Carbon nanohorn dahlia aggregates with a lateral size of hundreds of nanometers and a height of a few nanometers were prepared utilizing commercial carbon nanohorn powder. In the mid-infrared spectral region (@ 2845 nm), the saturable absorption property of the carbon nanohorn was experimentally investigated. It was employed as a saturable absorber in a Er:Lu2O3 laser. Laser pulses with a shortest pulse duration of 255 ns were yielded at a repetition rate of 149 kHz under the maximum pump power of 6.5 W. To the best of our knowledge, this is the first time that the saturable absorption property of carbon nanohorn was investigated and pulsed lasers were generated with YAG-based CNH in the mid-infrared. Our investigation indicates that carbon nanohorn exhibits a comparable nonlinear optical performance to carbon nanotube in the mid-infrared region. Further investigation of mid-infrared mode-locking lasers can be realized with carbon nanohorn by decreasing the thickness of the carbon nanohorn.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Optical responses of low dimensional sp2-bonded low-dimensional nanocarbon materials have attracted great interests for their unique optical properties. Previous researches show that the optical nonlinearity of the low dimensional material is highly related to their nanostructures and microscopic morphology. The sp2-bonded fullerene was regarded as a unique optical limiting material. The graphene and single wall carbon nanotube (SWCNT) are of the great significance as a saturable absorber among those allotropes for ultrafast recovery time and adjustable nonlinearity. Hexagonal graphene exhibits a Dirac cone band structure, which supports an ultrabroadband optical absorption (visible to terahertz) . Multiple Q-switched and mode-locked lasers were realized in a broad optical spectral from visible to mid-infrared dominated by the Pauli-blocking effect [2–5]. SWCNT have typical cylinder diameter of 1-2 nm. By mixing different diameters of the SWCNT, it can be a superior saturable absorber for different spectral regions. Up to now, many high-performed pulsed laser operations were realized (716 nm to 2.8 μm) [6–9]. Liu et al. realized ultrafast laser centered at 1540, 1550 and 1560 nm in an erbium doped laser applying carbon nanotube . Solodyankin et al. obtained 1.32-ps laser pulses in a Tm-doped silicon fiber laser system employing carbon nanotube as saturable absorbers . The transformation characteristic from saturable absorption to reverse saturable absorption (0.7, 1 and 1.8 μm)  also means such materials can be a promising optical limiter. Unfortunately, further investigation of this material is restricted by the complex preparation process and the low production rate at room temperature. Based on aforementioned investigations, the diversity of the sp2-bonded nanocarbons and the structure-related optical nonlinearity indicate that an easily fabricated novel nanocarbon with comparable optical nonlinearity as carbon nanotube can be applied for further pulsed laser investigations.
Carbon nanohorn (CNH) is an elongated closed sp2-hybridized carbon cage with a typical diameter of 2-5 nm and a length of 40-50 nm . The structural similarity between CNH and carbon nanotube (CNT) reveals that same chemistry characteristics are shared. In many investigations, CNH is regarded as the replacement candidate for CNT [13,14]. It should be noted that CNH exhibits many superior properties than CNTs. In the fabrications, CNT can be fabricated without any toxic metal catalyst and can be massively produced at room temperature . In the microscopic structure, CNHs present more diversity spatial structures, which is constructed by pentagons, hexagons and heptagons carbon rings, in which, dahlias [15,16] are one of the most famous CNH. In stabilities, the nano-windows on the CNH surfaces can be thermally reversed. Now that the optical properties of the CHT has been massively reported, CNH, which features many superiorities, also exhibit superior in optical responses. At 532 and 1064 nm, the materials shows the optical limiting properties , while in mid-infrared wavelength range, its optical properties were less studied.
