Download Three-dimensional photonic crystals for optical wavelengths

APPLIED PHYSICS LETTERS
VOLUME 81, NUMBER 17
21 OCTOBER 2002
Three-dimensional photonic crystals for optical wavelengths assembled
by micromanipulation
Kanna Aokia)
Semiconductors Laboratory, RIKEN, Saitama 351-0198, Japan
Hideki T. Miyazaki
Materials Engineering Laboratory, National Institute for Materials Science, Ibaraki 305-0047, Japan
Hideki Hirayama
Semiconductors Laboratory, RIKEN, Saitama 351-0198, Japan
Kyoji Inoshita and Toshihiko Baba
Department of Electrical and Computer Engineering, Yokohama National University, Kanagawa 240-8501,
Japan
Norio Shinya
Materials Engineering Laboratory, National Institute for Materials Science, Ibaraki 305-0047, Japan
Yoshinobu Aoyagi
Semiconductors Laboratory, RIKEN, Saitama 351-0198, Japan
共Received 31 May 2002; accepted 27 August 2002兲
We have established a profitable fabrication technique for three-dimensional 共3D兲 photonic crystals
for optical wavelengths. In our method, two-dimensional photonic plates, which serve as unit parts
for 3D structures, are prepared by the semiconductor nanofabrication technique. Then, these plates
are assembled into 3D structures by micromanipulation. Accurate lamination of the plates is assured
by linking fiducial holes of neighboring plates with matching microspheres. With this technique, we
have succeeded in fabricating 3D photonic crystals with one to four layers of woodpile structures.
From scanning electron microscope observation of the crystals, the periodic error was determined to
be within 50 nm. The optical properties of the crystals indicate existence of the photonic band gap
at the expected wavelength of 3– 4 ␮m. © 2002 American Institute of Physics.
关DOI: 10.1063/1.1515117兴
Since the concept of the photonic band gap was
presented,1,2 photonic crystals have attracted much attention
in the field of optelectronics, because they have the potential
to realize integrated optical devices with, for example,
thresholdless lasers,3 high-reflectivity mirrors, and polarizing
beam splitters. Three-dimensional 共3D兲 photonic crystals, in
particular, are considered to be the most desirable material
for such applications since they can control the flow of light
tridimensionally. However, recent studies have mostly involved two-dimensional 共2D兲 structures, even though these
structures allow the leakage of light in out-of-plane directions, because the realization of intricate 3D structures is
beyond the capability of current fine structure technologies.
The realization of 3D photonic crystals was triggered by the
achievement of centimeter-order Yablonovite4 and woodpile
structures,5 and the structure size was gradually scaled down.
For realization of the aforementioned photonic devices, techniques for introducing intricate defects and active regions are
a prerequisite. However, the existing techniques4 –15 have
mostly been developed for attaining defectless crystals,
which consequently gives rise to the problem of a lack of the
free design capability necessary to produce the complicated
structures required in device production. Also, they have
a兲
Electronic mail: [email protected]
other problems peculiar to each method, such as deterioration of photonic patterns by severe etching6,11–15 or the limitation of the ordered region.7,8
In our method, 2D photonic plates, which serve as unit
components for 3D crystals, are prepared by only a single
integrated circuit 共IC兲 processing procedure. Then, these
plates are assembled by micromanipulation. To ensure correct lamination of each layer, fiducial holes with a diameter
corresponding to twice the thickness of the plate are prepared
in the frame of the plates, and spheres with a diameter equal
to that of the holes are inserted into the holes as stoppers.
Since neighboring plates have fiducial holes in the same position, layers are automatically fixed at the proper position by
inserting spheres into the holes. With this method, a considerable number and a wide variety of 2D plates can be prepared through only one IC processing procedure, thus good
accuracy of the photonic structure is maintained throughout
the process, and much less processing is required than in
angle etching6 and layer-by-layer11–15 methods. In addition,
precisely aligned 3D lattices are automatically obtained regardless of the complexity of the crystal pattern and material.
Moreover, microscopic components, such as lasing elements,
can be laid out at arbitrary positions. Our technique can satisfactorily overcome the problems of existing techniques. In
this work, we demonstrate our technique by fabricating
woodpile structures. The crystals obtained are evaluated by
their optical properties.
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© 2002 American Institute of Physics
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Appl. Phys. Lett., Vol. 81, No. 17, 21 October 2002
Aoki et al.
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FIG. 1. SEM images of air-bridge photonic plates. 共a兲 Surface view. 共b兲
Bird’s-eye view.
FIG. 2. Schematic of plate-assembly procedure. 共a兲 Microspheres are inserted into holes of a substructure. Arrows indicate the positions where
spheres were inserted. 共b兲 An air-bridge plate is separated from a substrate.
共c兲 The plate is superposed on the substructure in 共a兲. 共d兲 The plate is fixed
on the substructure by inserting another microsphere into a residual hole,
and two more spheres are inserted for the next stacking. 共e兲 Cross section
through fiducial holes and spheres of the structure in 共c兲. 共f兲 Completed
crystal.
