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The safety of railway transportation is crucial to social and economic development. Therefore, real-time monitoring of the rail is particularly necessary. The current track circuit structure is complex and costly, posing challenges to monitoring broken tracks using alternative methods. As a non-contact detection technology with a lower environmental impact, electromagnetic ultrasonic transducers (EMATs) have become a concern. However, traditional EMATs have problems such as low conversion efficiency and complex modes, which can limit their effectiveness for long-distance monitoring. Therefore, this study introduces a novel dual-magnet phase-stacked EMAT (DMPS-EMAT) design comprising two magnets and a dual-layer winding coil arrangement. The magnets are positioned at a distance equal to the wavelength of the A0 wave from each other, while the center distance between the two sets of coils beneath the transducer is also equal to the wavelength. After analyzing the dispersion curves of the rail waist, it was determined that the optimal frequency for long-distance rail monitoring is 35 kHz. At this frequency, adjusting the relative positions of the two magnets and the coil directly underneath to be one A0 wavelength can effectively excite a constructive interference A0 wave in the rail waist. The simulation and experimental results show that DMPS-EMAT excited a single-mode A0 wave, resulting in a 1.35-times increase in amplitude.

Surface-enhanced Raman spectroscopy (SERS) can be regarded as a powerful tool for probing chemical molecules by effectively enhancing Raman signals. However, the enhancement factors depend on the SERS template, the probed molecular structures, and the excitation laser wavelength. Herein, we proposed a simple and easily fabricated nanostructured template for SERS and analyzed the wavelength-dependent factors. Three types of golden nanopillar arrays on silicon wafers were designed and manufactured. The SERS signals of the Rhodamine 6G (R6G) molecules were extracted. Three laser sources, a blue 17 mW 458 nm diode laser, a green 20 mW 532 nm laser, and a red 6 mW 633 nm laser, were employed as the excitation laser sources. The 458 nm laser was located far from the resonate spectrum of R6G. The optical intensity distributions for the different SERS templates excited by three laser beams were also simulated. The enhancement factors (EFs) of R6G on the three nanostructured templates were measured and compared. The photoluminescence spectrum of the nanostructured templates and SERS signals of R6G were also measured. In addition, the experimental results concerned optical simulations. The analysis tool that was used was a convolution profile of multiple Lorentzian line shapes with a Gaussian profile. It is helpful to understand the SERS signals when the excitation laser wavelength is located out of the resonance region of molecules. It can also provide a new design approach to fabricate an SERS Template with a nanopillar array for different excitation wavelengths.

This proposed optical imaging method is a nondestructive, real-time and high-resolution approach to distinguish healthy and injured temporomandibular joint (TMJ) tissues. And the TMJ health index was invented. TMJ pathologies are commonly and reported frequently. It could be associated with the damage of collagen, cartilage and bone tissue. The second harmonic generation images could be obtained by a femtosecond laser pulses, so the aligned information of the collagen fibers in all directions for the TMJ disorders was collected. The disorder degree of collagen fibers was quantified and ranked using a fast Fourier transform (FFT) method. The TMJ health index can effectively present the TMJ healthy condition and the disorder degree of collagen fibers, a valuable objective tool for tissue characterization for TMJ healthy condition. Integrated with the staining methods, we can provide the scaling information at different injury degree.

Precision laser micromachining plays an important role in the biomedical, electronics, and material processing industries. During laser drilling, precision depth detection with micrometer-level resolution is required, particularly with blind-hole or heterogeneous structures. We present an optical detection system utilizing an optical confocal structure, experimentally confirmed to achieve a >95% accuracy for micron-diameter holes that are tens-of-microns deep. This system can be easily integrated into commercial laser micromachining processes, and can be employed in laser drilling and three-dimensional active-feedback laser printing.

