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Terahertz (THz) quantum-cascade lasers (QCLs) are currently unparalleled for high-resolution spectroscopy of very sharp absorption lines (linewidths below 100 MHz) in the range between 2 and 5.4 THz. Since the frequency range accessible by a single QCL is determined by its typically very limited tuning range, a particular QCL has to be fabricated for each specific application. We quantitatively analyze the frequencies of the modes in THz QCLs with a Fabry–Pérot resonator as a function of its length taking into account waveguide dispersion. Based on these results, we develop a process based on mechanical polishing of the front facet to adjust the emission frequency with a precision of 1 GHz. The demonstrated process makes it possible to reliably fabricate THz QCLs for the spectroscopy of very sharp absorption lines. [ABSTRACT FROM AUTHOR]

We have studied the intrinsic frequency tuning of several terahertz (THz) quantum-cascade lasers (QCLs) based on a hybrid design. With increasing driving current, most QCLs exhibit either a redshift or a blueshift, while some QCLs show both. An oscillator model describes the observed behavior. The accuracy of the model is affected by the complexity of the current-dependent gain spectra of the THz QCLs. Numerical simulations demonstrate a cavity pulling effect insofar as the tuning behavior depends on the resonator losses and on the width of the gain spectra. [ABSTRACT FROM AUTHOR]

We have investigated high-performance GaAs/AlAs terahertz (THz) quantum-cascade lasers (QCLs) with respect to the long-term stability of their operating parameters. The output power of lasers that contain an additional, thick AlAs refractive-index contrast layer underneath the cascade structure decreases after three months by about 35%. The deterioration of these lasers is attributed to the oxidation processes in this contrast layer starting from the facets. However, GaAs/AlAs THz QCLs with an Al0.9Ga0.1As refractive-index contrast layer exhibit long-term stability of the operating parameters over many years even when they are exposed to atmospheric conditions. Therefore, these lasers are promising high-power radiation sources in the terahertz spectral region for commercial applications. [ABSTRACT FROM AUTHOR]

We present scattering rates for electrons at longitudinal optical phonons within a model completely formulated in the Fourier domain. The total intersubband scattering rates are obtained by averaging over the intrasubband electron distributions. The rates consist of the Fourier components of the electron wave functions and a contribution depending only on the intersubband energies and the intrasubband carrier distributions. The energy-dependent part can be reproduced by a rational function, which allows for the separation of the scattering rates into a dipole-like contribution, an overlap-like contribution, and a contribution which can be neglected for low and intermediate carrier densities of the initial subband. For a balance between accuracy and computation time, the number of Fourier components can be adjusted. This approach facilitates an efficient design of complex heterostructures with realistic, temperature- and carrier density-dependent rates. [ABSTRACT FROM AUTHOR]

A phenomenoμogicaμ scattering-rate modeμ introduced for terahertz quantum cascade μasers (QCLs) [Schrottke et aμ., Semicond. Sci. Technoμ. 25, 045025 (2010)] is extended to mid-infrared (MIR) QCLs by incμuding the energy dependence of the intersubband scattering rates for energies higher than the μongitudinaμ opticaμ phonon energy. This energy dependence is obtained from a phenomenoμogicaμ fit of the intersubband scattering rates based on pubμished μifetimes of a number of MIR QCLs. In our approach, the totaμ intersubband scattering rate is written as the product of the exchange integraμ for the squared moduμi of the enveμope functions and a phenomenoμogicaμ factor that depends onμy on the transition energy. Using the modeμ to caμcuμate scattering rates and imposing periodicaμ boundary conditions on the current density, we find a good agreement with μow-temperature data for current-voμtage, power-current, and energy-photon fμux characteristics for a QCL emitting at 5.2 μm. [ABSTRACT FROM AUTHOR]

We present a self-consistent model for carrier transport in periodic semiconductor heterostructures completely formulated in the Fourier domain. In addition to the Hamiltonian for the layer system, all expressions for the scattering rates, the applied electric field, and the carrier distribution are treated in reciprocal space. In particular, for slowly converging cases of the self-consistent solution of the Schrödinger and Poisson equations, numerous transformations between real and reciprocal space during the iterations can be avoided by using the presented method, which results in a significant reduction of computation time. Therefore, it is a promising tool for the simulation and efficient design of complex heterostructures such as terahertz quantum-cascade lasers. [ABSTRACT FROM AUTHOR]

