Plasmonic Laser Antennas and Related Devices

Abstract

This paper reviews recent work on device applications of optical antennas. Localized surface plasmon resonances of gold nanorod antennas resting on a silica glass substrate were modeled by finite difference time-domain simulations. A single gold nanorod of length 150 or 550 nm resonantly generates enhanced near fields when illuminated with light of 830 nm wavelength. A pair of these nanorods gives higher field enhancements due to capacitive coupling between them. Bowtie antennas that consist of a pair of triangular gold particles offer the best near-field confinement and enhancement. Plasmonic laser antennas based on the coupled nanorod antenna design were fabricated by focused ion beam lithography on the facet of a semiconductor laser diode operating at a wavelength of 830 nm. An optical spot size of few tens of nanometers was measured by apertureless near-field optical microscope. We have extended our work on plasmonic antenna into mid-infrared (mid-IR) wavelengths by implementing resonant nanorod and bowtie antennas on the facets of various quantum cascade lasers. Experiments show that this mid-IR device can provide an optical intensity confinement 70 times higher than that would be achieved with diffraction limited optics. Near-field intensities ~ 1 GW/cm2 were estimated for both near-infrared and mid-IR plasmonic antennas. A fiber device that takes advantage of plasmonic resonances of gold nanorod arrays providing a high density of optical ldquohot spotsrdquo is proposed. Results of a systematic theoretical and experimental study of the reflection spectra of these arrays fabricated on a silica glass substrate are also presented. The family of these proof-of-concept plasmonic devices that we present here can be potentially useful in many applications including near-field optical microscopes, high-density optical data storage, surface enhanced Raman spectroscopy, heat-assisted magnetic recording, and spatially resolved absorption spectroscopy. Engineering and Applied Sciences Version of Record

