G. Strasser

Reversing the pump dependence of a laser at an exceptional point.

When two resonant modes in a system with gain or loss coalesce in both their resonance position and their width, a so-called exceptional point occurs, which acts as a source of non-trivial physics in a diverse range of systems. Lasers provide a natural setting to study such non-Hermitian degeneracies, as they feature resonant modes and a gain material as their basic constituents. Here we show that exceptional points can be conveniently induced in a photonic molecule laser by a suitable variation of the applied pump. Using a pair of coupled microdisk quantum cascade lasers, we demonstrate that in the vicinity of these exceptional points the coupled laser shows a characteristic reversal of its pump dependence, including a strongly decreasing intensity of the emitted laser light for increasing pump power.

Ultrastrong Light-Matter Coupling Regime with Polariton Dots

The regime of ultrastrong light-matter interaction has been investigated theoretically and experimentally, using zero-dimensional electromagnetic resonators coupled with an electronic transition between two confined states of a semiconductor quantum well. We have measured a splitting between the coupled modes that amounts to 48% of the energy transition, the highest ratio ever observed in a light-matter coupled system. Our analysis, based on a microscopic quantum theory, shows that the nonlinear polariton splitting, a signature of this regime, is a dynamical effect arising from the self-interaction of the collective electronic polarization with its own emitted field.

High-temperature performance of GaAs-based bound-to-continuum quantum-cascade lasers

GaAs-based quantum-cascade lasers based on a bound-to-continuum transition have been realized and characterized. This band structure design combines the advantages of the well known three-well and superlattice active regions. We observed lasing of Fabry–Perot lasers in pulsed mode up to a temperature of 100u200a°C. Multimode emission with a pulsed peak power of 340 mW is observed at room temperature and 42 mW at 80u200a°C. Further, from aging tests we expect a lifetime of over 60 years for these devices.

GaAs/AlGaAs superlattice quantum cascade lasers at λ≈13 μm

We report the realization of an injection laser based on intraband transitions in a finite AlGaAs/GaAs superlattice. The active material is a 30 period sequence of injectors/active regions made from AlGaAs/GaAs quantum wells. By an applied electric field, electrons are injected into the second miniband of a chirped superlattice and relax radiative to the lowest miniband. At a heat-sink temperature of 10 K, the laser emission wavelength is 12.9 μm with peak optical powers exceeding 100 mW and a threshold current density of 9.8u200akA/cm2. The maximum operating temperature is 50 K. For this device, a waveguide consisting of heavily doped GaAs cladding and low doped core layers has been used as a plasma-enhanced confinement.

Continuous-wave operation of distributed feedback AlAs/GaAs superlattice quantum-cascade lasers

We report on continuous-wave operation of first-order distributed feedback quantum-cascade lasers at λ=11.8 μm, based on interminiband transitions in a chirped AlAs/GaAs superlattice. Short devices operate in continuous-wave up to ∼30 K. The single-mode emission wavelength is continuously tunable with the temperature. A metallized surface-relief grating is used for feedback to achieve single-mode emission.

GaAs/AlGaAs distributed feedback quantum cascade lasers

We report on the realization of distributed feedback quantum cascade lasers in the GaAs/AlGaAs material system. A metallized surface relief grating is used for feedback. Both single mode and double mode emission is observed at λ≈10 μm. The coupling coefficient is measured from the mode spacing for double mode emission to be 24 cm−1. The emission wave number can be tuned with the temperature at a rate of dν/dT≈0.048u2002cm−1/K.

Magnetic-field-enhanced quantum-cascade emission

We have observed an enhancement of terahertz intersubband electroluminescence in a quantum cascade structure in the presence of a magnetic field applied normal to the epitaxial layers. At a field of B=7.2 T the emission efficiency doubles. This effect is attributed to the suppression of nonradiative Auger–intersubband transitions caused by Landau-quantization of the in-plane electron motion. The magnetic field dependence of the luminescence intensity shows strong oscillations. These magnetointersubband oscillations are caused by the modulation of the transition rate via resonant inter-Landau-level transfer.

Room-temperature emission of GaAs/AlGaAs superlattice quantum-cascade lasers at 12.6 μm

We report on the lasing properties of quantum-cascade lasers that combine the advantage of strong electron confinement with the superior performance of superlattice structures. To achieve an optimum band offset between wells and barriers, we chose an Al content of the AlGaAs barriers of 45%. We fabricated both conventional Fabry–Perot lasers, as well as single-mode-emitting distributed-feedback lasers. In pulsed mode, the lasers work well up to temperatures as high as 40u200a°C. The emission wavelength at this temperature is 12.6 μm. At 78 K, some lasers show a low threshold current density of 1.6 kA/cm2, while others show a high optical peak power of 240 mW.

GaAs/AlGaAs-based microcylinder lasers emitting at 10 μm

The realization of electrically pumped GaAs/AlGaAs quantum cascade microcylinder lasers is reported. Design and fabrication of special resonator shapes (microcylinder and ridge waveguide) are presented. Threshold characteristics and optical output of different resonators of the same quantum cascade laser material emitting at 10 μm are investigated. A low threshold current of 318 mA is obtained for a microcylinder resonator with circular cross section. The maximum working temperature of the microcylinder lasers is 165 K. Single mode emission is detected with a side mode suppression ratio better than 20 dB.

High performance InGaAs/GaAsSb terahertz quantum cascade lasers operating up to 142 K

We report on the demonstration of a maximum operating temperature of 142u2009K for InGaAs-based terahertz quantum cascade lasers. This result is achieved by using the alternative material combination In0.53Ga0.47As/GaAs0.51Sb0.49, lattice-matched to InP, which exhibits fabrication advantages over standard In0.53Ga0.47As/In0.52Al0.48As due to more suitable material parameters. An active region, based on a three-well phonon depletion design, with improved injection and extraction tunneling coupling, was designed. The devices exhibit threshold current densities of 0.75u2009kA/cm2 and provide peak optical powers up to 9 mW. A broad spectral emission range between 3.3 and 4 THz is measured.