The study of strong field-driven nonlinear responses from high harmonic generation (HHG) in two dimension (2D) materials has been recognized as a valuable tool for probing ultrafast electron dynamics in the condensed matter systems. The laser polarization dependence of HHG in solids provides significant scientific insights into the crystal and band structure of the generation medium. In this work we experimentally investigate the impact of HHG in MoS2 monolayer by changing the laser polarization from linear to circular for different orientations relative to the symmetry axes of the crystal. The polarization analysis of harmonic emission reveals ellipticity and helicity dependent enhancement of even order harmonics which is strongly asymmetric near the symmetry directions of the crystal.
Bound states in the continuum (BICs) represent dark modes trapped in the radiation continuum. BICs received significant attention in optics and photonics as a simple tool to achieve giant quality factors by transforming them into quasi-BICs. Here, we report the observation of high-harmonic generation in dielectric metasurfaces hosting BICs. The metasurface is composed of a square lattice with parallel Si bars of a slightly different width placed on a transparent substrate. The structure is engineered to support a quasi-BIC in the mid-IR with a high quality factor. We tune the metasurface asymmetry to enable the optimal coupling condition that provide the highest high-harmonic generation efficiency. In the experiment, we demonstrate the generation of odd optical harmonics from the 3rd to the 11th order in the BIC regime and study their polarization dependence. We measure the dependence of the high-harmonic signal on the input intensity. The concept of metasurfaces with highly localized light boosted by BIC resonances provides a new degree of freedom to control experimentally strong nonlinear optical response.
X-ray microscopy has proven its advantages for resolving nanoscale objects. High Harmonic Generation (HHG) sources allow performing nanoimaging experiments at the lab scale and their femtosecond pulse duration and synchrony to an optical laser renders them useful for studying dynamic processes. HHG sources regularly provide high average photon flux but relatively low single-shot flux limiting time-resolved applications to adiabatic processes. Here, we show that soft X-ray lasers (SXRL) in turn provide high flux due to an X-ray lasing transition, but the coherence of an SXRL operating in the amplified-spontaneous-emission scheme is limited. The coherence properties of an SXRL seeded by an HHG source can be significantly improved allowing single-shot nanoscale imaging. In combination with ptychography, source properties are measured with high fidelity. This is applied to study the plasma dynamics of SXRL amplification in unprecedented quality.
Imaging of biological specimen is one of the most important tools to investigate structures and functionalities in organic components. Improving the resolution of images into the nanometer range call for short wavelengths light sources and large aperture optics. Subsequently, the use of extreme ultraviolet light in the range of 2 nm to 5 nm provides high contrast and high resolution imaging, if it is combined with lensless imaging techniques. We describe important parameters for high resolution lensless imaging of biological samples and specify the required light source properties. To overcome radiation based damage of biological specimen, we discuss the concept of ghost imaging and describe a possible setup towards biological imaging in the extreme ultraviolet range.
We evaluated the capabilities of an intense ultrafast high-harmonic seeded soft X-ray laser at 32.8 nm wavelength regarding single-shot lensless imaging and ptychography. Additionally the wave front at the exit of the laser plasma amplifier is monitored in amplitude and phase using high resolution ptychography and backpropagation techniques.Characterizing the laser plasma amplifier performance depending on the arrival time of the seed pulse with respect to pump pulses provides insight into the light plasma interaction in the soft X-ray range.
In cancer treatment, it is highly desirable to classify single cancer cells in real time. The standard method is polymerase chain reaction requiring a substantial amount of resources and time. Here, we present an innovative approach for rapidly classifying different cell types: we measure the diffraction pattern of a single cell illuminated with coherent extreme ultraviolet (XUV) laser-generated radiation. These patterns allow distinguishing different breast cancer cell types in a subsequent step. Moreover, the morphology of the object can be retrieved from the diffraction pattern with submicron resolution. In a proof-of-principle experiment, we prepared single MCF7 and SKBR3 breast cancer cells on gold-coated silica slides. The output of a laser-driven XUV light source is focused onto a single unstained and unlabeled cancer cell. With the resulting diffraction pattern, we could clearly identify the different cell types. With an improved setup, it will not only be feasible to classify circulating tumor cells with a high throughput, but also to identify smaller objects such as bacteria or even viruses.