Full text

Page 1

1448IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 14, NO. 6, NOVEMBER/DECEMBER 2008
Plasmonic Laser Antennas and Related Devices
Ertugrul Cubukcu, Student Member, IEEE, Nanfang Yu, Student Member, IEEE, Elizabeth J. Smythe, Laurent Diehl,
Kenneth B. Crozier, Member, IEEE, and Federico Capasso, Fellow, IEEE
(Invited Paper)
Abstract—This paper reviews recent work on device applica-
tions of optical antennas. Localized surface plasmon resonances
of gold nanorod antennas resting on a silica glass substrate were
modeledbyfinitedifferencetime-domainsimulations.Asinglegold
nanorod of length 150 or 550 nm resonantly generates enhanced
near fields when illuminated with light of 830 nm wavelength. A
pair of these nanorods gives higher field enhancements due to ca-
pacitive coupling between them. Bowtie antennas that consist of
a pair of triangular gold particles offer the best near-field con-
finement and enhancement. Plasmonic laser antennas based on
the coupled nanorod antenna design were fabricated by focused
ion beam lithography on the facet of a semiconductor laser diode
operating at a wavelength of 830 nm. An optical spot size of few
tens of nanometers was measured by apertureless near-field opti-
cal microscope. We have extended our work on plasmonic antenna
intomid-infrared(mid-IR)wavelengthsbyimplementingresonant
nanorodandbowtieantennasonthefacetsofvariousquantumcas-
cade lasers. Experiments show that this mid-IR device can provide
an optical intensity confinement 70 times higher than that would
be achieved with diffraction limited optics. Near-field intensities
∼1 GW/cm2were estimated for both near-infrared and mid-IR
plasmonic antennas. A fiber device that takes advantage of plas-
monic resonances of gold nanorod arrays providing a high density
of optical “hot spots” is proposed. Results of a systematic theo-
retical and experimental study of the reflection spectra of these
arrays fabricated on a silica glass substrate are also presented. The
family of these proof-of-concept plasmonic devices that we present
here can be potentially useful in many applications including near-
fieldopticalmicroscopes,high-densityopticaldatastorage,surface
enhanced Raman spectroscopy, heat-assisted magnetic recording,
and spatially resolved absorption spectroscopy.
Index Terms—Gold nanoparticles, near-field optics, surface-
enhanced Raman spectroscopy (SERS), surface plasmons (SPs).
I. INTRODUCTION
O
PTICAL applications of metals have been limited to re-
flectors, due to their large free electron densities [1],
Manuscript received October 7, 2007; revised October 29, 2007. First pub-
lished April 11, 2008; current version published December 24, 2008. This work
was supported by the Center for Nanoscale Systems (CNS) at Harvard Univer-
sity,amemberoftheNationalNanotechnologyInfrastructureNetwork(NNIN).
The work of E. Cubukcu, N. Yu, and F. Capasso was supported by the Air Force
Office of Scientific Research (AFOSR) under a Multidisciplinary University
Research Initiative (MURI) Program (Contract FA9550-05-1-0435). The work
of E. J. Smythe, K. B. Crozier, and F. Capasso was supported by the De-
fense Advanced Research Project Agency (DARPA) (Center for Micromechan-
ical and Plasmonic Systems) under Contract HR0011-06-1-0044. The work of
