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Abstract
The nonlinear absorption and ultrafast dynamic process of WS2 nanosheets were investigated by using a broadband (ranging from 450 to 700 nm) nanosecond Z-scan technique. Z-scan measurements showed that WS2 nanosheets exhibit saturable absorption (SA), and the magnitude of SA is wavelength-dependent. Besides, the ultrafast dynamics process of WS2 was also investigated with femtosecond transient absorption spectrum. It was found that there is a double-exponential energy relaxation in the process. The investigation shows that WS2 nanosheets can be used for ultrashort pulse generation and a wide spectral range optical absorber.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Inspired by the discovery of graphene, layered transition metal dichalcogenides (TMDCs) have engaged more and more attention due to their excellent electrical and optical properties [1–4]. For example, tungsten disulphide (WS2), tungsten diselenide (WSe2), molybdenum disulphide (MoS2) and molybdenum tungsten disulphide (Mo(1-x)WxS2) have been applied in photonic devices such as mode-locker, optical limiter and optical switcher [2,5–9]. Particularly, as potential material for broadband saturable absorber, WS2 has been studied widely. In 2014, Xiuli Fu et al. reported the nonlinear optical (NLO) properties of WS2 nanoplates measured by Z-scan technology excited by 532 nm, 25 ps pulsed laser with 10 Hz repetition rate. They found that the nanoplates have greatly saturable absorption but inappreciable nonlinear refraction, and they proposed that the mechanisms of NLO response are the bleaching of the ground-state plasma, and the effect usually comes from a strong one-photon absorption [10]. In 2015, Xin Zheng et al. studied the NLO properties of WS2 with Z-scan using 800 nm, repetition rate of 1 kHz, 100 fs pulsed laser. They measured the nonlinear refractive index of monolayer WS2, and found that at higher input pump intensities, nonlinear absorption switches from saturable absorption (SA) to reverse saturable absorption (RSA), which is as a result of two-photon absorption (TPA) [11]. In 2015, Saifeng Zhang et al. investigated the NLO properties of monolayer and few-layer WS2 using Z-scan technique with 515 nm, 800 nm, and 1030 nm, 340 fs pulsed laser. The effect of layer number on the nonlinear absorption of WS2 films has been systematically studied. The huge nonlinearity of WS2 is attributed to 2D constraint, large exciton effect and the band edge resonance of two-photon absorption [12]. In 2015, Ningning Dong et al. studied WS2 by using Z-scan technique with ns pulsed laser at 532 nm. They found that the WS2 exhibit SA and nonlinear scattering(NLS) [13]. In 2015, Hui Long et al. studied WS2 by using Z-scan technique with ns pulsed laser at 532 nm, 8 ns. They found that nonlinear optical absorption properties can be tuned over a wide range by controlling its size and thickness [14]. In 2016, Rongfei Wei et al. made WS2 comprised into PMMA, and studied the nonlinear absorption using Z-scan technology under the excitation of 130 fs pulses at 800 nm. They found that the sample shows excited-state absorption (ESA) or TPA [15]. In 2016, Ningning Dong et al. studied WS2 films using Z-scan measurements with 100 Hz, 340 fs laser pulses at 1040 nm. The results proved that these films exhibit TPA response. It was also found that these films show TPA properties under 1040 nm fs pulse excitation, and they found that the third-order nonlinear optical parameters decrease with the increase of sample thickness [16]. In 2018, Jun Wang et al. investigated the nonlinear absorption of monolayer WS2. They found the sample shows huge TPA which is 3-4 times larger than traditional semiconductor [17]. Furthermore, they also investigate some important effects such as the layer number dependent nonlinear absorption, the nonlinear refractive index of monolayer and the few-layer TMDCs in a wide wavelength range (Vis-NIR). The results show that the multilayer films have SA effect and the monolayer films have obvious TPA effect. The nonlinear refractive index of WS2 films was studied at 1040 nm under fs excitation. Dispersion of nonlinear refractive index exists in WS2 films, which changes from positive refractive index of monolayer to negative refractive index of bulk [18].
All researches of WS2 nanosheets above were carried out with picosecond and femtosecond laser pulses at the wavelength of 532 nm, or in infrared region (800 nm, 1040 nm and 1550nm). To our knowledge, only Ningning Dong et al. and Hui Long et al. studied the nonlinear properties of WS2 nanosheets by using 532 nm nanosecond pulse [13,14]. In fact, nonlinear properties and ultrafast dynamics process are strongly width-dependent and wavelength-dependent. Therefore, it is necessary to investigate the NLO properties with other wavelength and pulse-width.
In this paper, the nonlinear absorption properties of WS2 nanosheets were systematically studied by using broadband (450-700 nm) open aperture (OA) Z-scan technique. Besides, the ultrafast dynamics process of WS2 was also investigated with femtosecond transient absorption technique.
