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A new substrate composing of Ag nanoflowers on graphene@Cu net (AgNFs/G@Cu) was fabricated on the flexible woven copper net. Graphene, an ideal 2D plane, provides better dispersion of regular electrodeposited AgNFs arrays. Graphene nanogap was used to enhance the EM coupling between AgNFs and metal Cu substrate. Regular AgNFs arrays offer denser “hotspots”, which further enhance the EM coupling. High sensitivity (rhodamine 6G (R6G): 10−14 M, crystal violet (CV): 10−12 M) and good homogeneity are realized in our experiment and proved by the COMSOL simulation. Cu net, remarkable flexibility and practicality, allows for a close contact to the sample surface by swabbing. Malachite green (MG) as low as 10−11 M on the surface of the marine fish can be detected. This work may offer novel and practical SERS applications in the areas of pesticide residue, food security and biotechnology.

Recently, biological materials have attracted more and more attentions on preparing the surface-enhanced Raman spectroscopy (SERS) substrate with the conspicuous SERS performance. Herein, we demonstrate a simply facile and inexpensive approach to fabricate a novel natural SERS substrate based on graphene oxide-Ag nanoparticles-Mytilus coruscus (GO-AgNPs-M.c.). The surface-enhanced resonance Raman spectroscopy (SERRS) signals of the Rhodamine 6G (R6G) on the GO-AgNPs-M.c. hybrids exhibit the much better SERRS performances in terms of the sensitivity, the signal reproducibility and the antioxidant stability than that on the AgNPs-M.c. one. The well electric field enhancement contribution is likewise confirmed in theory via the COMSOL software. Moreover, the successful detection of trace malachite green (MG) and methylene blue (MB) indicates the well feasibility for multi-molecules detection on the GO-AgNPs-M.c. hybrids. Thus, this inexpensive and high-efficiency natural SERS substrate have great potential for the considerable biochemical SERS applications and broaden the way for preparing multiple SERS platforms derived from other natural materials.

For application in practice, reducing the cost and simplifying the steps in fabrication of SERS substrates with excellent property are still the important factors. Here, a three-dimensional (3D) Ag nanoparticles (NPs) obtained by facile thermal evaporation is obtained for SERS. The Raman enhancement effect gradually become strong as the number of Ag NPs layers increase and attain the best until four layers Ag NPs deposited for the finite penetration depth of incident laser. The intensity of Raman signal collected from the four-layer Ag NPs substrate is around 11.3 times stronger than that from one-layer Ag NPs. Besides, the reproducibility of the SERS substrate was also significantly promoted as the layers of Ag NPs increases. The SERS substrate exhibits temporal stability and its enhancement factor attains 6.8 × 108. Furthermore, the SERS substrate can easily combine with some other blocks with superior SERS property and show better SERS ability. Additionally, the in-situ trace analyses in aquatic products were realized by flexible Ag NPs/PET substrate.

We proposed a high sensitivity and a high resolution surface plasmon resonance sensor composed of graphene–WS2 hybrid nanostructure and Au–Ag bimetallic-layers film. The bimetallic-layer configuration uses the high sensitivity of gold and the narrow full width at half maximum (FWHM) of silver to improve the sensor's sensitivity and resolution respectively. Graphene and WS2 serve as an effective light absorption medium. Once they cover the bimetallic-layer, the sensitivity can be further improved significantly. Graphene–WS2–Au–Ag model shows a higher sensitivity and higher resolution than that of the conventional sensing scheme where pure Au thin film is used. In our paper, we mainly use Fresnel equations and the transfer matrix method (TMM) to study the reflectivity and sensitivity as well as the full width at half maximum (FWHM) of the SPR sensor.

