Terahertz Surface Emission from Layered MoS2 Crystal: Competition

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Terahertz Surface Emission from Layered MoS Crystal: Competition Between Surface Optical Rectification and Surface Photocurrent Surge Yuanyuan Huang, Lipeng Zhu, Zehan Yao, Longhui Zhang, Chuan He, Qiyi Zhao, Jintao Bai, and Xin Long Xu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b09723 • Publication Date (Web): 11 Dec 2017 Downloaded from http://pubs.acs.org on December 13, 2017

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The Journal of Physical Chemistry

Terahertz Surface Emission from Layered MoS2 Crystal: Competition between Surface Optical Rectification and Surface Photocurrent Surge Yuanyuan Huang1, Lipeng Zhu1, Zehan Yao1, Longhui Zhang1, Chuan He1, Qiyi Zhao1, Jintao Bai1, Xinlong Xu1,∗ 1

Shaanxi Joint Lab of Graphene, State Key Lab Incubation Base of Photoelectric Technology and Functional Materials, International Collaborative Center on Photoelectric Technology and Nano Functional Materials, Institute of Photonics & Photon-Technology, Northwest University, Xi'an 710069, China.

Abstract Terahertz (THz) radiation of layered molybdenum disulfide (MoS2) crystal under femtosecond laser irradiation was observed using THz surface emission spectroscopy under variable angle transmission configuration. Although MoS2 demonstrates inversion symmetry, surface-symmetry–breaking will introduce the resonant optical rectification, which is consistent with the incident polarization and azimuthal angle dependences of the THz radiation from MoS2. However, the surface depletion field induced THz radiation will make important contribution under oblique incidence, which is consistent with the radiation saturation due to the electrostatic screening effect by photoexcited carriers. This pump dependent saturable THz radiation can be fitted well by the calculation from Maxwell equations with electromagnetic boundary condition. The maximum of surface depletion field is estimated to be 1.45×104 V/cm with 130 nm in depth under -40o incidence. Interestingly, when the incident angle is tuned from -40o to 0o, the optical rectification contribution varies from 40% to 90%. In addition, MoS2 is diagnosed to be p-type from THz waveforms by comparison with GaAs (100). The results afford not only comprehensive understanding of THz radiation from layered materials like MoS2, but also put forward THz emission spectroscopy

∗

for

characterizing

the

surface

and

Corresponding author. Fax: +86 29 88303336

E-mail address: [email protected] (X. Xu)

ACS Paragon Plus Environment

interface

properties

of

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two-dimensional materials.


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1. INTRODUCTION Transitional metal dichalcogenides (TMD) are a kind of two-dimensional (2D) layered materials with tunable band gap between gapless graphene and insulating hexagonal boron nitride. TMD materials are environmentally stable and can be exfoliated from natural crystals owing to the weak van der Waals forces between layers. As one of the most typical TMD materials, molybdenum disulfide (MoS2) has aroused extensive attentions owing to its striking properties for photonic and optoelectronic applications, such as transistors,1 photodetectors,2 solar cell,3 memory cells4 and valleytronic devices.5 These applications are based on the unique electronic and optical properties, such as saturable absorption,6 large on/off ratio (~108),1 valleyand spin-dependent properties,7 and excitonic effect.8 Recently, with the photonic and optoelectronic devices speeding up from gigahertz (GHz) to terahertz (THz), MoS2 raises the prospect of high-speed (>1 THz) applications. For instance, the photoconductivity response time of MoS2 can reach 350 fs in THz region,9 while the momentum scattering time of multilayer MoS2 varies from ~90 fs at 300 K to ~1.46 ps at 30 K with the corresponding carrier mobility of ~257 and ~4200 cm2V-1s-1, respectively.10 Besides, the ultrafast negative photoconductivity of monolayer MoS2 can be induced by the trionic effect in THz region.11 However, the THz property investigation of MoS2 basically utilizes THz time domain spectroscopy or optical-pump THz-probe spectroscopy with few works on the THz radiation properties. THz surface emission spectroscopy can not only provide the THz radiation



