Plasmonics (2016) 11:493–501
DOI 10.1007/s11468-015-0075-3
Analyzing Photothermal Heat Generation Efficiency
in a Molecular Plasmonic Silver Nanomatryushka Dimer
Arash Ahmadivand 1 & Nezih Pala 1
Received: 2 June 2015 / Accepted: 10 September 2015 / Published online: 22 September 2015
# Springer Science+Business Media New York 2015
Abstract We report on the numerically and analytically investigated plasmonic and photothermal responses of a
nanomatryushka structure composed of silver concentric
nanoshells which exhibited strong plasmon resonance localization in the optical frequencies. Illuminating an isolated silver nanomatryushka in an aqueous system, we calculated the
photothermal response of the structure and quantified the
absorbed optical power and generated photothermal heat. In
addition, it is shown that a couple of nanomatryushka structures as a symmetric molecular dimer in weak and strong
coupling regimes are able to support strong plasmon resonances in the visible to the near-infrared region. Utilizing
strong near-field coupling in the metallic nanostructures and
hybridization of plasmons, and also employing silver as a
highly absorptive material at the visible spectrum, we increased the energy dissipation per unit volume almost three
orders of magnitude in comparison to the other analogous
subwavelength structures. Employing numerical methods,
we showed that a symmetric metallic nanomatryushka dimer
is able to generate enough photothermal heat which could
result in a remarkable amount of temperature change (ΔT=
140 K) at the picosecond time scale. According to hybridization theory, the symmetric dimer is able to support strong
bonding and antibonding plasmon resonant modes. Utilizing
concentric nanoshells with high geometrical tunability facilitates using all of the surfaces and center of nanoparticles to
generate heat with a large temperature change within a short
* Arash Ahmadivand
[email protected]
1
Department of Electrical and Computer Engineering, Florida
International University, 10555 W Flagler St., Miami, FL 33174,
USA
relaxation time. This understanding opens new avenues to
utilize simple nanoparticle orientations to generate significant
heat power in an extremely short time scale for cancer therapy,
photothermal therapy, and biological applications.
Keywords Plasmonics . Nanomatryushka dimer .
Photothermal heat . Plasmon hybridization . Numerical
modeling
Introduction
Nanomatryushka (NM) was introduced as one of the strategic
nanoparticle (NP) arrangements to characterize the plasmon
resonance hybridization theory in simple and complex colloidal NP aggregates in molecular dimensions [1–3]. As a pioneer work, Prodan et al. [3] have investigated the plasmon
response of a gold (Au) NM during strong localization of
plasmon resonances across the visible spectrum to the nearinfrared region (NIR). In terms of physical properties, a NM
consists of two concentric metallic nanoshells embedded and
filled with a dielectric material (e.g., SiO2), entirely. The attractive behavior of a NM originates from the controllability
of plasmon resonances in a wide range of optical spectrum by
changing its geometrical and chemical properties. Having
large number of independent and tunable geometrical parameters, this configuration yields a remarkable geometrical tunability, which can be exploited to enhance the electromagnetic
(EM) field localization and hybridization in subwavelength
dimensions. In terms of chemical characteristics, Au has widely been used for designing and fabricating of NM units for
various purposes [1–4]. As another conventional noble metal,
silver (Ag) has been proposed as a suitable material for plasmonic applications in the visible to the NIR, which is wellknown for its significant light absorption across the wide
494
range of spectrum [5, 6]. These features make the Ag as a
proper choice in designing molecular clusters and structures
to utilize in bio-chemical sensing [7], light harvesting and
photovoltaic devices [8, 9], and surface-enhanced Raman
spectroscopy (SERS) [10].
Considering the light-matter interaction effect in nanoscale
regime, a simple metallic NM can be tailored to support strong
bonding and antibonding plasmon resonances. It is wellknown that a bright mode can be excited via light-matter interactions directly with a net dipole moment, whereas an antibonding mode has no net dipole moment and, therefore,
cannot be excited by regular light-matter interactions [11,
12]. Adjusting two equivalent nanoparticle structures in a
close proximity to each other results in the formation of a
molecular dimer. In a cluster-type structure, the appeared
bonding and antibonding modes of each one of particles (here
is NM) can be interfered with each other in destructive and
constructive regimes. In the retarded (quasi-static) limit, a
weak and destructive interference between opposite modes
(bonding and antibonding modes) gives rise to the formation
of pronounced Fano resonant dips in the extinction profile.