In this paper, systematic investigations on CNH samples preparation were described firstly. The microscopic size was characterized by the morphology characterization. The broadband optical transmittance and the nonlinear optical response at highlighted wavelength were systematically characterized. Employed as a saturable absorber in Er:Lu2O3 laser, a stable Q-switched laser was generated. Under the maximum pump power of 6.5 W, the maximum average output power of 599 mW was realized applying the T = 3% output coupler. Laser pulses with 255-ns pulse duration was yield at a repetition rate of 149 kHz corresponding to a pulse energy of 4.02 μJ and a peak power of 15.8 W. To the best of our knowledge, this is the first time, that pulsed lasers were realized with YAG-based CNH in the mid-infrared region.
2. Preparation and characterization
2.1 Material preparation
The sample was prepared by purifying commercial carbon nanohorn (Nanjing XFNANO Materials Tech Co.) in ethyl alcohol. 7-mg CNH powder was dispersed in the 7-mL ethyl alcohol at first. The large carbon particle was sedimentation at the bottom of the test tube by centrifugated at 4000 rpm for 5 min. Then, the top liquid sample was transferred into a new test tube and recentrifuged the collected mixtures at 8000 rpm for 5 min to purify the residual impurity and deduce the CNH concentration in the solutions. Finally, the supernatant was collected for the following microscopic morphology characterizations, optical properties characterizations and Q-switched laser experiments.
2.1 Morphology characterizations
Systematic characterizations were performed to investigate the morphology and the three-dimensional size of the prepared samples. Transmission electron microscopy (TEM) characterization was performed at first, which can give a referable lateral morphology and roughly laterals size of the sample. The test samples were dropped on the carbon membrane support. As shown in Fig. 1 (a), the CNH samples gather as irregular-shaped with a lateral size of few micrometers, which is mainly caused by the coffee-ring effect. During the volatilization of the solvent, the CNH samples gradually gathered into the involatile solvent, and finally assembled as an aggregate. The morphology of a single aggregate was shown in Fig. 1 (b). It is clear that our CNH collections are consisted of the free stand dahlias CNH and the CNH clusters formed by the coffee-ring effect during the drying. The clusters can deduce the uniformity of the samples, and thus affect the optical response of the CNH. Such disadvantages can be conquered by applying the spin-coating method. As shown in Fig. 1 (c), the free stand dahlia CNH is circular with a typical diameter of 100 nm. And the wrinkle on the surface is the individual CNH.
The height information and accurate lateral size of the prepared CNH sample were also characterized by the atom force microscopic (AFM). The test sample solvent was prepared by spin-coating 15 mL solvent collection on the 1×1 cm2 silicon wafer and dried for 5 min. The morphology of an 8×8 μm2 silicon-based CNH section is shown in Fig. 2 (a). As can be seen, the spin-coated CNH aggregate was bedded on the wafer, homogeneously. Detail investigations on the microscopic size were performed by systematic section analysis. The lateral size and height of a sample along line 1 and line 2 (see Fig. 2 (a)) are shown in Fig. 2 (b). The height and laterals size of the sample is around 4.2 nm and 1 μm, separately. The ratio of the transverse dimension and height is around 250:1. To support a better morphology comprehensive of the prepared sample, the analysis covering 6 samples was performed and shown in Fig. 2 (c). From the visualized lateral size and the height, a conclusion can be made that the homogeneous CNH samples with few-nanometers height and hundred-nanometers lateral size were prepared.
Raman spectrum was also applied to identify the molecular variations of CNH. As shown in Fig. 2(d), two Raman shift peaks (1341 and 1593 cm-1) have emerged under the excitation of a 532-nm laser. The D-band 1341 cm-1 peaks are ascribed to A1 g symmetry modes attributed to the loss of the basal symmetry. The G-band 1593 cm-1 is assigned to the E2g-liked vibrations with the sp2 hybridized carbon atoms . The good agreement between the illustrated results and the previous research results prove the CNH samples were prepared successfully.
