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Abstract
With the development of miniaturization of high-energy laser systems, a new Faraday rotator material must be studied to realize the miniaturization and integration of optical isolators. In this paper, high-quality (TbBi)3Fe5O12 (TbBiIG) and Ca-doped (TbBi)3Fe5O12 (Ca: TbBiIG) single crystal films with hundreds of microns thickness were grown by liquid phase epitaxy method on (111) oriented garnet substrate. The crystal structure, magneto-optical (MO), optical and laser induced damage properties were investigated in detail. We found that the (TbBi)3Fe5O12 film has outstanding magneto-optical and laser-induced damage properties. Optical and MO properties indicate that TbBiIG films have a high specific faraday rotation angle of 1452 deg/cm at 1310 nm, and 2812 deg/cm at 1064 nm, absorption coefficient (α) is 5.63 cm-1 and 15.7 cm-1 at 1310 nm and 1064 nm, respectively. The laser-induced damage threshold (LIDT) of TbBiIG irradiated by a multi-frequency laser is 8.91 J/cm2. The light absorption has a significant impact on LIDT value. Rare-earth ion doped iron garnet (RIG) material is a very potential MO material, which can greatly reduce the size and weight of optical isolators in the 1064 nm band.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
As an essential component in optical communication systems or high-power laser systems, the optical isolators (OIs) are usually applied behind the light source or amplifier to make the light pass in one direction by blocking the reflected light [1–3]. With the development of laser medical treatment, information transmission, laser processing, space detection, integrated photonics and other fields [4–7], the power carrying capacity of OIs has also become an important parameter in addition to isolation and insertion loss. In that case, extensive use of high-power system requires OIs to withstand higher power lasers, which require high laser-induced damage thresholds (LIDT) MO material [8,9]. The MO material is the essential component of the OIs to change the polarization plane of the incident light and garnet single crystal materials are the most important kind of MO material used in the near-infrared (NIR) band, 1064 nm and 1260 nm ∼ 1570 nm are two commonly used wavelength bands in NIR [10,11]. Rare-earth ion doped iron garnet (RIG) film is widely used in 1260 nm ∼ 1570 nm band because of its high Verdet constant, low absorption coefficient (α), and high specific Faraday rotation angle (θF) [12,13]. Terbium gallium garnet (TGG) crystals and terbium aluminum garnet (TAG) crystals are the traditional MO materials used for 1064 nm with high LIDT [14,15]. However, the specific Faraday rotation angle of TAG and TGG is small, it is difficult to apply in compact high-power laser systems because of theirs long optical path and large external magnetic field. Therefore, the purpose of this study is to explore the application feasibility of RIG materials in compact systems in the 1064 nm band and its anti-laser damage mechanism.
Unlike the application of RIG film in optical communication of 1260 nm ∼ 1570 nm, high LIDT and high thermal stability are more critical to MO materials used at 1064 nm wavelength. Comparing with TGG and TAG, RIG has a higher absorption coefficient at 1064 nm, resulting in a low laser-induced damage threshold. This is due to the electronic energy level transitions of Fe2+, Fe4+ and Pb2+ in the RIG lattice resulting in an absorption peak around 900 nm, and the tailing effect of the absorption peak increases the light absorption at 1064 nm [16–19]. On the other hand, the θF of RIG is much higher than that of TGG and TAG, the optical path length is reduced from a few centimeters to several hundred microns, and the driving magnetic field is reduced from several thousand to several hundred oersteds, which can greatly reduce the size and weight of the OIs. Therefore, it will be a very potential magneto-optical isolator material as long as we reduce the loss of RIG film in the 1064 nm band. It is very important to figure out the effects of ion doping and transition mechanism in RIG on LIDT.
In this paper, a specific formulation of RIG material, (TbBi)3Fe5O12 (TbBiIG) thick films [20], were prepared by liquid phase epitaxy (LPE) method [21,22], and the thicknesses were able to rotate the laser polarization plane by 45 degrees at 1064 nm. To study the effect of absorption coefficient on LIDT, the Ca2+ was doped into the crystal lattice. The effect on the ion transition mechanism and the transmittance in the NIR band has been investigated. The micro structure, optical, magneto-optical and LIDT properties of the thick film material are studied in detail.
