Optical dipole antennas with varying length and width are fabricated using e-beam lithography. Antennas with wider width are shown to exhibit stronger scattering while preserving the same resonance frequency

Choo, Hyuck

Jamshidi, Arash

Lakhani, Amit

Miller, Owen

Seok, Tae Joon

Wu, Ming C.

Yablonovitch, Eli

Yu, Kyoungsik

Caltech Authors - Main

Characterization of Extended Width Optical  
Dipole Antennas 
Tae Joon Seok, Arash Jamshidi, Amit Lakhani, Kyoungsik Yu, Hyuck Choo, Owen Miller,  
Eli Yablonovitch, and Ming C. Wu 
Department of Electrical Engineering and Computer Sciences, University of California, Berkeley, CA 94720, USA 
Contact e-mail: tjseok@eecs.berkeley.edu 
 
Abstract: Optical dipole antennas with varying length and width are fabricated using e-beam 
lithography. Antennas with wider width are shown to exhibit stronger scattering while preserving 
the same resonance frequency.  
©2010 Optical Society of America 
OCIS codes: (240.6695) Surface-enhanced Raman scattering; (350.4238) Nanophotonics and photonic crystals 
1. Introduction 
Optical antennas offer interesting solutions in a variety of applications including biochemical sensing, imaging, data 
storage, wavelength conversion, and energy harvesting [1].  The ability to focus the light beyond the diffraction limit 
with optical antenna is especially important in improving the sensitivity of optical sensing techniques such as 
fluorescent detection [2] and surface-enhanced Raman spectroscopy (SERS) [3].  For example, bowtie optical 
antenna is employed to confine the incident light in the tight focal volume of the optical antenna, and the focused 
electromagnetic wave interacts with analyte molecules within the small focal volume [1, 2, 4].  However, in typical 
sensing applications, analytes are adsorbed uniformly on the substrates surface, and most analyte molecules lie 
outside the optical antennas focal volume because of low aerial density of the hot spots.  In this paper, we 
investigate the effect of antenna width variation on the antenna resonance wavelength.  By measuring the optical 
scattering spectra from antenna arrays, we found that wider antennas exhibit much stronger scattering intensity 
while maintaining the same resonance frequency.  By increasing the antenna width, we can effectively increase the 
hot spot volume which is important for SERS and other sensing applications. 
2. Optical Antenna Design and Fabrication 
Optical dipole antenna arrays with various lengths (220, 280, 340, and 400 nm) and widths (70, 100, 130, 150, and 
180 nm) are fabricated on an indium-tin-oxide (ITO)-coated glass substrate using high-resolution electron beam 
lithography and lift-off technique.  A 3-nm germanium layer is evaporated as an adhesion layer, and followed by 30 
nm gold evaporation. The Ge seed layer reduces the roughness of the gold surface from 3 to 1 nm, similar to the 
silver results in [5].  Each array contains identical dipole antennas with a 1-m pitch.  Figure 1(a) and (b) show 
examples of the patterned dipole antenna arrays and individual antenna with different dimensions.  According to our 
scanning electron microscope (SEM) measurements, antennas have ~16 nm wide feed gaps. 
 
