Broadband terahertz wave generation from an epsilon-near-zero material
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Jia et al. Light: Science & Applications (2021)10:11 Official journal of the CIOMP 2047-7538
https://doi.org/10.1038/s41377-020-00452-y www.nature.com/lsa
ARTICLE Open Access
Broadband terahertz wave generation from an
epsilon-near-zero material
Wenhe Jia1, Meng Liu2, Yongchang Lu2, Xi Feng2, Qingwei Wang2, Xueqian Zhang2, Yibo Ni1, Futai Hu1, Mali Gong1,
Xinlong Xu3, Yuanyuan Huang3, Weili Zhang 4, Yuanmu Yang1 and Jiaguang Han 2
Abstract
Broadband light sources emitting in the terahertz spectral range are highly desired for applications such as
noninvasive imaging and spectroscopy. Conventionally, THz pulses are generated by optical rectification in bulk
nonlinear crystals with millimetre thickness, with the bandwidth limited by the phase-matching condition. Here we
demonstrate broadband THz emission via surface optical rectification from a simple, commercially available 19 nm-
thick indium tin oxide (ITO) thin film. We show an enhancement of the generated THz signal when the pump laser is
tuned around the epsilon-near-zero (ENZ) region of ITO due to the pump laser field enhancement associated with the
ENZ effect. The bandwidth of the THz signal generated from the ITO film can be over 3 THz, unrestricted by the phase-
matching condition. This work offers a new possibility for broadband THz generation in a subwavelength thin film
made of an ENZ material, with emerging physics not found in existing nonlinear crystals.
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Introduction GaP16, and LiNbO317, with a femtosecond laser typically
Terahertz (THz) radiation spanning from 0.1 to 10 THz operating in the near-infrared range. However, the
falls between the microwave and infrared spectral ranges1. intensity and bandwidth of the generated THz signal, as
In recent years, THz technology applications have been well as the pump wavelength, are often limited by the
rapidly expanding2,3 in areas including nondestructive phase-matching condition in the bulk nonlinear crystal.
material evaluation4,5, imaging6,7, sensing8,9, and wireless This problem has recently stimulated growing interest in
communication10,11. THz radiation can be generated developing ultrathin THz emitters with thicknesses down
through the nonlinear downconversion of optical signals to even a few atomic layers. In particular, optical meta-
or through the nonlinear upconversion of microwave materials composed of split-ring resonators with magnetic
signals, among which the nonlinear optical rectification dipole resonances have been identified as excellent non-
method is particularly popular for the generation of linear THz sources, with a spectral bandwidth unrest-
broadband THz pulses for spectroscopy-related applica- ricted by the phase-matching condition, as well as the
tions. Despite the recent discoveries of broadband THz material absorption in the Reststrahlen region of con-
generation in air plasma12,13 and liquids14, THz emission ventional nonlinear crystals18,19. However, the highly
is more routinely generated by pumping solid-state non- sophisticated nanofabrication process and the low laser
centrosymmetric nonlinear crystals, such as ZnTe15, damage threshold have largely hindered their wide
adoption. Nanoscale THz emitters have also been realized
with monolayer graphene20 and tungsten disulfide21,
Correspondence: Yuanmu Yang (ymyang@tsinghua.edu.cn) or
although with limited efficiency. Furthermore, surface
Jiaguang Han (jiaghan@tju.edu.cn)
1
State Key Laboratory of Precision Measurement Technology and Instruments, THz emission from semiconductors such as InAs, InSb,
Department of Precision Instrument, Tsinghua University, Beijing 100084, China
2
and GaAs has been investigated22,23, although efficient
Center for THz Waves and College of Precision Instrument and
THz emission has only been shown in the reflection
Optoelectronics Engineering, Tianjin University, Tianjin 300072, China
Full list of author information is available at the end of the article configuration.
