Picosecond Laser Pulses Research Articles

Composition-dependent optical limiting behavior of all-inorganic halide perovskite quantum dots

Abstract Nonlinear optical nanostructured materials are gaining interest as optical limiters for various applications. Here, all-inorganic halide perovskite quantum dots (PQDs, SiO2@CsPbX3 (X = Cl, Br, I) QDs) with tunable composition were prepared via ligand assisted reprecipitation, using 3-aminopropyltriethoxysilane as a ligand. The formation of the SiO2@CsPbX3 QDs was confirmed by transmission electron microscopy, X-ray diffraction, infrared spectroscopy, ultraviolet–visible absorption spectroscopy and photoluminescence spectroscopy. The composition-dependent optical limiting (OL) behavior of the SiO2@CsPbX3 QDs was observed using nanosecond and picosecond laser pulses at a wavelength of 532 nm. The SiO2@CsPbBr3 QDs exhibit favorable OL properties. Their OL threshold for the nanosecond laser pulse is 1.68 J/cm2, which is comparable to that of carbon nanotube suspension (a benchmark optical limiter). The presence of chloride or iodide in the SiO2@CsPbX3 QDs leads to weaker OL performance. The OL behavior of the SiO2@CsPbBr3 QDs is attributed to the narrow band gap, large average size, abundant charge carriers under laser excitation and high stability. The OL mechanism was investigated using the open-aperture and closed-aperture techniques. The OL behavior of the SiO2@CsPbX3 QDs is largely attributed to the combined mechanisms of nonlinear absorption and nonlinear refraction. These results reveal the physical processes of the composition-dependent OL properties of the CsPbX3 QDs. These findings provide a means to tailor the OL response of PQDs by controlling the halide ion ratio.

Hot electron energy relaxation time in vanadium nitride superconducting film structures under THz and IR radiation

The paper presents the experimental results of studying the dynamics of electron energy relaxation in structures made of thin (d ≈ 6 nm) disordered superconducting vanadium nitride (VN) films converted to a resistive state by high-frequency radiation and transport current. Under conditions of quasi-equilibrium superconductivity and temperature range close to critical (~ Tc), a direct measurement of the energy relaxation time of electrons by the beats method arising from two monochromatic sources with close frequencies radiation in sub-THz region (ω ≈ 0.140 THz) and sources in the IR region (ω ≈ 193 THz) was conducted. The measured time of energy relaxation of electrons in the studied VN structures upon heating of THz and IR radiation completely coincided and amounted to (2.6–2.7) ns. The studied response of VN structures to IR (ω ≈ 193 THz) picosecond laser pulses also allowed us to estimate the energy relaxation time in VN structures, which was ~ 2.8 ns and is in good agreement with the result obtained by the mixing method. Also, we present the experimentally measured volt-watt responsivity (S~) within the frequency range ω ≈ (0.3–6) THz VN HEB detector. The estimated values of noise equivalent power (NEP) for VN HEB and its minimum energy level (δE) reached NEP@1MHz ≈ 6.3 × 10–14 W/√Hz and δE ≈ 8.1 × 10–18 J, respectively.

Picosecond pulsed digital off-axis holography for near-nozzle droplet size and 3D distribution measurement of a swirl kerosene spray

Near-nozzle spray formation determines the spray structure of downstream dilute spray, and thus plays an important role in evaluating the performance of air blast atomizer. To capture the exact structure of the atomization field in near-nozzle region, the spray cones from the exit of a two-stage swirler to the 9 mm downstream area with the gas pressure of 0.5 kPa, 1.0 kPa, 3.0 kPa and the fuel pressure of 0.03 MPa and 0.71 MPa, are investigated using a picosecond pulsed laser digital off-axis holography (DOH) system. The spray structures and droplet characteristics in near-nozzle region are visualized with clear image of nonspherical droplets and then quantified. Semi-spray cone angles from the front view (x-y) projections of particle fields are obtained and results indicate that the angle increases with the increase of gas pressure within test range. From the overall view of spray field, the droplet peak diameter decreases with gas pressure, from about 80 μm at 0.54 kPa to about 30 μm at 3.05 kPa. In addition, cumulative volume distribution of droplet obtained in near-nozzle region agrees well with modified Rosin–Rammler distribution. From the local view, spatial distributions of the normalized droplet number density and Sauter mean diameters (SMD) ranging from 30 μm to 480 μm of the swirl spray field are statistically analyzed. Results indicate that as the gas pressure increases, the SMD distribution is more uniform. Under all gas pressure conditions, SMD distributions tend to be larger in the near-exit area and the region inside mist cone edge. The work also demonstrates that picosecond pulsed DOH is a powerful tool for 3D diagnostics of spray.

