Femtosecond UV Laser Research Articles

Formation of quasi unipolar pulses in nonequilibrium magnetized plasma channels

The possibility of controlling both the spectral and polarization properties of THz pulses propagating in strongly nonequilibrium extended magnetized plasma channels formed by intense UV femtosecond laser pulses in nitrogen (air) is analyzed. The formation of quasiunipolar pulses with a nonzero electric area and a specific state of polarization is discussed. The transformation of such pulses upon leaving the region of a static magnetic field is analyzed.

Laser engineered architectures for magnetic flux manipulation on superconducting Nb thin films

Custom shaped magnetic flux guiding channels have been fabricated on superconducting Nb thin films by laser nanopatterning of their surface. Preferential pathways are defined by suitable combination of imprinted anisotropic pinning domains through laser-induced periodic surface structures (LIPSS). Generated by the selective energy deposition of femtosecond UV laser pulses, quasi-parallel ripple structures are formed under optimized irradiation conditions. On average, each domain is formed by grooves with a lateral period of 260–270 nm and a depth about 80 nm. By combination of scanning and transmission electron microscopy, magneto-optical imaging, and conductive atomic force microscopy techniques, we conclude that the boundaries of the LIPSS-covered domains play a prominent role in the magnetic flux diversion process within the film. This is confirmed by dedicated modeling of the flux dynamics, combined with the inversion of the magneto-optical signal. The created metasurfaces enable control of the flux penetration process at the microscale.

Electronic relaxation in PLD TiO2 films processed with femtosecond UV-laser

Modifications of shallow electron trap states near conduction band was analyzed after surface structuring of TiO2 thin films prepared via pulsed laser deposition method. Laser-induced periodic surface structures (LIPSS) were produced on the large-area films after irradiation with femtosecond KrF laser (λ = 248 nm). While spatial period of LIPSS Λ ≈ 190 ± 10 nm was not affected by laser fluence, coherence was increased with an increase of laser dose. Time-resolved microwave conductivity measurements accompanied by theoretical modelling revealed energetic positions of shallow electron trap states, connected to LIPSS. A possibility to alter the trap states energy via surface structuring of the material was shown.

Dynamics of phase transformations in Si and Ge upon strong excitation with UV femtosecond laser pulses

Femtosecond laser processing of semiconductors has evolved into a mature, high-precision fabrication technique, enabling a wide range of applications. While initially most studies have employed pulses at near infrared wavelengths, the interest in using UV laser pulses is constantly increasing due to the different excitation conditions as a consequence of the much shorter optical penetration depth, leading to an improved resolution. In this context, fundamental studies on the temporal dynamics of phase transformations triggered by such pulses are necessary in order to comprehend and eventually control the complex phase transformation pathways. Here, we report a detailed time-resolved study on the phase transformation dynamics of crystalline silicon and germanium upon irradiation with single 400 nm, 100 fs laser pulses in the moderate and high excitation regime. To this end, we have employed fs-resolved optical microscopy with a probe wavelength of 800 nm to study the reflectivity evolution of the irradiated surface over a temporal window ranging from 100 fs up to 20 ns. At moderate excitation fluence, the data reveals the entire sequence of laser-induced processes, starting from the generation of a free-electron plasma, non-thermal melting, ablation onset and expansion of a semi-transparent ablation layer with sharp interfaces. At excitation with peak fluences more than 30 times the ablation threshold, an anomalous transient high-reflectivity state is observed, which might be indicative of a recoil pressure-induced liquid–liquid phase transition. Moreover, 70 nm-thick amorphous surface layers are formed in both materials after irradiation at moderate fluences. Overall, our results provide relevant information on both, transformation dynamics and final state of both materials for fs-pulse excitation in the near-UV wavelength range.

Excitation and ionization of OCS molecules in strong UV and NIR laser fields

Abstract Rydberg state excitation (RSE) is a highly non-linear physical phenomenon that is induced by the ionization of atoms or molecules in strong femtosecond laser fields. Here we observe that both parent and fragments (S, C, OC) of the tri-atomic molecule carbonyl sulfide (OCS) can survive strong 800 nm or 400 nm laser fields in high Rydberg states. The dependence of parent and fragment RSE yields on laser intensity and ellipticity is investigated in both laser fields, and the results are compared with those for strong-field ionization. Distinctly different tendencies for laser intensity and ellipticity are observed for fragment RSE compared with the corresponding ions. The mechanisms of RSE and strong-field ionization of OCS molecules in different laser fields are discussed based on the experimental results. Our study sheds some light on the strong-field excitation and ionization of molecules irradiated by femtosecond NIR and UV laser fields.

