Surface Optical Rectification from Layered MoS2 ... - ACS Publications

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Surface Optical Rectification from Layered MoS2 Crystal by THz TimeDomain Surface Emission Spectroscopy Yuanyuan Huang, Lipeng Zhu, Qiyi Zhao, Yaohui Guo, Zhaoyu Ren, Jintao Bai, and Xinlong Xu* 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 and Photon-Technology, Northwest University, Xi’an 710069, China S Supporting Information *

ABSTRACT: Surface optical rectification was observed from the layered semiconductor molybdenum disulfide (MoS2) crystal via terahertz (THz) time-domain surface emission spectroscopy under linearly polarized femtosecond laser excitation. The radiated THz amplitude of MoS2 has a linear dependence on ever-increasing pump fluence and thus quadratic with the pump electric field, which discriminates from the surface Dember field induced THz radiation in InAs and the transient photocurrentinduced THz generation in graphite. Theoretical analysis based on space symmetry of MoS2 crystal suggests that the underlying mechanism of THz radiation is surface optical rectification under the reflection configuration. This is consistent with the experimental results according to the radiated THz amplitude dependences on azimuthal and incident polarization angles. We also demonstrated the damage threshold of MoS2 due to microscopic bond breaking under the femtosecond laser irradiation, which can be monitored via THz time-domain emission spectroscopy and Raman spectroscopy. KEYWORDS: molybdenum disulfide (MoS2), optical rectification, terahertz (THz) time-domain surface emission spectroscopy, femtosecond laser, second-order susceptibility

1. INTRODUCTION Terahertz (THz) technology as the cutting-edge technology1 shows great prospects for applications in imaging,2 medicine biology,3 high-speed communication,4 and spectroscopy. It is an interdiscipline between ultrafast nonlinear optics, condensed matter physics, and optoelectronics. Recently, broadband THz spectroscopy approaches have been used to study fundamental physics in various materials such as the excitation of surface plasmons in metallic grating,5 electrostatic coupling between particles in GaAs,6 carrier drift and diffusion characteristics in InAs,7 exciton-like trap states in TiO2 nanotubes,8 and dynamic THz polarization in single-walled carbon nanotubes.9 Especially, among the spectroscopy approaches, THz time-domain emission spectroscopy under femtosecond laser excitation offers a contactless and sensitive method for the surface and interface properties of the semiconductors.10 When the ultrafast femtosecond pulse laser impinges onto the semiconductor surface, the polarity, amplitude, and phase information on the generated THz pulses offer a powerful tool © 2017 American Chemical Society

to deduce the linear and nonlinear optical properties of semiconductors, such as the carrier mobility, doping concentration, polarity of the static field,10 photo-Dember field,11 plasmon field enhancement,12 ultrafast carrier dynamics,13 optical rectification,14 and photon-drag current.15 Additionally, THz time-domain emission spectroscopy can be steered conveniently via external magnetic field, 16 temperature control,17 angle-dependent incidence,18 and other experimental configurations. Thus, the THz surface emission spectroscopy of various semiconductors such as GaAs, GaSb, InAs, InSb, and InP10 are systematically studied, and these spectroscopy information in turn can be used for the rapid improvement of THz sources, which once were the bottleneck for the THz community. Received: November 2, 2016 Accepted: January 13, 2017 Published: January 18, 2017 4956

DOI: 10.1021/acsami.6b13961 ACS Appl. Mater. Interfaces 2017, 9, 4956−4965

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ACS Applied Materials & Interfaces

Figure 1. Characterization of MoS2 crystal for (a) simplified energy band diagram, the blue and black arrows indicate the direct and indirect transition from the valence band maximum to the conduction band minimum; (b) Raman spectrum under 514.5 nm excitation; (c) X-ray diffraction spectrum.

