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

Electron states in heterostructures K ||, cm E, eV z, A E, eV AlInAs GaInAs 80 A AlInAs 8-band k  p method (4 bands x 2 spins) Ga 0.47 In 0.53 As Bulk semiconductors Quantum wells

z, A E, eV Optical transitions in quantum wells K ||, cm E, eV AlInAs GaInAs 80 A AlInAs absorption frequency interband intersubband

1 2 3 Intersubband transitions: dipole moment Dipole matrix element: Typical values ~ A Compare with atomic transitions ~ A

1 2 3 Intersubband transitions: selection rules - Dipole matrix element: f 1 and f 3 are even -> z 13 = 0 - Only TM-polarization (E  QW plane)

Intersubband transitions in asymmetric coupled QWs Control of the optical response by engineering the shape (symmetry) of envelope functions and energies of ISBT High optical nonlinearities: e.g.  (2) ~ 10 6 pm/V Short relaxation time ~ 1 ps: possibility of an ultrafast modulation Add advantages of a semiconductor medium: electron transport and Stark effect under applied voltage, integration with other components Saturation easily reached: Large coherence can be excited Rich nonlinear dynamics measured since 1980s

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

60 nm 520 meV 3 2 active region injector (n-doped) e active region QC lasers J. Faist, F. Capasso, et al. Science 264, 553 (1994) Control of lifetimes: phonons, tunneling; need t32 > t2 Cascading: high power when t_stim approaches T 1 From sawtooth to staircase potential E 21 = E phonon

From sawtooth to staircase potential V = 0 V = V th

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

Rui Yang’s talk

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

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”

HITRAN Simulation of Absorption Spectra ( &  m) NO: 5.26  m CO: 4.66  m CH 2 O: 3.6  m NH 3 : 10.6  m O 3 : 10  m N 2 0, CH 4 : 7.66  m CO 2 : 4.3  m CH 4 : 3.3  m COS: 4.86  m Frank Tittel et al.

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 C y 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.

Air Pollution: Houston, TX

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: ppb Appl. Opt. 41, 6018 (2002) NH 3 : marker of kidney and liver diseases Need fast and compact sensors

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

Generation in the THz range Why THz range is important ~  m, f ~ 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 imaging 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

Terahertz QCLs Highest 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 design GaAs/AlGaAs Belkin et al.

Free carriers help to reduce losses!

Metal-metal waveguide

z x Active region GaAs substrate Gold  m (a) (b) Gold Active region  m GaAs substrate 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 (H y ) is plotted. Heavily doped GaAs

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

Heterogeneous Cascades (multi- generation) 9.5  m active region 9.5  m active region 8.0  m active region Distance Energy Current flows in series

Design of the ultrabroadband quantum cascade laser Active waveguide core Shorter wavelengths generation Longer wavelengths generation

Ultrabroadband (6 - 8  m) spectrum

 (2) ~ 10 5 pm/V Maximizing the product of dipoles d 23 d 34 d 24 Quantum interference between cascades I and II Monolithic integration of quantum-cascade lasers with resonant optical nonlinearities

Frequency down-conversion to the THz range Difference frequency generation Stokes Raman and cascade lasing Parametric down-conversion Three ways to achieve using nonlinear optics: ~  m, f ~ THz Current THz semiconductor lasers require cryogenic temperatures They 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, (2008)

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