Field-Free Two-Direction Alignment Alternation of Linear Molecules by Elliptic Laser Pulses

We show that a linear molecule subjected to a short specific elliptically polarized laser field yields postpulse revivals exhibiting alignment alternatively located along the orthogonal axis and the major axis of the ellipse. The effect is experimentally demonstrated by measuring the optical Kerr effect along two different axes. The conditions ensuring an optimal field-free alternation of high alignments along both directions are derived.

Figure 1

Expectation values of the observables ${cos}^2{\theta }_z$ (full line) and ${cos}^2{\theta }_y$ (dashed line) for $a^2=1/3$, $\xi =11.1$, and $\tilde{T}=20$, as a function of normalized time.

Figure 2

Representation of the molecular state in spherical coordinates {$r\equiv |\psi ({\theta }_z,{\varphi }_z;t){|}^2$, ${\theta }_z,{\varphi }_z$} for $a^2=1/3$, $\xi =11.1$, $\tilde{T}=20$, at times $t={\tau }_{\mathrm{rot}}/4$ (left panel), $t\approx 3{\tau }_{\mathrm{rot}}/8$ (middle panel), and $t=3{\tau }_{\mathrm{rot}}/4$ (right panel). The polarization ellipse of the field $\overrightarrow{\mathcal{E}}$ is sketched in the $(x,y)$ plane.

Figure 3

Maximum over time of $⟨{cos}^2{\theta }_z⟩$ (dashed line), $⟨{cos}^2{\theta }_y⟩$ (full thick line), and $⟨{cos}^2{\theta }_x⟩$ (full thin line) with $\xi =11.1$ and $\tilde{T}=20$, as a function of $a^2$.

Figure 4

Field-free alignment of an ensemble of ${\mathrm{CO}}_2$ molecules ($T=296\mathrm{K}$, $P=4\times {10}^4\mathrm{Pa}$) produced by an elliptically polarized pump field [$a^2\approx 1/3$, see Eq. (1)] of peak intensity $25\mathrm{TW}/{\mathrm{cm}}^2$ and pulse duration ${\tau }_{\mathrm{FWHM}}=100\mathrm{fs}$. Cross defocusing signal recorded as a function of delay with the probe field linearly polarized along (a) the $y$ axis (major field axis) and (c) the $x$ axis (minor field axis). In (a) and (c), the theoretical results (thick lines) are shown as mirror images of the experimental ones. Corresponding observables (b) $⟨{cos}^2{\theta }_y⟩$ and (d) $⟨{cos}^2{\theta }_x⟩$. (e) $⟨{cos}^2{\theta }_z⟩$ along the propagation $z$ axis deduced from (b) and (d). The isotropic values $1/3$ are indicated with dashed lines.