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Abstract
Nonlinear optical properties of nanocomposites exhibit wide nonlinear optical applications, but the nonlinear coefficient is not sufficiently high that it limits the potential application. Thus, we improve the nonlinear optical response of Nd:YAG by dual implantation with Zn and Ag ions to induce the formation of Ag nanoparticles (NPs) in the surface of a crystal. And the structural and optical properties of the NPs embedded in the Nd:YAG have been investigated. The surface plasmon resonance (SPR) absorption of Ag NPs in Nd:YAG crystal shows ~3 times enhancement with a narrower spatial distribution of NPs. Meanwhile, third-order nonlinear coefficients of the crystal embedded with NPs increase to 8 orders of magnitude larger than that of pure Nd:YAG due to the SPR effect. Furthermore, annealing can further modify SPR and nonlinear optical properties owing to improved crystalline structure. The present work provides an alternative route for enhancing the nonlinear optical properties and potential application in optical devices.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In the recent decades, nanocomposites embedded metal nanoparticles (NPs) attracted tremendous attention and showed broad application prospects due to their unique optical linearity and nonlinearity properties. It has been demonstrated that metal NPs embedded in silica can fabricate molecule detectors and waveguides owing to the surface plasmon resonance (SPR) effect [1,2], etc. Nonlinear optical properties of noble metallic nanocomposites can be promising for the development of nonlinear optical applications, including optical modulator, optical switcher and optical limiter [3–5]. For instance, saturation absorption characteristics of metal NPs (such as Ag, Au, Cu) can realize Q-switched pulsed laser [6–8]. Graphene oxide by hybridizing Ag NPs serves as saturable absorber with higher modulation depth [9]. Thus, it is crucial to fabricate nanocomposites with excellent linear and nonlinear optical properties.
Compared with other noble metal NPs, the plasmonic enhancement response of Ag NPs are superior [10]. Up to now, monometallic NPs and alloy or binary compounds embedded in SiO2 have been widely studied [11–13]. Nevertheless, few researches reported on optical crystals [14–16]. Currently, Ag NPs embedded in KTiOPO4 (KTP) and Bi4Ge3O12 (BGO) crystals as well as Ag NPs and Ag-Cu alloy in LiNbO3 crystal have been investigated [16–19]. Moreover, the BGO and LiNbO3 crystals embedded with Ag and Au NPs exhibit excellent nonlinear absorption response [18,20,21]. Neodymium-doped yttrium aluminum garnet (Nd:YAG), as one of the most widely used laser crystal, with excellent optical and thermal properties, such as wonderful fluorescence response, high gain and high damage threshold [22–24], is selected as the matrix for the formation of NPs. It was also found that the Nd:YAG crystal embedded with the small-sized spherical Ag NPs shows giant enhancement of nonlinear optical response [25]. Meanwhile, Au NPs embedded in Nd:YAG could enhance optical nonlinearity for efficient pulsed lasing operation [26].
Ion implantation, as one of the important material modification method, has been proved to be the most promising technology owing to its advantages of less selectivity to materials and good controllability [27,28]. Moreover, pre-implantation can affect the surface structure, spatial distribution and optical property of NPs during the following ion implantation, which is an effective means to improve the optical response [29]. In present, numerous studies have been performed on the dual implantation [30,31]. Particularly, our earlier research illustrated that the structural and optical properties were effectively tailored by prior Zn, Cu and Ti ions implantation into silica [32–34].
In this work, we fabricated the Ag NPs/ Nd:YAG nanocomposites by ion implantation and significantly improved the nonlinear optical response of the Nd:YAG crystal. Low-energy Zn and Ag ions were dually implanted in Nd:YAG crystal together with subsequent annealing. The structural and optical properties of Ag NPs/ Nd:YAG nanocomposites were studied in detail. Linear and nonlinear optical properties can be significantly enhanced, which is crucial for photonic devices.
