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Abstract
Hematite (α-Fe2O3), as one of the most prevalent transition metal oxides (TMOs) on the Earth, has been the hot spot of intense research for recent decades. Understanding the optical signatures in hematite is essential for the design of α-Fe2O3-based devices in the photoelectrochemical and photonic fields. In the present work, we successfully synthesized hematite nanoflakes by the facile oil bath method. The small signal transmission of the prepared few-layered hematite nanoflakes was 76% and the modulation depth was determined as 11.2%. Subsequently, the α-Fe2O3 nanoflakes were successfully implemented as the saturable absorber in a passively Q-switched Tm:YLF laser around 2 µm. The achieved shortest pulse duration was 293 ns. The long-term pulse-pulse fluctuation was < 3% in root-mean square error (RMSE), indicating the Q-switching pulses are highly stable. The present work will assist in understanding the nonlinear optical features and their interpretation in stable Q-switching pulses generation.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The mid-infrared pulsed lasers especially operating at 2 µm have been widely used in laser radars, high-resolution spectroscopy, material processing and so on [1,2]. Simplest realizations of the short optical pulse are based on different saturable absorbers (SAs) [3–5]. Recently, two-dimensional (2D) materials have occupied the main market of SAs, such as graphene [6,7], black phosphorus (BP) [8], transition metal dichalcogenides (TMDs) [9–11], because of their excellent electrical, carrier transport characteristics and easy preparations. However, the current 2D materials have low damage threshold, and the output Q-switched pulse can only maintain stable in a short period of time, which greatly limits its application. Hence, seeking the novel 2D materials with high stable performance is still the main task for the optical society.
Besides TMDs materials, another kind of material group, i.e. transition metal oxides (TMOs), has recently become a new research hotspot because of its large third-order nonlinear effect, superfast carrier recovery time and excellent thermal stability [12–16]. Hematite (α-Fe2O3) is extremely stable in the air and has been extensively studied for their possible applications as the light harvesting materials [17–20]. Indeed, the linear optical absorption features have been highlighted by many research groups. Until recently, the nonlinear optical properties have launched an investigation wave. In 2015, nanosecond and ultrafast optical power limiting properties of Fe2O3 hexagonal nanomorphotype were demonstrated [21]. Subsequently, the broadband saturable absorption of Fe2O3 nanoparticles was exhibited [22]. Based on the Fe2O3 nanoparticles as the nonlinear SA in the fiber laser, optical pulses in the near-infrared regime were produced in the µs level [22]. More recently, based on the hematite nanosheets as the SA, the passively Q-switched Tm:CGLA laser was realized with a pulse duration of 402 ns [23]. However, the pulse stability and the peak power still need to be improved for possible applications. When the hematite was exposed in the ambient atmosphere, oxygen molecules are absorbed on the surface in terms of the physical and chemisorption progress. The oxygen splices induced point defects would further modify the electronic band structure of the hematite, leading to the defect levels in the band. Consequently, the optical absorption features are significantly enhanced in the mid-infrared region.
In this paper, we prepared and characterized α-Fe2O3 nanoflakes with simple facile oil bath method. Employing the balanced twin-detector measurement technique studied its nonlinear absorption properties at 2 µm. The modulation depth and saturable pulse fluence were 11.2% and 17.5 mJ/cm2, respectively. Then a passively Q-switched Tm:YLF solid-state laser with α-Fe2O3 nanoflakes as the saturable absorber was demonstrated at 2 µm, delivering a shortest pulse duration of 293 ns with a repetition rate of 104 kHz. Under all three different transmissions of output mirrors, high stable pulse outputs could be obtained and the power stability was less than 5%. Our work clearly revealed the high stable modulation characteristics of the prepared α-Fe2O3 nanoflakes and its excellent potential as SA in mid-infrared pulse laser. We believe that the present work help to understand the nonlinear optical properties which could benefit the possible applications in MIR photonics. Moreover, the ultrastability feature makes it more competitive in wide applications such as military, metrology and free-space telecommunications.
