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Abstract
A technique for inkjet-printing of graphene saturable absorbers (SAs) for ~2-μm bulk and waveguide lasers is presented. Based on distillation-assisted solvent exchange to fabricate high-concentration graphene inks, this technique is capable of producing few-layer graphene films of arbitrary shape. Absorption saturation of graphene printed on glass is demonstrated at ~1.56 µm for picosecond and femtosecond pulses indicating a large fraction of the saturable losses. Inkjet-printed transmission-type graphene SAs are applied in passively Q-switched nanosecond thulium (Tm) microchip and planar waveguide lasers. The Tm microchip laser generates 136 ns / 1.2 µJ pulses at 1917 nm with a repetition rate of 0.37 MHz with a Q-switching conversion efficiency reaching 65%. The planar waveguide laser generates 98 ns / 21 nJ pulses at 1834 nm at a repetition rate in the MHz-range. The inkjet-printing technique is promising for production of patterned SAs for waveguide lasers.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Graphene is a two-dimensional layer of carbon atoms arranged in a honeycomb lattice [1]. In recent years, it has been recognized as an attractive material for saturable absorbers (SAs) employed to passively modulate the losses in near-IR lasers [2,3]. Graphene has a unique zero-bandgap feature with the hollow cone electron and hole bands meeting at the Dirac point. This property leads to a broadband linear (low-signal) absorption (at 0.4 – 3 µm) which is anomalously strong, about 2.3%, for a single atomic layer [4]. Graphene also exhibits broadband (at 0.8 – 2.8 µm) ultrafast saturable absorption due to the finite number of electronic states in the valence band [5,6]. It features a relatively low saturation fluence (intensity), a fast recovery of the initial absorption (with two characteristic time constants, ~200 fs and ~1 ps [2], respectively) and a reasonably low laser-induced damage threshold. The latter is related to very high thermal conductivity of graphene [7]. The modulation depth of graphene SAs can be altered by stacking several carbon layers [5,6,8]. These properties have motivated multiple studies of graphene SAs in passively Q-switched (PQS) [9–11] and mode-locked (ML) [12–15] bulk and waveguide lasers emitting in the near-IR spectral range.
The application of graphene in pulsed lasers emitting at ~2 µm is particularly attractive. Due to the graphene band structure, the absorption saturation properties of graphene are wavelength-dependent, i.e., the saturation fluence (intensity) decreases at lower photon energy [6]. The ~2 µm laser emission is eye-safe, typically achievable with thulium (Tm3+, 3F4 → 3H6 transition) or holmium (Ho3+, 5I7 → 5I8 transition) ions. Pulsed (nanosecond) ~2 µm lasers are used in range-finding, environmental sensing, wind mapping, spectroscopy and medicine. Regarding ML lasers at ~2 µm, graphene can be a competitor of the GaSb-based semiconductor SAs (SESAs) where the technology is far from being mature. Indeed, recently, a graphene SA was successfully employed in a bulk ML Tm laser generating for the first time sub-100 fs pulses at ~2 µm [15]. As for PQS lasers emitting at ~2 µm, graphene can be treated as a “fast” SA leading to the generation of nanosecond pulses at high repetition rates [10,16]. Similar to the case of ML lasers, it was argued that graphene SA is a competitor of the SESAs in such lasers as well [17]. Note that there also exist efficient “slow” SAs for PQS ~2 µm lasers (e.g., Cr2+:ZnS(e) crystals and ceramics) which, however, operate in a different regime providing high energy pulses at low repetition rates [18]. Graphene is attractive for PQS waveguide lasers since it can be used in different geometries (transmission-type [19] or evanescent-field interaction) [11]. Such lasers are of interest in integrated optics for sensing applications.
