Open Access
Add to BibSonomy
Abstract
We report an all-fiber scheme for the second harmonic generation (SHG) by embedding gallium selenide (GaSe) nanosheets into a suspended-core fiber (SCF). Based on modes analysis and theoretical calculations, the phase-matching modes from multiple optional modes in the SHG process and the optimal SCF length are determined by calculating the effective refractive index and balancing the SHG growth and transmission loss. Due to the long-distance interaction between pumped fundamental mode and GaSe nanosheets around the suspended core, an SHG signal is observed under a milliwatt-level pump light, and exhibits a quadratic growth with the increased pump power. The SHG process is also realized in a broad wavelength range by varying the pump in the range of 1420∼1700 nm. The SCF with the large air cladding and suspended core as an excellent platform can therefore be employed to integrate low-dimensional nonlinear materials, which holds great promise for the applications of all-fiber structures in new light source generating, signal processing and fiber sensing.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
All-fiber frequency conversion devices with miniaturized structure, high conversion efficiency and low loss are now widely used in a variety of nonlinear optics applications. Second harmonic generation (SHG) in typical silica optical fibers has been a research focus. Even if weak signals from the surface SHG can be detected in pristine optical fiber, the centrosymmetric nature of silica fiber is still a critical factor limiting the conversion efficiency of second-order nonlinear processes in conventional silica fibers. Although subsequently emerged solutions including optical polarization [1], thermal polarization [2] and electric polarization [3] improve the SHG conversion efficiency for practical applications, the demerits such as harsh fabrication conditions and high cost and narrow operation bandwidth hinder the above methods’ wide implementations. With the advanced manufacturing technique, diverse fibers like micro/-nano fiber (MNF) [4–6], photonic crystal fiber (PCF) [7–9] and hollow-core fiber (HCF) [10,11] provide flexible platforms to enhance the in-fiber second-order parameter processes. Due to the disturbance of integrated material on the guided modes is negligible, the layered nanomaterial-integrated fibers are effective to enhance and manipulate nonlinear optical response [12–14]. For instance, we have previously reported the enhancement of SHG in gallium-selenide (GaSe) nanosheets coated MNF [5] and filled HCF with high reliability and stability [10]. The strong SHG accompanied by weak third-harmonic generation (THG) has been also observed in the molybdenum disulfide (MoS2)-grown HCF with the assistance of chemical vapor deposition (CVD) method [11].
The layered material-assisted MNF frequency-doubled devices have excellent performances, but require precise phase-matching diameter and easily susceptible to environmental disturbances such as airflow vibration, dust capture and material oxygenation. The morphology of layered materials filled in the HCF will affect the high-quality transmission modes of the waveguide to a certain extent. Compared with other schemes by nonlinear materials integrated silica fibers, the suspended-core fiber (SCF) possesses the advantages of robust HCF in addition to being able to excite strong evanescent field of MNF. The suspended core provides a good platform to couple the guided modes with the embedded nanomaterials inside the large air cladding for a long light-matter interaction length. The outer silica cladding acts as an effective package of the suspended core and inner-filled nonlinear materials for enhancing the immunity from the environmental disturbance and device stability.
In this work, we demonstrate an SCF frequency-converter embedded with GaSe nanosheets via a highly stable second-order nonlinear process. The GaSe nanosheets deposition on the surface of suspended core plays an important role for the enhancement of SHG, with the merits of the ultrahigh nonlinear susceptibility χ(2). By tuning the pump wavelength in the range of 1420∼1700 nm, we observe a tunable SHG signal in a broadband spectrum. Considering the popularity of functional optical fiber devices, the integration of layered materials in optical fibers has great significances in fiber optics and optoelectronics. The integration scheme of SCF with nonlinear materials provides an alternative for new light source generation and frequency conversion.