We apply a rate equation model in order to simulate the measured change of the reflectance of an epitaxially grown Ge2Te2Sb5 (GST) film during optically induced switching between the amorphous and crystalline phases of the material. The amorphization and the re-crystallization process are considered to exhibit different switching rates, which depend on the actual value of the energy fluence of the applied laser pulse. The measured time dependence of the reflectance of the GST film as a measure of its degree of crystallinity is qualitatively reproduced by the rate equation model demonstrating that both processes can occur simultaneously during the switching process, but with different rates. [ABSTRACT FROM AUTHOR]

We investigate the effect of mid-infrared (MIR) pumping on the transport properties of GaAs/(Al,Ga)As terahertz (THz) quantum lasers (TQLs), which rely on quantum coherence effects of intersubband transitions. Aiming at THz lasing at elevated temperatures, we extend the concept of THz gain with and without population inversion of a single, MIR-pumped, electrically driven THz stage proposed by Waldmueller et al. [Phys. Rev. Lett. 99, 117401 (2007)] to an entire TQL. However, experiments using a CO2 as well as a free-electron laser and numerical simulations show that this resonant MIR pumping causes a negative differential conductivity (NDC) in addition to the NDC caused by sequential tunneling. Lasing of these TQLs is prevented by the formation of electric-field domains below the resonance field strength for gain of each single THz stage. [ABSTRACT FROM AUTHOR]

The slope efficiency is, in addition to the threshold current density and operating voltage, a decisive target value for the operation of quantum-cascade lasers (QCLs) in order to achieve an optimal total efficiency for the conversion of electrical input power into optical output power. We analyze the light-current characteristics for a set of similar, high-power, bound-to-continuum terahertz QCLs. The calculated internal slope efficiency shows a clear dependence on the height and thickness of the barriers. In contrast to the simulations, we found experimentally a significant difference in the threshold current densities and slope efficiencies for QCLs with nominally identical layer structures, which we mainly attribute to different line broadening. [ABSTRACT FROM AUTHOR]

We study the anisotropic in-plane strain in M-plane GaN films on LiAlO2 by using photoreflectance, photoluminescence, Raman, and time-resolved pump-and-probe spectroscopy. We find a partial and anisotropic relaxation of these highly strained films under intense optical pumping. The strain relaxation is observed as a redshift of the E2-Raman line and as a redshift of the fundamental interband transition energies in the photoreflectance spectra. The photoluminescence intensity in the exposed areas of the sample is significantly reduced compared to the one for unexposed areas. This observation suggests that the strain relaxation is probably due to the introduction of defects, which act as nonradiative recombination centers. [ABSTRACT FROM AUTHOR]

The gain coefficient of several GaAs/AlxGa1-xAs quantum-cascade lasers (QCLs) with a nominal Al content of x=0.45 has been calculated as a function of transition energy and electric field strength. We solve the Schrödinger and Poisson equations self-consistently in the framework of a linear scattering-rate model with periodic boundary conditions. The actual layer thicknesses as well as the Al content of the barriers have been obtained from x-ray diffraction. The calculated gain characteristics exhibit a large range of transition energies and a corresponding range of possible operating field strengths. These results may provide an explanation for the observation of a rather large range of the lasing energies for QCLs with nominally identical cascade structures. The comparison with the experimental values of the lasing energy reveals that most of the lasers operate below the designed field strength. The analysis of the operating field strengths and the threshold current densities indicates larger losses for higher field strengths probably due to leakage currents. [ABSTRACT FROM AUTHOR]