Nanogap-rich silver nanoislands with fascinating optical properties are desirable substrates for surface-enhanced Raman scattering (SERS). Here, we propose a simple and high-throughput approach through the laser molecular beam epitaxy (LMBE) technique for preparing silver nanoislands containing large numbers of intra-nanogaps on a silicon wafer (6×6 cm2). By optimizing the deposition time, the enlarged silver nanoislands with ∼5 nm interstitial gaps of abundance and homogeneity were formed. Remarkably, the optimized SERS substrate with high-density hotspots demonstrated a high analytical enhancement factor (AEF) as large as 1.17×105, excellent reproducibility with relative standard deviation (RSD) of 7.76% over the entire substrate, and good stability after storage for 21 days. The electromagnetic field distribution of the optimized SERS substrate was simulated using the software COMSOL Multiphysics based on the actual SEM image of the fabricated sample, and the calculated enhancement factor (EF) is as high as 109. Furthermore, it can enable sensitive and quantitative detection of malachite green at concentrations as low as 10−8 M. This simple fabrication of silver nanoislands with homogeneous ∼5 nm interstitial gaps provides a practical solution for wafer-scale, sensitive, and reproducible SERS substrates.

We report a three-dimensional (3D) SERS substrate with different numbers of silver nanoparticle (Ag NP) layers using multilayer graphene oxide (GO) as a spacer. The SERS performance of the 3D nanostructure was investigated and it was found that the SERS effect increased as the number of Ag NP layers increased, and showed almost no change for more than four layers. We found that the SERS performance of the 3D nanostructures can be mainly attributed to the topmost hot spots which are closely related to the Ag NP layers in the 3D nanostructure. Furthermore, we explored 3D nanostructures with different Ag NP layers using the finite difference time domain method (FDTD). The 3D SERS substrates also exhibit excellent detection capability. The limit of detection (LOD) was calculated down to 10−15 M for R6G and 10−12 M for CV. In addition, the reproducibility of the 3D SERS substrate was attributed obviously to the increasing number of Ag NP layers. Based on these promising results, the highly sensitive detection of molecules such as malachite green was demonstrated for food safety inspection.

Passively Q-switched mode-locked dual-wavelength Nd:GYSGG laser using graphene oxide saturable absorber is demonstrated. The output spectrum peaked at 1057.28 nm and 1060.23 nm with wavelength spacing of 3 nm has been measured. At the pump power of 5 W, the maximum average output power of 0.189 W was obtained with the optical conversion efficiency of 3.79%. The mode-locked pulse inside the Q-switched envelope has a repetition rate of 100 MHz and its pulse width is estimated to be about 441 ps. In addition, a simple way to fabricate few layers graphene oxide saturable absorbers has been reported.

A dual-wavelength self-Q-switched operation of Nd:GYSGG laser with different output couplers is proposed and demonstrated. In self-Q-switched operation, two laser lines between 1056.86 nm and 1060.23 nm were found. Under the pump power of 4 W, the shortest pulse width of 2.02 μs was obtained with a maximum average output power of 565 mW and the optical conversion efficiency of 14.13%. The pulse repetition rate and single pulse energy were 50.2 kHz and 11.25 μJ, respectively. The system is very stable and suitable for generation of multi-wavelength pulses.

A homogeneous titanium oxide device is fabricated by Molecular Beam Epitaxy and the excellent memristive behavior could be observed. The resistance ratio between high resistance state and low resistance state increases as the thickness of titanium oxide or the oxygen content reduces. The forming voltage is lower than the previous report. Based on the experimental results, we suggest a double-interface conductive filament model and calculate the current–voltage curve of the device in agreement with experimental measurements. The result indicates that the forming voltage of the device will rise with increasing of the chemical potential differences across the two interfaces. In other words, the switching voltage could be adjusted by selecting a proper electrode and oxide materials.

