Rectification is the process of turning alternating current (AC) signals into direct current (DC), and it underpins technologies such as wireless power transfer, photodetection, terahertz sensing, and energy harvesting. To generate a one-directional current, a material normally needs a built-in directional preference for electron motion. This preference usually arises from broken inversion symmetry, meaning that the material is not identical under spatial inversion. For example, graphene has inversion symmetry, while materials with inequivalent sublattices, intrinsic electric dipoles, surfaces, or interfaces naturally break it. When inversion symmetry is broken, electrons can respond differently to opposite directions of an applied light field, allowing oscillating optical fields to generate a DC current.

In this work, the researchers show that this rule has an important exception. Even centrosymmetric bulk materials can rectify light through third-order nonlinear optical effects. Linear optical responses scale with the applied light field. Second-order responses depend on the square of the electric field and can mix frequencies or generate harmonics, but they are usually forbidden in centrosymmetric bulk materials. Third-order responses, however, are symmetry allowed and can generate DC photocurrents even when inversion symmetry is present. These currents are controlled by the shape of the Fermi surface, disorder, and the geometry of electronic bands. This means that materials once considered unsuitable for bulk optical rectification, including common metals, doped systems, and two-dimensional materials, can be engineered to convert oscillating light into DC electrical signals.

This research expands the material platform for optical rectification. Instead of relying only on noncentrosymmetric crystals, it shows that common centrosymmetric materials can also be used for light-to-DC conversion through third-order nonlinear response. This opens a route to energy-efficient photodetectors, terahertz technologies, and future energy-harvesting devices based on materials such as graphene.

Do you want to learn more about this topic?

Balancing selectivity and permeability in nanofluidic membranes for osmotic power generation Han Qian et al. (2025)

The post Rethinking rectification for future energy technology appeared first on Physics World.

In condensed matter physics, driving a material with an external stimulus can push it into new nonequilibrium states that reveal hidden properties or create entirely new, potentially useful behaviours. These stimuli can include optical driving, where strong oscillating light is applied to the material, or periodic forcing, which refers to any repetitive push such as acoustic waves, modulated electric fields, or oscillating magnetic fields. 

In this work, the researchers wanted to understand how shining bright, oscillating light on a material can make it behave in unexpected ways, sometimes even resembling a superconductor. They used a theoretical model to study what happens when the system is periodically driven, allowed to exchange energy with a heat bath, and coupled to electromagnetic fields. When the drive is strong enough, the system can spontaneously organise into different kinds of ordered states: uniform order, spatially patterned order, or time oscillating order. 

Ordered phases can repel magnetic fields in the same way a superconductor does, through the Meissner effect, where the electromagnetic field behaves as if the photon has gained an effective mass and therefore cannot propagate inside the material. In some driven phases, however, not all of the magnetic field is expelled: part of it enters the material but only as a standing wave, forming a hybrid light-matter excitation known as a Meissner polariton. Additionally, strong fluctuations near the onset of ordering can make the material’s optical conductivity appear superconducting, causing experiments to detect superconducting like signals even when no true superconducting phase is present, helping explain why light‑driven systems sometimes show ambiguous signs of superconductivity. 

Overall, the researchers developed a unified theoretical picture showing how periodic driving can create or mimic superconducting behaviour, including predicting a new hybrid light-matter mode (the Meissner polariton), offering insight into puzzling experimental results in light driven materials.

Do you want to learn more about this topic?

Laser-induced magnetization dynamics and reversal in ferrimagnetic alloys by Andrei Kirilyuk, Alexey V Kimel and Theo Rasing (2013)

The post Driving matter into new states appeared first on Physics World.