Transcript Group 2 - Index of

Quantum Cascade Laser Function
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Quantum cascade lasers are a fairly new
technology that is promising for it’s many
incredible properties.
High powered, wide wavelength range, and room
temperature operation in pulsed mode.
Pulsed mode: current is sent in a nanosecond
bursts, emitting radiation in pulses. Peak power
range: several Watts
Continuous mode: a constant bias is applied to
the cascade laser. Peak power range: tens of
milliwatts.
The structure of the QCL: an array of quantum
wells gradually dropping in height.
Two regions: active well and injector region
Structure: A gradually slanted array of quantum
wells in the conduction region of a semiconductor
material.
When a bias is applied to the falling array or
“cascade,” a beam is emitted.
Electrons tunnel through one injector region and
are confined in a quantum well. This confinement
forces the electrons to obey wave mechanics,
with a quantized vertical motion. The electron
drops in energy level emitting a photon, it then
tunnels through the thin injector region to the next
quantum where it again is confined and drops in
energy level. (Figure 1).
The electron continues this quantum mechanicsdescribed movement, releasing photons as it
moves through the lattice of quantum wells
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Detailed Cascading Scheme
Wavelength and Power
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Once the array of quantum wells is
formed, the power is based on step
number and wavelength is based on size
of quantum Wells.
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The chart to the right shows the relations
between wavelength and optical power.
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Image of Range of Wavelength against
generated power. A true flexible range of
wavelength is thus realized.
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Maximum QCL energy is proportional to
the amount of cascading stages.
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Graph of optical power versus current to
the right, notice the effect step number
has on maximum power.
Design advantages to traditional lasers
Conventional Laser
Quantum Cascade Laser
Holes and electrons exhausted at
each emission
Quantum wells are not exhausted per photon
emission
Rely on a electron hole and
emitting an available photon.
QC lasers rely only on the one type of carrier,
they are the electrons.
Photon emission relies on intraband transitions
Photon emission relies on a photon between quantized conduction band states in
avaliablity(1X).
quantum wells.
Wavelength dependant by the
Material Band gap. Different
wavelength requires different
material
Wavelength dependant on the size of the
Quantum wells and super lattices
Critical Design Aspects
• The heterojuction interface determines wavelength
• Electron tunneling from injector to the active region
• Megahertz frequency can be obtained using varying width
heterostructures
• Terahertz frequencies require heterostructure + waveguides
• Max Temperature of operation (GaAs/AlGaAs): 280 K.
• Peak Power (GaAs/AlGaAs) is above 1 W at 77 K.
• High power comes at the sacrifice of convenience, cryogenic
freezing is required to cool the QCL for high power beams.
• At room temperature average beam power drops, even though peak
power stays the same. Encouraging thermo-electrical cooling to
keep the Pulsed Cascade laser at “quasi-room temperature.”
Heterostructure development
Molecular Beam Epitaxy
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The superlattice of “GaAs/AlGaAs”
is grown through a process of
growing high-purity epitaxial layers
of compound semiconductors
called Molecular Beam Epitaxy.
This process is performed thru the
use of elements of a
semiconductor in the form of
‘molecular beams’ deposited onto
a heated crystalline substrate to
form thin epitaxial layers. To
obtain high-purity layers, it is
critical that the material sources
be extremely pure and that the
entire process be done in an ultrahigh vacuum environment.
See the diagram to the right of a
MBE system.
Terahertz QCL Operation
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QCLs were originally developed for use in the megahertz operation. There are
considerable challenges taking this concept into the THz region.
Carrier-carrier scattering makes it difficult to achieve terahertz frequency. Design
through heterostructures generates an inversion problem.
To solve this problem a waveguide is required creating what is know as a
Distributed feedback (DFB) QC laser
Distributed feedback (DFB) QC lasers
Designed For spectroscopy on gases, single-mode, narrow linewidth lasers with well-defined, precise
tunability are required.
On order to achieve these goals, scientists fabricated quantum cascade lasers with a periodic
waveguide structure built in the cavity.
