Decreasing Pulse Duration Research Articles

Copper Functionalized SnSe Nanoflakes Enabling Nonlinear Optical Features for Ultrafast Photonics.

This study enhances the ultrafast photonics application of tin selenide (SnSe) nanoflakes via copper (Cu) functionalization to overcome challenges such as low conductivity and weak near-infrared (NIR) absorption. Cu functionalization enhances concentration, induces strain, and reduces the bandgap through Sn substitution and Sn vacancy filling with Cu ions. Demonstrated by density functional theory calculations and experimental analyses, Cu-functionalized SnSe exhibits improved NIR optical absorption and superior third-order nonlinear optical properties. Z-scan measurements and femtosecond transient absorption spectroscopy reveal better performance of Cu-functionalized SnSe in terms of nonlinear optical properties and shorter carrier relaxation times compared to pristine SnSe. Furthermore, saturable absorbers based on both SnSe types, when integrated into an erbium-doped fiber laser, show that Cu functionalization leads to a decrease in pulse duration to 798 fs and an increase in 3dB spectral bandwidth to 3.44nm. Additionally, it enables stable harmonic mode-locking of bound-state solitons. This work suggests a new direction for improving wide bandgap 2D materials by highlighting the enhanced nonlinear optical propertiesand potential of Cu-functionalized SnSe in ultrafast photonics.

Compression and self-compression of frequency modulated spherical photon density waves in anisotropic scattering media

We report on the results of a simulation of the photon density waves with pulse amplitude modulation by a complex frequency modulated signal. The problem is considered for the optical properties typical for sea water with anisotropy factor values varying from 0.75 to 0.93 at source-detector distances up to 120 m. It is shown that multiple scattering in a medium does not prevent effective compression of a signal registered using matched detection. Two competing phenomena affecting the detected pulse duration and depending on the central frequency of the modulation signal are discussed. The effect of faster attenuation of high harmonics in a complexly modulated signal leads to the detected signal duration increase as a consequence of multiple scattering. On the other hand, anomalous dispersion of photon density waves in media with scattering anisotropy leads to the pulse self-compression. The simulation results presented in the paper demonstrate the prevalence of different phenomena depending on the central frequency of the modulation signal resulting in a pulse duration decrease or increase in different frequency ranges covering the band from 107 to 2⋅109 Hz. The effect of the phase function shape on the observed effect is also discussed.

Enhancement of the photoacoustic effect during the light-particle interaction.

Exogenous photoacoustic contrast agents such as gold nanoparticles are widely utilized in photoacoustic imaging. Enhancing the photoacoustic performance of gold nanoparticles is pivotal for improving the quality and expanding the application scope of photoacoustic imaging. In this work, the photothermal and photoacoustic responses of gold nanospheres surrounded by water excited with a pulsed laser are obtained via a two-temperature model. The interplay between pulse duration and interface thermal resistance and its effect on the photothermal and photoacoustic performances are uncovered quantitatively. The results reveal that, as the pulse duration decreases, increasing the interfacial thermal conductivity can substantially enhance heat transfer between the gold nanosphere and the surrounding water. However, this approach does not effectively enhance the photoacoustic performance. Interestingly, when increasing the thermal conductivity, it was found that there is an optimal pulse duration within the range of 10 ps-10 ns. Employing an incident pulse laser with this optimal pulse duration can maximize the enhancement of the photoacoustic signal.

High-Frequency Shift and Extension of the Terahertz Radiation Spectrum up to 10 THz During Optical Rectification of High-Power Few-Cycle Near-Infrared Femtosecond Pump Radiation in a BNA Crystal

The generation of terahertz radiation in a BNA crystal pumped by 1.24-µm femtosecond laser radiation from a Cr:forsterite laser system with a pulse duration of 100 and 35 fs and a pump density of 10 mJ/cm2 has been realized. The achieved generation efficiency is 0.1%. It is found that a decrease in the pump pulse duration from 100 to 35 fs leads to the generation of high-frequency components in the ranges of 2.5–6.5 THz and 9‒10.5 THz in the generated radiation spectrum. Simulation of the terahertz radiation generation based on the solution of Maxwell’s equations by the finite-difference time-domain method has made it possible to adequately describe the measured spectra. The generation of broadband high-frequency terahertz radiation in the BNA crystal pumped by the Cr:forsterite laser system allows one to consider this schematic as an alternative to sources based on the BNA crystal pumped by a Ti:sapphire laser system.

