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 (χ(2)=30pm/V)^[1–5]. A wide range of high-performance device applications, such as waveguide/resonator optical frequency convertors, high-speed Mach–Zehnder modulators, and multiplexers, have been demonstrated due to the rapid developments in the ion-slicing technique and LNOI nanofabrication technology^[6–23]. Among the various devices, optical waveguides with ultra-low propagation loss and high refractive index contrast are building elements for the realization of large-scale PICs^[24–28]. The propagation losses in optical waveguides are susceptible to sidewall roughness induced by the fabrication imperfection. To reduce the sidewall roughness of the waveguide, diamond-blade dicing, precision cutting, focused ion beam milling, argon ion milling, and chemo-mechanical polishing (CMP) have been successively used to etch the lithium niobate (LN) thin film into waveguides with smooth sidewalls to improve propagation losses^[29–34]. For instance, an ultra-smooth sidewall roughness as small as 0.1 nm has been achieved by CMP etching, leading to an ultra-low propagation loss of ridge waveguides of 0.027 dB/cm^[25,35]. However, the minimum propagation loss of the waveguide determined by the intrinsic material absorption is only 0.001 dB/cm^[36]. There is still room to reduce the propagation loss by improving the quality of LN thin film.

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^[1,4,37]. Particularly, the internal crystal lattice damage induced by the ion-slicing fabrication of the LN thin film has long been recognized, which represents the ultimate limit of the loss achievable in the traditional ion-sliced LNOI wafer^[37,38]. To avoid ion-induced lattice damage, CMP is proposed to produce the LNOI thin-film wafer and ultra-high Q LNOI microdisk. The Q factor as high as 1.23×108 at 1550 nm wavelength, which is close to the intrinsic material absorption limit, has been reported^[38]. The microring resonators, compared with microdisk resonators, have great advantages due to their superior characteristics such as flexibility in dispersion engineering, capability of electrical integration, and potential for large-scale integration. However, LN, as a difficult-to-cut material, is difficult to be fabricated into a ridge waveguide with low loss.

Here, we challenge the status quo and show that the LN ridge waveguides can be fabricated with a propagation loss as low as 0.0034±0.0001dB/cm 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^[24,25]. The loaded Q factor of the fabricated microring is measured as 0.8×108, corresponding to an intrinsic Q factor of 108. Remarkably, the microring loss is close to the intrinsic material absorption limit of LN crystals.

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. [38]. 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^[39]. Sixth, chemical etching was performed to remove the Cr mask. Finally, a secondary CMP was carried out for thinning the microrings. The main advantage of our fabrication method relies on the optimized process of CMP for thinning LN crystal, alleviating the crystal damage and surface damage.

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 Q factors^[40]. As a cost of a small wedge angle, the bottom of the microring is relatively large, which is 20 µm. The waveguide width can be further decreased by choosing thinner film wafers with sub-micron thicknesses^[41].

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 xyz-θx-θy 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(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^[42,43] and identified to certain modes according to the free spectral ranges of the modes and the resonant wavelengths of the modes. The refractive indices used in the simulation are calculated using Sellmeier coefficients for congruently grown LN^[44]. The two numbers in the subscript of the modes denote the radial (first number) and azimuthal (second number) mode numbers. The free spectral range between modes TE12,736 and TE12,737 is 1.69 nm, which agrees well with the calculated value of 1.69 nm. The field distributions of the modes TE12,736 and TM12,769 are plotted in the insets of Figs. 3(d) and 3(e), respectively. The Q factors of the modes TE12,736 and TM12,769 were characterized using the wavelength tunable laser scanning around the resonant wavelengths, as shown in Figs. 3(d) and 3(e), respectively. The loaded Q factor of the mode TE12,736 was measured to be (4.3±0.2)×107, indicating an intrinsic Q factor of (5.8±0.3)×107. The mode TM12,769 shows a mode splitting with two loaded Q factors of (5.9±0.3)×107 and (8.0±0.3)×107 under the condition of near critical coupling, which should be caused by the back reflections inside the ring. Considering the transmission rates of modes, the intrinsic Q factors (Qi) are (0.85±0.04)×108 and (1.08±0.04)×108, respectively, agreeing well with the result of the LNOI microdisks^[38]. Remarkably, the highest intrinsic Q factor close to the material intrinsic absorption limit of LN is about one order of magnitude higher than the best-reported values in LNOI microrings^[24,25]. The effective index neff of the mode TM12,769 was determined to be 2.10. The corresponding waveguide propagation loss LP is 0.0034±0.0001dB/cm, which is calculated by Lp=10·α·loge=10·(2πneff/(Qiλ))·loge,(1)where α is the material absorption coefficient, and Qi and λ are the intrinsic Q factor and resonant wavelength of the mode, respectively.

In conclusion, we demonstrate an ultra-high Q microring resonator in LNOI without ion-induced lattice damage by CMP. The intrinsic Q factor above 108 was experimentally demonstrated at the 1550 nm wavelength band, while the waveguide propagation loss is only 0.0034±0.0001dB/cm. This microresonator can be fully integrated with silicon nitride ridge waveguides on chip by vertical coupling^[45] or side coupling^[46] in the future. This ultra-high Q multimode microring can be used as electro-optic tunable photonic devices, such as low-threshold Kerr frequency combs, narrow-linewidth Brillouin lasers^[46], and high-brightness quantum light sources^[19], to name a few. Such a low-loss LNOI ridge waveguide shows great potential for loss-less cascaded electro-optic modulators, high-speed information processing, and large-scale photonic integration on LNOI wafers.