A few-layer graphitic carbon nitride (g-CN) nanosheet film on an yttrium aluminum garnet substrate was fabricated and employed as saturable absorber for a passively Q-switched Ho,Pr:LiLuF4 laser at 2.95 μm. Under an absorbed pump power of 3.89 W at a pump wavelength of 1.15 μm, a maximum average output power of 101 mW was realized with a pulse duration of 420 ns and a repetition rate of 93 kHz. Even shorter pulse durations of 385 ns were obtained at a reduced output coupler transmission.
© 2017 Optical Society of America
Mid-infrared (mid-IR) lasers operating near 3 μm are highly demanded in medical and sensing technologies, owing to the strong absorption of O-H . Pulsed mid-IR lasers can be further used as a pump source for driving optical parametric processes for the generation of coherent mid-IR light in the 3-19 μm range . Furthermore, such high pulse energy sources also find broad application in defense, spectroscopy and atmospheric monitoring .
Among various active ions for the mid-IR lasers, Er3+ has been studied comprehensively in both fiber and solid-state lasers at 3 μm based on the 4I11/2→4I13/2 transition . Also the Ho3+-ion provides a transition in this wavelength range. However, compared to Er3+, the progress in Ho3+ lasers operating on the 5I6→5I7 transition is limited. This is due to the considerably longer fluorescence lifetime of the lower level (5I7) compared to the upper level (5I6). This condition normally prohibits laser operation. Thus, despite energy transfer upconversion (ETU) processes contributing to a depopulation of the lower level (5I7), lasing was only obtained in a self-pulsed regime and the output power levels obtained in this way are very low . An efficient approach to quench the lifetime of the 5I7 multiplet is the co-doping with Pr3+-ions. A resonant energy transfer to the 3F2 multiplets of Pr3+ allows for a fast, phonon-supported decay via the 3H6 multiplet into the ground state, which efficiently depopulates the 5I7 multiplet of Ho3+ . In this way, continuous wave (cw) laser operation could be obtained in Ho,Pr:LiLuF4 crystals .
Graphitic carbon nitride (g-CN) represents a new class of sp2 hybridized metal-free polymeric semiconductors. It has been widely applied in photo- and organocatalytic processes [8–13]. The intrinsic optical response of this material is limited to the ultraviolet due to the large bandgap energy of ~2.7 eV. However, the photoresponsivity of g-CN can be strongly influenced by facile doping, defect control, etc . Furthermore, the absorption spectral curve of ultrathin g-CN nanosheets has previously been found to extend to visible or even IR region [15,16]. Since 2016, g-CN has been verified to be a broadband saturable absorber (SA), with the advantages of a low-cost synthesis and a high chemical and thermal stability. The saturable absorption of g-CN has been demonstrated experimentally by Q-switched or mode-locked lasers at 1.1, 1.3, and 2.8 μm [4,17,18].
Here, we report on the preparation of few-layer g-CN nanosheets on an yttrium aluminum garnet (YAG) substrate based on a vertical evaporation technique and their application for passive Q-switching of a Ho,Pr:LiLuF4 laser at 2.95 μm. We realized stable pulses with pulse durations as short as 385 ns. Under an absorbed pump power of 3.89 W, an average output power of 101 mW was obtained with a pulse energy of 1.1 μJ.
2. Preparation and characterization of the g-CN saturable absorber
g-CN was synthesized by thermal condensation of melamine . 5 g melamine powder (99%, Aldrich) was tempered at 550 °C in an alumina crucible under N2 atmosphere for 4 h. After cooling down, 2.2 g of light yellow bulk g-CN powder was collected and ground for subsequent preparation.
The crystal structure of the synthesized bulk g-CN powder was examined by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). In the SEM image presented in Fig. 1(a), graphite-like morphology of the bulk sample in the μm-range is visible. As seen in Fig. 1(b), the bulk sample exhibits two XRD peaks located at 13.1° and 27.6°, which can be identified as (100) and (002) diffraction planes. The weak (100) diffraction results from the in-planar structural packing motif and the strong (002) peak reveals the graphite-like stacking of the conjugated aromatic segments with an interplanar distance of 0.326 nm .
Atomically thin g-CN nanosheets possess a higher charge carrier density compared to bulk g-CN material. This is caused by an increased density of states at the conduction band edge . As a result, few-layer g-CN nanosheets possess optical absorption exceeding far into the IR range, permitting their application as SAs in the mid-IR range [4,15–18].
In order to prepare such g-CN nanosheets, we utilized the liquid phase exfoliation (LEPx) technique. 10 mg of the synthesized bulk g-CN powder was added to 10 ml aqueous solution and ultrasonicated for 7 h. After centrifugation at 4000 rpm for 15 min, the top two-thirds of the dispersions were collected for further processing.