Nonlinear optical properties of gold with intense femtosecond laser excitations above the damage threshold

The nonlinear optical properties of gold thin films have been studied under the irradiation of destructive laser pluses. The transmissivity and the reflectivity of a 100-nm-thick gold film are measured with femtosecond laser irradiations up to 106 J/m2 fluence, which is much higher than the damage threshold of gold. The nonlinearity is calculated with Fresnel loss equations. The real part of the complex refractive index is calculated to have a peak at ∼10 kJ/m2, which is close to the damage threshold. The complex third-order nonlinear susceptibility χ3 is estimated to be (4.95 − 2.02i) × 10−21 m2/V2, which agrees with the trend in previous reports that it decreases as the pulse duration decreases. This nonlinearity is further studied with a plasmon-photon exchange (PPE) model, where laser induced plasmons and absorbed photons are strongly correlated. The result of the simulation with the PPE model is in good agreement with the experimental transmissivity above the damage threshold. The model predicted an interaction time between photons and plasmons to be about 500 fs. This model is found to be essential in understanding such nonlinear optical processes under both extremely high and conventionally low laser irradiations.

High-Frequency Shift and Extension of the Terahertz Radiation Spectrum up to 10 THz During Optical Rectification of High-Power Few-Cycle Near-Infrared Femtosecond Pump Radiation in a BNA Crystal

The generation of terahertz radiation in a BNA crystal pumped by 1.24-µm femtosecond laser radiation from a Cr:forsterite laser system with a pulse duration of 100 and 35 fs and a pump density of 10 mJ/cm2 has been realized. The achieved generation efficiency is 0.1%. It is found that a decrease in the pump pulse duration from 100 to 35 fs leads to the generation of high-frequency components in the ranges of 2.5–6.5 THz and 9‒10.5 THz in the generated radiation spectrum. Simulation of the terahertz radiation generation based on the solution of Maxwell’s equations by the finite-difference time-domain method has made it possible to adequately describe the measured spectra. The generation of broadband high-frequency terahertz radiation in the BNA crystal pumped by the Cr:forsterite laser system allows one to consider this schematic as an alternative to sources based on the BNA crystal pumped by a Ti:sapphire laser system.

Liquid crystal resonator as a nonlinear reflector for passive Q-switching and mode locking

We carry out a theoretical study of a Fabry–Perot resonator with a central layer of nematic liquid crystal as a nonlinear polarization rotation (NPR) device for passive Q-switching and mode locking. We develop a self-consistent procedure for calculating the device dynamical response as a function of the input light intensity and polarization. We show how a positive feedback effect for the optical reorientation of the liquid crystal director makes the resonator abruptly switch to high-reflectivity state. We demonstrate that the time-delay between the pulse initiation and the resonator switch can be avoided by tilted liquid crystal molequles at the resonator boundaries. We show that such a NPR optical device is suitable for Q-switching operation with μs pulses and hundreds of kHz repetition rates. Further decrease of pulse duration and increase of the pulse repetition rate is possible by either increasing the pulse power or by increasing the Fabry–Perot resonator finesse. Finally, we show that the same Fabry–Perot resonator can be used for Q-switching and mode locking at tens of MHz repetition rates and ns pulse durations by using an electro-optic uniaxial–biaxial transition as an alternative to NPR in order to reduce pulse duration and increase repetition rate.

An improved theory of thermoelasticity for ultrafast heating of materials using short and ultrashort laser pulses

