Coherent Emission Research Articles

Generation of ultrashort pulses in the THz frequency range

There are results for the spontaneous coherent super-radiative undulator emission in the terahertz frequency range from a short (as compared to the wavelength of the radiated wave) dense electron bunch. If the group velocity of the wave is close to the bunch velocity, this is a process of spontaneous radiation followed by amplification of a single wave cycle. Despite the Coulomb repulsion of electrons inside the bunch, its compactness is provided by the compression of the bunch under the action of its own radiation fields. As a result, formation of an ultra-short (several cycles long) powerful wave packet occurs when the bunch moves through several undulator periods with high (∼20% in optimized systems) efficiency of extraction of the electron energy and high intensity (∼ 100 MV/m) of the peak wave field.

Steerable Coherent Terahertz Radiation Using Sheet Electron Beams to Excite 2-D Composite Arrays of Sub-Wavelength Holes

Steerable coherent terahertz wave emission is attractive for various applications. Aiming for that, we propose to use a uniformly moving sheet electron beam to drive a 2-D composite array of sub-wavelength holes (SHs). Each SH is an independent resonator in the terahertz region, and a set of SHs with different orientations form a composite radiating unit. Excited by the electron beam, the emissions from all the units in the array constructively interfere in specified directions via the combination of the famous Cherenkov radiation (CR) and Smith–Purcell radiation (SPR) effects, leading to a directional coherent terahertz radiation, which can be real-timely steered by rotating the array. The detailed theoretical analyses of this radiation are performed and verified by simulations. This radiating scheme promises novel types of multi-functional terahertz sources and devices.

The population of M dwarfs observed at low radio frequencies

Coherent low-frequency ($\lesssim 200$ MHz) radio emission from stars encodes the conditions of the outer corona, mass-ejection events, and space weather. Previous low-frequency searches for radio emitting stellar systems have lacked the sensitivity to detect the general population, instead largely focusing on targeted studies of anomalously active stars. Here we present 19 detections of coherent radio emission associated with known M~dwarfs from a blind flux-limited low-frequency survey. Our detections show that coherent radio emission is ubiquitous across the M~dwarf main sequence, and that the radio luminosity is independent of known coronal and chromospheric activity indicators. While plasma emission can generate the low-frequency emission from the most chromospherically active stars of our sample, the origin of the radio emission from the most quiescent sources is yet to be ascertained. Large-scale analogues of the magnetospheric processes seen in gas-giant planets likely drive the radio emission associated with these quiescent stars. The slowest-rotating stars of this sample are candidate systems to search for star-planet interaction signatures.

Replica symmetry breaking in coherent and incoherent random lasing modes.

We investigate intensity fluctuations of a weakly scattering optofluidic random laser having coherent and incoherent emission fractions. The coherent part comprises random spikes, whereas the incoherent part forms a broad pedestal in the emission spectra. Evaluating the fractional ratio of the coherent and incoherent parts of the emission, a replica symmetry breaking phase transition is observed independently in both coherent and incoherent parts of the intensity. Also, the incoherent component has higher non-zero correlation values compared to those of the coherent part, implying a larger contribution to mode coupling. Moreover, survival function analysis reveals a significant contribution of the incoherent part on determining the decay profile of lasing intensity.

Imaging and Localization of Single Emitters near Plasmonic Particles of Different Size, Shape, and Material.

Colloidal plasmonic materials are increasingly used in biosensing and catalysis, which has sparked the use of super-resolution localization microscopy to visualize processes at the interface of the particles. We quantify the effect of particle–emitter coupling on super-resolution localization accuracy by simulating the point spread function (PSF) of single emitters near a plasmonic nanoparticle. Using a computationally inexpensive boundary element method, we investigate a broad range of conditions allowing us to compare the simulated localization accuracy to reported experimental results. We identify regimes where the PSF is not Gaussian anymore, resulting in large mislocalizations due to the appearance of multilobed PSFs. Such exotic PSFs occur when near-field excitation of quadrupole plasmons is efficient but unexpectedly also occur for large particle–emitter spacing where the coherent emission from the particle and emitter results in anisotropic emission patterns. We provide guidelines to enable faithful localization microscopy near colloidal plasmonic materials, which indicate that simply decreasing the coupling between particle and molecule is not sufficient for faithful super-resolution imaging.

Coherent up-conversion in Erbium-doped silica microspheres enhanced by thin-film metal deposition

Combination of micro-cavity and nano-metals using plasmonic effect may create novel optical devices for communications and optical sensing. In this paper, we study the green coherent emission from an erbium-doped silica microsphere pumped at infrared laser beam with wavelength of 976 nm, which can enhance by a metal coating such as gold, platinum, and aluminum. The case of aluminum coating is studied and the high enhancement shown to decrease with time. The threshold of narrow line-width up-conversion emission from erbium-doped Al-coated silica microsphere is of some tens nano-watt pump power. But after one month kept in air the green light emission threshold of this one increases up to mili-watts, that the enhancement is decreased down to some thousand times in comparison with platinum-coated microsphere as a film of aluminum oxide is formed. In addition, the spectra of green light emission from degraded aluminum-coated microsphere changed from single peak to double peaks that mean the cavity of silica microsphere may make from multi-layers.

