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Abstract
A passively Q-switched Er3+ doped fiber laser has been realized by using Zn doped hexagonal CuGaO2 (CGZO) nanoplates (NPs) as a saturable absorber (SA) for the first time. The CGZO NPs SA film exhibits strong saturable absorption property, meanwhile with a small nonsaturable loss of 5.179%, and the modulation depth is up to 40.821%. A stable passively Q-switched laser, which was centered at 1559.75 nm, was achieved, and the threshold was as low as 42 mW. With an increase of the pump power from 42mW to 361mW, the pulse duration decreases from 36 μs to 1.71 μs, and the maximum output power of 12.1 mW is achieved. Particularly, the optical-optical conversion efficiency of the Q-Switched laser based on CGZO NPs reached 3.76%. Due to whispering-gallery-mode (WGM) resonance in CGZO NPs, the nonlinear optical response of CGZO NPs has been enhancement. These findings demonstrate that CGZO NPs are promising SA for fabricating high-efficiency and low-threshold pulse lasers.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In order to achieve pulse lasers, passively Q-switching and mode-locking are the widely used techniques in commercial laser systems due to the advantages of easy implementation, low cost and excellent mechanical stability [1–5]. Nonlinear optical modulators which based on saturable absorber (SA) (utilize the nonlinear optical effects of materials) are play a key role in these laser systems. Nonlinear optical effects have an important role in various photonic fields, and originate when electronic movement in an intensity electromagnetic field cannot be considered harmonic. Consider to the saturable absorption depends on the third-order nonlinear processes, the materials with large third-order optical nonlinearity can be used to fabricating efficient SAs desirably [6–10].
Up to the present, various nonlinear SAs, such as noble metal nanostructure and two-dimensional (2D) materials, have been studied to replace the current semiconductor saturable absorption mirrors (SESAMs) whose operation bandwidth is no more than 100 nm and costly fabrication requirements [11–14]. The noble metal nanostructure (gold, silver and copper) have been used as SAs for constructing short pulse lasers which is ascribed localized surface plasmon resonances (LSPRs). However, the high optical loss (caused by optical scattering and absorption) and strong photo-thermal effect limit noble metal nanostructure to become efficient SAs [15–17]. The 2D materials, such as graphene, topological insulators, black-phosphorous (BP) and transition metal dichalcogenides (TMDs), also can exhibit saturable absorption, and they indeed have been used for mode-locked or passively Q-switching pulse generation in the infrared region [18–24]. However, the small density of state near the Dirac point presents as a bottleneck for both linear and nonlinear absorption for the Dirac materials, such as graphene and topological insulators. Furthermore, for the BP and TMDs with flake-like morphology, the inefficient light-matter interaction which associated with the intrinsic subnanometer thickness is the main issues that should be carefully handled.
Due to the quantum confinement effect arising from small size, semiconducting nanomaterials offer interesting characteristics and have been consistently explored for applications ranging from bio-medicine to photonics. In particular, the electron motion has been confined in nanoscale materials below a critical size by quantum confinement effect which could enhance their interaction with intense optical fields, leading to much strengthen nonlinear optical response [25–27]. For instance, Cu-based binary and ternary colloidal nanocrystals (Cu2-xS, Cu-Sn-S) (NCs) have been demonstrated as an attractive saturable absorption propriety and realized mode-locked or passively Q-switched pulse generation [28–30]. Furthermore, transparent conducting oxides (TCOs) material exhibits strong optical nonlinearity in spectral region where the dielectric permittivity approaches zero, known as epsilon-near-zero (ENZ) region. These TCOs materials with ENZ region have been regarded as a new paradigm of SAs that may have significant influences for nonlinear optics. Recently, the indium tin oxide (ITO) NCs have been proven to be an excellent candidate as SAs for mode-locking lasers [31]. However, passively Q-switching fiber lasers using TCOs material as SA have not been demonstrated experimentally. Moreover, in order to make full use of such excellent nonlinear optical properties, SA with tunable nonlinear optical properties fabricated by means of growing heterojunction is one of efficient way. Although this method provides a new idea to develop suitable SA for different application scenarios, almost all of the well-known TCOs such as Al-doped ZnO and ITO are n-type semiconductor oxide. Therefore, the intrinsic p-type s TCOs with saturable absorption properties is highly desirable and yet to be developed.