Several methods have been proposed for the fabrication of graphene on various substrates (e.g., Si, SiC, glass, metals, etc.). Mostly investigated are the mechanical and liquid phase exfoliation of graphite or graphite oxide, epitaxial growth on SiC and metals, and chemical vapor deposition (CVD) [20]. Among these, the exfoliation methods yield graphene in the form of small-size graphene flakes which can then be deposited as thin films with tailorable optical and electrical properties [21]. In this relation, the inkjet printing technique has gained a lot of research interest as it enables the deposition of various functional materials, e.g., graphene nanostructures (graphene, graphene oxide, carbon nanotubes, etc.) [22–24] and other 2D materials (e.g., black phosphorus) [25]. This method offers simplicity, cost-efficiency, scalability and versatility provided by the direct patterning feature, allowing fabrication of functional devices and thin films through the controlled ejection of droplets of liquid phase material (ink) from the nozzle onto a substrate.
It is crucial to fine-tune certain properties of graphene inks for obtaining a precise and reliable jetting. These include relevant fluidic properties (viscosity and surface tension), acceptable concentration of graphene flakes while avoiding aggregation and restacking, low toxicity and small lateral dimensions of the graphene flakes to avoid nozzle clogging [26]. Efficient techniques for inkjet-printing of graphene have been proposed [23,27].
To date, inkjet-printed graphene and graphene oxide devices have been mostly intended for electronics applications, such as sensors, transparent conductors, supercapacitors, thin film transistors, etc [24,28,29]. We report herein on the application of an inkjet-printing technique for the size-scalable production of high-quality graphene SAs for PQS ~2 µm lasers in bulk and waveguide geometry.
2. Experimental
2.1 Inkjet-printing of graphene SA
The graphene ink was prepared through the distillation-assisted solvent exchange technique [23]. First, graphene flakes were obtained through the exfoliation by ultrasonication of graphite in dimethylformamide (DMF). Next, 8 mg/ml of ethyl cellulose (22 cP for 5 w/v% in 80:20 toluene:ethanol, Sigma-Aldrich, product no. 200697) and 2 mg/ml of ethyl cellulose (4 cP for 5 w/v% in 80:20 toluene:ethanol, Sigma-Aldrich, product no. 200646) were added to stabilize the graphene flakes and the solvent (DMF) was exchanged with terpineol through distillation. Last, the dispersion was diluted with ethanol at the volume ratio of 3:1 (terpineol: ethanol) thus obtaining the final ink with an estimated graphene concentration of ~0.5 mg/ml.
The graphene film was printed with a commercial inkjet printer (Dimatix Materials Printer, DMP 2800, Dimatix-Fujifilm Inc.) equipped with 10 pl cartridges (DMC-11610). The graphene films were fabricated with 20 printing passes, a drop spacing of 40 µm, a jetting voltage of 27 V and a plate temperature of 45 °C. The substrate was a microscope glass slide (SuperFrost Slides, VWR, product no. 631-0114, 1-1.2 mm thick). After printing, the films were dried at 80 °C for 1 h and annealed at 400 °C for 1 h.
2.2 Material characterization
The morphology of the graphene film was studied with a Scanning Electron Microscope (SEM, Zeiss Ultra 55). The small-signal transmission spectra were measured with a Varian CARY-5000 spectrophotometer. The Raman spectra were measured with a calibrated home-made spectrometer (KTH) using an Ar+ ion laser (514 nm) as an excitation source.
The absorption saturation of the inkjet-printed graphene SA was studied by an open-aperture Z-scan method. The sample was translated along the focused beam of a pulsed laser providing variation of the peak fluence F incident on the sample. We used a ML Er-doped fiber laser emitting at 1560 nm with a repetition rate of 100 MHz (Menlo T-Light). The pulse duration Δτ was adjusted with an external dispersion-managed fiber path to 65 fs or 1 ps (determined as full width at half maximum, FWHM, under Gaussian shape assumption). The Gaussian radius of the laser beam in the focus wL was 9 μm and the Rayleigh length zR was 0.16 mm. The peak laser fluence is F = 2E/(πwL2), where E is the pulse energy. For the designed set-up, the variation of F was in the range 1...400 µJ/cm2.
