Investigation of low pulse repetition passively mode-locked Nd:YVO4 lasers via multi-pass periodic trajectories in a confocal cavity.

We derive the parametric equations for the geometric rays of a periodic orbit inside a confocal cavity. Based on the derived formula, we demonstrate a passively mode-locked solid-state laser with a low pulse repetition rate to obtain a pulse train traveling along zigzag multi-pass trajectories. We achieve a stable mode-locked pulse train with a pulse repetition rate of 18 MHz by designing the cavity to satisfy the dual-M trajectory. Furthermore, by precisely adjusting cavity mirrors under the same experimental setup, we can reach pulse repetition rates of 12 and 9 MHz for the mode-locked laser. It is believed that the numerical calculation and the developed experiment can provide a straightforward and convenient way to achieve a low pulse repetition rate for passively mode-locked lasers.

Tunable plasmonic effects arising from metal-dielectric nanorods.

We have investigated the plasmonic effects in a two-dimensional periodic array of metallodielectric nanorods with and without the rotational angle, in which the integration of the localized surface plasmon resonance (SPR) and hollow plasmon resonance (HPR) properties is performed. Four patterns of nanostructures are investigated. We make use of the three-dimensional finite element method to obtain the simulation results, which demonstrate that the localized SPR and HPR in metallodielectric nanorods enhance the near-field intensity and increase the depth of the transmittance dip, providing an additional degree of freedom in the control of the light wave at the nanoscale. Numerical results show that the depth of the transmittance dip and sensitivity of case 1 and case 2 can be elevated to a value of 83.21% and 6.7 times, respectively, when the rotational angle of metal-dielectric nanorods varies from 0° to 90°. The sensitivity of case 3 and case 4 can be raised to the magnitude of 700-1091 nm/RIU (where RIU is the refractive index unit), and the characteristics enable the extensive applications for nanophotonic devices with high performance in a predictable manner.

Interplay of mutual electric and magnetic couplings between three-dimensional split-ring resonators.

We experimentally and theoretically study the interplay between capacitive electric and inductive magnetic couplings in infrared metamaterials consisting of densely-packed three-dimensional (3D) meta-atoms. The meta-atom is made of metal-stress-driven assembled 3D split-ring resonators to exhibit strong bi-anisotropy, where electric and magnetic resonances occur simultaneously. By varying the spatial arrangement of the arrayed meta-atoms, the mutual coupling between meta-atoms dramatically modifies their mode profiles and resultant spectral responses. The corresponding numerical simulations evidently retrieved current densities and magnetic field strengths, as well as the transmittance, to reveal the important resonant behavior in the coupled meta-atom systems. We conclude that the mutual electric coupling between the neighboring meta-atoms plays a crucial role to the scattering behaviors of the bi-anisotropic metamaterials.

Confined migration of induced hot electrons in Ag/graphene/TiO2 composite nanorods for plasmonic photocatalytic reaction.

Confined migration of hot electrons is presented in nanorods of layered Ag/graphene/TiO2 structure for highly efficient plasmonic photocatalytic water treatment. The light-illuminating titanium dioxide (TiO2) nanorods provide a large amount of high-energy hot electrons for the generation of highly-active superoxide radical (*O2 -) that leads to the degradation of organics in water. Comparison between photocatalytic processing efficiency by photocatalysts with various composite materials were presented based on the preferred propagation path of induced hot electrons that leads to generation of *O2 -. The best results done by Ag/graphene/TiO2 nanorods showed that the sandwiched layer of graphene on TiO2 nanorods collects the induced hot electrons and results in high efficiency photocatalytic reaction.

Special Issue on the State-of-the-Art Optical Properties and Applications of Metallic Nanostructures in Asia.

With developments in nanofabrication technology, the optical properties and applications of metallic nanostructures have attracted increased research interest in recent years [...].

Effects of external light in the magnetic field-modulated photocatalytic reactions in a microfluidic chip reactor.