These authors contributed equally: Wenhe Jia, Meng Liu
© The Author(s) 2021
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Jia et al. Light: Science & Applications (2021)10:11 Page 2 of 8
More recently, materials that exhibit a vanishing real frequency of ITO further follows the formula
pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
part of their permittivity in certain spectral ranges24,25, ωp ¼ Ne e2 =meff ε0 , where Ne is the carrier density, ε0 is
commonly known as epsilon-near-zero (ENZ) materials, the permittivity in vacuum, and meff is the ensemble-
have drawn much attention in the field of nonlinear averaged effective electron mass. The carrier density of ITO
optics26–29 for applications such as second- and third- is typically in the range of 1020–1021 cm−3, leading to an
harmonic generation30–32, all-optical switching33,34, and ENZ wavelength λENZ typically in the near-infrared region.
tuneable absorption35. The enhancement of nonlinear
optical responses in materials within a subwavelength In this work, we show THz generation from commer-
thickness, induced by the ENZ effect, can be partially cially available ITO glass by leveraging the ENZ effect. We
explained by the amplification of the pump laser field due observe THz emission from a 19 nm-thick ITO film when
to the continuity of the normal displacement field at the pumped around its ENZ wavelength. The bandwidth of
boundary of the ENZ material and the background med- the generated THz pulse is ~3 THz, limited only by the
ium, following the formula εENZ·EENZ = ε0·E0, where εENZ bandwidth of the pump laser and the detection crystal.
and ε0 are the permittivities of the ENZ material and We measure the pump wavelength, power, and polariza-
background medium, respectively, and EENZ and E0 are the tion dependence to confirm that the THz emission ori-
normal components of the electric fields at the material ginates from surface optical rectification in the ITO film
boundary. When εENZ approaches zero, the electric field in and is enhanced by the ENZ effect. Moreover, we observe
the ENZ material can be significantly enhanced. The that the most efficient THz generation initially occurs at a
enhanced nonlinearity in the ENZ material can be alter- pump wavelength slightly blueshifted from the static bulk
natively explained by the small group velocity, also known λENZ of ITO, which redshifts as the pump fluence
as the slow-light effect36, in the Ferrell–Berreman mode or increases, which may be associated with the nonlocal
the ENZ mode supported in the ENZ thin film. effect as well as the unique hot-electron dynamics in the
ENZ effects occur in a wide range of materials, ITO film.
including transparent conducting oxides (TCOs)27,
doped semiconductors37, and polar dielectrics38. Indium Results
tin oxide (ITO) is one of the most widely available ENZ Sample characteristics and measurement setup
materials. It has been used as transparent electrodes in A schematic of the commercially available ITO on a
solar cells and consumer electronics. The dispersion of glass substrate (PGO GmbH) is depicted in Fig. 1a. The
the permittivity of ITO follows the Drude–Lorentz film is found to be amorphous from the X-ray diffraction
formula ε¼ε1 þω2p;b = ω20;b ω2 iγ b ω ω2p =ωðω þ iγ f Þ, measurement, with a root mean square roughness of
0.35 nm determined from the atomic force microscopy
where ε∞ is the high-frequency permittivity, ωp,b, ω0,b, measurement (see Supplementary Fig. 1 and Supple-
and γb are the plasma frequency, resonance frequency, mentary Note 1 for details). The real and imaginary parts
and plasma damping rate of bound electrons, respec- of the relative permittivity and the thickness of the ITO
tively, and ωp and γf are the plasma frequency and plasma film are measured through spectroscopic ellipsometry, as
damping rate of free electrons, respectively39. The plasma shown in Fig. 1b (see Supplementary Note 2 for details).