3. Characterization of the inkjet-printed graphene SA
3.1 Morphology
In Fig. 1, we show a secondary electron high-contrast SEM image. Graphene flakes with lateral dimensions of <1 µm are observed. During their deposition by inkjet printing, they are arranged with different orientations forming a thin film with an almost uniform thickness. The flakes located at the surface have a tendency of protruding, increasing the surface area of the sample. Only a small amount of ethyl cellulose residue in the form of tiny white particles can be noticed, confirming that annealing of the sample was efficient. The thickness of the graphene thin film was measured by a profilometer [29] to be around 27 nm with a roughness Ra of 3-5 nm, partially affected by inherent roughness of the glass substrate.
Fig. 1 Scanning Electron Microscope (SEM) image of the inkjet-printed graphene film exhibiting sharp graphene flakes.
3.2 Linear transmission
The low-signal transmission (T0) spectrum of the film is shown in Fig. 2. The value of T0 is determined by subtracting the Fresnel losses from the substrate, i.e. T0 = T (graphene + substrate) / T(substrate).
Fig. 2 Small-signal transmission, T(graphene + substrate) / T(substrate), spectrum of the inkjet-printed graphene film, inset – photograph of the SA; the spectrum for a CVD-grown single carbon layer graphene SA is shown for comparison [10].
The graphene film exhibits a broadband linear absorption. At ~1 μm, T0 = 88.8% (small-signal absorption, α'0 = 1 – T0 = 11.2%). It is known that the absorption of a single carbon layer is about πα ≈2.3% where α ≈1/137 is the fine structure constant [4] and it scales linearly with the number of carbon layers n [6]. Indeed, for a commercial graphene SA containing a single carbon layer deposited by the CVD method [10], T0 is 97.7% at ~1 µm (Fig. 2). Thus, we can estimate that the effective number of carbon layers ‹n› for the studied sample is between 4 and 5. Near 2 μm, T0 for the studied sample is 90.5%.
3.3 Raman spectroscopy
The presence of the two-dimensional graphene structure was confirmed by Raman spectroscopy, see Fig. 3. The excitation wavelength was 514 nm. In Fig. 3, the spectrum of the single layer CVD-grown graphene SA is also shown for comparison (not in scale). For inkjet-printed graphene, the peak at 1582 cm−1 (G) exhibits a double-peak structure due to the double-degenerated (in-plane transverse optical, iTO, and longitudinal optical, LO) phonon mode E2g at the Brillouin zone center. The peak at 2702 cm−1 (2D, also referred as G') is due to a second order process of the zone boundary phonons (two iTO phonons at the K point). The D peak with the frequency of about 1/2 of that of the 2D one (1354 cm−1) arising from one iTO phonon and a defect is visible in the spectrum of the inkjet-printed graphene film. Its observation is due to the structure defects inherent to the finite size of the graphene flakes. The weak D' peak (1623 cm−1) is also due to the defects. The weak and broad peak at ~2450 cm−1 (G*) originates from a combination of the zone boundary in-plane longitudinal acoustic, iLA, and iTO phonon modes. The interpretation of the Raman spectra is according to the previous papers [30,31].
Fig. 3 Raman spectrum of the inkjet-printed graphene film, the excitation wavelength is 514 nm. The spectrum for a CVD-grown single carbon layer graphene SA is shown for comparison [10].
The increase of n (number of layers) affects strongly the Raman spectra, e.g., it modifies the spectral shape of the 2D band and the intensity ratio of the 2D and G ones [30,31]. From the broadening of the 2D band and its shift to higher frequencies, we estimate n for the inkjet-printed graphene film as 3-5 [30]. The corresponding ratio I(2D)/I(G) is ~0.45 supporting this conclusion (compare with I(2D)/I(G) = 2.2 for the single carbon layer CVD-grown graphene). As a conclusion, the Raman observations are in good agreement with the absorption spectroscopy.