Spatial Self-Organization of Laser-Induced Graphite Nanonetwork in Diamond

The interior of spontaneously growing graphitized microstructures formed in diamond bulk by picosecond laser pulses has been studied. Longitudinal and cross sections of microstructures, prepared using mechanical polishing and focused ion beam, have been studied by Raman spectroscopy and scanning electron–ion microscopy. The laser irradiation of diamond bulk is found to induce formation of a graphite nanonetwork in the processed region and its subsequent spontaneous growth towards the laser beam by a distance of ~60 µm. The graphite nanonetwork consists of many ~130-nm-thick graphite sheets clustered into segments. It is shown that the thickness of graphite sheets decreases with a decrease in the laser fluence at the modification front, whereas the segment length (~2.6 µm) is independent of the laser fluence at the modification front. A three-dimensional structural model of microsegment has been constructed based on the analysis of several longitudinal and cross sections of microstructures. The mechanism of graphite nanosheet clustering into microsegments is discussed.

Picosecond stimulated Raman scattering at 3000 and 3430 cm-1 OH vibrations without optical breakdown.

For the first time, to the best of our knowledge, stimulated Raman scattering (SRS) of picosecond laser pulses without optical breakdown has been detected simultaneously (as the first Stokes and anti-Stokes paired components) at ∼3430 and ∼3000cm-1 vibrations of water OH band. These components were generated coaxially to the pump beam in the forward direction as axial and conical ring beams, respectively, when the pump beam was focused at the water-air interface. We suggest an explanation of these new SRS phenomena by non-collinear four-wave parametric interaction.

Frequency up-conversion of intense, ultrashort laser pulses at maximal atomic coherence

We present experimental data on adiabatically driven frequency conversion with ultrashort (picosecond) laser pulses towards the vacuum-ultraviolet regime, enhanced by coherent population return and preparation of maximal atomic coherences. We generate the sum frequency of an intense pump pulse and a probe pulse via a two-photon resonance in xenon. When we slightly detune the pump laser from two-photon resonance, the atomic populations are adiabatically driven forth and back from the atomic ground state to an excited state. This coherent population return (CPR) prepares the medium in a transient state of maximal atomic coherence, which enhances frequency mixing with a probe laser. We thoroughly study the variation of CPR-enhanced frequency conversion with experimental parameters and compare CPR with conventional, resonantly enhanced frequency conversion. In particular, we investigate the temporal evolution of the atomic coherence and the effect of inevitable ac Stark shifts. We find that ac Stark shifts induce an asymmetry in the spectral characteristics of CPR, which nevertheless permits enhanced frequency conversion. For the case of resonantly enhanced frequency conversion, we show that the atomic coherences are maintained for several tens of picoseconds after the pump pulse, which permits time-delayed frequency conversion. Moreover, we analyze the pressure dependence of the atomic coherence' lifetime and observe a free induction decay at the two-photon coherence with the generation of a second harmonic field. These findings shall push applications of adiabatic light-matter interactions also to the regime of nonlinear optics, driven by intense laser pulses.

Morphogenesis of mesoscopic surface patterns formed in polarized two-photon etching of diamond

Two-photon etching of diamond surfaces is a novel technique for providing controlled removal of atomic layers and nano-patterning the surface. However, the details of the etching mechanism and pattern development are not well understood so that the full capability of the process is unclear. Here, a comprehensive investigation into the dynamics of nanopattern formation on the (100), (110) and (111) facets is reported as a function of polarization, for CVD and HPHT diamond, and for nanosecond and picosecond laser pulse durations. A diverse array of behaviour is observed including sub-wavelength scale structuring with morphologies characteristic of the facet and the selected polarization. As etching proceeds, the anisotropies in the quantum photon-lattice interaction are imprinted on the emergent patterns on nano and mesoscopic scales. The sum of the results leads to rules for predicting pattern type, roughness and etch rate. We also argue that they support a mechanism for carbon ejection based on the photo-ejection of carbonyl groups from carbonyl-supporting surfaces, defects and atomic-level step edges. The results provide guidance for optically manipulating diamond surfaces and comprise a major step towards a complete model for the process to aid laser direct-write structuring of functional diamond surfaces.