In cancer treatment it is highly desirable to identify and /or classify individual cancer cells in real time. Nowadays, the
standard method is PCR which is costly and time-consuming. Here we present a different approach to rapidly classify
cell types: we measure the pattern of coherently diffracted extreme ultraviolet radiation (XUV radiation at 38nm
wavelength), allowing to distinguish different single breast cancer cell types. The output of our laser driven XUV light
source is focused onto a single unstained and unlabeled cancer cell, and the resulting diffraction pattern is measured in
reflection geometry. As we will further show, the outer shape of the object can be retrieved from the diffraction pattern
with sub-micron resolution. For classification it is often not necessary to retrieve the image, it is only necessary to
compare the diffraction patterns which can be regarded as a spatial fingerprint of the specimen. For a proof-of-principle
experiment MCF7 and SKBR3 breast cancer cells were pipetted on gold-coated silica slides. From illuminating each
single cell and measuring a diffraction pattern we could distinguish between them. Owing to the short bursts of coherent
soft x-ray light, one could also image temporal changes of the specimen, i.e. studying changes upon drug application
once the desired specimen is found by the classification method. Using a more powerful laser, even classifying
circulating tumor cells (CTC) at a high throughput seems possible. This lab-sized equipment will allow fast classification
of any kind of cells, bacteria or even viruses in the near future.
Light beams carrying an isolated point singularity with a screw-type phase distribution are called an optical vortex (OV). The fact that in free space the Poynting vector of the beam gives the momentum flow leads to an orbital angular momentum (OAM) of the photons in such a singular beam, independent on the spin angular momentun1. There are many applications of optical OAM shown in literature that would benefit from the availability of optical vortex beams in all spectral regions. For example it was shown that transitions forbidden by selection rules in dipole approximation appear allowed when using photons with the additional degree of freedom of optical OAM2. However, the common techniques of producing new light frequencies by nonlinear optical processes seem problematic in conserving the optical vortex when the nonlinearity becomes large. We show that with the extremely nonlinear process of High Harmonic Generation (HHG) it is possible to transfer OVs from the near-infrared to the extreme ultraviolet (XUV)3 at wavelengths down to ~30 nm. The observed XUV light was examined spatially and spectrally. The spatial profile showed the expected singular behavior, a dark region in the center. A comparison of the far-field fringe pattern caused by a thin wire with corresponding simulations suggests that the XUV vortex beam carries a unit topological charge. A screw-like phase evolution around the profile was also verified by employing a Hartmann type measurement. The generated spectrum revealed that in all Harmonic orders an OV was present. The profile, however, looked the same in all orders, indicating identical topological charge, which runs counterintuitive to the assumption that the phase of exp(–ilφ) is multiplied by the harmonic order in a frequency up-conversion experiment.
The dependence of the yield of high-order harmonic generation (HHG) on several important experimental parameters
has been successfully modeled in the last 20 years by taking into account the single atom response and propagation
effects. We extended this description by adding a stimulated emission process and named it x-ray parametric
amplification (XPA). Beyond the super-quadratic increase of the XUV signal, which can be explained only in a limited
pressure range by HHG theory, other observed characteristics like exponential growth, gain narrowing, strong blue-shift,
beam divergence, etc. and their dependence on laser intensity and gas pressure can be explained accurately only by the
new XPA model. We experimentally demonstrated XPA in Argon in the spectral range of 40-50 eV in excellent
agreement with the theory. XPA holds the promise to realize a new class of bright x-ray sources for spectroscopy.
Taking advantage of the non-adiabatic blue-shift of high-order harmonics generated by a fixed frequency Nd:Glass laser
system, we are able to report more than 50 % coverage of the XUV spectral range between 18 nm and 35 nm. The
generated harmonic lines are capable of seeding Ni-like Y, Zr and Mo soft x-ray lasers and others.
