Deana Rafizadeh

FDTD microcavity simulations: design and experimental realization of waveguide-coupled single-mode ring and whispering-gallery-mode disk resonators

We investigate the properties of high-Q, wide free-spectral-range semiconductor microcavity ring and disk resonators coupled to submicron-width waveguides. Key optical design parameters are characterized using finite-difference time-domain (FDTD) solutions of the full-wave Maxwells equations. We report coupling efficiencies and resonant frequencies that include the effects of waveguide dispersion and bending and scattering losses. For diameters of 5 /spl mu/m, the microcavity resonators can have Qs in the several thousands and a free spectral range of 6 THz (50 nm) in the 1.55 /spl mu/m, wavelength range. Studies of the transmittance characteristics illustrate the transition from single-mode resonances to whispering-gallery-mode resonances as the waveguide width of the microring is increased to form a solid microdisk. We present nanofabrication results and experimentally measured transmission resonances of AlGaAs/GaAs microcavity resonators designed in part with this method.

Waveguide-coupled AlGaAs/GaAs microcavity ring and disk resonators with high finesse and 21.6-nm free spectral range

We report the realization and demonstration of novel semiconductor waveguide-coupled microcavity ring and disk resonators. For a 10.5-microm-diameter disk resonator, we measure a finesse of 120, a resonant linewidth of 0.18 nm, and a free-spectral range of 21.6 nm in the 1.55-mum-wavelength region. We present the nanofabrication methods and the experimental results for 10.5- and 20.5-mum-diameter ring and disk resonators to show the feasibility of such devices.

Propagation loss measurements in semiconductor microcavity ring and disk resonators

We report the measurement of cavity propagation losses in nearly single-mode semiconductor waveguide-coupled ring and disk microcavity optical resonators. Using a novel 10.5-/spl mu/m-diameter ring resonator, we measure transverse electric (TE) and transverse magnetic (TM) field intensity losses in 0.35-/spl mu/m-wide ring waveguide cavities in the 1.55-/spl mu/m-wavelength region. We present the experimental results for nanofabricated AlGaAs-GaAs 10.5-/spl mu/m-diameter ring and disk resonators to quantify cavity losses and to show the feasibility of these promising and robust submicron-scale devices.

Numerical analysis of vectorial wave propagation in waveguides with arbitrary refractive index profiles

Abstract We demonstrate an efficient numerical method for calculating transverse electric (TE) or transverse magnetic (TM) modal properties in a waveguide with an arbitrary refractive index distribution. We approximate a planar waveguide cross-section by a finite number of thin dielectric layers and use 2 × 2 transfer matrices to relate adjacent layers. Using a back-propagation method that is simple and computationally efficient, we solve for the vectorial field across the multi-layered waveguide and plot the mode profile. There is rapid convergence to a high-accuracy solution. We also obtain the number of guided modes and the propagation constant of each mode.

Temperature tuning of microcavity ring and disk resonators at 1.5-/spl mu/m

We report the temperature-tuning of novel microcavity resonators with wide free spectral range /spl Delta//spl lambda//sub fsr/, and high finesse. We have fabricated and demonstrated AlGaAs/GaAs ring and disk resonators with diameters of 10.5 and 20.5 /spl mu/m evanescently coupled to single-mode waveguides. Strong lateral waveguide confinement allows micron-size cavities with negligible bending loss and the resonators can be fabricated with reasonably low loss using nanofabrication techniques, resulting in a high finesse. The 10.5-/spl mu/m-diameter disk resonator yields a finesse as high as 120 (Q=8600) and /spl Delta//spl lambda//sub fsr/ of 21.6 nm. The temperature tuning is 1.3 nm/10/spl deg/C, suitable for a thermally-controlled switch or widely tunable filter.

High-Q microcavity ring and disk resonators: FDTD analysis of resonance and coupling characteristics

Micron-size photonic and optoelectronic components can be designed end optimized using finite-difference time-domain (FDTD) solutions of Maxwells equations. We have developed FDTD models to characterize key optical design parameters of passive microcavity ring and disk resonators proposed for VLSI-scale photonic integrated circuits (PICs). We present FDTD-computed coupling factors, resonance frequencies, and quality factors (Q) of microcavity ring and disk resonators coupled to strongly-guiding waveguides. These structures offer high Qs and wide free spectral range. Key mechanisms which affect the Q are radiative waveguide bending loss, surface-roughness scattering loss, and the coupling between the cavity and adjacent input/output waveguides. All of these effects are included in our simulations. The strong lateral waveguide confinement due to the large contrast between the semiconductor core and air cladding allows ring and disk diameters to be as small as 2 /spl mu/m with negligible bending loss. To model the side-wall roughness, we use stepped-edge geometries in the Cartesian grid. The FDTD method naturally takes into account phase variations, which is crucial for investigating the coupling phenomenon. We also present an example of our nanofabrication results for devices designed in part with this method.

Suppression of higher-order radial whispering gallery modes in waveguide-coupled microcavity disk resonators

In comparison to single-mode microring resonators, microcavity disk resonators supporting whispering gallery modes (WGMs) have the advantage of less side-wall scattering loss and higher Q. However, the microdisk resonator has multiple sets of resonances corresponding to fundamental and higher-order radial WGMs of the disk. Suppression of the higher-order modes is necessary in order to use microdisks as single-mode laser sources or as WDM devices with low crosstalk across a wide spectrum. We have investigated methods for suppressing the higher-order modes using FDTD simulations. As the radial order of the WGM increases, the mode width increases and the peak of the mode shifts toward the center of disk. Therefore, we can manipulate the resonances by taking advantage of the spatial characteristics of the WGMs.