properties of the materials, but also open a new window on the optoelectronic measurements of surfaces and interfaces of materials. When femtosecond (fs) optical pulses impinge on the semiconductors, the radiated THz pulses are sensitive to the properties such as mobility, carrier density, nonlinear susceptibility, surface and interface properties, structural symmetry and so on. And the THz pulses carry the information such as polarization, amplitude, phase, and polarity, which can reflect the linear and nonlinear physical process occurring on semiconductor surfaces and interfaces, such as diffusion and drift carrier,12 photo-Dember effect,13 coherent optical absorption,14 optical rectification15 and photon drag effect.16 With the development of new advanced materials, this spectroscopic method has been used for the optoelectronic property characterizing of emergent materials such as the ultrafast surface photocurrents of topological insulator Bi2Se3,17 the transient currents of semimetal graphite,18 photocurrent surge along the nanotubes,19 dynamic photon drag effect of graphene,16, 20 and enhanced THz radiation from vertical grown graphene.21 However, the THz radiation from the semiconductors are not trivial, as several competing physical processes happen simultaneously in the same condition.22 The THz radiation contributions depend on the crystalline orientation, doping density, pumping fluence, applied extrinsic or intrinsic electric field as well as the surface and interface properties.23 Take conventional GaAs as an example, the carrier drift and diffusion process can be described based on the current surge effect,24 while the evolution of nonlinear susceptibility of GaAs near the band gap can be observed from the optical rectification process.25 Similarly, the ultrafast evolution of photocurrents in


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Bi2Se3 are understood from the surface shift current surge analysis,17 while the rapid relaxation of electronic states are involved in the optical rectification.26 In terms of MoS2, we have observed THz radiation mainly from the optical rectification in surface region by THz reflection emission spectroscopy.27 However, the reflection configuration has a limitation on steering the incident angle, while the new physical process emerges under different incident angles.22 Based on the condensed matter physics, the surface depletion field by the band bending at the surface will result in THz radiation due to the photogenerated carriers acceleration. Fundamental physical processes become even more complicated and yet interesting if there exists a competition between the surface-symmetry–breaking induced optical rectification and surface depletion field induced photocurrent surge. The corresponding experiments are especially important for TMD materials as there are no dangling bonds, strain, and few defects at their surfaces due to the weak van der Waals interactions between layers. In this work, we have investigated angular dependence of THz radiation from layered MoS2 crystal under femtosecond laser with a transmission configuration. The results suggest that both the surface-symmetry-breaking induced resonant optical rectification and the surface depletion field induced photocurrent surge have taken effect under the oblique incidence. The THz radiation contribution from resonant optical rectification varies from 40% to 90% when the incident angle is tuned from -40o to 0o. THz radiation also presents saturable dependence on the pump fluence, which originates from the photocarrier screening effect and can be fitted according to



the analysis of electromagnetic boundary conditions. In view of solid-state physics, the surface depletion field and width can be estimated under oblique incidence. In addition, MoS2 is diagnosed to be p-type by comparing THz radiation from n-type GaAs (100). This investigation paves the way for the full understanding of THz radiation mechanism from the layered TMD materials as well as optical physics characterizing of the new functional materials by THz surface emission spectroscopy.

2. EXPERIMENTAL SETUP The free-standing MoS2 sample from SPI Supply is 90 µm in thickness, and the corresponding optical and structural characterization can be found in our previous work.27 Figure 1a demonstrates the THz surface emission spectroscopy system under the transmission configuration. A Ti: sapphire regenerative amplifier (Spectra-Physics, Spitfire) with 35 fs duration, 800 nm central wavelength, and 1 KHz repetition rate is employed as a laser source. The infrared (IR) pulses are divided into a pump beam and a probe beam by a beam splitter. The pump beam is focused onto the sample with a 3 mm beam diameter after a time delay line. Then the generated THz radiation is collected and collimated by an off-axis parabolic mirror and focused onto the ZnTe (110) crystal with another parabolic mirror. It is worth noting that the focus of the pumping beam is behind the sample to get rid of the laser damage as well as the THz radiation by air plasma. A combination of a 10-mm-thick polyethylene and 0.5-mm-thick silicon plates are placed after the sample in the optical path, which can attenuate the pump beam via the polyethylene plate and block the remnant IR pulses


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with the silicon plate. However, the generated THz pulses can pass through these plates with a small absorption. At the same time, the probe beam is also focused onto the ZnTe. This beam is fixed at s-polarization by a Glan-Taylor prism (GTP), then the power of the beam can be controlled via a half-wave plate (HWP) in front of the GTP. Afterwards, the probe beam is overlapped with the generated THz pulses and the THz radiation can be recorded by the electro-optical sampling technique.28

Figure 1. (a) Illustration of experimental setup. HWP: half-wave plate, GTP: Glan-Taylor prism, WGP: wire-grid polarizer, combined plates: polyethylene and silicon plates. (b) Simplified diagram of angle-resolved THz measurement. XYZ and X'Y'Z' indicate the laboratory and crystalline coordinates, respectively. Incident angle and polarization angle are labeled as θ and α, respectively. The normal directions of rotatable sample are labeled by n.

