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Terahertz radiation generation from liquid water has long been considered impossible due to strong absorption. A few very recent works reported terahertz generation from water, but the mechanism is not clear and the efficiency demands to be enhanced. We show experimentally that strong single-cycle terahertz radiation with field strength of 0.2MVcm−1 is generated from a water line (or column) of approximately 200μm in diameter irradiated by a mJ femtosecond laser beam. This strength is 100-fold higher than that produced from air using single-color pumping. We attribute the mechanism to the laser-ponderomotive-force-induced current with the symmetry broken around the water-column interface. This mechanism can explain our following observations: the radiation can be generated only when the laser propagation axis deviates from the column center; the deviation determines its field strength and polarity; it is always p polarized no matter whether the laser is p or s polarized. This study provides a simple and efficient scheme of table-top terahertz sources based on liquid water.
Intense terahertz (THz) radiation sources, the frequency of which lies between far-infrared waves and microwaves in the electromagnetic spectrum, are significantly important for THz sciences and applications in many interdisciplinary fields [1]. Currently THz radiation with energies of >100 μJ/pulse is usually obtained with huge-sized accelerators [2]. Laser-plasma interactions provide a unique opportunity to achieve tabletop high-field THz radiation sources. With the relativistic electron beams accelerated by laser wakefields in gas targets, Leemans et al. have obtained ∼0.3 μJ THz pulses through transition radiation [3].
Strong THz radiation from laser-solid interactions has attracted much interest [4,5]. Compared with gas targets, fast electron beams from solid foil targets have much higher charge, up to nC or even near μC. Usually the bunch length of the electron beam accelerated by a femtosecond laser pulse is of the order of ~10 μm, which is smaller than the wavelength of THz radiation. Therefore, the forward fast electrons will induce coherent transition radiation (CTR) in the THz regime when crossing the rear surface-vacuum boundary (see Figure 1). This has so far not yet been verified experimentally.
Figure 1: Illustration of the THz generation due to the CTR of fast electron beams at the rear surface of a foil target irradiated by intense laser pulses.
In our recent work [6], we have experimentally demonstrated intense coherent THz transition radiation by laser-driven, relativistic electron beams crossing the rear surface of a thin solid foil. The experiment was carried out on the femtosecond laser system at the Laboratory for Laser Plasma, Shanghai Jiao Tong University. From the rear side of a 5 μm thick metal foil irradiated by a 2 J/ 30 fs laser pulse, we obtain an intense THz pulse with an energy of ~400 μJ, which is comparable to the energy level of the conventional accelerator based THz sources [2]. The measured THz radiation covers a bandwidth up to 30 THz [see Figure 2(a)], and has an asymmetric “double-wing-like” angular distribution [see Figure 2(b)]. Both CTR-based theoretical calculations and two-dimensional particle-in-cell simulations can well reproduce the experimental measurements.
Figure 2: [click on the image to view with higher resolution] (a) Experimentally measured (blue circle dashed) and simulated (black solid) frequency spectra of the THz radiation from the metal foil. (b) Angular distributions of the THz radiation measured (blue circle), simulated (black dashed), and calculated with CTR model (red solid), all of which are normalized by the THz intensity at 75°.
The CTR model predicts that the THz radiation intensity is closely dependent on the target parameters, for example, the size and dielectric property of the target. To verify this, several types of targets are adopted to understand the THz generation. For the mass-limited metal targets, the observed dependence of THz intensity on the target sizes [see Figure 3(a)] can be explained by the CTR model modified by diffraction effect [7]. For the metal-PE double layered targets, we find that there exists an optimal PE thickness when increasing the thickness of the PE layer from 15 μm to 500 μm [see Figure 3(b)]. This can be explained by the CTR model considering the formation-zone effects [8]. Compared with the THz radiation from the PE targets, we find the THz intensity from the targets with a 5 μm thick metal coating at the target rear is dramatically enhanced by over 10 times [see Figure 3(c)]. This is a solid evidence for transition radiation.
Figure 3: [click on the image to view with higher resolution] (a) Experimentally measured THz intensity (blue circles) and theoretically calculated diffraction modification factor D (curves) as a function of target sizes. (b) Measured THz intensity at 75° (black square) and -75° (blue circle) from the metal-PE targets as a function of the thickness of the PE layer. (c) Comparison of the THz signals measured from the 40 μm thick PE targets with or without a 5 μm metal coating at the rear.
