Short-wavelength Radiation Sources Research Articles

Advanced Studies for the Dynamics of High Brightness Electron Beams with the Code MILES

High brightness electron beams enable a wide spectrum of applications ranging from short wavelength radiation sources to high gradient wakefield acceleration. The rich dynamics that are intrinsic in charged particles accelerated in complex systems require a careful description in the analysis and design of a given machine, particularly regarding its stability. Numerous computer codes are in use by the accelerator community for such purposes. In particular, MILES is a simple tracking code we have developed that allows fast evaluations of collective effects in RF linacs. In this paper we extend the simple models previously developed to describe specific, diverse applications that can benefit from the fast simulation tools developed in MILES. Examples of this kind include particle driven acceleration schemes in a plasma where driver and witness beams propagate in the “comb” pulse-train configuration. Specifically, we investigate the self-induced fields excited within the X-band rf-linac stage of EuPRAXIA@SPARC_LAB. Further, we discuss additional advanced topics such as resistive wall wakefield effects in planar FEL undulators and their impact on the radiation emitted.

High-resolution imaging enabled by 100-kW-peak-power parametric source at 5.7 THz

Similar to x-ray imaging, THz imaging will require high power and high resolution to advance relevant applications. Previously demonstrated THz imaging usually experiences one or several difficulties in insufficient source power, poor spectral tunability, or limited resolution from a low-wavelength source. A short-wavelength radiation source in the 5–10 THz is relatively scarce. Although a shorter wavelength improves imaging resolution, widely used imaging sensors, such as microbolometers, Schottky diodes, and photoconductive antennas, are usually not sensitive to detect radiation with frequencies above 5 THz. The radiation power of a high-frequency source becomes a key factor to realize low-noise and high-resolution imaging by using an ordinary pyroelectric detector. Here, we report a successful development of a fully coherent, tunable, > 100-kW-peak-power parametric source at 5.7 THz. It is then used together with a low-cost pyroelectric detector for demonstrating high-resolution 5.7-THz imaging in comparison with 2-THz imaging. To take advantage of the wavelength tunability of the source, we also report spectrally resolved imaging between 5.55 and 5.87 THz to reveal the spectroscopic characteristics and spatial distribution of a test drug, Aprovel.

InSn plasma penetration through protective single-walled carbon nanotube-based membranes

Laser-produced plasma sources of short-wavelength (1–20-nm) radiation are actively used nowadays in numerous applications, including water-window microscopy and extreme ultra-violet lithography. Suppression of laser-plasma debris (responsible for damaging optics) is crucial for the lifetime prolongation of optical systems operated with the short-wavelength radiation. Here, we examine the capability of single-walled carbon nanotube (SWCNT)-based membranes to decrease an InSn plasma flux containing both ions and atoms. Faraday cup measurements show that 40- and 90-nm-thick SWCNT membranes reduce the total charge transition by 20 and 130 times, respectively. The ion analyzer measurements demonstrate that ions pass through the membrane mainly due to the collisionless (ballistic) mechanism. Using scanning electron microscopy, we estimate a decrease in a plasma (ions + atoms) flux to be of 18 and 140 times for 40- and 90-nm-thick SWCNT-based membranes, respectively. The average plasma flux attenuation coefficient of SWCNT membranes is calculated as k = 0.063 nm−1.

Increasing the brightness of harmonic XUV radiation with spatially-tailored driver beams

Bright high harmonic sources can be produced by loosely focussing high peak power laser pulses to exploit the quadratic scaling of flux with driver spot size at the expense of a larger experimental footprint. Here, we present a method for increasing the brightness of a harmonic source (while maintaining a compact experimental geometry) by spatially shaping the transverse focal intensity distribution of a driving laser from a Gaussian to supergaussian. Using a phase-only spatial light modulator we increase the size and order of the supergaussian focal profiles, thereby increasing the number of harmonic emitters more efficiently than possible with Gaussian beams. This provides the benefits of a loose focussing geometry, yielding a five-fold increase in harmonic brightness, whilst maintaining a constant experimental footprint. This technique can readily be applied to existing high harmonic systems, opening new opportunities for applications requiring bright, compact sources of coherent short wavelength radiation.

Modelling a laser plasma accelerator driven free electron laser

Free-electron lasers (FEL) are the brightest, coherent sources of short wavelength radiation from the VUV into the x-ray. There is much research interest in reducing the cost and the size of FELs by utilising new accelerator techniques. Laser-plasma accelerator (LPA) are a promising accelerator for next generation compact FEL light sources with many potential advantages due to the high acceleration gradient and large peak currents they offer. The electron beams of a LPA typically have a smaller transverse emittance, a large energy spread and tend to be of shorter duration and higher current than conventional Radio Frequency (RF) accelerators. In this paper, a FEL driven by an electron beam from a typical LPA was simulated using the 3D FEL simulation code Puffin. It is shown that lowering the homogenous electron beam energy spread increases the radiation energy output in a short undulator and , as become less than the FEL, or Pierce parameter (), then the peak radiation energy increases and the saturation length reduces significantly as expected.

