Waveguide Bragg Microcavity

Design of Bragg-grating based photonic waveguide microcavities using FDTD Solutions

Waveguide-based photonic microcavities and their associated high quality factor resonances are of increasing importance for a number of technological applications, including filtering and sensing. An example of these types of photonic microcavities is a six air-slot microcavity, with the central semiconductor tooth intentionally widened to introduce a transmission resonance within the stop band of the underlying Bragg structure. This particular photonic microcavity is designed to be supported by a high index substrate, unlike the alternative air membrane photonic microcavities.

Step 1: Construct the waveguide-based Bragg microcavity in the FDTD Solutions layout editor

The layout editor shows the positioning of all of the simulation objects. Different classes of objects (physical primitives, radiation sources, monitors) are color coded for easy identification. Objects can be moved and resized with simple mouse movements.

  • the multilayer waveguide is composed of different refractive index regions denoted by varying shades of blue
  • the orange region is the computation area, bounded by absorbing (PML) boundary conditions and an asymmetric boundary to speed the simulation and analysis process
  • yellow lines and symbols show measurement monitors - freely positioned by the end-user to collect simulation data of interes

 schematic of the Bragg waveguide microcavity in FDTD Solutions

Three-dimensional FDTD Solutions model of the Bragg waveguide microcavity.

Step 2: Use the built-in mode solver to calculate and launch a waveguide mode into the microcavity

The microcavity is excited by launching a TE-polarized guided mode of the input optical waveguide. The MODE Solutions mode solver, which is integrated within the FDTD simulation environment, allows the end user to easily select the desired waveguide mode for launch from the complete set of guided modes. This waveguide mode is incident upon the photonic microcavity, and the frequency response of the microcavity is measured using field profile simulation monitors.

built-in waveguide mode solver in FDTD Solutions

Built-in waveguide mode solver in FDTD Solutions allows you to solve for waveguide modes and use them as excitations in the FDTD model.

Step 3: Use broadband excitation to locate the cavity resonance

By placing a time monitor at the output of the waveguide microcavity, we can monitor how long it takes for the radiation to resonate within the structure and radiate away. By looking at the data to the left, we can see that the vast majority of the optical energy has radiated away 1000 fs after the start of the simulation. Knowing this allows us to terminate the simulation at the appropriate time without truncating the simulation data. Fast Fourier transform (FFT) of the time-domain signal shows that the Bragg structure has a large stop band which extends from 185 to 215 THz, with a defect transmission peak within the stop band.

time signal of waveguide cavity transmission

Time signal of waveguide cavity transmission.
fast Fourier transform of broadband signal, showing waveguide cavity resonance
FFT of broadband signal, showing waveguide cavity resonance, Bragg cavity stopgaps and transmission band around 1.5 microns.

Step 4: Measure the Q-factor of the waveguide resonance

Closer examination of the microcavity transmission resonance, as shown on the left, allows for one to measure the quality (Q) factor of the transmission peak defined as the ratio of the peak center frequency to its full-width half-maximum (FWHM).

microcavity quality factor simulated by FDTD Solutions

Microcavity quality factor resonance as simulated by FDTD Solutions.

Step 5: Measure the CW field enhancement on (top figure) and off (bottom figure) resonance

To measure the field enhancement within the microcavity, frequency-domain (CW) measurement monitors can be used to measure the steady-state field distribution. Further, frequency-domain monitors provide this data at multiple frequency points within a single simulation, offering tremendous advantage of a time-based simulation technique like FDTD over frequency-domain simulation tools.

steady-state (CW) field profile of microcavity on resonance

Steady-state (CW) field profile of microcavity on resonance extracted from frequency-domain profile monitor.

steady-state (CW) field profile of microcavity off resonance

Steady-state (CW) field profile of microcavity off resonance.  Most of the excitation light is reflected back into the input waveguide.