Picosecond Laser Pulses Research Articles

Observation of the ponderomotive effect in non-valence bound states of polyatomic molecular anions

The ponderomotive force on molecular systems has rarely been observed hitherto, despite potentially being extremely useful for the manipulation of the molecular properties. Here, the ponderomotive effect in the non-valence bound states has been experimentally demonstrated, for the first time to the best of our knowledge, giving great promise for the manipulation of polyatomic molecules by the dynamic Stark effect. Entire quantum levels of the dipole-bound state (DBS) and quadrupole-bound state (QBS) of the phenoxide (or 4-bromophenoxide) and 4-cyanophenoxide anions, respectively, show clear-cut ponderomotive blue-shifts in the presence of the spatiotemporally overlapped non-resonant picosecond control laser pulse. The quasi-free electron in the QBS is found to be more vulnerable to the external oscillating electromagnetic field compared to that in the DBS, suggesting that the non-valence orbital of the former is more diffusive and thus more polarizable compared to that of the latter.

W-Band MMIC Chip Assembly Using Laser-Enhanced Direct Print Additive Manufacturing

In this article, the first W-band laser-enhanced direct print additive manufacturing (LE-DPAM) monolithic microwave integrated circuit (MMIC) chip-carrier assembly is demonstrated. Acrylonitrile butadiene styrene (ABS) is 3-D printed using fused deposition modeling to form substrate layers, and DuPont CB028 conductive paste is microdispensed to form transmission lines and on-chip interconnects. These processes are combined with picosecond-pulsed laser micromachining using two wavelengths in the infrared (IR) (1064 nm) and ultraviolet (UV) (355 nm) regions to create features as small as 5 μm for printed layers with a high 6:1 aspect ratio and to improve the effective conductivity of the printed traces. The laser machining process is also adopted to fabricate vertical interconnections (vias) over multilayer substrates while also suppressing higher order modes. Four different dc-to-110-GHz transmission-line configurations have been investigated, including a coplanar waveguide (CPW), a grounded CPW (GCPW), a novel multilayer CPW-microstrip transition, and a GCPW-microstrip transition with attenuation 10 dB. This process is extended to embed a W-band MMIC low-noise amplifier (LNA) die inside a laser-machined cavity with a carrier-chip gap less than 5 μm. Also, RF/dc interconnects were microdispensed to connect the transmission lines on the substrate and MMIC die. Chip-to-board transition loss as low as 0.03 dB per interconnect with return loss >7.5 dB was achieved with performance that is comparable to that of the probed MMIC die. The MMIC chip-carrier assembly provides a low-cost, versatile yet simple on-demand laser-enhanced additive manufacturing process to package a wideband MMIC die using a single automated LE-DPAM platform.

Nanoparticle Fragmentation Below the Melting Point Under Single Picosecond Laser Pulse Stimulation.

Understanding the laser-nanomaterials interaction including nanomaterial fragmentation has important implications in nanoparticle manufacturing, energy, and biomedical sciences. So far, three mechanisms of laser-induced fragmentation have been recognized including non-thermal processes and thermomechanical force under femtosecond pulses, and the phase transitions under nanosecond pulses. Here we show that single picosecond (ps) laser pulse stimulation leads to anomalous fragmentation of gold nanoparticles that deviates from these three mechanisms. The ps laser fragmentation was weakly dependent on particle size, and it resulted in a bimodal size distribution. Importantly, ps laser stimulation fragmented particles below the whole particle melting point and below the threshold for non-thermal mechanism. We propose a framework based on near-field enhancement and nanoparticle surface melting to account for the ps laser-induced fragmentation observed here. This study reveals a new form of surface ablation that occurs under picosecond laser stimulation at low fluence.

