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SUPPLEMENTARY INFORMATIONDOI: 10.1038/NNANO.2014.27

NATURE NANOTECHNOLOGY | www.nature.com/naturenanotechnology 1

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Third harmonic upconversion enhancement from a single

semiconductor nanoparticle coupled to a plasmonic antenna

Heykel Aouani, Mohsen Rahmani, Miguel Navarro-Cía and Stefan A. Maier

This document includes the following supplementary information:

1- Fabrication of the ITO nanoparticles decorated with plasmonic dimers

2- Characterization of structures with Fourier transform infrared spectroscopy

3- Experimental setup and measurements

4- THG measurements from hybrid plasmonic antennas

5- Use of third harmonic signal for probing plasmonic hot spots

6- Effective third order susceptibilities and conversion efficiencies of the nonlinear upconversion

nanosystems

7- Current factors limiting the nonlinear upconversion efficiency rates

8- Additional linear and nonlinear simulations

Third-harmonic-upconversion enhancement from a single semiconductor nanoparticle coupled to a plasmonic antenna

© 2014 Macmillan Publishers Limited. All rights reserved.

http://www.nature.com/doifinder/10.1038/nnano.2014.27

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1- Fabrication of the ITO nanoparticles decorated with plasmonic dimers

The nonlinear upconversion systems investigated in this work were fabricated via

combination of etch-down and lift-off approaches accompanied with precise alignments.

Unlike conventional hybrid structures which can be fabricated with dual lift-off methods, the

fabrication procedure developed in this work allows employing non-directional deposition

methods such as sputtering for fabricating objects with characteristic dimensions down to 25

nm. Indeed, a pure lift-off approach cannot be employed here as the non-directional plasma

blocks define nanoscale windows on the resist before the material reaches the substrate. These

limitations hinder sputtered nanomaterials to be fabricated via lift-off, which prevent the use

of many promising materials in nanophotonics. Here, we tackle this issue by combining etch-

down and lift-off approaches. In a first step, a quartz substrate was covered with a 40 nm ITO

film using sputtering. Subsequently, high resolution ITO nanodots and alignment markers

were defined in negative resist (HSQ) by electron-beam lithography. Ion beam etching (Ar

ions) of the ITO layer was then performed to generate the ITO nanoparticles. Next, the

substrate was coated with PMMA resist and the antennas’ shapes were defined in PMMA

around the ITO nanodots with a nanometric alignment precision. Hereafter, the sample was

covered with a 2�nm Cr adhesion layer and a 40�nm Au film by thermal evaporation. A

final lift-off step enables to obtain the nonlinear upconversion nanosystem made by a single

25 nm ITO dot at the gap of a gold plasmonic dimer. We emphasize that this procedure leads

to a precise control of the distance between the ITO nanoparticle and the gold structure. A

schematic description of the fabrication process is presented in Supplementary Fig. 1. Let us

point out that ITO has been chosen for our nonlinear investigations given its high third

harmonic susceptibility.


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Supplementary Fig. 2: Schematic representation of the fabrication process developed to manufacture the nonlinear upconversion system made by a single ITO nanoparticle decorated with a plasmonic gold dimer.

2- Characterization of structures with Fourier transform infrared spectroscopy

The extinction spectra of the fabricated structures (defined as 1 – transmission) were

measured by Fourier transform infrared spectroscopy (FTIR, Bruker Hyperion 2000) through

an array of plasmonic antennas (approximately 120 structures, with a pitch of 2 μm) with and

without an ITO nanoparticle at their gap at normal incidence under linear polarization. The 2

μm pitch ensured insignificant coupling between neighbouring nanostructures, as confirmed

by numerical simulations. For measurements in the near-infrared regime, we used an InGaAs

detector cooled with liquid nitrogen. As the extinction spectra presented in Fig. 1f were

measured for structures under parallel polarized excitation, i.e., electric field parallel to the

dimer axis, the extinction spectra for a 35 nm gap nanorod dimer with / without an ITO

nanoparticle under perpendicular polarized excitation are presented in Supplementary Fig. 2.

