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1. Planar chiral metamaterials for biosensing applications
Sangeeta Murugkar*a
, Israel De Leona
, Matthew Hortona
, Hammam Qassima
, Jonathan Leachb
and
Robert W. Boyda, c
a
Department of Physics, University of Ottawa, Ottawa ON K1N 6N5 Canada; b
School of
Engineering & Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, United Kingdom;
c
The Institute of Optics, University of Rochester, Rochester, New York 14627 USA
ABSTRACT
There has been a considerable effort recently in the development of planar chiral metamaterials. Owing to the lack of
inversion symmetry, these materials have been shown to display interesting physical properties such as negative index of
refraction and giant optical activity. However, the biosensing capabilities of these chiral metamaterials have not been
fully explored. Ultrasensitive detection and structural characterization of proteins adsorbed on chiral plasmonic
substrates was demonstrated recently using UV-visible circular dichroism (CD) spectroscopy. Second harmonic
generation microscopy is an extremely sensitive nonlinear optical probe to investigate the chirality of biomaterials. In
this study, we characterize the chiral response of chiral plasmonic metamaterials using second harmonic generation
microscopy and CD spectroscopy. These planar chiral metamaterials, fabricated by electron-beam lithography, consist of
right-handed and left-handed gold gammadions of length 400 nm and thickness 100nm, deposited on a glass substrate
and arranged in a square lattice with a periodicity of 800nm.
Keywords: chiral plasmonic metamaterials, nanostructures, second harmonic generation, circular dichroism
1. INTRODUCTION
There is a growing interest in the development of plasmonic nanostructured surfaces or metamaterials for applications in
biosensing [1]. The coupling of light to surface plasmons results in a strong enhancement of the local electromagnetic
field, commonly referred to as “hotspots”. This significantly enhances the light-matter interaction at the surface of the
plasmonic metamaterial leading to applications involving surface enhanced Raman scattering (SERS) [2] and surface
enhanced coherent anti-Stokes Raman scattering (SECARS) [3]. Moreover, frequency of the surface plasmon resonances
excited by the incident light are highly sensitive to the dielectric environment surrounding the metal. This property of the
change in plasmon frequency as a function of the refractive index of the metal and the surrounding dielectric medium is
used in surface plasmon resonance (SPR) based biosensors [1].
Planar chiral plasmonic metamaterials (CPMs) have been extensively studied in the past decade for their interesting
optical properties such as negative index of refraction [4, 5] and giant optical activity [6, 7]. They consist of a two-
dimensional array of periodically arranged, chiral structures. These structures are subwavelength in size and are typically
made up of a metal-on-dielectric structure and written by electron-beam lithography or ion beam milling. The surface
plasmon resonances excited in the chiral nanostructured grating give rise to these unique optical properties. In fact, in
addition to the usual enhancements of electromagnetic field intensity at the surface of the plasmonic metamaterial, chiral
electromagnetic fields are induced therein such that the optical chirality is much higher than that of circularly polarized
light [8-10]. This increases the interaction of the incident light with chiral material adsorbed on the CPM leading to
enhanced sensitivity of a chiroptical measurement. Based on this principle, ultrasensitive detection and structural
characterization of proteins adsorbed on CPMs was recently demonstrated using UV-visible circular dichroism (CD)
spectroscopy [11,12].
Second harmonic generation (SHG) is the second-order nonlinear optical response of a material in the presence of a
strong electromagnetic field. In this process, light at twice the frequency of the incident optical field is generated due to
the induced second-order nonlinear polarization [13].
*smurugka@uottawa.ca
Plasmonics in Biology and Medicine X, edited by Tuan Vo-Dinh, Joseph R. Lakowicz,
Proc. of SPIE Vol. 8597, 85970Y · © 2013 SPIE · CCC code: 1605-7422/13/$18
doi: 10.1117/12.2004748
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2. 0 nnn800 nm
(a)
80 nmj
80 nm
. ->
800 nm
(b)
However, in the electric-dipole approximation, SHG is only allowed in materials that lack centrosymmetry and since
surfaces and interfaces inherently satisfy this condition, they are ideally suited for characterization based on SHG [14,
15]. In addition to the breaking of symmetry at the surface, symmetry is also broken in chiral molecules due to the lack
of inversion symmetry. This has led to polarization-dependent SHG spectroscopy becoming a very sensitive tool for
characterizing two-dimensional chirality [16-18]. The two enantiomers of the chiral molecules can be distinguished
based on this technique since the SHG signal from the chiral thin film has a different response when excited by left-
handed circular polarized (LCP) and right-handed circular polarized (RCP) light at the fundamental frequency. In
addition to spectroscopy, SHG microscopy has emerged as a viable in vivo imaging modality for label-free visualization
of structural changes to tissue for early disease diagnosis [19].