DFB Technology
Surface gratin…. This periodic variation of the refractive index or the gain leads to a certain amount of coupling
between the back- and forth-traveling waves. The coupling becomes strongest if the periodicity is a integer
multiple of half the laser wavelength in the cavity, according to the following formula: L = l/2neff
Here, L is the grating periodicity, l the laser wavelength in vacuum, and Neff the effective refractive index of the
waveguide. Because feedback occurs along the whole cavity and not only on the mirrors, these devices are
called distributed feedback lasers.
cross-section through the laser waveguide
SEM picture of a QC DFB laser
Current Applications
for Quantum Cascade
Lasers:
Gas Sensing – Uses the QCL for direct
absorption spectroscopy in the mid-IR
region. Able to detect trace amounts of gas
in real time. Environmental, Industrial,
Military/Defense, and Health Sciences are
only a select few of the many possible
applications.
Spectroscopy – The study of spectra by use
of spectroscope, the QCL is excellent as a
spectroscopic tool
Nonlinear Light – The incorporation of
Raman Scattering to QCLs yields a very
innovative nonlinear light technology.
Spectroscopy: Inter- and Intra
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Interpulse:
“Ultra short” current pulses sent to the
laser, laser is tuned to the spectroscopic
transition with an additional current or
a temperature ramp
Spectral resolution is limited by
frequency chirps generated by this
pulsation process
Tuning range: 1 – 2 cm^(-1)
[wavenumber]
Repetition rates: 10 Hz – 1 KHz
Room temperature operation
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Intrapulse: Higher Resolution &
Repetition Rate
“ultra short” current pulses sent
through laser, again generating
frequency chirps
Chirp utilized to sweep swiftly through
the frequencies of interest
Frequency downchirps 4 – 6 cm^-1
wide are generated by microsecond
long current pulses several amps above
lasing threshold
The downchirps are produced by the
subsequent heating within the laser
High Resolution: .01 cm^-1
[wavenumber]
Repetition rates: up to 100 kHz
Why are QCLs so Perfect for Gas Sensing?
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Wavelength of Quantum Cascade is based on quantum well thickness and not
on band gap, leaves an open range for lasers that are not material dependant.
The particular Range that is important is the Mid-IR range (3 to 20
micrometers), where most molecular absorption bands are.
QCLs are the only semiconductor laser able to produce mid-IR wavelength
beams at or above room temperature.
Excellent for sensing trace gases: sensors based on QCL can have sensitivities
in the range of parts per trillion
Because QCLs can operate in pulsed mode up to and above room
temperature, consumables associated with deep cooling processes (such as
liquid N2 for cryogenics) are no longer necessary.
This also significantly reduces the bulk of the sensor, reducing its size to a
more practical one.
Distributed Feedback (DFB) emits single frequency high powered radiation
that is ideal for the spectroscopy required for gas sensing.
Schematic for gas sensing
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Thermoelectric cooling keeps laser at
quasi room temperature
Function generator produces pulses for
operation in pulsed mode
Laptop used to monitor the system
Laser is tuned to specific wavelength
that coincides with an absorption line
of a specific molecule (i.e. Carbon
Monoxide, NH3, etc.)
In this way, the cascade uses direct IR
absorption spectroscopy to monitor
levels of a specific molecule at a high
repetition rate.
The only component that requires
cryogenic cooling in the shown
schematic is the detector, making for a
very convenient and relatively low
maintenance gas sensor.
Nonlinear Light: The Raman Chip
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The first electrically driven Raman
laser ever created
Raman scattering occurs between
quantum wells in active region of
QCL
Raman Conversion of 30%:
Emission begins at 6.7 um and is
Raman shifted to 9 um
Size: 10 um x 6 um x 2 mm
Driven by a DC power source
Cooled Thermoelectrically
Designed with a InGaAs/InAlAs
heterostructure
Max Temperature of operation:
170 K
What is Raman Scattering?
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When photons gain or loss energy due to interaction
with molecules, causing a frequency shift
Inelastic interaction that can either amplify or decrease
energy
As opposed to Rayleigh scattering where energy photon
energy and wavelength is not altered
Energy states below Rayleigh level are called “Stokes
Lines”
States above Rayleigh level are “Anti-Stokes Lines”