4 inch 6 inch lithium tantalate wafer PIC-- Lithium tantalate waveguide on low loss insulator for nonlinear photonics on chip
Abstract: We have developed a lithium tantalate waveguide on 1550 nm insulator with a loss of 0.28 dB/cm and a toroidal resonator quality factor of 1.1 million. The application of χ(3) nonlinearity in nonlinear photonics is studied.
1. Introduce
Waveguide technology based on lithium niobate insulators (LNoI) has made great progress in the field of ultra-high speed modulators and on-chip nonlinear photonics due to their favorable χ(2) and χ(3) nonlinear properties and the strong optical limiting effect generated by the "on-insulator" structure [1-3]. In addition to LN, lithium tantalate (LT) has also been studied as a nonlinear photonic material. Compared with LN, LT has a higher optical damage threshold and a wider optically transparent window [4, 5], although its optical parameters are similar to those of LN, such as refractive index and nonlinear coefficient [6,7]. LToI is therefore another strong material candidate for high-optical power nonlinear photonics applications. In addition, LToI is emerging as a major material for surface acoustic wave (SAW) filter parts for high-speed mobile and wireless applications. In this context, LToI chips may become a more common material for photonic applications. However, only a few LTOI-based photonic devices have been reported to date, such as microdisk resonators [8] and electro-optical phase shifters [9]. In this paper, we introduce a low loss LToI waveguide and its application in ring resonators. In addition, the χ(3) nonlinearity of the LToI waveguide is provided.
Highlight
Provide 4 "-6" LTOI wafer, thin film lithium tantalate wafer, top thickness of 100nm-1500nm, domestic technology, mature process
Other products;
LTOI; Lithium niobate's most powerful competitor, thin film lithium tantalate wafers
LNOI; The 8-inch LNOI supports the mass production of lithium niobate thin films on a larger scale
LT fabrication on insulator waveguides
In this study, we used 4-inch LTOI wafers. The top LT layer is a commercial 42° rotary Y-cut LT substrate for SAW devices that directly bonds to a Si substrate with a 3 µm thick thermal oxide layer and performs an intelligent cutting process. Figure 1(a) shows the top view of the LToI wafer, where the top LT layer has a thickness of 200 nm. We evaluated the surface roughness of the top LT layer using atomic force microscopy (AFM)
Figure 1. (a) Top view of the LToI wafer, (b) AFM image of the top LT layer surface, (c) PFM image of the top LT layer surface, (d) schematic cross section of the LToI waveguide, (e) calculated outline of the basic TE mode, And (f) SEM image of LToI waveguide core before SiO2 coating deposition.
As shown in Figure 1 (b). The surface roughness is less than 1 nm, and no scratch lines are observed. In addition, we examined the polarization of the top LT layer using a piezoelectric response force microscope (PFM), as shown in Figure 1 (c). Even after the bonding process, we confirmed that uniform polarization was maintained.
Using the LTOI substrate, we fabricate the waveguide as follows. First, we deposit a metal mask layer for subsequent LT dry etching. We then perform electron beam (EB) lithography to define the waveguide core pattern on top of the metal mask layer. Next, we transferred the EB resist pattern to the metal mask layer by dry etching. After that, the LToI waveguide core is formed by electron cyclotron resonance (ECR) plasma etching. Finally, we removed the metal mask layer by a wet process and deposited the SiO2 cover layer by plasma enhanced chemical vapor deposition. Figure 1 (d) shows the schematic cross-section of the LToI waveguide. The total core height, plate height and core width are 200, 100 and 1000 nm respectively. Note that to facilitate fiber coupling, the core width is extended to 3 µm at the waveguide edge. Figure 1 (e) shows the calculated distribution of light wave intensity for the basic transverse electric field (TE) mode at 1550 nm. Figure 1 (f) shows a scanning electron microscope (SEM) image of the LToI waveguide core before the SiO2 coating was deposited.
Waveguide characteristic
First, we evaluate the linear loss properties by feeding TE polarized light from an amplified self-emitting light source at 1550 nm into LToI waveguides with varying lengths. The propagation loss is obtained from the slope of the relationship between the length of the waveguide and the transmittance of each wavelength. The measured propagation losses are 0.32, 0.28 and 0.26 dB/cm at 1530, 1550 and 1570 nm, respectively, as shown in Figure 2 (a). The manufactured LToI waveguides exhibit fairly low loss performance similar to the most advanced LNOI waveguides [10].
We then evaluate χ(3) nonlinearity through the wavelength conversion generated by the four-wave mixing process.
We fed a 1550.0 nm continuous wave pump light wave and a 1550.6 nm signal light wave into a 12 mm long waveguide. As shown in Figure 2 (b), the phase conjugated (idle) light wave signal strength increases with increasing input power. The illustration in Figure 2 (b) shows a typical output spectrum for four-wave mixing. From the relationship between the input power and the conversion efficiency, we can estimate the nonlinear parameter (γ) to be about 11 W-1m
Figure 3. (a) Microscope image of the fabricated ring resonator. (b) Transmission spectrum of a ring resonator with various gap parameters. (c) Measurements of a ring resonator with a gap of 1000 nm and Lorentzian fitting transmission spectra
Applied to ring resonators
Next, we fabricated an LTOI ring resonator and evaluated its characteristics. Figure 3 (a) shows an optical microscope image of the fabricated ring resonator. The ring resonator has a "runway" configuration consisting of a curved area with a radius of 100 µm and a straight area with a length of 100 µm. The gap width between the ring and the bus waveguide core varies in increments of 200 nm, i.e. 800, 1000, and 1200 nm. Figure 3 (b) shows the transmission spectrum for each gap, showing that the extinction ratio varies with the gap. From these spectra, we determined that the 1000 nm gap provides almost critical coupling conditions, as it has a maximum extinction ratio of -26 dB. Using a critically coupled resonator, we estimate the factor of quality (Q factor) by fitting the linear transmission spectrum through Lorentzian, and obtain an internal Q factor of 1.1 million, as shown in Figure 3 (c). To our knowledge, this is the first demonstration of a waveguide coupled LToI ring resonator. In particular, the Q-factor value we obtained is much higher than that of the fiber-coupled LToI microdisk resonator [9]
Conclusion
We have developed an LTOI waveguide with a loss of 0.28 dB/cm at 1550 nm and a ring resonator Q value of 1.1 million.
The performance obtained is comparable to that of the most advanced LNoI low-loss waveguides. In addition, the χ(3) nonlinearity of fabricated LTOI waveguides in on-chip nonlinear applications is also studied.
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