Quantum cascade lasers

GaAs/AlxGa1-xAs; GaxIn1-xAsyP1-y/AlxIn1-xAs on InP; InAs1-xSb/AlGa1-xSb on GaSbElectron states in heterostructures

K||, cm-101071.6E, eV1.20.4-0.025

z, AE, eV1.540.1610200100-0.3AlInAs GaInAs 80 A AlInAs

8-band kp method(4 bands x 2 spins)Ga0.47In0.53AsBulk semiconductorsQuantum wells

z, AE, eV1.540.1610200100Optical transitions in quantum wells

K||, cm-101071.6E, eV1.20.4-0.025-0.3AlInAs GaInAs 80 A AlInAs

absorptionfrequency

interbandintersubband

123Intersubband transitions: dipole moment

Dipole matrix element:

Typical values ~ 10-100 ACompare with atomic transitions ~ 0.2-0.5 A

123Intersubband transitions: selection rules

- Dipole matrix element: f1 and f3 are even -> z13 = 0- Only TM-polarization (E QW plane)

Rui Yangs talkHigh voltage to align levels, high current => high heat dissipation

60 nm520 meV32activeregioninjector (n-doped)injector (n-doped)eactiveregionQC lasersJ. Faist, F. Capasso, et al. Science 264, 553 (1994)Control of lifetimes: phonons, tunneling; need t32 > t2

Cascading: high power when t_stim approaches T1From sawtooth to staircase potential

E21 = EphononFrom sawtooth to staircase potential

V = 0

V = Vth

9Fabrication: MBE or MOCVD TEM / SEM image

55 nm

0.9 nm thickwell and barrier

Rui Yangs talk

Rui Yangs talkMid-Far Infrared lasersIV-VI lead-salt diode lasers: 3-30 m, low-TType II lasersInterband cascade lasersIntersubband (quantum) cascade lasersWhat makes the QC-laser special?Wavelength agilitylayer thicknesses determine emission wavelength

Demonstrated applications in mid/far-IR gas sensing

High optical power ~ 1W, room-T operationcascading re-uses electrons

Ultra-fast carrier dynamicsno relaxation oscillations

Pure TM-polarization efficient in-plane light couplingMicro-lasers

Small linewidth enhancement factorIntrinsic design potential

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HITRAN Simulation of Absorption Spectra (3.1-5.5 & 7.6-12.5 m)NO: 5.26 mCO: 4.66 mCH2O: 3.6 mNH3: 10.6 mO3: 10 mN20, CH4: 7.66 mCO2: 4.3 mCH4: 3.3 mCOS: 4.86 mFrank Tittel et al.15Wide Range of Gas Sensing ApplicationsUrban and Industrial Emission MeasurementsIndustrial PlantsCombustion Sources and Processes (eg. early fire detection)Automobile and Aircraft EmissionsRural Emission MeasurementsAgriculture and Animal FacilitiesEnvironmental Gas MonitoringAtmospheric Chemistry of Cy gases (eg global and ecosystems)Volcano Gas Emission Studies and Eruption ForecastingChemical Analysis and Industrial Process ControlChemical, Pharmaceutical, Food & Semiconductor IndustryToxic Industrial Chemical DetectionSpacecraft and Planetary Surface MonitoringCrew Health Maintenance & Advanced Human Life Support TechnologyBiomedical and Clinical Diagnostics (eg. non-invasive breath analysis)Forensic Science and SecurityFundamental Science and PhotochemistryLife Sciences

Frank Tittel et al.16Air Pollution: Houston, TX

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Non-invasive Medical Diagnostics:Breath analysis

NO: marker of lung diseases

Concentration in exhaled breath for a healthy adult: 7-15 ppb For an asthma patient: 20-100 ppb

Appl. Opt. 41, 6018 (2002)NH3: marker of kidney and liver diseasesNeed fast and compact sensorsNASA Atmospheric & Mars Gas Sensor Platforms

Tunable laser sensors for earths stratosphereAircraft laser absorption spectrometersTunable laser planetary spectrometerFrank Tittel et al.19Generation in the THz rangeWhy THz range is important ~ 100-1000 m, f ~ 0.3-3 THz

T-rays allow you to see through any dry optically opaque cover: envelope, clothing, suitcase etc, and locate non-metallic things, even read letters.

