Interband vs Intersubband Optics: Fundamentals and Applications M.F. Pereira Theory of Semiconductor Materials and Optics Materials and Engineering Research Institute Sheffield Hallam University S1 1WB Sheffield, United Kingdom

Outline Motivation/Technological Applications Highlights of Nonequilibrium Many Body Methods Applied to Quantum Cascade Lasers Summary

Relevance of Intersubband Devices for THz and MIR Intersubband-based devices allow the generation of long wavelengths not possible with conventional interband optics!!!

Tricorder: 23rd Century End Users Keldysh Nonequilibrium Green's Functions are required to design new materials and custom- designed effects for those devices!!

Disease detection by breath analysis? MoleculeFormulaBiological/Pathology Indication PentaneCH 3 (CH 2 ) 3 CH 3 Lipid peroxidation, oxidative stress associated with inflammatory diseases, transplant rejection, breast and lung cancer EthaneC2H6C2H6 Lipid peroxidation and oxidative stress, lung cancer CO 2 isotope ratio 13 CO 2 / 12 CO 2 Marker for Helicobacter pylori infection, Gastrointestinal and hepatic functions Carbonyl SulfideCOSLiver disease and acute rejection in lung transplant recipients ( ppb?) Carbon disulfideCS 2 Schizophrenia AmmoniaNH 3 Liver and renal diseases, fasting response, hepatic encephalopathy FormaldehydeHCHOCancerous tumors, breast cancer ( ppb) Nitric OxideNOInflammatory and immune responses (e.g., asthma) and vascular smooth muscle response (6-100 ppb) Hydrogen PeroxideH2O2H2O2 Airway Inflammation, Oxidative stress (1-5 ppb) Carbon MonoxideCOSmoking response, CO poisoning, vascular smooth muscle response, platelet aggregation ( ppb) EthyleneH 2 C=CH 2 Oxidative stress, cancer AcetoneCH 3 COCH 3 Fasting response, diabetes mellitus response, ketosis Courtesy of F.K. Tittel

Possible with off-axis integrated cavity output spectroscopy (ICOS) based gas sensing using a QCL mid-IR source Courtesy of F.K. Tittel, Rice University

Wide Range of Gas Sensing Applications for mid-IR QCLs 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, Spacecraft and Planetary Surface Monitoring (NH 3, CO, CH 4, C 2 H 4, N 2 O, CO 2 and H 2 CO) Crew Health Maintenance & Advanced Human Life Support Technology Atmospheric Chemistry (eg ecosystems and airborne) Volcanic Emissions Chemical Analysis and Industrial Process Control (NO, NH 3, H 2 O) Chemical, Pharmaceutical, Food & Semiconductor Industry Toxic Industrial Chemical Detection Biomedical and Clinical Diagnostics (eg. breath analysis) (NO, CO, COS, CO 2, NH 3, C 2 H 4 ) Forensic Science and Security Fundamental and Life Sciences

Security Scanning (THz) discriminate different types of drugs, explosives and automated identification of toxins such as nerve and mustard gas, anthrax.

Would you like to detect an otherwise undetectable cancer under the skin??

Medical Imaging and Diagnostics (THz) diseased tissue healthy tissue

Wide Range of Gas Sensing Applications for THz QCLs IR astronomy Space Missions Dental Imaging Pharmaceutical applications Proteomics Postal scanning Plastic Landmine detection Semiconductor imaging Environmental sensing and monitoring Point sensors Last mile high bandwidth (Telecoms) Collision avoidance radar, remote sensing, telemetry, physics, isotope ratios, industrial process control

Limitations of Current THz Sources Device ConceptPhysical MechanismLimitation Semiconductor electronic devices: transistors, Gunn oscillators, Schottky-diode frequency multipliers and photomixers. Based on freely moving electrons.Limited by the transit time and parasitic RC time constants; Power level decreases as 1/f 4, or even faster as the frequency increases above 1 THz. Photonic or quantum electronic devices.Oscillating bound charge carriers (leading to an oscillating displacement current), which are thus not limited by the transient and/or the RC time constants. Conventional bi-polar (interband-transition- based devices) are restricted to the frequencies corresponding to the band gap, which is larger then 10 THz even for narrow- gap lead-salt materials. Four-wave mixing in semiconductor lasers.Sidebands up to 4 THz, stemming from a modulation of the carrier plasma at the difference frequency of the two laser modes. The four-wave mixing high order process essentially limits the maximum achievable power to a very low sub-nW range.

Teraview Scanner ~ £250K

Physical Principle Behind the Teraview Scanner

The Challenge: Intersubband Lasing It took 23 years to achieve this laser!

The Quantum Cascade Laser

Current THz sources: why quantum cascade lasers? Details of the theory. Current challenges for further THz QCL development: how can the theory help address them? Summary Outline

Self-Consistent Solution of the Dyson Equations

Energy and Spatially Resolved Populations Courtesy of S.-C. Lee

Voltage-Current Characteristics of a THz QCL Operating at 77 K

Importance of Quantum Transport

Light Generation and Amplification

M.F. Pereira Jr. and A. Wacker, phys. stat. sol. (c) 4, 356 (2007).

Many-body effects are stronger at THz than at midIR

M.F. Pereira Jr, S.-C. Lee, and A. Wacker., Phys. Rev. B69, (2004).

Many-body effects are stronger at THz than at midIR M.F. Pereira, et al IEEE Conference Proceedings, pp ISBN – 6 Based on the structure introduced by B. Williams et al, APL 82, 1015 (2003).. 10 x less doping than in typical mid-IR designs.

Importance of Many-Body Effects in Absorption and Gain Coulomb enhanced cross absorption can even eliminate the gain. Coulomb-induced shift of the same order of magnitude of the transition energies in the THz domain. Carrier-carrier scattering may dominate the broadening in THz QCLs. Strong interplay of bandstructure and Coulomb effects leading to extra features in the absorption spectra.

Micro-Transmission Experimental Set Up QCL ridge MCT Detector λ ~ 1.5 – 12.5 µm (100 – 800 meV) Broad band globar light from FTIR spectrometer Measurements in very broad mid-IR range Diameter of focused spot ~ 50µm CW operation in the cryostat (T=7-300K) TE, TM polarization Reflecting optics; better than 100:1 signal to noise contrast D.G. Revin, L.R. Wilson, J. Cockbun, A.B. Krysa, J.S. Roberts, and R. Airey. Appl. Phys. Lett. 88, (2006).

MidIR QCL Spectroscopy: Theory vs. Experiments Lattice-matched InGaAs/InAlAs/InP operating around λ7.7 μm.

Theory vs Experiments (10K) TheoryExperiments

Theory vs Experiments (78K) M.F. Pereira Jr, R. Nelander, A. Wacker, D.G. Revin, M.R.Soulby, L.R. Wilson, J.W. Cockburn, A.B. Krysa, J.S. Roberts, and R.J. Airey, Journal of Materials Sience: Materials in Electronics 18, 689 (2007).

R.Nelander et. al., to appear in JAP. Fingerprints of charge transfer between injectors and active regions

Summary Leading edge nonequilibrium many body theory applied to semiconductor materials. Software development. MidIR laser development for medical applications. THz laser development for security applications.