Domain engineering in ferroelectric crystals is an increasingly important technique for applications in different areas including photonics and integrated optics, liquid manipulation, particles trapping and recently these crystals are emerging also for biotechnology applications. Among those, periodically poled lithium niobate (PPLN) continues to attract interest, and most efforts have been devoted to one-dimensional and two-dimensional PPLN crystals. These are ferroelectric materials broadly used in nonlinear optics frequency conversion processes by quasi-phase-matched (QPM). Fabrication of periodically reversed ferroelectric domains is usually achieved by an electric field poling (EFP) process. The period of the ferroelectric domain patterns usually range from hundreds of µm down to nm, depending on the specific applications. The fabrication of submicron periodic domain structures in ferroelectric crystals allows different types of optical components, such as electrically controlled Bragg reflectors, beam-steering devices, and narrow band filters. A variety of methods have recently been investigated to improve the pattern fidelity of the reversed domains. Technique of domain patterning in PPLN uses a lithographically patterned insulating layer and lithium chloride liquid electrodes contained in a chuck, sandwiching the sample between two O-ring gaskets. Upon application of an external electric field exceeding the coercive field of the material, domains first nucleate along the edges of the electrodes, due to the increase in field strength caused by the fringing field, and then spread laterally underneath the adjacent insulating layer. Usually, the external electric field is applied through appropriate waveforms or pulses that critically depend on many parameters depending on the material. However one main difficulty in achieving multi-periodicity in inverted ferroelectric domains is that when there are photoresist masks with different sizes patterns: it is very difficult to calibrate and modulate the electric field in order to avoid overpoling of shorter pitches without incurring in under-poling of larger structures. Thus, fabricating multi-periods PPLN is very arduous by a standard EP process that uses photoresist masks. The only way to have the chance to create the poled with freely patterns is by point-wise scanning techniques such as e-beam, electric tip AFM. Nonetheless, even if such scanning probe techniques allow getting patterns with whatever geometry they are very time-consuming in respect to the flexibility offered by single-shot classical EP process. In order to evaluate the poling process several attempts have been performed and new tools have been explored. In this work was adopted the interferometric approaches based on digital holography (DH) in microscopy configuration that allows at same time non-destructive inspection, full-field analysis and quantitative evaluation for assessing the quality of the poling process. This work presents the TEFP process as a tool for fabricating multi-scale PPLN samples avoiding cross drawbacks between over-poling and under-poling. The two EFP steps were applied with two different photoresist gratings. Since the pitch of the two gratings have periods with different scales, the final result is intriguing as it allows one to fabricate a sort of satellite structures.
Two-step electric field poling process for engineering multiperiodic microstructure in LN
Vito Pagliarulo
2017
Abstract
Domain engineering in ferroelectric crystals is an increasingly important technique for applications in different areas including photonics and integrated optics, liquid manipulation, particles trapping and recently these crystals are emerging also for biotechnology applications. Among those, periodically poled lithium niobate (PPLN) continues to attract interest, and most efforts have been devoted to one-dimensional and two-dimensional PPLN crystals. These are ferroelectric materials broadly used in nonlinear optics frequency conversion processes by quasi-phase-matched (QPM). Fabrication of periodically reversed ferroelectric domains is usually achieved by an electric field poling (EFP) process. The period of the ferroelectric domain patterns usually range from hundreds of µm down to nm, depending on the specific applications. The fabrication of submicron periodic domain structures in ferroelectric crystals allows different types of optical components, such as electrically controlled Bragg reflectors, beam-steering devices, and narrow band filters. A variety of methods have recently been investigated to improve the pattern fidelity of the reversed domains. Technique of domain patterning in PPLN uses a lithographically patterned insulating layer and lithium chloride liquid electrodes contained in a chuck, sandwiching the sample between two O-ring gaskets. Upon application of an external electric field exceeding the coercive field of the material, domains first nucleate along the edges of the electrodes, due to the increase in field strength caused by the fringing field, and then spread laterally underneath the adjacent insulating layer. Usually, the external electric field is applied through appropriate waveforms or pulses that critically depend on many parameters depending on the material. However one main difficulty in achieving multi-periodicity in inverted ferroelectric domains is that when there are photoresist masks with different sizes patterns: it is very difficult to calibrate and modulate the electric field in order to avoid overpoling of shorter pitches without incurring in under-poling of larger structures. Thus, fabricating multi-periods PPLN is very arduous by a standard EP process that uses photoresist masks. The only way to have the chance to create the poled with freely patterns is by point-wise scanning techniques such as e-beam, electric tip AFM. Nonetheless, even if such scanning probe techniques allow getting patterns with whatever geometry they are very time-consuming in respect to the flexibility offered by single-shot classical EP process. In order to evaluate the poling process several attempts have been performed and new tools have been explored. In this work was adopted the interferometric approaches based on digital holography (DH) in microscopy configuration that allows at same time non-destructive inspection, full-field analysis and quantitative evaluation for assessing the quality of the poling process. This work presents the TEFP process as a tool for fabricating multi-scale PPLN samples avoiding cross drawbacks between over-poling and under-poling. The two EFP steps were applied with two different photoresist gratings. Since the pitch of the two gratings have periods with different scales, the final result is intriguing as it allows one to fabricate a sort of satellite structures.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
