Search

Recent years have witnessed a rapid development of sources for intense terahertz (THz) pulses and an increasing number of their applications. The advent of laser pulses with intensities in excess of $10^{18}$ W/cm2 offers an immense opportunity for generating extremely energetic THz pulses from laser-produced plasmas. This paper will review recent progress in the study of THz radiation generated from relativistically intense laser–plasma interactions, mainly focusing on the underlying THz generation models under different laser and plasma parameters, albeit still not yet fully explored. Various THz generation scenarios are discussed associated with laser-excited waves in plasmas, the transport and emission of laser-accelerated electrons, and structured targets.

A full characterization of fast electrons is important for understanding laser-plasma physics. Conventionally, the total number, angular distribution and energy spectra of escaping electrons are diagnosed experimentally with image plates and electron spectrometers, which however are destructive for electrons themselves. In contrast, the terahertz (THz) radiation, generated by electrons crossing the target surface, enables a non-invasive in-situ diagnostic for electrons. A hybrid model involving the high-current electron emission and transient sheath dynamics at the target rear is proposed to account for the THz generation from laser-foil interactions. With this, the measured THz radiation have been employed in turn to quantitatively infer some properties (like the total charge and dynamics) of the escaping electrons and transient ion acceleration sheath, in good agreement with experimental measurements and theoretical calculations.

Controlling the time-domain oscillation of a terahertz (THz) wave offers promising capabilities for THz-based all-optical particle acceleration and strong-field THz nonlinear physics. However, the lack of highly efficient and frequency-modulable multicycle THz sources is impeding the spread of strong-field THz science and applications. Here, we show that by simply adding an echelon into a single-cycle THz source based on optical rectification in lithium niobate crystals via the tilted pulse-front technique, multicycle THz pulses can be efficiently generated with an 800 nm-to-THz efficiency of 0.1% at room temperature. The radiated THz properties can be engineered by precisely designing the echelon structure. Our proposed multicycle THz generation method has the advantages of high efficiency, ease of operation, and quick switching between single-cycle and multicycle working modes, all of which are important in the application of high-field THz radiation.

To integrate a terahertz pump into an ultrafast electron diffraction (UED) experiment has attracted much attention due to its potential to initiate and detect the structural dynamics both directly. However, the deflection of the electron probe by the electromagnetic field of the terahertz pump alters the incident angle of the electron probe on the sample, impeding it from recording structural information afterwards. In this article, we studied this issue by a theoretical simulation of the terahertz-induced deflection effect on the electron probe, and came up with several possible schemes to reduce such effect. As a result, a terahertz-pump-electron-probe UED experiment with a temporal resolution comparable to the terahertz period is realized. We also found that MeV UED was more suitable for such terahertz pump experiment.

An ever-increasing number of strong-field applications, such as ultrafast coherent control over matter and light, require driver light pulses that are both high power and spectrally tunable. The realization of such a source in the terahertz (THz) band has long been a formidable challenge. Here, we demonstrate, via experiment and theory, efficient production of terawatt (TW)-level THz pulses from high-intensity picosecond laser irradiation on a metal foil. It is shown that the THz spectrum can be manipulated effectively by tuning the laser pulse duration or target size. A general analytical framework for THz generation is developed, involving both the high-current electron emission and a time-varying electron sheath at the target rear, and the spectral tunability is found to stem from the change of the dominant THz generation mechanism. In addition to being an ultrabright source (brightness temperature of about 10^{21} K) for extreme THz science, the THz radiation presented here also enables a unique in situ laser-plasma diagnostic. Employing the THz radiation to quantify the escaping electrons and the transient sheath shows good agreement with experimental measurements.

When comets interacting with solar wind, straight and narrow plasma tails will be often formed. The most remarkable phenomenon of the plasma tails is the disconnection event, in which a plasma tail is uprooted from the comet’s head and moves away from the comet. In this paper, the interaction process between a comet and solar wind is simulated by using a laser-driven plasma cloud to hit a cylinder obstacle. A disconnected plasma tail is observed behind the obstacle by optical shadowgraphy and interferometry. Our particle-in-cell simulations show that the difference in thermal velocity between ions and electrons induces an electrostatic field behind the obstacle. This field can lead to the convergence of ions to the central region, resulting in a disconnected plasma tail. This electrostatic-field-induced model may be a possible explanation for the disconnection events of cometary tails.

