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Abstract
Au nanocages (Au-NCs) have attracted wide attention as low-dimensional materials with broadband absorption, ultrafast optical response, large third-order optical nonlinearity coefficient, and high photothermal stability and thermal tolerance. By employing Au-NCs as saturable absorbers, we demonstrate a widely tunable passively Q-switched erbium-doped fluoride fiber laser at the wavelength of 2.8 µm. When operates at 2778.0 nm, this laser delivers stable Q-switched pulses with a maximum average power of 584.6 mW at a pulse repetition rate of 80.6 kHz. The minimum pulse duration attained was 1.16 µs corresponding with the single pulse energy of 7.25 µJ. Our results present onefold increase in pulse energy over previously published values achieved from Au nanoparticles based 3-µm passively Q-switched fiber lasers. By introducing a plane ruled grating, a tuning rage of 57.0 nm from 2753.0 to 2810.0 nm is achieved, while maintaining stable Q-switched operation. To our knowledge, this is the first time to demonstrate that Au-NCs can realize mid-infrared pulsed laser. Our research results show that Au-NCs are promising broadband nonlinear modulators for mid-infrared pulse generation.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Pulsed lasers operating in the 3 µm mid-infrared (mid-IR) spectral region are promising sources for applications in a variety of scientific and industrial fields including molecular spectroscopy, laser surgery, material processing, remote sensing, and mid-IR supercontinuum source generation [1–5]. Compared with other types of mid-IR pulsed lasers, fiber lasers feature several advantages due to their compactness, high efficiency, ruggedness, and outstanding beam quality. While a variety of modulation techniques are currently being considered for mid-IR ns-ms pulsed laser operation, including actively Q-switching [6–8] and gain-switching [9,10], there is a particular demand for passively Q-switching approaches, due to their compactness and simplicity with the absence of additional electronic equipment. This has led to significant sustained research effort to reliably generate energetic pulses from a number of mid-IR passively Q-switched fiber lasers. A nonlinear optical modulator with saturable absorption behavior in the mid-IR waveband plays an important role in the efficient pulse generation from a mid-IR passively Q-switched fiber laser. Saturable absorbers (SAs) that have been employed in the 3-µm mid-IR pulsed fiber lasers include semiconductor saturable absorption mirrors (SESAMs) [11–13], Fe2+:ZnSe crystals [14–17], low-dimensional materials such as single-walled carbon nanotubes (SWCNTs) [18,19], graphene [20], topological insulators (TIs) [21,22], black phosphorus (BP) [23], transition metal dichalcogenides (TMDs) [24], and antimonene [25]. Nevertheless, SESAM has narrow absorption band, slow response time, and requires a rather complex fabrication process. Moreover, introducing bulk SESAM and Fe2+: ZnSe crystal in the laser cavity compromises the simplicity and compactness of the fiber laser while also introduces stringent alignment requirements. The newly emerging low-dimensional nanomaterials have gained much attention in the pulsed mid-IR laser community for their fascinating advantages including broadband nonlinear absorption performance and ultrafast recovery time. However, practical applications of them still suffer from some drawbacks. For example, SWCNTs have to achieve a tradeoff between broadband operation and high non-saturable loss. Graphene exhibits weak absorption (∼2.3% per layer) and low modulation depth. TIs require complicated preparation process that severely limits their application in optoelectronic devices. Due to the large band gap of TMDs, suitable defects need to be introduced when TMDs are used for mid-IR optoelectronics and the fabrication process might become extremely difficult. BP is easily oxidized. Although antimonene is known for its high stability, it is rather difficult to be fabricated with tailored parameters. In addition, artificial SAs including nonlinear polarization rotation and nonlinear amplifying loop mirror have also been widely used in ultrafast pulses generation [26–29], nevertheless, the artificial SAs are extremely sensitive to polarization changes and perturbations of ambient environment.
