Frictionless multiphasic interface for near-ideal aero-elastic pressure sensing.

Conventional pressure sensors rely on solid sensing elements. Instead, inspired by the air entrapment phenomenon on the surfaces of submerged lotus leaves, we designed a pressure sensor that uses the solid-liquid-liquid-gas multiphasic interfaces and the trapped elastic air layer to modulate capacitance changes with pressure at the interfaces. By creating an ultraslippery interface and structuring the electrodes at the nanoscale and microscale, we achieve near-friction-free contact line motion and thus near-ideal pressure-sensing performance. Using a closed-cell pillar array structure in synergy with the ultraslippery electrode surface, our sensor achieved outstanding linearity (R2 = 0.99944 ± 0.00015; nonlinearity, 1.49 ± 0.17%) while simultaneously possessing ultralow hysteresis (1.34 ± 0.20%) and very high sensitivity (79.1 ± 4.3 pF kPa-1). The sensor can operate under turbulent flow, in in vivo biological environments and during laparoscopic procedures. We anticipate that such a strategy will enable ultrasensitive and ultraprecise pressure monitoring in complex fluid environments with performance beyond the reach of the current state-of-the-art.

Zinc-Air Battery-Based Self-Powered Sensor with High Output Power for Ultrasensitive MicroRNA let-7a Detection in Cancer Cells.

Self-powered sensors do not require a power supply and are easy to miniaturize, which have potential for constructing wearable, portable, and real-time detection devices. However, it is challenging for the detection of low abundant targets due to the low output power density of fuel cells and much interference of complex biological environment. Herein, a new kind of photocatalytic zinc-air battery-based self-powered electrochemical sensor (ZAB-SPES) was constructed for the detection of microRNA let-7a (miRNA let-7a) by combining magnetic nanobeads (MBs) with a metal-organic framework loaded with glucose oxidase (MOFs@GOX). Poly(1,4-di(2-thienyl))benzene (PDTB) was used as the photocathode material, and the proposed ZAB-SPES had a high power density of 22.8 µW/cm2, which was 2-3-fold of commonly used photofuel cells. MBs can capture and separate miRNA from complex samples quickly with a high separation efficiency of 99% within 60 s. The competitive reaction of oxygen reduction reaction between PDTB and MOFs@GOX would change the output power density of the ZAB-SPES. Based on the relationship between output power density and target concentration, the ZAB-SPES realized ultrasensitive detection of miRNA let-7a with a detection limit down to 1.38 fM. Furthermore, the successful detection of miRNA let-7a in A549 cancer cells indicated the great prospects of ZAB-SPES in clinical analysis and early diagnosis of cancers.

Photocatalytic Fuel Cell-Assisted Molecularly Imprinted Self-Powered Sensor: A Flexible and Sensitive Tool for Detecting Aflatoxin B1.

The self-powered electrochemical sensor has gained big achievements in energy and devices, but it is challenging in analytical application owing to its low energy conversion efficiency and limited selectivity caused by the plentiful interference in actual samples. Herein, a new self-powered biosensor was constructed by the integration of a photocatalytic fuel cell (PFC) with a molecular imprinting polymer (MIP) to achieve sensitive and specific detection of aflatoxin B1 (AFB1). Compared with other fuel cells, the PFC owns the advantages of low cost, high energy, good stability, and friendly environment by using light as the excitation source. MoS2-Ti3C2Tx MXene (MoS2-MX) served as the photoanode material for the first time by forming a heterojunction structure, which can enhance the photocurrent by about 3-fold and greatly improve the photoelectric conversion efficiency. Aiming at the poor selectivity of the self-powered sensor, the MIP was introduced to achieve the specific capture and separation of targets without sample pretreatment. Using the MIP and PFC as recognition and signal conversion elements, respectively, the proposed self-powered biosensor showed a wide dynamic range of 0.01-1000 ng/mL with a detection limit of 0.73 pg/mL, which opened opportunities to design more novel self-powered biosensors and promoted its application in food safety and environmental monitoring.

Optical vortex switch based on multiplexed volume gratings with high diffraction efficiency.

Systems of controllable orbital angular momentum (OAM) require more compact, higher conversion efficiency and more tolerable wavelength or polarization. We introduce an optical vortex switch based on a multiplexed volume grating (MVG). The MVG recorded in a piece of photo-thermo-refractive (PTR) glass exhibits high diffraction efficiency (DE, also known as conversion efficiency in transporting), sensitive angular selectivity, and polarization-insensitivity. The effects of the incident divergence angle and polarization on the DE and the far-field diffraction profiles are demonstrated and investigated. It turns out that the divergence angle of the probe beam can greatly affect the DE. The fluctuation of the DE caused by polarization variation is less than 1.59%. This switch can be potentially applied in vortex tweezers, optical communication, and high power systems.