We discuss the mechanisms for negative differential conductance (NDC) and bistable behavior observed in the current-density–electric-field (j-F) characteristics of undoped GaAs/Al0.45Ga0.55As quantum-cascade structures (QCSs). While the j-F characteristic of a QCS with a single period exhibits NDC without bistability, NDC and a bistable behavior are observed for a QCS with 20 periods of the same design. Calculations using a scattering-rate model neglecting any field inhomogeneities show that the interplay of resonant tunneling between the states in the QCS with resonant scattering of longitudinal optical phonons can lead to NDC in both structures. However, the bistable behavior in the QCS with 20 periods can only be explained if an inhomogeneous field distribution due to charge accumulation is taken into account in addition to the NDC. The abrupt decrease of the current density at the field strength of the bistability is attributed to a rapid decrease of the accumulated charge, which is confirmed by electric-field-dependent photoluminescence spectroscopy. [ABSTRACT FROM AUTHOR]

The population of the laser levels in undoped GaAs/AlxGa1-xAs quantum-cascade structures (QCSs) is investigated by interband photoluminescence spectroscopy. We compare similar QCSs with different barrier heights (x=0.33 and x=0.45), for which the calculated population ratios ρpca are equal. While the experimental value ρpme for x=0.45 agrees with the theoretical one, ρpme for x=0.33 is much smaller than ρpca. At the same time, the threshold current densities jth are significantly smaller for x=0.45 than for x=0.33. In the framework of a linear rate equation model, we estimate the effect of the experimentally observed reduction of the population ratio on jth. We show that the increased value of jth for x=0.33 cannot only be attributed to a larger leakage current due to the lower barriers, but also to the reduced population ratio. [ABSTRACT FROM AUTHOR]

We have investigated negative differential conductivity (NDC) and laser level population in undoped GaAs/(Al,Ga)As quantum-cascade structures using current–voltage characteristics and interband photoluminescence spectroscopy. While for both GaAs/Al0.33Ga0.67As and GaAs/Al0.45Ga0.55As structures a strong, bistable NDC is observed, a weaker NDC without bistability appears only in the GaAs/Al0.45Ga0.55As structure, which is due to the resonant coupling between injector and upper laser level. Although the bistable NDC is connected with a significant population of the laser levels, it cannot be explained by resonant coupling between electronic states in the active region. We believe that the bistable NDC is caused by an interplay of resonant coupling effects within the injector with the carrier redistribution in the vicinity of the optically active region. Furthermore, a still unidentified state, which exhibits a strong photoluminescence signal, may play an important role for the bistability. [ABSTRACT FROM AUTHOR]

GaAs/Al0.33Ga0.67As quantum-cascade lasers with plasmon-assisted waveguides exhibit a decreasing threshold current density jth with increasing wave number ν0 of the laser line, which changes as a function of the injector doping density. We have developed an analytical approach based on the effective dielectric tensor component for the p-polarized light emitted from a quantum-cascade laser, which explains the observed dependence of jth(ν0) in terms of losses due to free-carrier absorption predominantly in the doped waveguides αWG(ν0). A contribution to the losses by free-carrier absorption in the quantum-cascade structure itself and subsequently to jth can be neglected except for very high injector doping densities. The calculated values for αWG(ν0) are in good agreement with the experimental data. Our approach quantitatively predicts the observed decrease of jth from 17 to 7 kA cm-2 with increasing ν0 between 900 and 1100 cm-1. In addition to achieving a direct physical insight into the influence of free-carrier absorption on the laser performance, the proposed analytical approach provides a simple tool for the determination of the waveguide losses for any quantum-cascade laser without adopting a numerical solver. [ABSTRACT FROM AUTHOR]

We have investigated the current density-electric field ( j-F) characteristics and photoluminescence (PL) spectra of several GaAs/(Al,Ga)As quantum-cascade structures (QCS's) up to the threshold field strength to obtain information on the carrier distribution and field inhomogeneities. Due to the complex structure of each single period of the cascaded device, the dark j-F characteristics of undoped QCS's exhibit distinct current maxima, resulting in regions of negative differential conductivity, while a plateaulike feature appears under illumination. For doped QCS's, a plateau with sawtoothlike structures appears, which are well known from doped, weakly coupled superlattices under electric-field-domain formation. However, in contrast to the splitting of the PL line that is observed for doped, weakly coupled superlattices, no splitting has been observed in the doped QCS's. The j-F characteristic of an undoped QCS with thicker barriers than the original QCS exhibits a much more pronounced current maximum in the dark. Under illumination, the j-F characteristic of this structure shows a clear plateaulike feature, which contains additional fine structure indicating the existence of electric-field inhomogeneities within each period due to the separation of electrons and holes. The PL spectra of the undoped and doped QCS's show emission from only the widest quantum well (QW) because the holes accumulate very quickly in this QW resulting from the strong coupling within the entire QCS. However, in the PL spectra of the undoped QCS with thicker barriers, PL lines from several quantum wells can be identified. The carrier distribution process in QCS's is also discussed as a function of the applied electric field and compared for the different structures. [ABSTRACT FROM AUTHOR]

