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Abstract
With the development of laser technology, nonlinear optics plays a crucial role in frequency conversion. However, the generation of second harmonics in nonlinear optical crystals is generally subject to rigorous phase-matching conditions that hinder the performance of broadband tunability. It is believed that introducing disorders in nonlinear optical materials is helpful to overcome this obstacle. In this work, we have prepared a nonlinear microcrystal-doped glass (NMG) composite material, allowing for tunable and polarization-independent nonlinear conversion from visible to near-infrared. The linear dependence of SHG intensity versus sample thickness indicated the facilitation of random quasi-phase matching by using the NMG. Our results provide a more stable and promising platform for disordered nonlinear photonic materials and suggest the possibility of more efficient nonlinear conversions using the NMG composite glass fibers in future.
© 2024 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
In the field of nonlinear optics (NLO), second-harmonic generation (SHG) is a basic method for producing new optical frequencies [1,2]. In contrast to rare earth (RE) ions that emit at certain discrete energies, NLO materials have the ability to emit at any wavelength with high color purity (monochromatic light) [3]. Moreover, SHG is an instantaneous nonlinear optical parametric process that prevents photobleaching or other kinds of photodamage [3,4]. However, their use is bounded by optical dispersion, which imposes rigorous phase-matching conditions to ensure optimal nonlinear conversion [5,6]. Although many phase-matching techniques have been developed [7,8], it is challenging to achieve broadband nonlinear conversion using these methods. [9]. Compared to the efficient phase-matching of ordered materials, disordered materials are considered helpful for relaxing phase-matching conditions at a cost of conversion efficiency [10]. For disordered materials, the SHG intensity is impervious to factors such as pump wavelength, polarization, and incident angle, providing an alternative platform to bulk crystals [11]. In the case of disordered nonlinear (χ2) crystalline domain structures, the random nonlinear light interference does not disrupt the global signal [12], and the overall SHG intensity increases linearly with the thickness of the disordered material, i.e. a phenomenon known as random quasi-phase-matching (RQPM) [11]. So far, the RQPM has been implemented in disordered polycrystals [11,13,14], stacked powders [15], and bottom-up assembled disordered photonic media [12,16].
In recent years, glass ceramics or glasses rather than single crystals have attracted great interest for SHG [17–19]. However, the high symmetry limits their applications for even-order nonlinear processes, which can be improved by introducing crystal particles of non-centrosymmetric structures into the glass matrix. Various SHG signals have been observed in in-situ crystallization glass ceramics, such as LaBGeO5 [20], LiNbO3 [17,21], Ba2TiSi2O8 [4] and other systems [22]. However, their nanoscale crystal particles in in-situ crystallized glass ceramics are normally smaller than the coherence length Lc (micrometer scale for NLO materials), which weakens the conversion efficiency. Compared to the in-situ crystallization, the ex-situ approach, i.e. synthesizing crystal and glass respectively and then integrating them into a uniform composite, shows great flexibility in developing new ‘glass ceramic’ materials [23]. The ex-situ approach permits the embedded crystals of specific sizes and volume fractions, especially the crystals that are hard to synthesize by in-situ crystallization. In this work, we have successfully prepared transparent nonlinear microcrystal-doped glass (NMG) composite material through the ex-situ approach. As illustrated in Fig. 1(a), the nonlinear crystals are distributed in the glass matrix in a disordered manner, forming an isotropic photonic material.
Fig. 1. (a) Schematic of NMG material, orange dots are micron-sized nonlinear crystals. (b) The diagram of LiTaO3 ferroelectric phase structure.
2. Experiment section
LiTaO3 (LTO) powders were prepared by grinding LTO single crystal using an agate mortar. LTO powders were graded by standard sieves to obtain the particles with the desired size range (30-50 µm), and then transferred to a vacuum drying oven to facilitate the subsequent doping process.
Precursor glass with a composition of TeO2-Bi2O3-BaO was fabricated using the conventional melt-quenching technique. The raw materials were thoroughly mixed and melted in an alumina crucible under an ambient atmosphere at 850 °C for 20 min. The melt was quenched by pouring onto a steel plate to obtain a dense precursor glass. Putting the crucible lid during the melting process is not proposed, because the low valent Bi ions produced in an oxygen-deficient environment may cause undesirable absorption in the visible range [24]. The precursor glass was then melted at 750 °C for 10 min, slowly reduced to 640 °C at a rate of 5 °C/min, and kept for 5 min. Next, dispersed LTO powders were added to the crucible and stirred continuously with a stirring quartz rod for 3 min at 640 °C. Finally, the melt was annealed in a preheated graphite mold, then slower cooling to room temperature at a rate of 0.1 K/min. The NMG materials were synthesized at 640 °C with doping concentrations ranging from 1% to 4 wt%.
