Towards mid-infrared on-chip sensing utilizing a bi-functional quantum cascade laser/detector

Abstract

The miniaturization of spectroscopic setups is a fascinating research topic, gaining momentum during the last couple of years. Quantum cascade lasers are known as one of the most important and influential sources in the mid-infrared region. In the so called "fingerprint" region (3-20μm) most molecules have their resonances, that can be observed by optical absorption or a change of the refractive index of the chemical substance. Many chip-scale sensing concepts have been demonstrated utilizing quantum cascade lasers [1][2][3]. However, all of these concepts have been demonstrated with external optics and detectors.Our recently demonstrated bi-functional quantum cascade laser/detector (QCLD) [4] opens up a novel approach for monolithically integrated chemical and biological sensors. Simply by changing the applied bias, the structure switches between lasing (58kV/cm) and detection (0kV/cm). We have shown that it is possible to match the wavelength of the laser and the detector by a spectial design approach to decrease the intrinsic Stark-shift. In the QCLD the laser injector also acts as an efficient phonon-ladder extractor at zero bias. As the design is rather complicated, we have developed an optimizer based on a semi-classical Monte-Carlo transport simulator. With this simulator we are able to calculate gain/absorption, as well as the responsivity. By considering non-parabolic in-plane masses in the transport calculation, we can observe the temperature shift of the responsivity, which has to be considered to match the wavelength of the laser and the detector at room-temperature. 0 0.2 0.4 0.6 0.8 1 0 5 10 15 20 norm. opt. Power Current Density [kA/cm 2 ]T=300K on-chip ext. DTGS Fig. 1 Sketch of a monolithically integrated sensor utilizing our bi-functional QCLS (left). Optical power of the laser measured with the on-chip detector at the back facet and with an external DTGS with a lens at the front facet. We have observed a peak detector signal of 191.5mV at room-temperature. The small difference is due to atmospheric absorption between the laser and the DTGS and due to the fact that the DTGS measures the average power of the pulse, where the fast on-chip QCD can observe the peak power. Fig. 1 (left) sketches a mid-infrared on-chip sensor based on our bi-functional QCLD. We have integrated a laser and a detector on the same chip, observing a peak detector signal of 191.5mV at room-temperature. The light power versus current density plot of the laser, comparing the on-chip detector with an external DTGS detector is shown in Fig 1. (right). By separating the contacts of the laser and the detector we were able to reduce the electric cross-talk below 2mV at laser threshold. The remaining part is mainly due to external measurement setup and can be eliminated by signal post processing. By eliminating the need of any external lens and detector, we move a significant step towards monolithic integrated mid-infrared sensors. The structure can be used as a mid-infrared sensor when combined with microfluidic channels or gas f...