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Quantum-Cascade Laser
Laser a cascata quantica (QCL)
Docente: Mauro Mosca
(www.dieet.unipa.it/tfl)
A.A. 2014-15
Ricevimento: alla fine della lezione o per appuntamento
Università di Palermo – Facoltà di Ingegneria (DEIM)
The inventor of QCL
Federico Capasso
1994
MBE
Laser property depends
on quantum-well width,
NOT on materials!!
possibility of more than one photon per electron!!
Banda di conduzione in un QCL
The emission
wavelength
independent
of
the
most
favourable
material
The For
energy
cannot
beisgreater
than
lower
level
lifetime
has to
be the
population
inversion:
Other relaxation the band
gap
of the
materials
but energies
dependent
system,
AlGaAs/GaAs,
conduction
band
offset level
shorter
than
the
upper
lifetime
mechanisms, such
on limited
the width
the tenths
quantum
are
to aoffew
of 1wells
eV
as electron-electron
scattering, become
important at such low
subband separations
tULL
In mid-IR (5-20 µm)
injectorenergy of the laser is lowered by widening
If
the
emission
rapid
relaxation
from the LLL
If the ULL-LLL
regiontransition energy is reduced below the
If the
resonant
phonon
emission
can be used
to depopulate
the
quantum
well
so
that
the
ULL-LLL
energy
separation
to
DPL is achieved
by
making
the
phonon energy then veryactive
careful engineering of
the LLLLLL-DPL
energy
separation
the
same
as
the LLL thenphonon
it can also
depopulate
the
ULL! separation
approaches
resonance,
the
LLL-DPL
region
DPL
energy
level
is
necessary
to
ensure
the scattering
the
longitudinal
optical
phonon
energy
so
that
the
will naturally
move
off theinversion.
resonance, leaving a very short
rate
will
allow
population
probability of relaxation by single phonon emissionDPL
is high. LLL
t
upper lifetime and a relatively longer lower lifetime.
Wannier-Stark ladder
the “cascade”
This isan
theapplied
“quantum”.
Now…
Under
field the
miniband
is not flat!...
achieved
In electron
by grading
transport
the problems
thicknesses
of
the
electron-electron
layers
and the
theand
composition
electronAn electron leavingofthe
DPL
enters
miniband
states
withinand
the
-phonon
scattering
so thatthrough
under
have the
of the next injector region
islayers,
transported
toend
be
bias
considered
the band
superlattice into the ULL zero
at theapplied
other
offsets resemble more of
LLL
a saw-toothULL
structure
The miniband breaks up into a series of discrete
states
than a square well
DPL
called a Wannier-Stark
ladder
miniband
It is necessary to design the injector region so that the
electron states of the interacting quantum wells align
Thethe
electrons
are of
re-used
in the lasing
process,
under
action
an electric
field
to produce a mini
unlike the conventional diode laser where the band
band
that is essentially
recombination
removes theflat
electron from the process
Differenze con i laser tradizionali
The devices are unipolar
Population inversion between sub-bands
of a quantum well system rather than
between electron and hole states
no dependence of the lasing wavelength
on band gap
Emission wavelengths extend from the
near IR to the very far IR, close to 100 µm
Guadagno in un QCL (unipolare) e
in un laser convenzionale (bipolare)
Bipolar devices have widely separated maximum and
minimum transition energies caused by the separation
of the Fermi levels
In QCL the in-plane dispersion in each sub-band is
of the same sign, unlike bipolar devices, so even if
transitions occur between states at in-plane k ≠ 0 the
wavelength is very similar to transitions at k = 0 and
the gain spectrum is correspondingly narrow
Similitudini con i laser tradizionali
Light emission occurs perpendicular to the
direction of current flow
Optical confinement is achieved through
the use of wave guiding structures
The cavity reflectors can be formed by
cleaving or through the fabrication of
distributed Bragg reflectors
Anti-crossing
• Different schemes for active regions
• Transitions could be either vertical, or diagonal
Panoramica di QCL
Minibande nei super-reticoli:
modello di Kronig-Penney
We assume that the barrier thickness tends to zero in
the limit as U0 tends to infinity and also that the effective
masses are both unity
Minibande nei super-reticoli
For smaller values of P the bands will merge at lower values of
kA a, but there will always be an energy gap at low energies
kA a
Stati super-reticolari
in una struttura periodica GaAs/AlGaAs
1,7 mm
70 meV
5,3 mm
- As the well width increases
above 5.3 nm, the second
- There
are fixed
no band
gaps
barrier
width
band
descends
into
the
well
at zero well-width
of 5 mm
-At
zero
the into
- The
firstbarrier
band width
descend
band
should
conformbut
to the
the well
immediately
the
GaAs
structure,
and the
top of band
the band
descends
band
extends
overthe
the whole
into the
well when
energy
range
of nm
the well.
