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Abstract
We demonstrate the large spectral range nonlinear optical response of a Co:ZnO epitaxial thin film (with single crystal structure) with the absorption range from 1.0 μm to 2.0 μm, for the first time to our knowledge. The third-order nonlinear optical properties and saturable absorption of Co0.15Zn0.85O (Co:ZnO) were studied with the modulation depth of 0.68% and saturation fluence of 1.532 μJ/cm2. Passive Q-switched lasers at the wavelengths of a 1.4 μm Nd:YGG laser and a 1.9 μm Tm:LuVO4 laser were realized with the Co:ZnO epitaxial thin film as the saturable absorber. In the case where the wavelength is 1.4 μm, we obtained the shortest pulse width, the largest pulse energy and the highest peak power, which were recorded at 89 ns, 2.3 μJ and 25.8 W, respectively. Moreover, when the laser output wavelength is 1.9 μm, we got the smallest pulse width of 274 ns. This work provides a promising saturable absorber with a large spectral range response.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Saturable absorbers have important applications in the pulsed lasers as passive Q-switchers or mode-lockers. It has been theoretically prospected and experimentally identified that the 3dN –ions can exhibit saturable absorption if they are placed in arrangements of a tetrahedral symmetry [1, 2]. Up to now, the 3dN –ions include Cr4+(3d2), Cr5+(3d1), Co2+(3d7), Cr2+(3d4), V3+(3d2), etc., doped crystals, glass and ceramics have been developed aiming at the particular wavelength bands [2–23]. As a successful case, the yttrium aluminum garnet (YAG), as a very important matrix crystal material for laser, has a very wide range of applications and has a cubic lattice structure. When the tetravalent Cr4+ ions are mixed into the YAG, the Cr4+ ion may be substituted for the trivalent Al3+ position on the oxygen regular tetrahedral. It has been confirmed that the saturated absorption characteristic of widespread concern is due to the role of Cr4+ ions in Cr4+:YAG crystal [24]. So far, Cr4+:YAG has already been widely used in passive optical switchers in the wavelength of 1.0 μm and has been commercialized [4, 25–27]. Similarly, using YAG as a matrix, a tetrahedral trivalent vanadium-ion-doped yttrium aluminum garnet crystal (V3+:YAG) is a relatively new material that exhibits saturable absorption in the near-infrared spectral range [28]. Q-switched pulses have been successfully obtained by using this crystal in cw diode-pumped neodymium crystalline lasers of 1.06, 1.34, and 1.44 μm [19–22, 29, 30]. And the Cr5+-doped crystals can also be used as saturable absorbers in 1.0 μm [6, 8]. The optical absorption and luminescence properties of Cr5+-doped GdVO4, Li3VO4, ZrSiO4, YVO4, YPO4, and YVO4 crystals where the Cr5+ ions substitute for the tetrahedrally coordinated pentavalent ions have also been reported [6, 8, 31–35]. Co2+ ions also belong to 3dN-series, crystals doped with Co2+ ions occupy a dominant position at the wavelengths of 1.3 μm and 1.5μm. At present, many Co2+-doped crystals have been used as saturable absorbers, such as Co:LiGa5O8 [7], Co:LaMgAl11O19 [9], Co:MgAl2O4 [10, 11], Co:YSGG [18], Co:ZnS [12], and Co:ZnSe [13, 14]. It has also been confirmed that Co2+ ions enter into the matrix of the above crystals, occupying tetrahedrally coordinated site symmetry positions. In addition, the presence of Cr2+ as another form of 3dN-series of Cr ion, the Cr2+-doped crystals such as Cr2+:ZnS, Cr2+:ZnSe and Cr2+:Cd0.55Mn0.45Te have been reported as the saturable absorbers Q-switches for 2.0 μm lasers [14–17, 23]. By employment of their wide vibronic shifts, the absorption bands of the ions shown above have been broadened in the near- and mid-infrared wavelength bands [2]. In fact, the electrons of the 3dN –ions are not shielded, indicating that their absorption bands are sensitive to the host lattices. Therefore, the investigation of the atypical 3dN –ion in suitable host materials has attracted plenty of attention and the hosts which can further broaden the saturable absorption bands of 3dN –ions covering the near- and even mid-infrared range are favorable by the developed lasers.
