A study on the optical nonlinearity of thermally poled planar fused silica plates and development of QPM SHG devices

• Main Authors: Huai-Yi Chen, 陳准義
• Other Authors: Shiuh Chao
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/ncx8wy

博士 === 國立清華大學 === 電機工程學系 === 91 === Silica-based glasses are important materials in optical fiber communication and planar light-wave circuits. Silica glasses are, however, amorphous with macroscopic inversion symmetry and have inherently no second-order nonlinearity (SON) or linear electro-optic coefficient, which makes them available only in passive devices. In 1986, Osterberg and Margulis discovered that Ge-doped fiber irradiated by intensive 1064nm laser displays SON and its second-harmonic conversion efficiency is as high as 5%. Later, R. A. Myers et al. found that large and stable second-order nonlinear susceptibility χ(2) (~1 pm/V) could be created in fused silica by means of thermal poling. Since then, there are intensive researches on the poling of glass and fibers. The mechanism behind the formation of SON in poled fused silica is not yet fully understood. One given explanation is related to alignment of dipole/bond orientation which breaks the symmetry of structure and the other is that χ(2) comes from third-order susceptibility χ(3) and built-in space charge field Edc induced by poling, i.e. χ(2) =3χ(3) Edc . The built-in electric field Edc is usually located in an extremely thin (about 4~20 m) region near the anodic surface. The reason for the existence of Edc is that positive ions such as Na+，H+ or H3O+ drift toward the cathode side during the poling process, leaving positive/negative space charge at the anode side. Leshe and Margulis et al. found that under the attack of diluted hydrofluoric (HF) acid the etching rate of poled fused silica is slower than that of un-poled one, which is connected with the effect of electric field applied to fused silica. The destination of all these researches is to put poled glass and fibers in application of active devices, e.g. electro-optic modulator, switch, frequency converter, and parametric oscillator etc. The intensity of SON in thermally poled fused silica (TPFS) can be erased by heating, electron beam, ultraviolet (UV) or near infrared (IR), which could be utilized to implement quasi-phase-matched second-harmonic generation (QPM SHG). Bonfrate et al. had succeeded in developing periodic UV erasure of the nonlinearity for QPM SHG in optical fibers. However, photonics devices and circuits directly implemented in the silicon dioxide layer on silicon chips, such as planar light-wave circuits, have not been realized yet. In this dissertation, periodically poled structure on type-III UV-grade planar fused silica was realized by periodic UV erasure of second-order nonlinearity of the thermally poled fused silica plate. Poling and UV-erasure characteristics are given. The second order nonlinearity profile of thermally poled fused silica was found to correlate with a buried Gaussian function. The nonlinear depth, calculated by curve fitting of Maker’s’s fringes with assumption of a step-like nonlinear profile, corresponded to the location of the 1/e peak in buried Gaussian function. The etching rate in hydrofluoric acid of the UV-bleached thermally poled fused silica was found to be the same as that of the un-poled fused silica along the direction of the poling field but was larger along the direction perpendicular to the poling field. We present here for the first time to our knowledge QPM SHG device implemented on planar fused silica plate by UV-assisted periodic thermal poling. This technique could lead to practical frequency conversion and electro-optic devices on silicon-based planar light-wave circuits. First order QPM SHG of 1064nm to 532nm in thermally poled planar fused silica plate with periodically UV erasure of the second-order nonlinearity was successfully implemented. We obtained a 1:2.9 ratio of d31: d33 for UV grade fused silica in support of the proposed mechanism for electric-field-induced second order nonlinearity in this material. The content of this paper has been accepted by Optics Letters, Journal of Applied Physics and submitted to IEEE CLEO/PR (2003). We are now preparing to submit all the extended results for further publication.