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Abstract
In this paper, high quality few-layer MXene-Ti3C2Tx was fabricated by the LPE method and successfully applied as saturable absorber for a passively Q-switched (PQS) Ho,Pr:LLF laser operating at 2.95 µm. The maximum average output power was determined to be 105 mW with a slope efficiency of 5%. The shortest pulse width and the largest pulse repetition rate were determined to be 266.7 ns and 83.24 kHz, corresponding to the single pulse energy and pulse peak power of 1.26 µJ and 4.73 W, respectively. It is the first demonstration of MXenes applied in mid-infrared (MIR) PQS solid-state bulk lasers, to the best of our knowledge. The results not only verify the broadband nonlinear saturable absorption properties of MXenes, but also pave the way for exploring their applications in photonic devices.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
3.0 µm wavelength-band MIR pulsed lasers possess the advantages including but are not limited to the following: First, 3.0 µm laser overlaps with the strong absorption peak of water (absorption coefficient ∼104 cm-1) [1], leading to the capable applications in laser medicine; Second, the pulsed 3.0 µm laser can be used as the pump source for optical parametric oscillators (OPOs) to generate long wave infrared laser [2]; Third, the molecular resonance peaks of the most gases locate at 3.0 µm wavelength band, making it exhibit great potential for trace gas detections [3]. Therefore, they have attracted an increasing interest and played a significant role in the fields of laser medicine, industry, military, scientific research, and so on. This kind of laser source can usually be obtained by the following two methods: OPOs pumped by the proven 1.0 µm laser and directly emitted from ions-doped (e.g., Er3+, Ho3+) laser crystals [4–8]. Compared with OPOs, the latter one has the merits of more robust and compact, higher conversion efficiency and better laser beam quality.
Saturable absorber (SA), a key element for the PQS and mode-locked pulsed lasers, plays a significant role in ultrafast optical modulation for turning the laser operation from continuous wave (CW) into the pulsed regime. Since the first demonstration of a graphene-based SA in 2009 [9], two-dimensional (2D) materials have recently been widely studied due to their unique nonlinear optical (NLO) properties, including fast response, broadband operation, and easy fabrication and integration with lasers, making them to be excellent SAs for pulsed laser generation. To date, various 2D materials including graphene, transition metal dichalcogenides (TMDs), topological insulators (TIs), black phosphorus (BP), as well as van der Waals (vdW) heterostructures have proved to have excellent saturable absorption properties and widely used for PQS and mode-locked pulse generation [9–17].
Among 2D materials, MXenes, a latest new class of 2D materials, are transition metal carbides, carbonitrides and nitrides with the chemical formula of MnXn+1Tx (n = 1, 2 or 3), where M represents the transition metal (e.g., Ti, Ta, Cr, Mo, etc), X is C and/or N, and T is surface functional group (e.g., hydroxyl (H), oxygen (O) or fluorine (F)) [18]. MXenes are structurally analogous to graphene and can be synthesized by etching MAX phases with removing the “A” elements, resulting in the typical structure in the forms of [MX]nM with n layers of X covered by n + 1 layers of M. Being discovered recently, MXenes have already been intensively studied both theoretically and experimentally [18–21]. They have proved to exhibit great potentials for the applications in magnetic, electric, thermoelectric, optical and sensing devices. In particular, they have been demonstrated to possess distinct nonlinear saturable absorption properties. Due to the metallic electronic band structure, their optical absorption can cover a wide frequency band ranging from near-infrared to MIR region, indicating the broadband SA applications [22–24]. Y.I. Jhon et.al reported a stable mode-locked Er-fiber laser with a pulse width of 660 fs at 1157 nm [23]. Liu’s group demonstrated a MXene (Ti3C2Tx) based PQS Nd:YAG ceramic laser operating at 1.06 µm with the shortest pulse width of 359 ns [25]. A highly efficient CW mode-locked solid-state laser by using Ti3C2Tx as SA, generating the pulse duration of 316 fs with the output power of 0.77 W has been realized [26]. However, the saturable absorption applications in the 3.0 µm MIR region have rarely reported.
In this paper, high quality few-layered titanium carbide (Ti3C2Tx), a typical member of MXenes family, was successfully fabricated by liquid phase exfoliation (LPE) method and applied as a SA for PQS Ho,Pr:LLF laser operating at 2.95 µm. The shortest pulse width of 266.7 ns was obtained with the repetition rate of 83.24 kHz and average output power of 105 mW, corresponding to the pulse peak power and single pulse energy of 4.73 W and 1.26 µJ, respectively. The results indicate that Ti3C2Tx is an outstanding SA for MIR pulsed laser generation.
