We report a tunable passively Q-switched fiber laser at the wavelengths near 3 μm, using large aspect ratio gold nanorods (LAR-GNRs) as a saturable absorber (SA) for the first time. The GNRs with a large average aspect ratio of up to ~20 were prepared using the seed-mediated growth method, which yielded a strong absorption band of 2.2–3 μm with a peak at ~2600 nm, stemming from longitudinal surface plasmon resonance (SPR). The corresponding nonlinear absorption was characterized using 2.87 μm ultrafast pulses, giving the modulation depth of 8.89%, saturation intensity of 14.9 MW/cm2, and nonsaturation loss of 39.9%. When introducing the material into a tunable Ho3+/Pr3+ codoped ZBLAN fiber laser as a SA, stable Q-switched pulses with a tunable wavelength within 2.83–2.88 μm were achieved. The largest output power of 30.8 mW, repetition rate of 78.12 kHz, and narrowest pulse width of 2.18 μs were simultaneously attained when tuned to ~2.865 μm at the pump power of 307.2 mW, while the largest pulse energy of 0.48 μJ was obtained at the longest tuning edge of 2.88 μm. Our work indicates that LAR-GNRs are a type of versatile broadband SA material available for the mid-infrared region.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Pulsed version of fiber laser operating at 3 μm mid-infrared waveband has attracted increased attention in the past decade because of its active demand in a wide range of applications e.g., surgery, material processing, remote sensing, infrared countermeasures, etc . Q-switching, as one of the widely used methods to generate ns–ms scale short pulse with high pulse energy, was always paid much attention. Thereinto, the passive scheme resorting to saturable absorbers (SAs) was much more advantageous on the aspect of compactness, simplicity and low cost in design compared to the active scheme based on some externally controlled modulators. SESAM which maybe the current most mature commercial SA, was earliest introduced in a Ho3+/Pr3+ codoped ZBLAN fiber laser at ~3 μm to realize pulse generation in 2012 . The shortest pulse widths of 720 ns (Q-switching) and 24 ps (mode-locking) were gained. Despite its excellent performance and the ability of customizing some of its parameters e.g., modulation depth, non-saturation loss, recovery time, etc., mainly attributable to well-developed semiconductor technologies such as bandgap and defect engineering and growth, the narrow operation band of hundreds of nanometers and the limited effective wavelength of <3.2 μm hinder its utilization at longer wavelengths. Although Fe2+:ZnSe crystal could cover some shortages of SESAM to some extent , its bulk nature doesn’t match with the development tendency of all-fiber scheme in the future. With the development of material science and technology, novel two-dimensional (2D) materials e.g., graphene, topological insulators (TIs), transition mental dichalcogenides (TMDs), and black phosphorus (BP), antimonene, etc., have drawn our attention because of their distinct broadband saturable absorption properties and been also used to Q-switch or mode-lock Er3+- or Ho3+-doped ZBLAN fiber lasers around 3 μm [4–11]. In 2013, C. Wei et al demonstrated the first graphene based Q-switched Er3+-doped ZBLAN fiber laser at 2.78 μm. The maximum output power of 70 mW was obtained with a pulse width of 2.9 μs . After that, we used TI as the SA to realize a passively Ho3+-doped ZBLAN fiber laser at 2.98 μm giving an output power of 327.4 mW and a pulse width of 1.37 μs . Then, BP was also introduced into an Er3+-doped fiber laser at 2.8 μm by Z. P. Qin et al to generate stable pulses [6,8]. The shortest pulse widths of 1.18 μs (Q-switching) and 42 ps (mode-locking) were achieved. Recently, we presented a passively Q-switched Ho3+/Pr3+ codoped ZBLAN fiber laser at 2.87 μm using multi-layer antimonene. The maximum output power and shortest pulse width were 112.3 mW and 1.74 μs, respectively . But all have their own shortages. For example, graphene has weak absorption (~2.3% single layer) and low modulation depth. TIs require complex preparation process. TMDs possess a large bandgap and need complex control of defects when operating in the mid-infrared region. BP is easily oxidized under ambient condition while antimonene is difficultly tailored on the aspect of parameters.
