Natural Science
This passage is adapted from "The Truth Behind Lightsaber Technology" by Matt Gluesenkamp (©2010 General Electric).
There are a lot of myths and legends about lasers that Hollywood has generated or perpetuated over the years, but perhaps the most well-known instance of "lasers" in cinema are the lightsabers from the Star Wars1 saga. I put quotes around "lasers" because the way lightsabers behave in these movies is quite a bit different from the way lasers behave in real life.
In the Star Wars2 universe, lightsabers are typically custom-built by Jedi and Sith warriors, and all have several common elements. Each has a power source, a lightsaber crystal, one or more focusing crystals, and a stabilizing emitter system. The power source is typically a diatium power cell, often with a capacity of several megawatt-hours. The lightsaber crystal converts the power cell's energy into a plasma that is then passed through and directed by the focusing crystals. Finally, the emitter system stabilizes the plasma into a blade shape using a mix of power modulation and magnetic field containment.
Did that make sense to anyone?3 Good, then I'm not alone. Science fiction is typically a blend of materials or physical laws that exist and some that don't. Although real-life battery technology is coming along great, we are a long way off from creating handheld batteries with capacities like the ones found in the lightsaber's diatium power cell. Perhaps the key lies in discovering this fictional "diatium" material.4 And although crystals do have many useful optical and piezoelectric properties, I don't know of one that could magically create plasma from electricity. (However, I read that the crystals must be "attuned to the Force" by a Jedi or Sith in a meditation ritual that can take days, so maybe we should start there.)
Where the explanations of lightsaber technology get really convoluted is when they start talking about how the blade is shaped and contained. Magnetic fields are currently used to contain plasmas, but they are generated by machinery that must also surround it—generating such a magnetic envelope from a single, unidirectional source would likely require some new laws of physics. There are no crystals that can "direct" a plasma. In fact, a plasma being "directed" by a crystal lens doesn't even make any physical sense. A plasma is really just an ionized gas—a gas in which the electrons have been stripped from their atomic nuclei.5 We see plasmas all the time. They make up and are emitted from every star, like our solar wind and solar flares. The interaction of the solar wind with Earth's magnetic field produces the aurora borealis6, or northern lights—another form of plasma. Plasmas are also the stuff of every spark and lightning bolt.
Plasmas can be created by bringing gases up to a high energy level. The higher the energy, the more atoms will be stripped of their electrons, and the better quality plasma we will have. It's completely possible7 that one could create a plasma by producing a large enough voltage difference—as with lightning—or a powerful enough laser focus. However, enormous amounts of energy are required with either of these approaches, and it would be extremely difficult to control the plasma's shape. An electrical arc can have wild shifts in direction, and it can hardly be controlled without being surrounded by magnets. A laser will go in a straight line… but, of course, it doesn't stop. A laser-based lightsaber would require a block or a couple of mirrors floating in midair, moving in sync with the hilt, which is largely impossible. On top of that, they would certainly melt in the presence of such a plasma anyway. Furthermore, all of this says nothing about what the actual quality of the plasma would be and how reliably or quickly8 it would cut through objects.
So it seems quite impossible to create a lightsaber, as seen in the Star Wars9 films, using existing technologies, materials, and physical laws. But given the enthusiasm of Star Wars10 superfans out there, I wouldn't be surprised if people are trying.