3.4 Absorption saturation
The open-aperture Z-scan absorption saturation curves of the inkjet-printed graphene SA are shown in Fig. 4. The intrinsic sample transmission T (i.e., after subtracting the Fresnel losses) can be represented as T(F) = 1 – α'(F), where α' is the sample absorption. For F ≈0 (low-signal regime), T0 = 1 – α'0 and α'0 = α'S + α'NS, where α'S and α'NS are the saturable and non-saturable absorption, respectively. Both pulse durations are shorter than the characteristic recovery time of initial absorption of graphene SA τrec, ~1.3 ps, as measured by the pump-probe method at ~1.5 µm [32]. Thus, the following expression can be used to model the measured absorption saturation curves [33,34]:
where Fsat is the saturation fluence which corresponds to the reduction of the saturable part of absorption by a factor of 1/e, and the two-photon absorption (TPA) is neglected.Fig. 4 (a,b) Absorption saturation curves for the inkjet-printed graphene SA measured at 1.56 µm with (a) 1 ps and (b) 65 fs pulses: circles – experimental data, curves – their fitting with Eq. (1).
For the 1 ps pulses, Fig. 4(a), the best-fit parameters are α'S = 6.5% and Fsat = 102 µJ/cm2. The corresponding saturation intensity Isat is 170 MW/cm2. Note that Fsat corresponds to the reduction of the saturable part of absorption by a factor of 1/e and Isat – by a factor of ½, so there is no straightforward relation between these parameters. The ratio of the saturable to the total (low-signal) absorption α'S/α'0 is 0.68. For the 65 fs pulses, Fig. 4(b), α'S = 6.8% and Fsat = 60 µJ/cm2 (Isat = 1.36 GW/cm2), so that α'S/α'0 = 0.71. For both measurements shown in Fig. 4, when the laser spot size on the sample 2wL was larger than 230 µm (for F < 20 µJ/cm2), the laser beam was slightly distorted due to the sample inhomogeneity. For smaller beam spot sizes, the signal was free of parasitic modulation.
For bulk solid-state lasers PQS by graphene SAs, the characteristic time of formation of a single Q-switched pulse is between few hundreds of ns to few µs [10]. This is longer than τrec of graphene, so that it is considered as a “fast” SA. It will be saturated by the instantaneous pulse intensity I = F/Δτ*, Δτ* ≈1.06Δτ [35]. Under the “slow” SA condition (i.e., for pulse duration Δτ shorter than τrec) satisfied in graphene ML lasers, the absorption is saturated (bleached) by the pulse fluence.
Previously, Isat for graphene SAs containing few carbon layers was measured to be about 1 MW/cm2 for ns-long pulses and for a relatively large laser spot size of hundreds of µm [6]. A similar value was estimated from modelling the laser performance of graphene PQS lasers [16,35]. However, as it was shown in Ref [6]. for small laser spot sizes, the measured Isat can increase by a factor of ~10 due to the effect of the carrier recombination beyond the excitation region induced by carrier diffusion. Even a further increase of Isat is expected when the pulse duration Δτ approaches the electron-hole recombination time for graphene (~1.67 ps), so that the photo-carrier density dynamics is in a transient state [6]. Such enhancement of Isat up to GW/cm2-range due to the small laser spot size and short Δτ is indeed observed from our experiments.
4. Passive Q-switching of thulium lasers
The fabricated inkjet-printed graphene SA was used in two types of Tm lasers: a microchip (bulk) and a planar waveguide geometry for PQS. The transmission-type SA was inserted at a normal incidence of the mode propagation in both lasers resulting in stable passive Q-switching. As a gain material, we used Tm3+-doped monoclinic double tungstates (MDTs) [36], KRE(WO4)2 (shortly KREW), where RE = Lu or Y. These crystals are known for their advantageous spectroscopic properties when doped with Tm3+ ions [36,37] allowing for efficient laser operation in the CW [36,38] and the PQS regime [39]. MDTs are optically biaxial, so that the optical and spectroscopic properties are described in the optical indicatrix frame with the principal axes Np, Nm and Ng.
