BACKGROUND OF THE INVENTION

(A) Field of the Invention

The present invention relates to a method for preparing a periodically poled structure, and more particularly, to a method for preparing a periodically poled structure by controlling the charge into a ferroelectric single crystal.

(B) Description of the Related Art

The periodically poled structure having poled domains in a ferroelectric single crystal such as lithium niobate (LiNbO₃), lithium tantalite (LiTaO₃) and potassium titanyl phosphate (KTiOPO₄) may be widely used in the optical fields such as optical storage and optical measurement. There are several methods for preparing the periodically poled structure such as the proton-exchanging method, the electron beam-scanning method, the electric voltage applying method, etc.

U.S. Pat. No. 6,002,515 discloses a method for manufacturing a polarization inversion part on a ferroelectric crystal substrate. The polarization inversion part is prepared by steps of applying a voltage in the polarization direction of the ferroelectric crystal substrate to form a polarization inversion part, conducting a heat treatment for reducing an internal electric field generated in the substrate by the applied voltage, and then reinverting polarization in a part of the polarization inversion part by applying a reverse direction voltage against the voltage that was previously applied. In other words, the method for preparing a polarization inversion part disclosed in U.S. Pat. No. 6,002,515 requires performing the application of electric voltage twice.

SUMMARY OF THE INVENTION

One aspect of the present invention provides a method for preparing a periodically poled structure by controlling the charge into a ferroelectric single crystal.

A method for preparing a periodically poled structure according to this aspect of the present invention comprises the steps of providing a ferroelectric substrate and performing a poling process by applying a poling current to at least one portion of the ferroelectric substrate according to a current waveform. The current waveform includes a major phase and a tailed phase accompanying the major phase, and the major phase has at least one peak current (Ip) and terminates when the current drops substantially to Ip/e. The charge delivered to the portion of the ferroelectric substrate during the major phase is larger than that delivered during the tailed phase.

Another aspect of the present invention provides a method for preparing a periodically poled structure comprising the steps of providing a ferroelectric substrate and performing a poling process by applying a poling electric field to at least one portion of the ferroelectric substrate according to an electric field waveform including a nucleation phase and a spreading phase accompanying the nucleation phase, wherein the nucleation phase is configured to generate nucleation sites in the portion of the ferroelectric substrate and the spreading phase is configured to increase the size of the nucleation sites.

A further aspect of the present invention provides a method for preparing a periodically poled structure comprising the steps of providing a ferroelectric substrate, performing a pre-poling process by applying a reiterating electric field to at least one portion of the ferroelectric substrate according to a reiterating waveform to let the portion reiterate domain inversions, and performing a poling process by applying a poling electric field to the portion of the ferroelectric substrate according to an electric field waveform including a nucleation phase and a spreading phase accompanying the nucleation phase. In the latter process the nucleation phase is configured to generate nucleation sites in the portion of the ferroelectric substrate and the spreading phase is configured to increase the size of the nucleation sites.

BRIEF DESCRIPTION OF THE DRAWINGS

The objectives and advantages of the present invention will become apparent upon reading the following description and upon reference to the accompanying drawings in which:

FIG. 1 to FIG. 7 illustrate a method for preparing a periodically poled structure 10 according to one embodiment of the present invention;

FIG. 8 illustrates another electric field waveform according to another embodiment of the present invention; and

FIGS. 9 and 10 illustrate another electric field waveform according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 to FIG. 7 illustrate a method for preparing a periodically poled structure 10 according to one embodiment of the present invention. First, a ferroelectric substrate 12 consisting of a plurality of first domains 16 and second domains 18 positioned between the first domains 16 is provided. The ferroelectric substrate has an upper surface 12A and a bottom surface 12B, and a plurality of upper electrodes 14A are formed on the upper surface 12A and a plurality of bottom electrodes 14B are formed on the bottom surface 12B. The original polarization direction of the ferroelectric substrate 12 is from âˆ’Z to +Z, as shown by the arrows in FIG. 1.

Referring to FIG. 2, a pre-poling process is performed by applying a reiterating voltage difference (V) between the upper electrodes 14A and the bottom electrodes 14B. The ferroelectric substrate 12 is prepared by epitaxy process and may include some crystal defects therein, which inhibit the quality of a formal poling process. The iterating voltage difference (V) applied in the pre-poling process generates reiterating electric field according to a reiterating waveform to let the second domains 18 of the ferroelectric substrate 12 reiterate domain inversions, which can reduce the inhibition effect of the crystal defects. The reiterating waveform may include a plurality of waves such as square waves, as shown in FIG. 3. The amplitude (the reiterating electric field) of the square waves is larger than the coercive field (Ec) of the ferroelectric substrate 12, for example, between 18 and 21 kV/mm.

