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We present a detailed analysis of GaAs/AlGaAs terahertz quantum cascade laser in the presence of an intense external magnetic field. One of the objectives in further development of THz quantum cascade laser is the realization of structures operating at higher temperatures. This is difficult to obtain as the operating photon emission energy is smaller than the longitudinal-optical phonon energy in the semiconductor material. With increased temperature, electrons in the upper radiative state gain sufficient in-plane energy to emit an longitudinal-optical phonon, which represents a non-radiative scattering and reduces the optical gain. By applying strong magnetic field, two-dimensional continuous energy subbands become split into series of discrete Landau levels, and at particular values of B it is possible to quench these non-radiative channels. Numerical simulations are performed on two-well design quantum cascade laser operating at 4.6 THz, implemented in CaAs/Al(0.15)Ga(0.85)As, and the magnetic field is perpendicular to the epitaxial layers. Strong oscillations of carrier lifetimes for the upper state of the laser transition, as a function of magnetic field are observed, which can be attributed to interface roughness scattering and longitudinal-optical phonon scattering between Landau levels. 12th Annual YUCOMAT Conference, Sep 06-10, 2010, Herceg Novi, Montenegro

in generating optimal structures which emit radiation at specified wavelengths corresponding to absorption fingerprints of particular harmful pollutants found in the atmosphere. We also illustrate another interesting prospective application of quantum cascade laser-type structures: the design of metamaterials with tunable complex permittivity, based on amplification via intersubband transitions. In this case, the role of the magnetic field is to assist the attainment of sucient optical gain (population inversion), necessary to eectively manipulate the permittivity and fulfill the conditions for negative refraction (left-handedness).

Single-mode frequency-tuneable semiconductor lasers based on monolithic integration of multiple cavity sections are important components, widely used in optical communications, photonic integrated circuits and other optical technologies. To date, investigations of the ultrafast switching processes in such lasers, essential to reduce frequency cross-talk, have been restricted to the observation of intensity switching over nanosecond-timescales. Here, we report coherent measurements of the ultrafast switch-on dynamics, mode competition and frequency selection in a monolithic frequency-tuneable laser using coherent time-domain sampling of the laser emission. This approach allows us to observe hopping between lasing modes on picosecond-timescales and the temporal evolution of transient multi-mode emission into steady-state single mode emission. The underlying physics is explained through a full multi-mode, temperature-dependent carrier and photon transport model. Our results show that the fundamental limit on the timescales of frequency-switching between competing modes varies with the underlying Vernier alignment of the laser cavity., Single-mode, tuneable monolithic semiconductor lasers are important light sources for integrated photonics. Here, Kundu et al. observe the switch-on dynamics and mode competition of a terahertz quantum cascade laser and explain the behaviour with a carrier and photon transport model.

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We make a systematic study to find the dependence of the threshold current density of a mid-infrared quantum-cascade laser on the waveguide ridge width. The optical and thermal parameters of the waveguide are calculated with a finite-element method, and gain is found from a self-consistent rate equation model with energy-balance. The results show the existence of an optimal ridge width that minimizes the threshold current density of the laser. The influence of the interface roughness parameters on laser performance is also discussed.

We start from the fourth-order nonparabolic and anisotropic conduction band bulk dispersion relation to obtain a one-band effective Hamiltonian, which we apply to an AlGaSb symmetric double-barrier structure with resonant energies significantly (more than 200meV) above the well bottom. The spin-splitting is described by the k3 Dresselhaus spin-orbit coupling term modifying only the effective mass of the spin eigenstates in the investigated structure. Apart from the bulk-like resonant energy shift due to the band nonparabolicity, we obtain a substantial shift depending on the choice of boundary conditions for the envelope functions at interfaces between different materials. The shift of resonant energy levels leads to the change of spin-splitting and the magnitude of the dwell times. We attempt to explain the influence of both the nonparabolicity and boundary conditions choice by introducing various effective masses.

We report on the results of our simulations of Γ−X scattering in GaAs/AlGaAs heterostructures, discussing the importance of the mole fraction, doping density, and lattice and electron temperature in determining the scattering rates. We consider three systems, a single quantum well (for the investigation of Γ−X scattering), a double quantum well (to compare the Γ−X−G and Γ−Γ scattering rates), and an example of a GaAs/AlGaAs mid-infrared quantum cascade laser. Our simulations suggest that Γ−X scattering can be significant at room temperature but falls off rapidly at lower temperatures. One important factor determining the scattering rate is found to be the energy difference between the Γ and X-states.

