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Synthetic two-dimensional transition metal dichalcogenides such as tungsten disulphide (WS2), tungsten diselenide (WSe2), molybdenum disulphide (MoS2) as well as mixed molybdenum tungsten disulphide (Mo0.5W0.5S2) single crystals were grown by the chemical vapor transport method using halogens (bromine or chlorine) as transport agents. Multi-layer samples were cleaved from the single crystals, and their nonlinear optical (NLO) properties were obtained from both open aperture and closed aperture Z-scan measurements using a picosecond mode-locked Nd:YAG laser operating at a wavelength of 1064 nm, with pulse duration of 25 ps and 20 Hz repetition rate. Both WS2 and MoS2 exhibited nonlinear saturable absorption (SA), whereas WSe2 and Mo0.5W0.5S2 showed nonlinear two-photon absorption (2PA). A large 2PA coefficient β as high as + 1.91x10−8 cm/W was obtained for the Mo0.5W0.5S2, and an index of refraction coefficient γ = −2.47x10−9 cm2/W was obtained for the WSe2 sample.
© 2016 Optical Society of America
Corrections
S. Bikorimana, P. Lama, A. Walser, R. Dorsinville, S. Anghel, A. Mitioglu, A. Micu, and L. Kulyuk, "Nonlinear optical responses in two-dimensional transition metal dichalcogenide multilayer: WS2, WSe2, MoS2 and Mo0.5W0.5S2: publisher’s note," Opt. Express 24, 26998-26998 (2016)https://opg.optica.org/oe/abstract.cfm?uri=oe-24-23-26998
3 October 2016: Corrections were made to the author affiliations and abstract.
8 November 2016: Corrections were made to the funding section.
1. Introduction
Two-dimensional materials like transition metal dichalcogenides (TMD) such as, tungsten disulphide (WS2), tungsten diselenide (WSe2), molybdenum disulphide (MoS2) and molybdenum tungsten disulphide (Mo(1-x)WxS2) have great potential as an emerging new generation of photonic nanomaterials. The use of these two dimensional materials for optical switching, Q-switching, mode-locking, optical limiting, nanoelectronics and micromechanics has produced devices with high performance and unique functions [1–6]. Strong inplane bonds and weak van der Waals forces hold the layers together in the TDM crystals. Due to the weak interacting van der Waals like forces that keep the layers together, a single crystal of a few layers (or a monolayer) can be obtained by a micromechanical cleaving process, which results in changing the bulk TMD materials from an indirect band-gap to a direct band-gap semiconductor material [7–11]. Extensive studies have revealed that for certain applications, the unique electron, optical and mechanical properties of the TMD materials are better suited than those of graphene [12–18].
Several studies based on inplane carrier mobility, optical absorption, photoconductivity, and the photoluminescence of bulk and nanosheets of TMDs have shown that TMDs are potentially great materials for use in the fabrication of nonlinear and optoelectronic devices [19–21]. It is therefore imperative that the nonlinear properties of TMD materials be carefully investigated in order to learn how they can be effectively used in the development of the next generation of nonlinear devices, applications and techniques.
Efforts to study the nonlinear properties of TMDs have focused primarily on monolayer samples. For example, the nonlinear second order susceptibility of a TMD MoS2 monolayer has been used as a direct probe to study its electronic and structural dynamics. In the way of devices, monolayers of MoS2 have exhibited very large second order susceptibility, which can be used to characterize the ultrafast optical pulse from an ultrafast light source [22]. In addition, the ultrafast nonlinear absorption properties of MoS2 have led to the development of MoS2-based optical fiber saturable absorber devices for mode-locking applications [23,24]. Similarly, monolayers of WS2 have shown a shift from saturable absorption to reverse saturable absorption at high excitation intensities [25], and have been used to make a micro fiber-based WS2-film saturable absorber [26,27] for Q-switching and mode-locking applications in fiber lasers [28].