Photocatalytic reactions and their magnetic-field enhancement present significant potential for practical applications in green chemistry. This work presents the mutual enhancement of plasmonic photocatalytic reaction by externally applied magnetic field and plasmonic enhancement in a micro optofluidic chip reactor. The tiny gold (Au) nanoparticles of only a few atoms fixed on the surface of titanium dioxide (TiO2) nanoparticles lead to mutually boosted enhancement photocatalytic reactions under an external magnetic field and plasmonic effects. The dominant factor of adding green light to the photocatalytic reaction leads to the understanding that it is a plasmonic effect. The positive results of adding ethanol alcohol (EA) in the experiments further present that it is a hot electron dominant path photocatalytic reaction that is positively enhanced by both the external magnetic field and plasmonic effects. This work offers great potential for utilizing magnetic field enhancement in plasmonic photocatalytic reactions.

Bacteria Contaminants Detected by Organic Inverter-Based Biosensors.

The importance of bacteria detection lies in its role in enabling early intervention, disease prevention, environmental protection, and effective treatment strategies. Advancements in technology continually enhance the speed, accuracy, and sensitivity of detection methods, aiding in addressing these critical issues. This study first reports the fabrication of an inverter constructed using crosslinked-poly(4-vinylphenol) (C-PVP) as the dielectric layer and an organic complementary metal-oxide semiconductor (O-CMOS) based on pentacene and N,N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide (PTCDI-C13) as a diagnostic biosensor to rapidly detect bacterial concentration. Bacteria including Escherichia coli O157, Staphylococcus aureus ATCC25922, and Enterococcus faecalis SH-1051210 were analysed on the inverters at an ultra-low operating voltage of 2 V. The high density of negative charge on bacteria surfaces strongly modulates the accumulated negative carriers within the inverter channel, resulting in a shift of the switching voltage. The inverter-based bacteria sensor exhibits a linear-like response to bacteria concentrations ranging from 102 to 108 CFU/mL, with a sensitivity above 60%. Compared to other bacterial detectors, the advantage of using an inverter lies in its ability to directly read the switching voltage without requiring an external computing device. This facilitates rapid and accurate bacterial concentration measurement, offering significant ease of use and potential for mass production.

Surface plasmon-enhanced photoluminescence of DCJTB by using silver nanoparticle arrays.

It is demonstrated that photoluminescence of DCJTB can be enhanced by surface plasmons occurred in silver nanoparticle arrays on glass substrates fabricated by using nanosphere lithography (NSL) combined with reactive ion etching (RIE). By changing the size of the seed polystyrene nanosphere with fixed thickness of SiO(2) film as a buffer layer between silver nanoparticles and fluorescent dye, we systematically studied the interaction between surface plasmons in Ag nanostructures and fluorescent dye by measuring the photoluminescence and time-resolved photoluminescence (TRPL) of the samples. As compared with pure DCJTB, it is observed that PL enhancement as high as 9.4 times and life time shortening from 0.966 ns shortened to 0.63 ns can be achieved with polystyrene nanosphere 430 nm in diameter. The physical origin due to plasmonic excitation has been clarified from 3D finite element simulations, as well as the assistance of UV-visible reflectance spectrum.

Multi-level surface enhanced Raman scattering using AgOx thin film.

Ag nanostructures with surface-enhanced Raman scattering (SERS) activities have been fabricated by applying laser-direct writing (LDW) technique on silver oxide (AgOx) thin films. By controlling the laser powers, multi-level Raman imaging of organic molecules adsorbed on the nanostructures has been observed. This phenomenon is further investigated by atomic-force microscopy and electromagnetic calculation. The SERS-active nanostructure is also fabricated on transparent and flexible substrate to demonstrate our promising strategy for the development of novel and low-cost sensing chip.

Imaging layer number and stacking order through formulating Raman fingerprints obtained from hexagonal single crystals of few layer graphene.

