Using Air to Change the Frequency of Light

Shifting the frequency of broadband light is typically a demanding task, involving special modulators and requiring tailor-made adaptations to the specific case. We present an elegant and relatively simple way to shift the frequency of a light pulse by an order of THz, using only the molecules of air. Our idea opens the way to observe the Rotational Doppler shift that is many orders of magnitude higher than previously observed for mechanically-controlled methods.  Two independent experiments support our theory, making it not only novel, but also directly useful for interpreting several optical phenomena encountered experimentally.

Nowadays, femtosecond lasers can be used to rotate molecules in a dense gas. They can be spun coherently in the same sense. We assert that when circularly polarized light encounters such a gas, while the molecules are aligned, the rotating birefringent axis generates light with the opposite circular polarization. The exchange of angular momentum and energy between the light and the molecules induces a nonresonant and field-free frequency shift of the light by twice the molecular rotation frequency.

When a laser ignites rotation of molecules using the “laser induced molecular propeller” method, the gas is not isotropic anymore, but consists of aligned molecules that rotate on average in one direction (even after the laser pump is over). The rotational Doppler effect is manifested when a circularly polarized probe follows in the path of the pump pulses. The effective oscillation period of the electric field that the molecules experience is changed due to the rotation. In analogy, one may think of the question: in a watch, why do the slow hours dial (the rotating molecules) coincide with the fast minutes dial (the probe’s electric field) about every 65 minutes and not exactly every hour? The relative motion is what determines the frequency.
We started from the paraxial approximation of Maxwell’s equation, and formulated a theory that accounts for the optical effects through a chiral time-dependent refraction index [1]. We also participated in the design and analysis of experiments conducted by the Prior group [2]. Later, the Milner group (at UBC) performed other experiments that further corroborated our findings [3], as demonstrated in the following figure (top: theory, bottom: experiment).

 

References:

  1. U. Steinitz, Y. Prior, and I. Sh. Averbukh, "Optics of a Gas of Coherently Spinning Molecules", Phys. Rev. Lett. (in press); (2013).
  2. O. Korech, U. Steinitz, R. J. Gordon, I. Sh. Averbukh and Y. Prior, Nature Photonics 7, 711 (2013).
  3. A. Korobenko, A. A. Milner and V. Milner, "Complete control, direct observation and study of molecular super rotors", (2013).