In this paper, we report optical modulation by using a p-type TCOs material: Zn doped hexagonal CuGaO2 (CGZO) nanoplates (NPs) as a new type of effective SA for passively Q-switching fiber laser, which is characterized of large modulation depth and low nonsaturable loss. Besides, due to the unique hexagonal morphology compared with continuous thin film and the NCs, the optical-optical conversion efficiency of passively Q-switching fiber laser is further enhanced by whispering-gallery-mode (WGM) resonance. Our results indicate that Zn doped CGO NPs could be used as SA for constructing high efficiency pulse lasers. Furthermore, it provides a basis for fabricating heterojunction SA with tunable saturation characteristics.
2. Experiment
2.1 Synthesis of Zn doped CuGaO2 (CGZO) nanoplates (NPs)
A facile hydrothermal reaction modified from our previous report was used to synthesis Zn doped CGO NPs [32]. As shown in Fig. 1, 0.05 mmol Zn(C4H6O4)∙2H2O, 0.95 mmol Ga(NO3)2∙9H2O and 1 mmol Cu(NO3)2∙3H2O were dissolved in 15 mL deionized water and heated for 30 minutes. Then 6 mmol KOH which dissolved in 5 mL deionized water was poured into this prepared solution and reheated for 30 minutes. The resulting deep blue solution was diluted with ethylene glycol and deionized water (capacity ratio of ethylene glycol and deionized water = 1/3), and then transferred into a 100mL teflon bomb (filled 70% volume capacity) heating at 190°Cfor 56h. After the reactions, the products were cooled to room temperature naturally, and washed thoroughly with ethanol, wajter, dilute HNO3 and ammonia water. Finally, the CGZO NPs with good purity have been fabricated. The average dimensions of the CGZO NPs is about 2-5 µm.
2.2 Fabricate of CGZO saturable absorber (SA)
Sodium carboxymethyl methylcellulose (NaCMC) is weighed about 1.5g by precision balance and the 15mg/mL NaCMC aqueous solution is prepared for 24 hours with magnetic stirrer. 2mL Zn-Doped CGO NPs dispersions with 3mL NaCMC aqueous in a centrifuge tube, and then the mixture is processed for another 3 hours by ultrasonic. After that, the uniform mixture is dropped onto the surface of a 1cm × 1cm cover glass substrate which is ultrasonic cleaned with acetone, anhydrous ethanol and deionized water respectively. The substrate is dired naturally under room temperature for 1~2 days. Finally, the high quality CGZO NPs SA films are obtained, as shown in Fig. 2. The average thickness of the SA film is about 5 µm which measured by Profile-system.
2.3 Passively Q-switched EDF laser generation
CGZO SA film was placed between two fiber connectors forming a fiber compatible SA, and then integrated into a ring cavity which play the role in an Er3+-doped fiber laser, as shown in Fig. 3. The laser system consist of a 980 nm single mode diode, a wavelength-division multiplexer (WDM) is employed for coupling the pump power into the ring cavity. The utilized gain fiber was erbium-doped fiber (Liekki, Er 80-8/125) with the length of 35 cm, a polarization controller (PC) and a polarization-independent isolator (PI-ISO) are employed for adjusting the polarization state and ensure unidirectional propagation; a 10% optical coupler (OC) is used to output the signal. The time and frequency-domain of the signal can be detected by a high-speed oscilloscope and a photodetector (DOSV-084A Keysight, DET08 Thorlabs). The spectra have been detected by the spectrograph (AQ6370D, YOKOGAWA).
