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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.
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 developmentMolecular 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 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.
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 Spectroscopy: Inter- and Intra-
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.
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.
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
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”