We demonstrate the use of molybdenum disulfide (MoS2)-based saturable absorber to develop a passively Q-switched Nd:YVO4 laser. The few-layer MoS2 saturable absorber was prepared by mixing MoS2 with alcohol and then evaporating the mixture on a substrate. Using an MoS2 saturable absorber, stable Q-switching operation was achieved with a laser pulse width of 327 ns and a repetition rate of 5 kHz under a pump power of 2.23 W. Due to the simple manufacturing process and high saturated absorption characteristics, MoS2 seems to be a suitable material as a saturable absorber for solid-state lasers.

We have proposed a synthetic approach to produce self-supported and bendable surface-enhanced Raman scattering (SERS)-based 3D chemical sensors with high adsorptivity. Such 3D substrates consist of foam-like graphene macrostructures obtained by template-directed chemical vapour deposition on nickel foams (interconnected 3D scaffold of nickel) and uniform and high-density Ag nanoparticles wrapping around the foam graphene, via seed-mediated in situ growth process. Such 3D AgNPs/G@Ni foam substrates show high-quality SERS performance in terms of Raman signal reproducibility and sensitivity for the analyte, resulting from the high density and homogeneity of “hot spots” on AgNPs/G@Ni foam, multiple cascaded amplication (localized surface plasmon mode and optical standing waves or optical refraction) of incident laser to the 3D foam structures and powerful support from nickel scaffold. Moreover, in virtue of the high adsorptivity and sensitivity of AgNPs/G@Ni foam, the low-concentration crystal violet molecules can be easily traced in the curvilinear fish surface, by simply swabbing the surface to achieve molecules concentration effect in the practical applicability. This work shows promising potential in developing the applications of SERS in the foodstuffs processing and security field.

Metal film is an essential part of the electrically pumped organic semiconductor lasers. But the large loss is the most important factor restricting the electrical pumping. In this paper, we investigate optically pumped amplified spontaneous emission (ASE) in the presence of metal films. The ASE threshold of device with metallic film is reduced by 2.5 times in comparison with that of the metal-free devices. The SiO2 space layer with optimizing thickness between gain media and metal film can effectively prevent absorption loss but also provides a proper waveguide effect. Furthermore, the metal film can prevent the light leaking to the substrate and reflect the lights back into the media, which increases the intensity of pumping and emission again.

The contribution of the Green's function to the scattered magnetic field is not well expressed in the traditionally used magnetic field integral equation (MFIE) when the source and field points lie on the same plane. This decreases the accuracy of MFIE for objects with flat surfaces. To solve this problem, the normal magnetic field integral equation resulting from the use of the normal boundary condition is considered. It is then combined with the traditionally used MFIE into a new MFIE formulation, named the combined magnetic field integral equation (CMFIE). Through the use of an appropriate combination parameter, the omitted contribution of the Green's function to the scattered magnetic fields in the traditionally used MFIE can be recovered in this new CMFIE. Numerical results validate the analysis and show that the proposed MFIE formulation is much more accurate than the traditional one for small objects with flat surfaces.

Passively Q-switched mode-locking performance of Nd:GdTaO4 crystal using molybdenum disulfide (MoS2) as a saturable absorber at 1066 nm was demonstrated for the first time. The MoS2 saturable absorber was prepared simply by transferring the MoS2 suspension onto a quartz substrate. By inserting the MoS2 saturable absorber into the Nd:GdTaO4 laser, stable Q-switched modelocked operation can be achieved. At the pump power of 4 W, the maximum average output power of 0.156 W was obtained with the optical conversion efficiency of 3.9%.

The performance of a diode end-pumped passively Q-switched dual-wavelength Nd:GYSGG laser operating at 1057.28 and 1060.65 nm with graphene oxide as the saturable absorber was demonstrated. The maximum dual-wavelength average output power of 521 mW was achieved under the absorbed pump power of 5.4 W, corresponding to the optical-to-optical conversion and slope efficiency of 9.8% and 21%, respectively. The minimum pulse width was 115 ns with a pulse repetition rate of 338 kHz.