Through the analysis of several kinds of crystals doped with 3dN -ions, it naturally drives us to think of Co:ZnO and it has been proved that Co:ZnO nanocrystals as saturable absorbers [44, 45]. For Co:ZnO epitaxial thin film (with single crystal structure), it has been proved experimentally that Co2+ ions enter the hexagonal wurtzite structure of ZnO and substitute the position of Zn2+ ions forming a tetrahedral structure in ZnO [36]. So the Co:ZnO epitaxial thin film should have saturable absorption properties and should be a promising material as saturable absorber for near- and mid-infrared lasers. In this paper, we have not only confirmed that Co:ZnO epitaxial thin film has a saturation absorption characteristic, and also confirmed that the epitaxial thin film has a wide band response characteristics. The passive Q-switching of a fiber-coupled laser-diode end-pumped Nd-doped ytterbium gallium garnet (Nd:YGG) and Tm-doped vanadate (Tm:LuVO4) lasers at 1.4 μm and 1.9 μm were testified using the Co:ZnO epitaxial thin film as the saturable absorber.
2. The characterization of prepared Co:ZnO epitaxial thin film
In this experiment, a 15% Co-doped Co:ZnO epitaxial thin film was used as the saturable absorber which was obtained by the molecular beam epitaxy method. Co:ZnO epitaxial films were grown on Al2O3(0001) substrates by radio-frequency oxygen-plasma-assisted molecular beam epitaxy (RF-MBE). Metal fluxes were provided by evaporating high purity elemental solid sources (5N cobalt and 6N zinc). Oxygen flux was supplied in the form of active oxygen (5N5) radicals by a radio-frequency plasma source. Before deposition, the Al2O3 substrates were thermally annealed at 700°C for 10 min in the growth chamber with a base pressure of 1 × 10−9 mbar. A 40 nm ZnO buffer layer was first grown to relax the lattice mismatch, and then a Co:ZnO epilayer 200 nm thick was deposited. During growth, Co:ZnO epilayers were grown at the relatively low temperature of 400 °C, and the partial oxygen pressure was reduced to 10−7 mbar using a liquid nitrogen trap [36–38]. At room temperature, the transmittance spectrum of the prepared Co:ZnO epitaxial thin film sample was measured from 200 nm to 3000 nm with a V-570 JASCO UV/VIS/NIR spectrophotometer and presented in Fig. 1(a). For comparison, the transmittance spectrum of pure ZnO was also measured as shown in Fig. 1(a). From this figure, it can be seen that there was an absorption peak near 661 nm and a little absorption at the wavelengths from 1.0 μm to 2.0 μm in comparison with pure ZnO. The transmittances at the wavelengths of 1.4 μm and 1.9 μm were about 81% and 84.6%, respectively, which indicates the Co:ZnO epitaxial thin film may be used as the saturable absorber for visible and near infrared Q-switched pulse lasers. It has been proved experimentally that Co2+ ions enter the hexagonal wurtzite structure of ZnO and substitute the position of Zn2+ ions forming a tetrahedral structure in ZnO, and the transmission spectrum of Co:ZnO is related to the substitution of Co2+ ions [36, 37]. An energy-level diagram of Co2+ ions in a tetrahedral crystal was presented in the inset of Fig. 1(a). The strong absorption band at 661 nm can be assigned to the spin and electric-dipole-allowed 4A2 (4F) →4T1 (4P) transition. The broad near infrared absorption band at 1 - 2 μm is assigned to the 4A2 (4F) →4T1 (4F) transition [24].
Fig. 1 (a) The transmission spectra of Co:ZnO epitaxial thin film at room temperature. (Inset). An energy-level diagram of Co2+ ions in a tetrahedral crystal. (b) The transmittance versus the fluence at the wavelength of 1.55 μm. (c) Experimental setup for Z-scan technique.
The Z-scan technique is an effective way to study the nonlinear properties of materials [39, 40]. And it was used to measure the saturable absorption of the Co:ZnO epitaxial thin film. The experimental setup for Z-Scan technique as shown in Fig. 1(c). The sample was irradiated by laser with the center wavelength of 1.55 μm, pulse width of 400 fs and repetition rate of 43 MHz. The average output power of the laser used in the setup could be tuned by adjusting the input current of the laser. The laser was divided into two beams by a beam splitter. One beam of the laser was used as a reference, and a focus lens with a focus length of 50 mm focused the other beam. The Co:ZnO sample was placed on a computer controlled displacement platform (NRT150/M. THORLABS. Inc.) that could be moved along the z-axis with the face perpendicular to the incident beam. Two energy meters (PM320E.THORLABS. Inc.) were used to detect the optical energy of two beams simultaneously. In the experiment, the nonlinear absorption of the sample could be obtained by measuring the transmittance at different positions in different z values.
Fitted by the Eq. (1), the typical relationship between transmission and peak intensity of Co:ZnO epitaxial thin film measured by the Z-scan technique were achieved and demonstrated in Fig. 1(b).