To determine the precursor glass transition temperature, the differential scanning calorimetry (DSC) system (STA449 CNETZSCH) was used to record the differential scanning calorimetry curve of the glass. The crystalline phases of the samples were identified by an X-ray diffractometer (XRD, PANAlytical X’pert PRO) with Cu Kα radiation (λ = 0.15418 nm). The refractive indices of the samples were measured by a prism coupling apparatus (Metricon Model 2010). The transmission spectra of the precursor glass and NMG were measured by Perkin Lambda 900/UV/Vis/NIR spectrophotometer, with a range of 350-2000 nm. The bonding structure of the sample was obtained by Raman spectroscopy (Renishaw inVia) using a 532 nm laser as the excitation source. The microcrystals distribution and thermal-induced corrosion were inspected by using a scanning electron microscope (SEM). A femtosecond Ti: sapphire laser with a repetition rate of 80 MHz and a pulse of 100 fs serves as the pumping source for exciting SHG, and the SHG signal is captured by a CCD camera/spectrometer.
3. Result and discussion
3.1 Design principle of NMG materials
For NMG prepared from the ex-situ crystallization method, the fundamental physical/chemical characteristics between precursor glass and crystal should be matched. Priority should be given to the following factors [23,25]: (1) The refractive index of precursor glass should be close to the embedded crystals to avoid composite material opacity; (2) the density of the glass and crystals should also be matched to avoid the microcrystal particles sinking to the bottom of the molten glass, resulting in poor homogeneity of the composite; (3) the melting temperature must be low enough to prevent the thermal-induced corrosion of embedded crystals; (4) the precursor glass melt viscosity should be as low as possible in the doping temperature range to disperse the crystal particles. Based on these requirements, tellurite glass is an appropriate precursor glass for its high refractive index (>2), and low melting temperature (<600 °C). Excellent thermal stability will be given top priority when choosing doping crystals. Lithium tantalate (LiTaO3) has received considerable attention due to its robust thermal/chemical stability, high laser damage threshold, and significant nonlinear optical coefficient [26], making it one of the most important lead-free ferroelectric materials in the perovskite family (ABO3) [27,28]. It is commonly recognized that LTO will maintain a ferroelectric phase below the Curie temperature(Tc, ∼610 °C). Li+ and Ta5+ will respectively deviate from the oxygen plane and the center of the oxygen octahedron along the c-axis, forming spontaneous polarization and a non-centrosymmetric structure (Depicted in Fig. 1(b)). Higher Curie temperature allows the sample to generate SHG signals even when exposed to a high-temperature environment. Therefore, we chose to prepare NMG material by embedding LTO, whose refractive index is 2.176 at 633 nm, into a tellurite glass matrix.
A range of tellurite glass compositional families, including the most well-known ternary compositions TeO2-ZnO-Na2O, have been used to prepare crystal-in-glass composite materials through the ex-situ crystallization method [29–32]. Unfortunately, none of them are appropriate for doping with perovskite-type crystals, for the refractive index of the tellurite glass is only 1.9 [30], lower than that in LTO. Adding heavy metal oxides (such as Bi2O3\ WO3\ PbO\ BaO) to the glass is a feasible option for increasing its refractive index while increasing the glass density [33,34]. For this purpose, we prepared a series of tellurite glasses with heavy metal oxides (TBBs), as shown in Table 1. The refractive index of the glass increases from 2.044 to 2.130 while the density increases from 5.874 to 6.145 g/cm3. The refractive index of TBB-2 is found to be the closest to that of the crystal LTO ($\Delta n$<0.05). Meanwhile, the parameter $\Delta T$ implies that TBB-2 is less susceptible to crystallization ($\Delta T$=Tx-Tg > 100 °C, where Tx is the onset of crystallization, and Tg is the glass transition) [35], which is essential for the NMG since adverse crystallization causes severe scattering and lowers the transmittance of the glass. Therefore, we chose TBB-2 as the precursor glass. The DSC data of TBBs are presented in Fig. S1 in Supplement 1.