Coherent radio emission from a population of RS Canum Venaticorum systems

Coherent radio emission from stars can be used to constrain fundamental coronal plasma parameters, such as plasma density and magnetic field strength. It is also a probe of chromospheric and magnetospheric acceleration mechanisms. Close stellar binaries, such as RS Canum Venaticorum (RS CVn) systems, are particularly interesting as their heightened level of chromospheric activity and possible direct magnetic interaction make them a unique laboratory to study coronal and magnetospheric acceleration mechanisms. RS CVn binaries are known to be radio-bright but coherent radio emission has only conclusively been detected previously in one system. Here, we present a population of 14 coherent radio emitting RS CVn systems. We identified the population in the ongoing LOFAR Two Metre Sky Survey as circularly polarised sources at 144 MHz that are astrometrically associated with known RS CVn binaries. We show that the observed emission is powered by the electron cyclotron maser instability. We use numerical calculations of the maser’s beaming geometry to argue that the commonly invoked ‘loss-cone’ maser cannot generate the necessary brightness temperature in some of our detections and that a more efficient instability, such as the shell or horseshoe maser, must be invoked. Such maser configurations are known to operate in planetary magnetospheres. We also outline two acceleration mechanisms that could produce coherent radio emission, one where the acceleration occurs in the chromosphere and one where the acceleration is due to an electrodynamic interaction between the stars. We propose radio and optical monitoring observations that can differentiate between these two mechanisms.

Kinematics of Crab Giant Pulses

Abstract The Crab Pulsar’s radio emission is unusual, consisting predominantly of giant pulses, with durations of about a microsecond but structure down to the nanosecond level, and brightness temperatures of up to 1037 K. It is unclear how giant pulses are produced, but they likely originate near the pulsar’s light cylinder, where corotating plasma approaches the speed of light. We report observations in the 400–800 MHz frequency band, where the pulses are broadened by scattering in the surrounding Crab Nebula. We find that some pulse frequency spectra show strong bands, which vary during the scattering tail, in one case showing a smooth upward drift. While the banding may simply reflect interference between nanosecond scale pulse components, the variation is surprising, as in the scattering tail the only difference is that the source is observed via slightly longer paths, bent by about an arcsecond in the nebula. The corresponding small change in viewing angle could nevertheless reproduce the observed drift by a change in Doppler shift, if the plasma that emitted the giant pulses moved highly relativistically, with a Lorentz factor γ ∼ 104 (and without much spread in γ). If so, this would support models that appeal to highly relativistic plasma to transform ambient magnetic structures to coherent gigahertz radio emission, be it for giant pulses or for potentially related sources, such as fast radio bursts.

Annular coherent wake emission

Coherent wake emission may be generated only by sufficiently high-contrast driving laser pulses. When the laser contrast is too low, the formed long-scale-length plasma cannot support its generation. In this Letter, we show how, by gently spoiling a pristine laser contrast in an engineered way, coherent wake emission becomes inhibited in the center of the irradiated substrate only, thus forming an annular-shaped source of coherent extreme ultraviolet (XUV) pulses. We present an analytical model that describes the phenomenon and validation of its predictions in the experiment and the simulation. We also show how the ion-acoustic velocity dependency on the laser intensity may be obtained from the emission patterns and offer examples for future applications.

Subset of Fluorophores Is Responsible for Radiation Brightening in Viromimetic Particles.

In certain conditions, dye-conjugated icosahedral virus shells exhibit suppression of concentration quenching. The recently observed radiation brightening at high fluorophore densities has been attributed to coherent emission, i.e., to a cooperative process occurring within a subset of the virus-supported fluorophores. Until now, the distribution of fluorophores among potential conjugation sites and the nature of the active subset remained unknown. With the help of mass spectrometry and molecular dynamics simulations, we found which conjugation sites in the brome mosaic virus capsid are accessible to fluorophores. Reactive external surface lysines but also those at the lumenal interface where the coat protein N-termini are located showed virtually unrestricted access to dyes. The third type of labeled lysines was situated at the intercapsomeric interfaces. Through limited proteolysis of flexible N-termini, it was determined that dyes bound to them are unlikely to be involved in the radiation brightening effect. At the same time, specific labeling of genetically inserted cysteines on the exterior capsid surface alone did not lead to radiation brightening. The results suggest that lysines situated within the more rigid structural part of the coat protein provide the chemical environments conducive to radiation brightening, and we discuss some of the characteristics of these environments.

