Aug. 1, 2024

Two RIKEN physicists have discovered why a special material consisting of alternating organic and inorganic layers exhibits a large difference to left and right circularly polarized light¹. This discovery could help to develop materials for new optoelectronic devices.

Polarized sunglasses work by using special chemicals to block light polarized in one direction, thereby eliminating about 50% of the light.

Light can also be ‘circularly’ polarized, such that the axis of polarization rotates either clockwise (right circularly light) or counterclockwise (left circularly light) as the light travels.

Scientists are keen to harness circularly polarized light to produce novel devices.

“Circularly polarized light adds an extra degree of freedom for optical information processing using photons,” notes Daichi Okada of the RIKEN Center for Emergent Matter Science (CEMS). “So a simple system that can discriminate or generate circularly polarized light efficiently will be highly useful for future optical information technology.”

Some organic chemicals are chiral, namely they exist in two mirror images. The mirror images of chiral molecules absorb left and right circularly light by different amounts depending on their chirality.

But this effect is very feeble—typically less than 1%. A much bigger difference is observed for so-called nonlinear optical effects that occur at high light intensities in special materials.

One class of such special materials consists of alternating layers of chiral organic molecules and of an inorganic compound known as a halide perovskite. They have many potential uses. “Chiral organic–inorganic halide perovskites are promising for solar cells, memory devices, LEDs, chiral sensors and quantum computers,” notes Okada.

However, it was not clear why chiral organic–inorganic halide perovskites exhibit such a large difference between left and right circularly polarized light for nonlinear optical effects.

Now, Okada and Fumito Araoka, also of CEMS, have uncovered its cause–a transition involving magnetic dipoles generated by optical stimulation.

While the magnetic dipole transition is well known to be related to nonlinear optical responses in other materials, its role in the nonlinear response of chiral organic–inorganic halide perovskites had not been explored previously.

“We’ve revealed the larger contribution of magnetic dipole transition in nonlinear optical effects leads to larger anisotropic response to circular polarized light,” says Okada. “It provides an important platform for material design and a system to achieve an efficient chiral nonlinear optical response.”

One surprise of the study was just how large the effect was. “Our materials exhibit the largest anisotropic response to the handedness of circularly polarized light among chiral materials to date,” says Okada.

The team is now longer at ways to increase the efficiency of the nonlinear optical effect in their material.

Reference

1. Okada, D. & Araoka, F. Magneto-chiral nonlinear optical effect with large anisotropic response in two-dimensional halide perovskite. Angewandte Chemical International Edition e202402081 (2024). doi: 10.1002/anie.202402081