We present a sub-2-cycle laser system combining high average power, pulse energy and repetition rate with CEP-stable operation. The laser system creates 300 fs pulses with 1.8 mJ pulse energy that are nonlinearly post-compressed down to few optical cycles in two subsequent multipass cells (MPC). A pulse duration of 5.8fs (sub-2-cycle) at a pulse energy of 1.1mJ in combination with 110W average power (100 kHz) is achieved. This corresponds to the shortest pulses and highest compressed average power for few-cycle MPCs. Furthermore, the carrier-to-envelope-phase stability amounts to 300 mrad for frequencies above 2 kHz as measured by stereo—above-threshold-ionization (ATI).
Here we will present a reliable (experimentally and numerically proved) technique for multi-spot pattern formation in the focus of a lens (i.e. in the artificial far field). This was done using large square-shaped and/or hexagonal optical vortex (OV) lattices generated by spatial light modulators. Experimental and numerical results showing a controllable far-field beam reshaping when such lattices are generated in the Fourier plane will be discussed. Even more interesting bright structures can be obtained by combining OV lattices (of any type) with different node spacings. We show that the small-scale structure of the observed patterns results from the OV lattice with the larger array node spacing, whereas the large-scale structure stems from the OV lattice with the smaller array node spacing. The orientation of the mixed far-field structures is proven to rotate by 180° when all TCs are inverted.
Optical coherence tomography (OCT) is an established method for non-invasive cross-sectional imaging of biological samples using visible and near infrared light. The axial resolution of OCT only depends on the coherence length l_c∝λ_0^2/Δλ_FWHM, with the central wavelength λ_0 and the spectral width Δλ_FWHM of the light source. For OCT, the axial resolution is in the range of a few micrometers.
XUV coherence tomography (XCT) extends OCT into the extreme ultraviolet range. The significant reduction of the coherence length of a broadband XUV source allows nanoscale axial resolution. The usable spectral bandwidth in XCT is mainly limited by absorption edges of the sample under investigation. For example, the so-called silicon transmission window allows cross-sectional imaging of silicon-based samples like semiconductors.
A laboratory-based XCT setup has been implemented by using XUV radiation from a laser-driven high-harmonic source. By averaging harmonic combs generated by different fundamental wavelengths, a quasi-supercontinuous spectrum, which is well-suited for XCT, is generated.
The radiation is focused onto the sample and the reflected radiation is recorded. Interferences due to reflections at structures in different depths result in a modulated spectra that can be used to reconstruct the axial structure of the sample. Experimentally, we achieve an axial resolution of 24 nm.
In the XUV range, focusing with high numerical aperture (NA) is extremely expensive. Therefore, XCT uses low-NA optics, which limits the lateral resolution to the micrometer range. A combination of XCT with coherent diffraction imaging would provide improved lateral resolution. We present first results a proof-of-concept experiment at a synchrotron source.
ELI-ALPS in Hungary, one of the three pillars of the Extreme Light Infrastructure, aims at providing diverse light sources, including energetic attosecond pulses at the highest possible repetition rates. One of the main laser systems for driving plasma and gas-based HHG stages, is a state-of-the-art 1 kHz few-cycle laser called SYLOS. Targeted pulse parameters are an energy of 100 mJ and a duration shorter than two optical cycles (<6 fs), with outstanding energy, phase and pointing stability as well as high spatiotemporal quality.
The first phase of the laser system has already set a new standard in kHz laser system engineering and technology. The performance and reliability of the SYLOS laser have been consistently tested over the course of a six-month trial period. During this time the system was running at least 8 hours a day at full power for more than 5 months. The current output parameters are 5 TW peak power, 45 mJ pulse energy with 9 fs duration and 300 mrad CEP stability, while the spectrum spans over 300 nm around 840 nm central wavelength. The layout follows the general scheme NOPCPA architecture with a passively CEP-stabilized front-end. The pulses are negatively chirped for the amplification process and compressed by a combination of large aperture bulk glass blocks and positively chirped mirrors under vacuum conditions at the output.
