Refractive-Index Engineering of Planar
Waveguides with Subwavelength Gratings
Jens H. Schmid, Pavel Cheben, Przemek J. Bock, Jean Lapointe, Siegfried Janz,
Dan-Xia Xu, Adam Densmore, André Delâge and Trevor J. Hall
In integrated photonic circuits, the refractive-index contrast is usually
set by the choice of the material platform. For example, for silicon photonic
circuits operating at a wavelength near
l= 1. 55 μm, the waveguide core and the
cladding indices are given by the material constants of silicon (n = 3. 5) and
silicon dioxide (n = 1. 44), and waveguide
devices must be designed within the
constraint of these fixed values.
From free-space optics, we know
that periodic dielectric structures with
a periodicity smaller than one half of
the wavelength do not diffract any light.
Instead, such so-called subwavelength
gratings (SWGs) act as homogeneous
effective media with spatially averaged
refractive index. 1 We have recently
demonstrated the first use of SWGs for
refractive-index engineering in micro-photonic waveguides, providing a powerful method for controlling the refractive
index of a waveguide core in any specific
location of a photonic chip. Importantly,
our method only relies on standard fabrication techniques and can be implemented without any modifications to the
chip fabrication process flow.
The structure shown in (a) exemplifies
refractive-index engineering of a silicon
photonic wire waveguide. By etching
periodic gaps of a well-defined width w
and pitch L into a standard silicon photonic wire, an SWG waveguide is formed
with an effective core index determined
by the duty ratio w/L. Calculation of
the dispersion relation of the segmented
waveguide and comparison with the dispersion of an equivalent photonic wire
waveguide with identical cross section
and a core index of n = 2.65, as shown in
(b) confirms theoretically the concept of
spatial refractive-index averaging.
Experimentally, we have observed
waveguiding in such SWG structures
with a propagation loss as low as
SWG Wire 10 l = 1. 55 mm 31014
f [s– 1]
b [mm– 1]
(a) SEM image of an SWG waveguide. (b) Dispersion relation of an SWG waveguide and
an equivalent photonic wire waveguide with core refractive index of 2.65 (TE polarization).
(c) SWG waveguide crossings.
2. 1 dB/cm, comparable to the best
photonic wire waveguides reported,
and with a low and nearly wavelength-independent group index, as predicted
by theory. 2 Although consistent with
Bloch theory, it is fascinating to observe
light propagating almost unperturbedly
through so many strong discontinuities. 3
Among the applications of SWG
waveguides4 is an SWG slab waveguide
structure that simultaneously acts as
a lateral cladding for a photonic wire
waveguide in a novel microspectrometer
design and an efficient in-plane fiber-chip coupling structure. The coupler
structure works by gradual modification
of the waveguide core index, leading
to mode-size transformation between a
high-index photonic wire and the low-index optical fiber. Measured coupling
loss is 0.9 dB for TE and 1. 2 dB for
TM polarization. SWG waveguides
were also implemented for highly efficient waveguide crossings, 5 such as
those shown in (c).
Having the ability to intersect waveguides with low loss and crosstalk is an
important prerequisite for designing
complex high-density photonic circuits.
SWG waveguide loss per crossing was
measured to be as low as 0.02 dB with
polarization-dependent loss of less then
0.02 dB and crosstalk less than 40 dB.
These applications demonstrate the
obvious advantages of having the new
degree of freedom in photonic circuit
design afforded by SWG refractive-index engineering. t
Jens Schmid ( email@example.com), Pavel
Cheben, Jean Lapointe, Siegfried Janz, Dan-Xia
Xu, Adam Densmore and André Delâge are with
the National Research Council Canada in Ottawa,
Canada. Przemek Bock and Trevor Hall are with the
University of Ottawa.
1. S. M. Rytov. Sov. Phys. JE TP 2, 466-75 (1956).
2. P. J. Bock et al. Opt. Express 18( 19) 20251-62 (2010).
3. F. Morichetti. Spotlight on optics summary: www.optic-
4. P. Cheben et al. Opt. Lett. 35( 15), 2526-8 (2010).
5. P. J. Bock et al. Opt. Express 18( 15), 16146-55 (2010).
24 | OPN Optics & Photonics News