1.46 ps at 30 K, with the corresponding carrier mobility of 4200 cm2 V−1 s−130 in THz spectroscopy. Virtually, although the THz time-domain transmission/reflection spectroscopy has been widely used to measure the linear THz properties of MoS2, it is limited to achieve the surface research on the optoelectronic semiconductors. However, THz time-domain surface emission spectroscopy under reflection configuration can make up for the limitation of the THz technology effectively, as this technique focuses on the nonlinear properties of MoS2 as a THz source. In this report, we present investigations of THz pulse radiation from layered MoS2 crystal under the linearly polarized femtosecond laser excitation with the reflection configuration. The linear dependence of radiated THz amplitude on incident pump influence can confirm the second-order nonlinear process. Interestingly, we can figure out the damage threshold of MoS2 due to the microscopic chemical bond breaking under the femtosecond laser irradiation by THz time-domain emission spectroscopy and Raman spectroscopy. Moreover, theoretical analysis based on azimuthal and incident polarization angle-dependent THz radiation agrees well with the experimental results, revealing the underlying radiation mechanism is surface optical rectification. The investigation can not only supplement the nonlinear property characterization of MoS2 but also deepen the understanding of the surface optical rectification effect of these layered semiconductors.

Recently, with the development of advanced materials science, THz spectroscopy has also been used to understand the photoelectric properties in some new applied materials such as the effect of the surface states on topological insulator Bi2Se3,19 built-in electric field along the aligned carbon nanotubes,20 plasmon enhancement in monolayer graphene,21 and dynamical photon-drag effect in multilayer graphene.22 Among them, the emergence of two-dimensional (2D) layered materials provides a new route for not only compact 2D THz devices but also remarkable photoelectric properties understanding of layered materials. Recent experiments mainly focus on the THz generation from graphite,23 plasmon-enhanced single-layered graphene,21 and multilayered graphene.22 However, graphene and graphite are gapless semiconductors, which limit the further optoelectronic applications in THz field. Molybdenum disulfide (MoS2) as an emerging 2D layered material possesses extraordinary physical, electrical, and optical properties owing to its particular energy band structure.24 As an indirect semiconductor with band gap energy Eg = 1.29 eV, the bulk MoS2 can be transformed into direct semiconductor of monolayer MoS2. Therefore, MoS2 is an attractive candidate for various photonic and optoelectronic applications, such as phototransistors,25 photodetectors,26 and heterojunction solar cells.27 As the working frequency of devices improves gradually from GHz to THz region, the unique advantages of MoS2 in THz field are supposed to be accentuated. For the static optical response of MoS2 in the THz region, the dielectric properties have been studied, which suggest high absorption in THz region with carrier densities as high as 1015 cm−3.28 As for the dynamic optical response, the transient THz photoconductivity of trilayer MoS2 can reach ultrafast picosecond (ps) scale.29 The momentum scattering time in multilayer MoS2 approaches

2. EXPERIMENTAL SECTION The free-standing MoS2 sample from SPI Supply is 90 μm in thickness, and the size of the sample is approximately 8 × 8 mm. As an indirect band gap semiconductor, the simplified energy band diagram of MoS2 4957


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Figure 2. Schematic illustration of (a) experimental setup and (b) THz radiation in reflection configuration. XYZ and X′Y′Z′ represent the laboratory and crystal coordinate, respectively. HWP: half-wave plate, wire-grid polarizer P1 and P2 are perpendicular and 45° aligned. The ppolarized and s-polarized excitation are along the X-axis and Y-axis; (c) and (d): X- and Y-components of generated THz electric field under ppolarized (red curves) and s-polarized (black curves) IR laser illumination. crystal (Figure 1a) is calculated from the software Quantum Espresso, the in-plane lattice constants a = 3.166 Å and c = 18.41 Å. For monolayer MoS2 with the band gap of approximate 1.9 eV, the second conduction band minimum (CBM) lies at the symmetry point between K and Γ,31 as shown in Figure 1a with a dash line. When the valence band maximum (VBM) and CBM both locate at K point, the direct transition indicated with a blue arrow occurs in monolayer MoS2. However, the splitting caused by the interlayer interaction in multilayer and bulk MoS2 forces the CBM to a lower-energy position compared with the monolayer structure.32 Besides, the energy of valence edge at Γ point increases and eventually becomes the VBM instead of K point in the monolayer structure. Hence, the indirect transitions of electrons occur from VBM at Γ point to the CBM in the middle of K and Γ points, as shown by the black-arrow lines in Figure 1a. Additionally, the Raman spectrum (microscope model inVia from Renishaw Company) of MoS2 crystal under 514.5 nm excitation indicates that in-plane E2g1 and the out-of-plane A1g modes occur at 383.6 and 409.4 cm−1 (Figure 1b). The two characteristic peaks manifest high purity of the MoS2 crystal. The typical polytypes of layered MoS2 crystal are hexagonal 2H- and rhombohedral 3R-MoS2 differing from stacking orders,33 belong to D6h and C3v space groups, respectively. Among them, the 2H polytype is centrosymmetric, while the inversion symmetry of 3R polytype is broken. We take the X-ray analysis (2θ-scanning) of MoS2 crystal via X-ray diffraction (XRD, HaoYuan Instrument) to confirm which polytype the sample is. The XRD primary peaks shown in Figure 1c suggest that the MoS2 crystal is 2H polytype holding 6-fold screw axis of symmetry (JCPDS no. 03065-1951) with a good crystalline quality. However, similar to graphite,34 as the surface breaking the bulk symmetry, the surface of MoS2 crystal has one 3-fold axis of symmetry, which plays an essential effect in our reflection configuration experiment.

