Coherent Radiation Research Articles

Free-electron terahertz radiation based on silicon gratings with in-plane waveguide emission

We present a method utilizing the coupling between a pre-bunched electron beam and a silicon subwavelength grating to generate coherent terahertz waves. The grating that is connected to two opposite-traveling in-plane waveguides functions as a resonator. An example operating around 2 THz shows that, when the velocity and repetition frequency of the electron bunches respectively match the phase velocity and resonant frequency of the Bragg resonance in the grating, the strong electron-wave coupling leads to coherent radiation through the waveguide. The repetition frequency of the electron bunches can be halved by using its second harmonic to match the resonant frequency. This study might offer a potential approach for on-chip terahertz sources.

Generation of coherent THz Cherenkov radiation by electron bunch tilt control

We propose a new method that generates coherent Cherenkov radiation by controlling the tilt of the electron bunch. In this scheme, the wavelength band of coherent radiation depends on the transverse size of the electron bunch. This study demonstrates a coherent Cherenkov radiation (CChR) pulse in the THz region corresponding to the beam size. To generate the CChR, we used a 4.8-MeV electron bunch from a photocathode radio-frequency (rf) gun and controlled the angle through an rf transverse deflecting cavity. Our new method increased the pulse intensity by a factor of 7.3 compared to the case without tilting and the THz pulse characteristics well agreed with the prediction.

All-metal wakefield accelerator: Two-dimensional periodic surface lattices for charged particle acceleration

Interest in ``all-metal'' surface periodic structures to control charged particle beam dynamics and properties, and to generate coherent radiation in the THz frequency range has recently risen exponentially. While such structures have the benefits of metal, they are capable of providing similar properties to a dispersive dielectric. All-metal structures allow good temperature and space charge management. Transportation of high energy charge particle beams in the vicinity of the structure without risking material degradation makes them very attractive for particle acceleration and terahertz radiation generation applications. Here, the results of theoretical studies (via numerical modeling) of the electron beam dynamics and wakefield excitation and evolution along the cylindrical two-dimensional periodic surface structure will be presented, and the properties of the wakefields generated will be discussed. The wakefield acceleration of the witness bunch in the potential reaching $1\text{ }\text{ }\mathrm{GV}/\mathrm{m}$ will be demonstrated. The challenges will be identified and discussed, and the concept of the cascade accelerator based on such all-metal wakefield accelerator (AMWA) structures will be presented.

Reconstructing the Semiconductor Band Structure by Deep Learning

High-order harmonic generation (HHG), the nonlinear upconversion of coherent radiation resulting from the interaction of a strong and short laser pulse with atoms, molecules and solids, represents one of the most prominent examples of laser–matter interaction. In solid HHG, the characteristics of the generated coherent radiation are dominated by the band structure of the material, which configures one of the key properties of semiconductors and dielectrics. Here, we combine an all-optical method and deep learning to reconstruct the band structure of semiconductors. Our method builds up an artificial neural network based on the sensitivity of the HHG spectrum to the carrier-envelope phase (CEP) of a few-cycle pulse. We analyze the accuracy of the band structure reconstruction depending on the predicted parameters and propose a prelearning method to solve the problem of the low accuracy of some parameters. Once the network is trained with the mapping between the CEP-dependent HHG and the band structure, we can directly predict it from experimental HHG spectra. Our scheme provides an innovative way to study the structural properties of new materials.

Frequency stabilisation and SI tracing of mid-infrared quantum-cascade lasers for precision molecular spectroscopy

The advancement of technologies for the precise interrogation of molecules offers exciting possibilities for new studies in the realms of precision spectroscopy and quantum technologies. Experiments in these domains often address molecular vibrations in the mid-infrared (MIR) spectral region, thus generating the need for spectrally pure and accurate MIR laser sources. Quantum-cascade lasers (QCLs) have emerged as flexible sources of coherent radiation available over a wide range of MIR frequencies. Here, we demonstrate a robust approach for the simultaneous linewidth narrowing, frequency stabilisation, and absolute frequency referencing of MIR QCLs all of which are prerequisites for precise spectroscopic experiments. Following upconversion of its radiation to the visible domain, we implement a phase lock of the QCL to a linewidth-narrowed optical frequency comb which is referenced to a remote SI-traceable primary frequency standard via a fibre link for absolute frequency calibration. To achieve a reliable frequency counting of the beat note between the QCL and the OFC, we employ redundant tracking oscillators and demonstrate a frequency instability of at 1 s and at 1000 s integration time, limited by the accuracy of our remote reference.