This paper introduces an improved thermoelastic theory designed specifically to address the challenges posed by fast and ultrafast heating of materials. The proposed theory incorporates a time delay in the temperature gradient and introduces the rate of temperature gradient as an additional term in the heat equation. This term accounts for the phase lag and the precedence of the temperature gradient and heat flux vector. By leveraging the sign of the temperature gradient rate, the theory automatically determines the precedence of either the temperature gradient or the heat flux vector. To verify the effectiveness of the proposed theory, experiments reported in the literature were simulated using several theories, namely the classical, Lord-Schulman (LS), Green-Lindsay (GL), dual-phase-lag (DPL), and the presented theory. The simulations considered the effects of relaxation times and pulse duration on the results. The findings reveal that for pulse durations shorter than 50 ps, the second-order time derivative of the temperature gradient becomes a crucial term in the energy equation for accurately predicting temperature profiles. Additionally, as the pulse duration decreases, the sensitivity to relaxation times increases. Regarding the surface temperature corresponding to the material ablation threshold, it is observed that for an 80 fs laser pulse, the ablation temperature falls between the boiling and critical temperatures, significantly higher than the boiling temperature associated with the ablation by a 270 ps pulse duration. Lastly, the simulation results exhibit a high level of agreement with the experimental data, particularly in terms of displacements, further validating the accuracy of the presented simulations.

Optimization design of laser-EMAT and its application in high-temperature forgings detection

In high-temperature continuous forging process, according to the real-time monitoring of workpiece thickness and flaws, the processing parameters can be adjusted accordingly, so we can remove defective components in time, which has essential research value for avoiding the interruption of production line and improving their yield and quality grade. We established a finite element (FE) model of the carbon steel’s laser-electromagnetic acoustic transducer (laser-EMAT) testing process. Based on the simulation model, we analyzed the effects of laser parameters, EMAT parameters, and sample thickness on the detected ultrasonic signal amplitude, and we also achieved the optimized Laser-EMAT design parameters. Subsequently, we developed a high-temperature resistant spiral coil EMAT and measured the high-temperature forging with a thickness of 100 mm and temperatures from 300 °C to 730 °C. Based on the experiments, we researched the effect of specimen temperature on the received ultrasonic wave amplitude. The results show that the excitation efficiency of laser-induced ultrasonic waves improves by decreasing pulse duration, decreasing laser spot radius, and increasing pulse laser energy. The receiving efficiency of the shear wave (SW) detected by the EMAT enhances when reducing the diameter of the EMAT wire and increasing the permanent magnet height. When the radius of the permanent magnet is equal to the radius of the EMAT coil, the receiving efficiency of SW is the highest. As the sample thickness increases, the size of the EMAT should increase accordingly to the acoustic beam divergence for obtaining a higher ultrasonic wave intensity. The amplitude of the SW signal received by the EMAT increases by 679% after the optimization design. With rising carbon steel forging temperature, the SW signal amplitude increases first and then decreases sharply, reaching its maximum at 617 °C, which is 29% higher than at room temperature, and the signal-to-noise ratio (SNR) of the SW is 20.5 dB.

Correlating armature and needle dynamics with voltage waveforms of solenoid-actuated GDI injector

The injector voltage hump that appears near the needle closing has been used for the real-time monitoring and feedback control of fuel injection duration in modern engines. This voltage hump has been thought to result from the abrupt change in electromagnetic induction by the stoppage of needle motion but detailed electromagnetic processes and associated armature and needle dynamics during the needle closing have not been thoroughly investigated in a wide range of injection conditions, which knowledge is crucial for the delicate control of fuel injection based on the voltage hump. The current study analyzes the transient armature and needle dynamics of a solenoid-actuated gasoline direct injection injector using an X-ray phase-contrast imaging technique. Then, the results are correlated with voltage waveforms during the needle closing transient under various injection pressures, injection pulse durations, and dwell times of split injections. The time derivatives of voltage waveforms showed lower and upper peaks in order in the regime of the voltage hump. Inconsistent with conventional understandings, the lower peak timing of the voltage derivative did not match with the timing of needle closing (end of injection) but rather matched with the abrupt descent timing of the armature and needle. The inflection timing and upper peak timing of the voltage derivative matched with the timings of actual needle closing and armature closing respectively. The amplitude of the voltage hump was near linearly dependent on the needle closing speed. The needle closing speed decreased upon the decrease of injection pulse duration and injection pressure which made it difficult to detect the voltage humps in ballistic injection regimes and low injection pressures. In split injection conditions, the voltage hump of the first injection was not detectable if the dwell time was shorter than the needle closing delay, the time from the current cut-off to the actual needle closing.

