1. Light-Matter Interaction
Einstein’s Theory of
Light-Matter Interaction

2. Light-Matter Interaction
Einstein postulated that the existence of thermal equilibrium between light and matter can be explained by three basic interaction processes.
Stimulated absorption
Spontaneous emission
Stimulated emission

3. Light-Matter Interaction
Let us consider a sample of atoms that have two levels of energy E1 and E2 and a nearly monochromatic field of frequency  and irradiance I is incident on it.
The two levels are nearly resonant with the incident light. E2 – E1  h

4. Stimulated Absorption
h  E2 – E1 = ho
The rate of occurrence per unit volume of stimulated absorption:
B12 is the Einstein B coefficient for stimulated absorption.

5. Stimulated Emission h  E2 – E1 = ho
The rate of occurrence per unit volume of stimulated emission:
B21 is the Einstein B coefficient for stimulated emission.
The emitted photon has the same energy, direction, phase and polarization of the incident photon.

6. Spontaneous Emission h  E2 – E1 = ho
The rate of occurrence per unit volume of spontaneous emission:
A21 is the Einstein A coefficient for spontaneous emission.

7. Einstein A and B Coefficients
What is the relation between Einstein’s A and B coefficient in the case of thermal equilibrium between the radiation field and an assembly of atoms?
Downward transition rate=upward transition rate

8. Einstein A and B Coefficients
What is the ratio of the overall rate of stimulated emission to that of stimulated absorption?
The Boltzmann distribution can be used:
Stimulated absorption will occur more often than stimulated emission when atoms are in thermal equilibrium!

9. Einstein A and B Coefficients
In order for an assembly of atoms to amplify an incident EM field, pump energy must be supplied to the atoms in order to drive the atom out of thermal equilibrium and even preferentially populate the upper energy level so that:
The amplifying or gain medium plays a central role in laser action to help achieve population inversion.

10. Essential Elements of a Laser
Pump
Gain medium
Resonator
Cooling system

11. Essential Elements of a Laser
An external energy source that produces a population inversion in the laser gain medium.
Without the pump energy, light wave would be attenuated during each pass through the medium.
Pumps can be optical, electrical, chemical, or thermal in nature, so long as they create energy that would provide the necessary population inversion.
Laser
Pump

12. Essential Elements of a Laser
A laser system is named by the gain medium that makes it up.
The gain medium can be a gas, liquid or solid.
The gain medium determines the wavelength of the laser radiation.
Laser action has been observed over half of the known elements  more than 1,000 laser transition in gases alone.
In some lasers, the amplifying medium consists of two parts; the laser host medium and the laser atoms.
A gain medium must be able to support a population inversion.
Laser
Gain medium

13. Essential Elements of a Laser
A resonator, is an optical “feedback device” that directs photons back and forth through the laser (amplifying) medium.
A basic form of a resonator consists of a pair of carefully aligned plane or curved mirrors centered along the optical axis of the laser system.
One of the mirrors is chosen with a reflectivity as close to 100% as possible. The other reflectivity is somewhat less than 100% to allow part of the internally reflecting beam to escape and become the useful laser output beam.
Laser
Resonator

14. Essential Elements of a Laser
Resonator

15. Essential Elements of a Laser
A laser cavity consisting of two flat mirrors separated by an optical distance d will only support standing wave modes of wavelength m and frequency m that satisfy the condition:
Therefore, the frequencies of the modes of such a cavity are:
The resonator will support fields with a narrow range of frequencies given by the above equation.
Laser
Resonator

16. Essential Elements of a Laser
A laser resonator acts as a feedback device and also a frequency filter.
Only EM fields that have frequencies near the resonant frequency of the lasing transition (and so experience significant gain) and very near a standing wave frequency of the cavity (and so experience low loss) will be present in the laser output.
Laser mirrors can have plane or spherical surfaces.
The geometry of the mirrors and their separation determine the mode structure of the EM field within the laser cavity.
Higher-order modes can be suppressed to obtain a single fundamental mode TEM00 (Gaussian Shape)
Laser
Resonator

17. Essential Elements of a Laser
Resonator

18. Essential Elements of a Laser
The overall efficiency of a laser system is given by:
Typical efficiencies range from a fraction of a percent to 25% or so.
Pump energy that does not result in laser output inevitably degrades into thermal energy.
If heat energy is not removed from the system, the components of the system will be damaged or degraded.
Cooling jackets are used with water or cooling oil flowing through it.
Laser
Cooling system

19. Simplified Description of Laser Operation

20. Simplified Description of Laser Operation