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TECHNOLOGY LASERS


Such calculations can reveal the voltage swing needed to produce a desired change in output frequency. Meanwhile, waveguide modelling allows engineering of the optical mode and an estimation of internal loss based upon free-carrier absorption. It is important to carry out this modelling for


both the engineering of the epitaxial layer stack, and for determining the widths of the emitter and base mesa, which control lateral mode confinement. The last aspect of modelling, that of the electrical transport characteristics, is performed to enable a prediction of the current-voltage


characteristic of the device.


So far we have only published results from device modelling, due to an initial focus on design and concept validation. To prove that our device is capable of fulfilling its promise, is must be capable of generating stimulated emission, which requires a population inversion – the presence of more carriers in the excited state than the ground state.


Calculations of electron wavefunctions have enabled us to determine state transition rates, and from this state population densities. The good news is that the population densities for the two most important states for lasing action converge to values that will allow a population inversion (see Figure 4). We have also undertaken waveguide modelling for our laser design, determining the expected mode profile (see Figure 5). Further data showing device operation is being prepared and will be soon be published.


Figure 4. Modelling results reveal the electron sheet density in the two most important states for lasing action – the upper and lower lasing states. The simulation shows convergence toward an electron population that is higher in the upper lasing state than the lower lasing state – a necessary condition for laser operation.


The great potential of our device, and the encouraging preliminary results from our modelling efforts, are providing motivation for us to continue to pursue the development of a TI-QCL. Supported by funding from National Science Foundation, we will focus on device design refinement, fabrication, and characterization, with the primary goal of achieving a performance suitable for portable systems. This system- enabling work will focus on design iterations to improve device performance, demonstrate operating wavelengths that extend through terahertz frequencies, show extended tuning range, and expand the ability to modulate at high speeds.


Figure 5. Two-dimensional optical waveguide modelling of the TI-QCL showing mode confinement provided by the upper and lower cladding layers, as well as the emitter ridge. The confinement factor and the layer doping levels are used to provide an estimate of the optical losses due to free carrier absorption.


Beyond this initial device-level work, we will explore, in collaboration with partners, the use of our novel QCL in specific applications such as chemical sensing and biomedical imaging. We look forward to a future where these devices are broadly deployed to help maintain air and drinking water quality, ensure process efficiency in chemical and semiconductor manufacturing, and help provide early detection of illnesses such as cancer.


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Copyright Compound Semiconductor October 2014 www.compoundsemiconductor.net 69


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