2. Ti3C2Tx-SA fabrication and characterization
Similar to other 2D materials, Ti3C2Tx is held together by strong in-plane covalent bonds and weak out-of-plane van der Waals forces. Here, high quality few-layered Ti3C2Tx nanosheets were fabricated by LPE method. The commercial available high purity Ti3C2Tx powder (99.99%) was firstly dispersed into the alcohol solvent. Then it was ultra-sonicated for 6 hours to break down the inter-layer van der Waals bonding and form the thin nanosheets. After that, the solution was centrifuged at 3000 rmp for 10 mins. Followed by 10 mins’ standing, the supernatant liquor was collected and transferred onto a sapphire substrate by spin-coating method to form the uniform 2D nanosheets. Finally, the sample was placed under an infrared oven lamp for 5 hrs. Thus, Ti3C2Tx-SA was successfully fabricated.
The surface morphologies of the prepared Ti3C2Tx-SA were examined by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The SEM image with a scalar bar of 5.0 µm shown in Fig. 1(a) exhibits Ti3C2Tx nanosheets are uniformly dispersed on the substrate. Figure 1(b) shows the AFM results of the 2D Ti3C2Tx nanosheets, giving the average thickness of ∼6.9 nm. Figure 1(c) shows a typical transmission electron microscopy (TEM) image of the prepared Ti3C2Tx nanosheets, appearing to be ultrathin nanosheet with size up to ∼600 nm. The high-resolution TEM (HRTEM) image (inset of Fig. 1(c)) shows the obviously high crystalline quality, where the lattice space is determined to be ∼10 Å corresponding to (002) facet. Figure 1(d) shows absorbance spectra of the Ti3C2Tx-SA measured by an UV/VIS/NIR spectrophotometer. The prepared Ti3C2Tx-SA exhibit a smooth absorption throughout the Visible-MIR region with a absorptivity absorbance of ∼38%, illustrating its ability for broadband optical modulating. The fluctuating absorbance peaks at approximately 2.6 µm might be caused by sapphire surface functional groups.
Fig. 1. (a) SEM image of the prepared Ti3C2Tx nanosheets with a scalar bar of 5.0 µm; (b) AFM measurement of the Ti3C2Tx nanosheets, giving the average thickness of 6.9 nm; (c) TEM and HRTEM (inset) images of the Ti3C2Tx nanosheets; (d) Visible-MIR absorptivity of the Ti3C2Tx-SA, pure sapphire is served for the reference.
In order to determine the saturable absorption parameters of the prepared Ti3C2Tx-SA, the nonlinear transmission curve as a function of the incident light intensity was measured with a balanced twin-detector system. The laser source was a home-made nanosecond OPO laser operating at 3.0 µm with a pulse width of ∼100 ns and repetition rate of ∼50 kHz. The nonlinear transmission curve is shown in Fig. 2(a). It should be noted that the Fresnel reflection losses of the substrate were not subtracted. The prepared Ti3C2Tx-SA exhibits a typical saturable absorption response with the transmission increases as increasing the incident light intensity. By using the model of a two-level SA to fit the nonlinear transmission curve, the non-saturable loss (ΔRns), the saturable intensity and the modulation depth are calculated to be 17%, 5.56 µJ/cm2 and 17.2%, respectively.
Fig. 2. (a) The nonlinear transmission as a function of the incident light energy intensity; (b) The experimental setup of the PQS Ho,Pr:LLF laser.
3. PQS Ho,Pr:LLF laser operating at 2.95 µm based on the Ti3C2Tx-SA
In order to obtain a high efficient laser, the SA must be saturated earlier than that of the laser gain medium, which is the so-called second threshold condition. The criterion can be simplified and described as [27]:
The experimental setup of the PQS laser is shown in Fig. 2(b). Ho,Pr:LLF crystal was chosen as the laser gain medium, which has proved to be an efficient MIR laser crystal for 3.0 µm laser generation due to the advantages of: First, co-doped with Pr3+ ions can effectively de-excite the lower laser level (5I7) to give rise of a population inversion between 5I6 and 5I7 levels and achieve high efficiency laser output; Second, the LiLuF4 fluoride crystal has relatively lower phonon energy (400-600 cm-1) and lower refractive index, which makes them more suitable for Ho3+ ions 3.0 µm laser operation. The Ho,Pr:LLF crystal used in our experiment was cut along a-axis with dimensions of 2×5×15 mm3 and doping concentrations of 0.185 at.% of Ho3+ and 0.079 at.% of Pr3+, respectively. The pump source was an 1150-nm Raman fiber laser with output power of 50 W. The pump beam passed through an isolator and then was focused into the laser crystal with a radius of ∼80 µm by a focus lens (f = 150 mm). To mitigate the heat effect, Ho,Pr:LLF crystal was wrapped with indium foil and mounted in a red copper heat sink cooled at a temperature of 18 °C. The input mirror M1 was with a radius of curvature of 100 mm and high transmission (HT) coated at the pumping wavelength of 1100-1200 nm (T > 95%), and high reflectivity (HR) coated with the emitting wavelength of 2800-3000 nm (R > 95%). The plane mirror with a transmission of 1% at wavelength of 2800-3000 nm was used as the output coupler.
