Anisotropic Metasurface for Ultrafast Polarization Control via

All-Optical Modulation

Giulia Crotti

1,2

, Mert Akturk

, Andrea Schirato

1,2

,Vincent Vinel

, Remo Proietti Zaccaria

2,4 a

Margherita Maiuri

1,5

, Anton A. Trifonov

, Ivan C. Buchvarov

7,8

, Dragomir N. Neshev

9 b

Giuseppe Leo

, Giulio Cerullo

1,5 c

and Giuseppe Della Valle

1,5,6 d

Department of Physics, Politecnico di Milano, 20133 Milano, Italy

Istituto Italiano di Tecnologia, 16163, Genova, Italy

Laboratoire Mat

eriaux et Ph

enom

enes Quantiques, MPQ UMR 7162, Universit

e de Paris, CNRS, 75013, Paris, France

Cixi Institute of Biomedical Engineering, Ningbo Institute of Industrial Technology,

Chinese Academy of Sciences, Ningbo 315201, China

IFN, Consiglio Nazionale Delle Ricerche, 20133, Milano, Italy

INFN, Sezione di Milano, I-20133 Milano, Italy

John Atanasoff Center for Bio and Nano Photonics (JAC BNP), 1164 Soﬁa, Bulgaria

Faculty of Physics, St. Kliment Ohridski University of Soﬁa, 5 James Bourchier Boulevard, 1164 Soﬁa, Bulgaria

ARC Centre of Excellence for Transformative Meta-Optical Systems (TMOS), Research School of Physics,

Australian National University, Acton, ACT 2601, Australia

Keywords:

Ultrafast Photonics, All-Optical Polarization Control, Reconﬁgurable Metasurfaces.

Abstract:

The ability to manipulate light polarization at an ultrafast speed is a challenging goal in the ﬁeld of Photonics:

sub-picosecond control of polarization is a fundamental functionality for a variety of applications, includ-

ing the developement of free-space optical links for robust information encoding. To this aim, an important

paradigm consists in employing metasurfaces as platforms which can be reconﬁgurable by all-optical means,

i.e. upon illumination by an energetic femtosecond laser pulse. Here, we present giant all-optical modulations

of dichroism in an anisotropic AlGaAs metasurface. An optimized design allows to exploit a sharp extended

resonance of the nanostructure in the desired spectral range, where the pump-induced band-ﬁlling effect is the

dominant process presiding over the ultrafast change of permittivity.

1 INTRODUCTION

Polarization is one of the fundamental degrees of free-

dom of light. Ultrafast, active control of it is a cru-

cial functionality for a plethora of applications in vari-

ous ﬁelds, ranging from classic and quantum informa-

tion manipulation and encoding (Stanciu et al., 2007;

Flamini et al., 2018) to modulation of material pro-

cesses, such as molecular orientation (Fleischer et al.,

2011) and lattice excitation (F

orst et al., 2011), to

probing polarization-sensitive chemical and biologi-

cal systems (e.g. chiral molecules).

https://orcid.org/0000-0002-4951-7161

https://orcid.org/0000-0002-4508-8646

https://orcid.org/0000-0002-9534-2702

https://orcid.org/0000-0003-0117-2683

In order to develop reconﬁgurable devices, oper-

ating in the THz speed regime, the most promising

approach consists in adopting all-optical schemes. In

these, an energetic femtosecond laser pulse is em-

ployed to trigger a third-order optical nonlinearity,

which provides modiﬁcation of the material permit-

tivity of the system, thus modulating the optical re-

sponse on a sub-picosecond timescale.

Within this framework, some possible platforms

have been identiﬁed for efﬁcient polarization con-

trol. Polarization switching with an extinction rate of

91 and a switch-on time of 800 fs has been demon-

strated in a perfect absorber consisting of an In-doped

cadmium-oxide ﬁlm with a gold capping (Yang et al.,

2017); synthesis and switching of light polarization

with up to 60

◦

rotation of the polarization ellipse have

been achieved in a nonlinear metamaterial based on

Crotti, G., Akturk, M., Schirato, A., Vinel, V., Proietti Zaccaria, R., Maiuri, M., Trifonov, A., Buchvarov, I., Neshev, D., Leo, G., Cerullo, G. and Della Valle, G.

Anisotropic Metasurface for Ultrafast Polarization Control via All-Optical Modulation.

