Coherent Radiation Research Articles

Spatial coherence of synchrotron radiation degraded by grating monochromators

Fourth-generation synchrotron sources promise an enormous increase in the spatial coherence of X-ray radiation. In the EUV to soft X-ray range, the spatial coherence could reach almost 100% in both the horizontal and vertical directions. Identifying and understanding potential sources of degradation in the spatial coherence of X-rays transported along the beamline is critical to enable optimal performance for the experiments at the beamlines. Grating monochromators are an essential optical component of most EUV and soft X-ray beamlines. Recently, we have found that the spatial coherence is strongly degraded by the gratings used in these monochromators. In this work, we present a detailed physical and theoretical description of the origin and underlying effects that cause this degradation and describe the influence of the grating parameters and the exit slit of the monochromator. The theoretical analysis is presented in the framework of statistical optics. It is important to note that the described effects in the paper are distinct from the decoherence effects based on optics vibrations and the resulting virtual source broadening or wavefront degradation caused by surface irregularities and optical roughness.

Low temporal radiation coherence of noise-like pulses

Abstract It is quite well-known that coherence is an intrinsic property of laser radiation. However, after the advent of noise-like pulses, it turns out that the temporal coherence of their radiation is low and the electromagnetic field oscillations in adjacent noise-like pulses are not correlated. This work experimentally studies for the first time the degree of coherence of electromagnetic field oscillations inside a noise-like pulse. We measure the visibility of interference fringes at the exit from a Michelson interferometer when its optical path difference is scanned around zero up to and exceeding the length of the noise-like pulse. The experimental data indicate close to zero temporal coherence of radiation within noise-like pulses.

Peniotron: A Promising Microwave Source with Potential That Has Yet to Be Realized

The peniotron is a fast-wave vacuum tube that can generate coherent microwave radiation in the millimeter-wave range. Although it uses a beam of gyrating electrons like other gyro-devices (gyrotron, gyro-TWT, gyro-BWO, etc.), its operating principle is completely different from that of electron cyclotron masers. The theory predicts a very high efficiency (about 95%) of the peniotron mechanism of interaction and energy transfer from the electron beam to the wave. However, this extremely attractive and advantageous property of peniotrons has not yet been realized in practice. In this paper, we present the current state of research on this class of devices and give an overview of the theory and experimental results of peniotrons and gyro-peniotrons with different configurations. We also discuss the main problems and the reasons for the lower efficiency and finally evaluate the potential for solving the problems and revitalizing the work on this promising device.

Efficient generation of multi-order Raman radiation in aqueous solutions derived from -CH2 and -NH2 vibrations via cascaded four-wave mixing and Stokes processes.

Multiple coherent radiations are achieved in a water-3-aminopropanol (3AP) mixed solution through cascaded four-wave mixing (C-FWM) and cascaded Stokes (C-Stokes) processes, both driven by stimulated Raman scattering (SRS) in this work. The O-H vibration peak from water is replaced by the emergence of the -NH2 symmetric stretching Raman peaks from 3AP, with intensity approaching that of the -CH2 symmetric stretching peak. The dual-wavelength SRS signals for the -NH2 and -CH2 stretching vibrations have a relatively small frequency interval of about 400 cm-1 (16 nm). By varying the 3AP concentration and pump energy, these two peaks from 3AP act as new pump sources, enabling the successful generation of up to 16th-order Stokes and 5th-order anti-Stokes radiation through C-FWM and C-Stokes processes. The resulting spectrum spans a broad wavenumber range from -3318 to 6629 cm-1 (452 to 822 nm), offering a new approach for broadband coherent light sources and multi-order Raman spectra in liquid media.