Table 1. Composition (mol%) and properties of TBB glasses
3.2 Doping strategy and microstructure of NMG materials
As previously reported, the number of embedded Ce: YAG phosphor particles decreases with increasing sintering temperatures through the low-temperature co-sintering method (one of the ex-situ methods) [36]. However, ferroelectrics are less thermally stable than phosphors. To prevent thermal-induced corrosion, we chose to prepare NMG using the direct doping method rather than the common co-sintering method. Dwell temperature in the direct doping process is also essential for the corrosion and dispersion of LTO microcrystals in molten glass. Firstly, the raw materials were heated at a higher temperature for a short time to obtain precursor glass and prevent residual bubbles and component volatilization. The resulting homogenous precursor glass was then remelted at a low temperature and allowed to cool until it reached the perfect temperature for microcrystals direct doping. As shown in Fig. 2(a), the NMG transmittance increases with the doping temperature from 620 to 720 °C. This could be related to a drop in scattering loss due to the decrease in the total number of LTO crystals, which is consistent with Fig. S2(a-d) in Supplement 1. The number and size of embedded microcrystals drastically decrease at temperatures over 680 °C for 3 min, whereas this phenomenon is not observed at 620-660 °C. The low transmittance at 620 °C is caused by microcrystals aggregation in certain areas. Therefore, a balance must be struck between aggregation and degradation of crystals. Furthermore, we noticed that considerable thermal-induced corrosion may occur when the dwell time reaches 10 min at 640 °C (see Fig. S2(e) in Supplement 1). Considering the possibility of thermal-induced corrosion and microcrystals aggregation, we believe that 640 °C/3min is an ideal doping condition, allowing for microcrystals preservation and homogeneous dispersion. We prepared a series of NMG samples at 640 °C with doping concentrations varying from 1% to 4 wt%. As shown in Fig. 2(b), even with large microcrystal sizes, all NMG samples exhibit good transparency owing to the refractive index matching. The transmittance for the 1 wt% doped sample can almost reach 70%. To understand the influence of crystals on the refractive index of NMG, we have provided the refractive indices of pure glass and NMG in Table S1 and Fig. S3 in Supplement 1. It is worth mentioning that higher transmittance glasses can be obtained with shorter dwell times using direct doping compared to the common low-temperature co-sintering method (Section S4 in Supplement 1).
Fig. 2. Basic characterization of NMG materials. (a) Comparison of UV/VIS/NIR transmittance from NMG prepared at different doping temperatures. (b) Transmittance of NMG doping 1 wt% LTO at 640 °C for 3 min and precursor glass. The inset shows photographs of NMG doped with 1,2,3,4 wt% LTO crystals, respectively. (c) XRD pattern of 4 wt% NMG material prepared at 640 °C for 3 min. (d) SEM images of 2 wt% NMG prepared at different temperatures (640 to 720 °C for 3 min). (e) EDS mapping of an individual LTO particle embedded in the glass matrix. (f) Comparison of Raman spectra of TBB-2 precursor glass, LTO crystals, and NMG material prepared at 640 °C for 3 min.
To further understand the microstructure of the NMG material, we performed a qualitative analysis. The XRD pattern in Fig. 2(c) indicates that NMG contains several sharp peaks aligned with the LTO sample and several intrinsic amorphous glass peaks, demonstrating the successful integration of LTO. Scanning electron microscopy (SEM) observations on the NMG samples containing 2 wt% LTO microcrystals were carried out to investigate the microstructure variation with increasing melting temperatures. According to Fig. 2(d), the available quantity of LTO particles decreases as the melting temperature rises, which is consistent with previously discussed transmittance spectra. As seen in the EDS mapping (Fig. 2(e)), the glass matrix and LTO particles have distinct boundaries, demonstrating less elemental diffusion. Furthermore, it can be seen that 30-50 µm sized microcrystals are homogeneously distributed in the glass matrix in Fig. S2(f) in Supplement 1, also indicating that no significant thermal corrosion has occurred. K. TANAKA et al found that only the paraelectric phase rather than the ferroelectric phase of BaTiO3 (Tc${\approx} $ 120 °C) was observed in glass ceramic at room temperature since the paraelectric-to-ferroelectric phase transition around Tc of BaTiO3 would be dimensionally hampered by the glass matrix below its Tg [37–39]. While, it is designed in our system that the glass transition temperature here is below the Tc of LTO. That is, the low-Tg glass here would not affect the phase of LTO crystals embedded in the glass matrix, which is of great advantage for second harmonic response [22]. To better understand the structure of NMG material, we investigated the Raman vibration modes of NMG material. The Raman spectra of NMG material presented in Fig. 2(f) can be considered a mixture mode from pure TBB-2 glass and LTO crystals. The Raman peaks at 446 and 648 cm-1 are related to the stretching and antisymmetric vibrations of Te-O bonds in [TeO4], [TeO3], and [TeO3+δ], which occur in the presence of network modifiers. The Raman peak at 742 cm-1 is related to the stretching vibrations between tellurium and non-bridging oxygen (NBO) atoms [40,41]. For typical LTO crystals, when the temperature increases, the intensity of the transverse vibrations of the A1 modes (206, 252, 316, and 597 cm-1) will be weakened or even disappear (above Tc) [42]. As seen by the clear A1 vibration modes in the NMG material, we can confirm that the embedded LTO crystals were ferroelectric phase with the second harmonic response.
