Speaker
Seyed Ghasem Razavipour
Title
Design, Analysis, and Characterization of Indirectly-pumped Terahertz Quantum Cascade Lasers
Abstract
Quantum
cascade
laser
(QCL),
as
a
unipolar
semiconductor
laser
based
on
intersubband
transitions
in
quantum
wells,
covers
a
large
portion
of
the
Mid
and
Far
Infrared
(IR)
electromagnetic
spectrum.
The
frequency
of
the
optical
transition
can
be
determined
by
engineering
the
layer
sequence
of
the
heterostructure.
The
focus
of
this
work
is
on
Terahertz
(THz)
frequency
range
(frequency
of
1
-
10
THz
and
photon
energy
of
~
4
-
40
meV),
which
is
lacking
of
high
power,
coherent,
and
efficient
narrowband
radiation
sources.
THz
QCL,
demonstrated
in
2002,
as
a
perfect
candidate
of
coherent
THz
source,
is
still
suffering
from
the
empirical
operating
temperature
limiting
factor
of
T?h?/k_B,
which
allows
this
source
to
work
only
under
a
cryogenic
system.
Most
of
high
performance
THz
QCLs,
including
the
world
record
design
which
lased
up
to
~
200
K,
are
based
on
a
resonant
phonon
(RP)
scheme,
whose
population
inversion
is
always
less
than
50%.
The
indirectly-pumped
(IDP)
QCL,
nicely
implemented
in
MIR
frequency,
starts
to
be
a
good
candidate
to
overcome
the
aforementioned
limiting
factor
of
RP-QCL.
A
rate
equation
(RE)
formalism,
which
includes
both
coherent
and
incoherent
transport
process,
will
be
introduced
to
model
the
carrier
transport
of
all
presented
structures
in
this
thesis.
The
second
order
tunneling
which
employed
the
intrasubband
roughness
and
impurity
scattering
was
implemented
in
our
model
to
nicely
predict
the
behavior
of
the
QCL
designs.
This
model
which
is
easy
to
implement
and
fast
to
calculate
could
help
us
to
engineer
the
wavefunction
of
the
structure
with
an
optimization
tool.
We
developed
a
new
design
scheme
which
employs
the
phonon
scattering
mechanism
for
both
injecting
carriers
to
the
upper
lasing
state
and
extracting
carriers
from
lower
lasing
state.
Since
there
is
no
injection/extraction
state
to
be
in
resonance
with
lasing
states,
this
simple
design
scheme
does
not
suffer
from
broadening
due
to
the
tunneling.
Finally,
three
different
THz
IDP-QCLs,
based
on
phonon-photon-phonon
(3P)
scheme
were
designed,
grown,
fabricated,
and
characterized.
The
performance
of
those
structures
in
terms
of
operating
temperature,
threshold
current
density,
maximum
current
density,
output
optical
power,
lasing
frequency,
differential
resistance
at
threshold,
intermediate
resonant
current
before
threshold,
and
k_B
T/h?
factor
will
be
compared.
We
could
improve
the
k_B
T/h?
factor
of
the
3P-QCL
design
from
0.9
in
first
iteration
to
1.3
and
the
output
optical
power
of
the
structure
from
0.8
mW
in
first
design
to
3.4
mW.
The
performance
of
the
structure
in
terms
of
intermediate
resonant
current
and
the
change
in
differential
resistance
at
threshold
was
improved.