Simulation of the impact of an additional corrugated structure impedance on the bursting dynamics in an electron storage ring

In the case of a single-digit picosecond bunch length, synchrotron light sources produce intense coherent radiation up to the THz range. The reduction of the bunch length by lowering the momentum compaction factor (low-α) gives rise to the microbunching instability, which is on the one hand a crucial roadblock in the x-ray range during to the resulting effective bunch lengthening, but on the other hand also an opportunity for the generation of intense THz radiation if it can be controlled appropriately. In the KIT storage ring KARA (Karlsruhe Research Accelerator), two parallel plates with periodic rectangular corrugations are planned to be installed as a proof of principle in an electron storage ring. These plates create an additional longitudinal impedance based on their geometry, which can affect the beam dynamics. The resulting impedance manipulation will be used to study and control the longitudinal electron beam dynamics and the emitted coherent synchrotron radiation. This paper presents the results of systematic studies in simulation of the impact of additional corrugated plate impedances on the longitudinal beam dynamics using the example of the KARA storage ring. If the periodicity of the wake function of the corrugated plates matches the size of the substructures in the longitudinal bunch profile, the instability threshold can be effectively manipulated. This extends intense THz radiation to different beam current regimes. Published by the American Physical Society 2024

Hybrid dark-field and attenuation contrast retrieval for laboratory-based X-ray tomography

X-ray dark-field imaging highlights sample structures through contrast generated by sub-resolution features within the inspected volume. Quantifying dark-field signals generally involves multiple exposures for phase retrieval, separating contributions from scattering, refraction, and attenuation. Here, we introduce an approach for non-interferometric X-ray dark-field imaging that presents a single-parameter representation of the sample. This fuses attenuation and dark-field signals, enabling the reconstruction of a unified three-dimensional volume. Notably, our method can obtain dark-field contrast from a single exposure and employs conventional back projection algorithms for reconstruction. Our approach is based on the assumption of a macroscopically homogeneous material, which we validate through experiments on phantoms and on biological tissue samples. The methodology is implemented on a laboratory-based, rotating anode X-ray tube system without the need for coherent radiation or a high-resolution detector. Utilizing this system with streamlined data acquisition enables expedited scanning while maximizing dose efficiency. These attributes are crucial in time- and dose-sensitive medical imaging applications and unlock the ability of dark-field contrast with high-throughput lab-based tomography. We believe that the proposed approach can be extended across X-ray dark-field imaging implementations beyond tomography, spanning fast radiography, directional dark-field imaging, and compatibility with pulsed X-ray sources.

Experimental Demonstration of Mode-Coupled and High-Brightness Self-Amplified Spontaneous Emission in an X-Ray Free-Electron Laser.

X-ray free-electron lasers (FELs) are modern research tools with applications in multiple scientific fields. Standard x-ray FEL pulses are produced by the self-amplified spontaneous emission (SASE) mechanism. SASE-FEL pulses have high power, short duration, and excellent transverse coherence but exhibit poor temporal coherence with power and spectral profiles consisting of multiple randomly distributed spikes. Here, we present the demonstration of two modes that enhance the temporal coherence of SASE-FEL radiation: mode-coupled and high-brightness SASE. Both schemes are based on delaying the electron beam with magnetic chicanes placed between the undulator modules of the FEL facility. First, we show the generation of frequency combs with tunable peak separation via the mode-coupled SASE scheme. Second, we present a proof-of-principle demonstration of the high-brightness SASE mechanism, producing FEL pulses with a bandwidth reduction up to a factor of 3 with respect to standard SASE pulses. The demonstration was done at Athos, the soft x-ray beamline of SwissFEL, for photon energies between 500 and 600eV. Our work will benefit current applications and may open up new research areas requiring frequency combs or narrow bandwidths.

Poynting flux transport channels formed in polar cap regions of neutron star magnetospheres

Context. Pair cascades in polar cap regions of neutron stars are considered to be an essential process in various models of coherent radio emissions of pulsars. The cascades produce pair plasma bunch discharges in quasi-periodic spark events. The cascade properties, and therefore also the coherent radiation, depend strongly on the magnetospheric plasma properties and vary significantly across and along the polar cap. Importantly, where the radio emission emanates from in the polar cap region is still uncertain. Aims. We investigate the generation of electromagnetic waves by pair cascades and their propagation in the polar cap for three representative inclination angles of a magnetic dipole, 0°, 45°, and 90°. Methods. We use two-dimensional particle-in-cell simulations that include quantum-electrodynamic pair cascades in a charge-limited flow from the star surface. Results. We find that the discharge properties are strongly dependent on the magnetospheric current profile in the polar cap and that transport channels for high intensity Poynting flux are formed along magnetic field lines where the magnetospheric currents approach zero and where the plasma cannot carry the magnetospheric currents. There, the parallel Poynting flux component is efficiently transported away from the star and may eventually escape the magnetosphere as coherent radio waves. The Poynting flux decreases with increasing distance from the star in regions of high magnetospheric currents. Conclusions. Our model shows that no process of energy conversion from particles to waves is necessary for the coherent radio wave emission. Moreover, the pulsar radio beam does not have a cone structure; rather, the radiation generated by the oscillating electric gap fields directly escapes along open magnetic field lines in which no pair creation occurs.

