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Abstract
We report the first passive Q-switching operation at 1.95 µm utilizing the disordered Tm:CaLu0.1Gd0.9AlO4 (Tm:CLGA) crystal and the hematite (α-Fe2O3) nanosheets as the saturable absorber. The nonlinear saturable absorption properties of the hematite nanosheets were investigated by the conventional Z-scan technology. The modulation depth of 14.3% with the low saturation intensity of 205 kW/cm2 was obtained, indicating that the hematite could be a suitable saturable absorber for the mid-infrared pulse generation. Using the disordered Tm:CLGA crystal as the gain medium, the passive Q-switching operation could be realized with the hematite nanosheets as the saturable absorber, producing the shortest pulse duration of 402 ns with a repetition rate of 76 kHz. The experimental results convinced us that the hematite nanosheets could be of great interest in the optical pulse generation in the mid-infrared region.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Eye-safe lasers at ∼ 2 µm are of great interest in the applications such as medical diagnostics, LIDAR, material processing, surgery and so on. Nowadays, these laser sources are under rapid development since they could be efficient tools for the investigations in physics, chemistry and other optical research [1–3]. The special demands for the eye-safe lasers emitting near 2 µm require the large gain, high thermal conductivity and other excellent physicochemical features. Therefore, a lot of efforts have been made to explore the more suitable laser host. Up to date, using the Tm- or Ho-doped bulk gain media, including crystals, ceramics or glass, many groups have been realized the ultrafast lasers with ultrafast pulses, high peak power and broad spectrum [4–6]. Among the different laser hosts, the disordered crystals have drawn intense attention owing to the excellent broadened spectrum and the relatively large thermal conductivities. In disordered crystals, the inhomogeneous broadening effect plays an import role in the lasing linewidth, resulting in the ultrashort pulses operation [7–11]. Recently, Tm-doped tetragonal calcium aluminates (space group: I4/mmm) aroused extensive concerns because of the negative thermo-optic coefficients and large thermal conductivities [12–14]. The additional introduced Lu3+ ions in the CaGdAlO4 (CGA) are expected to further improve the spectral bandwidth. The previous studies have proven that the novel disordered Tm:Ca(Gd,Lu)AlO4 (Tm:CLGA) crystal could be a promising candidate for the continuous-wave and Q-switching operations [12,15].
It is well known that the saturable absorbers are of great importance in the Q-switching and mode-locking operations. Since the low photon energies near 2 µm, researchers have great expectations of the two-dimensional (2D) materials with very small band gaps. Up to date, lots of 2D materials have been implemented as the saturable absorber to Q-switch or mode-lock the lasers [16–24]. However, the conventional 2D materials have their own drawbacks and the long-term running stability needs to be improved. Hematite is of great stability in the environment and has been used as the photon-absorption materials [25–27]. Until recently, the nonlinear optical characteristics of the hematite was emphasized [28,29]. The previous work [28] demonstrated the passive Q-switching operation in a fiber laser with Fe2O3 nanoparticles saturable absorber at 1942nm, producing the µs pulses.
In this paper, we report the first passively Q-switched Tm:CaLu0.1Gd0.9AlO4 (Tm:CLGA) laser with the hematite nanosheets as the saturable absorber. A 5 at.% Tm:CLGA disordered crystal was grown by the CZ method with the Ar atmosphere. The spectroscopic features of the grown disordered crystal were studied. By applying the open Z-scan technology, the nonlinear absorption properties of the as-prepared hematite saturable absorber were demonstrated. The modulation depth and the saturation intensity were 14.3% and 205 kW/cm2, respectively, indicating the good saturable absorption features. The minimum pulse duration of 402 ns with a repetition rate of 76 kHz can be obtained from the Q-switching operation.
2. Characterization of the disordered Tm:CLGA crystal
Figure 1 shows the absorption and the stimulated emission cross sections of the as-grown 5 at.% Tm:CLGA disordered crystal. For the absorption from the 3H6 to 3H4 transition, the maximum absorption cross section was 8.67×10−21 cm2, corresponding to an absorption coefficient of 7.05 cm-1 at 793 nm. For the stimulated emission spectrum, the maximum FWHM was measured as 240 nm for the σ polarization, showing the great potential in the mode-locking operation. In the meantime, the lifetime of the laser upper level was as large as ∼ 3.5 ms, indicating that the as-grown disordered crystal can be suitable for the energy storage. The estimated gain cross section σg is shown in Fig. 1(b). For the different inversion level β, the gain cross section peaked at different wavelength. For the small gain with β < 0.1, the maximum gain located near 1950nm, indicating that Tm:CLGA laser oscillated around 1.95 µm.