2. Experimental
In this study, YAG crystals of 2 mm in thickness doped with 1 atom % Nd3+ ions were used as specimens. Such specimens were singly or sequentially implanted with 70 keV Zn and 120 keV Ag ions at the same fluence of 5.0 × 1016 cm−2. The projected ranges of 70 keV Zn and 120 keV Ag ions were calculated to be about 33.2 and 37.9 nm, respectively, according to SRIM 2013 simulation code [35]. Thus, four kinds of samples were provided, which were named as the virgin Nd:YAG, Zn, Ag and Zn + Ag, respectively. During implantation, the flux density was controlled at ~4 μA/cm2 to avoid heating effect and the target plate was set to rotate in a constant speed to insure uniformity of implantation. After implantation, all as-implanted samples annealed isochronally for 1 h from 100 °C up to 700 °C under a flowing N2 atmosphere.
Optical absorption (OA) responses in a wavelength range from 200 to 800 nm were measured with a double-beam spectrophotometer (UV-3600PC). During measurements, a beam of light pass the virgin Nd:YAG crystal while another beam pass the as-implanted sample to subtract the absorption of the original crystal. Cross-sectional transmission electron microscope (XTEM) was performed with a Tecnai G2 F20 S-Twin microscope operating at an acceleration voltage of 200 kV to characterize the spatial and size distribution and the structure of the formed NPs. Moreover, Rutherford backscattering spectrometry (RBS) and RBS/channeling (RBS/C) with a 2.0 MeV He+ beam and a 160° scattering angle were used to evaluate depth profile of the implanted elements and ion beam induced damage in crystal structure. Finally, Z-scan technology were carried out by using passive-active mode-locked Nd:YAG laser (EKSPLA, PL2210A) with a repetition rate of 1 kHz and pulse width of 20 ps to analyze third-order nonlinear optical (NLO) properties of these samples. The excitation wavelength and beam-waist radius were 532 nm and 22.5 μm, respectively.
3. Results and discussion
3.1 Linear optical absorption properties of Ag NPs/ Nd:YAG
The OA spectra of the Zn, Ag and Zn + Ag samples before and after annealing at different temperatures were exhibited in Fig. 1. The absorption peak at 287 nm can be observed for the Zn implanted sample in Fig. 1(a), which can be related to formation of Zn NPs in the crystal. Subsequent annealing at high temperature causing the OA peak intensity to rise first and then decrease. The decreased absorption is most probably related to the decomposition of Zn NPs and diffusion of Zn atoms towards the substrate surface after annealing at 500 °C. When the annealing temperature reaches 700 °C, the absorption peak disappears, indicating that numerous Zn atoms escape towards the substrate surface. In Fig. 1(b), the absorption peak around 505 nm is observed in the Ag sample, and it can be assigned to the SPR of Ag NPs [25]. And the absorption peak continuously shifts towards the shorter wavelength and the peak intensity constantly enhances with the increasing temperature from 300 °C to 700 °C. According to the Ostwald mechanism [36], it can be attributed to the formation of larger-size NPs after annealing. For the Zn + Ag sample (see Fig. 1(c)), the absorption peak is significantly enhanced accompanied with a blueshift of about 37 nm compared with the pure Ag sample. After annealing at 300 °C, the OA peak intensity increases slightly indicating the thermal growth of NPs. Annealing temperature up to 500 °C causes the decrease of absorption peak with a redshift of 14 nm. It is ascribed to the decomposition of Zn-related NPs and diffusion of Zn atoms towards the substrate. Importantly, the SPR absorption peak reaches maximum after 700 °C annealing, showing ~3 times enhancement than the pure Ag sample without thermal annealing.
Fig. 1 OA spectra of the (a) Zn, (b) Ag and (c) Zn + Ag samples before and after annealing at different temperatures.
3.2 Structure and spatial distribution of Ag NPs/ Nd:YAG
In order to identify formation of NPs as well as their structure and spatial distribution, XTEM measurements have been selectively conducted for the Ag and Zn + Ag samples without annealing (Fig. 2). As shown in Fig. 2(a), numerous spherical NPs are mainly distributed into a region of about 65 nm from the sample surface after Ag ion implantation. The sizes of NPs change from ~1 to ~6 nm with an average diameter of 2.18 ± 0.95 nm (Fig. 2(b)). The selected area electron diffraction (SAED) pattern inserted in Fig. 2(a) and high-resolution TEM (HRTEM) of NPs together with the fast Fourier transform (FFT) pattern (Fig. 2(c)) confirm the formation of Ag NPs with the face-centered cubic crystal structure (FCC).
