As a binary oxide, β-Ga2O3 is a promising host material with high thermal conductivity compared with the traditional laser materials. In this work, Co2+-doped β-Ga2O3 single crystal was successfully grown by the edge-defined film-fed growth (EFG) method and firstly designed as a near-infrared (NIR) optical modulator by doping with cobalt. The doping concentration of Co2+ ions was determined to be 0.029 at.% by X-ray fluorescence. The thermal conductivity was determined to be 13.0 W·m-1·K-1 along a* direction. There were two typical broadband absorption peaks around 1173 nm and 1588 nm related to the transitions of transitions of 4A2(4F)→4T1(4F). By means of Z-scan technique, the third-order nonlinear refractive index of Co2+:β-Ga2O3 was measured to be 9.14×10−13 (esu), which was much larger than that of Cr4+:YAG crystal. It was further successfully employed as a saturable absorber for passively Q-switched lasers at 1342 nm. In the Q-switched regime, the maximum average output power of 35 mW was obtained with the shortest pulse width of 280 ns and repetition rate of 181 kHz. The results indicated that Co2+:β-Ga2O3 crystals have important potential to be used as optical modulators for passive Q-switching laser generation at the NIR band.
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Pulsed lasers have high power and could be used in the fields of material processing, medicine communication and micro-machining due to the advantages of high energy density and short pulse width . Lasers operating at 1.3 µm are safe to the eye and belong to low loss communication window of silicate glass fiber . Therefore, the obtain of 1.3 µm pulsed lasers are meaningful and have been attracted a lot of attention in recent years. As a compact, low cost, reliable and efficient technique, the passive Q-switching is an important and popular method for the generation of micro- and nano-second pulsed lasers. The saturable absorbers (SAs) are the key elements in passively Q-switched lasers. At present, many SAs have been studied such as dyes, bulk semiconductors, 2D materials and transition metal ions doped crystals [3–7]. Among all of them, the transition metal ions doped crystals such as Co2+ doped LiGa5O8, MgAl2O4 and LaMgAl11O19 have been widely used due to the advantages of broad absorption band, large absorption cross section, low saturable intensity [8–11]. However, the demand of higher power lasers put higher request on thermal management of the devices. Therefore, new crystalline SAs with higher thermal conductivity are desirable.
As an ultra-wide bandgap semiconductor (4.8 eV), β-Ga2O3 has attracted a lot of attentions in high-voltage electronics and ultraviolet optoelectronics . Similar to sapphire, β-Ga2O3 is also supposed to be used in the laser [13,14]. The thermal conductivity of pure β-Ga2O3 is as high as 27.9 W·m-1·K-1 along <010> axis which is much better than that of pure YAG (11.7 W/mK) . The ionic radius of Ga3+ in octahedral coordination is 0.62 nm which is suitable for the doping of transition metal ions even rare earth ions [14,16]. β-Ga2O3 is congruent melting and large size crystals have been grown by melt methods including Czochralski method, EFG method, Bridgman method and so on [15,17].
In this paper, Co2+:β-Ga2O3 single crystal was designed as a saturable absorber (SA) for the first time. Single crystal of Co2+:β-Ga2O3 was grown by EFG method. The thermal properties and absorption spectrum were studied. The optical Kerr nonlinearity was analyzed systematically by using Z-scan technique. In addition, the passive Q-switched operations at 1.3 µm were demonstrated by using Nd:GdVO4 as the gain medium and Co2+:β-Ga2O3 crystal as the saturable absorber.
2. Preparation and characterization of Co2+:β-Ga2O3 single crystal
2.1 Single crystal growth of Co2+:β-Ga2O3
The Co2+:β-Ga2O3 crystals were grown by EFG method with RF induction heating. β-Ga2O3 powder (purity 99.99%) and CoO powder (purity 99.5%) were used for the crystal growth of Co2+:β-Ga2O3. The raw materials were loaded into an iridium crucible with 60 mm in diameter and 60 mm in height. The crystal was grown under the atmosphere of 50% Ar, plus 50% CO2 and the entire crystal growth process was under atmospheric pressure. After all the materials melted, the melt was transported from the crucible to the top surface of the die through slit by capillary action. A seed with <010> orientation was used for the growth and the crystal was pulled up with the speed of 10 mm/h. As the crystal was grown to the predetermined length, it was separated from the top of the die at a high pulling speed. After the growth, the crystal was cooled down to the room temperature at a rate of 15-30 °C/h. X-ray fluorescence (XRF) test was carried out on the grown crystals, and the doping concentration of Co2+ ions was determined to be 0.029 at.%.
2.2 Thermal properties of Co2+:β-Ga2O3
The specific heat of the Co2+:β-Ga2O3 crystal was measured over the temperature range of 20-300°C at a heating rate of 5°C/min with a differential scanning calorimeter (Perkin-Elmer Diamond model DSC-ZC). The accuracies of calorimetry and temperature for the instrument were 0.1% and ± 0.01°C, respectively. The thermal diffusivities of Co2+:β-Ga2O3 was measured to 650°C with laser pulse method by Netzsch Nanoflash model LFA 457 apparatus. The single crystal sample for thermal measurements was shown in the Fig. 1(a). As can be seen, the thermal diffusivities declined as the increase of temperature in Fig. 1(a). The specific heat and density of crystal were mainly affected by the crystal components. Due to the doping concentration of cobalt was only 0.029 at.%, the thermal conductivity of Co2+:β-Ga2O3 was calculated according to the specific heat and density of pure β-Ga2O3 by the following formula: 1(b). The thermal conductivity of Co2+:β-Ga2O3 crystal was lower than that of pure β-Ga2O3 crystal, but it still had a great advantage over traditional laser crystals.
