Lasers are intense beams of colored light. Depending on their color and other properties, they can scan your purchases, cut through metal, eradicate tumors, and even trigger a nuclear meltdown. But not all laser colors are available with the right properties for a specific job. To fix that, scientists have found a variety of ways to convert one color of laser light to another. In a study recently published in the journal Applied Physical ReviewUS Department of Energy (DOE) Brookhaven National Laboratory scientists demonstrate a new color change strategy that is simple, efficient, and highly customizable.
The new method is based on the interactions between the laser and vibrational energy in the chemical bonds of materials called “ionic liquids”. These liquids are made of only positively and negatively charged ions, like ordinary table salt, but they flow like viscous fluids at room temperature. Simply shining a laser through a tube filled with a particular ionic liquid can reduce the energy of the laser and change its color while preserving other important properties of the laser beam.
“By adding a certain ion that has a particular vibrational frequency, we can engineer a liquid that changes the laser light at that vibrational frequency,” 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, then we can change one ion and put in another that has a different vibrational frequency. The component ions can be mixed and matched to change the colors of the laser to different degrees as needed.”
The paper describes the method’s use to achieve color changes that have been difficult to produce with other methods, including a change from green to orange laser light, long sought after for medical applications such as treatment of skin and eye conditions. .
Giving lasers good vibes
The idea arose from a project to increase the capabilities of a unique, high-powered carbon dioxide (CO2) laser at Brookhaven Lab’s Accelerator Test Facility (ATF). Scientists use ATF, a DOE Office of Science user facility, to explore innovative concepts ranging from laser-activated particle accelerators to beam sources X compact and bright.
ATF CO2 the laser is the world’s only long-wavelength, ultra-short-pulse laser; there are experiments you can do there that you can’t do anywhere else,” said study co-author Rotem Kupfer, a former postdoctoral fellow at ATF. “Replacing the pump method of this commonly used electrical discharge laser to optical excitation should improve beam quality and repetition rate to enable even better experiments.”
To create a laser with the right wavelength (also known as color) for optical pumping, the scientists sought to change the wavelength of an existing laser. They chose the general approach of stimulated Raman scattering, which takes advantage of the vibrational frequencies of molecules in a solid, liquid, or gas.
“Basically, the laser deposits energy into molecular vibrations: the squashing and stretching of the chemical bonds that make up the material. Then the photons (particles of light) that come out have the original energy minus the energy of those vibrations.” Kupfer said. Lower energy photons have a longer wavelength, or in other words, a different color.
In gases, the process is quite simple because you are dealing with individual molecules. But those molecules have limited vibrational frequencies, which limits the types of changes. And diffuse gas molecules mean that the scattering efficiency is low. Solids, with more closely packed molecules, could improve efficiency. But their more complex vibrational frequencies complicate the recipe for growing such materials with the desired properties, so manufacturing these materials is expensive.
“The liquids are somewhere in between,” Wishart said. “It’s still individual molecules, but denser, which means higher efficiency than gases. And with ionic liquids, you can engineer the molecules to get the frequency you need.”
Optically clear ionic liquids also make it easier to avoid background light absorption, and their higher viscosity prevents laser scattering from acoustic waves, which competes and lessens the color-shift effect in low-viscosity liquids like water.
While the scientists worked on choosing an ideal ionic liquid to pump the CO2 laser, they realized that the color change approach using ionic liquids had even greater appeal. In the paper, they describe its use in additional color changes, including the elusive change from green to orange.
“There are a lot of tricky ways to do the Raman shift. But for this one, we just filled a tube with a correctly selected ionic liquid, fired a laser from one end, and got the color we wanted, without any fine tuning,” Wishart said. .
“Other methods of achieving that color change require complex optical setups or the use of toxic materials such as dyes dissolved in solvents,” Kupfer said. “In addition, those other processes ‘break down’ the molecules; they wear out and need to be replaced. In our case, it’s a balance. The molecules remain unscathed.”
Wishart agreed: “It shakes the molecules but it doesn’t break them.”
The scientists say there are a number of improvements that could streamline the process, but overall, custom-built ionic liquids provide a platform for efficient, simple, adjustment-free laser color change for many industrial and technological purposes.
This research, which was conducted entirely at Brookhaven Lab, was funded by the laboratory’s Laboratory-Directed Research and Development grants.