The angle-resolved THz measurement with different pump polarization and THz radiation polarization is shown in Figure 1b. The polarizations of incident pulses and



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radiated THz pulses are controlled via a HWP and a wire-grid polarizer (WGP) as shown in Figure 1a. The p- and s-polarizations of incident waves are aligned along the X- and Y-axis, respectively (Figure 1b). While the EX and EY components of THz radiation are portrayed in accordance to our previous work.27 As THz EY component is much weaker than the EX component (see the THz waveforms in Figure S1 in Supporting Information), only THz EX component is considered in the following parts. The incident angle θ is between incident wave-vector and the normal of sample with the sign shown in Figure 1b. The radiated directions of the generated THz field can be calculated by a generalized Fresnel law:29 nair sin θ = nopt sin θ opt = nTHz sin θTHz

(1)

where θopt is the refraction angle between normal direction of the sample and the IR pulses. θTHz is the refraction angle of the generated THz pulses, which can be given as:

θTHz = sin −1 [sin(θ ) / nTHz ] . nopt and nTHz are refraction indices of the IR pulses and THz pulses propagating inside the sample.

3. RESULTS AND DISCUSSION 3.1 Surface-symmetry-breaking induced optical rectification During the measurement, the polarization of the pump IR pulse is fixed at p-polarization and the azimuthal angle of MoS2 crystal has been optimized. As shown in Figure 2a, the THz radiations from MoS2 under -40o and 0o incidences demonstrate subpicosecond duration following with some weak oscillations. The peak amplitude of THz radiation under -40o-incidence is four times larger than that under 0o-incidence.


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We also show the THz radiation from GaAs (100) crystal under the same experimental setup. On one hand, THz radiation by optical rectification under oblique incidence from GaAs (100) is confirmed by the two-fold rotational symmetry of azimuthal angle.30 On the other hand, current surge by the surface depletion field can also contribute to the THz radiation. However, there is no optical rectification contribution under normal incidence. As the surface depletion field is perpendicular to the surface, the THz radiation by current surge from GaAs (100) (magenta dash line in Figure 2a) cannot be detected under normal incidence, which is consistent with the previous report.31 Unlike the GaAs (100), there is obvious THz emission from the MoS2 (blue line in Figure 2a) even under the normal incidence, which suggests the resonant optical rectification may contribute to the THz radiation in MoS2 when the incident photon energy is above the band gap (1.55 eV>1.29 eV). Resonant and non-resonant optical rectification are typical second-order nonlinear optical processes with incident photon energy above and below the band gap of materials, respectively. Resonant optical rectification of GaAs (111) was first investigated by Zhang et al. in the early time.32 Then Zotova et al. used the resonant optical rectification to describe the THz emission from InAs due to the azimuthal angle dependent THz generation.33 Martin et al. used the resonant optical rectification to understand the second-order susceptibility

of

bacteriorhodopsin.34

This

classification

catches

the

main

characterization such as azimuthal angle dependent and resonant enhancement in the same language of optical rectification. From the photocurrent view point, Driel et al.35 and Sipe et al.36 suggested that the non-resonant optical rectification is just virtual



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current, while resonant optical rectification can produce real current due to spatial shift of the center of charge and leads to ‘shift’ current, which has been investigated in GaAs,37 GaP,36 Bi2Se3,17 GeS38 and so on. From the photovoltaic view point, Sturman and Fridkin attributed the resonant optical rectification to the photovoltaic effect phenomenologically.39 These investigations focus on the same phenomenon from different perspectives and descriptions. At the oblique incident angle, such as -40o-incidence (Figure 2a), both GaAs and MoS2 demonstrate increscent THz radiation. One feature is that the polarities of THz waveforms are flipped over, which suggest that the directions of the surface depletion fields in these samples are just opposite. Figure 2b shows the peak-to-valley values of THz radiation as a function of incident angle. The THz radiations of both crystals are proportional to the incident angle in either positive or negative directions (Figure 1b). The THz radiation will reverse in polarity when the angle is switched from positive to negative direction. Notably, THz radiation amplitude of GaAs under normal incidence is zero, while the radiation from MoS2 is above the zero baseline (as the arrow indicating in Figure 2b). This suggests that although MoS2 is centrosymmetric, optical rectification still happens in MoS2 as the surface breaks the symmetry, which holds one three-fold symmetry.27 The surface-symmetry-breaking induced optical rectification can be expressed by the dielectric nonlinear polarization as:

Pi(2) = χ ijk(2)eff (0; −ω , ω ) : E j (−ω )E k (ω )

(2)

where χ ijk(2)eff is the effective second-order nonlinear susceptibility. The THz radiation from the resonant optical rectification can be given as:27


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ETHz ∝ − PX cos θTHz + PZ sin θTHz

(3)

Here PX and PZ are X- and Z- components of dielectric nonlinear polarization. In this situation, the dielectric nonlinear polarization has the components paralleled to the surface of sample under normal incidence and the THz radiation from MoS2 can still be probed at normal incidence.

Figure 2. (a) THz pulses generated from MoS2 under -40o (red solid line) and 0o (blue solid line) incidences. For comparison, THz pulses from GaAs are also shown with the values divided by ten (dash lines) in the same experimental setup. (b) The peak-to-valley values of THz radiations as a function of incident angle for MoS2 (red dots) and GaAs (black dots).

THz pulses generated from MoS2 under p-polarized and s-polarized (Pin and Sin) excitations are portrayed in Figure 3a and 3b for 0o-incidence and -40o-incidence, respectively. The THz radiations under Pin and Sin illuminations are basically identical in amplitude and reversed in polarities under normal incidence. While the amplitude maximum of THz radiation under Pin illumination is six times larger than the value of



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Sin situation for -40o-incidence. Moreover, the amplitude maximum of THz radiation under -40o is two times larger than that under normal incidence for Pin incidence. In order to quantitatively confirm the relationship between THz radiation and incident polarization states, the polarization angle (α in Figure 1b) is tuned from 0o to 360o (0o, 180o, 360o correspond to Pin, 90o and 270o correspond to Sin). The THz radiation at normal incidence presents maximum with positive and negative values at α=0o (Pin) and α=90o (Sin), respectively (Figure 3c). Thus the THz radiation exhibits two-fold rotational symmetry with the change of α in the whole range. Although THz radiation under -40o-incidence also has two-fold rotational symmetry (Figure 3d), the amplitude increases and significantly shifts towards the positive direction (as the red arrow indicating in Figure 3d), maximum THz radiation appear only at α=0o and 180o (Pin) and the minimum THz radiation appear at α=90o and 270o (Sin). The polarization angle dependent THz radiation can be given after the coordinate transformation:27 ETHz ∝ A cos α + B sin α + C cos(2α ) + D sin(2α ) + E

(4)

The polarization angle dependences of THz radiation at both normal and oblique incidences agree well with the fitting results as shown in Figs. 3c and 3d. The difference of THz radiations under 0o and -40o incidences mainly originate from the fitting constants A, B, C, D and E, which are related to the nonzero susceptibility terms. Among them, E = −2cos θTHz d31 + sin θTHz d33 is independent to α and can give rise to THz amplitude shift for -40o-incidence. However, the susceptibility terms d31 and d33 of bulk MoS2 are too small to induce such great amplitude shift of -40o-incidence (please see the second-order nonzero susceptibility terms of MoS2


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crystal in Supporting Information). This suggests the shift mainly stems from the photocurrent surge induced THz radiation.

Figure 3. THz pulses generated from MoS2 illuminated by p- (Pin, black) and s-polarizations (Sin, red) under (a) 0o and (b) -40o incidences. THz amplitude maximums as a function of incident polarization angle under (c) 0o and (d) -40o incidences. The experimental and fitting results are depicted with dot and solid lines, respectively. The red arrow indicates the amplitude shift from zero baseline.

