FTh1C.7.pdf
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Intense broadband THz pulse generation from relativistic laserplasma interaction S. Mondal, Q. Wei, H. A. Hafez, M. A. Fareed, A. Laramée, S. Sun, T. Ozaki INRS-Énergie Matériaux Télécommunications,1650, boulevard Lionel-Boulet, Varennes, QC, J3X 1S2, Canada Email:
[email protected]
Abstract: We demonstrate intense broadband terahertz pulse generation from the interaction of high intensity femtosecond laser with solid density plasmas. We measure substantial enhancement of terahertz pulse energy (32 J/pulse) by using aligned copper nanorod targets. OCIS codes: 140.0140, 190.0190, 320.0320, 020.2649, 350.5400
Introduction: Laser-plasma interaction [1,2] is known to be an excellent source of highly energetic photons [3], electrons [4], protons and ions [4]. Interesting researches are being carried out in laboratories around the world to develop tabletop sources of highly energetic charged particles and photons [5]. Recently, it has been found that intense laser-plasma interaction can also generate intense broadband terahertz (THz) pulses [6–9]. THz radiation is very useful for imaging and probing physical systems because of its nonionizing behavior, and also since it can pass through many materials that are opaque in the visible range. It also has strong interaction with many materials. Intense broadband THz pulses can open up new scientific areas to explore, such as nonlinear optics in the THz domain, single-shot THz spectroscopy and single-shot THz imaging. Intense THz pulses are also useful in developing the THz streak camera for the temporal measurement of femtosecond bunches of highly energetic electrons generated in high-intensity femtosecond laser plasma interaction and other techniques. Basic theory: To explain energetic THz pulse generation from high-intensity laser plasma interaction, mode conversion theory has been proposed [10]. Plasma waves oscillate at THz frequencies. For example plasma at 1018 cm-3 electron density oscillates at 9 THz frequency. If somehow such electrostatic plasma waves could convert to electromagnetic pulses, energetic THz pulses are generated. The plasmas generated by high-intensity laser solid interaction have density profiles that vary from zero (on the vacuum side) to solid density (1023 cm-3), and thus the generated THz pulses will be broadband. For obliquely incident high-intensity lasers irradiating a solid surface, oscillating surface current can also play an important role in intense THz pulse generation. THz pulse energy generated via this mechanism peaks at relatively large incidence angles (67o). Experimental setup: Fig. 1 shows a schematic diagram of the experimental setup. A multi-TW laser pulse from the 10 Hz beam line of the Advanced Laser Light Source (ALLS) with high contrast (contrast ratio ~ 10 -7) is focused on to a copper target (size: 5cm 5cm 3mm) using an f/3 off-axis parabolic mirror on to a circular spot size of 20 µm. The target is mounted on a motorized XYZ translation stage so that each laser pulse interacts with a fresh target surface. The translation stage is placed inside the vacuum chamber. The 10 Hz beam line at ALLS can deliver laser pulses with energies of 240 mJ maximum in 40 fs after compression, which corresponds to peak intensity of about 31018W/cm2. Ultra-broadband THz and infrared (IR) pulses are generated as a result of the laser-plasma interaction. The generated THz pulses are then collimated by using a goldblended off-axis parabolic mirror and then guided out of the vacuum chamber through a plastic window. The generated THz pulse is then refocused on to a pyroelectric detector by another off-axis parabolic mirror. The energy of the THz pulses generated by this process is measured by this calibrated pyroelectric detector in a single-shot measurement. THz pulse energy is collected over a solid angle of 0.0873 sr. To remove the 800 nm driving laser, we have used two high- Fig. 1: Schematic diagram of the experimental setup. resistivity float-zone silicon (HRFZ-Si) filters that have flat
FTh1C.7.pdf
CLEO:2015 © OSA 2015
transmission with a cut-off around 1.5 µm (200 THz). We take 10 shots average to reduce the statistical errors. Then the THz pulse energy from the plain target is compared with that from the copper nanorod target coated on a bulk copper substrate. Results and discussions: Besides characterizing the THz source, our aim is to increase the THz pulse energy by using copper nanorod coated targets. The results are shown in the Fig. 2 below.
Fig. 2 THz energy per pulse from nanorod targets of different length.
Fig. 3 SEM image of Cu nanorod targets
The results are summarized in the table below. Material
Aspect ratio
THz pulse energy at the detector
THz enhancement
Cu tape on bulk Cu
N.A.
1.13 µJ
1
5 µm long Cu nanorod
25
3.2 µJ
2.8
10 µm long Cu nanorod
50
2.9 µJ
2.6
20 µm long Cu nanorod
100
10 µJ
8.8
50 µm long Cu nanorod
250
32 µJ
28.3
Table 1: THz pulse energy enhancement with structural properties of Cu nanorod.
With an increase in the aspect ratio of nanorod, the THz pulse energy increases, and a maximum enhancement of about 28 times (compared with the THz energy from a copper tape target of bulk copper) and a THz pulse energy of 32 µJ is achieved for the nanorod target with an aspect ratio 250. Recently, we have also performed experiments with another low-pass window, which transmit THz pulses only up to 20 THz. Detailed analysis of the experimental results will be given. References: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
P. Gibbon, "Short Pulse Laser Interactions with Matter ", (Imperial College Press London, 2005). G. Ravindra Kumar, "Intense, ultrashort light and dense, hot matter", Pramana 73, 113 (2009). G. Kulcsar, D. AlMawlawi, F. Budnik, P. Herman, M. Moskovits, L. Zhao, and R. Marjoribanks, "Intense picosecond X-Ray pulses from laser plasmas by use of nanostructured "Velvet" targets", Phys. Rev. Lett. 84, 5149 (2000). S. P. Hatchett, et al., "Electron, photon, and ion beams from the relativistic interaction of Petawatt laser pulses with solid targets", Phys. Plasmas 7, 2076 (2000). D. Umstadter, "Relativistic laser plasma interactions", J. Phys. D. Appl. Phys. 36, R151 (2003). H. Hamster, A. Sullivan, S. Gordon, W. White, and R. W. Falcone, "Subpicosecond, electromagnetic pulses from intense laser-plasma interaction ", Phys. Rev. Lett. 71, 2725 (1993). X. Dong, Z. Sheng, H. Wu, W. Wang, and J. Zhang, "Single-cycle strong terahertz pulse generation from a vacuumplasma interface driven by intense laser pulses", Phys. Rev. E 79, 046411 (2009). A. Gopal, et al., "Observation of energetic terahertz pulses from relativistic solid density plasmas", New J. Phys. 14, 083012 (2012). C. Li, et al., "Effects of laser-plasma interactions on terahertz radiation from solid targets irradiated by ultrashort intense laser pulses", Phys. Rev. E 84, 036405 (2011). H.-C. Wu, Z.-M. Sheng, and J. Zhang, "Single-cycle powerful megawatt to gigawatt terahertz pulse radiated from a wavelength-scale plasma oscillator", Phys. Rev. E 77, 046405 (2008).