3. Results and discussion
The elemental compositions, morphology and structure of CuGa1-xZnxO2 NPs were characterized by using the scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). Figure 4(a) shows that well-crystallized Zn doped CGO NPs have hexagonal morphology and six sharp corners. Furthermore, Fig. 4(b)-4(f) displays the elemental maps of individual CuGa1-xZnxO2 NPs. Cu, Ga, O and Zn are homogeneously distributed in the nanoplates. The elemental mappings prove that Zn has been doped into the CGO NPs thoroughly.
The doping and phase purity of the as-fabricated CuGa1-xZnxO2 NPs have been further confirmed by XRD characterization. Figure 5 shows the XRD pattern for pure CGO, 1% and 5% Zn doped CGO NPs which spin-coated on Si substrate. All diffraction peaks of three samples can be indexed to the standard JCPDS card (No. 41-0255, with hexagonal lattices of a = b-2.9748 Å, c = 17.155 Å). However, only the 003, 006 and 1010 diffraction peaks can be detected obviously for spin-coated CGO NPs, the other diffraction peaks are much weaker than that of powder CGO NPs (Fig. 11, Appendix). This result is demonstrating that the CGO NPs which through spin-coated form a highly directional thin film and align on the substrate with their c-axis perpendicular to the substrate surface [33]. Furthermore, the XRD peaks of the CuGa1-xZnxO2 NPs entire shift toward lower angles with the increase of Zn doping, as can be seen in the magnified 006 diffraction peak expressed in inset of Fig. 5. This phenomenon can be ascribe to local lattice distortion which leading by the partial substitution of Zn2+ (0.74 Å) with large ionic radii into Ga3+ (0.62 Å) sites with small radii [34].
Fig. 5 The XRD patterns of the pure CGO NPs, 1% and 5% Zn doped CGO NPs, inset expresses the enlarged XRD pattern at 006 peak.
The linear optical response of pristine CGO NPs and 5% Zn doped CGO NPs thin film on quartz substrate were measured by absorption spectra, as shown in Fig. 6(a). For the unitary CGO NPs film, an obvious UV light absorption peak appears at 330 nm which can be attributed to direct bandgap absorption, and the absorption intensity is rather weak in visible and infrared region. This apparent optical property is the reason that CGO can be used at TCOs and the most promising p-type material for ultraviolet (UV) optoelectronic devices. Interestingly, besides the UV absorption, the absorption of Zn doped CGO NPs is much higher than that of the CGO in visible and infrared region. This phenomenon is related to the change of energy band and electrical properties which caused by Zn ions substitute the Ga ions in the reaction process. Due to some doping levels could form in the forbidden band, the absorption in visible region is caused by the absorption transition of these carriers which located at defect levels. Furthermore, Zn doping will generate more hole carriers in valence band, and increased carrier concentration would cause LSPR absorption [28]. Therefore, the absorption intensity of Zn doped CGO NPs is much higher than pure CGO NPs in infrared region can be ascribed to collective oscillation of free carrier which introduced by Zn doping [29]. In addition, the absorption intensity of Zn doped CGO NPs is increase with the doping ratio increasing in infrared region. Consider that the hole carrier concentration is increase with the doping ratio increasing. When the injection light provides enough energy to generate localized plasma oscillations, a large amount of incident light can be localized on the surface of the structure which is ascribed to the strong limitations of LSPR. Therefore, LSPR absorption could be the main reason for the infrared absorption enhancement. Figure 6(b) investigated the dependence of absorption intensity on the wavelength of pristine CGO NPs and Zn doped CGO NPs. The absorption of Zn doped CGO NPs was found to be 2-10 times higher than that of CGO NPs from 400 to 2000 nm. It is worth noting that the absorption intensity has been enhanced almost 10 times in infrared region (around 1200-1600 nm) through the Zn doping, and also an evidence to proof that Zn ions have been doped into the CGO NPs which could caused the carrier concentration enhancement. Therefore, LSPR absorption could be produced in infrared region by the carrier concentration increasing [35].
