what are their (femto, pico second pulses ) advantages over nano pulses.
that depends on what you want to do with your laser. In general the shorter the pulse the higher the power density per pulse as P = E/t.
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I agree, this answer is too general. Here are some more practical arguments.
- If you want to picture a moving car with a camera you need to expose the film no more than about 1/100th of a second for the picture to be sharp. If you would like to picture the flying bullet of a firearm you would live the camera open in a dark room and shine the bullet with a nanosecond flash of light at the time it passes in front of the camera. When exploring the fastest dynamics of electrons in semiconductors you need femtosecond light pulses, and you want to explore the fastest dynamics of electrons because you want computer to run ever faster.
- This argument does not apply to attosecond light pulses because even the light objects like electrons do move too slowly at an attosecond time scale. The second strong argument favouring short pulses goes as follows. If you can put as much energy in an attosecond light pulse than in a nanosecond light pulse then the peak power of the attosecond pulse is a billion times larger than the peak power of the nanosecond pulse. When we will be able to produce pulses with peak powers larger than 1020 W/cm2 then we will be able to observe the interaction of two visible photons. In the mean time we will explore the field of photo induced nuclear reactions.
There are certainly things you can do with picosecond, femtosecond, or attosecond laser pulses that you cannot do with nanosecond pulses. For example, electron orbits about the nucleus of an atom occur on attosecond time scales. Thus to probe this you need the time resolution which attosecond pulses provide. As for picosecond and femtosecond pules, there are also many specific applications. Specifically, my interest in them is to produce intensities which exceed the relativistic threshold (>10^18 W/cm^2). At these intensities, it becomes possible to use a laser to perform nuclear science. Laser accelerated MeV ion beams, GeV electron beams, etc.. The physical mechanisms for generating these beams occur on picosecond time scales. Any laser energy deposited into samples after a few picoseconds have elapsed does not contribute.
To be clear, it is possible to generate ultra high intensities by adding more energy to nanosecond pules, but the cost is prohibitive to most countries. Shorter pulse duration with less energy can achieve the same intensity for costs affordable to universities and businesses. As an example, the National Ignition Facility combines 192 nanosecond lasers pules on a single target sample with a total energy of about 2 megajoules. That is an average pulse power of a petawatt. The cost of this facility far exceeded 6 billion US dollars. Also, the repetition rate of the laser is slow because of all the heat produced in the amplifiers at that energy. You get roughly 1 pulse every 3 to 5 hours. Compare this with a laser which produces 500 joule pules in 500 fs. Such lasers exist and cost closer to a couple of million dollars and produces pulses every hour or less.
Of course nanosecond pules have applications which shorter pulse lasers are not the best choice. Again referring to the National Ignition Facility. The fusion experiment for which it was built requires megajoules of energy.To date, no one has figured out how to compress a megajoule, short pulse laser in a way that is cost effective.
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