The nonlinear third-order response in TMD materials leads to processes such as third harmonic generation and two-photon absorption, but more importantly to an intensity-dependent refractive index, which is the basis of most nonlinear optical phenomena in optical switching devices. Currently, there has not been enough study of the nonlinear optical responses in multilayer TMD materials, specifically, their intensity-dependent nonlinear refractive indices.
In this paper, we study the nonlinear absorption (β) and nonlinear refractive index (γ) in multilayer TMD materials such as, tungsten disulphide (WS2), tungsten diselenide (WSe2), and molybdenum disulphide (MoS2) as well as mixed TDM materials, like molybdenum tungsten disulphide (Mo0.5W0.5S2), which exhibits a large two-photon absorption (2PA) coefficient β as high as + 1.91x10−8 cm/W. Moreover, the nonlinear index of refraction coefficient γ = −2.47x10−9 cm2/W was obtained for the WSe2 sample. The multilayer TMD samples, WS2 and MoS2, exhibited nonlinear saturable absorption, whereas WSe2 and Mo0.5W0.5S2 showed nonlinear two-photon absorption. These values were determined from Z-scan measurements using a pico-second Q-switched Nd: YAG laser operating at a wavelength of 1064 nm, well outside the direct linear absorption band of the TMD samples, with pulse duration of 25 ps and 20 Hz repetition rate.
2. Experimental
2.1 Material and sample preparation
Tungsten disulfide (WS2), tungsten diselenide (WSe2), molybdenum disulfide (MoS2) and mixed molybdenum tungsten disulphide (Mo0.5W0.5S2) single crystal samples were grown by the chemical vapor transport method using halogens (bromine or chlorine) as transport agents. As starting materials, Mo, W and S/Se were used. The chemical elements were allowed to react in an evacuated sealed quartz ampoule of 8 mm inner diameter and 120 mm length in order to avoid the increase of the amount of material transported per unit time throughout the growth. The tubes were slowly heated up to the synthesis temperature of 1000°C for two days and maintained under these conditions for two more days. Halogen molecules were used as a transport agent in the concentration of 5 mg/cm3. Subsequently, the ampoules with the polycrystalline material were placed into a two-zone tube furnace and provided with the appropriate temperature profile. Depending on the TX2 system, the crystallization chamber temperature at the charge zone and growth zone was set at values according to reference [29,30]. The ampoules were held inside the furnace for a period of up to 6 days, after which they were slowly cooled to room temperature. The temperature inside the tube furnace was precisely controlled in order to prevent its value from oscillating.
Due to weak interacting van der Waals forces existing between the monolayers of the single crystals, the multilayer samples were easily cleaved to obtain smooth and clean surfaces. In this way, no chemical etching or mechanical polishing was required. The thickness of each crystal was of an order of a few tens of microns with diameters varying from 1 to 3 mm. The chlorine-transported crystals obtained were very thin compared to the bromine-transported crystals. The WS2 and WSe2 single crystals had a bright bluish and a greenish, reflective metallic look to them, respectively. Single and few layer flakes of tungsten disulfide (WS2) have been obtained by mechanical exfoliation of bulk 2H-WS2 (the hexagonal 2H polytype of tungsten disulfide) single crystals grown using chemical vapor transport with bromine as the transport agent. Samples obtained in this way are naturally n type. Hall measurements on bulk crystals reveal an electron density ne ≈1016 cm−3 at room temperature, which decreases rapidly at lower temperatures and is only ≈1012 cm−3 at T = 140 K. All samples were transferred and attached to one inch diameter glass substrates (Edmund Optics: glass window 02-105) for material and Z- scan characterization measurements.
