Investigation of molecular chirality on a sub-femtosecond time scale

A new nonlinear, all-optical method for detecting chiral molecules has been proposed and demonstrated. It is significantly more sensitive than conventional all-optical methods and also allows a sub-femtosecond temporal resolution of chiral dynamics.

Two enantiomers of a chiral molecule are just like our left and right hands: they are mirror images of each other. They have identical physical properties, except when interacting with chiral light. In linear chiroptical spectroscopy, the chiral response, often referred to as "chiral dichroism," is very low, in the range of 10-4 - 10-5 of a normal linear optical response, such as light absorption. This poses a major challenge to time-resolved measurements. This challenge was addressed in the paper by Cireasa et al. in which a new approach to chiroptical recognition is demonstrated and analyzed.

The new approach is based on high-harmonic spectroscopy. High-harmonic generation occurs when an intense femtosecond laser pulse is focused into a gas. It can be understood as a sequence of three steps: ionization in a strong infrared field, laser-induced acceleration of the released electron, and its recombination with the parent ion, all within one laser cycle. The recombination results in the emission of coherent radiation which extends from the vacuum ultraviolet to the soft X-ray region.

Fig. Calculations of the ellipticity dependence of the high-harmonic signal in S-epoxypropane (a) and R-epoxypropane (b) confirm that the chiral-sensitive signal is particularly strong around harmonics 41-43, where the primary (chiral -insensitive) High-harmonic channels XX and AA destructively interfere. For each harmonic, the signal was normalized with its corresponding ellipticity-dependent maximum.

R. Cireasa et al. investigated how the chiral structure of the molecule influences this process. While the released electron is driven by the laser field, the same thing happens with the electron hole. Moreover, the laser-driven hole motion is chiral and enantio-sensitive due to the chiral structure of the molecule. When the returning electron recombines with the hole, the enantio-sensitive nature of the hole motion makes the emitted light enantio-sensitive. As a result, a very small ellipticity of the drive laser field, in the range of about 1%, is sufficient to distinguish between the harmonics emitted by left-handed and right-handed molecules, with signals being 2-3% different.

High-harmonic generation can be regarded as pump-probe spectroscopy. Ionization has the function of pumping and starts the electron-hole dynamics. Recombination serves as a probe that maps the electron-hole dynamics by means of the emitted light. The pump-probe delay is controlled by the vibrations of the laser field which drives the electron. The energy of the returning electron depends on how much time it has spent in the field. Therefore, harmonics with different energies are emitted at different times, allowing an assignment between the harmonic order and the pump-probe delay. In short, the harmonic emission records a film of the recombining system, each harmonic representing a frame of the film. A huge bandwidth of the harmonic spectrum leads to a very high temporal resolution, about 0.1 fs or better. R. Cireasa et al. used this property to reconstruct the chiral component of the hole dynamics from the experimentally measured chiral dichroism with a resolution of 0.1 fs.

Original publication

Probing molecular chirality on a sub-femtosecond timescale

R. Cireasa, A. E. Boguslavskiy, B. Pons, M. C. H. Wong, D. Descamps, S. Petit, H. Ruf, N. Thire, A. Ferre, J. Suarez, J. Higuet, B. E. Schmidt, A.F. Alharbi, F. Legare, V. Blanchet, B. Fabre, S. Patchkovskii, O. Smirnova, Y. Mairesse, V. R. Bhardwaj

Nature Physics 11 (2015) 654-658