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Template for Abstract Submission – A Concrete Example of the Connection between Basic Physics and the Detection of Explosives and CBRN Agents using THz Radiation M.F. Pereira1 1 Materials and Engineering Research Institute, Sheffield Hallam University, S1 1WB, Sheffield, United Kingdom IEEE Abstract- Interconduction band gain without global inversion is obtained by engineering conduction band dispersion relations with dilute N resonant centres. This may lead to high temperature operation of THz Quantum Cascade Lasers, which are candidate sources for Detection of Explosives and CBRN, using Terahertz radiation. The abstract with this format should have a maximum length of two pages. I. INTRODUCTION The main focus of this workshop is on novel materials, concepts and device designs for generating and detecting THz (0.3 THz to 10 THz) and Mid Infrared (10 THz to 100 THz) radiation for the detection of explosives and CBRN agents. CBRN is an initialism for chemical, biological, radiological and nuclear. CBRN agents, explosives and illegal drugs can be detected by their characteristic absorption spectra at THz frequencies with high selectivity and resolution in applications fields as industrial quality inspection control, customs inspection and security screening. Moreover, MIR and THz radiation has no endangering effects on human beings and enables higher contrast for “soft matter” than x-rays. In comparison to standard optical technologies for wavelengths up to about 2μm, sources and detectors for MIR and THz have not yet reached this level of maturity and there is still a large gap for features like wavelength tunability, spectral purity, high power and room temperature operation, which all are necessary for commercial applications. generation of THz radiation; Millimetre wave and microwave generation and detection – bridging the gap to THz; Quantum Cascade Lasers; Quantum Cascade Detectors; Intersubband Transport and Optics; THz and MIR Metamaterials; Intersubband Polaritons and Antipolaritons. Intersubband (ISB) devices like the quantum cascade laser QCL [1], can unleash the potential for many applications in the far infrared, notably in the THz range [2], notably detection of explosives and CBRN, but THz QCLs still require cryocooling. The main difficulty at high temperatures is achieving population inversion and ISB lasing without inversion (LWI) may be a solution for this microscopic bottleneck [3-5]. Modeling of the optical and THz response of semiconductors can be achieved using Nonequilibrium Many Body Simulations, proven to deliver realistic results for both interband and intersubband transitions [6-14]. Here we exploit extreme nonparabolicity in the conduction bands to engineer LWI. This can be achieved, e.g., in dilute nitride quantum wells. Indeed, we have recently demonstrated the possibility of interconduction band gain without global inversion by engineering the conduction band effective masses so that the upper lasing subband has an effective mass considerably smaller than the lower lasing subband that could not be obtained in conventional III-V materials. Next we summarize the main steps of the technique used to address the ISB LWI case [7]. II. Papers are requested on ether direct THz detection of CBRN agents and explosives, or on basic or applied science with a clear introduction on how the investigation can be useful for CBRN and explosives detection. Furthermore, detection using THz radiation implies that people are exposed to it, thus progress in understanding the interaction of THz radiation with bio matter is also of relevance. The underlying topics include, but are not restricted to: Detection of explosives and CBRNs; Interaction of MIR or THz radiation with living biological material; Applications of MIR and THz to medical diagnosis MIR and THz imaging; Integration of THz and MIR with optical fibers; Our of the box methods for MIR and THz generation and detection; Lasers without inversion; Nonlinear MIR and THz optics; Difference frequency and other nonlinear methods of NUMERICAL RESULTS AND DISCUSSION The strong interaction between the N resonant states and the conduction band edge means that the conventional eight-band k·p method cannot be applied directly to GaAsN and related heterostructures. One must include the interaction between the N resonant states and the conduction band edge to describe the variation of the zone-center conduction band edge energy with N. In this paper we use a ten-band k·p Hamiltonian to describe the average interaction of the N, NN-pairs and other N-cluster states with the host material. The absorption α(ω), or equivalently the gain spectra g(ω) are evaluated within the context of a NGF approach. The susceptibility is directly