Near-infrared Femtosecond Laser Pulses Research Articles

Super-regular femtosecond laser nanolithography based on dual-interface plasmons coupling

Abstract Advances in femtosecond laser-material interaction facilitate the extension of maskless optical processing to the high efficiency and deep-subwavelength scale. Here, a hybrid plasmon lithography technique has been demonstrated by irradiating near-infrared femtosecond laser pulses onto the Si material coated with thin Cr films in a vacuum chamber, and superior nanograting structures are found to deeply penetrate through the thin Cr film into the underlying Si substrate. In stark contrast to the common ripple structures formed on the Si surface, the Cr-layer mediated Si nanograting structures not only exhibit the spatially super-regular arrangements with a deep-subwavelength period of 355 nm but also present the nonsinusoidal sharp-edged groove geometry with a large depth-to-width aspect ratio of 2.1. Theoretical analyses and calculations reveal that the anomalous structure characterizations are physically ascribed to the excitation of dual-interface coupled plasmons in the thin metal layer, which possess the squeezed spatial wavelength and the periodic columnar intensity distributions. Moreover, the further deepening of periodic nanostructures into the Si substrate is also elucidated by the simulation of electric field enhancements at the bottom of shallow grooves under irradiation of subsequent laser pulses. In combination with a wet etching process, the Si nanograting structures can be modified into the smooth and narrow-mouthed V-profiles, whose optical measurements show a near omnidirectional antireflection especially in the visible range of 565–750 nm, which is expected for the design of advanced photonic devices.

Single Particle Color Switching by Laser-Induced Deformation of Liquid Metal-derived Microcapsules

Active controlling of optical properties of metallic particles holds great promise for nonlinear nanophotonics and compact optoelectronic devices. Except for the electronic and chemical tuning of their properties, active control through fast and reversible shape modulation remains a significant challenge. Here, we report on the concept for changing the color and brightness of single particles by reversible/irreversible tuning of their shapes. As a family of plasmonic materials with low melting points and high flexibility, we synthesized liquid metal microparticles with different interior (dense/hollow) and morphology from Ga and its alloys (GaNi, GaCu). Utilizing near-infrared femtosecond laser pulses, we achieve two regimes for reversible/irreversible optical tuning due to consequent weak/strong perturbation of the microcapsules (MC) shapes. The chemical composition and MCs morphology significantly affect the tuning of color and brightness, as well as the rigidity of the MCs to extreme laser conditions.

Controlled pDNA Release in Gemini Cationic Lipoplexes by Femtosecond Laser Irradiation of Gold Nanostars.

The design of nanovectors able to overcome biological barriers is one of the main challenges in biomedicine. Gemini cationic lipids are considered potential candidates for gene therapy due to their high biocompatibility and capacity to condense nucleic acids safely in the form of lipoplexes. However, this approach presents difficulties regarding genetic unpacking and, therefore, control over this process becomes crucial to ensure successful transfection. In this work, gemini cationic lipoplexes were prepared in the presence of plasmonic gold nanostars (AuNSs) to afford a nanovector that efficiently releases plasmid DNA (pDNA) upon irradiation with near-infrared femtosecond laser pulses. A critical AuNSs concentration of 50 pM and optimized laser power density of 400 mW led to successful pDNA release, whose efficiency could be further improved by increasing the irradiation time. Agarose gel electrophoresis was used to confirm pDNA release. UV-Vis-NIR spectroscopy and transmission electron microscopy studies were performed to monitor changes in the morphology of the AuNSs and lipoplexes after irradiation. From a physicochemical point of view, this study demonstrates that the use of AuNSs combined with gemini cationic lipoplexes allows control over pDNA release under ultrafast laser irradiation.

Imaging Plasmons with Sub-2 nm Spatial Resolution via Tip-Enhanced Four-Wave Mixing.