In recent years, low-dimensional noble metal materials such as Au nanoparticles (Au-NPs) have attracted considerable attention and been proposed as SAs in Q-switched pulsed lasers attributed to their broadband absorption, large third-order optical nonlinearity coefficient (∼10−6 esu), and ultrafast optical response (a few picoseconds) and recovery time caused by the localized surface plasmon resonance (LSPR) [30–35], which refers to a strong electromagnetic near-field effect related to the collective oscillation of conduction electrons. The LSPR absorption peak can be tuned readily and conveniently from visible to mid-IR spectral region [36–45]. By adjusting their shape, size, and internal structure, Au-NPs can exhibit wide absorption based on the LSPR, compared with some other saturable absorption materials like MXene, Bismuthene, etc. [46–48]. These superior properties indicate that Au-nanomaterials can be promising SAs for pulse generation in mid-IR fiber lasers. Up till now, gold nanorods [39,41], gold nanospheres [42], gold nanobipyramids [43], gold nanocrystals [44], gold nanostars [45] have been served as SAs for realizing pulsed fiber lasers at mid-IR wavelength region. Compared with these Au-NPs, Au nanocages (Au-NCs) possess some remarkable properties such as large absorption cross section (∼1.13 × 10−14 m2) [49], surface-enhanced Raman scattering [50], high photothermal stability, high thermal tolerance, and fast thermal diffusion time (tens of picoseconds) [35] thanks to their irregular structure with hollow interiors and porous walls, which effectively increase the surface/volume ratio. Therefore, in contrast to other solid nanoparticles, Au-NCs are less likely to suffer from the influence of thermal coagulation that might lead to the decrease of absorption efficiency and the deviation of the absorption peak. Besides, the hollow structure of Au-NCs promises the encapsulation of other materials, which might attribute to new saturable absorber synthesis [49]. Furthermore, Au-NCs with the characteristic of easy synthesis and the ability to precisely tune their desirable SPR peak position, can also conveniently adjust their scattering and absorption cross-sections [51]. All these outstanding properties guarantee Au-NCs to be broadband SAs for pulse generation in fiber lasers. Au-NCs have been utilized as SAs to realize Q-switched Yb-doped fiber lasers at 1.06 µm [52,53]. However, there have been no demonstration concerning a passively Q-switched fiber laser based on Au-NCs to date.
In this paper, we experimentally demonstrate a stable passively Q-switched erbium (Er3+) doped ZBLAN fiber laser with a 57.0 nm tuning range from 2753.0 to 2810.0 nm based on Au-NCs SAs. When the laser operates at 2778.0 nm, stable Q-switched pulses with an average output power of 584.6 mW and a pulse energy of 7.25 µJ were obtained at the launched pump power of 6.86 W, representing the highest level of Au-NPs based 3-µm passively Q-switched fiber laser systems. The shortest pulse duration was 1.16 µs at the repetition rate of 80.6 kHz. To our best knowledge, this is the first time that Au-NCs are used as SAs to realize pulsed laser around 3 µm.
2. Preparation and characterizations of Au-NCs SA
The Au-NCs were synthesized by the galvanic replacement reaction between Ag nanocubes and HAuCl4, and provided by Nanjing XFNANO materials Tech Co., Ltd. The Au-NCs were dispersed in an aqueous citrate solution to create a suspension with a density of 50 µg/mL. We collected the supernatant after centrifugation at 2000 r/min for an hour, and then drop-coated it onto a gold mirror (acting as the SA) and a CaF2 substrate (for linear absorption measurement), respectively. The Au-NCs coated gold mirror and CaF2 substrate were then dried at room temperature for 12 hours.
The morphology and size of the as-prepared Au-NCs sample were measured with a transmission electron microscopy (JSM-7500F), as shown in Fig. 1(a). The average diameter of the Au-NCs is about 100 nm, and the color of the Au-NCs suspension is light blue, as shown in the inset of Fig. 1(a). The ununiform morphology might be due to the adding amount of HAuCl4 during preparation, indicating high porosity and thin wall thickness [54,55]. The structure not only provides Au-NCs with fast thermal diffusion time, high thermal tolerance and high photothermal stability but also contributes to the red-shift of absorption peak position [49,51]. The linear absorption spectrum of the Au-NCs film on the CaF2 substrate was captured by a Fourier transform infrared spectroscopy (FTIR) spectrometer (Nicolet iS50R) and indicated in Fig. 1(b). The absorbance of the CaF2 substrate, which had been measured in advance, was subtracted from the absorption spectrum of the film. It can be seen that Au-NCs exhibit wideband absorption with two obvious peaks. One absorption peak at around 1.5 µm is caused by the LSPR of single Au-NC with high porosity and thin wall [38]. The other absorption peak extended to around 3 µm could be ascribed to the aggregation and assembly resulted from the overlapping of Au-NCs [56–58], which is helpful for maintaining good modulation depth in 3-µm region.