We present a 445-nm InGaN blue diode laser in wavelength-tunable Littrow-type external cavity with a single longitudinal mode output, and a linewidth of 4.7 MHz. Its tuning range was estimated at 4 nm. The maximum output power was up to 20 mW, and the slope efficiency was approximately 0.36 W/A. The external cavity-diode laser linewidth was measured as 4.7 MHz with a lasing threshold to 8.1 MHz at higher applied current. The slope of the linewidth broadening with applied currents was calculated as 57 kHz/mA. And the wavelength tuning range was approximately 4.3 nm at low applied current and narrowed to approximately 2.6 nm at high applied current. A blue external cavity diode laser can deliver a single frequency beam within entire operation range.

The mode evolution of an InGaN blue laser diode with operating current and diode temperature was investigated. The spectra of a 445-nm InGaN blue laser diode with operating currents and diode temperatures was recorded using an ultrahigh resolution spectrometer and a scanning Fabry-Perot interferometer. The blue laser diode can deliver a mW-level single frequency beam with about 20 MHz linewidth at nearly above the lasing threshold. And the temperature-dependent wavelength drift was estimated as 0.31 nm for single longitudinal mode operation region. The spectra showed the number of the longitudinal modes increased with raising operating current. Analysis of the mode evolution of a blue laser diode with operating current and diode temperature is important when how to choose the laser diode for specific applications.

We present here an optical scheme for a trace methane gas detection system using mid-infrared (MIR) light emitting diode (LED) (λ = 3.3 μm) in a hollow-core fiber (HCF). When light from the MIR LED was focused on the HCF, the latter confines the interaction of light and methane gas within a fiber core of 0.5 mm diameter. The 1-m-long HCF provided enhanced optical path, and optimum light-gas interaction. By employing direct absorption spectroscopy, the detection limit of methane gas concentration was extended to as low as 17 ppb, and the dynamic range was estimated as 42 dB. The 400-nm bandwidth of a MIR LED can cover all strong absorption lines in the vicinity of 3.3 μm. This scheme provides an engineering perspective to realize a low-cost, hand-held methane detector for remote and harsh environments.

Recently, tunable high absorptance from various nanophotonic structures has been demonstrated. However, most of these structures require nano-lithography, which is expensive and slow. Lithography-free tuneable absorbers are rarely explored for tuneable visible and near-infrared photonics. Herein, we demonstrate a gold (Au)/chalcogenide dual-layer that is resonantly coupled to Au nanoparticles (NPs). The structure exhibits angle and polarisation-independent high absorptance. At resonance, waveguide cavity-like modes are excited between the film and NPs whilst gap plasmon modes are excited between the NPs. Coalescence of the waveguide cavity-like modes, the gap plasmon modes, and the highly absorbing chalcogenide semiconductor not only leads to perfect absorptance but also a reconfigurable response via reversible structural phase transitions in the chalcogenide film. In the amorphous state, the design provides nearly perfect absorptance for both p- and s-polarisation states at an incident angle of 20°. However, after switching to the crystalline state, the peak absorptance spectrally broadens and redshifts from 980 to 1520 nm. This experimental observation was theoretically validated by the finite element method. Thermal-electric modeling was performed to show that the structural transition from crystalline to amorphous states is possible in just 5 ns, thus allowing high-speed reconfigurable perfect absorbers.

Recent developments of wavefront shaping make the multimode fiber (MMF) as a promising tool to deliver images in endoscopy. However, previous studies using the MMF were limited to monochromatic light or polychromatic light with narrow bandwidth. The desires for colored imaging stimulate us to deliver multi-wavelength light that covers the entire visible spectrum through the MMF. In this work, we demonstrated delivering targeted color light through the MMF by mixing three primary colors (red, green, and blue) with a single spatial light modulator. The optimum phase map that considers all three colors was generated through field synthesis (FS), which requires every pixel of the SLM to partially account for all colors. With both theoretical and numerical approaches, we showed that FS exhibited much better performance than the previously developed spatial segmentation method that employs different pixels to represent different colors. Moreover, by computationally adjusting the compositions of the weight for each color, the colors of the delivered focus can be switched at video framerate. We anticipate that our work paves a way for future applications of delivering color images through the MMF in endoscopy.

