Picture a ring of light on a camera screen, dotted with bright patches like the numerals on a clock face. Nudge the colour of the laser ever so slightly, and the whole pattern turns, as if someone had reached in and twisted the dial.
That, in essence, is what a University of Glasgow-led team of physicists has demonstrated. It may offer a new way of answering one of the most basic questions in laser science: exactly what colour is this beam, and is it staying put?
The work, led by Professor Sonja Franke-Arnold of the university's School of Physics & Astronomy, is published in the journal Optica, the university announced on 29 September.
Why lasers need keeping in tune
A laser's colour is really its frequency: how rapidly its light waves oscillate. Atomic clocks, GPS and the quantum sensors now emerging from laboratories all depend on lasers whose frequency is held remarkably steady.
The standard trick is called frequency locking. Scientists shine the laser through a gas of atoms, which respond only to very particular frequencies, and use that response as a fixed reference point. Electronics then keep nudging the laser back on target, rather like a musician tuning a string against a reference note.
"Atoms respond to light at very specific, characteristic frequencies, and this property has been exploited for a long time as an absolute frequency standard in systems which require absolute accuracy, like atomic clocks," Professor Franke-Arnold said in a statement released by the university.
"Almost every technique for measuring frequency currently in use involves reading a single trace on a photodiode detector, providing a one-dimensional trace to which a laser is locked."
From a single line to a whole picture
The Glasgow team's approach swaps that single line for an image.
Two laser beams travel in opposite directions through a small glass cell of rubidium vapour. The first, a conventional beam, lines up the atoms, herding around 90 per cent of them into the same quantum state, according to the university.
The second is so-called structured light: a ring-shaped "vector beam" whose polarisation, the direction in which its light waves vibrate, changes at every point around the circle.
How strongly the prepared atoms act on this ring depends on how close the laser is to their natural resonance. When the team split the emerging light by polarisation and photographed it, bright "lobes" appeared around the ring. As the frequency shifts, the lobes rotate; as absorption changes, they brighten or dim.
The result is a visual readout, in one snapshot, of how far the laser has drifted. A shift of one megahertz produced a rotation of almost six degrees in the image.
"In this new research, however, we probe our atoms with an image, and we watch how that image changes as the frequency changes," said the paper's first author, Richard Aguiar Maduro, also of the School of Physics & Astronomy. "An image contains far more information than an individual signal, so this opens up a completely new way of determining frequencies."
Early days
It is worth being plain about where this stands. The university says the technique currently lags behind the most accurate methods, which can detect frequency changes at the kilohertz level, far finer than the sub-megahertz sensitivity shown in the paper. The researchers say they are working to sharpen it and are confident it can be improved.
This is a laboratory demonstration of a new principle, not a device heading for the shops.
What it could lead to
Because the pattern also responds to magnetic fields, the team suggest it could underpin a new kind of magnetometer able to map magnetic fields in three dimensions from a single image. It could also help create storage media for "hybrid entanglement" of light, for use in secure quantum networks. The paper also names image-based laser locking and high-precision spectroscopy as possible applications.
"What excites us most is that we've tied together three different degrees of freedom with a light beam: its shape, its polarisation, and its frequency," Professor Franke-Arnold said.
"Normally a spatial light pattern shouldn't be affected by frequency or magnetic fields at all, but for us it does, which is a powerful correlation. These results give us a handle on a new method that hasn't been explored before, and we're excited to explore how far it will take us."
Co-authors include Glasgow's Dr Sphinx Svensson and Craig Millar, alongside researchers from the Glasgow-based Fraunhofer Centre for Applied Photonics, Germany's national metrology institute, the Physikalisch-Technische Bundesanstalt, and TU Braunschweig.
The paper, "Spatio-spectral vector light created by optical activity in rubidium vapor", is published in Optica, volume 13, pages 1951-1957 (DOI: 10.1364/OPTICA.606117).
