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

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.

Design advantages to traditional lasersConventional 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

Critical Design AspectsCritical 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

• 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.

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

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

SEM picture of a QC DFB laser 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.

Current Applications Current Applications for Quantum Cascade for Quantum Cascade

Lasers:Lasers:Gas Sensing – Uses the QCL for Gas Sensing – Uses the QCL for direct absorption spectroscopy in direct absorption spectroscopy in the mid-IR region. Able to detect the mid-IR region. Able to detect trace amounts of gas in real time. trace amounts of gas in real time. Environmental, Industrial, Environmental, Industrial, Military/Defense, and Health Military/Defense, and Health Sciences are only a select few of Sciences are only a select few of the many possible applications.the many possible applications.Spectroscopy – The study of Spectroscopy – The study of spectra by use of spectroscope, spectra by use of spectroscope, the QCL is excellent as a the QCL is excellent as a spectroscopic toolspectroscopic toolNonlinear Light – The Nonlinear Light – The incorporation of Raman incorporation of Raman Scattering to QCLs yields a very Scattering to QCLs yields a very innovative nonlinear light innovative nonlinear light technology.technology.

Spectroscopy: Inter- and Spectroscopy: Inter- and Intra-Intra-

Interpulse:Interpulse: ““Ultra short” current pulses Ultra short” current pulses

sent to the laser, laser is sent to the laser, laser is tuned to the spectroscopic tuned to the spectroscopic transition with an additional transition with an additional current or a temperature current or a temperature rampramp

Spectral resolution is limited Spectral resolution is limited by frequency chirps by frequency chirps generated by this pulsation generated by this pulsation processprocess

Tuning range: 1 – 2 cm^(-1) Tuning range: 1 – 2 cm^(-1) [wavenumber][wavenumber]

Repetition rates: 10 Hz – 1 Repetition rates: 10 Hz – 1 KHzKHz

Room temperature operation Room temperature operation

Intrapulse: Higher Resolution Intrapulse: Higher Resolution & Repetition Rate& Repetition Rate

““ultra short” current pulses ultra short” current pulses sent through laser, again sent through laser, again generating frequency chirpsgenerating frequency chirps

Chirp utilized to sweep swiftly Chirp utilized to sweep swiftly through the frequencies of through the frequencies of interestinterest

Frequency downchirps 4 – 6 Frequency downchirps 4 – 6 cm^-1 wide are generated by cm^-1 wide are generated by microsecond long current microsecond long current pulses several amps above pulses several amps above lasing thresholdlasing threshold

The downchirps are produced The downchirps are produced by the subsequent heating by the subsequent heating within the laserwithin the laser

High Resolution: .01 cm^-1 High Resolution: .01 cm^-1 [wavenumber][wavenumber]

Repetition rates: up to 100 Repetition rates: up to 100 kHzkHz

Why are QCLs so Perfect for Gas Sensing?Why are QCLs so Perfect for Gas Sensing?

Wavelength of Quantum Cascade is based on quantum well Wavelength of Quantum Cascade is based on quantum well thickness and not on band gap, leaves an open range for lasers thickness and not on band gap, leaves an open range for lasers that are not material dependant.that are not material dependant.

The particular Range that is important is the Mid-IR range (3 to The particular Range that is important is the Mid-IR range (3 to 20 micrometers), where most molecular absorption bands are.20 micrometers), where most molecular absorption bands are.

QCLs are the only semiconductor laser able to produce mid-IR QCLs are the only semiconductor laser able to produce mid-IR wavelength beams at or above room temperature.wavelength beams at or above room temperature.

Excellent for sensing trace gases: sensors based on QCL can Excellent for sensing trace gases: sensors based on QCL can have sensitivities in the range of parts per trillionhave sensitivities in the range of parts per trillion

Because QCLs can operate in pulsed mode up to and above Because QCLs can operate in pulsed mode up to and above room temperature, consumables associated with deep cooling room temperature, consumables associated with deep cooling processes (such as liquid N2 for cryogenics) are no longer processes (such as liquid N2 for cryogenics) are no longer necessary.necessary.

This also significantly reduces the bulk of the sensor, reducing This also significantly reduces the bulk of the sensor, reducing its size to a more practical one.its size to a more practical one.

Distributed Feedback (DFB) emits single frequency high Distributed Feedback (DFB) emits single frequency high powered radiation that is ideal for the spectroscopy required powered radiation that is ideal for the spectroscopy required for gas sensing.for gas sensing.

Schematic for gas Schematic for gas sensingsensing

Thermoelectric cooling keeps Thermoelectric cooling keeps laser at quasi room temperaturelaser at quasi room temperature

Function generator produces Function generator produces pulses for operation in pulsed pulses for operation in pulsed modemode

Laptop used to monitor the Laptop used to monitor the systemsystem

Laser is tuned to specific Laser is tuned to specific wavelength that coincides with an wavelength that coincides with an absorption line of a specific absorption line of a specific molecule (i.e. Carbon Monoxide, molecule (i.e. Carbon Monoxide, NH3, etc.)NH3, etc.)

In this way, the cascade uses In this way, the cascade uses direct IR absorption spectroscopy direct IR absorption spectroscopy to monitor levels of a specific to monitor levels of a specific molecule at a high repetition rate.molecule at a high repetition rate.

The only component that requires The only component that requires cryogenic cooling in the shown cryogenic cooling in the shown schematic is the detector, making schematic is the detector, making for a very convenient and for a very convenient and relatively low maintenance gas relatively low maintenance gas sensor.sensor.

Nonlinear Light: The Nonlinear Light: The Raman ChipRaman Chip

The first electrically driven The first electrically driven Raman laser ever createdRaman laser ever created

Raman scattering occurs Raman scattering occurs between quantum wells in between quantum wells in active region of QCLactive region of QCL

Raman Conversion of 30%: Raman Conversion of 30%: Emission begins at 6.7 um Emission begins at 6.7 um and is Raman shifted to 9 and is Raman shifted to 9 umum

Size: 10 um x 6 um x 2 mmSize: 10 um x 6 um x 2 mm Driven by a DC power Driven by a DC power

sourcesource Cooled ThermoelectricallyCooled Thermoelectrically Designed with a Designed with a

InGaAs/InAlAs InGaAs/InAlAs heterostructureheterostructure

Max Temperature of Max Temperature of operation: 170 Koperation: 170 K

What is Raman What is Raman Scattering?Scattering?

When photons gain or loss energy due to When photons gain or loss energy due to interaction with molecules, causing a frequency interaction with molecules, causing a frequency shiftshift

Inelastic interaction that can either amplify or Inelastic interaction that can either amplify or decrease energy decrease energy

As opposed to Rayleigh scattering where energy As opposed to Rayleigh scattering where energy photon energy and wavelength is not altered photon energy and wavelength is not altered

Energy states below Rayleigh level are called Energy states below Rayleigh level are called “Stokes Lines” “Stokes Lines”

States above Rayleigh level are “Anti-Stokes States above Rayleigh level are “Anti-Stokes Lines”Lines”