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Abstract
In this letter we present the first study on the electrode surface topology enhanced KTN deflector, in which one of the electrodes contains sharp edges. In this case, the electric field can be enhanced near the sharp edge. This can increase the injected charges, which in turn results in the increased deflection angle for the space charge controlled KTN deflector. Both the conducted simulations and experimental investigations agreed well. In the experiments, the sharp edge structures were created by two methods: nanofabrication and roughing with a sandpaper. Both cases experimentally obtained increased deflection ranges. This study would be beneficial to many deflector applications because it can enlarge the deflection angle without the need to increase the driving voltage.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Although relaxor ferroelectric potassium tantalate niobate (KTN) crystal has been discovered over six decades ago, with the recent advent of high quality sizable KTN crystal that is suitable for the device fabrication, it still attracts a considerable amount of interest due to its large linear and quadratic electro-optic (EO) coefficients. The current efforts can be classified into two aspects. One aspect is to fabricate practical devices such as switchable waveguides [1], fast speed KTN deflectors [2,3], optical coherence tomography [4], and variable focal length lens [5]. The other aspect is to explore new physical mechanisms and methods to enhance its material properties and device functionality, such as improving conductivity and dielectric property via UV illumination [6], achieving bi-directional beam scanning via UV-assisted electron and hole injections [7], enhancing the linear EO coefficient and transparency via domain engineering [8], and increasing the deflection range via electric field induced high permittivity [9].
In this letter, we report the enhanced deflection range by harnessing the electrode surface topology to further improve the performance of a KTN deflector. It should be note that, electrode surface topology has been used to improve the charge injection in a variety of materials [10–13] and device functionality [14]. However, this is the first time to use this technique for enhancing the deflection range of a KTN deflector.
The physical mechanism of the electrode surface topology enhanced KTN deflector can be summarized as follows: the local electric field can be enhanced by the electrode surface topology, which contains sharp edges. The enhanced local electric field results in an increased injected charge density. This in turn enhances the beam deflection angle of a KTN deflector under the same applied field because the beam deflection angle is proportional to the injected charge density.
Two types of electrode surface topologies were investigated in this paper. The first type of topology had a rectangularly shaped periodic structure that contained sharp edges. This structure was selected due to the following reasons: First, the analytic form of electric field existed in this kind regularly shaped structure so that an accurate electric field enhancement factor could be obtained. Second, this type of structure could be precisely fabricated by the micro/nano lithographic process. This allowed us to quantitatively compare the analytical and experimental results. The second type of topology was achieved by roughing the surface with high hardness (e.g., silicon carbide) sandpaper. This is a simple and low-cost approach although it is hard to get precise analytic solutions for electric field distributions.
To ensure that the electrode surface topology could indeed be harnessed to enhance the deflection range of a KTN deflector, we initially conducted the following theoretical analyses. Figure 1 shows an illustration of a x-y plane cross-section of an electrode surface topographically modified KTN deflector.
Fig. 1. An illustration of an electrode surface topology enhanced KTN deflector, which contains a rectangularly shaped periodic structure.
The surface on the side of the anode contained sharp knife-edges with a gap size of 2a between the fins and a depth of h for each fin. Titanium/Gold (Ti/Au) was used as the electrode material, which was deposited along the fins. The electric field was applied along the y direction. In this case, the electric field near the corner (i.e., the area near A and B marked in Fig. 1 can be derived as [15]:
Figure 2 is a simplified sketch of a KTN beam deflector, containing a topologically structured Ti/Au electrode. In the drawing, we assume that the incoming laser beam propagates along the y direction; the electric field is applied along the x direction; and the crystal axis is along the x direction, the polarization direction of the incoming light is aligned along the electric field direction.
Fig. 2. A sketch of laser interaction with a surface topologically modified KTN deflector with Ti/Au electrodes.
In this case, under the trapped charge model, the deflection angle, $\theta (x)$, can be derived as [2]:
To obtain a quantitative electric field enhancement factor for the electrode having a realistic structure, we quantitatively computed the electric field distribution of an electrode surface topologically modified KTN crystal, containing a periodic rectangularly shaped structure by COMSOL software. In the simulation, we assumed that the dimension of each rectangular structure was $2.5\mu m$ and the separation between adjacent structures was also $2.5\mu m$. The height of each structure was $1\mu m$. The distance between the top anode and the bottom cathode was 1 mm and the applied voltage was 250 V. Figure 3 depicts the computed result. The electric field is around 250 V/mm without the electrode surface topology. However, with the electrode surface topology, the electric field was enhanced to 330 V/mm near the bottom sharp edge. This simulation result indicates that a substantial increase in the electric field can be obtained near the bottom sharp edge, which in turn enhances the charge injection.
Fig. 3. The quantitatively computed electric field distribution of an electrode surface topologically modified KTN crystal, containing periodically shaped rectangular nanostructure.
To validate the above theory and simulation result, we conducted the following two experiments, marked as Experiment 1 and Experiment 2, respectively. In Experiment 1, the electrode surface topology was created by nanofabrication, which had a rectangularly shaped structure, as depicted in Fig. 1. In Experiment 2, the electrode surface topology was created by roughing the polished surface with silicon carbide (SiC) sandpaper.
