Coupled spiral-shaped microdisk resonators with
asymmetric non-evanescent coupling
Xianshu Luo, Jonathan Y. Lee, and Andrew W. Poon
Photonic Device Laboratory, Department of Electronic and Computer Engineering,
The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong SAR, China
Tel: (852) 2358 7905; Fax: (852) 2358 1485; e-mail: email@example.com
Abstract: We demonstrate coupled spiral-shaped microdisk resonators with asymmetric non-evanescent
coupling between the two microdisks in silicon nitride. Initial experiments reveal reciprocal
throughput-port transmissions with high-Q. Simulations show input-direction-dependent drop-port
transmissions and asymmetric mode-field distributions.
OCIS code: (230.5750) Resonators, (230.3990) Microstructure devices.
Spiral-shaped microdisk resonators for unidirectional lasing have recently been proposed and demonstrated
by Chern et al. . It has been recognized that the unidirectional lasing emission is non-evanescently
out-coupled from the spiral notch. Inspired by this non-evanescent coupling to a microdisk resonator, we
previously proposed and demonstrated silicon-nitride spiral-shaped microdisk resonator-based three-port
channel filters, with a seamlessly butt-coupled waveguide at the spiral notch and an evanescently-coupled
waveguide at the cavity sidewall . Moreover, novel structures with interesting optical switching
mechanisms that are based on high-Q preserving non-evanescent coupling between a spiral-shaped
microdisk oscillator and a half-circular microdisk amplifier has also been recently proposed and
In this summary, we report our initial experimental demonstration of coupled spiral-shaped microdisk
resonators with asymmetric non-evanescent coupling between the two spirals via seamlessly jointed spiral
notches on a silicon nitride-on-silica substrate. Our experiments reveal that the coupled spiral-shaped
microdisks preserve high-Q with reciprocal waveguide throughput-port transmissions. Our
two-dimensional (2-D) finite-difference time-domain (FDTD) simulations suggest that the drop-port
transmissions, and the asymmetric mode-field distributions in the coupled microdisks, depend on the sense
of the lightwave circulations.
Fig. 1 Schematics of the coupled spiral-shaped microdisk resonator-based channel drop filters with input-coupling to the
first cavity (a) counterclockwise (CCW) orbits, and (b) clockwise (CW) orbits. Insets: zoom-in schematics of the
asymmetric non-evanescent coupling between the coupled-spirals. The blue waveforms depict the tail-to-tail mode
spatial overlap induced weak transmission. The red waveforms illustrate the nearly peak-to-peak mode spatial overlap
induced preferential transmission.
Figures 1(a) and 1(b) show the schematics of the coupled spiral-shaped microdisk resonator-based
channel drop filters. The structure comprises two identical spiral-shaped microdisk resonators that are
seamlessly jointed at their notches without interfaces, and two evanescently side-coupled waveguides for
input- and output-coupling. The spiral shape is defined in terms of the azimuthal angle dependent radius
r(φ) as : r(φ) = r0(1 - εφ/2π), where r0 = r(φ=0), and ε is a deformation parameter giving a notch
junction width of r0ε. The lightwave can be input-coupled to either the counterclockwise (CCW) or
clockwise (CW) orbits of the first cavity. The cavity light can partially transmit between the two
microcavities via the notch junction. The mode spatial overlaps between the two spirals at the notch
junction are not identical for the CW and CCW circulations because of the geometrical asymmetry at the
notch junction (insets of Fig. 1). Thus, the light transmissions from the first spiral to the second spiral
(likewise from the second spiral to the first spiral) are asymmetric between the two senses of circulations.
Specifically, the CCW circulation in the first spiral allows relatively weak partial transmissions to the
second cavity (blue dashed arrows), while light from the second cavity feedbacks to the first cavity CCW
orbits with relatively large partial transmissions (blue dashed arrows). Whereas, for CW circulation in
the first spiral, the lightwave can be preferentially transmitted to the second spiral (red solid arrow), while
light from the second cavity is weakly coupled back (red dashed arrow).
We fabricate the filters using standard silicon microelectronics processes on a silicon nitride-on-silica
substrate with a 1.1-μm-thick silicon nitride device layer on a 1.5-μm-thick silica under-cladding layer.
Figure 2(a) shows the top-view scanning electron micrograph (SEM) of the coupled-spiral microdisk filter
with measured spiral radius of ~25 μm, and the notch width of ~400 nm. The insets show the zoom-in
view SEMs of the notch-coupling and waveguide evanescent-coupling regions. The width of the
side-coupled waveguide is ~450 nm, and the gap spacing between the cavity sidewall and the waveguide
is ~300 nm.