3. RESULTS AND DISCUSSION THz radiation from layered MoS2 crystal is proceeded at room temperature. Figure 2a illustrates the experimental setup and key elements. The infrared (IR) wave emits from a modelocked Ti: sapphire regenerative amplifier (Spectra-Physics, Spitfire) with 800 nm central wavelength, 35 fs pulse duration, and 1 kHz repetition rate. The laser beam is divided into two parts by a beam splitter. The pump wave with the primary energy distribution illuminates the sample at 45° incident angle (angle θ in Figure 2b) and 3 mm in diameter. THz radiation from a sample is collected and collimated by an off-axis parabolic mirror. Afterward, the THz radiation is focused onto ZnTe crystal (110) via another parabolic mirror. The electrooptical sampling35 via changing time delay between THz pulses and probe beam is utilized to portray the THz waveforms. We fixed the polarization of the probe beam at s-polarization with a Glan-Taylor prism, and the power is fixed at 1.2 mW. The polarization of THz radiation is fixed via a pair of wire-grid polarizer (WGP), and the optimal crystalline direction of probe ZnTe crystal has been optimized via rotating the crystal.36 A 10 mm-thick polyethylene plate and a 0.5 mm-thick silicon plate are closely fixed in a holder and placed after the sample. The polyethylene plate can not only attenuate the infrared wave but also avoid the THz radiation from silicon after the residual excitation laser.37 Then the silicon plate can totally block the residual infrared wave across the polyethylene, while the THz wave can pass through the plates. There are no multiple 4958


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ACS Applied Materials & Interfaces reflections from the plates, as we can distinguish the multiple reflections from the time-domain windows. The polarization states of the pump beam can be varied using a half-wave plate (HWP).The EX component of the generated THz electric field can be extracted with a perpendicularly aligned WGP (P1 in Figure 2a), while the EY component can be obtained after adding another 45° WGP (P2 in Figure 2a) in the optical path and after simple mathematical operation without changing the probe beam and ZnTe crystal.38 Then both THz components defined in Figure 2b can be portrayed completely. We present the THz waveforms of two components under p-polarized and s-polarized excitation (Pin and Sin) in Figure 2c,d. The absolute values of the THz electric field can be calculated according to the current measurement from Lock-in amplifier as following:36 ΔI Iprobe

=

ωn3E THzr41L c

Table 1. Physical Properties of MoS2 and InAs sample

MoS2

InAs

band gap (eV) electron mobility μe (cm2/(V·s)) hole mobility μh (cm2/(V·s)) electron mass me* hole mass mh* refractive index (at 800 nm) refractive index (at 1 THz) penetration depth (at 800 nm)

