Journal of Physics: Condensed Matter
J. Phys.: Condens. Matter 31 (2019) 415502 (6pp)

https://doi.org/10.1088/1361-648X/ab2e9f

Photon energy dependence of Kerr rotationin GeTe/Sb2Te3 chalcogenide superlattices
Takara Suzuki1, Richarj Mondal1 , Yuta Saito2 , Paul Fons2,
Alexander V Kolobov2,3 , Junji Tominaga2, Hidemi Shigekawa1
and Muneaki Hase1
1

Department of Applied Physics, Faculty of Pure and Applied Sciences, University of Tsukuba,
1-1-1 Tennodai, Tsukuba 305-8573, Japan
2
Nanoelectronics Research Institute, National Institute of Advanced Industrial Science and Technology,
Tsukuba Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan3
Department of Physical Electronics, Faculty of Physics, Herzen State Pedagogical University,
St. Petersburg, 191186, Russia
E-mail: mhase@bk.tsukuba.ac.jp
Received 7 May 2019, revised 21 June 2019
Accepted for publication 2 July 2019
Published 15 July 2019
Abstract

We report on pump-probe based helicity dependent time-resolved Kerr measurements
under infrared excitation of chalcogenide superlattices, consisting of alternately stacked
GeTe and Sb2Te3 layers. The Kerr rotation signal consists of the specular inverse Faraday
effect (SIFE) and the specular optical Kerr effect (SOKE), both of which are found tomonotonically increase with decreasing photon energy over a sub-eV energy range. Although
the dependence of the SIFE can be attributed to the response function of direct third-ordernonlinear susceptibility, the magnitude of the SOKE reflects cascading second-order nonlinear
susceptibility resulting from electronic transitions between bulk valence/conduction bands andinterface-originating Dirac states of the superlattice.
Keywords: chalcogenide superlattices, inverse Faraday effect, Optical Kerr effect,
ultrafast spectroscopy(Some figures may appear in colour only in the online journal)

1. Introduction

there has been no direct experimental evidence for a Diraclike electronic band structure in CSLs. To further examine
the topological properties of CSLs and to further spintronics
applications, experimental investigation of the band structureof CSLs is required.
TI-like characteristics can be discerned optically using
circularly polarized light due to the fact that the angularmomentum of circularly polarized photons can induce spinselective excitations to/from topological surface states, which
is a key signature of topological character [7–10]. Under excitation by circularly polarized light, some solid materials show
helicity dependent optical activity, such as polarization rotations and circular dichroism [11–14]. In this study, the authors
focus on nonlinear optical effects induced by femtosecond
laser irradiation, these include the inverse Faraday effect (IFE)[15, 16] and the optical Kerr effect (OKE) [17], both of whichlead to polarization rotation. The former rotation occurs due

Structures fabricated from the alternate stacking of GeTe and
Sb2Te3 layers, referred to as interfacial phase change memory(iPCM) or chalcogenide superlattices (CSL), are known toform either Weyl or Dirac semimetal systems, which haveclosely related topological insulator (TI) properties, dependingon the thickness of the constituent layers, and are believed to
be excellent candidates for power-saving nonvolatile memory
applications [1–4]. Ab initio band structure simulations of
CSL structures show a gapless band intersection, a so-called
Dirac cone, is formed for the so-called inverted Petrov phase(Dirac semimetal phase) due to the hybridization of the electronic wave functions across the interfaces between the GeTeand Sb2Te3 layers [5]. Although recent angle-resolved photoemission spectroscopy (ARPES) study have examined theband structure of the chalcogenide alloy (Ge2Sb2Te5) [6],1361-648X/19/415502+6$33.00

1© 2019 IOP Publishing Ltd Printed in the UK

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J. Phys.: Condens. Matter 31 (2019) 415502

