Since the first laser was invented in 1960, they have given off heat, whether used as a tool in industry or as a fictional way to vanquish intergalactic enemies.
But the concentrated beams of light have never been successfully able to cool liquids – until now.
Researchers have used an infrared laser to drop the temperature of water by around 2°C (36°F) in a process that has been described as ‘a major breakthrough in the field’.
‘Typically, when you go to the movies and see Star Wars laser blasters, they heat things up’, senior author and assistant professor of materials science and engineering at the University of Washington Peter Pauzauskie explained.
‘It was really an open question as to whether this could be done because normally water warms when illuminated.’
The discovery could help ‘point cool’ tiny areas with a focused point of light, which would be a big step forward in manufacturing.
Microprocessors, for instance, might someday use a laser beam to cool specific components in computer chips to prevent overheating and enable more efficient information processing.
Scientists could also use a laser beam to precisely cool a portion of a cell as it divides or repairs itself to slow these rapid processes down and give researchers the opportunity to see how they work.
They could even cool a single neuron in a network to see how its neighbours bypass the silenced cell and rewire themselves.
‘There’s a lot of interest in how cells divide and how molecules and enzymes function, and it’s never been possible before to refrigerate them to study their properties,’ Professor Pauzauskie continued.
‘Using laser cooling, it may be possible to prepare slow-motion movies of life in action.
‘And the advantage is that you don’t have to cool the entire cell, which could kill it or change its behaviour.’
The team chose infrared light for its cooling laser so it could be used on delicate biological cells.
By comparison, visible light can cause damaging ‘sunburn’.
The researchers showed that the laser could refrigerate saline solution and cell cultures that are commonly used in genetic and molecular research.
To come up with the invention, the scientists used a material commonly found in commercial lasers but essentially ran the laser process in reverse.
They illuminated a single microscopic crystal suspended in water with infrared laser light to excite a unique kind of glow that has slightly more energy than that amount of light absorbed.
This higher-energy glow carries heat away from both the crystal and the water surrounding it.
While the laser refrigeration process was first demonstrated in vacuum conditions at Los Alamos National Laboratory in 1995, it has taken nearly 20 years to demonstrate the same process in liquids.
Typically, growing laser crystals is an expensive process that requires lots of time and money, costing thousands of dollars to produce just a single gram of material.
But the team also came up with a low-cost hydrothermal process that can be used to manufacture a well-known laser crystal for laser refrigeration applications in a faster, inexpensive and scalable way.
As well as this, they designed an instrument that uses a laser trap, similar to a microscopic tractor beam, to ‘hold a single nanocrystal surrounded by liquid in a chamber and illuminate it with the laser.
This instrument projected the particle’s ‘shadow’ in a way that allowed the researchers to observe minute changes in its motion.
As the surrounding liquid cooled, the trapped particle slowed down, allowing the team to clearly observe the refrigerating effect.
The crystal changed from a blueish-green to a reddish-green colour as it cooled, like a built-in colour thermometer, according to the study published in the journal PNAS.
So far, the team has only demonstrated the cooling effect with a single nanocrystal, because exciting multiple crystals would require more laser power.
The next step is to improve the laser refrigeration process’ efficiency so that one day the technology could be used for higher-power lasers for manufacturing, telecommunications or defence applications.
‘Few people have thought about how they could use this technology to solve problems because using lasers to refrigerate liquids hasn’t been possible before,’ he said.
‘We are interested in the ideas other scientists or businesses might have for how this might impact their basic research or bottom line.’