There aren't many new, actual bona-fide discoveries in classical optics these days.
I saw this news item in Phys.org: Observation of twisted optical beam traveling slower than the speed of light
The researchers first noticed the slow speed of twisted light when conducting experiments with Gaussian laser light and light with 10 twists. "We realized that the two beams didn't arrive at the detector at the same time," Karimi said. "The twisted light was slower, which was surprising until we realized that the twists make the beam tilt slightly as it propagates. This tilt means that the twisted light beam doesn't take the straightest, and thus fastest, path between two points."
In waveguides, for example few-mode optical fibers with very small index differences, each mode propagates at a different velocity. The differences can be big - a few percent. Roughly speaking, the more complex the mode looks, the more transverse k, the less longitudinal k.
The same is true with defined beams in free space. As soon as you start adding spatial structure, by focusing a wide beam to a narrow beam, or making more complex shaped beams, anything where you have more transverse k vector, the beam slows down, and it is 100% consistent and calculable from Maxwell's equations.
It may be quite difficult to verify for experimental reasons, I can understand that, and it's always good to verify something that hasn't been verified before for many reasons - student research experience, putting the expensive equipment to the test, increment publication list, etc. It's good.
My Question: Is it something that is absolutely expected to happen - any structured beam in space will be slower than an infinite plane wave in space? Wouldn't any structured beam of light be expected to travel slower than a plane wave?
The Phys.org article links to the paper Observation of subluminal twisted light in vacuum Frédéric Bouchard, Jérémie Harris, Harjaspreet Mand, Robert W. Boyd, and Ebrahim Karimi Optica Vol. 3, Issue 4, pp. 351-354 (2016)
Subluminal means slower than the speed of plane wave light (in the same medium), and my point is that any finite beam of light will always be subluminal - travel slower than a plane wave in the medium.
edit: I just noticed this at the bottom. "...their calculations have predicted may travel around 1 femtosecond faster than the speed of light in a vacuum" so apparently they don't think so!
If it's possible to slow the speed of light by altering its structure, it may also be possible to speed up light. The researchers are now planning to use FROG to measure other types of structured light that their calculations have predicted may travel around 1 femtosecond faster than the speed of light in a vacuum.
Read more at: http://phys.org/news/2016-03-optical-slower.html