Enhanced nonlinear effects in femtosecond laser-inscribed depressed cladding waveguides in sapphire crystals.

High-performance depressed cladding waveguides can be fabricated in crystals using ultrafast laser inscription. The investigation of nonlinear phenomena, which manifest during the transmission of high peak power femtosecond pulses within waveguides, holds significant importance for their practical integration into waveguide lasers and waveguide-based components, among other pioneering applications. In this study, the depressed cladding waveguides were successfully prepared in sapphire crystal by a femtosecond laser. The nonlinear phenomena occurring in this waveguide were investigated. The experimental results show that the nonlinearity in the depressed cladding waveguides is significantly enhanced compared with that of the bulk. This enhancement notably manifests through augmented nonlinear losses (NLs) and the third harmonic (TH) blueshift increase. Meanwhile, we theoretically investigate the influence of nonlinear effects on the TH, such as self-phase modulation (SPM), cross-phase modulation (XPM), and group delay. Our results reveal that the phase mismatch between the TH and the pump pulses is the main reason for the asymmetric broadening and blueshift of the TH spectrum. Our study reveals the unique nonlinear properties of the waveguides and lays the foundation for further relevant studies and applications of such waveguides.

High-performance laser power converters with resistance to thermal annealing.

High-performance laser power converters are crucial for laser wireless power transmission systems. Through the optimization of the resistive thermal annealing temperature applied to the laser power converter, the conversion efficiency reaches 55.0%. For 830 nm laser irradiation, the conversion efficiency further elevates to 59.3%. The potential for improvement remains substantial, with an anticipated increase to 63.8% achievable through the optimization of current matching at this specific wavelength. Moreover, the reliability of the laser power converter is demonstrated by its ability to 1,000 hours of operation at an elevated temperature of 180°C.

InP-based tunnel junctions for ultra-high concentration photovoltaics.

To enhance the performance of multi-junction photovoltaics, we investigated three different InP-based tunnel junction designs: p++-InGaAs/n++-InP tunnel junction, p++-InGaAs/i-InGaAs-/n++-InP tunnel junction, and p++-InGaAs/i-InGaAs/n++-InGaAs tunnel junction. The p++-InGaAs/i-InGaAs/n++-InGaAs tunnel junction demonstrated a peak tunneling current density of 495 A/cm2 and a resistivity of 9.3 × 10-4 Ωcm2, allowing the tunnel junction device to operate at a concentration over 30000 suns. This was achieved by inserting an undoped InGaAs quantum well at the p++-InGaAs/n++InGaAs junction interfaces, which enhanced its stability within the operating temperature range of multi-junction solar cells. Moreover, the p++-InGaAs/i-InGaAs/n++-InGaAs tunnel junction exhibited the lowest resistance.

High-power 970†nm semiconductor disk laser: erratum.

An erratum is presented to modify a calculating error in our published manuscript ["High-power 970 nm semiconductor disk laser" Opt. Express31, 43963 (2023)10.1364/OE.506462 [CrossRef]]. All results throughout the manuscript and its conclusions are unaffected by this correction and remain valid.

Helical hollow channel waveguide in YAG fabricated by femtosecond laser enhanced wet etching.

Three-dimensional optical waveguides with hollow channels have many advantages, such as strong mode confinement and excellent dispersion control ability. Femtosecond laser enhanced wet etching is widely used to fabricate hollow channel waveguides in transparent dielectric materials. We propose a method for fabricating hollow channel waveguides in YAG using femtosecond laser enhanced wet etching with a simpler fabrication process and shorter etching time compared with the previous work. After 90 h of etching, a series of helical hollow channel waveguides with a length of 5 mm and a radius of 32 µm were successfully fabricated. At a pitch of 3 µm, the waveguide exhibited a loss (including coupling loss and transmission loss) as low as 0.68 dB at 1030 nm. The helical hollow channel waveguide also exhibited exceptional isotropic light confinement capability and remarkable supercontinuum-generating properties. Moreover, helical hollow channel waveguides with a radius of 2 µm were successfully fabricated. According to simulations, waveguides of such size can effectively control dispersion. Our work presents, to our knowledge, a novel approach to fabricating hollow channel waveguides with arbitrary lengths using femtosecond laser-enhanced wet etching.

High-performance laser power converts for wireless information transmission applications.

Laser Power Converters (LPCs) are components of the laser wireless power transmission (LWPT) system receiving laser power. This paper proposes a comprehensive test method that employs continuous, pulse-pause, and short-time techniques to evaluate the performance of six-junction GaAs LPCs operating with an optical input at 808 nm. Additionally, we investigate the performance of LPCs with different areas and achieve a conversion efficiency over 60%. Furthermore, we apply LPCs with varying areas to wireless information transmission and successfully achieve a response time of 1.7 µs.