Four-wave mixing at plasmonic tip-sample nanojunctions may be used to visualize plasmonic fields with sub-2 nm spatial resolution under ambient laboratory conditions. We illustrate the latter using a gold-coated atomic force microscopy probe irradiated with a pair of near-infrared femtosecond laser pulses and used to image plasmonic gold nanoplates and silver nanocubes. Through diagnostic polarization-dependent tip-only measurements, we illustrate that the four-wave mixing signal is localized to the tip apex. The apex-bound signal is further enhanced when the tip is located at specific locations near plasmonic nanoparticles. Overall, this work paves the way for visualizing chemical transformations as well as coherent electronic and vibrational dynamics with joint femtosecond temporal and few-nanometer spatial resolution under ambient conditions.

Intensity modulation of the population of atomic excited states by using near-infrared femtosecond laser pulses

We have theoretically investigated the modulation structures of the excited-state population of an argon atom exposed to a linearly polarized 800-nm laser pulse in the tunneling region. The numerical result identifies a nearly opposite phase modulation structure between the low-lying n = 4–7 states and over all higher $$n>7$$ states. This phase-dependent alternating population phenomenon is found to be related to the periodic channel-closing effect that results from ac-Stark-shifted multiphoton resonances. Moreover, the angular momentum states attributed to different parities have a similar alternating effect, which confirms a multiphoton process in the tunneling region. We introduce a mechanism of two-photon $$\Lambda $$ -type Raman transitions involving Rydberg and continuum states with a simultaneous redistribution of states to account successfully for this observed out-of-phase angular momentum state population.

Strong-field induced fragmentation and isomerization of toluene probed by ultrafast femtosecond electron diffraction and mass spectrometry.

We investigate the fragmentation and isomerization of toluene molecules induced by strong-field ionization with a femtosecond near-infrared laser pulse. Momentum-resolved coincidence time-of-flight ion mass spectrometry is used to determine the relative yield of different ionic products and fragmentation channels as a function of laser intensity. Ultrafast electron diffraction is used to capture the structure of the ions formed on a picosecond time scale by comparing the diffraction signal with theoretical predictions. Through the combination of the two measurements and theory, we are able to determine the main fragmentation channels and to distinguish between ions with identical mass but different structures. In addition, our diffraction measurements show that the independent atom model, which is widely used to analyze electron diffraction patterns, is not a good approximation for diffraction from ions. We show that the diffraction data is in very good agreement with ab initio scattering calculations.

Selective induction of targeted cell death and elimination by near-infrared femtosecond laser ablation

The techniques for inducing the death of specific cells in tissue has attracted attention as new methodologies for studying cell function and tissue regeneration. In this study, we show that a sequential process of targeted cell death and removal can be triggered by short-term exposure of near-infrared femtosecond laser pulses. Kinetic analysis of the intracellular accumulation of trypan blue and the assay of caspase activity revealed that femtosecond laser pulses induced immediate disturbance of plasma membrane integrity followed by apoptosis-like cell death. Yet, adjacent cells showed no sign of membrane damage and no increased caspase activity. The laser-exposed cells eventually detached from the substrate after a delay of >54 min while adjacent cells remained intact. On the base of in vitro experiments, we applied the same approach to ablate targeted single cardiac cells of a live zebrafish heart. The ability of inducing targeted cell death with femtosecond laser pulses should find broad applications that benefit from precise cellular manipulation at the level of single cells in vivo and in vitro.

Direct Production of CH(A2Δ) Radical from Intense Femtosecond Near-IR Laser Pulses.