Referring to FIG. 3, a poling process is performed by applying a poling current to the second domains 18 of the ferroelectric substrate 12 according to a current waveform. The current waveform includes a major phase and a tailed phase accompanying the major phase, wherein the major phase has at least one peak current (Ip) and terminates when the current drops substantially to Ip/e, i.e., I₁=Ip/e. The charge delivered to the second domains 18 of the ferroelectric substrate 12 during the major phase is preferably larger than that delivered during the tailed phase, i.e., Q1 is larger than Q2. For example, the ratio of the charge delivered to the second domains 18 of the ferroelectric substrate 12 during the major phase to the charge delivered during the tailed phase is between 1:1 and 1:9. The duration (t₁) of the major phase to the duration (t₂âˆ’t₁) of the tailed phase is preferably between 1:1 and 1:5. The tailed phase terminates at a tail current (I₂), and the peak current (Ip) to the tail current (I₂) is preferably larger than 5.

Referring to FIG. 5, the current waveform can be implemented by applying a poling electric field to the second domains 18 of the ferroelectric substrate 12 according to an electric field waveform including an initiating phase, a nucleation phase, a spreading phase and a stabilizing phase. The nucleation phase is configured to generate nucleation sites in the second domains 18 of the ferroelectric substrate 12 and the spreading phase is configured to increase the size of the nucleation sites in the second domains 18. The nucleation phase includes at least one spike-shaped nucleation wave. The spike-shaped nucleation wave has a spike field (E_S1) configured to generate nucleation sites in the second domains 18 of the ferroelectric substrate 12. The nucleation phase terminates as the electric field decreases from the spike field (E_S1) to a poling field (E_P1) to initiate the spreading phase configured to drive the nucleation sites to spread. The electric field of the spreading phase decreases from the poling field (E_P1) to a stabilizing field (E₀) in a stepped manner. The electric field of the stabilizing phase is maintained at the stabilizing field (E₀) for predetermined duration and decreases to zero to complete the periodically poled structure 10 with the polarization direction of the second domains 18 being reversed, as shown in FIG. 6.

For example, the coercive field (E_C) of MgO doped congruent lithium niobate (MgOCLN) crystal is about 1.7 kV/mm at 150Â° C., the spike field (E_S1) is preferably larger than the coercive field (E_C), for example, between 18 and 21 kV/mm, and the duration of the spike-shaped nucleation wave is preferably between 50 and 200 microseconds. Since there are different sizes of nucleation sites (grains) in the second domains 18 during the spreading phase, the larger grains will experience a smaller electric field and the smaller grain will experience a stronger electric field under the same external voltage difference, and the spreading rate of the grain is proportional to the applied electric field, as shown in FIG. 7.

The decrease of the applied electric field in the spreading phase in the stepped manner (or in the ramp-shaped manner) contributes to adjust the spreading rate of the grains with different sizes to get a better uniformity. Preferably, the initiating phase is performed by applying an initiating field (E₀) to the second domains 18 before the nucleating phase, and the stabilizing phase is performed subsequent to the spreading phase by applying a stabilizing field the same as the initiating field (E₀) to the second domains 18. In particular, both the spike field (E_S1) and poling field (E_P1) are preferably larger than the coercive field (E_C) of the ferroelectric substrate 12, and the initiating field (E₀) and the stabilizing field (E₀) is preferably smaller than the coercive field (E_C).

FIG. 8 illustrates another electric field waveform according to another embodiment of the present invention. Compared to the electric field waveform having one spike-shaped nucleation wave as shown in FIG. 5, the electric field waveform shown in FIG. 8 has a plurality of spike-shaped nucleation waves. The single spike-shaped electric field can be replaced by multiple spike-shaped electric fields to increase the uniformity of the nucleation site distribution. In addition, the electric field in the spreading phase decreases in a stepped manner as shown in FIG. 5, while the electric field in the spreading phase decreases in a ramp-shaped manner, as shown in FIG. 8.

FIG. 9 illustrates another electric field waveform according to another embodiment of the present invention, and FIG. 10 shows the hysteresis behavior of the ferroelectric substrate 12. Compared to the electric field waveform having a plurality of spike-shaped nucleation waves in FIG. 8, the electric field waveform in FIG. 9 has a plurality of trapezoid nucleation waves. The amplitude of the trapezoid electric field is smaller than that of the spike-shaped electric fields. In addition, the amplitude of the trapezoid electric field in the nucleation phase is smaller than that in the spreading phase. Preferably, the amplitude of the trapezoid electric field is smaller than the coercive field (E_C) but larger than or equal to the stabilizing field (E₀) such that a portion of the ferroelectric substrate 12 has reversed polarization, as shown in FIG. 10. In other words, there are few nucleation sites with reversed polarization direction in the second domains 18 in view of the hysteresis behavior. The amplitude of the trapezoid electric field is preferably between 1.3-1.5 kV/mm for about 50-1000 microseconds.

The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the scope of the following claims.