We present an improvement design of a two-dimensional (2D) angular-resolved proton spectrometer for wide-angle measurement of proton beams from high-intensity laser-solid interactions. By using a 2D selective entrance pinhole array with different periods in orthogonal axes, the angular resolution along one dimension is improved by a factor of 6.7. This improvement provides the accessibility to detect the spatial fine structures of the proton energy spectrum.

The bremsstrahlung x-ray emission profile from high intensity laser-solid interactions provides valuable insight to the internal fast electron transport. Using penumbral imaging, we characterise the spatial profile of this bremsstrahlung source as a function of laser intensity by incrementally increasing the laser focal spot size on target. The experimental data shows a dual-source structure; one from the central channel of electrons, the second a larger substrate source from the recirculating electron current. The results demonstrate than an order of magnitude improvement in the intensity contrast between the two x-ray sources is achieved with a large focal spot, indicating preferable conditions for applications in radiography. An analytical model is derived to describe the transport of suprathermal electron populations that contribute to substrate and central channel sources through a target. The model is in good agreement with the experimental results presented here and furthermore is applied to predict laser intensities for achieving optimum spatial contrast for a variety of target materials and thicknesses.

Significance Terahertz (THz) radiation, with frequencies spanning from 0.1 to 10 THz, has long been the most underdeveloped frequency band in electromagnetic waves, mainly due to the dearth of available high-power THz sources. Although the last decades have seen a surge of electronic and optical techniques for generating intense THz radiation, all THz sources reported until now have failed to produce above-millijoule (mJ) THz pulses. We present a THz source that enables a THz pulse energy up to tens of mJ, by using an intense laser pulse to irradiate a metal foil., Ultrahigh-power terahertz (THz) radiation sources are essential for many applications, for example, THz-wave-based compact accelerators and THz control over matter. However, to date none of the THz sources reported, whether based upon large-scale accelerators or high-power lasers, have produced THz pulses with energies above the millijoule (mJ) level. Here, we report a substantial increase in THz pulse energy, as high as tens of mJ, generated by a high-intensity, picosecond laser pulse irradiating a metal foil. A further up-scaling of THz energy by a factor of ∼4 is observed when introducing preplasmas at the target-rear side. Experimental measurements and theoretical models identify the dominant THz generation mechanism to be coherent transition radiation, induced by the laser-accelerated energetic electron bunch escaping the target. Observation of THz-field-induced carrier multiplication in high-resistivity silicon is presented as a proof-of-concept application demonstration. Such an extremely high THz energy not only triggers various nonlinear dynamics in matter, but also opens up the research era of relativistic THz optics.

A multichannel calorimeter system is designed and constructed which is capable of delivering single-shot and broad-band spectral measurement of terahertz (THz) radiation generated in intense laser–plasma interactions. The generation mechanism of backward THz radiation (BTR) is studied by using the multichannel calorimeter system in an intense picosecond laser–solid interaction experiment. The dependence of the BTR energy and spectrum on laser energy, target thickness and pre-plasma scale length is obtained. These results indicate that coherent transition radiation is responsible for the low-frequency component (${1 THz) of BTR. It is also observed that a large-scale pre-plasma primarily enhances the high-frequency component (${>}$3 THz) of BTR.

Traditional electron accelerators usually use the radio-frequency field as the driving field. In contrast, the terahertz radiation, with a shorter wavelength and a higher accelerating gradient, has the capability to work as the driver of compact electron accelerators in the future. In addition, the terahertz pulse can also provide an ultrafast modulated field to compress and measure the electron pulse width. Recently, the interactions between terahertz filed and electrons have attracted increasing attention. Intense laser-solid interactions can produce both a high-energy terahertz pulse and a high-charge electron beam simultaneously. This advantage enables a unique platform for the terahertz control of electrons and the terahertz pump-electron probe experiments. Here, by taking a feasible experimental layout as an example, we first investigate the interactions of a terahertz pulse with a co-propagating electron beam. Systematic discussions on the influences of several parameters on the terahertz field-induced electron deflection are presented via numerical simulations. We find that a strong terahertz electric field or the long pulse duration will enhance the deflection. Furthermore, the deflection depends strongly on the terahertz waveform. A preliminary experiment is performed to qualitatively demonstrate the relevant analysis.

