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Terahertz surface emission from layered semiconductor WSe2
Accepted Manuscript Full Length Article Terahertz surface emission from layered semiconductor WSe2 Keyu Si, Yuanyuan Huang, Qiyi Zhao, Lipeng Zhu, Longhui Zhang, Zehan Yao, Xinlong Xu PII: DOI: Reference:
S0169-4332(18)31065-1 https://doi.org/10.1016/j.apsusc.2018.04.117 APSUSC 39116
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
13 January 2018 31 March 2018 10 April 2018
Please cite this article as: K. Si, Y. Huang, Q. Zhao, L. Zhu, L. Zhang, Z. Yao, X. Xu, Terahertz surface emission from layered semiconductor WSe2, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc. 2018.04.117
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Terahertz surface emission from layered semiconductor WSe2 Keyu Si1, Yuanyuan Huang1, Qiyi Zhao1, Lipeng Zhu1, Longhui Zhang1, Zehan Yao1, and 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 Ultrafast laser interaction with the layered semiconductors has attracted wide interest due to not only the fundamental physical understanding of the light-matter interaction in these advanced materials, but also the potential optoelectronic devices from visible region to THz region based on these emergent semiconductors. Herein, we investigated the THz radiation property from the layered WSe 2 due to the d-d photo-transition by an ultrafast laser excitation. We observed strong broadband p-polarized THz radiation under different pump polarization and an evident THz radiation saturation with the pump fluence. The THz radiation demonstrated a cosine function with the polarization angle of the pump beam. Angular dependent THz radiation had a polarity reverse with the opposite incident angle and could be fitted well with a dipole approximation model. These results reveal that the dominant mechanism of THz emission is due to the photocarrier surging under the surface field. The azimuthal angle dependence of THz radiation suggested that the dominant contribution is due to the surface depletion field rather than surface field induced optical rectification. In addition, we inferred that the laser damage threshold for the WSe2 crystal is 3.11 mJ/cm2 confirmed by both THz emission spectroscopy and Raman Spectroscopy. Our results could provide the fundamental light-matter interaction data for the layered WSe2 and promise the potential applications of this
semiconductor for THz devices. KEYWORDS Tungsten diselenide (WSe2), layered semiconductor, terahertz (THz) radiation, surface field effect, femtosecond laser
1 Introduction When ultrafast laser pulses interact with the semiconductors, THz radiation pulses can be generated by either nonlinear optical processes such as photon-drag effect[1-3] and optical rectification effect[4, 5] or linear optical processes such as the surface[6, 7] and interface[8, 9] field induced surge current and photo-Dember current.[10] The light-matter interaction promotes the development of THz science, which is an interdiscipline among photonics, electronics and material science. THz time-domain spectroscopy as well as optical-pump THz-probe spectroscopy[11] are this kind of spectroscopic methods which have been developed based on the ultrafast laser for the understanding of THz properties of materials. Previous work focuses mainly on the ultrafast laser pulses interaction with the traditional semiconductors such as GaAs,[12] ZnTe,[5] InAs[13] and so on. With the development of materials, THz radiation from the advanced materials such as graphene,[1, 3] carbon nanotubes,[11] plasmonic materials,[14, 15] and transition metal dichalcogenides(TMDs)[7, 16] have received considerable interest in THz community because of their potential implementation in the next-generation optoelectronics. Among them, TMDs are important because their bandgap covers the region from visible to infrared, which is desirable for the photovoltaic and switchable devices[17]. These layered TMDs materials demonstrate van der Waals (vdW) interaction between layers. Photoexcitation of d-band electrons in TMDs usually introduces the d-d electron transition instead of p-s photo-transition in traditional III-V group semiconductors.[18] This d-d electron transition demonstrates fast recombination rates and ultrafast photoconductivity response,[19] which have the potential applications in ultrafast broadband THz emitters.
WSe2 is a typical stable TMDs, which demonstrates ultrafast transient THz conductivity,[19] layer-dependent photoluminescence,[20] enhanced nonlinear optics response,[21, 22] and so on. Bulk WSe2 is an indirect semiconductor with the bandgap of approximately 1.21 eV, and there exists an indirect bandgap to direct bandgap transition with the decreasing of layers. The bandgap for the monolayer is approximately 1.64 eV.[23] WSe2 also demonstrates quantum emission (photon anti-bunching) property due to the edge imperfections.[24] Owing to these properties, WSe2 is widely used in tunnel field effect transistors(TFETs),[25, 26] light-emitting diodes (LEDs),[27, 28] photoelectrochemistric photoelectrodes,[29] and so on. With the device working frequency going up to THz, the understanding of the THz properties of WSe2 as well as the THz generation with the WSe2 are desirable. However, to our best knowledge, THz emission due to the d-d photo-transition from WSe2 directly under a femtosecond laser has not been discussed yet. THz radiation pulses carry the information of amplitude, phase, polarity, polarization etc, from which we can determine much information such as the transient carrier mobility, doping concentration, surface field property, crystal symmetry structure, and so on.[6] Thus a full understanding of THz radiation can not only enrich the light-matter interaction property of WSe2 but also help to find new THz radiation sources based on TMDs. In this paper, we observed THz radiation from layered WSe2 crystal under the linearly polarized femtosecond laser irradiation in both reflection and transmission configurations. The THz radiation mechanism of WSe2 is based on the current surge
by surface field effect according to the investigations of incident angle and polarization angle dependence of THz signals. We also investigated azimuthal dependent THz radiation, which reveals that the contribution of surface depletion field is much higher than that of the nonlinear process. Additionally, The pump fluence dependent THz generation process demonstrates a saturation phenomenon with the increasing of pump fluence, which is mainly due to the screening effect of the surface depletion field. Furthermore, we can get the laser damage threshold of the WSe2 by THz emission spectroscopy combined with Raman spectroscopy. The results could promote not only the THz radiation property understanding of the layered TMDs, but also the THz devices such as THz emitters.