Quantitative mapping of layer number and stacking order for CVD-grown graphene layers is realized by formulating Raman fingerprints obtained on two stepwise stacked graphene single-crystal domains with AB Bernal and turbostratic stacking (with ~30°interlayer rotation), respectively. The integrated peak area ratio of the G band to the Si band, A(G)/A(Si), is proven to be a good fingerprint for layer number determination, while the area ratio of the 2D and G bands, A(2D)/A(G), is shown to differentiate effectively between the two different stacking orders. The two fingerprints are well formulated and resolve, quantitatively, the layer number and stacking type of various graphene domains that used to rely on tedious transmission electron microscopy for structural analysis. The approach is also noticeable in easy discrimination of the turbostratic graphene region (~30° rotation), the structure of which resembles the well known high-mobility graphene R30/R2(±) fault pairs found on the vacuum-annealed C-face SiC and suggests an electron mobility reaching 14,700 cm(3) V(-1) s(-1). The methodology may shed light on monitoring and control of high-quality graphene growth, and thereby facilitate future mass production of potential high-speed graphene applications.

Enhancement of luminous efficacy of polymer light-emitting diodes with silver-nanoparticles by oxygen-plasma treatment.

Silver-nanoparticles deposited on indium tin oxide (AgNPs/ITO) with different O2 -plasma treatment times are used as the anode window substrate for polymer light-emitting diodes (PLED). When AgNPs/ITO with an O2 -plasma treatment time of 10 min is used for PLED, a maximum current efficiency of 3.33 cd/A is realized, which is notably higher than that of a reference PLED (1.00 cd/A). Compared to those of the reference PLED, the mean current efficiency and electroluminescence intensity of the optimal PLED are enhanced by 3.24 times and 480%, respectively. O2 -plasma treatment is an easy method for optimizing the localized surface plasmon resonance effect of metal nanoparticles, exhibiting advantages of scalable mass production and high suitability for applications in related optoelectronic components.

Sub-wavelength GaN-based membrane high contrast grating reflectors.

The GaN-based membrane high contrast grating (HCG) reflectors have been fabricated and investigated. The structural parameters including grating periods, grating height, filling factors and air-gap height were calculated to realize high reflectivity spectra with broad bandwidth by the rigorous coupled-wave analysis and finite-difference time-domain method. Based on the optimized simulation results, the GaN-based membrane HCGs were fabricated by e-beam lithography and focused-ion beam process. The fabricated GaN-based membrane HCG reflectors revealed high reflectivity at 460 nm band with large stopband width of 60 nm in the TE polarization measured by using the micro-reflectivity spectrometer. The experimental results also showed a good agreement with simulated ones. We believe this study will be helpful for development of the GaN-based novel light emitting devices in the blue or UV region.

Fabrication of plasmonic devices using femtosecond laser-induced forward transfer technique.

Using femtosecond laser-induced forward transfer techniques we have fabricated gold dots and nanoparticles on glass substrates, as well as nanobumps on gold thin film. The surface morphologies of these structures with different laser fluences and film thicknesses are investigated. We also study the focusing and defocusing properties of the nanofence-an arranged nanobump pattern-by the total-internal reflection microscope. Observations reveal that surface plasmon waves can be highly directed and focused via this nanofence pattern. Results are in good agreement with the simulation results using the finite-element method and demonstrate the potential applications of these nanophotonic devices. Furthermore, we utilize high laser energy to fabricate plasmonic waveguides, and also succeed in transferring the waveguides to another substrate. The attenuation rates of the light propagating in the waveguides are observed to achieve 0.31 dB µm(-1) and 0.48 dB µm(-1) on the target and receiver sides, respectively.

Biosensing on a Plasmonic Dual-Band Perfect Absorber Using Intersection Nanostructure.

