A lithium-niobate-on-insulator (LNOI) wafer is considered as an important candidate platform for photonic integrated circuits (PICs), owing to its outstanding material properties featuring a broad transparency window (350 nm to 5 µm), a linear electro-optic effect, and a large second-order nonlinearity susceptibility ()[
In addition to the sidewall-roughness scattering, some other factors, particularly the ion-induced lattice damage caused by ion slicing/milling, should also be taken into account to further reduce the propagation loss[
Here, we challenge the status quo and show that the LN ridge waveguides can be fabricated with a propagation loss as low as through suppressing the fabrication imperfection during thin-film production and nanostructuring by CMP, which is one order of magnitude lower than the previously reported results[
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2. Fabrication Methods
The manufacturing process for fabricating the LN microrings by CMP is schematically illustrated in Fig. 1, which mainly consists of two consecutive procedures including production of monocrystalline high-quality LN thin-film wafers and nanostructuring of LNOI microrings. To fabricate such LNOI microrings, first, an X-cut LN crystal was bonded to a silica buffer layer at room temperature, where the silica layer with a thickness of 2 µm was deposited on another LN bulk crystal by plasma enhanced chemical vapor deposition. Then, high-temperature annealing at 500°C was performed to enhance the bonding strength. Second, the top bulk crystal was thinned into a 4-µm-thick thin film via CMP, considering the trade-off between the surface evenness and thickness, as shown in Ref. . Thus, combined with the techniques of step 1 and step 2, an LNOI thin-film wafer was formed. Third, to pattern the LN thin film into microrings by CMP etching, a 600-nm-thick chromium (Cr) layer was coated on the LNOI wafer by magnetron sputtering. Fourth, the Cr layer was ablated into a microring-pattern hard mask by femtosecond laser direct writing with a scan speed of 10 cm/s and a pulse width of 190 fs. The laser focal spot was approximately 1 µm, and the thickness of the ablation layer was controlled to be as small as 400 nm by choosing the average power of the laser. Fifth, the sample underwent CMP to etch the exposed LN thin film, leading to the pattern transferring from the Cr layer to LN thin film[
Figure 1.Illustration of the fabrication flow of the microrings.
3. Characteristics of LN Microrings
The optical microscope image and the magnified scanning-electron-microscope (SEM) image of the microrings are shown in Figs. 2(a) and 2(b), respectively, indicating the LN microring with a diameter of 200 µm and an ultra-smooth surface. To accurately measure the wedge angle and the height of the microrings, a small slit is cut through the microring with a focused ion beam, as shown in Fig. 2(c), showing a wedge angle of 9° and a height of 720 nm. Interestingly, the small wedge angle will drive the modes far from the edge of the microrings, benefiting higher factors[
Figure 2.(a) Optical microscope image of the fabricated microring. (b) Magnified scanning-electron-microscope (SEM) image of the fabricated microring. (c) The SEM image shows that a small slit is cut through the microring with a focused ion beam. (d) The optical microscope image of the ridge waveguide on other LNOI chips for coupling of the microring.
To couple light into and out of the microring, a ridge waveguide with top width, bottom width, and height of 2.11 µm, 9.68 µm, and 700 nm, respectively, is fabricated on a second LNOI wafer (produced by ion slicing) by CMP etching, as shown in Fig. 2(d). The experimental setup is schematically illustrated in Fig. 3(a). The ridge waveguide was adjusted to be parallel with the top surface of the microrings by an rotatability stage and came into contact with the microring to gain optimum coupling, as shown in Fig. 3(b). Lensed fibers were used to couple the light signal into and out of the ridge waveguide by end-fire coupling with a coupling efficiency of 10% per facet. A narrow-linewidth wavelength tunable laser with a linewidth less than 200 kHz (model: TLB-6728, New Focus Inc.) was used as light source with an effective in-coupled power of 5 µW [tuned by a variable optical attenuator (VOA)] in the microring to avoid the thermal and nonlinear optical effects. An inline fiber polarization controller (PC) was used to adjust the polarization of the input light. The output optical signal was coupled out of the microring by the same ridge waveguide and lensed fiber and sent into a photodetector (PD, model: 1811, New Focus Inc.). The transmission spectrum was real-time analyzed by an oscilloscope (model: Tektronix MDO04) when scanning the wavelength of the optical signal. Whispering gallery modes were excited when the optical signal was resonant with the microring, resulting in a spectrum of sharp dips in the transmission spectrum.
Figure 3.(a) Experimental setup for mode characterization. (b) Optical micrograph of the waveguide coupled with the microring. (c) The measured transmission spectrum. (d) and (e) Q factors of the modes fitted by Lorentz-shape curves; insets: the corresponding field distributions of the modes, where the direction presents the radial direction.
Figure 3(c) shows the transmission spectra of the microring at the wavelength ranging from 1566 nm to 1570 nm, exhibiting two sets of high-order transverse electric (TE) and transverse magnetic (TM) modes. The modes were simulated by a finite-element method[
In conclusion, we demonstrate an ultra-high microring resonator in LNOI without ion-induced lattice damage by CMP. The intrinsic factor above was experimentally demonstrated at the 1550 nm wavelength band, while the waveguide propagation loss is only . This microresonator can be fully integrated with silicon nitride ridge waveguides on chip by vertical coupling[
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