Recently generation of strong magnetic (B) fields has been demonstrated in capacitor coils heated by high power laser pulses [S. Fujioka et al., Sci. Rep. 3, 1170 (2013)]. This paper will present a direct measurement of B field generated with an open-ended coil target driven by a nanosecond laser pulse using ultrafast proton radiography. The radiographs are analyzed with particle-tracing simulations. The B field at the coil center is inferred to be ∼50 T at an irradiance of ∼5 × 1014 W·cm−2. The B field generation is attributed to the background cold electron flow pointing to the laser focal spot, where a target potential is induced due to the escape of energetic electrons. Keywords: Strong magnetic field, Laser-driven coil targets, Proton radiography, PACS codes: 52.70.Ds, 52.50.Jm, 52.38.Fz

The interaction of solar wind with a dipole magnetic field is modeled in a laboratory setting with a small cylindrical permanent magnet and magnetized plasma driven by intense lasers. The result shows a potential application in the understanding of Earth's magnetosphere near the pole region. Some significant features are observed in our experiments, such as magnetic reconnection and repulsion, which agree well with magnetohydrodynamics (MHD) simulation results.

Physical review letters 120(20), 204801 (2018). doi:10.1103/PhysRevLett.120.204801, A dual ion species plasma expansion scheme from a novel target structure is introduced, in which a nanometer--thick layer of pure deuterium exists as a buffer species at the target--vacuum interface of a hydrogen plasma. Modeling shows that by controlling the deuterium layer thickness, a composite H+ /D+ ion beam can be produced by target normal sheath acceleration (TNSA), with an adjustable ratio of ion densities, as high energy proton acceleration is suppressed by the acceleration of a spectrally peaked deuteron beam. Particle in cell modeling shows that a (4.3 +/- 0.7) MeV per nucleon deuteron beam is accelerated, in a directional cone of half angle 9 degrees. Experimentally, this was investigated using state of the art cryogenic targetry and a spectrally peaked deuteron beam of (3.4 +/- 0.7) MeV per nucleon was measured in a cone of half angle 7 degrees-9 degrees, while maintaining a significant TNSA proton component., Published by APS, College Park, Md.

We have systematically studied strong THz radiation from solid targets driven by relativistic femtosecond and picosecond laser pulses. The THz generation is due to coherent transition radiation of relativistic laser-driven electron beams when they pass the solid-vacuum boundary. The THz radiation with pulse energy of >mJ has been observed with picosecond pulses. Such high energy THz pulses can not only trigger various nonlinear dynamics in matter, but also open up a new research field of relativistic THz optics.

Nuclear fusion reactions are the most important processes in nature to power stars and produce new elements, and lie at the center of the understanding of nucleosynthesis in the universe. It is critically important to study the reactions in full plasma environments that are close to true astrophysical conditions. By using laser-driven counter-streaming collisionless plasmas, the fusion $d+d{\ensuremath{\rightarrow}}^{3}\mathrm{He}+n$ is studied in a Gamow-like window around 27 keV. The results give hints that astrophysical nuclear reaction yields can be modulated significantly by the self-generated electromagnetic fields and the collective motion of the plasma. This plasma-version minicollider may provide a novel tool for studies of astrophysics-interested nuclear reactions, as well as a useful tool to constrain the models of plasma colliding dynamic.

A novel method of initiating nuclear fusion reactions in a full plasma environment was suggested, and a proof-of-concept experiment was carried out with the $$\hbox {D}+\hbox {D } \rightarrow \hbox { n}\,+\,^3$$ He reaction. In this new approach, two plasma jets generated by high-intensity lasers collide head-on-head. The center-of-mass energy of the nuclei increases accordingly, and therefore, reaction products can be significantly enhanced, especially in the sub-Coulomb barrier ranges. As a result of the fusion reaction, up to $$\sim 7.6\times 10^{5}$$ neutrons had been observed. This new type of “plasma collider” could provide an innovative tool to study nuclear reactions under astrophysical conditions.

