Coffee Ring Effect Enhanced Surface Plasmon Resonance Imaging Biosensor via 2-λ Fitting Detection Method.

SPR biosensors have been extensively used for investigating protein-protein interactions. However, in conventional surface plasmon resonance (SPR) biosensors, detection is limited by the Brownian-motion-governed diffusion process of sample molecules in the sensor chip, which makes it challenging to detect biomolecule interactions at ultra-low concentrations. Here, we propose a highly sensitive SPR imaging biosensor which exploits the coffee ring effect (CRE) for in situ enrichment of molecules on the sensing surface. In addition, we designed a wavelength modulation system utilizing two LEDs to reduce the system cost and enhance the detection speed. Furthermore, a detection limit of 213 fM is achieved, which amounts to an approximately 365 times improvement compared to traditional SPR biosensors. With further development, we believe that this SPR imaging system with high sensitivity, less sample consumption, and faster detection speed can be readily applied to ultra-low-concentration molecular detection and interaction analysis.

Tensile Strain-Dependent Ultrafast Electron Transfer and Relaxation Dynamics in Flexible WSe2/MoS2 Heterostructures.

Understanding and controlling carrier dynamics in two-dimensional (2D) van der Waals heterostructures through strain are crucial for their flexible applications. Here, femtosecond transient absorption spectroscopy is employed to elucidate the interlayer electron transfer and relaxation dynamics under external tensile strains in a WSe2/MoS2 heterostructure. The results show that a modest ∼1% tensile strain can significantly alter the lifetimes of electron transfer and nonradiative electron-hole recombination by >30%. Ab initio non-adiabatic molecular dynamics simulations suggest that tensile strain weakens the electron-phonon coupling, thereby suppressing the transfer and recombination dynamics. Theoretical predictions indicate that strain-induced energy difference increases along the electron transfer path could contribute to the prolongation of the transfer lifetime. A subpicosecond decay process, related to hot-electron cooling, remains almost unaffected by strain. This study demonstrates the potential of tuning interlayer carrier dynamics through external strains, offering insights into flexible optoelectronic device design with 2D materials.

Thermal stability and decomposition kinetics of mixed-cation halide perovskites.

Organic-inorganic halide perovskites (OIHPs) have emerged as one of the most efficient photovoltaic materials due to their superior properties. However, improving their stability remains a key challenge. Herein, we investigate the thermal decomposition properties of OIHP FAxMA1-xPbI3 with mixed cations of formamidinium (FA) and methylammonium (MA). Using thermogravimetric analysis together with Fourier transform infrared spectroscopy, we identify and monitor the gaseous decomposition products as a function of temperature and cation composition. Thermal decomposition products of both MA and FA cations were observed at all stages of the thermal decomposition process, contrary to previous expectations. The yield, release sequence and kinetics of the organic gaseous products were found to depend strongly on the ratio between FA and MA cations. Furthermore, cesium ion doping was investigated as a potential strategy to improve the thermal stability of mixed cation perovskites. These results provide new insights into the effect of cation mixing on perovskite stability, suggesting that optimizing the cation ratios and decomposition pathways can guide approaches to boost the stability and performance of mixed cation perovskites.

Research Progress in Tunable Fiber Lasers Based on Multimode Interference Filters.

Tunable fiber lasers have the advantages of good beam quality, high integration, and adjustable output wavelength, and they are widely used in fields such as optical fiber communication and optical fiber sensing. The fiber filter is one of the key components of tunable fiber lasers. Among the various filters currently used, multimode interference filters have the advantages of simple structure, convenient implementation, flexible tuning methods, and convenient spectral range design. The structures of multimode interference filters based on multimode fibers, no-core fibers, multi-core fibers, tapered fibers, and other special fibers are introduced in this paper. The working principles and tuning methods are analyzed and the research progress of tunable fiber lasers based on these filters is summarized. Finally, the development trend of tunable fiber lasers based on multimode interference filters is discussed. The rapid development and applications of multimode interference filters can help improve the performance of continuous and pulse lasers as well as promote the practicality of tunable fiber lasers.

Intensity Interrogation-Based High-Sensitivity Surface Plasmon Resonance Imaging Biosensor for Apoptosis Detection in Cancer.

