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Abstract
The novel two-dimensional MXenes have been investigated in the photonics field in areas such as broadband nonlinearity, pulse laser generation and passive photonic diode, and so on. In this contribution, the nonlinear optical response at the visible band and the demand pulse laser generation based on MXenes was initially realized. The few-layer MXene Ti3C2Tx was fabricated and utilized as a saturable absorber (SA) to realize passive Q-switched visible bulk laser covering the spectral range of orange (607 nm), red (639 nm), and deep red (721 nm). At the three wavelength, the maximum average output powers and the shortest pulse widths were (111 mW, 426 ns) for 607 nm, (150 mW, 264 ns) for 639 nm, (115 mW, 328 ns) for 721 nm, respectively. Our experimental results indicate the MXene Ti3C2Tx SA could be an efficient and promising optical modulator in the visible domain.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Visible lasers are fascinating not only due to their numerous applications in our daily life such as entertainment and education, but also their important requirements in a wide range of topical areas such as microscopy, display technology, medicine, material processing, and scientific research [1–3]. Trivalent praseodymium (Pr3+) doped materials can generate visible laser directly owing to its suitable emission transitions. Compared with visible lasers achieved by nonlinear optical conversion, direct lasing has the advantages of compact structure, high stability, large conversion efficiency. In the laser regime, pulse lasers can provide much higher peak power for various applications that are not available for continuous-wave lasers. An efficient optical modulator is an key element for a pulse laser. However, the shortage of suitable optical modulators in the visible region impeded the development of directly generated visible pulse lasers until the low dimension saturable absorbers (SAs) emerged [4–10]. In just recent years, graphene [11], transition metal dichalcogenides (TMDs) [12–15], black phosphorus (BP) [16], topological insulators (TIs) [17], Au nanorods [18], and CdTe/Cds quantum dots [19] have been successfully employed as SAs in visible passive Q-switched lasers. Unfortunately, the output power of these relevant pulse visible laser were still limited in tens of milliwatts. The development of efficient optical modulator in visible range is still a longstanding goal.
MXene, as a new member of the 2D material family, has been attracted much attention because of its outstanding optical, thermal, and electronic properties [20–22]. The general formula of MXene is Mn+1XnTx, where M is an early transition metal, X is C and/or N, T is the surface terminations(hydroxyl, oxygen or fluorine), and n = 1, 2, or 3. The MXene can be fabricated by the etching out of the A layers from MAX phases, due to weaker M-Ametallic bonds compared to the covalent/ metallic/ionic mixed M-X bond. Currently, more than 70 types of MAX phases have been reported, representing the rich variety of the MXene family. Typical 2D MXene titanium carbide (Ti3C2Tx) could be a promising SA for pulse lasers owing to its various excellent optical properties. First, The nonlinear absorption coefficient (βeff) is found to be on the order of 10−21 m2/V2, which is comparable with graphene and is much higher than the values reported in MoS2, black phosphorus and other 2D materials, indicating a strong optical switch capability [23]. Moreover, zero-gap band structure of Ti2C3 (or Eg < 0.2 eV of Ti3C2Tx) makes its potential suitable for broadband absorbers. What's more, MXene Ti3C2Tx has high damage thresholds of 70 mJ/cm−2, which is more resilient than other 2D materials [24]. Recently, Jiang et al. reported mode-locked fiber laser at the wavelength of 1.55 and 1.0 µm using Ti3C2Tx as a SA [23]. Afterwards, passive Q-switched lasers at 1.0 µm [25] and 1.3 µm [26], passive mode-locked lasers at 1.0 µm [27] based on Ti3C2Tx SAs have been reported. However, visible pulse laser based on Ti3C2Tx SA have been rarely reported. In this paper, the MXene Ti3C2Tx was characterized and the saturable absorption of MXene in the visible range was investigated. Using the MXene Ti3C2Tx SA, the efficient passive Q-switched lasers were achieved at the wavelength of 607 nm, 639 nm, and 721 nm.
2. Morphology characterization of the Ti3C2Tx nanosheets
MXene Ti3C2Tx was prepared by aqueous acid etching method as previous report [20]. The obtained MXene power was then dissolved into isopropyl alcohol (IPA) and sonicated for 1 hour. After the ultrasonic process, the dispersed solution of MXene was centrifuged at 4000 rmp for 20 minutes. The supernate was taken for later investigation. Using a scanning electronic microscope (SEM), the accordion-like structures of delaminated Ti3C2Tx were clearly observed, as shown in Fig. 1(a). The surface morphology and lattice structure were further determined by transmission electron microscope (TEM) and high-resolution TEM (HRTEM), shown in Fig. 1(b) and Fig. 1(c). The atoms arrangement for Ti3C2Tx nanosheets was hexagonal with an equiangular lattice spacing of ∼2.7 Å, which matched well with the previous research result (∼2.6 Å) [28], and further confirmed by the selected area electron scattering (SAED) image. Figure 1(d) shows the side-view of HRTEM, indicating the layer thickness was 1 nm.
