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Résultats exploitables
The main goals of the NEMIS project were the development of an Sb-based technology platform for electrically pumped VCSELs in the 2.0-3.5 microns wavelength range, the realization of application suited and packaged devices at 2.33 microns and 2.7 microns as well as demonstrator sensing applications using these lasers.
Two device concepts for the electrically pumped GaSb-VCSELs were investigated. One is a modified version of the buried-tunnel-junction (BTJ) concept that has proven very successful on 1.3-2.3 microns InP-based VCSELs and has now been extended to the GaSb-based materials. The second approach is based on mesa-confined VCSEL structures.
The consortium first succeeded in realizing GaSbbased 2.3 microns VCSELs with both approaches. These comprise very low threshold current densities comparable to edge-emitting devices, but also excellent cw-performance with the BTJ-structure was obtained.
The output power is of the order of several hundred microwatts which well which is sufficient for most sensing applications. Also displayed are the room temperature emission spectra for different laser currents clearly showing single-mode operation with an SMSR exceeding 25dB (1:300) and an electro-thermal tuning range of about 10 nm. WMS- and PAS-based sensor systems have been realized and first measurements with the GaSb- VCSELs showed promising results for CO detection around 2.33 microns.
Further VCSELs were also developed for the 2.7microns (+/- 100nm) wavelength regime. Considerable progress has been achieved in spite of the extremely challenging technology. Excellent layer and crystal quality has been achieved even though the total thickness for this large wavelength is of the order 17 microns.
In addition, complete VCSELs were made and characterized in this wavelength range. Using the BTJ-structure with a top dielectric DBR, cw operation and single-mode emission was reproducibly obtained around room-temperature. As the gain-DBR adjustment was not yet perfect, because of the blue-shift occurring during epitaxial overgrowth, higher threshold currents were obtained than for the 2.3 microns counterparts.
Extensive numerical modelling accompanied the experimental work and helped to understand device performances better and to straightforwardly establish optimized designs. In the frame of the NEMIS project a full 'multiphysics'' VCSEL model that simultaneously accounts for the detailed optical fields, the complex current transport, and the local heat generation and dissipation was developed. This ''multiphysics'' VCSEL model was applied to simulate the output characteristics, i.e. output power and wavelength versus current for the buried-tunnel-junction (BTJ) VCSEL, with and without a surface relief. As a particular feature, the diffraction losses occurring in a VCSELs because of the localized transverse waveguiding have been analysed in dependence on the relevant technological parameters.
In summary, the main achievements gained in the NEMIS project are:
- worldwide first realisation of electrically pumped single-mode and tuneable GaSb- VCSELs in the 2.3-2.7 microns wavelength range that operate at room-temperature and above
- successful demonstration of WMS- and PAS-based pilot gas-sensors for carbon monoxide and ammonia using packaged GaSb-VCSELs
- establishing a technology platform for GaSb-based VCSELs in the GaSb material system enabling the realisation of VCSELs in the entire 2.0-3.5µm wavelength range
- development of a comprehensive opto-electro-thermal modelling suite for design and analysis of the VCSELs
- improving and completing the data base on optical constants of GaSb-based compounds.
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