Simple Approach to Laser Color Conversion Uses SRS in Ionic Liquids

Scientists from Brookhaven National Laboratory showed that ionic liquids provide an efficient means to convert one color of laser light into another. The discovery could lead to a way to create lasers with desired colors for a range of medical, scientific, and technological applications.

The method is based on the interaction between the laser and different types of ionic liquids (also known as liquid salts). The vibrational energy in the chemical bonds in the ionic liquid cause the laser’s energy to shift and change color.

“By adding a certain ion that has a particular vibrational frequency, we can design a liquid that shifts the laser light by that vibrational frequency,” chemist James Wishart said. “And if we want a different color, then we can switch out one ion and put in another that has a different vibrational frequency. The component ions can be mixed and matched to shift laser colors by different degrees as needed.”

Shooting a green laser through a tube filled with a particular ionic liquid (right side of photo) can easily convert the green laser light to orange (upper left) — a long-sought color for medical applications. The method can be tailored for different color shifts by choosing different ionic liquids. Courtesy of Brookhaven National Laboratory.

The new approach to changing laser wavelengths has its roots in a project to boost the capabilities of a CO₂ laser at Brookhaven’s Accelerator Test Facility. To improve the laser’s beam quality and repetition rate, the researchers wanted to pump the laser using optical excitation instead of electric discharge.

To create a laser with the appropriate wavelength for optical pumping, the researchers used stimulated Raman scattering (SRS) to shift the wavelength of an existing laser. SRS can be used to harness the vibrational frequencies of molecules in a solid, liquid, or gas form.

“Basically, the laser deposits energy into the molecular vibrations — the squishing and stretching of the chemical bonds that make up the material,” researcher Rotem Kupfer said. “Then the photons (particles of light) that come out have the original energy, minus the energy of those vibrations.” The lower-energy photons have a longer wavelength and, thus, a different color.

The researchers demonstrated that 1-ethyl-3-methylimidazolium dicyanamide (EMIM DCA), an ionic liquid, was an effective medium for converting 532-nm pulses from a Q-switched Nd:YAG laser to 603 nm. This corresponded to an approximate 2200 cm⁻¹ shift, which could be used to generate mid-infrared radiation for optical pumping of CO₂ lasers.

Members of the research team (left to right): Igor Pogorelsky (Accelerator Test Facility, ATF), Mark Palmer (ATF), James Wishart (Chemistry Division), Marcus Babzien (ATF), Mikhail Polyanskiy (ATF), Navid Vafaei-Najafabadi (ATF and Stony Brook University), Furong Wang (Chemistry), and Luca Cultrera (Instrumentation Division). Not pictured: Rotem Kupfer (ATF) and Triveni Rao (Instrumentation). Courtesy of Brookhaven National Laboratory.

While choosing the best ionic liquid for pumping the CO₂ laser, the researchers considered that their color-shifting approach could have a broader use.

In a proof-of-principle, single-pass conversion setup, the researchers obtained a threefold-higher Raman conversion efficiency in the ionic liquid compared with water under identical conditions, resulting in an efficient generation of high-quality orange laser pulses in a wavelength region that is difficult to access at high energies.

“There are a lot of hard ways to do Raman shifting. But for this one, we just filled a tube with a properly selected ionic liquid, shot a laser in from one end, and we got the color we wanted out — without any fine tuning,” Wishart said.

Kupfer said that other methods for achieving a shift in laser color require complex optical setups or the use of toxic materials. “Plus, those other processes ‘break’ the molecules; they wear out and have to be replaced,” he said. “In our case, it is a balance sheet. The molecules stay unharmed.”

The researchers determined that ionic liquids provide a framework to engineer liquids suitable for wavelength conversion over a broad spectral range. Careful selection of the molecular structures of the ionic liquid anions and cations lead to specific characteristics, such as a desirable Raman shift, low Brillouin scattering, and good optical transmission in the pump and Stokes wavelengths. An ionic liquid can interact with photons while offering a high density of energy-swapping sites.

Gas molecules have limited vibrational frequencies, and diffuse gaseous molecules mean scattering efficiency is low. Solids have more tightly packed molecules, making them more efficient, but their complex vibrational frequencies make them costly to produce.

“Liquids are somewhere in between,” Wishart said. “You’re still dealing with single molecules, but denser, meaning higher efficiency than gases. And with ionic liquids, you can engineer the molecules to give you the frequency you need.”

The large number of ionic liquids available makes it possible to precisely tune the energy loss caused by the ionic liquid-photon interaction, providing greater selective control over color. Optically transparent ionic liquids prevent background absorption of light. In addition, their viscosity prevents laser scattering from acoustic waves, which can diminish the color-shifting effect in low-viscosity liquids.

Although further improvements could optimize the process, the researchers said that overall, the made-to-order ionic liquids provide a suitable platform for efficient, simple, adjustment-free laser color shifting using SRS.

The research was published in Physical Review Applied (www.doi.org/10.1103/PhysRevApplied.19.014052).