Lattice design for an improved angular dispersion-induced microbunching scheme

In this paper, an improved angular dispersion-induced microbunching (ADM) scheme [1] is studied to generate fully coherent soft x-ray radiation pulses in an electron storage ring light source with a limited energy modulation amplitude. This scheme releases the coupling between the dispersion function and the momentum compaction in the original ADM scheme. The lattice structure of the improved ADM is simple, which only adds a chicane following the dogleg compared to the original one. With the chicane, even higher harmonic radiation can be generated. Numerical simulation results show that soft x-ray radiation with photon energy covering the water window range can be produced.

Plasma emission induced by ring-distributed energetic electrons in overdense plasmas

According to the standard scenario of plasma emission, escaping radiations are generated by the nonlinear development of the kinetic bump-on-tail instability driven by a single beam of energetic electrons interacting with plasmas. Here, we conduct fully-kinetic electromagnetic particle-in-cell simulations to investigate plasma emission induced by the ring-distributed energetic electrons interacting with overdense plasmas. Efficient excitations of the fundamental (F) and harmonic (H) emissions are revealed with radiation mechanism(s) different from the standard scenario: (1) The primary modes accounting for the radiations are generated through the electron cyclotron maser instability [for the upper-hybrid (UH) and Z modes] and the thermal anisotropic instability [for the whistler (W) mode]; the F emission is generated by the nonlinear coupling of the Z and W modes and the H emission by the nonlinear coupling of the UH modes. This presents an alternative mechanism of coherent radiation in overdense plasmas.

Hints of a universal width–energy relation for classified fast radio bursts

The total available sample of fast radio bursts (FRBs) has been growing steadily in recent years, facilitating the study of FRBs from a statistical point of view. At the same time, the classification of FRBs is currently an imperative issue. We propose that the brightness temperature of bursts can serve as an ideal criterion for classification. In this work, we gather the available data for all localized FRBs and we find a positive relation between the intrinsic pulse width and burst energy, Ti ∝ Eν0.25, for three repeating FRBs that is similar to that of our previous work using FRB 20121102A data alone. The critical line TB, cri is found to vary for different FRBs, which may reflect the differences in source properties. This relation can put strong constraints on mainstream radiation mechanisms. It is evident that neither the coherent curvature radiation or synchrotron maser radiation have the capability to reach the high brightness temperature required to reproduce this relation.

The plasma suppression effect can be ignored in realistic FRB models invoking bunched coherent radio emission

ABSTRACT One widely discussed mechanism to produce highly coherent radio emission of fast radio bursts (FRBs) is coherent emission by bunches, either via curvature radiation or inverse Compton scattering (ICS). It has been suggested that the plasma oscillation effect can significantly suppress coherent emission power by bunches. We examine this criticism in this paper. The suppression factor formalism was derived within the context of radio pulsars in which radio waves are in the low-amplitude, linear regime and cannot directly be applied to the large-amplitude, non-linear regime relevant for FRBs. Even if one applies this linear treatment, plasma suppression is not important for two physical reasons. First, for an efficient radiation mechanism, such as ICS, the required plasma density is not high so that a high-density plasma may not exist. Secondly, both bunched coherent mechanisms demand that a large global parallel electric field (E∥) must exist in the emission region in order to continuously inject energy to the bunches to power an FRB. In order to produce typical FRB duration via coherent curvature or ICS radiation, a parallel electric field must be present to balance the acceleration and radiation back reaction. The plasma suppression factor should be modified with the existence of E∥. We show that the correction factor for curvature radiation, fcur, increases with E∥ and becomes 1 when E∥ reaches the radiation-reaction-limited regime. We conclude that the plasma suppression effect can be ignored for realistic FRB emission models invoking bunched coherent radio emission.