K. B. Crozier was supported by the Draper Laboratory.
The authors are with the School of Engineering and Applied Sciences,
Harvard University, Cambridge, MA 02138 USA (e-mail: cubukcu@fas.
harvard.edu; nyu@fas.harvard.edu; smythe@fas.harvard.edu; ldiehl@deas.
harvard.edu; kcrozier@deas.harvard.edu; capasso@deas.harvard.edu).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/JSTQE.2007.912747
even in the optics laboratories until the middle of the last
century. Surface plasmons (SPs) and surface plasmon po-
laritons (SPPs) have provided a basis for new applications
[2]–[7]. Nowadays, they are of interest in a broad range of re-
search fields including near-field optical microscopy [8]–[12],
chemical and biological sensing [13]–[16], single molecule
spectroscopy [17]–[20], subwavelength optics [20]–[33], left-
handed metamaterials [35]–[46], and high-density optical data
storage [47]–[54] among many others.
The SPP modes are physical solutions of Maxwell’s equa-
tions at an interface between a metal and a dielectric. These
interface waves are possible since a metal has a negative
dielectric constant whereas that of the dielectric is posi-
tive [55]. This yields a mode that decays evanescently into
both materials and propagates, accompanied by a signifi-
cant longitudinal electric field component along the inter-
face, with a propagation length associated with losses in the
metal.
For frequencies below the terahertz range, in which met-
als behave like a perfect conductor, the SPs exhibit a linear
dispersion since most of the energy is in the tail of the SPs
that decays into the dielectric side of the interface. However,
at optical frequencies, the SPs have unique dispersion char-
acteristics; they can have extremely large propagation vectors
(i.e., ultraviolet wavelengths at visible frequencies) since met-
alshavestronglyfrequency-dependent dielectricconstants[55].
This property makes SPs important for nanophotonics since
it allows light to be concentrated to subwavelength regions
[3], [4].
ExcitingSPPsonmetalsurfacesrequirescareatopticalwave-
lengths [13]. First, the surface-parallel wavevector mismatch
needstobecompensatedfor.Themostcommonlyusedschemes
for exciting SP modes are prism and grating coupling configu-
rations [7]. The latter can compensate for the momentum mis-
match by taking advantage of the periodicity of the grating. The
former usually involves a glass prism. When light is incident on
the prism–metal interface at the correct angle, the SPs on the
top metal–air interface will be excited since the wave vector in
the glass prism can be as large as that of the SP mode. Sen-
sors based on prism coupling can provide single atomic layer
sensitivity [14], [15] since SP resonances are very sensitive to
changes in the vicinity of surface due to their evanescent tails.
This technology is commercially available for applications in
life sciences from Biacore AB [56].
Ebbesen’s pioneering work [32] on subwavelength hole ar-
rays in silver films demonstrated that the power transmitted
throughsuchaholearrayishigherthanthatisimpingingdirectly
on the total area of the holes. This “extraordinary” transmission
1077-260X/$25.00 © 2008 IEEE
Authorized licensed use limited to: IEEE Xplore. Downloaded on January 14, 2009 at 01:32 from IEEE Xplore. Restrictions apply.

