Lasers (Unit 2) Flashcards
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 stimulated emission lead to light that is coherent?
-The emitted photon has the same frequency, phase, direction of travel and polarisation direction as the passing photon. (light emitted is therefore coherent)
What is a LASER?
-Light Amplification by the Stimulated Emission of Radiation.
-A LASER is a coherent light source.
What is a population inversion?
-A population inversion is a situation in which a higher energy state in an atomic system is more heavily populated than a lower energy state
Why do we need population inversion?
- N2 > N1
-we need more electrons in an excited state
-more will drop and emit photons.
-We need stimulated emission to be more likely than absorption so that the laser beam increases in INTENSITY (i.e., the light is amplified).
-EXTRA PHOTON PRODUCED
What happens if you don’t use a population inversion? (more electrons at ground state)
-when we produce a lot of LIGHT
– the very photons that we produce are the actual photons that can be absorbed (they have the correct energy to produce both effects).
- If photons are being absorbed all the time then our laser beam isn’t getting any stronger.
How do you create a population inversion?
-Pumping
- feeding energy into the amplifying medium of a laser to produce a population inversion.
(energy given to electrons)
What is spontaneous emission?
- when an electron drops spontaneously (and randomly) from an excited state to a lower energy state and a photon is emitted.
What the problem with a two level energy system?
- population inversion is not usually possible
- the best that can be achieved here is N2 = N1
-the incoming FLOOD OF PHOTONS is just as likely to cause an electron to drop (stimulated emission) as it is to cause an electron to rise (absorption). - as well as spontaneous emission rapidly depleting the excited state
How is population inversion obtained in a 3 level energy system?
- 1) Pumping. Electrons are promoted from the ground state, E1, to a higher energy level, E3.
- 2)Electrons drop quickly ( E3 has a short lifetime ) through SPONTANEOUS EMMISION to the metastable energy level, E2.
- 3)Electrons stay in this level long enough for a photon of the LASER FREQUENCY to cause stimulated emission.
-4) This is the transition that produces the laser photons so we must have N2 > N1 (more likely to be emitted than absorbed)
What is meant by a metastable energy level?
it has a long lifetime and electrons stay there for a relatively long time
How is population inversion possible in a 3 and 4 level energy system?
-the pumping frequency is higher than the laser frequency
Problem with a 3 level energy system?
-at least half of the electrons in the lower level must be promoted to ensure there is a higher number in level E2 (so stimulated emission occurs). This takes a lot of energy
How is population inversion obtained in a 4 level energy system?
1 ) -Pumping. Electrons are promoted from the ground state, E1, to a higher energy level, E4.
2) Fast drop to the metastable state, E3.
3) This is the LASER LIGHT PRODUCING TRANSITION so this time N3 > N2. Because E1 is the ground state, E2 is practically empty initially so obtaining population inversion is far, far easier. (There is no need to pump half of the electrons than with a 3 level energy system)
4)Another fast transition, so E2 has a short lifetime. This is because we want E2 to be empty so that we have a population inversion (if N2 is small it’s easier for N3 to be larger than N2).
How is a 4 level energy system better than a 3 level energy system?
-LASER LIGHT PRODUCING TRANSITION is E3 so N3 > N2 There is no need to pump half of the electrons as E1 is the ground state, E2 is practically empty initially so obtaining population inversion is far, far easier (therefore more efficient) as less pumping is required
