Water Window Region Research Articles

Modeling generation of harmonics in the water window region in hollow core waveguides by mid-infrared femtosecond pulses

We numerically investigate generation of harmonics in the water window region (down to 2.8 nm) by 2 μm femtosecond pulses propagating in hollow core waveguides filled with high pressure He. Numerical calculations are based on a three dimensional macroscopic model, which solves the pulse propagation by a split-step method, uses the strong field approximation to evaluate the single atom response, and integrates it coherently to obtain the harmonic field. Two configurations for the waveguides are considered: the standard one with a constant diameter of 70 μm and a conical one with a decreasing diameter from 70 to 50 μm. We demonstrate that harmonic field enhancement can be obtained in spectral domains of great practical interest, from 2.8 to 20 nm, and identify quasi-phase matching induced by multimode beating as the mechanism responsible for this enhancement.

Numerical experiment of phase-matched ion-based high-order harmonic generation in the water-window x-ray spectral region via electromagnetic envelope equation.

We present a phase-matching scheme for efficient high-order harmonic generation in the water-window x-ray spectral region using a 405-nm driving pulse. A high-intensity pulse (∼1016 W/cm2) is used to produce He1+ ions as the target medium, increasing the cutoff photon energy to the water-window x-ray spectral region. By adjusting the driving pulse divergence, the positive dipole phase variation balances the negative plasma dispersion and geometrical phase shift, achieving phase matching. Using the electromagnetic envelope equation coupled with the Keldysh ionization model, numerical experiments identify the optimal conditions. Results show that the relative conversion efficiencies of the 95th harmonic (4.26 nm) and the 169th harmonic (2.4 nm) reach 66% and 79% of the perfect phase-matching conditions, respectively.

Interference-induced generation of a chirp-free short isolated attosecond pulse in the water window region with multicolor laser fields.

Compensating for the intrinsic attosecond chirp (atto-chirp) of wideband high-order harmonics in the water window region is a significant challenge, in order to obtain isolated attosecond pulses (IAPs) with a width of tens of attoseconds (as). Here, we propose to realize the generation of IAP with duration as short as 20 as, central energy of 365 eV, and bandwidth exceeding 150 eV from chirp-free high harmonics generated by a four-color driving laser, without the necessity for atto-chirp compensation with natural materials. Unlike any other gating methods that an IAP arises from only one electron ionization event, we take advantage of the interference between harmonic radiation produced by multiple ionizing events. We further demonstrate that such chirp-free short IAP survives after taking account of macroscopic propagation effects. Given that the synthesized multicolor laser field can also effectively increase the harmonic flux, this work provides a practical way for experiments to generate the broad bandwidth chirp-free IAPs in the water window region.

Generating high repetition rate X-ray attosecond pulses in a diffraction limited storage ring

To steer and track electron motion in atoms, molecules, and nanostructures, light pulses with attosecond duration and high repetition rate are required. In this paper, we use the angular dispersion-induced microbunching scheme and a few-cycle laser within a straight section (a few meters) of a diffraction-limited storage ring to generate a coherent high-flux attosecond pulse in the water window region. Simulation results based on the Southern Advanced Photon Source indicate that the proposed method can generate a chirp-free Fourier transform limited pulse with a minimum duration of 50 as, a maximum repetition rate of a few MHz, and a maximum average flux of about 4.4times 10^{11} photons/s/1%Bw.

Generation of Highly Charged Au Ion in Laser-Produced Plasma for Water Window X-ray Radiation Sources

Highly charged ions in the plasma produced by high-power laser can radiate bright and short-pulse X-rays. Owing to the unresolved transition array (UTA) from the high-Z elements, laser produced plasma has been applied for developing X-ray sources. In particular, X-rays in the water-window (WW) region (2.3–4.4 nm) is utilized as the light source of the X-ray microscopy to observe living organisms under high contrast and resolution. In this work, WW X-rays radiated from a laser (1064 nm, 6.2 ns) produced Au-plasma has been studied. UTA spectrum in the WW range has been observed through a grazing incident spectrometer (GIS). Dependence of Au-ion charge state distribution on laser intensity has been experimentally investigated and evaluated by a transition probability data calculated by the flexible atomic code. The integrated soft X-ray emission has been observed through a pinhole camera with a 1.0-μm Ti-filter, combined with a 2-D plasma radiation scanning achieved by the GIS. An intense WW emission region 200-μm away from the target surface has been observed, which indicates a more effective area is possible to be utilized for a practical use.