We study the quantum efficiency (η) and transition energy (E[sub t]) as a function of excitation density and temperature in (In,Ga)N/GaN multiple quantum wells (MQWs) fabricated by molecular-beam epitaxy (MBE) and metal-organic chemical-vapor deposition (MOCVD), as well as in an MBE-grown GaN/(Al,Ga)N MQW. A method based on cathodoluminescence spectroscopy is proposed to be suitable for a reproducible measurement of the power dependence of η and E[sub t]. The experimental results are fit to a recently developed model allowing for a distinction of localization and electric-field effects for η and E[sub t], as well as for the extraction of the localization energy, density of localization centers, and radiative recombination rate of localized excitons. In the (In,Ga)N/GaN MQWs grown by MBE and MOCVD, we found a value of the localization energy of 34 and 100 meV, respectively. In the MBE-grown GaN/(Al,Ga)N MQW, the exciton recombination is dominated by quasifree excitons even at low temperatures. [ABSTRACT FROM AUTHOR]

Photoreflectance excitation (PRE) spectroscopy is shown to be a suitable method to investigate semiconductor materials in the near-infrared to near-ultraviolet spectral range. The investigation of a GaAs and a GaN bulk film demonstrates the applicability of this method even up to room temperature. For both materials, the line shape analysis suggest a nonlinear relation between the PRE and the absorption line shapes, leading to a reduction of the excitonic peak height in the PRE spectra compared to the absorption spectra. To keep this reduction as small as possible, the pump power should be on the order of the probe power or smaller. [ABSTRACT FROM AUTHOR]

The amplitude and the signal-to-noise ratio of photoreflectance (PR) spectra are experimentally and theoretically investigated as a function of the pump and probe intensity. The model calculations of the PR amplitude and the signal-to-noise ratio based on a simple transport model taking the shot noise of the photodetector as the only noise source confirm the experimentally observed dependencies. Increasing the probe light intensity leads to a decrease of the absolute PR background noise. At the same time, the PR amplitude decreases. This may, in particular for comparable probe and pump intensities, result in a decrease of the signal-to-noise ratio. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

For transport studies on semiconductor superlattices under optical excitation, samples with highly Si-doped (Al,Ga)As contact layers are used due to their optical transparency. These samples exhibit a giant increase of the resistance at temperatures below 200 K. Likewise, the current plateau in the I–V characteristics, which contains the region of current self-oscillations, is shifted towards higher voltages. However, the oscillation frequencies remain unchanged. The current decreases during cooling by as much as seven orders of magnitude. The giant resistance can be compensated for at low temperatures by applying a high dc voltage or by weak illumination. The optically induced current transients are shown to be controlled by thermally activated processes. On the basis of the activation energies derived, it is suggested that the dramatic resistance changes are mainly due to deep donors, i.e., well-known DX centers, in the (Al,Ga)As contact layers. The effects are negligible, if GaAs contact layers are used. Possible DX centers at the AlAs/GaAs interfaces inside the superlattice structure can be therefore neglected. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The photoreflectance signal from GaN films is sensitive to the ambient medium. A large decrease in the photoreflectance amplitude is observed, when the ambient medium is changed from air to vacuum. This effect is attributed to ultraviolet-light-induced desorption of oxygen from the sample surface leading to a reduction of the surface barrier height. The effect is absent, when a thin Ti layer is deposited on top of the GaN film. A simple model is used to demonstrate that the surface photovoltage can be strongly reduced with a decrease of the surface barrier height. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Provides information on a study which examined the temperatures and excitation intensities in gallium arsenide-aluminum arsenide superlattices on gallium arsenide substrates. Methods; Results and discussion; Conclusion.

