Field-free molecular alignment induced by elliptically polarized laser pulses: Noninvasive three-dimensional characterization

An investigation of field-free molecular alignment produced by elliptically polarized laser pulses is reported. Experiments are conducted in ${\text{CO}}_2$ at room temperature. A noninvasive all-optical technique, based on the cross defocusing of a probe pulse, is used to measure the alignment along two orthogonal directions which is sufficient to provide a three-dimensional characterization. The field-free molecular alignment produced by a laser of elliptical polarization is in good agreement in terms of amplitude and shape with theoretical predictions. It turns out to be almost equivalent to the superposition of the effects that one would obtain with two individual cross-polarized pulses. The investigation highlights notably the occurrence of field-free two-direction alignment alternation for a suitably chosen degree of ellipticity. The analogy between this specific ellipticity and the well-known “magic angle” used in time-resolved spectroscopy to prevent rotational contributions is discussed.

Figure 1

(Color online) Experimental setup. $L$, lens; $\lambda ∕2$, half wave plate; $\lambda ∕4$, quarter wave plate; SC, static cell; BS, beam stop; PM, photomultiplier.

Figure 2

Cross defocusing signal (Expt.) measured with the probe field polarized along the $y$ axis (left-hand column) and along the $x$ axis (right-hand column) with the calculation (Theor.) shown reversed for different ellipticities defined by $a^2\simeq 0$ (a), (b), 1/4 (c), (d), 1/3 (e), (f), and 1/2 (g), (h). The corresponding observables $⟨{\text{cos}}^2{\theta }_y⟩\left(t\right)-1∕3$ and $⟨{\text{cos}}^2{\theta }_x⟩\left(t\right)-1∕3$ are depicted in the upper part of each panel. The alignment along the third axis (orthogonal to the ellipse’s plane), i.e., $⟨{\text{cos}}^2{\theta }_z⟩\left(t\right)-1∕3$, can be deduced from Eq. (2).

Figure 3

(Color online) Cross defocusing signal (full line) measured with the probe field polarized along the $y$ axis (left-hand column) and along the $x$ axis (right-hand column) for $a^2\simeq 0$ (a), (b), 1/4 (c), (d), 1/3 (e), (f), and 1/2 (g), (h). The calculations, shown as dashed lines, are multiplied by an adjusted scaling factor.

Figure 4

Maximum of signal at the first revival as a function of $a^2$ with the probe field polarized along the $y$ axis (a) and along the $x$ axis (b). The measurements are normalized with respect to the linear polarization $a^2=0$. Calculation is shown as a solid line.