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Abstract
Previously, PbS/CdS core/shell quantum dots with excellent optical properties have been widely used as light-harvesting materials in solar cell and biomarkers in bio-medicine. However, the nonlinear absorption characteristics of PbS/CdS core/shell quantum dots have been rarely investigated. In this work, PbS/CdS core/shell quantum dots were successfully employed as nonlinear saturable absorber (SA) for demonstrating a mode-locked Er-doped fiber laser. Based on a film-type SA, which was prepared by incorporating the quantum dots with the polyvinyl alcohol (PVA), mode-locked Er-doped operation with a pulse width of 54 ps and a maximum average output power of 2.71 mW at the repetition rate of 3.302 MHz was obtained. Our long-time stable results indicate that the CdS shell can effectively protect the PbS core from the effect of photo-oxidation and PbS/CdS core/shell quantum dots were efficient SA candidates for demonstrating pulse fiber lasers due to its tunable absorption peak and excellent saturable absorption properties.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Er-doped passively mode-locked fiber lasers have attracted more and more attention due to their wide practical applications and abundant optical nonlinear phenomena [1–18]. Previously, numerous nonlinear absorption materials have been employed as SAs for demonstrating passively mode-locked Er-doped fiber lasers [1]. Thereinto, graphene [2–5] and graphene-like two-dimensional materials (topological insulators (TIs) [6–11], black phosphorus (BP) [12,13], layered metal dichalcogenides [14–21]) have been extensively investigated due to their advantages of wide absorption band, ultra-fast recovery time and high damage threshold [1–21]. However, their layered-dependent properties also bring inconvenience to the flexibly designment of the nonlinear absorption parameters. Besides the mentioned two-dimensional materials, metal nanoparticals have also been used as SAs for demonstrating Er-doped mode-locked lasers [22,23]. In comparison with the graphene and graphene-like materials, metal nanoparticals have the most obvious advantage of a tunable absorption peak, which can be adjusted by controlling the aspect ratio of nanoparticals [22–24]. In addition, the nonlinear saturable absorption of metal nanoparticles is contributed by the surface plasmon resonance (SPR) of the materials. But, the metal particles have the phenomenon of thermal condensation, which will results in the deviation of the SPR absorption peak and the decreases of the absorption efficiency.
Recently, IV-VI group quantum dots have been preliminarily investigated as SAs for achieving pulse fiber laser operations. In comparison wtih the mentioned two-dimensional materials, quantum dot have the obvious advantage of tunable nonlinear absorption peak, cost-effectiveness, low cost and low saturable absorption intensity. Lee et al have reported on the demonstration of a passively Q-switched 1.55 µm fiber laser by utilizing a colloidal PbS quantum dot thin film as a SA [25]. To our knowledge, this is the first pulsed fiber laser demonstration based on quantum dots as SA. In their work, PbS quantum dots with a low saturable absorption intensity of 0.047 W/μm2 and a absorption peak of 1524 nm were prepared. The maximum average output power was 19.4 mW under a pulse repetition rate of 24.2 kHz. In addition, A passively mode-locked Yb fiber laser using PbSe colloidal quantum dots as saturable absorber was reported by Wei et al. The non-saturable loss, modulation depth, and saturable intensity of the PbSe SA were 23%, 7%, and 12 MW/cm2, respectively [26]. Their results also proved that PbS or PbSe quantum dot have excellent performance for achieving pulse fiber laser operations due to the mentioned advantages of tunable nonlinear absorption peak, cost-effectiveness, and low saturable absorption intensity. Additionally, unlike two dimensional materials, IV-VI group quantum dots also have the uniqe advantages of wide coverage of the absorption peak, lager Bohr exciton radius (PbS@18 nm, PbSe@46 nm), high third-order nonlinear coefficients and unique multiple exciton generation [27–30]. Besides the mentioned nonlinear optical applications, IV-VI group quantum dots also have wide applications in the fields of solar cell, biomedical science and so on [28–30].
