Stable -switched and mode-locked erbium-doped fiber lasers (EDFLs) are first demonstrated by using the novel layered palladium disulfide (), a new member of group 10 transition metal dichalcogenides (TMDs)-based saturable absorbers (SAs). Self-started -switched operation at 1567 nm was achieved with a threshold pump power of 50.6 mW. The modulation ranges of pulse duration and repetition rate were characterized as 12.6–4.5 μs and 17.2–26.0 kHz, respectively. Meanwhile, a mode-locked EDFL was also obtained with a pump power threshold of 106.4 mW. The achieved pulse duration is 803 fs, corresponding to a center wavelength of 1565.8 nm and 4.48 nm 3 dB bandwidth. To the best of our knowledge, the achieved pulse duration of the mode-locked EDFL in this work is the narrowest compared with all other group 10 TMD SA-based lasers.
1. INTRODUCTION
Pulsed lasers are extensively being applied in diverse modern technology fields, ranging from material processing [1,2], medical treatment [3,4], sensing, metrology [5], to scientific research [6–8]. Depending on the application, either the -switching or mode-locking technique is utilized to produce laser pulses with nano- to femtosecond duration. An acousto-optic modulator (AOM) [9] or an electro-optic modulator (EOM) [10] is used to generate laser pulses actively. However, these active modulators are bulky and costly. Comparatively, passive switching and mode locking based on saturable absorbers (SAs) would be a cost-effective technique for laser pulse generation.
An SA is a nonlinear optical (NLO) device whose transmittance can be modulated passively according to the incident light intensity. Semiconductor saturable absorber mirrors (SESAMs, e.g., InGaAs [11]) and doped crystal (e.g., [12]) were typically utilized for inducing passively mode-locked and -switched pulses, respectively. Nevertheless, their disadvantages of limited bandgap energy and complicated and expensive synthesis processes still limit the development of the pulsed laser [13]. Therefore, the exploration of substitutional SA materials with new features is a sustained quest in the NLO field. Graphene is one of the most prominent demonstrations of a broadband SA [14,15]. It also motivated the study of other graphene-like two-dimensional (2D) material SAs, for instance, graphene oxide [16,17], black phosphorus (BP) [18,19], as well as transition metal dichalcogenides (TMDs) [20]. TMDs are 2D materials that consist of molecular structure, where M is the group 4–10 transition metal element (Nb, Mo, W) and X is the chalcogenide element (S, Se, Te). In recent years, TMDs have attracted huge interest due to their unique atomic structure, distinctive electrical and photonic properties, strong mechanical strength [21], high charge carrier mobility [22], and tunable electronic and photonic properties [23,24]. TMDs have been widely applied in various optoelectronic applications, ranging from photocatalysts [25] to field-effect transistors (FETs) [26] to photodetectors [27–30], and especially SAs (e.g., [31,32] and [33]).
Recently, the newly developed platinum (Pt)- and palladium (Pd)-based group 10 layered materials have drawn enormous attention due to the merits of high mobility, remarkable NLO properties, and high air stability within an ambient environment [34]. The literature has demonstrated the ultrastable operations of - [26,27] and - [30,35,36] based optoelectronic devices in ambient conditions, which suggested the group 10 TMDs are a promising candidate for laser photonic applications. For the SA application, the works of Pt-based TMDs–SAs have been well demonstrated, including of [37,38], [13,34,39,40], and [41]. However, there is no report of ultrafast laser performance based on the Pd TMDs; therefore, it is worthwhile to further explore and investigate the nonlinear saturable absorption properties of these layered Pd TMDs. Palladium disulfide () is one of the most competitive candidates, with an interesting crystal structure and material properties. As shown in Fig. 1, has a pentagonal crystal structure different from the hexagonal crystal structure of other TMD materials, such as , , and . Wang et al. reported the monolayer (ML) has a 1.6 eV indirect bandgap [42]. Bilayer (BL) to bulky exhibits the semimetallic property [43]. This unique electronic structure granted the multifunctional applicability. For instance, structure has been proposed as a channel in the logical junction [43]. Meanwhile is predicted to possess a strong photocatalytic ability for hydrogen and oxygen evolution reactions [44]. In this work, passively -switched and mode-locked EDFLs based on are successfully demonstrated. To the best of our knowledge, this is the first time for utilizing Pd-based TMDs as SAs for both -switching and mode-locking operations.
