Lasers are intense beams of colored light. Depending on their color and other characteristics, they can clear your groceries, cut metal, eliminate tumors, and even trigger nuclear fusion. However, not every laser color has the right characteristics for a particular job. To fix this, scientists have found a variety of ways to convert one color of laser light into another. In a study just published in the journal Physical review was appliedScientists at the US Department of Energy’s (DOE) Brookhaven National Laboratory have demonstrated a new color-changing strategy that is simple, effective, and highly customizable.
The new method relies on the interactions between lasers and vibrational energy in the chemical bonds of materials called “ionic liquids”. These liquids consist only of positively and negatively charged ions, like regular table salt, but flow like viscous liquids at room temperature. Simply shining a laser through a tube filled with a specific ionic liquid can lower the laser’s power and change its color while preserving other important properties of the laser beam.
“By adding a specific ion that has a certain vibrational frequency, we can design a liquid that deflects laser light through the frequency of the vibrations,” said Brookhaven Lab chemist James Wishart, an expert in ionic liquids and a co-author on the paper. “And if we want a different color, we can swap out an ion and put in another ion that has a different vibrational frequency. And the component ions can be mixed and matched to change the laser colors to different shades as needed.”
The paper describes using the method to achieve color changes that would have been difficult to produce with other methods, including switching from green laser light to orange — a long-awaited prospect for medical applications such as treating skin and eye diseases.
Give the laser positive feedback
The idea arose from a project to enhance the capabilities of high-energy unique carbon dioxide (CO2) lasers at Brookhaven Lab’s Accelerator Test Facility (ATF). Scientists use the ATF, a user facility of the Department of Energy’s Office of Science, to explore innovative concepts ranging from laser-powered particle accelerators to compact, bright X-ray sources.
ATF’s Corporation2 The laser is the only laser in the world with short and long wavelength pulses; “There are experiments you can do there that you can’t do anywhere else,” said study co-author Rotem Kupfer, a former ATF postdoctoral fellow. Replacing the method of pumping this laser from the commonly used electrical discharge to optical excitation should improve beam quality and repetition rate to allow for better experiments.”
To create a laser with the appropriate wavelength (aka color) for optical pumping, scientists sought to alter the wavelength of the existing laser. They chose the general approach of stimulated Raman scattering, which harnesses the vibrational frequencies of molecules in a solid, liquid or gas.
“Essentially, the laser deposits energy into molecular vibrations — the crushing and stretching of the chemical bonds that make up matter. Then the photons (particles of light) that come out have the original energy minus the energy of those vibrations,” Kupfer said. Low-energy photons have a longer wavelength, or in other words, a different color.
In gases, the process is fairly simple because you’re dealing with single molecules. But these particles have limited vibration frequencies, which limits the types of transitions. And diffused gaseous particles mean that the dispersion efficiency is low. Solids, with particles packed more tightly, can improve efficiency. But their more complex vibrational frequencies complicate the recipe for cultivating such materials with the desired properties, so making these materials is expensive.
“Liquids are somewhere in between,” Wishart said. “You’re still dealing with single molecules, but they are much denser, which means higher efficiency than gases. And with ionic liquids, you can engineer molecules to give you the frequency you need.”
Transparent ionic liquids also make it easy to avoid background light absorption, and their high viscosity avoids laser scattering of acoustic waves, which competes with and reduces the color-changing effect of low-viscosity liquids such as water.
Where scientists worked to choose an ideal ionic liquid for pumping carbon dioxide2 lasers, they realized that the color-changing approach using ionic liquids had a broader appeal. In the paper they describe its use in additional color changes, including an elusive shift from green to orange.
“There are a lot of tricky ways to do Raman shifting. But for this method, we just filled a tube with a properly chosen ionic liquid, fired a laser at one end and got the color we wanted—without any tuning fine,” Wishart said.
“Other methods for achieving such a color shift require complex optical formulations or the use of toxic materials such as dyes dissolved in solvents,” Kupfer said. “Plus, these other processes ‘break’ the molecules; they wear out and have to be replaced. In our case, it’s a budget. The molecules stay intact.”
Wishart agreed: “It shakes up the molecules but doesn’t break them.”
Scientists say there are a range of improvements that could improve the process, but in general, custom-made ionic liquids are the platform for efficient, simple, tuning-free laser color conversion for many industrial and technological purposes.
This research, which was conducted entirely at the Brookhaven Lab, was funded by the lab’s research and development grants.