targeted tissue is achieved while leaving surrounding tissue intact by choosing appropriate femtosecond laser exposure with multimodal optical microscopy monitoring in real time.