Tailoring the Optical Dipole Force for Molecules by Field-Induced Alignment

We report on the ability to tailor the optical dipole force for molecules by tuning their effective polarizability with strong field alignment using polarized fields. We have measured a difference of 20% in the dipole force on cold ${\mathrm{CS}}_2$ molecules when changing from linear to near-circular polarization using peak field intensities of $5.7\times {10}^{11}\mathrm{W}{\mathrm{cm}}^{-2}$. A variation in the focal length with laser polarization of a molecular-optical lens formed by a single focused laser beam was also measured. This provides a new way of modifying this force for many molecules.

Figure 1

Effective polarizability of ${\mathrm{CS}}_2$ for linearly polarized [dark gray (blue) line] and circularly polarized light [light gray (red) line] as a function of temperature at an intensity of $5.7\times {10}^{11}\mathrm{W}{\mathrm{cm}}^{-2}$. The inset graph shows the average effective polarizability of ${\mathrm{CS}}_2$ as a function of intensity calculated for $T_R=2\mathrm{K}$ (black), 12 K [light gray (red)], and 35 K [dark gray (blue)]. The solid lines are for linearly polarized light, and the dashed curves are for circularly polarized light. The average polarizability for randomly oriented molecules is shown by the horizontal short dashed line.

Figure 2

The experimental setup used to measure the velocity of ${\mathrm{CS}}_2$ within a time-of-flight mass spectrometer along the $x$ axis. The molecular beam propagates along the $x$ axis where it is crossed by a focused nonresonant laser beam (colored red) and is subject to the dipole force. The molecules experience a force along the $x$ and $y$ axis, shown as the green arrows. Velocity changes along the $x$ axis are detected by ionizing the molecules using a probe beam (colored blue) and recording their time-of-flight on a microchannel plate (MCP).

Figure 3

(a) Typical TOF distribution of molecules ionized by the UV probe beam $11\mu \mathrm{m}$ downstream from the center of the IR laser. The black curve is the TOF distribution with no IR field. The dark gray/blue (linear polarized light) and light gray/red (circular polarized light) distributions show the difference between the two polarizations. (b) Measurements of velocity along the $x$ axis of the IR beam. Dark gray (blue) circles are for linearly polarized light and light gray (red) circles for near-circularly polarized light. Solid curves are best fits for linearly and circularly polarized light at 35 K. The dashed curve simulates the effect of increasing the intensity of the circularly polarized beam by 6% to be equal to the linearly polarized beam. (c) Measurements of molecular velocity in a hot beam. Dark gray (blue) circles are measurements for linearly polarized light and light gray (red) circles for near-circularly polarized light.

Figure 4

Molecular density in the focal region of the molecular-optical lens along the $x$ axis. The linearly polarized light [dark gray (blue) circles] produces a focal length of $600\mu \mathrm{m}$, and the circularly polarized light [light gray (red) circles] produces a focal length of $700\mu \mathrm{m}$.