Physicists fires laser in college hallway, breaks record

Physi­cists have just set a new record by trap­ping a self-focused laser pulse in an air cage along a 45-meter-long uni­ver­si­ty corridor.

Led by physi­cist Howard Milch­burg of the Uni­ver­si­ty of Mary­land (UMD), the new exper­i­ment breaks new ground in con­fin­ing light in chan­nels called air waveguides.

The results of this research have been accept­ed by Phys­i­cal Review X and are avail­able on the arX­iv preprint serv­er. This result may pro­vide new hints for real­iz­ing long-dis­tance com­mu­ni­ca­tion using lasers and advanced weapons tech­nol­o­gy using lasers.

“Our results show that if we had a longer wave­guide, we could have tuned the laser,” says UMD physi­cist Andrew Tartaro.

“But we did the right thing with the pas­sage guid­ance we have now.”

Lasers can be used in a vari­ety of appli­ca­tions, but an ordered, coher­ent beam of light must be some­how col­lect­ed and focused. If left unat­tend­ed, the laser will scat­ter, reduc­ing pow­er and efficiency.

Wave­guide, as the name sug­gests, is a tech­nol­o­gy that guides elec­tro­mag­net­ic waves to a spe­cif­ic path and pre­vents scattering.

One exam­ple is fiber optics. It con­sists of a glass tube that allows elec­tro­mag­net­ic waves to pass through. The out­er cladding of the tube has a low­er refrac­tive index than the cen­ter of the tube, so light that would oth­er­wise be scat­tered is deflect­ed inside the tube, keep­ing the beam along its length.

In 2014, Milch­berg and col­leagues suc­cess­ful­ly demon­strat­ed what they call an air wave­guide. Instead of using a phys­i­cal struc­ture like a tube, they used laser puls­es to focus the laser light. 

As a result, they found that a pulsed laser gen­er­ates a plas­ma that heats the air on its path, leav­ing a path of less dense air. As the less dense air expands, it cre­ates tiny thun­der­ous sounds that fol­low the laser to form what is called a filament.

Less dense air has a low­er refrac­tive index than the sur­round­ing air, like the cladding of a fiber optic tube. So, by launch­ing these fil­a­ments in a spe­cif­ic arrange­ment to “cra­dle” the laser light, it is pos­si­ble to form a wave­guide out of the air.