Therefore, tremendous localization of plasmon resonances at
the Fano dip frequency is expected, and such a huge amount
of hot electron energy can be controlled effectively based on
the behavior of Fano resonant mode [13, 14]. Additionally,
intense localized surface plasmon resonance (LSPR) modes at
the resonance frequency (ωLSPR) can result in scattering and
absorption of optical power at this point, while the ratio of
scattering to absorption cross sections depends on the structural and chemical properties of a metallic nanostructure. By
careful selection of these parameters, the scattering and absorption efficiencies can be controlled effectively.
On the other hand, conversion of light into usable
photothermal heat energy in an extremely short time scale in
plasmonic components has been considered as one of fundamental mechanisms in cancer therapy [15, 16], photothermal
therapy and imaging [17–20], and vapor generation in the
aqueous systems [21, 22]. For instance, in cancer therapy applications, illuminating metallic NPs with an external infrared
laser delivers optical power to a targeted zone as a treatment
mechanism [23]. Therefore, developing a process to design
noble metallic (e.g., Ag, Au, Cu) structures to harvest large
amount of incident light helps to fabricate numerous efficient
and practical nanoscale devices for clinical applications.
Moreover, the shape and size of NPs, their orientation, and
surrounding medium have fundamental impacts on the enhancement of the light into heat generation efficiency. Very
recently, Toroghi et al. [24] have studied the photothermal
response of compositional (Au-Ag-Au) nanospheres as a heterogenous molecular trimer, wherein the proposed nanostructure shows twofold of absorption and heat generation in a
finite and isolated subwavelength structure. In terms of lightmatter interactions, an oscillating free electron gas
Plasmonics (2016) 11:493–501
interchanges the heat energy with the metallic part of the
structure via electron-photon scattering in few picoseconds.
The result of this process is generation of photothermal heat
energy in subwavelength dimensions from the absorbed power [25, 26]. Due to the ultra-fast conversion of the absorbed
photons into heat power and vice versa (cooling) in nanoscale
structures, they have a significant potential for realizing beneficial devices for photothermal clinical applications. This
mechanism can be understood by considering the
photothermal effect in plasmonic structures, wherein the
strong localization and hybridization of resonant modes can
be ensued by magnifying the heating energy in a molecular
system. Most of the earlier works have utilized cascaded plasmon resonances to enhance the photothermal effect as well as
intensifying the EM field for heat generation with
photothermal heat variations in the range of ~100 K [24, 27].
In this study, we utilize a couple of molecular silver
nanomatryushka (Ag-NM) units in a dimer orientation with
the experimentally determined Palik constants [28] to generate
significant photothermal heat to achieve as high as possible
photothermal heat via hybridization of plasmon resonances.
Assuming a controllable thermal atmosphere (a liquefied environment) as the ambience for our nanostructure, we numerically calculate extinction spectra to determine the produced
heat energy in the light into heat conversion process. Moreover, by comparing the temperature of the produced heat for
an isolated Ag-NM and a couple of Ag-NM units (dimer), we
demonstrate the effect of structural dimensions on the light
into heat conversion mechanism by using Mie [29], plasmon
hybridization [3, 11], and discrete dipole approximation
(DDA) theories [30].
Theory
Heat generation in nanoscale dimensions in an extremely
short time in a fluid medium includes numerous events,
which make the entire process tremendously complex to
analyze theoretically and numerically [31–33]. The
photothermal response and produced temperature in a
light-matter interaction in subwavelength dimensions can
be calculated by considering the physical and optical properties of NP as well as the characteristics of the ambient
in analytical computations [34, 35]. Using this scenario,
one would be able to determine the amount of incident
light absorption by a plasmonic nanostructure and the efficiency of light into heat conversion. Currently, there are
two major methods that can be employed to study the
photothermal response and heat generation for the presented Ag-NM structure. The general mathematical model to
describe the relation between temperature changes (ΔT)
and absorption coefficient can be understood by using
time-dependent Fourier law equation for thermal
Plasmonics (2016) 11:493–501
495
conductivity as q(t)=−κ∇T(t) W m−2 [36]. Consequently,
using the latest equation, for the quasi-static EM field we
have [37]:
∂ΔT
1
C abs
ð∇:κÞ∇ΔT þ
¼
ρNM cNM
∂t
ρNM cNM
ð1Þ
where κ is the thermal conductivity (W m−1K−1),cNM is
the specific heat capacity of a sample NM (J kg−1K−1), t
is the illumination time, ΔT is the temperature changes in
a specific time scale (K), ρNM is the density of the
nanosize structure (kg m−3), and C abs is the absorption
coefficient for the Ag-NM unit that can be obtained by
calculating the absorption spectra under illumination by a
light source or laser pump exposure. More than the described parameters in the differential equation above, also,
the intensity of the incident EM field (laser source) plays
a major role in the light into heat conversion efficiency.