Page 2

CUBUKCU et al.: PLASMONIC LASER ANTENNAS AND RELATED DEVICES1449
can be explained by coupling of SPs on either side of the metal
film and scattering by the hole array [34].
Metallic nanoparticles offer near-field enhancement due to
their localized SP resonances [57]. Gold and silver colloidal
nanoparticles are responsible for the red and yellow colors in
medievalstainedglasswindows[57].AlthoughFaradaystudied
metallic nanoparticles, the first rigorous theoretical treatment
was introduced by Gustav Mie in 1908 [58]. Mie presented
an exact solution to Maxwell’s equations that accounts for the
extinction spectra of metallic nanospheres of arbitrary size [59].
Whenametallicnanosphereisilluminatedwithaplanewave,
a conduction current oscillating at the same frequency as the
incident light will be induced in the nanoparticle. If we take a
snapshot of the charge distribution in the particle, we would see
electronspilingupononesideandpositivelychargedionsonthe
other. In the near field of the particle, the electric field is larger
than the incident field due to the Coulomb field generated by
thesechargesanddominatedbythenonpropagatingcomponents
that decay rapidly with distance. This singular behavior of point
dipoles accounts for the very short range of the optical near
fields [57].
In recent years, a variety of schemes taking advantage of
localized near fields generated by metallic nanoparticles have
been proposed to use them as optical nanoantennas [60]–[69].
Optical antennas consisting of nanometer size metallic parti-
cles can be used to improve the size mismatch between the
diffraction limited spot of the excitation light and fluorescent
molecules that are much smaller than the excitation wave-
length [65]. Optical antennas can produce very high near-
field intensities when they are optically excited with a wave-
length suitably matched to the antenna size due to their lo-
calized SP resonances [66]. Passive optical antennas were first
demonstrated in the microwave regime by Grober and cowork-
ers [70]. Also, mid-infrared (mid-IR) passive antennas [60]
and bowtie antenna-based bolometers [71], [72] have been
implemented.
Of particular interest are resonant strongly coupled nanorod
opticalantennasthatleadtoalargeintensityenhancementinthe
gap.Aizpuruaetal.[73]carriedoutacomprehensivetheoretical
analysis of field enhancement with single and coupled metallic
nanorods. More recently, resonances of nanorod optical anten-
nas designed for near-infrared wavelengths were characterized
using two-photon photoluminescence generated in gold [66]. A
scanningprobemicroscopewithabowtieantennaprobeforvisi-
blespectrumwasalsoutilizedforimagingsinglesemiconductor
quantum dots [67], [68].
Near-field enhancement generated by optical antennas is
useful for surface-enhanced Raman spectroscopy (SERS)
[17]–[19], [64]. Recently, metallic nanoparticles have been
utilized in biosensing [74], cancer treatment [75], and near-
field probes [10]. Optical nanoantennas can also increase
the excitation and emission rates of fluorescent molecules
[11], [12].
This paper is organized as follows. In Section II, we dis-
cuss our simulation results on single and double gold optical
antennas. Section III discusses our experimental results with
near-infrared and mid-IR active plasmonic laser antennas. In
Fig. 1.
(n = 1.46) substrate.
Simulated geometry: single gold nanorod resting on a SiO2
Section IV, a fiber device based on plasmonic resonances of
gold nanorod antenna arrays is proposed, and measurements of
SP resonances in these arrays fabricated on glass substrate are
also presented.
II. OPTICAL ANTENNA DESIGN AND MODELING
A. Single and Coupled Nanorods
We first investigated plasmonic resonances of single gold
nanorods on a silica glass substrate as shown in Fig. 1. A com-
mercialfinitedifferencetime-domain(FDTD)codewasusedfor
the simulations. For the modeling, perfectly matched absorbing
boundary conditions were introduced in all directions in order
to eliminate spurious reflections. The single plasmonic nanorod
antenna is illuminated from the substrate side by a monochro-
matic plane wave excitation of 830 nm free-space wavelength
polarized along the nanorod length.
When the plane wave is incident on the gold nanorod,
the conduction electrons in the metal respond to the elec-
tric field by generating a polarization field that leads to
charge accumulation at the opposite ends of the nanorod. This
charge buildup, which is quasistatic in the dipole approxi-
mation, results in a strong optical near-field enhancement at
the antenna ends [73]. We have calculated the peak intensity
enhancement at the same level as the top nanorod surface. By
varying the length L of a single nanorod, we determined the
resonant antenna lengths for which the near-field enhancement
is maximum. The ends of the nanorods are realistically rounded
off with a 20 nm radius of curvature. The nanorod has a square
cross section of 40 nm × 40 nm. A Drude model, fitted to
the experimental literature values [76], is used for the dielec-
tric constant of gold. The refractive index of the substrate is
n = 1.46.
When the length of the nanorod is an odd integer multiple of
half the SP wavelength corresponding to the incident radiation,
the nanorod acts like a low-quality factor resonator for the SPs
induced in the nanorod [24], [78]. The current density has its
maximum in the middle of the nanorod and gradually dies off
toward the antenna ends for resonant antenna lengths. Since for
a monochromatic field, the charge density is proportional to the
gradient of the current density via the continuity equation, the
charges will pile up around the edges generating the enhanced
Authorized licensed use limited to: IEEE Xplore. Downloaded on January 14, 2009 at 01:32 from IEEE Xplore. Restrictions apply.

