Fs Pulses Research Articles

Femtosecond multi-10-mJ pulses at 2 µm wavelength by compression in a hollow-core fiber.

High-energy few-ps pulses from a Ho:YLF chirped pulse amplifier operating at a 1 kHz repetition rate are compressed in a two-stage arrangement to sub-90-fs duration. The energy of the compressed pulses is more than 20 mJ at an average power of 20 W. In the first stage, the duration of the 2.8 ps, 40 mJ pulses at 2.05 µm wavelength was reduced to 1.4 ps by using nonlinear propagation in air. Subsequently, the pulses were further compressed to 86 fs after spectral broadening in a 3-m-long Kr-filled stretched flexible hollow-core fiber. The high photon flux, peak power, and excellent beam quality and stability make this light source highly attractive for fs pulse generation in the extreme ultraviolet (XUV) to x-ray spectral range for time-resolved XUV spectroscopy or measurements of structural dynamics in solids.

Study of quasimonoenergetic electron bunch generation in self-modulated laser wakefield acceleration using TW or sub-TW ultrashort laser pulses

This work presents a study on laser wakefield electron acceleration in the self-modulated regime (SM-LWFA) using 50-fs laser pulses with energy on the mJ scale, at λ = 0.8 µm, impinging on a thin H2 gas jet. Particle-in-cell simulations were performed using laser peak powers ranging from sub-terawatt to a few terawatts and plasma densities varying from the relativistic self-focusing threshold up to values close to the critical density. The differences in the obtained acceleration processes are discussed. Results show that bunched electron beams with full charge on the nC scale and kinetic energy in the MeV range can be produced and configurations with peak density in the range 0.5–5 × 1020 atoms/cm3 generate electrons with maximum energies. In this range, some simulations generated quasimonoenergetic bunches with ∼0.5% of the total accelerated charge and we show that the beam characteristics, process dynamics, and operational parameters are close to those expected for the blowout regime. The configurations that led to quasimonoenergetic bunches from the sub-TW SM-LWFA regime allow the use of laser systems with repetition rates in the kHz range, which can be beneficial for practical applications.

Investigation of supercontinuum generation in photonic crystal fiber using bursts of femtosecond pulses

Results of experimental and numerical investigation of supercontinuum generation in polarization maintaining photonic crystal fiber using a burst of two orthogonal polarization femtosecond pump pulses are presented. The initial pump pulse source was a mode-locked Yb:KGW laser generating 52 nJ energy 90 fs duration pulses at 1033 nm with 76 MHz repetition rate. Analysis of cross-correlation frequency-resolved optical gating traces and numerical simulation results reveal that there are slight changes in supercontinuum generation when temporal delay between pump pulses in the burst is changed. Weak dependence of supercontinuum characteristics on temporal distance between the pump pulses is related to supercontinuum generation dynamics which is analyzed in detail.

Strong-field ionization of water: Nuclear dynamics revealed by varying the pulse duration

Polyatomic molecules in strong laser fields can undergo substantial nuclear motion within tens of femtoseconds. Ion imaging methods based on dissociation or Coulomb explosion therefore have difficulty faithfully recording the geometry dependence of the field ionization that initiates the dissociation process. Here we compare the strong-field double ionization and subsequent dissociation of water (both H$_2$O and D$_2$O) in 10-fs and 40-fs 800-nm laser pulses. We find that 10-fs pulses turn off before substantial internuclear motion occurs, whereas rapid internuclear motion can take place during the double ionization process for 40-fs pulses. The short-pulse measurements are consistent with a simple tunnel ionization picture, whose predictions help interpret the motion observed in the long-pulse measurements.

Mid-Infrared Ultra-Short Pulse Generation in a Gas-Filled Hollow-Core Photonic Crystal Fiber Pumped by Two-Color Pulses

We show numerically that ultra-short pulses can be generated in the mid-infrared when a gas filled hollow-core fiber is pumped by a fundamental pulse and its second harmonic. The generation process originates from a cascaded nonlinear phenomenon starting from a spectral broadening of the two pulses followed by an induced phase-matched four wave-mixing lying in the mid-infrared combined with a dispersive wave. By selecting this mid-infrared band with a spectral filter, we demonstrate the generation of ultra-short 60 fs pulses at a 3–4 µm band and a pulse duration of 20 fs can be reached with an additional phase compensator.