to magnetization induced by the circularly polarized lightirradiation while the latter is a result of birefringence througha third-order nonlinear optical process governed by the nonlinear susceptibility χ(3).
In a pump-probe experiment, the polarization rotationof the incident probe beam in a material is sensitive to thehelicity of excitation pulse. For instance, simple metals [18],magnetic materials [16] or chalcogenide compounds [19, 20]
exhibit specular IFE (SIFE) or specular OKE (SOKE). It isnoteworthy that, in CSL, the SOKE signal becomes larger
than the SIFE even though the SIFE is usually larger in normal
insulators such as chalcogenide alloys [20]. This is believedto arise from the large second-order susceptibility χ(2) of the
topological Dirac state since third-order nonlinear optical processes are constituted from not only direct third order processes but also cascading second-order nonlinear processes
as expressed by χ(2) · χ(2) [19, 20]. In a previous experiment
by Mondal et al, only near-infrared 830 nm femtosecond laser
pulses were used [20]. To explore the relationship between the
SOKE (SIFE) signal and the band structure of CSLs, pumpprobe measurements over a wide wavelength (or photon
energy) range are required. Here, we report on the photon
energy dependence of the SIFE and SOKE signals in CSL
by observing the polarization rotation of a linearly polarizedprobe beam. In particular, 1200  ∼  1600 nm IR pulses wereused to investigate the TI band structure in detail. We foundthat both the SIFE and SOKE signals were strongly enhanced
when the photon energy approached the energy required for
direct excitation from core valence band states to topological
Dirac states [5, 21, 22].

Figure 1. (a) Optical setup for transient Kerr rotation measurement
is illustrated. (b) The time resolved Kerr rotation data obtained
at 1300 nm. Strong peak was observed at zero time delay. Left
(α = 70 °) and right-handed elliptically polarized light excitation(α   =  130°) results in different peak sign (±).

respectively. A quarter wave plate (QWP) was inserted in thepump optical path to allow the pump helicity to be varied continuously from linear to circular, and data was recorded forevery 10-degree rotation of the QWP. Analyzing the opticalresponse as a function of the pump helicity offers insight into
the role of topological Dirac state in the optical response. The
Kerr rotation was recorded as a function of time delay between
the pump and probe over a range of 20 ps by means of a horizontally shaking mirror with a scanning frequency of 10 Hz[23, 24]. The pump power was kept constant at 20 mW over
the wavelength range used, which corresponds to a fluence of≈16 mJ cm−2, a value calculated using a focused beam diameter of ≈70 µm (measured using a knife-edge). An optical
penetration depth of ≈100 nm at 1550 nm was calculated fromthe absorption coefficient [25], suggesting negligibly small
inhomogeneous excitation effects along the sample depth.
The (GeTe)2/Sb2Te3 sample used was fabricated by radiofrequency magnetron sputtering at 230 °C and was deposited onto a Si (1 0 0) substrate. For better z-axis orientation[26, 27], the Sb2Te3 layers were grown first to a thickness of
3 nm at room temperature and subsequently GeTe (0.8 nm)
and Sb2Te3 (1.0 nm) layers were alternately deposited for 20
cycles (∼36 nm) at elevated temperature. To prevent oxidization, a 20 nm-thick ZnS-SiO2 layer was deposited on top of the
as-grown CSL without breaking the vacuum. The ZnS-SiO2
layer is transparent to near infrared light and therefore makes
no contribution to the observed signal. The samples arehighly oriented poly-crystals. Based on an image of the

2. Experimental
Reflection geometry pump-probe measurements were carriedout to obtain the transient Kerr rotation of the reflected probe
pulse at room temperature [19]. The reflected probe pulse wasdecomposed into two beams polarized at +45◦ and −45◦,
respectively, by a Wollaston prism and detected by balanced
Si-PIN photo-detectors as illustrated in figure 1(a). The intensity difference of the two beams was defined as the Kerr rotation of the probe polarization: ∆θk . Femtosecond laser pulses(repetition rate, 80 MHz; pulse width, 20 fs; and average
power, 525 mW) were generated by a mode locked Ti3+ : sapphire laser oscillator. Its output was amplified by a regenerative amplifier system (RegA9040) to produce pulses of 40 fs
duration with an average power of 500 mW at a 100 kHz repetition rate. Finally, the amplified pulses were sent to an opticalparametric amplifier (OPA9580) to generate IR pulses, whose
wavelength was tuned from 1200 to 1600 nm (1.03–0.775 eV).
600 nm visible photons were also generated by doubling the1200 nm photon energy using second harmonic generation
with a 1 mm-thick β -barium borate (BBO) crystal. The pumpand probe wavelengths were identical in the present experiment. The output beam was split into intense pump and weakprobe beams. The pump and the probe beam were co-focused
onto the sample to a spot size of about 70 µm with an incident
angle of about 15◦ and 10◦ with respect to the sample normal,
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T Suzuki et al

