Probing the optical near-eld interaction of Mie nanoresonators with atomically thin semiconductors Ana Estrada-Real12 Ioannis Paradisanos32 Peter R. Wiecha4 Jean-Marie Poumirol5

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Probing the optical near-ﬁeld interaction of Mie nanoresonators

with atomically thin semiconductors

Ana Estrada-Real1,2, Ioannis Paradisanos3,2, Peter R. Wiecha4, Jean-Marie Poumirol5,

Aurelien Cuche5, Gonzague Agez5, Delphine Lagarde2, Xavier Marie2, Vincent Larrey6,

Jonas M¨uller4, Guilhem Larrieu4, Vincent Paillard5, and Bernhard Urbaszek1,2

1Institute of Condensed Matter Physics,

Technische Universit¨at Darmstadt, 64289 Darmstadt, Germany

2Universit´e de Toulouse, INSA-CNRS-UPS, LPCNO,

135 Avenue Rangueil, 31077 Toulouse, France

3Institute of Electronic Structure and Laser (IESL),

Foundation for Research and Technology-Hellas (FORTH), 70013, Heraklion-Crete, Greece

4LAAS-CNRS, Universit´e de Toulouse, 31000 Toulouse, France

5CEMES-CNRS, Universit´e de Toulouse, Toulouse, France and

6CEA-LETI, Universit´e Grenoble-Alpes, Grenoble, France

Abstract

Optical Mie resonators based on silicon nanostructures allow tuning of light-matter-interaction

with advanced design concepts based on CMOS compatible nanofabrication. Optically active

materials such as transition-metal dichalcogenide (TMD) monolayers can be placed in the near-

ﬁeld region of such Mie resonators. Here, we experimentally demonstrate and verify by numerical

simulations coupling between a MoSe2monolayer and the near-ﬁeld of dielectric nanoresonators.

Through a comparison of dark-ﬁeld (DF) scattering spectroscopy and photoluminescence excitation

experiments (PLE), we show that the MoSe2absorption can be enhanced via the near-ﬁeld of a

nanoresonator. We demonstrate spectral tuning of the absorption via the geometry of individual

Mie resonators. We show that we indeed access the optical near-ﬁeld of the nanoresonators, by

measuring a spectral shift between the typical near-ﬁeld resonances in PLE compared to the far-

ﬁeld resonances in DF scattering. Our results prove that using MoSe2as an active probe allows

accessing the optical near-ﬁeld above photonic nanostructures, without the requirement of highly

complex near-ﬁeld microscopy equipment.

arXiv:2210.14058v1 [physics.optics] 25 Oct 2022

Introduction.— Optical resonators are essential in many applications such as laser sys-

tems and sensing. The physical size and properties of the resonator are adapted to the

speciﬁc application and to the relevant part of the electromagnetic spectrum [1]. Optical

resonators that are capable of amplifying optical ﬁelds in very small nanoscopic volumes, for

addressing individual nanocrystals or molecules in the near-ﬁeld, are called nanoresonators

[2–13]. They can be fabricated by bottom-up techniques, such as growth of metallic nanopar-

ticles, or top-down approaches, such as Si-nanoresonators on CMOS compatible substrates

[14–19]. Whereas the resonance energies of a resonator with macroscopic dimensions, such

as a laser cavity, are directly accessible in a standard optical far-ﬁeld measurement, the

situation for nanoresonators is more challenging. It has been shown experimentally and in a

substantial body of theory work that there is a shift between the optical resonance energy in

the near-ﬁeld compared to the measured resonance energies in the far-ﬁeld. The exact near-

ﬁeld resonance is key for applications for example in sensing of molecules directly placed in

the near-ﬁeld [20] and it has been accessed up to now either in sophisticated tip-enhanced

experiments or through extrapolation from far-ﬁeld data [21–25].

Here we show that by placing an atomically thin semiconductor directly in the near-ﬁeld

of individual dielectric nanoresonators, we have access to the near-ﬁeld resonance energies

without the use of complex near-ﬁeld spectroscopy techniques. We compare the near-ﬁeld

resonance energies with far-ﬁeld resonance energy measurements on the same resonators and

observe a clear blue-shift of the near-ﬁeld energies in our far-ﬁeld results. We show tuning of

the optical absorption of the atomically thin semiconductor MoSe2through the interaction

with the nanoresonator near-ﬁeld and our results are well reproduced by model calculations

of the shift between near-and far-ﬁeld resonances.

Results and Discussion.— We fabricated two sets of Si/SiO2nanoresonator arrays on

silicon-on-insulator (SOI) substrates. The nanoresonators are cylindrical pillars, arranged

as close-packed heptamers as sketched in Fig. 1e and 1f. The diameters of the individual

cylinders increase from 50 nm to 300 nm with steps of 50 nm. The gaps between neighbour-

ing pillars are 50 nm, 100 nm or 300 nm. The structures are fabricated as arrays of seven

discs to match approximately the size of a diﬀraction limited, focused laser beam, in order to

maximize the experimental signal associated with the Si-NRs. On the ﬁnal nanostructures,

MoSe2monolayers are aligned and transferred on top of the nanoresonators using a micro-

manipulator system [26]. A description of the fabrication process is given in the supporting

Diameter 250 nm

Gap 100 nm

Diameter 300 nm

Gap 100 nm

Nanoresonators Nanoresonators with MoSe2

NR1

NR2

Gap

Diameter Diameter

Gap

Darkfield Darkfield

FIG. 1: dark-ﬁeld scattering images and spectra with and without MoSe2(a) dark-ﬁeld

microscope image of SiO2/Si nanoresonators, the 15 structures on the top part of the image are

hexamers (six pilars) with diameters varying from D=100-300 nm from left to right and the gap

between pillars varying G=50-100-300 nm from top to bottom. The 15 structures on the bottom

are heptamers (seven pillars) with the same variation. (b) Dark-ﬁeld Scattering intensity of the

two nanoresonators highlighted. (c) dark-ﬁeld microscope image with a MoSe2monolayer on top.