Enhanced recombination next to laser-patterned lines in CIGSe solar cells revealed by spectral and time-resolved photoluminescence

Laser-based patterning of CIGSe solar cell absorber layers for monolithic series connection by nanosecond (ns) and picosecond (ps) laser pulses introduces material modifications in the vicinity of the patterned lines, which affect the CIGSe composition, the electrical properties, the extent of the dead area and finally the efficiency of the complete solar cell. Here, spectral and time-resolved photoluminescence is used to show that Cu vacancies and anti-sites are formed next to the edges of the scribe lines, which lead to the formation of additional energy states in the band gap near the valence band edge. A significant reduction in charge carrier lifetime is noted as the scribed lines are approached from unaffected areas of the material. Since this effect is absent in the mechanical needle patterning, it can clearly be attributed to the influence of the laser pulses. Despite the laser-induced material modification, laser-based CIGSe patterning outperforms needle-based patterning. Advantages regarding the width of the affected edge areas were observed when using ns pulses instead of ps pulses, which could be due to the lower peak intensities.

Relativistic magnetic reconnection in laser laboratory for testing an emission mechanism of hard-state black hole system.

Magnetic reconnection in a relativistic electron magnetization regime was observed in a laboratory plasma produced by a high-intensity, large energy, picoseconds laser pulse. Magnetic reconnection conditions realized with a laser-driven several kilotesla magnetic field is comparable to that in the accretion disk corona of black hole systems, i.e., Cygnus X-1. We observed particle energy distributions of reconnection outflow jets, which possess a power-law component in a high-energy range. The hardness of the observed spectra could explain the hard-state x-ray emission from accreting black hole systems.

Recast-Free Helical Drilling of Fused Silica Using SHG Picosecond Laser Pulses

Recast-Free Helical Drilling of Fused Silica Using SHG Picosecond Laser Pulses

Fabrication and Characterization of Biplasmonic Substrates Obtained by Picosecond Laser Pulses

Bimetallic nanostructures have the potential to become the new generation candidates for applications in catalysis, electronics, optoelectronics, biosensors and also for surface-enhanced Raman Spectroscopy (SERS). The bimetallic nanocrystals offer additional properties over the single metal components such as improved electromagnetic properties and corrosion protection. This work presents a simple and inexpensive method to fabricate large area biplasmonic (bimetallic) substrates, employing DC magnetron sputtering, picosecond laser pulses and a digital galvanometric scanner. The aim of this study was to achieve large area homogeneous substrates while having a good and predictable signal amplification by SERS effect. Gold thin films with 200 nm thickness were deposited on optical polished substrates and then irradiated in atmospheric air with λ = 1064 nm wavelength laser pulses with 8 ps pulse duration and 500 kHz fixed repetition rate. Various laser fluences and laser irradiation speeds were employed in order to optimize the Laser-Induced Periodic Surface Structures (LIPSS) formed on the substrate. The results are presented comparatively for the standalone Cu substrates and for the Cu-Au substrates using Raman spectral analysis on a single signal peak of a Rhodamine 6G solution.

Graphene Oxide-Based Silico-Phosphate Composite Films for Optical Limiting of Ultrashort Near-Infrared Laser Pulses.

The development of graphene-based materials for optical limiting functionality is an active field of research. Optical limiting for femtosecond laser pulses in the infrared-B (IR-B) (1.4–3 μm) spectral domain has been investigated to a lesser extent than that for nanosecond, picosecond and femtosecond laser pulses at wavelengths up to 1.1 μm. Novel nonlinear optical materials, glassy graphene oxide (GO)-based silico-phosphate composites, were prepared, for the first time to our knowledge, by a convenient and low cost sol-gel method, as described in the paper, using tetraethyl orthosilicate (TEOS), H3PO4 and GO/reduced GO (rGO) as precursors. The characterisation of the GO/rGO silico-phosphate composite films was performed by spectroscopy (Fourier-transform infrared (FTIR), Ultraviolet–Visible-Near Infrared (UV-VIS-NIR) and Raman) and microscopy (atomic force microscopy (AFM) and scanning electron microscopy (SEM)) techniques. H3PO4 was found to reduce the rGO dispersed in the precursor’s solution with the formation of vertically agglomerated rGO sheets, uniformly distributed on the substrate surface. The capability of these novel graphene oxide-based materials for the optical limiting of femtosecond laser pulses at 1550 nm wavelength was demonstrated by intensity-scan experiments. The GO or rGO presence in the film, their concentrations, the composite films glassy matrix, and the film substrate influence the optical limiting performance of these novel materials and are discussed accordingly.

