Mr. Yu-Chun Chang Profile

Yu-Chun Chang

at National Cheng Kung Univ.

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Proceedings Article | 15 March 2024 Presentation + Paper

Lithium niobate long-period waveguide gratings integrated with bismuth ferrite (BiFeO3) resistive random access memory

Ricky Chuang, Yu-Chun Chang, Cheng-Liang Huang

Proceedings Volume 12887, 128870D (2024) https://doi.org/10.1117/12.3004487

KEYWORDS: Waveguides, Lithium niobate, Electrodes, Silver, Ferroelectric materials, Fabrication, Optical gratings, Optical transmission, Optical testing, Electrical conductivity

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We report the miniaturization of Ag/BiFeO₃/ITO resistive random access memory (ReRAM) in the form of long-period waveguide grating, all fabricated entirely on z-cut lithium niobate (LiNbO₃) substrate. The electric characterization vividly reveals a selector-like threshold-switching (TS) characteristic. It is well accepted that there are two filament formation mechanisms governing the operation of Ag/BiFeO₃/ITO ReRAM, a two-side TS characteristic is typically generated when the positive and negative bias voltages are applied during the cycles. It is reasonable to believe the TS characteristic exists because the Joule heat produced during device operation cannot be dissipated effectively, resulting in the rupture of the conductive filament. To mitigate this shortcoming, replacing the BiFeO₃ (BFO) layer of the original grating shape with a planar-layer structure is beneficial for heat dissipation, and this, in turn, would help to improve the characteristics of the ReRAM device by delivering the memory-switching characteristics as intended. As already mentioned, this ReRAM structure with the ITO bottom electrode fabricated over the lithium niobate waveguide can jointly serve as the long-period waveguide grating. Furthermore, when the Ag/BFO/ITO ReRAM structure's device area shrinks to 200 μm², the growth path of the silver conductive filament is confined immediately above the lithium niobate waveguide. The corresponding spectral measurement shows that as the increasing number of ReRAM grating fingers are in the set state (silver conductive filaments formed), the energy of the transmission dips would decrease gradually in return, and during the reset state, the energy of the transmission dip would rise accordingly.

Proceedings Article | 17 March 2023 Poster + Paper

Long period waveguide grating filters on lithium niobate

Ricky Chuang, Han-Lin Chen, Yu-Chun Chang, Yu-Shun Chiu

Proceedings Volume 12424, 124241Q (2023) https://doi.org/10.1117/12.2655507

KEYWORDS: Waveguides, Cladding, Channel waveguides, Refractive index, Tunable filters

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Steric acid is used to fabricate cladding and core layers on z-cut lithium niobate (LiNbO3) substrate by maintaining the solution melt at 280°C for 4 and 2 hours, respectively. After completing the two-step proton exchange (PE), the refractive indices of the core and cladding layers are ascertained by using the prism coupling technique, and with this information at hand, the grating period Λ of 50 μm is deduced by solving a system of transcendental waveguide equations with MATLAB. There are three methods adopted to fabricate the gratings. The first one is to utilize the proton-exchange method by directly diffusing ions into LiNbO₃ to realize phase grating while keeping the solution melt at 280°C for 0.5 hours. The second one relies on using a Shipley S1813 photoresist as the corrugation grating via standard lithography. The third approach is to deposit and subsequently pattern silver metal as corrugation grating. A series of measurements would show that the maximum dip contrast of the phase grating could reach up to 31.188 dB, and the corresponding full width at half maximum (FWHM) is about 0.77 nm. In comparison, the maximum dip contrast of the photoresist corrugation grating attains up to 28.44 dB with an FWHM of approximately 1.18 nm. On the other hand, the maximum dip contrast ratio of the silver corrugation grating is determined to be around 8.15 dB with an FWHM of about 0.6 nm. The thermal dependency of the phase grating is also probed by increasing the temperature from 40 to 60°C and the corresponding dips have appeared to be blue-shifted. All of these devices have managed to demonstrate the multiple rejection bands, which is believably due to the multimode interference (MMI) phenomenon.

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