Based on the coupled Ginzburg-Landau equations and Jones matrices of the waveplates considered, a numerical model of an all-normal-dispersion fiber laser mode-locked by nonlinear polarization rotation has been proposed. The operating characteristics of the fiber laser discussed were studied numerically. It has been found that the proposed all-normal-dispersion mode-locked fiber laser (AND-MLFL) could deliver dissipative solitons (DSs) with a M-shaped and U-shaped spectrum, the splitting pulse with a divided spectrum and the amplifier similaritons. The evolution of the intra-cavity pulse and spectrum has been calculated under different regimes and the effects of group velocity dispersion (GVD) and nonlinearity are analyzed. When the fiber laser delivers DSs or causes pulse splitting, nonlinear effects dominate the pulse evolution. With the increase of the accumulated nonlinear phase shift, the operation states change from DS with a M-shaped spectrum to a U-shaped spectrum, and then to the splitting pulse. In the case of amplifier similaritons, both the GVD and nonlinearity play important roles in pulse evolution. The effect of nonlinear polarization rotation and filtering on the pulse reshaping has been analyzed. When the fiber laser delivers DSs with a M-shaped spectrum, the filter has a very weak effect on the pulse and on spectral reshaping. However, when the fiber laser operates in the amplifier similariton state, the filter plays a key role in pulse and spectral reshaping, whereas the nonlinear polarization rotation become less dominant. The dependence of the operational states on the filter bandwidth, fiber length, small signal gain coefficient and orientation of waveplates has also been calculated. A Yb-doped doubled-cladding fiber laser, mode-locked by nonlinear polarization rotation, has also been demonstrated and all of the four pulse regimes are obtained experimentally.

We demonstrate a real-time microanalysis approach to the evaporation rate of a (sub)picoliter liquid drop, measured over a 7.85-0.38 picoliter volume, using an off-axis fiber interferometer and achieving the best displacement resolution of 24 nm.

The welding scale of laser welding now went to micrometer size. Under the view of optical microscopy, the high-precision laser welding systems can be achieved. But the welding processes were monitored by human eyes, and the light scattering may interfere the observation. We presented a visual-assisted laser microwelding system. This work combined laser microwelding technology, thermal imaging process, and the active feedback control loop to achieve the real-time microwelding quality control. The thermal imaging camera and the near infrared CCD camera were used to monitor the welding process. The active feedback control loop was employed, and linked the thermal distribution of the thermal images with the duty cycle of laser operations. 0.1-s response time can make sure the laser welding under control without manually tuning. So that welding technicians with this optical system can control the quality, improve the production yields, and shorten the whole process time. With this optical system, we successfully welded the carbon strip and tens-of-µm carbon fiber with aluminum plate. It will be useful for soft, and flexible carbon-based electrical devices.

In this paper, we present and discuss two methane gas detection systems using a mid-infrared (MIR) ( $\lambda =3.3\,\,\mu \text{m}$ ) light emitting diode (LED). Although methane is a major greenhouse gas, it is widely used in modern daily life. The need to effectively monitor the methane concentration has always been emphasized. As a first of its kind, the direct absorption method was employed. It included a light source, an MIR LED, an infrared photodetector, and a 20-mm gas cell with a proper gas mixer system. The output signal could be amplified using a signal amplifier or a lock-in technique. The detection limit was estimated as 49 ppm using the signal amplifier and 2.3 ppm using the lock-in technique. The MIR light is invisible to the naked eye but can be seen with a thermal camera. The optical alignment is time consuming. A microstructured infrared multimode fiber was adopted instead of a gas cell. The fiber surface was machined by a high-power Q-switched laser, and the interaction of light and methane molecules occurred on this micromachined fiber surface. The detection limit was estimated as 6.1 ppm. The dynamic range was estimated as 26 dB.