Electron beam and betatron x-ray generation in a hybrid electron accelerator driven by high intensity picosecond laser pulses

We experimentally studied electron beams and synchrotron x-ray radiation generated from the interaction of high-intensity picosecond laser pulses of 1–3 × 1019 W/cm2 with underdense plasmas of 1–8 × 1019 cm−3. Electron beams with maximum energy over 120 MeV, hundreds mrad divergence and tens of nC charge were generated in the experiments. The measured x-ray spectra were fit well with a synchrotron radiation spectrum combination with a bremsstrahlung spectrum. When laser pulses with intensity of 2.38 × 1019 W/cm2 interacting with pure Nitrogen gas with density of 2.35 × 1018 cm−3, we obtained betatron x-ray radiation with critical energy 3–4 keV and photon number up to 1013 photons/Sr per shot, the energy conversion efficiency is about 3 × 10−5. The high photon yield and large energy conversion efficiency are due to large amounts of electrons trapped by self-modulated laser wakefield and mainly accelerated by direct-laser-acceleration mechanism. Such high yield betatron x-ray sources have widely application prospect as backlight or probe in large scale of laser facilities used for high energy density or fusion sciences.

Ion acceleration from microstructured targets irradiated by high-intensity picosecond laser pulses.

Structures on the front surface of thin foil targets for laser-driven ion acceleration have been proposed to increase the ion source maximum energy and conversion efficiency. While structures have been shown to significantly boost the proton acceleration from pulses of moderate-energy fluence, their performance on tightly focused and high-energy lasers remains unclear. Here, we report the results of laser-driven three-dimensional (3D)-printed microtube targets, focusing on their efficacy for ion acceleration. Using the high-contrast (∼10^{12}) PHELIX laser (150J, 10^{21}W/cm^{2}), we studied the acceleration of ions from 1-μm-thick foils covered with micropillars or microtubes, which we compared with flat foils. The front-surface structures significantly increased the conversion efficiency from laser to light ions, with up to a factor of 5 higher proton number with respect to a flat target, albeit without an increase of the cutoff energy. An optimum diameter was found for the microtube targets. Our findings are supported by a systematic particle-in-cell modeling investigation of ion acceleration using 2D simulations with various structure dimensions. Simulations reproduce the experimental data with good agreement, including the observation of the optimum tube diameter, and reveal that the laser is shuttered by the plasma filling the tubes, explaining why the ion cutoff energy was not increased in this regime.

Qualification of Operational Amplifier used in Satellite Subsystem using Picosecond Pulsed Laser System

Picosecond Pulsed Laser System (PPLS) was used to simulate the single event effects (SEE) on satellite electronic components. Single event transients effect induced in an operational amplifier (LM324) to determine how transient amplitude and charge collection varied with pulsed laser energies. The wavelength and the focused spot size are the primary factors generating the resultant charge density profile. The degradation performance of LM324 induced by pulsed laser irradiation with two wavelength (1064nm, 532nm) is determined as a function of laser cross section. The transient voltage changed due to pulsed laser hitting specific transistors. This research shows the sensitivity mapping of LM324 under the effect of fundamental and second harmonic wavelengths. Determine the threshold energy of the SET in both wavelength, and compare the laser cross section of 1064 nm beam and 532 nm beam.

Multichannel-coupled compressed ultrafast photography

Compressed ultrafast photography (CUP) is currently the fastest receive-only ultrafast imaging technique with snapshot. CUP can capture non-repetitive transient events through three-dimensional (3D) image reconstruction based on a compressed sensing (CS) algorithm. However, CUP is a lossy imaging technique via image encoding and decoding, so how to further improve the image reconstruction accuracy has always been a goal pursued by scientists. Here, we develop a novel design of multichannel-coupled CUP (i.e. MC-CUP) to improve the image reconstruction accuracy. MC-CUP can not only increase the sampling rate, but also keep the snapshot of CUP. To demonstrate the advantage of MC-CUP in the image reconstruction accuracy, we measure a spatially modulated picosecond laser pulse and a 3D stepped structure, and these experimental results confirm that MC-CUP can significantly improve the spatial and temporal resolutions compared to CUP. This study will further promote the practical applications of CUP in ultrafast imaging, especially for biomedical imaging with high spatial resolution.