However, as is known, due to the trap states at the surface, quantum dots suffer from the effect of photo-oxidation and exhibit unstable state, which limited its practical applications. Cover-coating an inorganic shell on the initial core has become an effective technical method for passivating trap states at the core surface and improving the optical stability of quantum dots [31]. PbS/CdS core/shell quantum dots have a type-I band alignment, in which, the band gap of the CdS shell encloses that of the PbS core. So that, the electron and hole states are confined in the core [32,33]. Thus the CdS shell can effectively passivate trap states at the surface of PbS core and improve the optical stability of the core [35]. Previously, PbS/CdS core/shell quantum dots have been widely reported as light-harvesting materials in solar cell and biomarkers in biological monitoring. However, the nonlinear absorption applications of PbS/CdS core/shell quantum dots have been rarely reported [32–38].
In this letter, PbS/CdS core/shell quantum dots with a absorption peak of 1583 nm was prepared and employed as SA for demonstrating an passively mode-locked Er-doped fiber laser. The saturation intensity and modulation depth of the film-type SA were 6.29 MW/cm2 and 4.95%, respectively. Mode-locked operation with a minimum pulse width of 54 ps under a pulse repetition rate of 3.302 MHz was obtained.
2. Preparation and characteristics of PbS/CdS core/shell quantum dots
Figure 1 shows the transmission electron microscope (TEM) image of the PbS/CdS core/shell quantum dots which was recorded by a JEM-2100 microscope with an optical resolution of 20 nm. As is shown, the PbS/CdS core/shell quantum dots have relatively uniform sizes. The corresponding size distribution histogram is shown in the insert of Fig. 1, the average diameter of the quantum dots was about 6.99 nm.
Fig. 1 The TEM image of the PbS/CdS core/shell quantum dots. Insert of Fig. 1. The corresponding size distribution histogram.
The high resolution transmission electron microscope (HRTEM) image under the resolution of 5 nm is shown in Fig. 2(a). As is shown, the PbS/CdS quantum dots exhibited obvious lattice fringes. A d-spacing of 0.295 nm which corresponded to the (200) dominant lattice planes of the PbS core quantum dots was described in Fig. 2(b).
The Raman spectrum of the prepared the PbS/CdS core/shell quantum dots was recorded by a Horiba HR Evolution 800 Raman microscope system. As is shown in Fig. 3, obvious Raman shift peak at 234.6 cm−1 corresponding to the P-type surface phonon mode of the PbS/CdS core/shell quantum dots was observed. The sharp Raman peak at 234.6 cm−1 shows that the prepared quantum dots have a relatively perfect crystal morphology. In addition, the unusual rising base of the Raman spectrum was caused by the fluorescence of the quantum dots.
The crystal structure of the PbS/CdS quantum dots was also studied by X-ray Diffraction (XRD) (D8 advance Bruker). As is shown in Fig. 4. The XRD pattern exhibits multiple diffraction peaks, which indicates that PbS/CdS quantum dots with excellent crystallinity were prepared in our works. Additionally, due to the the small grain size of the synthesized sample, the diffraction peaks of the sample were low and wide. High diffraction peaks at (001), (200), (220), (311), (400), (420) planes can be observed in the spectrum, which were in agreement with reported results [28].
The absorption and photoluminescence spectra of the PbS/CdS core/shell quantum dots are provided in Figs. 5(a) and 5(b), respectively. The PbS/CdS core/shell quantum dots showed a narrow photo-luminescence peak around 1600 nm (Fig. 5(b)). Additionally, The PbS/CdS core/shell quantum dots exhibited a broad absorption peak around 1583 nm, corresponding to a Stoke shift of 17 nm (Fig. 4(a)). As previously reported, the relationship between the position of the first absorption peak wavelength and the size of the PbS quantum dot can be expressed as [28]
where d is the size of the quantum dots and λ is the first absorption peak wavelength. As is mentioned, the absorption peak wavelength was 1583 nm. Finally, the corresponding size of the PbS core quantum dots was calculated to be about 6.13 nm. In addition, the average diameter of the PbS/CdS core/shell quantum dots was 6.99 nm, indicating the existence of the CdS shell and the thickness of the CdS shell was about 0.43 nm.Fig. 5 (a). The absorption spectra of the PbS/CdS core/shell quantum dots. 4(b) The photoluminescence spectra of the PbS/CdS core/shell quantum dots.