In this formula, T is the transmission of the used Co:ZnO epitaxial thin film, A represents a normalization constant, δT represents the depth of modulation, I and Isat represent the incident intensity and saturation intensity, respectively. Fitted by the above formula, the modulation depth was 0.68% and the saturation fluence was 1.532 μJ/cm2. The above results shown that Co:ZnO epitaxial thin film could be used as the saturable absorber in the passive Q-switched pulsed lasers at the wavelengths from 1 μm to 2 μm.
The above analyses show that Co:ZnO can be used as a saturable absorber. Besides, the zinc has been substituted for cobalt, the bond length and crystal lattice parameters of the crystal structure have been changed due to the different radii of the two atoms. And there are also many defects in the crystal, such as oxygen vacancies, zinc vacancies, and zinc gap filling [41, 42]. Moreover, with the increase of Co-doping concentration, more disorder of the crystal lattice and defects will be induced, and the surface roughness of the crystal will also increase. In our view, it is precisely because of the defects and disorders of the Co:ZnO epitaxial thin film that the absorption wavelength of the epitaxial thin film can be extended to 2 μm.
3. Experiment results and discussion
The experimental configuration was shown in Fig. 2(a). For the wavelength of 1.4 μm laser, a 1 at. % Nd-doped YGG crystal was used as the gain mediums with the dimensions of 3 × 3 × 7 mm3. Both of its end faces were polished without coating. The pump source used in this experiment was a fiber-coupled laser diode (LD) with the emission wavelength of 808 nm. The diameter of the fiber is 200 μm, corresponding to the numerical aperture (NA) of 0.22. The pump light was focused on the Nd:YGG crystal through a coupling system with a beam compression ratio of 2:1 and a focal length of 20 mm. M1 was the input mirror which was a flat mirror, the mirror was coated thin film of antireflection (AR) at the wavelength (808 nm) of pump source on the side of near the pump light and highly reflective (HR) coating at the laser wavelength (1.4 μm) on the other side of the mirror. M2 was a concave coupler, which was a concave mirror with a curvature radius of 50 mm with the transmission of 3% at 1400-1500 nm. The Nd:YGG crystal was fixed in a copper block which was filled with circulating water with a temperature of 12 °C. The cavity length was optimized by optimizing the output power in cw, and the optimal cavity length 42 mm was obtained. With the same experimental device, the cw and Q-switched pulse lasers at the wavelength of 1.9 μm has also been obtained. The pump source was a LD with the wavelength of 795 nm. Input mirror M1 was also a plane mirror with the plated film, which was highly permeable to the pump light (795 nm) and was highly reflective for the output laser (1.9 μm). M2 was a concave mirror with a radius of curvature of 50 mm. Its transmission was 3% at 1.9 μm, and in the intracavity side, the mirror was coated a high reflection film at the wavelength of 795 nm. By optimizing the output power in cw, the optimum cavity length was 45 mm.
Fig. 2 (a) Schematic diagram of experimental laser setup. (b) The characterizations about the cw and pulsed laser average output power versus the absorbed pump power of the cw and pulsed 1.4 μm Nd:YGG lasers.
For Q-switched pulsed lasers, the Co:ZnO epitaxial thin film was inserted into the cavity between M2 and Nd:YGG or Tm:LuVO4 for the wavelengths of 1.4 μm and 1.9 μm, respectively. In order to reduce the influence of pump light, the Co:ZnO optical modulator was placed as close as possible to the output coupler. The output power was measured by a power meter (1918-R, Newport, Inc.), and the pulse repetition frequency (PRF) and pulse width were measured by a TDS3012 digital oscilloscope (100MHz bandwidth and 1.25GS/s sample rate, Tektronix, Inc.).
The cw laser output power of Nd:YGG crystal at the wavelength of 1.4 μm has been shown in Fig. 2(b). The threshold absorbed pump power was 1.8 W corresponding to a slope efficiency of 8%. When the pump power reached 12.2 W, with the increasing of the absorbed pump power, the cw output power tended to saturate, which may be due to the thermal lens appearance in the laser cavity. The maximum output power of the cw laser was 775 mW under the absorbed pump power of 15.6 W. Pulsed laser had been obtained when the saturable absorber of Co:ZnO epitaxial thin film was put into the cavity under the absorbed pump power of 2.1 W. The average output power of pulsed Nd:YGG laser at the wavelength of 1.4 μm versus the absorbed pump power was also shown in Fig. 2(b) with a slope efficiency of 4.2%. The maximum average output power was 410 mW when the absorbed pump power was 15.6 W. The characteristics of the Q-switched pulse laser were also recorded with an oscilloscope and shown in Fig. 3. It can be seen from the Fig. 3(a) that the PRF increased while the pulse width decreased with the increasing of absorbed pump power, which was a typical feature of the passive Q-switching [43]. When the absorbed pump increased to 15.6 W, the pulse width reduced to 89 ns, which was the shortest pulse width as shown in Fig. 3(b). The PRF was also shown in Fig. 3(a). It can be seen that the PRF increased with the increasing of the absorbed pump power and the maximum PRF was 198 kHz when the absorbed pump power was 13.8 W. By calculating, the maximum single pulse energy and peak power can be calculated to be 2.36 μJ and 25.8 W, respectively. Pulse train was also recorded and shown in Fig. 3(c) in the experiment. As can be seen from the Fig. 3(c) the pulses train was uniform and stable. In the experiment, the spectra of the Q-switched pulse laser was measured with an optical spectrum analyzer (OSA205, ThorLabs, Inc.) with a center wavelength of 1416.3 nm as shown in Fig. 3(d).