4.1 Laser set-ups
The scheme of the microchip (bulk) laser is shown in Fig. 5. This laser was based on a 5 at.% Tm:KLuW crystal grown by the Top-Seeded Solution Growth (TSSG) method [36]. The laser crystal was cut for light propagation along the Ng-axis enabling microchip laser operation [38]. The 4.1 mm-thick active element (aperture: 3(Nm) × 3(Np) mm2) was polished to laser quality, wrapped with In foil and mounted in a Cu-holder water-cooled down to 12 °C.
The crystal was pumped through the pump mirror (PM) by a fiber-coupled AlGaAs laser diode (fiber core diameter: 200 µm, numerical aperture, N.A. = 0.22) emitting at ~802 nm (3H6 → 3H4 transition of Tm3+). The unpolarized pump radiation was collimated and focused into the crystal by a lens assembly (1:1 reimaging ratio, 30 mm focal length). The pump spot size 2wp in the crystal was 200 μm and the pump absorption under lasing conditions was 54%.
The plano-plano laser cavity comprised a flat PM, antireflection coated for 0.78–1.0 µm and high-reflection coated for 1.84–2.1 µm, and a flat output coupler (OC) with a transmission TOC of 5%, 9%, 20% or 30% at the laser wavelength. The graphene SA was inserted between the active element and OC. All the optical elements in the cavity were placed next to each other in order to minimize the cavity roundtrip time and to shorten the pulses in the PQS regime. The calculated size of the laser mode on the SA was ~70 µm.
The second laser studied was a planar waveguide Tm laser, see Fig. 6. It was based on a thin film with a composition of KY0.61Gd0.22Lu0.12Tm0.05(WO4)2 (abbreviated: 5 at. % Tm:KYW) grown by the Liquid Phase Epitaxy (LPE) method on an undoped (010)-oriented 2-mm thick undoped KY(WO4)2 substrate. The substrate itself was grown by the TSSG method. The top surface and both edge faces of the Tm:KYW/KYW epitaxy were polished to laser quality and remained uncoated. After polishing, the active layer thickness was 18 μm, as determined with ESEM measurement. The layer was oriented for light propagation along the Ng-axis with a length of 5.0 mm. The waveguide propagation losses were estimated from the Caird analysis for CW operation mode as 0.7 ± 0.2 dB/cm [40]. The epitaxy was mounted on a BK7 glass block and it was passively-cooled.
The active layer was pumped by a CW Ti:Sapphire laser at 802 nm. The pump polarization corresponded to E || Nm in the active layer. The pump was coupled into the waveguide by a 10 × microscope objective lens (N.A. = 0.28, focal length: 20 mm) resulting in a spot diameter of 20 μm. ~48% of the incident pump power was launched into the active layer and >85% of that was absorbed under lasing conditions.
The laser cavity was composed by a flat PM and a flat OC with TOC = 30% at the laser wavelength (see the details above). This OC was selected because it was difficult to achieve stable passive Q-switching for lower TOC. The graphene SA was inserted between the active layer and OC minimizing the air gaps.
4.2 Microchip Tm laser
At first, we studied the CW performance of the microchip Tm laser (with SA removed from the cavity). The best output performance corresponded to TOC = 9%, namely 1.38 W at 1946 nm with a slope efficiency η of 60% (with respect to the absorbed pump power Pabs). The laser threshold was at Pabs = 0.61 W. For CW experiment, we limited Pabs to ~3 W because Q-switching was not stable for higher pump powers. With increase of the output coupling, the emission wavelength experienced a blue-shift from 1945 to 1960 nm (for TOC = 5%) to 1923 nm (for TOC = 30%).