1.29 82544 155 0.47 me 0.63 me 4.8328 2.95 5.33 μm43

0.36 30000 240 0.027 me 0.33 me 3.73 3.78 142 nm

gap, the MoS2 crystal has a wide indirect band gap. As for the carriers, the great differences of mobility and mass between electrons and holes in InAs are essential elements to form transient photo-Dember field,41 which further induce intense THz radiation. However, the mobility and mass of electrons and holes in MoS2 are in the same order of magnitude. In addition, the penetration depth of MoS2 crystal can be calculated from the absorption coefficient in reference43 at 1.55 eV (800 nm). The depth of MoS2 is much larger than that of InAs. The differences between two crystals suggest that THz radiation from MoS2 is not from the photo-Dember effect as it is in InAs. Similar to 2H-MoS2, another typical layered material graphite is a centrosymmetric crystal possessing hexagonal symmetry23 as well. It also exhibits peculiar thermal, electrical, and optical properties. In order to compare, we present the THz radiation pulses from MoS2, graphite, and p-type InAs (100) (carrier concentration: 1.5 × 1017cm−3) crystals. These samples are mounted in the same holder and manipulated in the same experimental condition. Notably, the penetration depth of MoS2 is much larger than that of other two crystals, the incident IR wave would transmit a penetration depth and cause a transverse shift (Figure S2 in Supporting Information) on MoS2. Thus, the combined effect of penetration depth and rugged surface of MoS2 crystal would lead to the longer pulse delay of THz radiation as shown in Figure 3a. Generally, the single-cycle THz pulses generated from three samples have similar waveforms. Among them, the maximum THz amplitude of MoS2 and graphite are about 7.6% and 3.8% of the values generated from InAs, respectively. Thus, the radiations from MoS2 and graphite are expanded 15 times that of the original values to guide the eyes in Figure 3a,b. For analogous comparison in the frequency domain, the Fourier-transformed spectra of three crystals are shown in Figure 3b. The central frequency and bandwidth of the spectrum originating from MoS2 are approximately 0.74 and 2.4 THz, which is similar to the other two crystals. To further confirm the nonlinear process, we determined the dependence between peak-to-peak amplitude of THz components and the incident pump fluence. In the experiments, the excitation laser beam is always p-polarized. Besides, the laser focus point is behind the sample, which can prevent not only damage from the intense focal laser beam on sample but also the air ionization process. After measuring the beam diameter on the samples (d = 3 mm) with a scale card, the pump fluence can be calculated from the measured average optical power P and the repetition rate f = 1 kHz of the laser system as I = P/(f × S) (mJ/cm2). Where S is the beam area on the samples. The dependences of EX and EY THz components with pump fluence are depicted in Figure 3c,d. For E X component, the experimental data can be divided into two parts. One can be

(1)

Here, the nonzero electro-optic coefficient (ZnTe) r41 = 3.9 pm/V, the refractive index of ZnTe in the infrared region n = 2.8.39 c and ω are the speed of light in vacuum and the circular frequency, and the length of the crystal L = 2 mm. In the experiment, Iprobe is 9.4 μA, and the peak amplitude of measured ΔI/Iprobe = 1.3 × 10−4; therefore, we can calculate the peak value of THz electric field ETHz = 96.72 V/m. According to the previous demonstration of organic crystals DSTMS and OH1, 40 this value could be enhanced after systematic optimization such as adjusting the spot size on samples and THz spot size on detectors. Strikingly, the EX (Figure 2c) and EY (Figure 2d) components present giant amplitude differences, suggesting the primary THz energy distributes along EX component regardless of the incident polarization states. For the EX component, the radiation amplitude from Sin excitation are three times less than that from Pin, and the polarity of generated pulses will reverse under different polarization (Pin and Sin) excitations. Similarly, the THz polarity of EY component under such two polarization excitations are also reversed, but the amplitudes are approximately equal. The polarization-induced polarity reversal is related to the THz radiation mechanism. Generally, the THz radiation polarity are hardly influenced by the incident polarization from the photoconductivity mechanism either by intrinsic electric field or by extrinsic electric field under linear process. For instance, the THz polarity from the surface depletion field are decided by the doping type not by the excitation polarization in semiconductor, while the THz polarity from the photo-Dember induced field are determined by the discrepant mobility between electrons and holes instead of doping type.41 However, the THz polarity of EX and EY components from MoS2 are definitely influenced by the incident polarization states. Similar to the previous demonstration of graphene,21 the resemblance of polarity reversal under different excitation polarization stems from the nonlinear process. Thus, we tentatively attribute the THz radiation mechanism of MoS2 crystal to the nonlinear process. The further quantitative investigation of THz radiation on incident polarization states is proceeded to confirm which nonlinear process occurs in MoS2 crystal in the following part. To our best knowledge, the representative semiconductor InAs is one of the most effective THz generators. According to its basic physical properties, the comparison between MoS2 and InAs42 is shown in Table 1 to further explore the radiation mechanism. Different from the InAs with narrow direct band 4959