Fig. 1. (a) TEM image of Au-NCs on the scale of 100 nm. (b) Linear absorption spectrum of the Au-NCs.
The nonlinear transmission of the Au-NCs was characterized by a typical balanced twin detector system elaborated in [24]. A home-made 2850 nm SESAM based mode-locked Ho3+/Pr3+ co-doped ZBLAN fiber laser with a repetition rate of 17.6 MHz and pulse duration of 22 ps was utilized as the laser source. The parameters of the Au-NCs SA are fitted with the following equation $T(I )= 1 - \Delta T \cdot \exp ({{{ - I} \mathord{\left/ {\vphantom {{ - I} {{I_{sat}}}}} \right.} {{I_{sat}}}}} )- {T_{ns}}$, where T(I) indicates the transmittance, I is the incident intensity, ΔT is the modulation depth, Isat is the saturation peak intensity and Tns represents the non-saturable loss. As described in Fig. 2(a), the modulation depth, non-saturable loss, and saturation peak intensity are determined to be 10.83%, 3.24%, and 2.38 µJ/cm2, respectively. The sample thickness was characterized with an atomic force microscopy (AFM). The AFM image and corresponding height were exhibited in Fig. 2(b) and 2(c), respectively. The thickness varies from 98 to 190 nm, which indicates the overlapping or the self-assembly of the Au-NCs, since the particle diameter measured by TEM was merely about 100 nm.
Fig. 2. (a) Nonlinear transmission of the Au-NCs sample. (b) AMF image and (c) the corresponding height profile of Au-NCs.
3. Experimental setup
The schematic diagram of the experimental setup of the passively Q-switched Er3+ doped ZBLAN fiber laser based on Au-NCs SA is shown in Fig. 3. The fiber was pumped by a commercially available 976 nm laser diode (LD) (BTW, Beijing). The pump light was first collimated by an aspheric condenser lens (L1) with a focal length of 16 mm and then focused into the first cladding of a fluoride fiber using a CaF2 plano-convex lens (L2) with a focal length of 20 mm. We used a 3.0 m long 6 mol.% double-cladding Er3+ doped ZBLAN fiber (Fiberlabs, Japan), which has a circular core with a diameter of 33 µm (NA = 0.12) and a first cladding with a diameter of 300 µm (NA = 0.51), providing efficient pump absorption. The front fiber end was perpendicularly cleaved to provide one cavity feedback, while the other end of the fiber was cleaved at an angle of 10° to eliminate the influence of the Fresnel reflection. An anti-reflection CaF2 plano-convex lens (L3) with a focal length of 20 mm was employed to collimate the laser beam, and a CaF2 plano-convex lenses (L4) with same focal length was used to focus the laser onto the Au-NCs-coated gold mirror. Two gold mirrors (GM1 and GM2) were employed to enhance the compactness of the laser system.
Fig. 3. Schematic diagram of the passively Q-switched Er3+ doped ZBLAN fiber laser using the Au-NCs-based SA.
A dichroic mirror (DM) placed between L1 and L2 at an angle of 45° to the incidence was served as the output coupler. The total length of the free-space propagation segment in this cavity was about 70 cm. A power meter (Laserpoint) was utilized to measure the average power of the output laser beam along with an IR bandpass filter (FB2750-500, Thorlabs). The pulse train and waveform were recorded by a 2 ns response time InAs detector connected with a 500-MHz-bandwidth digital oscilloscope. A monochromator that employ a liquid nitrogen cooled InAs photodiode (Princeton instrument Acton SP2300) was utilized to measure the laser spectrum with a scanning resolution of 0.1 nm.