2. Synthesis and characterization of hematite nanoflakes
We fabricated the α-Fe2O3 nanoflakes by the facile oil bath technique. 1.2 g ferric chloridehexahydrate (FeCl3·6H2O), 7.0 g tetrabutylammonium bromide (TBAB), and 2.5 g urea were added into 200 ml ethylene glycol in order. Subsequently, the mixture was vigorously stirred until fully dissolved. The solution was then heated to 175 °C by a rotary evaporator for 2 hours. The precipitate was collected and washed by ethanol for several times. Put the collections in an oven and dried in vacuum for 24 hours. To ensure the α-Fe2O3 precipitates, we heated the collections at 400 °C in the air for 4 hours. The α-Fe2O3 powder was dispersed in pure 1-Methyl2-pyrrolidinone (NMP) and followed by 18 h water-bath sonication at a sonication power of 100 W. The dispersions were centrifuged at 2000 rmp for 15 min to remove the unexfoliated and large-sized α-Fe2O3 powders. After that, we transferred them onto a UV fused silica substrate with a surface area of 2 cm × 2 cm. Finally, the sample was placed in a vacuum oven at 60°C for 24 h. Thus, the α-Fe2O3 nanoflakes based SA was prepared.
In order to more directly reflect the morphology and structure of the prepared α-Fe2O3 nanoflakes, we have carried out scanning electron microscopy (SEM) imaging from JEOL (JSM5610LV, 0.5–35 kV) and high-resolution transmission electron microscopy (HRTEM) imaging by a Philips Tecnai 20U-Twin HRTEM at an acceleration voltage of 200 kV, as shown in Figs. 1(a), 1(b) and 1(c). It is clearly observed that the nanostructures have flake-like structures and have uniform size of ∼20 nm. Figure 1(d) demonstrates the X-ray diffraction (XRD) pattern, in which all the diffraction peaks agree well with that of hematite α-Fe2O3 (JCPDS File no. 33-0664). The sharp diffraction peaks indicate the good crystallinity. The Raman spectra of the prepared α-Fe2O3 nanoflakes was determined by a LabRAM HR800 Raman spectrometer (Horiba Jobin Yvon) with a range from 150 to 2000 cm−1 with a spectral resolution of 2 cm−1. As shown in Fig. 1(e), there are five distinct vibration modes, which appeares at 226 cm−1, 291 cm−1, 405 cm−1, 618 cm−1, 1375 cm−1, respectively. The result agreed well with the previous work [23]. Figure 1(f) shows the broad spectrum absorption of the prepared α-Fe2O3 nanoflakes by an UV-VIS-NIR spectrophotometer (Hitachi U-4100), revealing its potential as a saturable absorber of mid-infrared laser. The thickness of the as-synthesized α-Fe2O3 nanoflakes was studied using atomic force microscopy (AFM) as depicted in Fig. 1(g), which was important for the optical absorption. It was measured to be about 5.7 nm, corresponding to ∼5 layers, considering the single sheet thickness of 1.1 nm [24].
Fig. 1. Characterization of α-Fe2O3 nanoflakes: (a) SEM, (b) TEM, (c) HRTEM, (d) XRD pattern, (e) Raman spectra, (f) Linear absorption spectrum, (g) AFM image, (h) Height along the blue line in (g).
