Bursts of Terahertz Radiation from Large-Scale Plasmas Irradiated by Relativistic Picosecond Laser Pulses

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 $100\mu \mathrm{J}$ per steradian in the laser specular direction at an optimal preplasma scale length around $40–50\mu \mathrm{m}$. 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.

Figure 1

Schematic view of the experimental setup. The inset shows the transmittance curve of HRFZ Si calibrated with a Fourier transform infrared spectrometer (red dash dotted) and the total effective responsivity of the combination of the filters and detector (blue solid).

Figure 2

Dependence of the THz energy or relative THz intensity on (a) the laser energy, (b) preplasma scale length $L$, and (c) different frequency regions. The experimental data are an average of about 5 shots. The data in (b) and (c) are obtained at $E_{\text{laser}}\sim 5\mathrm{J}$ with a shot-to-shot fluctuation of 10%.

Figure 3

Schematic diagram showing the THz radiation via LMC in a nonuniform plasma. When the intense picosecond laser pulse is incident obliquely into a nonuniform underdense plasma, EPWs are produced in a wide region along the laser propagation by SRBS and SM LWF. Note even though these two kinds of EPWs show distinctly different wavelengths, the radiation frequency is determined by the local plasma frequency.

Figure 4

(a) Spatial-temporal plots of the THz radiation field from PIC simulations. The plasma density rises exponentially from $x=50{\lambda }_0$ to $x=234{\lambda }_0$ with $L=40{\lambda }_0$. The white solid lines show the vacuum-plasma boundary. The laser pulse runs along $t=x$, indicated by the red dashed line. The white dashed line shows $t=x+{\tau }_L/2$. (b)–(d) The time-integrated frequency spectra of the $p$-polarized THz emission and (e)–(f) backward scattered laser light in different temporal stages, detected at $x=45{\lambda }_0$ in vacuum. (g) The waveform of the laser pulse transmitting the plasma slab, detected at $x=244{\lambda }_0$, and the initial envelope of laser pulse. The spectrum for $75–260{\tau }_0$ has been multiplied by a factor of 4.