2.3 Spectroscopy and third-order nonlinearity
Absorption spectrum of optically polished Co2+:β-Ga2O3 was shown in Fig. 2. There were two typical broadband absorption peaks around 1173 nm and 1558 nm assigned to the Co2+ transitions of 4A2(4F)→4T1(4F) similar with the other Co2+ doped materials .
In this work, the third-order nonlinear properties of Co2+:β-Ga2O3 was studied by using a close aperture Z-scan technique. A Ti:sappire laser with a pulse width of 190 fs and pulse repetition rate of 20 Hz at 800 nm was used in the Z-scan measurement. Figure 3 shows the close aperture experimental results. The open aperture Z-scan transmittance variations can be calculated following the fitting formula: 
3. Q-switching operation
The laser experimental set-up was shown in Fig. 4. A simple concave-plano cavity of 20 mm was used to investigate the passive Q-switching Nd:GdVO4 laser by using the Co2+:β-Ga2O3 single crystal as the SA. The pump source was fiber-coupled 808 nm diode laser with a core diameter of 105 µm and numerical aperture of 0.22. The pump beam was focused into the Nd:GdVO4 crystal through a 1:1 coupling optics system with the working distance of 46 mm. The Nd:GdVO4 crystal was 3×3×7 mm3 in dimension and high-transmission coated at 808 nm. The Nd:GdVO4 crystal was wrapped with a indium foil and mounted in a water-cooled copper holder maintained at a temperature of 16 °C to alleviate the thermal lensing effect. The input plane mirror M1 was high reflection coated at 1342 nm and high transmission coated at 808 nm. M2 was concave mirror with radius of curvature of 100 mm and transmission of 3.8% at 1342 nm. A polished (010)-faced Co2+:β-Ga2O3 crystal with the thickness of 5 mm was used as the SA without water cooling. The laser pulse profile was recorded by a Tektronix DPO7104 digital oscilloscope (1GHz bandwidth, 5 Gs/s sampling rate) and a power meter (POWERMAX 500D).
The continuous-wave (CW) operation was firstly realized without Co2+:β-Ga2O3 crystal. The pump threshold power was 0.86 W. The maximum output power was 198 mW under the pumped power of 1.9 W with the optical conversion efficiency of 10.4%, as shown in Fig. 5(a). The passively Q-switched laser was achieved by inserting Co2+:β-Ga2O3 SA into the cavity. Under the absorbed pump power of 1.9 W, the maximum output power of 35 mW was achieved with the Q-switched efficiency (from CW to passive Q-switching operation) of 17.7%. The pulse width and pulse repetition were shown in Fig. 5(b). As can be seen, with the increase of absorbed pump power, the repetition rate increased and the pulse width declined, respectively. The shortest pulse width of 280 ns with the repetition rate of 181 kHz were obtained under the absorbed pump power of 1.9 W. The train of pulse under the absorbed pump power of 1.9 W was shown in Fig. 5(c).
In summary, Co2+:β-Ga2O3 was prepared and used as the SA for 1.3 µm passively Q-switched lasers. Two broadband absorption peaks around 1.0 and 1.5 µm were observed and assigned to the transitions of 4A2(4F)→4T1(4F) of Co2+ ion. As a simple sesquioxide, the thermal conductivity along a* direction in Co2+:β-Ga2O3 was 13.0 W·m-1·K-1 which was still larger than that of pure YAG indicating a better heat management ability. It was interesting that the nonlinear refractive index of Co2+:β-Ga2O3 single crystal was as large as 9.14 × 10−13 (esu) which was much larger than 2.7 × 10−13 (esu). The large nonlinear refractive index indicated that Co2+:β-Ga2O3 had potential for kerr-lens mode-locked laser. Passively Q-switched laser at 1342 nm based on Co2+:β-Ga2O3 SA was experimentally demonstrated for the first time. In the Q-switched operation, the maximum average output power of 35 mW was obtained under the absorbed pump power of 1.9 W. At the maximum output power, the Q-switched efficiency was 17.7%. The corresponding shortest pulse width is 280 ns with the repetition rate of 181 kHz. Combined with the high thermal conductivity, the Co2+:β-Ga2O3 is a protentional SA at the NIR band.
National Key Research and Development Program of China (2016YFB1102201, 2018YFB0406502); the 111 Project 2.0 (BP2018013); Guangdong Basic and Applied Basic Research Foundation (2019A1515110857); Special Project for Research and Development in Key areas of Guangdong Province (2020B010174002); China Postdoctoral Science Foundation (2019M652379); Key Technology Research and Development Program of Shandong (2018CXGC0410); National Natural Science Foundation of China (51932004, 61975098).
The authors declare no conflicts of interest.
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