Andreas Wittmann

External cavity quantum cascade laser tunable from 7.6 to 11.4 μm

We present the development of a broad gain quantum cascade active region. By appropriate cascade design and using a symmetric active region arrangement, we engineer a flat gain and increase the total modal gain in the desired spectral range. Grating-coupled external cavity quantum cascade lasers using this symmetric active region are tunable from 7.6 to 11.4 μm with a peak optical output power of 1 W and an average output power of 15 mW at room-temperature. With a tuning of over 432 cm−1, this single source covers an emission range of over 39% around the center frequency.

Room temperature terahertz quantum cascade laser source based on intracavity difference-frequency generation

We report on our progress in the development of a terahertz quantum cascade laser source based on intracavity terahertz difference-frequency mixing in a dual-wavelength mid-infrared quantum cascade laser with the active region engineered to possess giant second-order nonlinear susceptibility. In this letter, we demonstrate devices that operate in mid-infrared at λ1=8.9μm and λ2=10.5μm and produce terahertz output at λ≈60μm via difference-frequency generation with 7μW output power at 80K, 1μW output at 250K, and still approximately 300nW output at 300K.

Intersubband linewidths in quantum cascade laser designs

We present a model to a priori calculate the temperature and field dependent intersubband linewidth of the optical transition in quantum cascade laser designs. Besides intra- and intersubband lifetime broadening, it comprises interface roughness scattering based on the approach of Tsujino et al. [Appl. Phys. Lett. 86, 062113 (2005)]. We verified our model with experimental data of quantum cascade lasers having different linewidths. Excellent agreement with the experiment was found for the two-phonon resonance design. Linewidths are slightly overestimated in the bound-to-continuum design. Differential gain and threshold current density are in excellent agreement for the two-phonon resonance design. Although the slope efficiency is somewhat underestimated at low temperatures, there is still reasonable agreement with the experiment.

Broadband Distributed-Feedback Quantum Cascade Laser Array Operating From 8.0 to 9.8

An ultra-broadband distributed-feedback quantum cascade laser array was fabricated, using a heterogeneous cascade based on two bound-to-continuum designs centered at 8.4 and 9.6 mum. This array emitted in a range over 220 cm-1 near a 9-mu m wavelength, operated in pulsed mode at room temperature. The output power of the array varied between 100- and 1100-mW peak intensity.

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Based on the bound-to-continuum active region design, we shall present a high performance continuous-wave (CW) quantum-cascade laser. In contrast to high performance lasers based on a two-phonon resonance transition and a narrow linewidth (< 165 cm-1), the device presented here exhibits a spontaneous emission full-width at half-maximum as large as 295 cm-1. Thus, such devices are very suitable for broadband tuning. At 30degC, it shows a maximum output power and slope efficiency of 188 mW and 500 mW/A, as well as a threshold current density of only 1.79 kA/cm2. Furthermore, at this temperature, the device demonstrates an internal differential quantum efficiency of 71% and a wall plug efficiency of 2.0%. The maximum CW operation temperature reached is 110degC. A thermal resistance of 4.3 K/W was attained by epi-down mounting on diamond submounts. The waveguide losses of 14 cm-1 are explained by intersubband absorption in addition to free-carrier absorption.

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Single-mode lasers operating at lambda ap 9 mum in continuous wave up to 423 K (150degC) were achieved by the combination of strong distributed-feedback coupling, a narrow gain active region design, low intersubband, and free-carrier losses as well as a good thermal management. Tuning of 10 cm-1 or 0.9% of the center frequency was achieved by heating the device. The threshold current density varies from 1.1 kA/cm2 at 303 K to 2.4 kA/cm2 at 423 K. Other devices with low electrical power consumption of 1.6 and 3.8 W for an optical output power of 16 and 100 mW have been demonstrated at 263 K.

High-Performance Bound-to-Continuum Quantum-Cascade Lasers for Broad-Gain Applications

A heterogeneous high-performance quantum-cascade laser gain chip comprising two bound-to-continuum active region designs emitting at 8.2 and 9.3 mum is presented. Its extrapolated gain spectrum has a full-width at half-maximum (FWHM) of 350 cm - 1. Though a broad gain bandwidth invariably results in a reduced gain cross section, devices with a high-reflection coated back facet still lase continuous-wave (CW) up to a temperature of 50 degC and demonstrates output powers in excess of 100 mW at 30degC. Such high performance was achieved by designing the waveguide in a buried heterostructure fashion and epi-down mounting on a diamond submount, resulting in a thermal resistance of only 4.8 K/W. In pulsed mode, we reached a peak output power of 1 W at room temperature. Finally, in order to prove the usability for broad-band tuning, this chip was antireflection coated on the front facet with a residual reflectivity of < 2.5 x 10-3 and used in our external cavity (EC) setup operated at room temperature. In pulsed mode, we were able to tune the gain chip over 292 cm -1, which is 25% of center frequency. In CW, we reached a coarse tuning range of 201 cm-1 (18%) and an output power in excess of 135 mW at the gain maximum at 15degC. This gain chip enabled CW room temperature EC tuning with output powers in excess of 20 mW over 172 cm -1.

Distributed-Feedback Quantum-Cascade Lasers at 9

Lasing properties of room temperature, continuous wave operated distributed feedback (DFB) quantum cascade lasers are reported. A bound-to-continuum active region was used to generate a broad gain spectrum. As a result, first-order DFB lasers employing different periods allowed us to achieve single mode continuous wave emission at several wavelengths ranging from 7.7to8.3μm at a temperature of +30°C. The frequency span corresponds to 8% of the center frequency.

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Room-temperature, continuous-wave operation of an external-cavity quantum cascade laser (EC-QCL) is reported. Single-mode tuning range of 120 cm(-1) was achieved, from 7.96 to 8.84 microm. The gain chips utilized are based on the bound to continuum design and were fabricated as buried heterostructure lasers. Gap-free tuning (mode hops only on the external-cavity modes) is demonstrated for an antireflection-coated laser, just by grating rotation. The EC-QCL was implemented in a Littrow setup and an average power of 1.5 mW was obtained at 20 degrees C, while a peak power of 20 mW was obtained for a modified Littrow setup with the back extraction of light.

m Operating in Continuous Wave Up to 423 K

A model of sequential resonant tunneling transport among two-dimensional subbands that takes into account explicitly elastic scattering is investigated. It is compared to transport measurements performed on quantum cascade lasers, where resonant tunneling processes are known to be dominating. Excellent agreement is found between experiment and theory over a large range of current, temperature, and device structures.