Ionic Liquids Turn Green Light into Orange

Green laser light

Researchers at Brookhaven National Laboratory, USA, fired green laser light through a tube of ionic liquid (seen on the right) to produce orange light (upper left) via stimulated Raman scattering. They say that tailoring the ionic liquid can yield a range of wavelength shifts. [Image: Brookhaven National Laboratory]

Short, high-energy pulses of yellow-orange laser light are ideal for numerous applications, among them medical procedures such as corrective eye surgery and treatments for certain skin conditions. But they are hard to create, whether directly from a laser or by applying a nonlinear optical process to light of another wavelength.

Now, scientists in the United States have shown how to generate such pulses efficiently, simply by sending green light along a tube containing an ionic liquid and collecting the Raman-scattered output (Phys. Rev. Appl., doi: 10.1103/PhysRevApplied.19.014052). They say that the technique could be used to produce high-energy pulses at a range of visible and infrared wavelengths.

Exploiting Raman scattering

The research grew out of work on a high-power CO2 laser at the Brookhaven National Laboratory in Upton, NY, USA. That laser is used to study advanced types of accelerator technology, including laser-driven particle accelerators and secondary radiation sources. But its beam quality and repetition rate are limited by the inefficient electrical discharge currently used to pump it.

Rotem Kupfer and colleagues at Brookhaven set out to substitute the electrical discharge with a source of orange laser pulses that, when combined with green light, could produce the mid-infrared radiation needed for optical pumping. Their idea was to exploit ionic liquids to convert green laser light to orange via Raman scattering—the inelastic process that involves incoming photons losing energy to specific molecular vibrations, leading to a well-defined reduction in their frequency.

As the researchers explain, Raman scattering can be achieved by directing light at a solid. In principle, this ensures a high density of scattering molecules, but the collective oscillations involved are difficult and expensive to engineer. Using a gas as the scattering medium, on the other hand, reduces complexity but also the efficiency of wavelength conversion.

Liquids provide a happy compromise between the two, involving straightforward scattering from single molecules that are present at relatively high densities. Ionic liquids in particular have the added advantage that their constituent molecules can be engineered to shift light by a given frequency. Such liquids are molten salts at room temperature—artificially constructed combinations of specific cations and anions.

Increased efficiency with ionic liquids

First, the Brookhaven group established which material might be best for the job by preparing around a dozen different ionic liquids and then measuring both their Raman shift and optical transmission spectra. The winner was 1-ethyl-3-methylimidazolium dicyanamide, or EMIM DCA for short.

Kupfer and colleagues argue that their new technique provides a practical and efficient way of converting the wavelengths from existing laser systems

To put that liquid to the test, Kupfer and co-workers filled a 63-cm long tube with it and subjected the liquid to laser pulses having a length of 10 ns, energy of 115 mJ and wavelength of 532 nm. They then measured the beam profile and energy of the input pump pulses and the output Stokes pulses at 603 nm.

The researchers also repeated the exercise with water, which they used as a convenient reference liquid. They found that, compared with water, the EMIM DCA was able to transfer energy from the pump to the Stokes pulses at least three times more efficiently. They also showed that the higher viscosity of the ionic liquid meant far less energy being wasted as sound waves, while the liquid’s broader region of optical transparency allows the use of other pump sources in the near infrared.

In addition, the researchers tested two other ionic liquids with different Raman shifts. One contained bistriflimide anions, which confer chemical stability and low viscosity. The other instead contained pyridinium cations. In both cases, the scattering process yielded several orders of Stokes shifts—implying a significant scattering cross section and efficient conversion of laser wavelength.

More practical, less toxic

Kupfer and colleagues argue that their new technique provides a practical and efficient way of converting the wavelengths from existing laser systems, pointing out that it requires no precise phase matching (as is the case with optical parametric amplification) and involves no toxic materials (such as dyes dissolved in solvents). “We anticipate that this method could allow flexible and convenient generation of high-energy laser radiation in spectral regions that are useful for various scientific and medical applications but are challenging to attain with other techniques,” they write.

However, the researchers acknowledge that the system needs to be worked on. Potential improvements, they say, include optimizing the laser path length and demonstrating subtraction of the orange laser’s frequency from that of the green laser to yield the mid-infrared radiation needed for optical pumping of the CO2 laser. They add that they plan on testing the setup with a smaller tube, which will cut the cost of the ionic liquid needed to fill it.

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