Two-Dimensional MXene Flakes with Large Second Harmonic Generation and Unique Surface Responses.

MXenes are a family of two-dimensional (2D) materials with broad and varied applications in biology, materials science, photonics, and environmental remediation owing to their layered structure and high surface area-to-volume ratio. MXenes have exhibited significant nonlinear optical characteristics, which have been primarily explored in the context of photonics applications, yet the second-harmonic generation (SHG) behavior of MXenes remains an unexplored aspect of their optical properties. Herein, we demonstrate and quantify large second-order responses of 2D Ti3C2Tx MXenes both in aqueous solutions and on a silicon substrate for the first time. MXene flakes showed strong second-harmonic scattering (SHS) in a dilute suspension with a sensitivity of less than 0.1 μg/mL. Angle-dependent SHS experiments further found that the second-order responses originate from coherent 2D dipole radiation. Through confocal and atomic force microscopies, we found that the intense SHG signal from free-standing MXene flakes increases exponentially with decreasing thickness, while two-photon fluorescence increases linearly with thickness. The second-order susceptibility of the MXenes was determined to be 3.6 pm V-1 with a thickness of 10 nm, almost twice of that for an often-used SHG crystal, beta barium borate. We further explored surface properties of the MXene sheets by investigating the SHS responses upon addition of organic dye molecules to the system. It was found that the adsorption of crystal violet (CV) obeys a Langmuir adsorption model while the addition of malachite green (MG) resulted in almost no change in SHG intensity, even though the adsorption capacities for both CV (61.3 ± 1.7 mg/g) and MG (54.8 ± 2.8 mg/g) are similar. Such a stark difference in adsorption characteristics between cationic organic CV and MG dyes is likely due to their distinct orientational orderings on the MXene surfaces. This work opens many possibilities for the further employment of the family of 2D materials in photonics, optics, and surface catalysis applications.

Coherent high-harmonic generation with laser-plasma beams

Active energy compression scheme is presently being investigated for future laser-plasma accelerators. This method enables generating laser-plasma accelerator electron beams with a small, ∼10−5, relative slice energy spread. When modulated by a laser pulse, such beams can produce coherent radiation at very high, ∼100th harmonics of the modulation laser wavelength, which are hard to access by conventional techniques. The scheme has a potential of providing additional capabilities for future plasma-based facilities by generating stable, tunable, narrow-band radiation. Published by the American Physical Society 2024

Coherent synchrotron radiation instability in low-emittance electron storage rings

High-frequency longitudinal impedances, particularly at wavenumbers k≫1/σz (where σz is the bunch length), can drive microwave instability and potentially limit the performance of modern low-emittance electron storage rings. In such rings, the coherent synchrotron radiation (CSR) emerges as a prominent contributor to these high-frequency impedances. This paper undertakes a systematic investigation into the effects of CSR on electron rings, utilizing the fourth-generation storage ring light source Elettra 2.0 and the low-energy ring of the SuperKEKB e+e− circular collider as illustrative examples. Our work revisits theories of microwave instability driven by CSR impedance, extending the analysis to encompass other high-frequency impedances, such as resistive wall and coherent wiggler radiation. Through instability analysis and numerical simulations conducted on the two aforementioned rings, the study explored the impact of high-frequency impedances and their combined effects with broadband impedances caused by discontinuities in vacuum chambers. Published by the American Physical Society 2024

Instantaneous and Retarded Interactions in Coherent Radiation.

In coherent radiation of an ensemble of electrons, the radiation field from electrons resonantly drives the other electrons inside to produce stimulated emission. The radiation reaction force on the electrons accounting for this stimulated radiation loss is classically described by the Liénard-Wiechert potential. Despite its being the foundation of beam physics for decades, we show that using the "acceleration field" in Liénard-Wiechert potential to describe radiative interactions leads to divergences due to its implicit dependence on instantaneous interactions. Here, we propose an alternative theory for electromagnetic radiation by decomposing the interactions into an instantaneous part and retarded part. It is shown that only the retarded part contributes to the irreversible radiation loss and the instantaneous part describes the space charge related effects. We further apply this theory to study the coherent synchrotron radiation energy loss, which hopefully will reshape our understanding of coherent radiation and collective interactions.