Considerable interest exists for sensing and imaging technologies in the terahertz (THz) spectral range, in particular for the interrogation of materials of an organic or biological nature. Development in THz quantum cascade lasers is seeing higher operating temperatures and peak output powers in pulsed mode, accentuating their place as the preferred source of coherent THz frequency radiation. Technological development of interferometric sensing schemes continues to take advantage of practical improvements in THz quantum cascade lasers. In this Summary, we give a brief overview of some recent developments in this regard.

The aim of this work is to calculate carrier relaxation rates from the upper laser level due to electron–electron interactions in three and four-level quantum cascade lasers (QCLs) in a strong magnetic field. The comparison between calculated results and previously obtained values for acoustical and optical-phonon scattering processes indicates that carrier–carrier scattering might have noticeable influence on laser output properties, depending on the structural design. Numerical results are presented for two λ ∼ 9 μm GaAs-based QCLs in magnetic fields between 20 T and 60 T and the band nonparabolicity is taken into account.

Experimental results showing room temperature normal incidence mid-infrared detection by AlGaN/GaN quantum well infrared photodetectors are presented. Designed structures have intersubband transitions corresponding to wavelengths in the region of 3 and 4 µm, where strong absorption in a sapphire substrate dominates. The intersubband spectra, therefore, were characterized by electronic Raman scattering and infrared photocurrent spectroscopy. The absorption spectra agree well with theoretical predictions. Details of device fabrication are presented with sensitivity estimates for the devices.

We present experimental and theoretical investigations of the bias-dependent spectral shift of the photoresponse in InAs/InxGa1?xAs quantum-dots-in-a-well structures. Experimental results show that the wavelength response of the transition from the quantum dot ground state to quantum well states can be Stark-shifted by ~15% by changing the applied bias between ?1?V and +1?V. A theoretical model based on the 8-band k???p method fits our experimental data well using realistic dot parameters. We also demonstrate an increase in the operating wavelength and a reduced bias-dependent spectral shift for samples containing dots formed by depositing less InAs during growth.

We have further improved our procedure for the optimization of multilayer semiconductor nanostructures, based upon diluted magnetic semiconductors, developed previously with the goal of maximizing their spin-filtering properties. The new optimization scheme relies on the application of a modern class of evolutionary algorithms for global optimization, specifically the genetic algorithm. Its fitness function is set to select the best possible spin-polarization properties within the chosen range of bias voltages and with a fixed value of external magnetic field. Numerical calculations are presented for the ZnSe/Zn1−xMnxSe based semiconductor system, and the obtained results predict an enhanced spin-diode performance over the existing designs.

Simulations of an optically pumped intersubband laser in magnetic field up to 60 T are performed within the steady-state rate equations model. The electron-polar optical phonon scattering is calculated using the confined and interface phonon model. A strong oscillatory optical gain vs. magnetic field dependence is found, with two dominant gain peaks occurring at 20 and 40 T, the fields which bring appropriate states into resonance with optical phonons and thus open additional relaxation paths. The peak at 20 T exceeds the value of gain achieved at zero field.

Recent research shows that terahertz quantum cascade lasers are well-suited to high speed free space communication. The results of both theoretical and laboratory work indicate the devices are able to deliver bandwidths in the gigahertz to tens of gigahertz range without the burden of relaxation oscillations found in diode lasers. Using a novel rate equation model we explore the frequency response characteristics of a real device and report on the finding of a strongly peaked bias current-dependent response.

We developed an accurate model accounting for electron-phonon interaction in colloidal\ud quantum dot supercrystals that allowed us to identify the nature of charge carriers and the\ud electrical transport regime. We find that in experimentally analyzed CdSe nanocrystal solids the electron-phonon interaction is sufficiently strong that small polarons localized to single dots are formed. Charge-carrier transport occurs by small polaron hopping between the dots, with mobility that decreases with increasing temperature. While such a temperature dependence of mobility is usually considered as a proof of band transport, we show that the same type of dependence occurs in the system where transport is dominated by small polaron hopping.