With inserted graphene SA, a stable Q-switching regime was achieved. For all output couplers (OCs), there existed an upper power limit for the Q-switching stability related to heating of the SA by the residual (non-absorbed) pump power [10]. This limit clearly increased with TOC, probably due to the corresponding reduction of the intracavity peak intensity. Below this limit, no damage of the graphene SA was observed. The results in terms of output power and laser emission spectra are shown in Fig. 7. The highest average output power corresponded to the highest studied TOC of 30%, namely 454 mW at 1917 nm with η = 34%. The laser threshold was at Pabs = 1.12 W. The Q-switching conversion efficiency with respect to the CW mode ηconv was 65% indicating a moderate insertion loss of the SA. With the increase of the output coupling, a blue-shift of the emission wavelength from 1940 nm for TOC = 5% to 1917 nm for TOC = 30% was observed due to the quasi-three-level nature of the Tm3+ laser in agreement with the gain spectra of Tm:KLuW for light polarization E || Nm [36]. The emission spectrum of the PQS laser typically contained one single intense line. The laser output was linearly polarized (E || Nm) for all OCs; the polarization was naturally selected by the anisotropy of the gain. The output beam corresponded to a TEM00 mode (measured M2 parameter: <1.2).
Fig. 7 (a,b) Diode-pumped Tm:KLuW microchip laser passively Q-switched by inkjet-printed graphene SA; (a) input-output dependences, η – slope efficiency (laser polarization: E || Nm); (b) typical laser emission spectra measured at maximum Pabs.
The pulse duration Δτ (determined as FWHM) and the pulse repetition frequency (PRF) were measured directly, and the pulse energy was determined as Eout = Pout/PRF, see Fig. 8. The pulse characteristics were dependent on the absorbed pump power: Δτ decreased, PRF increased, and Eout first increased and then saturated with Pabs. Such a behaviour is typical for quasi-three-level lasers PQS by “fast” SAs and it is related to the dynamic bleaching of the SA [41]. For TOC = 30%, the pulse duration shortened from 556 to 136 ns, and the PRF increased almost linearly from 111 to 372 kHz. The maximum pulse energy was 1.2 µJ. This corresponded to the maximum peak power Ppeak = Eout/Δτ of 8.3 W. With increasing TOC, a tendency to shorten the pulse duration and to increase the pulse energy was observed.
Fig. 8 (a-c) Diode-pumped Tm:KLuW microchip laser PQS by inkjet-printed graphene SA: (a) pulse duration (FWHM); (b) pulse energy; (c) pulse repetition frequency (PRF). TOC is the transmission of the output coupler.
The oscilloscope traces of the single Q-switched pulses obtained for TOC = 30% at different absorbed pump powers are shown in Fig. 9(a). The pulses exhibit a Gaussian temporal shape. The corresponding pulse train for the maximum Pabs of 2.40 W is shown in Fig. 9(b). The intensity fluctuations are <20% and the room-mean-square (rms) pulse-to-pulse timing jitter is <15%. These instabilities are attributed to heating of the SA by the residual pump.
Fig. 9 Oscilloscope traces of (a) single Q-switched pulses for different absorbed pump powers Pabs and (b) the corresponding pulse train for Pabs = 2.40 W for the diode-pumped Tm:KLuW microchip laser PQS by inkjet-printed graphene SA.
4.3 Planar waveguide Tm lasers
The input-output dependences and the typical laser emission spectra of the CW and PQS Tm:KYW planar waveguide laser are shown in Fig. 10. In the CW regime, the maximum output power was 132 mW at 1839.2 nm with a slope efficiency η of 52.2%. The laser threshold was at Pabs = 104 mW.
Fig. 10 (a,b) Ti:Sapphire laser pumped Tm:KYW planar waveguide laser: CW operation and PQS by the inkjet-printed graphene-SA: (a) input-output dependences, η – slope efficiency (the laser polarization is E || Nm), inset – typical spatial profile of the laser mode in the far-field (CW regime), (b) typical laser emission spectra measured at maximum Pabs.
With inserted graphene SA into the laser cavity, a stable Q-switching regime was observed. The maximum average output power was 26.7 mW at 1834.2 nm with η = 11.3%. The laser threshold was at Pabs = 170 mW and the Q-switching conversion efficiency ηconv was 20%. No damage of the SA was observed. A profile of the elliptic output laser beam (multimode) captured in the far-field is shown as an inset in Fig. 10(a). The laser output was linearly polarized (E || Nm). The shorter emission wavelength observed for the waveguide laser is due to its higher losses compared to the microchip laser and it agrees with the gain spectra of Tm:KYW [37].