As expected, the extinction spectra do not exhibit plasmonic resonances in this spectral

window when the structures are excited under perpendicular polarization.


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Supplementary Fig. 2: Experimental extinction cross section of the 35 nm gap nanorod dimers with (red dots) and without (black dots) a 25 nm ITO nanoparticle at their gaps under perpendicular polarized excitation.

3- Experimental setup and measurements

The inverted microscope developed for this work uses a Yb:KGW femtosecond PHAROS

laser system as a pump of a collinear optical parametric amplifier ORPHEUS with a LYRA

wavelength extension option (Light Conversion Ltd, Lithuania, pulse duration 140 fs,

repetition rate 100 kHz). The excitation beam is reflected by a shortpass dichroic mirror

(Thorlbas DMSP805 and DMSP1000) and focused on the sample plane by a dry microscope

objective (Nikon S Plan Fluor x40, 0.6 NA). For nonlinear experiments, the fundamental

incident wavelength is set at 1500 nm, and the backward-emitted third harmonic generation at

500 nm is collected via the same objective. The third harmonic signal is then directed to a

70:30 cube beamsplitter that separates the beam towards a spectrograph (PI Acton SP2300 by

Princeton Instruments) for spectral measurements and towards an avalanche photodiode

(MPD PDM Series by Picoquant) for nonlinear imaging. Accurate positioning of the sample

at the laser focus spot is ensured by a multi-axis piezoelectric stage (Nano-Drive, Mad City

Labs). For all experimental measurements, the excitation power was set below 50 μW (peak


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intensities of 45.7 GW/cm2) in order to prevent sample damage. A schematic representation

of the nonlinear microscope developed in this work is presented in Supplementary Fig. 3.

For measurements on ITO nanoparticles, the third harmonic generation spectra at λ= 500 nm

were obtained by integrating the nonlinear signal for 1800 s with an average incident power

set at 50 μW thought an array of 10 × 10 nanoparticles (pitch of 200 nm). As the signal to

noise ratio is dramatically improved in presence of a plasmonic dimer, the third harmonic

generation spectra of the nonlinear upconversion nanosystems were integrated for 5 s with an

average excitation power set at 50 μW. All the experimental spectra presented in the main

text have been normalized by the integration time and by the area of the ITO nanoparticle in

order to present a third harmonic intensity expressed in photon.s-1.μm2.

Supplementary Fig. 3: Schematic description of the nonlinear microscope used for our investigations.


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4- THG measurements from hybrid plasmonic antennas

The third harmonic generation signal from the plasmonic dimers without ITO nanoparticle at

their gap has been carefully evaluated in order to quantify the intrinsic nonlinear gold

background. Typical raw spectra determined for a 35 nm gap nanorod dimer with and without

ITO nanoparticle at its gap and excited under parallel and perpendicular polarizations are

presented in Supplementary Fig. 4 (average excitation power of 50 μW). As can be seen, a

strong polarization dependence of the THG signal is highlighted in both cases. Under parallel

polarization, the THG response of the ITO nanoparticle + plasmonic dimer combined system

is more than 16 times higher than the intrinsic THG background of the plasmonic dimer itself.

Under perpendicular polarization, the THG response of the ITO nanoparticle + plasmonic

dimer is similar (the nonlinear signal from the ITO nanoparticle is not enhanced under

perpendicular polarization), thus demonstrating that the THG signal from the ITO

nanoparticle + plasmonic dimer under parallel polarized excitation mainly comes from the

ITO nanoparticle. This is further corroborated with the nonlinear simulations in

Supplementary Section S9. For each nonlinear upconversion system investigated, we have

taken into account the intrinsic nonlinear third harmonic from the gold dimer without ITO

nanoparticle at its gap by considering it as a background, which was subtracted from the

spectra of hybrid upconversion nanosystem responses presented in the main body of the

paper. Taking into account the dimensions of the 25 nm ITO particle and the dimensions of

the 35 nm gap nanorod dimer (2 nm × 100 nm × 280 nm), and using the data presented in

Supplementary Fig. 4, we determined a third harmonic generation upconversion efficiency 3

orders of magnitude higher for the ITO nanoparticle coupled into the dimer’s gap compared

to the dimer itself (third harmonic upconversion of 7×10-6 for the ITO nanoparticle at the gap

of the nanorod dimer and 6.2×10-9 for the nanorod dimer itself).


