Imagine holding up two crystals that look almost identical. They are mirror images of each other, like your left and right hands. Shine ordinary light on them, and they behave the same. But shine light that twists in a spiral—and suddenly, one crystal comes alive while its twin barely responds. Scientists at Hebrew University of Jerusalem have found a new way to watch this happen without touching the materials at all.
Dr. Joanna Dehnel and Dr. Igal Levine led a team that wanted to understand what happens inside these special crystals when light twists in different directions. The materials are called chiral 2D perovskites, named for their hand-like mirror-image structure. The problem was that traditional testing required building the crystal into a full electronic device first, complete with metal contacts. Those contacts can introduce tiny flaws or other effects that muddy the results, making it hard to know what the crystal itself is really doing.
So the team invented a new technique. They call it circularly polarized time-resolved surface photovoltage, or CP-TRSPV for short. Instead of wiring a crystal into a device, the method lets researchers watch what happens inside the material using only light. Think of it as listening to what the crystal has to say before wiring it into a machine.
What they found surprised them.
When the researchers switched between right-twisting and left-twisting light, the two mirror-image crystals responded in opposite ways. The R form lit up most strongly under right-twisting light, while the S form responded best to left-twisting light. A mixed sample with no preferred handedness showed no difference between the two types of light. The crystal structure itself was acting like a microscopic filter, determining which electrons could move through most easily.
But the most striking finding was just how large the effect was. The electrical response differed by about 1,000 times more than what you would expect from light absorption alone. The team measured a number called the photovoltage anisotropy factor, reaching values of minus 0.7 in the R material and 0.17 in the S material. Something more interesting was happening than simple light absorption—electrons with different quantum properties called spin were being sorted and separated by the crystal's twisted structure.
This aligns with an effect known as chiral-induced spin selectivity, or CISS. The twisted crystal acts as a kind of filter for electrons, favoring those with one spin direction over another. The initial spin sorting happens almost instantly, in tiny fractions of a second. But the electrical charge difference created by this sorting can last much longer, from billionths of a second all the way to thousandths of a second.
The method could ultimately help scientists develop new mirror-image materials for technologies that use twisted light, electrical charge, and electron spin. It offers a faster, cleaner way to test promising materials before building them into real devices, eliminating the noise that metal contacts and other parts introduce.
For the team at Hebrew University, the work opens a window into behaviors that were previously hidden behind layers of contacts and construction. Now, the crystals can speak for themselves.