American scientists use ionic liquids as the medium to easily turn green light into orange light!

Recently, scientists have found that by simply adjusting ionic liquids (penetrating liquids), lasers can produce a series of wavelength changes, thus producing a "color change" effect that is simple, efficient and highly customizable.

Researchers at the Brookhaven National Laboratory of the United States Department of Energy (DOE) emit green laser through an ionic liquid tube, and then generate orange light through stimulated Raman scattering. Their new technology provides a practical and effective way to convert the wavelength of existing laser systems.

(Image source: Brookhaven National Laboratory)

As we all know, the yellow-orange laser pulse has the characteristics of higher power, and is an ideal choice in many applications such as clinical medicine and surgery, including ophthalmic corrective surgery and the treatment of some skin diseases and other medical treatment processes, and can be used for medical blood control, industrial spectral analysis, sensing and compact wearable devices. However, it is difficult to produce them, whether directly using laser or using nonlinear optical process to process light of another wavelength.

Recently, American scientists have shown how to effectively generate this kind of pulse, and simply send green light along the tube containing ionic liquid and collect Raman scattering output. They said that this technology can be used to generate high-energy pulses in the range of visible and infrared wavelengths.

Using Raman scattering
It is reported that this research is based on a unique high-power carbon dioxide (CO2) laser in the Accelerator Test Facility (ATF) in the United States. This kind of laser is used to research 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 discharge currently used for pumping.

In the research and observation, Rotem Kupfer of Brookhaven University and his colleagues gradually replaced the discharge with an orange laser pulse source. When it is combined with green light, it can produce the mid-infrared radiation required for optical pumping.

Their idea is to use ionic liquids to convert green laser into orange through Raman scattering. In this process, the incident photon loses energy in the specific molecular vibration, resulting in a significant reduction in its frequency.

As the researchers explained, Raman scattering can be achieved by directing light to the solid. In principle, this ensures the high density scattering of molecules, but it is not only difficult but also costly to form collective oscillation. On the other hand, using gas as the scattering medium reduces the complexity and efficiency of wavelength conversion.

Ionic liquids have become the best mediator between the two, and in this process, relatively high density single molecule direct scattering occurs. The constituent molecules of ionic liquids are designed to move light at a specific frequency. This liquid is equivalent to molten salt at room temperature, which is synthesized by specific cations and anions.

Improve efficiency with ionic liquids
First of all, Brookhaven's team determined which material might be most suitable by preparing about 12 different ionic liquids and measuring their Raman shifts and optical transmission spectra (the results show that EMIM DCA is the most suitable).

To test this liquid, Kupfer and his colleagues filled a 63cm long tube with this liquid and placed it in a laser pulse with a length of 10 nanoseconds, an energy of 115 megajoules and a wavelength of 532 nm. Then, they measured the beam profile and energy of the input pump pulse and the output Stokes pulse at 603 nm.

The researchers also used water as the reference liquid and repeated the experiment. They found that the efficiency of EMIM DCA ionic liquid to transfer energy from pump to Stokes pulse is at least three times higher than that of water. The results of this comparative experiment also show that, based on the high viscosity characteristics of ionic liquids, the energy loss in the liquid acoustic wave is less, and the wider optical transparency region of the liquid allows the use of other pump sources in the near infrared.

In addition, the researchers also tested two other ionic liquids with different Raman shifts. In two different cases, the scattering process produces several Stokes shifts, which means an important scattering cross section and effective conversion of laser wavelength.

More practical and less toxic
Kupfer and his colleagues believe that their new technology provides a practical and effective way to convert the wavelength of the existing laser system, and it does not require accurate phase matching (as in the case of optical parameter amplification), nor does it produce toxic substances (such as dyes dissolved in the solvent). Researchers predict that this method can flexibly and conveniently generate high-energy laser radiation in the spectral region, which is valuable for various scientific and medical applications.

Next, they plan to optimize the length of the laser path and demonstrate how to subtract the frequency of the orange laser from the frequency of the green laser to generate the mid-infrared radiation required for the optical pumping of the carbon dioxide laser. In addition, they plan to test the device with a smaller tube, which will reduce the cost of filling it with ionic liquid.

Source: OFweek