Surface-plasmon mode hybridization in subwavelength microdisk lasers

Abstract

Hybridization of surface-plasmon and dielectric waveguide whispering-gallery modes are demonstrated in a semiconductor microdisk laser cavity of subwavelength proportions. A metal layer is deposited on top of the semiconductor microdisk, the radius of which is systematically varied to enable mode hybridization between surface-plasmon and dielectric modes. The anticrossing behavior of the two cavity mode types is experimentally observed via photoluminescence spectroscopy and optically pumped lasing action at a wavelength of ϳ 1.3 m is achieved at room temperature. © 2009 American Institute of Physics. ͓doi:10.1063/1.3266843͔In wavelength-scale lasers, the very small number of optical modes and small volume of gain material allows one to probe the subtle and often interesting properties of lasing action. 1 Semiconductor microdisk lasers, in particular, have been actively studied due to their simple geometry and amenability to planar chip-scale integration with microelectronics. [2][3][4] More recently there has been great interest in using surface-plasmon ͑SP͒ modes at a semiconductormetal interface for guiding as well as high intensity and subwavelength optical confinement. 5 There has been significant work on the incorporation of SP waveguides that also act as electrical contacts in mid-infrared quantum cascade lasers, 6 in increasing SP propagation lengths using SP-dielectric waveguide mode hybridization, 7 as well as in creating ultrasmall laser cavities. 8,9 In miniaturizing semiconductor lasers to the nanoscale one encounters several design challenges that must be addressed, such as thermal management, 10,11 proximity of metal contacts to the optical cavity, surface states, 12 and demanding tolerance levels in fabrication. In this letter, we investigate the purposeful integration of a metal contact into a subwavelength whispering-gallery microdisk laser. We show that whispering-gallery SP and dielectric modes hybridize into low loss modes. We predict and map out this hybridization using finite-element-method ͑FEM͒ simulations, and experimentally measure the properties of fabricated microdisk laser cavities with varying levels of mode hybridization.Simulation of the hybrid laser cavities is performed using fully-three-dimensional FEM simulations with azimuthal symmetry. 13 A 250 nm thick semiconductor disk with index n disk = 3.4 and radius R d = 0.65 m is simulated with a centered metal contact of varying radius ͑R m ͒. A schematic of the microdisk device is shown in Fig. 1͑a͒. Silver with a complex refractive index of n Ag = 0.11− i9.5 at = 1.3 m is chosen for the metal layer due to its low optical loss. 14 For this disk size in the 1300 nm wavelength band there occurs a near degeneracy of the transverse-electric-like ͑TE-like͒ whispering-gallery mode ͑WGM͒ with dominant electric field polarization in the plane of the disk and the transversemagnetic-like ͑TM-like͒ mode with dominant electric field normal to the disk plane. A plot of the wavelength and Q-factor of these two resonances is shown in Fig. 2͑a͒ a...

Coupling of plasmonic and optical cavity modes in quasi-three-dimensional plasmonic crystals

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