2.2 Z-scan experimental setup
The study of the nonlinear optical response of the TMD multilayer samples was quantantively and analytically carried out by using the open aperture (OA) and closed aperture (CA) Z-scan technique [31, 32]. The Z-scan technique was performed with the experimental setup depicted in Fig. 1. The Z-scan experimental setup system mainly consists of a picosecond Nd: YAG laser operating at a wavelength of 1064 nm, with pulse duration of 25 ps and 20 Hz repetition rate. The incident laser power was controlled by rotating a half-wave plate placed between two polarizers (polarizer and analyzer). Carbon disulfide (CS2) solution contained in a cuvette of 1mm thickness was utilized as a reference standard sample to accurately calibrate the Z-scan experimental setup. The CS2 reference sample was replaced by the TMD multilayer samples in order to perform the OA and CA Z-scan measurements. The reference beam and transmitted beam intensities, before and after the sample, were monitored by two photodiode detectors, D1 and D2, respectively, as the sample moved through the focal point located at 20 cm of lens 1 along the laser beam propagation direction. The optical nonlinearities in the sample manifested when the incident intensity of the focused laser beam was intense enough, and the sample changed its transmittance as it passed through the lens 1 focus.
In order to perform the OA Z-scan measurement, the aperture was fully kept open resulting in the aperture linear transmittance (S = 1) [31, 32]. During the OA Z-scan measurement, the transmitted laser beam through the sample was collected and collimated by the lens 2 on to the detector D2. For the CA Z-scan measurement, the aperture was closed with a small opening of 1.5mm diameter (S≈0.1), and the laser beam after the aperture was focused onto the detector D2 as well. During the course of the OA Z-scan measurement, the transmitted laser intensity arriving onto the detector D2 was mainly sensitive to the nonlinear absorption occurring in the sample. While, the CA Z-scan measurement was subject to beam distortion due to nonlinear refraction as well as to nonlinear absorption. The pure nonlinear refraction Z-scan data after removing the nonlinear absorption contribution was obtained through dividing the CA Z-scan measurement data by the corresponding OA Z-scan measurement data (i.e.CA/OA Z-scans). The normalized transmittance OA Z-scan measurements were used to determine the nonlinear absorption coefficient, β, while the nonlinear refraction γ coefficient for each sample was obtained from the division of the normalized transmittance CA/OA Z-scan measurements. All the Z-scan measurements were carried out by changing the sample position from z = −20 mm to z = + 20 mm.
3. Results and discussion
The TMD multilayer sample thicknesses for WS2, WSe2, MoS2 and Mo0.5W0.5S2 were 20, 22, 25, and 22 µm, respectively, which implies that they are layered bulk indirect band-gap TMD materials. The cross section and surface SEM images of the TMD samples are illustrated in Figs. 2(a)-2(d) and Figs. 2(e)-2(h), respectively. Figures 3(a)-3(d) and Figs. 3(e)-3(h) show the microscopic images and photographs of the TMD samples transferred onto the glass substrates. The optical absorbance of each sample was measured with a double beam UV/VIS/NIR (Perkin Elmer Lambda 19) spectrometer. The linear absorbance at 1064 nm excitation wavelength or energy of 1.1652 eV for WS2, WSe2, MoS2 and Mo0.5W0.5S2 was 0.2543, 0.1551, 0.5032, and 0.0622, respectively. Figure 4 shows the absorption spectra of the TMD multilayer samples, WS2, WSe2, MoS2 and Mo0.5W0.5S2, at room temperature, which are in good agreement with the ones obtained by [33–36] in similar samples at room temperature.
Fig. 2 (a-d) Illustrate the SEM images of the cross sections of the multilayer WS2, WSe2, MoS2 and Mo0.5W0.5S2 samples, with layers that are 20, 22, 25, and 22 μm thick, respectively. (e-h) show the SEM images of the top surface of each of the TMD multilayer samples.
Fig. 3 (a-d) Show a microscopic view of the interface between the substrate and each of the multilayer TDM samples WS2, WSe2, MoS2 and Mo0.5W0.5S2. (e-h) show photographs of the multilayer WS2, WSe2, MoS2 and Mo0.5W0.5S2 samples transferred on to the glass substrates in preparation for Z-scan characterization.
Fig. 4 Absorption spectra of the glass substrate and TMD multilayer samples (WS2, WSe2, MoS2 and Mo0.5W0.5S2).
