Tilting light's polarization plane to spatially separate the nonlinear optical response of chiral molecules on ultrafast timescales

Abstract

Distinguishing between the left-and right-handed versions of a chiral molecule (enantiomers) is vital, but also inherently difficult. Traditional optical methods using elliptically or circularly polarized light rely on weak linear effects which arise beyond the electric-dipole approximation, posing major limitations for time resolving ultrafast chiral molecular dynamics. Here we show how, by tilting the plane of polarization of an ultrashort burst of intense elliptically polarized light, towards its propagation direction, we can turn the light field into a highly efficient chiro-optical tool. This "forward tilting" can be achieved by focusing the beam tightly, creating structured light which exhibits a nontrivial polarization pattern in space. We demonstrate that our structured field allows us to realize an interferometer for efficient chiral recognition that separates the nonlinear optical response of left-and right-handed molecules in space. Our work provides a simple, yet highly efficient, way of spatially structuring the polarization of light to image molecular chirality, with extreme enantio-sensitivity and on ultrafast time scales.Chirality plays key roles in nature, from dictating the behaviour of subatomic particles 1 to selecting the mating partners of snails 2 . In general, an object is chiral when it cannot be superimposed to its mirror image, with our hands being the typical example. In chemistry, the left-and right-handed versions of a chiral molecule are called enantiomers. Their handedness is essential in molecular recognition, and thus distinguishing between opposite enantiomers is vital in many different fields, including bio-medicine, organic chemistry or materials science. However, chiral distinction is challenging, as opposite molecular enantiomers behave identically unless they interact with another chiral object, such as another chiral molecule or chiral light.Cutting-edge laser technology creates exciting opportunities for studying molecular chirality, allowing us to access the natural temporal and spatial scales of molecules with unprecedented sub-femtosecond and sub-Angstrom resolution 3 . Important breakthroughs include the real-time observation of electronic currents in atoms 4-6 , molecules [7][8][9] , and solids [10][11][12] . Yet, despite these groundbreaking achievements, imaging the three-dimensional chiral currents governing enantio-sensitive chemical reactions is still very challenging, as natural chiral light is ill-suited for this purpose.Circularly polarized light is a standard tool for chiral recognition. In photo-absorption circular dichroism, one measures the differential absorption of left-and right-handed circularly polarized photons, ∆I = I L − I R , in a chiral medium 13 . While ∆I has opposite sign in opposite in opposite molecular enantiomers, it is only a small fraction of the total intensity of the optical response (I L I R ), making chiral recognition challenging, especially on ultrafast time scales. The reason behind this weak enantio-sensitivity is that circularl...