a b λENZ= 1400 nm c
Pump pulse THz pulse 8
2 Real (ε)
Z
Imag (ε) 0.3
Y 6
1
(Ez,ITO/Ez,incident)2
X
Permittivity
0.2 4
Rp/Rs
0
19 nm
2
–1 0.1
–2 0
ITO
1000 1100 1200 1300 1400 1500 1600 1700 1000 1100 1200 1300 1400 1500 1600 1700
Glass substrate
Wavelength (nm) Wavelength (nm)
Fig. 1 Static optical response of the ITO film. a Schematic of the ENZ sample composed of a 19-nm-thick ITO layer and a 1.1 mm-thick glass
substrate. b Real and imaginary parts of the permittivity of the ITO film as a function of wavelength measured via spectroscopic ellipsometry. The ENZ
wavelength is marked with a grey dashed line. c Rp/Rs and (Ez,ITO/E0,incident)2 measured (dashed line) and calculated by the transfer matrix method
(solid lines) as a function of the wavelength with an incident angle of 40°, where Rp and Rs are the reflectance of p-polarized and s-polarized light,
respectively, and Ez,ITO and Ez,incident are the normal components of the electric field in the ITO film and the incident electric field, respectively
Jia et al. Light: Science & Applications (2021)10:11 Page 3 of 8
The real part of its permittivity crosses zero at a wave- wavelength tuneable from 1100 to 1600 nm in both the
length of 1400 nm, with an imaginary part of 0.35. The p- transmission and reflection configurations (see ‘Materials
polarized linear reflectance spectrum of the sample is and Methods’ for details). Figure 2b, c illustrate the
measured at an incident angle of 40°, showing a resonance measured THz time-domain signals. We observe THz
dip near its λENZ as a result of the excitation of the emission from the ITO sample in both the transmission
Ferrell–Berreman mode25,40, in close agreement with the and reflection configurations. In contrast, no THz emis-
calculation, as illustrated in Fig. 1c. The corresponding sion is observed from a bare glass substrate under iden-
electric field enhancement in ITO near its λENZ is also tical optical excitation conditions. We attribute the origin
calculated using the transfer matrix method. of the THz generation from the ultrathin ITO film to the
The key elements of the THz time-domain emission surface second-order optical nonlinearity. Similar obser-
spectroscopy system in both the transmission and vations on the surface second-order optical nonlinearity
reflection configurations are schematically shown in of ITO have been made in previous literature studying the
Fig. 2a. We use the output of an optical parametric second-harmonic generation of ITO films around their
amplifier (OPA) to excite the ITO sample with a ENZ wavelength39,41,42.
a
Ti:sapphire amplifier
Time delay
Balanced
Parabolic mirror
photodiodes
QWP
Probe 800 nm 35 fs 1 kHz
Splitter
ZnTe Wollaston
Lens crystal prism
Pump Mirror
OPA
1100 nm–1600 nm
100 fs 1 kHz
Sample
b c 0.8
0.8
Transmission ITO film 0.6 Reflection ITO film
Glass Glass
0.6
THz amplitude (norm.)
THz amplitude (norm.)
0.4
0.4
0.2
0.2
0.0
0.0
–0.2
–0.2
–0.4
–0.4
–2 –1 0 1 2 3 4 –2 –1 0 1 2 3 4
Time delay (ps) Time delay (ps)
Fig. 2 Experimental setup and time-domain THz signals from the ITO film. a Schematics of the THz generation and detection setup in both the
transmission and reflection configurations. b, c Time-domain THz signal measured by pumping the ITO film (red) and 0.5 mm-thick bare glass
substrate (blue) with pump wavelengths of 1350 and 1400 nm in the transmission and reflection configurations, respectivelyJia et al. Light: Science & Applications (2021)10:11 Page 4 of 8
Pump wavelength dependence of the THz generation conduction band electrons in ITO quickly thermalize into
To further confirm that the THz emission from ITO is a hot Fermi distribution with a maximum electron tem-
enhanced by the ENZ effect, we measure the pump perature Te. Electrons then cool down and relax back to
wavelength dependence of the THz generation. In Fig. 3a, the conduction band minimum through energy exchange
we plot the peak-to-peak amplitude of the THz signal as a with phonons33,34. The meff of ITO is a function of the
function of the pump wavelength, with pump fluences of electron distribution and Te and is therefore time
0.78, 3.12, and 6.25 mJ/cm2 in the transmission config- dependent, owing to the nonparabolicity of the conduc-
uration. A redshift of the THz generation peak from 1340 tion band. According to the Drude–Lorentz formula, an
to 1400 nm is observed as the pump fluence increases. increase in meff leads to a decrease in ωp and a redshift in
At the low pump fluence, the THz generation peak is the ITO’s λENZ. The subpicosecond electron dynamics are
blueshifted from the static λENZ of the bulk ITO film comparable to the dwell time of the pump pulse inside the
measured through spectroscopic ellipsometry. This may ITO cavity, which leads to the pump pulse interacting
be attributed to the nonlocal effect occurring in the with a time-variant ITO cavity44. As a result, we observe
ultrathin ITO film, as the electron doping concentration the spectral redshift of the THz generation peak from the
at the surface of the ITO film, where the THz wave is static λENZ of ITO. We develop a quantitative model
generated, may be higher than that in the bulk ITO based on the hot-electron dynamics of ITO to derive a
film39,43. However, as the pump fluence increases, due to static λENZ of 1286 nm at the surface of the ITO film,
the photoinduced heating of conduction band electrons which redshifts to 1340 nm under a pump fluence of
and the consequent time-dependent λENZ of ITO, the 0.78 mJ/cm2. Moreover, we calculate λENZ as a function of
THz generation peak redshifts. As schematically shown in the pump fluence (see Supplementary Note 3 for details),
Fig. 3b, upon sub-bandgap photoexcitation, the which explains the larger redshift when the ITO film is
a λENZ b
1.0
THz amplitude (norm.)
