Controlled light localization and nonlinear-optical interactions of ultrashort laser pulses in micro-and nanostructured fibers with a tunable photonic band gap

Physical principles behind the control of light localization and nonlinear-optical interactions in micro-and nanostructured fibers are demonstrated. Transmission measurements on the cladding of nanostructured fibers having a form of a two-dimensional periodic structure with a pitch less than 500 nm have revealed the existence of a photonic band gap tunable within the range from 930 to 1030 nm. The influence of the structure of the holey-fiber cladding on the effective area of the waveguide mode and the spectral broadening of Ti:sapphire and Cr:forsterite femtosecond laser pulses is experimentally studied. It is shown that the increase in the air-filling fraction of a holey-fiber cladding results in a considerable enhancement of spectral broadening of short laser pulses due to the increase in the light localization degree in the fiber core.     

Double-resonance plasmon and polarization effects in a SERS fiber sensor with a grid nanostructure

We presented a systematic study of surface enhanced Raman scattering (SERS) fiber sensor with a grid nanostructure. The plasmonic resonance peak is stable when measuring gas and double-resonance plasmon can be effectively excited; meanwhile local electric field can be strongly enhanced with the metal coated nanostructured fiber facet. Studies on the influence of polarization effects, the plasmon resonance wavelength shift is relatively small in our structure.     

Efficient light confinement with nanostructured optical microfiber tips

Nanostructured optical microfiber tips are proposed and experimentally demonstrated to efficiently confine light beyond the diffraction limit at high powers. Focused ion beam milling was used for the nanostructuring of gold-coated optical microfiber tips with both single-ramp and wedge geometries. Small apertures were formed by flat cutting or hole drilling and optical spot sizes of ∼λ/10 with high transmission efficiency were achieved. Numerical simulations were carried out to optimize the device design with circularly polarized light. Enhanced transmission efficiencies (higher than 10⁻²) were recorded by optimizing the overall light throughput along the fiber tips. The tip thermal behavior was investigated by launching high powers into the device and recording the tip position in a scanning near-field optical microscopy set-up. This nanostructured optical microfiber tip has the potential for applications in optical recording, scanning near-field optical microscopy and lithography.     

Preparation and Dielectric Properties of Nanostructured ZnO Whiskers

By a novel controlled combustion synthesis method, a large number of nanostructured ZnO whiskers with different morphologies, such as tetra-needles, long-leg tetra-needles and multi-needles, are prepared without any additive in open air at high temperature. The morphologies and crystalline structures of the as-prepared ZnO nanostructured whiskers are investigated by SEM and XRD. The possible growth mechanism on the nanostructured ZnO whiskers is proposed. The experimental results indicate that the dielectric constants and losses of the nanostructured ZnO whiskers are very low, demonstrating that the nanostructured ZnO whiskers are low-loss materials for microwave absorption in X-band. However, obvious microwave absorption in nanostructured ZnO whiskers is observed. The quasi-microantenna model may be attributed to the microwave absorption of the ZnO whiskers.     

Broad and tunable multiwavelength fiber laser at the assistance of modulation-instability-assisted four-wave mixing

Based on a piece of highly-nonlinear near-zero-dispersion-flattened photonic crystal fiber (PCF), a broadly tunable multiwavelength erbium-doped fiber laser is proposed by using a bi-directionally pumping scheme. This kind of PCF induces the modulation-instability-assisted four-wave mixing to generate new wavelengths. The proposed laser with excellent stability is tunable and switchable by adjusting the fiber Bragg gratings and the variable optical attenuators. The outstanding merits of the proposed multiwavelength laser are the flexible tuning and the ultrabroad spectral range over 150 nm. Especially, the proposed laser source can work at the wavelength of less than 1460 nm, overcoming the limit of gain bandwidth of erbium-doped fiber.

     

SIZE AND GRAIN-DISTRIBUTION EFFECTS IN ELECTRONIC PROPERTIES OF TWO-DIMENSIONAL NANOCRYSTALLINE RECTANGLE SYSTEMS

According to the structural characters of real nanostructured systems, a model for two-dimensional nanostructured systems has been proposed. By developing a renormalization-group Green's function scheme, local electronic density of states at a type of crystallire sites is presented on the pure hopping model. It is found that the changes of all the grain-distribution rules, the average atomic number and the atomic interaction are connected with the spectral structures of nanostructured systems. However, bule shifts of the spectra of nanostruetured systems seem to be modulated mainly by the changes of the bond-parameters which are correlated with the lattice distortions of the nano-sized grains.     