The pulse characteristics of the PQS Tm:KYW planar waveguide laser are shown in Fig. 11(a,b). Similarly to the case of the microchip Tm laser, they varied with Pabs. The pulse duration Δτ decreased from 178 to 98 ns, the pulse energy Eout increased from 5.8 to 21 nJ, and the PRF increased almost linearly from 0.89 to 1.27 MHz. The maximum peak power Ppeak thus reached 0.2 W. The observed higher PRFs as compared to the microchip Tm laser are due to the small size of the laser mode in the waveguide laser. The latter limits the pulse energy as expected from the theory of graphene PQS lasers [41]. The oscilloscope traces of the shortest single Q-switched pulse and the corresponding pulse train are shown in Fig. 11(c,d). The intensity fluctuations are <25%.
Fig. 11 (a-d) Ti:Sapphire laser pumped Tm:KYW planar waveguide laser PQS by the inkjet-printed graphene SA: (a) pulse duration (FWHM) and pulse energy; (b) pulse repetition frequency (PRF); (c,d) oscilloscope traces of (c) the shortest single Q-switched pulse and (d) the corresponding pulse train for Pabs = 0.35 W. TOC = 30%.
In Table 1, we compare the PQS output characteristics of the microchip and waveguide Tm lasers studied in the present work employing an inkjet-printed graphene SA with those reported recently for a commercial single carbon layer CVD-grown graphene SA [10,19]. The results obtained in the present work are superior in comparison to the previous studies. We attribute this improvement to a higher modulation depth of the inkjet-printed graphene film (Fig. 4) and, possibly, to the larger surface area of the protruded graphene flakes, Fig. 1.
Table 1. Output Characteristics of Thulium Lasers PQS by Commercial CVD-Grown and Inkjet-Printed Graphene SAs
5. Conclusion
We report on the successful application of inkjet-printing for the fabrication of high-quality few carbon layer graphene SA. Its nonlinear absorption characteristics were studied by the Z-scan technique at 1.5 µm using fs- and ps-long pulses resulting in a relatively high fraction of the saturable losses. The suitability of the inkjet-printed graphene SA for passive Q-switching in the ~2 μm spectral range was demonstrated using Tm3+-doped microchip (bulk) and planar waveguide lasers capable of generating nanosecond pulses at high repetition rates (hundreds of kHz to MHz). The microchip (thermally guided) diode-pumped Tm:KLuW laser and the Ti:Sapphire laser pumped planar waveguide (index guided) Tm:KYW / KYW laser both passively Q-switched by the transmission-type inkjet-printed graphene SA generated 1.2 μJ / 136 ns and 98 ns / 21 nJ pulses at 0.35 MHz and 1.27 MHz repetition rates, respectively. Inkjet-printing can be applied to deposit graphene films directly on crystal faces, e.g., for passive Q-switching of ~2 μm (Tm and Ho) waveguide lasers by evanescent field interaction. Further efforts will be devoted to the application of inkjet-printed graphene SAs in mode-locked ~2 μm lasers. The inkjet-printing technique can be also applied to deposit different nanostructured absorbers, e.g. single-walled carbon nanotubes (SWCNTs) or 2D materials on dielectric substrates.
Funding
Spanish Government (MAT2016-75716-C2-1-R, (AEI/FEDER,UE) TEC 2014-55948-R); Generalitat de Catalunya (2017SGR755); National Science Centre (NCN), Poland (UMO-2015/18/E/ST7/00296).
Acknowledgments
E. K. acknowledges financial support from the Generalitat de Catalunya under grants 2016FI_B00844 and 2017FI_B100158. F. D. acknowledges additional support through the ICREA academia award 2010ICREA-02 for excellence in research. M. K. acknowledges the doctoral fellowship from the National Science Centre (NCN, Poland) under grant No. UMO-2017/24/T/ST7/00234. P. L. acknowledges financial support from the Government of the Russian Federation (grant No. 074-U01) through ITMO Post-Doctoral Fellowship scheme.
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