The laser-plasma-based THz transition radiation presented here could be a promising tabletop high-energy THz source. Moreover, it may provide a potential diagnostic to infer the spatiotemporal distribution of the high-flux fast electron beams generated in laser-solid interactions. References: [1] M. Tonouchi, “Cutting-edge terahertz technology”, Nature Photonics, 1, 97 (2007). Abstract. [2] Ziran Wu, Alan S. Fisher, John Goodfellow, Matthias Fuchs, Dan Daranciang, Mark Hogan, Henrik Loos, Aaron Lindenberg, “Intense terahertz pulses from SLAC electron beams using coherent transition radiation”, Review of Scientific Instruments, 84, 022701 (2013). Abstract. [3] W. P. Leemans, C. G. R. Geddes, J. Faure, Cs. Tóth, J. van Tilborg, C. B. Schroeder, E. Esarey, G. Fubiani, D. Auerbach, B. Marcelis, M. A. Carnahan, R. A. Kaindl, J. Byrd, M. C. Martin, “Observation of terahertz emission from a laser-plasma accelerated electron bunch crossing a plasma-vacuum boundary”, Physical Review Letters, 91, 074802 (2003). Abstract. [4] G. Q. Liao, Y. T. Li, C. Li, L. N. Su, Y. Zheng, M. Liu, W. M. Wang, Z. D. Hu, W. C. Yan, J. Dunn, J. Nilsen, J. Hunter, Y. Liu, X. Wang, L. M. Chen, J. L. Ma, X. Lu, Z. Jin, R. Kodama, Z. M. Sheng, J. Zhang, “Bursts of terahertz radiation from large-scale plasmas irradiated by relativistic picosecond laser pulses”, Physical Review Letters, 114, 255001 (2015). Abstract. [5] A. Gopal, S. Herzer, A. Schmidt, P. Singh, A. Reinhard, W. Ziegler, D. Brömmel, A. Karmakar, P. Gibbon, U. Dillner, T. May, H-G. Meyer, G. G. Paulus, “Observation of Gigawatt-class THz pulses from a compact laser-driven particle accelerator”, Physical Review Letters, 111, 074802 (2013). Abstract. [6] Guo-Qian Liao, Yu-Tong Li, Yi-Hang Zhang, Hao Liu, Xu-Lei Ge, Su Yang, Wen-Qing Wei, Xiao-Hui Yuan, Yan-Qing Deng, Bao-Jun Zhu, Zhe Zhang, Wei-Min Wang, Zheng-Ming Sheng, Li-Ming Chen, Xin Lu, Jing-Long Ma, Xuan Wang, Jie Zhang, “Demonstration of coherent terahertz transition radiation from relativistic laser-solid interactions”, Physical Review Letters, 116, 205003 (2016). Abstract. [7] C. B. Schroeder, E. Esarey, J. van Tilborg, W. P. Leemans, “Theory of coherent transition radiation generated at a plasma-vacuum interface”, Physical Review E, 69, 016501 (2004). Abstract. [8] Luke C. L. Yuan, C. L. Wang, H. Uto, “Formation-zone effect in transition radiation due to ultrarelativistic particles”, Physical Review Letters, 25, 1513 (1970). Abstract.
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.
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 the >10 THz or <3 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.
Yet it has the potential for numerous applications. In Physical Review Letters, scientists at Jülich together with their international partners present a new concept that uses short-pulse lasers to expand the capabilities of terahertz sources currently being developed. An important part was played by calculations performed on Jülich's supercomputer JUQUEEN. Using detailed simulations, the scientists showed how the wavelengths and polarization of the generated terahertz radiationcan be controlled via a strong externalmagnetic field. In the electromagnetic spectrum,terahertz wavesoccupy a band between microwaves and infrared radiation. The range from 0.1 THz to 30 THz, the so-called "terahertz gap", sits right between electronics and optics and is therefore not accessible through conventional electric devices or optical sources such as antennas and lamps. However, its special properties make THz radiation interesting for a variety of reasons: on the one hand, it penetrates textiles and plastics, while on the other, it is absorbed by many materials in a characteristic manner. The application spectrum ranges from non-invasive early cancer screening to food controls and body scans as well as ultrafast wireless connections.