Presents a study that examined the effect of exciton diffusion and localization on the cathodoluminescence (CL) intensity distribution in a single quantum well and a multiple quantum well structure prepared by growth interrupted molecular beam epitaxy. Use of scanning electron microscope; Background on CL investigations of quantum well structures.

A super-superlattice structure containing a conventional superlattice, a graded-gap superlattice, and a wide quantum well in its unit cell is analyzed. It is designed in such a way that alternating high- and low-electric-field domains appear due to the presence of positive and negative space charges. The positive space charge is formed by ionized donors, while electrons, which are trapped in the wide quantum well, provide the negative space charge. Since the low-field domain spans the conventional superlattice and the high-field domain is located in the graded-gap superlattice, the flatband condition can be simultaneously achieved for the two superlattices. The self-consistent solution of the Poisson and Schrödinger equations using a simplified scattering-rate approach demonstrates that the conventional superlattice exhibits an inversion of the intra-miniband population. Such structures may be useful for terahertz lasers, since the large dipole matrix element of the intra-miniband transitions allow for a rather low doping density and hence a reduction of scattering processes. Furthermore, they permit the study of superlattices under flatband conditions with a nonequilibrium population, which is achieved without optical excitation. [ABSTRACT FROM AUTHOR]

We have determined the interface profiles of GaAs/Al0.25Ga0.75As and GaAs/AlAs terahertz quantum-cascade laser structures using transmission electron microscopy (TEM). The experimental composition profiles are directly extracted from the analysis of the diffracted intensity distribution of g002 dark-field TEM images. Simulated composition profiles, which have been obtained using a Fourier-transform-based model, are in remarkably good agreement with the experimental results so that the determined interface parameter can be used for designing terahertz quantum-cascade lasers. Due to the interface grading, these actual profiles reveal a significantly lower aluminum content of the Al0.25Ga0.75As and AlAs barriers than the nominal values. [ABSTRACT FROM AUTHOR]

We have investigated a [1120]-oriented A-plane GaN film on R-plane sapphire, where the c axis of GaN lies in the film plane, by polarized photoreflectance (PR) spectroscopy. Near the fundamental energy gap of GaN, the PR spectrum with the probe light polarized perpendicular to the c axis exhibits one feature corresponding to a single transition labeled T1. For a polarization parallel to c axis, two different features labeled T2 and T3 are observed at higher energies than the transition T1. In order to explain the origin of these three features, we compare the measured energies with calculations of the transition energies and oscillator strengths of the three band-to-band transitions of GaN near its fundamental gap for an anisotropic in-plane strain in the A plane. The analysis shows that the observed transition energies and polarization properties of the three transitions can be explained by the presence of an overall compressive, anisotropic in-plane strain in the film. [ABSTRACT FROM AUTHOR]

We analyze experimental and simulated photoreflectance spectra of excitonic transitions in semiconductors with a redefined set of fitting parameters in Aspnes' line shape function. The redefinition, based on a comparison with the first derivative of a Gaussian, allows one to directly relate these parameters to the observed experimental spectrum, making them independent of the exponent m. None of the original information, including the underlying physics involved as determined by m, is lost in this process. The physical significance of the fitting parameters is explained. Their usefulness lie in making line shape fitting easier and the possibility of comparing different sets of samples exhibiting inhomogeneities with regard to their broadening, oscillator strengths, and hence, quality. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Focuses on a study which investigated Wannier-Stark localization in an asymmetric double-well superlattice. Effect of electric fields on semiconductor nanostructures; Details on the sample structure; Information on stark ladders.

Presents a study that investigated the temperature dependence of the cathodoluminescence intensity from a gallium arsenide-AlGaAs single quantum well single quantun well prepared by growth interrupted molecular beam epitaxy. Function of photoluminescence and cathodoluminescence; Cathodoluminescence images of the quantum well; Function of cathodoluminescence in heterostructures like quantum wells.