T-rays have enough specificity to distinguish big molecules; they can be used to detect explosives, drugs, etc. THz spectroscopy and imagingThree different drugs: MDMA (left), aspirin (center), and methamphetamine (right), have different images in T-raysK. Kawase, OPN, October 2004Q. Hu, QCL Workshop

Q. Hu, QCL Workshop

Terahertz QCLsHighest operating temperature ~ 175 K in pulsed regime

Narrow tunability

Q. Hu (MIT), F. Capasso (Harvard), J. Faist (ETH), A. Tredicucci (Pisa)

Terahertz QCLs: 3 QW designGaAs/AlGaAsBelkin et al.

Free carriers help to reduce losses!

Metal-metal waveguidezxActive regionGaAs substrate

Gold01075150m(a)(b)GoldActive region01075150mGaAs substrateFig. 2. Schematic representation of (a) the semi-insulating surface-plasmon waveguide (b) the metal-metal plasmon waveguide, used in THz QCLs. The component of the magnetic field of the mode parallel to the layers of the active region (Hy) is plotted.Heavily doped GaAsHeterogeneous Cascades (multi-l generation)

Homogeneous cascade: single stack of ~ 30 identical active regions & injectors Stacked cascades: Interdigitated cascades: Cooperative cascades: Different electric field across sub-stacksCharge transport between stagesHow to design cooperation

So far:Now:28So far all QC lasers have homogeneous cascades;but intersubband transitions can do more:transparent on both energy sides, no cross abs.unipolar, cascading between stages-> can make heterogeneous cascades:several typs:stacked (different E-fields across substacks OK)interdigitated cascades (NEXT TALK Trinesha)these two: two lasers dont interfere negatively with each otherstep forward: cooperative cascades: supercontinuum gen.Heterogeneous Cascades (multi-l generation)

9.5 mmactive region9.5 mmactive region8.0 mmactive regionDistanceEnergyCurrent flows in seriesDesign of the ultrabroadband quantum cascade laser

Active waveguide coreShorter wavelengths generationLonger wavelengths generation

IEEE LEOS LECTUREUltrabroadband (6 - 8 mm) spectrum

31Above threshold spectra;measured at several pulsed currents;full 6 - 8 mm spectrum above 5 Aheat sink temperature ~ 50 K

get continuous spectrum from 6 - 8 mm (+ 5.5 mm)

short wavelength cut-off likely due from 3-1 absorption, can be corrected with other active region types (e.g. SL)

(2) ~ 105 pm/V Maximizing the product of dipoles d23d34d24 Quantum interference between cascades I and IIMonolithic integration of quantum-cascade lasers with resonant optical nonlinearities

Frequency down-conversion to the THz rangeDifference frequency generationStokes Raman and cascade lasingParametric down-conversionThree ways to achieve using nonlinear optics: ~ 100-1000 m, f ~ 0.3-3 THzCurrent THz semiconductor lasers require cryogenic temperaturesThey are not tunable

Difference frequency generation in two-wavelength QCLs

M. Belkin, F. Capasso, A. Belyanin et al. Nature photonics 1, 288 (2007).M. Belkin, F. Xie et al., APL 96, 201101 (2008)

Difference frequency generation in two-wavelength QCLs

123qpcladdingLaser1 sectionSide contact layerLaser 2 sectionsubstrateM. Belkin, F. Capasso, A. Belyanin et al. Nature photonics 1, 288 (2007).

M. Belkin, F. Xie et al., 2008Results obtained by Feng Xie in Harvard in summer 200736

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Fig. 11. Schematics of the THz DFG process with population inversion in our devices (a) and the schematic view of the THz source based on intra-cavity DFG in dual-wavelength mid-IR QCL. The active region (shown in red) generates light output at mid-IR frequencies 1 and 2 through the laser action and light output at THz frequency THz through the DFG process.
Electron population
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