In order to confirm that the measured optical nonlinear response came from the actual samples, the Z-scan measurements were also performed on the glass substrate, and no obvious contribution from the glass substrate was observed at the excitation irradiance levels. The WS2, WSe2, MoS2 and Mo0.5W0.5S2 samples were excited at 1064 nm wavelength with incident irradiance at focus of around 0.023 GW/cm2, 0.085 GW/cm2, 0.0566 GW/cm2 and 0.0085 GW/cm2, respectively. The complex third-order nonlinear susceptibility χ(3), Eq. (1), of each TMD multilayer sample was obtained from the nonlinear refraction coefficient γ, and nonlinear absorption coefficient β, which are related to its real part, χR(3), and imaginary part, χIm(3), given by Eq. (2) and Eq. (3), respectively.
The normalized transmittance OA Z-scan measurement was used to determine the nonlinear absorption coefficient β from the theoretical curve fitting obtained with Eq. (4) [31, 32].
where ω is the optical frequency, no is linear index of refraction, and c is the speed of light in vacuum.where β is the nonlinear absorption coefficient, which can be assigned a positive or negative sign [32, 37, 38], Io is the on-axis irradiance of the laser beam at focus(i.e. z = 0), Leff is the effective length of the sample with thickness L, Leff = (1-eαL)/α, αL is the linear absorbance, α is the linear absorption coefficient, z is the sample position, zo = πωo2/λ is the Rayleigh diffraction length of the laser beam, λ is the laser center wavelength and ωo is the focused laser beam waist radius, all in free space, and the parameter S is the aperture linear transmittance, S = 1-exp(−2ra2/wa2), with ra and wa stand for the aperture and beam radius at the aperture, respectively.Typically, only the first few terms are needed for numerical evaluation, if the series converges. Thus, the highest m-order of three was used for the theoretical curve fitting with Eq. (4). Nonlinear saturable absorption (SA), with negative β value was observed in the TMD multilayer samples of WS2, and MoS2, Figs. 5(a) and 5(b), respectively, whereas nonlinear two-photon absorption (2PA), with positive β value, was observed in WSe2 and Mo0.5W0.5S2 multilayer samples, Figs. 5(e) and 5(f), respectively,(see Table 1.). Dividing the normalized transmittance CA(S<1) by OA (S = 1) Z-scan measurements removes the nonlinear absorption component from the normalized transmittance CA Z-scan measurement which results in leaving only the component related to the nonlinear refractive index of the sample.
Fig. 5 Illustrates the normalized transmittance Z-scan measurements (dotted lines) and the theoretical results (solid lines) from the TMD multilayer samples of WS2, MoS2,WSe2, and Mo0.5W0.5S2 with an incident irradiance at the focus of around 0.023 GW/cm2, 0.0566 GW/cm2, 0.085 GW/cm2 and 0.0085 GW/cm2, respectively. (a) and (b) are open aperture (OA) data showing saturable absorption (SA) in WS2, and MoS2, respectively. (e) and (f) are open aperture (OA) data showing two-photon absorption (2PA) in WSe2, and Mo0.5W0.5S2. (c) and (d) are the division of closed aperture (CA) by open aperture (OA) data showing self-focusing in WS2 and MoS2, respectively. (g) and (f) are the division of closed aperture (CA) by open aperture(OA) data showing self-defocusing in WSe2 and Mo0.5W0.5S2, respectively.
Table 1. Nonlinear Absorption and Nonlinear Refraction Coefficients of the TMD Multilayer Samples at 1064nm Excitation Wavelength
The nonlinear refraction index (γ) was obtained from a theoretical curve fit of Eq. (5) to the measured normalized transmittance where the closed aperture Z-scan measurement is divided by the open aperture measurement. In Eq. (5) ΔΦo = kγIoLeff is the phase distortion at the exit surface of the sample due to the nonlinear refraction response within the sample [31, 32], k = 2π/λ is the propagation wave vector, and λ is the laser center wavelength.
The γ coefficient is the intensity-dependent nonlinear refractive index coefficient, which changes at high optical intensities due to the Kerr effect within the medium. Furthermore, the nonlinear refractive index change can arise from the contribution of both bound and free electrons [38].