Terahertz-frequency quantum cascade lasers (THz QCLs) based on bound-to-continuum active regions are difficult to model owing to their large number of quantum states. We present a computationally efficient reduced rate equation (RE) model that reproduces the experimentally observed variation of THz power with respect to drive current and heat-sink temperature. We also present dynamic (time-domain) simulations under a range of drive currents and predict an increase in modulation bandwidth as the current approaches the peak of the light-current curve, as observed experimentally in mid-infrared QCLs. We account for temperature and bias dependence of the carrier lifetimes, gain, and injection efficiency, calculated from a full rate equation model. The temperature dependence of the simulated threshold current, emitted power, and cut-off current are thus all reproduced accurately with only one fitting parameter, the interface roughness, in the full REs. We propose that the model could therefore be used for rapid dynamical simulation of QCL designs. (C) 2015 AIP Publishing LLC

We demonstrate coherent three-dimensional terahertz imaging by frequency modulation of a quantum cascade laser in a compact and experimentally simple self-mixing scheme. Through this approach, we can realize significantly faster acquisition rates compared to previous schemes employing longitudinal mechanical scanning of a sample. We achieve a depth resolution of better than 0.1 μm with a power noise spectral density below -50 dB/Hz, for a sampling time of 10 ms/pixel.

A quantum-cascade laser (QCL) thermal model is presented. On the basis of a finite-difference approach, the model is used in conjunction with a self-consistent carrier transport model to calculate the temperature distribution in a near-infrared InGaAs/AlAsSb QCL. The presented model is used to investigate the effects of driving conditions and device geometries on the active-region temperature, which has a major influence on the device performance. A buried heterostructure combined with epilayer-down mounting is found to offer the best performance compared with alternative structures and has thermal time constants up to eight times smaller. The presented model provides a valuable tool for understanding the thermal dynamics inside a QCL and will help to improve operating temperatures.

In this work we report on modeling the electron transport in n-Si/SiGe structures. The electronic structure is calculated within the effective-mass complex-energy framework, separately for perpendicular (Xz) and in-plane (Xxy) valleys, the degeneracy of which is lifted by strain, and additionally by size quantization. The transport is described via scattering between quantized states, using the rate equations approach and tight-binding expansion, taking the coupling with two nearest-neighbour periods. The acoustic phonon, optical phonon, alloy and interface roughness scattering are taken in the model. The calculated U/I dependence and gain profiles are presented for a couple of QC structures.

We investigate theoretically the possibility of lasing in the terahertz in a suitably designed quantum cascade structure comprising dilute magnetic semiconductors. quantum cascade laser. Tunability is based on the spin-dependent potential induced by a magnetic field. The Boltzmann equation approach was used in order to predict the dependence of the gain and current on magnetic and electric fields. The electron dynamics were modeled by taking electron-longitudinal optical phonon, electron-longitudinal acoustic phonon and electron-electron scattering into account. Tunability of the emission energy between 10 meV and 26 meV, and 38 meV and 42 meV, for transitions of spin-down and spin-up electrons respectively, may be achieved by varying a magnetic field up to 5 T, at a temperature of 1.5 K. Comparison of the calculated optical gain, and losses in the appropriate waveguide, indicates the prospect of achieving laser operation. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A symmetry based method for the efficient calculation of single-particle states in hexagonally symmetric GaN/AlN quantum dots within the framework of a k · p model is presented. The envelope functions are expanded into a plane wave basis on a hexagonal lattice and the group projector method is used to adapt the basis to exploit the symmetry, resulting in block diagonalization of the corresponding Hamiltonian matrix into six matrices and classification of the states by the quantum number of total quasi-angular momentum. The method is applied to the calculation of the electron and hole energy levels in a quantum dot superlattice and the intraband optical absorption matrix elements. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A theoretical model describing the electron transport in InAs/GaAs quantum dot infrared photodetectors, modelled as ideal quantum dot superlattices, is presented. The carrier wave functions and energy levels were evaluated using the strain dependent 8-band k · p Hamiltonian and used to calculate all intra- and inter-period transition rates due to interaction with phonons and electromagnetic radiation. The interaction with longitudinal acoustic phonons and electromagnetic radiation was treated perturbatively within the framework of Fermi's golden rule, while the interaction with longitudinal optical phonons was considered taking into account their strong coupling to electrons. The populations of energy levels were then found from a system of rate equations, and the electron current in the superlattice was subsequently extracted. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Theoretical modelling of the intraband absorption spectrum in InAs/GaAs quantum dot infrared photodetectors is performed for several typical structures reported in the literature. The calculations are performed within the framework of the two methods: a simple and so far widely used effective mass method with the values of conduction band offset and the effective mass modified to take account of the effects of strain and band mixing on average and the more realistic eight-band k × p method with the strain distribution taken into account via the continuum mechanical model. Both methods give qualitatively the same results; however, the peak positions obtained within the effective mass approach are blue shifted and the absorption cross sections are overestimated, compared to the more accurate k × p approach.

