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.

Broadband 1.0†µm emission in Nd3+/Yb3+ co-doped phosphate glasses and fibers for photonic applications.

In this work, the spectroscopic properties of 1.0 µm emission in Nd3+/Yb3+ co-doped phosphate glasses were systematically investigated under 808 nm excitation. Notably, broadband 1.0 µm emission with a full width at half maximum (FWHM) of 96 nm was obtained in the phosphate glass doped with 2 mol.% Nd2O3 and 1 mol.% Yb2O3. In addition, the energy transfer microscopic parameter and transfer efficiency were analyzed. What is more, multimaterial fibers with Nd3+/Yb3+ co-doped phosphate glass core and silicate cladding were successfully drawn by using the molten core method. An intense 1.0 µm amplified spontaneous emission (ASE) can be realized in a 3 cm long multimaterial fiber. More importantly, the FWHM of the ASE can reach as large as 60 nm when excited at 976 nm. These results demonstrate that the Nd3+/Yb3+ co-doped phosphate glasses and fibers are promising gain materials for amplifier and laser applications in photonics.

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.

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.

Broadband near-infrared amplified spontaneous emission of Er3+-doped germanate glass fiber.

Er3+-doped glass and fiber are very attractive for near-infrared (NIR) lasers and photonic applications. In this work, the full width at half maximum (FWHM) of NIR fluorescence emission of the Er3+-doped germanate glass can be broadened from 72 to 99 nm when Al2O3 was added. In addition, the spectroscopic properties, including absorption and emission spectra, Judd-Ofelt intensity parameters, absorption and emission cross sections, gain coefficient, and fluorescence lifetime, of the Al2O3-modified germanate glass were systematically investigated. What is more, silicate-clad heavily Er3+-doped germanate core multimaterial fibers were successfully drawn by a rod-in-tube method. Notably, broadband NIR amplified spontaneous emission (ASE) with an FWHM of 120 nm was achieved in this new fiber. To the best of our knowledge, this is the largest FWHM reported for Er3+-doped germanate glass fibers. These results suggest that the as-drawn Er3+-doped germanate glass fiber with superior performances is a promising candidate for broadband optical amplification.

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.

Enhanced 2-µm and upconversion luminescence properties regulated by network structure in Ho3+/Yb3+ co-doped germanate laser glasses.

Rare-earth (RE) ions doped laser glass has attracted the interest of many researchers because of its numerous potential applications in planar waveguides and fiber lasers. In this work, the 2-µm and upconversion luminescence properties of Ho3+ are simultaneously enhanced through the design of components used to regulate the network structure of the germanate glass. Furthermore, the thermal, structural, and spectroscopic properties of the Ho3+/Yb3+ co-doped germanate laser glass are systematically investigated. It is noted that the calculated gain coefficient of the Nb2O5 modified germanate laser glass can reach as high as 3.05 cm-1 at 2047 nm. These results suggest that the prepared germanate laser glass with superior performances is a promising candidate for 2-µm mid-infrared laser materials applications.

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.

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 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.

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.

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.

A strategy for fabrication of controllable 3D pattern containing clusters and nanoparticles inside a solid material.

Directly controlling the growth process of clusters and nanoparticles is an effective way to tune their specific properties, which has been considered as a significant issue lying at the heart of nanotechnology. For technological applications, great strides have been made in the assembly of clusters and nanoparticles. However, controllable synthesis of clusters and nanoparticles inside a bulk solid-state medium remains a tremendous challenge, which is important for integrated devices. Here we report a strategy for space-selective control of elemental tellurium (Te) precipitation as clusters or nanoparticles in glass by femtosecond (fs) laser irradiation. After irradiation by a 1 kHz fs laser at 800 nm, Te2 clusters, which emit near-infrared (NIR) light, are observed at the focal point of the laser beam inside the glass sample. By shifting the repetition rate to 250 kHz, a temperature field forms around the focal area that facilitates transformation of Te clusters into nanoparticles. Raman mapping shows that the clusters are localized in the center of the laser-induced microstructure, while the nanoparticles exhibit an annular distribution. The possible mechanisms of generation and distribution of different species are discussed. We have also demonstrated optical data storage and embedded micro-grating by using this technique.