Fig. 1. (a) SEM images of delaminated Ti3C2Tx. (b) TEM image charactering the surface morphology oTi3C2Tx. (c) The space lattice characterized by HRTEM. Inset is the SAED image of Ti3C2Tx. (d) Side-view image of HRTEM.
The linear transmittance spectra of MXene film from 350 to 1500 nm is shown in Fig. 2(a) . We believe that three part contributes to the broadband absorption, i.e., ground state transition, exciton effect and surface plasmon. The nonlinear optical absorption at 640 nm was characterized by the open-aperture Z-scan technique. The ultrashort pulse was generated by a femtosecond Ti:sapphire laser with a pulse of 100 fs and a repetition rate of 1 kHz. The near-parallel light was shaped into a symmetric Gaussian beam by a f = 150 mm plano-convex lens, where the diameter of the beam waist was around 6 µm. The beam was divided into two parts (30:70), one was set as the reference, and the remaining large part was used to inject into MXene sample. The absorption property was determined by collecting the transmitted light when moving the computer-controlled stage along the “Z” axis, and the sample was fixed at the stage. As indicated from previous investigation, MXene shows broadband optical response [29]. At the visible band, ground state bleaching is the main mechanism contributes to the nonlinearity. A quartz was used as substrate to support the MXene thin film. The Z-scan result is shown in Fig. 2(a), giving strong saturable absorption under low intensity of 0.92 µJ. The ground state bleaching induced saturable absorption can be understood by the Pauli blocking for the excessive photon incidence. Besides, the thermo-induced refection index change also modifies some refraction loss, while it is weak and negligible. To confirm the saturable absorption was from the material, pure quartz was also measured under the highest intensity, which gave no nonlinear absorption, as the band gap for that is high to 9 eV. The transmittance versus the incident optical is shown in Fig. 2(b). The data is fitted by $T = 1 - \Delta T \times \exp ( - I/{I_{sat}}) - {T_{ns}}$, where ΔT, Isat and Tns are modulation depth, saturable optical intensity and saturation loss [30]. From which, the modulation depth, saturation fluence, were fitted to be 13.5%, 43MW/cm2.
Fig. 2. (a) Linear transmission of the MXene film. (b) Normalized transmittance of MXene film versus the “Z” axis. (c) Transmittance versus the incident optical intensity.
3. Experimental setup for passive Q-switched laser
The passive Q-switched laser experimental setup is shown in Fig. 3. The pump source is a commercially available InGaN blue laser diode with the maximum output power of 3 W, which emits at the wavelength of 444 nm. After a 75 mm (focal lens) focusing lens with anti-reflection coating in the visible range, the pump beam was focused into the laser gain medium. A 3×3×6 mm a-cut Pr: LiYF4 with 0.5 at% doped was used as the laser gain medium. The Pr: LiYF4 crystal was wrapped by an indium foil and mounted in a copper block cooled by the circulating water with a temperature of 17 ℃. Three two-mirror plane-concave cavities were individually configured for the three lasers, and the length of the laser cavity was optimized to be about 45 mm. Three flat mirrors M1, used for input mirrors (IMs), were coated with antireflection (AR) coated for the pump wavelength of 444 nm and highly reflective (HR) for the three laser wavelengths. For the orange laser cavity, the IM has a transmission of about 98% at 639 nm; for the deep red nm laser cavity, the IM has a transmission of 67% at 639 nm aiming at suppressing the high-gain emission lines. All the output couplers (OCs) M2 have the radii of curvature of 50 mm. The transmission of three OCs were optimized to choose 2.0% for 607 nm, 2.0% for 639 nm, 1.9% for 721 nm. In the pulse laser, the MXene Ti3C2Tx SA was inserted into the laser cavity as close as possible to the laser crystal to modulate the cavity loss.