Intensity interrogation-based surface plasmon resonance imaging (ISPRi) sensing has a simple schematic design and is the most widely used surface plasmon resonance technology at present. In this study, we report the successful development of a novel high-sensitivity ISPRi biosensor and its application for apoptosis detection in cancer cells. By optimizing the excitation wavelength and excitation angle, we achieved a refractive index resolution (RIR) of 5.20 × 10-6 RIU. Importantly, the biosensor has been tested and validated for high-throughput and label-free detection of activated caspase-3 with its specific inhibitor Z-DEVD-FMK in apoptotic cells. Therefore, this study describes a novel molecular imaging system to monitor apoptosis in cancers for disease diagnosis and/or evaluation of therapeutic efficacy of anti-cancer drugs.

Quasi-phase extraction-based surface plasmon resonance imaging method for coffee ring effect monitoring and biosensing.

Wavelength interrogation surface plasmon resonance imaging (WSPRi) sensing has unique advantages in high-throughput imaging detection. The refractive index resolution (RIR) of WSPRi is limited to the order of 10-6 RIU. This paper demonstrates a novel WSPRi sensing system with a wavelength scanning device of an acousto-optic tunable filter (AOTF) and a low-cost speckle-free SPR excitation source of a halogen lamp. We developed a sensitive quasi-phase extraction method for data processing. The new technique achieved an RIR of 8.84×10-7 RIU, which is the first WSPRi system that has an RIR in the order of 10-7 RIU. Moreover, we performed a real-time recording of the formation of the coffee ring effect during brine evaporation and enhanced the biosensor performance of SPR for the first time. We believe the higher RIR and accuracy of the system will benefit more potential applications toward exploring the biomolecules' behaviors in biological and biochemistry studies.

Observation of a transient intermediate in the ultrafast relaxation dynamics of the excess electron in strong-field-ionized liquid water.

A unified picture of the electronic relaxation dynamics of ionized liquid water has remained elusive despite decades of study. Here, we employ sub-two-cycle visible to short-wave infrared pump-probe spectroscopy and ab initio nonadiabatic molecular dynamics simulations to reveal that the excess electron injected into the conduction band (CB) of ionized liquid water undergoes sequential relaxation to the hydrated electron s ground state via an intermediate state, identified as the elusive p excited state. The measured CB and p-electron lifetimes are 0.26 ± 0.02 ps and 62 ± 10 fs, respectively. Ab initio quantum dynamics yield similar lifetimes and furthermore reveal vibrational modes that participate in the different stages of electronic relaxation, with initial relaxation within the dense CB manifold coupled to hindered translational motions whereas subsequent p-to-s relaxation facilitated by librational and even intramolecular bending modes of water. Finally, energetic considerations suggest that a hitherto unobserved trap state resides ~0.3-eV below the CB edge of liquid water. Our results provide a detailed atomistic picture of the electronic relaxation dynamics of ionized liquid water with unprecedented time resolution.

Black Arsenic-Phosphorus Nanosheets for Highly Responsive Photodetection and Dual-Wavelength Ultrafast Pulse Generation at Telecommunication Bands.

Black arsenic-phosphorus (b-AsP), an alloy containing black phosphorus and arsenic in the form of b-AsxP1-x, has a broadly tunable band gap changing with the chemical ratios of As and P. Although mid-infrared photodetectors and mode-locked or Q-switched pulse lasers based on b-AsP (mostly b-As0.83P0.17) are investigated, the potential of this family of materials for near-infrared photonic and optoelectronic applications at telecommunication bands is not fully explored. Here, we have verified a multifunctional fiber device based on b-As0.4P0.6 nanosheets for highly responsive photodetection and dual-wavelength ultrafast pulse generation at around 1550 nm. The fiber laser with a saturable absorber (SA) based on b-As0.4P0.6 nanosheets can output dual-wavelength mode-locking pulses with a larger bandwidth and spectral separation than those based on other two-dimensional (2D) materials. Remarkably, it is found that the b-As0.4P0.6-based photodetector can achieve a high responsivity of 10,200 A/W at 1550 nm and a peak responsivity of 2.29 × 105 A/W at 980 nm. Our work suggests that b-As0.4P0.6 shows great potential in ultrafast photonics, dual-comb spectroscopy, and infrared signal detection.

Femtosecond laser nanostructuring on a 4H-SiC surface by tailoring the induced self-assembled nanogratings.

