Observation of Enhanced Field-Free Molecular Alignment by Two Laser Pulses

We show experimentally that field-free alignment of iodobenzene molecules, induced by a single, intense, linearly polarized 1.4-ps-long laser pulse, can be strongly enhanced by dividing the pulse into two optimally synchronized pulses of the same duration. For a given total energy of the two-pulse sequence the degree of alignment is maximized with an intensity ratio of $1:3$ and by sending the second pulse near the time where the alignment created by the first pulse peaks.

Figure 1

The degree of alignment vs time for the first pulse only (squares), the second pulse only (triangles), and both pulses (filled circles). The lines connecting the points serve to guide the eye. For a sample of randomly oriented molecules $⟨{cos}^2\theta ⟩=1/3$. The measured intensity shapes of the alignment pulses are given by the dotted line (arb. units).

Figure 2

The alignment dynamics due to a single pulse with intensity $5.5\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ (diamonds) and due to the two-pulse sequence illustrated in Fig. 1 (filled circles). The lines connecting the points serve to guide the eye. The total energy of the two-pulse sequence is the same as the energy for the single intense pulse.

Figure 3

(a) $⟨{cos}^2\theta ⟩$ measured 2 ps after the peak of the second pulse as a function of the time separation between the two pulses. The intensity ratio is $1:3$ and $I_1=1.4\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$, $I_2=4.1\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$. (b) The degree of alignment created by the first pulse only, measured at the time when the second pulse is sent. The data displayed in (a) and (b) were recorded for a higher rotational temperature than the data displayed in Figs. 1, 2. This explains why the overall degree of alignment is lower in Fig. 3 than in Figs. 1, 2.

Figure 4

The optimal degree of field free double pulse alignment, measured 2 ps after the peak of the second pulse, as a function of the intensity ratio, $I_1/I_2$, for a fixed total energy. For $I_1/I_2=0$, $I_2=5.5\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$, and for $I_1/I_2=1$, $I_1=I_2=2.6\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$.