2.3 Optical properties characterization
The testing sample was prepared by spin-coating the collected solvent on the YAG windows substrate. The high-quality YAG substrate can support high optical transmittance (∼87%) in a broad-spectrum region. The linear optical transmittance of YAG-based CNH aggregates was identified by the spectrophotometer. As shown in Fig. 3 (a), removing the influence of the YAG-substrate, the pure CNH aggregates shows high optical transmittance (> 80%) from 0.5 μm to 3.2 μm. In the visible and near-infrared region (500 to 1500 nm), the optical transmittance gradually increases with the growth of the wavelength. In the mid-infrared range, the material shows a smooth optical transmittance, and the optical transmittance was 82.3% at 2845 nm.
The optical nonlinearity of the YAG-based CNH at 2845 nm was characterized by the chopper Q-switched solid state Er:Lu2O3 laser. A laser pulse with a width of ∼50 ns was delivered at 4 kHz. By placing the sample at the focus point, we got the nonlinear optical response of the YAG-based CNH samples. Different from the reported optical limitation effect in the visible and near-infrared region, the YAG-based CNH shows a typical saturable absorption property at mid-infrared 2845 nm (Fig. 3 (b)). It needs to be noted that the displayed transmittances were contributed by both CNH and YAG-substrate, which is more close to the real transmittance as saturable absorption in laser cavity. The optical nonlinearity can be fitted by the following formula:
The systematical investigations on the morphology characterizations and optical nonlinearity response show that spin-coated YAG-based CNH has a saturable absorption property in the mid-infrared region. A comparison of the nonlinearity parameters of the CNH, the conventional sp2-hybridized graphene and the CNT is summarized in Table 1. In the mid-infrared region, the YAG-based CNH has the deepest modulation depth and the lowest saturable photon flux intensity. Multiple research results show that the value of the optical nonlinearity parameters can be tuned by adjusting the morphology of the nanomaterials. Indicated by the above results, the easily fabricated and massive produced CNH can be a promising candidate as the saturable absorber in the mid-infrared region.
3. Laser experiment
To investigate the optical modulation performance of the YAG-based CNH sample in the mid-infrared region, an Er:Lu2O3 laser was established shown in Fig. 4. A 3×3×10 mm3 Er:Lu2O3 crystal, which was wrapped in the copper sink and cooled at 10 °C, was placed in a 30-mm linear concave-plane cavity. The concave mirror (ROC = 1000 mm), anti-reflected coated in 960∼990 nm and high reflected coated in 2.7∼2.9 μm, was employed as input coupler (IC). The plane mirror coated by the fitted reflective film in 2.7∼2.9 μm was employed as output coupler (OC). In this manuscript, three mirrors with T = 1%, 3% and 5% were selected to find the optimized laser operations. The pumping source is the laser diode emitting at 976 nm. The pump is firstly coupled by fiber with the numerical aperture of 0.15 and the core diameter of 100 μm. The pump laser was further refocused in the crystal by the lens group with a couple ratio of 1:2. To block the residual pump light, a filter was placed behind the output coupler. Output power was measured by the PM 100D and visualized by the S314C powerhead (Thorlabs Inc., USA). The laser pulses were detected by the photodetector with a response time of 1 ns (PVI-4TE-4, VIGO system S. A.) and recorded by a digital phosphor oscilloscope (DPO 4102B-L, Tektronix Inc. USA).
4. Laser performance and discussion
The continuous wave (CW) laser was realized in the aforementioned laser system without YAG-based CNH saturable absorbers. By applying three OCs, the highly performed CW laser was realized. The corresponding variation of the output power versus the incident pump was shown in Fig. 5(a). The pumping thresholds of T = 1%, 3% and 5% lasers are 1.01, 1.23 and 1.35 W, respectively. By further scaling up the pump power, the output powers linearly increase with slope efficiencies of 13.6%, 16.8% and 16.2%. The maximum output power of 0.9 W was realized in the T = 3% lasers under a pump power of 6.5 W. The output power can be highly improved by employing high-quality IC. A spectrum of the ultimate CW laser was characterized and shown in the inset of Fig. 1(a). It is clear that the peak of the spectrum is 2845 nm.