A free electron laser (FEL) amplifier with an elliptical waveguide and a planar wiggler is proposed. A set of coupled operating equations for this FEL operating in the Raman regime is derived using three-dimensional nonlinear theory. The numerical simulation code is developed, and the characteristics of this FEL amplifier are analysed numerically. Our numerical simulations are conducted to model a 34.6 GHz amplifier with an electron beam with an energy of 3.1 MeV. The results indicate that the elliptical waveguide is more suitable for a FEL with a planar wiggler system than a rectangular waveguide.

A novel Smith–Purcell (SP) free-electron laser (FEL) with axial-symmetrical structure is proposed. The distribution of the high frequency field in the resonator is analyzed and the operating frequency and output power are also studied by the use of particle-in-cell (PIC) simulation method. Results indicate that coherent radiation with output peak power up to 58 MW at millimeter wavelengths can be generated by using relativistic electron beams of moderate energy.

The compact free electron maser (FEM), with the advantages of small size and low cost, is important for realistic applications in medicine and industry. In order to utilize the planar wiggler to the best advantage and simultaneously to reduce the necessary electron beam voltage and the size of FEM device, we propose to utilize an elliptical waveguide as the high frequency structure of planar wiggler FEM. A set of universal operating equations suitable for elliptical waveguide and rectangular waveguide FEMs is derived by using the nonlinear theory. The characteristics of the FEMs are numerically analyzed, comparatively.

Dispersion equation of planar-wiggler free-electron laser with background plasma is derived using linear fluid theory. The characteristic of this plasma FEL is analyzed in detail. It is shown that reasonable background plasma density tends to improve FEL growth rate. We also find there exits a critical plasma density for FEL operating. When the background plasma density is higher than critical density, the FEL can no longer be excited.

The physical phenomenon, surface-enhanced Raman scattering (SERS), is mainly based on the local electromagnetic fields enhancement located at the nano-gaps between metal nanostructures attributed to localized surface plasmon resonance. Therefore, nano-gaps are very important for obtaining high-density hot spots and optimal and uniform SERS signals. However, it remains a challenge to form the three-dimensional ultra-narrow nano-gaps. Here, a gyrus-inspired Gyrus-SERS substrate was fabricated with the nanostructure of Ag gyrus/graphene/Au film using an extremely simple method. The lateral and vertical hot spots respectively were obtained from the dense nano-gaps (~3 nm) between gyrus and the coupling of Ag gyrus and Au film in bilayer graphene nano-gaps (0.68 nm), which were demonstrated in experiment and theory. The proposed Gyrus-SERS platform performs an excellent SERS activity (EF~5 × 109), high sensitivity (the minimum detected concentration of R6G and CV respectively is 10−13 and 10−12 M), and outstanding reproducibility (RSD~7.11%). For practical application, the in situ detection of Malachite green (MG) residue on prawn skin was executed using the prepared flexible Gyrus-SERS substrate, which shows the wide potential in food safety field.

We experimentally demonstrated a laser diode-pumped Q-switched Nd:GdTaO4 crystal laser at 1066 nm using a multilayer graphene oxide as the saturable absorber (GOSA). The GOSA is fabricated by transferring the liquid-phase-exfoliated GO nanosheets onto a K9 glass substrate. When the GOSA was inserted into the plano–plano laser cavity, a stable Q-switched laser operation is achieved with a maximum average output power of 0.382 W and repetition rate of 362 kHz. The shortest pulse duration is 194 ns and the single pulse energy is about 1.05 μJ.

We describe the development of computer visualization tools that display topographic maps of neuromagnetic data on a realistic head shape. Two topographic display modes are implemented — a "raw" mode that shows the actual position and orientation of the detection coils with respect to the head shape, as well as the magnetic field sensed by each coil, and a "cooked" mode, where the magnetic data is interpolated onto the scalp. False coloration is used to represent magnetic field strength. The topographic displays are combined with waveform displays in order to integrate spatial and temporal visualization of the data. Dipole fitting routines have also been incorporated into the software.