All That Matters

In the absence of GPS, a compass is the best option to find your way around. However, although the earth’s magnetic field is a great way to find your own position, doing the reverse, measuring magnetic fields with a high accuracy — on an atomic scale — remains a challenge. Sure, there are electron microscopes, which are great instruments that can image single atoms and other physical objects. However, when it comes to measuring magnetic fields, the achievable resolution is much worse than the size of an atom.

The schematic of a Lorentz microscope. Differently oriented magnetic domains deflect an electron beam in opposite directions, creating an image contrast at the border between magnetic domains.

The problem is even bigger in the fourth dimension, time, when we like to know how magnetic fields evolve over time on a microscopic scale. Then, the best resolution that can be achieved is about 500 times worse than what is possible in the imaging of atoms using state-of-the-art electron microscopes. The group of Ahmed Zewail at Caltech in Pasadena, California has now developed a technique that could significantly enhance the resolution of time-resolved measurements of magnetic fields.

There are of course a number of methods to measure a magnetic field with high spatial resolution. One is magnetic force microscopy, where essentially the tip of a magnetic needle gets moved across the surface of a magnetic material. The force between tip and sample is then a measure of the magnetic field. If the tip is atomically sharp, resolutions of a couple of tens of nanometers can be achieved.

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Gold heptamer structures show strong Fano resonances. The gold nanoparticles are only about 150 nanometers in diameter. (c) Sven Hein, Na Liu, Harald Giessen, University of Stuttgart, Germany.

Photonics is all about light. Processing of light for applications ranging from holograms and displays to optical telecommunications. Thanks to a better theoretical understanding and to advances in fabrication technology, photonic devices and gadgets have become increasingly versatile and powerful.

But photonics also has a dark side. In many light-processing devices and structures there are dark modes — oscillations of the light wave that while not forbidden cannot be directly excited by a given experimental configuration. In a violin for example, the strings are best sounded by drawing the bow perpendicular to their length.

However, the dark modes in photonic devices are not lost to applications. In the past months, researchers have developed new approaches that can make use of the dark modes by using interference effects with the allowed, bright modes. The resonances created by this interference are called Fano resonances, after Ugo Fano who first described them in 1961.

“Fano resonances are cool because they are the manifestations of dark modes that cannot be excited directly. In any system there are only a few bright modes but an infinite number of dark modes. The Fano resonance is the interference between a bright mode and one of the dark modes,” explains Peter Nordlander, a physicist from Rice University in Houston, Texas.

Fano resonances are very sharp, with a spectral shape that is much narrower than what can be achieved with comparable regular oscillators and their Lorentzian lineshape. This advantage makes them attractive for sensing applications. Because the spectra of Fano resonances are so narrow, the tiniest changes to the local environment of the resonator structures lead to noticeable shifts of the resonance wavelength. Even the presence of a single molecule could be detected.
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