Beam Material Interaction

Question 1

What is absorption?

Answer 1

• Amount absorbed depends upon the material, the incident wavelength, power, etc
• Absorption produces: Heat, Luminescence, Photochemical reactions
• Beam stops are designed to absorb laser radiation
  Lesser power is outputted than inputted.

Question 2

What is Transmission?

Answer 2

An amount of radiation will be transmitted by an absorber depending upon: material
type, wavelength.
* A filter can be used to absorb some wavelengths and transmit others
Lesser power is outputted than inputted.

Question 3

What is reflection?

Answer 3

Whether a laser beam is reflected or
absorbed by a particular surface is
governed by the condition of the surface on which it impinges and the wavelength of the laser beam itself.

Question 4

What is the equation for reflectivity of opaque and transparent materials in regard to absoptivity?

Answer 4

Essentially, for opaque materials:
Reflectivity = 1 – Absorptivity
And for transparent materials:
Reflectivity = 1 – (Transmissivity + Absorptivity)

Question 5

What is light scattering?

Answer 5

Light can scatter when it interacts with water droplets, dust particles and other
molecules in air. This can change the direction and other properties of the light.

Question 6

What are 4 examples of scattering?

Answer 6

• Rayleigh: scatter from small particles (&laquo_space;λ). Gives uniform scatter direction but is highly wavelength dependent. Shorter wavelengths are scattered more.
• Mie: scatter from large particles (≈ λ). Gives non-uniform scatter direction but is not wavelength dependent.
• Brillouin: very very small wavelength (frequency) shift from solid surfaces.
• Raman: very small changes in intensity caused by change in energy from interacting with particles.

Question 7

What is Rayleigh scattering?

Answer 7

As white light interacts with molecules
and water droplets in the air it is scattered. Blue is scattered the most and the sun looks yellow (red+green) leaving the shortest wavelength (green) to scatter so the sunset looks red.

Question 8

What is polarisation?

Answer 8

• The polarization of light refers to the plane in which the waveform and electric force field lies.
• Most light is unpolarized - The waveforms and electric force field on a given ray keep jumping around in orientation while always perpendicular to the ray.
• Scattered or reflected light is polarized in particular planes.
• This could be deliberate by the design of our optics, or cause a disadvantage
  when processing specific materials.

Question 9

What is refraction?

Answer 9

The redirection of a wave as it passes from one medium to another.

Question 10

What are the lenses affecting refraction?

Answer 10

Positive - Plano convex & Bi-Convex
Negative - Plano-concave and Bi concave

Question 11

What is diffraction?

Answer 11

Diffraction is the interference or bending of waves around the corners of an obstacle or through an aperture.

As the laser is monochromatic and
coherent it can interfere with itself to
cause diffraction patterns.

Famous double slit experiment →
This also occurs in reflection from
surfaces to give a speckle pattern.

Question 12

What is diffraction for beam shaping?

Answer 12

Can cause holograms allowing the shape of the laser to change.
Can control the interference pattern to create a desirable shape

Question 13

What are the 3 main absorption mechanisms?

Answer 13

Fresnel Absorption
Photochemical Scission
Inverse Bremsstrahlung absorption

Question 14

What is fresnel absorption?

Answer 14

1. The photons of the laser beam interact with the electrons in the material, causing them to vibrate and, in turn, produce heat.
   2.The beam produced by infrared lasers has relatively long wavelengths so the photon energy is small. Meaning many shall be absorbed by the material to casue the materials molecules to dissociate by melting or vaporisation = heat gen

Question 15

What is the Beer-Lambert Law for thermal absorption?

Answer 15

• As the laser beam propagates through the workpiece, the energy is absorbed.
• The amplitude of the laser beam reduces exponentially.
• The temperature of the workpiece increases as the energy from the laser beam is absorbed.

Question 16

What are wavelengths effect on absorption?

Answer 16

At shorter wavelengths, more photons can be absorbed by a greater number of bound electrons. Thus, the absorption typically increases as the wavelength reduces.

Question 17

Angle and polarisation effect on absorption?

Answer 17

as the angle of incidence increases the reflectivity shall decrease then rapidly increases around 70/80deg

Question 18

What is the roughness effect on absorption?

Answer 18

If a surface is rough, then the light that is reflected from the surface is not reflected normally.
This means that more light is absorbed.
Thus the rougher the surface the higher the absorption.
A specular or reflective surface has
surface irregularities which are small
relative to λ Specular reflections retain much of original power of incident beam and can be hazardous

Question 19

What is photo chemical absorption?

Answer 19

Because of this similarity between UV laser wavelength and the excitation energy of the bonds holding most materials together, the UV output (typically of excimer lasers) has the ability to directly break these bonds.

This process is called scission and is a photo-chemically induce occurrence does not include melting or vaporisation.

microfabrication is cold machining or cold working.

Question 20

Fresnel Absorption was the repeated absorption of many photons to to induce heat.
What conditions are required for a single photon to break chemical bonds?

Answer 20

A photon with sufficient energy and certain wavelength to overcome the bond energy.

Question 21

What is the bond energy?

Answer 21

Bond energy is the strength of a chemical
bond between atoms, expressed as the
amount of energy required to break it
apart. A higher bond energy, therefore,
means a stronger bond.
Bond energies are usually expressed in
kilojoules per mole (kJ/mol)

Question 22

How to calculate the energy required per bond?

Answer 22

Divide bond energy by Avogadro constant 6.022×1023 mol-1 to calculate energy required per bond.

Question 23

How to convert from J to eV?

Answer 23

To compare energies we use the electron volt (eV) value.
1 electron volt = 1.602×10−19J.

Question 24

What is inverse Bremsstrahlung absorption?

Answer 24

Plasma Absorption
In inverse Bremsstrahlung absorption, the photons of a laser beam interact with free electrons in an ionized gas (plasma). The photons transfer energy to the plasma. The plasma acts like a black body, absorbing all the incoming light.
Plasma electrons can then emit photons across the spectrum (Bremsstrahlung radiation = why plasma looks white hot)

Question 25

What are advantages of Inverse Bremsstrahlung ?

Answer 25

Advantage = Keyhole plasma
Use the 100% absorption to increase the heat. This is the mechanism for
the absorption of the laser within the “keyhole”. This is very important to
cutting and drilling!

Question 26

What are disadvantages of Inverse Bremsstrahlung ?

Answer 26

Disadvantage = Plasma Shielding
In laser materials processing, this is the shroud gas or ejected materials in
the form of a plasma. Not desirable as it blocks the laser beam from the
surface!