We demonstrate the feasibility of high-resolution terahertz gas spectroscopy based on the external optical feedback effect in a quantum-cascade laser. Tuning the frequency of the quantum-cascade laser across a molecular absorption line of CH3OH leads to a reduction of the optical feedback, which can be detected by monitoring the voltage across the quantum-cascade laser. This method provides a high spectral resolution of ν/Δν=106ν/Δν=106 and a sensitivity comparable to that obtained with a cryogenically cooled Ge:Ga photoconductive detector. [ABSTRACT FROM AUTHOR]

We report on a fast self-mixing approach for real-time, coherent terahertz imaging based on a quantum-cascade laser and a scanning mirror. Due to a fast deflection of the terahertz beam, images with frame rates up to several Hz are obtained, eventually limited by the mechanical inertia of the employed scanning mirror. A phase modulation technique allows for the separation of the amplitude and phase information without the necessity of parameter fitting routines. We further demonstrate the potential for transmission imaging. [ABSTRACT FROM AUTHOR]

The far-field distribution of the emission intensity of terahertz (THz) quantumcascade lasers (QCLs) frequently exhibits multiple lobes instead of a single-lobed Gaussian distribution. We show that such multiple lobes can result from selfinterference related to the typically large beam divergence of THz QCLs and the presence of an inevitable cryogenic operation environment including optical windows. We develop a quantitative model to reproduce the multiple lobes. We also demonstrate how a single-lobed far-field distribution can be achieved. [ABSTRACT FROM AUTHOR]

We have realized GaAs/AlAs quantum-cascade lasers operating at 4.75 THz exhibiting more than three times higher wall plug efficiencies than GaAs/Al0.25Ga0.75As lasers with an almost identical design. At the same time, the threshold current density at 10K is reduced from about 350 A/cm2 for the GaAs/Al0.25Ga0.75As laser to about 120 A/cm2 for the GaAs/AlAs laser. Substituting AlAs for Al0.25Ga0.75As barriers leads to a larger energy separation between the subbands reducing the probability for leakage currents through parasitic states and for reabsorption of the laser light. The higher barriers allow for a shift of the quasi-continuum of states to much higher energies. The use of a binary barrier material may also reduce detrimental effects due to the expected composition fluctuations in ternary alloys. [ABSTRACT FROM AUTHOR]

We report on the observation of an approximately linear reduction in the maximum operating temperature with an increasing emission frequency for terahertz quantum-cascade lasers between 4.2 and 5.4 THz. These lasers are based on the same design type, but vary in period length and barrier height for the cascade structure. The sample emitting at the highest frequency around 5.4 THz can be operated in pulsed mode up to 56 K. We identify an additional relaxation channel for electrons by longitudinal optical phonon scattering from the upper to the lower laser level and increasing optical losses toward higher frequencies as major processes, leading to the observed temperature behavior. [ABSTRACT FROM AUTHOR]

An efficient model is proposed to evaluate the impact of interface grading on the properties of semiconductor heterostructures. In the plane-wave approximation, the interface grading is taken into account by simply multiplying the Fourier components of the potential by a Gaussian function, which results only in a very small increase of the computation time. We show that the interface grading may affect the transition energies, the field strength for resonant coupling of subbands, and even the miniband formation in complex systems such as quantum-cascade lasers. This model provides a convenient tool for the incorporation of interface grading into the design of heterostructures. [ABSTRACT FROM AUTHOR]

A low-temperature scanning near-field optical microscope has been adapted to achieve a high topographical sensitivity. This setup allows us to resolve morphological features on semiconductor surfaces of monolayer height, which is demonstrated for two different material systems, monolayer steps on an epitaxial GaN film and growth islands in a GaAs single quantum well (SQW) structure. Complementary photoluminescence measurements for the SQW structure reveal the potential of combining high-resolution topography with spatially resolved optical spectroscopy for investigating semiconductor nanostructures. [ABSTRACT FROM AUTHOR]

