Single Femtosecond Research Articles

Assessment of OH femtosecond thermally assisted fluorescence thermometry for hydrogen non-premixed flames

In this work, we further developed the planar temperature measurement method based on thermally assisted laser-induced fluorescence (TALF) with a single femtosecond (fs) laser and investigated the temperature distribution in the nitrogen-diluted oxygen–hydrogen cross-jet non-premixed flames at 1 kHz. Fs-TALF thermometry was firstly calibrated and assessed in a series of unconfined methane/air laminar flames at atmospheric pressure. In the calibration process, although the calibrated energy difference between OH v′ = 0 and 1 energy level was different from the theoretical value, the measured fluorescence ratio accurately reflected the temperature change from 1600 K to 2200 K in different methane flames. By comparing the calculated OH fluorescence spectrum, we believed that the main reason was the overlap of the OH (1-1) and (0-0) fluorescence bands. After the calibration, experiments were conducted with changing hydrogen flow velocity from 60 to 100 m/s and constant oxidizer flow velocity, where the dilution ratio, D varies from 0.46 to 0.7, and the equivalence ratio, ϕ, varies from 0.5 to 0.9. It is found, within the calibration range, fs-TALF thermometry can correctly measure the flame temperature. However, with ϕ further increases, the difference between the measured and theoretical temperatures gradually increases. At the same time, it was noted that the error in the measured temperature rises gradually as D decreases. In conclusion, fs-TALF thermometry can measure turbulent non-premixed hydrogen flames accurately in a range of temperature with acceptable temporal and partial resolution, while the errors within 200 K and spatial resolution is 170μm. The main error source should be the collisional effect due to the different local concentration of N2 and H2O. The results of this work will contribute to the further development the temperature measurement method hydrogen turbulent flames.

Single-shot picosecond interferometry for the characterization of laser-driven shock waves

In conventional laser-driven shock experiments, an out-of-plane shock wave is launched and is typically detected interferometrically after it propagates through the sample. In such experiments, the target materials are unavoidably optically damaged at each laser shot. This necessitates changing targets after laser exposure, lowering the shot-to-shot reproducibility and data quality. Here we present a Sagnac interferometer combined with an echelon that can split a single femtosecond probe into many beams, very well adapted for single-shot interferometric characterization of laser-induced shock waves. The echelon provides a 10 ps time resolution and a full time window of about 150 ps. The simplicity, stability, and sensitivity of the single-shot Sagnac interferometer technique ease the thorough characterization of picosecond to nanosecond shock waves, specifically for samples available in limited quantities or for samples that are not uniform from one region to the next.

Selective Detection in Impulsive Low-Frequency Raman Imaging Using Shaped Probe Pulses

Impulsive stimulated Raman scattering (ISRS) using a single short femtosecond pump pulse to excite molecular vibrations offers an elegant pump-probe approach to perform vibrational imaging below $200\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$. One shortcoming of ISRS is its inability to offer vibrational selectivity as all the vibrational bonds whose frequencies lie within the short pump-pulse bandwidth are excited. To date, several coherent control techniques have been explored to address this issue and selectively excite a specific molecular vibration by shaping the pump pulse. There has not been any systematic work that reports an analogous shaping of the probe pulse to implement preferential detection. In this work, we focus on vibrational imaging and report vibrational selective detection by shaping the probe pulse in time. We demonstrate numerically and experimentally two pulse-shaping strategies with one functioning as a vibrational notch filter and the other functioning as a vibrational low-pass filter. This enables fast ($25\phantom{\rule{0.2em}{0ex}}\text{\ensuremath{\mu}}\mathrm{s}/$pixel) and selective hyperspectral imaging in the low-frequency regime ($<200\phantom{\rule{0.2em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$).

Dynamics of single cell femtosecond laser printing.

In the present study, we investigated the dynamics of a femtosecond (fs) laser induced bio-printing with cell-free and cell-laden jets under the variation of laser pulse energy and focus depth, by using time-resolved imaging. By increasing the laser pulse energy or decreasing the focus depth thresholds for a first and second jet are exceeded and more laser pulse energy is converted to kinetic jet energy. With increasing jet velocity, the jet behavior changes from a well-defined laminar jet, to a curved jet and further to an undesired splashing jet. We quantified the observed jet forms with the dimensionless hydrodynamic Weber and Rayleigh numbers and identified the Rayleigh breakup regime as the preferred process window for single cell bioprinting. Herein, the best spatial printing resolution of 42 ± 3 µm and single cell positioning precision of 12.4 µm are reached, which is less than one single cell diameter about 15 µm.

