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Abstract
We report on the synthesis and features of copper (Cu) nanoparticles (NPs) embedded into LiNbO3 crystal. A linear optical absorption that peaked at 613 nm is observed, which correlates to the localized surface plasmon resonance (LSPR) effect. In addition, the Cu NPs embedded LiNbO3 (CuNP:LN) possesses ultrafast saturable absorption properties at a wavelength of 1µm. Based on these enhanced nonlinear optical properties, CuNP:LN is applied as a saturable absorber (SA) for pulsed laser generation in a waveguide laser system. Under an optical pump, the 8.6 GHz fundamentally Q-switched mode-locked laser operation has been efficiently implemented based on a Nd:YAG cladding waveguide fabricated by femtosecond laser writing. The measured pulse duration is as short as 55 ps and the slope efficiency is up to ∼22.7%. This work suggests the promising application of LiNbO3 crystal embedded Cu NPs for ultrashort pulse generation.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Optical waveguides confine light fields and propagation within small channels of various configurations, serving as the fundamental elements for integrated photonic systems. Waveguide lasers, which are based on the guiding cavity of gain media, operate in both continuous wave and pulsed regimes. In comparison to traditional bulk counterparts, waveguide lasers possess a number of advantages, such as reduced lasing thresholds, enhanced efficiency, and capability for on-chip integration with other optical devices [1–3]. The first step to realize waveguide lasers is to construct waveguide cavity of gain media. The commonly used techniques to fabricate waveguides are epitaxial layer deposition, ion/proton exchange, chemical vapor deposition (CVD), ion-beam implantation/irradiation, pulsed laser deposition (PLD), femtosecond (fs) laser writing, etc. Especially, fs-laser writing has emerged as one of the most powerful techniques for direct 3D microfabrication of various structures [4–8]. On the basis of waveguide platforms, short-pulse lasers could be obtained by Q-switching or mode-locking technology, which uses optical modulator to modulate the quality value in the laser cavity so as to output pulsed laser with a short pulse and a high output peak [9,10]. Compared with other types of modulators in pulsed lasers, SAs allow the generation of Q-switched and mode-locked pulses with controllable pulse parameters without the need for complex and expensive electrical or Kerr modulators [11–13]. Low-dimensional materials with excellent nonlinear optical responses and wide saturated absorption bandwidth [14–16], i.e. graphene, carbon nanotubes, and MoS2, have deployed to effective SAs to generate ultrafast lasers in a wide spectral region based on Q-switched [17–21] or mode-locked [22–25] waveguide platforms. In particular, mode-locked lasers with multi-GHz repetition rates have attracted increasing interests for researchers [26–38]. For examples, Choi et al. reported the mode-locked laser experiment in a Yb:YAG channel waveguide using carbon nanotube as SA, and obtained output laser with 2-GHz repetition rate [26]. Grivas et al. realized 21.25-GHz mode-locked waveguide laser in Ti:sapphire crystal [33].
Lithium niobite (LiNbO3) crystal is one of the most widely used electro-optic crystals owing to its remarkable electro-optic, nonlinear optical, and photorefractive properties. In particular, the applications of LiNbO3 crystals to be multifunctional substrates for the manufacture of integrated optical devices (e.g., nonlinear optical frequency converters, optical modulators, optical filters, and dielectric waveguides) have arouse great scientific attentions [39,40]. However, as the pure LiNbO3 crystals have lower laser intensity threshold, which easily induces in optical damage, the nonlinear optical properties of LiNbO3 crystals get restrained. Formation of NPs within LiNbO3 crystal is able to resist this limitation of pure LiNbO3 crystals because the obtained nanocomposite possesses enhanced third-order nonlinear optical responses induced by the LSPR effect of metallic ions [41]. Recently, Pang et al. reported that LiNbO3 crystal embedded gold (Au) NPs by ion implantation was employed as a SA to efficiently modulate 6.4 GHz passive Q-switched mode-locked (QML) pulsed laser generation [42], proving the feasibility of modifying LiNbO3 crystals by embedding NPs. One of the most popular methods of embedding NPs in dielectric crystals is ion implantation, which shows unique advantages, such as low material limitations, long-term retention of NPs, and no deterioration of substrate compared to other methods like chemical synthesis [43–45]. Besides, the size and the shape of NPs can be controlled under different ion implantation conditions, such as implantation angle, fluence, energy, etc., producing different optical response characteristics [46–49]. Owing to the excellent optical nonlinear enhancement of metallic NPs, it is of great sense to explore the applications of metallic NPs-based materials to serve as SAs in mode-locked waveguide systems.
