Mode evolution and nanofocusing of grating-coupled surface plasmon polaritons on metallic tip

We present a detailed analysis on mode evolution of gratingcoupled surface plasmonic polaritons （SPPs） on a conical metal tip based on the guidedwave theory. The eigenvalue equations for SPPs modes are discussed, revealing that cylindrical metal waveguides only support TM01 and HEm1 surface modes. During propagation on the metal tip, the gratingcoupled SPPs are converted to HE31, HE21, HE11 and TM01 successively, and these modes are sequentially cut off except TM01. The TM01 mode further propagates with drastically increasing effective mode index and is converted to localized surface plasmons （LSPs） at the tip apex, which is responsible for plasmonic nanofocusing. The gapmode plasmons can be excited with the focusing TM01 mode by approaching a metal substrate to the tip apex, resulting in further enhanced electric field and reduced size of the plasmonic focus.

Plasmonic tip internally excited via an azimuthal vector beam for surface enhanced Raman spectroscopy

The synergy of a plasmonic tip and fiber-based structure light field excitation can provide a powerful tool for Raman examination. Here, we present a method of Raman spectrum enhancement with an Ag-nanoparticles(Ag-NPs)-coated fiber probe internally excited via an azimuthal vector beam(AVB), which is directly generated in a few-mode fiber by using an acoustically induced fiber grating. Theoretical analysis shows that gap mode can be effectively generated on the surface of the Ag-NPs-coated fiber probe excited via an AVB. The experimental result shows that the intensity of Raman signal obtained with analyte molecules of malachite green by exciting the Ag-NPs-coated fiber probe via an AVB is approximately eight times as strong as that via the linear polarization beam(LPB), and the activity of the AVB-excited fiber probe can reach 10^-11 mol∕L, which cannot be achieved by LPB excitation.Moreover, the time stability and reliability are also examined, respectively.

Generation of polarization and phase singular beams in fibers and fiber lasers

Cylindrical vector beams and vortex beams,two types of typical singular optical beams characterized by axially symmetric polarization and helical phase front,possess the unique focusing property and the ability of carrying orbital angular momentum.We discuss the formation mechanisms of such singular beams in few-mode fibers under the vortex basis and show recent advances in generating techniques that are mainly based on long-period fiber gratings,mode-selective couplers,offset-spliced fibers,and tapered fibers.The performances of cylindrical vector beams and vortex beams generated in fibers and fiber lasers are summarized and compared to give a comprehensive understanding of singular beams and to promote their practical applications.

PM_(2.5) in an industrial district of Zhengzhou,China:Chemical composition and source apportionment

Zhengzhou is a developing city in China, that is heavily polluted by high levels of particulate matter. In this study, fine particulate matter （PM2.5） was collected and analyzed for their chemical composition （soluble ions, elements, elemental carbon （EC） and organic carbon （OC）） in an industrial district of Zhengzhou in 2010. The average concentrations of PM2.5 were 181, 122, 186 and 211 μg/m3 for spring, summer, autumn and winter, respectively, with an annual average of 175 μg/m3, far exceeding the PM2.5 regulation of USA National Air Quality Standards （15 μg/m3）. The dominant components of PM2.5 in Zhengzhou were secondary ions （sulphate and nitrate） and carbon fractions. Soluble ions, total carbon and elements contributed 41%, 13% and 3% of PM2.5 mass, respectively. Soil dust, secondary aerosol and coal combustion, each contributing about 26%, 24% and 23% of total PM2.5 mass, were the major sources of PM2.5, according to the result of positive matrix factorization analysis. A mixed source of biomass burning, oil combustion and incineration contributed 13% of PM2.5. Fine particulate matter arising from vehicles and industry contributed about 10% and 4% of PM2.5, respectively.