The NLO response values listed in Table 1 were obtained from the WS2, WSe2, MoS2 and Mo0.5W0.5S2 samples excited at 1064 nm wavelength with incident irradiances at the focus of around 0.023 GW/cm2, 0.085 GW/cm2, 0.0566 GW/cm2 and 0.0085 GW/cm2, respectively.
The sign and magnitude of the nonlinear refraction coefficient γ was directly deduced from the peak-valley sequence of the normalized transmittance CA/OA Z-scan curves [31, 32]. The negative sign of the nonlinear refraction coefficient γ is defined by a sequence of a peak followed by a valley, while the positive one is due to a valley-peak sequence in the Z-scan trend. Positive nonlinear refraction response with positive γ value was observed and obtained in the TMD multilayer samples of WS2, and MoS2, Figs. 5(c) and 5(d),whereas negative nonlinear refraction response with negative γ value was observed in WSe2 and Mo0.5W0.5S2 multilayer samples, Figs. 5(g) and 5(h), respectively.
The nonlinear absorption in the TMD samples is typically due to saturable absorption, reverse saturable absorption (RSA) and two-photon absorption, where both RSA and 2PA cause an increase in absorption while the SA leads to a decrease in absorption of the samples with an increase in the irradiance intensity levels. The large nonlinear optical (NLO) responses obtained in the TMD multilayer samples are generally linked to the exciton effect, two-dimensional confinement and the band edge resonance of 2PA [1, 25]. The beam excitation photon energy of 1.1652 eV (i.e. 1064nm) utilized in the measurements is less than the indirect band-gap energy of the multilayer TMD samples (see Fig. 4), which implies that a nonlinear 2PA response should arise in each of the four samples under investigation, where the bulk indirect band-gap energy of WS2, MoS2, WSe2 and Mo0.5W0.5S2 were determined to be around 1.35, 1.23, 1.2, and 1.4 eV, respectively [9, 36]. However, for the multilayer WS2 and MoS2 samples no 2PA was observed, rather they exhibited nonlinear SA responses, which should normally occur when the excitation single-photon energy is greater than or equal to the band-gap energy of the TMD materials. Normally, the nonlinear SA response is caused by the free-carrier excitation from valence to conduction band because of the Pauli-blocking effect. The nonlinear SA behavior observed in the multilayer WS2 and MoS2 samples suggests that single-photon absorption takes place at the excitation single-photon energy of 1.1652 eV, which could be due to sub-bandgap absorption or a shift of the single-photon resonance caused by edge states and induced defect states in the TMD materials [4, 5, 27]. In the case of the WS2 TDM sample, at low incident irradiance levels the nonlinear absorption is predominately SA, as shown in Fig. 6(a). As the irradiance level is increased (around 0.0862 GW/cm2), reverse saturable absorption (RSA) becomes the dominant effect, similar to what is observed in a monolayer WS2, sample [25]. The change from SA to RSA is usually attributed to the excited state absorption process [39, 40].
Fig. 6 Shows the normalized transmittance open aperture measurements and corresponding nonlinear absorption coefficients of the TMD multilayer samples of WS2, MoS2, WSe2, and Mo0.5W0.5S2 at different incident irradiance levels, I (GW/cm 2). (a, b) and (c, d) are the open aperture (OA) results and nonlinear absorption coefficients, β (cm/GW), showing saturable absorption (SA) in WS2, and MoS2, respectively. (a) WS2 shows reverse saturable absorption (RSA) above 0.2071 GW/cm2. (e, f) and (g, h) are the open aperture (OA) results and nonlinear absorption coefficients, β (cm/GW), showing two-photon absorption (2PA) in WSe2, and Mo0.5W0.5S2, respectively. The fitting curves (solid lines) of the OA Z-scan measurements in (a, c, e, and g) and the β (cm/GW) values shown in (b, d, f, and h) were obtained using Eq. (4).
