Laser diodes lasing threshold

As in any other laser, the lasing threshold in a semiconductor laser diode is reached when the gain of the active material overcomes the losses of the laser cavity. These losses have two basic origins, namely the finite reflectivity of the mirrors mid distributed losses due to scattering and parasitic absorption in the active medium. In contrast to other lasers, the mirrors in typical semiconductor lasers are simply formed by cleaved or etched crystal facets. Therefore, the reflectivity (Fresnel reflectivity) is rather low, about 20% in the case of the nitrides. 

Nevertheless, the threshold current density of GaN-based laser diodes is still almost one order of magnitude higher than in other III-V material systems. Being one of the most basic laser properties, this highlights the present lack of understanding of the lasing mechanism in nitride-based semiconductors. 

The broad spectral coverage for luminescence from the ultraviolet (UV) to the NIR offered via the tunability of composition, size and shape in semiconductor nanocrystals, presents an obvious advantage for the use of such materials as tunable optical gain media, and in laser applications. Furthermore, low lasing thresholds are predicted for QDs and quantum wires, compared with two-dimensionally confined quantum wells, form the basis of the present semiconductor diode laser devices which are ubiquitous in information and telecommunication technologies [86, 87]. 

Several diode laser parameters are temperature dependent, among them threshold current and slope efficiency. The threshold current increases with increasing temperature, eventually making lasing action impossible. The slope efficiency decreases with increasing temperature, and the leakage current around... 

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