Diffraction-limited coherent wake emission

The plasma mirror is a relativistic optical element that under certain irradiation conditions generates intense attosecond extreme-ultraviolet light pulses, in a process known as coherent wake emission (CWE). CWE has been previously characterized by its high spatial divergence, originating from an intrinsic intensity-dependent phase accumulation. In this Letter, we show that the transverse variations of the plasma expansion can completely cancel the CWE intrinsic phase. Accordingly, we experimentally demonstrate nearly diffraction-limited CWE with unprecedented divergence, under 6 mrad. We validate our analytical model with particle in cell simulations. This understanding facilitates the development of plasma mirrors as applicable ultraviolet light sources.Received 9 March 2021Accepted 5 August 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.L032059Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHigh intensity laser-plasma interactionsHigh-order harmonic generationPhysical SystemsRelativistic plasmasAtomic, Molecular & OpticalPlasma Physics

Photon-number squeezing in nano- and microlasers

Based on theoretical predictions on the appearance of antibunching before the laser threshold at the nano- and microscale, we analyze the amount of photon-number squeezing naturally produced in the laser emission. Up to 3 dB photon number noise reduction is obtained in comparison with the coherent emission, with output power in the range of pW and with negligible effects due to pump fluctuations. The scheme requires a moderately high Q cavity and holds promise for the construction of a simple and effective photon-number squeezed source.

Detection of coherent low-frequency radio bursts from weak-line T Tauri stars

In recent years, thanks to new facilities such as LOFAR that are capable of sensitive observations, much work has been done on the detection of stellar radio emission at low frequencies. Such emission has commonly been shown to be coherent emission, generally attributed to electron-cyclotron maser (ECM) emission, and has usually been detected from main-sequence M dwarfs. Here we report the first detection of coherent emission at low frequencies from T Tauri stars, which are known to be associated with high levels of stellar activity. Using LOFAR, we detect several bright radio bursts at 150 MHz from two weak-line T Tauri stars: KPNO-Tau 14 and LkCa 4. All of the bursts have high brightness temperatures (1013 − 1014 K) and high circular polarisation fractions (60–90%), indicating that they must be due to a coherent emission mechanism. This could be either plasma emission or ECM emission. Due to the exceptionally high brightness temperatures seen in at least one of the bursts (≥1014 K), as well as the high circular polarisation levels, it seems unlikely that plasma emission could be the source; as such, ECM is favoured as the most likely emission mechanism. Assuming this is the case, the required magnetic field in the emission regions would be 40–70 G. We determine that the most likely method of generating ECM emission is plasma co-rotation breakdown in the stellar magnetosphere. There remains the possibility, however, that it could be due to an interaction with an orbiting exoplanet.

Coherent super-radiative undulator emission of ultra-short THz wave pulses

We study spontaneous coherent super-radiative undulator emission in the terahertz frequency range from a short (as compared to the wavelength of the radiated wave), dense electron bunch. Since the group velocity of the wave is close to the bunch velocity, this is a process of spontaneous radiation followed by amplification of a single wave cycle. Despite the Coulomb repulsion of electrons inside the bunch, its compactness, which is necessary to ensure the spontaneous coherent character of the radiation process, is provided by the compression of the bunch under the action of its own radiation fields. As a result, formation of an ultra-short (several cycles long) powerful wave packet occurs when the bunch moves through several undulator periods with high (∼20% in optimized profiled systems) efficiency of extraction of the electron energy and high intensity (∼100 MV/m) of the peak wave field.

The Habitable-zone Planet Finder Detects a Terrestrial-mass Planet Candidate Closely Orbiting Gliese 1151: The Likely Source of Coherent Low-frequency Radio Emission from an Inactive Star

Abstract The coherent low-frequency radio emission detected by LOFAR from Gliese 1151, a quiescent M4.5 dwarf star, has radio emission properties consistent with theoretical expectations of star–planet interactions for an Earth-sized planet on a 1- to 5-day orbit. New near-infrared radial velocities from the Habitable-zone Planet Finder (HPF) spectrometer on the 10 m Hobby–Eberly Telescope at McDonald Observatory, combined with previous velocities from HARPS-N, reveal a periodic Doppler signature consistent with an m sin i = 2.5 ± 0.5 M ⊕ exoplanet on a 2.02-day orbit. Precise photometry from the Transiting Exoplanet Survey Satellite (TESS) shows no flares or activity signature, consistent with a quiescent M dwarf. While no planetary transit is detected in the TESS data, a weak photometric modulation is detectable in the photometry at a ∼2-day period. This independent detection of a candidate planet signal with the Doppler radial velocity technique adds further weight to the claim of the first detection of star–exoplanet interactions at radio wavelengths and helps validate this emerging technique for the detection of exoplanets.