Electronic‐Resonance‐Enhanced Coherent Raman Spectroscopy with a Single Femtosecond Laser Beam

AbstractCoherent Raman scattering (CRS) spectroscopy holds versatile applications in many fields ranging from measuring temperature and concentration in reacting flows to imaging biological molecules in living cells. Although the powerful spectroscopic technique has been studied for a few decades, significant attention is still paid to developing state‐of‐the‐art CRS spectroscopy with the simpler design and higher sensitivity for practical applications in harsh or complex environments. Herein, a novel electronic‐resonance‐enhanced coherent Raman scattering (ERE‐CRS) technique is developed with a single femtosecond laser beam. Such single‐beam ERE‐CRS scheme is accomplished with the narrowband air lasing signal naturally generated in the interaction region as a probe source. The developed technique not only avoids fine spatio‐temporal control in the conventional multibeam configuration, but also accomplishes 1–2 orders of magnitude enhancement of Raman signal due to the resonance of air lasing with electronic transition of the target gas. The dramatic enhancement effect is also manifested through the comparative measurements in CO2 isotopes. The single‐beam ERE‐CRS spectroscopy shows an unprecedented opportunity for the high‐sensitivity standoff detections of gas‐phase molecules and plasmas.

Raman Study of the Diamond to Graphite Transition Induced by the Single Femtosecond Laser Pulse on the (111) Face.

The use of the ultrafast pulse is the current trend in laser processing many materials, including diamonds. Recently, the orientation of the irradiated crystal face was shown to play a crucial role in the diamond to graphite transition process. Here, we develop this approach and explore the nanostructure of the sp2 phase, and the structural perfection of the graphite produced. The single pulse of the third harmonic of a Ti:sapphire laser (100 fs, 266 nm) was used to study the process of producing highly oriented graphite (HOG) layers on the (111) surface of a diamond monocrystal. The laser fluence dependence on ablated crater depth was analyzed, and three different regimes of laser-induced diamond graphitization are discussed, namely: nonablative graphitization, customary ablative graphitization, and bulk graphitization. The structure of the graphitized material was investigated by confocal Raman spectroscopy. A clear correlation was found between laser ablation regimes and sp2 phase structure. The main types of structural defects that disrupt the HOG formation both at low and high laser fluencies were determined by Raman spectroscopy. The patterns revealed give optimal laser fluence for the production of perfect graphite spots on the diamond surface.

Temporal modulation toward femtosecond laser-induced nonlinear ionization process.

The temporal chirp of single femtosecond (fs) pulses will affect the laser-induced ionization process. By comparing the ripples induced by negatively and positively chirped pulses (NCPs and PCPs), the growth rate showed a significant difference, resulting in a depth inhomogeneity of up to 144%. A carrier density model tailored with temporal characteristics showed that NCPs could excite a higher peak carrier density, contributing to a highly efficient generation of surface plasmon polaritons (SPPs) and overall advancement of the ionization rate. Such distinction originates from their contrary incident spectrum sequences. Current work reveals that temporal chirp modulation can control the carrier density in ultrafast laser-matter interaction, which possibly brings an unusual acceleration for surface structure processing.

Simultaneous detection of three chemical species (NO, O, O2) using a single broadband femtosecond laser

Simultaneous nitric oxide (NO) laser-induced fluorescence (LIF), atomic oxygen (O) two-photon LIF (TPLIF), and molecular oxygen (O2) LIF were demonstrated using a single broadband femtosecond (fs) laser in methane- and hydrogen-fueled flames. An amplified Ti:Sapphire laser with ∼80-fs pulse duration was used to pump an optical parametric amplifier (OPA) to generate 226.1-nm pulses having 2.3 nm (∼450 cm−1) bandwidth to excite all three species simultaneously. The specific excitation transitions are NO A-X (0,0) system, O 3p3P←←2p3P two-photon transition, and O2B3Σu−−X3Σg− Schumann-Runge system. Detailed emission characterization was performed using a 1D imaging spectrometer to identify suitable emission bands for interference-free detection of each species. A simultaneous detection system consisting of a visible and a UV camera provided the optimal detection of all three species. Imaging studies revealed high concentrations of NO, O, and O2 at the edge of the flame cone of Bunsen flames because NO and O are produced at the hot flame front. O2 LIF signal was present only at the flame front because of the strong temperature dependence of the O2 Schumann-Runge system. Direct imaging of O TPLIF enabled single-shot as well as high-fidelity shot-averaged line images in stable laminar flames. Equivalence ratio scans in the CH4/air Hencken flames showed good agreements for O and O2 with Cantera equilibrium predictions performed using GRI-Mech 3.0. Although the NO measurements deviated from the equilibrium calculations on the fuel-rich side, it agreed well with previously reported NO LIF measurements in a similar flame. This discrepancy is likely due to Prompt NO, which was not considered in the simulation. NO LIF measurements as a function of height-above-the-burner in the H2/air Hencken flames agreed well with calculated NO mole fractions using the UNICORN flame code. The present study lays the foundation for single-laser imaging of three critical flame species in NO formation pathways in combustion systems.