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Figure 3. Generated THz pulses in (a) time and (b) frequency domain from layered MoS2 (red solid curve), graphite (blue dash curve), and InAs crystal (black dash curve). The THz radiations from MoS2 and graphite are expanded 15 times compared with the original values to the guide the eyes. (c),(d) Dependences between peak-to-peak amplitude of EX and EY components of generated THz radiation from MoS2 and pump fluence. The experimental and fitting results are depicted with dots and solid curves, respectively.

Figure 4. Raman spectra (514.5 nm excitation) of layered MoS2 crystal. (a) The spectra taken on one pristine spot (black curve) and two ablated spots under low damage level (red curve) and high damage level (blue curves); (b) Optical image of MoS2 with dash circles depicting two ablated spots; (c) Variation of prominent vibrational modes E2g1 and A1g of three measured spots. The green arrows are labeled to depict the intensity variation.

fitted linearly up to 5.66 mJ/cm2, and the remaining data can be fitted linearly with a different slope, as shown in Figure 3c. Similarly, the fluence dependence of THz radiation for EY component is consistent with that of EX component until the critical point at 4.95 mJ/cm2. As the values are much lower than that of EX component, the EY component could be easily influenced by laser fluctuation and the ambient environment. Moreover, this pump-fluence-increasing process is irreversible once the fluence exceeds the critical point. Thus, we can rule

out other effects such as free carrier absorption influencing the variation of pump-fluence-dependent slopes. We can reasonably infer that the decline of slopes are caused by the surface damage of MoS2, and 4.95 mJ/cm2 ∼ 5.66 mJ/cm2 is the laser-damage threshold for MoS2 crystal. This threshold is less than the reported value ∼15 mJ/cm2 under 200 fs Ti: sapphire laser with 100 μm laser diameter on the sample.45 The divergence could be attributed to the combined differences from samples, the exposure time, and the ambient environment. The linear fitting 4960


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Figure 5. THz radiation dependence on the azimuthal angle. (a) Diagram of the crystalline surface under p-polarized illumination (incoming arrow). The THz radiation (outgoing arrow) generates in the reflective direction; (b) Microscopic illustration of MoS2 crystal with the crystalline azimuthal angle φ; (c) THz electric field dependence on azimuthal angle φ under fixed p-polarized excitation. The experimental and fitting results are depicted with dot and solid curves, respectively.

between pump fluence I and both THz components are consistent with the second-order nonlinear effect (optical rectification) for I ∝ |Eω|2, as this effect is quadratic with the incident electric field: ETHz ∝ χ(2) · Eω · Eω. This suggests that the strength of optical rectification (OR) effect is decided by the interaction between incident electric field and the susceptibility of nonlinear media. Comparing the bulk and few layer MoS2, the different bandgap will lead to the different absorption of the excitation infrared laser. Besides, it will also lead to different nonlinear optical dispersion relations, which result in different nonlinear dielectric polarization at the same wavelength. From the solid-state physics point of view, the monolayer MoS2 has D3h symmetry, and its inversion symmetry is broken. However, the symmetry of the bulk structure is related with stacking orders with different susceptibility. All these distinctions could lead to the discrepancy in OR process between the bulk and few-layer MoS2. Although the monolayer MoS2 can also generate THz radiation under the average 200 mW infrared illumination in the same experimental configuration (Supporting Information), the primary THz radiation mechanism would be essentially different from the bulk structure. Previously, there were still few experimental data on the damage threshold and ablation for MoS2, to our best knowledge. However, the optical damage threshold is very important for the nonlinear optical responses under ultrafast laser illumination, especially for the performance of nonlinear photonic devices as well as the nonlinear THz devices. Moreover, the THz emission mechanism by nonlinear optics from the MoS2 is still unclear near the damage threshold. Interestingly, we found the THz radiation can continue to increase even after the ablation. This phenomenon is rarely discussed in the nonlinear process. Our further investigation on damage and ablation embodies the evolution of the surface and structure during this laser heating process, which can be effectively characterized via Raman spectroscopy. As shown in

