Lasers Flashcards
What does laser stand for?
Light Amplification by Stimulated Emission of Radiation.
What is photon absorption?
An electron is excited to a higher energy level by absorbing the energy of an incoming photon. The energy of the incoming photon must be equal to the difference between the two energy levels in order for it to be absorbed.
What is photon emission?
The decay of an electron from one energy state to a lower energy state, resulting in the creation of a photon.
What is spontaneous emission?
A photon which is created spontaneously. The frequency of the emitted light is determined by the difference in energy levels.
What is stimulated emission?
The emission of a photon from an excited atom, triggered by a passing photon of energy equal to the energy gap between the excited state and a state of lower energy in the atom.
How does the incoming photon and the emitted photon compare in stimulated emission?
They have the same wavelength, exit in phase and in the same direction.
What is a population inversion?
A situation in which a higher energy state in an atomic system is more heavily populated than a lower energy state of the same system.
What is pumping?
Feeding energy into the amplifying medium of a laser to produce a population inversion.
Describe the three energy level laser system
- Pumping. Electrons are promoted from the ground state (E1) to E3 by an external light source or electron collisions.
- Electrons drop quickly to the metastable (E2).
- Electrons drop from E2 to E1 to produce a laser photon so we must have N2>N1. We need to ensure that pumping exceeds the stimulated emission to maintain a population inversion.
What is a metastable state?
One which has a long lifetime, and electrons stay there for a long time.
Describe the four energy level laser system
- Pumping. Electrons are promoted from the ground state (E1) to E4.
- Fast drop to the metastable E3.
- Electrons drop from E3 to E2, emitting a laser photon. We need to have N3>N2 and as E2 is practically empty, obtaining a population inversion is much easier.
- Another fast transition from E2 to E1 as E2 has a short lifetime. This is because we want E2 to be empty so we have a population inversion.
Why is a four level laser better than a three level laser?
E2 is as depopulated as possible which means less pumping is required so it is easier to maintain a population inversion and therefore the laser is more efficient.
How does light from a laser differ from normal light?
It is polarised, coherent, monochromatic and a parallel beam.
Describe how a laser works
Photons are reflected back and forth between two mirrors stimulating more photons and eventually escape through the partially reflective mirror.
What is an amplifying medium?
The region where the population inversion exists.