   
(a) (b) (c) 
Fig. 1 (a) SEM of an optical dipole antenna array.  Identical dipole antennas are equally distributed with a 1-m pitch in one array.  (b) Magnified 
SEMs of individual optical dipole antennas.  Left and right column representatively show the dimension variation in antenna length and width, 
respectively.  Each antenna has ~16 nm gap separation.  (c) Electric field magnitude distribution of the dipole antennas excited by linearly 
polarized plane waves.  White arrows represent the polarization of the plane wave, incident from the surface normal direction. 
       To understand the resonant coupling to optical antennas, we simulate antenna arrays using a finite element 
method (FEM)-based computer software.  Dipole antenna arrays with a 1-m pitch are modeled with a periodic 
boundary condition and excited by plane waves from the top.  Figure 1(c) shows typical electric field profiles at 
resonance with two different polarizations.   The electric field is greatly enhanced only when the input polarization 
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is aligned with the antenna optical axis.  Our simulation also shows that the antenna resonance mainly depends on 
the antenna length. 
3. Experiments and Results 
Optical scattering from the antenna arrays are measured under broadband dark field illumination to characterize the 
antenna resonance in the visible and near-infrared wavelength ranges.  An inverted microscope with a 40x objective 
(Nikon TE2000) and a spectrometer are used to collect and analyze the scattered light from the optical antennas. 
       Figure 2(a) shows the measured and simulated scattering spectra for antennas with 220 nm and 280 nm length.  
Simulated scattering spectra are obtained by subtracting absorption cross sections of antennas from their extinction 
cross sections.  220 nm long antennas show measured resonance peaks at 763 nm and 280 nm long antennas show 
resonance peaks at 846 nm, which are red-shifted by 83 nm relative to the resonance peaks of 220 nm long antennas. 
Simulation results show similar trends but the resonance peak values are slightly red-shifted relative to the measured 
values. This is caused by the actual sample having an ITO-coated glass substrate which has a refractive index 
slightly smaller than ITO.  The substrate used for simulations is only an ITO layer with a higher refractive index, 
resulting in a red-shift in the resonance peak values. 
       Figure 2(b) shows the resonance wavelengths for 220 nm (black) and 280 nm (red) long antennas with various 
antenna widths.  Varying antenna width does not result in a considerable shift of resonance peak since kinetic 
inductance of antennas is not dominant in this large width regime.  Individual scattering spectra of 220 nm and 280 
nm long antennas are shown in Figure 2(c) and (d), respectively.  Dipole antennas with wider width exhibits 
stronger scattering signal.  The wider width also produces an extended hot spot that can greatly increase the surface-
enhanced Raman spectroscopy (SERS) signal. 
 
  
(a) (b) 
  
  
(c) (d) 
Fig. 2 (a) Experimental (solid line) and simulated (dashed line) scattering spectra of dipole antennas with 60 nm width and 220 nm length (black 
curves) and 280 nm length (red curves).  (b) Measured antenna resonance wavelength as a function of antenna width.  Black and red dots 
represent the antenna length of 220 nm and 280 nm, respectively.  (c) Measured antenna scattering spectra for 220 nm length and variable widths.  
(d) Measured antenna scattering spectra for 280 nm length and variable widths. 
4. References 
[1] P. Bharadwaj, B. Deutsch, and L. Novotny, Optical antennas, Advances in Optics and Photonics 1, 438-483 (2009). 
[2] A. Kinkhabwala, Z. Yu, S. Fan, Y. Avlasevich, K. Mullen, and W.E. Moerner, Large single-molecule fluorescence enhancements produced 
by a bowtie antenna, Nature Photonics 3, 654-657 (2009). 
[3] D. P. Fromm, A. Kinkhabwala, P.J. Schuck, and W.E. Moerner, Exploring the chemical enhancement for surface-enhanced Raman scattering 
with Au bowtie nanoantennas, J. Chem. Phys. 124, 061101 (2006). 
[4] P.J. Schuck, D.P. Fromm, A. Sundaramurthy, G.S. Kino, and W.E. Moerner, Improving the mismatch between light and nanoscale objects 
with gold bowtie nanoantennas, Phys. Rev. Lett. 94, 017402 (2005). 
[5] V.J. Logeeswaran, et al., Ultrasmooth silver thin film deposited with a Germanium nuclear layer, Nano Lett. 9, 178-182 (2009). 
Antenna Width Antenna Width 
       a2569_1.pdf  
 
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       CFI4.pdf 
 


Characterization of Extended Width Optical Dipole Antennas

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Characterization of Extended Width Optical Dipole Antennas

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