4. Results
4.1 Performance of the Q-switching operation at a fixed wavelength
Stable Q-switched operation was obtained when the launched pump power was increased to 1.85 W. It can be seen that the repetition rate of the Q-switched pulses was 32.5 kHz, as shown in Fig. 4(a). Stable Q-switched operation could maintain until the launched pump power exceeded 6.86 W. Figure 4(b) shows the Q-switched pulse train at the maximum launched pump power of 6.86 W. We can see that the fundamental repetition rate was 80.6 kHz. The pulse envelops of the Q-switched pulses at 1.85 W (red) and 6.86 W (blue) are described in Fig. 4(c), with corresponding pulse durations of 3.04 µs and 1.16 µs, respectively. Moreover, the rather small pulse intensity fluctuation shown in Figs. 4(a) and 4(b) indicates that the Q-switching regime is stable. This is further confirmed by the radio frequency (RF) spectra of the signal shown in Fig. 5(a), which was characterized with a RF spectrum analyzer (YIAI, AV4033A). The signal-to-noise ratio (SNR) was measured to be 38.2 dB at a resolution bandwidth (RBW) of 100 Hz with a 50-kHz scanning span. The optical spectrum measured under the maximum launched pump power is characterized by Fig. 5(b). The central wavelength is at 2778.0 nm, and the full width at half maxima (FWHM) is about 1.31 nm.
Fig. 4. Q-switched pulse trains (a, b) and single pulse waveforms (c) at the launched pump power of 1.85 W and 6.86 W, with corresponding repetition rate of 32.5 kHz and 80.6 kHz, respectively.
Fig. 5. (a) RF spectrum and (b) optical spectrum of the passively Q-switched Er3+ doped ZBLAN fiber laser using an Au-NCs based SA.
Figure 6(a) indicates the average output power and pulse energy as functions of the launched pump power. As the launched power increased from 1.85 W to 6.86 W, the output power increased almost linearly from 76.8 mW to 584.6 mW. The slope efficiency η of this laser system is 10.5%, and the low slope efficiency was mainly resulted from the scattering loss of Au-NCs, which can be diminished by reducing the size and/or increasing the porosity of Au-NCs [51]. The pulse energy grew from 2.36 µJ to 7.25 µJ. The repetition rates and pulse durations at various launched pump powers were also measured, as displayed in Fig. 6(b). The pulse repetition rate increased from 32.5 kHz to 80.6 kHz, while the pulse duration decreased from 3.04 µs to 1.16 µs, which is typical Q-switched laser behavior. A maximum average output power of 584.6 mW with a repetition rate of 80.6 kHz and a pulse width of 1.16 µs was obtained at the launched pump power of 6.86 W.
Fig. 6. (a) Output power and single pulse energy, (b) repetition rate and pulse duration as functions of the launched pump power.
When the launched pump power exceeded 6.86 W, Q-switched pulses started to become unstable with noticeable amplitude fluctuation and timing jitter. Moreover, once the incident pump power is greater than the damage threshold of 7.61 W (∼2.6 J/cm2), Q-switched operation could not be achieved anymore, even after decreasing the pump power. The SA might be damaged due to the photothermal effect occurring in the Au-NCs, but there was no visible deformation of the SA being observed during and after experiment.
4.2 Wavelength tunable Q-switching operation
A tunable passively Q-switched Er3+: ZBLAN fiber laser was achieved through replacing the gold mirror DM2 by a plane ruled grating (450 lines/mm, blazed at 3.1 µm, Thorlabs) in the aforementioned experimental setup as shown in Fig. 3. Rotating the grating at the pump power of 3.85 W, a widest tuning range of 57.0 nm spanning from the central wavelength of 2753.0 nm to 2810.0 nm could be obtained. Figure 7 depicts the optical spectra with corresponding average output powers of this wideband tunable passively Q-switching operation, which can be maintained within the tuning range. The output power varied from 136.4 mW to 268.7 mW, and the maximum output power was recorded at 2778.0 nm. It can be observed that the output power increases firstly, and then decreases from 2778.0 nm to 2803.0 nm, consistent with the gain spectrum of Er3+ doped ZBLAN fiber. The output power increase from 2803.0 nm to 2810.0 nm might due to the absorption peak of Au-NCs at around 3.0 µm. The tunable range limit might be primarily attributed to the high intracavity loss and the gain spectrum of the Er3+ doped ZBLAN fiber instead of the SA. By replacing the Er3+-doped fiber by a piece of Ho3+/Pr3+ co-doped fluoride fiber in the cavity and then pumping it with a commercially available 1150 nm LD (Eagleyard Photonics, Berlin) at an interval of more than 2 months, we also observed stable Q-switching operation at 2.9 µm. Besides, the long-term tolerance of illumination was demonstrated by exposing the SA to constant illumination for about 10 hours. Experimental results manifested that SA can serve as a broadband Q-switcher with high stability.