The nonlinear transmission property was measured by the balanced twin-dector measurement method. The laser source was a 2 µm AOM (QSG27-2000-3QE, CETC) Q-switched Tm:YAP laser, with a pulse duration of 300 ns and PRF of 1 kHz. By changing the laser energy density irradiated on the prepared α-Fe2O3 nanoflakes sample, the nonlinear transmittance versus the incident pulse fluence was depicted as Fig. 2. It was fitted by the following formula [25–27]:
where, ΔT is the modulation depth, Φ is the incident pulse fluence, Φs is the saturable pulse fluence and Tns is the non-saturable loss. The fitting data showed that the modulation depth was 11.2 ± 0.8%, the saturable pulse fluence was 17.5 ± 1.5 mJ/cm2 and the non-saturable loss was 13.8 ± 0.5%, which verified its potential application as the saturable absorber in mid-infrared laser.3. Saturable absorber for producing MIR pulses
The schematic of the α-Fe2O3 nanoflakes passively Q-switched Tm:YLF laser was depicted in Fig. 3. A simple plane-plane cavity with a length of 15 mm was adopted. The pump source was a 792 nm fiber-coupled diode laser, with fiber core diameter of 400 µm and the numerical aperture (NA) of 0.22. Through a 1:1 coupled focusing system, the pump beam was converged to the gain crystal. The crystal was a c-cut 3.0 at.% Tm:YLF with a dimension of 3 × 3 × 10 mm3 and it was cooled at 15 °C by a water-cooled copper sink to avoid thermal damage. The input mirror was high-reflection coated at 1900-2000 nm and high-transmission coated near 800 nm. Three output couplers (OC) with different transmissions of 1%, 3% and 5% were utilized to obtain the optimum output performance. The α-Fe2O3 nanoflakes were placed close to the OC to preserve it from thermal damage. A longpass filter was utilized to leach the residual pump laser. The output power was measured by a MAX 500AD (Coherent., USA) laser power meter. Pulse temporal behavior was recorded by a DPO 7104C digital phosphor oscilloscope (1 GHz bandwidth and 20 G samples s−1 sampling rate, Tektronix Inc., USA) and a superfast InGaAs photodiode detector (5 GHz bandwidth with a rising time of 70 ps, DET08CFC/M, Thorlabs Inc., USA).
Stable Q-switching laser operation could be achieved by aligning the mirrors. The curves of the average output power of continuous laser (CW) and passive Q-switched laser with the absorbed pump power were shown in the Figs. 4(a) and 4(b). The threshold absorbed pump power was 380 mW, 575 mW, 640 mW with T=1%, T=3% and T=5%, respectively. The highest output power of CW laser were 1.47 W at the absorbed pump power of 3.7 W with T=5%, corresponding to the slope efficiency of 51.5%. Inserting the α-Fe2O3 nanoflakes sample in the cavity, the threshold absorbed pump power was 1.02 W, 1.15 W and 1.27 W. Because of the small signal transmission of ∼75%, the insertion loss of the α-Fe2O3 nanoflakes was about 25%. The maximum average output power of the Q-switched laser were 331 mW, 356 mW and 390 mW at the absorbed pump power of 3.7 W with T=1%, T=3% and T=5%, respectively. The slope efficiencies were 12.6%, 14.1% and 15.9%, respectively. Greater slope efficiency was obtained with 5% output mirror. Moreover, we found the α-Fe2O3 nanoflakes began to be damaged when the incident pulse fluence on it exceeded 10 J/cm2, which was regarded as the damage threshold of the prepared α-Fe2O3 nanoflakes sample. To pursue the high stability of the output pluses, the maximum absorbed pump power was set to be 3.7 W. As displayed in Fig. 4(c), the pulse width decreased with the augment of absorbed pump power, while the repetition rate exhibited a rising trend. At the identical absorbed pump power of 3.7 W, the minimum pulse duration of 293 ns was obtained at the OC of 1% transmission, corresponding to the largest peak power of 10.8 W, as Fig. 4(d) showed. The single pulse energies were calculated and depicted in Fig. 4(e). We could see that firstly the single pulse energy increased nearly linearly versus the absorbed pump power and then tended to saturate state, which could be ascribed to the thermal effects happening on the SA and gain crystal due to the large quantum defects in Tm3+ doped crystal. Shorter pulse duration and higher single pulse energy could be expected by optimizing the cavity and the fabrication process of α-Fe2O3 nanoflakes. The highest repetition rate was 123 kHz, shown in Fig. 4(f), obtained with an OC of T=5% at the absorbed pump power of 3.7 W.