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Supplementary Fig. 4: a, Raw spectra of the third harmonic generation from a 35 nm gap nanorod gold dimer excited under parallel polarization with (red) and without (black) an ITO nanoparticle at its gap. b, Raw spectra of the third harmonic generation from a 35 nm gap nanorod gold dimer excited under perpendicular polarization with (red) and without (black) an ITO nanoparticle at its gap.

As the introduction of a dielectric nanoparticle at the gap of a metallic nanodimer shifts its

plasmonic resonance, a special attention has been devoted to characterize the third harmonic

responses from plasmonic dimers with and without an ITO nanoparticle at their gaps in a

much larger spectral window. In order to avoid any ambiguity regarding the origin of the

nonlinear signal, the fundamental excitation wavelength of the incident pulsed laser was

tuned from 1480 to 1630 nm, which correspond to the resonance peak in the extinction

spectra of the 35 nm gap nanorod dimer without and with ITO respectively, see Fig. 1f. The

evolution of the third harmonic generation intensity from the structures under investigation is

presented in Supplementary Fig. 5 for various fundamental excitation wavelengths (average

excitation power set at 20 μW). Please note that these experimental data have been processed

by taking into account the transmission / reflection coefficients of the different optical

elements and the quantum efficiency of the CCD detector. For each of these multi-wavelength

measurements, the THG intensity from the plasmonic dimer with an ITO nanoparticle at its

gap is 1 order of magnitude higher than the THG intensity from the plasmonic dimer without

ITO, thus confirming that the nonlinear signal originates from the ITO nanoparticle, and is

not due to a shift of the plasmonic resonance position.


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Supplementary Fig. 5: a, Evolution of the third harmonic generation from a 35 nm gap nanorod gold dimer excited under parallel polarization without an ITO nanoparticle at its gap for various fundamental wavelengths. b, Evolution of the third harmonic generation from a 35 nm gap nanorod gold dimer excited under parallel polarization with an ITO nanoparticle at its gap for various fundamental wavelengths.

Lastly, we have investigated the case when the relative position ITO nanoparticle is shifted

inside the gap of the 35 nm gap nanorod dimer. The SEM image of such configuration and its

corresponding third harmonic generation spectrum are presented in Supplementary Fig. 6. As

can be seen, the THG signal decreases when the ITO nanoparticle is shifted from the center to

the bottom edge of the gap. From these data, we computed an intensity enhancement inside

the ITO nanoparticle of about 76, in good agreement with the value of about 82 determined

by numerical simulations (see Section S8, Supplementary Fig. 9d for more details about

simulations).

Supplementary Fig. 6: Evolution of the third harmonic generation from an ITO nanoparticle coupled to a 35 nm gap nanorod gold dimer excited under parallel polarization when the relative position of the ITO nanoparticle is shifted inside the gap.


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5- Use of third harmonic signal for probing plasmonic hot spots

As mentioned in the main body of this paper, indium tin oxide was chosen as the nonlinear

material for our investigations. Under excitation at frequency ω, its third order susceptibility

χ3ω induces a nonlinear polarization:

P3ω= ε0χ3ωEω Eω Eω, (Eq. 1)

which in turn generates a radiation of intensity I3ω α |P3ω|2.

The third order polarization PH3ω of an ITO nanoparticle localized at the gap of a plasmonic

dimer can be expressed as:

PH3ω= ε0χ3ωEH

ω EHω EH

ω, (Eq. 2)

where EHω is the field inside the ITO nanoparticle at the gap of the plasmonic dimer

investigated. By introducing the excitation intensity enhancement factor η at the gap of

plasmonic dimers defined by η= (EHω/ Eω)2, Eq. 2 can be rewritten:

PH3ω= ε0χ3ω η3/2 Eω Eω Eω= η3/2P3ω, (Eq.3)

which implies:

η= (IH3ω/ I3ω)1/3, (Eq.4)

where I3ω and IH3ω are respectively the third harmonic intensity from the Ø 25 nm ITO

nanoparticle at the gap of a plasmonic dimer or isolated. The Eq. 4 and the experimental data

presented in the Fig. 2a and Fig. 3c,e enable the immediate probing of the hot spot intensity

inside the ITO nanoparticle at the gap of the plasmonic dimers investigated and presented in

Fig. 3d,f.