Pump (0.9 eV)
0.8
Static
0.6
Relaxation
0.4
Photoexcited
0.78 mJ/cm2
0.2 3.12 mJ/cm2
6.25 mJ/cm2
0.0 Static
1200 1300 1400 1500 1600
Pump wavelength (nm)
c d 1600
104
104
1550
THz amplitude (a.u.)
THz amplitude (a.u.)
1500
103
103
λENZ (nm)
1450
102 1400
1350 102
101 ITO film
1300
ZnTe crystal Calculation
10 0
1250 101
0.1 1 10 0.1 1 10
Pump fluence (mJ/cm2) Pump fluence (mJ/cm2)
Fig. 3 Pump wavelength and pump fluence dependence of THz generation. a Measured peak-to-peak THz amplitude as a function of the pump
wavelength with pump fluences of 0.78 (red), 3.12 (blue), and 6.25 mJ/cm2 (yellow) in the transmission configuration. The dashed curve denotes the
static ENZ wavelength of the bulk ITO film measured via spectroscopic ellipsometry. b Schematics of ultrafast electron dynamics in the ITO film,
which consist of photoexcitation, hot-electron redistribution, and relaxation. The dashed line represents the parabolic conduction band
approximation. c Measured peak-to-peak THz amplitude as a function of the pump fluence in the transmission configuration with a pump
wavelength of 1350 nm for the ITO film (red) and 1 mm-thick ZnTe crystal (blue). The dashed lines are from the linear fitting. d Calculated λENZ of the
ITO film (red) and THz amplitude (blue) as a function of the pump fluence in the transmission configurationJia et al. Light: Science & Applications (2021)10:11 Page 5 of 8
excited with a higher pump fluence and is in good crystal we use quickly declines beyond 3 THz47. To verify
agreement with the literature studying the ultrafast that this is indeed an issue, we change the ZnTe crystal
dynamics of ENZ materials34,44. with a 1 mm thickness to another ZnTe crystal with a
0.1 mm thickness and observe an increase in the mea-
Pump fluence dependence of the THz generation sured spectral bandwidth from 2.8 to 3.05 THz.
To investigate the underlying physical mechanism of
the THz generation in the ITO film, we also measure the Pump polarization and sample orientation dependence of
peak-to-peak THz amplitude as a function of the pump the THz generation
fluence, as shown in Fig. 3c. Under low pump fluence, the We further measure the peak-to-peak amplitude of THz
THz amplitude scales linearly with the pump fluence, in signals versus the pump pulse polarization angle θ, as
agreement with the linear scaling law of the second-order shown schematically in Fig. 4b. We define θ to be 0° and
nonlinear process of optical rectification. When the pump 90° when the pump light is s-polarized and p-polarized,
fluence exceeds 1.8 mJ/cm2, the THz amplitude deviates respectively. As shown in Fig. 4c, the p-component of the
from the linear scaling, which we again attribute to the THz amplitude well fits the sin2(θ) function, consistent
time-dependent λENZ of ITO. with the fact that the Ferrell–Berreman mode in ITO can
For optical rectification, the nonlinear THz polarization only be excited with p-polarized light. Moreover, the s-
ðTHzÞ
is PTHz ¼ χ eff E02 , where E0 is the incident electric field component of the THz amplitude isJia et al. Light: Science & Applications (2021)10:11 Page 6 of 8
p-component THz
a b X c 0° Fitting
Z
100 s-pol 100 330° 30°
ITO sample Y
80
Transmission Epump Pump pulse
THz amplitude (a.u.)