We have analyzed the spin-filtering effects of the electron current in asymmetric ZnSe/Zn1-xMnxSe multilayer structures, under the influence of both an external magnetic field and a bias voltage. In this type of semiconductor systems, conduction band electrons interact with 3d electrons of the magnetic Mn2+ ions. Because of this sp-d exchange interaction, an external magnetic field modulates the effective potential profile seen by spin-up and spin-down electrons, giving rise to a large Zeeman effect. It is found that the degree of spin polarization changes significantly when the electrical bias is switched from forward to reverse, thus the proposed structure displays obvious behavior of spin-filter diode. This originates from the effective “lifting” of the potential for spin-up electrons, which tunnel through actual potential barriers. Structural parameters optimization, with the goal of maximizing the spin-filtering coefficient, was performed by using simulated annealing algorithm. The described effect may be important for designing new tunable spin-based multifunctional semiconductor devices.

We present a symmetry-based method for the efficient calculation of energy levels in hexagonal GaN/AlN quantum dots within the framework of a kp model. The envelope functions are expanded into a plane wave basis on a hexagonal lattice and the group projector method is used to adapt the basis to exploit the symmetry, resulting in block diagonalization of the corresponding Hamiltonian matrix into six matrices and classification of the states by the quantum number of total quasi-angular momentum. The method is applied to the calculation of the electron and hole single-particle states in a quantum dot superlattice. The selection rules for absorption of electromagnetic waves in the dipole approximation are established and the intraband optical absorption matrix elements are found. Good agreement with the available experimental data on intraband optical absorption is found.

A first principles theory of electronic transport in quantum well infrared photodetectors (QWIPs) is presented. The model is based on the cascade scattering rate equations approach and uses the translation symmetry to reduce the computational cost of the problem. The model delivers a non-equilibrium electron distribution in both dark conditions and under illumination. The model was tested by simulating a non-conventional broadband InGaAs/AlGaAs QWIP and good agreement with experimental data available in the literature was achieved. The model can be applied to any QWIP structure and can be used for extending the range of materials employed.

We report on a comparative study of GaAs/Al0.45Ga0.55As mid-infrared quantum cascade lasers. Devices differ in the mechanisms for depopulating the lower laser level. The depopulation mechanisms rely on longitudinal optical (LO) phonon emission into a single subband, into two subbands or into a set of subbands forming a miniband. The influence of injector doping density and electron confinement on device characteristics is explored experimentally and theoretically. The mechanisms limiting high temperature operation and the dynamic range are investigated. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We describe the application of our computational model, developed for finding the optical gain in a mid-infrared quantum cascade laser subjected to a strong magnetic field, to two distinct λ ~ 9 µm GaAs-based structures. The additional carrier confinement induced by the field alters the transition rates for the optical- and acoustic-phonon scattering processes from the upper laser level, thus affecting the laser output properties, in particular the optical gain. Within this model, the gain is found by solving the system of rate equations, from which the carrier densities in each level are calculated. Numerical results are presented for magnetic fields between 10 and 60 T, and the band nonparabolicity is taken into account.