6- Effective third order susceptibilities and conversion efficiencies of the

nonlinear upconversion nanosystems

In order to investigate the nonlinear performances of the nonlinear upconversion systems

presented in this work, we started by determining the third harmonic conversion efficiency


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defined as the ratio of third harmonic intensity I3ω relative to the fundamental average

incident power. The experimental values determined for I3ω are summarized in

Supplementary Table 1.

Supplementary Table 1: Experimental third harmonic radiated intensity I3ω by the various nonlinear upconversion nanosystems investigated.

Using Eq. 3, we can introduce an effective third order susceptibility χΗ

3ω for the nonlinear

upconversion nanosystems defined by χΗ3ω= (χ3ω)3/2. In order to investigate the potential of

the hybrid systems as ultrabright nanosources of upconverted third harmonic light, we plotted

in Supplementary Fig. 7 the THG conversion efficiency and effective susceptibility for a 25

nm ITO nanoparticle decorated with the various plasmonic dimers. Upconversion efficiencies

between 0.4 and 7×10-4 % were experimentally achieved, which is 1000 folds greater than the

highest values reported for intrinsic second order processes from plasmonic antennas1. Third

order effective susceptibilities ranging from 249 to 3543 nm2/V2 were here computed for the

ITO-plasmonic hybrid systems, much larger than the 0.2 nm2/V2 recently reached with gold

nanostructures alone2. Altogether, the high third harmonic conversion efficiency and effective

susceptibility of ITO-plasmonic hybrid systems make them suitable candidates for nanoscale

nonlinear upconversion of light.


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Supplementary Fig. 7: Third order upconversion efficiency (solid bars) and effective susceptibility (grid bars) corresponding to an ITO nanoparticle decorated with the various plasmonic motifs under investigation.

7- Current factors limiting the nonlinear upconversion efficiency rates

Although, in theory, the nonlinear upconversion efficiency from the hybrid nanosystem can

be increased until converting all the fundamental red photons (λ= 1500 nm) into third

harmonic green photons (λ= 500 nm), the damage thresold prevents achieving upconversion

efficiencies greater than 0.0007 %. As we can see in Supplementary Fig. 8, an average

excitation power of 100 μW modifies the shape of the plasmonic dimer and destroys the ITO

nanoparticle at its gap. To overcome these limitations, and as a future work, we are planning

to embed our samples in a low refractive index superstrate in order to make the nonlinear

upconversion nanosystems more resistive to high incident powers.


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Supplementary Fig. 8: SEM picture of a 25 nm ITO particle at the gap of a nanocylinder dimer after excitation under low (left-side image, after average excitation power of 50 μW) or high laser power (right-side image, after average excitation power of 100 μW). Scale bar, 200 nm. In the right-side SEM image, the ITO nanoparticle is destroyed and one of the dimer’s arms is rotated because of the heat induced in the Cr adhesion layer between the gold film and the quartz substrate.

8- Additional linear and nonlinear simulations

As can be seen in Supplementary Fig. 9, the nanorod dimer without anything at the gap has its

maximum intensity at the interfaces metal-gap air. Because of the small gap, the peaks at both

sides of the gap are strongly coupled leading to a plateau of ~255 intensity enhancement

within the gap for the x-cut denoted by the white dotted line. However, when the ITO is

placed at the gap, the field distribution is changed in the surrounding of the ITO nanoparticle

and additional intensity peaks emerge at the interface ITO-gap air. Now, within the ITO

nanoparticle, there is an almost flat intensity enhancement of ~110. If the ITO nanoparticle is

shifted to the bottom edge of the gap, there is a significant reduction of the intensity

enhancement within the plateau generated by the ITO nanoparticle. In this case, the intensity

enhancement drops up to ~70. The perturbation on the field distribution, and thus, the change

on the intensity enhancement at the gap, depends on the geometry and optical dielectric

function of the probing element. Notice that quantum dots have high refractive index, and

thus, show even higher invasiveness3 than the here proposed ITO-based technique. Probing

elements with low refractive index would be necessary to pave the way for quantification of

the hot spot on gap-free plasmonic dimers. However, the technique proposed in the main

manuscript is highly relevant for plasmonic applications, i.e. to probe the excitation intensity