Selective Metallization of Polymers: Surface Activation of Polybutylene Terephthalate (PBT) Assisted by Picosecond Laser Pulses

The selective metallization of nonconductive polymer materials has broad applications in the fields of integrated circuit technology and metallized patterns. This work discusses a methodology to pattern metal tracks on polybutylene terephthalate substrates. The process consists of three steps: 1) surface patterning with picosecond laser pulses (1030 nm) in air, 2) Pd seeding via treatment in PdCl2solution, and 3) selective metallization via electroless copper deposition. Picosecond laser irradiation promotes not only surface roughening but also chemical modification to enable Pd seeding as the polymer surface acquires the ability to reduce Pd(II)‐chloride species to metallic Pd. The laser parameters, as well as the PdCl2concentration and seeding temperature, have an influence on the polymer surface morphology, the concentration and distribution of metallic Pd, and the copper layer properties. Homogeneous copper layers with well‐defined geometries, good coating‐substrate adhesion, and high electrical conductivity can be obtained. This is ascribed to the synergistic effect of the chemical surface activation and roughness development (from 0.13 to ≈1.6 μm). As the patterning and surface activation are performed in air, directly on the as‐received polymer substrate, this methodology shows great potential for metallization of electronic devices with 3D complex geometries.

Ultrafast coherent excitation of an ytterbium ion with single laser pulses

Experimental realizations of two qubit entangling gates with trapped ions typically rely on addressing spectroscopically resolved motional sidebands, limiting gate speed to the secular frequency. Fast entangling gates using ultrafast pulsed lasers overcome this speed limit. This approach is based on state-dependent photon recoil kicks from a sequence of counter-propagating, resonant, ultrafast pulse pairs, which can allow sub-microsecond gate speeds. Here, we demonstrate a key component of the ultrafast gate protocol, the coherent excitation of a 171Yb+ ion across the 2S1∕2-2P1∕2 transition with a single near-resonant short optical pulse at 369.53 nm. We achieve a maximum population transfer of 94(1)% using a picosecond pulsed laser that can be tuned across the 2S1∕2-2P1∕2 transition and 42.5(9)% with 190(7) GHz detuning.

Improved temporal contrast of streak camera measurements with periodic shadowing.

Periodic shadowing, a concept used in spectroscopy for stray light reduction, has been implemented to improve the temporal contrast of streak camera imaging. The capabilities of this technique are first proven by imaging elastically scattered picosecond laser pulses and are further applied to fluorescence lifetime imaging, where more accurate descriptions of fluorescence decay curves were observed. This all-optical approach can be adapted to various streak camera imaging systems, resulting in a robust technique to minimize space-charge induced temporal dispersion in streak cameras while maintaining temporal coverage and spatial information.

Nonlinear refraction and absorption of spectrally tuneable picosecond pulses in carbon disulfide

We analyze the spectral dependence of the nonlinear refractive index and nonlinear absorption coefficient of carbon disulfide (CS2) in the spectral range of 400–1100 nm using picosecond laser pulses. An increase of the nonlinear refractive index of CS2 at the shorter wavelength region compared with the near infrared region (9 × 10−14 cm2 W−1 (λ = 700 nm) and 4 × 10−14 cm2 W−1 (λ = 1000 nm), respectively) follows by a decrease of this parameter at the wavelengths below 600 nm. We compare the spectral dependences of the lower- and higher-order nonlinear absorption coefficients in carbon disulfide. The three-photon absorption coefficient was found to be maximal at λ = 1000 nm (β3PA = 2 × 10−19 cm3 W−2), while analyzing between 700 and 1100 nm. The maximal value of the two-photon absorption coefficient was determined at λ = 400 nm (1.8 × 10−9 cm W−1).

Manufacturing of Microfluidic Devices with Interchangeable Commercial Fiber Optic Sensors.

In situ measurements are highly desirable in many microfluidic applications because they enable real-time, local monitoring of physical and chemical parameters, providing valuable insight into microscopic events and processes that occur in microfluidic devices. Unfortunately, the manufacturing of microfluidic devices with integrated sensors can be time-consuming, expensive, and “know-how” demanding. In this article, we describe an easy-to-implement method developed to integrate various “off-the-shelf” fiber optic sensors within microfluidic devices. To demonstrate this, we used commercial pH and pressure sensors (“pH SensorPlugs” and “FOP-MIV”, respectively), which were “reversibly” attached to a glass microfluidic device using custom 3D-printed connectors. The microfluidic device, which serves here as a demonstrator, incorporates a uniform porous structure and was manufactured using a picosecond pulsed laser. The sensors were attached to the inlet and outlet channels of the microfluidic pattern to perform simple experiments, the aim of which was to evaluate the performance of both the connectors and the sensors in a practical microfluidic environment. The bespoke connectors ensured robust and watertight connection, allowing the sensors to be safely disconnected if necessary, without damaging the microfluidic device. The pH SensorPlugs were tested with a pH 7.01 buffer solution. They measured the correct pH values with an accuracy of ±0.05 pH once sufficient contact between the injected fluid and the measuring element (optode) was established. In turn, the FOP-MIV sensors were used to measure local pressure in the inlet and outlet channels during injection and the steady flow of deionized water at different rates. These sensors were calibrated up to 140 mbar and provided pressure measurements with an uncertainty that was less than ±1.5 mbar. Readouts at a rate of 4 Hz allowed us to observe dynamic pressure changes in the device during the displacement of air by water. In the case of steady flow of water, the pressure difference between the two measuring points increased linearly with increasing flow rate, complying with Darcy’s law for incompressible fluids. These data can be used to determine the permeability of the porous structure within the device.