We propose an optically pumped laser based on intersublevel transitions in InAs-GaAs pyramidal self-assembled quantum dots. A theoretical rate equations model of the laser is given in order to predict the dependence of the gain on pumping flux and temperature. The energy levels and wave functions were calculated using the 8-band k/spl middot/p method where the symmetry of the pyramid was exploited to reduce the computational complexity. Carrier dynamics in the laser were modeled by taking both electron-longitudinal optical phonon and electron-longitudinal acoustic phonon interactions into account. The proposed laser emits at 14.6 /spl mu/m with a gain of g/spl ap/ 570 cm/sup -1/ at the pumping flux /spl Phi/=10/sup 24/ cm/sup -2/ s/sup -1/ and a temperature of T=77 K. By varying the size of the investigated dots, laser emission in the spectral range 13-21 /spl mu/m is predicted. In comparison to optically pumped lasers based on quantum wells, an advantage of the proposed type of laser is a lower pumping flux, due to the longer carrier lifetime in quantum dots, and also that both surface and edge emission are possible. The appropriate waveguide and cavity designs are presented, and by comparing the calculated values of the gain with the estimated losses, lasing is predicted even at room temperature for all the quantum dots investigated.

A microscopic model of quantum well infrared photodetectors (QWIPs) is presented, based on quantum scattering rate equation approach. The macroscopic parameters and figures of merit such as current density, responsivity, capture probability and drift velocity, can be estimated directly from a first principles calculation. For conventional GaAs/AlGaAs QWIP device, a very good agreement was found between the calculated and the measured data taken from the literature. The model is general and can be applied to any other material system or QWIP structure.

The optical gain in the active region of quantum cascade laser in an external magnetic field is analyzed. When the magnetic field is applied in the direction perpendicular to the plane of the layers, electron dispersion is broken into series of discrete Landau levels. This additional confinement strongly modifies the lifetime of electrons in the upper state of the laser transition, which is controlled by electron-phonon scattering. Landau levels are magnetically tuneable and, depending on their configuration, phonon emission is either inhibited or resonantly enhanced. This translates into a strong modulation of the population inversion, and consequently of the optical gain by varying the magnetic field. Numerical results are presented for a structure previously considered by Smirnov et al. [Phys. Rev B 66 (2002) 125317] which is designed to emit radiation at λ~11.4µm, with the magnetic field varied in the range 10-60T. The effects of band nonparabolicity are taken into account in this model.

In this paper a procedure for the global optimization of mid-infrared GaAs/AlGaAs quantum cascade lasers that relies on the method of simulated annealing is presented. We propose a double longitudinal optical phonon resonance design obtained via a ladder of three states, with subsequent pairs separated by optical phonon energy. Addition of an extra level decreases the lower laser level population by enabling an efficient extraction into the injector region. The output characteristics of the optimized structures are calculated using the full self–consistent rate equation model, which includes all of the relevant scattering mechanisms. We also presented the experimentally measured output characteristics of an initial device, which are in agreement with the numerically calculated values, confirming the good design capabilities of the applied procedure.

Properly designed waveguides are of great importance for efficient operation of mid-infrared and tera-hertz quantum cascade semiconductor laser. Such waveguides comprise layers with suitably chosen doping, which determines refractive index, which enables bound modes to exist. It is of great interest to tailor the structure so to get minimal losses and maximal modal overlap with the active (gain) layer. In this paper we use the analogy between the Schrodinger and Helmholtz equations, and employ supersymmetric quantum mechanics, extended into the complex domain, in order to vary refractive index profile in search for better waveguide design. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Analysis of carrier transport in cascaded GaAs/AlGaAs, p-Si/SiGe and GaN/AlGaN terahertz structures is performed using self-consistent rate equations with kinetic energy-balance simulations. The electron transport in n-type structures is simulated using standard envelope function and nonparabolic effective mass approximations with all relevant electron–electron and electron–optical phonon scattering. In the case of GaAs-based structures the model is extended to include ionized impurity and interface roughness scattering as well. The hole subband structure in the p-type device is calculated using the 6 × 6 k p model, and then used to find carrier relaxation rates due to the alloy disorder, acoustic and optical phonon scattering, as well as hole–hole scattering. The simulation accounts for the in-plane k-space anisotropy of both the hole subband structure and the scattering rates. Results are presented for the original GaAs/AlGaAs THz quantum cascade laser and prototypes of GaN/AlGaN and Si/SiGe THz quantum cascade structures.