2. Experiment
2.1 Synthesis of RIG thick films
TbBiIG and Ca-doped TbBiIG (Ca: TbBiIG) thick films were grown on (111) oriented monocrystalline SGGG substrates by LPE method. The detailed LPE experimental procedure is as follows: Highly purified Tb4O7, Bi2O3, Fe2O3, and CaO or just Tb4O7, Bi2O3, Fe2O3 powders were mixed in the PbO-B2O3-Bi2O3 flux system as solute. All oxides were mixed and heated at 1050 °C in a platinum crucible until everything melted. The melt was stirred with a platinum stirrer and maintained at 1050 °C for 12 hours to homogenize. Fig. 1(a) shows that the SGGG substrate was immersed in the melt and rotated to grow films. The growing process would continue at a rate of 0.7 µm/min for 15 hours to ensure sufficient thickness of RIG film. The grown RIG film/SGGG wafer is shown in Fig. 1(b).
Fig. 1. TbBiIG film preparation schematic (a) LPE method to grow TbBiIG film (b) the 3-inch grown TbBiIG film/SGGG wafer (c) the grinding process of TbBiIG film/SGGG wafer (d) The TbBiIG film with coarse surface after grinding (e) the polishing process of TbBiIG (f) the 5×5 mm2 small TbBiIG pieces
2.2 Processing of RIG film/SGGG wafer
To obtain a RIG film that can accurately rotate the light polarization plane by 45 degrees, The RIG film/SGGG wafer was ground on a rotating copper plate with a uniform distribution of diamond suspensions. A heavy block was placed on the wafer to apply pressure. The copper plate was rotated at 60 rpm to remove the substrate. Figure 1(c) shows the grinding process of the wafer. After grinding, the RIG film with proper thickness and rough surface, as shown in Fig. 1(d). The film needs flattening by chemical mechanical polishing (CMP) on a rotating CMP pad that sprayed with silica suspension for 30 minutes. Figure 1(e) shows the polishing process of the RIG film. Finally, the RIG film was sliced into 5×5 mm2 for subsequent studies, as shown in Fig. 1(f).
2.3 Laser damage testing
The schematic experimental setup of the LIDT measuring system is shown in Fig. 2. Laser source was a multi-frequency Nd: YAG laser with the wavelength at 1064 nm, using a pulse duration of 45 ns and pulse repetition frequency of 1 K ∼ 100KHz, the power of the laser can be adjusted by a computer, provide a far-field circular Gaussian beam with a diameter of 25 µm at 1/e2 of the maximum intensity, as shown in the inset of Fig. 2. The laser beam was focused on the sample surface through focusing lens. The sample was placed on a platform which can adjust the beam’s position in three directions. A spectroscope and a power meter were used to monitor beam power and power data was displayed on the computer. Because the transmittance of the sample under the visible light is low, we observed the reflected light of the sample under white light to determine whether the sample was damaged by using a CCD camera. The damage threshold was assessed using R-on-1 testing modes; meanwhile, the R-on-1 testing mode was carried out at eight different film spots. The damage was estimated from the ablative marks on the sample surface.
2.4 Characterization of TbBiIG thick film
The crystal structure was measured using High-resolution X-ray diffraction (D8 Advance, Bruker) and a transmission electron microscope (TEM). The transmission spectra of the films were measured using an UV-VIS spectrometer (Lambda 750, PerkinElmer). Faraday rotation angle was measured by the experimental equipment set up by orthogonal extinction method. The composition of the films was tested by ICP-MS (7800, Agilent). The surface morphology after laser damage testing was observed by an optical microscope (STM6-LM, OLYMPUS) and scanning electron microscopy (Sigma 300, Zeiss).