2. Fabrication Procedures
2.1 Experiment 1 – Create topologically structured electrode by nanofabrication
The KTN crystal that we chose had a composition of $KT{a_{0.61}}N{b_{0.39}}{O_3}$. A KTN crystal, marked as sample A was prepared. Sample A was cut and optically polished in to a dimension of $L = 10mm$, $W = 1.7mm$, and $d = 0.8mm$ The Curie temperature of Sample A was ${T_c} = 22.6{}^ \circ C$. The electrode surface topology of Sample A was created by nanofabrication technique, which includes the following procedures, as illustrated in Fig. 4:
- • Spin coat MMA and PMMA photoresist on the top surface of sample A.
- • Create a periodic structure on the top surface by using an e-beam lithographic machine - Raith 5200 E-beam writer.
- • Develop the exposed photoresist with MIBK: IPA 1: 1 solution.
- • Coat a layer of 200 nm nickel (Ni) on the developed photoresist.
- • Lift off the coated Ni on the area with the photoresist so that a Ni hard mask is formed.
- • Dry etch the KTN crystal with the Ni hard mask by using Alcatel “Speeder 100SiO2” ICP etching system. An etching depth of 900 nm was achieved.
- • Remove the Ni hard mask in the wet bench. In this way, a periodic rectangularly shaped structure was created. Figure 5 shows the scanning electron microscope (SEM) image of the fabricated structure. The structure was designed to be 2.5 µm fin size and of 50/50 duty cycle. From this figure, one can observe that the experimental result agrees well with the design.
- • Deposit the Ti/Au electrode on the etched top surface with the Temescal E-beam evaporator. Ti/Au electrode was selected because it had a low work function so that charges could be readily injected into the KTN crystal. Note that, only the top surface had the structure.
2.2 Experiment 2 – Create topologically structured electrode by sandpaper roughing
A KTN crystal boule with a slightly different composition $KT{a_{0.62}}N{b_{0.38}}{O_3}$ was used in experiment 2. Two rectangularly shaped KTN crystal samples were cut from the boule and optically polished into the following dimensions: $L = 30\,mm$, $W = 1.7\,mm$, and $d = 1.7\,mm$. The Curie temperature of two samples was ${T_c} = 18{\,^o}C$. First, we fabricated a KTN deflector by coating the top and bottom surfaces with Ti/Au from one sample. The surface profile and the deflection angle as a function of applied voltage were measured. Then, we made another sample with topologically structured surface by roughing the top surface with a sandpaper, with the same thermal wax mounting and polished with the Logitech PM5 polishing system, only finalized with P4000 grit SiC sandpaper instead of going through the full nano-scale optical polish. According to the spec sheet of the sandpaper, P4000 was used because it had a grit size around $2.5\,\mu m$, which was similar to the gap size of the structure created by the nanofabrication.
The surface profiles of this sample, the optically polished sample, and the sample with the structure created by nanofabrication were obtained by using Zygo’s Nexview 3D Optical Profiler, as shown in Fig. 6.
Fig. 6. Profilometer measurements of topology modified KTN sample and conventional optically polished KTN sample.
3. Deflection angle tests and discussion
We measured the deflection angle as a function of the applied voltage for the samples created by both the nanofabrication and sandpaper roughing techniques. The measurement was conducted at the temperature ${T_c} + 5{\,^o}C$ by using the experimental setup, as depicted in Fig. 2. Note that the incident 633 nm He-Ne laser beam with ∼1mm2 beam size was aligned by the anode which was surface engineered. Figure 7 shows the experimentally measured deflection angle as a function of applied voltage for the sample created by nanofabrication. The blue curve with square marks is the case without electrode surface topology. The orange curve with circle marks is the case with electrode surface topology. It can be clearly seen that the deflection angle is indeed increased at the same applied voltage. This shows the advantage of using electrode surface topology.
Fig. 7. The experimentally measured deflection angle as a function of applied voltage for the sample with and without electrode surface topology created by nanofabrication.
Figure 8 shows the experimentally measured deflection angle as a function of applied voltage for the sample created by roughing with the sandpaper P4000. Again, the blue curve with square marks is the case without electrode surface topology. The orange curve with circle marks is the case with electrode surface topology. It can be clearly seen that the deflection angle is indeed increased at the same applied voltage. This further verified the advantage of using electrode surface topology.
Fig. 8. The experimentally measured deflection angle as a function of applied electric field for the samples with and without electrode surface topology created by roughing with a sandpaper.
4. Conclusion
In summary, the deflection range of a KTN deflector could be enhanced by using the electrode with sharp edge topological structures. This was because the electric field was enhanced near the sharp edges, which in turn increased the injected charges. This increased injected charge resulted in the increased deflection range for the space charge controlled KTN deflectors. In the experiment, the sharp edge structures were created using two methods. One method was based on the nanofabrication and the other method was achieved by roughing the surface with a sandpaper. The measured deflection ranges were indeed increased for the deflectors with sharp edge structured electrodes created by using the nanofabrication and roughing with a sandpaper. The micro- and nano-sized modifications displayed no degradation in the beam quality as the beam quality seen was similar to that shown in Ref. [17] despite being perpendicular to the c-axis. This study could be greatly beneficial to a variety of KTN deflector-based applications, such as high-speed beam scanning and fast speed optical coherence tomography, due to the enhanced deflection range while maintaining the same applied voltage.
Funding
Office of Naval Research (N00014-17-1-2571).
Acknowledgments
This research was sponsored and partially supported by the Office of Naval Research (ONR) under Grant Number N00014-17-1-2571. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressed or implied, of the Army Research Laboratory or the US Government. The US Government is authorized to reproduce and distribute reprints for Government purposes notwithstanding any copyright notation hereon.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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