Quasi-phase-matched high-harmonic generation in gas-filled hollow-core photonic crystal fiber

High-order harmonic generation (HHG) provides a promising tabletop source of coherent short wavelength radiation. However, the low generation efficiency limits the mean photon flux of HHG sources when driven with low average power, kHz repetition rate lasers. The HHG flux could be increased by employing MHz repetition rate driving lasers. However, the low pulse energy necessitates tight focusing, which limits the effective interaction volume. Generating harmonics in a gas-filled photonic crystal fiber (PCF) mitigates this problem, but for both free focus and PCF targets, the HHG conversion efficiency is optimized at technologically challenging multibar gas pressures. Here, we perform HHG with μJ-level driving pulses in a hollow-core PCF. We observe dramatic 60-fold enhancement of the HHG flux through a time-multiplexed multimodal quasi-phase-matching technique at low gas pressures. We also observe high harmonic photon energies up to 61.6 eV with continuous spectral tunability made possible by controlled ionization-induced blue shifting of the driving laser.

Plasma polarization grating for circularly polarized high-order harmonic generation

Polarization grating (PG) is a kind of optical element based on the Pancharatnam-Berry phase, providing a unique capability of manipulating the polarization properties of output beams [Bomzon et al., Opt. Lett. 27, 1141–1143 (2002) and Oh and Escuti, Opt. Lett. 33, 2287–2289 (2008)]. Here, we extend this concept into the nonlinear domain of high-order harmonic generation from the laser-solid interaction. Through theoretical analysis and numerical simulations, we proposed a scheme of two non-collinear relativistic lasers of opposite handedness interacting with a solid target to generate angularly isolated, circularly polarized harmonics of both left and right handedness simultaneously. The physical mechanism was described as the confluence of PG and relativistically oscillating mirror, which can be defined as a new mechanism: “relativistically oscillating polarization grating mechanism.” This work provides a useful method for developing bright, near-monochromatic, short-wavelength radiation sources with desirable polarization for a broad range of applications.

Emerging photon technologies for chemical dynamics.

I discuss the recent developments concerning emerging ultrafast sources of short wavelength radiation, as well as the new methods they generate. I then dwell on a few examples of recent results and the way the new sources will bring major advancements in our understanding of fundamental chemical dynamics.

Modelling single shot damage thresholds of multilayer optics for high-intensity short-wavelength radiation sources

The single shot damage thresholds of multilayer optics for high-intensity short-wavelength radiation sources are theoretically investigated, using a model developed on the basis of experimental data obtained at the FLASH and LCLS free electron lasers. We compare the radiation hardness of commonly used multilayer optics and propose new material combinations selected for a high damage threshold. Our study demonstrates that the damage thresholds of multilayer optics can vary over a large range of incidence fluences and can be as high as several hundreds of mJ/cm(2). This strongly suggests that multilayer mirrors are serious candidates for damage resistant optics. Especially, multilayer optics based on Li(2)O spacers are very promising for use in current and future short-wavelength radiation sources.

Imaging of nanostructures with sub-100 nm spatial resolution using a desktop EUV microscope

Laser-produced plasma sources of short-wavelength radiation offer an interesting alternative to synchrotron and free-electron laser installations. Recently, we reported on a newly developed desktop EUV microscope based on plasma generated from a gas-puff target and diffractive optics. The half-pitch resolution of the microscope approached 50 nm. Compared to analogous microscopes based on synchrotron sources, our system is compact and cost-effective. In this paper, we present the results of imaging experiments on a thin polycrystalline object that was carried out in order to further examine the applicability of the microscope. We have demonstrated here that EUV microscopy can provide structural information that cannot be accessed by conventional optical microscopy or SEM.

Properties of zinc oxide at low and moderate temperatures

The properties of zinc oxide are examined as an analog of gallium nitride over a wide range of temperatures and possible applications. Its economic and environmental advantages are noted, as well as its radiation hardness, compared to group III nitrides. Methods for growing films and nanostructures with high crystal perfection are proposed. In particular, a magnetron technique for layer-by-layer growth of films is implemented which makes it possible to obtain high structural perfection and substantial thicknesses unattainable by several other methods. The feasibility of producing monochromatic UV radiation from films excited by short-wavelength radiation and electrons is demonstrated; this means that they may be useable as short-wavelength radiation sources. Efficient field emission by ZnO nanostructures and films is demonstrated and opens up the prospect of their use in vacuum microelectronics equipment. Nitrogen-doped ZnO films, in particular, have been used to fabricate a phototransistor with a sensitivity two orders of magnitude higher than conventional detectors. The physical basis for creating LEDs for different colors based on ZnO films and solid solutions with CdO is discussed. The importance of studying the physics and technology of zinc oxide-based devices is emphasized.