As the material for the crystal, indium phosphide 共InP兲
was chosen because III–V semiconductors can be applied
directly to existing electronic devices. First, air-bridged 2D
photonic plates, which serve as unit parts for 3D crystals,
were prepared. Indium gallium arsenide 共InGaAs兲 with a
thickness of 1.5 ␮m, which serves as a sacrificial layer for
the InP layer, was deposited on an InP wafer by the metalorganic chemical vapor deposition method. Then, a 0.5-␮mthick InP layer was formed on the InGaAs layer. Subsequently, a metal mask was prepared on the sample by
evaporation followed by spin coating of the electron beam
resist. Then woodpile patterns with a lattice constant of a
⫽1.4 ␮ m and a pile width of d⫽0.37 ␮ m were drawn on
the wafer by electron-beam lithography. Next, the patterns on
the resist were transferred to the metal mask by electron
cyclotron resonance etching, and then transferred to the InP
layer by inductively coupled plasma etching. Finally, the
InGaAs layer was selectively etched off with sulfuric acid
etchant so that the photonic plates were supported in air by
narrow bridges. Figures 1共a兲 and 1共b兲 show top and bird’seye views of air-bridged photonic plates, respectively. The
size of one plate is 25⫻25 ␮ m2 , and that of one photonic
pattern is 15⫻15 ␮ m2 . The junctures of the bridges and the
plates were notched for easy neat separation of plates. Since
one period of the woodpile structure consists of four layers,
we prepared four types of air-bridge plates. For easy assembly, all plates were marked with their pattern type and row
number.
The 2D photonic plates were stacked by
micromanipulation,16 which was based on techniques established for microsphere assembly,17 in the specimen chamber
of a scanning electron microscope 共SEM兲. Unlike in the case
of microspheres, components of various sizes must be assembled in our case. Thus, we prepared various glass needles
which were coated with chromium and gold to prevent
chargeup. The plates were stacked on a double-polished InP
wafer patterned with fiducial holes so that we could measure
the transmission properties of the crystals. Figure 2 shows a
schematic of the procedure of crystal fabrication. First, two
polystyrene microspheres are inserted into the fiducial holes
etched on a substructure using a probe with a tip thickness of
0.5 ␮m 关Fig. 2共a兲兴. Then the bridges of the plates are broken
by pushing them using a probe with a tip thickness of 10 ␮m
关Fig. 2共b兲兴. Since several probes were in the system, we
could continue the assembly procedure without breaking
vacuum. The separated plates adhere to the probe or the substrate, because electrostatic and/or van der Waals forces have
a stronger influence than gravitational force. The plates’ affinity to the probe or the substrate was tuned by adjusting the
accelerating voltage of the SEM. When a plate approaches
the microspheres inserted in the substructure 关Fig. 2共c兲兴, the
tops of the microspheres catch the fiducial holes of the plate,
and guide the plate to the appropriate position since the diameters of microspheres and holes are the same. Then the
layer is fixed by inserting another sphere into a residual
opening, and two more microspheres are inserted into the
holes of the first layer for the next stack 关Figs. 2共d兲 and 2共e兲兴.
The assembly procedures are repeated and finally form
multilayer 3D crystals 关Fig. 2共f兲兴. In this study, structures
with one to four layers were fabricated. Figures 3共a兲–3共c兲
show the top view, magnification of fiducial holes with microspheres, and magnification of fiducial holes without microspheres, respectively, of a four-layered woodpile structure. Since the sizes of a hole and a microsphere are the
same, the microspheres and holes were tightly interlocked, as
seen in Fig. 3共b兲. From the SEM image of a hole without a
microsphere in Fig. 3共c兲, we could not identify positional
fluctuation because the edges of the two layers were tightly
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Aoki et al.
Appl. Phys. Lett., Vol. 81, No. 17, 21 October 2002
FIG. 3. SEM images of the four-layer woodpile structure. 共a兲 Surface view.
共b兲 Enlargement of fiducial holes with microspheres in the area enclosed by
the white rectangle in 共a兲. 共c兲 Enlargement of a fiducial hole without a
microsphere in the area enclosed by the white square in 共a兲.
joined. However, when we took the fluctuation of the photonic pattern which was caused by dry etching into account,
the positioning error was found to be within 50 nm.
The optical properties of crystals were evaluated using a
Fourier-transform infrared measurement system for wavelengths from 1.4 to 14 ␮m at room temperature. The sample
was characterized using a beam collimated within a divergent angle of 20°, with the incident angle tilted 20° from the
normal of the crystal surface. The area exposed to light was
fixed at 15⫻15 ␮ m2 , which fitted the photonic patterns exactly, using a rectangular slit. To determine absolute transmittance and reflectance, reference spectra were taken from a
bare double-side-polished InP wafer and a gold film deposited by sputtering on a double-side-polished InP wafer, respectively. The reflection and transmission spectra of the
crystals are shown in Figs. 4共a兲 and 4共b兲, respectively. As the
number of layers increased, the reflection peak at 3– 4.5 ␮m
became stronger and clearer, and reached 60% for the fourlayer crystal. In the measurement of transmittance, a dip appeared in the same wavelength range as that where the reflectance peaks appeared, and reached 30% for the four-layer
crystal. The intergradations of the spectra in accordance with
the formation of the woodpile structure indicate the development of a photonic band gap. However, the intensities of the
peaks and gaps are slightly weaker than those in other
reports.7,8,11–15 Two main reasons are conceivable for these
results. The first is the obstructive light from outside the
photonic patterns due to diffraction, because the wavelength
of such light and the size of the photonic pattern area are
comparable. The second reason is the limitation of the incidence angle of the system. Noda et al.15 investigated the influence of the incidence angle on the photonic band gap of a
woodpile structure, and found that the intensity decreases
dramatically as the incident angle changes from the 具001典 to
the 具110典 direction. These problems will be clarified in the
FIG. 4. Transmission and reflection spectra of one- to four-layer woodpile
structures. The dips at 4.2 ␮m in all spectra are attributed to absorption of
carbon dioxide in air. 共a兲 Reflection spectra. 共b兲 Transmission spectra.
near future. The assemblage of thicker crystals and the introduction of defects are in progress.
The authors thank T. Sato and H. Morishita for their
support. This work was supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan. One of the authors
共K.A.兲 gratefully acknowledges the fellowship, President’s
Special Research Grant, provided by RIKEN.
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