AlGaAs/GaAs Distributed Feedback Quantum Cascade Lasers
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ABSTRACT We report on the realization of distributed feedback quantum cascade lasers in the GaAs/AIGaAs material system. The use of a metallized surface relief grating for feedback allows a fabrication process without regrowth. A feature of this laser is that either single mode or double mode emission at X - 10 gm is achieved, which is typical for index coupled lasers. The coupling coefficient is measured from the mode spacing for double mode emission of a strong overcoupled laser to be K:z 24 cm"-. The emission wavenumber can be continuously tuned with the temperature at a rate of dv / dT ; 0.048 cm'/K, which is in close agreement to the temperature dependence of the refractive index of GaAs.
INTRODUCTION Quantum cascade lasers (QCLs) are a powerful light source in the mid infrared, a spectral region interesting for gas sensing. Since the demonstration of the QCLs in 1994 in the InGaAs/InAlAs system [1] grown on InP, the laser performance is improved dramatically yielding to room temperature operation [2]. The first QCLs in another material system, namly AIGaAs/GaAs grown on GaAs, were demonstrated by [3] and [4] with emission wavelengths of 9.6 pm and 13 pm respectivly. For chemical sensing, there is a need for continuously tunable single-mode sources. This has been achieved by distributed feedback (DFB) lasers [5],[6],[7],[8] and microcavity lasers [9],[1O] in the InGaAs/InAlAs material system. Microcavity lasers were also demonstrated in
the GaAs/AIGaAs material system [ 1I]. The emission wavelength of DFB and microcavity lasers is tuned by temperature with a tuning range around Av - 10 cm"-, limited by the highest
possible working temperature and the width of the gain peak. A wider tuning range of Av = 40 cm"- has been achieved by the use of the first order Stark effect [12]. These lasers are two-segment lasers, in order to control the emission wavelength and power individually. The emission spectra of these lasers show mulitimode behavior only.
EXPERIMENT The active zone of our laser material is basically the same as described in [3], with the
difference of a slightly lower Al fraction of 30 %. The emission wavelength is designed to be X 10 gm. Electrons are injected through a funnel injector into the third state of a three level % laser system, which is formed by three coupled quantum wells. The radiative transition takes place between level three and two, whereas fast depopulation of level two is achieved by assistance of LO phonon emission into level one. In order to get sufficient high gain, 30 periods of injector/active cell are cascaded. Good light confinement at this wavelength is achieved by a double plasmon enhanced optical waveguide [13]. This Al-free waveguide is made of a 3.5 pm thick, low doped GaAs as core layers and 1 pm thick, heavily n-doped GaAs as cladding layers. At an emission 141 Mat. Res. Soc. Symp. Proc. Vol. 607 0 2000 Materials Research Society
wavelength of 10 microns we compute waveguide losses of a 15 cm"], caused mainly by free electron absorption in the high doped claddin
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