High-power 970†nm semiconductor disk laser.

Semiconductor disk lasers (SDLs) have emerged at the frontier of laser technologies. Here, the chip design, packaging process, resonator, pumping strategy, etc. are optimized for the performance improvement of a 970 nm SDL. After optimization, a power of 70.3 W is attained under continuous wave (CW) operation, and the corresponding thermal resistance is around 0.49 K/W. The laser is highly efficient with a maximum slope efficiency of 58.2% and the pump threshold is only around 1.83 kW/cm2. Furthermore, the emission performances under quasi-continuous wave (QCW) pumping are also explored. Setting the duty cycle to about 11%, the chips can output a peak power of 138 W without thermal rollover, and the single pulse energy can reach about 13.6 mJ. As far as we know, they are the best results in terms of power/energy in this wavelength SDL. These explorations may help to understand the thermal characteristics in high-power SDLs and may also be regarded as an extension and enrichment of the earlier works on this topic.

Compact wavemeter incorporating femtosecond laser-induced surface nanostructures enabled by deep learning.

Miniature spectrometers have the advantage of high portability and integration, making them quick and easy to use in various working environments. The speckle patterns produced by light scattering through a disordered medium are highly sensitive to wavelength changes and can be used to design high-precision wavemeters and spectrometers. In this study, we used a self-organized, femtosecond laser-prepared nanostructure with a characteristic size of approximately 30-50 nm on a sapphire surface as a scattering medium to effectively induce spectral dispersion. By leveraging this random scattering structure, we successfully designed a compact scattering wavelength meter with efficient scattering properties. The collected speckle patterns were identified and classified using a neural network, and the variation of speckle patterns with wavelength was accurately extracted, achieving a measurement accuracy of 10 pm in multiple wavelength ranges. The system can effectively suppress instrument and environmental noise with high robustness. This work paves the way for the development of compact high-precision wavemeters.

Electrically controllable optical switch metasurface based on vanadium dioxide.

We report a voltage-tunable reflective gold wire grid metasurface on vanadium dioxide thin film, which consists of a metal-insulator-metal (MIM) structure. We excite surface plasmon polariton (SPP) modes on the gold surface by fabricating a one-dimensional structured gold wire grid. Joule heating of laser-induced graphene (LIG) can be controlled by the voltage at the bottom, allowing vanadium dioxide in the structure to complete the transition from the insulating state to the metallic state. The phase transition of vanadium dioxide strongly disrupts the plasmon modes excited by the gold wire grid above, thereby realizing a huge change in the reflection spectrum. This acts as a tunable metasurface optical switch with a maximum modulation depth (MD) of over 20 dB. We provide a more effective and simple method for creating tunable metasurfaces in the near-infrared band, which can allow metasurfaces to have wider applications in optical signal processing, optical storage, and holography.

Correction: Li et al. Fringe Projection Profilometry Based on Saturated Fringe Restoration in High Dynamic Range Scenes. Sensors 2023, 23, 3133.

The authors wish to make the following corrections to the original paper [...].

Fringe Projection Profilometry Based on Saturated Fringe Restoration in High Dynamic Range Scenes.

In high dynamic scenes, fringe projection profilometry (FPP) may encounter fringe saturation, and the phase calculated will also be affected to produce errors. This paper proposes a saturated fringe restoration method to solve this problem, taking the four-step phase shift as an example. Firstly, according to the saturation of the fringe group, the concepts of reliable area, shallow saturated area, and deep saturated area are proposed. Then, the parameter A related to the reflectivity of the object in the reliable area is calculated to interpolate A in the shallow and deep saturated areas. The theoretically shallow and deep saturated areas are not known in actual experiments. However, morphological operations can be used to dilate and erode reliable areas to produce cubic spline interpolation areas (CSI) and biharmonic spline interpolation (BSI) areas, which roughly correspond to shallow and deep saturated areas. After A is restored, it can be used as a known quantity to restore the saturated fringe using the unsaturated fringe in the same position, the remaining unrecoverable part of the fringe can be completed using CSI, and then the same part of the symmetrical fringe can be further restored. To further reduce the influence of nonlinear error, the Hilbert transform is also used in the phase calculation process of the actual experiment. The simulation and experimental results validate that the proposed method can still obtain correct results without adding additional equipment or increasing projection number, which proves the feasibility and robustness of the method.

Comparison study of the femtosecond laser-induced surface structures on silicon at an elevated temperature.