Fig. 1. (a): Absorption and stimulated emission cross sections of Tm:CLGA. (b): Gain cross section σg for the 3F4→3H6 transition of Tm:CLGA with different inversion levels β.
3. Preparation and characterization of hematite nanosheets
The conventional facile oil bath method was exploited to synthesize the hematite powder. 5 mmol FeCl3·6H2O, 22 mmol TBAB and 42 mmol Urea were added into the ethylene glycol ((CH2OH)2) one by one. Then the mixture was extensively stirred to obtain the solution. The solution was put into an oven to collect the red precipitate. In order to make the hematite quality, we tuned the temperature beyond 400 °C in the room atmosphere. Subsequently, the red powder we obtained was dissolved in the ethanol (C2H5OH). The mixture was sonicated for 12 hours. The dispersions were centrifuged at 5000 rmp for 5 minutes to reduce the hematite particles and improve the optical quality. We took 10 µL supernatants and spin-coated a UVFS substrate at a low speed of 1000 RPM. After the drying, the hematite nanosheets saturable absorber was successfully fabricated.
We implemented the high-resolution transmission electron microscopy to investigate the nanomorphology of the as-prepared hematite nanosheets. As shown in Fig. 2(a), the size of the hematite nanosheets ranges from 20 nm × 30 nm to 120 nm ×150 nm. Figure 2(b) shows the lattice structure of the hematite nanosheets, in which we can see two spacings of 0.25 nm and 0.21 nm, corresponding to the (110) and (006) crystal planes. The spacings in Fig. 2(b) agreed well with the previous study [30]. To confirm the nanostructure, the Raman spectrum was measured, illustrated in Fig. 2(c). Typical A1g and Eg modes were observed, consistent with the previous work [31]. Moreover, the strong peak at 1324 cm-1 was also obtained. However, no maghemite component was found, indicating the pure hematite with no magnetics. The linear absorption features were measured by a visible-NIR spectrometer. As shown in Fig. 2(d), the absorption peak is near 555 nm (A point), corresponding to a direct band gap of 2.23 eV, which agrees with the report [27]. Note that the band gap energy is large than the photon energy at 2 µm. However, owing to the edge effect and the quantum size effect, trap states intra the band gap can be expected, leading to the possible nonlinear absorption properties. To make sure the nonlinear optical features of the hematite nanosheets, an open-aperture Z-scan measurement was exploited with a balanced dual-detector system. The excitation source was an actively Q-switched Tm:YLF laser with a pulse duration of 150 ns and a repetition rate of 1 kHz. The nonlinear normalized transmission of the as-prepared hematite nanosheets was shown in Fig. 2(c), indicating that the hematite nanosheets possess good saturable absorption features. Figure 2(d) displays the transmission curve versus the incident pump intensity. By using the following fitting formula [32]:
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. In our case, the modulation depth, the saturable intensity and the non-saturable loss were 14.3%, 205 kW/cm2, and 11.4%, respectively.Fig. 2. Characteristics of the as-prepared hematite nanosheets: (a) HRTEM with a resolution of 200 nm; (b) HRTEM with a resolution of 5 nm; (c) Raman spectrum; (d) Absorbance from 400 -1100 nm; (e) Nonlinear normalized transmission versus the relative distance Z, dots for experimental results and red curve for the nonlinear fitting; and (f) The nonlinear transmission versus the incident pump intensity.
4. Experiment and results
A conventional simple plane-plane resonator (as shown in Fig. 3) was utilized to investigate the lasing characteristics of the Tm:CLGA disordered crystal and the hematite nanosheets. The pump laser was a fiber-coupled laser diode emitting 795 nm with a maximum output power of 10 W. The core diameter was 400 µm and the numerical aperture was 0.22. The pump beam was focused into the disordered crystal with a spot radius of 200 µm via an optical imaging system (1:1 imaging ratio). A 5 at.% Tm:CLGA disordered crystal with a dimension of 3×3×3 mm3 was cut along a-axis as the gain medium to obtain the large gain. During the experiment, the laser crystal was water-cooled at 15 °C to efficiently reduce the thermal lensing effect. The input mirror was with an anti-reflectivity coating at 795 nm and high-reflectivity coating from 1800 to 2100 nm. The output coupler (OC) had a partial reflectivity at 1800–2100 nm (T = 3.1% at 1950 nm). The hematite nanosheets saturable absorber was put as close as the OC. To reduce the residual pump power, a long pass filter was put behind the output coupler. The temporal pulse behavior was monitored by a digital oscilloscope (DPO 7104C, Tektronix Inc.) with a fast InGaAs photodiode (DET08CFC, Thorlabs Inc.). And the output power was measured by a laser power meter (MAX 500AD, Coherent Inc.).