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enhancement surrounding / inside a single emitter when coupled to a plasmonic antenna.

Indeed, the parameter of interest for plasmonic applications (sensing, photovoltaic to cite few

of them) is the intensity enhancement in presence of a localized nanoemitter (molecule,

quantum dot, nanoparticle...) and not the enhancement provided by an isolated plasmonic

antenna.

The conclusions of the manuscript are built on the assumption that any effect occurring at the

third harmonic (e.g. losses) has a negligible influence on the overall non-linear detection,

hence Eq. 4. To sustain the assumption, 3D FDTD nonlinear simulations were performed

using FDTD Solution v8.6. The same simulation setup as in the linear analysis (described in

Methods) is used unless anything else is stated next.

Supplementary Fig. 9: a, Intensity enhancement along the white dotted lines shown in the two-dimensional colour maps of panel b, c and d. b and c, FDTD intensity enhancement maps at the middle cross-sectional plane of the hybrid and nanodipole-only configuration, respectively. d, FDTD intensity enhancement maps at the middle cross-sectional plane of the hybrid configuration when the ITO nanoparticle is shifted to the bottom edge of the gap.


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Given the complexity of the analysis, the computation effort is reduced by considering: (i) the

nanodipole as two ideal rectangles, and (ii) the perfect ITO cylinder to be placed at the centre

of the gap. This allows us to use a twofold symmetry. See Supplementary Fig. 30 for the

sketch of the 35 nm gap dimer considered along with its dimensions.

The optical dielectric functions of Au, Cr, SiO2 are taken from tabulated data4 as in the linear

analysis of the main body of this manuscript. However, the dielectric dispersion of the

materials is fitted in the spectrum range from 400 nm to 1600 nm by a 6-coefficient model,

allowing a tolerance of 0.1 and enforcing passivity. In addition, the nonlinear response of Au

is considered with its intrinsic χAu(3) = 7.6 × 10-19 m2/V2. Meanwhile, ITO is characterized

with a constant εr = 2.89 and χITO(3) = 2.16 × 10-18 m2/V2 according to the literature5. We

investigate three limiting cases: when either χAu(3) or χITO

(3) is set to zero along with the

situation where χAu(3) and χITO

(3) are considered simultaneously.

The plane-wave excitation has an amplitude of 1 × 108 V/m, which has been chosen to fulfil

the condition χ(3)|E(t)|2 << εr, where E(t) is the temporal induced electric field within the ITO

particle. This excitation is a realistic narrowband temporal pulse with central wavelength of

1500 nm and spectral width ~23.65 nm (i.e. pulse length of 140 fs).

280 nm

45 nm40 nm

35 nm

100 nm

ITO: Ø20 nm

Supplementary Fig. 40: Sketch of the nano-dipole and nano-discs along with the geometrical parameters. The Cr layer shown in grey between the Au dimer and the SiO2 substrate has a thickness of 2 nm.


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The TFSF volume default grid is 7 nm × 7 nm × 3 nm. For the gap region, the volume with

dimensions 120 nm × 140 nm × 50 nm was discretized with a cubic grid of 2.5 nm × 2.5 nm ×

1.5 nm. An even smaller cubic grid of 0.3 nm × 0.3 nm × 1 nm is overridden in the region

enclosing the ITO cylinder.

The maximum simulation time is set to 1700 fs. The time stepping stability factor was set to

0.95, which corresponds to a time step of δt = 0.00061 fs.

ω3HI and ω3I are calculated as the total power flowing outward through a volume enclosing

the TFSF source when the Ø20 nm ITO nanoparticle is at the gap of a plasmonic dimer or

isolated, respectively.

