بخشی از پاورپوینت
--- پاورپوینت شامل تصاویر میباشد ----
اسلاید 1 :
Light Amplification by Stimulated Emission of Radiation
Stimulated emission
Spontaneous emission
اسلاید 2 :
Energy level diagram
- The possible energies which electrons in the atom can have is depicted in an energy level diagram.
اسلاید 3 :
The operation of the Laser
- In 1958, Charles Townes and Arthur Schawlow theorized about a visible laser, an invention that would use infrared and/or visible spectrum light.
- Light Amplification by Stimulated Emission of Radiation- (LASER).
- Properties of Lasers
–Produce monochromatic light of extremely high intensity.
اسلاید 4 :
The operation of the Laser
اسلاید 5 :
The operation of the Laser
Light: Coherent, polarized
The stimulating and emitted photons have the same:
frequency
phase
direction
اسلاید 6 :
- n1 - the number of electrons of energy E1
- n2 - the number of electrons of energy E2
اسلاید 7 :
Einstein’s coefficients
Probability of stimulated absorption R1-2
R1-2 = r (n) B1-2
Probability of stimulated and spontaneous emission :
R2-1 = r (n) B2-1 + A2-1
assumption: n1 atoms of energy e 1 and n2 atoms of energy e 2 are in thermal equilibrium at temperature T with the radiation of spectral density r (n):
n1 R1-2 = n2 R2-1 n1r (n) B1-2 = n2 (r (n) B2-1 + A2-1)
اسلاید 8 :
The probability of spontaneous emission A2-1 /the probability of stimulated emission B2-1r(n ):
1.Visible photons, energy: 1.6eV – 3.1eV.
2.kT at 300K ~ 0.025eV.
3.stimulated emission dominates solely when hn /kT <<1!
(for microwaves: hn <0.0015eV)
The frequency of emission acts to the absorption:
if hn /kT <<1.
اسلاید 9 :
Condition for the laser operation
If n1 > n2
- radiation is mostly absorbed
- spontaneous radiation dominates.
if n2 >> n1 - population inversion
- most atoms occupy level E2, weak absorption
- stimulated emission prevails
- light is amplified
Necessary condition:
population inversion
اسلاید 10 :
How to realize the population inversion?
Thermal excitation:
impossible.
The system has to be „pumped”
Optically,
electrically.