December 3, 2011

Laser -Degree-

Previously in our online 'course', we discussed about the basic principle of the laser technology. With the limited information provided, surely you must have come to a conclusion that stimulated emission is simply not sustainable without sufficient excited atoms.

And you may also have deduced that to achieve the required number of excited atoms is pretty complicated.

This is where population inversion comes into play. 

See, in lasers, light emission by stimulated emission occurs more than by spontaneous emission. To achieve this state, the number of atoms at high-energy levels must exceed the number of atoms at ground state. 

This situation is known as population inversion. To make population state possible, a state called metastable state is created. In the metastable state, the atom remains in the excited state a little longer than usual.

Which brings us to an example:

The Helium-Neon Laser.

A mixture of helium and neon gas in the ratio of 20:80 is contained in a tube with mirrors at both end. When a potential difference (Voltage) is applied across the tube, electrons will travel along it. 

Refer to the below diagram if you aren't a university graduate who probably should already know all this stuff.
Laser technology is taught even in the very basic of physics university courses.

Electrons colliding with the helium atoms cause them to be raised to energy level E2 (20.61eV). This value is close to the energy level of neon E4(20.66eV).

Collisions between the helium and neon atoms enables the neon atoms to attain energy level E4 easily, hence, the number of neon atoms at E4 is more than the number of neon atoms at E1.

And we have a Population Inversion.

Which brings us back to our previous course: 

The combination of all of this results in a highly coherent, intense beam of light.


  1. I think its rather fascinating the uses the laser has now days, from surgery to communication, I think its impressive how diverse it can be!

  2. For those interested especially in metastable energy states: these principles regarding power of a single atom also apply in general chemistry. Quantum Physics and Quantum Chemistry are subjects I find as probably the hardest out there, but at the same time - as the laser example above shows - extremely useful. Also, some of the data here is fascinating: the power joining two single - let's say carbon - atoms is so big that if you'd like to take apart atoms in - let's say: 1g of carbon - you'd need energy big enough to supply the electricity needs of a major city - like New York - for a full year. Of course, the latest scientific novelties - like antimatter - are just around the corner.


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