Application Cases

Experiment Name: Study of Periodic Polarization of Lithium Niobate Crystals and Thin Films

Testing Equipment: High-voltage amplifier, function generator, high-voltage probe station, dual-channel oscilloscope, etc.

Experiment Process:

Figure 1: Schematic Diagram of the Periodic Polarization Experiment of Lithium Niobate. HVA: High-Voltage Amplifier; AFG: Arbitrary Function Generator; OSC: Oscilloscope

The periodic polarization process is shown in Figure 1. The entire experimental setup consists of five parts: an arbitrary function generator (AFG), a high-voltage amplifier (HVA), a high-voltage probe station, three 100MΩ resistors connected in series, and a dual-channel oscilloscope (OSC).

A lithium niobate sample with comb electrodes is placed in high-voltage insulating oil to prevent air breakdown. The AFG provides a low-voltage pulse signal to the HVA, with the pulse waveform drawn by the AFG and a bias set to half of the given voltage peak, i.e., the minimum voltage is 0. The HVA amplifies the specified pulse signal 2000 times to create a high-voltage pulse signal. This signal is loaded onto one end of the comb electrodes via a probe, while the other end is connected to a probe that leads to a 300MΩ equivalent resistor and then grounded. Channel one of the dual-channel oscilloscope monitors whether the HVA has successfully amplified the AFG signal, and channel two monitors the pulse signal passing through the comb electrodes by voltage division through the 300MΩ resistor. Since the comb electrodes are not connected at both ends, a strong electric field is formed from the positive to the negative electrode. When the field strength of the strong electric field exceeds the coercive field of lithium niobate, the internal domains of the crystal reverse their polarization. The periodically distributed electrodes form a periodically varying electric field, ultimately resulting in a periodic polarization pattern.

Keeping the number of pulses at 8 constant, the polarization voltage is increased. The electrode spacing is 9.5μm, and the original peak voltages are 110mV, 115mV, and 120mV, corresponding to electric field strengths of 23.15kV/mm, 24.2kV/mm, and 25.2kV/mm, respectively. Figure 2 shows the SEM images of the polarization effects under these three conditions. It can be clearly seen that as the pulse peak increases, the size of the non-polarized region gradually increases, the domain wall expands outward, and the polarization duty cycle increases from 60% to 83%. Subsequently, the electrode spacing was reduced to 4μm, and a clear polarization phenomenon was observed at an electric field strength of 25kV/mm with 8 polarization pulses, as shown in Figure 3. Further increasing the electric field strength to 30kV/mm, the polarization effect obtained is different from that under a 10μm electrode pattern. As the polarization peak voltage increases, the polarized region does not significantly expand laterally. Depending on the electrode morphology, rounded electrodes and pointed electrodes exhibit different polarization states. However, with 8 polarization pulses, the polarized region can reach the negative electrode, completing the polarization of the entire area. The reason for the above phenomenon may be that the electrode spacing is too short and the electrodes are too wide, causing lateral domain growth to saturate and favoring longitudinal domain growth, i.e., extending the polarization depth. However, the polarization depth information cannot be seen from the SEM images.

Figure 2: SEM images obtained under different electric field strengths with 8 polarization pulses. (a) Electric field strength 23.15kV/mm; (b) Electric field strength 24.2kV/mm; (c) Electric field strength 25.2kV/mm

For the 10μm electrode spacing pattern, the field strength was changed from 27kV/mm to 33kV/mm, as shown in Figure 4. The lithium niobate crystal shows significant fragmentation, but a residual polarized region can still be seen. The electrodes also show clear fragmentation, with a larger damaged area and a more apparent residual polarization state. At an electric field strength of 27kV/mm, there are fewer polarized regions, and the domain wall position is blurred. At 30kV/mm, the residual is very obvious, with only a small polarized region at the positive electrode tip. At 33kV/mm, the positive electrode damage is very apparent, with a larger damaged area between the positive and negative electrodes. The polarization of the positive electrode region is completely lost. However, considering that the electric field strength is much higher than the coercive field strength of around 21kV/mm, there is a more apparent polarization phenomenon near the negative electrode. The region near the negative electrode is almost undamaged, and polarization can be completed successfully, with the polarized region being more completely retained.

Figure 3: SEM images obtained under different polarization conditions with 4μm electrode spacing and two different electrode morphologies. (a) Electric field strength 25kV/mm, rounded electrodes; (b) Electric field strength 25kV/mm, pointed electrodes; (c) Electric field strength 30kV/mm, rounded electrodes; (d) Electric field strength 30kV/mm, pointed electrodes

Figure 4: SEM characterization of electrode and lithium niobate damage and residual polarized regions under different polarization conditions with 10μm electrode spacing and 8 polarization pulses. (a) Electric field strength 27kV/mm; (b) Electric field strength 30kV/mm; (c) Electric field strength 33kV/mm

Experimental Results:

In this paper, domain reversal was achieved in lithium niobate crystals with 10μm and 4μm electrode spacings, creating periodically polarized lithium niobate regions with a polarization period of 20μm. For the 10μm electrode spacing, the reason for the severe fragmentation of lithium niobate crystals under an electric field strength far below the breakdown field strength of lithium niobate but greater than 1.5 times the coercive field strength still needs further experimental verification. In lithium niobate thin films, domain reversal was also achieved with a 10μm electrode spacing, creating periodically polarized lithium niobate regions with a polarization period of 5μm. At a high polarization field strength of 40kV/mm, the thin film did not show the fragmentation seen in the crystalline state, indicating that the material is one of the factors affecting lithium niobate polarization. To achieve a polarization duty cycle close to 50% for lithium niobate thin films, future work will focus on reducing the electrode duty cycle to 35% and reducing the polarization time.

High-Voltage Amplifier Recommendation: ATA-7050

Figure: Specification Parameters of the ATA-7050 High-Voltage Amplifier

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