Transcript Lasers - Rougemont School

Physics
WJEC AS Level
“Lasers”
Light
Amplification by
Stimulated
Emission of
Radiation
Light from a hot filament is…
Polychromatic – many
colours/wavelengths are emitted;
Incoherent – there is a random and
varying phase difference between the
individual photons;
Divergent – light is emitted in all
directions.
Light from a laser is…
Monochromatic – one single
colour/wavelength is emitted;
Coherent – the photons are all in phase;
Collimated – the beam is highly focussed.
Consequently, laser light has extremely high
intensity.
How is laser light generated?
By utilising:
Stimulated emission
A population inversion
A laser cavity
Absorption of Light…
…by an atom causes promotion of an electron to
a higher energy state.
Electron excited to
higher energy
Excited
state
Photon absorbed
Ground state
Note: the photon must have precisely the correct energy;
hf  E2  E1
Spontaneous Emission…
…occurs almost immediately afterwards.
Excited state
Electron drops to
Lower energy
Photon emitted
Ground state
Spontaneous emission by many atoms
within a material (e.g. a hot filament)
generates incoherent photons.
Stimulated Emission…
…occurs when one photon causes emission
of another.
Excited state
Electron stimulated
to drop by photon
Incoming photon and emitted photon
exit in phase and in the same direction
Ground state
As well as being identical, the two photons
are in phase.
Stimulated Emission: Problems
Excited atomic states are very short lived.
Stimulated emission is therefore very rare.
It is highly improbable that a photon will
collide with an excited atom.
More likely is that the photon will excite an
atom in its ground state, leading to
spontaneous emission.
Population Inversion
Consider two energy levels within a
system of atoms: E1 and the higher state
E2.
The population (occupancy) of E1 is N1;
the population of E2 is N2.
A population inversion is said to exist
when
N2  N1
Population Inversion
Once a population inversion is
established, stimulated emission becomes
more likely than absorption.
 A population inversion is highly unlikely to
occur naturally though.
Energy can be supplied to the system in
an attempt to achieve an inversion – this is
called pumping.
Pumping with Light
 Electrons are promoted by incoming photons:
Excited state (N2 = 0)
Pumping light
Ground state (N1 = 8)
 Unfortunately, the best that optical pumping can achieve in a two-level
system is N1 = N2:
Excited state (N2 = 4)
Pumping light
Ground state (N1 = 4)
The Three Energy-Level Laser
E3 is very short lived
electrons are
pumped from
E1 to E3
E3
E2: metastable state
(relatively long-lived)
2
E2
1
3
E1
Note: pumping photons cannot cause
excitation from E1 to E2.
N2  N1
Problems
Pumping photons may cause stimulated
emission from electrons in E3.
E3 must therefore be very short-lived to
avoid this.
More than half of the electrons from E1
must be pumped to E2 (via E3) in order to
obtain a population inversion .
The Four Energy-Level Laser
E4 is short lived
E4
E3: metastable state
2
electrons are
pumped from
E3
3
1
E1 to E4
E2
4
E1
E2 is short lived
N3  N2
Advantages
E4 is short-lived and therefore stimulated
emission by pumping electrons is unlikely.
E2 is practically empty at any time as it is
not the ground state and has a short
lifetime.
The population inversion is therefore very
easy to achieve.
There is certainly no need to pump over
half of the electrons.
Laser Structure
population inversion in cavity
Laser beam
Amplifying
medium
100% reflecting
mirror
Mirror: approximately 99% reflecting
(approx. 1% gets transmitted)
Stimulated emission causes a cascade of
photons, similar to a chain reaction.
Most photons remain within the cavity,
causing ever more stimulated emission.
Efficiency
Lasers are generally highly inefficient
(typically 0.01%) due to:
The high energy input required for
pumping;
The constant cooling required to prevent
overheating of the amplifying medium .
Laser Diodes
Semiconductor lasers can be made very
small – in a chip around 1.0 mm  0.5 mm
 0.5 mm.
metal
contact
‘sandwich’ of area with
population inversion i.e.
amplifying medium
light emission
not silicon, usually gallium arsenide.
This surface and its opposite are mirrors
due to the air-solid boundary
metal contact below also
How a Laser Diode works
p-type: electron-deficient material
n-type: electron-rich material
Surplus electrons from the n-type layer attach themselves
to atoms in the p-type layer – a photon is emitted each
time this occurs.
Efficiency of Laser Diodes
Reflection occurs at the air boundaries at
the edge of the chip – at around 40%,
much less than a conventional laser’s
99%+.
However, the population inversion is huge
– the internal amplification that results
more than compensates.
Efficiency can therefore be as high as
70%.
Advantages and Uses of Laser Diodes
Advantages
Some Uses
Cheaper
Smaller
More efficient
Easy to mass
produce
Inside DVD, CD and Blu-ray
players
Barcode readers
Telecommunications (via
optical fibres)
Image scanning
Laser surgery