By introducing YAG-based CNH-SA in these three laser cavities, Q-switched lasers were realized. Laser pulses with hundreds of nanoseconds durations were delivered from Q-switched lasers applying pump powers exceed 1.1, 1.42 and 1.66 W with T = 1%, 3% and 5% lasers, respectively. A systematic characterization of the pulsed laser was performed. As shown in Fig. 5(b), average output powers linearly increase with the absorbed pump power with slope efficiencies of 10.1% 12.1% and 10.3%, respectively. Compared with the CW lasers, Q-switched lasers show higher lasing thresholds and lower slope efficiency, which is the result from the losses caused by YAG-based CNH. The lasing spectrum of the Q-switched laser was depicted in the inset of Fig. 5(b) showing the laser spectrum is still located at 2845 nm.
The characterization of the main parameters of the Q-switched laser is shown in Fig. 6. The variation tendencies of the repetition rate, the pulse duration, the pulse energy and the peak power versus the enhanced pump power are similar for T = 1%, 3% and 5% lasers. The pulse duration gradually decreased with the increasing of the pump power. In contrary, the repetition rate, the pulse energy and the peak power increased via scaling up the pump power. Under the maximum pump power of 6.5 W, three shortest pulse durations of 267, 255 and 281 ns were delivered at a repetition rate of 142, 149 and 138 kHz from T = 1%, 3% and 5% lasers, respectively. The corresponding pulse energy and peak powers were 3.7, 4.0 and 3.6 μJ and 14.0, 15.8 and 12.6 W.
The shortest Q-switched pulses with a pulse duration of 255 ns were delivered at a repetition rate of 149 kHz. The corresponding pulse train and temporal pulse profile are shown in Fig. 7 (a) and (b), respectively. The Q-switched laser performance has experimentally shown the optical nonlinearity of the carbon nanohorn. In the mid-infrared region, many superior laser performances were realized with the nanomaterials-based saturable absorbers. Some of the highlighted Q-switched results of the nanocarbon and other superior saturable absorbers were shown in Table 2. For the points of the pulse duration and repetition rate, the CNH exhibits a more similar Q-switched laser performance as the graphene than other materials. This is powerful support for that the CNH can be a promising saturable absorber as the graphene in the mid-infrared region. Further, by improving the fraction of the coverage of the CNH membranes for preparation, high-performed Q-switched and mode-locked laser can be realized with the CNH in the mid-infrared region.
In summary, dahlia CNH samples were successfully extracted from the commercial CNH powders. By spin-coated on the YAG windows, CNH membrane with a height of few nanometers was prepared successfully. The saturable absorption property of the YAG-based CNH membrane in the mid-infrared was characterized. The results show the CNH possesses a wavelength-related optical nonlinearity. In the visible and near-infrared region, the material shows a reverse saturable absorption property, while in the mid-infrared region, the nanoflakes exhibits saturable absorption properties. Employed as a saturable absorber in Er:Lu2O3 laser, stable Q-switched laser pulses were realized. Under the maximum pump power of 6.5 W, laser pulses with a duration of 255 ns were delivered at a repetition rate of 149 kHz, corresponding to a pulse energy of 4 μJ. The comparable laser performance with the graphene shows the CNH, easily fabricated and massively produced at room temperature, can be a promising candidate as the saturable absorber in the mid-infrared region.
National Key Research and Development Program of China (2016YFB1102201); Natural Science Foundation of Shandong Province (ZR2020MF116); Natural Science Foundation of Shandong Province (ZR2019MF061); Qi Lu Young Scholars Program of Shandong University (Tao Li, Tianli Feng); National Natural Science Foundation of China (61605100); Young Scholars Program of Shandong University (Wenchao Qiao).
Tao Li acknowledge financial support from Qi Lu Young Scholars Program of Shandong University and Wenchao Qiao acknowledges financial support from Young Scholars Program of Shandong University.
The authors declare no conflicts of interest.
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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