Pulse Duration Effects on Solution-Phase Protein Desorption in Laser Electrospray Mass Spectrometry

The effect of laser pulse duration on the ablation of aqueous myoglobin is investigated using laser electrospray mass spectrometry (LEMS). Pulse durations of 55 femtoseconds (fs), 56 piscoseconds (ps), and 10 nanoseconds (ns) were used to ablate aqueous myoglobin from stainless-steel and quartz substrates. The integrated signal intensity of myoglobin increases with decreasing pulse duration for both substrates. Laser-induced thermal effects are assessed by the relative amount of solvent adduction and number of phosphate moieties adducted to myoglobin by each laser pulse duration. The mass spectra for 55 fs vaporization shows myoglobin with appreciable solvent and phosphate adduction and baseline elevation. The mass spectra for 10 ns ablation have minimal adduction and limited baseline elevation. Heat-induced conformation changes in myoglobin were used to measure the amount of thermal energy deposited by each laser pulse duration. Ablation using the 55 fs pulse revealed the highest ratio of unfolded to folded myoglobin in comparison to the 56 ps and 10 ns measurements due to increased droplet lifetime and consequent interaction with the acid in the electrospray solvent. Collisional activation and heated capillary temperature were employed to reduce the droplet lifetime and demonstrate that fs ablation preserves approximately 2 times more myoglobin folded conformation in comparison to ps and ns pulses.

Potential Climate Impacts of Hydrological Alterations and Discharge Variabilities of the Mura, Drava, and Danube Rivers on the Natural Resources of the MDD UNESCO Biosphere Reserve

This study investigated hydrological alterations in the sections of the Mura, Drava, and Danube rivers, which together form a unique river landscape proclaimed by UNESCO as the Transboundary Biosphere Reserve Mura, Drava, and Danube (TBR MDD). A coherent network of 12 major protected areas along the rivers highlights their ecological value, which could be endangered by climate change and consequent environmental changes. Statistical analyses, such as the homogeneity test, Mann–Kendall trend test of monthly and seasonal discharges, and empirical probabilities of daily discharges, were applied to discharge data series (1960–2019) from six hydrological stations prior to the calculation of indicators of hydrologic alteration (IHA). This method could be a helpful tool for recognizing the changes in hydrological regimes that can affect river ecosystems. The 33 indicators were organized into five groups. The results showed a decrease in low pulse duration and increase in rise/fall rates and the number of reversals. From an ecological perspective, the results obtained for the probabilities of long flooding periods were particularly significant. They drastically decreased for all three rivers on their stretches within the reserve. According to IHA modeling results, the river sections analyzed were moderately altered with global indicator values between 0.5 and 0.75. The most pronounced hydrological alterations were associated with the frequency and duration of low and high pulses and the rate and frequency of changes in water condition, which could have a significant impact on the ecological values of the TBR MDD. In addition, results show more pronounced climate impact versus human activities.

Formation Principle and Characteristics of Self-Supercharging Pulsed Water Jet

High-pressure pulsed water jet technology has considerable development potential in the field of rock fragmentation. To overcome the shortcomings of existing pulsed jets, a self-supercharging pulsed water jet (SSPWJ) generation method is proposed, which is based on the theory of the pulsed water jet and the principle of hydraulic boosting. The proposed method changes the flow direction of the fluid medium through the valve core to make the piston reciprocate in the cylinder and relies on the effective area difference between the front and rear chambers in the stroke stage of the piston to realize the organic combination of “pulse” and “supercharging” of the jet, thus forming an SSPWJ. On the basis of the formation principle of the SSPWJ, a SSPWJ testing platform was constructed, and tests were performed on the jet pressure acquisition, morphology capture, and granite erosion. Both the jet pressure and the jet morphology exhibited periodic changes, and a higher pulse pressure was obtained at lower inlet pressure. The error of the pressure ratio calculated according to the experimental results was <3% relative to the theoretical design value, confirming the feasibility of the method. The pulse pressure and pulse frequency are controllable; that is, as the inlet flow rate increases in the stroke stage of the piston, the pulse pressure and pulse frequency increase, and the pulse duration decreases. As the inlet flow rate increases in the backward-stroke stage of the piston, the pulse frequency increases, and the pulse pressure and pulse duration remain unchanged. Under the combined action of the water-hammer pressure, high-speed lateral flow, and high-frequency dynamic load of the SSPWJ, local flaky exfoliation was observed when the granite surface was eroded. The results of this study lay the foundation for enriching the theory of pulsed jet generation and expanding its application range.