Nanostructured substrate with nanoparticles fabricated by femtosecond laser for surface-enhanced Raman scattering

A simple and fast method to fabricate nanostructured substrates with silver nanoparticles over a large area for surface-enhanced Raman scattering (SERS) is reported. The method involves two steps: (1) dip the substrate into a silver nitrate solution for a few minutes, remove the substrate from the solution, and then air dry and (2) process the silver nitrate coated substrate by femtosecond (fs) laser pulses in air. The second step can create silver nanoparticles distributed on the nanostructured surface of the substrate by the photoreduction of fs multiphoton effects. This study demonstrates that an enhancement factor (EF) greater than 5×10⁵, measured by 10⁻⁶ M Rhodamine 6G solution, can be achieved. The proposed technique can be used to integrate the SERS capability into a microchip for biomedical and chemical analysis.     

Inspection of nano-sized SNOM-tips by optical far-field evaluation

High resolution optical microscopy has many interesting applications in solid state physics, low temperature physics, biology and semiconductor technology. Unfortunately, the lateral resolution of conventional microscopes is limited by the Rayleigh-limit. “Scanning nearfield optical microscopy” (SNOM) seems to be a promising new approach to characterize the properties of materials optically with a high lateral resolution of 50–100 nm. The most important part of such a microscope is the scanning probe (a special glass fiber tip). However, the quality of the optical fiber tip is of decisive importance. Since the production process of pulled and coated glass fiber tips is still highly empirical and error-prone, a technique would be useful to determine the tips’ quality before they are shipped to the user or mounted in the microscope. The tips’ apertures are smaller than λ/2 and therefore they cannot be measured in a non-destructive way by conventional optical microscopy. This paper discusses an easy and fast method for the optical characterization of common glass fiber SNOM tips. The effective aperture of the tip is measured from the far-field distribution of the emitted intensity recorded by a CCD target. A numerical model is introduced to solve this inverse task and a simple optical setup is presented to detect light emitted by the tip at an angle of up to 90° from the optical axis. Experimental investigation, near/far-field calculations and scanning electron microscope investigations show the working principle of this measurement technique for the analysis and evaluation of a typical nanostructured object.     

Fractal description of significant nano-effects in polymer composites with nanosized fillers. Aggregation,phase interaction,and reinforcement

The paper analyzes experimental data obtained on physical and mechanical properties of nanostructured particle-reinforced composites with elastomer matrices and nano- and microsized carbon-containing particles by scanning probe microscopy and nanoindentation with specialized 3D computer processing. The nano-effects observed in the elastomer matrices are described using the fractal approach. A fractal model of nanoparticle aggregation in a polymer matrix is proposed. Phase interactions in the nanostructured polymer materials are described and fractal relations that predict the reinforcing effect of this type of media are presented. It is shown that interphase regions in the nanostructured composites are the same reinforcing elements as a nanofiller for the medium. It is found that reinforcement of elastomer composites by nanosized particles is a true nano-effect.     

Low-threshold single-wavelength all-fiber laser generating cylindrical vector beams using a few-mode fiber Bragg grating

We propose and demonstrate a low-threshold single-wavelength all-fiber laser generating cylindrical vector beams using a few-mode fiber Bragg grating. Both radially and azimuthally polarized beams have been generated with very good modal symmetry and polarization purity higher than 94%. The radially and azimuthally polarized modes can be switched by simply adjusting the polarization controllers built in the fiber laser cavity. This fiber laser operates at a single wavelength of 1053 nm with a 3 dB linewidth of less than 0.02 nm, signal-to-background ratio of more than 55 dB, and a threshold as low as 16 mW. A new method for the polarization purity measurement is also proposed.     

基于环形芯光纤的超低差分模式增益涡旋光纤放大器

提出了一种基于环形芯铒离子部分掺杂光纤的涡旋光纤放大器。针对该掺铒光纤的放大特性,研究了光纤长度、掺铒浓度与抽运功率对信号模式增益特性的影响。研究结果表明,该光纤放大器能够支持22个轨道角动量模式稳定传输,且C波段(1530~1565 nm)所有信号模式增益大于23 dB,信噪比高于27 dB,差分模式增益小于0.015 dB。所提出的基于环形芯光纤的涡旋光纤放大器具有支持轨道角动量模式数量多、差分模式增益低、信噪比高的优势,对于OAM复用长距离传输系统中的在线放大具有重要参考价值。     

Field emission of electrons from a single carbon fiber with a nanostructured emitting surface

The field emission of electrons from a single fine carbon fiber with a nanostructured emitting surface is studied experimentally. It is found that the fiber can serve as an effective field emitter of electrons at voltages of ∼10²–10³ under the conditions of technical vacuum and the emission current density may reach ∼10² A/cm². At a certain threshold voltage, the fiber starts executing flexural mechanical vibrations. The onset of mechanical vibrations is accompanied by a change in the field emission conditions. Namely, in the absence of vibrations, the mode of steady current extraction is observed. When mechanical vibrations set in, the field emission switches to the mode of current periodic oscillations with a constant component.     