Figure 4a, the Raman spectra focus on one pristine spot and two ablated spots. As the optical image shown in Figure 4b, we can clearly observe the laser ablation (spot 1) on the surface with eyes when the pump fluence increases to 7.07 mJ/cm2. Then we keep on increasing the pump fluence until to 11.3 mJ/ cm2; subsequently, it can be observed that the ablated region expands, and the ablation gets more severe (the spot 2). The ablations are caused by local laser heating and may further oxidize the surface. The previous transformation of MoS2 to molybdenum trioxide (MoO3) does happen to the microcrystalline powder MoS2.46 However, the antisymmetric stretching mode (Ag) of MoO3 at 817 cm−1 is absent in our Raman measurement. This mode is a characteristic peak of MoO3 with bonding aligning along the a-axis direction.47 It is suggested that the laser-induced transformation from MoS2 to crystalline MoO3 does not happen. Comparing the three Raman spectra, a new peak appears at 284.9 cm−1 for ablated spot 1, and another new peak at 201 cm−1 appears after sequentially increasing pump fluence. These new Raman modes correspond to the B2g modes due to the δO(1)MoO(1) wagging mode and δO(2)MoO(2) scissor mode, respectively,47 where O(1) and O(2) are nonequivalent types of O atoms. These suggest that the new molecular Mo−O bonds form on the surface because of the Mo−S bond dissociation under intense pump fluence. The Mo−S bond breaking can also be traced when we focus on the characteristic E2g1 and A1g modes of the three spots. As shown in Figure 4c, the intensity of E2g1 decreases monotonously from pristine to ablated spot 2, while the intensity of A1g mode increases first and then decreases (as labeled with green arrows in Figure 4c). E2g1 is an in-plane vibrational mode related to both Mo and S atoms.48 It is easily influenced by the decrease of the number of scattering centers (Mo−S bonds) on the surface under the laser irradiation.45 As for A1g mode, it is an out-of-plane molecular vibrational mode and only relates to S atom. On one hand, we speculate that the new constituent Mo−O bonds possibly 4961


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Here, ε0 is the permittivity of the space and χ(2) ijk is the secondorder susceptibility tensor of MoS2. According to the symmetry on the surface of crystal, χ(2) ijk must be invariant under the same group of the symmetry operations49,50

protect the surface at the onset of laser irradiation. On the other hand, under the protection of Mo−O bonds, Mo−S bond breaking and atoms removal could enhance the A1g mode due to its special vibrational feature. Afterward, the persistent laser irradiation and subsequent surface damage lead to local lattice sublimation,45 which may further weakens the intensity of A1g mode. Although the laser irradiation can lead to surface bond breaking, atom removal, new constituent Mo−O bonds, and even the local lattice sublimation, the underneath lattice structure does not change. Accordingly, the MoS2 structure underneath the ablated top surface can still generate THz radiation persistently. We believe that the corresponding investigations can not only deepen the understanding of the optical rectification process and the THz radiation mechanism but also provide data for individuals who are interested in the microscopic variation of MoS2, including bond breaking and forming, atoms removal, and lattice sublimation.

χi(,ni)...′ i 1 2

∂ 2P(0, t )

1

2

n+1

1 2

n+1

(5)

∂ 2I(t )

E THz ∝ −PX cos θTHz + PZ sin θTHz

(6)

Here, the three components PX, PY, and PZ have been calculated according to the operated susceptibility tensor (Supporting Information), and then the THz radiation based on the azimuthal angle can be written as E THz ∝ 0.97[d 22 sin(3φ) − 2d15] + 0.24(d31 + d33)

(7)