Sub-THz and THz Cherenkov radiation source with two-dimensional periodic surface lattice and multistage depressed collector

We present the theory, concept and design of an efficient, megawatt coherent Cherenkov radiation source based on a two-dimensional periodic surface lattice (2D-PSL) cavity combined with a novel energy recovery system for the generation of highly efficient (> 50%) single-frequency radiation. We demonstrate the scalability of the transverse dimension of the 2D-PSL cavity of the Cherenkov source and thus the potential for efficient, continuous-wave, high-power (> 1 MW) operation; fundamental to the eventual realization of clean, fusion energy. These new sources, with the capacity to operate in the 0.1-10THz range, hold strong promise to address the long-standing “Terahertz gap”. By combining a Cherenkov oscillator driven by a non-gyrating beam with an innovative four-stage depressed collector energy recovery system, the overall device efficiency can be increased to be competitive with gyrotrons in the requirements for heating and current drive in fusion plasma. In these Cherenkov devices, the frequency independence of the magnetic guide field enables advantageous frequency scaling without deployment constraints, making them especially attractive for high-impact applications in fusion science, turbulence diagnostics, non-destructive testing and biochemical spectroscopy. The novel energy recovery techniques presented in this paper have broad applicability to many electron-beam driven devices, bringing revolutionary potential to future THz source technologies.

Prototype optical enhancement cavity for steady-state microbunching.

The innovative mechanism of steady-state microbunching (SSMB) promises a potent light source, featuring high repetition rate and coherent radiation. The laser modulator, comprising an undulator and an optical enhancement cavity, is pivotal in SSMB. A high-finesse prototype optical enhancement cavity for SSMB with an average power of 55 kW is described in this paper. Preliminary design of the laser modulator, experimental setup, and methods to address frequency degeneracy and power coupling issues are discussed. D-shaped mirrors are utilized to successfully suppress the modal instability. This study is the first to illustrate the finesse reduction caused by high-order mode damping during experiments. The experimental and simulation results match closely. A cavity power coupling model is established, and the experimental results verify the correctness of the coupling model. A method for estimating the absorption coefficient through thermal-induced evolution of cavity mode has been implemented. Experimental results demonstrate a high-average-power enhancement cavity with a finesse of 16 518 ± 103 and an estimated average absorption coefficient of 12 ppm for the cavity mirrors. The findings contribute to the advancement of SSMB by providing insights into the design and operation of high-power optical enhancement cavities.

On the Flux Density Spectral Property of High Linearly Polarized Signal from Pulsar J0332+5434

The polarization position angles (PPA) of time samples with high linear polarization often show two parallel tracks across the pulsar profile that follow the rotating vector model (RVM). This feature supports coherent curvature radiation (CCR) as the underlying mechanism of radio emission from pulsars, where the parallel tracks of the PPA represent the orthogonal extraordinary (X) and ordinary (O) eigenmodes of strongly magnetized pair plasma. However, the frequency evolution of these high linearly polarized signals remains unexplored. In this work, we explore the flux density spectral nature of high linearly polarized signals by studying the emission from PSR J0332+5434 over a frequency range between 300 and 750 MHz, using the Giant Metrewave Radio Telescope. The pulsar average profile comprises a central core and a pair of conal components. We find the high linearly polarized time samples to be broadband in nature, and in many cases, they resemble a narrow spiky feature in the conal regions. These spiky features are localized within a narrow pulse longitude over the entire frequency range, and their spectral shapes sometimes resemble an inverted parabolic shape. In all such cases, the PPA is exclusively along one of the orthogonal RVM tracks, likely corresponding to the X-mode. The inverted spectral shape can, in principle, be explained if the high linearly polarized emission in these time samples is formed due to the incoherent addition of CCR from a large number of charged solitons (charge bunches) exciting the X-mode.