Interactions of two counter-streaming plasmas driven by high power laser pulses are studied on Shenguang II laser facility. Filamentary structures were observed in the interaction region after the electrostatic shockwave decay. Theoretical analysis and observations indicate that the filaments are because of collisionless mechanisms, which are caused by the electromagnetic instability, such as the beam-Weibel instability. Collision experiments were also carried out for comparison and no filaments were generated.

With the rapid developing of high energy lasers and ultrashort high intensity lasers, high energy density conditions, which widely exist in astronomical environment and nuclear explosion, can be created in laboratories. This provides new opportunities for various physics fields and forms new interdisciplinary frontiers. In this review, recent studies of astronomic phenomena in a controllable way in laboratories with high power lasers will be first presented. Then prospects of the new physics driven by next generation ultra-relativistic lasers will be discussed.

We present a novel design of two-dimensional (2D) angular-resolved spectrometer for full beam characterization of ultrashort intense laser driven proton sources. A rotated 2D pinhole array was employed, as selective entrance before a pair of parallel permanent magnets, to sample the full proton beam into discrete beamlets. The proton beamlets are subsequently dispersed without overlapping onto a planar detector. Representative experimental result of protons generated from femtosecond intense laser interaction with thin foil target is presented.

Coherent transition radiation in the terahertz (THz) region with energies of sub-mJ/pulse has been demonstrated by relativistic laser-driven electron beams crossing the solid-vacuum boundary. Targets including mass-limited foils and layered metal-plastic targets are used to verify the radiation mechanism and characterize the radiation properties. Observations of THz emissions as a function of target parameters agree well with the formation-zone and diffraction model of transition radiation. Particle-in-cell simulations also well reproduce the observed characteristics of THz emissions. The present THz transition radiation enables not only a potential tabletop brilliant THz source, but also a novel noninvasive diagnostic for fast electron generation and transport in laser-plasma interactions.

We report a systematic study on backward terahertz (THz) radiation generation from laser-solid interactions by changing a variety of laser/plasma parameters. We demonstrate a high-energy (with an energy flux density reaching 80 μJ/sr), broadband (10 THz) plasma-based radiation source. The radiation energy is mainly distributed either in the10 THz or3 THz regions. A radial surface current formed by the lateral transport of low-energy electrons (LEE) is believed to be responsible for the radiation in the high-THz region (10 THz), while high-energy surface fast electrons (SFE) accelerated along the target surface mainly contribute to lower frequency (3 THz) radiation. The unifying explanation could be applied to backward THz radiation generation from solid targets with presence of relative small preplasmas.

Diagnosing fast electrons is important to understand the physics underpinning intense laser-produced plasmas. Here, we demonstrate experimentally that a Cherenkov radiation-based optical fibre can serve as a reliable diagnostic to characterize the fast electrons escaping from solid targets irradiated by ultra-intense laser pulses. Using optical fibre loops, the number and angular distributions of the escaping electrons are obtained. The data agrees well with measurements made using image plate stacks. The optical fibre can be operated at high-repetition rates and is insensitive to X-rays and ion beams, which makes it advantageous over other routinely-used fast electron diagnostics in some aspects.

Proton emissions from vacuum-gapped cascaded-ultrathin-foil targets irradiated with low-contrast intense picosecond laser pulses were measured. The maximum energy of the proton beam and the laser-to-proton energy conversion efficiency were both increased in comparison with those from the single-layer reference targets. A transition from plateau to exponential profile in proton energy spectral shape was found for the target with a front-foil thickness of above 500 nm. The measured annular x-ray emissions from both target front and rear sides indicate that the proton enhancement could be attributed to the modified preplasma distribution. A simple model and hydrodynamic simulations further show that the optimal acceleration occurs when the front shutter foil is right swelled onto the front surface of the rear source foil by the prepulses at the arrival of the main laser pulse. This cascaded thin-foil target design can be popularized in improving laser-driven proton beams for wide applications.

Recently Terahertz (THz) radiation from laser-produced plasmas has attracted much interest since plasmas can work at an arbitrarily high laser intensity. We will present the generation of strong THz radiation from femtosecond and picosecond relativistic laser-plasma interactions. THz pulses with energies up to >200 μJ/sr have been demonstrated. The temporal waveform, polarization, and angular distribution are measured. We find that the radiation depends on the preplasma density scale length. The THz radiation is probably attributed to the self-organized transient fast electron currents formed along the target surface when the plasma density profile is steep, while, the linear mode conversion when a large preplasma is formed.