DOI: 10.5220/0011926600003408

In Proceedings of the 11th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2023), pages 107-113

ISBN: 978-989-758-632-3; ISSN: 2184-4364

 2023 by SCITEPRESS – Science and Technology Publications, Lda. Under CC license (CC BY-NC-ND 4.0)

107

a gold nanorod array embedded in a Al

matrix

(Nicholls et al., 2017). Another possibility is repre-

sented by epsilon-near-zero materials-based devices,

such as in (Wang et al., 2021), where indium titanium

oxide is coupled to gold nanoantennas.

An alternative route is represented by meta-

surfaces, which are planar arrangements of tightly

packed nanoresonators, and, as such, are both ultra-

compact and highly tunable. Consequently, they con-

stitute the ideal candidate for the development of inte-

grated photonic devices. A relevant example of polar-

ization control is illustrated in a paper by some of the

present authors (Schirato et al., 2020), where a siz-

able dichroic response is induced in an isotropic plas-

monic metasurface via photo-induced excitation of

out-of-equilibrium carriers with a non-uniform spa-

tial pattern. In this sense, a complementary approach

consists in employing an anisotropic metasurface in

which pumping could induce a transient modulation

of static dichroism and birefringence (Della Valle

et al., 2017).

Here, following this last strategy, we present an

anisotropic AlGaAs metasurface, very efﬁciently re-

conﬁgurable by all-optical means, and showing gi-

ant modulations of its dichroic properties on an ul-

trafast timescale. We combine experiments with the-

oretical simulations, by performing ultrafast pump-

probe spectroscopy and multisteps modelling to ratio-

nalize the dynamics of the retrieved transient optical

response.

The paper is organized as follows. In section 2 the

design of the metasurface is described, and the results

of measurements are shown; an account of modelling,

as well as the discussion on experimental results, can

be found in section 3. Finally, in section 4, some con-

clusions are drawn about the presented picture, and

considerations about ongoing and future work are pre-

sented.

2 DESIGN AND EXPERIMENTS

The fabricated sample consists of two replicas of a

4 × 7 matrix of 70 µm × 70 µm AlGaAs nanowire

metasurfaces, each implementing different values of

the periodicity P and wires width W . The left panel

of ﬁgure 1 shows a sketch of one such metasurface,

as well as a pictorial depiction of the concept of ul-

trafast pump-probe spectroscopy. The left inset il-

lustrates the unit-cell vertical cross-section, while the

SEM image on the bottom right shows the top-view

of the horizontal xy−plane.

We report here measurements and results rela-

tive to the metasurface parameters P = 450 nm, W =

180 nm: this speciﬁc combination of parameters al-

lows to tune an extended resonance of the unper-

turbed structure in a spectral range comprised be-

tween 750 nm and 800 nm; such a range is of partic-

ular interest due to band-ﬁlling, which is the mecha-

nism presiding over the third-order optical nonlinear-

ity in semiconductors at the bandgap edge. Indeed,

in such region, the photoinduced permittivity modu-

lation following pump absorption is pronounced and

dominated by its real part; thus, nonlinear losses are

negliglible (Pogna et al., 2021). For the present case

of Al

0.2

0.8

As, the bandgap sits at around 745 nm.

The right panel of ﬁgure 1 shows the measured re-

ﬂection spectrum of the unperturbed sample for light

polarized parallel (TE) or perpendicular (TM) with re-

spect to the nanowires (see inset), impinging at an an-

gle of approximately ∼ 10

◦

. The diameter of the spot-

size (FWHM) of the laser beam employed for probing

is approximately 250 µm, to be compared with the

diagonal of the metasurface (∼ 100 µm). Thus, the

sample is almost uniformly illuminated.

Note that the strong anisotropy of the metasurface

directly translates into a markedly dichroic static re-

sponse. A resonance in the TE polarization is located

at around 775 nm, whereas no resonant features are

observed upon illumination with TM polarized light.

It is also to be remarked that, due to probe spot-size

dimensions, the dominant contribution to the spec-

tra comes from light reﬂected by the substrate. In-

deed, we also measured the substrate optical response,

which, since an AlOx buffer is present, is conﬁgured

as the typical Fabry-Perot etalon response. In any

case, due to symmetry of quasi-normal incidence, the

substrate reﬂection spectra are almost degenerate in

the two different polarizations, and don’t show any

narrow resonant feature.

With the aim of characterizing the photo-induced

modulation of the dichroic properties of the meta-

surface, we performed ultrafast pump-probe spec-

troscopy measurements. These consist in illuminat-

ing the sample with a high intensity pump pulse and

then interrogating it with a second (lower intensity)

probe pulse, to record the relative variation of the

sample reﬂection, ∆R/R =

′

−R

, where R

′

ad R are

the reﬂection of the perturbed and unperturbed sam-

ple, respectively. This quantity is a function of both

the probe wavelength and the time delay between the

two pulses.

