Applied Physics LettersMechanical properties of photopolymerized photonic crystal  PhC  structures having woodpile and
spiral three-dimensional architectures were examined using flat-punch indentation. The structures
were found to exhibit a foamlike response with a bend-dominated elastic deformation regime
observed at strain levels up to 10%. Numerical simulations of optical properties of these PhC
structures demonstrate the possibility of achieving a substantial and reversible spectral tuning of the
photonic stop gap wavelength by applying a mechanical load to the PhC

Juodkazis, Saulius

Misawa, Hiroaki

Mizeikis, Vygantas

Seet, Kock Khuen

Wegst, Ulrike G. K.

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Mechanical properties and tuning of three-dimensional polymeric photonic
crystals
Saulius Juodkazis, Vygantas Mizeikis, Kock Khuen Seet, and Hiroaki Misawaa
Research Institute for Electronic Science, Hokkaido University, CRIS Bldg., Sapporo 001-0021, Japan
Ulrike G. K. Wegst
Department of Materials Science and Engineering, Drexel University, 3141 Chestnut Street, Philadelphia,
Pennsylvania 19104, USA
Received 20 October 2007; accepted 19 November 2007; published online 11 December 2007
Mechanical properties of photopolymerized photonic crystal PhC structures having woodpile and
spiral three-dimensional architectures were examined using flat-punch indentation. The structures
were found to exhibit a foamlike response with a bend-dominated elastic deformation regime
observed at strain levels up to 10%. Numerical simulations of optical properties of these PhC
structures demonstrate the possibility of achieving a substantial and reversible spectral tuning of the
photonic stop gap wavelength by applying a mechanical load to the PhC.
© 2007 American Institute of Physics. DOI: 10.1063/1.2822825
Laser-induced photopolymerization in liquid resins and
solid photoresists attracts significant interest due to its appli-
cability for the fabrication of three-dimensional 3D micro-
structures. The recording of 3D microstructures consisting of
submicrometer sized features was demonstrated using direct
laser writing DLW.1–3 DLW can be used for the preparation
of elements of microfluidic and microelectromechanical sys-
tems, such as molecular sieves, thermal isolators, mi-
crodampers, and optomechanical converters.4,5 Spatially pe-
riodic polymeric microstructures can be utilized in photonics
and optoelectronics as photonic crystals PhC for the control
of optical phenomena via photonic band gap PBG and pho-
tonic stop-gap PSG effects.
Many of these applications would benefit from the pos-
sibility of dynamic and reversible adjustments of the struc-
tures’ parameters. For example, in PhC structures the central
wavelength of the PBG or PSG intervals scales with the lat-
tice period.6 An elastic deformation of the PhC would
modify the lattice period and tune the PBG or PSG regions.
So far, irreversible7 and reversible8 spectral tuning was dem-
onstrated only in synthetic opal PhC structures. Neither
mechanical properties of laser-polymerized microstructures
nor mechanical tuning of their optical response have been
addressed with respect to the tunability of PBG or PSB so
far. In this work, we study the mechanical properties and
deformability of 3D PhC structures having woodpile9 and
spiral architectures10,11 fabricated by DLW in the commercial
photoresist SU-8. Elastic properties determined from the
measured stress-strain curves, together with theoretical mod-
eling of the optical properties, suggest that a substantial
spectral tunability is obtainable with these structures in their
elastic regime.
Several 3D PhC structures were fabricated by femtosec-
ond DLW. Descriptions of the optical setup, SU-8 samples,
and their processing conditions can be found in our earlier
works1,12,13.
The architectures of woodpile W,9 square spiral S,11
and circular spiral C PhC structures are illustrated sche-
matically in Fig. 1a. Detailed descriptions of these archi-
tectures can be found in the above references. Specific to the
structures fabricated by DLW is the ellipsoidal elongation of
the recorded features along the direction of the fabricating
laser beam. Parameters of the fabricated samples are given in
the caption of Fig. 1.
The mechanical response of the structures was tested by
applying a flat-punch indentor with a micrometer diameter
along the z axis using a NanoIndenter XP MTS Company,
aElectronic mail: misawa@es.hokudai.ac.jp.
FIG. 1. a Schematic explanation of woodpile W, square spiral S and
circular spiral C architectures, and their parameters. For W, rod-to-rod
distance within the layer axy =2.0 m, layer-to-layer distance az=0.65 m,
and total number of layers N=12. For spiral structures, these parameters are
redefined. For both S and C, axy =1.0 m is the period of a square lattice in
the x-y plane on which spirals are centered, and az=2.64 m is the vertical
period of spirals. For S, Lxy =1.5 m denotes the projection of a straight
spiral segment on the x-y plane, for S, Lxy =1.5 m is the spiral diameter.
Total height of the S and C samples comprises N=4 spiral periods. All
structures consist of ellipsoidal SU-8 features elongated in the z-axis direc-
tion parallel to the laser beam during the DLW have diameters dxy
400 nm and dz960 nm, and on the x-y plane occupy an area of 60
60 m2. b SEM micrograph of a woodpile structure fabricated on a
glass substrate. The circular impression at the center is left by the flat punch
during the tests that exceeded the elastic deformation limit.
APPLIED PHYSICS LETTERS 91, 241904 2007
0003-6951/2007/9124/241904/3/$23.00 © 2007 American Institute of Physics91, 241904-1
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and recording the displacement. Figure 1b shows a scan-
ning electron micrograph of a woodpile PhC. Overall fabri-
cation quality can be assessed by examining its periphery.
The circular impression seen at the center is due to plastic
deformation that was ultimately induced by the flat punch.
The stress-strain curves of several PhC samples are pre-
sented in Fig. 2. Significant differences that can be seen in
Fig. 2 between the responses of nominally identical samples.
