An interesting principle of photonics which has potential military, aeronautic and astro/cosmonautic applications.
The principle is that when enough energy (in the form of photons) is moving through a small volume, the atoms in that volume become so excited that they are stripped of their electrons, becoming ionized. Higher still energy levels rips apart the molecules, forming plasma.
As a side effect of this, pressure (shock) waves form that can reach thousands of atmospheres and temperatures of up to 30,000 Kelvin.
The shock wave can be used as a form of propulsion. Scientists at the High Energy Laser System Test Facility(HELSTF) at the White Sands Missile Range in New Mexico have driven small test craft to heights of 100 feet in wire-guided tests by beaming a laser at the reflective lense-shaped bottom of a 'lightcraft'.
In space, you have to provide your own atoms to convert to plasma. Scientists at the JPL have theorized that it may be possible to focus a laser beam (source not specified) in to a reaction chamber with hydrogen, and using the resulting plasma for thrust.
This effect also has potential use as a weapon. Two lasers, which by themselves are not powerful enough to trigger the effect, can be focussed on a point at a distance - the combined power of the two lasers rip the atoms in to plasma and trigger the shock wave at the focus of the two lasers.
The major problem with building such a weapon is that the shock-wave travels up the path of the laser, potentially damaging the equipment. This is circumvented by firing the laser in many short powerful bursts - typically about 20 per second.
Towards the end of 1999 another potential military use was demonstrated. Instead of using the shock wave as a weapon, it exploited the fact that the molecules ionized by a laser conduct electricity. Two ultraviolet lasers were used to form paths to a target, which a large current/voltage could then be conducted through. The effect on the target is similar to that of a tazer, except more powerful. The current can penetrate thick clothing, and is powerful enough to trigger a severe heart attack, but does not burn skin. The apparatus (as of 1999) occupied one whole laboratory bench, not including the power supply.
Formal definition of the Inverse Bremsstrahlung effect:
Collisional absorption of energy that occurs in inertial confinement fusion systems when hydrodynamic expansion of the plasma causes an electron passing through the field of an ion to absorb radiation, raising the energy and temperature of the electron particle.