Design of broadband reflective quarter-wave plates in the water window region based on aperiodic Cr/Sc multilayer mirrors

The generation of a water window attosecond pulse with circular polarization is important for ultrafast research such as X-ray magnetic circular dichroism spectroscopy. In this paper, we propose a design method of broadband reflective quarter-wave plates in the water window region based on aperiodic Cr/Sc multilayer mirrors. Three broadband reflective quarter-wave plates are designed for achieving high circular polarization and circular reflection in the water window region with 10, 15, and 20 eV bandwidths. The results indicate that the reflected lights of the designed aperiodic Cr/Sc multilayer mirrors all show a more than 95% circular polarization degree and considerable circular reflection in their design band. The proposed broadband reflective quarter-wave plates can be used for controlling the polarization state of broadband water window region sources and generating the circularly polarized attosecond pulse by the linearly polarized water window region sources.

Generation of superintense isolated attosecond pulses from trapped electrons in metal surfaces

Generation of ultrabroadband isolated attosecond pulses (IAPs) is essential for time-resolved applications in chemical and material sciences, as they have the potential to access the spectral water window region of chemical elements, which has yet to be established. Here we propose a numerical scheme for highly efficient high-order harmonic generation and hence the generation of ultrabroadband IAPs in the XUV and soft-x-ray regions. The scheme combines the use of chirped pulses with trapped electrons in copper transition-metal surfaces and takes advantage of the characteristic features of an infrared (IR) single-cycle pulse to achieve high conversion efficiencies and large spectral bandwidths. In particular, we show that ultrabroad IAPs with a duration of 370 as and with a bandwidth covering the photon energy range of 50--250 and 350--450 eV can be produced. We further show that introducing an additional IR single-cycle pulse permits us to enhance the harmonic yield in the soft-x-ray photon energy region by almost seven orders of magnitude. Our findings thus elucidate the relevance of trapped electrons in metal surfaces for developing stable and highly efficient attosecond light sources in compact solid-state devices.

Design of a broadband reflection mirror for the water window region using a stacked multilayer with optimized structure

A broadband reflection mirror that can realize maximum uniform reflectivity is an important optical element in the water window region. In this paper, a stacked multilayer (SM) mirror with optimized structure for achieving broadband reflection in the water window region is presented. A genetic algorithm with a two-parametric merit function is employed to achieve maximum uniform reflection in the desired spectral band. A 4.90% average reflectivity with 0.25% root-mean-square deviation in the range of 370–390 eV is achieved by a Cr/Sc SM with 40 periodic multilayer structures. Compared with aperiodic multilayers, proposed SMs exhibit better optimization efficiency, design robustness, and thickness error tolerance. This research provides an efficient method for designing broadband reflection mirrors in the water window region, and may be helpful for steering broadband water window sources, such as high order harmonics and attosecond pulse.

Controlling water-window high-harmonic generation with sub-cycle synthesized waveforms

We present the first results concerning synthesizer-driven high-harmonic generation that reach the water-window region. This approach holds the promise of offering greater spectral tunability in the generation of isolated attosecond pulses and at the same time of achieving higher photon-flux, required for attosecondresolved soft X-ray transient absorption experiments.

Time and space waveform optimization to extend the harmonic cutoff and to produce the water window single attosecond pulse

Abstract Based on the three-step theory of high-order harmonic generation, the harmonic cutoff is very sensitive to the few-cycle laser waveform in both time and space regions. Therefore, in this paper, we propose the method to control the harmonic cutoff and to produce the water window attosecond pulse through the optimization of time and space waveform. It is found that, in the time region, by properly choosing the delay and phase of the few-cycle two-color pulse, not only the harmonic intensity is enhanced, but also the quantum path of the harmonic emission can be controlled. Further, with the introduction of the 3rd pulse (i.e., the infrared pulse or the unipolar pulse), the harmonic cutoff from the single harmonic emission peak can be extended, showing a water window harmonic plateau. In the space region, by using the positive spatial inhomogeneous effect, the harmonic cutoff from the basic two-color waveform can also be extended, which leads to a water window spectral continuum. Finally, by Fourier transformation of harmonics during the water window region, the ultrashort single 29 as pulses can be obtained.

Near-field imaging of dipole emission modulated by an optical grating

Attosecond measurements have been achieved in technically demanding pump-probe experiments by photoelectron streaking with stable infrared lasers and extreme-ultraviolet (XUV) instruments. Here, we demonstrate an efficient single-image all-optical measurement of an isolated attosecond pulse for its complete temporal characterization. We create the attosecond pulse with a 0.1-mJ, few-cycle, infrared pump beam and modulate it with an obliquely incident same-frequency weak beam. By refocusing the XUV beams, we obtain a spectrally resolved XUV image, showing the spectral phase of the attosecond pulse. Near-field imaging allows us to measure our pulse in 150 shots. This efficiency will be important for attosecond pulses in the water-window region. For complex systems, multi-electron dynamics is encoded in the temporal structure of attosecond pulses.