CH(A2Δ) radical formation was observed in bromoform and methanol vapor in argon plasma with near-infrared femtosecond laser pulses (43 fs, 1030 nm, 100 kHz, 250 μJ/pulse). The beam was focused with an achromatic lens, creating very high intensity in the plasma that caused Coulomb explosion (calculated intensity was ∼1.1 × 1016 W/cm2 in the focal point). The emitted fluorescence light was measured with high spectral (1-10 cm-1) and temporal resolution (5 ns) with an FT-Vis spectrometer. The step-scan technique allowed the reconstruction of the time-resolved fluorescence spectra from CH(A-X) emission. The emission from atomic lines such as H, Br, C, and O was observed and also from C+ cations and CH and C2 radicals. This indicates that in a significant portion of these organic molecules, all chemical bonds were cleaved in the Coulomb explosion. For both organics, the peak maximum of the CH(A) emission occurred at about 10 ns after excitation by the femtosecond pulse. After the maximum, a rapid emission decay was observed in the case of bromoform (monoexponential decay, t = 10 ns). The fluorescence decay was biexponential when methanol was used as the source for CH(A) generation. It can be assumed that CH(A) generation involved a fast and a slower path with some secondary reactions via the stepwise loss of hydrogen atoms from the CH3 group. The time constants were t1 = 7.8-8.3 ns and t2 = 78-82 ns for the fast and slow components, respectively, and very similar values were obtained at 10 and 25 mbar total pressures. However, in the case of bromoform, the C-Br bonds are significantly weaker; therefore, these atoms can be removed even in a single step via multiphoton absorption. The rotational temperature of CH(A) radicals generated from methanol decreased rapidly in the 30-55 ns time period from 2770 ± 80 to 1530 ± 50 K. The vibrational temperature increased from 3530 ± 450 to 9810 ± 760 K in the 30-80 ns time period and then started to decrease (the average temperatures were Trot = 910 ± 20 K and Tvib = 7490 ± 340 K at 100 ns). This initial increase of Tvib is thought to be the result of electron collision with the CH radicals. The high temperatures of the fragment may indicate the roaming reaction associated with the Coulomb explosion of the parent molecule. We demonstrated that CH(A) radicals can be produced from both organic compounds, and the step-scan technique is ideal for the characterization of their time-resolved spectra using the 100 kHz high repetition rate near-infrared femtosecond laser pulses. The FT/UV-vis step-scan technique can detect neutral species directly with high spectral and time resolution, thus it is a complementary technique to the experiments utilizing ion detection schemes, such as velocity map imaging.

Large ultrafast nonlinear optical response and excellent optical limiting behaviour in pyrene-conjugated zinc(II) phthalocyanines at a near-infrared wavelength

The design and synthesis with full structural utilization of a new type of nonlinear photonic material, Zinc(II) phthalocyanine macrocycle (Pc-pyrene), bearing binary oriented (up/down) pyrene units is reported. The novel pyrene-based macrocycle has been successfully obtained in high yield upon coupling multiple azido pyrene derivatives on a previously fabri-cated alkynyl-substituted macrocyclic platform (Pc1) via multiple tandem Cu(I)-catalysed azide-alkyne cycloaddition (CuAAC) reactions. The ultrafast nonlinear optical characteristics of both Pc1 and Pc-pyrene have been investigated in solution using a near-infrared femtosecond pulsed laser. The femtosecond Z-scan results reveal a low optical limiting threshold and a good set of figures of merit that make these macrocycles feasible for optical limiting as well as switching applications. In a major breakthrough, ultrafast laser-induced deformations in Pc-pyrene-embedded PMMA films results in a very large enhancement of the three-photon absorption coefficient at 800 nm by up to ~105 times that of Pc solutions. Furthermore, the deformed Pc-pyrene PMMA films having the advantage of flexibility and possess an excellent optical limiting threshold (0.0018 J/cm2), much lower than various previously reported values, which enables them to be one of the best optical limiters.

Blueshifts of high-order harmonic generation in crystalline silicon subjected to intense femtosecond near-infrared laser pulse

We present the generation of high order harmonics in crystalline silicon subjected to intense near-infrared 30fs laser pulses. The harmonic spectrum extends from the near infrared to the extreme ultraviolet spectral region. Depending on the pulsed laser intensity, we distinguish two regimes of harmonic generation: (i) perturbative regime: electron-hole pairs born during each half-cycle of the laser pulse via multiphoton and tunnel transitions are accelerated in the laser electric field and gain kinetic energy; the electron-hole pairs then recombine in the ground state by emitting a single high-energy photon. The resultant high harmonic spectrum consists of sharp peaks at odd harmonic orders. (ii) non-perturbative regime: the intensity of the harmonics increases, their spectral width broadens and the position of harmonics shifts to shorter wavelengths. The blueshifts of high harmonics in silicon are independent on the harmonic order which may be helpful in the design of continuously tunable XUV sources.

Extremely robust femtosecond written fiber Bragg gratings for an ytterbium-doped fiber oscillator with 5 kW output power.