2. Sample and experiment
WS2 nanosheets were synthesized by ultrasonic exfoliation in water solution. Details of the process can be found in previous works [14]. The surface morphology of WS2 nanosheets were taken with scanning electron microscopy (SEM). UV-visible absorption spectrum of the sample was measured by using Ocean Optics USB 4000 spectrometer.
The nonlinear optical absorption was investigated via Z-scan technique [19] using a 6 ns Q-switched Nd:YAG pulsed laser with 10 Hz repetition rate and an optical parametric oscillator (Continuum, APE OPO) to generate tunable laser with various wavelengths. The linear transmittance of the sample solution is 75% at 500 nm. In Z-scan experiments, the laser beam waist diameter was obtained to be ${\omega _0} = 200\mu \textrm{m}$ by using blade method. The laser beam was focused on a 2 mm quartz cuvette filled with WS2 nanosheets water solution using a lens with a focal length of 20 cm. The cuvette was placed in a translation stage which was controlled by a computer. The stage can move step by step precisely and transmit laser pulses for each z point are recorded too.
Transient absorption spectrum excited by femtosecond pulse was used to studied the ultrafast dynamics process. Detailed information could be inquired in other Refs. [20–22]. The 8mw excitation pulse at 400 nm emitted from mode-locked Ti: Sapphire laser (Mira 900, Coherent) with 130 fs and repetition of 1 kHz penetrated a 1 mm thick BBO crystal and was focused on the quartz cuvettes. The probe pulses from the supercontinuum white light (450 nm-755 nm) were produced by the beam through the delay system go through a 2 mm thick Ti: sapphire plate. The signal of photodetector was processed with a lock-in amplifier.
3. Result and discussion
Figure 1(a), is the SEM image of WS2 nanosheets. From the SEM image we can find that the size of WS2 nanosheets were about 400 nm and the WS2 nanosheets were multilayer structure with 6–10 layers. Figure 1(b) shows the visible−near infrared (vis−NIR) linear absorption spectra of WS2 nanosheets. In the Fig, at 500 nm and 790 nm we can observed two peaks which result from A and B excitonic transitions of the TMDC due to the energy split of valence-band and spin-orbital coupling [23–28].
WS2 nanosheets were researched by OA Z-scan measurements under the wavelengths from 450 nm to 700 nm. In the following, we presented ten representative results at 450 nm, 475 nm, 500 nm, 525 nm, 550 nm, 600 nm, 625 nm, 650 nm, 675 nm, and 700 nm, respectively in Figs. 2(a-j).
Fig. 2. Normalized transmission as a function of WS2 nanosheets position for open aperture Z-scan at different wavelengths (a) 450 nm, (b) 475 nm, (c) 500 nm, (d) 525 nm, (e) 550 nm, (f) 600 nm, (g) 625 nm, (h) 650 nm, (i) 675 nm, and (j) 700 nm. The scatters are experimental data while the solid lines are theoretical fit.
We can observe from Fig. 2, the normalized transmittance of WS2 nanosheets increases as they approach the focal point (z = 0), which implied that WS2 nanosheets show a SA property. The transmittance varies nonlinearly with the input wavelength. As we can see from Fig. 2(g), the smallest normalized transmittance of WS2 nanosheets represent at 625 nm, and the largest one at 500 nm due to the effect of resonance enhancement. Other wavelengths in Fig. 2 were all within normal range.
In previous reports, when WS2 nanosheets were heated by laser irradiation, the water evaporated and solvent evaporation will produce many micro-bubbles around the WS2 nanosheets. These micro-bubbles then cause thermally induced nonlinear scattering (NLS) with the appearance of RSA, which influence the NLO properties of the sample [13]. In our case, the sample was excited under 6 ns and a low repetition rate of 10 Hz laser pulse, the peak intensity for Z-scan is only ${I_0}$=350uJ (3.5×1013 W/m2) which was weaker than that in their study. Especially, WS2 shows SA which implies that the thermal effect does not play a significant role.
In this case, the experimental data can be visualized as follows, when irradiance was moderate, correlated with the light absorption, large amount of WS2 were under the excited state, resulting in a small amount of the ground state which is called bleaching of ground state plasmon [10]. Therefore, the more light is transmitted in the Z-scan measurement, the lower ground state absorption of WS2 nanosheets occurs, and SA performed in WS2 nanosheets. We considered the SA of WS2 nanosheets to be induced by one photon absorption which is similar to that in other investigations [29]. The change of laser intensity with optical distance can be expressed as [19]:
In Eq. (1), $I$ represents the laser intensity, $\textrm{z}$ represents short optical distance, $\alpha$ is absorption coefficient which can be expressed as Eq. (2) when absorption is SA. where ${\alpha _0}$ is the linear absorption coefficient of WS2 nanosheets. ${I_s}$ is the saturable intensity. In OA Z-scan experiment, ${I_{}}$ can be denoted as:
Table 1. Nonlinear Optical Parameters of WS2 nanosheets
Figure 3, shows the dependence of the saturable strength on wavelength. As a contrast, linear absorption spectra were also supplied using solid lines. In the Fig, we can find that the saturable strength changes with similar trend to that of linear absorption, which means that strong linear absorption results in strong saturable strength.
