1. Quantum Cascade Laser Function
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 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 cross-section through the laser waveguide
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”