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powder (Six Carbon Inc.) was first mingled with isopropyl alcohol (IPA) solvent under a 1 mg/mL ratio. Then the mixture was sonicated at 40 kHz ultrasonic frequency and 400 W power for around 20 h. Afterward, the supernatant was prepared by using the centrifugal method at 3000 r/min speed for 5 min, which could remove impurities and the bulk materials.
After the successful exfoliation, the samples were first characterized by utilizing field emission electron microscopy (FETEM, JEOL Model JEM-2100F). Figure 2(a) shows the transmission electron microscopy (TEM) observation of a randomly selected flake whose lateral dimensions are about 250 nm (short axis) and 260 nm (long axis), respectively. In addition, from the high-resolution TEM observation [Fig. 2(b)], individual crystal lattice planes of flakes were quantified at (112), (021), (133), and (132) with the corresponding d-spacing of 0.29, 0.26, 0.23, and 0.16 nm, respectively [45]. Meanwhile, the crystallinity of the fabricated samples was also characterized by the selected area electron diffraction (SAED) pattern. Figure 2(c) has revealed several polymorphic rings that represent the crystal lattice planes (112), (021), (204), and (132); this suggested that the obtained sample is polycrystalline. The measured crystal lattice planes are in good agreement with the database of the Materials Project [37]. The energy-dispersive X-ray spectroscopy (EDS) profile of samples was measured and is shown in Fig. 2(d). There are no observable impurities (the signal of Cu is detected due to the copper mesh used in the FETEM measurement).
Figure 2.(a) FETEM image and (b) corresponding high-resolution transmission electron microscopy (HRTEM) image of a randomly selected flake; (c) SAED pattern and (d) EDS profile of the sample.
Atomic force microscopy (AFM, Bruker Nanoscope 8) was utilized to measure the topological characteristics of the flakes. The topological data among 250 flakes were analyzed; the average values of the lateral size along the short axis, long axis, and layer thickness are 214, 280, and 53 nm, respectively [Figs. 3(a)–3(c)]. As shown in Fig. 3(d), two flakes (Flake A and Flake B) were randomly selected from a topology graph to exhibit their height profiles [Figs. 3(e) and 3(f)]. The lateral sizes along the short axis of Flake A and the long axis of Flake B are about 200 and 250 nm, respectively, which agrees well with the TEM observation.
Figure 3.Statistical distribution of lateral size along (a) short axis, (b) long axis, and (c) layer thickness (analyzed from 250 flake samples); (d) AFM image of measured flakes with respect to the height profile of (e) Flake A and (f) Flake B.
The setup of -based passively -switched and mode-locked EDFL is presented in Fig. 4. The ring cavity is composed of a 976/1550 wavelength division multiplexer (WDM), polarization-independent isolator (PI-ISO), polarization controller (PC), 1:9 output coupler, erbium-doped single-mode fiber (EDF), and single-mode fiber (SMF). The total cavity length is about 12.36 m, which consists of a 0.7 m long EDF [LIEKKI Er 110-4/125, group velocity dispersion (GVD), ] and an 11.66 m SMF (SMF-28; GVD, ). In the experiment, two types of -based SAs were tested; the results are discussed in the following section.
Figure 4.Experimental setup of passively -switched and mode-locked EDFL cavity.