Fig. 3. Schematic setup of the Tm:CLGA disordered crystal laser with the hematite nanosheets as the saturable absorber. LD: laser diode; LC: laser crystal; SA: saturable absorber; OC: output coupler.
The continuous-wave (CW) π-polarization Tm:CLGA laser was initially studied. As shown in Fig. 4(a), the threshold for the CW operation was 1.07 W. With the enhancement of the pump power, the CW output power increased with a slope efficiency of 10.2%. The highest CW output power was 306 mW under an absorbed pump power of 4.05 W. When the hematite nanosheets saturable absorber was inserted into the laser cavity, the passive Q-switching operation can be obtained by augmenting the pump power. The threshold pump power for the Q-switching (QS) operation was increased to 2.8 W. At the maximum absorbed pump power of 4.05 W, the highest Q-switching power we can achieve was 48 mW, corresponding to a linear slope efficiency of 3.6%. The relative low output power was attributed to three factors: (a). The strong water vapor absorption at 1.86 - 1.94 µm [33] would add some extra losses in the laser resonator; (b). The non-uniform size of the hematite and residual organic molecules also contributed to the large intracavity losses. Besides, the gain factor of this disordered crystal was relative low in our case. For the energy booster, MOPA could be an effective approach. Figure 4(b) presents the variation of the pulse duration and the pulse repetition rate versus the absorbed pump power. Obviously, the pulse duration decreased monotonically from 1640 ns to 402 ns, while the pulse repetition rate monotony increased from 40 kHz to 76 kHz. The stable pulse train at the absorbed pump power of 4.05 W is illustrated in Fig. 4(c) with a repetition rate of 76 kHz. The pulse-to-pulse fluctuation was estimated as < 5% RMS. The typical temporal pulse profile is demonstrated in Fig. 4(d) with a pulse duration of 402 ns FWHM.
Fig. 4. (a) Average output power versus the absorbed pump power, CW: continuous-wave operation, QS: Q-switching operation. (b) Pulse duration and pulse repetition rate versus the absorbed pump power. (c) Stable pulse train at an absorbed pump power of 4.05 W; and (d) The corresponding temporal pulse profile.
The output spectrum of the CW and QS operations from the Tm:CLGA disordered crystal laser was measured by the laser spectrometer (NQ52A, Ocean Optics Inc.). However, owing to low resolution (> 3 nm), we cannot observe the obvious spectral difference. Figure 5 shows the typical QS spectrum and the beam quality at an absorbed pump power of 4.05 W. Clearly, the spectral peak located at 1948nm. The beam quality M2 factor was measured by a 90/10 knife edge method. By using the binominal fitting, the beam quality M2 was 1.138 along x-direction and 1.177 along y-direction. The laser was running for three days, and no significant degradation was observed during the long term running.
5. Conclusion
In conclusion, we demonstrated the first operation of the disordered Tm:Ca(Lu0.1Gd0.9)AlO4 crystal laser with the hematite nanosheets saturable absorber. The Tm:Ca(Lu0.1Gd0.9)AlO4 disordered crystal possessed a broad bandwidth with a relative large lifetime, indicating that this crystal is suitable for the Q-switching and mode-locking operations. On the other hand, the hematite nanosheets exhibited a large modulation depth of 14.3%, making it potential for the Q-switching operation. Combination the features of the disordered crystal and the hematite nanosheets, we realized the stable passive Q-switching pulse train with a repetition rate of 76 kHz and a minimum pulse width of 402 ns. Our work can benefit the optical community of laser media and the 2D nonlinear optical materials.
Funding
National Natural Science Foundation of China (61575109); Fundamental Research Fund of Shandong University (2018TB044); Foundation of President of China Academy of Engineering Physics (YZJJLX2018005).
Disclosures
The authors declare no conflicts of interest.
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