Optical absorbers with multiple absorption channels are required in integrated optical circuits and have always been a challenge in visible and near-infrared (NIR) region. This paper proposes a perfect plasmonic absorber (PPA) that consists of a closed loop and a linked intersection in a unit cell for sensitive biosensing applications. We elucidate the physical nature of finite element method simulations through the absorptance spectrum, electric field intensity, magnetic flux density, and surface charge distribution. The designed PPA achieves triple channels, and the recorded dual-band absorptance reaches 99.64 and 99.00% nm, respectively. Besides, the sensitivity can get 1000.00 and 650 nm/RIU for mode 1 and mode 2, respectively. Our design has a strong electric and magnetic field coupling arising from the mutual inductance and the capacitive coupling in the proposed plasmonic system. Therefore, the designed structure can serve as a promising option for biosensors and other optical devices. Here, we illustrated two examples, i.e., detecting cancerous cells and diabetes cells.

Breaking the Symmetry of a Metal-Insulator-Metal-Based Resonator for Sensing Applications.

This article designed a novel multi-mode plasmonic sensor based on a metal-insulator-metal waveguide side-coupled to a circular-shaped resonator containing an air path in the resonator. The electromagnet field distributions and transmittance spectra are investigated using finite element method-based simulations. Simulation results show that an air path in the resonator's core would impact the transmittance spectrum of SPPs. Besides, the air path is crucial in offering efficient coupling and generating multiple plasmon modes in the sensor system. The proposed structure has the advantage of multi-channel, and its sensitivity, figure of merit, and dipping strength can reach 2800 nm/RIU, 333.3 1/RIU, and 86.97%, respectively. The achieved plasmonic sensor can also apply for lab-on-chip in biochemical analysis for detecting the existence or nonappearance of diabetes through the human glucose concentration in urine.

Theoretical Study of CO Adsorption Interactions with Cr-Doped Tungsten Oxide/Graphene Composites for Gas Sensor Application.

The present study explores the CO adsorption properties with graphene, tungsten oxide/graphene composite, and Cr-doped tungsten oxide/graphene composite using density functional theory (DFT) calculations. The results of the study reveal the Cr-doped tungsten oxide/graphene composites, g-CrW n-1O3n (n = 2 to 4), to have a lowered highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) energy gap, high surface reactivity, and a strong cluster-graphene binding energy, hence exhibiting a strong adsorption interaction with CO. The CO adsorption interaction shows physisorption properties by having a greater tendency for Mulliken and natural bond orbital (NBO) charge transfer supported by a strong physisorption interaction toward the g-CrW n-1O3n (n = 2 to 4) composite with HOMO-LUMO energy gaps of -0.638, -0.486, and -0.327 eV, respectively. The calculated photoelectron spectroscopy (PES) and infrared spectra combined with the visualized electrostatic potential and contour line confirm the population density of the physisorption interaction. The calculated results show that the g-CrW n-1O3n composite achieves a greater sensing ability by possessing the highest sensitivity, adsorption, and desorption characteristics for n = 2 (g-CrWO6 composite). In conclusion, Cr-doped tungsten oxide/graphene has high sensitivity toward CO gas.

Enhanced photoluminescence and shortened lifetime of DCJTB by photoinduced metal deposition on a ferroelectric lithography substrate.