3. Results and discussion
3.1 Structural analysis
The High-resolution X-ray diffraction (HRXRD) patterns are shown in Fig. 3(a). We use HRXRD scanning from 49.5° ∼ 51.5° with a step size of 0.005 degrees to measure the lattice matching and crystallinity of the films. The (111) oriented TbBiIG film and (111) oriented Ca-doped TbBiIG film used for XRD pattern have been ground to remove the entire substrate. It is shown that there is only one diffraction peak in XRD patterns of (111) SGGG substrate at 50.6°, the diffraction peak of RIG films overlaps with the diffraction peak of the (111) SGGG substrate, while the width of RIG films’ diffraction peak is wider than SGGG because the lattice constant of the RIG film has a slightly change in growth process, resulting in a peak broadening in the XRD patterns. The specific figure is shown in Fig. 3(a), the diffraction peak of TbBiIG film is in the range from 50.50° to 50.65°, and the diffraction peak of Ca: TbBiIG film is in the range from 50.55° to 50.75°. The XRD pattern indicates that the fabricated RIG films are single crystals. The highly-quality single crystal is illustrated by TEM images as shown in Fig. 3(b), the cross-section wafer was viewed along [11$\bar{2}$] direction. The selected area electron diffraction (SAED) pattern in Fig. 3(c) shows that the grown TbBiIG film is high-quality monocrystalline. Fig. 3(d) indicated that the TbBiIG film lattice matched well with the SGGG substrate.
Fig. 3. (a) XRD patterns of SGGG substrate, TbBiIG film and Ca: TbBiIG film (b) TEM pattern of TbBiIG film (c) SAED pattern of TbBiIG film (d) Zooming in pattern of the growth interface
3.2 Optical and magneto-optical properties
Figure 4(a) shows the transmission spectra and α of the TbBiIG film and Ca: TbBiIG film. Faraday rotation angle of the samples is 45° at 1310 nm and can be applied in OIs directly. It is obvious that α of Ca: TbBiIG is gradually increasing in the near-infrared band while α of TbBiIG is almost unchanged at the wavelength from 1200 nm to 1600 nm. In general, the transmittance of Ca: TbBiIG is lower than TbBiIG in the near-infrared band, and is most significant around 1100 nm. At the wavelength of 1064 nm, transmittance of the Ca: TbBiIG film is 18.5% and the TbBiIG film is 49.6%. The following equation calculates the absorption coefficient:
where n is the refractive index of the RIG film (n≈2.38), n0 is the refractive index of air, and d is the length of the light path through the crystal. In this measurement, light is normally incident on the crystal surface, so d is the thickness of the film. The calculated α of TbBiIG and Ca: TbBiIG are 15.7 cm-1 and 48.5 cm-1, respectively.Fig. 4. (a) Transmittance and absorption, (b) θF of TbBiIG film and Ca-doped TbBiIG film at 1310 nm and 1064 nm
The difference of α is closely related to the doped Ca2+ ions [16]. The elemental composition within the films was analyzed by ICP-MS to analyze the optical properties, as shown in Table 1. Table 1 shows the range of atomic percentage (at%) of different elements in TbBiIG and Ca: TbBiIG films, Pb and Pt in the melt and crucible entered the lattice in an undesired way, while the doping of Ca element resulted in the film contains more Pt and Pb elements. In the ICP test, Ca is abundant in the environment, resulting in a minor error in the measured value from the equipment itself, but the results of several tests have shown that there is more Ca2+ in Ca: TbBiIG than TbBiIG. According to the average at%, specific molecular formula of TbBiIG and Ca: TbBiIG are calculated, which is Tb2.03Bi0.9Ca0.03Pb0.02Pt0.02Fe5O12 and Tb1.81Bi1.05Ca0.08Pb0.03Pt0.03Fe5O12, respectively. Therefore, we can infer two main reasons for α change. Firstly, the radius of the Ca2+ ion (112 pm) is close to that of Bi3+ (117 pm) in the dodecahedral position, so it replaces part of the Bi3+ in the lattice. To maintain the valence balance, part of Fe3+ in the octahedral site is converted into Fe4+. Secondly, the element Pb exists as a Pb2+ ion in the crystal lattice, and the element Pt exists as a Pt4+ ion, these impurities ions induced the Fe3+ converted to Fe2+ and Fe4+ [23]. In Ca: TbBiIG crystal, Ca2+ ions are known to generate Fe4+ and Fe2+. Considering the triangular field and non-triangular field components caused by the crystal field distortion of the octahedral site, the energy level splitting of Fe2+ and Fe4+ is very complicated, resulting in more absorption peaks for Fe2+ and Fe4+ in the near-infrared band, such as the triangular field of Fe2+ from 5Eg1 transitions to 5Eg (1250 nm) or non-triangular field of Fe3+ transitions from A4 to A3 (1050 nm) [19]. In this kind of film, Ca2+, Pb2+ and Pt4+ also have intrinsic absorption peaks, while the peaks are not in the NIR band, and have no effect on the light absorption.