Recently, gold nanoparticles (GNPs) received our extra attention since their unique optical properties such as large third-order nonlinearity (~10−6 esu), broadband absorption induced by surface plasmon resonance (SPR), and fast response time of few picoseconds [12–16], making it an ideal SA for Q-switching and mode-locking. In 2012, Jiang et al experimentally demonstrated a passively Q-switched Er3+-doped fiber laser at ~1.5 μm using gold nanospheres (GNSs) as a SA, for the first time . However, the isotropic dimensions of GNSs lead to only one SPR band with an absorption peak at 523 nm, which originates from transverse SPR absorption. Although the aggregated GNSs have been experimentally identified to be effective in a passively Q-switched solid-state laser at ~3 μm very recently , it is difficult to control the aggregation as required. Compared to GNSs, the non-spherical symmetric structure of gold nanorodes (GNRs) results in an anisotropic SPR showing two absorption bands. One is the weak transverse SPR absorption, originating from electron resonance that is perpendicular to the axial of rod. The other one is the strong longitudinal SPR absorption, originating from electron resonance that is along the axial of rod. In contrast to the former one, the latter one can be tuned in a wide range by varying the aspect ratio of GNRs. Specifically, the transverse SPR absorption peak was almost fixed at 532 nm while the longitudinal SPR absorption peak was red-shifting with increasing the aspect ratio , making it a more flexible broadband SA for pulse generation compared to GNSs and some other saturable absorption materials (e.g., MXene, plasmonic semiconductors, etc [20–23]). Until now, Q-switched and mode-locked fiber lasers in a wide near-infrared wavelength region (i.e., ~1 μm [19,24], ~1.5 μm [24–31], and ~2 μm [30,32]) have been constructed using GNRs with different aspect ratios as SAs. However, there are no reports at the longer mid-infrared wavelengths near 3 μm. In the previous reports, most aspect ratios of the used GNRs were less than 10, corresponding to longitudinal SPR absorption peaks of <2 μm. To modulate the ~3 μm laser more efficiently, it needs to exploit larger aspect ratios GNRs with a longer absorption peak closer to ~3 μm.
In this paper, we adopted the seed-mediated growth approach to prepare GNRs with a large average aspect ratio of up to ~20, contributing to a longitudinal SPR absorption peak at ~2600 nm. The nonlinear absorption of the LAR-GNRs was first characterized at 2.87 μm. Then introducing them into a Ho3+/Pr3+ codoped ZBLAN fiber laser as a SA, stable Q-switched pulses were obtained at a tunable range of 2.83–2.88 μm. The dependences of output power, pulse energy, repetition rate, and pulse width on pump power and laser wavelength were studied. To our knowledge, this is the first pulsed laser around 3 μm using LAR-GNRs as a SA.
2. Preparation and characterizations of LAR-GNRs SA
2.1 Preparation of LAR-GNRs SA
The LAR-GNRs used in our experiment were synthesized based on the seed-mediated growth method [33,34], and the schematic of the synthesis procedure is displayed in Fig. 1. It starts with the preparation of the seed solution. First, 10 mL aqueous solution containing 0.2 mM HAuCl4 and 0.1 M cetyl trimethyl ammonium bromide (CTAB) was prepared in a beaker. Then, 0.5 mL 0.02 M freshly NaBH4 solution was introduced into the above solution while stirring vigorously. The color of the solution turned brown immediately, indicating the formation of gold particles. After stirring for 10 min, the solution was kept undisturbed for 2 h at room temperature to decompose excess borohydride by water and used for the synthesis of the LAR-GNRs. The size of the GNRs was measured by using the image measurement software and then doing scale conversion. The seed particles had sizes of a few nanometers. Subsequently, the growth solution was prepared by mixing 1.4 mL 0.01 M HAuCL4 solution, 52.5 mL deionized water and 2 g CTAB in a beaker. Next, 1 mL 0.5 M HNO3 solution was added into the above solution. After that, three flasks were prepared and labeled A, B, and C, respectively. 4.5 mL of the growth solution was added into flasks A and B, and 45 mL to flask C. Then 25 μL, 25 μL, and 250 μL of 0.1 M ascorbic acid (AA) were, respectively, introduced into flasks A, B, and C under vigorously stirred for 3 min until they became colorless. Finally, 400 μL gold seed solution was added to flask A and shaken slightly for 10 s. Immediately, 400 μL of the solution in flask A was transferred to flask B and shaken for 10 s. After that, 4 mL of the solution in flask B was transferred to flask C and shaken for 5 s. Flask C was left undisturbed for 12 h in order to make sure of the full growth of GNRs. The final solution was concentrated by centrifugation at 5000 rpm for 10 min twice and redispersed in 10 mL of deionized water. Then it was deposited onto a CaF2 substrate and an Au mirror for linear/nonlinear absorption measurements and acting as SA, respectively.