Effect of the interface on femtosecond laser damage of a metal-dielectric low dispersion mirror.

Metal-dielectric low dispersion mirrors (MLDM) have a promising application prospect in petawatt (PW) laser systems. We studied the damage characteristics of MLDM and found that the damage source of MLDM (Ag + Al2O3+SiO2) is located at the metal-dielectric interface. We present the effect of the interface on the femtosecond laser damage of MLDM. Finite element analysis shows that thermal stress is distributed at the interface, causing stress damage which is consistent with the damage morphology. After enhancing the interface adhesion and reducing the residual stress, the damage source transfers from the interface to a surface SiO2 layer, and the damage threshold can be increased from 0.60 J/cm2 to 0.73 J/cm2. This work contributes to the search for new techniques to improve the damage threshold of MLDM used in PW laser systems.

Low-dispersion mirror with a broad bandwidth and high laser damage resistance.

A low-dispersion mirror (LDM), an important component in ultrafast laser systems, requires both a broad low-dispersion laser-induced damage threshold (LIDT). It is difficult for a traditional quarter-wavelength-based dielectric LDM to achieve these characteristics at the same time. We propose a novel, to the best of our knowledge, low-dispersion mirror (NLDM) that combines periodic chirped layers at the top and alternating quarter-wavelength layers at the bottom. Low dispersion is achieved by introducing a large same group delay (GD) for different wavelengths, so the bandwidth is broadened greatly. In addition, owing to the staggered electric field intensity peak effect in the structure, the NLDM shows the potential for high laser damage resistance. The experiments demonstrated that the NLDM doubles the low-dispersion bandwidth, while the LIDT is also increased compared with the LDM. This novel concept results in improved performance and paves the way toward a new generation of the LDM for ultrafast bandwidth and a high laser applications.

Direct measurement of radial fluence distribution inside a femtosecond laser filament core.

Modulation and direct measurement of the radial fluence distribution inside a single filament core (especially less than 100 µm in diameter) is crucial to filament-based applications. We report direct measurements of the radial fluence distribution inside a femtosecond laser filament core and its evolution via the filament-induced ablation method. The radial fluence distributions were modulated by manipulating the input pulse diffraction through an iris. Compared with using a traditionally circular iris, a stellate iris substantially suppressed the diffraction effect, and laser fluence, intensity and plasma density inside the filament core were considerably increased. The radial fluence inside filament cores was also quantitatively measured via the filament drilling diaphragms approach. Furthermore, numerical simulations were performed to support the experimental results by solving nonlinear Schrödinger equations. The effects of the tooth size of the stellate iris were numerically investigated, which indicated that bigger tooth favors higher fluence and longer filament. In addition to being beneficial in understanding the filamentation process and its control, the results of this study can also be valuable for filament-based applications.

Accurate temperature measurement of a spectral beam combination grating based on VO2 film.

In a spectral beam combination system, temperature increase of the multilayer dielectric grating (MDG) worsens the far-field beam quality of the output laser. To accurately monitor the surface temperature of the MDG, this study deposits VO2 phase-change film on the lowest layer of multilayer dielectric films in the MDG and tests the transmittance with a probe laser. Based on this measurement, the surface temperature of the MDG can be calculated. Additionally, the study analyzes the influence of VO2 film on the surface electric field and the -1 diffraction efficiency of the MDG and presents a specific example of using VO2 film to test high reflector temperature. The study concludes that VO2 film is a feasible method of measuring temperature and better than an infrared thermal imager.

Fabrication of high-precision reflective volume Bragg gratings.

Reflecting Bragg gratings (RBGs) recorded in photo-thermo-refractive (PTR) glasses have been widely used in narrowing and stabilization of the laser emission spectrum. As the center wavelength of RBGs determines the final output wavelength of lasers, it is necessary to carefully control the center wavelength of RBGs during the fabrication process. In this paper, the fabrication process of high-precision RBGs was investigated. We developed a two-step method and demonstrated both theoretically and experimentally that it is effective and can be used to guide the fabrication process of high-precision RBGs. The experimental results show that the center wavelength of the fabricated RBG deviates from the target center wavelength within ±10 pm.