Insight on the regulation mechanism of the nanochannels in hard and brittle materials induced by sparially shaped femtosecond laser.

The efficient fabrication of nanochannels on hard and brittle materials is a difficult task in the field of micro and nano processing. We have realized nanochannel arrays on silica with characteristic scales varying from 50–230 nm using a single femtosecond Bessel beam pulse of 515 nm. By characterizing the surface openings, we found that the characteristic scales of the nanopore openings are inextricably linked to the surface energy deposition effect. We achieved not only three asymmetric channel profiles by adjusting the laser-sample interaction region, but also high aspect ratio nanochannels with characteristic scales about 50 nm and aspect ratios over 100. These results on hard and brittle materials provide a broader platform and application scenarios for smart particle rectifiers, DNA molecular sequencing, biosensors, and nanofluidic devices, which are also more suitable for future practical applications due to their low cost, good durability, and high productivity.

Chirp-dependent dual light emission in Na0.95Er0.05Nb0.9Ti0.1O3 perovskite

Polar Na0.95Er0.05Nb0.9Ti0.1O3 perovskite has been synthesized as a nanostructured material by microwave assisted hydrothermal method. The characterization indicates that erbium is a constituent of the crystal structure and is preferentially located in sodium positions. The compound combines the nonlinear optical properties of the host (NaNb0.9Ti0.1O2.95) and the fluorescent properties of the Er3+-dopant. Under excitation by a single femtosecond (< 10 fs) laser in the near-infrared region, simultaneous dual emission signals of second harmonic generation (SHG) and up-converting fluorescence (UCF) are observed and the nonlinear dependencies of the SHG and UCF intensities on the excitation intensity are measured. In addition, both emissions are shown to be sensitive to the chirp of the exciting pulses, and for UCF, it can be explained by means of a simple theoretical model based on the density matrix equations. These nano-structured particles with chirp-dependent dual behavior can be very advantageous when used in biological systems, since they can provide complementary information in different spectral ranges and tissues and are susceptible of coherent control, which can be both useful in optical microscopy and bioimaging.

On the importance of electron–electron and electron–phonon scatterings and energy renormalizations during carrier relaxation in monolayer transition-metal dichalcogenides

An ab initio based fully microscopic many-body approach is used to study the carrier relaxation dynamics in monolayer transition-metal dichalcogenides. Bandstructures and wavefunctions as well as phonon energies and coupling matrix elements are calculated using density functional theory. The resulting dipole and Coulomb matrix elements are implemented in the Dirac–Bloch equations to calculate carrier–carrier and carrier–phonon scatterings throughout the whole Brillouin zone (BZ). It is shown that carrier scatterings lead to a relaxation into hot quasi-Fermi distributions on a single femtosecond timescale. Carrier cool down and inter-valley transitions are mediated by phonon scatterings on a picosecond timescale. Strong, density-dependent energy renormalizations are shown to be valley-dependent. For MoTe2, MoSe2 and MoS2 the change of energies with occupation is found to be about 50% stronger in the Σ and Λ side valleys than in the K and K′ valleys. However, for realistic carrier densities, the materials always maintain their direct bandgap at the K points of the BZ.

Toric Intraocular Lens Calculation Considering Anterior Surgically Induced Astigmatism and Posterior Corneal Astigmatism

ABSTRACT Purpose To evaluate the prediction error (PE) after applying the Abulafia–Koch formula in an online calculator with and without consideration of anterior corneal surgically induced astigmatism (SIACornea). Methods SIACornea models were calculated with a historical database of 204 right eyes (REs) from a single surgeon, either for manual (2.2 mm) or femtosecond (2.5 mm) temporal clear corneal incisions. PE was assessed in 58 REs operated by the same surgeon with a monofocal toric IOL and calculated, considering the PCA estimation in an online calculator with the combination of each one of the following SIACornea calculation approaches: (A) considering only significant centroids after stratification, (B) all centroids after stratification and (C) a single centroid without stratification. Results The consideration of all centroids resulted in an underestimation of SIACornea in cases of preoperative against-the-rule astigmatism (ATR-A) and an overestimation in with-the-rule astigmatism (WTR-A). After stratification, SIACornea was only significant in preoperative ATR and oblique astigmatism cases for femtosecond incisions. PE considering PCA only was 0.03@160º. The combination with SIACornea resulted in a WTR-A surprise in preoperative ATR-A and WTR-A, however only being significant for preoperative ATR-A in calculation approaches B (0.29@84º) and C (0.21@80º). SIACornea addition to PCA estimation only reduced the centroid for oblique preoperative astigmatism. Conclusions Surgeons should consider the calculation of the SIACornea after stratification by astigmatism type when using the same incision location (i.e. temporal). However, SIACornea derived from the anterior corneal surface should not be combined with PCA estimation for IOL power calculations.