2 Experimental Setup
In our experiment, we used a mode-locked Ti: sapphire regenerative amplifier (Spectra-Physics, Spitfire) with 800 nm central wavelength, 35 fs pulse duration, and 1 kHz repetition rate. The infrared (IR) laser is split into two beams by a beam splitter for the THz radiation generation and detection. Figure 1(a) demonstrates the schematic of experimental setup in both reflection and transmission configurations, which can be switched by a reflecting mirror (M). The pump beam with the dominant energy distribution impinges on the sample at a fixed incident angle (θr in Figure 1 (b)) 45°with a spot diameter of 3 mm. Meanwhile, we also employ transmission configuration in our experiment and it is flexible to vary incident angle (θt in Figure 1 (b)), which is used to observe the incident angle dependence of the THz radiation. An
off-axis parabolic mirror is used to collect and collimate THz radiation from the sample successively, and then the THz radiation is focused onto the ZnTe crystal (110) by another parabolic mirror. Thus, we obtain THz waveforms by means of electro-optical sampling through changing time delay between two beams.[30] The probe beam can be fixed at s-polarization state by a Glan-Taylor prism and a half-wave plate (HWP), while the polarization state of the pump beam can be adjusted by a HWP, and the energy fluence can be regulated by a neutral density filter. The polarization states of the generated THz waves are controlled via a pair of wire-grid polarizer (WGP) and the detection crystal ZnTe has been rotated to the optimal crystalline direction. The probe beam and THz wave can pass through a quarter-wave plate and a Wollaston prism, and then be detected by a pair of balanced photodiodes as shown in Figure 1(a). If we want to extract the EX (p-polarized) signal of THz electric field, we will place a piece of perpendicularly-aligned WGP (WGP1 in Figure 1(a)) in the optical path, while the EY (s-polarized) THz signal could be obtained by inserting another 45° WGP(WGP2 in Figure 1(a)) and after simple mathematical calculations without changing both the probe beam and the detector crystal.[31] What’s more, we closely place a 10 mm-thick polyethylene plate and a 0.5 mm-thick silicon plate in holders after the sample for not only blocking the IR pulses but also ensuring THz radiation to transmit.
Figure 1. (a) Schematic of experimental setup. M is a reflection mirror which can switch from transmission to reflection configuration by removing the mirror. HWP: half-wave plate, WGP: a wire-grid polarizer. (b) THz radiation in both reflection and transmission configuration with different polarizations. XYZ and X′Y′Z′ represent the laboratory and crystalline coordinates, respectively. θr and θt mean the incident angle for reflection and transmission configuration, respectively.
3 Material Characterizations Figure 2(a) shows Raman spectrum (Model invia from Renishaw Company) excited by a laser with the wavelength of 514.5 nm. We can observe two distinct vibration modes E 2 g1 and A1g at the 247.8 cm-1 and 258.4 cm-1 respectively.[22, 32] The E 2 g1 mode, an in-plane vibration mode near 250 cm-1, originates from the out-phase vibrations of two selenium (Se) atoms relative to tungsten (W) atom. The A1g mode corresponds to out-of-plane vibrations near 260 cm-1, which is due to the vibration of Se atoms in an opposite direction to W atom[33]. Around 370 cm-1, there are high order oscillation modes: overtone mode 2 E 2 g1 , the combination mode E 2 g1 +LA(M) and the overtone longitudinal acoustic phonon branch at the M point 3LA(M).[34-36] The broad weak peaks are also observed in the spectral range of 450–900 cm-1, which illustrates the higher order overtones and combinational modes.[37] The experimental results are in agreement with the calculated result.[38] Figure 2(b) shows the X-ray diffraction (XRD) spectrum with four distinct peaks corresponding to (002), (006), (008), (0010) crystal orientation. This signifies that our sample belongs to
2H-structure (JCPDS:38-1388), which is a hexagonal structure.[39] The symmetry of 2H-WSe2 structure can be depicted by D 6h4 (P63/mmc) space group, which means this material possesses 6-fold screw axis of symmetry with a good crystalline quality.[16] To further investigate the electronic properties, we did the density functional theory (DFT)
calculation of the energy band, density of states (DOS) of bulk WSe 2 via
VASP (Vienna Ab-initio Simulation Package) software. Figure 2(c) shows the energy band structure, which suggest that bulk WSe2 is an indirect band gap semiconductor. There are four types of transitions taking place between the valence band maximum(VBM) and conduction-band minimum(CBM) such as K→K with a direct band gap of 1.25 eV (the red arrow in Figure 2(c)), K→Γ-K with an indirect band gap of 1.21 eV (the green arrow in Figure 2(c)), Γ→Γ-K of 1.48 eV and Γ→K of 1.52 eV.[23] Figure 2(d) presents the partial density of states consistent with the energy band structure. In Figure 2(d), energy of CBM, Fermi level (EF) and VBM is given out. Obviously, the main contribution to the CBM is d-orbital energy level of W atom and p-energy level of Se level, and the VBM is also occupied by these two energy states.[40]
Figure 2. Characterization of WSe2 crystal: (a) Raman spectra of pristine WSe2 crystal. (b) X-ray diffraction (XRD) spectra of WSe2 sample with (002), (006), (008) and (0010) crystal orientation. (c) Energy band diagram of WSe 2 crystal. The red and green arrows indicate the direct and indirect transition from VBM to CBM, respectively. (d) DOS of all atoms, tungsten atoms and selenium atoms of WSe2 crystal.
4 Results and Discussions
Generally, THz radiation by ultrafast transient polarization and transient photocurrent after ultrafast laser excitation can be described by the following formula:
(1) In Eq. 1, P is the nonlinear polarization, which is a source term of optical rectification (OR) based on nonlinear response.[5, 41, 42] The first term in Eq. 1 shows an azimuthal angle dependence with symmetry as the second order nonlinear polarization
relies on symmetry of crystal. The current density J is induced by photo-excited carriers surging under the intrinsic or extrinsic electric field. The second term illustrates that THz amplitude has a linear relation with the electric field induced by either surface depletion field[43] or photo-Dember effect.[10] Figure 3 shows a time domain THz spectrum and its Fourier-transformed spectrum from WSe2 under a pump fluence of 0.28 mJ/cm2. The central frequency and bandwidth of the spectrum are approximately 0.69 and 2.3 THz. We first compare the difference value of mobility between electrons and holes in InAs and WSe 2. InAs crystal as a typical THz emitter based on photo-Dember effect, has the hole mobility of 240 cm2/V·s and electron mobility of 30000 cm2/V·s.[13] By contrast, the hole mobility of WSe2 crystal is about 100 ~ 500 cm2/V·s, and electron mobility approaches 700 cm2/V·s.[44-47] Compared with InAs crystal, mobility difference of WSe2 is smaller that photo-Dember effect could not be the the main effect for THz radiation in WSe2.
Figure 3. Typical THz waveforms in (a) time domain and (b) frequency domain generated from layered WSe2.
Azimuthal angle dependence of THz radiation can be used to distinguish the THz radiation from nonlinear process and transient current. In Figure 4, we use the polar
coordination to illustrate the azimuthal angle dependence of THz amplitude (EX component) in the transmission configuration under p-polarized illumination. The experimental results show that WSe2 has an isotropic response with the azimuthal angle, which is quite different from MoS2 due to surface optical rectification.[16] As for
(P63/mmc) space group of 2H-WSe2 structure, the second-order nonlinear
susceptibility (χ(2)) vanishes.[48] Thus, the THz emission by the bulk optical rectification (OR) process should be negligible. According to the formula χ
-
-
, there exists THz radiation
generated by surface field induced optical rectification (SOR) at high excitation.[49, 50] Based on the nonzero third-rank susceptibility terms of WSe2, the azimuthal angle dependence of THz radiation from SOR can be deduced as: Herein, χ
, χ
and χ
sθ χ
χ
χ
sn θ
(2)
are nonzero elements of third-rank susceptibility tensor,
and θ2 is the refraction angle of THz wave, which is approximately 10 when the incident angle of pump beam is 45. Then Eq. (2) can be simplified as: - .
