Nanostructure Lasers Flashcards
What is ‘stimulated emission’?
A photon of a given energy cause an identical second photon to be produced by stimulating an electron to fall from a high to low energy level.
What is a population inversion?
More electrons at the bottom of the conduction band than remain at the top of the valence band.
Why is a population inversion required for lasing?
So the amplification process is greater than the absorption which would reduce the photon number.
What is required to reach the required population inversion for lasing?
Injecting a large number of electrons and holes into the device.
What is the threshold current?
The current required to start lasing (enough electrons and holes are injected).
Why is the threshold current relatively high in a semiconductor inter-band laser and why does it increase with temperature?
Electrons and holes only contribute to the lasing if they are in the bottom of the conduction band and the top of the valence band. If their thermal energy increases they can move away from these positions and so the threshold current must increase with temperature to make up for the lost electrons and holes.
How are electrons and holes restricted to the band edges where they contribute to the lasing process?
By reducing the number of states in higher energies by using a quantum well, wire or dot.
How do quantum cascade lasers work?
Electrons arrive at a suitable energy and tunnel through a barrier into n=2 level of quantum well 1. They then drop down to the n=1 level and release a photon before tunnelling into the second quantum well at the n=2 level. They drop to n=1 again, release a photon and tunnel out.
What is a drawback of cascade lasers?
The electrons can create vibrations when jumping between levels, not all transitions result in a photon, and can heat up the semiconductor. This can cause damage.
What is an application of cascade lasers?
Gas sensing. A laser tuned to the absorption band of a specific gas can detect the presence of the gas at very low levels.
Why is very long wavelength emission difficult to achieve?
Need larger band gaps so smaller distance between conduction and valence bands. Difficult practically.
