Synthesis and Characterization of Poly(<i>N</i>-isopropylacrylamide)-Coated Polystyrene Microspheres with Silver Nanoparticles on Their Surfaces^†

Dispersion copolymerization of styrene and a poly(N-isopropylacrylamide) macromonomer in ethanol−water media has been successfully carried out in the presence of AgNO3. Nearly monodisperse polystyrene microspheres with diameters ranging from 530 to 1250 nm were obtained. Nanoscopic silver particles were generated on their surfaces via in situ reduction of Ag+ by radicals generated from the initiator, 2,2‘-azobisisobutyronitrile (AIBN). The particle sizes of both polystyrene microspheres and silver nanoparticles were affected by the initial AIBN, AgNO3, and macromonomer concentrations. The diameters of the silvered microspheres and silver nanoparticles followed the relationships Dn ∝ [AIBN]0-0.107 [AgNO3]00.083[macromonomer]0-0.533 and dn ∝ [AIBN]00.027 [AgNO3]00.173 [macromonomer]0-0.137, respectively. Over 95.8% of the silver ions are converted into zerovalent metal and immobilized on the microspheres, according to atomic absorption spectroscopy measurements. The silvered microspheres were characterized by transmission electron microscopy, atomic force microscopy, and FTIR, UV−visible, and X-ray photoelectron spectroscopy. The surface-grafted PNIPAAm chains were found not only to serve as steric stabilizers to prevent the flocculation of the polystyrene particles but also to adsorb the Ag nanoparticles onto the surface of the microspheres. A mechanism for the formation of silvered polystyrene microspheres in dispersion copolymerization was presented

Theory of Transverse Mode Instability in Fiber Amplifiers with Multimode Excitations

Transverse Mode Instability (TMI) which results from dynamic nonlinear thermo-optical scattering is the primary limitation to power scaling in high-power fiber lasers and amplifiers. It has been proposed that TMI can be suppressed by exciting multiple modes in a highly multimode fiber. We derive a semi-analytic frequency-domain theory of the threshold for the onset of TMI under arbitrary multimode input excitation for general fiber geometries. We show that TMI results from exponential growth of noise in all the modes at downshifted frequencies due to the thermo-optical coupling. The noise growth rate in each mode is given by the sum of signal powers in various modes weighted by pairwise thermo-optical coupling coefficients. We calculate thermo-optical coupling coefficients for all $\sim$$10^4$ pairs of modes in a standard circular multimode fiber and show that modes with large transverse spatial frequency mismatch are weakly coupled resulting in a banded coupling matrix. This short-range behavior is due to the diffusive nature of the heat propagation which mediates the coupling and leads to a lower noise growth rate upon multimode excitation compared to single mode, resulting in significant TMI suppression. We find that the TMI threshold increases linearly with the number of modes that are excited, leading to more than an order of magnitude increase in the TMI threshold in a 82-mode fiber amplifier. Using our theory, we also calculate TMI threshold in fibers with non-circular geometries upon multimode excitation and show the linear scaling of TMI threshold to be a universal property of different fibers

Transport/Magnetotransport of High-Performance Graphene Transistors on Organic Molecule-Functionalized Substrates

In this article, we present the transport and magnetotransport of high-quality graphene transistors on conventional SiO₂/Si substrates by modification with organic molecule octadecyltrichlorosilane (OTS) self-assembled monolayers (SAMs). Graphene devices on OTS SAM-functionalized substrates with high carrier mobility, low intrinsic doping, suppressed carrier scattering, and reduced thermal activation of resistivity at room temperature were observed. Most interestingly, the remarkable magnetotransport of graphene devices with pronounced quantum Hall effect, strong Shubnikov-de Haas oscillations, a nonzero Berry’s phase, and a short carrier scattering time also confirms the high quality of graphene on this ultrasmooth organic SAM-modified platform. The high-performance graphene transistors on the solution-processable OTS SAM-functionalized SiO₂/Si substrates are promising for the future development of large-area and low-cost fabrications of graphene-based nanoelectronics

Suppressing transverse mode instability through multimode excitation in a fiber amplifier

High-power fiber laser amplifiers have enabled an increasing range of applications in industry, medicine and defense. The power scaling for narrow-band amplifiers is currently limited by the transverse modal instability. Various techniques have been developed to suppress the instability in a single or few-mode fiber in order to output a clean, collimated beam. Here we propose to use a highly multimode fiber and equal modal excitation to suppress the thermo-optical nonlinearity and instability. Our numerical simulations and theoretical analysis predict a significant reduction of dynamic coupling among the fiber modes with such excitation. When the bandwidth of a coherent seed is narrower than the spectral correlation width of the multimode fiber, the amplified light maintains high spatial coherence and can be transformed to any target pattern or focused to a diffraction-limited spot by a spatial mask at either input or output end of the amplifier. Our method simultaneously achieves high average power, narrow spectral width, and good beam quality, which are desired for fiber amplifiers in many applications

Magnetic Dipole Resonance and Coupling Effects Directly Enhance the Raman Signals of As-Grown Graphene on Copper Foil by over One Hundredfold

