Long-wavelength (9.5-11.5 μm) Microdisk Quantum-Cascade Lasers

Quantum-cascade whispering-gallery-mode disk lasers emitting at 9.5- μm and 11.5- μm wavelength are reported. Taking advantage of the high-quality resonator (Q ≈ 200), the threshold current density of disk lasers emitting at 9.5 μm is reduced below the value of the corresponding ridge waveguide geometry (J_th,disk = 2.39 kA·cm^-2 versus J_th,ridge = 3.0 kA·cm^-2). Additionally, the increase in wavelength compared to previously reported disk lasers at 5.0 μm is a significant step toward the microcavity regime (by an effective scaling factor of 2.5, comparing identical disk sizes), disk diameters from 125 μm down to 20 μm are used to study the approach to the microcavity regime by size reduction. Far-field pattern measurements identify scattering from the pedestal as an important outcoupling mechanism for microdisk lasers. An excellent agreement between the measured and calculated free spectral range of the whispering gallery modes allows us to estimate the beta factor of the microdisks, resulting in β ≈ 0.05 for a 20-μm diameter disk. A two-level rate equation model is evaluated for the quantum-cascade disk laser as a tool for a direct measurement of β. Nevertheless, the actual measurement is at present blurred by luminescence (light-emitting diode) from the disk center accompanied by an unbalanced carrier distribution between the whispering gallery laser and the center light-emitting diode.