This study aims to investigate the longitudinal mode evolution of a InGaN green laser diode. A spectrometer with a 3-pm resolution was employed to obtain the emission spectra of a green laser diode, at a wavelength of around 520 nm, as a function of applied current and temperature. The spectral behavior of the laser modes with applied current was investigated. Right above the lasing threshold, the green diode laser emitted single longitudinal mode output. With increasing applied current, the number of the longitudinal modes increased. Up to ten lasing modes oscillated within the entire gain profile when the applied currents were tuned to 2.2Ith. Subsequently, a multi-Lorentzian profile model was adopted to analyze the spectra and observe how the modes evolved with temperature and applied current.

We present a design of a single-frequency green InGaN external cavity diode laser (ECDL) with 40-mW output power and 10-MHz linewidth. The ECDL was mounted in the Littrow configuration, with an overall efficiency of approximately 52% and a tuning region of 8 nm. We analyzed the output efficiency of the ECDL under different polarization directions and emitted light patterns. We used a spectrometer to analyze the laser spectrum under the different applied currents and temperatures. Finally, we obtained the linewidth of the ECDL using a scanning Fabry-Perot interferometer and found that the linewidth varied with the wavelength.

Perfect absorbers that can efficiently absorb electromagnetic waves over a broad spectral range are crucial for energy harvesting, light detection, and optical camouflage. Recently, perfect absorbers based on a metasurface have attracted intensive attention. However, high-performance metasurface absorbers in the visible spectra require strict fabrication tolerances, and this is a formidable challenge. Moreover, fabricating subwavelength meta-atoms requires a top-down approach, thus limiting their scalability and spectral applicability. Here, we introduce a plasmonic nearly perfect absorber that exhibits a measured polarization-insensitive absorptance of ∼92% across the spectral region from 400 to 1000 nm. The absorber is realized via a one-step self-assembly deposition of 50 nm gold (Au) nanoparticle (NP) clusters onto a 35 nm-thick Ge2Sb2Te5 (GST225) chalcogenide film. An excellent agreement between the measured and theoretically simulated absorptance was found. The coalescence of the lossy GST225 dielectric layer and high density of localized surface plasmon resonance modes induced by the randomly distributed Au NPs play a vital role in obtaining the nearly perfect absorptance. The exceptionally high absorptance together with large-area high-throughput self-assembly fabrication demonstrates their potential for industrial-scale manufacturability and consequential widespread applications in thermophotovoltaics, photodetection, and sensing.

Perovskite microwires have a larger surface-to-volume ratio and better photoelectric conversion efficiency than perovskite films. The degree of crystallization also affects the optoelectrical performances of perovskite microwires. Laser annealing was regarded as a tool for crystallization. High light absorption induced fast heating process. A 405 nm violet laser located near the absorption peak of typical perovskite films was employed as the annealing laser. In an in situ experimental design, the annealing laser beam was combined into the micro Raman measurement system. Real-time information of the annealing and crystallization was provided. Many excellent works were done, and typically needed offline optoelectronic measurements. An mW-level continuous-wave laser beam can provide enough kinetic energy for crystalline in perovskite microwires. The thermal distribution of the perovskite microwire under the annealing laser beams was considered here. Polarized Raman signals can provide evidence of the perovskite microwires crystallization. This work offered the novel approach of an on-site, real-time laser-induced thermal annealing design for perovskite microwires. This approach can be used in other material procedures. Intensity-dependent conditions were crucial for the annealing processes and analyzed in detail. The substrate effect was found. This proposed scheme provided integrated novel, scalable, and highly effective designs of perovskite-based devices.