related to the carriers Green’s functions G, which satisfies a Dyson equation. The first step of the numerical scheme is the solution of the ten-band k·p Hamiltonian which includes the dilute nitrogen levels responsible for the extra nonparabolicity that gives rise to strong differences in effective subband masses. The Green’s functions and self-energies are expanded using eigenstates and eigenvalues of this Hamiltonian. The model system actually investigated in this paper is globally out of equilibrium but the electrons are assumed to be thermalized within each subband. For a feasibility study we control the total number of electrons in each subband, which can in practice be achieved, e.g., by optical pumping, selective doping or a combination of both methods. Thus, the full NGF scheme is simplified and reduces to the self-consistent evaluation of chemical potentials and self-energy matrix elements which lead to subband energy renormalization, dephasing constants, and occupation functions. Finally, absorption and gain are given by the solution of the integro-differential equation for the optical susceptibility obtained from the carriers Green’s function in linear response. The numerical matrix inversion technique used to solve this equation is similar to the interband methods used under different approximations for both absorption and gain for III-V and II-VI materials [6-14]. Figure 1 illustrates the ISB LWI effect for a 10 nm Ga0.98 N0.02As/Al0.3Ga0.7As QW. In summary, this paper shows that it is possible to engineer conduction subbands with substantial differences in effective masses which make gain without global inversion feasible in dilute nitride heterostructures. In the full presentation we shall extend the results of Ref. [7] for different structures in an effort to push the gain towards the THz domain, where the devices can be used for detection of explosives and CBRN agents. REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] Figure 1. Gain without global population inversion for a 10 nm Ga0.98 N0.02As/Al0.3Ga0.7As QW. The density for the top e2+ subband is the same in all curves. nup=1.0 ×1011 cm−2. From top to bottom, the equal density of subbands e1− and e3- increase by nlow=1.0, 1.1, 1.2, 1.3, 1.4, 1.5×10 11. The insets show occupations for nup = nlow and the corresponding bandstructure for the e1- (dashed), e3- (dot-dashed) and e2+ (dotted) subbands [9]. The curves with smaller mass have larger occupation. [13] [14] J. Faist, F. Capasso, D.L. Sivco, C. Sirtori, A.L. Hutchinson, A.Y. Cho, Science Cho, "Quantum Cascade Laser", vol. 264, no. 5158, pp. 553-556 (1994). R. Köhler, A. Tredicucci, F. Beltran, H.E. Beere, E.H. Linfield, A. Davies, D.A. Ritchie, R. Iotti and F. Rossi, "Terahertz semiconductor-heterostructure laser", Nature, vol. 417, no. 6885, pp. 156-159 (2002). J. Faist, F. Capasso, C. Sirtori, D.L. Sivco, A.L. Hutchinson, M. S. Hybertsen and A. Y. Cho,"Quantum cascade lasers without intersubband population inversion", Phys. Rev. Lett. 76, no.3, pp. 411-415 (1996). R. Terazzi, T. Gresch, M. Giovanni, N. Hoyler, F. Faist and N. Sekine,"Bloch gain in quantum cascade lasers" , Nat. Phys. vol. 3, no. 5, pp. 329-333 (2007). A. Wacker, "Coexistence of Gain and Absorption", Nat. Phys. vol. 3, no. 5, pp. 298-299 (2007). M.F. Pereira Jr.,"Intervalence transverse-electric mode terahertz lasing without population inversion", Phys. Rev. B. vol. 78, no. 24, pp. 245305-1 - 245305-5 (2008). M.F. Pereira and S. Tomić, “Intersubband gain without global inversion through dilute nitride band engineering,” Appl. Phys. Lett. 98, 061101 (2011). M.F. Pereira Jr., Intersubband antipolaritons: Microscopic approach, Phys Rev B75, 195301 (2007). R. Nelander, A. Wacker, M.F. Pereira Jr., D.G. Revin, M.R.Soulby, L.R. Wilson, J.W. Cockburn, A.B. Krysa, J.S. Roberts, and R.J. Airey, Fingerprints of spatial charge transfer in quantum cascade lasers, Journal of Applied Physics 102, 113104 (2007). M.F. Pereira Jr., R. Nelander and A. Wacker, Characterization of intersubband devices combining a nonequilibrium many body theory with transmission spectroscopy experiments, J. Mater Sci: Mater Electron 18, 689 (2007). M.F. Pereira Jr. and K. Henneberger, Microscopic Theory for the Optical Properties of Coulomb-Correlated Semiconductors, Physica Status Solidi (b) 206, 477 (1998). M.F. Pereira Jr. and K. Henneberger, Gain Mechanisms and Lasing in II-VI Compounds, Phys. Stat. Sol. B202, 751 (1997). M.F. Pereira Jr., R. Binder and S.W. Koch, Theory of nonlinear absorption in coupled band quantum wells with many-body effects, Appl. Phys. Lett. 64, 279 (1994). W.W. Chow, M.F. Pereira Jr., and S.W. Koch, Many-Body Treatment on The Modulation Response in a Strained Quantum Well Semiconductor Laser Medium, Appl. Phys. Lett. 61, 758 (1992).