Table 1. ICP-MS analysis results of TbBiIG and Ca: TbBiIG film
Figure 4(b) shows the Faraday rotation angle of TbBiIG film and Ca: TbBiIG film at the wavelength of 1310 nm. θF of the TbBiIG film and Ca: TbBiIG film is 1452 deg/cm at the saturation magnetic field. Figure 4(b) shows the Faraday rotation angle of TbBiIG film at the wavelength of 1064 nm, the θF is 2812 deg/cm. The saturation magnetic field of TbBiIG is 654 Oersteds, and at the same field, the specific Faraday rotation angle of TGG is only 1.46 deg/cm. Table 2 shows the θF of the other films prepared in our previous work and TGG [24–27], we found that the θF of TbBiIG is higher than any material other than (NdGdBi)3Fe5O12. The (NdGdBi)3Fe5O12 has an impressive MO property, but it is difficult to grow as its lattice constant is too large (12.560 Å) [28]. The TbBiIG has a much larger θF than TGG and can reduce the length of the optical path from several centimeters to several hundred microns. On the other hand, the TbBiIG can be grown easily to hundreds of microns. By controlling the thickness in the grinding, the TbBiIG film can be used both in 1064 nm and 1310 nm the optical communication band. Such improvements indicated that the TbBiIG is a potential MO material for use both in the 1064 nm band and optical communication band.
Table 2. MO and Optical properties of different RIG film and TGG bulk
3.3 Laser-induced damage tests
The LIDT of TbBiIG film at 1064 nm multi-frequency laser obtained by R-on-1 mode, 5000 pulses per spot irradiated with 50 KHz. By measuring the laser damage data at eight spots of each sample to ensure experimental accuracy. The samples discussed below have been processed by CMP if not otherwise specified. In comparison with TbBiIG samples, there is a small decrease in LIDT for Ca-doped samples. The data indicate that the higher α and more intensive crystal defects caused by more impurities (Pt and Pb) are the reason for lower LIDT due to both samples have the same thickness. This means that α and laser damage characteristics are positively correlated in RIG crystal.
To further investigate the laser damage properties of TbBiIG material, we measured the TbBiIG, Ca: TbBiIG at 1064nm in 2500, 5000, 7500 and 10000 pulses. Due to the θF of TbBiIG at 1064nm being much larger than the θF at 1310 nm, the film only needs 160 µm to rotate the polarization plane of the laser by 45 degrees. In the grinding process, 160 µm TbBiIG film and 150 µm SGGG substrate are retained to make its thickness equal to other samples. As shown in Fig. 5 the LIDT of the material polished decreases and then remains constant as the number of pulses rises. The data indicated that at low pulse counts, the heat accumulation from many pulses can cause surface damage to the material while as the number of pulses rises and the LIDT decreases. The peak laser power has a decisive role in the damage thermal accumulation does not determine whether damage occurs to the material but only the size of the shape of the damaged spot. The size of the laser effect zone is shown in Fig. 6(a) ∼ (c). It is shown in Fig. 5 that the LIDT of TbBiIG@1064 nm under the irradiation of 5000 pulses is 11.62 J/cm2, laser irradiation from the TbBiIG film surface. The difference in LIDT demonstrates that the thickness of the absorber in composite TbBiIG film has a significant effect on LIDT. In TbBiIG@1064 nm film, because SGGG barely absorbs the laser light, the 150 µm thick SGGG acts as a good buffer for the heat accumulation of the RIG material. The specific values of polished and unpolished films are shown in Table 3, indicated the key role of polishing on the increase of LIDT.
