Two- and Three-Dimensional Photonic Crystals Built with VLSI Tools
Demonstration of 2D Guided
The ability to bend light sharply and
effectively is important for optical signal
routing at 1.55 ꢁm. Based on the same slab
structure, a 60ꢃ photonic-crystal bend can
Modes and 2D Bandgaps
To find the absolute intrinsic transmit-
tance of a 2D crystal, a reference transmis-
sion spectrum is taken from an identical
waveguide with no 2D hole array built in
the middle section. By rationing transmis-
sion signals, taken with and without a
35
be successfully fabricated (see Figure 7c).
The bend has a compact bending radius of
ꢂ1 ꢁm and is useful for connecting differ-
ent optical components in a compact way.
Two 60ꢃ bends are used to form a double-
bend device so that the input and output
light are parallel to each other. The ridge
input and output waveguides are designed
to have a lateral width of 31 a to better
match the modal extent of the photonic-
crystal waveguide. The measured bend-
ing efficiency shows a clear maximum value
2
D crystal, intrinsic transmittance is ob-
tained. This procedure eliminates external
uncertainties associated with reflection
at both the waveguide–crystal and the
waveguide–air interfaces. Consequently,
the normalization procedure allows for a
better determination of intrinsic transmit-
tance of a 2D crystal slab.
/2
of ꢂ100% at ꢂ ꢀ 0.272, but with a nar-
In Figure 7a, the measured and calcu-
lated TE (transverse electric) transmission
spectra are plotted as black dots and a
solid line, respectively. The frequency is
max
row bandwidth, ꢅꢀ ꢂ 30 nm. Future ef-
forts must be concentrated on improving
the bending bandwidth to ꢅꢀ ꢂ 100 nm.
The coupling efficiency between the ridge
waveguide and the photonic-crystal wave-
guide must also be improved for practical
applications.
expressed in a reduced unit ꢂ (a /ꢀ) and is
0
controlled by varying ꢀ and a independ-
0
ently. Here, a ꢀ 400 nm, 430 nm, and
0
4
60 nm, and ꢀ is tuned to 1320–1380 nm,
1
520–1580 nm, and 1620–1685 nm, respec-
Summary
tively. In the allowed band, ꢂ ꢆ 0.245,
light is guided and propagates freely in
the 2D plane. In the bandgap, ꢂ ꢂ 0.27,
Basic 2D and 3D photonic-crystal struc-
tures operating at optical wavelengths
have been experimentally realized in Si
and GaAs. The next challenge in photonic-
crystal research is to integrate superior
photonic-crystal devices on-chip, com-
pactly and effectively. The resulting opti-
cal subsystems will have an enhanced
optical functionality to meet, for example,
the needs for high-bandwidth communi-
cations networks. Another equally impor-
tant challenge is in the integration of
optically functional materials with a pho-
tonic crystal to achieve active photonic-
crystal devices. Two distinct examples are
the introduction of nonlinear materials for
high-speed optical switching and the in-
filtration of a gain medium for highly
efficient light-emitting applications.
ꢈ4
transmittance as low as ꢂ 2 ꢇ 10 is ob-
served. This is the condition at which light
is guided vertically by strong index guid-
ing and controlled horizontally by the 2D
bandgap. It is in this sense that light can be
controlled in all three dimensions using
a 2D photonic-crystal-slab structure. The
upper and lower TE band edges occur
at ꢂ ꢂ 0.34 and ꢂ ꢂ 0.25, respectively,
1
2
yielding a large gap-to-midgap frequency
30
ratio of 30%.
Figure 7. (a) Transmittance of a bulk
D photonic-crystal slab (solid circles)
2
2
D Photonic-CrystalWaveguides
and guiding efficiency of the triple line
waveguide in (b) (red triangles). The
calculated theoretical values are shown
by the solid lines; measured values
are shown by the respective symbols.
VB indicates the valence band,
andWaveguide Bends
A photonic-crystal waveguide can be
created by introducing a triple line defect
into a periodic 2D hole array (Figure 7b).
The linear defect acts as a highly efficient
1D optical channel for light-guiding in the
CB indicates the conduction band.
References
(
b) SEM image of a photonic-crystal
1. E. Yablonovitch, Phys. Rev. Lett. 58 (1987)
p. 2059.
GaAs high-index layer. The defect hole di-
ameter (d ꢀ 0.8a, where a is the lattice pa-
rameter) is slightly bigger than that of the
regular holes (d ꢀ 0.6a). It is noted that a
single line defect also supports a guiding
mode, but its center frequency is too close
to the lower photonic-band edge. On the
other hand, a triple line defect structure
has an effective index lower than that of a
single line defect, thus pushing the guided-
linear waveguide in GaAs. The
waveguide consists of a triple line
defect with a hole diameter (d ꢀ 0.8a)
larger than that of the regular holes
2. S. John, Phys. Rev. Lett. 58 (1987) p. 2486.
3. E. Yablonovitch and T.J. Gmitter, Phys. Rev.
Lett. 63 (1989) p. 1950.
4. K.M. Ho, C.T. Chan, C.M. Soukoulis, R.
Biswas, and M.M. Sigalas, Solid State Commun.
9 (1994) p. 413.
(
d ꢀ 0.6a). (c) SEM image of 60ꢃ
waveguide bend. Two 60ꢃ bends are
used to form a double-bend device so
that the input and output light are
parallel to each other.
8
5. E. Ozbay, A. Abeyta, G. Turtle, M. Tringides,
R. Biswas, C.M. Soukoulis, C.T. Chan, and K.M.
Ho, Phys. Rev. B 50 (1994) p. 1945.
6
. S.Y. Lin, J.G. Fleming, D.L. Hetherington,
mode frequency away from the lower
band edge and more into the bandgap.31
B.K. Smith, R. Biswas, K.M. Ho, M.M. Sigalas,
photonic-bandgap attenuation is strong
ꢈ4
W. Zubrzycki, S.R. Kurtz, and J. Bur, Nature 394
In Figure 7a, the measured and calcu-
lated guiding efficiencies are shown as
red triangles and a red line, respectively.
An efficient guiding of light is observed
at ꢂ ꢀ 0.26–0.29, consistent with the
theoretical prediction. Moreover, while
(T ꢂ 4 ꢇ 10 ) at ꢂ ꢂ 0.265, a near-perfect
(
7
1998) p. 251.
guiding efficiency of ꢂ100% is observed.
This correlation confirms that guiding of
light in this device is caused by the forma-
tion of a triple line defect and by the exis-
tence of a photonic bandgap.
. J.G. Fleming and S.Y. Lin, Opt. Lett. 24 (1999)
p. 49.
8
. H. Sozuer and J. Dowling, J. Mod. Opt. 41
(1994) p. 231.
9. K.M. Leung, Phys. Rev. B 56 (1997) p. 3517.
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MRS BULLETIN/AUGUST 2001