χ
.
χ
χ
(3)
It can be seen that the SOR induced THz field is independent to the azimuthal angle. This is consistent with our experimental result (Figure 4 and Figure S5(a) in the supporting information). Eq.(3) and experimental results suggest that SOR can contribute to THz radiation at high pump intensity. The third-order nonlinear susceptibility of WSe2 is approximately 1.43×10-11 esu, [51] suggesting the susceptibility tensor elements are quite small. According to Eq. (3), the first term is negative and the second term is positive, which could weaken the SOR contribution to the THz radiation. Besides, the surface depletion field and SOR effect may both contribute to the THz emission from InAs,[52] and the third-order nonlinear susceptibility of InAs is in the order of 10 -7 esu,[53] which is four order in magnitude larger than that of WSe2. Thus, the SOR process in WSe2 is not strong enough to dominate THz radiation, and the contribution of SOR is much smaller than that of
surface depletion field.
Figure 4. Azimuthal angle dependence of THz amplitude of WSe2 crystal with the fixed p-polarized excitation in the transmission configuration.
Figure 5(a) demonstrates the pump energy fluence dependence of THz peak-to-peak (from positive peak to negative peak as shown in Figure 3(a)) amplitude in the transmission configuration. In our experiment, we set the pump beam to p-polarized state and keep the focal spot behind the sample in order to prevent not only damage from the intense laser beam but also the air ionization process. As shown in Figure 5(a), we can observe that THz amplitude gradually increases with the increasing of the pump fluence. However, when the pump fluence reaches 2 mJ/cm2, the THz amplitude starts saturation. When the pump fluence approaches 3.4 mJ/cm2, the amplitude keeps stable. To understand the THz radiation mechanism, we also measured THz radiation signal from the GaAs(100) in both time-domain and frequency-domain (Figure S1 in the Supporting Information) and the pump fluence dependence of GaAs(100) crystal (Figure 5(b)), which can generate THz by surface field under the oblique incidence.[54] Distinctly, we can see that THz radiation from both GaAs and WSe2 crystal in Figure 5(a) and (b) show the same fluence dependence and demonstrate the same saturation with the increasing of pump fluence. This
saturation phenomenon is quite different from that nonlinear process in other TMDs[16] and photo drag effect in graphene,[2] in which THz amplitude increases as a linear function of the pump fluence due to the second order nonlinear optics. Besides, if SOR process is responsible for THz emission, the THz amplitude will saturate at the fluence within the order of tens
f μJ/cm2.[55] However, in our
experiment (see Figure 5(a) and Figure S2 in the supporting information), it is clear that the saturation fluence is much higher than the order of tens of μJ/cm2. Thus, we conclude that the main cause of the saturation is attributed to the accumulation of massive photo-excited carriers at the surface of WSe2 after high intensity excitation, which will lead to electrostatic screening of THz field radiation.[54] Therefore, the dominant THz generation mechanism attributes to semiconductor surface transient current rather than nonlinear process.
Figure 5. (a) Pump energy fluence dependence of THz peak-to-peak value of WSe2 crystal in the transmission configuration. Squares and red curve represent experimental results and fitting result, respectively. (b) Pump energy fluence dependence of THz peak-to-peak amplitude of GaAs crystal.
We also measured the pump energy fluence dependence of THz peak-to-peak value in a reflection configuration (Figure S2 in the Supporting Information). The data also
shows a saturation effect until 3.11 mJ/cm2, which is consistent with the data taken in the transmission configuration. However, there is a jump with a different slope when we further increase the pump energy fluence. As the reflection configuration is more sensitive to the surface state of WSe2 than that of the transmission configuration, we can infer that there is an abrupt surface change after the 3.11 mJ/cm2 excitation. This slope change could be caused by the surface damage of sample with the laser damage threshold of approximately 3.11 mJ/cm2 for WSe2 (Figure S2). Literally, measurement on laser damage threshold is excessively essential for the nonlinear optical responses under ultrafast laser illumination in order to protect the sample from the damage by intensity laser. This is especially important for the performance of nonlinear photonic devices. Nevertheless, to our best knowledge, there are still no studies on the laser damage threshold of WSe2 crystal. To check the laser damage of the sample, we also took microscopic photo-image and Raman spectra (Figure 6(a) and (b)) after the damage. It is clear that there is an ablated region in the microscopic photo-image (Figure 6(a)) of which the morphology is different from other regions (spot1 and spot2 in Figure 6 (a)). In Figure 6(c), we can see that the E 2 g1 and A1g peaks shift close to each other that they are hardly distinguished when the sample is damaged, and there is a new weak peak emerging near 176 cm-1, which is E1g oscillation mode. This Raman spectrum change also happened with the decreasing of the thickness of layered WSe2.[32] This suggests that the surface of the WSe2 could be exfoliated by intensity laser. Near 800 cm-1, a pronounced pump emerges, which denotes an O-W-O stretching mode of tungsten trioxide (WO3). This suggest that transformation of WSe2
to WO3 could happen after intensity laser irradiation,[37, 56, 57] for WSe2 and WO3 could coexist simultaneously in the laser ablated sample. Take both THz emission data and Raman spectra into account, we can determine that the pump fluence of 3.11 mJ/cm2 is the laser damage threshold for WSe2 crystal.
Figure 6. (a) Microscopic photo image of the sample with two colored dash circles depicting two ablated spots, respectively. (b) The microscopic photo image of the pristine sample. (c) Raman spectra of two laser damaged spots of WSe2 crystal compared with that of the pristine sample.
In order to further confirm the dominant mechanism of THz generation, we conducted an angular dependent THz emission experiment in the transmission configuration. The incident angle can be tuned within -40° to 40° via rotating the sample holder (Figure 1(b)) around the axis. The incident angle obeys the generalized Fresnel law as n sin sin 45 n2 sin 2 . Here, n and n2 are the refraction index of the incident optical beam in the air and the THz beam through the sample. Incident angle is θ and the output (refracted) angle θ2. In Figure 7(a), the time-domain
waveforms of two opposite incident angles, -40° and 40°, are symmetric along abscissa axis and opposite in polarity. This is induced by the reverse of the direction of transient current due to the converse of the incident angle so that the directions of photo-excited carriers surging will change with the incident angle. Figure 7(b) demonstrates the peak-to-peak value of the THz waves under different incident. The polarity of THz radiations are opposite under every pair of opposite incident angles, but the THz amplitude at normal incidence is approximately zero, which is due to the direction of the photocurrent along the surface normal.[6] The angular dependence of THz radiation can be fitted by the following formula:[6]
E ( ) sin( )[1 (
tan( 2 ) 2 2cos( )sin( 2 ) ) ] tan( 2 ) sin( 2 ) cos( 2 )
(4)
Here, θ2 sin -1 sinθ n2 , while the refractive index of THz beam is 4.13 at 1 THz by THz time-domain transmission spectroscopy. The experimental data can be fitted well by Eq. 4, which is consistent to the phenomenon of THz radiation due to the surface field effect.
