Article Cite This: ACS Photonics 2018, 5, 1558−1565
Saturation of Two-Photon Absorption in Layered Transition Metal Dichalcogenides: Experiment and Theory Ningning Dong,†,‡ Yuanxin Li,†,‡ Saifeng Zhang,†,‡ Niall McEvoy,*,§ Riley Gatensby,§ Georg S. Duesberg,§,⊥ and Jun Wang*,†,‡,∥
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Key Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § Advanced Materials and BioEngineering Research (AMBER) Centre and School of Chemistry, Trinity College Dublin, Dublin 2, Ireland ∥ State Key Laboratory of High Field Laser Physics, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China ⊥ Institute of Physics, EIT 2, Faculty of Electrical Engineering and Information Technology, Universität der Bundeswehr München, Werner-Heisenberg-Weg 39, 85577 Neubiberg, Germany S Supporting Information *
ABSTRACT: The saturation of two-photon absorption (TPA) in four types of layered transition metal dichalcogenides (TMDCs) (MoS2, WS2, MoSe2, WSe2) was systemically studied both experimentally and theoretically. It was demonstrated that the TPA coefficient is decreased when either the incident pulse intensity or the thickness of the TMDC nanofilms increases, while TPA saturation intensity has the opposite behavior, under the excitation of 1.2 eV photons with a pulse width of 350 fs. A three-level excitonic dynamics simulation indicates that the fast relaxation of the excitonic dark states, the exciton−exciton annihilation, and the depletion of electrons in the ground state contribute significantly to TPA saturation in TMDC nanofilms. Large third-order nonlinear optical responses make these layered 2D semiconductors strong candidate materials for optical modulation and other photonic applications. KEYWORDS: two-photon absorption, saturable absorption, transition metal dichalcogenides, 2D semiconductor, optical modulation, nonlinear optics
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understood, and the characteristics and magnitudes of this third-order NLO response in various TMDCs with different thicknesses have not been studied in detail. On the other hand, few-layer TMDCs can act as saturable absorbers for Q-switched or mode-locked laser pulse generation above 1 μm, which is below the band gap of the TMDCs, the physical mechanism of which is still vague and the TPA saturation could be a rational explanation. In this work, we present an experimental and theoretical investigation of frequency-degenerate TPA in four TMDC nanofilms (MX2 = MoS2, WS2, MoSe2, WSe2). Our findings show that the TPA saturation process in these 2D semiconductors is similar, but depends significantly on the thickness of the TMDCs. The saturation of photon-excited carriers and depletion of the electron population in the ground state of these 2D semiconductors contributes to the saturation of the TPA. Additionally, it is theoretically demonstrated that
wo-dimensional (2D) layered transition metal dichalcogenides (TMDCs) exhibit unique nonlinear optical (NLO) features including layer-dependent second/thirdharmonic generation,1−3 ultrafast saturable absorption,4−6 optical limiting,7 etc., which have led to these nanomaterials being touted for intriguing and promising applications in the 2D photonic and photoelectric fields.8 Recently, two-photon effects9 in MoS2 and WS2 have also attracted considerable research interest. Ye et al. used two-photon absorption (TPA)induced luminescence spectroscopy to reveal the excitonic dark states in monolayer WS2.10 Our previous studies have demonstrated the large degenerate TPA coefficient in both monolayer and few-layer MoS2 and WS2.11,12 In addition, the saturation of the TPA process has been experimentally observed,11 which is one of the fundamental, yet most important, NLO mechanisms in TMDCs if these 2D semiconductors are to be considered as candidate materials for next-generation photonic and optoelectronic devices. However, TPA saturation in TMDCs is not yet fully © 2018 American Chemical Society
Received: January 3, 2018 Published: February 21, 2018 1558
DOI: 10.1021/acsphotonics.8b00010 ACS Photonics 2018, 5, 1558−1565
Article
ACS Photonics Table 1. Fitted Parameters of TPA Saturation in MX2 Nanofilms Using Model_homo sample
thickness (nm)
β0 (×102cm/GW)
Isat (GW/cm2)
T0 (%)
sample
thickness (nm)
β0 (×102cm/GW)
MoS2
1.5 ± 0.75 5.5 ± 0.75 18.7 ± 0.70 50.0 ± 0.75 0.65 ± 0.65 1.4 ± 0.70 5.8 ± 0.64
42.23 ± 0.62 10.65 ± 0.62 4.85 ± 0.11 4.99 ± 0.02 69.06 ± 0.39 42.87 ± 0.58 27.55 ± 1.33
41.56 ± 0.20 39.88 ± 0.45 52.61 ± 1.09 54.15 ± 0.22 35.06 ± 0.74 133.85 ± 3.00 94.68 ± 2.04
91.26 90.09 76.27 47.79 92.02 91.25 62.67
WS2
1.20 ± 0.65 7.5 ± 0.75 14.0 ± 0.70 30.0 ± 0.75 0.8 ± 0.65 1.6 ± 0.75 5.6 ± 0.80
40.85 ± 0.67 7.65 ± 0.44 7.88 ± 0.18 8.57 ± 0.20 71.02 ± 0.45 30.21 ± 0.47 23.59 ± 2.01
MoSe2
WSe2
Isat (GW/cm2)
T0 (%)
± ± ± ± ± ± ±
91.51 89.56 66.63 45.16 92.07 89.42 43.50
43.24 44.62 58.37 61.61 33.11 36.50 74.18
1.70 1.38 1.45 0.22 0.68 2.05 8.70
Figure 1. Nonlinear transmittance versus incident pulse peak irradiance for MX2 nanofilms. The solid lines are the fitting results obtained by numerically solving eq 6.
fs at 1030 nm (ℏω = 1.2 eV), generated from a mode-locked fiber laser, were used to excite the MX2 at a repetition rate of 100 Hz. The beam was focused down to a waist radius of ∼8.5 μm on the surface of MX2 by an f/10 cm lens. The pulse energy was tuned by a Glan-Taylor prism, allowing focused intensities to change from a few GW/cm2 to ∼200 GW/cm2. The reference beam from the fiber laser and the transmitted signal through the MX2 were detected by two high-precision photodetectors. The variation of the transmittance versus the incident intensities was recorded, which reflects the NLO response of the MX2. The NLO measurements were carried out on random spots of the samples and repeated at least three times for each spot to confirm that this point was not damaged (Figure S2 in SI). In addition, we carried out the NLO experiments at 10 Hz, 100 Hz, and 1 kHz pulse repetition and found the NLO responses had little difference among three excitation conditions (see Figure S4), which demonstrated the possible femtosecond laser heating has little effect on the nonlinear absorption. Figure 1 shows the normalized nonlinear transmittance (NLT) for four types of MX2 with different thickness. The solid circles with error bars show the experimental data, and the solid lines are the fitting curves obtained using the method discussed in the following text. All of the samples, except for 1.40 ± 0.70 and 5.8 ± 0.64 nm MoSe2, show very interesting NLT variation where it first decreases in the low-intensity region (the first half) and then increases after a certain intensity (the second
TMDCs can be promising candidates for optical modulation if the device is appropriately designed.
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RESULTS AND DISCUSSION Experimental Section. In this study, MX2 samples of high quality were prepared by a vapor-phase growth method,12−15 whereby transition metal films were first sputtered onto quartz substrates followed by sulfurization (or selenization) in a custom-built tube furnace. In this way MX2 films of different thickness can be produced, e.g., 1.5 ± 0.75, 5.5 ± 0.75, 18.7 ± 0.70, and 50.0 ± 0.75 nm for MoS2. The thickness was determined by cross-sectional transmittance electron microscopy for thin (