Ultrafast laser micromachining of crystalline silicon carbide (SiC) has great perspectives in aerospace industry and integrated circuit technique. In this report, we present a study of femtosecond laser nanostructuring on the surface of an n-type 4H-SiC single crystal. Except for uniform nanogratings, new types of large-area periodic structures including nanoparticle array and nanoparticle-nanograting hybrid structures were induced on the surface of 4H-SiC by scanning irradiation. The effects of pulse energy, scan speed, and the polarization direction on the morphology and periodicity of nanogratings were systematically explored. The proper parameter window for nanograting formation in pulse energy-scan speed landscape is depicted. Both the uniformity and the periodicity of the induced nanogratings are polarization dependent. A planar light attenuator for linear polarized light was demonstrated by aligning the nanogratings. The transition between different large-area periodic structures is achieved by simultaneous control of pulse energy and scan interval using a cross scan strategy. These results are expected to open up an avenue to create and manipulate periodic nanostructures on SiC crystals for photonic applications.

Trap-free exciton dynamics in monolayer WS2via oleic acid passivation.

Two-dimensional transition metal dichalcogenides have attracted a great deal of attention in the past few decades owing to their attractive optoelectronic properties. However, their widespread utility in photonic devices and components is still limited owing to their weak photoluminescence. While various treating methods are in place to improve the photoluminescence yield, the impact of these treatments on the excited state (especially exciton) dynamics in these two-dimensional materials remains ill defined. In this work, exciton dynamics in pristine and oleic acid-treated monolayer WS2 were comprehensively studied through various ultrafast experimental techniques. We demonstrate that oleic acid effectively passivates the defect states in as-fabricated WS2, resulting in trap-free exciton dynamics and exciton annihilation rate reduction, which leads to stronger steady-state photoluminescence and longer photoluminescence lifetime. These results provide valuable information on the intrinsic exciton dynamics in monolayer WS2, which could also be applicable in other two-dimensional transition metal dichalcogenides and help improve optoelectronic device performance.

Ultrafast nonequilibrium dynamic process of separate electrons and holes during exciton formation in few-layer tungsten disulfide.

Femtosecond transient absorption spectroscopy has been employed to unravel separate initial nonequilibrium dynamic processes of photo-injected electrons and holes during the formation process of the lowest excitons at the K-valley in few-layer tungsten disulfide. Charge carrier thermalization and cooling, as well as concomitant many-body effects on the exciton resonances, are distinguished. The thermalization of holes is observed to be faster than that of electrons. Both of them proceed predominantly via carrier-carrier scattering, as evidenced by the observed dependence of the thermalization time on pump fluences. The fluence dependent time constants also suggest that the subsequent cooling for electrons is probably dominated by acoustic phonons, whereas for holes it is mostly controlled by LO phonons. An extremely fast red- and blue-shift crossover followed by a slow blue-shift of exciton resonance was observed in the temporal evolution of exciton resonances by resonant exciton A excitation. The rapid red-shift could be due to the strong screening of the Coulomb interaction between quasi-free charge carriers in electron-hole plasma. The subsequent slow blue-shift is the net result of the competition among many-body effects in the hot-exciton cooling process. Our findings elucidate the carrier-selective ultrafast dynamics and their many-body effects, underpinning new possibilities for developing optoelectronic devices based on transport properties of a single type of carrier.

Polarization-dependent microstructural evolution induced by a femtosecond laser in an aluminosilicate glass.

Manipulation of femtosecond laser induced microstructures in glass by tuning the laser polarization has great potential in optics. Here we report two different polarization-dependent microstructures and their evolution with pulse repetition rate in an aluminosilicate glass induced by femtosecond laser irradiation. A V-shaped crack oriented parallel to the laser polarization plane is induced at the bottom of modified regions by pulses operated at 200 kHz, 1030 nm, and 300 fs. Further increasing the pulse repetition rate to 500 kHz leads to the formation of a dumbbell-shaped structure, which is elongated perpendicularly to the laser polarization, at the top of the modified region. The size of the coloration area and the dumbbell-shaped structure can be controlled by tuning the pulse duration. Further investigation indicates that higher numerical apertures are in favor of the presence of the polarization effects in femtosecond laser irradiation. The possible mechanism responsible for the formation of the two microstructures is discussed. These results could be helpful for understanding of ultrafast laser interaction with glass.