3.2 Surface depletion field induced THz radiation Photo-Dember effect has been ruled out as the mobility and mass between electrons and holes of MoS2 are in the same order of magnitude.27 Thus, the



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photocurrent surge of MoS2 is induced by the surface depletion field, of which the direction is opposite to the n-type GaAs (100) due to the reversed polarities of the THz radiations from two crystals (Figure 2a). This further indicates that MoS2 crystal is p-type, which is consistent with the previous summary as most layered MoS2 structures are p-type.40 This suggests that THz surface emission spectroscopy can be used to determine the carrier type of samples based on the surface depletion field, which would be superior to conventional characterization of carrier type via Hall effect measurement41 as no contact electrodes are needed. The band bending diagram of surface depletion field is shown in Figure 4a. Both conduction and valence bands bend downwards and a depletion field with the width ld form near the air-MoS2 interface. When the ultrafast laser with photon energy above the band gap impinges on the crystal, the electrons and holes move along opposite directions and separate in the surface field region, which further generate the THz radiation due to the carrier acceleration. The surface depletion field related to the distance x vertical to the surface can be expressed as:29

Ed ( x) = (eN / κ )(ld − x)

(5)

where κ =ε 0ε r , ε0=8.854 × 10-14 F/cm is the permittivity of vacuum, εr=4.83 is relative permittivity of MoS2 at 800 nm. N is doping concentration, ld is the depletion width and can be given as:

ld = (2κ / eN )[V − (kT / e)]

(6)

V=0.119 V is the potential barrier of MoS2,42 kT/e=0.026 V is the thermal energy of room temperature. For N=3×1015 cm-3,43 the depletion width is calculated as 0.13 µm,


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and maximum value of surface depletion field can reach approximately 1.45×104 V/cm.

Figure 4. (a) Band bending diagram of MoS2. Ec and Ev depict conduction and valence bands, respectively. (b) Schematic of the surface current and boundary conditions on electric and magnetic fields.

The schematic illustration of surface current and THz electromagnetic field is shown in Figure 4b. Time-dependent electric and magnetic fields of THz pulses radiating inward and outward the sample are represented by Ein(t), Eout(t), Hin(t), and

Hout(t). ∆S and h are basal area and the height of the cylinder in air-MoS2 interface. Surface photocurrent J s (t ) can be given by:44 δ

J s (t ) = ∫ J (t ) dz 0

(7)

Where J (t ) is current density, δ is the penetration depth of IR laser into MoS2, and

dz is the distance increment into the sample. The boundary conditions of the radiated fields can be deduced from the Maxwell's equation in normal direction.45 As shown in Figure 4b, the surface current coming from the floating charges can be expressed as: J s (t) = σ f ∆Sv . σ f = Q f / ∆S is the density of surface charges and v is the velocity



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of flowing charges. According to the continuity of magnetic fields in normal direction, the corresponding boundary condition can be given by:

nx × (H in (t) − H out (t)) = 0

(8)

n x is a unit vector along x-axis. The magnetic and electric fields are related by free space resistance Z0:

n Ein (t) Z0

(9)

1 Eout (t) Z0

(10)

nx × H in (t) = − and nx × H out (t) = −

From Eqs. (8)-(10), the surface current can be expressed as a function of inward radiated electric field as: J s (t ) = ε (1 − n)v∆SEin (t )

(11)

The surface current can also be given by Ohm's law: J s (t ) = σ s (t)[Ed + Ein (t )]

(12)

σ s (t) is the surface photoconductivity proportional to pump fluence, and its peak value is given by:44

σ s ,max =

e(1 − R) µtr Fopt hω

(13)

Where R is the reflectivity of MoS2 and µtr is the transient carrier mobility when

σ s (t) reaches to its maximum. Fopt is the optical pump fluence. From Eqs. (11) and (12), the peak surface current as a function of depletion field can be expressed as: J s ,max =

ε (n − 1)∆Svσ s ,max E ε (n − 1)∆Sv + σ s ,max d


(14)

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As the time of surface current increasing to its maximum is in the same order of magnitude with the pulse-width τp of the radiated field, the far-field THz radiation maximum can be given by: J  ∂J  ETHz,max ∝  s  ≅ s ,max τp  ∂t  max

(15)

From Eqs. (13)-(15), the THz radiation maximum as a function of pump fluence Fopt can be expressed as:

ETHz,max ∝ Here,

the

constant

terms

C1 ⋅ C2 Fopt C1 + C2 Fopt

C3 + C4 Fopt

C1 = ε (n − 1)∆Sv

,

(16) C2 = e(1 − R) µtr / hω

,

C3 = SE d ,max / (4πε c 2 z ) are related to the maximum of surface depletion field and constant terms of dipole oscillation radiation.44 S is the illuminating area of sample, z is the displacement along the radiated beam. C4 ∝ χijk(2) eff represents the THz radiation contribution from surface-symmetry-breaking induced optical rectification. As shown in Figure 5, the THz radiation under -40o-incidence as a function of pump fluence agrees well with the fitting results from Eq. (16) (black dots and red solid line). When pump fluence increases from 0.57 mJ/cm2 to 5.7 mJ/cm2, the peak-to-valley values of THz radiations keep increasing with a saturable effect. This mainly originates from the photoexcited carriers accumulation in the depletion field with the increasing of pump fluence, and further leads to the electrostatic screening of photocarriers. This photocarrier effect also results in the similar saturation effect in the THz radiation from n-type GaAs (100).46 The THz radiation amplitude at normal incidence is approximately 10 times smaller than that at -40o-incidence, and the results can be



