Carrier transport after ultrashort-pulse laser excitation in dielectric materials leads to $10\mathrm{MV}{\mathrm{m}}^{-1}$ electric fields: Transient birefringence and emission of terahertz radiation

Charge-carrier transport is investigated theoretically in a high-band-gap dielectric material excited by intense ultrashort laser pulses. Simulations of fused silica reveal that the photo-Dember effect causes a ${10}^7\mathrm{V}{\mathrm{m}}^{-1}$ electric field to build up inside the material. It is shown that this electric field can induce a significant transient optical anisotropy in the dielectric, which may help reconcile earlier reports of an ultrafast onset of birefringence in highly excited fused silica. The time-dependent electric field will also lead to emission of a broadband THz pulse propagating out from and along the dielectric surface.

Figure 1

Electron (blue) and hole (red) distributions in $k$ space, distorted by the Dember field $E_D$. Energies are measured from the vacuum level ${\epsilon }_{\text{vac}}$ while the quasichemical potentials ${\zeta }_i$ are measured from (to) the band edges for electrons (holes). The upper left inset shows the charge-density distribution at the dielectric surface (left) that creates the Dember field.

Figure 2

(a) Simulated initial number densities and temperatures of electrons ($N_e,T_e$) and holes ($N_h,T_h$) at $t=50\mathrm{f}\mathrm{s}$. Note that $N_e\approx N_h=N$ in this case. The dashed green curve shows, in relative units, the spatial profile of the Dember field $E_D$. (b) Spatial profile of the photo-Dember field on a logarithmic depth scale at selected times after excitation. (c) Time evolution of the temperatures and the Dember field at the surface ($z=0$). The shaded area centered at time zero illustrates the intensity envelope of the excitation laser pulse.

Figure 3

Simulated effects of the Dember field on electrons in laser-excited fused silica. (a) Crystal momentum imparted to an electron at time $t$ at a depth $z$ from the surface with ${\tau }_M=120\mathrm{f}\mathrm{s}$. (b) Time-dependent effective mass along $z$ ($m_{zz}$) and $x$ ($m_{xx}$), averaged over a 20 nm depth from the surface. (c) Transient reflectivities of $p$- and $s$-polarized light ($R_p$ and $R_s$) for a 700 nm probe pulse incident at $60{}^{\circ }$ to the surface. Measurements are from Ref. [13]; solid lines are calculations based on the model presented in the main text.

Figure 4

THz pulse emitted from fused silica. (a) Instantaneous electric-field strength in the far field, and (b) the associated power-spectral density (PSD). The peak of the 35 fs pump pulse strikes the material surface at $t=105\mathrm{f}\mathrm{s}$. (c) Polar plot of the emitted THz electric-field magnitude from the sample surface. The blue-shaded area represents the solid. The red arrow marks the direction of the transient photo-Dember current.