J. Phys.: Condens. Matter 31 (2019) 415502(GeTe)2/Sb2Te3 film taken using scanning probe microscopy,
domain sizes about 50 nm to 100 nm can be identified [28], avalue which is much smaller than the laser spot size (≈70 µm). Further, high-resolution TEM images revealed a van der
Waals (vdW) gap between the GeTe and Sb2Te3 layers [29],indicating the interface quality is high and that the effects ofdomain walls and lattice defects play a minor role.
3. Results and discussion
Time resolved Kerr rotation signals (∆θk ) as a function of theangle of the QWP (α ) of 70° and 130° for the excitation wavelength of 1300 nm (0.954 eV) are presented in figure 1(b). The
transient peak at time delay zero has the opposite sign anda different intensity level from the remainder of the spectra,
depending on the pump helicity. Both α = 70◦ and 130° correspond to an elliptically polarized pump pulse and exhibit thelargest positive and negative transient, respectively. The peakintensity of ∆θk is plotted as a function of the QWP angle
α in figure 2 for different photon energies. The peak signals
for the smaller photon energies shown in figure 2 exhibit asignificantly asymmetric sin(2α ) oscillation due to contrib­
utions from smaller sin(4α ) oscillations. The stronger sin(2α )oscillation observed in the present study for the CSL samples
compared to the previous report [20] may be a consequence ofthe two-orders larger pump fluence used, which is expected toinduce stronger SIFE arising from larger third-order susceptibility contributions. All the data were fit using equation (1) toextract the contributions from the sin(2α ) and sin(4α ) comp­onents, corresponding to SIFE and SOKE signals, respectively [18, 30]
Figure 2. QWP angle dependence of transient Kerr rotation. The
peak intensity of the time resolved Kerr rotation is plotted for eachphoton energy. The closed circles represent the experimental data
and the solid lines correspond to the fit using equation (1). The
reason why the oscillation behavior in QWP is reversed between
1.03 and 2.07 eV is believed to be the relatively larger contribution
from the sin4α oscillation in the case of 2.07 eV.

32π 2 Ipump∆θk (α) =[C sin(2α) + L sin(4α)] + D,
(1)c|1 + n|2where
 (2) (2) χ ·χ
,
L = Re(2)2n(1 − n)2

induced magnetization (SIFE) for the off-resonant case can be
simply expressed by [33],



χ(3)
.
C = −Im(3)n(1 − n)21,
χ(3) (ω; ω, ω, −ω) ∝ 2
(4)ω (ω − ω0 )Here, α is the angle of the quarter wave plate (QWP; λ/4plate), n is the refractive index of the medium, Ipump is thepump intensity, c is the speed of light, and D is related to apolarization-independent background. The photon energy
dependences of the C (SIFE) and L (SOKE) terms are shownin figure 3. As the photon energy decreased, the absolutevalues of C and L terms were found to sharply increase. The
steeper increase in L over C may be a consequence of TI properties [20]. To investigate the sharp increases in the value of
L and C shown in figure 3, the effect of the photon energy
dependent susceptibility function on the SIFE and SOKE is
examined, where second- and third-order nonlinear susceptibilities are expected to play major roles [19]. Based on theclassical Drude–Lorentz model and the quantum mechanicaldescription for the IFE [31, 32], the response function forwhere ω0 is the resonant frequency of the system, whichmeans that even though the SIFE is enhanced by being close
to resonance, it does not vanish away from the resonance.
Figure 3(a) shows a fit of |C| using equation (4) with the fitting
parameter of ω0 = 0.66 ± 0.03 eV. Equation (4) was foundto fit the experimental data (|C|) satisfactory. In contrast, for
the SOKE case, it was assumed that the response function for
L was dominated by cascading χ(2) · χ(2) contributions [19].
In quantum theory, the second-order nonlinear susceptibility
for the off-resonant case can be expressed by permutation ofindexes as [33, 34]:1
χ(2) (0; ω, −ω) = χ(2) (−ω; ω, 0) ∝,(5)(ω − ω0 )2
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J. Phys.: Condens. Matter 31 (2019) 415502

Figure 3. Photon energy dependence of (a) C and (b) L. The black solid curves are fits based on equations (4) and (6), respectively. The
dashed line at 0.75 eV represents the upper bound photon energy required for exciting electrons from BVB bands to the Dirac bands.