Cu NPs have excellent features such as high reactivity, high electrical conductivity and high surface to volume ratio, which enable Cu NPs for significant applications in various fields. For examples, photocatalysts, biomedical sensors, and information technology [50–52]. In particular, Cu NPs embedded in dielectric crystals have promising potential to be broadband SAs in pulsed laser generation due to broad absorption band of Cu NPs. In addition, compared with Au and silver (Ag) NPs, Cu NPs not only have the linear absorption peak closer to near-infrared band, indicating that the LSPR effect of Cu NPs at near-infrared band is more remarkable, but also have significant reduced cost [42,53]. In this work, we have successfully introduced Cu NPs into LiNbO3 wafer via ion implantation. The details of Cu NPs below the surface of LiNbO3 wafer were obtained by transmission electronic microscopy (TEM), and open-aperture Z-scan arrangement was utilized to investigate the ultrafast nonlinear optical responses of CuNP:LN nanocomposite on account of LSPR effect. We applied the as-prepared sample as an optical modulator to perform an efficient QML pulse laser operation and investigated the performances of the obtained pulsed laser.
2. Sample preparation and characterization
The LiNbO3 wafer with a size of 10 × 10 × 1 mm3 was optically polished. Then the wafer was implanted on 180 keV Cu+ ions at the influence of 5 × 1016 ions cm−2 by analytical-type ion implanter Lc22-IC0-01 at room temperature. The formation of Cu NPs was completed after ion implantation.
Figure 1(a) displays the optical image of CuNP:LN. To demonstrate clearly, ion distribution was calculated by the Stopping and Range of Ions in Matter (SRIM) software [42,47]. Figure 1(b) shows the simulated results of Cu+ ion distribution, and Fig. 1(c) demonstrates an enhanced broadband linear optical response of CuNP:LN. As shown, the absorption peak falls at 613 nm, which shows the contribution of the LSPR effect in the surrounding area of Cu NPs to the obvious enhanced linear absorption in visible and near-infrared band. The linear absorption spectrum of CuNP:LN was measured by UV-vis-NIR spectrophotometer (Hitachi, U-4100). Furthermore, with the employment of TEM (FEI Tecnai G2 F20 S-TWIN) measurement, more details about Cu NPs were obtained. More specific experimental procedures about TEM can refer to previous works [42]. The profile of Cu NPs below the surface of the sample is depicted by Cross-sectional Transmission Electronic Microscopy (XTEM) image as shown in Fig. 2(a), we can observe that the main distribution area of Cu NPs is between 53 nm and 150 nm below the surface of the sample, which is consistent with the results of our simulation showed in Fig. 1(b). The insert is Fast Fourier Transformation (FFT) image of the measured range. Figure 2(b) demonstrates clear appearances of Cu NPs by applying High-Resolution Transmission Electron Microscopy (HRTEM) and shows a fine formation of Cu NPs. The embedded image in the upper left corner labels with interplanar spacing d of single Cu NP, which is accordance with the data of Cu from standard Powder Diffraction File (PDF) cards. In addition, the diameter distribution of NPs in Fig. 2(c) indicates that the most frequent NP size is located at 24.4 nm. As revealed in Figs. 2(d) and (e), Selected Area Electron Diffraction (SAED) was carried out and proved superior crystallization of Cu NPs. Figure 2(f) exhibits element mapping of the sample, which can intuitively display the radius distributions of Cu NPs under the surface of LiNbO3 wafer.
Fig. 1. (a) The optical image of CuNP:LN. (b) Cu+ ion distribution simulated by the SRIM. (c) Measured linear absorption spectrum of CuNP:LN.
Fig. 2. (a) Image of XTEM. The insert is FFT image of measured range. (b) Image of HRTEM. The insert demonstrates interplanar space d of single Cu NP. (c) Diameter distributions of Cu NPs. (d)(e) SAED of single Cu NP. (f) Element Mapping of sample.
