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.
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Step 1: Construct the waveguide-based Bragg microcavity in the FDTD Solutions layout editor
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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 interest
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Step 2: Use the built-in mode solver to calculate and launch a waveguide mode into the microcavity
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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.
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Step 3: Use broadband excitation to locate the cavity resonance
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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.
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- to find the resonance, use broadband excitation and measure the time response
- via the built-in fast Fourier transform, look at the frequency content of the measured time signal through
simple mouse actions
- note the resonance occurs at a frequency of 197.3 THz
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Step 4: Measure the Q-factor of the waveguide resonance
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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). The resonance shown has a quality factor in excess of 300. As the cavity quality
factor represents the ratio of the energy storage to the energy loss, achieving higher quality requires minimizing
radiation loss.
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Step 5: Measure the CW field enhancement on (top figure) and off (bottom figure) resonance
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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.
- on resonance, a six-fold intensity enhancement is observed
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- off resonance, it is seen that negligible radiation propagates through the cavity, and most is reflected from the
Bragg stack
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