The experimental apparatus is constructed as fol-

lows: a Ti:sapphire laser that emits 100-fs pulses at

800 nm wavelength, with a repetition rate of 1kHz,

pumps a 1 mm Beta-Barium-Borate (BBO) crystal to

generate second harmonic pulses, centered at 400 nm.

These are directly employed as the optical pump in

NanoPlasMeta 2023 - Workshop on Nanophotonics, Plasmonics and Metamaterials

108

     























Figure 1: (Left). A depiction of one of the fabricated metasurfaces, together with the illustration of the concept of pump-probe

spectroscopy. Left inset shows a vertical cross-section of the unit cell of the structure, with the AlGaAs nanowire on top of

a 900 µm thick AlOx buffer and GaAs substrate. Bottom right inset is a SEM image showing the corresponding top-view

(horizontal plane). (Right). Reﬂection spectra of the unperturbed sample for light polarized parallel (TE) or perpendicular

(TM) with respect to the nanowires orientation, impinging at a small angle (θ ≃ 10

◦

the experiment. A non-collinear optical parametric

ampliﬁer is used to generate pulses at 1240 nm, to

pump in turn a YAG crystal in order to obtain the

probe pulse. An optical delay line is inserted along

the pump path and the pump is modulated at 500 Hz

by a mechanical chopper, so that it is possible to ob-

tain the differential reﬂection ∆R/R as a function of

the pump-probe delay time.

By properly adding polarizers and waveplates

to this conﬁguration, one can record polarization-

resolved two-dimensional maps of ∆R/R

, ∆R/R

Measurements are shown in ﬁgure 2. The transient

optical response is strongly dichroic, showing huge

modulations (up to ∼ 40%) of the reﬂected power for

the TE polarization at a spectral position which corre-

sponds to the neighbourhood of the unperturbed reso-

nant wavelength, whereas in TM the differential sig-

nal, though sizable, is much lower. These results are

remarkable considering the incident pump ﬂuence of

70 µJ/cm

, which can be esteemed a low - moderate

level of excitation with respect to the ones reported in

literature (Pogna et al., 2021; Mazzanti et al., 2021),

and is two orders of magnitude below the damaging

threshold of the sample.

The same measurements have been also per-

formed by illuminating the substrate only, i.e. in a

region with an AlOx thick ﬁlm on top of GaAs sub-

strate: in this case, the transient reﬂectivity response

is degenerate in polarization and amounts to less than

2%. Hence, the differential signal, and thus the mod-

ulation of dichroism shown in ﬁgure 2 is dominated

by the AlGaAs metasurface response.

In order to better visualize the complex spectral

features of the transient optical response, cross sec-

tions of the polarization-resolved 2D maps at ﬁxed

time delays of 3 ps, 6 ps and 12 ps can be inspected

in ﬁgure 3, with TE/TM polarizations color coded

as in previous ﬁgures. Concerning TE polarization,

one can observe that the peak of the differential re-

ﬂectance signal is clearly located at ∼ 770 nm, in

close correspondence with the static resonance; how-

ever, other extremely narrow features are present in

the transient map, which cannot be directly related to

the shape of the unperturbed spectrum. This could

be due to the experimental conditions in which the

static reﬂectivity measurements have been performed:

as it has already been remarked upon, the dimension

of probe spot-size diameter entails that the reﬂected

power shown in ﬁgure 1 is dominated by the substrate

contribution. Therefore, narrow spectral features be-

longing to the metasurface optical response are com-

pensated for or completely covered by substrate re-

ﬂection; instead, they are revealed in the transient

measurements, where high reﬂectivity of the back-

ground is almost fully rejected.

It is to be noted that the effect is in practice extin-

guished within 12 ps from pump arrival.

3 MODELLING AND

DISCUSSION

In order to rationalize the efﬁcient reconﬁguration

of the metasurface following photoexcitation, we de-

cided to adopt a simpliﬁed version of a multistep

model which describes the physical processes tak-

ing place following photoexcitation (Mazzanti et al.,

2021). We will now brieﬂy summarize such mech-

anisms and how they are taken into account in the

model, as schematically depicted in ﬁgure 4.

AlGaAs is a direct bandgap semiconductor with

Anisotropic Metasurface for Ultrafast Polarization Control via All-Optical Modulation

109

Figure 2: Measured 2D maps of differential reﬂection, retrieved as a function of probe wavelength and pump-probe time

delay, for the TM (left) and TE polarization (right).

