Mid-infrared (MIR) spectroscopy is a reliable tool for the identification of gaseous and liquid mixtures due to their unique and inherent absorption spectra. Quantum Cascade (QC) Lasers and Interband Cascade Lasers are modern reliable sources to penetrate the MIR spectral range.
To increase the functionality of QC devices we designed and optimized a QC material that can be used as a QC laser and as a QC detector for the very same MIR wavelength, respectively. Switching from laser to detector is achieved by biasing the semiconductor (lasing mode) or operate it without any electric field applied (detecting mode), respectively. Due to this functionality increase the on-chip integration of a designable QC light source, an interaction zone and a QC detector is now feasible and has been demonstrated recently.
In this talk we present improved bi-functional QC material for the integration and further development of sensor systems, as well as different cavity concepts for gas and liquid sensing scenarios. Proof of concept sensing examples to demonstrate the integrated sensor systems will be given. Multi mode and single mode lasers made from bi-functional materials show comparable performance to regular state of the art QC lasers and no performance drop due to the additional detection functionality.
While QC lasers are already accepted within the scientific community, QC detectors still need to be further promoted. Thus, in addition to the improvement of the bi-functional QC material, we demonstrated a single period quantum cascade photo-detector with a responsivity of up to 1.3 A/W.
Mid-infrared detection with semiconductor based pixel arrays attracted constant research interest over the past years. Remaining challenges for intersubband detectors are high device performance at elevated temperatures in combination with cost effective scalability to large pixel counts needed for applications in remote sensing and high resolution infrared imaging.
In this field, quantum cascade detectors may offer promising advantages such as photovoltaic room temperature operation at a designable operation wavelength with compatibility to stable material systems and growth technology.
We present a high performance InGaAs/InAlAs quantum cascade detector design suitable for pixel devices. The design is based on a vertical optical transition and resonant tunneling extraction. The 20 period active region is optimized for a high device resistance and thereby high detectivity up to room temperature. The pixels are fully compatible with standard processing technology and material growth to provide scalability to large pixel counts. An enhanced quantum cascade detector simulator is used for design optimization of the resistance and extraction efficiency while maintaining state of the art responsivity. The device is thermo-compression bonded to a custom read out integrated circuit with substrate bottom side illuminated pixels utilizing a metal grating coupling scheme. The operation wavelength is designed to align with the strong CO2 absorption around 4.3µm. A room temperature responsivity of 16mA/W and a detectivity of 5∙10^7 cm√Hz/W was achieved in good agreement with our simulation results. Device packaging and thermo-electric cooling in an N2 purged 16 pin TO-8 housing has been investigated.
The authors present a technique to reduce the facet reflectivity in quantum cascade lasers (QCLs) by tilted facets. In
order to minimize the Fabry-Pérot resonances, the feedback from the laser facets into the cavity must be minimized.
Due to intersubband selection rules, the light generated inside QCLs is TM polarized. This polarization purity in
QCLs enables the reduction of the facet reflectivity through the angle of light incidence at the laser facet. We
observed a maximum threshold current density when the facet is tilted 17° towards the surface normal. This is in
agreement with the calculated Brewster's angle for the QCL heterostructure.
The performance of quantum well infrared photodetectors (QWIP) can be significantly enhanced combining it
with a photonic crystal slab (PCS) resonator. In such a system the chosen PCS mode is designed to coincide
with the absorption maximum of the photodetector by adjusting the lattice parameters. However there is a
multitude of parameter sets that exhibit the same resonance frequency of the chosen PCS mode.
We have investigated how the choice of the PC design can be exploited for a further enhancement of QWIPs.
Several sets of lattice parameters that exhibit the chosen PCS mode at the same resonance frequency have
been obtained and the finite difference time domain method was used to simulate the absorption spectra of the
different PCS. A photonic crystal slab quantum well infrared photodetector with three different photonic crystal
lattice designs that exhibit the same resonance frequency of the chosen PCS mode were designed, fabricated and
This work shows that the quality factor of a PCS-QWIP and therefore the absorption enhancement can be
increased by an optimized PCS design. The improvement is a combined effect of a changed lattice constant, PC
normalized radius and normalized slab thickness. An enhancement of the measured photocurrent of more than
a factor of two was measured.