Polarization of magnetospheric curvature radiation in repeating fast radio bursts

ABSTRACT Fast radio bursts (FRBs) can exhibit a wide variety of polarization properties, not only between sources but also from burst to burst for a same one. In this work, we revisit the polarization characters of coherent curvature radiation from a bulk of charged bunches in the magnetosphere of a highly magnetized neutron star. FRBs have been observed to have a variety of polarization features, such as high levels of circular polarization (CP) or a sign change of CP. High linear polarization (LP) would appear when the line of sight is inside the emission beam (the on-beam geometry), whereas high CP would be present when it is outside (the off-beam geometry). By considering two scenarios of the ‘bulk shapes’ (thick versus thin), we apply the model to explain the polarization features of four repeating FRBs (FRB 20121102A, FRB 180916B, FRB 20190520B and FRB 20201124A). Most bursts are dominated by LP and negligible events have sign changes in CP, suggesting that such FRBs are most likely to be emitted by the ‘thin’ bulks with large opening angles. The higher probability of ‘thin’ bulks could be meaningful for understanding repeating FRB central engine, i.e. the sparking dynamics to produce different bulks of energetic bunches on a neutron star surface.

Ln2(SeO3)2(SO4)(H2O)2 (Ln=Sm, Dy, Yb): A Mixed‐Ligand Pathway to New Lanthanide(III) Multifunctional Materials Featuring Nonlinear Optical and Magnetic Anisotropy Properties

AbstractBottom‐up assembly of optically nonlinear and magnetically anisotropic lanthanide materials involving precisely placed spin carriers and optimized metal‐ligand coordination offers a potential route to developing electronic architectures for coherent radiation generation and spin‐based technologies, but the chemical design historically has been extremely hard to achieve. To address this, we developed a worthwhile avenue for creating new noncentrosymmetric chiral Ln3+ materials Ln2(SeO3)2(SO4)(H2O)2 (Ln=Sm, Dy, Yb) by mixed‐ligand design. The materials exhibit phase‐matching nonlinear optical responses, elucidating the feasibility of the heteroanionic strategy. Ln2(SeO3)2(SO4)(H2O)2 displays paramagnetic property with strong magnetic anisotropy facilitated by large spin‐orbit coupling. This study demonstrates a new chemical pathway for creating previously unknown polar chiral magnets with multiple functionalities.

Ln2 (SeO3 )2 (SO4 )(H2 O)2 (Ln=Sm, Dy, Yb): A Mixed-Ligand Pathway to New Lanthanide(III) Multifunctional Materials Featuring Nonlinear Optical and Magnetic Anisotropy Properties.

Bottom-up assembly of optically nonlinear and magnetically anisotropic lanthanide materials involving precisely placed spin carriers and optimized metal-ligand coordination offers a potential route to developing electronic architectures for coherent radiation generation and spin-based technologies, but the chemical design historically has been extremely hard to achieve. To address this, we developed a worthwhile avenue for creating new noncentrosymmetric chiral Ln3+ materials Ln2 (SeO3 )2 (SO4 )(H2 O)2 (Ln=Sm, Dy, Yb) by mixed-ligand design. The materials exhibit phase-matching nonlinear optical responses, elucidating the feasibility of the heteroanionic strategy. Ln2 (SeO3 )2 (SO4 )(H2 O)2 displays paramagnetic property with strong magnetic anisotropy facilitated by large spin-orbit coupling. This study demonstrates a new chemical pathway for creating previously unknown polar chiral magnets with multiple functionalities.

Coherent XUV Multispectral Diffraction Imaging in the Microscale

The rapid growth of nanotechnology has increased the need for fast nanoscale imaging. X-ray free electron laser (XFEL) facilities currently provide such coherent sources of directional and high-brilliance X-ray radiation. These facilities require large financial investments for development, maintenance, and manpower, and thus, only a few exist worldwide. In this article, we present an automated table-top system for XUV coherent diffraction imaging supporting the capabilities for multispectral microscopy at high repetition rates, based on laser high harmonic generation from gases. This prototype system aims towards the development of an industrial table-top system of ultrafast soft X-ray multi-spectral microscopy imaging for nanostructured materials with enormous potential and a broad range of applications in current nanotechnologies. The coherent XUV radiation is generated in a semi-infinite gas cell via the high harmonic generation of the near-infrared femtosecond laser pulses. The XUV spectral selection is performed by specially designed multilayer XUV mirrors that do not affect the XUV phase front and pulse duration.

