introduction to laser and introductory concepts
TRANSCRIPT
Laser matter interaction
PREPARED BY : HASNAIN JAVED BS HONS. PHYSICS DEPARTMENT OF PHYSICS UNIVERSITY OF GUJRAT
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
What is light?? Light is a transverse, electromagnetic wave that can be seen by
humans.
Propagation of light
angle of incidence = angle of reflectionθ =θ’
Snell´s Lawnsin(θ ) = n'sin(θ ' )
Water tank: Reflected and refracted lightcomponents!
Dispersion
Wave package:
group velocity:
phase velocity:
Vgroup = velocity of the whole wave package
Historical development
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
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 )
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.
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
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
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
Discovery of Stimulated Emission in 1917
Albert Einstein* 14.3.1879, Ulm / Germany† 18.4.1955, Princeton / USA
Definition of laserA 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
1960 First LASER Constructed
Theodore Harold Maiman* 11.7.1927, Los Angeles / USA† 5.5.2007, Vancouver / Canada
Lasers operate in the ultraviolet, visible, and infrared.
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
Stimulated Emission
•
It is pointed out by Einstein that: “Atoms in an excited state can be stimulated to jump to alower energy level when they are struck by a photon of incident lightwhose energy is the same as the energy-level difference involved inthe jump. The electron thus emits a photon of the same wavelength asthe incident photon. The incident and emitted photons travel awayfrom the atom in phase.” This process is called stimulated emission.
Stimulated Emission
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.
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.
Components of laser
ACTIVE MEDIUM
Solid (Crystal), Gas,
Semiconductor (Diode),
Liquid (Dye)
EXCITATION MECHANISM
Optical, Electrical, Chemical
OPTICAL RESONATOR
HR Mirror and
Output Coupler
Properties of Laser Light
Types of lasers
lasers
Solid state liquid Gas lasers
semiconductors
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
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..
Beer lambert Law of absorption
Laser ablation is the removal of material by direct absorption of laser light
Laser induced plasma / plume
Insulator Conductor(metals)
Semiconductors
Interaction with semiconductor
Interband transition
gEh n gEh n nanoseconds in GaAs
n-type
Intraband transitions
< ps in GaAs
Interband vs Intraband
Interband:Transision between the conduction and valence 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
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
CW, ns, ps/fs lasers
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.
CO2 lasers
CO2 laser is used to cut by , burning
melting and vaporizing. they
can’t cut metal but only engrave it.
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.
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