Numerical Study on Effect of Non-uniform CMAS Penetration on TGO Growth and Interface Stress Behavior of APS TBCs

The penetration of CaO-MgO-Al_(2)O_(3)-SiO_(2)(CMAS)is one of the most significant factors that induce the failure of air-plasma-sprayed thermal barrier coatings(APS TBCs).The direct penetration of CMAS changes the thermal/mechanical properties of the top coat(TC)layer,which affects the thermal mismatch stress behavior and the growth of thermally grown oxide(TGO)at the TC/bond coat(BC)interface,thereby resulting in a more complicated interface stress state.In the present study,a two-dimensional global model of APS TBCs with half of the TC layer penetrated by CMAS is established to investigate the effect of non-uniform CMAS penetration on the interface stress behavior.Subsequently,a local model extracted from the global model is established to investigate the effects of interface morphologies and CMAS penetration depth.The results show that non-uniform CMAS penetration causes non-uniform TGO growth in APS TBCs,which consequently causes the stress behavior to vary along the interface.Furthermore,the CMAS pen-etration depth imposes a significant effect on the TC/TGO interface stress behavior,whereas the interface roughness exerts a prominent effect on the stress level at the BC/TGO interface under CMAS penetration.This study reveals the mechanism associated with the effect of non-uniform CMAS penetration on the interface stress behavior in APS TBCSs.

Simple,stable and efficient nonlinear pulse compression through cascaded filamentation in air

Nonlinear compression has become an obligatory technique along with the development of ultrafast lasers in generating ultrashort pulses with narrow pulse widths and high peak power.In particular,techniques of nonlinear compression have experienced a rapid progress as ytterbium(Yb)-doped lasers with pulse widths in the range from hundreds of femtoseconds to a few picoseconds have become mainstream laser tools for both scientific and industrial applications.Here,we report a simple and stable nonlinear pulse compression technique with high efficiency through cascaded filamentation in air followed by dispersion compensation.Pulses at a center wavelength of 1040 nm with millijoule pulse energy and 160 fs pulse width from a high-power Yb:CaAlGdO_(4) regenerative amplifier are compressed to 32 fs,with only 2.4% loss from the filamentation process.The compressed pulse has a stable output power with a root-meansquare variation of 0.2% over 1 hour.

Widely tunable continuous-wave visible and mid-infrared light generation based on a dual-wavelength switchable and tunable random Raman fiber laser

Nonlinear frequency conversion of wavelength agile and high-power random fiber lasers can provide a promising way to generate continuous-wave(CW)visible and mid-infrared(MIR)light with unique properties such as the continuous modeless spectrum,low temporal/spatial coherence,and high temporal stability.Here,we report a dual-wavelength switchable and tunable random Raman fiber laser(RRFL)based on a phosphosilicate fiber that has two Raman gain peaks for the first time and demonstrate its superior capability to generate widely tunable CW visible and mid-infrared light via nonlinear frequency conversions.By using the combination of a tunable pump and two tunable gratings in Littrow configuration that can provide separated point feedback for the two Stokes wavelengths corresponding to silica-and phosphorus-related Raman peaks,the spectrum of an RRFL can be flexibly manipulated for the aim of nonlinear frequency conversions,including single-wavelength tunable emission at the 1.1μm or 1.2μm band for second-harmonic generation(SHG),dual-wavelength simultaneously tunable emission at the 1.1μm and 1.2μm bands for the sum-frequency generation(SFG),and dual-wavelength separation tunable emission for difference-frequency generation(DFG).As a result,with the combination of SHG and SFG in a periodically poled lithium niobate crystal array,we experimentally demonstrate the broadest tuning range(560-630 nm)of visible light generated from an RRFL,to the best of our knowledge.The tunable MIR light in the range of 10.7-12.3μm is also demonstrated through DFG of an RRFL operating in separation tunable dual-wavelength emission mode in a Ba Ga4Se7(BGSe)crystal,which is the first realization of>10μm CW DFG in the BGSe crystal.We believe the developed dual-wavelength switchable and tunable RRFL can provide a new compact,robust,and cost-effective platform to realize broadly tunable light in both the visible and MIR regions,which can also find potential applications in imaging,sensing,and temporal ghost imaging in various spectral bands.