These are most likely due to random size variations of the
recorded features caused by mechanical instabilities of the
DLW setup and laser intensity fluctuations. Nevertheless,
woodpiles generally have the lowest structural modulus,
whereas circular and especially square spirals have a higher
rigidity. Table I lists parameters of the structures determined
from the stress-strain curves.
The qualitative mechanical behavior of all samples is
similar. At strains below about 10%, the stress increases lin-
early, signifying the region of reversible elastic deformation,
most likely due to bending of SU-8 rods; the slope of the
stress-strain curves in this elastic regimes is the, structural
modulus E see Table I. At higher strains, elastic collapse
occurs at the stress el, followed by densification of the
structure. Deformations above el are largely plastic and ir-
reversible. Structural collapse most likely occurs via a com-
bination of elastic buckling and the formation of plastic
hinges. Finally, as the strain increases even further, a steep
stress-strain response signifies that the cells have collapsed to
such an extent that opposite sides touch and further strain
compresses the solid SU-8 rod material.
The observed response is similar to that of foams or
lattices where bending deformation is dominant.14 In these
systems, structural modulus and elastic collapse stress in-
crease, while densification stress decreases with an increase
in relative density.14,15
Reversible compression of PhC structures along the z
axis by approximately 10% should induce a blueshift of
comparable magnitude of the PSG regions existing along the
same direction. As, at present, it is difficult to verify this fact
experimentally, the spectral tunability was assessed from the-
oretical optical transmission calculations before and after the
compression. For this, finite-difference time-domain FDTD
analysis was used. The calculations were done using FDTD
SOLUTIONS software from Lumerical, assuming parameters
of the structures similar to those of the fabricated samples.
Figure 3 shows the calculated transmission spectra of a
square spiral structure before and after the compression by
7%, which is below the elastic limit in all square spiral
samples. Parameters of the model structure used for the cal-
culations are indicated in the figure. The calculated spectra
reveal optical attenuation regions at the wavelengths of about
3.5 m and 1.6–1.7 m, which correspond to a fundamental
and a second-order PSG regions, respectively.2,16 Both PSG
regions exhibit notable blueshifts due to the deformation.
From a practical viewpoint, the second-order PSG is more
interesting due to its spectral proximity to the optical tele-
communications spectral range 1.3–1.5 m, where cou-
pling of optical radiation in and out of the small PhC samples
as well as spectral measurements can be conveniently per-
formed using tools e.g., optical fibers, lasers, and spectrum
analyzers available in the telecommunications industry. The
inset in Fig. 3 emphasizes that elastic compression leads to a
blueshift of the second-order PSG by approximately 70 nm
2.7% .
Hence, mechanical tuning of PhC structures recorded in
SU-8 by DLW seems to be a realistic goal for experimental
investigations in the near future. It may also be possible to
improve the elastic performance of these structures. The
elastic response depends on the ratio between the spatially
FIG. 2. Color online Stress-strain characteristics for a woodpile struc-
tures, and b square and circular spiral structures.
TABLE I. Parameters extracted from compression tests, E structural
modulus, el and el elastic collapse stress and strain, respectively, and
z height decrease prior to elastic collapse.
Sample E MPa el MPa el % z m
W No. 1 170±5 17.9 12.7 0.4
W No. 2 147±5 17.5 13.8 0.44
S No. 1 258±11 37.3 17.5 1.43
S No. 2 528±14 32.8 7.9 0.77
S No. 3 494±17 32.5 7.5 0.71
C 191±8 15.8 11.4 1.18
FIG. 3. Color online Optical transmission of square spiral structures cal-
culated by FDTD technique before and after the compressive deformation
by 7% along the z-axis direction. Parameters of the spiral are indicated in
the plot. The inset shows in detail the attenuation regions near 1.7 m
wavelength.
241904-2 Juodkazis et al. Appl. Phys. Lett. 91, 241904 2007
Downloaded 28 Feb 2008 to 129.25.130.76. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp
averaged density  of the structure and the density of
monolithic SU-8 s the latter value should take into account
nonuniform density of SU-8 features fabricated using Gauss-
ian laser beams17,18. The structural modulus generally scales
with the ratio r=  /s as EEsr
2
,
14,15,19
were Es is the
Young modulus of monolithic SU-8. Thus, the elastic re-
sponse can be controlled via the relative display of SU-8
filling fraction.
In conclusion, we have investigated the mechanical
properties of periodic polymeric microstructures fabricated
by DLW in photoresist SU-8. Stress-strain tests conducted on
3D PhC structures having woodpile and spiral architectures
indicate that their overall mechanical response is similar to
that found in foams and lattices. All of the investigated struc-
tures exhibit an elastic deformation regime for strain up to
about 10%. According to numerical FDTD simulations con-
ducted during this work, this regime allows reversible me-
chanical tuning of the PSG wavelength at infrared wave-
lengths within the range of tens of nanometers or a relative
range of about 3%, thus illustrating the possibility to exploit
the relationship between mechanical and optical properties of
PhC structures, for example, in tunable PhC structures or in
PhC-based optical strain or position sensors.
The authors thank Dr. S. Enders and Dr. S. Orso, Max
Planck Institute for Metals Research, Stuttgart, Germany for
experimental assistance with mechanical testing and scan-
ning electron microscopy. S.J. acknowledges financial sup-
port provided by a Grant-in-Aid from the Ministry of Edu-
cation, Science, Sports, and Culture of Japan 19360322.
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Mechanical properties and tuning of three-dimensional polymeric photonic crystals

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