The results of this nonlinear analysis are shown in in Supplementary Table 2, 3 and 4. As it

can be seen in Supplementary Fig. 9, high intensity is displayed at the circular interface

between the ITO and air. Given the unavoidable numerical error induced by the hexahedral

mesh failing to map perfectly circular geometries, we provide the average intensity computed

from the stepwise approximated ITO cylinder and from the ITO-enclosing cuboid. These two

cases define the worst and best case scenarios, respectively, i.e. it is expected to have

( ω3HI / ω3I )1/3 between these two cases if our assumption is correct. Indeed, ( ω3

HI / ω3I )1/3 for

the case with χAu(3) and χITO

(3) falls within this range, which supports the assumption (that the

effects at 500 nm can be neglected) and analysis carried out in the main body of this

manuscript. Arguably, this is expected since 500 nm is below the localized surface plasmon

resonance in Au metallic nanoparticles. Therefore, the nanoantenna is not activated, and thus,

its interaction with the THG is reduced. When the nanoantenna operates at its resonance, its

effect cannot be ignored.

It is also relevant to point out that these nonlinear simulations confirm that the main

contributor to the THG is the ITO since the results for χAu(3) & χITO

(3) (Supplementary Table 2)

are comparable to the case with χAu(3) set to zero (Supplementary Table 3), whereas the case

with χITO(3) set to zero has a three orders of magnitude lower THG (Supplementary Table 4).


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Nano-dipole (χAu(3) & χITO

(3))

Simulated structure

Average intensity within

Scattered power at λ = 500 nm [a.u.]

the ITO cylinder at λ = 1500 nm

[a.u.]

the cuboid enclosing the ITO

cylinder at λ = 1500 nm

[a.u.]ITO cylinder 2.46 × 10-11 3.88 × 10-11 1.19 × 10-39 Dimer + ITO

cylinder 8.09 × 10-9 1.53 × 10-8 6.04 × 10-32

Intensity enhancement η = 329.43 η = 394.58

3/1

3

3

ω

ω

IIH = 370.31

Supplementary Table 2: Numerical results determined from nonlinear analysis (χAu(3) & χITO

(3)).

Nano-dipole (χAu(3) = 0 & χITO

(3))

Simulated structure




[a.u.]



[a.u.]ITO cylinder 2.46 × 10-11 3.88 × 10-11 1.19 × 10-39 Dimer + ITO

cylinder 8.17 × 10-9 1.55 × 10-8 6.82 × 10-32

Intensity enhancement η = 332.58 η = 398.84

3/1

3

3

ω

ω

IIH = 385.54

Supplementary Table 3: Numerical results determined from nonlinear analysis (χAu(3) = 0 & χITO

(3)).


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Nano-dipole (χAu(3) & χITO

(3) = 0)

Simulated structure




[a.u.]



[a.u.]ITO cylinder 2.46 × 10-11 3.88 × 10-11 0 Dimer + ITO

cylinder 1.00 × 10-8 1.63 × 10-8 2.20 × 10-35

Intensity enhancement η = 407.93 η = 419.26 -

Supplementary Table 4: Numerical results determined from nonlinear analysis (χAu(3) & χITO

(3) = 0). References: 1. Aouani, H. et al. Multiresonant broadband optical antennas as efficient tunable nanosources of

second harmonic light. Nano Lett. 12, 4997–5002 (2012).

2. Renger, J., Quidant, R., Van Hulst, N. & Novotny, L. Surface-enhanced nonlinear four-wave

mixing. Phys. Rev. Lett. 104, 046803 (2010).

3. Bermúdez Ureña, E. et al. Excitation enhancement of a quantum dot coupled to a plasmonic

antenna. Adv. Mater. 24, OP314–OP320 (2012).

4. Palik, E.D. Handbook of optical constants of solids (Academic Press, 1985).

5. Humphrey, J.L. & Kuciauskas, D. Optical susceptibilities of supported indium tin oxide thin films.

J. Appl. Phys. 100, 113123 (2006).


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