Figure 7. (a) Time-domain waveform of two opposite incident angles, -40° and +40°. (b) THz emission as a function of the incident angle with a fixed azimuthal angle and pump polarization. The experimental and fitting results are depicted with dot and solid curves, respectively.
Figure 8(a) and (b) demonstrate the polarization of THz wave (p-polarized
components EX and s-polarized components EY) with different polarization of pump beam (p-polarized and s-polarized components). Both of them are insensitive to polarization states of pump beam as the polarity and amplitude is almost the same under different polarization states of pump beam. However, the value of EX components is almost five times larger than that of EY components under the same polarization states of pump beam in the transmission configuration. When bulk WSe2 crystal is excited by the energy of 1.55 eV greater than the bandgap of sample (1.21 eV), surface build-in electric field can accelerate photo-excited carriers in the surface depletion region of sample. This transient current is usually parallel to the surface normal accelerated by the surface depletion field, which will introduce this polarization of THz radiation. Additionally, in the reflection configuration, we can also observe the amplitude of two components is similar under different polarization excitation, and two components show insensitive to the pump polarization states (Figure S3). However, the THz amplitude in the reflection configuration is slightly higher than that in the transmission configuration, which could result from the reabsorption in the transmission configuration and the reflection configuration is more sensitive to the surface state of WSe2.
Figure 8. Time domain waveforms of EX and EY components of WSe2 crystal under different polarization states of pump beam excitation: (a) under p-polarized pump laser beam irradiation; (b) under s-polarized pump laser beam irradiation. All the data are taken in the transmission configuration.
To further investigate the relation among polarization states of pump beam, the generated THz radiation dependence on the incident polarization angle α can be expressed as: [16] ETHz A cos B sin C cos(2 ) D sin(2 ) E
(5)
Here, A, B, C, D and E are fitting constants relating to the nonlinear optical coefficient d15, d22, d31 and d33. We calculated the nonlinear optical coefficient by the first-principle method (see Supporting Information), and the effective nonlinear optical coefficients dij were exhibited in Figure S4. The THz field is the cosine function of the pump polarization angle with a 2α period from Eq. 5. Meanwhile, we measured the EX and EY components of THz field by changing the pump polarization as shown in Figure 9(a) and (b), and the experimental data is consistent with the Eq. 5. In Figure 9(a) and (b), we can see that the amplitude of THz EX is relatively stronger than that of THz EY, which is consistent with that in Figure 8. Additionally, in the reflection configuration, the THz field is also a cosine function of the polarization angle with a 2α period (Figure S5(b) and (c)). Thus, the THz radiation exhibits 2-fold rotational symmetry with the change of α in the whole range.
Figure 9. (a) and (b) are X and Y components of THz electric field as a function of the incident polarization angle in the transmission configuration with a fixed azimuthal angle. The experimental and fitting results are depicted with dot and solid curves, respectively.
As discussed above, the dominant THz generation mechanism of the WSe2 is due to the acceleration of photo-excited carriers under surface depletion field. Mostly, a clean semiconductor surface may be occupied by surface states owing to the termination of lattice periodic arrangement at the surface. The interaction of unpaired electrons in the dangling bonds of surface forms an electronic state at the semiconductor band gap. Initially, in p-type semiconductors, the Fermi level of the bulk (EF-bulk) is lower than that of the surface (EF-surf。) (Figure 10(a)). To maintain equilibrium of the Fermi level inside and outside the semiconductors surface, electrons transfer from the bulk to the surface so that EF-bulk drops and EF-surf。rises until equilibrium achieved. Thus, the Fermi level is pinned by such surface state, forming a charge depletion region and thus a built-in surface electric field (Figure 10(b)).[58] We use ld denote the width of depletion layer, where the intensity of the built-in field are affected by both barrier potential and doping concentration. Usually, intrinsic WSe2 crystal is one kind of p-type semiconductors,[59] and the surface depletion field Ed could be described by:
Ed
eN D
ld
(5)
Here, e is electron charge. ND = 2.958×10l6 cm-3 denotes carrier concentration[60]. κ ε0 εr is absolute dielectric constant in which relative dielectric constant εr is
approximately 11.7, and ε0=8.85×10-14 F/cm is vacuum dielectric constant[61]. The width of depletion layer ld could be given by ld
2 (VD kT / e) . What’s more, the eN D
built-in barrier potential VD for the WSe2 is approximately 0.33V,[61] and the thermal potential kT/e is estimated as 0.026 V at room temperature. The width of depletion layer ld is approximate 115 nm, and the field strength of the surface depletion field Ed is 5.256×106 V∙m-1 by calculation. This calculation result shows that the surface depletion field strength of WSe2 are similar to that of InP crystal.[6] From this, we can infer that the surface field is strong enough to accelerate the photo-excited carriers and induce THz radiation.
Figure 10. Band diagram of the p-type bulk WSe2 crystal: (a) unequilibrium and (b) equilibrium between bulk and its surface state. The region between red dashed line and boundary is the surface depletion region, and ld is the width of this region.
5 Conclusions
In summary, we observed THz radiation from the WSe2 by THz time-domain emission spectroscopy in both transmission and reflection configurations. The THz radiation showed saturation with the incident pump fluence due to the screening effect of accumulation of photo-excited carriers. What’s more, we got the laser damage threshold of WSe2 crystal with the fluence of 3.11 mJ/cm2 by THz emission spectroscopy confirmed with Raman spectroscopy. Finally, we analyzed the formation of the surface depletion filed and calculate the field strength of 5.256×106 V∙m-1 and depletion layer width of 115 nm. Our results provide a fundamental result of the ultrafast laser interaction with the layered WSe2 and pave the way for the new kind of TMDs THz emitter based on surface field effect.
AUTHOR INFORMATION Corresponding Author * Fax: +86 29 88303336. E-mail: [email protected] (X. Xu)
ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No.11774288), and Natural Science Foundation of Shaanxi Province (2017KCT-01).
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GA
Highlights
Ultrafast laser interaction with the layered semiconductors has attracted wide interest.
WSe2 was a typical stable TMDs which is a kind of advanced materials.
