Publish Date: 19.01.2024

Category: News from the University

Photo: Zala Korenjak in doc. dr. Matjaž Humar

Soap bubbles, with their changing colors, impress both adults and children and are a source of inspiration for researchers in various fields - mathematics, physics, chemistry and biology. Among them are Zala Korenjak and Asst. Dr. Matjaž Humar from the Jožef Stefan Institute and the Faculty of Mathematics and Physics, University of Ljubljana, who were the first to show that soap bubbles can be used as lasers.  

In their research paper published in the journal Physical Review X, they showed that a dye in a soap bubble can amplify the light circulating in it and generate laser light when the bubble is excited by an external light source. This light is visible as a bright ring around the soap bubble. Such bubbles can be used as extremely sensitive pressure and electric field sensors.  

Photo: Zala Korenjak in doc. dr. Matjaž Humar Soup bubble laser, photo: Zala Korenjak and doc. dr. Matjaž Humar  

Since classic soap bubbles are unstable due to the evaporation of water, in the next phase they used soap-like molecules, but without the addition of water – smectic liquid crystals. These smectic bubbles had a completely uniform wall thickness and, most importantly, they were extremely stable. The bubbles were attached to the end of a capillary or were free floating in a container filled with carbon dioxide. The spectrum of the laser light emitted by the bubbles consisted of hundreds of evenly spaced sharp peaks. Using the spectrum of the emitted laser light and its change over time, they were able to measure subtle size changes of just ten nanometers in a millimeter-sized bubble. That is 10,000 times less than the thickness of a human hair.  

This incredible precision allowed the bubbles to be used as one of the best pressure and electric field sensors developed to date. In the future, they could also be used as an extremely sensitive microphone, as a magnetic field sensor and to study new phenomena in thin films.  

The research is presented in a video.    

Zala Korenjak in Matjaž Humar, Smectic and Soap Bubble Optofluidic Lasers, Physical Review X 14, 011002 (2024),