2. Theoretical model and device fabrication
2.1 Theoretical model and analysis
Figure 1(a) exhibits the schematic of the all-fiber frequency up-converter and its operation principle. A segment of SCF with GaSe nanosheets is inserted between a single-mode fiber (SMF) and a multimode fiber (MMF). The pumped fundamental wave (ω) from the SMF is incident into the SCF, and the second-harmonic (2ω) signal with the assistance of GaSe nanosheets is excited along the surface of the suspended core and collected into the MMF. The cross-section of employed SCF is displayed in the left-bottom corner of Fig. 1(a), and the region of the suspended core is partially enlarged. The effective diameter of suspended core is around 2.6 µm, which is defined as the diameter of the inscribed circle of the central triangle-like area [15]. The three parts of the air claddings are approximately circular with the curvature radius of 35 µm, and the air occupancy rate of the entire cladding of the optical fiber is close to 100%. There are three thin walls among the air holes, while their actual thicknesses are negligible.
Fig. 1. (a) Schematic of the GaSe nanosheets embedded SCF device for second harmonic generation (SHG). Right-top inset shows the energy diagrams of the SHG process. Left-bottom inset is cross-section image of SCF under an optical microscope, theoretical model of the effective core diameter is scaled up. SMF, single-mode fiber; MMF, multimode fiber; SCF, suspended-core fiber. (b) Calculation results about the field distributions of the fundamental mode (FM) and five higher-order modes (HM-1, 2, 3, 4 and 5) at the 1550nm pump wavelength. (c) Phase-matched fundamental mode at 1550 nm and high-order second-harmonic modes (SHMs) at 775 nm, including a pair of degenerate FMs (FM-1 and FM-2) and the 4-fold degenerate SHMs (SHM-1, 2, 3 and 4, originating from HM-3 at 775 nm).
Compared with other micro-structured optical fibers, the core of SCF is suspended and fixed in the solid shell by several thin walls, and the effective cladding is a circle of large air holes outside the fiber core, which run longitudinally along the axis of SCF. The structure parameters of SCF are derived utilizing the commercial software based on the full vectorial finite difference method. Importantly, the ideal dispersion properties of waveguide modes can be obtained by designing the number and width of the supporting walls [16]. The SCF can be controlled to reach smaller core sizes, thus offering the potential to achieve smaller mode field diameters and anomalous dispersion at much shorter wavelength [17,18]. In addition, a portion of the pump electric field is allowed to extend air cladding outside the suspended core. By virtue of the above merits, SCFs have been effectively adopted to supercontinuum generation [15,19,20] and optical fiber sensing systems [21–24].
To further obtain the modal distribution and the matched modes in details, we built an equivalent model of the SCF and numerically simulate the possible modes in the light propagation. The structural parameters are set referring to the model of SCF. Then we carried out a full vectorial finite element method (FEM) analysis to the internal suspended-core model, and concluded that the core allows multiple modes to be guided simultaneously in the SCF. Figure 1(b) shows the light field distributions of the fundamental mode (FM) and five adjacent higher-order modes (HM-1, 2, 3, 4 and 5).
The emphasis to achieve efficient SHG is meeting the phase-matching condition between the FM and specific second-harmonic modes (SHMs) [25,26]. When a light source is pumped to implement the SHG processes in the SCF, the relationship between the intensity of the FM (ω) and SHMs (2ω) with nonlinear interaction length L can be revealed as [4,6]
2.2 Device fabrication and characterizations
Figure 2(a) shows the optical microscope image of the fabricated SCF device. The fiber preforms with three-hole channel were fabricated from a high-purity quartz rod using ultrasonic drilled method [15,27]. And then, the preform is further drawn to the desired size in the fiber-drawing tower. Meanwhile, the preform needs to be connected with an external air pressure control device which ensures that the air holes in the SCF will not collapse. The numerical aperture (NA) is calculated as about 1.04, according to the effective refractive index of the suspended core. The employed SCF is precisely aligned with the SMF and MMF. As the presence of the embedded GaSe nanosheets, the SHG can be excited by the incident pump light from the SMF. Then, the SHG signal is collected by the MMF for higher transmission efficiency. Figure 2(b) exhibits the light propagation along the suspended core of SCF device in a dark field environment. Taking the transmission loss and SHG growth with the SCF length into account, the GaSe nanosheets embedded SCF is cut to 560 µm. When a 635 nm laser is incident into the device, we can observe a bright red line scattered from the SCF region, as shown in Fig. 2(b), indicating a relatively uniform GaSe nanosheets distribution on the suspended core. To hold and protect the SCF device, the effective region of the coupling structure is encapsulated inside a hollow quartz tube with an inner diameter of 180 µm.