As for the experimental results, the dependence between the maximum amplitude of THz radiation and azimuthal angle of MoS2 is depicted in Figure 5c. The observed experimental results agree well with the 3φ-dependence from the theoretical calculation in eq 7. It is also confirmed that although the space group of bulk 2H-MoS2 is D6h, the nonlinear effect on the surface obeys the 3-fold rotational symmetry. Additonally, the inversion symmetry on surface is broken, which is necessary in the optical rectification process. Additionally, both curves in Figure 5c obviously show that the polarity of the THz radiation can reverse when φ is rotated approximately 90° and change back again after rotating about 40°. The experimental results deviate slightly from theoretical curve at some angles, which mainly attribute to the imperfections such as stacking faults of MoS2 crystal.51 We have tested another MoS2 sample in the experiment and find that the THz electric field dependence on azimuthal angle φ under fixed p-polarized excitation follows the same tread as that in Figure 5c. However, the variations in the amplitude are different, which suggest the existence of stacking faults. This 3-fold azimuthal angle dependence of MoS2 is definitely different from graphite and InAs crystals. The THz radiation from the edge plane of graphite have one-fold rotational symmetry on azimuthal angles, and the polarity is reversed when graphite is rotated by approximately 180°. This mainly stems from the accumulated carriers moving along a preferential direction from stacking faults in graphite.23 While for p-type InAs (100), the slight dependence of THz radiation on the azimuthal angle is negligible, which is accordant with the previous investigation.7 Thus, we emphasize that the THz mechanism of MoS2 essentially is related to the crystalline orientation in optical rectification process, differing from the transient photocurrent induced THz generation in graphite and photo-Dember filed induced THz radiation in InAs. According to the former demonstration of classical semiconductor InP in the optical rectification process,52 the proposed theoretical model can explain most of the experimental observations and emphasize the importance of the azimuthal dependence of nonlinear signal. Thus, almost all the investigations of optical

(2)

(3)

∑ ε0χijk(2) (0, ω , −ω)Ej(ω)Ek*(ω) j,k

Ti1, j Ti2 , j ...Tin+1, j χj(n, j) ...j

E THz ∝ = χ (2) 2 . Thus, its amplitude is proportional ∂t 2 ∂t to the nonlinear dielectric polarization and the p-polarized THz radiation can be detected in reflection geometry41 as follows:

The spatial phase information on the electric field is divided into Z component kzZ and X component kXX. p̂ and ŝ represent the p- and s-polarized states. As a consequence, the components EX, EY, and EZ of the incident electric field can be deduced (Supporting Information). Notably, the Y component (s-polarized component) is negligible in this situation for the fixed p-polarized incidence. According to the incident electric field components, the nonlinear optical rectification process can be expressed via nonlinear dielectric polarization, which can be given as Pi(2)(0) =

∑

Under the rotation operation R(φ) normal to the crystal, we can obtain the transformed expression of susceptibility tensor (Supporting Information). Herein, the single variable φ is the azimuthal angle of MoS2 as shown in Figure 5b. In the optical rectification process, the generated THz radiation ETHz has a relationship with the nonlinear dielectric polarization as:

Here, θopt is the refraction angle between normal direction of the crystal’s surface and the infrared wave, θTHz is the refraction angle of the generated THz wave in the interface between sample and air. nopt and nTHz are refractive index when infrared and THz waves propagate through the MoS2 sample in its penetration depth. From the refractive index in Table 1, θopt and θTHz are estimated to be 8.422° and 13.876°, respectively. Besides, the phase matching is negligible in our reflection configuration due to the micrometer-order penetration depth of MoS2. The incident electric field illuminating the sample is labeled as E±(r), which is the summation of the upward (+) and downward (−) propagation of the p- and s- polarized waves34 E±(r ) = (s Ê ±s + p±̂ E±p)e±ikZZeikXX

=

j1 , j2 ...jn + 1

4. THEORETICAL CALCULATIONS For the quantitative analysis on crystalline orientation, we put forward a prototypical model referring to the second-order nonlinear harmonic generation.49 The model assumes that the rectification process is confined to an interactive volume called electric dipole sheet. This hypothetic region at Z = 0 above the crystalline surface is shown in Figure 5a. The surface is paralleled with X−Y plane in the laboratory coordinate. The incident IR wave paralleled to X direction (p-polarized) propagates along Z direction. The incident angle θ is fixed at 45°. Thus, the emitted THz radiation angle is also 45° and the refraction angles can be given according to the Snell’s Law41 in reflection configuration: nair sin θ = sin 45◦ = nopt sin θopt = n THz sin θTHz

n+1

(4) 4962


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Figure 6. THz radiation dependence on incident polarization angle. (a) Diagram of the structural surface under variable pumping polarization illumination (incoming arrow). The generated THz waves (outgoing arrow) propagate in the reflective direction; (b) THz electric field dependence on the incident polarization angle with a fixed azimuthal angle. The experimental and fitting results are depicted with dot and solid curves, respectively.