Ultrafast carrier relaxation dynamics of photoexcited GaAs and GaAs/AlGaAs nanowire array.

Femtosecond optical pump-probe spectroscopy is employed to elucidate the ultrafast carrier nonradiative relaxation dynamics of bare GaAs and a core-shell GaAs/AlGaAs semiconductor nanowire array. Different from the single nanowire conventionally used for the study of ultrafast dynamics, a simple spin coating and peeling off method was performed to prepare transparent organic films containing a vertical oriented nanowire array for transient absorption measurement. The transient experiment provides the direct observation of carrier thermalization, carrier cooling, thermal dissipation and band-gap energy evolutions along with the carrier relaxations. Carrier thermalization occurs within sub-0.5 ps and proceeds almost independently on the AlGaAs-coating, while the time constants of carrier cooling and thermal dissipation are increased by an order of magnitude due to the AlGaAs-coating effect. The concomitant band-gap evolutions in GaAs and GaAs/AlGaAs include an initial rapid red-shift in thermalization period, followed by a slow blue and/or red shift in carrier cooling, and then by an even slower blue shift in thermal dissipation. The evolution is explained by the competition of band-gap renormalization, plasma screening and band-filling. These findings are significant for understanding the basic physics of carrier scattering, and also for the development of flexible optoelectronic devices.

Ultrafast acoustic vibrations of Au-Ag nanoparticles with varying elongated structures.

Acoustic vibrations of Au and Ag elongated nano-objects with original morphologies, from Ag-Ag homodimers to Au@Ag-Ag heterodimers and Au@Ag eccentric core-shell spheroids, have been experimentally investigated by ultrafast time-resolved optical spectroscopy. Their frequencies, obtained by the analysis of time-dependent transient absorption changes, are compared with the results obtained from finite element modeling (FEM) numerical computations, which allow assignment of the detected oscillating signals to fundamental radial and extensional modes. FEM was further used to analyze the effects of morphology and composition on the vibrational dynamics. FEM computations indicate that (1) the central distance between particles forming the nanodimers has profound effects on the extensional mode frequencies and a negligible influence on the radial mode ones, in analogy with the case of monometallic nanorods, (2) coating Au with Ag also has a strong mass-loading-like effect on the dimer and core-shell stretching mode frequency, while (3) its influence on the radial breathing mode is smaller and analogous to the non-monotonic frequency dependence on the Au fraction previously observed in isotropic bimetallic spheres. These findings are significant for developing a predictive understanding of nanostructure mechanical properties and for designing new mechanical nanoresonators.

Coherent phonon dynamics in a c-plane sapphire crystal before and after intense femtosecond laser irradiation.

Femtosecond pump-probe experiments with a ∼6.4 fs time-resolution were performed to investigate the coherent phonon dynamics in a c-plane sapphire crystal before and after intense 800 nm femtosecond laser irradiation. The intense femtosecond laser induced defect/distortion and even re-crystallization of crystalline structures, which result in the appearance of new peaks and relative intensity change in coherent phonon and Raman spectra. The combination of these two spectra was found to be beneficial to evidence the variation of crystalline structure and further to differentiate the origins of new Raman peaks after irradiation. Further analysis of time-dependent differential absorbance with damped cosine function fitting and Fourier transfer calculation yields the vibrational parameters, including periods, damping times and initial phases, before and after irradiation. With these parameters, the defect-effects on damping time and the mechanism of coherent phonon generation were addressed.

Mixed-Solvent Polarity-Assisted Phase Transition of Cesium Lead Halide Perovskite Nanocrystals with Improved Stability at Room Temperature.

Cesium lead halide perovskite nanocrystals (NCs) have attracted enormous interest in light-emitting diode, photodetector and low-threshold lasing application in terms of their unique optical and electrical performance. However, little attention has been paid to other structures associated with CsPbBr3, such as CsPb2Br5. Herein, we realize a facile method to prepare dual-phase NCs with improved stability against polar solvents by replacing conventional oleylamine with cetyltrimethyl ammonium bromide (CTAB) in the reprecipitation process. The growth of NCs can be regulated with different ratios of toluene and ethanol depending on solvent polarity, which not only obtains NCs with different sizes and morphologies, but also controls phase transition between orthorhombic CsPbBr3 and tetragonal CsPb2Br5. The photoluminescence (PL) and defect density calculated exhibit considerable solvent polarity dependence, which is ascribed to solvent polarity affecting the ability of CTAB to passivate surface defects and improve stoichiometry in the system. This new synthetic method of perovskite material will be helpful for further studies in the field of lighting and detectors.