Figure 4. (a) The electronic band structure of the chalcogenide superlattice, (GeTe)2/Sb2Te3 inverted Petrov model, obtained by ab initio

calculations [36]. The solid arrows indicate the transition energies necessary to excite electrons from BVB states to topological Dirac states
above the Dirac point (DP) at 0.75 eV and that to BCB states at 1.0 eV. The dashed arrows represent possible transitions from BVB to near
the DP or that from near the DP to the BCB (both transition energies are ≈0.65 eV). (b) The crystal structure of the inverted Petrov model.
(c)–(g) Element projected band structures for each atomic layer shown in (b). The thicker bands represent stronger occupations.

The photon energy dependence of |L| in figure 3(b) was well fit
by equation (6) with ω0 = 0.62 ± 0.24 eV.
The good fit results seen in figure 3 using equations (4)and (6) suggest that the increase in both C (SIFE) and L(SOKE) terms arises from possible resonant excitationin the range ≈ 0.60–0.66 eV. According to the simulated
electronic band structure, one can see that photon energies less than 0.75 eV will promote electrons from thebulk valence band (BVB) to near the top of the Dirac band

where χ(2) (0; ω, −ω) represents optical rectification andχ(2) (−ω; ω, 0) represents the electro-optic effect. Thus,
SOKE signal generated via cascaded optical rectification andthe electro-optic effect, χ(2) (0; ω, −ω) · χ(2) (−ω; ω, 0), is
simply proportional to the square of the χ(2):

2
1
(6)
χ ·χ ∝.
(ω − ω0 )2
(2)

(2)

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J. Phys.: Condens. Matter 31 (2019) 415502

ORCID iDs

(DB), while for photon energies larger than 0.75 eV (e.g.
1.0 eV) a direct optical transition will occur from underlying BVB to the bulk conduction band (BCB) across the
DB (see figure 4). It has been reported that in a magnetic
topological insulator, the photo-induced current becomeslarger when electrons are excited to topological surface
states and reaches a maximum when electrons are excitedfrom one surface band to another [35]. We propose that thesteep increase in both C and L values toward lower photon
energies corresponds to the tail of the resonant excitation from the BVB to around the Dirac point (DP) or thatfrom near the DP to the BCB (both trans­ition energies ≈0.65 eV), as indicated by the dashed arrows in figure 4(a)[22]. Importantly, the steeper increase in L relative to C is
expected to be a consequence of cascaded optical rectification and the electro-optic effect, varying as  ∼ω −4 , where
spatial inversion symmetry at the interfaces is broken bystrain effects, leading a nonzero χ(2) [20]. Note that the
Dirac states in a CSL are either topological interface states[22] or topological bulk states [5], as predicted by ab initio
band structure simulations. The Kerr rotation associatedwith nonlinear susceptibilities may arise from the Berrycurvature dipole, as recently discussed for the photogalvanic effect [37] and for magnetoresistance [38]. Althoughfurther experiments using yet lower photon energies are
required to fully understand the enhancement of the Kerr
rotation in the topological chalcogenide superlattice, ourresults offer significant insights into the Dirac-cone oriented photon energy dependence of spin excitation inmodern two dimensional materials.
In summary, we have investigated the photon energy
dependence of the Kerr rotation in a chalcogenide superlattice using infrared (1200–1600 nm) femtosecond laser
pulses. The measured pump helicity dependence yields acontribution from both sin(2α ) and sin(4α ) components.
Phenomenologically, the former contribution arises from the
SIFE while the latter arises from SOKE. The photon energy
dependence of the relative contributions, labeled C and L
respectively, shows a monotonic increase with decreasing
photon energy. The energy dependence of C can be attributed
to the response function χ(3), while that of L can be attributed to the photon energy dependence of χ(2) · χ(2), where theresponse functions of χ(2) and χ(3) are derived from quantum
theory. The sharp increase is attributed to resonant-like electronic excitation from BVB to Dirac states or that from Diracstates to BCB.

Richarj Mondal https://orcid.org/0000-0001-9784-7467
Yuta Saito https://orcid.org/0000-0002-9576-1560
Alexander V Kolobov
https://orcid.org/0000-0002-81251172
Muneaki Hase https://orcid.org/0000-0003-4242-2546
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Acknowledgments
This work was supported by CREST (Nos. JPMJCR14F1
and JPMJCR1875), JST, Japan and the Japan Society for the
Promotion of Science (Grants-in-Aid for Scientific Research:
17H06088). We acknowledge Ms R Kondou for sample
preparation.

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