Fig. 4. Average output power of CW (a) and Q-switched laser (b), Pulse duration (c), peak power (d), single pulse energy (e), repetition rate (f) versus the absorbed pump power.
Figure 5 depicts the typical pulse profile and high stable pulse train of the Q-switched Tm:YLF laser with different OCs at the maximum absorbed pump power of 3.7 W. The pulse durations were 293 ns, 356 ns and 390 ns and the repetition rate were 104 kHz, 111 kHz and 123 kHz at T=1%, T=3% and T=5%, respectively. Under three OCs with different transmittances, high stable pulse output trains could be obtained and the long-term pulse-to-pulse instability of pulse train was < 3% root-mean-squared error (RMSE). During the experiment, we can obtain the most stable Q-switching operation with the OC of T = 3%. When the transmittance of OC was high, the photon density in the cavity was small, and the SAs could not reach the saturation state in time; when the transmittance was small, the photon density in the cavity was large, which would accumulate thermal effect in the cavity and affect the materials performances. Therefore, the most stable modulation characteristics could be obtained only under the suitable intracavity photon density with proper transmittance of OC. In order to exactly illustrate the stability of the laser operation, we have continuously detected the fluctuations for 40 minutes at the maximum output powers, as shown in Figs. 5(c), 5(f), and 5(i). The power instabilities were 2.0%, 1.2%, 1.5% for OCS of T=1%, T=3%, and T=5%, respectively. The great stability of output power indicated the stability of laser operation well. In comparison with our previous work [23], the present laser possessed much shorter pulse duration as well the long-term running stability. The spectrum of the Q-switched laser was measured by the spectrometer (Resolution: 0.5 nm, APE WaveScan, Germany) and the output spectrum was displayed in Fig. 6 with T=3% OC, and the center wavelength was 1914nm.
Fig. 5. Typical pulse profiles and pulse trains under the absorbed pump power of 3.73 W with different OCs, (a), (b) T=1%; (c), (d) T=3% and (e), (f) T=5%; (c), (f) and (i) describe the output power fluctuations versus time with different OCs.
In fact, when the hematite was exposed in the ambient atmosphere, the oxygen vacancy (OV) would be induced on the nanoflakes surface. In the previous work [28], a large amount (20.8%) of surface OV defects has been proven in our prepared Fe2O3 nanoflakes with the same fabrication method. These vacancies and defects can locate the electronic state, produce narrow bands, Mott transition, and even Anderson position, thus changing the band structure [29]. In fact, the introduction of the defects into the pure semiconductor would reduce the band gap [30]. Moreover, we also found that the defects in bismuthene quantum dots would introduce intermediate trap states and change the inter-bands recombination time and the recovery time of saturable absorbers [31]. Accordingly, the large amounts of surface oxygen vacancy defects in our prepared α-Fe2O3 nanoflakes changed the band structure and reduced the bandgap, leading to a better saturable absorption performance and optical modulation characteristic in the 2 µm waveband.
4. Conclusion
In conclusion, α- Fe2O3 nanoflakes were prepared by the facile oil bath method. Employing the twin-detector measurement technique, the modulation depth was fitted to be 11.2%. Based on the excellent nonlinear absorption property of α-Fe2O3 nanoflakes, a stable passively Q-switched Tm:YLF laser at ∼ 2 µm was demonstrated. The minimum pulse duration of 293 ns and the maximum pulse repetition rate of 123 kHz. The stability of the Q-switching pulses was also measured in the long term running. The advantages of α-Fe2O3 nanoflakes saturable absorber were benefited from the large amounts of surface oxygen vacancy defects. Our work definitely revealed that the as-grown α-Fe2O3 nanoflakes are the prominent saturable absorber for 2 µm pulses generation.
Funding
National Natural Science Foundation of China (21872084, 61575109); Fundamental Research Fund of Shandong University (2018TB044).
Disclosures
The authors declare no conflicts of interest.
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