Exploiting the FM-to-AM conversion to generate high-contrast picosecond laser pulses.

In this work, we simulate and demonstrate experimentally a new approach to generate picosecond laser pulses. This technique is based on optoelectronic FM-to-AM conversion: a transient radiofrequency phase modulation is applied on a continuous-wave fiber laser otherwise blocked by a bandstop filter. By leveraging the ability of modern electronic amplifiers to generate high voltage swings values on a short timescale, the presented setup induces a large and brief excursion out of the filter, hence generating 18 ps pulses with a time contrast of 38 dB. Detailed simulations pave the road toward pulses shorter than 1 ps by using an optimally shaped RF response.

Shaping of picosecond laser pulses with THz intensity modulation in the infrared, visible, and ultraviolet ranges.

The methods of shaping picosecond laser pulses with periodic intensity modulation tunable in frequency and depth are considered. Schemes for shaping modulated pulses "in-line" and with one output port are proposed. A picosecond modulation of the time envelope for IR laser pulses using a polarization interferometer is demonstrated experimentally. Shaping of modulated laser pulses of the UV range is shown by numerical modeling. The possibility to control the modulation depth of the fourth harmonic under the combined impact of material dispersion and nonlinear conversion in a classical collinear scheme of the fourth harmonic generation without distortion of the 3D pulse shape is demonstrated.

Theoretical study of soft X-ray emission and dynamics of laser-induced plasma of double stream gas puff

The double stream gas puff-based laser-produced plasma is studied as a source of soft X-ray radiation in nm wavelength spectral range. Dynamics of plasma induced by Nd:YAG laser beam and its emission is simulated with radiation-magnetohydrodynamic code Zstar. The modeling results for krypton gas stream in an annular helium jet as a circumferential gas for various picosecond and nanosecond laser pulses corresponding to the experiments are presented. The spatial–spectral features and temporal behavior of the soft X-ray and EUV emission are investigated. Under ps pulse, the gas is rapidly ionized in the laser beam channel, but it does not have enough time to shift sensibly during the pulse, and the plasma electron density grows against the background of almost constant ion density during the ionization in the laser radiation field. There is ionization instability only capable to be developed in ps range. At ns pulse, the gas ionization and heating leads to gas pushing out of the channel, and the formation of a divergent compression wave transforming into the shock wave. Behind the compression wave front, conditions arise for the development of Rayleigh–Taylor-type instability. The instability leads to the redistribution of plasma temperature and density, and to the formation of increased soft X-ray emission spots. Time evolution of spatial distributions and spectral characteristics of emitted SXR radiation is analyzed for different laser pulses. Transient effects in multicharged ion plasma are discussed, fundamental understanding of those is required for optimization of plasma radiation source. A conversion efficiency of laser energy into soft X-ray wavebands from krypton plasma is scanned by laser parameters and analyzed.

Wake dynamics of air filaments generated by high-energy picosecond laser pulses at 1 kHz repetition rate.

We investigated the filamentation in air of 7 ps laser pulses of up to 200 mJ energy from a 1.03 μm-wavelength Yb:YAG laser at repetition rates up to f=1kHz. Interferograms of the wake generated show that while pulses in a train of repetition rate f=0.1kHz encounter a nearly unperturbed environment, at f=1kHz, a channel with an axial air density hole of ∼20% is generated and maintained at all times by the cumulative effect of preceding laser pulses. Measurements at f=1kHz show that the energy deposited decreases proportional to the air channel density depletion, becoming more pronounced as the repetition rate and pulse energy increase. Numerical simulations indicate that contrary to filaments generated by shorter duration pulses, the electron avalanche is the dominant energy loss mechanism during filamentation with 7 ps pulses. The results are of interest for the atmospheric propagation of joule-level picosecond pulses from Yb:YAG lasers, of which average powers now surpass 1 kW, and for channeling other directed energy beams.

