Materials: One-way street for light

The closing ceremony of the International Year of Light took place last week. As a way to join in the chorus, and celebrate once more the importance and versatility of light-based research, it seems fitting to post this piece on a novel demonstration of directional light transparency. Long live fundamental physics!

 

Optical isolators – devices that allow the transmission of light in one direction only – are key components in systems such as laser sources. While isolators achieve one-way transparency by manipulating the properties of the incoming light, some materials exhibit electric and magnetic responses that are naturally dependent on the propagation direction of the electromagnetic radiation. This behaviour is known as nonreciprocal directional dichroism (NDD). A team from the University of Tokyo and Tohoku University in Japan reported the first experimental demonstration of one-way light transparency using a multiferroic crystal with large NDD.

In quantum mechanics, Fermi’s golden rule describes light-matter interactions in terms of electric and magnetic dipole transitions. The former usually dominates the latter at visible and infrared wavelengths, causing ordinary direction-independent absorption of light. Interference between the two dipole transitions is at the origin of NDD. In this case, the material can become transparent to light propagating in one direction provided that the electric and magnetic responses have the same amplitude.

Toyoda and collaborators chose multiferroic copper metaborate (CuB2O4) because it displays giant NDD at near-infrared frequencies. With the crystals of CuB2O4 cooled down to 4.2 K in order to enter an antiferromagnetic phase, the team shone linearly polarised light from a halogen lamp onto the samples while varying the intensity of an externally applied magnetic field. The absorption coefficients of CuB2O4 – one for each propagation direction of light – were then calculated from the intensity spectrum of the transmitted radiation.

The experimental data indicates that NDD appears above a critical magnitude for the magnetic field, with two distinct absorption peaks for each propagation direction of light due to Zeeman splitting. As the magnetic field becomes more intense (to about 50 T), the higher-energy absorption peak remains clearly visible for light propagating in one direction, while the radiation impinging on the crystals from the opposite direction experiences almost no absorption.

Further, the authors developed a theoretical quantum-mechanical description of NDD based on the wave functions for the ground state and the two excited levels for Cu2+ ions at a specific crystallographic site. In agreement with the experimental results, the model predicted one-way transparency of light in CuB2O4 for a given magnetic-field intensity.

The ability to describe and realise one-way transparency is an important insight into fundamental physics that may also lead to the design of novel optical isolators.

Phys. Rev. Lett. 115, 267207 (2015)

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