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