Quantum Cascade Laser Function

1
Quantum Cascade Laser Function

• Quantum cascade lasers are a fairly new
  technology that is promising for its 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
  mechanics-described movement, releasing photons
  as it moves through the lattice of quantum
  wells .

Detailed Cascading Scheme
2
Wavelength and Power

• ? Once the array of quantum wells is formed, the
  power is based on step number and wavelength is
  based on size of quantum Wells.
• The chart to the right shows the relations
  between wavelength and optical power.
• Image of Range of Wavelength against generated
  power. A true flexible range of wavelength is
  thus realized.
• Maximum QCL energy is proportional to the amount
  of cascading stages.
• Graph of optical power versus current to the
  right, notice the effect step number has on
  maximum power.

3
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 a photon avaliablity(1X). Photon emission relies on intraband transitions between quantized conduction band states in 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
4
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.

5
Heterostructure development Molecular Beam
Epitaxy

• 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 ultra-high vacuum
  environment.
• See the diagram to the right of a MBE system.

6
Terahertz QCL Operation

• 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

7
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.
8
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
9
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.

10
Spectroscopy Inter- and Intra-

• 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

• 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

11
Why are QCLs so Perfect for Gas Sensing?

• 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.

12
Schematic for gas sensing

• 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.

13
Nonlinear Light The Raman Chip

• 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

14
What is Raman Scattering?

• 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