Influence of Pulse Duration on X-ray Emission during Industrial Ultrafast Laser Processing.

Soft X-ray emissions during the processing of industrial materials with ultrafast lasers are of major interest, especially against the background of legal regulations. Potentially hazardous soft X-rays, with photon energies of >5 keV, originate from the fraction of hot electrons in plasma, the temperature of which depends on laser irradiance. The interaction of a laser with the plasma intensifies with growing plasma expansion during the laser pulse, and the fraction of hot electrons is therefore enhanced with increasing pulse duration. Hence, pulse duration is one of the dominant laser parameters that determines the soft X-ray emission. An existing analytical model, in which the fraction of hot electrons was treated as a constant, was therefore extended to include the influence of the duration of laser pulses on the fraction of hot electrons in the generated plasma. This extended model was validated with measurements of H (0.07) dose rates as a function of the pulse duration for a constant irradiance of about 3.5 × 1014 W/cm2, a laser wavelength of 800 nm, and a pulse repetition rate of 1 kHz, as well as for varying irradiance at the laser wavelength of 1030 nm and pulse repetition rates of 50 kHz and 200 kHz. The experimental data clearly verified the predictions of the model and confirmed that significantly decreased dose rates are generated with a decreasing pulse duration when the irradiance is kept constant.

High-power mesa-stripe semiconductor lasers (910 nm) with an ultra-wide emitting aperture based on tunnel-coupled InGaAs/AlGaAs/GaAs heterostructures

The characteristics of high-power semiconductor lasers with an 800 μm emitting aperture based on tunnel-coupled InGaAs/AlGaAs/GaAs heterostructures with three optically uncoupled laser sections are studied. The maximum power achieved under pumping by current pulses with an amplitude of 47 A and a duration of 1 μs is 110 W with the maximum active region heating not exceeding 4.7 °C. At a pulse duration of 860 μs, the maximum optical power is 22.6 W, and the decrease in the optical power to the pulse end reaches 6.7 %. A decrease in the laser pulse duration to 85 μs leads to an increase in the peak laser power to 41.4 W at a pump current amplitude of 20 A.

Formulation of diffraction efficiencies of binary phase gratings for array illumination with ultrashort pulse beams.

We present simple formulas for the diffraction efficiencies of a binary phase grating that performs array illumination with ultrashort pulse beams. Using scalar diffraction theory, we formulated the efficiencies as a function of pulse spectral width by Fourier-transforming the complex-modulated frequency spectra of diffracted pulses in the far-field region. From the analytical simulations, we found that pulse array uniformity departs from unity as the spectral width increases, or the pulse duration decreases, thereby limiting the attainable split counts. This finding can be considered in the design of gratings for delivering controlled amounts of pulse energies to diffraction orders of interest.

The cratering performance of concrete target under true triaxial confinements

It is of great significance to evaluate the cratering performances of concrete target subjected to steel projectile impacting. Different from the method proposed by Forrestal et al. (2003) to record the acceleration signals in steel projectile by single-channel acceleration data recorder, a new testing equipment is established in the present study to record the dynamic triaxial responses of concrete target by six lateral bars. The projectile impacts the target through a hollow square bar (one of six lateral bars), and the true triaxial confinements are applied on the target through six lateral bars which loaded by three servo-controlled cylinders. These three forces, i.e. end resistance, lateral reaming resistance, and lateral traction, acted on the projectile toe during cratering process are discussed respectively by wave signal εy- recorded on the left bar in y-axial, by wave signals εx+, εx-, εz+, and εz- recorded on the lateral bars, and by specially assigned strain gauges demonstrated in Appendix B. Series of experiments of concrete targets under various impact velocities and under various confinements are conducted accordingly. Two stages, e.g. the accelerating cratering stage, and the decelerating cratering and penetration stage, are observed in stress σy- profiles. Similar lateral reaming stages are observed in stress profiles of lateral bars. These stress profiles could be used to evaluate forces applied on projectile toe, and lateral tractions could be ignored. Comparing with the cratering process in the impact direction, there are about 10µs delaying for the lateral reaming process. Depth of crater and pulse duration decrease with increasing confinements, and they exhibit obvious velocity and confinement dependency. Three dimensionless factors, i.e. the inertial effect, the strength effect, and the deformation characteristics, are introduced to evaluate the crater depth. Strength of concrete target is proven to be the most important factor in the cratering process for concrete target under lower impact velocity. This work provides comprehensive and profound understanding of the effects of multiaxial confinement on the crater resistance for concrete-like materials.