Few femtosecond, few kiloampere electron bunch produced by a laser–plasma accelerator

Particle accelerators driven by the interaction of ultraintense and ultrashort laser pulses with a plasma(1) can generate accelerating electric fields of several hundred gigavolts per metre and deliver high-quality electron beams with low energy spread(2-5), low emittance(6) and up to 1 GeV peak energy(7,8). Moreover, it is expected they may soon be able to produce bursts of electrons shorter than those produced by conventional particle accelerators, down to femtosecond durations and less. Here we present wide-band spectral measurements of coherent transition radiation which we use for temporal characterization. Our analysis shows that the electron beam, produced using controlled optical injection(9), contains a temporal feature that can be identified as a 15 pC, 1.4-1.8 fs electron bunch (root mean square) leading to a peak current of 3-4 kA depending on the bunch shape. We anticipate that these results will have a strong impact on emerging applications such as short-pulse and short-wavelength radiation sources(10,11), and will benefit the realization of laboratory-scale free-electron lasers(12-14).

Relativistic laser plasmas for electron acceleration and short wavelength radiation generation

We consider here a few options to use relativistic laser plasmas for novel sources of short wavelength radiation. Electrons accelerated in underdense plasmas in the bubble regime wiggle in an ion channel. This leads to broadband incoherent synchrotron-like radiation bursts which are of femtosecond duration. The photon energies are in the kiloelectronvolt to megaelectronvolt energy range. However, this radiation is not coherent. To reach coherency, the electron bunch must have structure at the wavelength of the emitted x-rays. This can be achieved in principle by sending the laser-accelerated electron bunch through an external wiggler. However, to reach free electron lasing in the x-ray regime, the energy spread of the laser-accelerated electrons must be reduced dramatically. Another option is to use high harmonic generation at overdense plasma boundaries. The high harmonics are emitted in coherent sub-femtosecond flashes. The harmonic spectra decay as a power of the harmonic number, with an exponent that can be as low as −6/5. This can make the high harmonics potentially the brightest laser-driven short wavelength sources with unique properties.

Upper estimate of the conversion coefficient in a laser-plasma short-wavelength radiation source for nanolithography

Laser-generated plasma is considered as a source of extreme ultraviolet (EUV) radiation for nanolithography. The upper estimate of the coefficient of laser radiation conversion into EUV radiation in this source is obtained using the approximation of plasma emitting like a blackbody. Prospects for increasing the conversion efficiency in real plasmas used for this purpose are considered.

Use of short-wavelength synchrotron radiation to measure luminescence quantum yield and the luminescence excitation spectrum of polymers

We describe an optical diagnostics module and the instrumental and methodological features of ultrahigh vac- uum experiments investigating the optical characteristics of condensed media in the short-wavelength (hν ~ 3.5-25 eV) range of the spectrum of probing synchrotron radiation. We give a brief presentation of the re- sults of an experimental determination of the spectral dependence of the luminescence quantum yield and the luminescence excitation spectrum of ablatable polymer dielectrics on the Kurchatov synchrotron radiation source at values of the probing radiation power density (I0 ~ 10 12 photons/cm 2 ⋅sec) that are below threshold for extended surface vaporization and a surface temperature of the condensed targets equal to 77-300 K. Introduction. In photonic power systems with high power density (optical accelerators and plasmatrons, laser plasma injectors, thermionic plasma optical converters, etc.) (1-3), plasma active media, including optical ablation media, at high temperatures (Te ~ 1-5 eV), are a source of broadband short-wavelength radiation with maximum in- tensities in the UV (UVC) and vacuum UV (VUV) regions of the spectrum. The temperature of the structural ele- ments of such power systems in the cycle for different pulsed periodic conditions may vary from cryogenic to phase transition temperatures. Therefore to calculate the heat and mass exchange characteristics, here we need data on the spectral reflection and absorption coefficients, the luminescence quantum yield for the structural materials in the UV- VUV regions of the spectrum over a broad temperature range, taking into account the technologically attainable level of surface treatment. As we know, measurement of the optical characteristics of condensed media in the vacuum UV region of the spectrum presents significant experimental complications, first of all due to the lack of sufficiently bright (at least 1 μW/mrad 2 ⋅nm) and broadband radiation sources in this region of the spectrum and secondly due to the need to make precision measurements under vacuum. Spectroscopy studies of condensed media in the vacuum UV region of the spectrum are conducted mainly using gas-discharge sources with a line emission spectrum, which limits the experimen- tal options in this spectral range both with respect to the number of available frequencies and with respect to the spectral brightness of the probing sources (2, 3). Using synchrotron radiation helps solve this problem. The high spec- tral density, the continuous (from the IR to the x-ray region) emission spectrum, the sharp directionality, and the high degree of polarization, the possibility for accurate determination of the spectral and energy characteristics of the source of probing radiation enable efficient use of synchrotron emitters as a working tool in optics and metrology with the maximum degree of monochromatization of short-wavelength radiation for existing vacuum UV monochromators. The goal of this work was to analyze the instrumental and methodological features of study under ultrahigh vacuum of luminescence excitation spectra and luminescence quantum yield of polymers over a broad range of photon energies for UV-VUV radiation and over a broad temperature range.