SHG has been successfully applied for studying the microscopic and macroscopic second-order nonlinear response of
CPM [20-26] of various shapes such as L-shaped [21], G-shaped [22], star-shaped [23] structures and twisted-cross gold
nanodimers [24]. In this paper, we investigate the SHG response of CPMs with gammadion-shaped unit cell. We first
describe the design of such a CPM including a summary of the main results of numerical simulations of the electric field
distribution. We experimentally characterize the chiral response of the CPM in the presence of chiral molecules adsorbed
on the surface by means of UV-visible CD spectroscopy. Our objective is to study the effect on the SHG intensity when
chiral molecules are adhered to the surface and compare with CD measurements.
2. DESIGN OF CHIRAL PLASMONIC METAMATERIALS
The unit cell of our CPM consists of a right-handed (RH) gold gammadion as shown in Fig. 1a with arm length of 400
nm, width 80 nm and period of 800 nm arranged in a square lattice. The structure comprises of a 100 nm thick gold
layer, on top of a 5 nm adhesion layer deposited on the borosilicate glass substrate (Fig. 1b). We performed numerical
simulations of electromagnetic fields in the vicinity of the RH-CPM covered by water, using a commercial finite-
difference time domain (FDTD) package (Lumerical version 8) with a mesh size of 4.0 nm. Permittivity values for gold
and glass were taken from Ref. 27, while the permittivity of water was taken as 1.33.
The simulated transmission, reflection and absorbance spectra are shown in Fig. 2 for RCP and LCP light incident on the
RH-CPM. The three prominent peaks (dips) in transmission (reflection) spectrum are attributed to the excitation of three
different plasmon resonances at ~ 550 nm, 650nm and 800 nm in the CPM. It can be seen that there is a larger difference
between the intensity of RCP and LCP light transmitted or reflected at 800nm. This gives rise to a large CD signal at
800nm as described in Section 3.1 below. The higher values of the simulated absorbance seen for lower wavelengths in
in Fig. 2b, is consistent with the higher absorbance for metals in the visible wavelength region. There are two keys points
to be noted in Fig. 2b: (i) The higher absorbance at ~600nm than at ~ 800nm implies that a larger electric field intensity
exists at the surface of the gold at 600nm and (ii) The absorbance is higher when RCP light as compared to the LCP light
is incident on the RH-CPM. Fig. 3 illustrates the FDTD simulation of the distribution of the electromagnetic field at the
center of the RH-CPM for the plasmon resonance at 800 nm. It is evident that the electric field density is much stronger
when RCP light is incident on the RH-CPM (Fig. 3b). This is consistent with the higher absorbance for RCP light at
800nm as seen in Fig. 2b.
Figure 1. Top view (a) and side view (b) of the unit cell of the RH-CPM covered with water.
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3. Figure 2. The simulations of (a) transmission and reflection spectra and (b) absorbance spectra for RCP and LCP light
incident on the RH-CPM.
Figure 3. FDTD simulations of the electric field distribution at the center of the right-handed CPM at 800nm in the presence
of water, when excited by LCP (left) and RCP (right).
3. CHARACTERIZATION OF CHIRAL PLASMONIC METAMATERIALS
The CPM used for the experiments were fabricated by the group of Dr. Nikolaj Gadegaard at the University of Glasgow.
The structure consists of right-handed gold gammadia with same dimensions as in Section 2, and arranged in a squared
lattice geometry with a period of 800 nm. The fabrication is achieved on a glass substrate using electron-beam
lithography and subsequent gold deposition (100 nm) and lift-off.
3.1 Circular Dichroism spectroscopy
UV - Vis CD spectroscopy is a well-established linear optical technique for characterizing chiral samples and involves
the measurement of the differential absorbance of left and right circularly polarized light incident on a chiral material. A
commercial CD spectropolarimeter (Jasco - J810) was used to probe the optical properties of the RH-CPM in the
presence of water and various chiral biomolecules. The sample consisting of a 1x1 cm2
glass chip was placed in a
custom-made holder and inserted inside a quartz cuvette with a path length of 1 cm, such that ~ 4x 107
gammadia were in
the optical beam path. In all the CD measurements, a blank sample was first taken with the gammadia in the presence of
water, and then followed by the gammadia in the presence of chiral samples. Fig 4 shows the experimental data obtained
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4. on a RH-CPM in the presence of water, left handed (L-) Tryptophan and right handed (D-) Arabinose. It can be seen that
there is a difference in the wavelength shift when the water is replaced by (L-) Tryptophan as opposed to D-Arabinose
and is consistent with earlier results [11]. It is also evident that there is a good match between the experimental and
simulation results of the CD spectrum of the RH-CPM. The CD spectrum is calculated by taking the difference in the
transmitted RCP and LCP electric field intensity from the FDTD simulations and dividing by the total transmitted field
intensity.