Nonheating ozone suppression in pulsed air discharges: role of pulse duration and repetition rate

Facilitating the separate production of ozone (O3) and nitrogen oxides (NO x ) in air discharges without a thermal process is of most merit in diversifying plasma technology; in particular, it is a primary requirement in certain cold, heat-sensitive plasma applications. Here, we propose a new method of nonheating ozone suppression in air discharges. The present work demonstrates that controlling the plasma chemical kinetics by adjusting the duration (width) and/or repetition frequency of the high-voltage DC pulse is effective in suppressing ozone formation in a surface dielectric barrier discharge in static ambient air. The temporal development of each oxygen- and nitrogen-related species in air discharge is complicated and shows different trends in the time range <10 µs; relatively long-lived O3 and NO x are strongly governed by the temporal behavior of short-lived reactive species, such as excited N2(A) and N2(v). To quantify time-varying O3 and NO x , an in situ UV absorption spectroscopy is applied to our gas-tight plasma reactor, which is operated in air at 21 °C. With a fixed frequency at 10 kHz and decreasing pulse duration from 10 μs to 0.18 μs, ozone is quenched faster in the plasma reactor, resulting in an irreversible chemical mode transition from an O3- to NO-rich environment. From a different set of experiment (with a 200 ns pulse duration and a frequency range of 1–10 kHz), we can conclude that the off-pulse period also plays a crucial role in the temporal evolution of O3 and NO x ; the larger the applied driving frequency is, the earlier the ozone-free phenomenon appears over the discharge time. Our findings represent a breakthrough in expanding the usage of air discharges and their application in various fields of interest.

Spectral properties of partially coherent chirped Airy pulsed beam in oceanic turbulence

Spectral shifts and spectral intensities of partially coherent chirped Airy pulsed (PCCAP) beams propagating through oceanic turbulence are studied, where spectral properties of partially coherent chirped Gaussian pulsed (PCCGP) beams are also compared. The effects of optical parameters and oceanic turbulence parameters on spectral shifts at different observation positions are mainly discussed. It is shown that the spectral shift of PCCAP beam is richer than that of PCCGP beam, and there exists some critical positions for PCCAP beams where the spectral red-shift reaches its maximum. The spectral red-shift increases with increasing chirped parameter, correlation length or decreasing pulse duration. However, the increasing of spectral red-shift is accompanied by increasing dissipation rate of turbulence kinetic energy in unit mass liquid or decreasing relative intensity of temperature and salinity fluctuations, mean square temperature dissipation rate. A physical explanation of rapid spectral transition is also made.

Formation of near-surface melt films on glass ceramics due to ultrashort laser pulses

Glass ceramics are highly specialized composite materials, which have a partly polycrystalline and a partly glassy state. Due to their special properties such as good mechanical strength, low thermal expansion, and excellent thermal shock resistance, they are especially well known for their use in consumer goods industry. But also in the high-tech sector, like optics or microsystems technology, the applications for glass ceramics are constantly growing. Simultaneously, the continuing miniaturization of microelectronic components requires precise, high-resolution processing methods. While mechanical processing is limited due to the brittle-hard properties of the material, ultrashort pulse lasers can serve as an ideal tool for this purpose. The pulse durations in the pico- and femtosecond range are known to enable a highly precise and gentle processing with very small thermal load for the workpiece. Therefore, the process is often referred to as cold ablation. It has been known for some time, however, that if pulse energies and repetition rates are sufficiently high, heat accumulation effects can occur. In this article, we report on surface modifications on glass ceramics arising during femtosecond ablation. Using Low Temperature Co-fired Ceramics (LTCC) as an example, we show that even at low repetition rates of $${100}\,\hbox {kHz}$$ and moderate average laser power below $${3}\,\hbox {W}$$ several micrometer thick molten patterns can emerge. Another peculiarity of the observed phenomenon lies in the increase of the vitreous layer with decreasing pulse duration. The dependence of the effect on the material structure is investigated by means of X-ray diffraction (XRD) measurements.