Locked-energy at first-order Stokes stimulated Raman scattering for an aqueous sulfuric acid solution

We introduced a locked-energy coherent Stokes radiation scheme based on multi-mode stimulated Raman scattering (SRS) of aqueous sulfuric acid solutions. SRS of pure sulfuric acid has only one vibrational characteristic peak. Mixing sulfuric acid with water leads to the appearance of bisulfate, and the energy transfer between adjacent vibrational energy levels in a 17.0 mol/l aqueous sulfuric acid solution leads to generation of five different vibrational modes in a solution. The competition between these five SRS modes prevents the vibrational energy from being transferred to high-order Stokes waves, thereby locking most of the energy in the first-order Stokes SRS and outputting a very strong, broadband, and coherent Raman radiation ranging from 555 to 570 nm. This study demonstrates the possibility of producing an intense single longitudinal mode laser.

Temporal behavior of the high-power pulsed gas terahertz laser pumped by a fundamental mode TEA CO2 laser.

Optically pumped gas molecular terahertz (THz) lasers are promising for generating high-power and high-beam-quality coherent THz radiation. However, for pulsed gas THz lasers, the temporal behavior of the output THz pulse has rarely been investigated. In this study, the temporal behavior of a pulsed gas THz pumped by a fundamental-mode TEA CO2 laser has been presented for the first time both in simulation and experiment. A modified laser kinetics model based on the density matrix rate equation was used to simulate the temporal behavior and output pulse energy of a pulsed gas THz laser at different gas pressures. The results clearly show that the working gas pressure and pump pulse energy have critical influences on the output THz pulse shape. Three typical pulse shapes were obtained, and the THz pulse splitting caused by gain switching was quantitatively simulated and explained based on the laser dynamic process. Besides, with an incident pump pulse energy of 342 mJ, the maximum output THz pulse energy of 2.31 mJ was obtained at 385 µm, which corresponds to a photon conversion efficiency of approximately 56.1%, and to our knowledge, this is the highest efficiency for D2O gas THz laser. The experimental results agreed well with those of the numerical simulation for the entire working gas pressure range, indicating that our model is a powerful tool and paves the way for designing and optimizing high-power pulsed gas lasers.

Low- and High-Order Optical Nonlinearities of Quantum Dots

Various potential applications of quantum dots (QDs) require knowledge of their optical nonlinearities. In this review, the third-order nonlinearities responsible for the saturable absorption, two-photon absorption, reverse saturable absorption, and nonlinear refraction in QDs, as well as the high-order harmonics generation in the laser-induced plasmas containing QDs, are analyzed. The methods of QD synthesis and preparation strongly affect their optical nonlinearities. Above-mentioned nonlinear optical studies in QDs are analyzed for various potential applications (sources of coherent extreme ultraviolet radiation, optical limiters, mode-lockers, etc.).

Terahertz Light Sources by Electronic-Oscillator-Driven Second-Harmonic Generation in Cavities Featuring Extreme Confinement

The majority of sources of coherent optical radiation rely on laser oscillators driven by population inversion. Despite their technological importance in communications, medicine, industry, and other fields, it remains a challenge to access the spectral range of 0.1--10 THz (the ``terahertz gap''), a frequency band for applications ranging from spectroscopy to security and high-speed wireless communications. Here, we propose a way to produce coherent radiation spanning the THz gap by efficient second-harmonic generation in low-loss dielectric structures, starting from technologically mature electronic oscillators in the approximately 100 GHz range. To achieve this goal, we introduce hybrid THz-band dielectric cavity designs that combine (1) extreme field concentration in high-quality-factor resonators with (2) nonlinear materials enhanced by phonon resonances. We theoretically predict conversion efficiencies of $>{10}^{3}\mathrm{%}/$W and the potential to bridge the THz gap with 1 W of input power. This approach enables efficient, cascaded parametric frequency converters, and light sources extensible into the mid-IR spectrum and beyond.