Fig. 2 XTEM results of the ((a)-(c)) Ag and ((d)-(f)) Zn + Ag samples. (a), (d) are overall morphologies, and the insets represent the corresponding SAED patterns. (b), (e) are particle size distributions. (c), (f) are HRTEM micrographs and the corresponding FFT patterns of a region as marked with the white rectangle.
For the Zn + Ag sample, a large number of spherical NPs are mainly located in the area of about 35 nm in thickness, and 10 nm thick depleted layer is formed at the top of the sample where only few NPs can be found (see Fig. 2(d)). And the particle size distribution with a mean diameter of 3.15 ± 1.18 nm is shown in Fig. 2(e). It is not found any diffraction signal associated with NPs in the SAED image inserted in Fig. 2(d), which could be related to the prior Zn ion implantation producing numerous defects in the substrate. Figure 2(f) shows typical HRTEM micrograph and the FFT pattern, and the NPs with a lattice spacing of 0.236 nm match well with the Ag (111) plane whereas lattice spacing of 0.253 nm is considered to be the formation of Ag-Zn co-solvent. Therefore, the SPR absorption peak of the Zn + Ag sample is significantly enhanced and presents a blueshift comparing with that of the Ag sample, as shown in Figs. 1(b) and 1(c). These could be mainly ascribed to the formation of larger size Ag NPs and Ag-Zn compounds in the Zn + Ag sample. Compared with the Ag sample, the formed NPs show a narrower spatial distribution, larger mean size and better crystalline, which reveals that the pre-implanted ions could remarkably affect nucleation and growth of particles obtained by subsequent implantation. Due to the low electronegativity of Zn (~1.65) [37], some Zn atoms may react with the substrate during the implantation process. As a result, Zn clusters and Zn-related compounds and defects could be created, and they can provide nucleation sites for Ag NPs and promote the diffusion of Ag atoms. These nucleation centers are expected to further narrow down the spatial distribution of Ag NPs. In the meantime, the ion beam heating and the forward recoil effect caused by the posterior Ag implantation not only can significantly enhance the diffusion capacity of the Ag atoms and accumulation of Zn atoms, but also can induce Zn atoms to move towards deeper area from near the surface. The aggregation and migration of Zn atoms may promote the deposition of Ag atoms and form a depletion layer via carrying Ag atoms to precipitate into deeper region. The potential mechanisms may be similar to previous works [32].
As for the Zn + Ag sample after annealing at 700 °C, XTEM results (Fig. 3) reveal the effect of heat treatment on the growth of NPs and the repair of crystal structure. One can see from Fig. 3(a) that the spherical NPs are mainly located in the depth area from 6 to 60 nm below the surface. Among them, majority of NPs with large sizes are distributed in the shallower area while some small sizes are located in the deeper area, which can be ascribed to the accumulation and growth of NPs induced by heat treatment. As shown in Fig. 3(b), these NPs sizes range from 1 to 12 nm with a mean diameter of 5.25 ± 2.23 nm. Meanwhile, the SAED pattern (see Fig. 3(c)) only presents the diffraction signals of Ag NPs and HRTEM images (Fig. 3(d)) of typical NPs indicate that the crystalline structure of these NPs is excellent. The increase of average diameter of NPs and the disappearance of the Ag-Zn co-solvent could account for the enhancement of the SPR peak and the redshift after heat treatment (Fig. 1(c)).
Fig. 3 XTEM micrographs of the Zn + Ag sample after annealing at 700 °C. (a) Overall morphology, (b) particle size distributions, (c) SAED pattern, (d) typical HRTEM micrographs of NPs and (e) HRTEM micrograph of the rectangular areas labeled as A together with the corresponding FFT of amorphous areas and SAED pattern of virgin Nd:YAG crystal.