Fig. 2. (a) and (b) Optical microscope image of the fabricated SCF device embedded with GaSe nanosheets at bright-field and dark-field, when a 635 nm laser is incident into the device. (c) Atomic force microscope (AFM) and scanning electron microscope (SEM) image of the dispersed GaSe nanosheet. (d) Optical microscope image of SCF filled with GaSe nanosheets dispersion. (e) Dependence of the SHG and pump laser intensity on the effective length of the SCF. The inset is an enlarged view of the yellow area, corresponding to the 560 µm SCF device. (f) Transmission spectrum of 560 µm long SCF device pumped by a supercontinuum source in the spectral range of 1400∼1700 nm.
Layered GaSe materials have a low absorption coefficient (less than 1 cm-1) and extremely strong second-order susceptibility [28] in a wide spectral range covering from 650 nm to 1800 nm. The layered nanosheets embedded in the SCF are the ɛ-GaSe of the AB stacking sequence that belongs to the point group of D3h. The only nonzero element d22 in second-order nonlinear susceptibility tensor χ(2) is 180 pm/V [29], thus, the non-centrosymmetric of ɛ-GaSe has given great application in second-order nonlinear parameter processes. Few-layer GaSe nanosheets with excellent performances can generally be obtained using CVD technique [30–32]. The characterization of the GaSe nanosheets by an atomic force microscope (AFM) is shown in Fig. 2(b). The thickness of GaSe nanosheets is 3-5 nm, corresponding to about five atomic layers [5,30,33]. Additionally, the shape and size (0.05-1 µm) of GaSe nanosheets are shown in inset of Fig. 2(c), which is characterized by a scanning electron microscope (SEM).
The selected GaSe nanosheets are dispersed in a mixed solution of ethanol and deionized water with a concentration of 1 mg/ml. Then, the end face of SCF is immersed in the dispersion, and the GaSe nanosheets dispersion is filled into the air cladding by capillary phenomenon with the assistance of atmospheric pressure, as shown in Fig. 2(d). The average size of GaSe nanosheets is much smaller than the air cladding hole diameter of SCF. As a result, the GaSe material is allowed to relatively uniformly covered around the core surface of the SCF and interact well with the evanescent field of the guided modes. By the thermally assisted deposition method, the GaSe nanosheets can be attached onto the three supporting walls and the surface of the suspended core as the solvent of dispersion evaporates. In order to ensure higher coverage of GaSe nanosheets, the deposition procedure in multiple rounds is required.