Evolution of polarization dependent microstructures induced by high repetition rate femtosecond laser irradiation in glass.

We report the observation of an anomalous polarization dependent process in an isotropic glass induced by long time stationary irradiation of a high repetition rate near-infrared femtosecond laser. Two distinctive types of polarization dependent microstructures were induced at different irradiation stages. At early stage (a few seconds), a dumbbell-shaped structure elongated perpendicularly to the laser polarization formed at the top of the modified region, which was later erased by further irradiation. At later stage (above 30 s), bubbles filled with O2 formed by the irradiation, which were distributed along the laser polarization at a distance far beyond the radius of the laser beam. Based on a simple modeling of light reflection on boundaries, a thermal accumulation process was proposed to explain the formation and evolution of the dumbbell-shaped microstructure. The possible factors responsible for polarization dependent distribution of bubbles are discussed, which needs further systematic investigations. The results may be helpful in the development of femtosecond laser microprocessing for various applications.

Ultrafast saturable absorption in topological insulator Bi2SeTe2 nanosheets.

Topological insulator (TI) Bi2SeTe2 nanosheets with very regular hexagonal morphology were synthesized by a hydrothermal route. Open aperture (OA) z-scan method was performed to measure the saturable absorption (SA) characteristics of the as-prepared TI Bi2SeTe2 nanosheets. The measured modulation depth, saturation intensity and nonsaturable loss of the sample were 61.9%, 4.46 GW/cm2 and 4.5% respectively. An ultrafast intraband scattering time of ~50 fs was obtained through simulating the SA curve, which indicates the TI Bi2SeTe2 nanosheets may be a good candidate for mode-locking material.

Influence of laser-induced air breakdown on femtosecond laser ablation of aluminum.

We investigated the influence of laser-induced air breakdown on the femtosecond laser ablation of aluminum target using time-resolved pump-probe shadowgraphic imaging method. The early-stage plasma expanding dynamics and subsequent expanding behaviors of shockwaves and material ejection plume were analyzed through shadowgraphs recorded at different time delays. The dominated mechanisms were clarified at different stages during femtosecond laser pulses ablating aluminum, which provide very valuable information for ultrashort laser ablation of metals.

Grating-assisted generation of regular two-dimensional multicolored arrays in a tellurite glass.

A grating structure was inscribed in a tellurite glass after irradiation with high-repetition rate femtosecond laser pulses. High diffraction efficiency was obtained due to the large refractive index change, which was caused by the precipitation of Te crystals in the laser modified region. Two-dimensional multicolored arrays were generated by cascaded four-wave mixing (CFWM) together with the prefabricated grating structure, which showed much more superior than those induced by beam breakup.

Space-selective precipitation of ZnO crystals in glass by using high repetition rate femtosecond laser irradiation.

We report on three-dimensional (3D) precipitation of ZnO crystals inside a silicate glass by a 500 kHz femtosecond pulse laser. The precipitation and distribution of ZnO crystals in glass are confirmed and analyzed by Raman spectra and Raman mapping. Mirco- luminescence is observed in the laser modified region when excited by femtosecond pulse laser or Xenon lamp. The effect of laser average power on the precipitation of the ZnO crystals has also been investigated. The possibility of 3D optical data storage using the observed phenomena is demonstrated.

Fabrication of polarization-dependent light attenuator in fused silica using a low-repetition-rate femtosecond laser.

In this Letter, we have demonstrated the direct writing of polarization-dependent light attenuator inside fused silica by tailoring 1 kHz femtosecond (fs) laser induced self-organized nanogratings. Optical birefringence was observed to vary with the polarization plane azimuth of the fs laser and scanning direction. The formation of self-organized nanogratings was confirmed by scanning electron microscopy observation. A polarization-dependent light attenuator was fabricated by forming a plane consisting of nanograting lines inside fused silica by scanning the fs laser. The attenuation efficiency was improved by forming a multilayer nanograting structure. The technique may find important applications in micro-optical devices.