We present highly robust fiber Bragg gratings (FBGs) in passive large-mode-area fibers for kilowatt fiber laser systems. The gratings were inscribed directly through the fiber coating using near-infrared femtosecond laser pulses and then implemented in an all-fiber ytterbium-doped single-mode oscillator setup reaching up to 5 kW signal output power. The untreated cooled FBGs showed thermal coefficients as low as ${1}\;{\rm K}\;{{\rm kW}^{ - 1}}$1KkW-1, proving excellent qualification for the implementation into robust high-power fiber laser setups.

Springtail-Inspired Triangular Laser-Induced Surface Textures on Metals Using MHz Ultrashort Pulses

Considering the attractive surface functionalities of springtails (Collembola), an attempt at mimicking their cuticular topography on metals is proposed. An efficient single-step manufacturing process has been considered, involving laser-induced periodic surface structures (LIPSS) generated by near-infrared femtosecond laser pulses. By investigating the influence of number of pulses and pulse fluence, extraordinarily uniform triangular structures were fabricated on stainless steel and titanium alloy surfaces, resembling the primary comb-like surface structure of springtails. The laser-textured metallic surfaces exhibited hydrophobic properties and light scattering effects that were considered in this research as a potential in-line process monitoring solution. The possibilities to increase the processing throughput by employing high repetition rates in the MHz-range are also investigated.

Single nanowire defined emission properties of ZnO nanowire arrays

We report on stimulated emission from vertically aligned, vapor transport grown, ZnO nanowire arrays, and pumped by three-photon absorption in intense near-infrared femtosecond laser pulses. In respect to single nanowires, arrays have the advantage of a higher light absorption and emission rate. The intensity and bandwidth of the emitted ultraviolet radiation as a function of the pump intensity is compared for nanowire arrays with different wire lengths, diameters, and spacing. The measured lasing thresholds for all arrays can be well described by the geometry of individual nanowire lasers, showing that coupling effects between the individual emitters in the arrays are negligible, even for the smallest 100 nm diameter wires with an average distance of 200 nm.

Characterization of 20-fs VUV pulses by plasma-mirror frequency-resolved optical gating.

We demonstrate that vacuum ultraviolet (VUV) pulses (λ∼160 nm) with 20-fs duration can be fully characterized by plasma-mirror frequency-resolved optical gating. Plasma is formed as an ultrafast optical switch on a fused silica surface by irradiation of an intense near-infrared (NIR) femtosecond laser pulse. We measure the spectra of the VUV pulses reflected off plasma as a function of the delay with respect to the NIR pulse and reconstruct it to simultaneously extract a nearly Fourier-transform limited VUV pulse shape and a time-dependent reflectivity of plasma using the least-squares generalized projections algorithm. We show that there is almost no spatial dependence of the VUV pulse shape, whereas the plasma mirror reflectivity strongly depends on the spatial region.

Spectral broadening effects on metal photoemission by femtosecond laser pulses

Emitted electronic densities from an Al surface have been computed for various temporal shapes of near-infrared femtosecond laser pulses (less than or equal to 20 fs) over a wide range of low and intermediate laser intensities. At low intensities photoemission originates from the high-energy tail of the pulse spectrum leading to linear absorption in all cases, even if several photons are necessary to eject an electron at higher intensities. We propose an analytical method to calculate the transition intensity in which the behavior of emitted densities evolves from the single-photon ${I}_{m}$ slope to the multiphoton ${I}_{m}^{n}$ one (with ${I}_{m}$ the maximum intensity and $n$ the number of absorbed photons). We show that the use of a flat-top profile or at least of a super-Gaussian envelope enables strong electron emission at very low intensities for realistic pulse durations and should allow experimental observation of the transition between these two regimes of photoemission.

Open-path gas detection using terahertz time-domain spectroscopy

We report on the development of an experimental setup for remote sensing of gas atmosphere in open air conditions using broadband terahertz pulses. Terahertz time-domain spectroscopy technique is used to solve the problems of gas detection and recognition in the presence of water vapor absorption lines. Acetonitrile vapors were detected and gas concentration was successfully estimated. Organic crystal with a 2% conversion efficiency pumped with near-infrared femtosecond laser pulses was used to generate single-cycle terahertz pulses with energies up to 10 µJ. Application of a novel high-power source of broadband terahertz radiation (0.2–3 THz) opens up new perspectives for further increasing the open space distances in remote sensing problem.

