Download Wave / Particle Duality of Light

Brought to you by:
Jonathan E. Mace
What is a…
What make a laser so special?
History of Lasers
-S. N. Bose postulates that light does not behave classically in the
fact that statistically they will tend to travel together (1924).
- From this Albert Einstein uses photon statistics to predict
stimulated emission, the physical basis of lasers.
- Charles Townes and Arthur Shawlow produce first Maser from a
beam of ammonia and theorize existence of optical and IR lasers
(1958).
- T. H. Maiman produces the first optical laser from a ruby (1960).
- Gordon Gould eventually granted patent for components of
lasers.
Wave / Particle Duality of Light
Wave-like Properties
-Wavelength/Frequency
-Interference
-Phase
Particle-like Properties
-Photons
-Momentum
-Quantum mechanics
Lasers Use
Both!!!
Absorption and Emission of Light
Electrons can
only go to certain
energy levels.
They can only absorb
certain frequencies
of light.
Ground State
Electrons
Electrons emit lightExcited State
at the same discrete Electron s
frequencies at which
they absorb when
traveling between
energy levels.
The two types of emission
Excited atoms decay to a
lower energy state and emit
light in random directions.
The different atoms can emit
light at different times and
may not undergo the same
transitions.
Light from the decay of one
excited atom interacts with
another similarly excited
atom causing that element
to emit light
This light produced is: in
phase, directional, and
monochromatic.
Metastable States
Electron in an unstable excited state.
(lifetime in nanosecond timescale)
Electron drops to a lower excited
state through a “radiationless”
transition. (heat given off)
Electron is now in a metastable
state which is more stable than
pervious state (lifetime can be in
millisecond timescale)
Metastable states are required for
stimulated emission to have enough
time to be effective in producing
laser light.
To “Lase”?
- To lase is the verb form of what a laser does. Lasing, or laser action, is
when stimulated emission from atoms or molecules in excited states
overcomes spontaneous emission from excited atoms or molecules in the
medium, thus producing laser light.
- This sounds relatively simple, however a certain criteria has to be met….
- If only a few atoms or molecules are excited then it will be very hard to
have stimulated emission occur because the likelihood of a photon
encountering another excited atom or molecule would be small.
- Therefore, we just need to get a heck of a lot of atoms or molecules into an
excited state and we should get laser light…. right? Well, not exactly. Since
most atoms and molecules are in the grounds state we need a population
inversion.
Population Inversion
- A population inversion is achieved
when more atoms in a medium are
in a higher energy state than are in
the lower state.
- The second law of thermodynamics makes a
two level laser impossible since only half of
the atoms can be put in the excited state.
N2
E / kT

e
N1
- Three and four level lasers are possible when we take
advantage of metastable states. A three level laser was the first
created, but now four level lasers are the most common.
Three and Four Level Lasers
Electron pumped to
fourth energy level.
Electron pumped to
third energy level.
Electron quickly drops to
metastable state at third
level. Electrons build up
in third level. Population
inversion between third
and second levels
Electron quickly drops to
metastable state. Electrons build up in second
level. Population inversion between second
and first levels
Laser transition and thus
stimulated emission occurs
between third and second
levels.
Stimulated emission occurs
dropping electrons back to
ground state. Laser transition is between second and
first levels.
Electron quickly drops to
ground state.
Ground state
Optical Resonant Cavity
Pumping Source
An amplifier with a provision for positive feedback
is known as an oscillator, which is what a laser is.
The cavity allows the light to oscillate down its axis and
creates a more intense laser by effectively lengthening the cavity.
The cavity also spectral purity of the beam by only allowing
certain wavelengths of light to oscillate.
So in a way, this isAtoms
actually a LOSER, butMirror
who
Mirror
wants that?
Partial reflection
Total reflection
Vaporized silver or
dielectric coating up to
99.9999% reflective.
From 10-90% reflective.
Specialized for different
types of lasers.
Types of Lasers
- Solid state lasers - Insulator doped with a metal
- Ruby (Cr3+ in Al2O3), Nd:YAG, Erbium:Glass
- uses: Spectroscopy, industry, laser ranging, dentistry
- Gas lasers - Usually more than one type of atom involved.
- HeNe, CO2, ArF, Kr, N2
-uses: Eye surgery, holography, cutting/welding
- Semiconductor lasers - Novel mechanism for laser light.
- AlGaInP, GaAlAs, InGaAsP, GaAs, GaP
- uses: Laser pointers, checkouts, CD/DVD, communication
- Dye lasers - Need to be connected to another laser.
- Rhodamine, Coumarin, Stilbene
-uses: spectroscopy, scar reduction, tattoo removal
Solid State Lasers
The metal ions in the glass lattice are what produces the laser light.