Figure 3(e) displays the HRTEM of amorphous and crystalline interfaces together with the corresponding SAED pattern of Nd:YAG crystal and the FFT result of amorphous area. As presented in Fig. 2(a), it is clear that the Ag sample is divided into two regions, i.e., an amorphous layer about 135 nm in thickness, and a pure crystal part. The damaged area is much larger than the projected range of the Ag ions (37.9 ± 12.7 nm), which may be related to the cascade collision to cause secondary shift of atoms in the sample, but the underlying mechanism remains to be solved in further research. And the thickness of the damage layer is consistent with that of the Ag sample after the Zn pre-implantation (showed in Fig. 2(d)). For the Zn + Ag annealed sample, the thickness reduces by 10 nm (see Fig. 3(a)). Above of all, thermal annealing is crucial for the nucleation and growth of particles as well as the repair of lattice damage [19,38].
The RBS and RBS/C spectra of virgin Nd:YAG crystal as well as the RBS/C spectra of Ag and Zn + Ag samples before and after annealing are shown in Fig. 4. It can be seen that Zn ions pre-implantation causes the enhancement of intensity and the shift of position towards low channel of the Ag profile. This phenomenon reveals that the pre-implanted Zn ions can increase the precipitation of Ag implants and promote the Ag atoms deposited near the surface into deep region together with the generation of a depletion layer near the sample surface. Moreover, the measured Ag profile of the Zn + Ag as-annealed sample slightly decrease and diffuse towards the sample surface. Furthermore, the backscattering yield of yttrium signal in the as-implanted and as-annealed samples increase obviously, which indicates a damage layer formation caused by ion implantation [14]. The Zn + Ag sample exhibits a wider channel spectrum than that of Ag from 390 to 440 channel. In our opinion, it can be partially attributed to the dual ion implantation induced more serious damages in crystal and partially ascribed to the superposition of the Zn elements backscattering channels (at 395-415). Besides, the thickness of damage layer decreases for the Zn + Ag sample after annealing at 700 °C. It demonstrates that the recrystallization occurs after thermal treatment. As the annealing temperature rises, a certain amount of rearrangement may occur near the interface between crystalline and amorphous areas, making the region become a vacancy raising layer. These vacancies could offer space for atomic aggregation, and then accumulation of atoms provide the sites for recrystallization which further promote single crystal epitaxial growth. Above phenomenons are in consistent with the XTEM results.
Fig. 4 RBS spectra in random configuration of the virgin Nd:YAG as well as the RBS/C spectra in aligned configuration of virgin Nd:YAG, Ag and Zn + Ag samples before and after annealing at 700 °C.
3.3 Nonlinear optical properties of Ag NPs/ Nd:YAG
The third-order nonlinear absorption and refraction of virgin Nd:YAG crystal, as-implanted and as-annealed samples are measured by Z-scan method, and the results are displayed at Fig. 5. All normalized transmittance curves are theoretically fitted by [39]:
Among them, T is the normalized transmission, ΔΦ0 is the phase difference of the sample at the focus, z is the sample position down the lens axis and x = z/z0, where z0 is the scattering length of a Gaussian beam.Fig. 5 Normalized transmittance curves of virgin Nd:YAG, Zn, Ag and Zn + Ag samples before and after annealing at 700 °C: (a, c) open-aperture; (b, d) closed-aperture. Scattered points and solid lines represent the experimental values and the theoretical fitted curves, respectively.
As shown in Fig. 5(a), no any nonlinear absorption response of pure Nd:YAG while the as-implanted samples exhibit a symmetrical peak on the laser focal point (z = 0), revealing that these samples possess saturated absorption (SA) behavior corresponding to negative nonlinear absorption indices. Meanwhile, the closed-aperture normalized transmittance curve of the virgin Nd:YAG appears a valley-peak configuration reflecting self-focusing effect of crystal whereas the other curves exhibit a peak-valley shape indicating self-defocusing effect of as-implanted samples, i.e., negative nonlinear refraction values (see Fig. 5(b)). The nonlinear absorption response and transformation of the self-focusing effect are mainly attributed to the generation of NPs. Moreover, the open-aperture and closed-aperture results of the Zn + Ag sample are the strongest in all as-implanted samples. Besides, the results of as-annealed samples (showed in Figs. 5(c) and 5(d)) exhibit noticeable enhancement in SA performance and self-defocusing effect. This could be closely related to the fact that the SPR (as observed in the OA spectra in Figs. 1(b) and 1(c)) can greatly enhance the NLO properties of samples.