Multiphoton-excited dynamics of the trans or cis structural isomer of 1,2-dibromoethylene.

Photofragmentation dynamics of cis and trans isomers of 1,2-dibromoethylene (1,2-DBE) have been investigated by multiphoton excitation using a picosecond (ps) laser pulse. It has been found that the Br2 + product ion preferentially originates from the cis isomer rather than from trans. The Boltzmann-type isotropic low kinetic energy components of the Br+ and Br2 + product state distributions seem to be most likely from the unimolecular reactions of the vibrationally hot cationic ground state generated by the three-photon absorption at the photon energy below ∼38 000cm-1. The highly anisotropic kinetic energy components of Br+ and Br2 + start to appear at the photon energy above ∼38 000cm-1, where the Dn (n ≥ 1) - D0 transition is facilitated within the same ps laser pulse as the parent molecule is efficiently ionized by the two-photon absorption. The transition dipole moment of the D4 - D0 transition of the strongest oscillator strength has been theoretically predicted to be parallel to the C-Br bond or C=C bond axis for the trans or cis isomer, respectively. The fast anisotropic with the (β ∼ +2) component in the Br+ product distribution is thus likely from the trans isomer, whereas that of Br2 + (β ∼ -0.5) should be the consequence of the photodissociation of the cis isomer. The isomer-specific reactivity found here in the picosecond multiphoton excitation of 1,2-DBE provides a nice platform for the better understanding of the structure-reactivity relationship under the harsh condition of the strong or ultrashort optical field.

Synthesis and Third-Order Nonlinear Synergistic Effect of ZrO2/RGO Composites.

Tuning the third-order nonlinear properties of graphene by hybrid method is of great significance in nonlinear optics research. ZrO2/reduced graphene oxide (RGO) composites with different ZrO2 concentrations were prepared by a simple hydrothermal method. The morphology and structure show that ZrO2 nanoparticles were uniformly dispersed on the surface of graphene nanosheets. The nonlinear optical (NLO) characteristics of composites with different ZrO2 concentrations were studied by the Z-scan technique of 532 nm picosecond pulsed laser. The results showed that ZrO2/RGO composites had saturated absorption and positive nonlinear refraction characteristics. Meanwhile, the third-order nonlinear susceptibility of the ZrO2/RGO composite with a 4:1 mass ratio of ZrO2 to graphene oxide could reach 23.23 × 10−12 esu, which increased tenfold compared to RGO, and the nonlinear modulation depth reached 11.22%. Therefore, the NLO characteristics could be effectively adjusted by controlling the concentration of ZrO2, which lays a foundation for further research on the application of ZrO2/RGO composites in NLO devices.

Heat transfer behavior of as-processed and cleaned picosecond pulse laser processed copper

Improving the heat transfer characteristics of materials can be accomplished by increasing the surface area. This surface roughening can be accomplished efficiently and directly using pulsed-laser processing. Here, laser processing of copper using a picosecond laser pulse technique produced mound-like structures, with surface morphologies and subsurface microstructures dependent on laser pulse count, with higher pulse counts producing a unique layering of Cu and Cu2O in the mounds. Processing in ambient air resulted in the formation of surface oxides. The presence of oxides deleteriously influenced the heat transfer characteristics, with a lower heat transfer coefficient compared to unprocessed Cu. To remove the oxidation, the laser-processed copper was subjected to different acid treatments. It was found that treatment using citric acid resulted in efficient and effective removal of oxides both surface and subsurface. For low pulse count samples, an improvement in the heat transfer characteristics was observed after oxide removal, and outperformed the polished reference sample at heat fluxes above approximately 90 W/cm2. However, the removal of oxides using citric acid was found to actually decrease heat transfer characteristics for high pulse count samples with the onion-like subsurface layers, as oxide removal left behind voids in the structure which were detrimental to heat transfer. Thus, optimized laser surface processing which avoids subsurface onion-like layer formation enhances heat transfer performance.