Page 3

1450IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 14, NO. 6, NOVEMBER/DECEMBER 2008
Fig. 2.
nanorod(redline)andinthemiddleofa20nmgapofapairofcouplednanorods
(blue line) as a function of the antenna length. The two peaks correspond to
λeff/2 and 3λeff/2. The excitation wavelength isλ0= 830 nm.
Optical near-field intensity enhancement around the ends of a single
near fields. Fig. 2 shows the time-averaged intensity enhance-
ment at the nanorod end normalized to the incident intensity as
a function of the antenna length. The first resonance occurs for
a nanorod length of 150 nm and the second for 550 nm. These
resonances are analogous to the λ/2 and the 3λ/2 resonances,
respectively, of an RF antenna. For L = 150 nm, the near-field
intensity is ∼400 times the incident intensity and ∼180 times
for L = 550 nm.
Here, we can say that the two resonances at L = 150 nm and
L = 550 nmcorrespondtoanoddhalfintegermultipleoftheef-
fectivewavelengthdefinedasλeff= λ0/neff,whereλ0and neff
are the free-space wavelength (830 nm) and the effective in-
dex of the guided SPP mode of the nanowire, respectively
[78]–[80]. For L = 150 nm, the first dipolar resonance is at
λ0= 830 nmwhereasthefirstdipolarresonanceforthe550nm
antenna is at λ0≈ 2 µm, and the higher order dipole active
mode at λ0≈ 830 nm. Thus, we are exciting the higher order
dipolar mode of the 550 nm nanorod antenna when the incident
radiation is at the latter wavelength.
The second resonant length is not quite 3 times the first due
to the reactance of nanorod ends [80]. L = 550 nm resonance
yields a lower near-field enhancement than the L = 150 nm
one since bigger antenna dimensions mean higher radiation and
material losses for the same wavelength. When the resonance
condition is reached, there is a standing wave optical current
distribution present in the nanorod [82]. However, in the full-
wave antenna case for λeff= 350 nm, the opposite ends of the
nanorod have thesamesignofaccumulated charges.Thisyields
averyweakdipoleandnonear-fieldenhancementattheantenna
ends.Thesenanorodplasmonicantennasexhibitsignificantfield
components in all three directions in the near zone, as would be
the case near any dipole.
Forourpurpose,wheretheintensityenhancementinthenear-
field matters, both the λeff/2 and the 3λeff/2 antennas give sim-
ilar field distributions at the antenna ends but have different
far-field radiation patterns. Since nanorods are inherently elon-
Fig. 3.
minated with a plane wave polarized along them for different antenna lengths:
(a) L = λeff/2 and (b) L = 3λeff/2 where λ0= 830 nm.
Electric field amplitude enhancement when nanorod antennas are illu-
gated with a high-aspect ratio, the quadrupolar contribution is
insignificant [57]. The steady-state time-averaged x-polarized
field amplitude distributions around the nanorod antennas are
shown in Fig. 3. The field amplitudes are normalized to the
incident plane wave field amplitude.
To investigate coupling effects between nanorods, we studied
a pair of gold nanorods on glass substrate separated by a 20 nm
gap. When two nanorods are brought together, the near field
generated by one will affect the other. This near-field interac-
tion between the pair will change the resonant frequency of the
coupled system relative to the isolated nanorod case [73], [82].
In a given nanorod, the near-field generated by the neighbor-
ing nanorod counteracts the local electric field induced by the
incident field in the nanorod. This will in turn redshift the res-
onance wavelength [82]. Since the resonance shifts to longer
wavelengths with increased near-field coupling, a shorter an-
tenna length compared to an isolated nanorod is needed for
resonant enhancement at the wavelength of the single nanorod.
In our simulations for the coupled pair case, we kept all the
geometrical parameters constant except for the antenna length.
We have calculated the near-field intensity enhancement in the
gap as a function of the antenna length. Similar to the single
nanorod antenna case, we observed two resonances for antenna
lengths between 100 and 700 nm (Fig. 2). The first resonance
occurs at L = 130 nm and the second at L = 540 nm. The
enhancement for the first peak is about 800.
The sign of the interaction field between the two nanorods
can alternate as a function of the gap between them [82], [83].
When the latter is larger than a quarter of a wavelength, the
interaction is mostly due to radiative coupling, and the phase
of the radiated field will change as it propagates from one an-
tenna to another [83]. For short enough distances, the near-field
interaction dominates and the gap between the nanorods acts
as a nanocapacitor with opposite charges on facing antenna
ends [62]. Although the field enhancement is mostly in the
Authorized licensed use limited to: IEEE Xplore. Downloaded on January 14, 2009 at 01:32 from IEEE Xplore. Restrictions apply.