Spinal cord injury in zebrafish induced by near-infrared femtosecond laser pulses

BackgroundThe spinal cord is composed of a large number of cells that interact to allow the organism to function. To perform detail studies of cellular processes involved in spinal cord injury (SCI), one must use repeatable and specific methods to target and injure restricted areas of the spinal cord. New methodWe propose a robust method to induce SCI in zebrafish by laser light. With a 2-photon microscope equipped with a femtosecond near-infrared pump laser, we explored the effects of laser beam exposure time, area, and intensity to induce precise and repeatable SCI with minimized collateral damage to neighboring cells. ResultsThrough behavioral studies in zebrafish larvae, we assessed the functional outcome of intensive laser light directed at the spinal cord. Our experiments revealed that a laser pulse with wavelength 800 nm, duration 2.6 ms, and light intensity 390 mW was sufficient to induce controlled cell death in a single cell or a spinal cord segment. Collateral damage was observed if cells were exposed to laser pulses exceeding 470 mW. With these settings, we could induce precise and repeatable SCI in zebrafish larvae, resulting in loss of motor and sensory function. Comparison with existing method(s)Our method offers a simple and more controlled setting to induce SCI in zebrafish. We describe how the near-infrared femtosecond laser should be adjusted for achieving optimal results with minimal collateral damage. ConclusionsWe present a precise and robust method for inducing SCI in zebrafish with single-cell resolution using femtosecond near-infrared laser pulses.

High spatial coherence in multiphoton-photoemitted electron beams

Nanometer-sharp metallic tips are known to be excellent electron emitters. They are used in highest-resolution electron microscopes in cold field emission mode to generate the most coherent electron beam in continuous-wave operation. For time-resolved operation, sharp metal needle tips have recently been triggered with femtosecond laser pulses. We show here that electrons emitted with near-infrared femtosecond laser pulses at laser oscillator repetition rates show the same spatial coherence properties as electrons in cold field emission mode in cw operation. From electron interference fringes, obtained with the help of a carbon nanotube biprism beam splitter, we deduce a virtual source size of less than (0.65 ± 0.06) nm for both operation modes, a factor of ten smaller than the geometrical source size. These results bear promise for ultrafast electron diffraction, ultrafast electron microscopy, and other techniques relying on highly coherent and ultrafast electron beams.

Strong-field optoelectronics in solids

Perturbative optical nonlinearities induced by static electric fields1 have proven useful in visualizing dynamical function in systems including operating circuits2,3, electric and magnetic domain walls4, and biological matter5, and in controlling light for applications in silicon photonics6. Here, we extend field-induced second-harmonic generation to the non-perturbative regime. We demonstrate that static or transient fields up to terahertz (THz) frequencies applied to silicon and ZnO crystals generate even-order high harmonics. Images of the even harmonics confirm that static fields delivered with conventional electronics control the spatial properties of the high-harmonic emission. Extending our methodology to higher-harmonic photon energies7,8 paves the way for realizing active optics in the extreme ultraviolet and will allow imaging of operating electronic circuits9, of Si-photonic devices10 and of other functional materials11,12, with higher spatio-temporal resolution than perturbative methods. For THz spectroscopy, our method has the bandwidth to allow measurement of attosecond transients imprinted on THz waveforms. Near-infrared femtosecond laser pulses are sent to a Si or ZnO crystal to generate high-harmonic waves via static or transient field-induced optical nonlinearities. The beam profile of the high-harmonic emission is controlled by electronic methods.

Femtosecond laser-induced dissociation (fs-LID) as an activation method in mass spectrometry

We report the photolysis of biomolecules in a Fourier-transform ion cyclotron resonance mass spectrometer by intense near-infrared femtosecond laser pulses. Photo-fragmentation was accompanied by photo-ionization and created a large number and variety of charged fragments, which could be identified with high confidence due to the outstanding resolving power and mass accuracy of the mass spectrometer. Fragmentation patterns were sufficient for peptide sequence analysis. Fragments formed by non-ergodic cleavage retained labile post-translational modifications.