In the fabricated device, the transmission loss, including scattering and absorption of the GaSe coating and splicing points, is a key factor to the frequency-converted light generation and propagation. The selection of optimal length requires to make a tradeoff between the produced SHG intensity and the above total loss. To obtain further insight, we made a detailed analysis by constructing a theoretical model shown in Fig. 1(a) and calculating the SHG intensity and loss. From Eq. (1), the SHG intensity has the quadratic growth with the pump light intensity and the interaction length of the SCF. Whereas, the pump light and excited SHG intensity have an exponential attenuation with the propagation length due to scattering and adsorption losses caused by the longer GaSe-filled distance [10]. The coupling loss of SCF devices mainly caused by the mismatch of the mode field area and the NA between the SCF and SMF/MMF. To estimate the coupling loss of the SMF-SCF-MMF structure, we measured the transmission at 1550 nm using an infrared laser and an optical powermeter. The coupling efficiency $\eta$0 of the entire device with 950 µm is around 51.1%. Also, we made a compared measurement on the GaSe-filled SCF, and the transmittance $\eta$1 is reduced to 10.6%. Based on the Bouguer-Lamber law, we obtained the transmission loss coefficient $\alpha$ of 1.66 mm-1 by calibrating the coupling loss of the SCF before and after the deposition of GaSe nanosheets. And then, we differentiated Eq. (1) with respect to the parameter L under the perfect phase-match condition, thus, the output fundamental wave (ω) intensity and the change of the SHG intensity with the transmission distance L are determined by
According to the results of the above analysis, we plotted the changes of the pump light and SHG intensities with the transmission length, as shown in Fig. 2(e). With the increase of SCF length, the pump light intensity has an exponential decrease (green dashed line), while the SHG intensity is nonmonotonic change (orange solid line), showing a rapidly growth and then gradually decrease. As a result, there is an optimal length of SCF filled with GaSe nanosheets, which provides a guidance in the device fabrication and SHG signal collection. From Fig. 2(e), the optimal length of the SCF is 603 µm, corresponding to a maximum conversion efficiency of SHG. Actually, the over 90% of peak value shown in the enlarged yellow region can be acceptable, taking the fabrication difficulty into account. The fabricated device has a length of ∼560 µm, which is very close to the desired value. Moreover, we measured the transmission spectrum of the device using a supercontinuum source to evaluate its actual loss, indicating an average loss of about 3 dB in the spectral range of 1400 nm and 1700 nm, as shown in Fig. 2(f). The wrinkles in the transmission spectrum mainly originate from the intermodal interference in possible multimode propagation shown in Fig. 1(b).
3. Experimental arrangement and results
The experimental system for exciting and detecting SHG is shown in Fig. 3. The pump light from a 1550 nm picosecond laser is launched into the SCF through an SMF, and a strong light-matter interaction occurs between the pump evanescent field and filled GaSe nanosheets at the surface of the suspended core. The inset of Fig. 3 demonstrates the SCF with embedded GaSe nanosheets and the operation process of SHG. To maximize collection efficiency, an MMF is arranged to collect the excited SHG signal after the SCF. Subsequently, a simple filtering system mainly consist of a dichroic mirror and a reflector mirror is used to filter out the pump light. After filtering the interference signal, the pure SHG signal is incident into a spectrometer.
Fig. 3. Experimental system for exciting and detecting SHG of the fabricated SCF device. DM, dichroic mirror.
To examine the SHG signal, we selected a 1550 nm picosecond laser with a pulse width of 8.8ps and repetition rate of 18.5 MHz as the pump source to independently implement SHG signal at 775 nm. Figure 4(a) shows the SHG spectra from the GaSe nanosheets filled SCF devices at two different lengths of 560 µm and 950 µm, respectively, under a 9-mW pump power. The 560 µm device exhibit a stronger SHG signal, with the conversion efficiency calibrated as 3×10−5%/W [5,34]. To further evaluate the dependence of the SHG intensity on the pump power, we adjusted the output power of laser from 0 to 9 mW at an interval of 0.25 mW, and the spectral evolution of SHG is shown in Fig. 4(b). Log-log plotted Fig. 4(c) clearly reveals the quadratic relationship between SHG intensity and the input pump power, and the fitting slope is 1.933 ± 0.007, which agrees well with the theoretical prediction value 2 [30].
Fig. 4. (a) SHG spectra at 1550 nm wavelength with GaSe-filled SCF devices with different length of 560 µm and 950 µm. (b) Spectral evolution of SHG while the power of the 1550 nm laser is modulated from 0 to 9 mW. (c) Log-log plotted power dependence of SHG intensity, with a fitting slope of 1.933 ± 0.007.