We investigated GaAs/(Al,Ga)As structures for mid-infrared-pumped, electrically driven terahertz quantum lasers (TQLs) based on the concept of Waldmueller et al. [Phys. Rev. Lett. 99, 117401 (2007)]. We developed a design with the gain maximum appearing at a relatively low field strength of 6.5 kV/cm. The present TQL design avoids pumping-induced current instabilities, which prevented a previous design from operating at the required field strength as discussed by Giehler et al. [J. Appl. Phys. 110, 103104 (2011)]. We applied photocurrent (PC) spectroscopy to examine the pumping process. The measured PC spectra are found to be in agreement with simulated spectra in the vicinity of the desired operating field strength. We demonstrate that electric-field domain formation can be suppressed using a continuous-wave CO2 laser as the pump. [ABSTRACT FROM AUTHOR]

We investigate the effect of stimulated emission on the transport characteristics of terahertz quantum-cascade lasers operating in a frequency range between 4.3 and 4.6 THz. The impact of stimulated emission is varied by changing the mirror losses via damaging one or two facets. In the case of voltage-driven measurements, a reduction in the current density near the onset of stimulated emission is observed for the facet-damaged lasers as compared to the original one. The measurements are in qualitative agreement with results of numerical simulations including the effect of stimulated emission. [ABSTRACT FROM AUTHOR]

We report on a terahertz absorption spectrometer, which combines a grating monochromator, a quantum-cascade laser (QCL), and a microbolometer camera. The emission modes of the laser are spectrally resolved by the monochromator and imaged onto the camera. An absorption cell is placed between the QCL and the monochromator, and the absorption spectrum of methanol around 3.4 THz is measured by integrating simultaneously the signal of each of its Fabry-Pérot modes as a function of the laser driving current. The frequency coverage of the spectrometer is about 20 GHz. [ABSTRACT FROM AUTHOR]

For the full implementation of spintronic circuits, it is necessary to transmit spin information from one device to another. Electrons in semiconductors often suffer from high spin relaxation rates, making electrical transport of spin information highly inefficient. Here, we propose optical transport of spin information as an alternative. We demonstrate that the spin information associated with electrons injected from Co2FeSi and Fe layers into the quantum wells of spin light emitting diodes (spin-LEDs) can be transported optically in the form of circularly polarized light and deciphered electrically via the magnetic field dependence of the photocurrent in a distant detector spin-LED. [ABSTRACT FROM AUTHOR]

Using metalorganic chemical vapor deposition, we have grown ZnO nanowires (NWs) on ZnO(0001) as well as on Al2O3(0001) and Si(111) substrates. The optical properties of these NWs are studied by continuous-wave and time-resolved photoluminescence spectroscopy. Both the spectral and the temporal characteristics of the NWs are found to be virtually identical despite the different substrates and the different morphology of the NWs. This study thus provides strong experimental evidence for the hypothesis that NWs remove the constraints introduced by the substrate. [ABSTRACT FROM AUTHOR]

We investigated the influence of the barrier composition on the performance of GaAs-based terahertz (THz) quantum-cascade lasers (QCLs). Based on a nine-quantum-well active region design for 3–4 THz emission, QCLs with an Al content of x=0.15 and x=0.25 in the AlxGa1-xAs barriers are compared. We found a significantly reduced threshold current density for QCLs with x=0.25 as compared to QCLs with x=0.15, which is due to a weaker coupling of the subband states. The maximum output power and operating temperature of such lasers are reduced due to the onset of negative differential resistance. [ABSTRACT FROM AUTHOR]

Polarized in-plane and cross-sectional Raman spectra have been used to determine the crystal orientation of m-plane GaN grown on γ-LiAlO2(100) (LAO) using a three-step metalorganic vapor phase epitaxy process. The epitaxial relationship is found to be GaN(1100)∥LAO(100) and GaN[1120]∥LAO[001]. However, the stripes on the GaN surface are oriented parallel to [0001], i.e., perpendicular to the one found on striated m-plane GaN surfaces in previous studies. This unusual orientation is attributed to the changes in the Ga adatom kinetics due to the presence of a 2-nm-thick interlayer observed at the GaN/LAO interface in transmission electron microscopy. [ABSTRACT FROM AUTHOR]