10 60 300°
60°
40
1 20
p-pol
0 270° 90°
0.1 20
Detector: 40
0.01 0.1-mm-thick 120°
60 240°
ZnTe crystal THz pulse
s-comp 80
100 100 210° 150°
180°
THz amplitude (a.u.)
Transmission
p-comp Relative polarization angle θ
10
1
0.1 d s-component THz e
Detector: 0° 0°
0.01 1-mm-thick
8 330° 30° 330° 30°
ZnTe crystal 6
THz amplitude (a.u.)
THz amplitude (a.u.)
300° 60° 300°
100 Reflection 4 60°
10 2
0 270° 90° 270° 90°
1
2
0.1 4 120° 240° 120°
240°
0.01 Detector: 6
0.1-mm-thick 150° 210° 150°
ZnTe crystal 8 210°
180°
180°
0 1 2 3 4 5 6 7 8 Relative polarization angle Azimuth angle of ITO
Frequency (THz)
Fig. 4 Spectral bandwidth and polarization dependence of the THz generation. a THz spectra (red) and noise spectra (grey) generated from the
ITO film in both the transmission and reflection configurations, with 0.1 and 1 mm-thick ZnTe crystals serving as the detector. b Schematic illustration
of the pump light, ITO film, and THz emission. θ and φ represent the polarization angle and azimuthal angle of the ITO film, respectively. c, d Polar
graphs of the p-component and s-component THz amplitude as a function of the polarization angle θ in the transmission configuration. The p-
component THz amplitude well fits the sin2(θ) function, indicated by the dashed lines. e Azimuthal angle φ dependence of the THz generation in the
transmission configuration
larger field enhancement44, and by coupling the ENZ in the transmission configuration and 45° in the reflection
material to optical metasurfaces48. Moreover, although the configuration. We have calculated the electric field
ITO film is currently pumped by a solid-state OPA, the enhancement factor as a function of wavelength and
λENZ of ITO can be tailored by controlling the doping incident angle (see Supplementary Notes 5 and 6 for
concentration during film deposition and postdeposition details). There is no significant difference in the field
annealing30,49. For example, an ITO film with a λENZ near enhancement factor at 40° and 45° incidence angles. We
1550 nm can be directly pumped by a more compact fibre- did not use the larger incident angle due to the finite size
based femtosecond laser with a lower system cost. of the ITO sample (1 cm2) and the space limitations for
sample mounting in our current experimental setup. The
Materials and methods sample is placed at the focus of a parabolic mirror, and the
Optical measurement generated THz wave is focused on a < 110 > cut ZnTe
In the experiment, we use a Ti:sapphire amplifier system crystal by another parabolic mirror. The other portion of
with a central wavelength of 800 nm, a pulse duration of the laser beam from the amplifier system serves as the
35 fs, and a repetition rate of 1 kHz as the pump source. probe beam to implement electro-optical sampling of the
The laser beam is divided into two beams by a beam THz signal through the use of the ZnTe crystal, a quarter-
splitter to generate and detect the THz signal. The main wave plate (QWP), a Wollaston prism, and a pair of
portion of the laser beam is used to pump an OPA to balanced photodiodes. It is noteworthy that the ZnTe
produce infrared pulses with a pulse duration of crystal has the highest detection efficiency for the p-
approximately 100 fs and a wavelength tuneable from 1100 polarized THz signal. We can obtain the THz waveform by
to 1600 nm. The infrared pulse can be configured to excite changing the time delay between the generated THz beam
the sample in either the transmission or reflection geo- and the infrared probe beam. The polarization state of the
metry. The incident angle of the pump beam is fixed at 40° pump beam is changed by rotating a half-wave plate and aJia et al. Light: Science & Applications (2021)10:11 Page 7 of 8 polarizer. In the measurement, the time step is set to be 3. Ferguson, B. & Zhang, X. C. Materials for terahertz science and technology. Nat.
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