Figure 4. CD spectrum (experimental data) of RH-CPM in the presence of water (black), L-Tryptophan (blue) and D-
Arabinose (red). The modeled CD spectrum of RH-CPM in the presence of water is shown by the dashed line.
3.2 Second Harmonic Generation (SHG) Microscopy
The schematic of the experimental SHG microscopy setup is shown in Fig. 5. The light source is a femtosecond
Ti:sapphire laser (Tsunami, Spectra-Physics) producing ~100 fs pulses at a repetition rate of 80 MHz and with tunability
between 720 nm - 1000 nm. The diameter of the laser beam at 800 nm is increased using a pair of plano-convex lenses
and the light is guided into a home-built microscope. The sample is placed on a three-axis automated stage (Max341,
Thorlabs) that is computer controlled via Labview and can be scanned with a minimum step size of 60nm to generate an
image. An oil immersion objective (60x, 1.35NA, Olympus,) focuses the light into a spot size of ~400 nm at the sample.
The SHG light generated at the sample is collected in the backward direction by the same objective and is split off from
the incident pump light by means of a dichroic mirror (Chroma Technology) that reflects light less than 700 nm. This
light is filtered by a 700 nm short pass filter (Chroma Technology) to remove any residual excitation light while a band
pass filter centered at 400 nm with a 40 nm bandwidth is used to separate the SHG signal. This light is coupled into a
multimode fiber with a core diameter of 1mm and detected by a photomultiplier tube (H10721-20, Hamamatsu). The
power of the pump beam is controlled by means of a half wave plate and a polarization beam splitter and is ~ 7.5 mW at
the sample. A quarter wave plate inserted just before the objective enables excitation of the sample with left- or right-
handed circularly polarized light.
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5. Stage
i
sample
CP
Obj. lens
60X Oil
X/4 WP dichroic
mirror
telescope
V
lens
SHG +TPIF
SHG
I
IIII 1110,
I
SP filter
(700 nm)
1 1
PMT
BP filter
(400 nm)
beam
dump
X/2 WP
795 nm, 80MHz, 100 fs
Figure 5: Schematic of the SHG microscopy setup
The alignment of the SHG microscope was optimized by maximizing the SHG signal at 400nm generated in a nonlinear
KDP (Potassium Dihydrogen Phosphate) crystal. The KDP sample was then replaced by a sample of RH-CPM immersed
in water and sandwiched between two thin coverslips, such that it faced down towards the beam as illustrated in the inset
in Fig. 6a. As the sample height “z” is scanned through the beam focus, two SHG peaks are prominent in the z-scan as
seen in Fig. 6a and 6b. The peak at 2.48mm was confirmed to be the SHG signal from a single gold gammadion in the x-
y location of the focused light spot while the more intense SHG peak at 2.65 mm is from the glass-oil interface. It is
interesting to note that the SHG generated from the RH-gammadion by RCP light is twice as strong compared to the
SHG generated by LCP light. The SHG image of the RH-CPM obtained by scanning the stage when z = 2.48mm is
shown in Fig. 6c and Fig. 6d for incident RCP and LCP light, respectively. A periodic intensity distribution
corresponding to the SHG signal from the periodically spaced nanostructures in the CPM lattice can be clearly seen in
Figs. 6c and d. The pixel resolution in these images is 200nm and the false color scale indicates the voltage measured at
the PMT. Note that the background signal has been subtracted from the SHG images. Similar to the results shown in Fig.
6a and 6b, the SHG intensity maps of Fig. 6c and 6d indicate that there is ~30% stronger SHG signal when the RH-CPM
is excited by RCP light. This is consistent with the results of the simulation described earlier in Section 2.
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6. Figure 6. SHG intensity as a function of “z”, the distance of the sample from the microscope objective for (a) RCP light and
(b) LCP light. The schematic of the sample holder in the z-scanning configuration is shown in the inset of Fig. 6b. SHG
intensity map from RH-CPM when excited by (c) RCP light and (d) LCP light. The false color scale indicates the voltage
measured at the PMT and the background signal has been subtracted from the SHG images.