To get high performance gain medium of QML laser system, we fabricated depressed cladding waveguide by fs-laser writing in optical polished Nd:YAG crystal (doped by 1 at. % Nd3+ions) with dimensions of 10(x) mm × 9 (y) mm × 2(z) mm. An amplified Ti:Sapphire laser system (Universidad de Salamanca, Spain) [21], which generated linearly polarized 800 nm central wavelength with 1 kHz repetition rate and 120 fs pulse duration, was utilized to write the depressed cladding waveguide structures. The Nd:YAG crystal was placed on a three- dimensional PC-controlled translation stage. During micro-process, the precise value of pulse energy was set with a calibrated neutral density filter, a half-wave plate, and a linear polarizer. The laser beam was focused though one of the largest sample surfaces (10 × 9 mm2) by a 40 × microscope objective (N.A. = 0.65). The cladding waveguide structures with circular geometry and desired diameters were fabricated by parallel scan with ∼3 µm separation between neighboring damage tracks at different depths under the sample surface (maximum depth of ∼150 µm). The scan velocity was set to a relatively high constant speed of 500 µm/s. The pulse energy was set to be 2.52 µJ.
3. Nonlinear optical properties of the CuNP:LN system
Considering its application in the laser generation, it is necessary to obtain a further understanding of nonlinear optical responses of CuNP:LN. We carried out an open-aperture Z-scan arrangement, which is a widely used technique to record optical nonlinear responses. Figure 3 shows the schematic diagram of the experiment setup. The sample was placed on a platform, located on the slide rail along the Z-axis; that is, the direction of light propagation. The platform moved with the sample under the domination of the PC-controlled transfer stage to get different light intensities. The excitation laser source was mode-locked laser at 1030 nm with the repetition rate of 100 Hz and pulse duration of 340 fs, and could be considered as a Gaussian beam and possessed beam waist diameter of 30 µm at the focus. The incident pulse energies ranged from 50–200 nJ, tuned by attenuation slice. During the experiment, the transmitted light power and reference beam power of incident light was measured by two photodetectors.
Figure 4 shows the experimental results and corresponding fitting results under excitation pulse energies of 50 nJ, 100 nJ, and 200 nJ, respectively. As illustrated in the Figs. 4(a)–4(c), the curves of normalized transmittances as functions of the z coordinate values are symmetrical about the focus point (i.e., z = 0). As the sample approaches the focal point, normalized transmittance gradually increases, indicating that the absorption of CuNP:LN becomes saturated with the increase in the intensity of the incident pump; that is the typical performance of saturable absorber. In addition, with the increase of excitation light energy, the normalized transmittance of the sample at the focal point was also increased. As far as we know, pure LiNbO3 crystals do not exhibit apparent optical nonlinearities at near-infrared band [40,42], compared with pure LiNbO3 crystal, CuNP:LN system reveals manifest ultrafast saturable absorption at wavelength of 1 µm due to the LSPR effect of Cu NPs, which indicates Cu NPs formed by ion implantation played a significate role in nonlinear optical responses. Therefore, the sample of CuNP:LN is provided with potential applications in ultrafast photonics devices for the obvious saturable absorption properties it showed. There is the model which the experiment result was fitted [54]:
Fig. 4. (a–c) Measured and fitting results of open-aperture Z-scan at different energies. (d) The nonlinear absorber coefficients of CuNP:LN under different excitation pulse energies.
Equation (3) reveals solution of the normalized transmittance as the function of Z coordinate value. As shown in Fig. 4(d), the curve tendency indicates that the nonlinear absorber coefficient gradually increases to saturation as the excitation pulse energy increases, which is in keeping with the performance of saturable absorber effect. It is proved likewise that the LiNbO3 crystal embedded Cu NPs could be utilized as a SA to modulate the pulsed laser operation. To obtain the absorption intensity, we fitted the nonlinear optical responses with the following equation:
Where Is is the absorption intensity [54]. We obtained that the absorption intensity is 17.965 GW/cm2 at 1030 nm wavelength, the non-saturable loss is 24.87% and the modulation depth is 2.45%.4. Q-switched mode-locked laser operation
Based on the excellent nonlinear absorption, the efficient QML laser experiment applying LiNbO3 crystal embedded Cu NPs as a SA was implemented.
