Quantum-cascade laser
Quantum-cascade lasers (QCLs) are
Unlike typical interband
Intersubband vs. interband transitions
Within a bulk semiconductor
A QCL however does not use bulk semiconductor materials in its optically active region. Instead, it consists of a
Additionally, in semiconductor laser diodes, electrons and holes are annihilated after recombining across the band gap and can play no further part in photon generation. However, in a unipolar QCL, once an
Operating principles
Rate equations
QCLs are typically based upon a
In the steady state, the time derivatives are equal to zero and . The general rate equation for electrons in subband i of an N level system is therefore:
- ,
Under the assumption that absorption processes can be ignored, (i.e. , valid at low temperatures) the middle rate equation gives
Therefore, if (i.e. ) then and a population inversion will exist. The population ratio is defined as
If all N steady-state rate equations are summed, the right hand side becomes zero, meaning that the system is underdetermined, and it is possible only to find the relative population of each subband. If the total sheet density of carriers in the system is also known, then the absolute population of carriers in each subband may be determined using:
- .
As an approximation, it can be assumed that all the carriers in the system are supplied by
Active region designs
The scattering rates are tailored by suitable design of the layer thicknesses in the superlattice which determine the electron wave functions of the subbands. The scattering rate between two subbands is heavily dependent upon the overlap of the wave functions and energy spacing between the subbands. The figure shows the wave functions in a three quantum well (3QW) QCL active region and injector.
In order to decrease , the overlap of the upper and lower laser levels is reduced. This is often achieved through designing the layer thicknesses such that the upper laser level is mostly localised in the left-hand well of the 3QW active region, while the lower laser level wave function is made to mostly reside in the central and right-hand wells. This is known as a diagonal transition. A vertical transition is one in which the upper laser level is localised in mainly the central and right-hand wells. This increases the overlap and hence which reduces the population inversion, but it increases the strength of the radiative transition and therefore the gain.
In order to increase , the lower laser level and the ground level wave functions are designed such that they have a good overlap and to increase further, the energy spacing between the subbands is designed such that it is equal to the
Material systems
The first QCL was fabricated in the GaInAs/AlInAs material system lattice-matched to an InP substrate.[1] This particular material system has a conduction band offset (quantum well depth) of 520 meV. These InP-based devices have reached very high levels of performance across the mid-infrared spectral range, achieving high power, above room-temperature, continuous wave emission.[4]
In 1998 GaAs/AlGaAs QCLs were demonstrated by Sirtori et al. proving that the QC concept is not restricted to one material system.[5] This material system has a varying quantum well depth depending on the aluminium fraction in the barriers.[citation needed] Although GaAs-based QCLs have not matched the performance levels of InP-based QCLs in the mid-infrared, they have proven to be very successful in the terahertz region of the spectrum.[6]
The short wavelength limit of QCLs is determined by the depth of the quantum well and recently QCLs have been developed in material systems with very deep quantum wells in order to achieve short wavelength emission. The InGaAs/AlAsSb material system has quantum wells 1.6 eV deep and has been used to fabricate QCLs emitting at 3.05 μm.[7] InAs/AlSb QCLs have quantum wells 2.1 eV deep and electroluminescence at wavelengths as short as 2.5 μm has been observed.[8]
The couple InAs/AlSb is the most recent QCL material family compared to alloys grown on InP and GaAs substrates. The main advantage of the InAs/AlSb material system is the small effective electron mass in quantum wells, which favors a high intersubband gain.[9] This benefit can be better exploited in long-wavelength QCLs where the lasing transition levels are close to the bottom of the conduction band, and the effect of nonparabolicity is weak. InAs-based QCLs have demonstrated room temperature (RT) continuous wave (CW) operation at wavelengths up to with a pulsed threshold current density as low as .[10] Low values of have also been achieved in InAs-based QCLs emitting in other spectral regions: at ,[11] at [12] and at [13] (QCL grown on InAs). Most recently, InAs-based QCLs operating near with as low as at room temperature have been demonstrated. The threshold obtained is lower than the of the best reported InP-based QCLs to date without facet treatment.[14]
QCLs may also allow laser operation in materials traditionally considered to have poor optical properties.
Emission wavelengths
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QCLs currently cover the wavelength range from 2.63 μm [20] to 250 μm [21](and extends to 355 μm with the application of a magnetic field.[citation needed])
Optical waveguides
The first step in processing quantum cascade gain material to make a useful light-emitting device is to confine the
Two types of optical waveguides are in common use. A ridge waveguide is created by etching parallel trenches in the quantum cascade gain material to create an isolated stripe of QC material, typically ~10 um wide, and several mm long. A dielectric material is typically deposited in the trenches to guide injected current into the ridge, then the entire ridge is typically coated with gold to provide electrical contact and to help remove heat from the ridge when it is producing light. Light is emitted from the cleaved ends of the waveguide, with an active area that is typically only a few micrometers in dimension.