Dual-Wavelength Spectrum-Shaped Mid-Infrared Pulses and Steering High-Harmonic Generation in Solids

Mid-infrared(MIR)ultra-short pulses with multiple spectral-band coverage and good freedom in spectral and temporal shaping are desired by broad applications such as steering strong-field ionization,investigating bound-electron dynamics,and minimally invasive tissue ablation.However,the existing methods of light transient generation lack freedom in spectral tuning and require sophisticated apparatus for complicated phase and noise control.Here,with both numerical analysis and experimental demonstration,we report the first attempt,to the best our knowledge,at generating MIR pulses with dual-wavelength spectral shaping and exceptional freedom of tunability in both the lasing wavelength and relative spectral amplitudes,based on a relatively simple and compact apparatus compared to traditional pulse synthesizers.The proof-of-concept demonstration in steering the high-harmonic generation in a polycrystalline ZnSe plate is facilitated by dual-wavelength MIR pulses shaped in both spectral and temporal domains,spanning from 5.6 to 11.4μm,with multi-microjoule pulse energy and hundred-milliwatt average power.Multisets of harmonics corresponding to different fundamental wavelengths are simultaneously generated in the deep ultraviolet region,and both the relative strength of individual harmonics sets and the spectral shapes of harmonics are harnessed with remarkable freedom and flexibility.This work would open new possibilities in exploring femtosecond control of electron dynamics and light–matter interaction in composite molecular systems.

Multigigawatt 50 fs Yb:CALGO regenerative amplifier system with 11 W average power and mid-infrared generation

Lasers with high average and high peak power as well as ultrashort pulse width have been all along demanded by nonlinear optics studies,strong-field experiments,electron dynamics investigations,and ultrafast spectroscopy.While the routinely used titanium-doped sapphire(Ti:sapphire)laser faces a bottleneck in the average power upscaling,ytterbium(Yb)-doped lasers have remarkable advantages in achieving high average power.However,there is still a substantial gap of pulse width and peak power between the Ti:sapphire and Yb-doped lasers.Here we demonstrate a high-power Yb:CaAlGdO4(Yb:CALGO)regenerative amplifier system,delivering 1040 nm pulses with11 W average power,50 fs pulse width,and 3.7 GW peak power at a repetition rate of 43 k Hz,which to some extent bridges the gap between the Ti:sapphire and Yb lasers.An ultrabroadband Yb-doped fiber oscillator,specially designed spectral shapers,and Yb:CALGO gain medium with broad emission bandwidth,together with a double-end pumping scheme enable an amplified bandwidth of 19 nm and 95 fs output pulse width.To the best of our knowledge,this is the first demonstration of sub-100 fs regenerative amplifier based on Yb-doped bulk medium without nonlinear spectral broadening.The amplified pulse is further compressed to 50 fs via cascaded-quadratic compression with a simple setup,producing 3.7 GW peak power,which boosts the record of peak power from Yb:CALGO regenerative amplifiers by 1 order.As a proof of concept,pumped by the high-power,50 fs pulses,7.5–11.5μm midinfrared(MIR)generation via intrapulse difference-frequency generation is performed,without the necessity of nonlinear fiber compressors.It leads to a simple and robust apparatus,and it would find good usefulness in MIR spectroscopic applications.

Metal-organic framework-derived Co nanoparticles and single atoms as efficient electrocatalyst for pH universal hydrogen evolution reaction

Hydrogen release through water splitting is essential for reducing carbon emissions and promoting the hydrogen economy.One of the crucial challenges for industrial applications of water electrolysis is the manufacture of electrocatalysts which can reduce the kinetic energy barrier of the hydrogen evolution reaction(HER).Loading transition metal(TM)nanoparticles(NPs)or single atoms(SAs)into heteroatom-doped carbon materials(HCMs)is an effective method to improve electrochemical activity and stability.To this end,we synthesized N-doped porous carbon(NC)encapsulated Co NPs and isolated Co SA nanocatalysts(denoted as Co NPs@SAs-NC)using metal-organic frameworks(MOFs)as sacrificial precursors.The Co NPs@SAs-NC nanocatalysts displayed outstanding HER activity with a 110 mV overpotential in 1 M KOH,47 mV overpotential in 0.5 M H2SO4 and 171 mV in 0.5 M phosphate-buffered saline(PBS)to reach a current density of 10 mA·cm^(−2).In addition,the mechanism of the synergistic effect of Co NPs,Co SAs and N species was investigated in-depth using in situ shielding experiments and density functional theory(DFT)calculations.