3.3 SHG response characteristic
The schematic diagram of SHG signal measurement is shown in Fig. 3. The femtosecond laser beam (916 nm wavelength, 80 MHz) is collimated and reduced by two lenses and then injected into the NMG vertically. The transmitted signals were collected using another lens and delivered to a spectrometer/CCD for spectral analysis. Notably, the diameter of the beam is shrunk to about 1 mm, which is much larger than the size of the particles, ensuring sufficient particles are excited simultaneously. The samples used in the measurements were NMG doped with 4 wt% LTO.
Fig. 3. In the schematic diagram of SHG signal measurement, a half-wave plate is used to rotate the beam polarization, and a linear polarizer allows only a specific polarization angle to pass through.
In the experiment, significant blue bright spots were observed in the pump region of NMG, which was taken as an indication of SHG. The collected spectrum is displayed in Fig. 4(a), confirming that the output pulses are second harmonic. Moreover, power-dependent measurements were performed. Figure 4(b) revealed a quadratic dependence on the incident power of fundamental waves, which is in accordance with a second-order optical process. In the measurement, the maximum average power of the Ti: Sapphire laser going into the composite material is 400 mW, while the maximum average power of the SHG signal is 20 µW. It is worth noting that the SHG signal exhibits significant scattering, while the numerical aperture (NA) of the lens is limited, and only a portion of the SHG signal has been collected into the power meter. We expect that the actual SHG power would be higher. Therefore, by comparing the SHG signals of NMG with that of single crystal BBO, the SHG efficiency of NMG has been investigated. Under the same experimental conditions, the SHG intensity of the reference single-crystal BBO sample is eight times higher than that of the NMG doped with 4 wt% LTO (See Section S5 in Supplement 1 for a detailed derivation), indicating that NMG has a second-order nonlinear susceptibility χ2 close to BBO. In Table 2, the nonlinear coefficients and SHG features of different materials are presented. One should mention that NMG allows for generating SHG signals over a very broad bandwidth. Theoretically, any high-purity visible or even near-infrared light SHG emission can be achieved, as the variation of LTO crystal size is larger than twice the coherent length Lc of the wavelengths mentioned above (Section S6 in Supplement 1). Combining the characteristics of high nonlinearity efficiency and broadband response, NMG provides a feasible solution for broadband nonlinear conversion.
Fig. 4. Characterization of the SHG from the second-order nonlinear microcrystal-doped glass. (a) The emission spectrum compared to the pump spectrum. The inset shows the luminescence of the sample after pumping. (b) The dependence of the SHG on the pump power in NMG Material, R2 = 0.998. (c) SHG emission intensity under different incident polarizations, with a 10-degree difference in each polarization (Rotate the half-wave plate and remove the linear polarizer). (d) The intensity of the emission SHG signal at different polarization angles was recorded while maintaining the horizontal polarization of the pump light incident on the NMG material (Fix the first half-wave plate and rotate the linear polarizer). The emission signal is similar to elliptical polarization.