Systematic discrimination of heptane isomers with femtosecond laser linear time of flight mass spectrometry

The dissociative ionization processes of heptane molecule with nine isomers have been investigated and presented experimentally in detail by courtesy of femtosecond laser mass spectrometry technique with a fundamental wavelength of 800 nm and 90 fs pulse duration in this study. It has been aimed to show that the classification of heptane isomers using factor analysis as statistical method reducing the dimension of obtained data. The main mass ratio of heptane isomers is known as m/z 100 which is obtained. The dissociation process on the parent molecule was dominant at the laser intensity about 1.42 x 1014 W/cm2 used for all isomers. The ions obtained by the breakdown of the main ion peak were primarily determined as factors which were clearly distinguished from each other according to factor 1 and factor 2 axes by applying factor analysis.

Application of Pump-Probe Technique for Tracking of Charge Carrier Relaxation In Nanostructured Semiconductors

The pump-probe technique is a powerful tool for probing and characterizing the electronic and structural properties of short-lived excited states of materials. Upon the absorption of photons of the pump, excited states of the materials are established. Relaxation of these states reflects many physical aspects of the materials which can be tracked by a consequent beam – the probe. In this study, we present a conventional pump-probe technique at the University of Amsterdam and its application for tracking relaxation of charge carriers in thin films containing Si and Ge nanocrystals embedded in SiO2 matrix. The pump beam is obtained from a 150-fs laser pulse with the photon wavelength at 340 nm. The probe beam is constituted from a white-light beam in the wavelength range from approximately 900 - 1300 nm (0.9 - 1.4 eV). The photon-generated charge carriers feature with multi-exponent decay dynamics, involving to different physical characteristics. The fast decay components of about few ps time scale arise from defect-related trapping or Auger processes, while the slow decay components of about few hundred ps come from relaxation of the exciton left in the semiconductor nanocrystals. The deep-insight characterization of the materials involving to individual relaxation processes are presented and discussed.

1 kW, 10 mJ, 120 fs coherently combined fiber CPA laser system.

An ultrafast fiber chirped-pulse amplification laser system based on a coherent combination of 16 ytterbium-doped rod-type amplifiers is presented. It generates 10 mJ pulse energy at 1 kW average power and 120 fs pulse duration. A partially helium-protected, two-staged chirped-pulse amplification grating compressor is implemented to maintain the close to diffraction-limited beam quality by avoiding nonlinear absorption in air.

Comparison of the impact of nonlinearity in a p-i-n and an MUTC photodetector on electro-optic frequency combs.

We calculate the impact of nonlinearity in both a p-i-n photodetector (PD) and a modified uni-traveling carrier (MUTC) PD on an RF-modulated frequency comb generated using 100-fs optical pulses with a 50-MHz repetition rate. We take into account bleaching (nonlinear saturation) that is due to the high peak-to-average-power ratio and contributes to the device nonlinearity. Nonlinear impairment of an RF-modulated continuous wave is typically characterized by the second- and third-order intermodulation distortion products (IMD2 and IMD3). In contrast, an RF-modulated frequency comb must be characterized by a distinct IMD2n and IMD3n for each comb line n. We calculate IMD2n and IMD3n in both p-i-n and MUTC PDs and compare the results. We also calculate the ratio of the IMD2n power and the IMD3n power to the fundamental power Sin in both p-i-n and MUTC PDs. We find that nonlinear distortion has a greater impact at high frequencies in the MUTC PD than in the p-i-n PD.

Intra-oscillator high harmonic generation in a thin-disk laser operating in the 100-fs regime.

We demonstrate that Kerr lens modelocking is well-suited for operating an ultrafast thin-disk laser with intra-oscillator high harmonic generation (HHG) in the 100-fs pulse duration regime. Exploiting nearly the full emission bandwidth of the gain material Yb:YAG, we generate 105-fs pulses with an intracavity peak power of 365 MW and an intracavity average power of 470W. We drive HHG in argon with a peak intensity of ∼7⋅1013 W/cm2 at a repetition rate of 11MHz. Extreme-ultraviolet (XUV) light is generated up to the 31st harmonic order (H31) at 37 eV, with an average power of ∼0.4 µW in H25 at 30 eV. This work presents a considerable increase in performance of XUV sources based on intra-oscillator HHG and confirms that this approach is a promising technology for simple and portable XUV sources at MHz repetition rates.

Watt-level and sub-100-fs self-starting mode-locked 2.4-µm Cr:ZnS oscillator enabled by GaSb-SESAMs.

