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Droplet laser development could reduce optical communications costs

Researchers from the University of Tsukuba, Japan, have successfully demonstrated a new method of producing ionic liquid microdroplets that can work as flexible, long-lasting, and pneumatically tunable lasers. 

The scientists say that existing droplet lasers are unable to operate under ambient conditions without evaporating, whereas the new development – described in Laser & Photonics Reviews – may enable droplet lasers that can be used in everyday settings.

This could help lead to the development of new airflow detectors or more flexible and less expensive fibre-optic communication devices.

To produce the droplet laser, the researchers first selected an ionic liquid, 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4), due to its slow evaporation properties and relatively large surface tension. This liquid was then mixed with a special dye, which later enabled it to be used as a laser when excited with a pumping source. 

They then coated a quartz substrate with tiny fluorinated silica nanoparticles. Similar to the microscopic bumps on the surface of a lotus flower, these nanoparticles enable the substrate to repel liquids.

This meant that when the EMIBF4 was then deposited onto the coated substrate from a pipette, a tiny droplet would form that remains almost completely spherical. The researchers demonstrated that the droplet could remain stable for at least 30 days.

This stability enables the droplet to maintain an optical resonance when excited with a laser pumping source. The droplet can then be 'tuned' using blow nitrogen gas to slightly deform its shape, which can shift the laser peaks in a wavelength range of 645-662nm.

"This is, to our knowledge, the first liquid laser oscillator that is reversibly tunable by gas convections," says first author Professor Hiroshi Yamagishi.

To scale up production of the droplets, the researchers employed commercial inkjet printing apparatus equipped with a printer head that could work with a viscous liquid. The apparatus could then be used to print arrays of the 'laser droplets', without the need for further treatment.

The findings indicate that the production is highly scalable and easy to perform, so that it can be readily applied to manufacture laser droplets for inexpensive sensor or fibre optical communication devices. The laser droplets could also be used as very sensitive humidity sensors or airflow detectors, according to the researchers.


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