How indirect band semiconductors are used in lasers ?

[luvagnihotri]

Hi,
I would like to know how we can use indirect band semiconductors in lasers. Such type of semiconductors do not emit photons when transition takes place. Energy is given up as heat to the lattice.

Regards,

 

[genneth]

First of all, indirect band gap semiconductors certainly do emit photons when a transition occurs --- they emit/absorb photons and phonons simultaneously. Second, a material that does not produce photons would be pretty damn useless as a laser or a light-emitting anything!

 

[Cthugha]

As far as I know most of the interest in indirect bandgap lasing comes from people, who are interested in optical processing using silicon because silicon is an indirect bandgap material. As genneth already said, photon emission indeed happens at a transition. It is just not the only process.

Most lasers in silicon are not based on stimulated emission, but on stimulated Raman scattering if I remember correctly, so you need a pump laser in most cases anyway.

Another funny method I recently saw to achieve direct stimulated emission consists of pumping silicon so hard that the whole conduction band - even the states far from the band minimum - is filled with electrons. At some point even the point of the direct transition to the valence band will be filled, so that inversion can be present and lasing can be achieved. However this process is extremely inefficient and you need to pump so hard that two photon absorption losses can become critical and you are always close to destroy your piece of silicon due to the high power you fire at it. But in principle it works - or as my boss once said: If you pump it hard enough, you can achieve lasing in a slice of bread as well. ;)

 

[sokrates]

Most lasers in silicon are not based on stimulated emission, but on stimulated Raman scattering if I remember correctly, so you need a pump laser in most cases anyway.

Good question and answers. While you are at it, could you elaborate more on Raman scattering ?