4. SUMMARY
Tang and Cohen [8] introduced the concept of optical chirality to describe the degree to which the electric and magnetic
field vectors wrap around a helical axis at each point in space. By means of precisely sculpted standing waves, they
demonstrated the existence of superchiral light [9] with increased optical chirality as compared to circularly polarized
light. An alternate route to achieving superchiral light is by means of using CPM [10, 12]. Chiral electromagnetic fields
are induced around the chiral nanostructures such that the optical chirality is much higher than that of circularly
polarized light [8-12] at the surface. This increases the interaction of the incident light with chiral liquid molecules
adsorbed on the CPM leading to enhanced sensitivity of the CD measurement. As reported here, we have established
capabilities in the design and simulations involving CPM. The chiral response of samples of RH-CPM in the presence of
chiral liquids was characterized using UV-Vis CD spectroscopy.
The origin of SHG in CPMs is primarily associated with the surface local field enhancement (hot spots) at the
fundamental frequency due to the surface plasmon resonance and the electromagnetic coupling between nanostructures.
We have characterized the SHG response of RH-CPM in the presence of water, by means of SHG microscopy. We
observed a significant increase in the SHG signal when RH-CPM is excited by RCP light. This is consistent with the
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7. going work includes investigating the SHG response of the RH-CPM in the presence of chiral liquid molecules adsorbed
at the gold surface.
The optical response of the CPM can be tuned by designing not only the shape and size of the individual unit cell in the
CPM [25], but also by the ordering of the metamolecules in the CPM [26]. Future work will explore the possibility of
optimizing design parameters of the CPM in order to achieve a uniform distribution of the enhanced electric field and
optical chirality.
5. ACKNOWLEDGMENTS
The authors are grateful to Dr. Nikolaj Gadegaard at the University of Glasgow for providing the CPM samples. They
would like to acknowledge the support of Prof. Tito Sciano in the Department of Chemistry at the University of Ottawa
for the availability of the CD spectropolarimeter and acknowledge the technical support provided by Dr. Emilio Alarcon
in the preparation of the liquid chiral samples and the CD measurements. The authors acknowledge the support of Prof.
Hanan Anis in the Department of Engineering, University of Ottawa, for the availability of the Tsunami laser used for
the SHG measurements and Prof. Pierre Berini in the Department of Engineering, University of Ottawa, for the
availability of the wet bench essential for sample preparation. The authors would like to thank Mr. Majid Naji for
providing the KDP crystal samples. RWB acknowledges support by the US Defense Threat Reduction Agency. This
work is supported by the Canada Excellence Research Chairs (CERC) Program and the Natural Sciences and
Engineering Research Council of Canada (NSERC).
REFERENCES
[1] Anker, J N., Hall, W. P., Lyandres O., Shah, N. C., Zhao, J. and Van Duyne, R. P., “Biosensing with plasmonic
nanosensors”, Nat. Mat. 7, 442 (2008).
[2] Sharma, B., Frontiera, R. R., Henry, A., Ringe, E. & van Duyne, R. P. “SERS: materials, applications, and the
future”, Mater. Today 15, 16–25 (2012).
[3] Steuwe, C., Kaminski, C. F., Baumberg, J. J. and Mahajan, S., “Surface Enhanced Coherent Anti-Stokes Raman
Scattering on Nanostructured Gold Surfaces”, Nano Lett. (2011).
[4] Pendry J. B., “ A chiral route to negative refraction”, Science 306, 1353−1355 (2004).
[5] Zhang, S.; Park, Y.-S.; Li, J.; Lu, X.; Zhang, W.; Zhang, X. “Negative refractive index in chiral metamaterials”,
Phys. Rev. Lett. 2009, 102, 23901.
[6] M. Kuwata-Gonokami, N. Saito, Y. Ino, M. Kauranen, K. Jefimovs, T. Vallius, J. Turunen and Y. Svirko, “Giant
Optical Activity in Quasi-Two-Dimensional Planar Nanostructures”, PRL 95, 227401 (2005).
[7] M. Ren, E. Plum, J. Xu and N. I. Zheludev, “Giant nonlinear optical activity in a plasmonic metamaterial”, Nat.
Comm. 1 (2012)
[8] Y. Tang and A.E. Cohen, “Optical chirality and its interaction with matter”, Phys. Rev. Lett. 104, 163901 (2010).
[9] Y. Tang and A.E. Cohen, “Enhanced Enantioselectivity in Excitation of Chiral Molecules by Superchiral Light”,
Science 332, 333 (2011).