thickness
is 1.7
-As
the
barrier
fixedwidth
welldecreases
width
- The
band
energy
of 5band
mm
increases
the
becomes
with well width,
as does
the
progressively
narrower until
width of the band
eventually it becomes a
(the electrons spend more time in the well than in the barriers, so the probability
discrete state at ~ 70 meV.
of communication with neighboring wells is much smaller)
Laser a minibande o a super-reticolo
(continuum-to-continuum)
- If the transitions take place between minibands rather than
discrete subbands, the wavelength is determined principally
by the energy separation between the minibands
- Population inversion is easier to achieve in a superlattice
active region provided the electron temperature is low enough
for the bottom miniband to remain largely empty (if the electron
temperature is too high then all the states in the miniband will be
full and scattering between states will not occur)
- Similarly the doping level must be such that the Fermi level
lies well below the top of the lower miniband
-The depopulation of the LLL in a miniband is more rapid
because intra-miniband scattering times are usually shorter
Laser a minibande o a super-reticolo
(bound-to-continuum)
- As well as inter-miniband transitions, structures can be
designed so that transitions occur between a bound state and a
miniband
QW of varying thickness
- A chirped superlattice gives rise to minibands under the
influence of an electric field
- No separation of the active and the injection regions occurs, as
the lower laser level and the injector are in the same miniband
- However, a single narrow well placed at strategic points within
the superlattice gives rise to a discrete state which acts as the
upper laser level
- Fast depopulation rate of the lower laser level
- Relatively large linewidth
Guide d’onda nei QCL
- In waveguides based on the conventional refractive index
difference the optical penetration into the cladding layers is
heavily
doped
GaAs
perdite!!
proportional
to the wavelength
- If the cladding layer is not thick enough to contain the entire
optical field some irreversible leakage out of the guide will occur
- Therefore, structures several times thicker than the
wavelength need to be grown
- Heavily doped GaAs (n ≈ 5 × 1018 cm-3) has a plasma
frequency around 11 µm, and around this wavelength the real
heavily
doped GaAs
perdite!!
part of the refractive index decreases
- GaAs rather than AlGaAs has to be used because the
thickness of AlGaAs is limited by residual strain (max thickness
≈ 1.5 µm)
Guide d’onda nei QCL: plasmoni
- Waveguides for longer wavelengths utilise a particular
property of thin metal films that at long wavelengths have the
real part of the dielectric constant large and negative
- When bounded by a dielectric with a real and positive
dielectric constant, they support an electromagnetic mode that
propagates over large distances
- The condition on the dielectric constants is not so strict and
modes will propagate over a wide variety of wavelengths
- Surface charge density on the metal is induced by the
electric field and oscillates with it. It corresponds to the
collective oscillation known as a plasmon, hence the guide is
often called a surface plasmon guide
Guide d’onda nei QCL: plasmoni
Applicazioni
The high optical power output, tuning range and room temperature
operation make QCLs useful for spectroscopic applications:
- remote sensing of environmental gases and pollutants
in the atmosphere
- homeland security
- vehicular cruise control in conditions of poor visibility
- collision avoidance radar
- industrial process control
- medical diagnostics such as breath analyzers
- Thz sources
Sommario