Slits in Time...


The classic double-slit experiment leads to characteristic interference patterns. Credit: Russell Knightly/SPL

Topics: Modern Physics, Optics, Quantum Mechanics

A celebrated experiment in 1801 showed that light passing through two thin slits interferes with itself, forming a characteristic striped pattern on the wall behind. Now, physicists have shown that a similar effect can arise with two slits in time rather than space: a single mirror that rapidly turns on and off causes interference in a laser pulse, making it change color.

The result is reported on 3 April in Nature Physics1. It adds a new twist to the classic double-slit experiment performed by physicist Thomas Young, which demonstrated the wavelike aspect of light, but also — in its many later reincarnations — that quantum objects ranging from photons to molecules have a dual nature of both particle and wave.

The rapid switching of the mirror — possibly taking just one femtosecond (one-quadrillionth of a second) — shows that certain materials can change their optical properties much faster than previously thought possible, says Andrea Alù, a physicist at the City University of New York. This could open new paths for building devices that handle information using light rather than electronic impulses.

Romain Tirole, a quantum physicist at Imperial College London, and his collaborators shot an infrared laser at a surface made of layers of gold and glass with a thin coating of indium tin oxide (ITO), a material common in smartphone screens.

Under normal conditions, ITO is transparent to infrared light. But the researchers were able to make the material reflective using a second laser, which excited electrons in the material, affecting its optical properties. This could be done with pulses from the second laser that lasted for around 200 femtoseconds.

The researchers positioned a light sensor along the reflected beam. When they shot two ultrashort pulses separated by a few tens of femtoseconds — therefore turning the ITO mirror on twice in rapid succession — they saw that the waveform of the twice-reflected light changed in response. It went from a simple, monochromatic wave to a more complex one.

The results also showed that the ITO took less than ten femtoseconds to get excited — much faster than expected theoretically or from previous measurements. “The reason why everybody else thought it would be slower is that they used a different technique to measure the response time, which was limited to 50–100 fs,” says co-author Riccardo Sapienza, a physicist at Imperial College.

Light waves squeezed through ‘slits in time,’ Davide Castelvecchi, Nature

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