Few-Molecule Strong Coupling with Dimers of Plasmonic Nanoparticles Assembled on DNA.

Hybrid nanostructures, in which a known number of quantum emitters are strongly coupled to a plasmonic resonator, should feature optical properties at room temperature such as few-photon nonlinearities or coherent superradiant emission. We demonstrate here that this coupling regime can only be reached with dimers of gold nanoparticles in stringent experimental conditions, when the interparticle spacing falls below 2 nm. Using a short transverse DNA double-strand, we introduce five dye molecules in the gap between two 40 nm gold particles and actively decrease its length down to sub-2 nm values by screening electrostatic repulsion between the particles at high ionic strengths. Single-nanostructure scattering spectroscopy then evidence the observation of a strong-coupling regime in excellent agreement with electrodynamic simulations. Furthermore, we highlight the influence of the planar facets of polycrystalline gold nanoparticles on the probability of observing strongly coupled hybrid nanostructures.

Symmetries in cavity models: Beyond the rotating wave approximation

The interaction of confined atoms with a single mode radiation field is the main subject in the theory of cavity quantum electrodynamics. The constraints imposed by the cavity on matter and radiation fields give rise to collective phenomena. One possible outcome is the enhanced and coherent spontaneous emission of photons by the atoms: the superradiance. As predicted by Dicke, conservation laws are essential in superradiance and are derived from the matter-interaction Hamiltonian. Here, we consider N two-level ultracold atoms interacting with a single mode bosonic field, in the Dicke Hamiltonian, and trapped inside a non-dissipative optical cavity. Numerical and analytical results derived from finite size regime indicate the matter–radiation coupling strength, λ, is insufficient to draw the complete picture of the system. Instead, they support the relevance of U(1) symmetry, which prompts the study of (i) particle and angular momentum conservation, (ii) the constraints imposed to correlation functions and (iii) the influence of symmetries in the system dynamics. Further exploring the U(1) and rotational symmetries permits a simple interpretation of antirotating contributions as spin–orbit operators. As application, we show two species of ultracold clouds develop interactions due to antirotating operators.

Coherent curvature radiation: maximum luminosity and high-energy emission

ABSTRACT High brightness temperature radio transients such as pulsars and fast radio bursts (FRBs) require the coherent radiation of particles. The antenna class of coherent radiation models require a large number of charged particles radiating in phase, therefore the particles must be spatially confined and have well-aligned velocities. Given these necessary conditions, we look at the magnetic field induced by the currents associated with coherently emitting accelerated particles and consider the interaction between the radiating particles and the induced magnetic field. We find a maximum luminosity of coherent curvature radiation that depends on source parameters such as surface magnetic field and neutron star spin period. We find that coherent radio emission across all luminosities can be explained by coherent curvature radiation and suggest it could be universally responsible for both FRBs and extreme galactic sources. Using the Crab Pulsar as an example, we constrain the emission parameters and origin of the most extreme nanoshots to within 60 km of the pulsar surface assuming coherent curvature radiation. In agreement with recent observations, we also predict simultaneous X-ray emission from small-scale particle gyration due to the induced field.

Deep- to near-ultraviolet Raman frequency conversion pumped by femtosecond pulses in a hollow-core waveguide.

Broadband vibrational/rotational Raman generation ranging from deep ultraviolet (DUV) to blue wavelengths is demonstrated by using molecular hydrogen in a hollow-core waveguide as a Raman-active medium pumped by a femtosecond DUV laser. We find the high-order transient stimulated Raman scattering is drastically enhanced for input beams including a circularly polarized component; a circularly polarized input beam achieves the highest conversion efficiency. Coherent vibrational anti-Stokes Raman emission is observed only for a circularly polarized pump beam, indicating that the waveguide effect also contributes to the upconversion of a DUV pulse via transient stimulated Raman scattering.

Visible light photonic integrated Brillouin laser

Narrow linewidth visible light lasers are critical for atomic, molecular and optical (AMO) physics including atomic clocks, quantum computing, atomic and molecular spectroscopy, and sensing. Stimulated Brillouin scattering (SBS) is a promising approach to realize highly coherent on-chip visible light laser emission. Here we report demonstration of a visible light photonic integrated Brillouin laser, with emission at 674 nm, a 14.7 mW optical threshold, corresponding to a threshold density of 4.92 mW μm−2, and a 269 Hz linewidth. Significant advances in visible light silicon nitride/silica all-waveguide resonators are achieved to overcome barriers to SBS in the visible, including 1 dB/meter waveguide losses, 55.4 million quality factor (Q), and measurement of the 25.110 GHz Stokes frequency shift and 290 MHz gain bandwidth. This advancement in integrated ultra-narrow linewidth visible wavelength SBS lasers opens the door to compact quantum and atomic systems and implementation of increasingly complex AMO based physics and experiments.