H. T. Grahn

Lasing properties of GaAs/(Al,Ga)As quantum-cascade lasers as a function of injector doping density

The lasing properties of GaAs/Al0.33Ga0.67As quantum-cascade lasers are investigated as a function of injector doping concentration ns between 2×1011 and 1×1012 cm−2 per period. Lasing is observed for ns⩾3.5×1011 cm−2, with optimal lasing properties (minimum of the threshold current and maximum of the modified characteristic temperature) for nopt≈6×1011 cm−2. With increasing ns up to nopt, the lasing energy of 115 meV exhibits first a blueshift to 135 meV, followed by a redshift to 120 meV for higher doping levels. This shift of the lasing energy as a function of ns is discussed in terms of changes in the field distribution, occupation of additional levels above the upper laser level, and electron–electron interactions.

High-temperature, continuous-wave operation of terahertz quantum-cascade lasers with metal-metal waveguides and third-order distributed feedback.

Currently, different competing waveguide and resonator concepts exist for terahertz quantum-cascade lasers (THz QCLs). We examine the continuous-wave (cw) performance of THz QCLs with single-plasmon (SP) and metal-metal (MM) waveguides fabricated from the same wafer. While SP QCLs are superior in terms of output power, the maximum operating temperature for MM QCLs is typically much higher. For SP QCLs, we observed cw operation up to 73 K as compared to 129 K for narrow (≤ 15 μm) MM QCLs. In the latter case, single-mode operation and a narrow beam profile were achieved by applying third-order distributed-feedback gratings and contact pads which are optically insulated from the intended resonators. We present a quantitative analytic model for the beam profile, which is based on experimentally accessible parameters.

Frequency modulation spectroscopy with a THz quantum-cascade laser

We report on a terahertz spectrometer for high-resolution molecular spectroscopy based on a quantum-cascade laser. High-frequency modulation (up to 50 MHz) of the laser driving current produces a simultaneous modulation of the frequency and amplitude of the laser output. The modulation generates sidebands, which are symmetrically positioned with respect to the laser carrier frequency. The molecular transition is probed by scanning the sidebands across it. In this way, the absorption and the dispersion caused by the molecular transition are measured. The signals are modeled by taking into account the simultaneous modulation of the frequency and amplitude of the laser emission. This allows for the determination of the strength of the frequency as well as amplitude modulation of the laser and of molecular parameters such as pressure broadening.

Compact model for the efficient simulation of the optical gain and transport properties in THz quantum-cascade lasers

We present a compact model for the efficient simulation of the gain characteristics in THz quantum-cascade lasers (QCLs) based on the self-consistent solution of the Schrodinger and Poisson equations in the framework of a one-dimensional scattering-rate approach. The total intersubband scattering rates are factorized into the squared modulus of the respective dipole matrix elements and an energy-dependent factor, which we use as an approximation for the various scattering processes. Intended for designing THz QCLs, this model allows the efficient calculation of the gain characteristics and current densities due to a significantly reduced numerical effort compared to a full quantum transport theory. In view of the large parameter space, which causes even small variations of the parameters to have a complex influence on the lasing properties, the accuracy of the predicted results appears to be sufficient. We explore the benefits and limits of this approach by applying it to various THz QCLs based on either the bound-to-continuum design or the resonant-longitudinal-optical-phonon design. A remarkable agreement between the numerical and experimental results, in particular for the energy position of the gain maximum and lasing energy, is found. Finally, we use our model to design a THz QCL operating at a voltage close to the lower theoretical limit in order to reduce excess heating.

Evidence for frequency comb emission from a Fabry-Pérot terahertz quantum-cascade laser.

We report on a broad-band terahertz quantum-cascade laser (QCL) with a long Fabry-Pérot ridge cavity, for which the tuning range of the individual laser modes exceeds the mode spacing. While a spectral range of approximately 60 GHz (2 cm(-1)) is continuously covered by current and temperature tuning, the total emission range spans more than 270 GHz (9 cm(-1)). Within certain operating ranges, we found evidence for stable frequency comb operation of the QCL. An experimental technique is presented to characterize frequency comb operation, which is based on the self-mixing effect.

Multi-channel terahertz grating spectrometer with quantum-cascade laser and microbolometer array

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-Perot modes as a function of the laser driving current. The frequency coverage of the spectrometer is about 20 GHz.

Low-threshold terahertz quantum-cascade lasers based on GaAs/Al0.25Ga0.75As heterostructures

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.

Real-time terahertz imaging through self-mixing in a quantum-cascade laser

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.

Terahertz GaAs/AlAs quantum-cascade lasers

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 10 K 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.

Dependence of lasing properties of GaAs/AlxGa1−xAs quantum cascade lasers on injector doping density: theory and experiment

We investigate lasing and transport properties of GaAs/AlxGa1−xAs quantum cascade laser structures with varying injector doping density ne. Calculations from a nonequilibrium Green function theory show a linear dependence of the current density on ne in agreement with the experimental trend. We compare theoretical results for the threshold current density Jth and laser emission energy as a function of ne with previously obtained experimental results.