Combined effects of ambient gas pressures and magnetic field on laser plasma expansion dynamics

In this work,we investigated the influence of air gas pressures on the expansion features of nanosecond laser ablated aluminum plasma in the absence and presence of a nonuniform magnetic field using fast photography.A particular emphasis was given to the plume dynamics（shape,size） with the combined effects of ambient gas pressures and an external magnetic field.Free expansion,sharpening effect,and hemi-spherical structures of the aluminum plasma were observed without a magnetic field under different gas pressures.Analysis of the resulting plume images with the combined effects of air gas pressures and a magnetic field show significant changes,such as plume splitting,elliptical geometry changes,radial expansion,and plume confinement.Furthermore,the total size of the plasma plume with a magnetic field was measured to be smaller than the plasma plume without a magnetic field at several background pressures.

Theory of multiphoton photoemission disclosing excited states in conduction band of individual TiO_(2) nanoparticles

A theory of multiphoton photoemission is derived to explain the experimentally observed monotonic decrease with the wavelength in the electron yield of TiO_(2) nanoparticles(NPs)by as large as four orders of magnitude.It is found that the fitting parameter corresponds to the energy position of Ti3d e_(g) and t_(2g) states,and the derived theory is a novel diagnostic of excited states in the conduction band,very importantly,applicable to individual NPs.The difference between four-photon slope NPs and three-photon slope NPs is attributed to the difference in defect density.The success of the theory in solving the puzzling result shows that thermal emission from high-lying levels may dominate over direct multiphoton ionization in solids when the photon number larger than four is required.

Ultrafast plasmon dynamics in asymmetric gold nanodimers

We theoretically investigate the effect of symmetry breaking on the ultrafast plasmon responses of Au nanodisk(ND)dimers by varying the diameter of one of the constituent nanodisks.In the case of a single ultrafast laser pulse,we demonstrate that the ultrafast responses of Au ND homodimer can be significantly modified due to the effect of symmetry breaking.The symmetric dimer shows a single broad spectral peak,whereas the size-asymmetric dimer shows three spectral peaks.The first system displays at most one temporal maximum and no beats in ultrafast temporal,whereas the second system may have three temporal maxima and two beats due to a combination of broken symmetry and the coherent superposition between various plasmon modes induced by the ultra-short laser pulse.Moreover,the shape of temporal dynamics of the size-asymmetric dimer is significantly deformed due to the excitation of local plasmon modes with different wavelength components.Furthermore,the decay time of the amplitude of the local field is longer and oscillates with a high frequency due to the narrower linewidth and red-shifted spectral peaks.We show that the ultrafast plasmon responses of both dimers can be controlled by varying the relative phase and time delays between a pair of two pulses.Our results will open new paths to understanding ultrafast plasmon responses in asymmetric heterodimers with suitable properties for different applications.

Characteristics of ion debris from laser-produced tin plasma and mitigation of energetic ions by ambient gas

Extreme ultraviolet lithography is most promising for the next generation lithography. However, debris from laser-produced plasma, particularly energetic ions, severely decreases the lifetime of extreme ultraviolet optics. We measured the characteris- tics of ions from tin plasma by the time of flight method with a frequency-doubled Nd： YAG laser at the intensity of 3.5x1010 W/cm2 （532 nm, 8 ns）. Our measurement shows that the maximum and peak of tin ions energies from plasma under the above experimental parameters are about 4.2 and 1.8 keV, respectively. Moreover, it is found that kinetic energy angular distribution of tin ions can be fitted by cos0.8（θ）, where θ is the angle with respect to the target normal. We also investigated the mitigation effect of argon, helium gases to the tin ions, and found that tin ions from the plasma can be mitigated effectively at the pressure -38 mTorr for argon or -375 mTorr for helium, respectively.

