The optical view of magnetic cores
When doctors push their patients and chemists their samples into the tube, strong magnetic fields cause the atomic nuclei inside to dance. Knowing who rotates how reveals the structure of organs and molecules. The use of laser light as a sensitive measuring probe could significantly increase the accuracy of nuclear magnetic resonance methods in the future.
"What's the point of that?" entrepreneurs and politicians want to know when scientists need funding to indulge their curiosity. If you want to do research in times of tight budgets, it would be a good idea to be able to come up with a good reason when asked. Science should lead to applications – and fast. Innovations from the five-minute tureen are in demand, not dreamy experiments without a secure product return. Where would we be if research could just muddle around where it wants?
We're making progress. A look back at the history of natural science and technology teaches us that. Leaving aside such business-friendly innovations as portable phones that take pictures of the inside of our pockets as we walk, it turns out that the really big advances always started with completely pointless basic research. "Let's see," is the basis of the entire molecular biology, including a significant pharmaceutical part, as well as the relativity-corrected navigation technology. Real innovations traditionally start with curiosity and a playful instinct – the marketing plan comes later.
A nice example of a research direction that was originally useless and then not only became a marketable product itself, but also had a significant beneficial effect on other, completely foreign areas, is nuclear magnetic resonance. It's been sixty years since scientists were fascinated with experimenting with magnetic fields so strong they could align atomic nuclei like tiny compass needles. Additionally radiated radio waves even made the nuclei perform a wobbly dance. Fascinating and, at first glance, impeccably useless. It was only later that researchers recognized the potential of this effect and developed equipment that allows chemists to easily determine the structures of unknown molecules, physicians to examine the soft organs of the body without a scalpel or X-rays, and neurobiologists to watch people think.
The basis of all these applications is the magnetic moment of most atomic nuclei. It results from the spin properties of the protons, which, to put it simply, turn the nucleus into a tiny magnetic spinning top. The atomic nuclei react accordingly to a strong external magnetic field by neatly aligning themselves accordingly. If another magnetic field is added, which is perpendicular to the first, this tilts the nuclei a little to the side. They begin to precess, which means that their axis of rotation oscillates about a central axis like a tumbling top. The precession becomes particularly strong when the additional magnetic field is rhythmically switched on and off. If this happens in the right resonance frequency, the nuclei even lie completely on their side and follow the alternating field.
The dance of the nuclei has an analytical value because the respective resonance frequency (also called Larmor frequency) depends on several variables. Depending on the type of atom and the nature of its environment, its exact value shifts. If the resonance frequency can be determined, an experienced scientist can determine which element in which isotope variant is part of which chemical group in the apparatus. In short: the entire chemical structure of a molecule.
The only problem is finding out exactly when the magic frequency is hit. Usually, a coil is used to measure the energy that the precessing core gyroscopes radiate when they are allowed to briefly recover from the exciting magnetic field. A process that has practically not changed in the past sixty years. Mike Romalis from the US University of Princeton and his colleagues have now found a way to read out the information optically. With a laser they want to achieve spatial resolutions that would not be possible with coil technology.
In their demonstration experiments, they irradiated a solution of xenon in water with laser light. Its electromagnetic field rotated in lockstep, so to speak, and was influenced by the magnetic field of the electron spins, which in turn interacted with the nuclear spin. A magneto-optical coupling arose, known as the Faraday effect. As a result, the spin of the nuclei changed the direction of polarization of the laser light – measurably.
The new method called Nuclear-Spin Optical Rotation (NSOR) is not yet as sensitive as microcoils. To do this, it should achieve resolutions down to a few micrometers with focused laser light. And that with a simpler sensor technology, because individual photons are easier to measure than weak magnetic fields. Medical diagnostics could also benefit from this, because infrared laser light penetrates several centimeters deep into the tissue. And finally, optical and nuclear spin-based analyzes could be combined in chemistry. All-round screening of the samples in one go.
So we can still expect a lot from nuclear magnetic resonance in the future. What began as a playful curiosity has blossomed into one of the most important analysis tools and is far from being at the end of its development. It's a good thing that scientists are sometimes still allowed to do random research.