Effect of electric fields on the surface of nanostructured electrodes on charge formation

The processing of the atomic force microscopy images of specific nanostructured electrodes made of different metals shows that the shape of a nanostructure’s peak can be well approximated using a parabolic dependence. The local electric fields and injection currents are calculated using the data on the nanostructured surface of electrodes with the aid of the Fowler-Nordheim dependence. A qualitative analysis is performed and the applicability of the proposed approach in the construction of the electroconvection mass- and heat-exchange devices is demonstrated.     

A thermodynamic explanation for martensitic phase stability of nanostructured Fe-Ni and Co metallic materials

In order to explain the experimental facts that start temperatures of reverse martensite transformation (A_s) for nanostructured Fe-Ni and Co are lower than those for their coarse-grained counterparts, a thermodynamic model was established in this paper. The thermodynamic analysis shows that the decrease of A_s for nanostructured Fe-Ni and Co can be attributed to the difference of surface free energy for martensite and austenite. The correlation between the decrease of A_s and start temperature of martensitic transformation (M_s) in nanostructured material is further proposed.     

Intensity based erbium distribution for erbium doped fiber amplifiers

A method to calculate an optimum Erbium distribution to enhance the gain efficiency in EDFA is proposed. This method calculates Erbium distribution based on optical pump envelop, pump power and optical properties of Erbium ion. The intensity-based Erbium distribution is obtained for single-mode fiber and dispersion-shifted fiber types EDFA for pump power from 10 to 40 mW. All of the profiles have a Gaussian-like shape. For single-mode fiber type EDFA, high gain enhancement is obtained in a relatively short optimal length of fiber. Optimal length of EDFA, using intensity-based Erbium distribution, is increased by a factor of 1.5–3 with respect to the one using stepwise. This optimal length increasing factor is considerably smaller than that of the EDFA, using the one-fourth confinement Erbium profile.     

全光纤色散腔掺铒光纤激光器

报道用９７１ｎｍ半导体激光器泵浦的掺铒光纤激光器的一些实验研究结果。演示了由光纤环反射器和光纤光栅构成的全光纤色散掺铒光纤激光器，在１．５５μｍ波段获得了线宽小于０．０５ｎｍ的激光输出。     

长光纤环高性能光电振荡器研究

提出了采用多个长光纤环实现高性能光电振荡器的方法,通过合理的长度设计,多个长光纤环能够降低光电振荡器的相位噪声,同时提高边模抑制比。理论分析了多个长光纤环实现高性能光电振荡器的可行性,并构建了实验系统。在实验中,利用3个大于4km的长光纤环构建光电振荡器,实现了频率10GHz的微波信号,其相位噪声在频偏10kHz处达到了-130dBc/Hz,边模抑制比达到了60dBc。实验结果与理论分析一致,证明了该方法的正确性。     

基于两滤波器级联和饱和吸收体的单纵模双波长光纤激光器

提出和证实了一种光纤环形激光器. 将一个带通滤波器与一个带阻滤波器级联,用一段保偏掺铒光纤作为饱和吸收体,得到了一种单纵模双波长光纤激光器. 结果表明,该激光器波长间隔为0.4 nm,边模抑制比大于49 dB.如果将两个波长进行差拍,该激光器有潜力得到49.85 GHz的微波信号.     

Double-pass ytterbium-doped fiber amplifier with high gain coefficient and low noise figure

We have proposed and demonstrated a double-pass ytterbium-doped fiber amplifier using an optical circulator and a fiber Bragg grating as reflector. When the signal has passed through the ytterbium-doped fiber once, it reflects off a 0.2-nm passive fiber Bragg grating filter. This reduces amplified spontaneous emission (ASE) noise from the first pass. The input signal light is amplified both forward and backward through ytterbium-doped fiber. With this double-pass configuration, 1053.15-nm unsaturated signal gain of 28 dB, gain coefficient of 1.1 dB/mW, and noise figure of less than 4 dB are achieved at 977-nm pump power of 68 mW. It is also found that this double-pass configure provides enhancing gain coefficient and improving noise figure by comparison with single-pass configuration.     

基于椭圆孔包层和微型双孔纤芯的新型高双折射光子晶体光纤

设计了一种新型高双折射光子晶体光纤,即其包层引入椭圆形空气孔,且以三角晶格方式周期排列,纤芯引入亚波长尺寸(~0.16 μm)的微型双孔结构阵列.采用全矢量有限元法和各向异性完美匹配层边界条件分析了该型光子晶体光纤的双折射特性和色散特性,详细介绍了该光子晶体光纤在不同的椭圆率、椭圆归一化面积、微型双孔孔径、两小孔之间间距的情况下双折射和限制损耗随波长的变化曲线.模拟结果表明,通过同时在包层和纤芯引入非对称性,获得了较高的双折射(~10^-3量级)和极低(~10^-4 dB/km)的限制损耗.提供了一种新的光子晶体光纤设计方法,即通过同时在包层和纤芯引入新结构来同时获得高双折射和低损耗.