The Microstructure, Electric, Optical and Photovoltaic Properties of BiFeO3 Thin Films Prepared by Low Temperature Sol⁻Gel Method.

Ferroelectrics have recently attracted attention as a candidate class of materials for use in photovoltaic devices due to their abnormal photovoltaic effect. However, the current reported efficiency is still low. Hence, it is urgent to develop narrow-band gap ferroelectric materials with strong ferroelectricity by low-temperature synthesis. In this paper, the perovskite bismuth ferrite BiFeO3 (BFO) thin films were fabricated on SnO2: F (FTO) substrates by the sol-gel method and they were rapidly annealed at 450, 500 and 550 °C, respectively. The microstructure and the chemical state's evolution with annealing temperature were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), and the relationship between the microstructure and electric, optical and photovoltaic properties were studied. The XRD, SEM and Raman results show that a pure phase BFO film with good crystallinity is obtained at a low annealing temperature of 450 °C. As the annealing temperature increases, the film becomes more uniform and has an improved crystallinity. The XPS results show that the Fe3+/Fe2+ ratio increases and the ratio of oxygen vacancies/lattice oxygen decreases with increasing annealing temperature, which results in the leakage current gradually being reduced. The band gap is reduced from 2.68 to 2.51 eV due to better crystallinity. An enhanced photovoltaic effect is observed in a 550 °C annealed BFO film with a short circuit current of 4.58 mA/cm2 and an open circuit voltage of 0.15 V, respectively.

Self-assembled three-dimensional periodic micro-nano structures in bulk quartz crystal induced by femtosecond laser pulses.

In-volume, self-assembled, three-dimensional, periodic micro-nano structures are induced in quartz crystal by tightly focused, 500-kHz femtosecond laser pulses. With suitable pulse energy, three different types of periodic structures can be observed in modified regions using scanning electron microscopy. The first one with period (ΛE) of ~400 nm in the direction of the laser polarization, i.e. nanograting, shows indicative features similar to that in fused silica. The second one with period (ΛS) in the scan direction and the third one with period (Λk) in the laser propagation direction are both equally spaced by ~1 µm, which is close to the laser wavelength. Moreover, the structure with period (Λk) covers almost the whole cross-section of modified regions, which is distinctive to that observed in fused silica. Through the comparison of the structures induced by 1-kHz pluses and those by 500-kHz pluses, we deduce that the heat accumulation effect may have a positive influence on the formation of nanogratings in quartz crystal.

Broadband ultraviolet to near infrared conversion in Eu2+,Nd3+ co-doped SrAl2O4.

In this study, we investigated the quantum cutting (QC) mechanism in Eu2+-Nd3-co-doped SrAl2O4 microcrystals by fluorescence spectroscopy and decay lifetime analysis. In this material, the near-infrared (NIR) emissions of Nd3+ in the range of 800-1200 nm were enhanced under the excitation of the Eu2+:4f7 → 4f65d1 transition radiation. The lifetime of the 5d1 level of Eu2+ decreased with the increase in the Nd3+ concentration. These results verified the occurrence of cooperative energy transfer (CET) from the Eu2+:5d1 excited state to the Nd3+:4F3/2 level, by which one absorbed ultraviolet-visible photon was converted to two NIR photons with an optimal quantum efficiency (QE) of approximately 177.1%. Therefore, this broadband QC material paves the way for a further increase in the conversion efficiency of c-Si solar cells.

In situ measurement of the topological charge of a perfect vortex using the phase shift method.

We propose a method to determine the topological charge (TC) of a perfect vortex. With the phase shift technique, the perfect vortex and its conjugate beam exactly overlap and interfere. Consequently, the TC of a perfect vortex is determined by counting the number of interference fringes. This proposed method enables in situ determination of the TC of the perfect vortex without the need for additional optical elements, and it is immune to environmental vibration and parasitic interference.