The Technical University of Vienna, Austria, has improved its bifunctional 6.8 μm quantum cascade laser and detector (QCLD) technology [by Benedikt Schwarz et al. Published in Applied Physics Letters, Vol. 107, No. 071104]. The monolithic integration of quantum cascade lasers (QCLs) and quantum cascade detectors (QCDS) enables the preparation of compact spectroscopy systems for environmental monitoring and medical applications such as trace gas detection and serum analysis.
The integration of high-efficiency lasers and detectors on the same epitaxial material presents a challenge because one mode optimization design does not apply to the other. In addition, because the peak wavelength depends on the bias voltage, it is difficult to maintain spectral overlap between the laser radiation and the detector light response.
Viana University researchers commented: "Previous designs have shown that lasers and detectors can operate at room temperature, limited to low duty cycle operation due to the large threshold current density and low wall insertion efficiency." The team has been finely tuned to improve the laser Performance, and claims that "dual-function designs achieve pulsed performance compared to traditional quantum cascade lasers."
Quantum cascade lasers and detectors were grown on indium phosphide (InP) using molecular beam epitaxy (MBE). The design includes 35 cycles of active area and two low-doped indium gallium arsenide (InGaAs) confinement layers. Lasers and detectors are fabricated on 10 micron wide silicon nitride insulator ridges and annealed titanium / gold contacts. The device is mounted on an indium bonded copper surface.
The researchers reported that the new laser device has improved all the characteristic parameters in pulsed mode (100ns at 10kHz). Using a 3 mm Fabry-Perot ridge, the threshold current at room temperature is 3 kA / cm2 and the previous QCLD device technology threshold is 6 kA / cm2. At the same time, the light output is 470mW (formerly 200MW). The total wall insertion efficiency is 4.5%, which is one-half or one-third the efficiency of traditional QCL wall-inserts with waveguide structures. Some groups have achieved wall insertion efficiencies of up to 50%.
For the detector section, the 0.5mm long ridges have a peak response of 40mA / W at zero bias. The Johnson / Thermal Noise Equivalent Power (NEP) reaches 80 pW / √Hz at a peak wavelength differential resistance of 1.6 kΩ. The researchers pointed out that subject to the splitting process ridge length limit at 0.5mm. Lithography and dry etching can reduce the length of the ridge. The team commented: "The 15 μm ridge length optimizes Johnson noise to limit the equivalent power, while the 50 μm ridge length device has a higher response rate.
Chip structure configuration between the laser and the detector has a small air gap, the peak absolute unsaturated photocurrent 9mA.
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