Since the beginning of this century, femtosecond laser-based sources generating terahertz waves have been in use as comparatively compact and cheap alternatives to large particle accelerators. "This method works like a transformer that converts the high frequency of the incoming laser beam into the lower terahertz frequency range," explains Prof. Paul Gibbon from the Jülich Supercomputing Centre (JSC).
One particularly favourable technique, which is currently being refined, utilizes two ultrashort laser pulses of different frequencies which are aimed at a gas target. The gas is ionized and electrons are released. In this way, the much faster laser oscillationsare transformed into terahertz waves with lower frequencies. "The strong electromagnetic fields of the two lasers cause the electrons to oscillate but not quite harmonically - or sinusoidally - but instead asymmetrically, which when averaged over the laser cycle produces a type of direct current," says Humboldt fellow Dr. Wei-Min Wang, who also works at the Jülich Supercomputing Centre (JSC). The frequencies would otherwise remain in the higher range of lasers. "In this way, a terahertz pulse is generated that lasts exactly one cycle - and then radiates outwards," says Wang.
Conventional THz source (top): Two laser pulses with different frequencies are focused on a gas target. The electrons thus released have a slow drifting motion, which is emitted as THz radiation. New source (bottom): A multi-tesla magnetic field along the propagation direction of the laser forces the released electrons into a fast orbit and to thus function as a short-wave (sub-mm) radiation antenna. Credit: Forschungszentrum Julich
Together with researchers at the University of Strathclyde and the Institute of Physics in Beijing, which is part of the Chinese Academy of Sciences, the two researchers have now published a paper on their new concept, which would enable the generation of terahertz radiation with tunable wavelengths over several cycles with a narrow bandwidth - characteristics that are similar to those of lasers in the optical range. It utilizes a strong magnetic field which is applied externally to the ionized gas and forces the free electrons in the plasma to gyrate like in a particle accelerator. This orbit determines the wavelength as well as the direction of oscillation of the resulting radiation. The optical properties can be tuned as required by altering the strength of the magnetic field - which could open the door to a wide variety of new applications.
"Spectroscopic and imaging techniques - like, for example, those used to investigate the dynamics of large biomolecules such as DNA - could particularly benefit from such a radiation source as it promises better temporal and spatial resolving power," explains Wei-Min Wang. However, practically implementing it is not that easy, and an experimental verification has yet to be realised. "The concept requires a combination of powerful lasers and magnetic fields exceeding 100 tesla. Technically, this is extremely challenging, but it is possible within the given space and time scales," says Paul Gibbon. The scientists are performing complex simulation calculations on one of the fastest supercomputers in Europe - Jülich's supercomputer JUQUEEN - to explore the properties of the new terahertz source.
Optics Express, Vol. 22, Issue 10, pp. 11797-11803 (2014) http://dx.doi.org/10.1364/OE.22.011797
The interaction of 100-fs laser pulses with solid targets at laser intensities 1016-1018 W/cm2 has been investigated experimentally by simultaneous measurements of terahertz (THz) and second harmonic signals. THz yield at the front side of the target, which rises from the self-organized transient electron currents along the target surface, is found scaling linearly with the laser intensity basically. Measurements of specularly reflected light spectrum show clear evidence of resonance absorption. The positive effects of resonance absorption on surface current and THz radiation generation have been confirmed by two-dimensional (2D) particle-in-cell (PIC) simulations and angular-dependent experiments, respectively.
Wei-Min Wang, Yu-Tong Li, Zheng-Ming Sheng, Xin Lu, and Jie Zhang http://pre.aps.org/accepted/1d078R15L2619d0177d37fd730a6e235c87f880c7 Terahertz (THz) radiation via the two-color laser scheme is investigated theoretically and numerically when the second laser is at different harmonic orders of the main laser. It is found that THz radiation can be generated only when the second laser is an even harmonic and the THz field strength does not show a simple scaling with the two laser amplitudes, suggesting that it is different from a nonlinear optics processes. The THz strength generally tends to decrease with the increase of the even harmonic order. The strength is also sensitive to the carrier envelope (CE) phases of both the two laser pulses even if their durations are quite long. With different CE phases, the strength can be proportional to either sine or cosine of the relative phase displacement of the two lasers. For a given gas target except hydrogen, it increases with the laser amplitude between several saturation plateaus, each corresponding to a different ionization level. For a given main laser intensity, there is an optimized intensity of the second laser for the strongest THz radiation.
