E. Mujagić

Low divergence single-mode surface emitting quantum cascade ring lasers

We describe the fabrication and operation of surface emitting second-order distributed feedback quantum cascade ring lasers. The devices exhibit single-mode emission at a wavelength of 3.95μm with a side mode suppression ratio of 25dB. A linear tuning coefficient of 0.13cm−1∕K is observed. A single longitudinal mode in the ring shaped resonator results in a highly symmetric far-field pattern and a low beam divergence, represented by a full width at half maximum of ∼3°. Based on these characteristics the presented compact coherent light source may find its way into today’s midinfrared spectroscopy applications.

Vertically emitting terahertz quantum cascade ring lasers

We describe the fabrication and operation of vertically emitting distributed feedback quantum cascade ring lasers operating in the terahertz frequency range. A twofold increase in radiation efficiency is observed as compared to Fabry–Perot lasers. The emitters exhibit a robust single-mode operation around 3.2 THz with a side mode suppression ratio higher than 30 dB. Modal and threshold characteristics are investigated by performing finite element simulations with results in good agreement with experiments. The ring-shaped resonator facilitates beam collimation which results in a symmetric far-field profile.

Two-dimensional broadband distributed-feedback quantum cascade laser arrays

We present two-dimensional broadband quantum cascade laser arrays based on distributed-feedback (DFB) ring cavity surface emitting lasers. The 16-element arrays exhibit a linear tuning range of 180 cm−1 centered at a wavelength of 8.2 μm when operated in pulsed mode at room temperature. The devices show single-mode emission with a side mode suppression ratio of 30 dB. Given by the facetless nature of the single emitters, the spectral dependent threshold current densities and optical power reflect the gain profile of the incorporated material and are not impaired by the diversity of underlying DFB designs.

Impact of doping on the performance of short-wavelength InP-based quantum-cascade lasers

The effect of doping concentration on the performance of short-wavelength quantum-cascade lasers based on the strain-compensated InGaAs/InAlAs/AlAs heterostructure on InP, emitting at 3.8 μm, is investigated for average doping concentrations between 0.3 and 3.9×1017 cm−3 (sheet densities between 1.6 and 20.9×1011 cm−2). Although the threshold current density is rather independent of doping concentration, the maximum current density increases with doping and exhibits a saturation for the highest doping level. Other important performance characteristics such as differential quantum efficiency, peak optical emission power, slope efficiency, and maximum operating temperature are observed to be maximized for structures with an average doping of 2−3×1017 cm−3, corresponding to a sheet density of about 1.5×1012 cm−2.

Detectivity enhancement in quantum well infrared photodetectors utilizing a photonic crystal slab resonator.

We characterize the performance of a quantum well infrared photodetector (QWIP), which is fabricated as a photonic crystal slab (PCS) resonator. The strongest resonance of the PCS is designed to coincide with the absorption peak frequency at 7.6 µm of the QWIP. To accurately characterize the detector performance, it is illuminated by using single mode mid-infrared lasers. The strong resonant absorption enhancement yields a detectivity increase of up to 20 times. This enhancement is a combined effect of increased responsivity and noise current reduction. With increasing temperature, we observe a red shift of the PCS-QWIP resonance peak of -0.055 cm(-1)/K. We attribute this effect to a refractive index change and present a model based on the revised plane wave method.

Ring cavity induced threshold reduction in single-mode surface emitting quantum cascade lasers

We present ring cavity surface emitting (RCSE) quantum cascade lasers operating at temperatures as high as 380 K and above. A reduction in threshold current density and enhanced radiation efficiency are observed as compared to Fabry–Perot (FP) lasers. In continuous wave, the maximum operation temperature of RCSE lasers is 50 K higher than in FP emitters. The devices exhibit single-mode emission at a wavelength around 8 μm with a side mode suppression ratio of 30 dB at room temperature. A lithographic tuning of the resonance is achieved by a variation in the grating period. The emitters exhibit a low divergent ring-shaped beam pattern with a lobe separation of ∼1.5°.

Grating-coupled surface emitting quantum cascade ring lasers

We report on the fabrication and operation of quantum cascade ring lasers providing grating-coupled surface emission. The devices exhibit tunable far fields, ranging from spot- to ring-shaped symmetric beam cross sections, depending on the grating period. This—along with threshold current densities as low as for comparable Fabry–Perot lasers—demonstrates the compatibility of reduced beam divergence and two-dimensional integrability, resulting in an attractive light source for applications in midinfrared spectroscopy and imaging.

Midinfrared intersubband absorption in InGaAs/GaAsSb multiple quantum wells

We report on the observation of midinfrared intersubband (ISB) absorption in InGaAs/GaAsSb multiple quantum wells grown lattice-matched to InP substrates by molecular beam epitaxy. ISB absorption in a broad wavelength region (5.8–11.6 μm) is observed in quantum wells with well widths ranging between 4.5 and 12 nm. The conduction band offset at the InGaAs/GaAsSb heterointerface is measured to be 360 meV, giving an excellent agreement between the theoretically calculated ISB transition energies and the Fourier-transform infrared spectroscopy measurements over the whole range of well widths under investigation.

Intersubband optoelectronics in the InGaAs/GaAsSb material system

In this article the authors report on a novel material system for optoelectronic intersubband devices. Superlattices of In0.53Ga0.47As/GaAs0.51Sb0.49 were grown by molecular beam epitaxy. Layer thickness and quality was investigated by high-resolution x-ray diffraction measurements and high-resolution transmission electron microscopy images. Intersubband absorption measurements on In0.53Ga0.47As/GaAs0.51Sb0.49 superlattices, revealed at room temperature transition energies from 213 to 107 meV for In0.53Ga0.47As well widths of 4.5–11.5 nm at room temperature. These results were used to fit parameters for self-consistent superlattice band structure calculations. Finally, quantum cascade lasers with an emission wavelength of 11.3 μm and quantum well infrared photodetectors with a peak response near 5.5 μm were realized in this material system.

Tree array quantum cascade laser.

A monolithic coupling scheme for mid-infrared quantum cascade laser arrays is investigated with respect to brightness enhancement. The tree-shaped resonator enables parallel coupling of six laser elements into a single element by means of several Y-junctions. Phase-locking is observed on the basis of far field analysis, and leads to in-phase emission on both sides of the device. The experimental results match calculated far field profiles and demonstrate a high level of modal control when driven far above threshold. Whereas optical power measurements confirm negligible coupling losses, the slope efficiency is below the theoretically expected value, which is attributed to modal competition. Additional evaluation of near fields and spectral characteristics provides background on the modal dynamics of the sophisticated cavity and reveals limitations to coherent beam combining. The findings pave the way to improved coupling efficiency and brightness scaling of a single facet emitting compact quantum cascade laser array.