Femtosecond lasers with high peak power at wavelengths above 2 µm are of high interest for generating mid-infrared (mid-IR) broadband coherent light for spectroscopic applications. Cr2+-doped ZnS/ZnSe solid-state lasers are uniquely suited since they provide an ultra-broad bandwidth in combination with watt-level average power. To date, the semiconductor saturable absorber mirror (SESAM) mode-locked Cr:ZnS(e) lasers have been severely limited in power due to the lack of suitable 2.4-µm SESAMs.For the first time, we develop novel high-performance 2.4-µm type-I and type-II SESAMs, and thereby obtain state-of-the-art mode-locking performance. The type-I InGaSb/GaSb SESAM demonstrates a low non-saturable loss (0.8%) and an ultrafast recovery time (1.9 ps).By incorporating this SESAM in a 250-MHz Cr:ZnS laser cavity, we demonstrate fundamental mode-locking at 2.37 µm with 0.8 W average power and 79-fs pulse duration. This corresponds to a peak power of 39 kW, which is the highest so far for any saturable absorber mode-locked Cr:ZnS(e) oscillator.In the same laser cavity, we could also generate 120-fs pulses at a record high average power of 1 W. A comparable laser performance is achieved using type-II InAs/GaSb SESAM as well. These results pave the way towards a new class of high-power femtosecond SESAM mode-locked oscillators operating directly above 2-µm wavelength.

A STUDY OF FEMTOSECOND LASER POLISHING OF CVD NANOPOLYCRYSTALLINE DIAMOND FILMS

Femtosecond (fs) laser ablation has been recognized as an effective and promising technique for high-precision processing of natural and synthesized diamond. In this work, a study of femtosecond laser polishing for nanopolycrystalline diamond (NCD) films by chemical vapor deposition (CVD) is reported. The laser irradiation is induced by 200-fs laser pulses with a repetition frequency of 50[Formula: see text]MHz, and various laser fluences are employed to investigate their polishing effectiveness. The results show that the optimal laser fluence is 0.7[Formula: see text]J/cm2, at which the nanodiamond grains on top of the cauliflower-like clusters of NCD films can be ablated. With such laser fluence, the mean surface roughness of NCD films reduces from 73.84[Formula: see text]nm to 31.88[Formula: see text]nm, which presents a 57% reduction. Nevertheless, when the laser fluence rises beyond 0.7[Formula: see text]J/cm2, large amount of amorphous carbon (a-C) balls and porous lava-like morphology would come into being, resulting in severe degradation of the NCD surface.

Achromatic optical system with diffractive-refractive hybrid lenses for multifocusing of ultrashort pulse beams.

We report an achromatic cascade optical system for multifocusing ultrashort pulse beams with a diffractive beam splitter. Distortion compensation requires the removal of pulse front distortions from arrayed pulses, which originate from beam-radius-dependent group delay dispersions. The inclusion of hybrid diffractive-refractive lenses can effectively manage system dispersions. Simple design formulas are derived using the ray-matrix analysis and the designed system is evaluated using 20-fs pulses. We confirm that the hybridized system can remove not only chromatic aberrations but also pulse front distortions, hence improving the system spatio-temporal focusing resolutions. The proposed pulse delivery technique enhances the practicality of materials processing with ultrashort pulses.

Nonlinear amplification based on a tightly phase locked 750 MHz Yb:fiber frequency comb

In this paper, a nonlinear amplification system for a low noise 750 MHz ytterbium-doped fiber frequency comb is developed with a repetition rate stabilized to a continuous wave laser by employing a bulk electro-optic modulator. By managing the dispersion prior to the nonlinear amplifier, the proposed high-power frequency comb light source can deliver over 10 W of <90 fs pulse trains with an integrated relative intensity noise instability of 0.04%. Moreover, the noise characteristics of the proposed amplified comb, where the seed laser is a tightly stabilized 750 MHz fiber frequency comb, are analyzed in several representative states of different launched pump powers and pre-chirping group delay dispersions. The optical fiber frequency combs produced by nonlinear amplification may serve as an alternative for achieving high-power and ultrafast combs with good phase stability.

Ultrafast intermodal third harmonic generation in a liquid core step-index fiber filled with C2Cl4: erratum.

We provide a correction due to an erroneous repetition rate of one of the laser systems (90 fs pulse duration) in our previously published paper [Opt. Express28, 25037 (2020)10.1364/OE.399771].

Simulation Study on Supercontinuum Generation at Normal Dispersion Regime of a Carbon Disulfide-core Photonic Crystal Fiber

A photonic crystal fiber with a hollow core filled with carbon disulfide (CS2) is proposed as a new source of supercontinuum light. We numerically study guiding properties of modeled fibers including the dispersion and the effective mode area of the fundamental mode. As a result, octave spanning of the SC spectrum was achieved in the wavelength range of near-IR from 1.25 μm to 2.3 μm with 90 fs pulse and energy of 1.5 nJ at a pump wavelength of 1.55 μm. The proposed fibers are fully compatible with all-silica fiber systems, in particular, could be used for all-fiber SC sources and new low-cost all-fiber optical systems.