Application concepts for ultrafast laser-induced skyrmion creation and annihilation

Magnetic skyrmions can be created and annihilated in ferromagnetic multilayers using single femtosecond infrared laser pulses above a material-dependent fluence threshold. From the perspective of applications, optical control of skyrmions offers a route to a faster and, potentially, more energy-efficient new class of information-technology devices. Here, we investigate laser-induced skyrmion generation in two different materials, mapping out the dependence of the process on the applied field and the laser fluence. We observe that sample properties like strength of the Dzyaloshinskii–Moriya interaction and pinning do not considerably influence the initial step of optical creation. In contrast, the number of skyrmions created can be directly and robustly controlled via the applied field and the laser fluence. Based on our findings, we propose concepts for applications, such as all-optical writing and deletion, an ultrafast skyrmion reshuffling device for probabilistic computing, and a combined optical and spin–orbit torque-controlled racetrack.

Single Femtosecond Laser-Pulse-Induced Superficial Amorphization and Re-Crystallization of Silicon.

Superficial amorphization and re-crystallization of silicon in <111> and <100> orientation after irradiation by femtosecond laser pulses (790 nm, 30 fs) are studied using optical imaging and transmission electron microscopy. Spectroscopic imaging ellipsometry (SIE) allows fast data acquisition at multiple wavelengths and provides experimental data for calculating nanometric amorphous layer thickness profiles with micrometric lateral resolution based on a thin-film layer model. For a radially Gaussian laser beam and at moderate peak fluences above the melting and below the ablation thresholds, laterally parabolic amorphous layer profiles with maximum thicknesses of several tens of nanometers were quantitatively attained. The accuracy of the calculations is verified experimentally by high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (STEM-EDX). Along with topographic information obtained by atomic force microscopy (AFM), a comprehensive picture of the superficial re-solidification of silicon after local melting by femtosecond laser pulses is drawn.

Fabrication of an antireflection microstructure on AgClBr polycrystalline fiber by single pulse femtosecond laser ablation

Polycrystalline infrared (PIR) fibers are used for numerous applications, one of those being power delivery for CO2 −lasers. However, the fiber tip surface’s transmittance cannot be increased with conventional antireflection coatings due to the surface unevenness. Antireflection microstructures (ARMs) offer an alternative way of increasing transmittance. In this work, ARMs were fabricated on the fiber tip surface of an AgClBr fiber by single-pulse femtosecond laser ablation. A single-surface transmittance of 92.8% at 10.6 µm, a CO2 −laser operation wavelength, was achieved. The proposed method can help significantly improve the systems’ efficiency, where power delivery for CO2 −lasers or sources operating in the wide wavelength range is required.

Laser Printing of Chiral Silicon Nanoprotrusions by Asymmetric Donut-Shaped Femtosecond Pulses

Here, we showed formation of chiral nanoprotrusions upon direct laser ablation of bulk crystalline silicon (c-Si) wafer with single femtosecond (fs) pulses having asymmetric donut-shaped intensity profile. Breaking circular symmetry of the irradiating donut-shaped fs-pulse beam was demonstrated to switch the geometry of formed surface nanoprotrusions from regular to chiral, while the chirality of the obtained Si nanostructures was found to promote with a degree of asymmetry of the laser beam. The obtained experimental results explain, for the first time, the formation of previously reported chiral c-Si nanostructures produced via donut-shaped beam ablation in terms of uneven helical flow of laser-melted Si material caused by asymmetry of initial intensity and temperature pattern on laser-irradiated Si surface. Our findings open a pathway towards easy-to-implement inexpensive fabrication of chiral all-dielectric nanostructures for advanced nanophotonic applications.

Energy Efficient Control of Ultrafast Spin Current to Induce Single Femtosecond Pulse Switching of a Ferromagnet.