Tunable UV Laser for External Seeding of the High Repetition Rate Soft X-ray Free Electron Laser FLASH

We are currently developing a tunable high-repetition rate femtosecond UV laser system for external seeding superconducting soft X-ray Free Electron Laser FLASH. Initial results show excellent power and wavelength stability of the Optical Parametric Chirped Pulse Amplification system.

Laser nanostructured metasurfaces in Nb superconducting thin films

Superconducting Nb thin films have been nanostructured by means of a femtosecond UV laser. Laser induced periodic surface structures (LIPSS) with lateral modulation of ≈250nm and depth smaller than amplitude (≲20nm from crest to trough) are obtained for optimized laser scanning conditions over the film surface, i.e. power, frequency, scanning speed and polarization. This provides a fast and scalable procedure of surface control at the nanoscale. In thin films, control over the kinetics of the LIPSS formation process has been crucial. Untreated and laser-patterned samples have been characterized by electron and atomic force microscopy as well as by local and global magnetometry. The superconducting properties reveal anisotropic behavior in accordance with the observed topography. The imprinted LIPSS define channels for anisotropic current flow and flux penetration. At low temperatures, magnetic flux avalanches are promoted by the increased critical current density, though flux tends to be channeled along the LIPSS. In general, directional flux penetration is observed, being a useful feature in fluxonic devices. Scalability allows us to pattern areas of the order of cm2/min.

257 nm Deep UV Femtosecond Laser Ablation with Minimized Crack and Chipping on Display Ultra-Thin Glass

257 nm Deep UV Femtosecond Laser Ablation with Minimized Crack and Chipping on Display Ultra-Thin Glass

Mass-selected ion-molecule cluster beam apparatus for ultrafast photofragmentation studies.

We describe an apparatus for investigating the excited-state dissociation dynamics of mass-selected ion-molecule clusters by mass-resolving and detecting photofragment-ions and neutrals, in coincidence, using an ultrafast laser operating at high repetition rates. The apparatus comprises a source that generates ion-molecule clusters, a time-of-flight spectrometer, and a mass filter that selects the desired anions, and a linear-plus-quadratic reflectron mass spectrometer that discriminates the fragment anions after the femtosecond laser excites the clusters. The fragment neutrals and anions are then captured by two channeltron detectors. The apparatus performance is tested by measuring the photofragments: I-, CF3I-, and neutrals from photoexcitation of the ion-molecule cluster CF3I·I- using femtosecond UV laser pulses with a wavelength of 266nm. The experimental results are compared with our ground state and excited state electronic structure calculations as well as the existing results and calculations, with particular attention to the generation mechanism of the anion fragments and dissociation channels of the ion-molecule cluster CF3I·I- in the charge-transfer excited state.

SWIR anti-reflective nanostructures on nonlinear crystals by direct UV femtosecond laser printing

Nonlinear infrared (IR) crystals for radiation conversion are of paramount importance for realization of advanced laser spectrometers for medical diagnostics, environmental monitoring, and advanced sensing. However, performance of such crystals suffers from substantial surface reflectivity coming from rather high (over 2.5) refractive index of the key nonlinear materials used. Here, based on the example of promising BaGa4Se7 nonlinear crystal, we attested direct surface patterning with ultraviolet (257 nm) femtosecond laser pulses used to engrave anti-reflective microstructures (ARMs) directly on both output sides of the crystal. Imprinted surface nanotrenches arranged into a fish-net morphology with a periodicity down to 500 nm was found to increase transmittance of the crystals from 65% to 84% within a practically relevant shortwave IR spectral range. Formation of the ARMs with an optimized geometry is expected to weakly reduce the laser damage threshold of a pristine crystal material as it was also evidenced from supporting full-wave simulations and tests.

Control of Spectral and Polarization Properties of Quasiunipolar Terahertz Pulses in Strongly Nonequilibrium Magnetized Plasma Channels

The possibility to control both spectral and polarization properties of seed THz pulses in strongly nonequilibrium elongated magnetized plasma channels formed via intense UV femtosecond laser pulses in nitrogen (air) is analyzed. The physical mechanism of THz pulse control is based on cyclotron resonance, which can strongly reconstruct electrodynamical plasma features and, in particular, its ability to amplify the radiation of different spectral bands and polarization states. In particular, the formation of quasiunipolar pulses with a non-zero electric area and a specific polarization state is discussed. This study is based on the self-consistent solution of the kinetic Boltzmann equation for the electron velocity distribution function (EVDF) in the plasma channel and the second-order wave equation for THz pulse propagation.