Continuous-wave laser damage mechanism of a spectral combining grating.

With increased power of spectral beam combination, surface heat distortion of multilayer dielectric gratings (MDGs) could occur. In this study, the damage morphology of MDGs was initially analyzed under a continuous-wave laser irradiation. Subsequently, the surface distortion and temperature rise of different MDGs were tested experimentally. The experimental results showed that the initial damage of MDGs was caused by the thermal stress. Further, the thermal stress of the multilayer dielectric films on the MDG surface was analyzed theoretically. The calculated results were in good agreement with the experimental results. The conclusions indicated that with the increase of the MDG surface temperature, the stress in the HfO2 layers initially reached the stress damage threshold of the dielectric films and, therefore, the damage occurred.

Design and fabrication of multiplexed volume Bragg gratings as angle amplifiers in high power beam scanning system.

In this study, the realization of multiplexed volume Bragg gratings (VBGs) working as angle amplifiers in high power beam scanning system is theoretically and experimentally investigated. The design of the multiplexed VBG for the working wavelength of 1064 nm is described. We propose a cascaded multiplexed VBGs scheme that consists of 12 grating channels. Three 4-channel multiplexed VBGs were fabricated inside photo-thermo-refractive (PTR) glasses by multiple exposures and subsequent heat treatment. The test results show that this angle amplifier can achieve discrete angle deflection ranging from -45° to + 45°. The relative diffraction efficiency of all the grating channels is more than 80% and is almost polarization independent.

Study of the key factors affecting temperature of spectral-beam-combination grating.

Spectral beam combination is a promising method for high-radiance lasers with a good beam quality. With the increase of the combination power, the temperature of the multilayer dielectric grating (MDG) unavoidably increases, leading to surface heat distortion of the MDG. In this study, the temperature field equation of the MDG is derived, and the key factors influencing the MDG temperature are investigated. Furthermore, experiments are performed to confirm the calculation results. The results reveal that the increase of the thickness of the substrate can improve the power tolerance of the MDG but delays the stable output of beam laser; use of a substrate material with a large thermal conductivity can greatly reduce the temperature of the MDG.

Method for precise evaluation of refractive index modulation amplitude inside the volume Bragg grating recorded in photo-thermo-refractive glass.

In this paper, we present a method for achieving precise evaluation of amplitude of refractive index modulation (RIM) inside the volume Bragg grating (VBG) recorded in photo-thermo-refractive (PTR) glasses. The Gaussian divergence characteristics of the incident beam is theoretically considered when calculating the angular selectivity of VBG, and the profiles of experimental angular selectivity curves are utilized to determine the value of RIM with one step. The effectiveness of our proposed method is experimentally verified. This method is applicable even if the full width at half maximum (FWHM) of VBG's angular selectivity curve has the same order of magnitude as or is less than the beam divergence.

Dependence of temperature and far-field beam quality on substrate thickness of a spectral beam combining grating with 13.4 kW/cm2laser irradiation.

In previous research, the thermal distortion and far-field beam quality of a spectral beam combining grating were analyzed by theory and experiment under the irradiation of a high-power continuous-wave laser. It was concluded that the thermal expansion of the substrate was the main cause of the grating distortion and decrease in the beam quality. However, there was no further study to determine a method to decrease the heat deposition on the grating surface and far-field beam quality factor, M2. In this paper, we theoretically simulate the influence of the substrate thickness on the temperature field distribution and far-field beam quality of a multilayer dielectric grating. An experimental setup is proposed to verify the theoretical calculations. The experimental results are in good agreement with the calculations. The conclusions indicate that the temperature rise of the grating and M2 are effectively reduced by increasing the thickness of the substrate.

Polarization-independent almost-perfect absorber controlled from narrowband to broadband.

Achieving perfect absorption and controlling the absorption bandwidth are highly desirable for many applications. In this work, we design a narrowband almost-perfect absorber by using a metal-insulator-metal thin-film stack with absorption up to 99.67% at 0.58µm incident wavelength. The peak of absorption can be totally controlled by adjusting the thickness of the insulator layer. When the top metal layer is patterned by crossed grating nanostructure with optimized parameters, the absorber becomes broadband over 150nm bandwidth with average absorption exceeding 97% from 0.5µm to 0.65µm in the visible region. Both the narrowband and broadband absorbers are independent on polarization in specific incident angle range. This work opens up a promising new approach to control bandwidth of perfect absorption, which implicates many potential applications.