Fig. 7. Normalized spectra and the corresponding average output power of the tunable passively Q-switched Er3+: ZBLAN fiber laser.
5. Discussion
To further demonstrate the excellent properties of Au-NCs, laser performances of mid-IR passively Q-switched lasers employing varied Au-NPs as SAs are presented in Table 1. Compared with reported lasers employing other gold-nanomaterials Q-switchers, the Au-NCs based Er3+ doped ZBLAN fiber laser shows the highest average output power. Moreover, the pulse energy is almost twice of the greatest value from other reported results in the 3-µm mid-IR waveband, which might be attributed to the large surface/volume ratio and absorption cross section, high photothermal stability and thermal tolerance, fast thermal diffusion time of Au-NCs. The pulse duration and repetition rate are comparable to those from previous works with Au-NPs.
Table 1. Comparison of Laser Performance of Mid-IR Passively Q-switched Fiber Lasers with Different Gold Nanomaterials
Nanosecond level pulse width could be expected by optimizing the length and doping concentration of the Er3+ doped ZBLAN fiber, and/or enhancing the SA damage threshold. The damage threshold of the SA can be increased by employing passive cooling to the gold mirror substrate on which the Au-NCs were coated. Alternative approach to prevent the photothermal effect includes expanding the interactive area between the laser beam and Au-NCs material. For example, adjust the size of the focused beam spot on the Au-NCs, or adopt Au-NCs side-coated fluoride tapered fiber and make use of the evanescent field interaction. In addition, among all the gold nanomaterials served as SAs in Mid-IR region, Au-NCs exhibited great tunability with tunable range of 57.0 nm, compared with LAR-GNRs (46.5 nm) and GNRs (49.8 nm). The comparison and results indicate that Au-NCs are a kind of promising nonlinear material that can act as Q-switchers for mid-IR fiber lasers.
During our experiment, there was no stable continuous-wave mode-locking being observed, which might be caused by the un-optimized parameters of the SA and/or the current laser cavity design. A smaller size will result in a faster recovery time [35], and the wall thickness and porosity of Au-NCs can also influence the SA’s optical properties [49,51]. Moreover, the large insertion loss of the Au-NC-SA, which was primarily caused by the increased scattering loss, can prevent the fiber laser from operating in the mode-locking regime as well. The stability of Q-switching or mode-locking operation can be promoted through inserting an isolator into the cavity, and/or reducing the length of free-space propagation segment. Mode-locked operation of the Er3+ doped ZBLAN fiber laser by optimizing the linear and nonlinear absorption of the Au-NCs while maintaining relatively high intra-cavity laser intensity is currently underway.
6. Conclusion
In conclusion, we have investigated the nonlinear saturable absorption properties of the Au-NCs at 2.8 µm and demonstrated a tunable mid-IR fiber oscillator delivering mid-IR Q-switched pulses by employing the Au-NCs as SAs for the first time, to the best of our knowledge. The central wavelength of the Q-switched pulses could be tuned 57.0 nm (from 2753.0 to 2810.0 nm). This fiber laser system could produce stable Q-switched pulses at 2778.0 nm, with a maximum output power of 584.6 mW and pulse energy of 7.25 µJ, which are both the highest values compared with other reported Au-NPs based passively Q-switched fiber lasers at 3-µm waveband. The corresponding pulse duration and repetition rate were 1.16 µs and 80.6 kHz, respectively. The results indicate that Au-NCs are promising broadband SAs for generating stable laser pulses at 3-µm mid-IR spectral region and have great potential as ideal mid-IR SAs for longer wavelength.
Funding
National Natural Science Foundation of China (61875033, 61705147, 61435003, 61775031, 61421002); Chengdu Science and Technology Huimin Project (2016-HM01-00269-SF, 2016-HM01-00265-SF,); Fundamental Research Funds for the Central Universities (YJ201654).
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