THz surface emission from the surface of layered semiconductor WSe 2 was first investigated.
The laser damage threshold of WSe2 was investigated.
S0169-4332(18)31065-1 https://doi.org/10.1016/j.apsusc.2018.04.117 APSUSC 39116
To appear in:
Applied Surface Science
Received Date: Revised Date: Accepted Date:
13 January 2018 31 March 2018 10 April 2018
Please cite this article as: K. Si, Y. Huang, Q. Zhao, L. Zhu, L. Zhang, Z. Yao, X. Xu, Terahertz surface emission from layered semiconductor WSe2, Applied Surface Science (2018), doi: https://doi.org/10.1016/j.apsusc. 2018.04.117
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Terahertz surface emission from layered semiconductor WSe2 Keyu Si1, Yuanyuan Huang1, Qiyi Zhao1, Lipeng Zhu1, Longhui Zhang1, Zehan Yao1, and 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 Ultrafast laser interaction with the layered semiconductors has attracted wide interest due to not only the fundamental physical understanding of the light-matter interaction in these advanced materials, but also the potential optoelectronic devices from visible region to THz region based on these emergent semiconductors. Herein, we investigated the THz radiation property from the layered WSe 2 due to the d-d photo-transition by an ultrafast laser excitation. We observed strong broadband p-polarized THz radiation under different pump polarization and an evident THz radiation saturation with the pump fluence. The THz radiation demonstrated a cosine function with the polarization angle of the pump beam. Angular dependent THz radiation had a polarity reverse with the opposite incident angle and could be fitted well with a dipole approximation model. These results reveal that the dominant mechanism of THz emission is due to the photocarrier surging under the surface field. The azimuthal angle dependence of THz radiation suggested that the dominant contribution is due to the surface depletion field rather than surface field induced optical rectification. In addition, we inferred that the laser damage threshold for the WSe2 crystal is 3.11 mJ/cm2 confirmed by both THz emission spectroscopy and Raman Spectroscopy. Our results could provide the fundamental light-matter interaction data for the layered WSe2 and promise the potential applications of this
semiconductor for THz devices. KEYWORDS Tungsten diselenide (WSe2), layered semiconductor, terahertz (THz) radiation, surface field effect, femtosecond laser
1 Introduction When ultrafast laser pulses interact with the semiconductors, THz radiation pulses can be generated by either nonlinear optical processes such as photon-drag effect[1-3] and optical rectification effect[4, 5] or linear optical processes such as the surface[6, 7] and interface[8, 9] field induced surge current and photo-Dember current.[10] The light-matter interaction promotes the development of THz science, which is an interdiscipline among photonics, electronics and material science. THz time-domain spectroscopy as well as optical-pump THz-probe spectroscopy[11] are this kind of spectroscopic methods which have been developed based on the ultrafast laser for the understanding of THz properties of materials. Previous work focuses mainly on the ultrafast laser pulses interaction with the traditional semiconductors such as GaAs,[12] ZnTe,[5] InAs[13] and so on. With the development of materials, THz radiation from the advanced materials such as graphene,[1, 3] carbon nanotubes,[11] plasmonic materials,[14, 15] and transition metal dichalcogenides(TMDs)[7, 16] have received considerable interest in THz community because of their potential implementation in the next-generation optoelectronics. Among them, TMDs are important because their bandgap covers the region from visible to infrared, which is desirable for the photovoltaic and switchable devices[17]. These layered TMDs materials demonstrate van der Waals (vdW) interaction between layers. Photoexcitation of d-band electrons in TMDs usually introduces the d-d electron transition instead of p-s photo-transition in traditional III-V group semiconductors.[18] This d-d electron transition demonstrates fast recombination rates and ultrafast photoconductivity response,[19] which have the potential applications in ultrafast broadband THz emitters.
WSe2 is a typical stable TMDs, which demonstrates ultrafast transient THz conductivity,[19] layer-dependent photoluminescence,[20] enhanced nonlinear optics response,[21, 22] and so on. Bulk WSe2 is an indirect semiconductor with the bandgap of approximately 1.21 eV, and there exists an indirect bandgap to direct bandgap transition with the decreasing of layers. The bandgap for the monolayer is approximately 1.64 eV.[23] WSe2 also demonstrates quantum emission (photon anti-bunching) property due to the edge imperfections.[24] Owing to these properties, WSe2 is widely used in tunnel field effect transistors(TFETs),[25, 26] light-emitting diodes (LEDs),[27, 28] photoelectrochemistric photoelectrodes,[29] and so on. With the device working frequency going up to THz, the understanding of the THz properties of WSe2 as well as the THz generation with the WSe2 are desirable. However, to our best knowledge, THz emission due to the d-d photo-transition from WSe2 directly under a femtosecond laser has not been discussed yet. THz radiation pulses carry the information of amplitude, phase, polarity, polarization etc, from which we can determine much information such as the transient carrier mobility, doping concentration, surface field property, crystal symmetry structure, and so on.[6] Thus a full understanding of THz radiation can not only enrich the light-matter interaction property of WSe2 but also help to find new THz radiation sources based on TMDs. In this paper, we observed THz radiation from layered WSe2 crystal under the linearly polarized femtosecond laser irradiation in both reflection and transmission configurations. The THz radiation mechanism of WSe2 is based on the current surge
by surface field effect according to the investigations of incident angle and polarization angle dependence of THz signals. We also investigated azimuthal dependent THz radiation, which reveals that the contribution of surface depletion field is much higher than that of the nonlinear process. Additionally, The pump fluence dependent THz generation process demonstrates a saturation phenomenon with the increasing of pump fluence, which is mainly due to the screening effect of the surface depletion field. Furthermore, we can get the laser damage threshold of the WSe2 by THz emission spectroscopy combined with Raman spectroscopy. The results could promote not only the THz radiation property understanding of the layered TMDs, but also the THz devices such as THz emitters.
