laser matter inetraction basic concepts of light and laser.types of lasers, stimulated emmision,

1. Laser matter interaction
PREPARED BY : HASNAIN JAVED
BS HONS. PHYSICS
DEPARTMENT OF PHYSICS
UNIVERSITY OF GUJRAT

2. Outlines
 introduction to light
 Historical development in light
 Wave and particle nature of light
 Concept of photon
 Maxwell equation of EM field
 Definition of laser
 Mechanism of light emission
 Stimulated emission
 Components of laser (video)
 Types of lasers
 Laser interaction with solid or gas
 Laser ablation
 Semiconductors/ diode laser
 Gas laser
 Fiber laser
 Crystal laser

3. What is light?? Light is a transverse, electromagnetic wave that can be seen by humans.

4. Propagation of light
angle of incidence = angle of reflection
θ =θ’
Snell´s Law
nsin(θ ) = n'sin(θ ' )
Water tank: Reflected and
refracted light
components!

5. Dispersion
Wave package:
group
velocity:
phase velocity:
Vgroup = velocity of the
whole wave
package

6. Historical development

7. Wave Nature of Light
 Electromagnetic state of matter
 1. The charge density ρ (charge per unit volume)
 2. The polarization P (electric dipole per unit volume)
 3. The magnetization M (magnetic dipole per unit volume)
 4. The current density J

8. Wave Nature of Light
 The electric displacement vector D
D = εo E + P
 Electric susceptibility χ
P = χ εo + E
 Magnetic flux B
B = μo ( H + M )

9. Particle Nature of Light
Phenomena that reveals the particle nature of light:
 Photoelectric Effect
 Compton Scattering
 Blackbody Radiation
 Emission atomic lines
When the atoms radiate discrete values of energy, those small bundles of
energy are considered to be particles and are referred as photons.

10. Concept of photon
 A particle representing a quantum
of light.
 It carries energy proportional to
the radiation frequency
 it has zero rest mass.
The energy and momentum of a photon
E = h·n = p·c
p=h/λ

11. Maxwell’s Equations (static field)
1.Charges are the sources of electric fields
∇⋅D = ρ
∫∫D. dA = q(V)
2. Magnetic monopols do not exist
∇⋅ B = 0
∫∫ B.dA = 0

12. Maxwell’s Equations (dynamic field)
3.A changing magnetic field creates an
electric field
∇ × E = −∂B/∂t
4. Magnetic fields are created by electrical current
and by changing electric fields
∇ × B = J + ∂E/∂t

13. Discovery of Stimulated Emission in 1917
Albert Einstein
* 14.3.1879, Ulm / Germany
† 18.4.1955, Princeton / USA

14. Definition of
laser
A laser is a device that generates
light by a process called
STIMULATED EMISSION.
The acronym LASER stands for Light
Amplification by Stimulated
Emission of Radiation.
Semiconducting lasers are
multilayer semiconductor that
generates a coherent beam of
monochromatic light by laser action

15. 1960 First LASER
Constructed
Theodore Harold Maiman
* 11.7.1927, Los Angeles / USA
† 5.5.2007, Vancouver / Canada

16. Lasers operate in the ultraviolet,
visible, and infrared.

17. Mechanisms of Light Emission
 For atomic systems in thermal equilibrium with their surrounding,
the emission of light is the result of Absorption and
subsequently Spontaneous emission of energy
 Stimulated emission
There is another process whereby the atom in an upper energy level can triggered
or stimulated in phase with the an incoming photon. The process is Stimulated
emission
It is an important process for laser action

18. Stimulated Emission
•
It is pointed out by Einstein that:
“Atoms in an excited state can be stimulated to jump to a
lower energy level when they are struck by a photon of incident light
whose energy is the same as the energy-level difference involved in
the jump. The electron thus emits a photon of the same wavelength as
the incident photon. The incident and emitted photons travel away
from the atom in phase.”
This process is called stimulated emission.

19. Stimulated Emission

20. Population inversion
The atoms must be excited to the higher state. That is, an
inverted population is needed, one in which more atoms are
in the upper state than in the lower one, so that emission of
photons will dominate over absorption.

21. Metastable state
a state in which the electrons remain longer than usual so that the transition to the
lower state occurs by stimulated emission rather than spontaneously.

22. Components of
laser
ACTIVE MEDIUM
Solid (Crystal), Gas,
Semiconductor (Diode),
Liquid (Dye)
EXCITATION MECHANISM
Optical, Electrical, Chemical
OPTICAL RESONATOR
HR Mirror and
Output Coupler

24. Properties of Laser Light

25. Types of lasers
lasers
Solid state liquid Gas lasers
semiconductors

26. Light Absorption
Dominant interaction
– Photon absorbed
– Electron is excited to CB
– Hole left in the VB
• Depends on the energy
band gap (similar to
lasers)
• Absorption (a) requires
the photon energy to be
larger than the material
band gap

27. Interaction with solid or gas A laser beam (1012 w to 1015w) is focused onto a gas or solid target with focal spots of mm.
A high temperature plasma is produced..

28. Beer lambert Law of absorption

29. Laser ablation
is the removal of material by direct absorption of laser light

30. Laser induced plasma / plume

31. Insulator Conductor
(metals)
Semiconductors
Interaction with semiconductor

32. Interband transition
gEh n gEh n
  nanoseconds in
GaAs

33. n-type
Intraband transitions
  < ps in GaAs

34. Interband vs Intraband
Interband:
Transision between the conduction and
bands.
The devices are bipolar involving a p-n
junction.
Intraband:
quantum cascade lasers, are based on the
transitions between the sub-bands in the
conduction or valence bands.
The Intraband devices are unipolar.
C
V
C

35. Semiconductor vs solid-state
semiconductors
 Fast: due to short excited state lifetime (
ns)
 Direct electrical pumping
 Broad bandwidth
 Lack of energy storage
 Low damage threshold
Solid-state lasers,
 Need optical pumping
 Long storage time for high peak power
 High damage threshold

36. CW, ns, ps/fs lasers

37. CW, ns, ps/fs lasers
 The CW laser (far left) removes material primarily by melting, which creates a large
HAZ.
 The (ns) laser pulses (center) create a smaller HAZ and material is removed by
melt expulsion driven by the vapor pressure and the recoil pressure.
 With ultrafast pulses (ps/fs), the laser pulse duration is much shorter than the
timescale for energy transfer between free electrons and the material lattice.

38. CO2 lasers
 CO2 laser is used to cut by , burning
melting and vaporizing. they
can’t cut metal but only engrave it.

39. Fiber lasers
They generate beam by seed laser and
amplify it in glass fiber (l = 1.064 mm)
their intensity is up to 100 times higher
than CO2 laser. they used for metal
engraving plastic marking.

40. Crystal lasers
 Nd-YAG Nd; YVO
 They are doped by neodymium over
carrier crystal.
 Wavelength same as fibers 1.064 mm
used for marking metals
and plastics