Considering the characters of the crystals, and the saturable absorption parameters of the prepared Ti3C2Tx-SA, the concave-flat cavity was set to as short as 20 mm to satisfy the Eq. (1). The left side was calculated to be 1.12×106, which was much smaller than the right side (3.47×106), verifying the prepared Ti3C2Tx-SA was suitable for PQS laser generation with the as-designed laser resonator. By inserting the prepared Ti3C2Tx-SA into the laser cavity and adjusting the resonator, the laser operation was converted from continuous wave to the PQS operation regime. Figure 3(a) shows the average output power the PQS laser versus the absorbed pump power, giving the maximum output power of 105 mW and a slope efficiency of 5%. The CW output power was served as the reference. The conversion efficiency from CW to passive Q-switching operation was determined to be ∼44.3%. The laser output spectra were measured by a grating spectrometer and an InSb detector with the resolution of 2 nm. The results were shown in Fig. 3(b), where the Q-switched wavelength is blue-shifted to 2950 nm compared to the CW operation. This might be due to insertion loss introduced by Ti3C2Tx-SA, which leading to the rebuilding of the laser mode competition under the passive Q-switching operation. Figure 3(c) shows the pulse width and the repetition rate as a function of the absorbed pump power. With the increasing the pump power, the pulse width decreased while the repetition rate increased. The shortest pulse width of 266.7 ns was obtained with the corresponding pulse repetition rate of 83.24 kHz. The pulse width is comparable to that obtained with other 2D materials in 3.0 µm wavelength band, such as graphene (937.5 ns), black phosphorus (BP, 194.3 ns), tungsten disulfide (WS2, 1.73 µs), rhenium disulfide (ReS2, 324 ns), and so on [8,28–30].
Fig. 3. (a) CW and PQS output power versus the absorbed pump power; (b) Laser spectrum of the CW and PQS lasers; (c) and (d) are the pulse width (c), pulse repartition rate (c), pulse energy (d), and pulse peak power (d) as a function of the absorbed pump power.
With the average output power, the pulse width and the pulse repetition rate, the single pulse energy and pulse peak power can be calculated, which are shown in Fig. 3(d). The largest pulse energy and highest peak power were determined to be 1.26 µJ and 4.73 W. The typical pulse profile with the shortest pulse width and the pulse train of the largest pulse repetition rate are shown in Fig. 4. The pulse-to-pulse amplitude instability was recorded and calculated to be less than 3.5% in two hours. The laser beam quality factor M2 for the PQS laser operating at the maximum output power was measured to be 1.75 and 1.54 in the horizontal and perpendicular direction.
Fig. 4. The typical pulse profile with the shortest pulse width and pulse train of the largest repletion rate.
Table 1 summarizes the laser performances of PQS MIR lasers operating at 3.0 µm region with 2D materials based SAs. It should be pointed out that a reasonable cavity design and an excellent saturated absorber are the key to obtain high quality passive Q-switching. Up to now, the narrowest pulse width obtained at 3 µm based on a 2D material as a SA is 160.5 ns, which was realized by TiSe2 as SA. In this paper, a 266.7-ns PQS laser was realized based on a novel Ti2C2Tx-SA. This result can be considered superior to the results of MoS2, MoSe2, ReR2, Graphene. In comparison with other results, although the PQS operation based on ReSe2 or Bi2Te3/Graphene have a bit shorter pulses, the Ti2C2Tx-SA is an ideal mid-infrared saturated absorber as a whole.
Table 1. Comparison of ∼3 µm PQS Laser Performance Based on 2D Materials as SA
4. Conclusion
In this paper, high quality few-layered Ti3C2Tx-SA was fabricated by LPE and spin-coating methods. By using the prepared Ti3C2Tx-SA, PQS Ho,Pr:LLF laser operating at 2.95 µm was achieved. To the best of our knowledge, it is the first time that MXenes, particular of Ti3C2Tx, have been applied for the optical modulation in the MIR region. The maximum average output power of 105 mw was obtained with the shortest pulse width of 266.7 ns and pulse repetition rate of 83.25 kHz, corresponding to the single pulse energy of 1.26 µJ and pulse peak power of 4.73 W. This work not only verifies the nonlinear saturable absorption properties of Ti3C2Tx in the MIR region, but also provides a great platform for investigating the broadband nonlinear optical response of MXenes and their applications in nanophotonics.
Funding
National Natural Science Foundation of China (61575110); Beijing Municipal Science and Technology Commission (KM201811232007).
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