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PH 0101 UNIT-3 / LECT - 4
• SEMICONDUCTOR LASERS
• EXCIMER LASER
UNIT III Lecture 4
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SEMICONDUCTOR (Ga-As) LASERS
Introduction :
The semiconductor laser is today one of the most
important types of lasers with its very important application
in fiber optic communication.
These lasers use semiconductors as the lasing medium and
are characterized by specific advantages such as the
capability of direct modulation in the gigahertz region,
small size and low cost.
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Basic Mechanism :
The basic mechanism responsible for light emission
from a semiconductor is the recombination of
electrons and holes at a p-n junction when a current
is passed through a diode.
There can be three interaction processes
1)An electron in the valence band can absorb the
incident radiation and be excited to the conduction
band leading to the generation of eletron-hole pair.
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Contd.
2) An electron can make a spontaneous transition in which it
combines with a hole and in the process it emits radiation
3) A stimulated emission may occur in which the incident
radiation stimulates an electron in the conduction band to
make a transition to the valence band and in the process emit
radiation.
To convert the amplifying medium into a laser
Optical feedback should be provided
Done by cleaving or polishing the ends of the p-n
junction diode at right angles to the junction.
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Contd.
When a current is passed through a p-n junction under
forward bias, the injected electrons and holes will increase the
density of electrons in the conduction band.
 The stimulated emission rate will exceed the absorption rate
and amplification will occur at some value of current due to
holes in valence band.
 As the current is further increased, at threshold value of the
current, the amplification will overcome the losses in the cavity
and the laser will begin to emit coherent radiation.
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Simple structure (Homojunction) :
• The basic semiconductor laser structure in which the
photons generated by the injection current travel to the
edge mirrors and are reflected back into the active area.
• Photoelectron collisions take place and produce more
photons, which continue to bounce back and forth between
the two edge mirrors.
• This process eventually increases the number of
generated photons until lasing takes place. The lasing will
take place at particular wavelengths that are related to the
length of the cavity.
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Basic semiconductor laser structure
a) Side view, b) Projection Heterostructures
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Heterostructures :
The heterostructure laser is a laser diode with more
than single P and N layers. GaAs/AlGaAs is a
heterojunction laser. The notations P+ and N+ and P- and Nindicate heavy doping and light doping respectively. The
P-N structure consists of the two double layers, P+ - Pand N+ - N- .
A thin layer of GaAs is placed at the junction, the
active region. The substance is selected because the
electron-hole recombinations are highly radiative. This
increases the radiation efficiency.
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The P and N regions are lightly doped regions
that have an index of refraction n2 less than n1 of the
active region. These three layers, n2-n1-n2, form a
light waveguide much like the optical fiber, so that the
light generated is confined to the active region.
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Laser heterostructure (a) Schematic projection (b) Refractive index profile
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EXCIMER LASER :
•
An excimer laser or exciplex laser is a form of
ultraviolet chemical laser which is commonly used in eye
surgery and semiconductor manufacturing.
•
The term excimer is short for 'excited dimer',
while exciplex is short for 'excited complex'.
•
An excimer laser typically uses a combination of an
inert gas (Argon, krypton, or xenon) and a reactive gas
(fluorine or chlorine).
• Under the appropriate conditions of electrical
stimulation, a pseudo-molecule called a dimer is created,
which can only exist in an energised state and can give
rise to laser light in the ultraviolet range
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Contd.
•
Laser action in an excimer molecule occurs because
it has a bound (associative) excited state, but a repulsive
(dissociative) ground state.
•
This is because noble gases such as xenon and
krypton are highly inert and do not usually form chemical
compounds.
•
When in an excited state (induced by an electrical
discharge or high-energy electron beams, which produce
high energy pulses), they can form temporarily-bound
molecules with themselves (dimers) or with halides
(complexes) such as fluorine and chlorine.
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Contd.
The excited compound can give up its excess energy by
undergoing spontaneous or stimulated emission, resulting in a
strongly-repulsive ground state molecule which very quickly
(on the order of a picosecond) disassociates back into two
unbound atoms. This forms a population inversion between
the two states.
Most "excimer" lasers are of the noble gas halide type, for
which the term excimer is strictly speaking a misnomer (since
a dimer refers to a molecule of two identical or similar parts):
The correct but less commonly used name for such is
exciplex laser.
The wavelength of an excimer laser depends on the
molecules used, and is usually in the ultraviolet region
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EXCIMER WAVELENGTH
Excimer
Wavelength
ArF
193 nm
KrF
248 nm
XeBr
282 nm
XeCl
308 nm
XeF
351 nm
CaF2
193 nm
KrCl
222 nm
Cl2
259 nm
N2
337 nm

Excimer lasers are usually operated with a pulse rate
of around 100 Hz and a pulse duration of ~10 ns, although
some operate as high as 8 kHz and 30 ns.
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•
All commercial excimer lasers employ the modules.
Laser light is generated in the laser cabinet.
•
The electrical energy required by the laser to form
laser pulses is generated by the high voltage supply.
•
A gas supply and a vacuum pump are required to fill
the laser with the appropriate laser gas mixture.
•
The control computer is usually linked to the laser
cabinet and high-voltage supply by a fiber optic network. The
computer provides laser function user control.
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TYPICAL EXCIMER LASER CONFIGURATION
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USES :
The UV light from an excimer laser is well absorbed
by biological matter and organic compounds. Rather than
burning or cutting material, the excimer laser adds
enough energy to disrupt the molecular bonds of the
surface tissue, which effectively disintegrates into the air
in a tightly controlled manner through ablation rather than
burning.
Excimer lasers have the useful property that they can
remove exceptionally fine layers of surface material with
almost no heating or change to the remainder of the
material which is left intact.
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These properties make them useful for surgery
(particularly eye surgery), for lithography for
semiconductor
manufacturing,
and
for
dermatological treatment.
Excimer lasers are quite large and bulky devices,
which is a disadvantage in their medical applications,
although their size is rapidly decreasing with
ongoing development.
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Solved Problem (1) : Calculate the wavelength of emission from GaAs
semiconductor laser whose band gap energy is 1.44 ev (plank’s constant is
6.625 x 10-34 Js and charge of an electron is 1.6 x 10-19 C.
Given data : Band gap energy Eg = 1.44 ev (or) 1.44 x 1.6 x 10-19 Joules.
Solution :
We know Band gap energy (Eg) = h (or) hc/
we can write  = hc/Eg

= 6.625 x 10
= 8.6263 x 10
-7
= 8626.3 x 10
-34
8
x 3 x 10 ) / (1.44 x 1.6 x 10
-19
)
m
-10
m
 wave length of GaAs laser = 8626.3 A
o
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