4. Experimental results and discussion
Before inserting the saturable absorber inside the cavity, the continuous-wave (CW) laser were firstly realized and the output power characteristic are shown in Fig. 4(a)-(c). The maximum CW output power was 373 mW, 900 mW, 682 mW under the absorbed pump power of 2.52 W at the wavelength of 607 nm, 639 nm, and 721 nm, respectively. The corresponding optical conversion efficiency was 14.8%, 35.7%, 27.0%, respectively. The optical conversion efficiency at 607 nm was far lower than that at 639 nm and 721 nm, owing to a reabsorption phenomenon near the orange band in the optical absorption spectrum of Pr3+. The CW laser threshold was 0.95 W for the orange (607 nm) laser, 0.12 W for the red (639 nm) laser, 0.44 W for the deep red (721 nm) laser, respectively. After inserting the MXene Ti3C2Tx SA into the laser cavity, stable Q-switched pulse laser were achieved when the absorbed pump power reached 1.12 W for orange laser, 0.7 W for red laser, and 0.7 W for deep red laser, respectively. Figure 4(a)-(c) also show the average output power characteristic of Q-switched lasers. The maximum Q-switched output power was 111 mW for the wavelength of 607 nm, 150 mW for the wavelength of 639 nm, and 115 mW for the wavelength of 721 nm. The instability (output power, rms) of the Q-switched laser was measured to be less than 1% for three wavelength in 1 hour. The main reason for the instabilities may attribute to the sensitivity of the absorber. Figure 5 shows the variation trends of repetition rate and pulse width with the increasing of the absorbed pump power. The repetition rates displayed an increasing tendency and pulse width decreased with the absorbed pump power. The maximum repetition rate were 153 kHz, 163 kHz, and 140 kHz at the wavelength of 607 nm, 639 nm, and 721 nm, respectively and the shortest pulse width was 426 ns at 607 nm, 264 ns at 639 nm, and 328 ns for 721 nm. According to the above data, we can estimate the maximum single pulse energy to be about 0.73 µJ at 607 nm, 0.92 µJ at 639 nm, 0.82 µJ at 721 nm, and their corresponding peak power was 1.70 W, 3.48 W, and 2.50 W. Figure 6(a)-(c) show the typical Q-switched single pulse at the maximum output power, and their corresponding pulse trains were given in Fig. 7(a)-(c), which look uniform and stable. The output lasers were horizontal polarization, and the degree line polarization of about 99% were also detected by a polarization beam splitter (PBS). The optical spectrum of Q-switched laser were recorded and shown in Fig. 8(a)-(c), whose peaks were located at 607.0 nm with the full-width at half-maximum (FWHM) of 0.21 nm, 639.2 nm with the FWHM of 0.11 nm, and 720.8 nm with the FWHM of 0.25 nm. Figure 9 (a)-(c) show the far-filed beam profile at the maximum Q-switched output power, which indicate an excellent TEM00 transversal profile.
Fig. 4. Ouput power characteristics of CW and QS modes for the wavelengths at 607 nm (a), 639 nm (b), and 721 nm (c)
Fig. 5. Pulse width and repetition rate versus absorbed pump power at 607 nm (a), 639 nm (b), and 721 nm (c)
Fig. 6. The single pulse width at the maximum output power for the wavelengths at 607 nm (a), 639 nm (b), and 721 nm (c)
Fig. 7. The typical pulse train at the maximum output power for the wavelengths at 607 nm (a), 639 nm (b), and 721 nm (c)
Fig. 8. The pulsed laser spectrum with the center wavelength at 607 nm (a), 639 nm (b), and 721 nm (c)
Fig. 9. Beam profile of the laser at the maximum Q-switched output power at 607 nm (a), 639 nm (b), and 721 nm (c)
Finally, we compared the present results, in Table 1, with the previous reports on passive Q-switched visible Pr3+-doped laser using different low-dimension SAs. From Table 1, we can see that the obtained shortest pulse widths in this work are comparable with previous results. However, the Q-switched output power in our work exceeds 100 mW, which are the highest among the materials mentioned above. And the corresponding single pulse energies and peak powers are far better. These good output performance should be owed to the excellent optical properties of Ti3C2Tx SA. In order to achieve higher peak power pulse laser, we will increase the pump power by polarization multiplexing, optimize cavity and the modulation depth of SA.
Table 1. Comparison of the performance of the passively Q-switched Pr3+ visible laser with low-dimension materials as SAs
5. Summary
In conclusion, MXene Ti3C2Tx SA was utilized for passive Q-switched Pr3+: YLF visible lasers at 607 nm, 639 nm and 721 nm. For 607 nm, the shortest pulse width and the maximum repetition rate were 426 ns and 153 kHz, respectively, with the maximum output power of 111 mW. For 639 nm, the shortest pulse width and the maximum repetition rate were 264 ns and 163 kHz, respectively, with the maximum output power of 150 mW. For 721 nm, the shortest pulse width and the maximum repetition rate were 328 ns and 140 kHz, respectively, with the maximum output power of 115 mW. The present work indicates the MXene Ti3C2Tx can be used as an efficient optical modulator in the visible range.
Funding
Natural Science Foundation of Shandong Province, China (ZR2017LF024); the Postgraduate Innovation Development Fund Project of Shenzhen University (PIDFP-ZR2018004); Open Foundation of Shanghai Key Laboratory of All Solid-state Lasers and Application Technology (ADL-2017003); Youth Doctoral Cooperative Program of Qilu University of Technology (Shandong Academy of Sciences) (2017BSHZ018).
Acknowledgments
Qi Yang and Feng Zhang contributed equally to this work.
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