Figure 3: Cross-sections of the 2D maps of ﬁgure 2 at a time delay of 3 ps (left), 6 ps (centre) and 12 ps (right). Red (blue)

curve corresponds to TE (TM) polarization.

gap energy E

gap

≃ 1.65 eV. Absorption of the pump

pulse, which is centered at λ = 400 nm with an en-

ergy of 3.2 eV, generates a population of electron-

hole pairs in the conduction and valence band respec-

tively, via interband transitions.

The photoexcited population of electron-holes

then recombines through different channels: trap-

assisted nonradiative recombination, bimolecular and

Auger processes, the ﬁrst one being the dominating

due to the large surface-to-volume ratio of nanostruc-

tures (Shcherbakov et al., 2017). Nonradiative recom-

bination entails the emissions of phonons, and thus a

slow increase of the lattice temperature in the wires.

The aforementioned degrees of freedom of the

photoexcited system – namely, the density of e-h pairs

and the lattice temperature – and their evolution in

time can be described with the simple rate equation

system depicted in ﬁgure 4. This is the so-called Two

Temperature Model (TTM).

Electron-holes pairs can be conceived as a plasma,

contributing to an ultrafast change of AlGaAs permit-

tivity through two distinct mechanisms. One is the

typical Drude process, for intraband transitions, en-

tailing a modiﬁcation ∆ε

Drude

. The second effect is

the so called band-ﬁlling (hence the permittivity vari-

ation ∆ε

), which accounts for a saturation of the ab-

sorption channels: since the bottom of the conduction

band has been ﬁlled with electrons promoted follow-

ing photoexcitation, Pauli exclusion principle forbids

interband transitions upon arrival of the probe beam.

This entails a modiﬁcation of the imaginary part of the

material permittivity for probe wavelengths with en-

ergy above the bandgap; hence, modulation of the real

part of permittivity is determined through Kramers-

onig relations. Crucially, the probe beam having

an energy below the bandgap (i.e. in the present case

NanoPlasMeta 2023 - Workshop on Nanophotonics, Plasmonics and Metamaterials

110

at wavelengths longer than ∼ 745 nm) experiences a

permittivity modulation due to band ﬁlling which is

purely real. Therefore, in such spectral regions near

the bandgap, where band ﬁlling dominates, losses are

minimized. Finally, another contribution to permittiv-

ity modulation comes from the lattice heating through

thermo-optics effect, ∆ε

thermo

. We refer the reader

to (Mazzanti et al., 2021) for the exact expression of

each contribution to the transient permittivity change

as a function of the electron-hole couple density and

lattice temperature.

Therefore, by resolving the TTM and directly sub-

stituting the solution into the described semiclassical

formulas, it is possible to model the ultrafast permit-

tivity variation as a function of time delay and probe

wavelength. Transient optical response of the sam-

ple can then be retrieved by performing full vectorial

electromagnetic simulations.

Figure 4: Schematics of physical processes taking place

following photoexcitation, together with the steps of the

model: TTM to describe electron-hole pairs density and

lattice temperature as they evolve in time, semiclassical for-

mulas to compute permittivity variations due to Drude, band

ﬁlling and thermo-optics mechanisms.

This approach, though accurate, is also computa-

tionally demanding. We decided to start with a simple

estimate of the permittivity variation at the time delay

corresponding to the signal peak (3 ps) with respect

to the spectral region near the bandgap. The selected

timescale allows us to neglect the thermo-optics per-

mittivity variation, since the lattice heating is a slower

process. Instead, the most relevant mechanism is rep-

resented by band ﬁlling, which also dominates over

Drude effect (Mazzanti et al., 2021). As a further

simpliﬁcation, we also decided to set a ﬁxed (real val-

ued) permittivity change for all the interesting wave-

length range (700–800 nm), in order to highlight the

key features of the considered phenomenon. There-

fore, we solved the TTM using the same material pa-

rameters for AlGaAs as in (Mazzanti et al., 2021) and

considering a pump ﬂuence of 35 µJ/cm

– half the

experimental value – to correct the model’s permit-

tivity variation overestimation for a factor of two, as

explained in the same paper. Then, we chose the com-

puted value at λ = 745 nm, t = 3 ps as the uniform

permittivity variation, ∆ε = −0.08.

This is a rather coarse approximation, disregard-

ing both the imaginary part and spectral dependence

of ∆ε. However, the imaginary part of permittivity

change is nonzero only for wavelengths shorter than

745 nm, since band-ﬁlling effect dominates; more-

over, our model reveals that in this same spectral re-

gion the real permittivity variation is more dispersed,

whereas it is ﬂatter in the region between 745 nm and

800 nm. Thus, we should expect an acceptable quali-

tative accuracy in the interesting spectral range where

most important modiﬁcations of the optical response

occur.