Benefiting from the high index contrast of the micron-scale core and large air cladding, multiple modes are supported in the SCF hybrid waveguide, which relaxes the phase-matching requirement in the SHG processes over a broadband spectrum [15,19]. Additionally, the 560 µm long SCF device has a relatively low loss in a wide spectrum range shown in Fig. 2(f), which enables the broadband response of the SHG excitation. To verify this, a widely tunable optical parameter oscillator (OPO) was used to be a pump light. The OPO laser was tuned to seven individual wavelengths, including 1420 nm, 1460 nm, 1500 nm, 1550 nm, 1600 nm, 1650 nm and 1700 nm. Figure 5 illustrates the broadband response of SHG signals in the wavelength modulation process. The red dashed line in Fig. 5 indicates the variation of the pump power by switching the OPO laser among the seven wavelengths, leading to a significant difference of the SHG intensity at different wavelengths. The wavelength-dependent SHG signal therefore indicates the broadband operation feasibility of the device, covering the whole S/C/L bands.
Fig. 5. Broadband response of the SHG with the pump wavelength range of 1420∼1700 nm. Red dashed line depicts the fluctuation of the OPO laser power with wavelength conversions.
4. Conclusions
In summary, we have demonstrated the implementation of SHG in a parameter-optimized SCF device with embedded GaSe nanosheets. With phase-matching condition satisfied, we obtained efficient SHG due to the strong interaction between the light field of suspended core and the embedded GaSe nanosheets. Further investigation on the power dependence of SHG validates the quadratic relationship between the pump power and SHG intensity, and reveals the milliwatt-level excitation threshold of SHG. Owing to the high refractive index contrast between core of SCF and the air, multiple modes are allowed to exist in the hybrid SCF and satisfy the phase-matching condition approximately. Therefore, the relaxed phase-matching condition allows the SHG to be observed in a wide wavelength range over S/C/L telecom bands. The proposed GaSe-embedded SCF device for the frequency conversion has potential applications in new light source generation, frequency-doubled fiber laser devices, fiber communication and sensing.
Funding
National Natural Science Foundation of China (61975166, 11634010); Key Research and Development Program (2017YFA0303800).
Acknowledgments
The authors thank the Analytical & Testing Center of NPU for their assistance with the material and device characterizations.
Disclosures
The authors declare no conflicts of interest.
Data availability
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.
References
1. D. Z. Anderson, V. Mizrahi, and J. E. Sipe, “Model for second-harmonic generation in glass optical fibers based on asymmetric photoelectron emission from defect sites,” Opt. Lett. 16(11), 796–798 (1991). [CrossRef]
2. A. Canagasabey, C. Corbari, A. V. Gladyshev, F. Liegeois, S. Guillemet, Y. Hernandez, M. V. Yashkov, A. Kosolapov, E. M. Dianov, M. Ibsen, and P. G. Kazansky, “High-average-power second-harmonic generation from periodically poled silica fibers,” Opt. Lett. 34(16), 2483–2485 (2009). [CrossRef]
3. F. De Lucia, D. W. Keefer, C. Corbari, and P. J. A. Sazio, “Thermal poling of silica optical fibers using liquid electrodes,” Opt. Lett. 42(1), 69–72 (2017). [CrossRef]
4. J. Chen, J. Tan, G. Wu, X. Zhang, F. Xu, and Y. Lu, “Tunable and enhanced light emission in hybrid WS2-optical-fiber-nanowire structures,” Light: Sci. Appl. 8(1), 8 (2019). [CrossRef]
5. B. Jiang, Z. Hao, Y. Ji, Y. Hou, R. Yi, D. Mao, X. Gan, and J. Zhao, “High-efficiency second-order nonlinear processes in an optical microfibre assisted by few-layer GaSe,” Light: Sci. Appl. 9(1), 63 (2020). [CrossRef]
