Tunable Circularly Polarized Terahertz Radiation from Magnetized Gas Plasma

It is shown, by simulation and theory, that circularly or elliptically polarized terahertz radiation can be generated when a static magnetic ($B$) field is imposed on a gas target along the propagation direction of a two-color laser driver. The radiation frequency is determined by $\sqrt{{\omega }_p^2+{\omega }_c^2/4}+{\omega }_c/2$, where ${\omega }_p$ is the plasma frequency and ${\omega }_c$ is the electron cyclotron frequency. With the increase of the $B$ field, the radiation changes from a single-cycle broadband waveform to a continuous narrow-band emission. In high-$B$-field cases, the radiation strength is proportional to ${\omega }_p^2/{\omega }_c$. The $B$ field provides a tunability in the radiation frequency, spectrum width, and field strength.

Figure 1

[(a) and (b)] Snapshots of the terahertz electric fields ($\mathrm{MV}/\mathrm{cm}$) at the time of 0.7 ps, and (c) the field distributions on the axis ($y=0$) at 2 ps, where an external $B$ field of 178 T is imposed. The broken lines in (c) correspond to the case without the $B$ field.

Figure 2

(a) Temporal waveforms of the terahertz electric fields on the axis and (b) the corresponding spectra, where an external $B$ field of 178 T is imposed. The components below 1 THz are filtered.

Figure 3

(a) Temporal waveforms of the terahertz electric fields $E_y$ on the axis and (b) the corresponding spectra, where different lines correspond to different ${\omega }_c$ (${\omega }_c=1\mathrm{THz}$ corresponds to $B_0=35.7\mathrm{T}$).

Figure 4

(a) Temporal waveforms of the terahertz electric fields $E_y$ on the axis and (b) the corresponding spectra, where different lines correspond to different ${\omega }_p$ or gas densities. The $B$ field strength is fixed at 357 T. The inset in (a) shows the radiation fields $E_y$ and $E_z$ with ${\omega }_p=4\mathrm{THz}$ and the components below 2 THz are filtered.

Figure 5

[(a) and (c)] Temporal waveforms of the terahertz electric fields on the axis and [(b) and (d)] the corresponding spectra, where the gas density is taken as $1.96\times {10}^{17}{\mathrm{cm}}^{-3}$ (${\omega }_p=4\mathrm{THz}$) and different lines correspond to different ${\omega }_c$ (${\omega }_c=0.5\mathrm{THz}$ corresponds to $B_0=17.8\mathrm{T}$).