Q-switched fiber laser based on saturable absorption of ferroferric-oxide nanoparticles

We demonstrate that ferroferric-oxide(Fe_3O_4) nanoparticles exhibit nonlinear saturable absorption property at1.55 μm, and fabricate two filmy saturable absorbers by embedding the nanoparticles into a polyvinyl alcohol(PVA) film or polyimide(PI) film separately. In the Fe_3O_4-PVA(Fe_3O_4-PI) Q-switched fiber laser, the pulse repetition rate increases from 8.5 kHz(5.5 kHz) to 28 kHz(49 kHz) and the pulse duration decreases from 23.5 μs(47 μs) to 6 μs(3.5 μs) by varying the pump power from 25 m W(23 mW) to 150 m W(650 mW). Experiment results indicate that PI-based saturable absorbers can afford larger powers than PVA-based saturable absorbers, which can be attributed to the higher fusion point of the PI film. The Fe_3O_4-PI saturable absorber exhibits features of high damage threshold, low cost, and good flexibility, which could be applied in fields of near-infrared pulse generation and frequency conversions.

Silica nanocone array as a template for fabricating a plasmon induced hot electron photodetector

Plasmon induced hot electrons have attracted a great deal of interest as a novel route for photodetection and lightenergy harvesting. Herein, we report a hot electron photodetector in which a large array of nanocones deposited sequentially with aluminum, titanium dioxide, and gold films can be integrated functionally with nanophotonics and microelectronics. The device exhibits a strong photoelectric response at around 620 nm with a responsivity of 180 μA/W under short-circuit conditions with a significant increase under 1 V reverse bias to 360 μA/W. The increase in responsivity and a red shift in the peak value with increasing bias voltage indicate that the bias causes an increase in the hot electron tunneling effect. Our approach will be advantageous for the implementation of the proposed architecture on a vast variety of integrated optoelectronic devices.

Azimuthal vector beam exciting silver triangular nanoprisms for increasing the performance of surface-enhanced Raman spectroscopy

Surface-enhanced Raman spectroscopy(SERS)with high-sensitivity performance is a very necessary detection technology.Here,we present a method for increasing the performance of SERS based on silver triangular nanoprism arrays(ATNAs)vertically excited via a focused azimuthal vector beam(AVB).The ATNA substrates with different structural parameters are prepared based on the traditional self-assembled and modified film lift-off technique.Based on a theoretical model established adopting the structural parameters of the ATNA substrates,theoretical simulation results show that AVB excitation can achieve greater electric-field enhancement than linearly polarized beam(LPB)excitation.Experimental result indicates that SERS sensitivity obtained via AVB excitation is 10^-13M(1 M=1 mol/L)using rhodamine 6 G(R6 G)as the target analyte,which is 2 orders of magnitude lower than that of LPB excitation(10^-11M).Meanwhile,the uniformity and reproducibility of the ATNA substrates are examined using Raman mapping and batch-to-batch measurement,respectively,and the Raman enhancement factor is calculated to be^3.3×10^7.This method of vector light field excitation may be used to improve the SERS performance of the substrates in fields of ultra-sensitive Raman detection.

Affirming nonlinear optical coefficient constancy from z-scan measurement

Goodness of fit is demonstrated for theoretical calculation of z-scan data based on beams propagating in the nonlinear medium and the Fresnel–Kirchhoff diffraction integral in experiments with high nonlinear refraction and absorption. The constancy of nonlinear optical parameters is achieved regardless of sample thickness and laser intensity, which clarifies the physical significance of optical parameters. We have obtained γ = 2.0 × 10-19 m2/W and β = 5.0 × 10-13 m/W for carbon disulfide excited by a pulsed laser at 800 nm with pulse duration of 35 fs,which are independent of sample thickness and laser intensity. Affirming constancy of the extracted parameters to the incident light intensity may become a practice to verify the goodness of the z-scan experiment.