Fig. 6 (a) The absorption spectra of the pure CGO NPs and Zn doped CGO NPs. (b) The absorption intensity of the pure CGO NPs and Zn doped CGO NPs depend on wavelength.
Due to the CGZO NPs exhibited LSPR absorption in infrared region, the saturable absorption property of as-fabricated NPs has been investigated. Figure 7 expresses the relationship between the transmission ratio and the pump peak density (a 1560 nm pulsed fiber laser and pulse width of ~400 fs). The corresponding measured result clearly shows the character of saturable absorption, a small nonsaturable loss 5.179% and a large modulation depth 40.821% can be obtained for CGZO NPs. Fitting the measured data by using the equation, saturation power density is calculated as 38.7 MW/cm2:
where αS, αNS and IS are the modulation depth, nonsaturable loss, and saturation power density. The comparison result of saturable absorption based on different SAs is summarized in Table 1 [30,36–39]. The strong saturable absorption along with ultrafast transient bleach dynamics and highly efficient modulation stimulate us to explore pulsed lasers by using CGZO NPs as SAs.
Table 1. The comparison result of saturable absorption based on different SAs. αNS: Nonsaturable loss; αS: Modulation depth.
In order to demonstrate a possible optical switching function at the optical communication band (1550 nm) which can be achieved based on our CGZO NPs, the performances of the passively Q-switched EDF laser has been analyzed in detail. Increasing the pump power to 19 mW, output laser always works in continuous wave which centered at 1560 nm. Continue to increase the pump power to 42 mW, the intensity of the propagation light in the waveguide is large enough for CGZO film to trigger a Q-switching operation by adjusting the polarization controller, which changed the continuous wave into the pulses train output. The spectrum of Q-switched Er-doped fiber laser at a pump power of ~361 mW is shown in Fig. 8(a). The central wavelength of Q-switched Er-doped fiber laser was about 1559.75 nm. Figure 8(b) and 8(c) show the shape of output pulse and the train of the Q-switched Er-doped fiber laser under pump power of ~361 mW. The temporal width of a single pulse was as short as 1.71 μs, the interval between two adjacent pulses is about 31.75 μs and the corresponding repetition rate is about 31.5 kHz. To the best of our knowledge, this is the first report on Q-switched Er-doped fiber laser based on the CGZO NPs SA.
Fig. 8 The all-optical switching enabled by the CGZO NPs at optical communication band (C band). (a) The optical spectrum. (b) Temporal width of one pulse. (c) The modulated laser pulse train.
Figure 9(a) indicates the dependence of repetition rate and the pulse duration on the pump power. With increasing the pump power from 42 mW to 361 mW, the repetition rate increases from 6.7 kHz to 31.5 kHz and the pulse duration decreases from 36 μs to 1.71 μs, which shows a typical feature of passively Q-switched fiber lasers. Note that, the Q-switched Er-doped fiber laser exhibits an excellent stability at room temperature. Figure 9(b) shows the dependence of output power of Q-switched Er-doped fiber laser on the pump power. With the increase of pump power from 42 mW to 361 mW, the output power increases from 0.2 mW to 12.2 mW parabolically. Most interestingly, the corresponding slope efficiency is about 3.76%, such a value was found to be better than that of the Q-switched laser based on other SAs. The possible reason would be ascribed to the whispering-gallery-mode (WGM) resonance formed in CGZO NPs. This is the first report that the passively Q-switching fiber laser using TCOs (CGZO NPs) as SA, and Table 2 lists the compared performance parameters of those passively Q-switched fiber lasers developed using other nonlinear SAs [30,36–39]. The comparison result imply that the laser utilizing the CGZO NPs as a SA conducted shorter pulse duration, lower Q-switching threshold, larger output power and higher efficiency of the laser. Furthermore, due to the limitation of pump source driver (the maximum output power of the pump source is 361 mW), the CGZO NPs SAs still does not reach the damage threshold.