6. Z. Hao, B. Jiang, Y. Hou, C. Li, R. Yi, Y. Ji, J. Li, A. Li, X. Gan, and J. Zhao, “Continuous-wave pumped frequency upconversions in an InSe-integrated microfiber,” Opt. Lett. 46(4), 733–736 (2021). [CrossRef]
7. J. M. Ménard and P. St. J. Russell, “Phase-matched electric-field-induced second-harmonic generation in Xe-filled hollow-core photonic crystal fiber,” Opt. Lett. 40(15), 3679–3682 (2015). [CrossRef]
8. J. M. Ménard, F. Köttig, and P. St. J. Russell, “Broadband electric-field-induced LP01 and LP02 second harmonic generation in Xe-filled hollow-core PCF,” Opt. Lett. 41(16), 3795–3798 (2016). [CrossRef]
9. J. Yuan, X. Sang, Q. Wu, G. Zhou, F. Li, C. Yu, K. Wang, B. Yan, Y. Han, H. Y. Tam, and P. K. A. Wai, “Generation of second-harmonics near ultraviolet wavelengths from femtosecond pump pulses,” IEEE Photonics Technol. Lett. 28(16), 1719–1722 (2016). [CrossRef]
10. Z. Hao, Y. Ma, B. Jiang, Y. Hou, A. Li, R. Yi, X. Gan, and J. Zhao, “Second harmonic generation in a hollow-core fiber filled with GaSe nanosheets,” Sci. China Inf. Sci. 65(6), 162403 (2022). [CrossRef]
11. Y. Zuo, W. Yu, C. Liu, X. Cheng, R. Qiao, J. Liang, X. Zhou, J. Wang, M. Wu, Y. Zhao, P. Gao, S. Wu, Z. Sun, K. Liu, X. Bai, and Z. Liu, “Optical fibres with embedded two-dimensional materials for ultrahigh nonlinearity,” Nat. Nanotech. 15(12), 987–991 (2020). [CrossRef]
12. Y. Li, Y. Rao, K. F. Mak, Y. You, S. Wang, C. R. Dean, and T. F. Heinz, “Probing Symmetry Properties of Few-Layer MoS2 and h-BN by Optical Second-Harmonic Generation,” Nano Lett. 13(7), 3329–3333 (2013). [CrossRef]
13. X. Liu, Q. Guo, and J. J. A. M. Qiu, “Emerging low-dimensional materials for nonlinear optics and ultrafast photonics,” Adv. Mater. 29(14), 1605886 (2017). [CrossRef]
14. H. Liu, Y. Li, Y. S. You, S. Ghimire, T. F. Heinz, and D. A. J. N. P. Reis, “High-harmonic generation from an atomically thin semiconductor,” Nat. Phys. 13(3), 262–265 (2017). [CrossRef]
15. T. Cheng, X. Zhou, Y. Sun, X. Yan, X. Zhang, F. Wang, S. Li, T. Suzuki, and Y. Ohishi, “Supercontinuum-induced multi-wavelength third-harmonic generation in a suspended-core microstructured optical fiber,” Opt. Express 28(20), 28750–28761 (2020). [CrossRef]
16. A. Hartung, A. M. Heidt, and H. Bartelt, “Design of all-normal dispersion microstructured optical fibers for pulse-preserving supercontinuum generation,” Opt. Express 19(8), 7742–7749 (2011). [CrossRef]
17. J. K. Ranka, R. S. Windeler, and A. J. Stentz, “Visible continuum generation in air–silica microstructure optical fibers with anomalous dispersion at 800 nm,” Opt. Lett. 25(1), 25–27 (2000). [CrossRef]
18. L. Dong, B. K. Thomas, and L. Fu, “Highly nonlinear silica suspended core fibers,” Opt. Express 16(21), 16423–16430 (2008). [CrossRef]
19. I. Shavrin, S. Novotny, and H. Ludvigsen, “Mode excitation and supercontinuum generation in a few-mode suspended-core fiber,” Opt. Express 21(26), 32141–32150 (2013). [CrossRef]
20. U. Møller, Y. Yu, I. Kubat, C. R. Petersen, X. Gai, L. Brilland, D. Méchin, C. Caillaud, J. Troles, B. Luther-Davies, and O. Bang, “Multi-milliwatt mid-infrared supercontinuum generation in a suspended core chalcogenide fiber,” Opt. Express 23(3), 3282–3291 (2015). [CrossRef]
21. T. M. Monro, S. Warren-Smith, E. P. Schartner, A. François, S. Heng, H. Ebendorff-Heidepriem, and S. Afshar, “Sensing with suspended-core optical fibers,” Opt. Fiber Technol. 16(6), 343–356 (2010). [CrossRef]
22. Z. Shao, Q. Rong, F. Chen, and X. Qiao, “High-spatial-resolution ultrasonic sensor using a micro suspended-core fiber,” Opt. Express 26(8), 10820–10832 (2018). [CrossRef]
23. H. Su, Y. Zhang, K. Ma, Y. Zhao, and J. Wang, “High-temperature sensor based on suspended-core microstructured optical fiber,” Opt. Express 27(15), 20156–20164 (2019). [CrossRef]
24. X. Zhang, X. S. Zhu, and Y. W. Shi, “Fiber optic surface plasmon resonance sensor based on a silver-coated large-core suspended-core fiber,” Opt. Lett. 44(18), 4550–4553 (2019). [CrossRef]