2 Experimental Setup
In our experiment, we used a mode-locked Ti: sapphire regenerative amplifier (Spectra-Physics, Spitfire) with 800 nm central wavelength, 35 fs pulse duration, and 1 kHz repetition rate. The infrared (IR) laser is split into two beams by a beam splitter for the THz radiation generation and detection. Figure 1(a) demonstrates the schematic of experimental setup in both reflection and transmission configurations, which can be switched by a reflecting mirror (M). The pump beam with the dominant energy distribution impinges on the sample at a fixed incident angle (θr in Figure 1 (b)) 45°with a spot diameter of 3 mm. Meanwhile, we also employ transmission configuration in our experiment and it is flexible to vary incident angle (θt in Figure 1 (b)), which is used to observe the incident angle dependence of the THz radiation. An
off-axis parabolic mirror is used to collect and collimate THz radiation from the sample successively, and then the THz radiation is focused onto the ZnTe crystal (110) by another parabolic mirror. Thus, we obtain THz waveforms by means of electro-optical sampling through changing time delay between two beams.[30] The probe beam can be fixed at s-polarization state by a Glan-Taylor prism and a half-wave plate (HWP), while the polarization state of the pump beam can be adjusted by a HWP, and the energy fluence can be regulated by a neutral density filter. The polarization states of the generated THz waves are controlled via a pair of wire-grid polarizer (WGP) and the detection crystal ZnTe has been rotated to the optimal crystalline direction. The probe beam and THz wave can pass through a quarter-wave plate and a Wollaston prism, and then be detected by a pair of balanced photodiodes as shown in Figure 1(a). If we want to extract the EX (p-polarized) signal of THz electric field, we will place a piece of perpendicularly-aligned WGP (WGP1 in Figure 1(a)) in the optical path, while the EY (s-polarized) THz signal could be obtained by inserting another 45° WGP(WGP2 in Figure 1(a)) and after simple mathematical calculations without changing both the probe beam and the detector crystal.[31] What’s more, we closely place a 10 mm-thick polyethylene plate and a 0.5 mm-thick silicon plate in holders after the sample for not only blocking the IR pulses but also ensuring THz radiation to transmit.
Figure 1. (a) Schematic of experimental setup. M is a reflection mirror which can switch from transmission to reflection configuration by removing the mirror. HWP: half-wave plate, WGP: a wire-grid polarizer. (b) THz radiation in both reflection and transmission configuration with different polarizations. XYZ and X′Y′Z′ represent the laboratory and crystalline coordinates, respectively. θr and θt mean the incident angle for reflection and transmission configuration, respectively.
3 Material Characterizations Figure 2(a) shows Raman spectrum (Model invia from Renishaw Company) excited by a laser with the wavelength of 514.5 nm. We can observe two distinct vibration modes E 2 g1 and A1g at the 247.8 cm-1 and 258.4 cm-1 respectively.[22, 32] The E 2 g1 mode, an in-plane vibration mode near 250 cm-1, originates from the out-phase vibrations of two selenium (Se) atoms relative to tungsten (W) atom. The A1g mode corresponds to out-of-plane vibrations near 260 cm-1, which is due to the vibration of Se atoms in an opposite direction to W atom[33]. Around 370 cm-1, there are high order oscillation modes: overtone mode 2 E 2 g1 , the combination mode E 2 g1 +LA(M) and the overtone longitudinal acoustic phonon branch at the M point 3LA(M).[34-36] The broad weak peaks are also observed in the spectral range of 450–900 cm-1, which illustrates the higher order overtones and combinational modes.[37] The experimental results are in agreement with the calculated result.[38] Figure 2(b) shows the X-ray diffraction (XRD) spectrum with four distinct peaks corresponding to (002), (006), (008), (0010) crystal orientation. This signifies that our sample belongs to
2H-structure (JCPDS:38-1388), which is a hexagonal structure.[39] The symmetry of 2H-WSe2 structure can be depicted by D 6h4 (P63/mmc) space group, which means this material possesses 6-fold screw axis of symmetry with a good crystalline quality.[16] To further investigate the electronic properties, we did the density functional theory (DFT)
calculation of the energy band, density of states (DOS) of bulk WSe 2 via
VASP (Vienna Ab-initio Simulation Package) software. Figure 2(c) shows the energy band structure, which suggest that bulk WSe2 is an indirect band gap semiconductor. There are four types of transitions taking place between the valence band maximum(VBM) and conduction-band minimum(CBM) such as K→K with a direct band gap of 1.25 eV (the red arrow in Figure 2(c)), K→Γ-K with an indirect band gap of 1.21 eV (the green arrow in Figure 2(c)), Γ→Γ-K of 1.48 eV and Γ→K of 1.52 eV.[23] Figure 2(d) presents the partial density of states consistent with the energy band structure. In Figure 2(d), energy of CBM, Fermi level (EF) and VBM is given out. Obviously, the main contribution to the CBM is d-orbital energy level of W atom and p-energy level of Se level, and the VBM is also occupied by these two energy states.[40]
Figure 2. Characterization of WSe2 crystal: (a) Raman spectra of pristine WSe2 crystal. (b) X-ray diffraction (XRD) spectra of WSe2 sample with (002), (006), (008) and (0010) crystal orientation. (c) Energy band diagram of WSe 2 crystal. The red and green arrows indicate the direct and indirect transition from VBM to CBM, respectively. (d) DOS of all atoms, tungsten atoms and selenium atoms of WSe2 crystal.
4 Results and Discussions
Generally, THz radiation by ultrafast transient polarization and transient photocurrent after ultrafast laser excitation can be described by the following formula:
(1) In Eq. 1, P is the nonlinear polarization, which is a source term of optical rectification (OR) based on nonlinear response.[5, 41, 42] The first term in Eq. 1 shows an azimuthal angle dependence with symmetry as the second order nonlinear polarization
relies on symmetry of crystal. The current density J is induced by photo-excited carriers surging under the intrinsic or extrinsic electric field. The second term illustrates that THz amplitude has a linear relation with the electric field induced by either surface depletion field[43] or photo-Dember effect.[10] Figure 3 shows a time domain THz spectrum and its Fourier-transformed spectrum from WSe2 under a pump fluence of 0.28 mJ/cm2. The central frequency and bandwidth of the spectrum are approximately 0.69 and 2.3 THz. We first compare the difference value of mobility between electrons and holes in InAs and WSe 2. InAs crystal as a typical THz emitter based on photo-Dember effect, has the hole mobility of 240 cm2/V·s and electron mobility of 30000 cm2/V·s.[13] By contrast, the hole mobility of WSe2 crystal is about 100 ~ 500 cm2/V·s, and electron mobility approaches 700 cm2/V·s.[44-47] Compared with InAs crystal, mobility difference of WSe2 is smaller that photo-Dember effect could not be the the main effect for THz radiation in WSe2.
Figure 3. Typical THz waveforms in (a) time domain and (b) frequency domain generated from layered WSe2.
Azimuthal angle dependence of THz radiation can be used to distinguish the THz radiation from nonlinear process and transient current. In Figure 4, we use the polar
coordination to illustrate the azimuthal angle dependence of THz amplitude (EX component) in the transmission configuration under p-polarized illumination. The experimental results show that WSe2 has an isotropic response with the azimuthal angle, which is quite different from MoS2 due to surface optical rectification.[16] As for
(P63/mmc) space group of 2H-WSe2 structure, the second-order nonlinear
susceptibility (χ(2)) vanishes.[48] Thus, the THz emission by the bulk optical rectification (OR) process should be negligible. According to the formula χ
-
-
, there exists THz radiation
generated by surface field induced optical rectification (SOR) at high excitation.[49, 50] Based on the nonzero third-rank susceptibility terms of WSe2, the azimuthal angle dependence of THz radiation from SOR can be deduced as: Herein, χ
, χ
and χ
sθ χ
χ
χ
sn θ
(2)
are nonzero elements of third-rank susceptibility tensor,
and θ2 is the refraction angle of THz wave, which is approximately 10 when the incident angle of pump beam is 45. Then Eq. (2) can be simplified as: - .
