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利用SRIM软件模拟了离子辐照过程中原子核和电子能量的沉积过程,绘制了电子能量损伤Se和原子核能量损伤Sn随辐照深度变化关系,如图3(a)所示。可以看出,在0~17.6 µm深度范围内,Se显著高于Sn,在14 µm深度处,Se达到峰值1.8 keV/nm,然后逐渐降低,直至0。在0~16 μm深度范围内,Sn值为0,然后逐渐增大,在17.5 μm深度处达到最大值0.156 keV/nm,然后逐渐降低,在18 μm深度处Sn值降为0。通过比较Sn和Se的数值大小可以看到,电子能量损伤在引起波导折射率变化过程中占据主导地位。
图 3 (a)电子能量损伤(红线)和原子核能量损伤(蓝线)随C5+离子辐照深度变化关系;(b)重建的4 μm波长下MgF2脊形光波导的折射率随波导深度变化关系图
Figure 3. (a) Relation of electronic energy damage (red line) and nuclear energy damage (blue line) with the irradiation depth of C5+ ions; (b) Reconstructed relation of refraction index of MgF2 ridge waveguide with the waveguide depth at 4 μm wavelength
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由于制备的光波导结构无法采用暗模光谱法测量中红外波长下的折射率分布。为了计算波导中最大折射率变化值Δn,使用公式[18]:
$$ \Delta n=\frac{{{\rm sin}}^{2}{{{{\varTheta}} }}_{{\rm{m}}}}{2n} $$ (1) 式中:n = 1.3488为MgF2材料在波长为4 μm时的折射率;Θm为波导的最大受光角。通过端面耦合系统测量得到Θm = 8.4°,计算出了最大折射率变化Δn= 0.008。根据电子能量损伤随波导深度的变化,结合计算出的波导最大折射率变化,重建了波导折射率与波导深度之间的关系,如图3(b)所示[16]。
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利用显微镜拍摄制备出的脊形光波导的端面图像来确定波导的具体尺寸,波导宽度为14 μm,深度为17.5 μm,如图4(a)所示。通过端面耦合实验,发现MgF2晶体脊形光波导的近场模式呈现单模传输,如图4(b)所示。根据重建的折射率与波导深度之间的关系,使用Rsoft软件基于有限差分光束传播法(FD-BPM)模拟4 μm波长下MgF2脊形光波导近场模式,如图4(c)所示。通过比较图4(b)和图4(c)可以看出,二者图像吻合较好,证明重建的波导折射率分布和Rsoft软件模拟的结果是正确的。
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热退火处理可以消除离子辐照过程中产生的色心和点缺陷,从而进一步降低波导的传输损耗。基于端面耦合系统测量的波导入射功率和出射功率,计算每次退火处理后脊形光波导在4 μm波长下的传输损耗,如图5所示。可以看出,退火处理前波导的传输损耗为4.2 dB/cm,经过五次退火处理后,传输损耗依次降低到3.3 dB/cm、2.1 dB/cm、1.2 dB/cm、0.4 dB/cm和0.4 dB/cm。经第五次退火处理后,脊形光波导的传输损耗不变,表明脊形光波导在300~340 ℃具有良好的热稳定性。实验证明,热退火处理可以有效地优化波导的传输性能,降低传输损耗。
图 4 (a)脊形光波导横截面显微镜图像;(b)脊形光波导4 μm波长下近场模式分布实验图;(c)脊形光波导4 μm波长下近场模式分布模拟图
Figure 4. Sectional microscopic image of optical ridge waveguide; (b) Experimental diagram of near-field mode distribution at 4 μm wavelength; (c) Simulated diagram of near-field mode distribution at 4 μm wavelength of optical ridge waveguide
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图6为离子辐照制备的MgF2晶体波导层和衬底层的拉曼光谱。可以看出,波导层和衬底层的拉曼光谱的峰强、峰宽和峰位基本一致,证明C5+离子辐照对MgF2晶体并未造成较大的晶格损伤。
Fabrication and characterization of ridge waveguide in MgF2 crystal at mid-infrared 4 μm wavelength (invited)
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摘要: 为了研究离子与中红外晶体相互作用的机理,探索中红外晶体光波导的制备和性能,采用离子辐照技术结合精密金刚石刀切割,在MgF2晶体材料中制备了深度17.5 μm、宽度14 μm的脊形光波导。采用SRIM软件模拟了C5+离子辐照MgF2晶体的电子能量损伤和核能量损伤的过程,分析了波导的形成机理;模拟了波导的折射率变化,并对波导的近场模式进行了实验测量和理论模拟;采用热退火处理来降低波导的传输损耗,将传输损耗降低为0.4 dB/cm;微拉曼光谱证明离子辐照过程并未对MgF2晶体波导区造成较大的晶格损伤。该工作表明,离子辐照技术结合划片机精密切割是一种十分成熟的脊形波导制备手段,制备的MgF2晶体脊形光波导在中红外集成光学和光通讯领域具有广泛的应用前景。Abstract: To study the mechanism of interaction between ions and mid-infrared crystals and explore the preparation and properties of mid-infrared crystal optical waveguides, an optical ridge waveguide with a depth of 17.5 μm and a width of 14 μm was fabricated in MgF2 crystals by ion irradiation combined with precision diamond blade dicing. The SRIM software was used to simulate the process of electronic and nuclear stopping powers of MgF2 crystal irradiated by C5+ ions, and the mechanism of waveguide formation was analysed. The refractive index variation of the waveguide was simulated, and the near-field mode of the waveguide was experimentally measured and theoretically simulated. The propagation loss of the waveguide was reduced to 0.4 dB/cm by thermal annealing. The micro-Raman spectra show that there was no significant lattice damage in the waveguide region of the MgF2 crystal during ion irradiation. The results show that ion irradiation combined with diamond dicing is a very mature method to prepare ridge waveguides, and the prepared MgF2 crystal ridge waveguides have a wide application prospects in the field of mid-infrared integrated optics and optical communication.
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Key words:
- mid-infrared /
- MgF2 crystal /
- optical waveguide /
- ion irradiation
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图 3 (a)电子能量损伤(红线)和原子核能量损伤(蓝线)随C5+离子辐照深度变化关系;(b)重建的4 μm波长下MgF2脊形光波导的折射率随波导深度变化关系图
Figure 3. (a) Relation of electronic energy damage (red line) and nuclear energy damage (blue line) with the irradiation depth of C5+ ions; (b) Reconstructed relation of refraction index of MgF2 ridge waveguide with the waveguide depth at 4 μm wavelength
图 4 (a)脊形光波导横截面显微镜图像;(b)脊形光波导4 μm波长下近场模式分布实验图;(c)脊形光波导4 μm波长下近场模式分布模拟图
Figure 4. Sectional microscopic image of optical ridge waveguide; (b) Experimental diagram of near-field mode distribution at 4 μm wavelength; (c) Simulated diagram of near-field mode distribution at 4 μm wavelength of optical ridge waveguide
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