Disclosing transverse spin angular momentum of surface plasmon polaritons through independent spatiotemporal imaging of its in-plane and out-of-plane electric field components

The comprehensive capture of near field spatiotemporal information of surface plasmon polaritons(SPPs)is a prerequisite for revealing their physical nature.In this study,we first performed an independent,spatiotemporal imaging of the out-of-plane and in-plane components of SPP near fields in a femtosecond light-excited trench using an obliquely incident time-resolved photoemission electron microscopy(TR-PEEM).We did the capture by imaging of the interference patterns induced by a superposition of the p-or s-polarized probe light,with the out-plane or in-plane components of SPP near-fields,under the noncollinear excitation mode.The method may be used to reconstruct a 3D SPP spatiotemporal field.Moreover,we demonstrated that the fringe shift of the in-terference patterns between the captured in plane and out-of plane components of the SPP field in PEEM images corresponds to the 1/4 fringe period,which is attributed toπ/2 out of phase of the out-of plane and in-plane near-field components of SPP.The resulting TR-PEEM images are supported by a classical wave mode and FDTD simulations.Essentially,the measuredπ/2 phase difference between the in-plane and out-of plane components of the SPP indicated a rotating field component in the propagation plane,i.e,that the SPP exhibits an elliptically polarized electric field in the propagation plane.The experimental results presented herein provide direct evidence of SPP having the inherent attributes of transverse spin angular momentum.

Femtosecond laser micro-nano processing for boosting bubble releasing of gas evolution reactions

Coupling effect of chemical composition and physical structure is a key factor to construct superaerophobic electrodes.Almost all reports about superaerophobic electrodes were aimed at precisely controlling morphology of loaded materials(constructing specific structure)and ignored the due role of substrate.Nevertheless,in this work,by using high precision and controllable femtosecond laser,hierarchical micro-nano structures with superaerophobic properties were constructed on the surface of silicon substrate(fs-Si),and such special super-wettability could be successfully inherited to subsequent self-supporting electrodes through chemical synthesis.Femtosecond laser processing endowed electrodes with high electrochemical surface area,strong physical structure,and remarkable superaerophobic efficacy.As an unconventional processing method,the reconstructed morphology of substrate surface bears the responsibility of superaerophobicity,thus liberating the structural constraints on loaded materials.Since this key of coupling effect is transferred from the loaded materials to substrate,we provided a new general scheme for synthesizing superaerophobic electrodes.The successful introduction of femtosecond laser will open a new idea to synthesize self-supporting electrodes for gas-involving reactions.

Ultrafast spatiotemporal control of directional launching of surface plasmon polaritons in a plasmonic nano coupler

Ultrafast spatiotemporal control of a surface plasmon polariton(SPP)launch direction is a prerequisite for ultrafast information processing in plasmonic nanocircuit components such as ultrafast on–off of plasmonic switching and information recording.Here we realize for the first time,to the best of our knowledge,ultrafast spatiotemporal control of the preferential launch direction of an SPP at the nano-femtosecond scale via a plasmonic nano directional coupler.The spatiotemporal switching of the SPP field was revealed using time-resolved photoemission electron microscopy(TR-PEEM).Experimental results show that the extinction ratio of the SPP directional coupler can be substantially optimized by properly selecting the amplitude and time delay of the two incident light pulses in the experiment.More importantly,we demonstrate a solution for the launch direction of the SPP field,switched in a plasmonic nano directional coupler on the femtosecond timescale,by adjusting the instantaneous polarization state of the excitation light.The TR-PEEM images are supported by finite-difference time-domain(FDTD)simulations.We believe the results of this study can be used to develop high-speed,miniaturized signal processing systems.

Flexible manipulation of plasmon dephasing time via the adjustable Fano asymmetric dimer

It is highly desirable to flexibly and actively manipulate the dephasing time of a plasmon in many potential applications;however,this remains a challenge.In this work,by using femtosecond time-resolved photoemission electron microscopy,we experimentally demonstrated that the Fano resonance mode in the asymmetric nanorod dimer can greatly extend the dephasing time of a femtosecond plasmon,whereas the non-Fano resonance results in a smaller dephasing time due to the large radiative damping,and flexible manipulation of the dephasing time can be realized by adjusting one of the nanorods in the Fano asymmetric dimer.Interestingly,it was found that plasmon resonance wavelengths both appeared red-shifted as the length of the upper or lower nanorods increased individually,but the dephasing time varied.Furthermore,it also indicated that the dephasing time can be prolonged with a smaller ascending rate by increasing the length of both the nanorods simultaneously while keeping the dimer asymmetry.Meanwhile,the roles of radiative and nonradiative damping in dephasing time are unveiled in the process of nanorod length variation.These results are well supported by numerical simulations and calculations.