Generation of nanosecond cylindrical vector beams in two-mode fiber and its applications of stimulated Raman scattering

We present the generation of the nanosecond cylindrical vector beams(CVBs)in a two-mode fiber(TMF)and its applications of stimulated Raman scattering.The nanosecond(1064 nm,10 ns,10 Hz)CVBs have been directly produced with mode conversion efficiency of~18 d B(98.4%)via an acoustically induced fiber grating,and then the stimulated Raman scattering signal is generated based on the transmission of the nanosecond CVBs in a 100-m-long TMF.The transverse mode intensity and polarization distributions of the first-order Stokes shift component(1116.8 nm)are consistent with the nanosecond CVBs pump pulse.

Thermal energy dependent transient permittivity of epsilon-near-zero material

Transparent conductive oxides exhibit attractive optical nonlinearity with ultrafast response and giant refractive index change near the epsilon-near-zero(ENZ) wavelength, originating from the intraband dynamics of conduction electrons. The optical nonlinearity of ENZ materials has been explained by using the overall-effective-mass and the overall-scattering-time of electrons in the extended Drude model. However, their response to optical excitation is yet the last building block to complete the theory. In this paper, the concept of thermal energy is theoretically proposed to account for the total energy of conduction electrons exceeding their thermal equilibrium value. The time-varying thermal energy is adopted to describe the transient optical response of indium-tin-oxide(ITO), a typical ENZ material. A spectrally-resolved femtosecond pump-probe experiment was conducted to verify our theory. By correlating the thermal energy with the pumping density, both the giant change and the transient response of the permittivity of ITO can be predicted. The results in this work provide a new methodology to describe the transient permittivities of ENZ materials, which will benefit the design of ENZ-based nonlinear photonic devices.

A polarization-insensitive fishnet/spacer/mirror plasmonic absorber for hot electron photodetection application

A polarization-insensitive plasmonic absorber is designed consisting of Au fishnet structures on a TiO2 spacer/Ag mirror. The fishnet structures excite localized surface plasmon and generate hot electrons from the absorbed photons, while the TiO2 layer induces Fabry–Perot resonance, and the Ag mirror acts as a back reflector.Through optimizing the TiO2 layer thickness, numerical simulation shows that 97% of the incident light is absorbed in the Au layer. The maximum responsivity and external quantum efficiency of the device can approach 5 mA/W and ~1%, respectively, at the wavelength of 700 nm.

Acousto-optic tunable ultrafast laser with vector-mode-coupling-induced polarization conversion

Acousto-optic interactions, employed in the ultrafast laser regulation,possess remarkable advantages for fast tuning performance in a wide spectral range. Here, we propose an ultrafast fiber laser whose wideband tunability is provided by an acousto-optic structure fabricated with an etched single-mode fiber. Because of the laser polarization conversion induced by the coupling between the core and cladding vector modes in the etched fiber,a band-pass characteristic of the acousto-optic interaction is achieved to effectively regulate the inner-cavity gain range. Cooperating with a saturable absorber based on single-wall carbon nanotubes (SWCNTs) with polarization robustness,a soliton operating state is achieved in the tunable erbium-doped fiber laser.By controlling the acoustical wave frequency from 1.039 to 1.069 MHz, this soliton laser can be conveniently tuned in a wide spectral range from 1571.52 to 1539.26nm.Meanwhile, the laser pulses have near-transform-limited durations stably maintaining less than 2 ps at different wavelength channels,owing to the broadband nonlinear absorption of SWCNTs.

Polarization-modified Fano line shape spectrum with a single whispering gallery mode

Using the HE11Xand HE11Ymodes in a tapered fiber(TF)and the whispering gallery mode(WGM)in a microsphere resonator,Fano line-shape spectra were theoretically described and experimentally observed in this study.The line shapes of the spectra can be tuned to form a Lorentz notch,various Fano line shapes,and the Lorentz peak by controlling the polarizations at the input and output ports of the TF.The relative position or the size of the waveguide and WGM resonator do not need to be accurately controlled to produce these effects.The proposed configuration is suitable for mass production and will improve the performance of the devices in which it is applied.