Watt-level femtosecond Tm-doped "mixed" sesquioxide ceramic laser in-band pumped by a Raman fiber laser at 1627 nm.

We report on a semiconductor saturable absorber mirror mode-locked Tm:(Lu,Sc)2O3 ceramic laser in-band pumped by a Raman fiber laser at 1627 nm. The nonlinear refractive index (n2) of the Tm:(Lu,Sc)2O3 ceramic has been measured to be 4.66 × 10-20 m2/W at 2000 nm. An average output power up to 1.02 W at 2060 nm is achieved for transform-limited 280-fs pulses at a repetition rate of 86.5 MHz, giving an optical efficiency with respect to the absorbed pump power of 36.4%. Pulses as short as 66 fs at 2076 nm are produced at the expense of output power (0.3 W), corresponding to a spectral bandwidth of 69 nm. The present work reveals the potential of Tm3+-doped sesquioxide transparent ceramics for power scaling of femtosecond mode-locked bulk lasers emitting in the 2-µm spectral range.

Luminescence performance and vibronic behavior of Mn4+-activated Ca14-xKxAl10Zn6O35 deep-red phosphor.

In this study, a significant improvement of deep-red luminescence was successfully achieved via the substitution approach in the Mn4+-activated Ca14-xKxAl10Zn6O35 phosphor. The optimal Mn4+ doping level x was determined by studying luminescence concentration quenching behavior. The measured photoluminescence (PL) spectrum showed five distinct vibronic structures with the main peak centered at 712 nm. A theoretical simulation work was conducted for comparison, and the predominant phonon mode involving in the vibronic transition process was revealed. From the temperature-dependent PL spectra, an abnormal luminescence enhancement was observed at the temperature rising from T=100 to 340 K, and the underlying phonon-assisted luminescence mechanism was theoretically disclosed. Finally, we studied the temperature-dependent luminescence lifetime, and the primary phonon energy in the vibronic behavior was identified from the fitting work.

Steady-state and time-resolved upconversion photoluminescence in Yb3+-Er3+ co-doped transparent ceramics of YAG.

Transparent ceramics (TCs) represent a new family of functional hard materials. In this Letter, steady-state and time-resolved upconversion photoluminescence in Yb3+-Er3+ co-doped TC of yttrium aluminum garnet (TC-YAG) are reported for the first time, to the best of our knowledge. Under the excitation of near-infrared 940 nm laser at room temperature, the Yb3+-Er3+ co-doped TC-YAG emits intense multi-color luminescence consisting of cyan, green, and red groups of sharp lines. More excitingly, the green group of luminescence due to the transitions from 4S3/2 to 4I15/2 states of Er3+ are the prominent components with the average lifetime of ∼0.3ms. The internal quantum efficiency of the green luminescence is estimated to be 32.8%. A unique dual-resonance energy transfer from Yb3+ to Er3+ via the excited-state vibronic transitions is proposed as the principal mechanism of the strongest green luminescence of Er3+ ions in TC-YAG.

Mid-infrared supercontinuum generation in chalcogenide fibers with high laser damage threshold.

Laser damage thresholds (Ith) at 1.03 µm, as well as third-order nonlinear refractive indices (n2) and two photon absorption coefficients (ß) at 1.55 µm of a number of Ge-As-S glasses were measured and systematically studied. The glass with the composition Ge0.12As0.24S0.64 showed a high Ith and the maximum figure of merit (fm= n2/(ß·λ)), and therefore was selected as the core material for the fabrication of a step-index fiber. A compatible glass with the composition Ge0.18As0.1S0.72 was chosen as the cladding material. Based on the dispersion calculations, the fiber with a core diameter of ∼7-10 µm was designed. The designed fiber was fabricated by a multiple step rod-in-tube method. When the fiber with a core diameter of ∼9 µm and a length of ∼13.5 cm was pumped by ∼170 fs pulses (1 MHz) at 4.5 µm, the mid-infrared supercontinuum (SC) covering 1.3-8.1 µm was generated. These results demonstrate the good potential of Ge-As-S chalcogenide fibers for producing high-brightness broadband mid-infrared SC light sources.

Directly monitoring the active sites of charge transfer in heterocycles in situ and in real time.

Coherent degenerate infrared-infrared-visible sum frequency generation vibrational spectroscopy provides a powerful label-free sensitive probe for charge transfer active sites in heterocyclic molecules in situ and in real time.

Highly reproducible and sensitive silver nanorod array for the rapid detection of Allura Red in candy.

Allura Red (AR) is a highly stable synthetic red azo dye, which is widely used in the food industry to dye food and increase its attraction to consumers. However, the excessive consumption of AR can result in adverse health effects to humans. Therefore, a highly reproducible silver nanorod (AgNR) array was developed for surface enhanced Raman scattering (SERS) detection of AR in candy. The relative standard deviation (RSD) of AgNR substrate obtained from the same batch and different batches were 5.7% and 11.0%, respectively, demonstrating the high reproducibility. Using these highly reproducible AgNR arrays as the SERS substrates, AR was detected successfully, and its characteristic peaks were assigned by the density function theory (DFT) calculation. The limit of detection (LOD) of AR was determined to be 0.05 mg/L with a wide linear range of 0.8-100 mg/L. Furthermore, the AgNR SERS arrays can detect AR directly in different candy samples within 3 min without any complicated pretreatment. These results suggest the AgNR array can be used for rapid and qualitative SERS detection of AR, holding a great promise for expanding SERS application in food safety control field.

Study of in vitro RBCs membrane elasticity with AOD scanning optical tweezers.

