Photosensitivitiy refers to any permanent refractive index change in a material caused by ultraviolet(UV) exposure. Silica glass doped with $GeO_2$ has received much attention because of the UV-induced refractive index change that is responsible for the formation of distributed Bragg gratings. In spite of significant studies in the understanding of photosensitivity in glasses, the mechanisms of the effect are still not completely clear. Generally, it was accepted that the color-center model and densification model are the origin of the UV-induced refractive index change.
This work is concerned on the photo-induced refractive index change of the organically modified germanosilicate glasses fabricated by sol-gel method. Photosensitivity in the ORMOGSIL glasses having unreactable organic units and photo-polymerizable organic units was investigated.
ORMOGSIL glasses with Ge contents of 10, 15, and 20 mol% were prepared using a sol-gel method. Germanium isopropoxide(GI) and Diethoxydimethylsilane(DMDES) were used as the starting materials. An optically transparent ORMOGSIL glasses were obtained after sintering the gel at 250℃ degree for 8 hrs under a reduced atmosphere ($H_2/N_2=1/9$ volume ratio). UV light irradiation (220~260 nm) was performed with an Oriel 82511 Hg/Xe lamp, which gave a power density of 45 mJ/㎠.
The refractive index change of the ORMOGSIL glasses was about $10^{-2}$, which is compatible with that of the cold high pressure hydrogen soaking has led to germanium-doped fibers with the highest observed photosensitivitiy.
Photobleaching of optical absorption bands in the 5-eV region and change in refractive index of the ORMOGSIL glasses were examined using UV/VIS/NIR spectrophotometer and prism coupler, respectively. Intensity of the 5-eV absorption band was saturated with the UV fluence of 500 J/㎠. However, the refractive index increase continued with the UV fluence of 2500 J/㎠. For ORMOGSIL system, the refractive index change of 10 mol% Ge-doped ORMOGSIL is larger than that of 20 mol% Ge-doped ORMOGSIL glass. Also, the refractive index change with UV exposure is sensitive to the conditions of sol preparation even for the same composition of glass. Thus, these results indicate that the color-center model does not sufficiently account for the photosensitivity of ORMOGSIL glasses.
The density change, structural variations, and microstructure of the ORMOGSIL glasses with the UV fluence were investigated by Archimedes method, Raman spectroscopy, and TEM, respectively. Density increased linearly with UV fluence, the same behavior as was seen for the refractive index change. The calculated refractive index change using the measured density change from Lorentz-Lorenz equation is about $10^{-2}$, which is in good agreement with the experimental data. The shifts in frequency of the Raman bands measured at room temperature reveal structural densification by reduction of the average intertetrahedral bonding angle in the ORMOGSIL glasses. And also, calculated density change from bonding angle variation was about 0.06, which is in good agreement with the experimental data. Also, TEM image showed that the ORMOGSIL glass densified with the UV irradiation. Thus, it was found that high photosensitivity (over $10^{-2}$) in ORMOGSIL glasses may be dominated by the contribution of the glass density increase induced by the UV irradiation.
Refractive index change in ORMOGSIL glasses has a linear relationship with the change in thermo-optic coefficient (dn/dT). This is another evidence for the UV-induced densification because dn/dT is related to the glass network structure. Ge-undoped ORMOSIL glasses were prepared by sol-gel method. Refractive index and density were almost same with regardless of the UV irradiation. This result showed that the photosensitivity of the ORMOGSIL glasses was directly related to Ge atoms. And, the photosensitivity of the ORMOGSIL glasses is permanent at room temperature.
The shorter photo-imprinting time and the higher photosensitivity can be obtained by introducing photo-polymerizable organic units such as methacrylates into the ORMOGSIL glasses. Germanium isopropoxide(GI) and 3-(trimethoxysilyl)propylmethacrylate (MPTMS) were used as the starting precursors. Irgacure 369 was used as photoinitiator.
Photoinduced chain polymerization, decomposition of methacrylates, Ge-related photoinduced densification, and photoinduced condensation are important factors of the photosensitivity in ORMOGSIL glasses having photo-polymerizable organic units. And, the gradual decrease in thickness also coincides with the structural densification induced by UV exposure.
Photoinitiator “IRGACURE 369” concentration does not affect the photosensitive behavior in photo-polymerizable ORMOGSIL glasses. And also, the refractive index change in photoinitiator-absent ORMOGSIL glasses is smaller than that of the others by the difference of photo-polymerization.
Direct photo-patterning of ORMOGSIL glass was carried out through a quartz contact mask without further treatment such as wet etching. An AFM surface scans of the ORMOGSIL glasses showed that the direct volume compaction occurs in the UV illuminated regions. This presents direct evidence for the photoinduced densification in the ORMOGSIL glasses. And, the depth change from the original surface is larger in photo-polymerizable organic modified ORMOGSIL glasses rather than unreactable organic modified ORMOGSIL glasses. This result also indicates that the direct photo-patterning in methacrylates having ORMOGSIL glasses is more efficient rather than that in unreactable methyl group modified germanosilicate glasses.