The schematic diagram of QML laser operation is shown in Fig. 5. A linear polarized pump laser with 808 nm wavelength generated from a narrowband tunable Ti:sapphire laser (Coherent MBR PE) was employed as pump source light. The polarization of launched laser beam is changed by a half-wave plate. A spherical convex lens with 25 mm focal length was used to constitute end-face coupling system so that pump laser could couple into the cladding waveguide availably. A thin film with antireflection at ∼808 nm and reflectivity as high as >99.9% at ∼1064 nm was coated at the input mirror M1. The waveguide employed as gain medium in this work was behind M1. Our sample was set onto the output end-face of the waveguide. Afterwards, generated mode-locked laser was collected by a 25 × microscope objective lens (N.A. = 0.4). The obtained laser trains propagating in free space were detected by a High-Speed InGaAs Photodetector (New Focus, 1414 model) via coupling into a single mode fiber and investigated by a 25 GHz wide-bandwidth real-time digital oscilloscope (Tektronix, MSO 72504DX) with the rise time of 16 ps after a longpass filter (Thorlabs, FEL0850) which cleaned up the influence of stray light. In addition, an integrating sphere photodiode power sensor (Thorlabs, S142C) was used to record output light power, and a spectrometer (SGM100, 0.05nm resolution) was used to collect spectrum of obtained laser.
Fig. 5. Schematic diagram of QML pulsed laser operation modulated by LiNbO3 crystal embedded Cu NPs in Nd:YAG cladding waveguide.
According to the investigated performances of the obtained laser as shown in Fig. 6, it is proved that stable QML laser operation at 1064 nm by using CuNP:LN as SA was implemented successfully. Figure 6(a) reveals that, the threshold pump power is ∼143 mW, and the maximum average output power is ∼132 mW. In addition, the peak power is ∼340 mW and the pulse energy is 63.5 nJ. The corresponding slope efficiency is ∼22.7%. From the fitting curve we can notice that the polarization of the pump laser has no significant effect on the output power. Figure 6(b) demonstrates the pulsed laser emission spectrum whose central wavelength is 1064 nm. A single Q-switched envelope constituted by mode-locked pulse trains is shown in Fig. 6(c), the insert is QML pulses in the microsecond timescale. For TE-polarized lasering, the repetition rate of the Q-switching envelope of the QML pulses is 2.08 MHz. Figure 6(d) presents the mode-locked trains and Fig. 6(e) shows a single mode-locked trace whose pulse duration is 55 ps. Figure 6(f) depicts the radio frequency (RF) spectrum of the mode-locked pulses with 8.627 GHz central repetition rate and 51 dB signal-to-noise (SNR), as we can see, which indicates the feasibility of high-repetition-rate stable QML waveguide laser generation under the condition of CuNP:LN as SA.
Fig. 6. Performances of QML waveguide laser modulated by CuNP:LN. (a)The average output power as a function of lunched power. (b) Laser spectrum of the output pulsed laser at 1064 nm. (c) Q-switched envelope in the timescale of 100 ns/div, the insert is QML pulses in the microsecond timescale. (d) Mode-locked pulse trains. (e) Single pulse profile. (f) Radio frequency spectrum.
Table 1 demonstrates the comparisons of 1064 nm QML waveguide lasers based on several low-dimensional materials employed as SAs. As we can see, based on 2D materials such as graphene and WSe2 as well as nanocomposites as SAs, multi-GHz repetition and picosecond short pulse duration waveguide lasers were obtained. For the first time, LiNbO3 crystal embedded Cu NPs was used to efficiently modulate QML lasers in monolithic Nd:YAG waveguide platform. The laser performances are comparable with previous works which employed other low-dimensional materials as SAs, exhibiting feasibility of LiNbO3 crystal embedded Cu NPs to be SA.
Table 1. Comparisons of QML Waveguide Lasers at 1064 nm Based on Low-Dimensional Materials
5. Conclusion
In conclusion, the formation and characteristics of LiNbO3 crystal embedded Cu NPs via ion implantation were reported. The stable and efficient QML laser has been obtained reaching ∼8.6 GHz repetition rate and 55 ps pulse duration with CuNP:LN as the SA and Nd:YAG depressed cladding waveguide as the gain medium. This work indicates a promising application of LiNbO3 crystal embedded Cu NPs.
Funding
National Natural Science Foundation of China (11535008, 61875213); Project 111 of China (B13029); STCSM Excellent Academic Leader of Shanghai (17XD1403900).
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