The second waveguide type is a buried
Laser types
Although the quantum cascade gain medium can be used to produce incoherent light in a superluminescent configuration,[22] it is most commonly used in combination with an optical cavity to form a laser.
Fabry–Perot lasers
This is the simplest of the quantum cascade lasers. An optical waveguide is first fabricated out of the quantum cascade material to form the gain medium. The ends of the crystalline semiconductor device are then cleaved to form two parallel mirrors on either end of the waveguide, thus forming a
Distributed feedback lasers
A
External cavity lasers
In an external cavity (EC) quantum cascade laser, the quantum cascade device serves as the laser gain medium. One, or both, of the waveguide facets has an anti-reflection coating that defeats the optical cavity action of the cleaved facets. Mirrors are then arranged in a configuration external to the QC device to create the optical cavity.
If a frequency-selective element is included in the external cavity, it is possible to reduce the laser emission to a single wavelength, and even tune the radiation. For example, diffraction gratings have been used to create[26] a tunable laser that can tune over 15% of its center wavelength.
Extended tuning devices
There exists several methods to extend the tuning range of quantum cascade lasers using only monolithically integrated elements. Integrated heaters can extend the tuning range at fixed operation temperature to 0.7% of the central wavelength[27] and superstructure gratings operating through the Vernier effect can extend it to 4% of the central wavelength,[28] compared to <0.1% for a standard DFB device.
Growth
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The alternating layers of the two different
Applications
Fabry-Perot (FP) quantum cascade lasers were first commercialized in 1998,[29] distributed feedback (DFB) devices were first commercialized in 2004,[30] and broadly-tunable external cavity quantum cascade lasers first commercialized in 2006.[31] The high optical power output, tuning range and room temperature operation make QCLs useful for spectroscopic applications such as remote sensing of environmental gases and pollutants in the atmosphere[32] and security. They may eventually be used for vehicular cruise control in conditions of poor visibility,[citation needed] collision avoidance radar,[citation needed] industrial process control,[citation needed] and medical diagnostics such as breath analyzers.[33] QCLs are also used to study plasma chemistry.[34]
When used in multiple-laser systems, intrapulse QCL spectroscopy offers broadband spectral coverage that can potentially be used to identify and quantify complex heavy molecules such as those in toxic chemicals, explosives, and drugs.[clarification needed][35]
References
- ^ S2CID 220111282.
- ^ Kazarinov, R. F.; Suris, R. A. (April 1971). "Possibility of amplification of electromagnetic waves in a semiconductor with a superlattice". Fizika I Tekhnika Poluprovodnikov . 5 (4): 797–800.
- S2CID 240934073.
- S2CID 37864645.
- doi:10.1063/1.122812.
- S2CID 29073195.
- ISSN 0003-6951.
- S2CID 40872029.
- S2CID 46218942.
- ISSN 2304-6732.
- PMID 27505843.
- S2CID 126174361.
- S2CID 218844666.
- ISSN 2304-6732.
- S2CID 250846255. Retrieved 2007-02-18.
- PMID 11125134.
- ISSN 0003-6951.
- S2CID 120927848.
- S2CID 231602947.
- .
- .
- ^ Zibik, E. A.; W. H. Ng; D. G. Revin; L. R. Wilson; J. W. Cockburn; K. M. Groom; M. Hopkinson (March 2006). "Broadband 6 μm < λ < 8 μm superluminescent quantum cascade light-emitting diodes". Appl. Phys. Lett. 88 (12): 121109. .
- .
- doi:10.1063/1.119208.
- ^ "Quantum-cascade lasers smell success". Laser Focus World. PennWell Publications. 2005-03-01. Archived from the original on 2013-01-28. Retrieved 2008-03-26.
- .
- PMID 26698453.
- .
- ^ "Extrait du registre du commerce". Registre du commerce. Retrieved 2016-04-28.
- ^ "Alpes offers CW and pulsed quantum cascade lasers". Laser Focus World. PennWell Publications. 2004-04-19. Archived from the original on 2013-01-28. Retrieved 2007-12-01.
- ^ "Tunable QC laser opens up mid-IR sensing applications". Laser Focus World. PennWell Publications. 2006-07-01. Archived from the original on 2013-01-27. Retrieved 2008-03-26.
- ISSN 1043-8092. Archived from the originalon 2013-01-27. Retrieved 2008-01-25.
- S2CID 23963086.
- .
- ISSN 0740-2511. Archived from the originalon 2013-01-27. Retrieved 2008-01-25.