Ambipolar vertical diffusion of carriers generated in an Al0.3Ga0.7As barrier is investigated by cathodoluminescence (CL) spectroscopy in a system containing a sequence of GaAs-based quantum wells (QWs). The intensity distribution of the CL line scan exhibits a single exponential decay for the first QW of the sequence, reflecting a pure diffusion-limited transport. However, the CL line scans of the second, third, and fourth QWs are governed by diffusion only for large separations between the electron beam and the corresponding QW. For smaller distances, the CL intensity distribution is significantly influenced by the carrier capture into the intervening QWs. [ABSTRACT FROM AUTHOR]

We compare the spontaneous emission of C- and M-plane GaN quantum dots embedded in AlN. C-plane dots are characterized by an intense emission with an exceptionally long decay time up to room temperature. In contrast, M-plane dots exhibit a much weaker emission with a very short decay time. In addition, the emission of the C-plane dots temporally evolves on a timescale of seconds, while the emission of the M-plane dots is stable over time. These findings are correlated with the different growth mode and microstructure of C- and M-plane GaN quantum dots. [ABSTRACT FROM AUTHOR]

The authors present results of a perturbation theory study of the combined effects of composition and anisotropic in-plane strain on the optical polarization properties of the three interband transitions in the vicinity of the fundamental energy gap of wurtzite group-III-nitride alloy films, pseudomorphically grown on GaN substrates with nonpolar orientation such as M-plane GaN(1100). Valence band mixing induced by the anisotropic in-plane strain is shown to have a dramatic influence on the optical polarization properties. The results indicate that an increased efficiency of light emission in the visible spectral range can be achieved with compressively strained InxGa1-xN active layers. While AlxGa1-xN layers under tensile strain will exhibit a very poor light emission efficiency in the ultraviolet (UV) spectral range, efficient emission in the UV range can instead be achieved with InxAl1-xN films. These results also hold for alloy films on A-plane GaN(1120) substrates. [ABSTRACT FROM AUTHOR]

The authors investigate the in-plane optical polarization properties of [formula]-oriented (M-plane) GaN films on γ-LiAlO2(100) substrates by transmittance anisotropy spectroscopy (TAS). This technique is sensitive to the difference in the transmittance between light polarized parallel and perpendicular to the c axis of GaN, which for M-plane GaN lies in the film plane. The TAS spectrum exhibits a clear resonance in the vicinity of the fundamental bandgap. Simulations demonstrate that this resonance directly reflects the polarization-dependent shift of the bandgap. The zero crossings of the differential TAS spectrum are shown to be a measure for the polarization-dependent transition energies. [ABSTRACT FROM AUTHOR]

The authors present a photodetection configuration which allows for the determination of the optical in-plane polarization angle of the incident light beam with respect to a reference axis. The implementation for the ultraviolet spectral range is based on the intrinsic properties of M-plane GaN-based photodetectors on LiAlO2 substrates. This configuration can provide an infinite rejection ratio between the photodetector signal for light polarized perpendicular and parallel to the direction of maximum sensitivity. In addition, this configuration exhibits a bandpass response independent of the polarization state of the incident light beam with a bandwidth below 10 nm for M-plane GaN-based photodetectors. [ABSTRACT FROM AUTHOR]

The spectra of two-sectioned, coupled-cavity (TSCC) terahertz quantum-cascade lasers exhibit a number of longitudinal modes, which are periodically suppressed due to interference effects. The number of suppressed modes is equal to the ratio of the lengths of the two subcavities. At maximum laser output, the mode suppression disappears due to optical saturation, and a switching between TSCC and single-cavity-like mode features appears for certain delays between the current pulses through the individual subcavities. A transfer-matrix approach reproduces the mode spacings very well, but predicts a smaller modulation of the mode heights and does not describe the observed mode switching. [ABSTRACT FROM AUTHOR]

Using reflectance spectroscopy, the in-plane polarization behavior of unstrained C- and A-plane GaN films is experimentally investigated. While no in-plane polarization anisotropy is observed for all three band-gap related excitons (A, B, and C) in unstrained C-plane GaN films, the A exciton is completely linearly polarized perpendicular to the c axis in unstrained A-plane GaN films. However, the B and C excitons are only partially polarized. This observation is in excellent agreement with results based on band-structure calculations using the Bir-Pikus Hamiltonian for the wurtzite crystal structure. [ABSTRACT FROM AUTHOR]