25. Y. R. Shen, The Principles of Nonlinear Optics (Wiley-Interscience, 1984).
26. S. Liu, P. P. Vabishchevich, A. Vaskin, J. L. Reno, G. A. Keeler, M. B. Sinclair, I. Staude, and I. Brener, “An all-dielectric metasurface as a broadband optical frequency mixer,” Nat. Commun. 9(1), 2507 (2018). [CrossRef]
27. C. Yao, Z. Jia, Z. Li, S. Jia, Z. Zhao, L. Zhang, Y. Feng, G. Qin, Y. Ohishi, and W. Qin, “High-power mid-infrared supercontinuum laser source using fluorotellurite fiber,” Optica 5(10), 1264–1270 (2018). [CrossRef]
28. V. G. Voevodin, O. V. Voevodina, S. A. Bereznaya, Z. V. Korotchenko, A. N. Morozov, S. Y. Sarkisov, N. C. Fernelius, and J. T. Goldstein, “Large single crystals of gallium selenide: growing, doping by In and characterization,” Opt. Mater. 26(4), 495–499 (2004). [CrossRef]
29. E. Bringuier, A. Bourdon, N. Piccioli, and A. Chevy, “Optical second-harmonic generation in lossy media: Application to GaSe and InSe,” Phys. Rev. B 49(24), 16971–16982 (1994). [CrossRef]
30. X. Zhou, J. Cheng, Y. Zhou, T. Cao, H. Hong, Z. Liao, S. Wu, H. Peng, K. Liu, and D. Yu, “Strong second-harmonic generation in atomic layered GaSe,” J. Am. Chem. Soc. 137(25), 7994–7997 (2015). [CrossRef]
31. S. Lei, L. Ge, Z. Liu, S. Najmaei, G. Shi, G. You, J. Lou, R. Vajtai, and P. M. Ajayan, “Synthesis and photoresponse of large GaSe atomic layers,” Nano Lett. 13(6), 2777–2781 (2013). [CrossRef]
32. X. Li, M. W. Lin, A. A. Puretzky, J. C. Idrobo, C. Ma, M. Chi, M. Yoon, C. M. Rouleau, I. I. Kravchenko, D. B. Geohegan, and K. Xiao, “Controlled vapor phase growth of single crystalline, two-dimensional GaSe crystals with high photoresponse,” Sci. Rep. 4(1), 5497 (2015). [CrossRef]
33. Y. Zhou, Y. Nie, Y. Liu, K. Yan, J. Hong, C. Jin, Y. Zhou, J. Yin, Z. Liu, and H. Peng, “Epitaxy and photoresponse of two-dimensional GaSe crystals on flexible transparent mica sheets,” ACS Nano 8(2), 1485–1490 (2014). [CrossRef]
34. S. Mariani, A. Andronico, A. Lemaître, I. Favero, S. Ducci, and G. Leo, “Second-harmonic generation in AlGaAs microdisks in the telecom range,” Opt. Lett. 39(10), 3062–3065 (2014). [CrossRef]