The major problem correlating with the proposed method
above (Eq. 1) is the independency of the equation to the
volume of the structure [37]. It should be underlined that
the volume of metallic nanostructures has significant influence in light absorption and plasmon resonance localization. This problem can be mitigated by including the volume of a NP in the calculation of the absorption coefficient and photothermal heat generation. It is shown that
the equation above (Eq. 1) can be written in a simple and
new form as below [24], which directly depends on the
physical and chemical characteristics of the proposed AgNM unit:
ΔT
C abs
ð2Þ
¼
cNM V NM ρNM
φ
where φ is the optical flux of the incident light source
pulse (J m−2) (determined by the light source setting) and
V NM is the volume of a NM unit in Bm3.^ Therefore,
determining accurate geometrical dimensions for the proposed Ag-NM and illuminating the structure with strong
EM field could help generate remarkable heat during lightmatter interaction. In the current work, the behavior of
phonons correlating with the NP (hot object) and medium
(cool object) and their transport without appreciable interaction is used to characterize the heat generation process,
wherein mean free path of these phonons is on the same
order with the dimensions of the NPs. As an important
parameter in photothermal computations, the absorption
cross section can be defined by integration of the absorbed
optical power at the LSPR peak position over the volume
of the Ag-NM and then the obtained value is divided by
the simulation settings as below:
Z
ffi
. rffiffiffiffiffi
ε0 ⊥ 2
C abs ¼ 2 PLSPR dV nm
ð3Þ
E μ0 inc
NM
Where PLSPR is the optical power absorbed by the Ag-NM
unit(s) based on the ohmic losses at the resonance frequency,
nm is the refractive index of the surrounding medium, ε0and
μ0 are the permittivity and permeability of the vacuum or free
space, andĒ⊥incis the amplitude of the incident transverse electric field emitted by a laser source. The intensity of the incoming electric field is set to 1 mW μm−2 in all of the simulations.
More precisely, increasing both volume and the specific heat
capacity causes to the formation of undesired reduction in the
temperature of the produced photothermal heat (see Eq. 2).
Therefore, providing a method to absorb large amounts of
incident optical energy with a small volume of NP and also
using strong localization of plasmon resonances in a few
nanometer spot lead to the formation of remarkable thermal
power. In nanoplasmonic structures, the resonance frequency/
wavelength (λLSPR) has a significant impact on the behavior
of a nanoscale structure. In the quasi-static limit for a nanoscale structure [36], the absorption coefficient at the resonance
frequency is given by the following:
λ
LSPR
C abs
¼
18πV NM
n3m
λLSPR ImfεNM g
0
ð4Þ
0
0
is the complex permittivity of mewhere εNM ¼ εNM þ εNM
tallic NM. A simple NM facilitates excitation of surface plasmon resonances by using all of the outer and inner surfaces
and central spots of the NPs and hence allows for producing
thermal power with a significant temperature change. Also,
the geometrical tunability of a NM provides high efficiency
in optical power absorption, making it superior to simple
nanoparticles such as nanosphere, rod, disk, and shell.
Figure 1a illustrates three- and two-dimensional artworks for
the examined Ag-NM with the description of geometrical parameters that is filled and surrounded by a glass (SiO2) host
with the permittivity of ε=3.9, and the entire symmetric AgNM is completely immersed in a fluid system includes water
(H2O) with the refractive index of n=1.33, analogous to biological applications. The heat energy generation (Qh) can be
computed by considering the absorption cross-sectional coefficient at the resonance positions and the intensity of the incident spectrum as following equation [35]:
Qλh LSPR ¼
2
1
LSPR
nm c0 ε0 E ⊥inc C λabs
2
ð5Þ
where c0is the velocity of light in the free space (m s−1), and
the absorption coefficient is correlated with the dissipated
power coefficient at the LSPR wavelength. It is already verified that in Eq. 5, the term 0.5nc0ε0|E⊥inc|2refers to the properties of the propagating electric field in a free space ambient
[36, 37]. Thus, calculation of the optical power absorption for
the Ag-NM is enough for thermal heat quantification. To determine the absorption efficiency for an Ag-NM unit, we examined the optical response by modifying the size of NM
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Plasmonics (2016) 11:493–501
Fig. 1 a A schematic diagram of
the Ag-NM with the description
of the geometrical dimensions. b
Extinction spectra for the Ag-NM
under transverse electric
polarization mode excitation.