Page 4

CUBUKCU et al.: PLASMONIC LASER ANTENNAS AND RELATED DEVICES1451
Fig. 4.
nanorods for L = 130 nm and a gap of 20 nm.
Steady state near-field intensity distribution for a pair of coupled
antenna gap, there is also significant enhancement at the far
ends of the nanorods, as shown in Fig. 4.
Large near-field enhancements [84], [85] are of strong inter-
est for many applications. As shown earlier, one way of getting
such intensity enhancements in the near field of metallic par-
ticles is to have a pair of nanorods separated by a nanometric
gap. The aforementioned capacitor analogy explains this very
well as the capacitance is inversely proportional to the gap be-
tween the particles. For the case of nearly touching nanorods,
one expects huge near-field enhancements [73]. In our antenna
design we limit the interparticle gap to 20 nm that is feasible for
nanofabrication.
B. Triangular and Bowtie Antennas
Another way of increasing the near-fields is to engineer the
antenna terminations and shapes. It is well known in electro-
statics that sharp corners concentrate static charges, i.e., the
“lightning rod” effect. In this section, we will focus on triangu-
lar gold particles that have sharp corners and coupled pairs of
these particles so as to increase the near-field enhancement. We
studied resonances of 40-nm-thick gold triangular and bowtie
antennas on a silica glass substrate. The antennas are illumi-
nated by a plane wave polarized along the x-axis, which has a
wavelength of 830 nm. Same boundary conditions as the sin-
gle nanorod problem are used. The outline of these triangular
antennas and their optical near-field intensity enhancement dis-
tribution are shown in Fig. 5. The tip radius of curvature for
these particles is chosen to be 10 nm and the apex angle is 60◦.
The solid red curve in Fig. 6 shows the x-polarized near-field
intensity enhancement at the apex of triangular antennas as a
function of the antenna length L. The peak intensity enhance-
mentsoccurforL = 180 nmandL = 700 nm.Thesetworeso-
nancescorrespondtoanear-fieldenhancementof530and260at
the antenna apex, respectively. The conduction current induced
in the metallic antennas by the incident plane wave for these
resonances flows from the wide end of the particle toward the
apex of the particle, and then vice versa as the incident polariza-
tion changes sign during its normal cycle in time. This suggests
a dipolar nature for these L = 180 nm and L = 700 nm reso-
nances. In contrast to the isolated nanorod antenna case, trian-
gularantennasconcentratetheelectromagneticenergymostlyat
the apex of the particle owing to the “lightning rod” effect. The
ratioofthenear-fieldintensityonthewidebaseofthetriangleto
Fig. 5.
with L = 180 nm and (b) around a pair of triangular particles, or bowtie anten-
nas for L = 140 nm and a gap of 20 nm.
Near-field intensity distribution (a) around a triangular gold particle
Fig. 6.
and coupled triangular particles (bowtie antenna) versus antenna length L. The
inset shows the modeled geometry. The apex angle was kept constant at 60◦.
Near-field enhancement respectively at the tip and in the gap for single
that at the apex is small, whereas the nanorods provide identical
intensity enhancement on both ends of the particle due to their
symmetry.
The near-field response curve for the triangular antennas ex-
hibits another feature (solid red curve in Fig. 6); the resonance
peaks are broader compared with the nanorods and evolve into
shoulders between L = 250 and 400 nm as well as between 800
and 900 nm. For these length intervals, weak quadrupolar reso-
nances become more pronounced since the size of the triangular
particle in both x- and y-directions is comparable to the wave-
length of incident light [86], [87]. These quadrupolar modes
have significant near-field enhancement around the edges of the
particle as the induced current flows from one edge to the other.
Bowtie antennas, namely a pair of coupled triangular gold
particles, offer higher near-field enhancements and better
Authorized licensed use limited to: IEEE Xplore. Downloaded on January 14, 2009 at 01:32 from IEEE Xplore. Restrictions apply.