Transformation optics (TO) has been recognized as an established scheme that can precisely manipulate electromagnetic surface wave propagations. Utilising the advantages of the TO technique, we proposed a TO device which can control the propagation direction of surface waves. The device is based on a generalized Eaton lens and can be realized on curved surfaces. Such a device can be composed of stacked ring layers, where the refractive index of each ring segment can be individually engineered. By engineering the refractive index, the stacked ring structure can guide the propagation direction of the surface wave. It may open a new door to manipulate surface wave propagation on demand.

Hyperbolic metamaterials (HMMs) attract increasing attentions due to their unique optical properties and offer new approaches for realizing novel functionalities in emerging photonic meta-devices. Tunable is one of the most attractive optical properties since multifunction optical devices are one of the important research directions. So far, most active HMMs working in the visible region are based on the combination of metal and phase-change chalcogenides and the performance is limited by the optical losses of phase-change chalcogenides and interdiffusion of the metals with phase-change chalcogenides. In this work, incorporating α-phase molybdenum trioxide (α-MoO3) and Au, an active and low loss HMM device is proposed in the visible region and can effectively overcome the shortcoming. A tunable plasmonic biosensor based on prism coupled α-MoO3/Au HMM is further designed by enhancing Goos–Hänchen (GH) shift, since GH shift is highly sensitive to the refractive index of the substrate. The calculated refractive index sensitivity of this proposed biosensor is of the order of 106 nm/refractive index unit. The proposed approach offers new direction for potential application in the development of the active ultrasensitive biosensor operating at visible range.

We report the saturation spectroscopy of carbon dioxide inside a hollow glass waveguide (GW) using a tunable mid-infrared difference frequency generation (DFG) source. This DFG source was generated in a periodically poled lithium niobate crystal with a tuning range from 3.97 to 4. 71 μ m . Saturated spectroscopy was performed on the R(60) transition of CO 2 0001 ← 0000 band at 4. 2 μ m inside a hollow GW of diameter 300 μ m and length 1 m. The saturation dip signal was found with a pump power as low as 0.18 mW inside the hollow GW. The linewidth of the dip was determined to be 4.40 MHz.

We presented an approach to measure the strain on graphene by using the polarized Raman spectroscopy with two excited sources and lineshape analysis, and studied the strain effect due to golden nanoparticles on graphene flake with it. The surface enhanced Raman spectroscopy (SERS) can effectively enhance Raman signals, but depositing metallic films or nanoparticles on graphene is needed. We compared the Raman signal intensity using Raman spectroscopy and SERS, and fitted the lineshape into the combined Lorentzian profiles. The strain effect due to golden nanoparticles can be neglected in our experimental design. The distribution of golden nanoparticles can be regarded as random. The technique of SERS can enhance Raman signal and not cause strain changing. At the same time, the Raman signal intensity of graphene flake with 532-nm laser excitation is larger than one with 632-nm laser approximately 10 times. Expectedly, we used the way of 532-nm excited laser and technique of SERS to greatly enhance measuring efficiency.

Investigations of Raman spectra and surface enhanced Raman spectra (SERS) of supported and suspended bilayer graphene were realized. The ability of SERS to greatly enhance the Raman signals is regarded as useful tools, but the dopants induced by the metallic nanoparticle deposition may affect the electron scattering processes. The four 2D sub-bands of bilayer graphene are associated with four-step Stokes–Stokes double-resonance Raman electron scattering processes and can be analyzed by fitting the corresponding Raman peak with multi-Lorentzian functions. To extricate the dopant effect of the substrate from one of metallic nanoparticles, suspended graphene is adopted here. The enhancements of SERS over Raman spectra are also calculated. For the supported graphene, the SERS enhancement factors of the sub-bands obey 2D22 < 2D11 < 2D21 < 2D12 and exhibit an integrated intensity that is proportional to the Raman cross-section. For suspended graphene, such factors obey 2D11 < 2D12 < 2D21 < 2D22, with 2D12, 2D21, and 2D22 being close to each other because the rate of the scattering for some processes decreases and contributes to another process in the integrated area of the Raman signals when the decays happen. The reason for the factor difference is discussed through the presented analysis of the Raman and SERS signals of supported and suspended bilayer graphene.