We then performed full-vectorial electromagnetic

simulations employing commercial software COM-

SOL Multiphysics 6, to compute both the static op-

tical response of the metasurface (∆ε = 0) and the

perturbed one (∆ε = −0.08). We set the following

parameters for the structure geometry: periodicity

P = 435 nm, wires width W = 185 nm and wires

height H = 415 nm (nominal fabrication parameter

was H = 400 nm). These values are optimized to ﬁt

the static optical response to the experimental spectra,

in order to take into account fabrication defects and

boundary effects which cannot be included in simula-

tions with periodic boundary conditions.

Results are shown in right panel of ﬁgure 5. On

the left, for comparison, the experimental differen-

tial reﬂection curves at 3 ps are plotted with the same

vertical axis. For TM polarization (blue curves) the

agreement is remarkably good, with a slight quanti-

tative overestimation of the transient effects at short

wavelengths and between 780–800 nm. This can be

ascribed to the choice of ﬁxing the permittivity to the

value corresponding to λ = 745 nm and neglecting

losses in the range 700–745 nm. Something similar

happens also for TE polarization; notice however the

accurate estimation of the peak differential reﬂectiv-

ity at 769 nm (760 nm in the simulation). Difference

in narrow features in the range 760 – 780 nm (speciﬁ-

cally, the pronounced dip at 765 nm in the simulation)

has instead to be attributed to an imperfect ﬁt of the

geometrical parameters, which was performed to ac-

curately mimic the experimental sample response.

Nevertheless, notwithstanding the approxima-

tions, our model is capable of highlighting the origin

of such efﬁcient modulation of the sample dichroic

response, attributing it to the combination of two fac-

tors: (a) the dominant contribution of band ﬁlling to

permittivity modulation, which is purely real in the

spectral range of interest, thus allowing to minimize

losses, and (b) the presence of sharp resonances of

the metasurface in this precise spectral region near the

bandgap.

Anisotropic Metasurface for Ultrafast Polarization Control via All-Optical Modulation

111

Experiment, 3 ps

Simulation

Δε = -0.08

Figure 5: (Left). Experimental differential reﬂection at 3 ps. Red (blue) curve corresponds to a TE (TM) polarized probe.

(Right). Simulated differential reﬂection obtained for a uniform permittivity modulation ∆ε = −0.08, corresponding to the

value obtained by our model at t = 3 ps, λ = 745 nm. Color coding corresponds to the one of left panel. Vertical axis is shared

among the two panels.

4 CONCLUSIONS

We have presented an anisotropic metasurface based

on AlGaAs nanowires on top of a composite AlOx-

GaAs substrate, to be used as a platform for dichroism

modulation through all-optical schemes, to the aim of

obtaining ultrafast control of light polarization. We

experimentally demonstrated very high efﬁciency in

reshaping the optical response (with a differential re-

ﬂection peak of almost 40%) in the 700 – 800 nm

spectral range, with a moderate excitation ﬂuence of

70 µJ/cm

Our preliminary, simpliﬁed version of the multi-

physics modelling laid out in section 3 is capable of

capturing the key physical phenomena taking place

after photoexcitation and regulating the transient op-

tical response: speciﬁcally, high modulation is ob-

tained by exploiting a sharp resonance of the structure

in the spectral region near to the bandgap, and the

measured effect is to be attributed to band-ﬁlling as

the dominant mechanism presiding over the ultrafast

permittivity change. Since this contribution is purely

real for the interesting range of operation, losses are

extremely reduced. From the designer perspective,

this conclusion can suggest a suitable direction to fol-

low, with the goal of tailoring the static optical re-

sponse of devices by tuning narrow spectral features

near the bandgap region of the constituent metasur-

face semiconductor.

As for modelling, further steps (and work in

progress) include the employment of the full model to

elucidate the temporal dynamics of the transient opti-

cal signal. In addition, an interesting future perspec-

tive is represented by the possibility of studying the

reconﬁguration of the birefringent properties of such

metasurfaces, possibly tailoring them to the ambitious

goal of implementing ultrafast waveplate functionali-

ties.

ACKNOWLEDGEMENTS

This publication is part of the METAFAST project

that received funding from the European Union Hori-

zon 2020 Research and Innovation program under

grant agreement no. 899673. This work reﬂects only

authors’ view and the European Commission is not

responsible for any use that may be made of the infor-

mation it contains.

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