A systematic design of a GaN/AlGaN active region for a three-level intersubband laser emitting in the near-infrared is presented. The proposed system is based on a double coupled quantum well structure with the laser level energy separation corresponding to 1.55 µm (ΔE32~800 meV), a wavelength important for optical communications. The lower laser level depopulates mostly via longitudinal optical phonon resonance, requiring ΔE21~90 meV. The model for scattering rate calculations, including many-body carrier screening effects, was first tested by comparison with recent experimental measurements in GaN/AlGaN quantum wells, and a very good agreement was obtained. It was then employed to calculate the gain coefficient, a commonly used figure of merit for three level laser systems, and this in turn allowed the optimal structural parameters of GaN/AlGaN quantum well to be found.

The design of ZnCdSe-ZnMnSe-based quantum wells is considered, in order to obtain a large shift of the peak absorption wavelength in the far infrared range, due to a giant Zeeman splitting with magnetic field, while maintaining a reasonably large value of peak absorption. A triple quantum-well structure with a suitable choice of parameters has been found to satisfy such requirements. A maximal tuning range between 14.6 and 34.7 meV is obtained, when the magnetic field varies from zero to 5 T, so the wavelength of the absorbed radiation decreases from 85.2 to 35.7 /spl mu/m with absorption up to 1.25% at low temperatures. These structures might form the basis for magnetic field tunable photodetectors and quantum cascade lasers in the terahertz range.

A fully quantum mechanical model for electron transport in quantum well infrared photodetectors is\ud presented, based on a self-consistent solution of the coupled rate equations. The important macroscopic\ud parameters like current density, responsivity and capture probability can be estimated directly from this\ud first principles calculation. The applicability of the model was tested by comparison with experimental\ud measurements from a GaAs/AlGaAs device, and good agreement was found. The model is general and can\ud be applied to any other material system or QWIP design.

A method is proposed for finding optimal AlGaN-based quantum well profiles with respect to specified intersubband optical properties, e.g. nonlinear optical susceptibilities at resonance. It is based on applying the isospectral transformations (supersymmetric and coordinate transform) to an initial Hamiltonian in order to generate a family of parameters of controlled potentials, accompanied by position-dependent effective mass. By varying those control parameters, one call change the values of dipole matrix elements relevant for a particular effect, while energy levels, once obtained in the initial potential, remain unchanged. The internal electrostatic field, originating from piezoelectric and spontaneous polarization in the nitride alloy, has been taken into account. The use of the method is illustrated by maximizing second order nonlinear optical susceptibilities at wavelengths in the near infrared spectral range, and optimal smooth structures are eventually discretized to make their fabrication easier.

We present a Raman scattering study of two GaN/AlxGa1−xN multiple quantum well (MQW) structures with x = 0.3 and x = 0.4 and well widths of 6 nm and 4 nm respectively, they were nominally undoped but are expected to contain a low-density, n-type, carrier population due to the residual donors in the barriers. Polarization dependent Raman scattering was performed at room temperature, strong scattering due to intersubband transitions (ISBT's) in the GaN quantum wells was observed from both the e1–e2 and e1–e3 transitions for both samples. The first sample has an e1–e2 transition at 300 meV and e1–e3 at 480 meV whilst the second sample has an e1–e2 transition at 360 meV and e1–e3 at 580 meV. The full width half maximum of the Raman peaks is lower than that reported in infra-red absorption for similar, heavily doped, MQW structures. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A method is proposed for the design and optimization of structural parameters of GaN-AlGaN Bragg-confined structures with respect to peak intersubband absorption from the ground to the first excited state,1 /spl rarr/ 2 electronic transition, in the near infrared spectral range. An above-the-barrier bound state was used to extend the range of transition energies above the values available in conventional quantum wells. Intrinsic polarization fields and nonparabolicity effects were taken into account. The selection of optimal parameters, maximizing the absorption at wavelengths of 1.55 and 1.3 /spl mu/m, was performed by using a simulated annealing algorithm, and optimal structures with infinite superlattices as confinement regions were thus designed. These optimal parameters were then used to set realistic, finite structures with a small number of layers, the performance of which was re-evaluated by solving the Schrodinger-Poisson equation self-consistently for a few different levels and profiles of doping.