Inset is the schematic picture of
the fluid system. c Twodimensional snapshots for the
field intensity |E| in the NM for
the absorption peaks at 470, 620,
and 835 nm
under illumination in a liquid system. Our numerical studies
are performed by employing finite-difference time-domain
(FDTD) method (Lumerical FDTD Solutions 2015 package),
and the fundamental parameters are set as follows: the spatial
cell sizes are set to dx =dy =dz =0.1 nm, with 200,000 numbers
of cells, and perfectly matched layers (PMLs) as the boundary
condition with 128 layers. In addition, considering numerical
stability for the employed subwavelength structures, simulation time step is set to the 0.02 fs according to the Courant
stability condition. The light source is a Gaussian electric
pulse with a pulse length of 2.6533 fs, offset time of
7.5231 fs, the waist radius of the incident beam is 600 nm,
and the divergence angle is set to 9.04°.
Results and Discussion
The photothermal response of an Ag-NM in single and dimer
orientations can be achieved by using a theoretical model as
described already in the BTheory^ section. To this end, we
used Ag [38], SiO2 [39], and water [40] with the thermal
conductivity of (κ) of 429, 0.92, and 0.609 W m−1 K−1, respectively. In addition, to define the spectral response for the
proposed NM, we used a modified version of Drude model
[41]:
εAl ¼ ε∞ −
ω2B
ωðω þ iγ Þ
ð6Þ
where ε∞ is the high-frequency response (~5.7), ωB is the bulk
plasma frequency (1.3736×1016 rad/s), and γ is the damping
frequency (2.7335×1015 Hz), in which these parameters are
set to experimentally determined values by Murata et al. [42]
to calculate the effective dielectric function for an Ag-NP.
Figure 1b shows the numerically calculated extinction crosssectional profile for an isolated Ag-NM with the default size
of 35/75/95/115 nm in the wavelength range of λ=400–
1300 nm. To provide a detailed work, we examined the influence of all variable structural parameters (Ra/Rb/Rc/Rd) on the
absorption efficiency. Using these geometries, the net and effective volume of a concentric structure composed of two
nanoshells is given by the following: VNM = πh{(R2b −R2a)
−(R2d −R2c )}. The height of NPs are set to h=80 nm in all of
the future analysis. In the extinction diagram, three distinct
peaks are observed, which are correlated with the dipolar
and multipolar plasmon resonant modes at λ=470, 620, and
835 nm. This broad absorption characteristic including three
maxima in the visible to the NIR is the advantageous feature
of the proposed nanostructure. This absorption feature facilitates for heat generation in a wide range of spectrum with
minor differences in efficiencies. The inset in Fig. 1b is a
schematic for a liquid system under exposure by a laser diode,
while nanoparticles are immersed thoroughly in an aqueous
system. As it is expected, due to the inherent properties of Ag
material, the absorption behavior of the Ag-NM is dominant
in comparison to the scattering diagram in the extinction spectra. Figure 1c shows two-dimensional snapshots of the electric
Plasmonics (2016) 11:493–501
497
field intensity (|E|) and plasmon resonance excitation and hybridization inside the isolated Ag-NM at the plasmon resonance maxima under transverse polarization excitation, where
the role of the space (filled with dielectric substance) between
two nanoshells is obvious in all snapshots. To examine the
effect of geometrical sizes on the absorption profile, we compared the behavior of an Ag-NM in Fig. 2a by calculating
normalized absorption profile. For the proposed Ag-NM with
the size of 35/75/95/115 nm, we observed three distinct shoulders correlating with the absorption peaks at the visible to the
NIR spectra (λ~480, 650, and 840 nm). By increasing the size
of the inner shell (45/85/100/115 nm), we experienced a destructive reduction in the light absorption, and accordingly, the
absorption peaks blue-shifted to the shorter spectra. In contrast, for 45/85/110/135 nm, we increased the size of the outer
nanoshell which gives rise to the formation of strong red-shift
in the position of absorption peaks to the longer wavelengths,
where this shift includes a significant increase in the
normalized absorption amplitude. This absorption enhancement originates from the formation of bright modes during
light-matter interactions within the outer shell and leads to
support strong resonances by our metallic NM. Indeed, the
exterior and interior nanoshells interact with the incident
Gaussian beam, and the result of this interference is the formation of bonding plasmon modes in the energy continuum.