The temperature dependency of femtosecond laser induced surface structures opens up a new scenario for studying ultrafast laser-mater interaction on the surface and a novel method for controlling the features of these structures. The shape and crystallinity of micro/nano surface structures created by femtosecond laser irradiation of n-type silicon (100) at elevated temperatures were compared in this study. Low spatial frequency laser induced periodic ripples structures (LSFL), micrometer-sized grooves, and spikes occur at room temperature as the number of pulses increases. At 400 °C, however, the grooves parallel to the polarization are the dominant structures, notwithstanding the presence of LSFL. As the temperature rises, the periodicities of LSFL increase, which we believe is due to a reduction in the oscillation of the surface plasmon polaritons due to the increased damping rate at higher temperatures. Furthermore, Raman spectra reveal that surface structures generated at 400 °C have higher crystallinity than those formed at 25 °C. Our simulations show that the better crystallinity at high temperatures is due to a slower resolidification velocity which is caused by a smaller temperature gradient and higher energy absorption. Our findings demonstrate that the features of femtosecond laser induced surface structures, such as periodicity and crystallinity, can be controlled by adjusting the substrate temperature simultaneously, paving the way for high crystallinity surface micro/nano-structures.

High performance p++-AlGaAs/n++-InGaP tunnel junctions for ultra-high concentration photovoltaics.

A p++-AlGaAs: C/n++-InGaP: Te tunnel junction with a record peak tunneling current density of 5518 A/cm2 was developed. This was achieved by inserting a 6.6 Šundoped GaAs quantum well at the junction interface, and the numerical model demonstrated that trap-assisted tunneling contributes to the high peak tunneling current. Furthermore, we found that the p++-AlGaAs: C/n++-InGaP: Si + Te tunnel junctions have lower resistance and better stability than p++-AlGaAs: C/n++-InGaP: Te tunnel junctions in the operating temperature range of the multijunction solar cells, and the peak tunneling current density of the p++-AlGaAs: C/n++-InGaP: Si + Te tunnel junctions excess 3000 A/cm2 with a voltage drop of 7.5 mV at 10000 suns.

High-performance laser power converts for direct-energy applications.

Six-junction GaAs laser power converts (LPCs) were designed and fabricated. Each subcell is vertically connected by p++-AlGaAs: C/n++-AlGaAs: Si: Te (1:2) tunnel junction with good thermal stability and a record peak tunneling current density of 1867 A/cm2. The I-V characteristics of LPCs with an aperture of 10×10 mm2 were investigated as a function of laser power and temperature. Maximum conversion efficiency and output power of 57.7% and 15.4 W, respectively, and a continuous stable operation at 22.9 W for over 550 hours were demonstrated. The temperature coefficient of conversion efficiency and open-circuit voltage were -0.197%abs/°C and -8.15 mV/°C, respectively, under 808 nm laser illumination of 21.0 W. Furthermore, an array of 100 large-scale (41×46 mm2) LPCs with an output power of 179 W under 1 kW laser irradiation at 20 m wireless transmission was developed.

1064†nm InGaAs metamorphic laser power converts with over 44% efficiency.

InGaAs metamorphic laser power converters (LPCs) have the potential to deliver electrical energy over distances of several kilometers. In this study, metalorganic chemical vapor deposition (MOCVD) was used to grow InGaAs-based LPCs with an absorption wavelength of 1064 nm. At step thicknesses of 2800 nm, overshoot thicknesses of 6000 nm, reverse component and thicknesses of 2.4% and 700 nm, respectively, a surface roughness of 6.0 nm and InGaAs (24%) lattice relaxation of 93.7% of the InGaAs metamorphic buffer were obtained. The I-V characteristics of LPCs with 10 × 10 mm2 apertures were investigated as a function of laser power and temperature. The maximum conversion efficiency of 44.1% and 550 hours of continuous stable operation at 4 W were demonstrated. Under 1064 nm laser illumination of 4 W, the temperature coefficients for the conversion efficiency and open-circuit voltage were -0.1%abs/°C and -1.6 mV/°C, respectively, and the LPC output power fluctuation was less than 0.5% during 216 hours of continuous temperature change from 20 to 100°C.

An Improved Synthesis Phase Unwrapping Method Based on Three-Frequency Heterodyne.

An improved three-frequency heterodyne synthesis phase unwrapping method is proposed to improve the measurement accuracy through phase difference and phase sum operations. This method can reduce the effect of noise and increase the equivalent phase frequency. According to the distribution found in the phase difference calculation process, the Otsu segmentation is introduced to judge the phase threshold. The equivalent frequency obtained from the phase sum is more than those of all projected fringe patterns. In addition, the appropriate period combinations are also studied. The simulations and related experiments demonstrate the feasibility of the proposed method and the ability to improve the accuracy of the measurement results further.