Soft X‐ray Detectors Based on SnS Nanosheets for the Water Window Region

AbstractThe structural characteristics of biological specimens, such as wet proteins and fixed living cells, can be conveniently probed in their host aqueous media using soft X‐rays in the water window region (200–600 eV). Conventional X‐ray detectors in this area exhibit low spatial resolution, have limited sensitivity, and require complex fabrication procedures. Here, many of these limitations are overcome by introducing a direct soft X‐ray detector based on ultrathin tin mono‐sulfide (SnS) nanosheets. The distinguishing characteristic of SnS is its high photon absorption efficiency in the soft X‐ray region. This factor enables the fabricated soft X‐ray detectors to exhibit excellent sensitivity values on the order of at peak energies of ≈600 eV. The peak signal is found to be sensitive to the number of stacked SnS layers, with thicker SnS nanosheet assemblies yielding a peak response at higher energies and with peak sensitives of over 2.5 at 1 V. Detailed current–voltage and temporal characteristics of these detectors are also presented. These results showcase the excellent performance of SnS nanosheet‐based soft X‐ray detectors compared to existing direct soft X‐ray detectors, including that of the emerging organic–inorganic perovskite class of materials.

Laboratory-size x-ray microscope using Wolter mirror optics and an electron-impact x-ray source.

We developed a laboratory-size three-dimensional water-window x-ray microscope using condenser and objective grazing incidence Wolter type I mirrors, an electron-impact-type x-ray source, and a back-illuminated CCD. The imaging system was improved for practical applications in life science research fields. Using a new objective mirror with reduced figure errors, a resolution limit of 3.1 line pairs/μm was achieved for two-dimensional transmission images and sub-micrometer-scale three-dimensional structures were resolved. Incorporating a cryogenic stage into the x-ray microscope, we observed biological samples embedded in ice to evaluate the usefulness of observation in the water-window region and multi-energy observation was demonstrated using an x-ray source with multiple x-ray tubes.

X-Ray Crystalline Undulator Radiation in Water Window

The problem of radiation produced by the bunch of relativistic positrons, channeled into the crystalline undulator, is considered, taking into account the polarization of a crystal medium. If a bunch energy is above the threshold energy, then the radiation is generated in the limited frequency range, due to the medium polarization. Both soft and hard boundary photons are emitted at a zero angle. When the ratio of the bunch energy to the threshold energy approaches unity from above, the peak of the bunch radiation spectrum is near the resonant frequency and can fall into the soft X-ray frequency range due a certain choice of the parameters of the crystalline undulator. In that case, it is necessary to use a low energy relativistic bunch. Method is effective because a fairly dense beam of soft X-ray photons is generated. It is shown the possibility of obtaining monochromatic, intense, directed photon beams in the water window region, important for medical and biological applications.

Tuning the electronic band structure of metal surfaces for enhancing high-order harmonic generation

High-harmonic generation (HHG) from the condensed matter phase holds promise to promote future cutting-edge research in the emerging field of attosecond nanoscopy. The key for the progress of the field relies on the capability of the existing schemes to enhance the harmonic yield and to push the photon energy cutoff to the extreme-ultraviolet (XUV, 10-100eV) regime and beyond toward the spectral "water window" region (282-533eV). Here, we demonstrate a coherent control scheme of HHG, which we show to give rise to quantum modulations in the XUV region. These modulations are shown to be caused by quantum-path interferences and are found to exhibit a strong sensitivity to the delocalized character of bulk states of the material. The control scheme is based on exploring surface states in transition-metal surfaces and, specifically, tuning the electronic structure of the metal surface itself together with the use of optimal chirped pulses. Moreover, we show that the use of such pulses having moderate intensities permits us to push the harmonic cutoff further to the spectral water window region and that the extension is found to be robust against the change in the intrinsic properties of the material. The scenario is numerically implemented using a minimal model by solving the time-dependent Schrödinger equation for the metal surface Cu(111) initially prepared in the surface state. Our findings elucidate the importance of metal surfaces for generating coherent isolated attosecond XUV and soft-x-ray pulses and for designing compact solid-state HHG devices.

Apparatus for generation of nanojoule-class water-window high-order harmonics.