New methods to induce magnetization switching in a thin ferromagnetic material using femtosecond laser pulses without the assistance of an applied external magnetic field have recently attracted a lot of interest. It has been shown that by optically triggering the reversal of the magnetization in a GdFeCo layer, the magnetization of a nearby ferromagnetic thin film can also be reversed via spin currents originating in the GdFeCo layer. Here, using a similar structure, it is shown that the magnetization reversal of the GdFeCo is not required in order to reverse the magnetization of the ferromagnetic thin film. This switching is attributed to the ultrafast spin current and can be generated by the GdFeCo demagnetization. A larger energy efficiency of the ferromagnetic layer single pulse switching is obtained for a GdFeCo with a larger Gd concentration. Those ultrafast and energy efficient switchings observed in such spintronic devices open a new path toward ultrafast and energy efficient magnetic memories.

Simultaneous imaging of H and OH in flames using a single broadband femtosecond laser source

Atomic hydrogen (H) and hydroxyl radical (OH) are two key intermediate species in the combustion of hydrocarbon fuels. For example, the spatial and temporal distribution of H is useful to better understand the ignition/extinction processes, whereas OH distribution profiles can yield flame structure and reaction zone in different flames conditions. In the present study, simultaneous laser-induced fluorescence (LIF) measurements of H and OH are obtained using a single ultrashort femtosecond-duration laser pulse near λ=307.7 nm for 3pLIF of H. Measurements are performed in CH4-air and C2H4-air flames stabilized over the Hencken burner. Recently developed three-photon LIF (3pLIF) technique is used for H, whereas the same excitation laser pulse can simultaneously excite OH A2Ʃ+←X2Π (0,0) transition. Red-shifting the excitation wavelength to 307.7 nm as opposed to commonly used two-photon excitation using 205 nm deep-ultra-violet (DUV) laser pulses enables the reduction of absorption losses of excitation laser pulse energy when transmitting through thick optical windows of high-pressure vessels, for example. Furthermore, the efficiency of the fs wavelength generation scheme of commercial optical parametric devices is approximately a factor of two higher near 307.7 nm as compared to 292 nm for 3pLIF of H. In this study, H and OH number densities have been reported for a range of flame equivalence ratios in CH4/air and C2H4/air flames established over the Hencken burner. Simultaneous imaging of H and OH in the premixed Bunsen flame is presented. Prospective improvements for increasing the signal-to-noise and hence potential kHz-rate simultaneous single-shot imaging are also discussed.

Multi-wavelength annular optical pulses generated by double interferent femtosecond Bessel laser beams in silica glass

Double femtosecond Bessel laser beams (fs-BLBs) are used to interact with UV-visible silica glass and ultrashort annular beams (ABs) with six different peak wavelengths in the visible and near-infrared range are simultaneously generated while the super-continuum emission is totally suppressed. Experimental results show that as the time interval Δt between double fs-BLBs gradually approaches zero, the generation efficiency of the ABs increases and reaches the maximum at Δt = 0. When Δt equals ±18 fs, the intensity of the annular beam with a central wavelength of 605 nm decreases to half of its maximum. Compared with the case using single femtosecond Bessel laser beam, the ABs’ generation efficiency can be greatly enhanced due to the interference of double fs-BLBs. It is considered that the four-wave mixing (FWM) and corresponding cascaded parametric processes are responsible for the generation of these ABs, which can be confirmed by the numerical simulations. The colorful ABs have important applications in the imaging and communication fields, such as the stimulated emission depletion microscopy (STED), the high speed 3D live fluorescence imaging, and the multiplexing optical communications.

Simultaneous measurement of CO and OH in flames using a single broadband, femtosecond laser pulse

In this short communication, we report simultaneous measurement of CO and OH in flames using a single femtosecond (fs)-duration laser source. Two-photon excitation of CO A1Π ←←X1Ʃ+(3,0), (4,0) and (5,0) bands and single-photon excitation of OH A2Ʃ+←X2Π (1,0) band are achieved simultaneously using one fs-duration laser pulse near 283 nm. Subsequently, crosstalk-free, laser-induced fluorescence (LIF) emissions are detected from the v’=3, 4 and 5 upper vibrational levels of CO in the 200–230 nm region, and (1,1) and (0,0) bands of OH near the 310-nm region. A detailed spectroscopic investigation of CO in a gas cell is followed by simultaneous detection of CO and OH in C2H4/air flames. The measured CO and OH profiles as a function of flame equivalence ratio agree well with equilibrium calculations. The present study extends the applicability of broadband fs-LIF imaging of OH in flames, and reveal an alternate method to obtain heat release rate in turbulent flames via simultaneous OH and CO measurements.