Ultrashort Laser Pulses(<200 femtoseconds)

One of the exciting things about light is that it comes in different forms. Of course, we all know in different colors and different frequencies. The light can also be controlled in how its amplitude changes. We can have light pulses shorter than few attoseconds( s) or we can have continuous intensity wavetrains. As opposed to continuous wave(cw) laser light,  laser pulses(typically nanosecond and below) have interesting properties. The reason is that cw lasers typically have low powers and if you increase their power you would just burn the material at hand.  But with femtosecond or picosecond wide pulses, we have negligible thermal effects and therefore they can be employed to get non-linear optical effects with minimal damage to the material. Why do they have negligible thermal effects ? Because they are cool. (Thanks smartypants!) The reason is that when an ultrashort laser pulse(shorter than few picoseconds) hits the atoms, the electrons are excited faster than they transfer energy back to the lattice. Its like the solid did not even know what hit it and the laser pulse is gone, vanished.  The nonlinear effects are possible only when the peak powers are high.

So, how is power defined in terms of laser pulses ?

Power is the energy radiated every second from a laser. For a continuous laser, the power is defined in terms of intensity. So, in other words, the average power for a continuous laser will be defined by :

For instance, its power is characterized in two ways : Peak Power and Average Power.

Okay fine, we know how to talk about pulses in terms of power …

Now, how does a femtosecond or ultrashort pulse look like ? Gaussian pulse shape describes it quite well. People don’t quite agree with the functional form. Some use sech^2 shape to represent pulses from ultrashort oscillators(buzz word for the machine which produces short pulses using a physical process called mode-locking ) and a gaussian shape to the pulses from ultrashort amplifiers(buzz-word for the machine which amplifies the pulses which come from oscillators using regenerative amplification). This is how it looks like if you could measure its time varying electric field oscillations.

Typical Femtosecond Pulse(Adapted from Wikimedia)

A typical gaussian pulse is given as

That’s enough for this post. I hope this gets you excited to learn more about ultrashort pulses. Will soon be back with another post on these elusive short light bursts.