Fig. 5. LIDT of TbBiIG, Ca: TbBiIG and TbBiIG@1064nm under irradiation of 2500, 5000, 7500, 10000 pulses
Fig. 6. The optical microscope pictures of TbBiIG film irradiate by laser under different pulses: (a) 2500, (b) 5000, (c) 10000. The SEM pictures of TbBiIG film irradiate by laser under different pulses: (d) 2500, (e) 5000, (f) 10000
Table 3. LIDT of different TbBiIG film irradiated by laser with different pulses
3.4 Laser-induced damage morphology of TbBiIG films
The morphology of the laser-induced damage sites is photographed from the direction parallel to the direction of laser transmission by an optical microscope. The surface laser-induced damage morphologies are shown in Fig. 6. Figure 6(a) ∼ (c) shows the morphology of the polished TbBiIG films irradiated by 2500, 5000 and 10000 pulses, respectively. By comparing Fig. 6(a), (b) and (c), it can be obtained that the radius of the damage site of the material increases as the number of pulses rises. The laser-induced damage morphology of TbBiIG@1064 nm and the unpolished film is essentially the same as the damage pattern in Fig. 6(a) ∼ (c) and Fig. 6(d) ∼ (f), there is no particular appearance of damage, but different in size. It can be seen from Fig. 7(a) and (c) that the depth of laser irradiation center is higher and generates a small protrusion. It is because when a high-power laser irradiates the material, the material produces the plasma and it protects the material from further irradiation by subsequent laser [29].
Fig. 7. TbBiIG surface damage photographed by SEM (a) morphology of Ca: TbBiIG film irradiated by 8.36 J/cm2 laser (b) the specific shape of annular crack (c) the specific shape of the cleavage fracture (d) morphology of TbBiIG film irradiated by 9.54 J/cm2 laser (e) the explosion boundary (f) morphology of melting crystal
The damage spots of Ca: TbBiIG film irradiation at the fluence of 8.36 J/cm2 under 5000 pulses is shown in Fig. 7(a) photographed by SEM. Laser irradiation produced several large cracks through the damaged area, the cracks are 70 µm in length and across the irradiation point. The cracks indicated that the irradiation emergence a powerful shockwave caused fragmentation of material in unirradiated areas. By zooming in on Fig. 7(a), the central area of the damage has annular cracks that do not match the surrounding pattern as shown in Fig. 7(b). The diameter of the annular crack zone is around 20 µm, and the area diminishes with decreasing pulse numbers. The annular cracks are caused by the thermal stress extrusion of the single crystal. If the material absorbs higher laser energy in a short period of time, the damage will be more severe. The phenomenon that arises is increasing in this area. In Fig. 7(a), Branch-like cracks around the area can be observed, an enlarged image of it is shown in Fig. 7(c). By observing the enlarged image, it is inferred that the material occurs cleavage under heat accumulation. The branch-like cleavage fracture extends from the irradiation center to the surrounding area.
The damage spots of TbBiIG film irradiation at the fluence of 9.54 J/cm2 under 2500 pulses are shown in Fig. 7(d) photographed by SEM. As shown in Fig. 6 (d) ∼ (f), Due to the smaller number of pulses, the damage range is smaller. The annular crack zone which we mentioned above is almost unobservable. In this picture, it is easier to observe the melt pattern of the material. At the high-power damage center, the shock wave from the material explosion causes a regular grain arrangement of the single crystal material which is shown in Fig. 7(e). In the picture Fig. 7(e), a clear explosion boundary appears after the irradiation. The more clarity heat fusion morphology of Fig. 7(e) is shown in Fig. 7(f). No grains of complete shape appear in Fig. 7(f), illustrating that the crystal at the periphery of the laser irradiation center has heat damage only.
4. Conclusions
In conclusion, TbBiIG and Ca: TbBiIG single crystal film with hundreds of microns thickness was grown by the LPE method on (111) oriented SGGG substrates. Processing (TbBi)3Fe5O12/SGGG wafers into usable Faraday rotators by grinding and polishing process. XRD and TEM results indicate that the films are high-quality single crystals with lattice constants varying within a small range. Optical and MO properties indicate that TbBiIG films have a high θF and low driving magnetic field. The LIDT of TbBiIG irradiated by a multi-frequency laser is 8.91 J/cm2, smaller than TGG, but in the same order of magnitude. The study can provide a reference for the research on LID and MO properties of RIG single crystal films at 1064 nm. RIG material is a very potential MO material, which can greatly reduce the size and weight of OIs in the 1064 nm band.
Funding
International Cooperation Project (SQ2018YFE020560); National Natural Science Foundation of China (11972313, 51472046, 52072062).
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.
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