Calligraphic poling is a technique for generating an arbitrary, possibly complex pattern of localized reversal in the direction of permanent polarization in a wafer of LiNbO3 or other ferroelectric material. The technique is so named because it involves a writing process in which a sharp electrode tip is moved across a surface of the wafer to expose the wafer to a polarizing electric field in the desired pattern. The technique is implemented by use of an apparatus, denoted a calligraphic poling machine (CPM), that includes the electrode and other components as described in more detail below.
A capability for forming poling patterns is needed for research on, and development of, photonic devices that exploit the nonlinear optical properties of ferroelectric crystals. Specific poling structures in ferroelectric crystals can be engineered to variously amplify or suppress these nonlinear optical properties to effect such useful functions as optical switching, modulation, frequency conversion, and frequency filtering.
Outside of industry, poling machines have been available to only a few research laboratories and have been typified by high costs and limited flexibility. Prior poling techniques and machines are suitable for generating relatively simple patterns (e.g., straight lines) but are not suitable for generating the arbitrary patterns required in some applications. Each of the prior techniques involves one or more of the following: expensive fabrication of electrically conductive masks, expensive holograms, a cleanroom, high vacuum, and high temperature. In contrast, a CPM operates in air at room temperature with equipment readily available in a typical laboratory; hence, calligraphic poling costs less than does poling by the prior techniques.
In a CPM (see figure), the wafer to be poled is placed in the x–y (horizontal) plane on a smooth, flat substrate that serves as a lower electrode. Typically, the wafer is a crystal of LiNbO3 or other ferroelectric material that is z-cut. [In this case, "z-cut" signifies that the polarization axis of the wafer is perpendicular to its broad faces and thus coincides with the z (vertical) axis when the wafer is placed on the substrate]. A thin layer of saltwater is placed between the substrate and the wafer to ensure electrical contact between the nominally planar wafer and substrate surfaces. The thin layer of saltwater also serves to mechanically bind the wafer temporarily to the substrate. The electrode has a tungsten tip 0.5 μm wide and is biased at a suitable potential with respect to the substrate (typically, ≈1.5 kV for a 100-μm-thick LiNbO3 crystal). The writing electrode is mounted on an x–y translation stage, which is used to move the electrode across the wafer surface in the desired pattern.
By varying the magnitude and duration of the applied voltage in conjunction with the motion or stationarity of the electrode, one can exert some control over the sizes and shapes of the ferroelectric domains. For example, it was found that smooth lines having widths of the order of a few microns could be formed by applying potentials between 0.8 and 1.8 kV for times of the order of 10 s while the pen was moving, as illustrated in the left part of the figure. For another example, it was found that potentials between 2 and 3 kV applied for times <2 s while the electrode was held stationary yield hexagons (a consequence of the crystalline structure of LiNbO3), as illustrated in the right part of Figure 2.
Among the advantages of calligraphic poling is that it is possible to visually observe the domains in ordinary (that is, non-polarized) light as they are being formed. Light incident from above along the z-axis travels through the wafer and is reflected from its bottom surface. The poling electric field magnifies the gradient in the index of refraction between a +z- and –z-poled region to such an extent as to give rise to a dark outline, coinciding with the boundary between the regions, that is visible in the reflected light when viewed from above through a conventional optical microscope. In addition, it is possible to view the domains nondestructively after they have been formed, because a potential sufficient to generate the dark outline (typically 200 V) is much smaller than the domain-reversal potential.
This work was done by Makan Mohageg, Dmitry Strekalov, Anatoliy Savchenkov, Adrey Matsko, Lute Maleki, and Vladimir Iltchenko of Caltech for NASA's Jet Propulsion Laboratory.
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