Powerful terahertz (THz) radiation is observed from large-scale underdense preplasmas in front of a solid target irradiated obliquely with picosecond relativistic intense laser pulses. The radiation covers an extremely broad spectrum with about 70% of its energy located in the high frequency regime over 10 THz. The pulse energy of the radiation is found to be above $100\text{ }\text{ }\ensuremath{\mu}\mathrm{J}$ per steradian in the laser specular direction at an optimal preplasma scale length around $40--50\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$. Particle-in-cell simulations indicate that the radiation is mainly produced by linear mode conversion from electron plasma waves, which are excited successively via stimulated Raman scattering instability and self-modulated laser wakefields during the laser propagation in the preplasma. This radiation can be used not only as a powerful source for applications, but also as a unique diagnostic of parametric instabilities of laser propagation in plasmas.

With the development of ultraintense terawatt (TW) and petawatt (PW) laser systems, powerful terahertz (THz) radiation from laser–plasma interactions has been reported. Plasma-based THz systems, which are usually operated at extremely low repetition rates, call for single-shot diagnostics. In this paper, various state-of-the-art single-shot detection methods are introduced or designed for measurements and applications involved in high-power plasma-based THz sciences.

A bow shock is formed in the interaction of a high-speed laser-driven plasma cloud with a cylinder obstacle. Its temporal and spatial structures are observed by shadowgraphy and interferometry. The width of the shock transition region is ~ 50 μm, comparable to the ion–ion collision mean free path, which indicates that collision is dominated in the shock probably. The Mach-number of the ablating plasma cloud is ~ 15 at first, and decreases with time resulting in a changing shock structure. A two-dimension hydrodynamics code, USim, is used to simulate the interaction process. The simulated shocks can well reproduce the observed.

Recently Terahertz (THz) radiation from laser-produced plasmas has attracted much interest since plasmas can work at arbitrarily high laser intensity. This paper will discuss the generation mechanisms of plasma-based THz radiation.

The development of tabletop intense terahertz (THz) radiation sources is extremely important for THz science and applications. This paper presents our measurements of intense THz radiation from relativistic laser–plasma interactions under different experimental conditions. Several THz generation mechanisms have been proposed and investigated, including coherent transition radiation (CTR) emitted by fast electrons from the target rear surface, transient current radiation at the front of the target, and mode conversion from electron plasma waves (EPWs) to THz waves. The results indicate that relativistic laser plasma is a promising driver of intense THz radiation sources.

The terahertz (THz) radiation from the front of solid targets with a large-scale preplasma irradiated by relativistic picosecond laser pulses has been studied. The THz radiation measured at the specular direction nonlinearly increases with laser energy and an optimal plasma density scalelength is observed. Particle-in-cell simulations indicate that the radiation can be attributed to the model of mode conversion. While the THz radiation near the target normal direction is saturated with laser energy and plasma scalelength. Unlike the radiation in the specular direction' the transient current formed at the plasma-vacuum interface could be responsible for the radiation near the target normal.

We have studied the generation of terahertz (THz) radiation via the interaction of intense femtosecond laser pulses with solid targets at a small incidence angle. It is found that preplasma with a moderate density gradient can enhance the emission. We also observe saturation of the THz output with the driving laser energy. We find that THz emission is closely related to the 3/2 harmonics of the driving laser. Particle-in-cell simulations indicate that under the present experimental conditions, the THz emission could be attributed to the transient currents at the plasma-vacuum interface, mainly formed by the two-plasmon-decay instability.

Highly collimated MeV electron beam guiding has been observed along the target surface following the interaction of bulk target irradiation by femtosecond laser pulses at relativistic intensities. The beam quality is shown to depend critically on the laser contrast: With a ns prepulse, the generated electron beam is well concentrated and intense, while a high laser contrast produces divergent electron beams. In the case of large preplasma scale lengths, tunable guiding and acceleration of the target surface electrons is achieved by changing the laser incident angle. By expanding the preplasma scale length to several hundred micrometers, we obtained MeV spectrum-peaked electron beams with a 100 pC per laser pulse and divergence angles of only 3 degrees.. This technique suggests a stable method of injection of elections into a variety of accelerator designs.