Polarization-independent broadband beam combining grating with over 98% measured diffraction efficiency from 1023 to 1080 nm.

We report a grating solution for achieving broadband and polarization-independent properties that brings a laser combining system to much higher power levels. The grating, with a high-refractive-index-contrast bilayer ridge, was designed and successfully fabricated based on high-power laser coatings, lithography, and ion-beam etching technology. The measured -1st order non-polarized reflective diffraction efficiency of the grating exceeds 98% over the wavelength range of 1.023-1.08 µm, and the highest value is 99.15%.

Beam modulation due to thermal deformation of grating in a spectral beam combining system.

As the power of a spectral beam combining (SBC) system increases, the temperature of the multilayer dielectric grating (MDG) inevitably rises under the influence of high-power continuous-wave (CW) laser irradiation. Hence, thermal deformation of the MDG occurs, along with degeneration of the combined beam properties. In this study, we experimentally and theoretically investigate the influence of the MDG thermal deformation on the combined beam properties. An experimental setup is first proposed, in which beam quality M2, beam profile, and MDG wavefront deformation are investigated. The experimental results indicate that the beam quality clearly degrades and the MDG wavefront deformation becomes more significant with increasing pump-CW power density. On this basis, a calculation model for MDG thermal deformation in SBC systems is proposed. The results indicate that MDG wavefront deformation becomes more significant, combined beam profile becomes deformed, and beam quality of the combined beam degrades with increasing power density. Further, thermal expansion of the substrate is a crucial factor that induces MDG wavefront deformation and far-field intensity modulation.

Evaluation of femtosecond laser damage to gold pulse compression gratings fabricated by magnetron sputtering and e-beam evaporation.

In this study, two kinds of Au-coated gratings (ACGs) with a period of 1740 lines/mm were fabricated and evaluated. For these ACG samples, magnetron sputtering and e-beam evaporation were used as the gold deposition process, and the samples had a bandwidth of at least 170 nm, with the -1st-order diffraction efficiency exceeding 90% around the center wavelength of 800 nm. The one-on-one damage threshold of ACGs fabricated by magnetron sputtering and e-beam evaporation measured at a pulse width of 60 fs was 0.59 J/cm2 and 0.43 J/cm2 in the case of the beam normal fluence, respectively. The typical damage morphology of the former type of samples was melting of the gold film, whereas those of the latter type were blisters and peeling off of the gold film. In theory, the electromagnetic field, temperature field, and thermal stress field distribution in the ACGs were calculated using the finite element method. We demonstrated that the adhesion between the gold film and the photoresist played an important role in determining the damage behavior. Thus, the laser resistance of ACG can be improved by enhancing the adhesion between the gold film and the photoresist, and magnetron sputtering was an alternative method to obtain ACGs with much better adhesion.

Defect analysis of UV high-reflective coatings used in the high power laser system.

By considering the rapid change of standing-wave electric-field and assuming the interface defect distribution, an improved model is developed to analyze the defect density distribution and assess the damage performance of high-reflective coatings. Two kinds of high-reflective coatings deposited by e-beam evaporation (EBE) and ion beam sputtering (IBS) techniques are analyzed with this method. The lower overall damage threshold is the major feature for the coatings deposited by IBS method according to the defect parameters extracted from the model. Typical damage morphologies of coatings are also measured and analyzed. The assumption of interface defects is supported by the damage behavior. The damage mechanisms of two high-reflective coatings are attributed to the formation of molten pool and mechanical ejection. The influence of the incident angle on the damage probability is also considered and numerically calculated. The defect analysis model improved here is suitable for high-reflective coatings.

Near-field enhancement of the nanostructure on the fused silica with rigorous method.

A rigorous electromagnetic method is developed to analyze the resonance effect of near field caused by nanoscale subsurface defects, which play a key role in describing absorption enhancement during laser-matter interaction for transparent dielectric materials. The total electric field calculated with this new method is consistent with the result of finite-difference time-domain simulation. The concept of mode amplitude density spectrum is developed to analyze the specific modes of the total field. A new mode parameter is proposed to demarcate the contribution of the resonance. The frequency space is divided into four parts and the resonance effect is analyzed as well as the contributions of different modes to the total field. The influence of the structure parameters on the near-field modulation and energy transference is also discussed. It is found that the enhancement mechanism of the near-field and local absorption is the resonance effect caused by the total internal reflection on the sidewall of the nanostructure. In addition, the surrounding energy is mainly guided into the structure by the root of the structure via the energy flow analysis.