2.2 Characterizations of LAR-GNRs SA
Figure 2(a) displays the transmission electron microscopy (TEM) image of the LAR-GNRs, which was measured using a microscope (JEM-2100F). It is found that most of the particles are nanorods with an average length of 380 nm and a width of ~19 nm, corresponding to an aspect ratio of ~20. Its inset shows the photograph of the aqueous solution of LAR-GNRs, the color of the solution is buff. In order to investigate its linear absorption characteristics at 3 μm band, the absorption spectrum was measured using the spectrometer (Shimadzu, UV 3600) based on the previous prepared sample on a CaF2 substrate under natural drying to avoid the strong absorption band around 3 μm of liquid water as shown in Fig. 2(b). Two obvious SPR absorption peaks are observed. The longitudinal SPR absorption peak at ~2600 nm, longer than the theoretically predicted value (2320 nm) (λ = 95R + 420 nm; R: aspect ratio) , was primarily caused by the aggregation or self-assembling of these GNRs. Another longitudinal SPR absorption peak at ~980 nm was resulted from the rare small aspect ratio GNRs during the synthesized process or the cascaded multiple transverse SPR peaks. The absence of the transverse SPR band at 532 nm suggested high yield of nanorods without significant existence of spheres in our case.
Then the nonlinear absorption of the LAR-GNRs sample was characterized using the typical power dependent measurement scheme which involved a balance twin-detector arrangement. The detailed setup and operation descriptions have been given in . The laser source was a home-made SESAM based passively mode-locked Ho3+/Pr3+ codoped ZBLAN fiber laser at 2.87 μm. It had a pulse width of ~20 ps and a repetition rate of 17.86 MHz. Figure 3 shows the transmission as a function of the input pulse intensity. The relative parameters of the LAR-GNRs sample were obtained by fitting using the following formula:
3. Design of wavelength tunable passively Q-switched fiber laser
The schematic of our designed wavelength tunable passively Q-switched Ho3+/Pr3+ codoped ZBLAN fiber laser using LAR-GNRs as a SA is shown in Fig. 4. The pump unit included two commercially available laser diodes (LDs) (Eagleyard Photonics, Berlin) around 1150 nm. The laser was used to pump the gain fiber after polarization multiplexing through a polarized beam splitter (PBS) and then focusing using an uncoated CaF2 plano-convex lens (LA5315, Thorlabs) with a 20 mm focal length. A specifically designed dichroic mirror with a ~96% transmission around 1150 nm and a >95% reflection around 3 μm was placed between the PBS and CaF2 lens at an angle of 30° with respect to the pump beam to steer the laser. After that, a commercial 3 μm bandpass filter (FB3000-500, Thorlabs) with an operation bandwidth of 500 nm was used to block the residual pump. It had a measured transmittance of ~76% at ~2.87 μm. The gain fiber was a piece of commercial double-cladding Ho3+/Pr3+ codoped ZBLAN fiber (Fiberlabs, Japan) with an octangular pump core with a diameter of 125 μm and a NA of 0.5 and a circular core with a diameter of 10 μm and a NA of 0.2. It could lase at around ~3 μm from the 5I6→5I7 transition of Ho3+ ions. The concentrations of Ho3+ and Pr3+ were 30,000 and 2,500 ppm, respectively. The launching efficiency was estimated to be 82% . The selected fiber length was 5 m which could provide a ~90% pump absorption efficiency. The fiber end close to the pump was perpendicularly cleaved as one cavity feedback and the output coupler with the help of 4% Fresnel reflection. The other end of the fiber was cleaved at an angle of 8° to avoid parasitic lasing, from which the laser was collimated by a ZnSe objective lens (Innovation Photonics, LFO-5-6, 0.25 NA) with a 6.0 mm focal length onto a plane ruled grating (Thorlabs, 450 lines per mm, blaze wavelength λB = 3.1 µm, blaze angle θB = 32°). Then its first-order reflection was terminated by the LAR-GNRs coated Au mirror (Thorlabs) after focusing using another same ZnSe objective lens as before. According to the used lens, the minimum spot size on the Au mirror was estimated to be ~10 μm. The wavelength was tuned by rotating the plane ruled grating. An InAs detector with a response time of 2 ns connected with a 500 MHz bandwidth digital oscilloscope was used to capture temporal pulse trains and waveforms. A radio frequency (RF) spectrum analyzer (AV4033A) with a scanning range of 30 Hz-18 GHz was used to measure the pulsed RF spectrum. A monochromator with a minimum scanning resolution of 0.1 nm (Princeton instrument Acton SP2300) was utilized to measure the optical spectrum.