Sweat collection and real time monitoring of sweat rate play essential roles in physiology monitoring and assessment of an athlete&rsquo, s performance during exercise. In this paper, we report a micropump for sweat simulant collection based on the capillary&ndash, evaporation effect. An electrochemical sensor is integrated into the micropump, which monitors the flow rate in real-time by detecting the current using three electrodes. The evaporation rate from micropore array, equivalent to the sweat rate, was theoretically and numerically investigated. The designed micropump yields the maximum collection rate as high as 0.235 &mu, L/min. In addition, the collection capability of the micropump was validated experimentally, the flow rate through the microchannel was further detected in real-time with the electrochemical sensor. The experimental maximum collection rate showed good consistency with the theoretical data. Our proposed device shows the potential for sweat collection and real-time monitoring of sweat rate, which is a promising candidate for being a wearable platform for real-time physiology and performance monitoring during exercise.

We observe that silver atoms deposited by thermal evaporation deposition onto n-layer graphene films condense upon annealing to form nanoparticles with an average diameter and density that is determined by the layer numbers of graphene films. The optical microscopy and Raman spectroscopy were utilized to identify the number of the graphene layers and the SEM (scanning electron microscopy) was used to observe the morphologies of the particles. Systematic analysis revealed that the average sizes of the nanoparticles increased with the number of graphene layers. The density of nanoparticles decreased as the number of graphene layers increased, revealing a large variation in the surface diffusion strength of nanoparticles on the different substrates. The mechanisms of formation of these layer-dependent morphologies of silver nanoparticles are related to the surface free energy and surface diffusion of the n-layer graphenes.

All-fiber inclinometers and displacement sensors using tunable Mach-Zehnder interferometers with an extinction ratio of are produced by introducing two abrupt tapers in a short length of highly Er/Yb co-doped fiber. The abrupt tapers partially convert the core mode into cladding modes and interference occurs when the phase difference between the core and cladding modes accumulate to π after a certain propagation length. The maximal wavelength shift for the resonant transmission dips can reach 40.8 nm (from 1648 to 1607.2 nm) when the inclination angle, θ, varies from 0 ° to 32 °. The wavelength shift can reach 26 nm (from 1404.4 to 1430.4 nm) when the compressive strain e is 0.004.

We observed that a green diode laser shows single longitudinal mode output at low operation current, and its frequency can be tuned by adjusting the operation temperature and operational current. The behavior of laser modes with operational current was also investaigated.

We observed that photoacoustic imaging of a gelatin phantom, which consisted a graphite-filled inclusion with a 4-mm-diameter, illuminated by a microstructured plastic fiber guiding green laser light inside. This approach can effectively extend light propagation depth in tissue.

Dielectric porous anodic aluminum oxide films embedded with two-dimensional silver nanoparticles arrays (porous Ag/AAO) have been fabricated by electrochemical anodization and deposition, wherein the position and rate of the Ag nanoparticles within the nanopores can be properly controlled. These porous Ag/AAO films have photo-induced plasmonic coupling and can be applied toward photo-absorption sensors. The plasmonic absorption is clearly visible and its absorption maximum red-shifts with increasing particle diameters. An investigation of how the absorption spectrum is influenced by systematically changing the pore diameter and inter-pore spacing is laid out. This work provides an approach to precisely tune the peak wavelength and the bandwidth of plasmonic absorption of porous Ag/AAO films utilizing fine control of the pore diameters. The wavelength shift slope of absorption peak with the pore diameter is measured to be approximately 0.82 nm/nm (peak wavelength/pore diameter). These tuning curves may be useful in the design of photon-response sensors and 2D tunable plasmonic-based nanostructure is expected to enable applications such as tunable photon-induced electrical conductivity.