These can either be pulsed or continuous wave. May use Xe lamp or
diode array. Efficiency of 1% to 25%. Price $300 to $15,000.
Gas Lasers
Gas lasers are pumped by passing electrical discharge across the gas tube. In
HeNe lasers the He are excited and collide with Ne which then lase, allowing
continuous pop. inv. Argon lasers us a plasma of Ar ions moving in a helical
path and emit at a range of wavelengths. CO2 lasers use vibrational modes of
the molecule to produce light and require very long cavities. Efficiency 1-20%,
cost $500-$20,000.
Semiconductor Lasers Basics
Conducting Band
Conducting Band
} ΔE
} ΔE
Valence Band
p-type doped
When a potential difference is applied,
electrons move from the full valence
band to holes created by empty energy
levels in the atoms of the doping
substance, and the holes in the valence
band permit conductivity
Valence Band
n-type doped
Upon application of a potential
difference, electrons move from the
atoms on the doping substance to
the empty conducting band, and
are free to move and thus conduct
electricity
Semiconductor Lasers
As free electrons move from negatively doped region to holes in positively doped
region they emit light. The heavily doped active region ensures an abundance of
holes for electrons to move into. Flat polished ends and roughened sides trap
light inside the crystal to cause stimulated emission. These types of lasers are
the most common. They are cheap and easy to build and can be made very small
and are only continuous wave. 20-80% efficiency, cost negligible - $10,000
Dye Lasers
A laser or flash lamp is used to excite a dye consisting of a fluorescent
organic molecule in solution. This molecule then relaxes to a metastable
and re-emits the light at a different wavelength via stimulated emission.
Special optics can then be used to tune the frequency of the laser over a
distribution of wavelengths. Efficiency 20-50% cost $50-5000.
Laser Classification
Lasers are divided into classes depending on the power or energy of the beam and the
wavelength of emitted radiation. Laser classification is based on the potential for causing
immediate injury to the eye or skin and/or potential for causing fires from direct exposure
to the beam or from reflections from diffuse reflective surfaces.
Class 1 lasers - Considered to be incapable of producing damaging radiation levels, and
are exempt from most control measures or other forms of surveillance. Ex. laser printers.
Class 2 lasers - Emit radiation in the visible portion of the spectrum, and protection is
normally afforded by the normal human aversion response (blink reflex) to bright radiant
sources. May be hazardous if viewed directly for long periods of time. Ex. laser pointers.
Class 3a lasers - Normally would not produce injury if viewed only momentarily with
the unaided eye. May present a hazard if viewed using collecting optics, e.g., telescopes,
microscopes. Example: HeNe lasers between 1 and 5 milliwatts radiant power.
Class 3b Lasers - Can cause severe eye injuries if beams are viewed either directly or as
a result of specular reflection. Ex. visible or invisible lasers operating at power less than
500 milliwatts for continuous wave lasers or less than 10 J/cm2 for a 0.25 s pulsed laser.
Class 4 lasers - Hazard to the eye from direct beam and specular reflections and
sometimes even from diffuse reflections. Can also start fires and damage skin. Ex. Lasers
operating at power levels greater than 500 mW for continuous wave lasers or greater than
0.03 J for a pulsed system.
Optical Resonant Cavity
Pumping Source
Mirror
Total reflection
Atoms
Mirror
Partial reflection
Pump Cycle
Pumping Source
Excited Atoms
Emission and Lasing
Pumping Source
Frontiers of laser science
-Using blue light to read CD/DVD instead of red light… Shorter wavelength
means that way more information can be stored on a disc.
- Multiline lasers capable of producing different colors of light consecutively
hold much potential for spectroscopy.
- X-ray lasers will allow better surface characterization and biomolecule
imaging should be allow us to observe molecules undertaking key biological
processes while still in a cell.
- Tunable lasers for fiber optic networks will increase speed by moving data to
different wavelengths and making faster color laser printers.
- More precise and smaller band spreading lasers for dermatological surgery,
especially for ethnic skin.
- Quantum cascade lasers for precise measuring of fluids and gravity waves.
Some Laser Wavelengths
Medium
Type
Wavelength (nm)
Ruby
Solid State
694
HeNe
Gas
594, 612, 633, 1152
Nd:YAG
Solid State
1064, 532 (doubled)
ArF
Gas
193
XeCl
Gas
308
Er:Glass
Solid State
1540
CO2
Gas
10600
InGaAlP
Semiconductor
635-660
GaAs/GaAlAs
Semiconductor
780-905
Rhodamine
Dye
570-650
Ar
Gas
364, 514