4. Experimental results of passively Q-switched fiber laser
4.1 Wavelength fixed Q-switched operation
First, the grating angle was adjusted to maximize the output at the pump power of 307.2 mW, which was determined by controlling the pump power while monitoring the output behavior. For this grating position fixed, stable Q-switching regime was obtained as shown in Fig. 5(a) as the pump power increasing to 115.3 mW, at which the pulse width and repetition rate were 3.46 μs and 42.3 kHz, respectively. This regime could maintain until the pump power of 307.2 mW as plotted in Fig. 5(b) where the pulse width and repetition rate were 2.18 μs and 78.12 kHz, respectively. Once the pump was adjusted beyond this level, the pulse train began to become unstable and then reverted to CW operation with the further increased pump power to 387.6 mW. At this time, if the pump was adjusted back to ~305 mW, stable Q-switching appeared again indicating the LAR-GNRs was free from damage. This phenomenon was primarily caused by the excessive heat from Q-switched pulses, and also observed in our previous experiment . But Q-switching could not be obtained anymore if the pump power was increased beyond 387.6 mW no matter how to adjust the pump power, indicating the damage threshold of our used LAR-GNRs. At the pump power of 307.2 mW, the optical and RF spectra were measured as shown in Fig. 5(c) and 5(d), respectively. The fixed operation wavelength of 2864.3 nm matched well with the free running wavelength at the pump power, since it was selected to achieve the maximum output power. The FWHM of 0.6 nm was significantly smaller than that under free running state [9,11] due to the narrow operation linewidth of the plane ruled grating, but still limited by the distance of ~0.55 m between the grating and the Au mirror considering the scanning resolution of 0.3 nm of our monochromator around 3 µm. The captured RF spectrum yielded a signal-to-noise ratio (SNR) of 31.9 dB at the frequency of 78.12 kHz, which was located at the typical range of passive Q-switching based on some new SA materials [4–7,9,11]. Figures 6(a) and 6(b) show the pulse width, repetition rate, output power and pulse energy as a function of the pump power, respectively. The evolution tendency of both pulse width and repetition rate depending on the pump power matched with the case of typical passive Q-switching. Specifically, with the pump power increasing from 115.3 mW to 307.2 mW, the pulse width decreased from 3.46 µs to 2.18 µs, while the repetition rate increased from 42.3 kHz to 78.12 kHz. The output power increased from 6.8 mW to 30.8 mW at a slope efficiency of 12.0% while the pulse energy increased from 0.16 µJ to 0.38 µJ. The low slope efficiency was mainly resulted from three aspects: (1) non-saturation loss of the LAR-GNRs, (2) insertion loss (~1dB) introduced by the used pair of ZnSe objective lenses, and (3) diffraction loss from the plane ruled grating. Also the power stability of the Q-switched pulses was checked within 240 min as shown in Fig. 7, the low power fluctuation of about 0.8% indicated high stability of our Q-switched system. Recently (about two months after finishing the above experiment), the same SA was also introduced into another Er3+-doped ZBLAN fiber laser to achieve stable Q-switching at 2.8 µm. The results suggested that the prepared LAR-GNRs had high thermal and chemical stability.
4.2 Wavelength tunable Q-switched operation
Then wavelength tuning under Q-switching operation was performed by rotating the grating at the pump power of 307.2 mW. Figure 8 shows the optical spectra of the Q-switched pulses when tuned. The tuning range was about 46.5 nm spanning from 2834.5 nm to 2881.0 nm with a narrow FWHM of ~0.6 nm across the entire tuning range. It was just located within the absorption band of NH3 and NO2 , while avoiding the strong absorption of water vapor around 2.8 μm , hence better serving gas detection. Once tuning beyond this range, the laser began to operate at CW regime since the intra-cavity laser power was not strong enough to reach the saturation intensity of the LAR-GNRs anymore due to the decreased gain at two tuning edges. This point could be identified by the fact that the Q-switching appeared again if further increasing the pump power at this time. However, no laser could be attained if further tuning towards two sides, indicating that the intra-cavity gain was smaller than the loss outside of the range at the pump power of 307.2 mW. Figure 9(a) shows the evolutions of output power and repetition rate depending on the tuned wavelength at the pump power of 307.2 mW. It is observed that the output power increases from 18.3 mW to 30.8 mW and then decreases to 23.1 mW with the wavelength tuning from 2834.5 nm to 2881.0 nm, which matched with the gain spectrum of Ho3+/Pr3+ codoped ZBLAN fiber . While the pulse repetition rate exhibited a similar variation tendency to the output power since higher intra-cavity power led to faster blenching of SA hence larger repetition rate . The corresponding evolutions of pulse width and energy are shown in Fig. 9(b). In contrast to Fig. 9(a), the pulse width decreased first and then increased with tuning towards long wavelength direction, similar to the tendency observed in wavelength tunable passively Q-switched Er3+-doped fiber lasers around 1.5 μm [38,39]. While the pulse energy almost increased monotonously from 0.29 μJ to 0.48 μJ.