Fine and rough structure of the frequency spectrum of high-power laser diodes during slow degradation

It is shown that a fine and rough structure can be distinguished in the radiation spectrum of a powerful laser diode. The relationship between the spectrum characteristics and the internal parameters of the laser structure was established and experimentally verified, which was observed during the degradation of the device. The effect of losses in the resonator and coherence of laser radiation on both the fine and rough structure of its radiation spectrum is shown.

Dose increment in ultrashort particle irradiations via coherent stopping power

We present a relativistic self-consistent theory of the coherent stopping power (CSP) of ultradense charged particle beams propagating in a dense matter. CSP corresponds to a collective inelastic collision which adds to the ordinary stopping power of individual particles. Unlike the latter, which depends only on the particles' energy, CSP depends upon many more parameters such as the total charge of the ensemble and its charge density and shape. This paves the way for a broad variety of novel methods to tailor particle absorption and penetration in a dense matter. CSP losses can be explained both by collective excitations of single or multiple molecules and by the emission of coherent Cherenkov radiation. Here we demonstrate that the former mechanism is more relevant than the latter. We find that the coherent energy absorption of subpicosecond particle bunches in water occurs exciting a broad Debye process in the GHz range and an intermolecular stretching vibration mode in the THz region. We generalize the Bethe-Bloch stopping power formula to coherent effects including self-forces, dielectric screening, and absorption by the dense matter. For the sake of a self-consistent dynamical theory including phase space evolution, we have also generalized the Fermi-Eyges theory of particle diffusion to the presence of forces. Nonlinear dynamics is demonstrated, inducing nonlinear dose release, beam self-focusing, and self-enhancement of coherent losses. Given the advent of ultraintense particle sources and their use for biomedical and other relevant applications, our results may be of paramount importance for contemporary and future developments of science and technology. Published by the American Physical Society 2024

High-fidelity transfer of helical phase via hyper-Raman scattering in a monolayer graphene

Graphene is a fascinating two-dimensional optical material with a unique electronic band structure and a giant optical nonlinearity under the action of a strong magnetic field. Here we propose a scheme to transfer helical phase of the orbital angular momentum (OAM) light via hyper-Raman scattering in a monolayer graphene assisted by a magnetic field. Due to the unique electronic properties and selection rules near the Dirac point, it is found that high-fidelity transfer of phase structure and, in particular, of OAM from a pump field to the Raman field generated in a hyper-Raman scattering process occurs under the resonance condition. Interestingly, the obtained results allow us to control the intensity of Raman field with stable phase structure by varying the intensities of two pump fields. Hereby, the mechanism of efficient generation and manipulation of OAM Raman field may have exciting potential applications in the generation of short-wavelength coherent radiation, conversion of frequency as well as nonlinear spectroscopy in graphene materials.

K3Y3(BO3)4: A Potential UV Nonlinear-Optical Crystal Designed by a Chemical Substitution Strategy.

Nonlinear-optical (NLO) crystals capable of controlling and manipulating light to generate coherent radiation at challenging wavelengths are of significant interest. However, designing a new UV NLO crystal remains difficult due to the rigid requirements for structure and properties. Herein, we have successfully designed and synthesized a novel noncentrosymmetric (NCS) rare-earth borate UV NLO crystal, K3Y3(BO3)4, through the heterovalence substitution of YAl3(BO3)4. K3Y3(BO3)4 (KYBO) crystallizes in the NCS and polar space group of P63mc, with the structure formed by the interconnectioned BO3 triangles and YO8 polyhedra through corner-sharing and edge-sharing. The property measurements indicate that KYBO is second-harmonic-generation-active with a moderate response, ∼2 × KDP. Meanwhile, KYBO can exhibit a short UV cutoff edge (λcutoff < 190 nm), indicating its potential as a new UV or deep-UV NLO crystal.

Transverse echo effect induced multiple harmonic generation scheme in an electron storage ring

Multiple harmonic generation is a key issue for coherence radiation generated by an electron beam modulated by the seeding laser. A new multiple harmonic generation scheme by combining the angular dispersion-induced microbunching (ADM) with the transverse echo effect, which can be easily generated by using pulsed dipoles, a pulsed quadrupole and a pulsed multipole, is investigated in a multi-bend achromatic storage ring. Simulation results demonstrate a triple of harmonic number compared to the case of pure ADM, making it possible to generate coherent extreme ultraviolet radiation in storage rings.