In the experiment, the as-prepared samples were first incorporated into a polyvinyl alcohol (PVA) polymer to form a matrix composite. As shown in Fig. 4, the composite thin film was inserted between the end surface of two fiber connectors to act as a transmission-type SA. -switched pulses with a center wavelength of 1567 nm were detected as the optical pump power scaled beyond a threshold value of 50.6 mW. For the pump power over 81.7 mW, the -switched pulses vanished and switched to continuous wave (CW) operation, which is due to the oversaturation of the SA [18,46]. The measured average output power ranged from 0.138 to 0.393 mW, as shown in Fig. 5(a). Moreover, the modulation ranges of repetition rate and the full width at half-maximum (FWHM) of pulse duration are 17.2 to 26.0 kHz and 12.6 to 4.5 μs, respectively [Figs. 5(b)–5(d)]. Meanwhile, the achieved maximum single-pulse energy is around 15.1 nJ. The radio-frequency (RF) spectrum of the -switched operation was measured with a resolution bandwidth of 40 Hz and is shown in Fig. 5(e). The signal-to-noise ratio (SNR) is up to 50 dB, which suggests a highly stable -switched performance. As shown in Table 1, the achieved minimum -switched pulse duration of 4.5 μs in this work is comparable to other group 10 TMD SA-based results (, 4.2 μs [38]; , 0.9 μs [39]; , 5.2 μs [41]), and supports the notion that is a competitive candidate for use as a nonlinear SA device.
Figure 5.Optical performance of -switched operation. (a) Average output power; (b) repetition rate and pulse duration regarding various optical pump powers; (c) pulse train; (d) single-pulse profile; (e) RF spectrum; and (f) wavelength spectrum at the maximum average output power.
To further explore the nonlinear saturable absorption properties of the fabricated samples, an alternative SA synthesis method was also conducted in the experiment, in which the exfoliated samples were directly deposited onto the surface of a side-polished fiber (SPF) to form an SPF-based . The polarization-dependent loss (PDL) and the insertion loss of the SPF-based are about 2.9 and 10.1 dB, respectively. The PDL of the SPF-based is comparable to results in the previous literature, which utilized SPF and other low-dimensional materials to synthesize practical mode lockers, for instance, , 2.2 dB [47]/3 dB [48], and single-wall carbon nanotubes (SWCNTs), 5.56 dB [49]. According to the work of Zapata et al. [50], the PDL of SPF-based SA is affected by several parameters, including the polishing depth of SPF, the interaction length between the evanescent field and the SA materials, crystallinity or defect of the deposited SA materials, and especially the alignment direction of the 1D (SWCNTs) or 2D (TMDs) SA material [51]. To reduce the PDL, it can reduce the polishing depth of SPF or the material interaction length, and vice versa. Meanwhile, by using a balanced twin-detector system with a mode-locked laser source of 2.09 ps pulse duration and 1560 nm center wavelength, the saturable absorption characteristics of the SPF-based were quantified; they are shown in Fig. 6. The nonlinear transmission response of the SA was measured according to the different polarization conditions within the SPF, where the maximum and minimum recorded values of the modulation depth are shown in Figs. 6(a) and 6(b), respectively. The experimental data were fitted by using Eq. (1), where is the transmittance, is the saturable intensity, is the modulation depth, and is the nonsaturation loss [37]. The saturable intensity and modulation depth of the SPF based are around and 1.7%, respectively, measured in TE mode,
Figure 6.Nonlinear input intensity-dependent normalized transmittance curve of at 1564 nm, the recorded (a) maximum and (b) minimum modulation depth condition according to the variation of the polarization state of input light.
By modifying the intracavity polarization state with the PC, stable CW mode-locked operation was observed as the pump power scaled beyond 106.4 mW. As shown in Fig. 7(a), the highest achieved average output power is about 0.55 mW, corresponding to the pump power of 246 mW. The average output power can be further scaled up by modifying the length of the doped fiber or utilizing the postamplification system to amplify the output signal. The measured repetition rate of the output pulses [Fig. 7(b)] is well matched with the round-trip time and total cavity length, which proved the successful mode-locked operation. Meanwhile, from Fig. 7(c), the regular CW envelope of the output pulse train also suggested stable CW mode locking. The pulse duration was measured by using an autocorrelator (FR-103XL, Femtochrome Research, Inc.), and the result was fitted by using the squared hyperbolic secant function. As shown in Fig. 7(d), the FWHM is about 1.24 ps, which indicated that the real pulse duration is about 803 fs.