Heat flow dynamics in planar multilayer antennas excited by far-infrared picosecond laser pulses

Thin metallic buffer layers are commonly used between semiconductor substrates and noble metal films or nanostructures. We propose an extended two-temperature heat transfer model that can be used to investigate the heat flow in multilayer antennas and includes metallic layers, a dielectric substrate, and their respective interfaces. It is demonstrated how the heat flow dynamics in a layered material, excited by short laser pulses, can be controlled by the thermal properties of the buffer layer. A large buffer layer electron-phonon coupling was found to induce a non-intuitive reverse internal heat flow in the layered material and transient energy retention on time scales of a few tens of picoseconds, while the buffer layer-substrate Kapitza conductance was found to be the limiting parameter for the heat dissipation on nanosecond time scales.

The efficacy and the adverse reactions of laser-assisted tattoo removal - a prospective split study using nanosecond and picosecond lasers.

Laser pulses with nanosecond duration (NSL) have been the golden standard to destroy the pigment particles in skin. It is still controversially discussed whether picosecond pulses (PSL) are superior for tattoo removal. To compare the efficacy and the adverse reactions of nanosecond and picosecond laser pulses in a comparative study. The prospective study included 23 subjects with 30 black or coloured tattoos, which were split into two halves treated with either a new PSL (532, 1064 nm) or standard NSL (694 nm). The lasers were applied at regular time intervals of 4 weeks for up to eight treatments. Tattoo clearance (primary endpoint), pain and adverse reactions (secondary endpoints) were appraised by physicians, blinded observers, and by subjects. The extent and duration of adverse reactions were additionally assessed by using a questionnaire and photo-documentation after each treatment session. The tattoo clearance appeared to be more effective for PSL compared to NSL but without statistical significance (P > 0.05). Pretreated tattoos responded better to laser treatments than previously untreated tattoos. Subjects felt significantly less pain with PSL than with NSL (P < 0.001). Transient adverse reactions were statistically less pronounced lasting shorter for PSL as for NSL, especially blistering, pruritus, and burning sensation. Hypopigmentation appeared after NSL treatments only, whereas hyperpigmentation was caused by both lasers. No scarring was detected with either laser. Both laser systems enable acceptable clearance of most tattoos in the present study. PSL cause less collateral skin damage as compared to NSL.

Extending Single Cell Bioprinting from Femtosecond to Picosecond Laser Pulse Durations.

Femtosecond laser pulses have been successfully used for film-free single-cell bioprinting, enabling precise and efficient selection and positioning of individual mammalian cells from a complex cell mixture (based on morphology or fluorescence) onto a 2D target substrate or a 3D pre-processed scaffold. In order to evaluate the effects of higher pulse durations on the bioprinting process, we investigated cavitation bubble and jet dynamics in the femto- and picosecond regime. By increasing the laser pulse duration from 600 fs to 14.1 ps, less energy is deposited in the hydrogel for the cavitation bubble expansion, resulting in less kinetic energy for the jet propagation with a slower jet velocity. Under appropriate conditions, single cells can be reliably transferred with a cell survival rate after transfer above 95% through the entire pulse duration range. More cost efficient and compact laser sources with pulse durations in the picosecond range could be used for film-free bioprinting and single-cell transfer.

Fabrication of four-level hierarchical topographies through the combination of LIPSS and direct laser interference pattering on near-beta titanium alloy

Complex repetitive periodic surface patterns were produced on a near-beta Ti-13Nb-13Zr alloy, using two-beam Direct Laser Interference Patterning (DLIP) employing a picosecond-pulsed laser source with wavelengths of 355 nm, 532 nm and 1064 nm. Different types of Laser-induced periodic surface structures (LIPSS) are produced, including low and high spatial frequency LIPSS, which are observed frequently on top of the line-like DLIP microstructures, as well as quasi-periodic microstructures with periods greater than the laser wavelength. The feature size of the fabricated LIPSS features could be tuned as function of the utilized laser process parameters.

Influence of Free-Electron Density Distribution on Mechanical Strength in Micro-Welding of Glass by Picosecond Pulsed Laser

Influence of Free-Electron Density Distribution on Mechanical Strength in Micro-Welding of Glass by Picosecond Pulsed Laser