Finally, the PbS/CdS core/shell quantum dots dispersion (shown in Fig. 6(a)) and 4 wt% PVA solution were mixed at the volume ratio of 1:2. Afterwards, 80 μL mixed dispersion solution was spin coated on a sapphire substrate, the coated substrate was placed into a oven for 24 hours at 20°C. Then, a thin film-type SA was obtained. Finally, a 1*1 mm2 PbS/CdS-PVA film was cut off from the substrate and placed at the end of the fiber end for using as mode-locker (shown in Fig. 6(b)). The nonlinear absorption properties of the film-type SA were tested by using a power-dependent transmission technique which has been reported in our previous works [20]. the experimental setup is shown in the insert of Fig. 7. The pump source was a home-made nonlinear polarization rotation mode-locked Er-doped fiber laser with 560 fs pulses at 1580 nm with a repetition rate of 33.6 MHz. The experimental and fitting results are all shown in Fig. 7. The saturation intensity and modulation depth were 6.29 MW/cm2 and 4.95%, respectively.
3. Experimental details
Figure 8 shows the experimental construction of the PbS/CdS core/shell quantum dots based passively mode-locked Er-doped fiber laser. A 680 mW 976 nm laser diode (LD) was used as pump source and injected into the ring laser cavity through a 980/1550 nm wavelength division multiplexer (WDM). Polarization insensitive isolator (PI-ISO) and polarization controllers (PCs) were used to keep the laser unidirectional operation and adjust the polarization state of the laser, respectively. A 10:90 output coupler (OC) was used to output the laser through its 10% port. A 75 cm long erbium-doped fiber (EDF, Er110, 4/125) with a dispersion value of about −46 ps/(nm.km) was used as laser gain medium. The saturable absorber was inserted between the PI-ISO and PC. About 61.48 m long single-mode (SM) fiber are used to adjust the dispersion value of the cavity. The total cavity length is about 62.23 m with a net dispersion of −1.39 ps2.
4. Results and discussions
In the experiment, firstly, without inserting the SA into the laser cavity, only continuous-wave operation was detected by adjusting the pump power or the polarization controllers. In addition, when the SA was inserted into the laser cavity and a short-length (about 12 m) ring laser cavity was demonstrated, also, only continuous-wave operation was detected by adjusting the pump power or the polarization controllers (the angle of each polarizer converts between 0 and 180 degrees.). It should be noted that, in the experiment, traditional soliton mode-locked generations can be achieved by adding extra about 50 m SMF to compensate the net cavity dispersion into anomalous dispersion. The experimental results that the PbS/CdS core/shell quantum dots has a relative high value of normal dispersion.The optical emission spectrum of the mode-locked Er-doped fiber laser was recorded by a optical spectrum analyzer (AQ-6317) with a resolution of 0.02 nm and shown in Fig. 9(a). As is shown, obvious typical soliton-like spectrum shapes with characteristic Kelly-bandside peaks were obtained with a 3 dB spectrum bandwidth of 2.504 nm. The central wavelength was detected to be 1562.5 nm. As is shown, the separation of the first pair of Kelly side bands is about 6 nm, which in general is corresponding to small net dispersion [39], however, the net dispersion caused by the SMF and Er-doped fibers of the laser cavity was about −1.39 ps2, thus, the results proved that the PbS/CdS core/shell quantum dots has a relative high value of normal dispersion again, due to the high normal dispersion value of the SA, the net dispersion of the laser cavity was compressed from 1.39 ps2 to a small net dispersion. Figure 9(b) shows the relationship between the average output powers and pump powers. The maximum average output power was 2.71 mW under a pump power of 179 mW, corresponding to an optical conversion efficiency of 1.51%. Thereinto, based on an optical power meter, the total cavity loss of the the PbS/CdS-PVA SA was measured to be about 5.8 dB, whcih indicates that the low output power and optical-to-optical conversion efficiency were mainly due to the large intra-cavity loss produced by the SA.When the pump power was higher than 179 mW, the mode-locked operation became unstable which was mainly due to the saturated absorption of the SA. As is mentioned above, due to the trap states at the surface, PbS quantum dots suffer form the effect of photo-oxidation and exhibit unstable state, although cover-coating an inorganic CdS shell on the initial core can improve the optical stability of the PbS quantum dots, however, under a high pump power, due to the low non-saturable loss of the SA, the CdS shell and the PbS core will always affect by the thermal decomposition, which will cause the damage of SA. Therefore, the damage threshold of the PbS/CdS SA was different from that of the two-dimensional materials reported before [1–21]. Additionally, in our opinion, the CdS shell have little influence on the low non-saturable loss, as is known, low non-saturable loss is mainly caused by the crystal defect, thus, the experiment results shown that it is necessary to optimize the crystal structure of the PbS/CdS quantum dots in our future work.