In our experiment, no mode-locked pulses were observed mainly due to low intra-cavity laser intensity and large insertion loss of the SA, which made the system difficult to work in mode-locking regime. Here, the large insertion loss was primarily caused by the increased scattering loss with the enlarged aspect ratio. Moreover, it was also related to the solution concentration and could be reduced by further concentration optimizing while controlling the dispersibility. In the future, the polarization dependent loss property of GNRs  could be utilized as well to fabricate polarizer while acting as a SA. This new device would be useful for hybrid mode-locking combined with nonlinear polarization rotation (NPR) effect.
In this case, the maximum tunable range under Q-switching operation was about 46.5 nm (2834.5–2881.0 nm), which was significantly narrower than that under CW operation extended to ~150 nm (2825–2975 nm) . The primary reason was the low pump power allowable, which was not strong enough to make the intra-cavity net gains at longer or shorter wavelengths within the gain spectrum of Ho3+/Pr3+ codoped ZBLAN fiber reach the threshold levels. This was limited by the heat accumulated on the LAR-GNRs induced material damage. Thus further extending the tunable range could resort to improving the SA damage threshold by carefully expanding the spot focused onto the LAR-GNRs under Q-switching operation while imposing cooling on the Au substrate. Another effective approach to improve the SA damage threshold is to use the ZBLAN microfiber with the help of evanescent effect, which can enlarge the interactive area between laser and LAR-GNRs thus reducing the photothermal effect of the SA. Although the aspect ratio of ~20 was enough for efficiently modulating ~3 μm laser hence generating pulses, larger aspect ratio was demanded for the lasers at longer wavelengths (e.g., ~3.5 μm , ~3.9 μm ). This could be realized through changing the size of gold seeds or reaction time and concentration of growth solution, despite the uncontrollable yield of nanorods relative to byproducts such as spheres. Besides, the template approach  and assembly method  would be adopted to improve the ratio aspect of GNRs as well in the future.
In this paper, we demonstrated a wavelength tunable passively Q-switched Ho3+/Pr3+ codoped ZBLAN fiber laser at the range of 2.83–2.88 μm, using LAR-GNRs as a SA for the first time. Based on the seed-mediated growth approach, the GNRs with a large average aspect ratio of ~20 were prepared, contributing to a long longitudinal SPR band peaked at ~2600 nm. This is the current longest absorption peak of GNRs reported and suited to modulating ~3 μm laser. The nonlinear absorption of the LAR-GNRs was characterized at 2.87 μm as well, giving the modulation depth of 8.89%, saturation intensity of 14.9 MW/cm2, and non-saturation loss of 39.9%. Introducing the LAR-GNRs into a linear-cavity tunable Ho3+/Pr3+ codoped ZBLAN fiber laser as a SA, stable Q-switched pulses with a tunable wavelength within 2834.5–2881.0 nm were achieved. It was found that there was an optimal wavelength for getting the largest output power and repetition rate, and narrowest pulse width for a fixed pump power. While long wavelength operation was helpful for higher pulse energy. Finally, the largest output power of 30.8 mW, repetition rate of 78.12 kHz, and narrowest pulse width of 2.18 μs were obtained at the tuned wavelength of ~2.865 μm at the pump power of 307.2 mW, and the largest pulse energy of 0.48 μJ was gained at the longest tuning edge of 2.88 μm. This work indicates that GNRs are a kind of versatile nonlinear material for SA fabrication aiming at different operation wavelengths. In the future, more efforts would be paid on preparing larger aspect ratio GNRs for pulse generation at longer mid-infrared wavelengths.
National Natural Science Foundation of China (NSFC) (61435003, 61722503, 61605219, 61527823, and 61421002); Open Fund of Science and Technology on Solid-State Laser Laboratory, Fundamental Research Funds for the Central Universities (ZYGX2016J068); International Scientific Cooperation Project of Sichuan Province (2017HH0046); Joint Fund of Ministry of Education for Equipment Pre-research (6141A02033411); Science and Technology Project of Jilin Province (20160520085JH); Key Technology Research and Development Project of Jilin Province (20180201120GX); Youth Innovation Promotion Association, CAS.
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