Lasing in III-V microdisk core - TiO2 shell lasers
Microdisk lasers having III-V core coated with TiO2 shell are studied under optical pumping. Initial core microdisk lasers with 5-10 µm diameter comprising five layers of InAs/In0.15Ga0.85As quantum dots demonstrate room temperature lasing with excellent characteristics at the ground state optical transition in the 1.28-1.30 µm spectral range. Deposition of the TiO2 dielectric shell results in a decimation of the whispering-gallery-modes of the microdisk resonator and, for thicker (> 100 nm) shells, in a blue shift of the lasing wavelength. Using numerical analysis we demonstrate that coating III-V core microdisk with high-index shell causes strong penetration of the 2nd and 3rd radial modes into the shell, whereas the 1 st radial mode remains nearly undisturbed. Though optical loss added by TiO2 dielectric shell are low (it does not exceed 2 cm-1 even for 250 nm-thick TiO2 layer), mode leakage to TiO2 results in decrease of the optical confinement factor and increase in the threshold pump power. This effect is more pronounced in microlasers of the smallest diameter studied (5 µm). Thus, beside other applications, TiO2 shell can be used for mode selection in microdisk lasers or for selective outcoupling of the emission to waveguide structure, which requires proper adjustment of the TiO2 shell thickness and microdisk diameter.