Ripple period adjustment on SiC surface based on electron dynamics control and its polarization anisotropy

In this paper, the Mach Zehnder interferometer is utilized to generate a collinear dual optical path with 800-nm center wavelength, 1 kHz repetition rate, 120-fs pulse duration and an energy ratio of 1:1 to scan 6H-SiC crystal at a scanning speed of 500 μm/s, resulting in laser-induced periodic surface structure (LIPSS). From the perspective of instantaneous electron density level, variation of the period of low spatial frequency LIPSS (LSFL) with interpulse delay time and polarization angle is studied. The scanning electron microscope (SEM) characterization of the line-scanned ablation zone combined with fast Fourier transformation (FFT) spectrum analysis shows that the period of LSFL decreases with the increase of delay time, and remains stable when the delay exceeds 240 fs. Meanwhile, the period of LSFL increases with the increase of polarization angle under the same delay time, showing an anisotropy that dependent on the polarization angle. The variation trend of the maximum electron density level with delay time calculated by electron rate equation is roughly consistent with the periodic variation trend. Interaction between incident light and transient metal-like SiC surface is simulated based on finite-difference time-domain method (FDTD). The simulation results show that the surface of the sample has a periodic light field enhancement structure controlled by the transient electron density, which explains the phenomenon that the period decreases with increasing delay time. It is further confirmed that the formation of LSFL is due to the interference of incident light and surface plasmon, and the period of LSFL can be controlled by adjusting the electron density level by delay time. The periodic polarization dependence of LSFL may be related to the relative orientation change of the laser pulse front tilt (PFT) and laser polarization.

Compact and robust supercontinuum generation and post-compression using multiple thin plates

Abstract We report on compact and robust supercontinuum generation and post-compression using transmission of light through multiple thin solid plates at the SwissFEL X-ray free-electron laser facility. A single stage consisting of three thin plates followed by a chirped mirror compressor achieves compression of initially 30-fs pulses with 800-nm center wavelength to sub-10-fs duration. We also demonstrate a two-stage implementation to compress the pulses further to sub-5-fs duration. With the two-stage setup, the generated supercontinuum includes wavelengths ranging from 500 to 1100 nm. The multi-plate setup is compact, robust, and stable, which makes it ideal for applications at free-electron laser facilities such as pump-probe experiments and laser-arrival timing tools.

Single pulse laser removal of indium tin oxide film on glass and polyethylene terephthalate by nanosecond and femtosecond laser

Abstract Indium tin oxide (ITO) is the most important transparent conducting electrode to date and the candidate for ultrafast signal processing in telecommunication region. ITO is normally selectively removed in a multiple-steps process for device application. In this work, we aimed to study single pulse removal of ITO-coated glass and PET by using a nanosecond (ns) laser (266 nm) and a femtosecond (fs) laser (1,025 nm) where each process is dominated by either linear or nonlinear process. For ns laser, ITO was removed from PET substrate at 0.01 J/cm2. Detachment likely occurred via thermal-induced process because of the high absorption by both ITO and PET and the thermomechanical properties of PET. At higher laser fluence (∼0.04 J/cm2), the ITO films on both substrates were damaged, and at 1.34 J/cm2, ITO was ablated from the glass substrate. For fs laser removal via nonlinear process, ITO was removed from PET substrate at 0.3 J/cm2, but at 0.8 J/cm2, the PET substrate was also modified. ITO layer was partially removed from glass substrate by fs laser pulse at 0.3 J/cm2 and full removal only occurred at 1.7 J/cm2. Thus, the fluence range for single fs pulse removal of ITO/PET was 0.3–0.8 J/cm2 and >1.7 J/cm2 for ITO/glass.

Spatiotemporal focusing through a multimode fiber via time-domain wavefront shaping.

We shape fs optical pulses and deliver them in a single spatial mode to the input of a multimode fiber. The pulse is shaped in time such that at the output of the multimode fiber an ultrashort pulse appears at a predefined focus. Our result shows how to raster scan an ultrashort pulse at the output of a stiff piece of square-core step-index multimode fiber and in this way show the potential for making a nonlinear fluorescent image of the scene behind the fiber, while the connection to the multimode fiber can be established via a thin and flexible single-mode fiber. The experimental results match our numerical simulation well.