Due to the small distance between two concentric nanoshells
(<10 nm), dipolar and multipolar modes interfering with each
other includes possible destructive interferences between opposite modes (bonding↔ antibonding). The result of this destructive interference is significant intensification in the absorption coefficient. The other reason for the remarkable power absorption is related to the contribution of all of the surfaces
of the NM structure in the excitation process. Figure 2b compares the light absorption (nW nm−3) for the proposed NM
with two different geometries. The amount of the absorbed
power (P) by both NP systems is quite large due to the
Fig. 2 a Normalized absorption spectra of the Ag-NM with different
dimensions under transverse electric polarization excitation, b power
absorption snapshots for two Ag-NMs different dimensions, c a
comparison of the numerically calculated thermal power (Qh) generated
by Ag-NMs different dimensions, and d numerically calculated scattering
spectra for NM dimer for four different geometrical sizes
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Plasmonics (2016) 11:493–501
plasmon resonances localization and absorption at the resonance positions. This absorption spectra can be utilized efficiently in photothermal heat generation, and the produced heat
(Qh) correlated with the LSPR frequency/wavelength can be
estimated by using the modified version of Eq. 5, as:
¼
QTotal
h
2 X λ
1
LSPR
nm c0 ε0 E⊥inc C abs
2
λ
ð7Þ
LSPR
where λLSPR is the LSPR wavelength for an Ag-NM
under transverse polarization excitation. Figure 2c compares the computed entire photothermal heat for a single
Ag-NM with different geometrical sizes. The most important and effective parameter in the thermal heat generation spectroscopy is the resultant temperature variations. Employing Eq. 2, and using the heat capacity of
Ag cNM =233 J kg−1K−1 [43], we calculated the temperature change per optical fluence rate ΔT/φ=45 K μJ−1
mm2 for an Ag-NM which gives temperature changes as
ΔT=90 K for a pulse fluence of φ=20 nJ mm−2. Comparing the obtained temperature changes for an isolated
Ag-NM unit with the other types of NPs such as nanospheres and nanorods in a liquid system [24, 27, 44,
45], we proved a superior behavior for the examined
NMs. By far, we calculated the photothermal heat for
an isolated Ag-NM; however, this photothermal heat
generation can be further enhanced by forming a dimer
cluster using the examined NM. Figure 2d displays the
scattering cross-sectional profiles for the proposing dimer composed of similar NM units with four different
geometrical dimensions. As obvious as it is, for the
structure with following geometries: D 1 = (45/85/110/
135) nm, D2 =(35/75/95/115) nm, and D3 =(40/80/100/
115) nm, a distinct asymmetric Fano dip is performed
around λ~800 nm. However, this minimum can be even
enhanced using different geometries. Locating a couple
of NM units with the following dimensions: D4 =(45/85/
110/135) nm in a few nanometer distance from each
other (with the gap spacing of 10 nm), we induced an
asymmetric and deep Fano minimum including a redshift to the NIR spectra. Figure 3a illustrates a schematic picture of the Ag-NM dimer (Ag-NMD) with the
offset gap distance of Dg. Using the earlier mechanism
to illuminate an isolated Ag-NM, we examined the plasmon and photothermal responses of an Ag-NMD in a
liquid system. With the placement of two equivalent
Ag-NM units close to each other, we defined the effect
of gap distance alterations on the thermal heat generation numerically. It is well-accepted that the intensity
and strength of localized plasmon resonances can be
changed significantly by modifying the interparticle distance [46, 47]. In addition, it is shown that for high
values of Dg, the coupling strength is remarkably weak
Fig. 3 a A three-dimensional schematic diagram for an Ag-NMD. b–d
Numerically calculated scattering and absorption spectra for an Ag-NMD
under transverse electric polarization excitation
and hybridization cannot be achieved due to missing of
the required energy for dipolar and multipolar
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499
Fig. 4 a–c Two-dimensional
snapshots of the field intensity |E|
inside the Ag-NMD for the
absorption peaks. d–f Power
absorption spots snapshots for an
Ag-NMD under transverse
polarization excitation
interactions [48, 49]. Therefore, for small gap distances,
we expect strong coupling between dipolar and multipolar plasmon resonant modes, called strong coupling regime. Strong localization of plasmon resonances in a
molecular cluster composed of metallic NPs can be resulted with a large scattering of EM field. Formation of
bonding and antibonding plasmon modes and placement
of two metallic NPs in a close proximity to each other
Fig. 5 Numerically calculated thermal power (Qh) for an Ag-NMD with
three different offset gap distances
leads to intensification of plasmon modes (hybridization
mechanism). In this regime, a symmetry cancelation in
the structural properties directly gives rise to the formation of new collective antibonding modes and a destructive interference between bonding and antibonding
modes result in a Fano resonance dip as a hotspot
[49–51]. Our proposed Ag-NMD is a symmetric structure and formation of Fano resonances in such a symmetric molecular arrangement is challenging. However,
recently, Ahmadivand et al. [52] proposed a method to
obtain Fano dips in an Al/Al2O3 NM dimer in free
space conditions. In a simple dimer consisting of a couple of complex NM units, the outer shell supports the
bright mode via light-matter interactions and the inner
shell provides the required multipolar dark modes to
generate Fano dip via a weak and destructive interference between dark and bright plasmon energies. For our
Ag nanostructure, due to the strong absorption property
in the visible to the NIR spectra, we expect a distinct
Fano minimum as a hotspot in the scattering profile.