UV femtosecond laser cleaning of encrusted historical stained-glasses

UV femtosecond laser cleaning of encrusted historical stained-glasses

Femtosecond IR and UV laser induced periodic structures on steel and copper surfaces

We analyze in detail laser induced periodic surface structures (LIPSS) on steel and copper surfaces. The chosen metals exhibit extreme differences between their thermal conductivity values, which determines the rate of extinguishing of the laser-induced temperature modulation on the surface. Two wavelengths of femtosecond laser radiation sources in IR and UV spectral range were used to inscribe low spatial frequency LIPSS. Regular LIPSS were successfully formed on steel samples for both wavelengths and it was observed that their regularity decreased with shorter wavelength used to induce the structures. For copper samples, LIPSS of considerably lower regularity were formed and even absent for one of the used scanning method at UV wavelength. Experimental results were analyzed in view of the numerical solution of the 2D heat diffusion equation for induced temperature on the surface. It was found that the temperature modulation decreased considerably faster (lasting tens of picoseconds) in copper, and (additionally) the decrease of the modulation is faster at the higher modulation frequency.

Nondestructive inspection of surface nanostructuring using label-free optical super-resolution imaging

Ultrafast laser processing can induce surface nanostructurating (SNS) in most materials with dimensions close to the irradiation laser wavelength. In-situ SNS characterization could be key for laser parameter’s fine-tuning, essential for the generation of complex and/or hybrid nanostructures. Laser Induced Periodic Surface Structures (LIPSS) created in the ultra-violet (UV) range generate the most fascinating effects. They are however highly challenging to characterize in a non-destructive manner since their dimensions can be as small as 100 nm. Conventional optical imaging methods are indeed limited by diffraction to a resolution of approx 150 nm. Although optical super-resolution techniques can go beyond the diffraction limit, which in theory allows the visualization of LIPSS, most super-resolution methods require the presence of small probes (such as fluorophores) which modifies the sample and is usually incompatible with a direct surface inspection. In this paper, we demonstrate that a modified label-free Confocal Reflectance Microscope (CRM) in a photon reassignment regime (also called re-scan microscopy) can detect sub-diffraction limit LIPSS. SNS generated on a titanium sample irradiated with a lambda =257 nm femtosecond UV-laser were characterized with nanostructuring period ranging from 105 to 172 nm. Our label-free, non-destructive optical surface inspection was done at 180 upmum^2/s, and the results are compared with commercial SEM showing the metrological efficiency of our approach.

Femtosecond laser-assisted fabrication of piezoelectrically actuated crystalline quartz-based MEMS resonators

A novel technology for the precise fabrication of quartz resonators for MEMS applications is introduced. This approach is based on the laser-induced chemical etching of quartz. The main processing steps include femtosecond UV laser treatment of a Cr-Au-coated Z-cut alpha quartz wafer, followed by wet etching. The laser-patterned Cr-Au coating serves as an etch mask and is used to form electrodes for piezoelectric actuation. This fabrication approach does not alter the quartz’s crystalline structure or its piezo-electric properties. The formation of defects, which is common in laser micromachined quartz, is prevented by optimized process parameters and by controlling the temporal behavior of the laser-matter interactions. The process does not involve any lithography and allows for high geometric design flexibility. Several configurations of piezoelectrically actuated beam-type resonators were fabricated using relatively mild wet etching conditions, and their functionality was experimentally demonstrated. The devices are distinguished from prior efforts by the reduced surface roughness and improved wall profiles of the fabricated quartz structures.

Ultrashort pulsed Femtosecond UV laser for selective cleaning of significant Cretaceous flints

This work reports on studies aimed to evaluate the utilization of ultrashort 238 fs (fs) pulsed UV laser emission at 343 nm for eliminating colored crusts and surface deposits on significant Cretaceous flint surfaces, in an attempt to safeguard its aesthetic appearance and archaeological value. The results indicate that fs UV lasers may be an ideal, non-contact tool for selective surface cleaning of sensitive archaeological artefacts, since they enable contaminant desorption while avoiding photothermal damage.