Large-area graphene is commonly prepared through chemical vapor deposition (CVD); in situ and nondestructive methods for its characterization are desirable. In this paper, we demonstrate a practical methodexploiting magnetic dipole resonance and coupling effectswith which the Raman signals of graphene on copper (Cu) foil can be directly, faithfully, and greatly enhanced. The magnetic dipole resonance of a silicon nanoparticle (SiNP) can effectively couple its electromagnetic field with the Cu foil to induce an enormous electric field located solely at the position of the SiNP on the graphene. The coupled electromagnetic field can lead to hot spots of high electric field intensity (E2/E02 = 123.2) within the graphene. Even when we positioned only a few SiNPs upon the graphene/Cu foil, we obtained a Raman signal enhancement (ca. 206 times) much greater than that of graphene transferred onto a 300 nm oxide film (ca. 12 times). From a series of experiments comparing the Raman signals of graphene before and after removing the SiNPs on the graphene/Cu foil, we found that the coated SiNPs had almost no effect on the quality of the as-grown graphene. Furthermore, we have used the SiNP-enhanced Raman signals to distinguish the local quality of as-grown graphene at different areas and, therefore, the quality of the underlying Cu foil. Thus, this approach based on magnetic dipole resonance and coupling appears to be very useful for in situ and nondestructive characterization of as-grown graphene on Cu foil, without the need for transfer processes or harmful processing conditions

Exploiting spacetime symmetry in dissipative nonlinear multimode amplifiers for output control

Time-reversal symmetry enables shaping input waves to control output waves in many linear and nonlinear systems; however energy dissipation violates such symmetry. We consider a saturated multimode fiber amplifier in which light generates heat flow and suffers nonlinear thermo-optical scattering, breaking time-reversal symmetry. We identify a spacetime symmetry which maps the target output back to an input field. This mapping employs phase conjugation, gain and absorption substitution but not time reversal, and holds in steady-state and for slowly varying inputs. Our results open the possibility of output control of a saturated multimode fiber amplifier

Theory of Stimulated Brillouin Scattering in Fibers for Highly Multimode Excitations

Stimulated Brillouin scattering (SBS) is an important nonlinear optical effect which can both enable and impede optical processes in guided wave systems. Highly multi-mode excitation of fibers has been proposed as a novel route towards efficient suppression of SBS in both active and passive fibers. To study the effects of multimode excitation generally, we develop a theory of SBS for arbitrary input excitations, fiber cross section geometries and refractive index profiles. We derive appropriate nonlinear coupled mode equations for the signal and Stokes modal amplitudes starting from vector optical and tensor acoustic equations. Using applicable approximations, we find an analytical formula for the SBS (Stokes) gain susceptibility, which takes into account the vector nature of both optical and acoustic modes exactly. We show that upon multimode excitation, the SBS power in each Stokes mode grows exponentially with a growth rate that depends parametrically on the distribution of power in the signal modes. Specializing to isotropic fibers we are able to define and calculate an effective SBS gain spectrum for any choice of multimode excitation. The peak value of this gain spectrum determines the SBS threshold, the maximum SBS-limited power that can be sent through the fiber. We show theoretically that peak SBS gain is greatly reduced by highly multimode excitation due to gain broadening and relatively weaker intermodal SBS gain. We demonstrate that equal excitation of the 160 modes of a commercially available, highly multimode circular step index fiber raises the SBS threshold by a factor of 6.5, and find comparable suppression of SBS in similar fibers with a D-shaped cross-section

Supplementary_Figure_2 – Supplemental material for The irreversible HCV-associated risk of gastric cancer following interferon-based therapy: a joint study of hospital-based cases and nationwide population-based cohorts

Supplemental material, Supplementary_Figure_2 for The irreversible HCV-associated risk of gastric cancer following interferon-based therapy: a joint study of hospital-based cases and nationwide population-based cohorts by Chun-Wei Chen, Jur-Shan Cheng, Tai-Di Chen, Puo-Hsien Le, Hsin-Ping Ku and Ming-Ling Chang in Therapeutic Advances in Gastroenterology</p

Supplementary_Figure_5 – Supplemental material for The irreversible HCV-associated risk of gastric cancer following interferon-based therapy: a joint study of hospital-based cases and nationwide population-based cohorts

Supplemental material, Supplementary_Figure_5 for The irreversible HCV-associated risk of gastric cancer following interferon-based therapy: a joint study of hospital-based cases and nationwide population-based cohorts by Chun-Wei Chen, Jur-Shan Cheng, Tai-Di Chen, Puo-Hsien Le, Hsin-Ping Ku and Ming-Ling Chang in Therapeutic Advances in Gastroenterology</p

Additional file 13 of Two genomic regions of a sodium azide induced rice mutant confer broad-spectrum and durable resistance to blast disease

Additional file 13: Figure S4. Disease resistance study against rice bacterial blight (BB) pathogen Xanthomonas oryzae pv. oryzae (Xoo) XF-89b strai