[10]M. Schaferling, D. Dregely, M. Hentschel, and H. Giessen, “Tailoring Enhanced Optical Chirality: Design
Principles for Chiral Plasmonic Nanostructures”, Phys. Rev. X 2, 031010 (2012).
[11]E. Hendry et al., Nat. Nanotechnol. 5, 783 (2010).
[12]E. Hendry, R. V. Mikhaylovskiy, L. D. Barron, M. Kadodwala, and T. J. Davis, “Chiral Electromagnetic Fields
Generated by Arrays of Nanoslits”, Nano Lett. 12, 3640−3644 (2012).
[13]R. W. Boyd, Nonlinear Optics 3rd ed (Academic, 2008)
[14]Chen, C. K., de Castro, A. R. B. & Shen, Y. R. Surface-enhanced second- harmonic generation. Phys. Rev. Lett. 46,
145–148 (1981)
[15]P. Fischer and F. Hache, “Nonlinear Optical Spectroscopy of Chiral Molecules”, Chirality 17, 421 (2005)
[16]T. Verbiest, K. Clays, V. Rodriguez, “Second-Order Nonlinear Optical Characterization Techniques: An
Introduction”, CRC Press, (2009).
[17]L. Persechini and J. F. McGilp, “Chiral second-harmonic generation from small organic molecules at surfaces”,
Phys. Status Solidi B 249, No. 6, 1155–1159 (2012).
[18]S. Foerier, I. A. Kolmychek, O. A. Aktsipetrov, T. Verbiest, and V. K. Valev, “Optical Second Harmonic
Generation Chiral Spectroscopy”, ChemPhysChem, 10, 1431 – 1434 (2009).
increased distribution of “hot spots” observed in the FDTD simulations when RCP light is incident on the RH-CPM. On-
Proc. of SPIE Vol. 8597 85970Y-7
Downloaded From: http://proceedings.spiedigitallibrary.org/ on 03/05/2015 Terms of Use: http://spiedl.org/terms

8. [19]P. J. Campagnola, and C.-Y. Dong, “Second harmonic generation microscopy: principles and applications to disease
diagnosis”, Laser Photonics Rev. 5, No. 1, 13–26 (2011).
[20]M. Kauranen and A. V. Zayats, “Nonlinear plasmonics”, Nat. Phot. 6, 737 (2012).
[21]Canfield, B. K. et al. Linear and nonlinear optical responses influenced by broken symmetry in an array of gold
nanoparticles. Opt. Express 12, 5418–5423 (2004).
[22]V. K. Valev, X. Zheng, C.G. Biris, A.V. Silhanek, V. Volskiy, B. De Clercq, O. A. Aktsipetrov, M. Ameloot, N. C.
Panoiu, G. A. E. Vandenbosch, and V. V. Moshchalkov, “The origin of second harmonic generation hotspots in
chiral optical metamaterials”, Opt. Mater. Express 1, 36−45 (2011).
[23]Valev, V. K.; De Clercq, B.; Zheng, X.; Denkova, D.; Osley, E. J.; Vandendriessche, S.; Silhanek, A. V.; Volskiy,
V.; Warburton, P. A.; Vandenbosch, G. A. E.; Ameloot, M.; Moshchalkov, V. V.; Verbiest, T. “The role of chiral
local field enhancements below the resolution limit of second harmonic generation microscopy. Opt. Express 20,
256−264 (2012).
[24]M. J. Huttunen, G. Bautista, M. Decker, S. Linden, M. Wegener, and M. Kauranen, “Nonlinear chiral imaging of
subwavelength-sized twisted-cross gold nanodimers”, Opt. Mater. Express 1, 46 (2011).
[25]V. K. Valev, B. De Clercq, C. G. Biris, X. Zheng, S. Vandendriessche, M. Hojeij, D. Denkova, Y. Jeyaram, N. C.
Panoiu, Y. Ekinci, A. V. Silhanek, V. Volskiy, G. A. E. Vandenbosch, M. Ameloot, V. V. Moshchalkov, and T.
Verbiest. “Distributing the optical near field for efficient field-enhancements in nanostructures”, Adv. Mater.
(2012); DOI: 10.1002/adma.201201151
[26]H. Husu, R. Siikanen, J. Makitalo, J. Lehtolahti, J. Laukkanen, M. Kuittinen, and M. Kauranen, “Metamaterials with
Tailored Nonlinear Optical Response”, Nano Lett. 12, 673−677 (2012).
[27]E. D. Palik, Handbook of optical constants of solids (Academic Press, New York, 1985).
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