To determine the precise morphology and the thickness of the exfoliated g-CN, transmission electron microscopy (TEM) and atomic force microscopy (AFM) were utilized. The results are presented in Figs. 2(a) and 2(b), which clearly prove that the g-CN had been exfoliated to µm2-sized flakes surrounded by smaller particles. Figure 2(c) shows a height profile through line A and line B in Fig. 2(b). The height of the as-prepared g-CN flakes varies between 4 and 7.2 nm corresponding to roughly 10 to 20 layers considering the interplanar stacking distance of 0.326 nm .
The optical transmission spectrum of the g-CN nanosheets was detected by an ultraviolet-visible-near infrared spectrophotometer (U-4100, Hitachi, Japan). A test sample was prepared by dropping a certain amount of the stock dispersion onto a YAG substrate with dimensions of 40 mm × 20 mm. As seen in Fig. 3, the intrinsic absorption of the g-CN nanosheets exhibits a sharp decrease toward the visible band. However, the sample exhibits a broadband absorption. In the wavelength range between ~700 nm and ~3 μm, the sample provides a smooth and unstructured absorption of a few percent which can be utilized for SA purposes. The decrease of the transmission for wavelength exceeding 3000 nm is introduced by the basic primary and/or secondary amine groups, which were generated by the synthesis progress owing to the incomplete polycondensation [14,20]. The presence of amino groups does not influence the basic tri-s-triazine units of g-CN or affect the optoelectronic properties at 2.95 μm wavelength .
Various approaches have been explored for the fabrication of 2D-SAs, such as drop coating , spin coating , optical trapping , chemical vapor deposition , and vertical evaporation . In particular the latter is a simple and cost efficient way to uniformly transfer nano-dispersion onto a hydrophilic substrate . Here, we utilized this method to fabricate a homogeneous g-CN nanosheet film on a YAG substrate. The stock supernatant was poured into a beaker acting as a sample pool, and then a round YAG substrate with a radius of 12.7 mm was inserted in the sample pool vertically. After evaporating at 30 °C for 48 h, the YAG-based g-CN nanosheet film was ready for the application as a SA for mid-IR laser experiments. A photograph of the sample can be found in the inset of Fig. 4.
The nonlinear optical response of the g-CN-SA was investigated with a ~100 ns Q-switched Er:Lu2O3 laser at 2.84 µm . The result is shown in the inset of Fig. 3. The SA has a modulation depth of 2.6% and a saturated transmission of 91.4%. The transmission of the uncoated YAG substrate for the Er:Lu2O3 wavelength slightly deviated from the expected value of 92% assuming only Fresnel reflection at both surfaces  and was found to be 93.2%. This mismatch might be caused by Fabry-Perot effects in the plane-parallel surfaces of the substrate. Consequently, the nonsaturable loss of our g-CN film is estimated to be 1.8%. Considering the ultrafast recovery time intrinsic to 2D-SAs, we did not evaluate the saturation fluence of the g-CN-SA with the available ~100 ns laser pulses.
3. Laser experiments
3.1 Experimental setup
As shown in Fig. 4, a 2.5 cm long linear cavity configuration was utilized for the Ho,Pr:LiLuF4 laser. A fiber-coupled diode laser emitting at a wavelength of 1.15 µm with a core diameter of 400 μm and a numerical aperture of 0.22 was employed as the pump source. The pump beam was imaged into the laser crystal with a spot radius of 200 μm. An uncoated Ho,Pr:LiLuF4 crystal served as the gain medium. The slab shaped sample with dimensions of 5 mm × 1.5 mm × 10 mm was wrapped in indium foil and mounted in a copper block water-cooled to 10 °C. The doping concentrations of Ho3+ and Pr3+ in the sample were measured to be 0.710 wt. % (1.61 × 1020 ions/cm3) and 0.026 wt. % (0.07 × 1020 ions/cm3), respectively. With the co-doping of Pr3+-ions, the lifetime of the 5I7 manifold in Ho3+ is significantly decreased from 16 ms to 1.97 ms, and the emission intensity at 2.9 μm increases significantly . A flat IR fused silica mirror with an antireflection coating for the pump wavelength range between 1.1 and 1.2 μm (T > 95%) and a high-reflection coating for the laser wavelength range between 2.8 and 3.0 μm (R > 99.8%) was employed as the input coupler. Differently curved mirrors with transmission values T of 1% and 3% utilizing YAG substrates were used as output couplers (OCs). A filter was placed behind the OC to block the residual pump light. A PM200 power meter with a S470C power head (Thorlabs Inc., USA) was used for measuring the average output power. The laser spectra were measured by an optical spectrum analyzer with a spectral resolution of 0.12 nm (MS3504i, SOL Instruments, Belarus). The laser pulse train was detected by a fast HgCdTe IR detector with a response time of 1 ns (PVI-4TE-4, Vigo System S.A.) and recorded by a DPO 7104C digital phosphor oscilloscope with a rise time of 350 ps (1 GHz bandwidth and 20 GS/s sampling rate, Tektronix Inc., USA).
