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Strong sub-terahertz surface waves generated on a metal wire by high-intensity laser pulses.

Tokita S, Sakabe S, Nagashima T, Hashida M, Inoue S - Sci Rep (2015)

Bottom Line: Here, ultrafast field propagation along a metal wire driven by a femtosecond laser pulse with an intensity of 10(18) W/cm(2) is characterized by femtosecond electron deflectometry.From experimental and numerical results, we conclude that the field propagating at the speed of light is a half-cycle transverse-magnetic surface wave excited on the wire and a considerable portion of the kinetic energy of laser-produced fast electrons can be transferred to the sub-surface wave.The peak electric field strength of the surface wave and the pulse duration are estimated to be 200 MV/m and 7 ps, respectively.

View Article: PubMed Central - PubMed

Affiliation: 1] Advanced Research Center for Beam Science, Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan [2] Department of Physics, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo, Kyoto 606-7501, Japan.

ABSTRACT
Terahertz pulses trapped as surface waves on a wire waveguide can be flexibly transmitted and focused to sub-wavelength dimensions by using, for example, a tapered tip. This is particularly useful for applications that require high-field pulses. However, the generation of strong terahertz surface waves on a wire waveguide remains a challenge. Here, ultrafast field propagation along a metal wire driven by a femtosecond laser pulse with an intensity of 10(18) W/cm(2) is characterized by femtosecond electron deflectometry. From experimental and numerical results, we conclude that the field propagating at the speed of light is a half-cycle transverse-magnetic surface wave excited on the wire and a considerable portion of the kinetic energy of laser-produced fast electrons can be transferred to the sub-surface wave. The peak electric field strength of the surface wave and the pulse duration are estimated to be 200 MV/m and 7 ps, respectively.

No MeSH data available.


Related in: MedlinePlus

Results of numerical simulation of the electromagnetic field.The diameter of the tungsten wire is 0.3 mm. The length of wire between the position of electric charge emission and the end of the wire is 50 mm. (a) Snapshots of the distribution of the absolute electric field in cross sections along and across the wire axis. The x, y, and z coordinates correspond to those in Fig. 1. (b) Temporal waveforms of the radial electric field (solid lines) and azimuthal magnetic field (dashed lines) on the wire surface at the positions d = 12, 24, and 32 mm.
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f6: Results of numerical simulation of the electromagnetic field.The diameter of the tungsten wire is 0.3 mm. The length of wire between the position of electric charge emission and the end of the wire is 50 mm. (a) Snapshots of the distribution of the absolute electric field in cross sections along and across the wire axis. The x, y, and z coordinates correspond to those in Fig. 1. (b) Temporal waveforms of the radial electric field (solid lines) and azimuthal magnetic field (dashed lines) on the wire surface at the positions d = 12, 24, and 32 mm.