The elasticity of red cell membrane is a critical physiological index for the activity of RBC. Study of the inherent mechanism for RBCs membrane elasticity transformation is attention-getting all along. This paper proposes an optimized measurement method of erythrocytes membrane shear modulus incorporating acousto-optic deflector (AOD) scanning optical tweezers system. By use of this method, both membrane shear moduli and sizes of RBCs with different in vitro times were determined. The experimental results reveal that the RBCs membrane elasticity and size decline with in vitro time extension. In addition, semi quantitative measurements of S-nitrosothiol content in blood using fluorescent spectrometry during in vitro storage show that RBCs membrane elasticity change is positively associated with the S-nitrosylation level of blood. The analysis considered that the diminished activity of the nitric oxide synthase makes the S-nitrosylation of in vitro blood weaker gradually. The main reason for worse elasticity of the in vitro RBCs is that S-nitrosylation effect of spectrin fades. These results will provide a guideline for further study of in vitro cells activity and other clinical applications.

Reorganization of hydrogen bond network makes strong polyelectrolyte brushes pH-responsive.

Weak polyelectrolytes have found extensive practical applications owing to their rich pH-responsive properties. In contrast, strong polyelectrolytes have long been regarded as pH-insensitive based on the well-established fact that the average degree of charging of strong polyelectrolyte chains is independent of pH. The possible applications of strong polyelectrolytes in smart materials have, thus, been severely limited. However, we demonstrate that almost all important properties of strong polyelectrolyte brushes (SPBs), such as chain conformation, hydration, stiffness, surface wettability, lubricity, adhesion, and protein adsorption are sensitive to pH. The pH response originates from the reorganization of the interchain hydrogen bond network between the grafted chains, triggered by the pH-mediated adsorption-desorption equilibrium of hydronium or hydroxide with the brushes. The reorganization process is firmly identified by advanced sum-frequency generation vibrational spectroscopy. Our findings not only provide a new understanding of the fundamental properties of SPBs but also uncover an extensive family of building blocks for constructing pH-responsive materials.

Signature of coexistence of superconductivity and ferromagnetism in two-dimensional NbSe2 triggered by surface molecular adsorption.

Ferromagnetism is usually deemed incompatible with superconductivity. Consequently, the coexistence of superconductivity and ferromagnetism is usually observed only in elegantly designed multi-ingredient structures in which the two competing electronic states originate from separate structural components. Here we report the use of surface molecular adsorption to induce ferromagnetism in two-dimensional superconducting NbSe2, representing the freestanding case of the coexistence of superconductivity and ferromagnetism in one two-dimensional nanomaterial. Surface-structural modulation of the ultrathin superconducting NbSe2 by polar reductive hydrazine molecules triggers a slight elongation of the covalent Nb-Se bond, which weakens the covalent interaction and enhances the ionicity of the tetravalent Nb with unpaired electrons, yielding ferromagnetic ordering. The induced ferromagnetic momentum couples with conduction electrons generating unique correlated effects of intrinsic negative magnetoresistance and the Kondo effect. We anticipate that the surface molecular adsorption will be a powerful tool to regulate spin ordering in the two-dimensional paradigm.

In Situ and Real-Time SFG Measurements Revealing Organization and Transport of Cholesterol Analogue 6-Ketocholestanol in a Cell Membrane.

Cholesterol organization and transport within a cell membrane are essential for human health and many cellular functions yet remain elusive so far. Using cholesterol analogue 6-ketocholestanol (6-KC) as a model, we have successfully exploited sum frequency generation vibrational spectroscopy (SFG-VS) to track the organization and transport of cholesterol in a membrane by combining achiral-sensitive ssp (ppp) and chiral-sensitive psp polarization measurements. It is found that 6-KC molecules are aligned at the outer leaflet of the DMPC lipid bilayer with a tilt angle of about 10°. 6-KC organizes itself by forming an α-ß structure at low 6-KC concentration and most likely a ß-ß structure at high 6-KC concentration. Among all proposed models, our results favor the so-called umbrella model with formation of a 6-KC cluster. Moreover, we have found that the long anticipated flip-flop motion of 6-KC in the membrane takes time to occur, at least much longer than previously thought. All of these interesting findings indicate that it is critical to explore in situ, real-time, and label-free methodologies to obtain a precise molecular description of cholesterol's behavior in membranes. This study represents the first application of SFG to reveal the cholesterol-lipid interaction mechanism at the molecular level.

Structure and orientation of interfacial proteins determined by sum frequency generation vibrational spectroscopy: method and application.

In situ and real-time characterization of molecular structures and orientation of proteins at interfaces is essential to understand the nature of interfacial protein interaction. Such work will undoubtedly provide important clues to control biointerface in a desired manner. Sum frequency generation vibrational spectroscopy (SFG-VS) has been demonstrated to be a powerful technique to study the interfacial structures and interactions at the molecular level. This paper first systematically introduced the methods for the calculation of the Raman polarizability tensor, infrared transition dipole moment, and SFG molecular hyperpolarizability tensor elements of proteins/peptides with the secondary structures of α-helix, 310-helix, antiparallel ß-sheet, and parallel ß-sheet, as well as the methodology to determine the orientation of interfacial protein secondary structures using SFG amide I spectra. After that, recent progresses on the determination of protein structure and orientation at different interfaces by SFG-VS were then reviewed, which provides a molecular-level understanding of the structures and interactions of interfacial proteins, specially understanding the nature of driving force behind such interactions. Although this review has focused on analysis of amide I spectra, it will be expected to offer a basic idea for the spectral analysis of amide III SFG signals and other complicated molecular systems such as RNA and DNA.