Caution, automatic white-balance was used, colors do not directly compare to (a). (d) Dark-ﬁeld

Scattering intensity of the two nanoresonators after transfer of MoSe2monolayer. (e and f) Sketches

of the two selected nanoresonators NR1 and NR2, respectively (top view - see supplement for side

view).

information.

In ﬁgure 1a we show dark-ﬁeld (DF) images of the nanoresonators before MoSe2transfer,

ﬁgure 1c shows the same sample after transfer of an MoSe2monolayer ﬂake. A bright-ﬁeld

microscope image of the nanoresonators after MoSe2deposition is shown in the supplement

Fig. S3, where the MoSe2covered region is clearly visible. By varying the Si-NR gap

and diameter, diﬀerent colors appear in the DF images, which demonstrates the geometry-

dependent Mie resonance energy shifts of the individual resonators in the visible spectral

range.

DF spectra are collected using the same setup as the DF images. To this end, the

signal is sent to a spectrometer instead of the imaging camera. The setup for taking

dark-ﬁeld images and spectra is depicted in the supplement Fig. 4. We will focus in the

following on two selected nanoresonators, “NR1” (blue, Fig. 1e) and “NR2” (red, Fig. 1f).

These heptamers have respective diameters of 250 nm (NR1) and 300 nm (NR2), the

gap between the pillars is identical for both Si-NRs (100 nm). We selected these two

NRs because their main Mie resonances lie at diﬀerent, well distinguishable energies. We

measured the scattered light intensity before and after the monolayer transfer, shown in

Fig. 1b, respectively 1d. We observed a small, global blue-shift on the order of 10 meV for

the resonances when the MoSe2monolayer was on top of the nanoresonators, this small

energy-shift is at the limit of our detection accuracy. Knowing the size of this shift is

helpful for analyzing the spectral shift between measured near-ﬁeld and far-ﬁeld resonances

discussed below.

Spectral shift between near-ﬁeld and far-ﬁeld. In a second step, we want to analyze

the impact of the nanoresonators on the absorption of the MoSe2monolayer. We carry

out photoluminescence excitation (PLE) experiments [27] where temperature, position, and

optical power are kept constant and the excitation wavelength is varied from 450 nm to

650 nm (corresponding to 2.75 eV - 1.90 eV). The optical power of the laser is set to 100 nW,

after making sure that the MoSe2absorption is not saturated at this illumination power (see

also supplemental ﬁgure S4). We avoid tuning the laser close to the MoSe2exciton emission

peaks which occur around 745 nm (≈1.66 eV)[28–30], in order to remain in a non-resonant

excitation regime. During the entire measurements, the sample is kept in a closed-loop

cryostat at a temperature of 5 K. Typical PL spectra are shown in the supplemental Fig. 9,

where we also provide a comparison with room temperature measurements (T= 300 K).

An illustration of the optical setup can also be found in the SI Fig. 5.

The absorption and hence the PLE response of the bare MoSe2[27] is expected to vary

with the excitation energy. This variation is a result of the material’s band-structure, the

presence of high energy excitonic states as well as coupling to phonons [30–33]. Our goal is to

investigate how the absorption is modiﬁed by Mie resonances of the Si-NRs. To distinguish

eﬀects induced by Mie-resonances from material-related variations in the bare MoSe2, we

compare PLE measurements from MoSe2monolayers lying on the ﬂat substrate with MoSe2

on top of the nanoresonators. This is shown in Fig. 2for NR1 and NR2. The PL emission

FIG. 2: Photoluminescence excitation measurements. Accumulated counts of the pho-

toluminescence (PL) spectra measured on bare MoSe2(gray circles) and MoSe2on top of each

nanoresonator NR1 (blue squares) and NR2 (red triangles), excited with a continuum laser at 40

diﬀerent energies and at T=5 K.

spectra for each excitation laser wavelength are numerically integrated, by summation of

the counts per second in the spectral range around the emission peaks, from 1.5 eV - 1.7 eV

(see SI for raw spectra) [34]. Hence, each data point in Fig. 2is a separate PL measurement

for a speciﬁc laser excitation energy. The direct comparison of the PLE measurements from

MoSe2with and without Si-NR (c.f. Fig. 2), already reveals clear diﬀerences. To better

visualize the eﬀect of the nanoresonators on the PLE signal, we divide the integrated PL

intensities from MoSe2on the Si-NRs IP L,NR (blue squares and red triangles in Fig. 2) by

the signal from the bare MoSe2IP L,MoSe2(on ﬂat substrate, gray dotted line in Fig. 2):

ρ=IP L,NR

IP L,MoSe2

.(1)

This ratio ρgives an estimation of the absorption enhancement due to the presence of a

nanoresonator supporting Mie resonances. The Mie resonances locally amplify the optical

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Probingtheopticalnear-eldinteractionofMienanoresonatorswithatomicallythinsemiconductorsAnaEstrada-Real1;2,IoannisParadisanos3;2,PeterR.Wiecha4,Jean-MariePoumirol5,AurelienCuche5,GonzagueAgez5,DelphineLagarde2,XavierMarie2,VincentLarrey6,JonasMuller4,GuilhemLarrieu4,VincentPaillard5,andBernhardUrba...

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Probing the optical near-eld interaction of Mie nanoresonators with atomically thin semiconductors Ana Estrada-Real12 Ioannis Paradisanos32 Peter R. Wiecha4 Jean-Marie Poumirol5

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