Mentions: Numerical simulations are necessary to understand the mechanism of the surface wave generation. However, two- and three-dimensional particle-in-cell codes, which are the most common methods used in laser-plasma simulations, have not generally been used for such a large target (a few centimeters) because of computational limits. Here, using a finite-difference time-domain algorithm, we perform a three-dimensional simulation of surface wave generation and propagation with a simplified electric current that approximates the emission of fast electrons from the laser-spot. Typically, fast electrons generated by laser-plasma interactions with solids at the irradiation spot are emitted in almost all directions31, and have a wide energy distribution32. The average and total kinetic energies of the fast electrons strongly depend on the interaction mechanism, but are generally of the order of a hundred keV33 and ten to several tens of percent of the laser pulse energy31, respectively, for a laser intensity of 1018 W/cm2. It should be noted that the fast electron pulse emitted into the vacuum from the laser-spot has a short temporal duration but it expands rapidly due to wide velocity and angular distributions. Therefore, the pulse duration of an electromagnetic wave excited by the fast electron current could be longer than the laser pulse duration. However, the contribution to the surface wave excitation from the fast electrons at places far from the wire surface should be small, because the electric field distribution of a Sommerfeld wave is localized close to the wire surface. Taking these factors into account, we adopt a very simple model for a surface wave excitation which is driven by the radial component of the fast electron current flowing toward the vacuum from the surface of a wire target. We assume a negative charge moving with a constant velocity of 0.5c (corresponding to the velocity of a 79 keV electron). We neglect the electromagnetic forces, and the spatial charge density is Gaussian with a FWHM of 0.45 mm (3 ps in duration) in the direction of propagation. Figure 6(a) shows a snapshot of the calculated electric field distribution: the propagating electric field surrounding the wire is radially polarized and is nearly axisymmetric. The simulation, therefore, indicates that a Sommerfeld surface wave is excited by the moving electric charge. When we assume an amount of electric charge of 16 nC, the peak electric and magnetic fields at the surface of the wire reach 194 MV/m and 0.64 T, respectively. The temporal waveforms of the radial electric field and azimuthal magnetic field at positions d = 12, 24, and 32 mm are illustrated in Fig. 6(b), which show that the waveforms have a main peak and a long tail. The pulse duration of the main peak is 6.7 ps (FWHM), but that of the reflected pulse has increased to 11.2 ps. Also, the peak intensity of the reflected pulse is about half of the initial value. These simulation results are consistent with the experimental results. The total kinetic energy of the 16 nC moving electric charge is about 1.3 mJ, which corresponds to 2% of the laser pulse energy in our experiment. This ratio is reasonable in terms of energy balance, because it is lower than the typical laser-to-fast-electron energy conversion efficiency at a laser intensity of the order of 1018 W/cm231. The total energy of the surface wave in the simulation is calculated to be 410 μJ, which corresponds to 32% of the total kinetic energy of the moving charge, but this ratio might be overestimated because the energy loss of the moving charge is neglected. From the simulation, we conclude that a fast electron current in the radial direction can couple efficiently to a picosecond half-cycle surface wave. Moreover, additional simulations show that the pulse energy of the surface wave increases in proportion to the square of the amount of electron charge. The laser-to-surface-wave energy conversion efficiency is, therefore, expected to rise with increasing laser pulse energy, which is consistent with experimental results shown in Fig. 5(c).


Strong sub-terahertz surface waves generated on a metal wire by high-intensity laser pulses.

Tokita S, Sakabe S, Nagashima T, Hashida M, Inoue S - Sci Rep (2015)

Results of numerical simulation of the electromagnetic field.The diameter of the tungsten wire is 0.3 mm. The length of wire between the position of electric charge emission and the end of the wire is 50 mm. (a) Snapshots of the distribution of the absolute electric field in cross sections along and across the wire axis. The x, y, and z coordinates correspond to those in Fig. 1. (b) Temporal waveforms of the radial electric field (solid lines) and azimuthal magnetic field (dashed lines) on the wire surface at the positions d = 12, 24, and 32 mm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4317702&req=5