In our recent study [Fu et al., Commun. Phys. 3(1), 92 (2020)], we have developed an approach for energy-scaling of high-order harmonic generation in the water-window region under a neutral-medium condition. More specifically, we obtained a nanojoule-class water-window soft x-ray harmonic beam under a phase-matching condition. It has been achieved by combining a newly developed terawatt-class mid-infrared femtosecond laser and a loose-focusing geometry for high-order harmonic generation. The generated beam is more than 100 times intense compared to previously reported results. The experimental setup included two key parts: a terawatt mid-infrared femtosecond driving laser [Fu et al., Sci. Rep. 8(1), 7692 (2018)] and a specially designed gas cell. Despite the dramatic drop in the optimal gas pressure for phase-matching due to loose-focusing geometry, it still reached the 1bar level for helium. Thus, we have designed a double-structured pulsed-gas cell with a differential pumping system, which enabled providing sufficiently high gas pressure. Moreover, it allowed reducing gas consumption significantly. A robust energy-scalable apparatus for high-order harmonic generation developed in this study will enable the generation of over ten-nanojoule water-window attosecond pulses in the near future.

Efficient soft x-ray high-order harmonic generation via dual-pulse driving lasers in the overdriven regime

We theoretically investigate the high-order harmonic generation (HHG) driven by dual-pulse lasers. The first pulse of 800 nm can create plasma in the high-density gas target. Then, the second pulse of 1800 nm is spatiotemporally reshaped by the plasma and generates soft x-ray high-order harmonics in the overdriven regime. The simulation results show that adding the first pulse can promote the phase matching of HHG in the cutoff region. As a result, a cutoff-extended HHG is achieved, which is one order of magnitude more intense than that in one pulse scheme reported previously. In dual-pulse driving lasers, the harmonic yield and the cutoff photon energy can be optimized effectively by increasing the gas pressure inside the water window region.

Controlling three-step harmonic emission for intense attosecond pulses using water window harmonic spectra

ABSTRACT Based on the three-step model of high harmonic generation, we propose a scheme to improve the harmonic spectra and to produce the intense water window attosecond pulses. It is found that, firstly, by introducing the single chirp and two chirps modulations, the instantaneous frequency on the falling part of the laser field is decreased. As a result, the electron can obtain larger energy during the acceleration-recombination process, showing a broad harmonic plateau in the water window region. Secondly, by properly adding a third controlling IR or UV pulse, the ionization process can be controlled and a larger ionization probability can be found, which leads to the enhancement of the harmonic intensity by 1 or 2 orders of magnitudes, respectively. Most important, using this scheme, the harmonic plateau is only contributed by the intense single harmonic emission peak, which can support the generation of an intense 36 as pulse.

Spectroscopic studies of laser produced Bi-Pb alloy plasma

Generating soft X-rays in the water-window (WW) region is of great interest as a light source for cellular microscopy. The spectroscopic studies of laser-driven high-Z target plasma are important for parameters optimization in laser produced plasma (LPP) for efficient soft X-ray sources. A novel high-Z alloy target with very low melting point composed of five elements (Bi, Pb, In, Cd and Sn) was designed and used in this study. Two-dimensional (2D) spectral images were recorded and analyzed between 330 and 510 nm from LPP generated using Nd:YAG laser with 1064 nm wavelength in vacuum. The 2D spectral images showed that the Pb II and Bi II species had a higher velocity than neutral atoms. The emission lines were relatively dense in the short wavelength region of 330–380 nm and most came from Bi and Pb neutral species, which have higher atomic numbers among the five elements in the alloy target. The experimentally measured spectra were in good agreement with the theoretical results. The temporal and the spatial evolution of excitation temperature and electron density were investigated using Boltzmann plot method and Stark broadening of the Pb I spectral lines. The space resolved excitation temperature at 400 ns showed approximately similar values within the error bars regardless of laser energies. The electron density at higher laser energy was higher than that at low-energy laser (especially before 300 ns), but the difference among the three laser energies used was not significant at the later times. The assumption of Local Thermodynamic Equilibrium (LTE) was verified based on the three required criteria and the optically thin for the LPP was justified using the self-absorption analysis.

Sub-waveform optimization for producing water window single-order harmonic

Through the sub-waveform optimization of the laser field, a potential method to produce the water window single-order harmonic (SOH) has been proposed. First, by properly introducing the chirps of two-color field, the SOHs from 303th order to 616th order can be obtained. Theoretical analyses show that the folding region on the harmonic emission process, caused by the multiple accelerations, is responsible for the enhanced SOH. Moreover, the folding region is dependent on the neighbor two half-cycle profiles. Thus, through further controlling the sub-waveform of the folding region by using the unipolar pulse, the folding region on the harmonic emission process will be extended to the higher photon energy region, including the water window region. Finally, by properly choosing the combinations of chirps and unipolar pulses, the water window SOH from 446th order to 833th order (from 345 eV to 645 eV) can be obtained.