During the trial period, the laser system demonstrated outstanding reliability. Daily startup and shutdown procedures take only a few minutes, and the command-control system enables pulse parameters to be modified instantly. Controlling the delays of individual NOPCPA stages makes it possible to tailor the output spectrum of the pulses and tune the central wavelength between 770 nm and 940 nm. We performed several experimental tests to find out the pulse characteristics. Pulse duration was verified with Wizzler, chirp-scan, autocorrelation methods and a stereo-ATI independently. All of them confirmed the sub-9 fs pulse duration. We recorded the long-term waveform and pointing stabilities of the beam in order to find out the effect of the temperature load on optical elements. Excluding a short initial warm up time, stable signals were observed in general. The in-loop and out-of-loop CEP stability was cross-checked between f-to-2f and stereo-ATI devices. Moreover, the inherent CEP stability of the system without feedback loop was also found to be surprisingly robust thanks to the passive CEP stabilization of the front-end. The polarization contrast was better than 1000:1. The temporal contrast was also measured independently with Sequoia and Tundra cross-correlators, and on the ns scale with a fast photodiode and GHz oscilloscope as well. Results showed that the pulse pedestal generally consists of parametric superfluorescence below the 1E-7 level and about 100 ps long, well in accordance with the pump duration. Delaying the pump pulse allows us to shift the seed pulse to the front and reach a pre-pulse pedestal below 1E-11 at 30 ps before the pulse peak. Detailed findings on all the examined pulse characteristics of the SYLOS laser will be reported in this presentation.
In this work we report experimental measurement of an intentionally introduced pulse front tilt on femtosecond laser pulses by using an inverted field correlator/interferometer. The results obtained with a low-dispersion diffraction grating are in good qualitative agreement with the data from a previously developed analytical model and with these from an independent interferometric measurement.
Amrutha Gopal, Torsten May, Pushkar Singh, Sven Herzer, Wolfgang Ziegler, Gerhard Paulus, Albrecht Schmidt, Andreas Reinhard, Ulrich Dillner, Hans-Georg Meyer, Anupam Karmakar, Dirk Broemmel, Paul Gibbon
Here we report a laser plasma-driven source of T-rays with the highest pulse energy ever recorded in a laboratory. T-rays are emitted from the rear surface of a solid target in the non-collinear direction at incident laser intensities ~ 1019 W/cm2. Pulse energy measurements reported T-ray pulses with peak energies no less than 700 μJ. Temporal measurements using a single-shot electro-optic method showed the presence of sub-picosecond T-ray pulses with 570 fs duration, thus rendering the peak-power of the source higher even than that of state-of-the-art synchrotrons. A conversion efficiency of higher than 10−3 and an average power of 7 mW makes it the most efficient compact and powerful THz source known today. Spectral analysis revealed the presences of frequencies ranging from 0.1 − 133 THz, while most of the energy is localised in the low frequency region. The dependence of T-ray yield on incident laser energy is linear and shows no signs of saturation. The spatial distribution of the recorded T-rays indicates that most of the T-rays are emitted in the non-collinear direction from the rear-surface of a solid target and the contribution in the forward direction is very small. 2D particle-in-cell simulations show the presence of transient current at the target rear surface.
In this work we study the evolution and interaction of semi-infinite dark beams carrying edge-screw phase dislocations in self-focusing and self-defocusing local Kerr nonlinear media aiming to find appropriate conditions to control the process of fusion/crossing the dark beams in a way suitable for probe-beam cross-switching. We show that a quasi-infinite vortex dipole (dipole much longer than the background beam) evolves into a one-dimensional dark spatial soliton with vanishing transverse velocity. Single semi-infinite fractional dipole develops snake instability near the dark beam end. Depending on their phase profiles, four parallel semi-infinite fractional vortex dipoles aligned to initially form two dark stripes can evolve into two different ‘cross-connects’ able to branch and route probe optical beams. Perpendicular probe beam propagation in the optically-induced guiding structures is modeled and analyzed with respect to the branching efficiency to respective virtual output channels for both self-focusing and self-defocusing conditions.
Spatial characterization of high harmonics (HH) and XUV coherent radiation is of paramount importance, along
with its temporal characterization. For many applications it will be necessary to accurately measure the beam
properties, just as it is important to know the beam characteristics for many laser experiments. For example,
high harmonics and attosecond pulses are being proposed as a front-end for the next generation X-ray free
electron lasers. This oscillator-amplifier-like arrangement will require well characterized high harmonic sources.
On the other hand, the electromagnetic radiation carries the combined signature of underlying quantum physical
processes at the molecular level and of the cooperative phase matching. For example, accurate reconstruction of
the high harmonic spatial wavefront, along with its temporal profile, gives us a complete range of tools to apply
to the fundamental quantum properties and dynamics associated with high harmonic generation. We present
a new concept of frequency resolved wavefront characterization that is particularly suitable for characterizing
XUV radiation. In keeping with tradition in the area we give it an acronym - SWORD (Spectral Wavefront
Optical Reconstruction by Diffraction). Our approach is based on an analysis of the diffraction pattern of a slit
situated in front of a flat-field spectrometer. As the slit is scanned, the spectrally resolved diffraction pattern
is recorded. Analyzing the measured diffractogram, we can reconstruct the wavefront. The technique can be
easily extended beyond the XUV spectral region. When combined with temporal characterization techniques all
information about the beam can be measured.