related to the dielectric polarization components PX, PY, and PZ. The amplitudes of these three components can be changed by the variable incident polarization angle α according to the mathematical expressions (Supporting Information). Thus, the amplitudes as well as polarities of detected THz radiations are further influenced. It is determined from these results that the THz radiation mechanism of MoS2 in reflection configuration is surface optical rectification in reflection configuration.

rectification process take the in-depth studies on the azimuthal angles of materials. From the investigations, the (110) zincblende crystals InP, GaAs, CdTe,14 and (111)-oriented InP10 also show 3φ-dependence on azimuthal angles due to their 3fold rotation symmetry. The incident polarization states can definitely influence the THz radiation in the optical rectification process. For transmission configuration, the wave vector of incident beam is perpendicular to the interface of sample, hence the variation of incident polarization can be realized via changing azimuthal angle of sample.53 However, the rotational symmetries of azimuthal and incident polarization angles are different in the reflection configuration. Thus, we rotate the incident polarization angle α (see Figure 6a) from 0° to 360° (0° and 90° correspond to p-polarized and s-polarized, respectively) via a HWP instead of rotating sample. The incident angle θ is still 45°. After the investigation of azimuthal angle dependence, the azimuthal angle φ is fixed at the optimal angle with the strongest radiation efficiency (negative maximum value in Figure 5c). Similar to the above analysis, the nonlinear dielectric polarization can be given on the basis of the interactions between susceptibility tensor and incident electric field. After the coordinate transformation (Supporting Information) based on the polarization angle, the generated THz radiation has the dependence on α as

5. CONCLUSIONS In summary, we observe that 2H-MoS2 crystal can generate THz radiation efficiently ranging from 0.1−3.5 THz under linearly polarized femtosecond laser excitation. The linear dependence of THz amplitude on incident pump fluence indicates the second-order nonlinear process. We take the theoretical analysis of optical rectification mechanism based on the space symmetry. The calculated results fit well with the results from experiments with respect to the dependences on azimuthal and incident polarization angles. These consistent dependences of two angles verify the THz radiation of MoS2 by surface optical rectification. Furthermore, the bond breaking of MoS2 can be monitored by combining THz emission and Raman spectra after the laser-damage threshold 5.66 mJ/cm2. The investigations on layered MoS2 crystal can not only deepen the understanding of nonlinear optical rectification process but also potentially provide a sensitive and noninvasive method to characterize the surface and interface properties of MoS2 by THz surface emission spectroscopy.

E THz ∝ A cos α + B sin α + C cos(2α) + D sin(2α) + E (8)

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A, B, C, D, and E are fitting constants related with nonzero susceptibility terms and have been calculated (Supporting Information). The experimental results along with the fitting results from eq 8 are shown in Figure 6b. The experimental results fit well with the calculated results, which further confirm the validity of the theoretical analysis. From α = 0° at ppolarization, the THz amplitude decreases monotonously until α reaches to 90° at s-polarization. Then the amplitude increases to maximum again after α reaches to 180° at p-polarization once more. Consequently, the THz amplitudes exhibit 2-fold rotational symmetry with the changing polarization angle in the whole process. In addition, the polarity of THz radiation also shows the 2-fold symmetry. It reverses sign when α is in the range of 60° ∼ 120° and 240° ∼ 300°, which are adjacent to the s-polarization. The reversed polarities of THz radiations are

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* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b13961. Dependence of THz radiation on azimuthal angle, the dependence of THz radiation on incident polarization angle, THz radiation from monolayer MoS2, and THz surface emission considering the penetration depth (PDF)

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AUTHOR INFORMATION

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*E-mail: [email protected]. Fax: +86 29 88303336. 4963


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ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 11374240), Ph.D. Programs Foundation of Ministry of Education of China (No. 20136101110007), National Key Basic Research Program (2014CB339800), and Postgraduate Innovative Talent Training Program of Northwest University (YZZ15031).

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