Disrupting phosphatase SHP2 in macrophages protects mice from high-fat diet-induced hepatic steatosis and insulin resistance by elevating IL-18 levels.

Chronic low-grade inflammation plays an important role in the pathogenesis of type 2 diabetes. Src homology 2 domain-containing tyrosine phosphatase-2 (SHP2) has been reported to play diverse roles in different tissues during the development of metabolic disorders. We previously reported that SHP2 inhibition in macrophages results in increased cytokine production. Here, we investigated the association between SHP2 inhibition in macrophages and the development of metabolic diseases. Unexpectedly, we found that mice with a conditional SHP2 knockout in macrophages (cSHP2-KO) have ameliorated metabolic disorders. cSHP2-KO mice fed a high-fat diet (HFD) gained less body weight and exhibited decreased hepatic steatosis, as well as improved glucose intolerance and insulin sensitivity, compared with HFD-fed WT littermates. Further experiments revealed that SHP2 deficiency leads to hyperactivation of caspase-1 and subsequent elevation of interleukin 18 (IL-18) levels, both in vivo and in vitro Of note, IL-18 neutralization and caspase-1 knockout reversed the amelioration of hepatic steatosis and insulin resistance observed in the cSHP2-KO mice. Administration of two specific SHP2 inhibitors, SHP099 and Phps1, improved HFD-induced hepatic steatosis and insulin resistance. Our findings provide detailed insights into the role of macrophagic SHP2 in metabolic disorders. We conclude that pharmacological inhibition of SHP2 may represent a therapeutic strategy for the management of type 2 diabetes.

Laser paint removal monitoring based on time-resolved spectroscopy.

Laser paint removal is a new cleaning technology with high efficiency. Dynamic monitoring and closed-loop control of the laser paint removal process are key to reducing the risk of metal substrate damage and to achieving the best cleaning. In this paper, the time-resolved characteristics of the elemental peaks in the laser-induced breakdown spectrum of paints and substrate were studied by using the combination of a monochromator (or a bandpass filter) and a photomultiplier tube (PMT) detector. The results show that the intensity of the elemental spectrum peak of the paint has a sudden drop while the intensity of the elemental spectrum peak of the substrate has a sudden increase when the paint is removed. The time-resolved signals can be fitted by double exponential functions, which are combinations of exponential functions with a longer and a shorter lifetime, respectively. The relative ratios of the coefficients of the shorter and longer lifetimes (A s h o r t /A l o n g ) at the wavelength correspond to the elements in paint increasing suddenly while decreasing suddenly at the wavelength corresponding to the substrate. The intensity of the elemental spectrum peaks of paints and substrate and the ratio (A s h o r t /A l o n g ) can be used to monitor the laser paint removal process in real time to reduce the damage risk of the metal substrate and achieve the purpose of efficient cleaning with low cost.

Experimental and FDTD study of silicon surface morphology induced by femtosecond laser irradiation at a high substrate temperature.

Substrate temperature is an important parameter for controlling the properties of femtosecond laser induced surface structures besides traditional ways. The morphology on silicon surface at different temperatures are studied experimentally. Compared to those formed at 300 K, smoother ripples, micro-grooves and nano/micro-holes are formed at 700 K. A two temperature model and FDTD method are used to discuss the temperature dependence of surface structures. The results show that the increased light absorption at elevated temperature leads to the reduction of surface roughness. The type-g feature in the FDTD-η map at 700 K, which corresponds to the energy deposition modulation parallel to the laser polarization with a periodicity bigger than the wavelength, is the origin of the formation of grooves. This work can benefit both surface structures based applications and the study of femtosecond laser-matter interactions.

Role of nanoparticles generation in the formation of femtosecond laser-induced periodic surface structures on silicon.

An initial roughness is assumed in the most accepted Sipe-Drude model to analyze laser-induced periodic surface structures (LIPSS). However, the direct experimental observation for the crucial parameters is still lacking. The generation of nanoparticles and low-spatial frequency LIPSS (LSFL) (LIPSS with a periodicity close to laser wavelength) on a silicon surface upon a single pulse and subsequent pulses irradiation, respectively, is observed experimentally. Finite-difference time-domain (FDTD) simulation indicates that the nanoparticles generated with the first pulse enhance the local electric field greatly. Based on the experimental extrapolated parameters, FDTD-η maps have been calculated. The results show that the inhomogeneous energy deposition, which leads to the formation of LSFL, is mainly from the modulation of the nanoparticles with a radius of around 100 nm.