Optics: Cooling enlightenment

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Optics: Cooling enlightenment
Optics: Cooling enlightenment

Chilling Enlightenment

Temperatures usually rise in headlights. Not so when laser light falls on materials containing erbium atoms. The element then uses the warmth of the body to emit the captured energy plus a bit more as fluorescence - and it's already cooler.


Light makes you warm – the lizard on the bright wall knows that much, the sheep in the shade and (at least for a short time) the moth on the light bulb. It is just as familiar to the paddling pool in the garden as it is to the car steering wheel in the unfavorable outdoor parking lot and the expensive stainless steel railings on the balcony. From the Inuit boy to the New York broker to the Zulu tribal elder, everyone can confirm it. Only one group beneath the firmament seems to be ignorant of this truism: the chemical elements of the rare earth family. Because if you shine warming infrared light on materials with such admixtures - the thermometer drops!

The story seems unbelievable because it is so blatantly contrary to our everyday experience. What is even more unbelievable is that scientists have known about this phenomenon since the 1950s. At the time, Alfred Kastler of the École Normale Superieure in Paris theorized that under certain circumstances materials with low levels of rare earth metals should exhibit this counter-intuitive behavior. And in fact, researchers have now been able to experimentally demonstrate the surprising effect on materials with ytterbium and thulium. In one case, they even managed to lower the temperature by 85 degrees Celsius.

The trick to chilling bright is to spend more than you get. What is very easy with the household budget is much more difficult at the level of the atoms. They usually swallow light by an electron absorbing the energy of a photon and moving to a higher energy level, which is usually a little further away from the atomic nucleus. It rarely lasts long in this suburb.

Its re-descending begins in small increments at first as it slides down the sub-levels, emitting thermal radiation as it does so. Once the edge of the suburb is reached, a deep fall follows back to the more central area where the electron originated. Whatever energy was left leaves the atom as fluorescent light. This is a bit redder, so it has less energy, because the material was heated with part of the original light energy.

The rare earth metals in certain substances would love to play along with this interplay of energetic rise and fall - but unfortunately the incident light energy is not sufficient to stimulate them. However, so that the electron still makes it to the suburb, its atom scrounges the missing energy from the neighborhood. It uses the vibrational energy of the material or, to put it in less physical terms: the thermal energy. Because at the atomic level, heat is nothing more than vibrations. And so the electron gets the energy of a photon from the light and also a little heat from the body. Bottom line, enough to briefly get to a higher level and then immediately emit everything as fluorescent light. In photons, which this time, thanks to stolen thermal energy, are a little bluer and therefore more energetic. Scientists call this reversal of conventional energy trading anti-Stokes emissions.

Scientists would be only too happy to send the element erbium into the hot battle for clear cooling. Due to its energy structure, erbium would have to be excited with infrared light at a wavelength of around 1500 nanometers – a range that is well known in modern glass fiber optics. Unfortunately, the energy level storage of erbium is so complicated that some researchers have questioned whether an anti-Stokes effect could be achieved with this element at all.

These doubters have now convinced Angel Garcia-Adeva, Rolindes Balda and Joaquin Fernandez of the University of the Basque Country in San Sebastian with new experiments of the qualities of erbium as a coolant. Using laser light, they lowered the temperature of potassium lead chloride crystals and a heavy metal chloride fluoride glass, each of which contained erbium ions. Although the effect, which the researchers recorded with an infrared camera, was small - it was only 0.7 degrees Celsius for the crystal and 0.5 degrees for the glass - it is still sufficient for proof in principle.

Erbium coolers would need to be improved a lot if they were to find their way into our overheated everyday life, and even then we will hardly see them as soft drink refrigerators. But in electronic circuits they could one day really dissipate excess heat in a targeted manner and thus cool detectors or computer chips, for example. Because things get warm by themselves…

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