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GaAs/Al x Ga 1-x As; Ga x In 1-x As y P 1-y /Al x In 1-x As on InP;

GaAs/Al x Ga 1-x As; Ga x In 1-x As y P 1-y /Al x In 1-x As on InP; InAs 1-x Sb/AlGa 1-x Sb on GaSb. 1.54. 1. E, eV. AlInAs GaInAs 80 A AlInAs. Ga 0.47 In 0.53 As. 0.16. -0.3. 0. 100. 200. z, A. 1.6. 1.2. E, eV. 0.4. 8-band k  p method (4 bands x 2 spins). -0.025. 0.

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GaAs/Al x Ga 1-x As; Ga x In 1-x As y P 1-y /Al x In 1-x As on InP;

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  1. GaAs/AlxGa1-xAs; GaxIn1-xAsyP1-y/AlxIn1-xAs on InP; InAs1-xSb/AlGa1-xSb on GaSb

  2. 1.54 1 E, eV AlInAs GaInAs 80 A AlInAs Ga0.47In0.53As 0.16 -0.3 0 100 200 z, A 1.6 1.2 E, eV 0.4 8-band kp method (4 bands x 2 spins) -0.025 0 K||, cm-1 107 Electron states in heterostructures Quantum wells Bulk semiconductors

  3. 1.54 1 E, eV AlInAs GaInAs 80 A AlInAs 0.16 -0.3 0 100 200 z, A 1.6 1.2 E, eV 0.4 -0.025 0 K||, cm-1 107 Optical transitions in quantum wells interband absorption frequency intersubband

  4. 3 2 1 Dipole matrix element: Intersubband transitions: dipole moment Typical values ~ 10-100 A Compare with atomic transitions ~ 0.2-0.5 A

  5. 3 2 1 Intersubband transitions: selection rules - Only TM-polarization (E  QW plane) - Dipole matrix element: f1and f3 are even -> z13 = 0

  6. Saturation easily reached: Large coherence can be excited Intersubband transitions in asymmetric coupled QWs Control of the optical response by engineering the shape (symmetry) of envelope functions and energies of ISBT Short relaxation time ~ 1 ps: possibility of an ultrafast modulation High optical nonlinearities: e.g. (2) ~ 106 pm/V measured since 1980s Rich nonlinear dynamics Add advantages of a semiconductor medium: electron transport and Stark effect under applied voltage, integration with other components

  7. High voltage to align levels, high current => high heat dissipation Rui Yang’s talk

  8. injector (n-doped) active region e 3 injector (n-doped) 2 active region 520 meV 60 nm J. Faist, F. Capasso, et al. Science 264, 553 (1994) QC lasers From sawtooth to staircase potential • Control of lifetimes: phonons, tunneling; need t32 > t2 • Cascading: high power when t_stim approaches T1 E21 = Ephonon

  9. From sawtooth to staircase potential V = 0 V = Vth

  10. Fabrication: MBE or MOCVD TEM / SEM image 0.9 nm thick well and barrier 55 nm

  11. Rui Yang’s talk

  12. Rui Yang’s talk

  13. Mid-Far Infrared lasers • IV-VI lead-salt diode lasers: 3-30 m, low-T • Type II lasers • Interband cascade lasers • Intersubband (quantum) cascade lasers

  14. What makes the QC-laser special? • Wavelength agility • layer thicknesses determine emission wavelength • Demonstrated applications in mid/far-IR gas sensing • High optical power ~ 1W, room-T operation • cascading re-uses electrons • Ultra-fast carrier dynamics • no relaxation oscillations • Pure TM-polarization – efficient in-plane light coupling • Micro-lasers • Small linewidth enhancement factor • Intrinsic “design potential”

  15. HITRAN Simulation of Absorption Spectra (3.1-5.5 & 7.6-12.5 m) CO2: 4.3 m COS: 4.86 m CH2O: 3.6 m CO: 4.66 m CH4: 3.3 m NO: 5.26 m O3: 10 m NH3: 10.6 m N20, CH4: 7.66 m Frank Tittel et al.

  16. Wide Range of Gas Sensing Applications • Urban and Industrial Emission Measurements • Industrial Plants • Combustion Sources and Processes (eg. early fire detection) • Automobile and Aircraft Emissions • Rural Emission Measurements • Agriculture and Animal Facilities • Environmental Gas Monitoring • Atmospheric Chemistry of Cy gases (eg global and ecosystems) • Volcano Gas Emission Studies and Eruption Forecasting • Chemical Analysis and Industrial Process Control • Chemical, Pharmaceutical, Food & Semiconductor Industry • Toxic Industrial Chemical Detection • Spacecraft and Planetary Surface Monitoring • Crew Health Maintenance & Advanced Human Life Support Technology • Biomedical and Clinical Diagnostics (eg. non-invasive breath analysis) • Forensic Science and Security • Fundamental Science and Photochemistry • Life Sciences Frank Tittel et al.

  17. Air Pollution: Houston, TX

  18. 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 NH3: marker of kidney and liver diseases Need fast and compact sensors Appl. Opt. 41, 6018 (2002)

  19. NASA Atmospheric & Mars Gas Sensor Platforms Frank Tittel et al. Aircraft laser absorption spectrometers Tunable laser planetary spectrometer Tunable laser sensors for earth’s stratosphere

  20. Generation in the THz range  ~ 100-1000 m, f ~ 0.3-3 THz Why THz range is important

  21. THz spectroscopy and imaging 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. Three different drugs: MDMA (left), aspirin (center), and methamphetamine (right), have different images in T-rays K. Kawase, OPN, October 2004 Q. Hu, QCL Workshop

  22. Q. Hu, QCL Workshop

  23. Terahertz QCLs Highest operating temperature ~ 175 K in pulsed regime Narrow tunability Q. Hu (MIT), F. Capasso (Harvard), J. Faist (ETH), A. Tredicucci (Pisa)

  24. Terahertz QCLs: 3 QW design GaAs/AlGaAs Belkin et al.

  25. Free carriers help to reduce losses!

  26. Metal-metal waveguide

  27. 150m 75 10 0 150m 75 10 0 GaAs substrate Active region Gold (a) Heavily doped GaAs z x GaAs substrate Active region Gold (b) Fig. 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.

  28. Heterogeneous Cascades (multi-l generation) So far: single stack of ~ 30 identical active regions & injectors Homogeneous cascade: Now: Stacked cascades: Interdigitated cascades: Cooperative cascades: Charge transport between stages How to design cooperation Different electric field across sub-stacks

  29. Energy Distance Heterogeneous Cascades (multi-l generation) 9.5 mm active region 8.0 mm active region 9.5 mm active region Current flows in series

  30. Shorter wavelengths generation Longer wavelengths generation Design of the ultrabroadband quantum cascade laser Active waveguide core IEEE LEOS LECTURE

  31. Ultrabroadband (6 - 8 mm) spectrum

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

  33. Frequency down-conversion to the THz range • Difference frequency generation • Stokes Raman and cascade lasing • Parametric down-conversion  ~ 100-1000 m, f ~ 0.3-3 THz Current THz semiconductor lasers require cryogenic temperatures They are not tunable Three ways to achieve using nonlinear optics:

  34. 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)

  35. cladding Laser1 section Side contact layer Laser 2 section substrate Difference frequency generation in two-wavelength QCLs 3 ωq 2 ωp 1 Results obtained by Feng Xie in Harvard in summer 2007 M. Belkin, F. Capasso, A. Belyanin et al.Nature photonics 1, 288 (2007). M. Belkin, F. Xie et al., 2008

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