Table 2. The comparison of nonlinear coefficients and features of NMG composite with that of other approaches
To further understand the nonlinear light-matter interaction in NMG, we measured the polarization characteristics of SHG signals. By rotating the half-wave plate in front of the sample, the polarization of the pump beam was changed, and the corresponding emission SHG signals were collected, as shown in Fig. 4(c). There is no significant change in the emission SHG intensity under different pump beam polarizations, which is consistent with results of thin films of randomly oriented nanocrystals [46]. Generally, phase matching conditions are critical for second-order nonlinear optical processes, for example, SHG. Various strategies, including the quasi-phase matching strategy, were applied to compensate for momentum discrepancies between fundamental and second-harmonic wavelengths. In single-domain nonlinear material, SHG is sensitive to polarization of the pump beam because phase-matched SHG is obtained by birefringence. It was noted that such dispersed crystals in the NMG sample were spatially homogenous and arbitrarily orientated, generating perfectly random domains. In the case of random multi-domain nonlinear materials, the corresponding reciprocal vector G becomes quasi-continuous in momentum space, leading to a SHG pattern independent of pump polarization. Meanwhile, it also demonstrates the isotropy of the NMG sample. Furthermore, fixed the pump beam polarization and measured the polarization of the SHG signal by rotating the polarizer, as shown in Fig. 4(d). For the horizontally polarized pump beam, the SHG was found to be similar to elliptical polarization and the main component was also horizontal (A detailed explanation of the phenomenon is given in S7 in Supplement 1). This can also be explained by the quasi-continuous distribution of the reciprocal vector G in momentum space. Due to the spatially homogenous and arbitrary orientation of dispersed crystals in the NMG, the polarized emission under linear polarization excitation is the cumulative result of SHG generated by each grain, as opposed to the linearly polarized emission of phase-matched SHG in single-domain nonlinear material.
3.4 RQPM in NMG materials
Similar to multiple scattering, our disordered photonic NMG materials consist of random assemblies of nonlinear optical crystals, where each grain generates second harmonic, and the overall signals are the consequence of the interference of the light produced by each grain. The clearest visualization of interference in multiple scattered light is often demonstrated by the phenomenon of light speckles [47]. As shown in Fig. 5(a), interference-enhanced areas are brightly spotted in NMG composite material, forming nonlinear speckles. These nonlinear speckles are caused by a combination of SHG generated by a large number of nonlinear particles (see Section S8 in Supplement 1 for an assessment of the number of grains). To validate the occurrence of RQPM in NMG, we gathered the SHG intensities and transmittances of NMG samples with varying thicknesses, and obtained the SHG intensity scaling with the sample thickness after correcting the scattering effects using Eq.S14 in Supplement 1. Thanks to the advantages offered by the direct doping method in the fabrication of the NMG, the distribution of crystal phases is uniformly achieved, which helps reduce testing errors. As illustrated in Fig. 5, we noticed that SHG intensity exhibits a noticeable increase with increasing sample thickness. We have determined the optimal fitting parameter α to be 1.02 through a power function fitting. This observation aligns with prior reports on the disordered assembly of materials [12,16]. Therefore, the established linearity substantiates the existence of RQPM, a phenomenon hitherto unreported in glass composite materials. Although RQPM in NMG materials is less efficient than single crystal materials, as an alternative method to remove destructive interference in optical frequency conversion, it provides numerous advantages, including extremely loose frequency selectivity, low cost, ease of processing, and loose constraints on incident polarization. Owing to the good thermal stability of TBB-2 glass, another advantage of the NMG material is that it can be prepared as a core into glass fiber, the fiber waveguide structure can significantly enhance the SHG signal conversion efficiency and the beam quality. Meanwhile, the fiber length is easily adjustable, facilitating the implementation of RQPM along with more efficient SHG.
Fig. 5. SHG power versus sample thickness. Each measurement point is the average of many different areas, and the error bars are given by the standard deviation of these measurement values. The inset shows the emission spot image captured by a CCD camera from NMG composite material and SHG speckle can be observed from the image.
4. Conclusion
In summary, through the direct doping method, we have fabricated isotropic transparent nonlinear microcrystal-doped glass with well-preserved and uniformly distributed LTO micro-crystals. This approach shows great convenience in the fabrication and application of NMG materials. By adjusting the pump wavelength, dispersed micrometer LTO crystals in NMG can achieve high purity and intensity emission from visible to near-infrared wavelengths. In addition, we have demonstrated the existence of random quasi-phase matching in multiple scattering second-order nonlinear glass composite material, where the emission SHG intensity is linearly related to the sample thickness. Compared to powder assembly, crystals embedded within a glass matrix can be well protected from the external environment, which provides excellent operational stability. We are currently in the process of introducing NMG materials into glass fiber, as the superior optical waveguide structure will produce stronger nonlinear light.
Funding
National Natural Science Foundation of China (12174116, 52002131); The 2021 Talent Revitalization Plan Project for New High Performance Material Industry in Qingyuan City (2021YFJH02001); Fundamental Research Funds for the Central Universities (2023ZYGXZR002, xzy012023167).
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.
Supplemental document
See Supplement 1 for supporting content.
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