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fitted linearly as the THz radiation is quadratic with the incident electric field in resonant optical rectification process (magenta dots and blue solid line in Figure 5).

Figure 5. Peak-to-valley values of THz radiations as a function of pump fluence under -40o (black) and 0o (magenta) incidences. The experimental and fitting results are depicted with dot and solid lines, respectively.

3.3 Evolution of THz radiation contributions Combining the optical rectification and surface depletion field, the far-field THz radiation from MoS2 related to the real and virtual currents can be expressed by a general expression:47 E THz ∝

∂ 2 P ∂J + ∂t 2 ∂t

(17)

The first term is related to the susceptibility and can be influenced by the crystalline symmetry, while the second term is usually independent of the azimuthal angle. Thus, the THz radiation contributions of these two processes can be estimated from the azimuthal angle dependence of THz radiation. As shown in Figure 6a and 6b, THz radiations at 0o-and -40o incidences both exhibit three-fold rotational symmetry as the


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azimuthal angle φ changing from 0o to 360o. The experimental data are consistent with the fitting results from the theory of optical rectification27: ETHz ∝ [ d 22 sin(3ϕ ) − 2d15 ]cos θTHz + ( d31 + d33 ) sin θTHz

(18)

According to Eq. (18) and Figure 6, the THz radiation under different incident angles are noticeable as variable θTHz can give rise to differences in amplitude or polarity. On one hand, the maximum amplitude of THz radiation under -40o-incidence is more than two times larger than that under normal incidence. On the other hand, the THz radiation polarity under normal incidence can flip-over (Figure 6a: black and red dots represent positive and negative amplitudes, respectively) and also holds 3φ rotational symmetry, while the THz radiation polarity under -40o-incidence keeps positive in the whole angle range (Figure 6b). Therefore, the THz radiation contribution ratio from optical rectification can be calculated by (Emax-Emin)/2Emax with values extracted from Figure 6.13 The ratio is more than 90% under normal incidence (for more information, please see the THz radiation contribution ratio of MoS2 crystal in the Supporting

Information), and decreases to approximately 40% under -40o-incidence. This stems from the surface depletion field induced THz radiation contribution under oblique incidence.



Figure 6. THz radiation maximums as a function of azimuthal angle in a polar coordinate for (a) 0o and (b) -40o incidences. The experimental and fitting results are depicted with dot and solid lines, respectively. The black and red dots (lines) in (a) represent positive and negative values, respectively.

4. CONCLUSION In summary, we have observed the angular dependent THz radiation from MoS2 under femtosecond laser excitation in a transmission configuration. The THz radiation comes from the surface-symmetry-breaking induced resonant optical rectification combined with the surface depletion field under oblique incidence. The maximum of this surface electric field is 1.45 × 104 V/cm with 0.13 µm field width for -40o-incidence, and MoS2 is diagnosed to be p-type after comparing with the THz waveforms from n-type GaAs. Besides, the saturable property of THz radiation with the pump fluence agrees well with the theoretical analysis from boundary conditions of electromagnetic fields. According to the azimuthal angle dependence of THz radiation, the contribution ratio of resonant optical rectification changes from 40% to


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90% with incident angle changing from -40o to 0o. The results pave the wave for the understanding of THz radiation from layered materials and raise the THz surface emission spectroscopy as a promising way for characterizing the surface and interface properties of emergent materials.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 11774288,

11374240),

Natural

Science

Foundation

of

Shaanxi

Province

(2017KCT-01), Excellent Doctoral Dissertation Training Project of Northwest University (YYB17005).

ASSOCIATED CONTENT Supporting Information. THz EX and EY components; Azimuthal angle dependence of THz radiation from GaAs (100); THz radiation contribution ratio of MoS2 crystal; Second-order nonzero susceptibility terms of MoS2 crystal.

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Terahertz Surface Emission from Layered MoS2 Crystal: Competition

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