LIMITS ON PRODUCTION OF NARROW BAND PHOTONS FROM INVERSE COMPTON SCATTERING

In using the inverse Compton scattering (ICS) interaction as a high brilliance, short wavelength radiation source, one collides two beams, one an intense laser, and the other a high charge, short pulse electron beam. In order to maximize the flux of photons from ICS, one must focus both beams strongly, which implies both use of short beams and the existence of large angles in the interaction. One aspect of brilliance is the narrowness of the wavelength band emitted by the source. This paper explores the limits of ICS-based source brilliance based on inherent wavelength broadening effects that arise due to focal angles, laser energy density, and finite laser pulse length effects. It is shown that for a nominal 1% desired bandwidth, that one obtains approximately one scattered photon per electron in a head-on collision geometry.

Source of far and vacuum ultraviolet radiation based on a transverse high-frequency discharge

We present the results of an investigation of a short-wavelength radiation source (Δλ = 130–350 nm) with excitation by a transverse high-frequency (f = 1.76 MHz) discharge based on a mixture of argon and chlorine (p = 100–500 Pa). We have studied the spectral characteristics of the plasma, the oscilloscope traces of the voltage, the current and emission of the discharge, the dependence of the power of the emission on the electrical power of the discharge, and also the pressure and partial composition of the Ar/Cl2 mixture. The UV-VUV source emits in a system of broadened and overlapping ArCl(B/X), Cl2(D′/A′), and Cl**2 molecular bands.

Multifrequency third-harmonic generation by red-shifting solitons in a multimode photonic-crystal fiber

While the standard scenario of third-harmonic generation (THG) by a dispersive-wave pump involves the emission of light with a frequency 3omega, thrice the frequency omega of the input pump field, solitons undergoing a continuous shift of their central frequency omega due to the Raman effect in a multimode optical fiber can generate the third harmonic in a different fashion. In the experiments reported here, we provide the first direct experimental evidence of THG by a continuously red-shifting soliton pump by studying the third-harmonic buildup in relation to the spectral evolution of the soliton pump field in a silica photonic-crystal fiber (PCF). We show that solitons excited in a PCF by unamplified femtosecond pulses of a Cr:forsterite laser sweep through the spectral range from 1.25 to 1.63 microm , scanning through a manifold of THG phase-matching resonances with 3omega dispersive waves in PCF modes. As a result, intense third-harmonic peaks build up in the range of wavelengths from 370 to 550 nm at the output of the fiber, making PCF a convenient fiber-format multifrequency source of short-wavelength radiation. Time-resolved fluorescence measurements with photoexcitation provided by the third-harmonic PCF output are presented, demonstrating the high potential of PCF sources for an ultrafast photoexcitation of fluorescent molecular systems in physics, chemistry, and biology.

Signatures of direct double ionization under xuv radiation

In anticipation of upcoming two-photon double ionization of atoms and particularly Helium, under strong short wavelength radiation sources (45 eV), we present quantitative signatures of direct two-photon double ejection, in the photoelectron spectrum (PES) and the peak power dependence, that can be employed in the interpretation of related data. We show that the PES provides the cleanest signature of the process. An inflection (knee) in the laser power dependence of double ionization is also discernible, within a window of intensities which depends on the pulse duration and cross sections

Gamma-ray transitions induced in nuclear spin isomers by X-rays

Because of the high density of energy storage and the large cross section for its release there is considerable significance of the triggering of induced gamma emission from nuclear spin isomers to efforts to develop intense sources of short-wavelength radiation. The work reported here describes the current experimental focus and results recently obtained.