Figure 7.Optical performance of mode-locked operation. (a) Average output power regarding various optical pump powers; (b) and (c) are pulse trains with different time scales; (d) autocorrelation trace of mode-locked pulse; (e) RF spectrum; and (f) wavelength spectrum at 0.55 mW output power.
In Fig. 7(e), the fundamental repetition rate of the RF spectrum is given as about 12.1 MHz. The SNR is up to 53 dB, which also indicated that the system consists high operation stability. The wavelength spectrum illustrated the central wavelength and 3 dB bandwidth of 1565.8 and 4.48 nm, respectively, as shown in Fig. 7(f). Meanwhile, the time bandwidth product was estimated as 0.44, indicating that the pulse duration can be further improved. The output stability of the SA-based mode-locked laser was also studied, where the fluctuation of the average output power was less than 5% within the 30–45 min measuring period. To the best of our knowledge, this is the first demonstration of mode-locked EDFL based on palladium-TMD SAs. As shown in Table 1, the achieved pulse duration in this work is the narrowest among all other group 10 TMD SA-based results, indicating that the fabricated in this work is an advanced and promising NLO device. Wang et al. [52] demonstrated that the nonlinear optical absorption (NOA) response of , which has a tunable energy bandgap from 1.2 eV (indirect) of the monolayer to 0 eV of around 55 layers, is attributed to both single-photon absorption (1PA), two-photon absorption (2PA), and excited-state absorption (ESA) effects, as shown in Fig. 8. For the in a few-layer condition (4–7 layers, 0.87 to 0.59 eV), saturable absorption is recorded as the sample interacting with a 515 nm excitation light (340 fs) in the Z-scan measurement. Nevertheless, under the excitation with a 1030 nm light, the few-layered had demonstrated reverse saturable absorption (RSA) instead. Under the optical excitation with low photon energy, 2PA and the induced ESA will be dominant in the overall NOA effect and thus demonstrate the RSA response eventually. However, as the layer thickness increased to around 55 layers, where the transits into semimetal behavior due to the suppression of 2PA and the domination of the 1PA effect, the NOA response of the sample switched to saturable absorption as the incident fluence reached saturation intensity. This NOA mechanism can also be applied to the case of the semimetallic in this work, where the multilayered SA is expected to have extensive saturable absorption response within the infrared region. In fact, we obtained a preliminary result of applying the multilayered -based SA in a 1 μm ytterbium-doped fiber laser system recently. And it is worth further exploring the applicability of the in different laser systems, as well as deeper study of the excited carrier dynamics of this novel material in future work.
Figure 8.Schematic diagram illustrating the photon absorption process within a four-energy level model, where the , , , and are the ground state, first, second, and third excited state, respectively. 1PA and 2PA represent the single-photon absorption and two-photon absorption, respectively. ESA stands for the excited-state absorption.
Passively -switched and mode-locked EDFLs were first demonstrated by using . Self-starting -switching operation at 1567 nm was achieved with a threshold pump power of 50.6 mW. The modulation ranges of pulse duration and repetition rate were 12.6 to 4.5 μs and 17.2 to 26.0 kHz, respectively. The achieved highest average output power was 0.393 mW, corresponding to 15.1 nJ maximum single-pulse energy. Meanwhile, a mode-locked EDFL was also obtained with a pump power threshold of 106.4 mW. The achieved pulse duration of the mode-locked EDFL in this work was 803 fs, which is the narrowest result among all other group 10 TMD-SA-based lasers. This suggests the is a promising 2D materials candidate for use in NLO applications.