χ
.
χ
χ
(3)
It can be seen that the SOR induced THz field is independent to the azimuthal angle. This is consistent with our experimental result (Figure 4 and Figure S5(a) in the supporting information). Eq.(3) and experimental results suggest that SOR can contribute to THz radiation at high pump intensity. The third-order nonlinear susceptibility of WSe2 is approximately 1.43×10-11 esu, [51] suggesting the susceptibility tensor elements are quite small. According to Eq. (3), the first term is negative and the second term is positive, which could weaken the SOR contribution to the THz radiation. Besides, the surface depletion field and SOR effect may both contribute to the THz emission from InAs,[52] and the third-order nonlinear susceptibility of InAs is in the order of 10 -7 esu,[53] which is four order in magnitude larger than that of WSe2. Thus, the SOR process in WSe2 is not strong enough to dominate THz radiation, and the contribution of SOR is much smaller than that of
surface depletion field.
Figure 4. Azimuthal angle dependence of THz amplitude of WSe2 crystal with the fixed p-polarized excitation in the transmission configuration.
Figure 5(a) demonstrates the pump energy fluence dependence of THz peak-to-peak (from positive peak to negative peak as shown in Figure 3(a)) amplitude in the transmission configuration. In our experiment, we set the pump beam to p-polarized state and keep the focal spot behind the sample in order to prevent not only damage from the intense laser beam but also the air ionization process. As shown in Figure 5(a), we can observe that THz amplitude gradually increases with the increasing of the pump fluence. However, when the pump fluence reaches 2 mJ/cm2, the THz amplitude starts saturation. When the pump fluence approaches 3.4 mJ/cm2, the amplitude keeps stable. To understand the THz radiation mechanism, we also measured THz radiation signal from the GaAs(100) in both time-domain and frequency-domain (Figure S1 in the Supporting Information) and the pump fluence dependence of GaAs(100) crystal (Figure 5(b)), which can generate THz by surface field under the oblique incidence.[54] Distinctly, we can see that THz radiation from both GaAs and WSe2 crystal in Figure 5(a) and (b) show the same fluence dependence and demonstrate the same saturation with the increasing of pump fluence. This
saturation phenomenon is quite different from that nonlinear process in other TMDs[16] and photo drag effect in graphene,[2] in which THz amplitude increases as a linear function of the pump fluence due to the second order nonlinear optics. Besides, if SOR process is responsible for THz emission, the THz amplitude will saturate at the fluence within the order of tens
f μJ/cm2.[55] However, in our
experiment (see Figure 5(a) and Figure S2 in the supporting information), it is clear that the saturation fluence is much higher than the order of tens of μJ/cm2. Thus, we conclude that the main cause of the saturation is attributed to the accumulation of massive photo-excited carriers at the surface of WSe2 after high intensity excitation, which will lead to electrostatic screening of THz field radiation.[54] Therefore, the dominant THz generation mechanism attributes to semiconductor surface transient current rather than nonlinear process.
Figure 5. (a) Pump energy fluence dependence of THz peak-to-peak value of WSe2 crystal in the transmission configuration. Squares and red curve represent experimental results and fitting result, respectively. (b) Pump energy fluence dependence of THz peak-to-peak amplitude of GaAs crystal.
We also measured the pump energy fluence dependence of THz peak-to-peak value in a reflection configuration (Figure S2 in the Supporting Information). The data also
shows a saturation effect until 3.11 mJ/cm2, which is consistent with the data taken in the transmission configuration. However, there is a jump with a different slope when we further increase the pump energy fluence. As the reflection configuration is more sensitive to the surface state of WSe2 than that of the transmission configuration, we can infer that there is an abrupt surface change after the 3.11 mJ/cm2 excitation. This slope change could be caused by the surface damage of sample with the laser damage threshold of approximately 3.11 mJ/cm2 for WSe2 (Figure S2). Literally, measurement on laser damage threshold is excessively essential for the nonlinear optical responses under ultrafast laser illumination in order to protect the sample from the damage by intensity laser. This is especially important for the performance of nonlinear photonic devices. Nevertheless, to our best knowledge, there are still no studies on the laser damage threshold of WSe2 crystal. To check the laser damage of the sample, we also took microscopic photo-image and Raman spectra (Figure 6(a) and (b)) after the damage. It is clear that there is an ablated region in the microscopic photo-image (Figure 6(a)) of which the morphology is different from other regions (spot1 and spot2 in Figure 6 (a)). In Figure 6(c), we can see that the E 2 g1 and A1g peaks shift close to each other that they are hardly distinguished when the sample is damaged, and there is a new weak peak emerging near 176 cm-1, which is E1g oscillation mode. This Raman spectrum change also happened with the decreasing of the thickness of layered WSe2.[32] This suggests that the surface of the WSe2 could be exfoliated by intensity laser. Near 800 cm-1, a pronounced pump emerges, which denotes an O-W-O stretching mode of tungsten trioxide (WO3). This suggest that transformation of WSe2
to WO3 could happen after intensity laser irradiation,[37, 56, 57] for WSe2 and WO3 could coexist simultaneously in the laser ablated sample. Take both THz emission data and Raman spectra into account, we can determine that the pump fluence of 3.11 mJ/cm2 is the laser damage threshold for WSe2 crystal.
Figure 6. (a) Microscopic photo image of the sample with two colored dash circles depicting two ablated spots, respectively. (b) The microscopic photo image of the pristine sample. (c) Raman spectra of two laser damaged spots of WSe2 crystal compared with that of the pristine sample.
In order to further confirm the dominant mechanism of THz generation, we conducted an angular dependent THz emission experiment in the transmission configuration. The incident angle can be tuned within -40° to 40° via rotating the sample holder (Figure 1(b)) around the axis. The incident angle obeys the generalized Fresnel law as n sin sin 45 n2 sin 2 . Here, n and n2 are the refraction index of the incident optical beam in the air and the THz beam through the sample. Incident angle is θ and the output (refracted) angle θ2. In Figure 7(a), the time-domain
waveforms of two opposite incident angles, -40° and 40°, are symmetric along abscissa axis and opposite in polarity. This is induced by the reverse of the direction of transient current due to the converse of the incident angle so that the directions of photo-excited carriers surging will change with the incident angle. Figure 7(b) demonstrates the peak-to-peak value of the THz waves under different incident. The polarity of THz radiations are opposite under every pair of opposite incident angles, but the THz amplitude at normal incidence is approximately zero, which is due to the direction of the photocurrent along the surface normal.[6] The angular dependence of THz radiation can be fitted by the following formula:[6]
E ( ) sin( )[1 (
tan( 2 ) 2 2cos( )sin( 2 ) ) ] tan( 2 ) sin( 2 ) cos( 2 )
(4)
Here, θ2 sin -1 sinθ n2 , while the refractive index of THz beam is 4.13 at 1 THz by THz time-domain transmission spectroscopy. The experimental data can be fitted well by Eq. 4, which is consistent to the phenomenon of THz radiation due to the surface field effect.