f6: Results of numerical simulation of the electromagnetic field.The diameter of the tungsten wire is 0.3 mm. The length of wire between the position of electric charge emission and the end of the wire is 50 mm. (a) Snapshots of the distribution of the absolute electric field in cross sections along and across the wire axis. The x, y, and z coordinates correspond to those in Fig. 1. (b) Temporal waveforms of the radial electric field (solid lines) and azimuthal magnetic field (dashed lines) on the wire surface at the positions d = 12, 24, and 32 mm.
Mentions: Numerical simulations are necessary to understand the mechanism of the surface wave generation. However, two- and three-dimensional particle-in-cell codes, which are the most common methods used in laser-plasma simulations, have not generally been used for such a large target (a few centimeters) because of computational limits. Here, using a finite-difference time-domain algorithm, we perform a three-dimensional simulation of surface wave generation and propagation with a simplified electric current that approximates the emission of fast electrons from the laser-spot. Typically, fast electrons generated by laser-plasma interactions with solids at the irradiation spot are emitted in almost all directions31, and have a wide energy distribution32. The average and total kinetic energies of the fast electrons strongly depend on the interaction mechanism, but are generally of the order of a hundred keV33 and ten to several tens of percent of the laser pulse energy31, respectively, for a laser intensity of 1018 W/cm2. It should be noted that the fast electron pulse emitted into the vacuum from the laser-spot has a short temporal duration but it expands rapidly due to wide velocity and angular distributions. Therefore, the pulse duration of an electromagnetic wave excited by the fast electron current could be longer than the laser pulse duration. However, the contribution to the surface wave excitation from the fast electrons at places far from the wire surface should be small, because the electric field distribution of a Sommerfeld wave is localized close to the wire surface. Taking these factors into account, we adopt a very simple model for a surface wave excitation which is driven by the radial component of the fast electron current flowing toward the vacuum from the surface of a wire target. We assume a negative charge moving with a constant velocity of 0.5c (corresponding to the velocity of a 79 keV electron). We neglect the electromagnetic forces, and the spatial charge density is Gaussian with a FWHM of 0.45 mm (3 ps in duration) in the direction of propagation. Figure 6(a) shows a snapshot of the calculated electric field distribution: the propagating electric field surrounding the wire is radially polarized and is nearly axisymmetric. The simulation, therefore, indicates that a Sommerfeld surface wave is excited by the moving electric charge. When we assume an amount of electric charge of 16 nC, the peak electric and magnetic fields at the surface of the wire reach 194 MV/m and 0.64 T, respectively. The temporal waveforms of the radial electric field and azimuthal magnetic field at positions d = 12, 24, and 32 mm are illustrated in Fig. 6(b), which show that the waveforms have a main peak and a long tail. The pulse duration of the main peak is 6.7 ps (FWHM), but that of the reflected pulse has increased to 11.2 ps. Also, the peak intensity of the reflected pulse is about half of the initial value. These simulation results are consistent with the experimental results. The total kinetic energy of the 16 nC moving electric charge is about 1.3 mJ, which corresponds to 2% of the laser pulse energy in our experiment. This ratio is reasonable in terms of energy balance, because it is lower than the typical laser-to-fast-electron energy conversion efficiency at a laser intensity of the order of 1018 W/cm231. The total energy of the surface wave in the simulation is calculated to be 410 μJ, which corresponds to 32% of the total kinetic energy of the moving charge, but this ratio might be overestimated because the energy loss of the moving charge is neglected. From the simulation, we conclude that a fast electron current in the radial direction can couple efficiently to a picosecond half-cycle surface wave. Moreover, additional simulations show that the pulse energy of the surface wave increases in proportion to the square of the amount of electron charge. The laser-to-surface-wave energy conversion efficiency is, therefore, expected to rise with increasing laser pulse energy, which is consistent with experimental results shown in Fig. 5(c).

Bottom Line: Here, ultrafast field propagation along a metal wire driven by a femtosecond laser pulse with an intensity of 10(18) W/cm(2) is characterized by femtosecond electron deflectometry.From experimental and numerical results, we conclude that the field propagating at the speed of light is a half-cycle transverse-magnetic surface wave excited on the wire and a considerable portion of the kinetic energy of laser-produced fast electrons can be transferred to the sub-surface wave.The peak electric field strength of the surface wave and the pulse duration are estimated to be 200 MV/m and 7 ps, respectively.

View Article: PubMed Central - PubMed

Affiliation: 1] Advanced Research Center for Beam Science, Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan [2] Department of Physics, Graduate School of Science, Kyoto University, Kitashirakawa, Sakyo, Kyoto 606-7501, Japan.

ABSTRACT
Terahertz pulses trapped as surface waves on a wire waveguide can be flexibly transmitted and focused to sub-wavelength dimensions by using, for example, a tapered tip. This is particularly useful for applications that require high-field pulses. However, the generation of strong terahertz surface waves on a wire waveguide remains a challenge. Here, ultrafast field propagation along a metal wire driven by a femtosecond laser pulse with an intensity of 10(18) W/cm(2) is characterized by femtosecond electron deflectometry. From experimental and numerical results, we conclude that the field propagating at the speed of light is a half-cycle transverse-magnetic surface wave excited on the wire and a considerable portion of the kinetic energy of laser-produced fast electrons can be transferred to the sub-surface wave. The peak electric field strength of the surface wave and the pulse duration are estimated to be 200 MV/m and 7 ps, respectively.

No MeSH data available.


Related in: MedlinePlus