Figure 3(b), (c), and (d) exhibit the scattering and absorption diagrams for an Ag-NMD with different offset
gap distances Dg =25/10/5 nm. Due to superior behavior
of Ag in comparison to Al for plasmonics purposes,
effective support of strong plasmon resonances and interactions between the opposite modes are possible. In
the calculated scattering profiles, we obtained a noticeable minimum around λ~800 nm by reducing the size
500
of offset gap distance between two NP arrangements.
Bringing two NMs closer to each other causes to the
formation of a significant hotspot including a subtle redshift in the position of Fano dip. The peak of the calculated normalized absorption profile is observed at the
same position with the Fano resonances and originates
from the strong absorption of plasmon resonances by
two Ag-NMs. Comparing the absorption figures for
Ag-NMD with an isolated Ag-NM, a noticeable enhancement in the absorption efficiency is achieved with
a broadening of the absorption peak. Figure 4a–c are
numerically calculated simulation snapshots for the plasmon resonance excitation and hybridization in an AgNMD in an aqueous system for different gap distances
(Dg) from 25 to 5 nm, which resembles a transmutation
of weak coupling of resonant modes to the strong coupling regime. Figure 4d–f depicts the power absorbed
by Ag-NM units under illumination by a laser pump.
Increasing the intensity of the coupling of the plasmon
resonances (hybridization regime) gives rise to a dramatic power absorption enhancement in a molecular
system that is in accordance with the absorption and
scattering profiles (see Fig. 3b–d). Quantifying generated photothermal heat for the studied Ag-NMD with different gap sizes can help to understand the effect of
hybridization on the absorption of the incident light
and photothermal heat generation. Using the mechanism
that has been presented in the earlier section, we determined the photothermal heat power (Qh) as demonstrated in Fig. 5. The amount of photothermal power increases by the transmutation from weak to strong coupling regime. The obtained thermal power for the dimer
structure is almost twofold higher than the that of the
previous regime and even higher than that of the analogous dimers and trimers. Finally, we computed the
temperature change ratio per optical fluence of the incident pulse (J m-2) as ΔT/φ= 70 K μJ-1 mm2 for the
dimer. Assuming a Gaussian pulse fluence (laser pump
setting) of φ =20 nJ mm-2, the temperature variations
that can be obtained by an Ag-NMD colloidal structure
is quantified as ΔT=140 K.
Plasmonics (2016) 11:493–501
nanoparticles in photothermal heat generation. It is verified
that hybridization of plasmon resonances in such a symmetric
and molecular structure composed of NM units can be resulted by pronounced Fano dip as a hotspot in short time scale.
We exploited these unique features to achieve extremely fast
heating up to 140 K and fast cooling down to the liquefied
environmental temperature. Unlike regular spherical NP aggregates, concentric nanoshells support strong plasmon and
Fano resonances in the visible to the NIR spectra during
photothermal heating process. The results imply that the studied dimer is a strong candidate for various biological applications that requires short relaxation time in the range of
picoseconds.
Acknowledgments This work is supported by the NSF CAREER program with the award number: 0955013 and by the Army Research Laboratory (ARL) Multiscale Multidisciplinary Modeling of Electronic Materials (MSME) Collaborative Research Alliance (CRA) (Grant No.
W911NF-12-2-0023, Program Manager: Dr. Meredith L. Reed).
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Plasmonics Online first article doi: 10.1007/s11468-014-9868-z