Page 5

1452IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, VOL. 14, NO. 6, NOVEMBER/DECEMBER 2008
spatial confinement. When two triangular particles are placed
side by side with the apices facing one another, the capaci-
tive coupling between them will generate even higher fields
than those generated by an isolated particle. The blue curve
of Fig. 5 shows how the near-field intensity enhancement in
the gap of a bowtie antenna varies as the length of the an-
tenna is increased. There are four discernible features in this
curve. For L = 140 nm and L = 660 nm, the enhancement is
1200 and 700, respectively. These are the dipolar resonances
and their resonant lengths are slightly shorter than those of the
isolated triangle. This is consistent with the expected redshift
whenthenearfieldofthepairiscoupledcapacitivelyviathegap.
The peaks at L = 360 nm and L = 860 nm originate from the
quadrupolar resonances of the isolated triangular antenna.
Compared to the other antenna designs, bowties seem to of-
fer the largest near-field enhancements and the smallest spatial
extent for the field, i.e., a single sharp optical spot. As shown
in Fig. 6, most of the electromagnetic energy is localized in
the gap of the bowtie unlike the coupled nanorods where there
is significant field enhancement on the far ends. This property
of bowties is crucial for applications requiring a very intense
spatially confined optical spot.
III. PLASMONIC LASER ANTENNAS
A. Near-Infrared Antennas
Recently, active optical devices that can generate a subwave-
lengthopticalspothavebeenofinterestforvariousapplications.
Very small aperture lasers that consist of a laser diode with its
facet coated by a metal film on which a subwavelength aper-
ture is etched by focused ion beam (FIB) milling have been
demonstrated [47]. Panasonic, Inc., has introduced a vertical
cavity surface-emitting laser (VCSEL) based on a “SP mirror,”
namely, a silver nanohole array [88]. However, these devices
suffer from limited throughput even though the transmission is
enhanced when normalized to the aperture area [89], [90].
A C-aperture design has also been utilized [49]–[54]. The
C-aperture offers several orders of magnitude higher power
transmission compared to a square or circular aperture [54].
When a C-aperture is used to achieve a small optical spot in its
near field, which requires an aperture area much smaller than
the wavelength of light, its throughput is still not substantial.
In this section, we discuss a photonic device, the plasmonic
laser antenna, which consists of a resonant optical antenna in-
tegrated on the facet of a laser diode [91]. This device relies on
the localized SP resonances of the optical antenna that yields
large localized near-field enhancements rather than an aperture
that offers limited transmission. Such a compact laser source
with subwavelength spatial resolution provides distinct advan-
tages in a number of applications including microscopy, spec-
troscopy, optical data storage, lithography, and laser processing.
AschematicofthedeviceisshowninFig.7.Abowtieantennaor
a pair of coupled nanorods separated by a very small gap, which
is less than 30 nm in this paper, is defined on one of the facets of
a diode laser by FIB milling. Since the output of the laser diode
is polarized parallel to the quantum wells, the optical antenna
was oriented accordingly to achieve resonant excitation of SP in
Fig. 7.
integrated on the facet of a semiconductor laser diode. Shown is also a scanning
electronmicroscope(SEM)imageofthefabricatedbowtieantenna.Theantenna
concentrates the laser mode to a nanoscale area defined by the antenna gap.
Schematic of a plasmonic laser antenna with a gold bowtie antenna
the nanorods. The large field enhancement in these structures,
compared to that of gold nanospheres, is due to the substan-
tially reduced SP losses, an effect caused by the suppression of
interband damping at near-infrared wavelengths [92].
Inplasmoniclaserantennas,thesizeoftheintenseopticalspot
is largely determined by the size of the gap; a major advantage
of this scheme is that it does not suffer from limited throughput
as no subwavelength apertures are involved. With an aperture,
the size of the optical spot can be equally small, but the power
that can be transmitted through such a small subwavelength
aperture in a perfect metal will decay as the fourth power of the
aperture diameter when the latter is smaller than one-quarter of
the illumination wavelength [89].
We modeled optical antennas on alumina substrate in the
same fashion as the work described in the previous section.
The antennas consist of a pair of gold nanorods separated by a
20 nm gap. For a design wavelength of 830 nm, we determined
the resonant antenna lengths for gold nanorods having a 50 nm
× 50 nm square cross section and a bulk index of refraction of
0.188 + 5.39i [76].
Ourcalculationsindicatethatthenear-fieldintensityenhance-
ment in the gap peaks at a value ∼800 for an antenna arm length
of 130 nm. The nanorod edges are rounded off so as to mimic
the fabricated structures (Fig. 8). This length corresponds to the
first dipolar mode of this geometry. For this first resonance, the
steady-state time-averaged total electric field intensity distribu-
tion is very similar to that in Fig. 4. Also, the incident plane
wave illumination is polarized along the antenna length.
We fabricated an optical antenna structure that comprises a
pair of gold nanorods on a facet of a commercial edge-emitting
laser diode (Sanyo, Inc.) operating at a wavelength of 830 nm
[91]. This laser package incorporates a photodiode to monitor
thelaserpoweremergingfromthebackfacet.First,wedeposited
a 280-nm-thick alumina insulating layer onto the laser facet in
an electron-beam evaporator so as to prevent electrical shorting
of the laser and the monitor photodiode in the laser package. A
goldlayerwasthenelectron-beamevaporatedinacustom-made
high-vacuum-evaporation system onto the alumina film. This
high-vacuum electron-beam evaporation and the evaporation
Authorized licensed use limited to: IEEE Xplore. Downloaded on January 14, 2009 at 01:32 from IEEE Xplore. Restrictions apply.