Nonlinear absorption (β) and refractive (n2) indices can be calculated according to the following equation [40]:
where I0 is the laser beam intensity at the focus, Leff = (1-e–αL)/α, and L are the effective and practical thickness of the NPs layer, respectively. α is the linear absorption coefficient. ΔTpv represents the peak-to-valley difference of the normalized transmittance, ΔΦ0 = kn2I0Leff, S = 0.25, k = 2π/λ is wave vector.Based on the indices of β and n2, the imaginary and real parts of the third-order nonlinear susceptibility (χ(3)) can be obtained [39]:
Where c is the velocity of light and n0 is the linear index of refraction of the substrate. The nonlinear optical parameters for the as-implanted and as-annealed samples are presented in Table. 1.
Table 1. Nonlinear optical parameters of the samples that before and after annealing at 700 °C
It is found from Table 1 that the absolute values of β, n2 and χ(3) of the Zn + Ag as-implanted sample are up to ~4 times than those of the single Ag as-implanted sample. Moreover, the |χ(3)| values of those as-annealed samples display a significant increase compared to the as-implanted state. Simultaneously, the Zn + Ag sample after annealing at 700 °C shows the largest third-order nonlinear susceptibility. In our opinion, the enhanced nonlinear values could benefit from large-size NPs, repaired dielectric environment and strong SPR absorption [41,42], as shown in Fig. 3 and Fig. 1. Notably, the β value of the Zn + Ag sample after annealing at 700 °C is up to ~1.1 × 10−4 cm/W, which is approximately 8 orders of magnitude larger than pure crystal with a value of ~5.2 × 10−12 cm/W. Moreover, it is about 100 times as much as Ag NPs within Nd:YAG (~2 × 10−6 cm/W) reported by Li et al. [25]. For the nonlinear refractive index n2, NPs-embedded crystal is enhanced by nearly 6 orders of magnitude compared with the pure crystal. Therefore, dual ion implantation is an effective way to enhance the NLO properties. The increase of third-order NLO properties could be attributed to the SPR and the quantum size effect of NPs, as reported in previous works [42]. Furthermore, the SA effect of NPs can acts as the optical switcher in pulse modulation. For example, the Nd:YAG embedded with Au NPs, the β value enhanced nearly 5 orders of magnitude, serves as a saturable absorber in a Pr:LuLiF4 crystal laser cavity for realizing efficient pulse laser [26]. Hence, we will study the applications of NLO properties in next work.
4. Conclusion
The structural and optical properties of Ag NPs/ Nd:YAG nanocomposites have been researched. Our results demonstrated that prior Zn ions implantation could provide nucleation sites for post Ag implants so that spherical Ag NPs with narrower spatial distribution formed. As a result, SPR absorption of the NPs shows ~3 times enhancement accompanied with a blueshift of about 37 nm owing to the formation of the Ag-Zn co-solvent in the Zn + Ag sample. Based on the Z-scan system, it has been displayed that the β and n2 coefficients of the Zn + Ag sample present ~4 times increase due to the greatly enhanced SPR response compared with the Ag singly implanted sample. Moreover, Ag NPs/Nd:YAG nanocomposites possess large β and n2 values, which are enhanced by nearly 8 and 6 orders of magnitude with reference to virgin Nd:YAG crystal, respectively. Furthermore, the SPR response and third-order NLO properties of the fabricated nanocomposites are strongly improved after annealing at 700 °C, which can be ascribed to the generation of large size NPs with improved crystalline structure. This work could provide an available method for enhancing the optical properties of Nd:YAG crystal by embedded Ag NPs and further expand the optical nonlinear applications.
Funding
National Natural Science Foundation of China (NSFC) (11535008, 11675120).
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