Nanosecond illumination source for speckle-free liquid crystalmicroscopy

ABSTRACT We present a setup for the speckle-free, low-light microscopy imaging at ~5 ns exposure times. The design is based on stroboscopic principle and uses fast and incoherent fluorescent emission from a dye, excited by a picosecond laser pulse. The basic spatial, spectral, and temporal beam properties are characterised, modelled and compared for two different designs of the source: (i) Rhodamine 6 G dye dissolved in ethanol and put in a glass cuvette, and (ii) random laser based on the dispersion of 300 nm TiO2 nanospheres in an ethanol solution of Rhodamine 6 G dye. The proposed solution of Rhodamine 6 G dye in ethanol put in a glass cuvette gives an excellent image quality with high contrast, excellent stability and tunable coherence. It delivers ~30 nJ of completely incoherent light per excitation laser pulse energy of ~40 μJ. It allows for the photographic measurements with 5 ns exposure time, demonstrated in imaging of thermally quenched 5CB liquid crystal. The set-up is used in studies of Kibble-Zurek mechanism of topological defect nucleation and growth at sub-microsecond time resolution and extremely fast cooling rates of ~40,000 K/s.

The SiPM revolution in time-domain diffuse optics

Time-domain diffuse optics is a powerful non-invasive, non-ionizing and label-free technique based on the use of picosecond pulsed laser light to probe highly scattering media like biological tissues down to a depth of few centimeters to obtain functional and compositional information. This technique is opening new perspectives in various fields spanning from oncology (e.g. characterization of breast or thyroid lesions, etc.) to neurology (e.g. diagnosis and monitoring of traumatic brain injuries, functional brain imaging, etc.), as well as in non-biomedical fields (e.g. characterization of fruits, wood, etc.). Time-domain diffuse optics is nowadays undergoing fascinating technology advancements, permitting for the first time the design of low-cost compact/wearable high performance systems. This revolution has been made possible also taking advantage from Silicon PhotoMultiplier (SiPM) progresses, originally driven by other applications, since time-domain diffuse optics is highly demanding in terms of performance, in particular requiring single-photon detectors with large collection area, high fill-factor, high single-photon timing resolution, low power dissipation and compact high-throughput front-end electronics. This work will review the recent advancements introduced by SiPMs in time-domain diffuse optics, mostly thanks to the support of different running EU H2020 projects (e.g. SOLUS -G.A.731877-, LUCA -G.A.688303-, BITMAP -G.A.675332-, ATTRACT -G.A.777222-, Laserlab-Europe -G.A.654148-), showing their present performances in this field, the inherent advantages that allowed the design of innovative diffuse optical imaging systems, as well as highlighting their present limitations in order to push forward the research towards the perfect SiPM for time-domain diffuse optics.

3-Dimensional analysis of layer structured samples with high depth resolution using picosecond laser-induced breakdown spectroscopy

3-Dimensional (3D) analysis is crucial for many materials and can be used to study their structure and properties. Laser-Induced Breakdown Spectroscopy (LIBS) is a versatile tool to get depth information quickly, but the poor depth resolution and in general a difficult quantification are the two main drawbacks. To solve these problems, a LIBS method based on picosecond-laser pulses is introduced. The ablation depth is measured and associated with the laser pulse number. A series of layer-structured graphite samples was tested by this method and the 2D and 3D layer structures of these samples were identified with a resolution of up to 24 and 102 nm for Mo and C elements under a residual pressure of 1 × 10−5 Pa, respectively. This shows the great potential of picosecond Laser-Induced Breakdown Spectroscopy (ps-LIBS) in the field of depth analysis.

Laser-induced nanofibrous titania film electrode: A new approach for energy storage materials

Abstract The research described here proposes a novel use of titanium oxide nanofibrous film as advanced electrode material for high performance supercapacitors. High frequency picosecond laser pulses were employed to generate a film of oxidized Ti on Ti substrate. Physico-chemical characterizations and morphological studies revealed the formation of fibrous TiO2 micro/nano-structures. The electrochemical measurements of the generated nanostructures showed their improved electrochemical properties and proved the feasibility of using laser pulses for fabrication of advanced electrodes with high specific capacitance. Also, our results showed by double laser processing, the average diameter of the generated TiO2 fibers decreased from 300 nm to the range of 60–70 nm. Combination of having higher porosity (smaller fiber diameters) and more oxidized Ti results in better supercapacitance behaviours. These studies demonstrated the usefulness of laser generated nanofibrous film as a novel electrode material for high performance supercapacitors. It can lead to promising methods for fabrication of advanced electrodes with high energy and power density with excellent cyclic stability.