Figure 7. (a) Time-domain waveform of two opposite incident angles, -40° and +40°. (b) THz emission as a function of the incident angle with a fixed azimuthal angle and pump polarization. The experimental and fitting results are depicted with dot and solid curves, respectively.
Figure 8(a) and (b) demonstrate the polarization of THz wave (p-polarized
components EX and s-polarized components EY) with different polarization of pump beam (p-polarized and s-polarized components). Both of them are insensitive to polarization states of pump beam as the polarity and amplitude is almost the same under different polarization states of pump beam. However, the value of EX components is almost five times larger than that of EY components under the same polarization states of pump beam in the transmission configuration. When bulk WSe2 crystal is excited by the energy of 1.55 eV greater than the bandgap of sample (1.21 eV), surface build-in electric field can accelerate photo-excited carriers in the surface depletion region of sample. This transient current is usually parallel to the surface normal accelerated by the surface depletion field, which will introduce this polarization of THz radiation. Additionally, in the reflection configuration, we can also observe the amplitude of two components is similar under different polarization excitation, and two components show insensitive to the pump polarization states (Figure S3). However, the THz amplitude in the reflection configuration is slightly higher than that in the transmission configuration, which could result from the reabsorption in the transmission configuration and the reflection configuration is more sensitive to the surface state of WSe2.
Figure 8. Time domain waveforms of EX and EY components of WSe2 crystal under different polarization states of pump beam excitation: (a) under p-polarized pump laser beam irradiation; (b) under s-polarized pump laser beam irradiation. All the data are taken in the transmission configuration.
To further investigate the relation among polarization states of pump beam, the generated THz radiation dependence on the incident polarization angle α can be expressed as: [16] ETHz A cos B sin C cos(2 ) D sin(2 ) E
(5)
Here, A, B, C, D and E are fitting constants relating to the nonlinear optical coefficient d15, d22, d31 and d33. We calculated the nonlinear optical coefficient by the first-principle method (see Supporting Information), and the effective nonlinear optical coefficients dij were exhibited in Figure S4. The THz field is the cosine function of the pump polarization angle with a 2α period from Eq. 5. Meanwhile, we measured the EX and EY components of THz field by changing the pump polarization as shown in Figure 9(a) and (b), and the experimental data is consistent with the Eq. 5. In Figure 9(a) and (b), we can see that the amplitude of THz EX is relatively stronger than that of THz EY, which is consistent with that in Figure 8. Additionally, in the reflection configuration, the THz field is also a cosine function of the polarization angle with a 2α period (Figure S5(b) and (c)). Thus, the THz radiation exhibits 2-fold rotational symmetry with the change of α in the whole range.
Figure 9. (a) and (b) are X and Y components of THz electric field as a function of the incident polarization angle in the transmission configuration with a fixed azimuthal angle. The experimental and fitting results are depicted with dot and solid curves, respectively.
As discussed above, the dominant THz generation mechanism of the WSe2 is due to the acceleration of photo-excited carriers under surface depletion field. Mostly, a clean semiconductor surface may be occupied by surface states owing to the termination of lattice periodic arrangement at the surface. The interaction of unpaired electrons in the dangling bonds of surface forms an electronic state at the semiconductor band gap. Initially, in p-type semiconductors, the Fermi level of the bulk (EF-bulk) is lower than that of the surface (EF-surf。) (Figure 10(a)). To maintain equilibrium of the Fermi level inside and outside the semiconductors surface, electrons transfer from the bulk to the surface so that EF-bulk drops and EF-surf。rises until equilibrium achieved. Thus, the Fermi level is pinned by such surface state, forming a charge depletion region and thus a built-in surface electric field (Figure 10(b)).[58] We use ld denote the width of depletion layer, where the intensity of the built-in field are affected by both barrier potential and doping concentration. Usually, intrinsic WSe2 crystal is one kind of p-type semiconductors,[59] and the surface depletion field Ed could be described by:
Ed
eN D
ld
(5)
Here, e is electron charge. ND = 2.958×10l6 cm-3 denotes carrier concentration[60]. κ ε0 εr is absolute dielectric constant in which relative dielectric constant εr is
approximately 11.7, and ε0=8.85×10-14 F/cm is vacuum dielectric constant[61]. The width of depletion layer ld could be given by ld
2 (VD kT / e) . What’s more, the eN D
built-in barrier potential VD for the WSe2 is approximately 0.33V,[61] and the thermal potential kT/e is estimated as 0.026 V at room temperature. The width of depletion layer ld is approximate 115 nm, and the field strength of the surface depletion field Ed is 5.256×106 V∙m-1 by calculation. This calculation result shows that the surface depletion field strength of WSe2 are similar to that of InP crystal.[6] From this, we can infer that the surface field is strong enough to accelerate the photo-excited carriers and induce THz radiation.
Figure 10. Band diagram of the p-type bulk WSe2 crystal: (a) unequilibrium and (b) equilibrium between bulk and its surface state. The region between red dashed line and boundary is the surface depletion region, and ld is the width of this region.
5 Conclusions
In summary, we observed THz radiation from the WSe2 by THz time-domain emission spectroscopy in both transmission and reflection configurations. The THz radiation showed saturation with the incident pump fluence due to the screening effect of accumulation of photo-excited carriers. What’s more, we got the laser damage threshold of WSe2 crystal with the fluence of 3.11 mJ/cm2 by THz emission spectroscopy confirmed with Raman spectroscopy. Finally, we analyzed the formation of the surface depletion filed and calculate the field strength of 5.256×106 V∙m-1 and depletion layer width of 115 nm. Our results provide a fundamental result of the ultrafast laser interaction with the layered WSe2 and pave the way for the new kind of TMDs THz emitter based on surface field effect.
AUTHOR INFORMATION Corresponding Author * Fax: +86 29 88303336. E-mail: [email protected] (X. Xu)
ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (No.11774288), and Natural Science Foundation of Shaanxi Province (2017KCT-01).
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GA
Highlights
Ultrafast laser interaction with the layered semiconductors has attracted wide interest.
WSe2 was a typical stable TMDs which is a kind of advanced materials.
THz surface emission from the surface of layered semiconductor WSe 2 was first investigated.
The laser damage threshold of WSe2 was investigated.