Improved designs of surface plasmon waveguides for use in GaAs/AlGaAs terahertz quantum cascade lasers are presented. Modal losses and confinement factors are calculated for TM modes in metal-variably doped multilayer semiconductor and metal-intermetallic compound layer clad structures and compared with those obtained in recently realized metal-highly doped semiconductor clad layer structures. Considerable improvements of the mode confinement factors are predicted, and guidelines for choosing the confinement layer parameters are given.

A procedure is proposed for finding the globally optimal semiconductor quantum well profile in respect to the second order intersubband susceptibilities at resonance. It relies on the combined use of simulated annealing and variational calculus (i.e. the optimal control theory), and effectively performs a free variation of the quantum well (QW) profile, searching for the global optimum. The proposed method does not depend on the choice of input ("seed") potential and includes the variation of a continuous function instead of a set of scalar parameters, so it should lead to the globally optimal QW profile for a particular application, unconstrained to any particular class of functional forms.

We explore the single-band envelope-function model in the entire Brillouin zone for electronic structure calculation of nanostructures that involve semiconductors with one or more significant valleys in the conduction band. Comparison with the numerically more demanding empirical pseudopotential calculation shows reasonably good accuracy of the method described here, which correctly predicts the quantized state energies, splittings in multivalley semiconductor nanostructures, as well as the intersubband transition derived optical properties. This makes it potentially useful for modeling nanostructures and nanodevices based on Si and other materials.

A procedure is proposed for finding the optimal profile of a semiconductor quantum well to obtain maximal value of the optical rectification coefficient. It relies on the variational calculus, i.e. the optimal control theory, combined with the method of simulated annealing, and should deliver a globally optimized profile, unconstrained to any particular class of functional forms. For the purpose of illustration, the procedure is applied to the optimized design of AlxGa1−xAs based quantum wells, for rectification of ω = 116 meV (CO2 laser) radiation. The optimal smooth profile may eventually be discretized to make the structure fabrication easier.

The design of rectangular GaN/AlGaN quantum wells for maximal intersubband absorption in the near infrared wavelength range 1.3–2 μm, on the 0→1 electronic transition is considered, taking into account the effects of internal polarization fields and nonparabolicity. The nonlinear optimization method based on solving a system of nonlinear equations is employed in finding the structural parameters which give the maximal dipole matrix element while keeping the absorption peak at the desired wavelength.

There is considerable interest in the interrogation of biological tissue at terahertz (THz) frequencies, largely due to the contrast in the optical properties of different biological tissues which occur in this electro-magnetic radiation band. Of particular interest are THz biomedical images, which have the potential to highlight different information than those acquired in other frequency bands, thereby providing an augmented picture of biological structures. In this work, we demonstrate the feasibility of an interferometric biological imaging technique using a THz quantum cascade laser (QCL) operating at 2.59 THz to perform coherent imaging of porcine tissue samples. We show the potential of this new THz biomedical imaging technique for in vivo studies, by virtue of its reflection geometry and useful tissue penetration depth enabled by the large THz powers emitted by the quantum cascade laser used in this work. The observed clustering of interferometric tissue signatures opens a pathway towards automatic techniques for the discrimination of healthy tissue types for the study of normal physiology and possible therapeutic approaches. Refereed/Peer-reviewed

In this paper, we introduce the self-mixing phenomenon in terahertz quantum cascade lasers (THz QCLs) and present recent advancements in the development of coherent THz imaging and sensing systems that exploit the self-mixing effect. We describe an imaging method which utilises the interferometric nature of optical feedback in a THz QCL to employ it as a homodyning transceiver. This results in a highly sensitive and compact scheme. Due to the inherently low penetration depth of THz radiation in hydrated biological tissue, imaging of superficial skin is an ideal application for this technique. We present results for imaging of excised skin tissue, showing high-contrast between different tissue types and pathologies.

Recently, we demonstrated an interferometric materials analysis scheme at terahertz frequencies based on the self-mixing effect in terahertz quantum cascade lasers. Here, we examine the impact of variations in laser operating parameters, target characteristics, laser-target system properties, and the quality calibration standards on our scheme. We show that our coherent scheme is intrinsically most sensitive to fluctuations in interferometric phase, arising primarily from variations in external cavity length. Moreover we demonstrate that the smallest experimental uncertainties in the determination of extinction coefficients are expected for lossy materials.