We report on the realization towards a compact, pulsed XUV source for high temporal and spatial resolution pumpprobe
spectroscopy. The system will be based on intracavity high harmonic generation in a Ti:sapphire oscillator. An
oscillator with repetition rate of 20 MHz has been realized, which operates in the net negative (near zero) dispersion
regime with intracavity pulse energy up to 280 nJ. The cavity has been extended with a secondary focus, where the high
harmonic generation can take place. In the recent state, the oscillator is capable to generate XUV harmonics up to 35 eV.
The PHELIX laser at the GSI Helmholtz center for heavy-ion research is dedicated to provide high energy, ultra-intense laser pulses for experiments in combination with energetic ion beams. Development of x-ray lasers is targeting a number of applications in this context, including x-ray laser spectroscopy of highly-charged ions, and Thomson scattering diagnostics of heavy-ion driven plasmas. Recent developments centered on the application of a novel double-pulse
pumping scheme under GRIP-like, non-normal incidence geometry for both the pre- and the main pulse for transient pumped Ni-like lasers. This scheme considerably simplifies the set-up, and provides a very stable pumping situation even at low pump energies close to the lasing threshold. The technique was scaled to pulse energies above 100 J for the pumping of shorter wavelength x-ray lasers. In addition, a slightly tunable high-harmonic source using a split-off beam from the Nd:Glass pre-amplifier of PHELIX was developed as a seeding source.
Using adaptive temporal pulse shaping of the driving 800 nm laser pulses, we demonstrate for the first time the complete control over the XUV spectrum of high-order harmonics, generated in a gas-filled hollow fiber. We achieve both the enhancement and the suppression of single or several selected harmonic orders. These arbitrarily shaped soft-x-ray spectra will allow for important modifications of the resulting harmonic pulses in the temporal domain. Experiments are in progress to determine the time structure of the shaped harmonics using a cross-correlation technique. This constitutes first steps
towards direct attosecond pulse shaping in the soft-x-ray domain. Moreover, we show that harmonic generation in a hollow-core fiber can be enhanced by coupling into a single fiber mode using a feedback-controlled adaptive two-dimensional spatial light modulator. Temporal and spatial tailoring of harmonics opens the way towards optimal control in the soft-x-ray domain.
Damage experiments of absorbing filters (Schott BG18 and BG36) were performed with Ti:sapphire laser pulses with durations from 30 fs to 340 fs (800 nm, 1 kHz) in air. The direct focusing technique was employed under single- and multi-pulse irradiation conditions. Ablation threshold fluences were determined from a semi-logarithmic plot of the ablation crater diameter vs. laser fluence. The damage threshold fluence decreases for shorter pulse durations. In the investigated pulse duration range, the measured multi-pulse ablation threshold fluences are practically similar to those of undoped glass material (~1 Jcm-2). That means that the multi-pulse ablation threshold is independent on the doping level of the filters. For more than 100 pulses per spot and all pulse durations applied, the threshold fluence saturates. This leads to technically relevant damage threshold values in the femtosecond laser pulse duration domain.
Laser-micromachining of barium aluminum borosilicate glass, fused silica and stainless steel has been extended down to a pulse duration of 20 fs generated by a Ti:sapphire laser system at a wavelength of 0.8 micrometer. A systematic study shows that, below 100 fs, an enhanced precision and a substantial decrease of the ablation threshold fluence in comparison to pulse laser processing with pulses in the picosecond and nanosecond range could be achieved. The physical mechanism and the technical relevance of this novel microtechnology is discussed.
Optical thin film structures exhibiting high reflectivity and nearly constant negative group- delay dispersions, or optionally superimposed higher-order dispersions over frequency ranges beyond 80 THz are presented. This attractive combination makes these special laser mirrors well suited for intra- or extracavity dispersion control in broadband femtosecond laser systems. We address design issues and the principle of operation of these novel devices. Spectrally resolved white-light interferometry with high time and spectral resolution was used to measure the dispersion of the deposited mirrors. Experimental results on the operation of femtosecond laser systems utilizing dispersion compensating mirrors are given.
Phase and amplitude control during multiterawatt, ultrashort-pulse amplification is discussed. Methods for efficient energy extraction and scaling to 100-TW peak powers are outlined.
An overview of the basic principles, major theoretical aspects and technological issues of self- mode-locked solid state lasers is presented, with particular emphasis on recent advances in femtosecond Ti:sapphire lasers.
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