Prediction of attosecond light pulses in the VUV range in a high-order-harmonic-generation regime

Attosecond light pulses within the vacuum ultraviolet (VUV) energy range are predicted by solving the time-dependent Schrödinger equation (TDSE) for a model neon atom in short laser pulses of different field polarization states. We compare high-order harmonic generation in linearly polarized laser pulses to the method of polarization gating and find attosecond pulses that approach the Fourier limit of 700 as given by an indium filter, spectrally centered at 15 eV. At such low energies, harmonic generation has low sensitivity to ellipticity, which enables the generation of elliptically polarized attosecond pulses. We also show that emission at the atomic transition energies is strongly damped by including intensity averaging.

Figure 1

HHG spectra for the neon model generated with a linearly polarized IR field for two different CEPs $\varphi =0.0$ (a) and $\varphi =0.5\pi$ (b). Shown are the results from the SAR (blue dashed line) and after intensity averaging (red line). The black dotted line shows the indium filter. The arrow indicates the cutoff for the peak intensity, calculated as $I_p+3.2U_p$, with the ponderomotive potential $U_p$.

Figure 2

HHG spectra generated from the neon model irradiated with a linearly polarized pulse as a function of the CEP $\varphi$ after intensity averaging (logarithmic color scale). The dependence on the CEP for energies above 40 eV, in particular in the cutoff, can be clearly seen.

Figure 3

Gabor distributions $G(\omega ,t)$ for the linearly polarized IR field with CEP $\varphi =0$ (logarithmic color scale). (a) Single-atom response (SAR), (b) after intensity averaging.

Figure 4

(a) Temporal duration of the predicted VUV pulses (FWHM, red solid line and RMS, green dotted line) from a linearly polarized IR field for the intensity averaged signal. (b) IR field with $\varphi =0$ (gray dotted) and the VUV-pulse intensities predicted for a single atom (SAR, blue solid line, downwards and down-scaled for clarity) and the averaging model (red solid line). Also shown for comparison is the XUV pulse obtained from filtering the cutoff region (black dashed line).

Figure 5

HHG spectra generated with the polarization gating method ($\delta =3/4T_L$ and $\varphi =0$) polarized along the main axis (a) and minor axis (b). Shown are the result from the SAR (blue dashed line) and after intensity averaging (red solid line). The black dotted line shows the indium filter. The arrow indicates the cutoff $I_p+3.2U_p$ for the peak intensity.

Figure 6

Temporal duration of the VUV pulses [FWHM (a) and RMS (b)] from the polarization gating for the intensity averaged signal. (c)–(e) Absolute value of the ellipticity of the IR field for $\varphi =0$ (thin black line) and the intensity profiles of the VUV pulses. Delays are (c) $\delta =5/4T_L$ (red solid line), (d) $\delta =3/4T_L$ (blue dashed line), and (e) $\delta =1/4T_L$ (green dotted line).

Figure 7

(a) Electric field components of the VUV pulse generated with the polarization gating method ($\delta =5/4T_L$, $\varphi =0$). Shown are the components polarized along the main axis [$E_x\left(t\right)$, red solid line], minor axis [$E_y\left(t\right)$, blue dashed line], and their envelope [$\sqrt{|E_x{\left(t\right)|}^2+{\left|E_y\left(t\right)\right|}^2}$, green dotted line]. (b) Ellipticities of the VUV pulse (red solid line) and the IR field (blue dashed line) calculated from Eq. (5). (c) and (d) Same as (a) and (b) but for $\varphi =0.8\pi$.