For the TDM samples WSe2 and Mo0.5W0.5S2 the 2PA nonlinear response was observed as expected for the below band-gap excitation energy of 1.1652 eV. For Mo0.5W0.5S2 sample we observed an enhancement in the 2PA response that can be attributed to its single-photon absorption of around 1.4 eV being close to the two-photon excitation energy of 2.3304 eV (i.e., 532 nm) needed for the effect. As an alloy material, Mo(1-x) W(x) S2, one can engineer its band-gap energy in order to obtain even stronger NLO responses (e.g., 2PA enhancement) for different optical or photonic applications [41].
The large nonlinear refraction coefficient γ observed in the multilayer WSe2 is attributed to enhancement via its single-photon resonance at 1.2 eV, which is the nearest to the excitation photon energy of 1.1652 eV among the other studied samples (see Fig. 4). Note that the nonlinear refraction coefficient γ attained in multilayer WSe2 is about two orders of magnitude smaller than that obtained in a few layer graphene sample excited at 800 nm wavelength (i.e., 1.55 eV) [42]. However, the obtained value for γ is more than one order of magnitude of the value obtained in the topological insulator TI: Bi2Se3 that has a γ almost six orders of magnitude larger than that obtained in most bulk dielectrics [43].
The irradiance dependence of the nonlinear absorption responses from the TMD multilayer samples of WS2, MoS2, WSe2, and Mo0.5W0.5S2 were performed using the OA Z-scan at different incident irradiances, Fig. 6. The WS2 sample exhibited a change from the SA to RSA process when the incident irradiance was increased to 0.0862 GW/cm2, Fig. 6 (a), and its nonlinear absorption coefficient, β (cm/GW), decreased and remained nearly constant at irradiance above 0.2 GW/cm2, Fig. 6 (b). The MoS2 showed the SA response at all incident irradiance levels, Fig. 6 (c), where its β (cm/GW) decreased and remained nearly constant above the incident irradiance of 0.43 GW/cm2, Fig. 6 (d).
The WSe2, and Mo0.5W0.5S2 samples both showed 2PA at different irradiance levels, Fig. 6 (e) and Fig. 6 (g), where their corresponding nonlinear absorption coefficients decreased and remained nearly constant as the incident irradiance was increased above 0.2 GW/cm2, Fig. 6 (f) and Fig. 6 (h), respectively. The largest nonlinear absorption coefficient of 1.91 ± 0.78x10−8 cm/W was obtained for the Mo0.5W0.5S2 multilayer sample at the incident irradiance of 0.0023 GW/cm2.
The corresponding damage thresholds of the WS2, MoS2, WSe2 and Mo0.5W0.5S2 multilayer samples were about 0.55, 0.86, 0.75, 0.46 GW/ cm2, respectively.
4. Conclusion
The Z-scan measurements at 1064 nm excitation wavelength from a picosecond Q-switched Nd: YAG laser were performed in order to investigate the nonlinear absorption and non-linear refractive index in multilayer of synthetic two-dimensional transition metal dichalcogenide samples: tungsten disulphide (WS2), tungsten diselenide (WSe2), molybdenum disulphide (MoS2), as well as mixed TDM materials, such as Mo0.5W0.5S2 . A large two-photon absorption coefficient β and large index of refraction coefficient γ are obtained for the Mo0.5W0.5S2 and WSe2 samples, respectively. Both Mo0.5W0.5S2 and WSe2 samples showed nonlinear two-photon absorption, on the other hand WS2 and MoS2 samples exhibited nonlinear saturable absorption. The measured nonlinear optical properties from the TMD multilayer samples show great promise in their use as the building blocks for the next generation of photonic nanomaterials used in the development of high performance optical switching, Q-switching, mode-locking, optical limiting and optoelectronic devices.
Funding
Our work was partially supported by Corning Incorporated. Moreover, S. Anghel, A. Mitioglu, A. Micu, and L. Kulyuk acknowledge financial support provided by the Russian Science Foundation (Agreement No.14-12-01080).
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