The photodeposition of metallic nanostructures onto ferroelectric surfaces could enable new applications based on the assembly of molecules and patterning local surface reactivity by enhancing surface field intensity. DCJTB (4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran) is an excellent fluorescent dye and dopant material with a high quantum efficiency used for OLED displays on the market. However, how to raise the photoluminescence (PL) and reduce the lifetime of DCJTB in a substrate remain extraordinary challenges for its application. Here, we demonstrate a tunable ferroelectric lithography plasmon-enhanced substrate to generate photo-reduced silver nanoparticles (AgNPs) and achieve enhanced PL with a shortened lifetime depending on the substrate's annealing time. The enhanced PL with shortened lifetimes can attribute to the localized electromagnetic (EM) wave produced by the nanotextured AgNPs layers' surface and gap plasmon resonances. The simulation is based on the three-dimensional finite element method to explain the mechanism of experimental results. Since the absorption increases, the remarkable enhanced PL of DCJTB can attain in the fabricated periodically proton exchanged (PPE) lithium niobate (LiNbO3) substrate. Furthermore, the proposed fabrication method demonstrates to help tune the surface EM wave distribution in the substrate, which can simultaneously achieve the significantly shortened lifetime and high PL intensity of DCJTB in the substrate. Compared with the un-annealed substrate, the PL intensity of DCJTB in the assembly metallic nanostructures is enhanced 13.70 times, and the PL's lifetime is reduced by 12.50%, respectively. Thus, the fabricated substrate can be a promising candidate, verifying chemically patterned ferroelectrics' satisfaction as a PL-active substrate.

Fabrication of phase-change Ge2Sb2Te5 nano-rings.

Phase-change material Ge2Sb2Te5 rings with nanometer-scale thickness have been fabricated using the photo-thermal effect of a focused laser beam followed by differential chemical etching. Laser irradiation conditions and etching process parameters are varied to control the geometric characteristics of the rings. We demonstrate the possibility of arranging the rings in specific geometric patterns, and also their release from the original substrate.

Local electrical characterization of laser-recorded phase-change marks on amorphous Ge2Sb2Te5 thin films.

Amorphous thin films of Ge(2)Sb(2)Te(5), sputter-deposited on a thin-film gold electrode, are investigated for the purpose of understanding the local electrical conductivity of recorded marks under the influence of focused laser beam. Being amorphous, the as-deposited chalcogenide films have negligible electrical conductivity. With the aid of a focused laser beam, however, we have written on these films micron-sized crystalline marks, ablated holes surrounded by crystalline rings, and other multi-ring structures containing both amorphous and crystalline zones. Within these structures, nano-scale regions of superior local conductivity have been mapped and probed using our high-resolution, high-sensitivity conductive-tip atomic force microscope (C-AFM). Scanning electron microscopy and energy-dispersive spectrometry have also been used to clarify the origins of high conductivity in and around the recorded marks. When the Ge(2)Sb(2)Te(5) layer is sufficiently thin, and when laser crystallization/ablation is used to define long isolated crystalline stripes on the samples, we find the C-AFM-based method of extracting information from the recorded marks to be superior to other forms of microscopy for this particular class of materials. Given the tremendous potential of chalcogenides as the leading media candidates for high-density memories, local electrical characterization of marks recorded on as-deposited amorphous Ge(2)Sb(2)Te(5) films provides useful information for furthering research and development efforts in this important area of modern technology.

A multichannel color filter with the functions of optical sensor and switch.

This paper reports a multichannel color filter with the functions of optical sensor and switch. The proposed structure comprises a metal-insulator-metal (MIM) bus waveguide side-couples to six circular cavities with different sizes for filtering ultra-violet and visible lights into individual colors in the wavelength range of 350-700 nm. We used the finite element method to analyze the electromagnetic field distributions and transmittance properties by varying the structural parameters in detail. The designed plasmonic filter takes advantage of filtering out different colors since the light-matter resonance and interference between the surface plasmon polaritons (SPPs) modes within the six cavities. Results show that the designed structure can preferentially select the desired colors and confine the SPPS modes in one of the cavities. This designed structure can filter eleven color channels with a small full width at half maximum (FWHM) ~ 2 nm. Furthermore, the maximum values of sensitivity, figure of merit, quality factor, dipping strength, and extinction ratio can achieve of 700 nm/RIU, 350 1/RIU, 349.0, 65.04%, and 174.50 dB, respectively, revealing the excellent functions of sensor performance and optical switch, and offering a chance for designing a beneficial nanophotonic device.