Waveguide and luminescent properties of thin-film Pb-salt injection lasers

Abstract

We describe the waveguide properties of the asymmetric dielectric slab formed by a very-high-index film on a low-index substrate. The analysis is appropriate for Pb-salt films grown on fluorite structure substrates. The mode reflectivities, surface scattering losses, and gain enhancements for the low-order TE and TM modes are considered. The stronger confinement of the TM modes leads to larger gain enhancements than for the TE modes. For the film thicknesses of interest, 2–4 μm, the TE and TM mode reflectivities are comparable. Using the theory of Tien, the TE modes show substantially lower surface scattering losses than TM modes of the same order. The magnitude of this loss, however, depends on an unknown parameter σ, the variance of the surface height. Experimental results are presented for thin-film diode lasers made with Pb Schottky barriers on p-type PbTe (index 6.4) grown epitaxially on BaF2 substrates (index 1.42). The spontaneous emission at 77°K is shown to agree with the band-band recombination model, assuming k conservation. Laser emission at 6.5 μm, both pulsed and cw, is observed at 10–15°K. Well above threshold the spectra generally show two dominant modes spaced by ≃ 1.4 meV. This splitting is attributed to a strain-induced shift of the energy bands. Using the deformation potentials for PbTe the observed splitting leads to an estimate −2 × 10−4 of the strain component in the [111] direction normal to the film surface. All observed laser lines are TE polarized. An analysis is made of the gain and loss parameters at threshold. The optical gain is estimated from the measured current and quantum efficiency, while the free carrier and reflection losses are calculated from the device parameters. The remaining losses can then be estimated from the threshold condition. If these losses are attributed to surface scattering, the analysis can be used to give an upper limit on σ of 0.03–0.06 μm. In every case the surface scattering loss is dominant in establishing the laser threshold, which is consistent with the strong preference shown for TE modes.