Fig. 9 Q-switched pulse output characterization in Er-doped fiber laser cavity with CGZO NPs SA. (a) The pulse repetition rate and pulse width versus pump power; (b) output power versus pump power.
Table 2. Output performances comparison of different passively Q-switched fiber lasers. λ: Central wavelength (nm); PQth: Q-switching threshold (mW); fr: Repetition rate range (kHz); t: Shortest pulse duration (µs); Pmop/Ppp: Max. output power (mW)/pump power (mW); η: Efficiency (%).
Due to the hexagonal morphology and six sharp corners as shown in Fig. 4, CGZO NPs could be employed as natural resonance microcavity. Consider the microcavity resonance have the capability to trap photons by total internal reflection, these exceptionally long photon lifetimes could enhanced the light-matter interactions, leading to strengthen nonlinear optical response [40]. To investigate the resonance behaviors theoretically, the optical field distribution and waveguide mode transmission were numerical simulated by finite difference time domain (FDTD) method, as shown in Fig. 10. Figures 10(a)-10(d) demonstrates fundamental mode electric field E(x, y) propagates along with the hexagonal boundary at first, and then fill entire surface of hexagon. With the pump power increasing, corresponding radiative mode expressed in Fig. 10(e). From the simulation, the optical modes could be confined in the hexagonal CGZO NPs, and the corresponding standing wave pattern of the resonance was formed by three sides of the NPs cavity. Perfect standing wave field distribution and the wave beam reflected by three inner walls could prove that the microcavity forms the WGM resonance mechanism [41,42]. Due to the photons could be trapped inside the cavity body by WGM resonance, the loss caused by scattering and reflection will be reduced. This could be one of reasons that a small nonsaturable loss can be obtained (αNS = 5.179%). In addition, the light-matter interactions has been enhanced by resonance which causing the CGZO NPs SA with large modulation depth (αS = 40.821%). This phenomenon promotes bleaching power of CGZO NPs SA, thereby increasing the optical-optical conversion efficiency of the laser.
4. Conclusions
In summary, utilize CGZO NPs as SA to realize a passively Q-switched Er3+ doped fiber laser have been demonstrated for the first time. The CGZO NPs SA film exhibits strong saturable absorption property, meanwhile with small nonsaturable loss 5.179% and the modulation depth is up to 40.821%. Stable Q-switched laser which centered at 1559.75 nm was achieved at a low threshold pump power of 42 mW. The maximum repetition rate of 31.5 kHz and the shortest pulse duration of 1.71 μs were obtained under 361 mW. Most interestingly, the optical-optical conversion efficiency is about 3.76%, which is relatively better than that of the Q-switched laser based on colloidal NCs, topological insulators, BP and TMDs. The possible reason would be ascribed to the WGM resonance which formed in CGZO NPs microcavity. Consider the resonance have the capability to trap photons by total internal reflection, these exceptionally long photon lifetimes could enhanced the light-matter interactions, leading to strengthen nonlinear optical response. This study indicates that Zn doped CGO NPs could be used as SA for constructing high efficiency and low threshold pulse lasers.
Appendix
The X-ray diffraction (XRD) pattern of powder CGO NPs
Figure 11 expressed the XRD pattern of powder CGO NPs. The XRD pattern confirmed the phase purity of the as-prepared CGO NPs, which matches well with the standard JCPDF card (No. 41-0255). The CGO NPs have the rhombohedral delafossite crystal structure, and suggesting the high purity of the as-synthesized products.
Funding
National Natural Science Foundation of China (61804014, 61805023, 61804013); The Excellent Youth Foundation of Jilin Province (20180520194JH); Planning Project of Jilin Provincial Education Department (JJKH20181112KJ); Changchun University of Science and Technology Youth Science Foundation (XQNJJ-2017-20); Innovation Fund of Changchun University of Science and Technology (XJJLG-2017-08).
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