Partially coherent conical refraction promises new counter-intuitive phenomena

In this paper, we extend the paraxial conical refraction model to the case of the partially coherent light using the unified optical coherence theory. We demonstrate the decomposition of conical refraction correlation functions into well-known conical refraction coherent modes for a Gaussian Schell-model source. Assuming randomness of the electrical field phase of the input beam, we reformulated and significantly simplified the rigorous conical refraction theory. This approach allows us to consider the propagation of light through a conical refraction crystal in exactly the same way as in the classical case of coherent radiation. Having this in hand, we derive analytically the conical refraction intensity both in the focal plane and in the far field, which allows us to explain and rigorously justify earlier experimental findings and predict new phenomena. The last include the counterintuitive effect of narrowing of the conical refraction ring width, disappearance of the dark Poggendorff’s ring in the Lloyd’s plane, and shift of Raman spots for the low-coherent conical refraction light. We also demonstrate a universal power-law dependence of conical refraction cones coherence degree on the input correlation length and diffraction-free propagation of the low-coherent conical refraction light in the far field.

A 0.68–0.72-THz 2-D Scalable Radiator Array With –3-dBm Radiated Power and 27.3-dBm EIRP in 65-nm CMOS

A novel and compact 2-D scalable architecture of coupled harmonic oscillator array is proposed for high-power radiation beyond 600 GHz. The compact and symmetric scalable unit cell comprises two oscillators with two slot antennas radiating the third-harmonic power. Each unit cell is horizontally coupled out-of-phase and vertically in-phase with adjacent cells at the fundamental frequency. Therefore, coherent radiation and power combining are achieved at the third harmonic. A <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$4\times4$ </tex-math></inline-formula> array prototype (32 radiating elements) is designed and fabricated in a 65-nm CMOS technology. An elliptical Teflon lens is attached at the backside of the chip for a highly directive beam. An effective isotropic radiated power (EIRP) of 27.3 dBm and an output power of −3 dBm are measured at 694-GHz with 754-mW power consumption under 1.2-V supply voltage, implying a dc-to-terahertz (THz) efficiency of 0.066%. The core design occupies a chip area of 0.61 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , and the entire chip area is 0.97 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , leading to a 0.52-mW/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> area efficiency. The peak EIRP of 27.8 dBm and the radiated power of −2.4 dBm are measured at 699 GHz under 1.3-V supply voltage. A frequency tuning range of 5.26% from 679.4 to 716.1 GHz is measured by varying bias and supply voltages. The measured phase noise at 1-MHz offset is −73 dBc/Hz at 694 GHz. To the best of our knowledge, the designed array has the highest radiated power, radiated power per area, EIRP, frequency tuning range, and dc-to-THz efficiency among silicon-based scalable coherent radiator arrays operating beyond 600 GHz.

Implementation of digital holographic interferometry for pulsed plasma studies

The study of low-temperature plasma generated in pulsed mode at atmospheric pressure was carried out. The purpose of the presented research is to develop a method of digital holographic interferometry for registration and evaluation of parameters of low-temperature plasma at atmospheric pressure in pulsed mode. This type of plasma is currently applied in medicine and biology. Thus, there is a need to control the exposure dose and plasma environment formation regimes. As plasma parameters, it can be considered its electron concentration which can be calculated through the estimation of the refractive index of plasma pulse in relation to unperturbed state. The plasma pulses were activated in Helium. The plasma pulse frequency was 5 kHz and its duration was 750 ns. During an investigation a laboratory set-up for recording holographic images of plasma pulses was developed. Holograms are acquired on a digital camera and a pulsed laser INNOLAS SpitLight Hybrid II at a wavelength of 532 nm with pulse duration of 10 ns is used as a source of coherent radiation. In order to record plasma pulses, the laser, plasma generator and digital camera were strictly synchronized to each other. During the experiment, a series of about 500 holograms were acquired, and the reconstruction of the phase of the object field was calculated. Analysis of the sequence of holograms allowed calculations of phase difference (interferograms) related to the refractive index of low-temperature pulsed plasma in Helium. It is known that low-temperature plasma leads to low phase delay which forms low phase contrast of the evaluated interferograms. For this purpose, we carried out preliminary experiments with plasma-arc that has similar temporary parameters, however, with a higher phase contrast of the interferograms. The paper presents experimental results obtained by studying the phase contrast of the refractive index of pure Helium, plasma-arc and plasma pulses in Helium. Thus, the effectiveness of both the experimental set-up and the method to evaluate the interferograms related to the refractive index of the plasma pulse was verified. The data obtained can then be used to estimate the electron concentration of the plasma. However, it needs to increase the sensitivity of the method in order to enhance phase contrast. Increase of sensitivity can be done by means of extension of the spectral range, for example, toward to infra-red.