Fig. 3 (a) Dependence of the pulse width and repetition rate at 1.4 μm on the incident pump power. (b) The single pulse profile with a width of 89 ns. (c) The pulse train with the repetition of 66.6 kHz. (d) The pulsed laser spectrum with the center wavelength of 1416.3 nm.
The variation of the cw laser output power of Tm:LuVO4 crystal at 1.9 μm with the increase of the absorbed pump power was shown in Fig. 4(a). The laser appeared at the absorbed pump power was 3.5 W, and the maximum output power of the cw laser was 225 mW under the absorbed pump power was 6 W with the slope efficiency was 9.1% by fitting. Pulsed laser would appear when the saturable absorber which was used in previous experiment was put into the cavity under the absorbed pump power was 3.96 W. The average output power corresponded to the change of the absorbed pump power as shown in the Fig. 4(a). The slope efficiency of the fitting was about 5.46% and the maximum average output power was 116 mW when the absorbed pump power was 6 W. The relationship between pulse width and PRF and absorbed pump power was presented in Fig. 4(b). As can be seen from the figure that the pulse width decreased first and then increased with the increasing of the absorbed pump power, the smallest pulse width was 274 ns as shown in the Fig. 4(c) under the absorbed pump power of 4.7 W. With the same optical spectrum analyzer (OSA205, ThorLabs Inc), the spectra of the pulsed laser were measured with a center wavelength of 1932.8 nm as shown in Fig. 4(d).
Fig. 4 (a) The cw and pulsed laser average output power versus the absorbed pump power. (b) The pulse width and repetition rate versus the absorbed pump power. (c) The pulsed laser spectrum with the center wavelength of 1932.8 nm. (d) Pulse profile with the width of 274 ns.
By comparing the two groups of experimental data, it could be seen that when the wavelength was 1.4 μm, the experimental results were relatively good, the pulse width was shorter under the same absorbed pump power, and the average output power was larger than that of 1.9 μm. The reason for these results is the absorption intensity of the Co:ZnO epitaxial thin film at the wave band of 1.4 μm is stronger than that at 1.9 μm, which could be seen from the Fig. 1(a).
4. Conclusion
In conclusion, the 15 at. % Co:ZnO epitaxial thin film (with single crystal structure) was firstly used as a saturable absorber for the passively Q-switched 1.4 μm Nd:YGG laser and 1.9 μm Tm:LuVO4 laser. The saturation fluence was measured to be 1.532 μJ/cm2. A Q-switched pulse width of 89 ns at the wavelength of 1.4 μm has been obtained with a peak power of 25.8 W when absorbed pump power was 15.6 W. For the wavelength of 1.9 μm, the minimum pulse width was 274 ns when the absorbed pump power was 4.7 W. The experimental results show that the Co:ZnO epitaxial thin film can be used as a large spectral range saturable absorber at the wavelengths of 1.4 μm and 1.9 μm. It can be inferred that the Co:ZnO epitaxial thin film can be used as a saturable absorber at the wavelengths of 1.0 μm - 2.0 μm. Compared with several other crystals doped with 3dN -ions, Co:ZnO has a very wide absorption band, as shown in Fig. 5. We believe that the results of the present study can stimulate further research on Co:ZnO epitaxial thin film and explore a wider range of applications.
Fig. 5 The comparison of absorption bands between Co:ZnO and other crystals doped with 3dN –ions.(In the case of the same doping ions and the different matrix, it is selected that the crystal has a wider absorption band.)
Funding
National Key Research and Development Plan of China (2016YFB1102301); National Natural Science Foundation of China (NSFC) (51632004, 51772173); Natural Science Foundation for Distinguished Young Scholars of Shandong Province (JQ201415); Taishan Scholar Foundation of Shandong Province, China.
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