Lasers are intense beams of colored light. Depending on their color and other properties, they can scan your groceries, cut through metal, eradicate tumors, and even induce nuclear fusion. But not all laser colors are available with the right properties for a particular job. To fix that, scientists have found different ways to convert one color of laser light into another. In a study just published in the journal Physical Review implementedscientists at the US Department of Energy’s (DOE) Brookhaven National Laboratory demonstrate a new dye transfer strategy that is simple, effective and highly adaptable.
The new method depends on interactions between the laser and vibrational energy in the chemical bonds of materials called “ionic liquids.” These liquids are made only of positively and negatively charged ions, like common table salt, but flow like a viscous fluid at room temperature. Shining a laser through a tube filled with a special ionic liquid can reduce the energy of the laser and change its color while maintaining other important properties of the laser beam.
“By adding a certain ion with a specific vibrational frequency, we can design a liquid that shifts the laser light according to that vibrational frequency,” said Brookhaven Lab chemist James Wishart, an expert in ionic liquids and co- author of the paper. “And if we want a different color, we can change one ion and put in another with a different vibrational frequency. The component ions can be mixed and matched to gradually change laser colors as it is necessary.”
The paper describes how to use the method to achieve color changes that have been difficult to achieve using other methods, including switching from green to orange laser light — which has long been sought for applications medical such as treating skin and eye conditions.
Give your laser good vibes
The idea grew out of a project to increase the capacity of a unique high-powered carbon dioxide (CO2) laser at Brookhaven Lab’s Accelerator Test Facility (ATF). Scientists use the ATF, a DOE Office of Science user facility, to explore innovative concepts from laser-powered particle accelerators to compact and bright x-ray sources.
“ATF CO2 laser is the only ultra-short-pulse, long-wavelength laser in the world; 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. the quality of the beam and the repetition rate to allow for even better experiments.”
To create a laser with the appropriate wavelength (aka color) for optical pumping, scientists tried to change the wavelength of an existing laser. They chose the general approach of stimulated Raman scattering, which exploits the vibrational frequencies of molecules in a solid, liquid or gas.
“Essentially, the laser stores energy in the molecular vibrations — pushing and stretching 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 the vibrations that,” Kupfer said. The low energy photons have a longer wavelength, or in other words, a different color.
In a gas, the process is quite simple because you are dealing with individual molecules. But those molecules have limited vibrational frequencies, which limits the types of shifts. And diffusing gaseous molecules means that scattering efficiency is low. Solids, with more closely packed molecules, may improve efficiency. But their more complex vibrational frequencies complicate the recipe for growing such materials with the desired properties, so making these materials is expensive.
“Liquids are somewhere in between,” Wishart said. “You’re still dealing with individual molecules, but more closely, which means higher efficiency than gases. And with ionic liquids, you can engineer the molecules to give you the frequency you need.”
Optically transparent ionic liquids also avoid background absorption of light and their higher viscosity prevents laser scattering from acoustic waves, which competes with and reduces the color shift effect in low-viscosity liquids such as water.
As the scientists worked on choosing an ideal ionic liquid to pump the CO2 laser, they realized that the color-changing approach using ionic liquids had even wider appeal. In the paper they describe its use in further color changes, including the impossible green-to-orange transition.
“There are a lot of hard ways to shift Raman. But for this one, we just filled a tube with a properly chosen ionic liquid, shot a laser in from one and got the color we wanted — without no fine. tuning,” Wishart said.
“Other methods require complex optical setups or the use of toxic materials such as dyes dissolved in solvents to achieve such a color change,” Kupfer said. “Also, those other processes ‘break’ the molecules; they wear out and need to be replaced. In our case, it’s a balance sheet. The molecules remain unharmed.”
Wishart agreed: “It shakes the molecules but doesn’t break them.”
The scientists say there are a range of improvements that could optimize the process, but overall, ionic liquids provide a platform for efficient, simple and adjustment-free laser color transfer for numerous industrial and technological purposes.
This research, conducted entirely at Brookhaven Lab, was funded by Laboratory Directed Research and Development grants.