Typical pulse trains of the mode-locked laser under different bandwidth are shown in Fig. 10(a) and the insert of Fig. 10(a). As is shown, the pulse-to-pulse time was 302.8 ns, corresponding to an pulse repetition rate of 3.302 MHz, which agrees well with the total length cavity of 62.23 m. Additionally, the auto-correlation trace is shown in Fig. 10(b), the full width half maximums (FWHMs) was 54 ps, thus, the time-bandwidth product (TBP) is about 16.63, which is higher than the theoretical limit value (0.315), indicating that the optical pulse is highly chirped. In our opinion, such a wide pulse width was related to the lager net dispersion of the laser cavity and the long relaxation time of the phonon interaction in the PbS/CdS quantum dots. Thus, in our future work, we will compress the pulse width by optimizing the parameters of the resonant cavity (total cavity length or the length of the Er-doped fiber) and the SA (the saturation intensity and modulation depth).
Fig. 10 (a). Typical pulse trains of the mode-locked laser under different bandwidth. (b) The auto-correlation trace.
Based on a spectrum analyzer (R&S FPC1000), the radio frequency (rf) spectrum was recorded and shown in Fig. 11. The radio frequency spectrum located at the fundamental repetition rate of 3.032 MHz with a bandwidth of 2 MHz and a resolution of 100 Hz, the signal-to-noise ratio is about 50 dB. In addition, the radio frequency spectrum within a wide bandwidth of 1 GHz is shown in the insert of Fig. 11. All the radio frequency results exhibit that mode-locked pulses with high stability were obtained in our work.
Fig. 11 The RF spectrum of the mode-locked laser located at 3.302 MHz. Insert of Fig. 11. The RF spectrum with a bandwidth of 1 GHz.
Additionally, as mentioned earlier [25,26], Lee et al have demonstrated a PbS quantum dots based passively Q-switched Er-doped fiber laser. The maximum average output power was as high as 19.4 mW corresponding to an optical-to-optical conversion efficiency of 20.2%. Such high output power and conversion efficiency proved the advantageous optical and mechanical properties of PbS quantum dot SA [25]. Also, Wei et al have also proved that PbSe quantum dot also have excellent performance using as SA [26]. However, as is stated, due to the trap states at the surface, quantum dots without core-shell construction suffer form the effect of photo-oxidation and exhibit unstable state, which means that long-time stable laser operation with quantum dots as SA is a difficult challenge. Unfortunately, in previous works [25,26], long-time characteristics of the quantum dots based laser operations have not been described. In our work, long-time stable mode-locked operation was obtained. The output spectra was recorded continuously at a 2 h interval over 10 h. Figure 12 shows the central wavelength and the 3 dB spectrum bandwidth of the recorded spectra, the central wavelength fixed within a range of less than 0.49 nm. Meanwhile, the variety of the 3 dB spectrum bandwidth was less than 0.26 nm, indicating that PbS/CdS core/shell quantum dots based passively mode-locked laser operation exhibits excellent stable long-time characteristics. Finally, our results indicate that the CdS shell can effectively protect the PbS core from the effect of photo-oxidation and PbS/CdS core/shell quantum dots have excellent nonlinear optical properties.
In summary, PbS/CdS core/shell quantum dots were successfully employed as SA for demonstrating a passively mode-locked Er-doped fiber laser. Morphological and nonlinear saturable absorption properties of the PbS/CdS core/shell quantum dots were investigated. Stable mode-locked pulses generation with a maximum output power of 2.71 mW was achieved. The minimum pulse width was 54 ps at a pulse repetition rate of 3.302 MHz. Our results proved that PbS/CdS core/shell quantum dots have excellent performance in obtaining stable pulse generations.
Funding
China Postdoctoral science Foundation (2016M602177), Shandong Provincial Natural Science Foundation (ZR2016FP01, ZR2014FM028), National Natural Science Foundation of China (11747149, 11474187).
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