3.2 Results and discussion
cw laser operation was achieved by employing the curved OCs of T = 1% and 2%. The cw laser characteristics are shown in Fig. 5(a). The threshold pump powers amounted to 0.69 and 0.81 W at T = 1% and 2%, respectively. A maximum output power of 155 mW was achieved with T = 2% at an absorbed pump power of 3.89 W. The slope efficiencies decreased when the absorbed pump power exceeded ~1.7 W, which had been observed in previous reports as well [7,26]. Current studies suggest that this is a result of the complex transition processes in Ho3+, in particular an endothermic ETU process from the upper level 5I6 [25,26]. We excluded thermal lensing as the main origin of the decreased slope efficiency by utilizing two OCs of T = 1% with different radii of 50 and 500 mm in the cw laser experiment. As shown in Fig. 5(a), in both cases the efficiency decreased at the same absorbed pump power of ~1.7 W. This reveals that the thermal lensing effect in the laser crystal is not the main reason for the decrease in efficiency. Further investigations in this respect are in progress. The beam quality at the maximum output power was measured by the 90/10 scanning knife-edge method. As shown in Fig. 5(b), the beam quality factors (M2) were calculated to be 1.52/1.38 in the tangential/sagittal plane. A typical laser spectrum is provided in the inset of Fig. 5(b). The central laser wavelength was located at 2954.8 nm with a full width at half-maximum (FWHM) of 1.1 nm.
Stable passively Q-switched operation was realized by inserting the g-CN-SA into the cavity. The average output power as a function of the absorbed pump power for different output coupler transmissions is depicted in Fig. 6(a). At T = 1% and 2%, the threshold pump powers were 0.92 and 1.05 W, respectively. The decrease of slope efficiency in the Q-switched laser occurred at somewhat lower pump power compared to the cw laser. Given the higher inversion level in a Q-switched laser, this might be an indication that the energy transfer process Ho3+(5I7) → Pr3+(3F2) is not efficient enough. A higher deactivator (Pr3+) concentration might further enhance the depopulation process of the 5I7 manifold, avoid the decrease in the efficiency, and possibly allow in general for higher efficiencies . The maximum average output power reached 101 mW, i.e. the extraction efficiency was reduced by only 35% compared to cw operation. These results indicate that our g-CN-SA possesses lower insertion losses compared to spin-coated graphene-SA on sapphire substrates . As depicted in Fig. 6(b), the M2 factor of the Q-switched laser at maximum average output power was measured to be 1.55/1.41 in the tangential/sagittal plane. As shown in the inset of Fig. 6(b), the emission wavelength centered at 2954.7 nm remained nearly unchanged compared to the cw experiments, whereas the FWHM slightly reduced to 0.8 nm.
The pulse characteristics of the passively Q-switched laser are presented in Fig. 7. The pulse repetition rate and the pulse duration exhibit a strong dependence on the absorbed pump power, indicating a fast recovery time of the g-CN-SA. Utilizing the output coupling mirror with T = 2%, the pulse repetition rate increased from 50 to 93 kHz and the pulse duration decreased from 1.29 µs to 420 ns with increasing pump power. The shortest pulse duration of 385 ns was observed for the lower output coupler transmission of T = 1% at a pulse repetition rate of 104 kHz. This is the shortest pulse duration for any passively Q-switched Ho3+-doped laser at 3 μm based on either crystals or fibers.
We also calculated the evolution of the pulse energy vs. the absorbed pump power. It can be seen that the pulse energies saturate at 1.1 µJ and 0.8 µJ for T = 1% and 2%, respectively, for this configuration, cf. the top part of Fig. 7. Finally, the temporal shape of the pulse train and a single pulse were recorded for each output coupling mirror. The pulse shape can be well fitted by a Gaussian and the corresponding peak powers were calculated to be in the range of few watts.
In conclusion, we presented the first passively Q-switched Ho,Pr:LiLuF4 laser emitting at 2.95 µm utilizing a home-made graphitic carbon saturable absorber (g-CN-SA). The laser generated pulses as short as 385 ns, which are the shortest pulse duration obtained from any passively Q-switched Ho3+-doped laser in the 3 μm range. An average output power of 101 mW at a repetition rate of 93 kHz was achieved under an absorbed pump power of 3.89 W corresponding to a pulse energy of 1.1 μJ. According to our results, g-CN with more uniform large-area films and a well-defined film thickness can be considered as suitable saturable absorber material for Q-switched and even ultrafast lasers in the 3 μm spectral range. A fine tuning of the saturable absorber properties seems feasible by atomic modification such as facile doping or defect control.
National Key Research and Development Program of China (2016YFB1102201); Bundesministerium für Bildung und Forschung (BMBF) (13N13050, 13N14192), National Natural Science Foundation of China (NSFC) (61675116, 61405101).
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