On the Possibility of Intense Unipolar THz Pulses Formation in Nonhomogeneous Nonequilibrium Nitrogen Plasma Channels

We developed a 3D, fully self-consistent model for analysis of the ultrashort THz unipolar pulse formation accompanied by its amplification in a nonequilibrium plasma channel induced in nitrogen by a femtosecond UV laser pulse. The model is based on a self-consistent numerical solution of the second-order wave equation in cylindrical geometry and the kinetic Boltzmann equation for the electron velocity distribution function (EVDF) at different points of the spatially inhomogeneous nonequilibrium plasma channel. Rapid relaxation of the electron velocity distribution function in the plasma channel results in the amplification of the leading front of the THz pulse only, while its trailing edge is not amplified or even absorbed, which gives rise to the possibility of the formation of pulses with a high degree of unipolarity. The evolution of the unipolar pulse after its transfer from the channel to open free space is analyzed in detail.

Development of an integrated microperfusion-EEG electrode for unbiased multimodal sampling of brain interstitial fluid and concurrent neural activity

Objective. To modify off-the-shelf components to build a device for collecting electroencephalography (EEG) from macroelectrodes surrounded by large fluid access ports sampled by an integrated microperfusion system in order to establish a method for sampling brain interstitial fluid (ISF) at the site of stimulation or seizure activity with no bias for molecular size. Approach. Twenty-four 560 µm diameter holes were ablated through the sheath surrounding one platinum–iridium macroelectrode of a standard Spencer depth electrode using a femtosecond UV laser. A syringe pump was converted to push–pull configuration and connected to the fluidics catheter of a commercially available microdialysis system. The fluidics were inserted into the lumen of the modified Spencer electrode with the microdialysis membrane removed, converting the system to open flow microperfusion. Electrical performance and analyte recovery were measured and parameters were systematically altered to improve performance. An optimized device was tested in the pig brain and unbiased quantitative mass spectrometry was used to characterize the perfusate collected from the peri-electrode brain in response to stimulation. Main results. Optimized parameters resulted in >70% recovery of 70 kDa dextran from a tissue analog. The optimized device was implanted in the cortex of a pig and perfusate was collected during four 60 min epochs. Following a baseline epoch, the macroelectrode surrounded by microperfusion ports was stimulated at 2 Hz (0.7 mA, 200 µs pulse width). Following a post-stimulation epoch, the cortex near the electrode was stimulated with benzylpenicillin to induce epileptiform activity. Proteomic analysis of the perfusates revealed a unique inflammatory signature induced by electrical stimulation. This signature was not detected in bulk tissue ISF. Significance. A modified dual-sensing electrode that permits coincident detection of EEG and ISF at the site of epileptiform neural activity may reveal novel pathogenic mechanisms and therapeutic targets that are otherwise undetectable at the bulk tissue level.

Direct Laser Ablation of Glasses with Low Surface Roughness Using Femtosecond UV Laser Pulses

Direct Laser Ablation of Glasses with Low Surface Roughness Using Femtosecond UV Laser Pulses

Femtosecond UV Laser Ablation Characteristics of Polymers Used as the Matrix of Astronautic Composite Material.

Ultrafast laser processing has recently emerged as a new tool for processing fiber-reinforced polymer (FRP) composites. In the astronautic industry, the modified epoxy resin (named 4211) and the modified cyanate ester resin (known as BS-4) are two of the most widely used polymers for polymer-based composites. To study the removal mechanism and ablation process of different material components during the ultrafast laser processing of FRPs, we isolated the role of the two important polymers from their composites by studying their femtosecond UV laser (260 fs, 343 nm) ablation characteristics for controllable machining and understanding the related mechanisms. Intrinsic properties for the materials’ transmission spectrum, the absorption coefficient and the optical bandgap (Eg), were measured, derived, and compared. Key parameters for controllable laser processing, including the ablation threshold (Fth), energy penetration depth (δeff), and absorbed energy density (Eabs) at the ablation threshold, as well as their respective “incubation” effect under multiple pulse excitations, were deduced analytically. The ablation thresholds for the two resins, derived from both the diameter-regression and depth-regression techniques, were compared between resins and between techniques. An optical bandgap of 3.1 eV and 2.8 eV for the 4211 and BS-4 resins, respectively, were obtained. A detectable but insignificant-to-ablation difference in intrinsic properties and ablation characteristics between the two resins was found. A systematic discrepancy, by a factor of 30~50%, between the two techniques for deriving ablation thresholds was shown and discussed. For the 4211 resin ablated by a single UV laser pulse, a Fth of 0.42 J/cm2, a δeff of 219 nm, and an Eabs of 18.4 kJ/cm3 was suggested, and they are 0.45 J/cm2, 183 nm, and 23.2 kJ/cm3, respectively, for the BS-4 resin. The study may shed light on the materials’ UV laser processing, further the theoretical modeling of ultrafast laser ablation, and provide a reference for the femtosecond UV laser processing characteristics of FRPs for the future.