We study theoretically the non-phase-matched degenerate four-wave mixing of type ωs = 2ω1 ωω2 , involving beams
carrying two-dimensional spatial phase dislocations in the form of singly-charged optical vortices (OVs).
Accompanying third-order nonlinear processes in the non-resonant nonlinear medium (NLM), which are accounted
for, are self- and cross-phase modulation. In the case of pump OV beams with identical topological charges the
model predicts the generation of signal beams carrying OVs of the same charge. If the pump beams carry OVs with
opposite charges, the generated signals are predicted to carry triply charged vortices which, in the case of a nonnegligible
initial free-space propagation from the plane of vortex generation to the NLM, decay inside the NLM into
three singly-charged vortices with highly overlapping cores.
In this work we study the evolution of dark beams of finite length carrying edge-screw phase dislocations in selffocusing
Kerr nonlinear media aiming to find appropriate conditions to control the process of filamentation of the
background beam. In the case of a single fractional vortex dipole, geometry-controlled conditions for changing the
intensity ratio of the peaks and their offset are found. Depending on their orientation, two parallel or two in-line mixed
phase dislocations carried by a common background beam are predicted to perturb it and to initiate filamentation of
different number of peaks with different spatial distributions.
We experimentally generate optical vortices in the output beam of a Ti:sapphire laser emitting 20-fs pulses. Screw phase dislocations are imposed in each spectral component of the short pulses by aligning computer generated hologram within a 4f-setup. The analysis shows that phase dislocations can be generated also at the stage of femtosecond pluse compression.
We experimentally generate optical vortices in the output beam of Ti:Sapphire laser emitting 20-fs pulses. Screw phase dislocations are imposed in each spectral component of the short pulses by aligning computer-generated hologram within a 4f-setup.
We present a 100 kHz femtosecond amplifier system delivering pulses with a duration of 35 fs and an energy of 7 μJ at 800 nm. The system does not include a stretcher, since the large amount of dispersion accumulated during the amplification process is sufficient to prevent self-focusing. Compensation in all orders is achieved through a combination of a special prism compressor, chirped mirrors, and a liquid-crystal modulator.
Square and hexagonal lattices of optical vortices are generated in a saturable nonlinear medium. If the topological charges of the vortices are of the same sign the lattice exhibit rotation, while if alternative, stable propagation of the structures is observed. In a nonlinear medium vortex lattices induce periodic modulation of the refractive index. Diffraction of a probe beam by this optically-induced phase grating is observed.
Multiple-charged optical vortex solitons (OVSs) are generated in a thermal nonlinear medium with saturation. The respective soliton constants are found to be linearly proportional to the topological charges. The linear stability analysis and the numerical simulations indicate a rich variety of instability scenaria depending on the type of perturbation. The saturation of the nonlinearity is shown to be able to slow down the decay of multiple charged dark beams at an intermediate evolution stage and to prevent their ultimate decay into charge-one OVSs. This concept is experimentally verified by the experimental observation of a partial decay of a triple- charged optical vortex beam.
Various types of dark optical beams with respect to their phase dislocation are discussed and their propagation dynamics is analyzed. On the basis of numerical results 1D and 2D optical couplers are proposed. The respective energy efficiencies for each of the output channels are estimated. Multiple charged optical vortices and their topological instability are presented. Through a proper type of perturbation applied to these vortices they could be split into several sub-beams, thus giving the opportunity of proposing all-optical branching schemes. Two output channel distribution schemes for such an optical brancher are discussed.
Theoretical and experimental results on optical ring dark solitary waves are presented, emphasizing the interplay between initial dark beam contrast, phase-shift magnitude, background-beam intensity and saturation of the nonlinearity. An enhanced RDSW transverse dynamics originating in the interaction with a second coaxial dark beam is analyzed theoretically in a good agreement with the experiment. The results may open the way to construct a parallel all-optical three-position radial switch.
The tilt of the pulse front caused by misalignment in stretcher-compressor devices which are used in chirped pulse amplification should be carefully considered in the design of femtosecond laser systems. We present a convenient procedure for online measurement and minimization of the tilt in a grating stretcher/compressor setup. In addition, we present a theoretical model for the autocorrelation signal in the presence of pulse front distortion. The influence of the pulse front tilt to the autocorrelation function is numerically simulated and compared with the case for pulses with fourth order chirp.
In this work we analyze the nonlinear evolution of mixed edge-screw phase dislocations and provide arguments on the existence of 1D dark spatial solitary waves of finite length in bulk Kerr nonlinear media. The characteristic phase gradients force the dark beam to steer in space. An all- optical switching scheme is proposed and critically evaluated with respect to stability and deflection control. Experimental results are reported on the decay of quasi-2D dark spatial solitons into finite-length 1D dark spatial solitons. The role of the saturation of the nonlinearity is discussed.
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