Fig. 2 (a) Top-view SEM of our fabricated device on a silicon nitride chip, with r0 ~ 25 μm, ε ~ 0.016, β = 36o, w ~ 0.45
μm, and g ~ 0.3 μm. Insets: zoom-in view SEMs of the notch-coupling region and the lateral evanescent-coupling
region. (b) Measured TE-polarized throughput-port multimode spectra. CCW: blue dashed line. CW: red solid line.
(c) Measured TE-polarized drop-port multimode spectrum. The dotted vertical lines denote the corresponding
resonances between the throughput- and drop-port spectra. (d)-(e) Top-view scattering images at resonance wavelength
of 1550.1 nm for the CCW and CW orbits. The white arrows denote the input-coupled lightwave directions.
Figure 2(b) shows the measured TE-polarized (electric-field in plane) throughput-port multimode
spectra of the coupled-spiral microdisk filter. The throughput-port spectra corresponding to the CCW-
and CW-orbits in the first cavity are nearly identical, suggesting reciprocal transmissions despite the
asymmetric non-evanescent coupling between the two cavities. We measure a free-spectral range (FSR)
of ~7.3 nm, which is consistent with a single microcavity circumference. The highest measured Q-factor
is ~15,000, suggesting the non-evanescent coupling preserves high Q . The transmission intensity is
normalized to the intensity end-fired to the waveguide input-port. Figure 2(c) shows the measured
TE-polarized drop-port multimode spectrum of the coupled-spiral microdisk filter corresponding to the
CW-orbits in the first cavity. We note that most of the modal features find corresponding resonances in
the throughput-port spectra in Fig. 2(b). We also image the out-of-plane scattering with a top-view
microscope and a near-IR camera. Figures 2(d) and 2(e) show the top-view images of the scattering
intensity at resonance wavelength of 1550.1 nm (labeled by a star in Fig. 2(b)) for the two senses of
lightwave circulations. In both cases, the on-resonance images show pronounced scattering near the
cavity rim for the coupled-spirals, indicating that the light is indeed coupled from the first spiral to the
second spiral and forming resonances.
w = 0.3 μm, g = 0.3 μm, and refractive index n = 2.0. Figures 3(a) and 3(b) show the simulated
We also perform 2-D FDTD numerical simulations on a small-sized device, with r0 = 5 μm, ε = 0.06,
TE-polarized throughput- and drop-port spectra. The throughput-port spectra corresponding to the
CCW- and CW-orbits in the first cavity are nearly matched, which is consistent with the measurements
and suggesting reciprocal transmissions. However, the corresponding drop-port spectra only show
identical resonance wavelengths yet with distinct relative resonance peak heights and extinction ratios.
We attribute the asymmetric drop-port transmissions between the two senses of circulations to the
asymmetric non-evanescent coupling between the two microdisks.
of circulations at resonance wavelength of 1533.2 nm (labeled by a star in Fig. 3(a)). The modal
distributions show pronounced asymmetry between the two senses of circulations. While the CCW case
shows a preferred confinement in the first cavity, the CW case shows relatively high amplitude and evenly
distributed mode-field pattern along the coupled-cavity rims and the notch junction. This is due to the
mode spatial overlap induced asymmetric transmissions between the coupled-spirals as discussed in Fig. 1.
Figures 3(c) and 3(d) show the FDTD-simulated steady-state mode-field patterns for these two senses
Fig. 3 FDTD-simulated TE-polarized multimode (a) throughput- and (b) drop-port spectra of a small-sized coupled-spiral
microdisk filter. CCW: blue dashed line. CW: red solid line. (c), (d) Simulated steady-state mode-field patterns at
resonance wavelength of 1533.2 nm. Due to the asymmetric transmissions between the coupled spirals, the mode-field
patterns are highly asymmetric between the two senses of lightwave circulations.
silicon nitride-on-silica substrate. Our initial measurements showed that this structure can preserve high
Q-factors in multimode reciprocal throughput-port spectra. Our 2-D FDTD simulations suggest
reciprocal throughput-port spectra yet with asymmetric drop-port spectra and asymmetric modal
distributions between the two senses of lightwave circulations. We attribute such asymmetry to the
asymmetric non-evanescent coupling at the notch junction region between the two cavities. We envision
potential applications of such coupled-spiral microdisk structures for novel optical switching mechanisms
in coupled silicon Raman oscillator and amplifier [3-5].
In summary, we experimentally demonstrated the first coupled-spiral resonator-based filters in
resonators. The research is substantially supported by a grant from the Research Grants Council of The
Hong Kong Special Administrative Region, China (Project No. 618506).
The authors thank Prof. Richard K. Chang from Yale University for his insight on spiral microdisk
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