Fig. 3. The solid lines are the linear absorption spectra. The scatters are theoretical fit Saturable strength ${I_s}$ of WS2 vs. wavelength.
The ultrafast dynamics process of WS2 nanosheets was studied using pump-probe experiments from which femtosecond transient absorption spectra were obtained and shown in Fig. 4(a). Representative transient absorption spectra at different delay times (0 ps, 5 ps, 20 ps, 50 ps, 100 ps) were shown in Fig. 4(b). The signal of 0 ps delay time was reference signal obtained when the WS2 nanosheets have not been excited. The exciton bleaching peak of WS2 appeared nearby 560 nm arising from the bleaching of ground state plasma. The dynamic curve in Fig. 4(g) shows that bleaching signals at 550 nm decays 22% in 20 ps. With the increase of delay time, the signal decays slowly. And the phenomenon is observed as SA in Z-scan experiment [11,12].
Fig. 4. (a) Time and wavelength resolved transient absorption data of WS2 nanosheets. (b) Transient absorption spectra for WS2 nanosheets at different delay time. (c-l) Normalized dynamics curves for WS2 nanosheets at different wavelength, respectively (The scatters are experimental data while the solid lines are theoretical fit generated).
In Figs. 4(c-l) the dynamics curves at ten different probe wavelengths were shown. We can find that all of the ten curves show rapidly ascend and decay processes. All transient absorption signals indicated extremely rapid rise because of electron excitation, then decayed at alternative time regions attributed to electron-electron (e-e) and electron-phonon (e-ph) coupling. The large amount of the WS2 were pumped to the excited state, resulting in a small number of populations at the ground state, this brought out the bleaching in the ground-state absorption band. Initially, optical pulse created electron-hole pairs at the excitation photon energy. Subsequently, the decay process involving a fast and a slow component happen. Because of limitations in resolution and response time of our measurements, the rapid ascend process and initial relaxation cannot be identified accurately [34,35]. Then, process of slower dynamics was taken into account. The transient changes of the optical response at ultrashort time regions less than 10 ps are due to the photo-excited carriers and their following non-radiative decay via Auger processes [30–33]. In other researchers’ investigations, at delay time above 100 ps, a finite carrier population is also appeared in the system, as detected in time-resolved photo luminescence experiments [36,37]. At later time-scales, thermal effects will be the primary effect in pump-probe absorption-type measurements. Phonon dynamics can be explained largely as the electron–phonon process. The following slow decay process of the lattice when heated and cooled to the surrounding medium is due to phonon–phonon interaction [34,35,38,39]. Mechanism for the photo-excited carriers in TMDC multilayers is generally nonradiative. It is logical to say that the energy of the pump pulse is mainly metastasized from the carrier to the phonon system [30,40,41].
Generally speaking, the two-temperature model (TTM) is constantly use to analyze the process if only one decay occurs. In the present cases, the photodynamic curves can be fitted through double-exponential functions with two decay process as shown in Eq. (5):
Table 2. Fitting results for the decay processes for WS2 nanosheets at different wavelengths.
Figure 5 represent the decay constant (${\tau _1}$, ${\tau _2}$) versus change plotted of probe laser wavelength which result according to Table 2. The time constants of two decay parts ${\tau _1}$ and ${\tau _2}$ were represent in this picture separately. We can find that the trend of the two values were almost the same with the change of wavelength.
Fig. 5. The decay constant (${\tau _1},\, {\tau _1}$) versus change plotted as a function of probe laser wavelength.
4. Conclusion
In summary, the NLO properties of WS2 nanosheets was studied by using broadband OA z-scan technique. The experiment results indicated that the NLO absorption of WS2 nanosheets was susceptibly wavelength-dependent. Besides, the response time is important in applications. Ultrafast dynamics process of WS2 nanosheets was investigated via femtosecond time-resolved transient absorption measurements. The experiment results show that the relaxation processes involve two decay process of time scales, a double-exponential energy relaxation with a fast decay (the coupling of electron-phonon) and a slow decay component (the coupling of phonon-phonon) was found, and a strong dependence on the wavelength was observed. These nonlinear optical properties of WS2 nanosheets is indicative of the potential applications for broadband saturable absorber in photonic devices.
Funding
Natural Science Foundation of Heilongjiang Province (F2018027); Science and Technology Project of Heilongjiang Education Department (11531283, 12511424); Heilongjiang University Graduate Innovation Fund (YJSCX2018-015HLJU); East University of Heilongjiang Scientific Research Fund (HDFHX170115, HDFKY190106).
Acknowledgment
The authors would like to thank Shuang Chen for significant discussions and assistance input in experiment. The authors also acknowledge the technical support from Prof. Yachen Gao.
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