Lithium niobate microring with ultra-high Q factor above 108

Renhong Gao; Ni Yao; Jianglin Guan; Li Deng; Jintian Lin; Min Wang; Lingling Qiao; Wei Fang; Ya Cheng

doi:10.3788/COL202220.011902

Abstract

We demonstrate integrated lithium niobate (LN) microring resonators with

Q

factors close to the intrinsic material absorption limit of LN. The microrings are fabricated on pristine LN thin-film wafers thinned from LN bulk via chemo-mechanical etching without ion slicing and ion etching. A record-high

Q

factor up to

10^{8}

at the wavelength of 1550 nm is achieved because of the ultra-smooth interface of the microrings and the absence of ion-induced lattice damage, indicating an ultra-low waveguide propagation loss of

\sim 0.0034 dB / cm

. The ultra-high

Q

microrings will pave the way for integrated quantum light source, frequency comb generation, and nonlinear optical processes.

Keywords

lithium niobate microcavities waveguide

1. Introduction

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 ( $χ^{(2)} = 30 pm / V$ )^[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 $1.23 \times 10^{8}$ 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 $0.0034 \pm 0.0001 dB / cm$ 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 $0.8 \times 10^{8}$ , corresponding to an intrinsic $Q$ factor of $10^{8}$ . Remarkably, the microring loss is close to the intrinsic material absorption limit of LN crystals.

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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. [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.

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 $Q$ 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].

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 $x y z - θ_{x} - θ_{y}$ 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^[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 ${TE}_{12, 736}$ and ${TE}_{12, 737}$ is 1.69 nm, which agrees well with the calculated value of 1.69 nm. The field distributions of the modes ${TE}_{12, 736}$ and ${TM}_{12, 769}$ are plotted in the insets of Figs. 3(d) and 3(e), respectively. The $Q$ factors of the modes ${TE}_{12, 736}$ and ${TM}_{12, 769}$ 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 ${TE}_{12, 736}$ was measured to be $(4.3 \pm 0.2) \times 10^{7}$ , indicating an intrinsic $Q$ factor of $(5.8 \pm 0.3) \times 10^{7}$ . The mode ${TM}_{12, 769}$ shows a mode splitting with two loaded $Q$ factors of $(5.9 \pm 0.3) \times 10^{7}$ and $(8.0 \pm 0.3) \times 10^{7}$ 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 ( $Q_{i}$ ) are $(0.85 \pm 0.04) \times 10^{8}$ and $(1.08 \pm 0.04) \times 10^{8}$ , 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 $n_{eff}$ of the mode ${TM}_{12, 769}$ was determined to be 2.10. The corresponding waveguide propagation loss $L_{P}$ is $0.0034 \pm 0.0001 dB / cm$ , which is calculated by $L_{p} = 10 \cdot α \cdot \log e = 10 \cdot (2 π n_{eff} / (Q_{i} λ)) \cdot \log e,$ (1)where $α$ is the material absorption coefficient, and $Q_{i}$ and $λ$ are the intrinsic $Q$ factor and resonant wavelength of the mode, respectively.

4. Conclusion

In conclusion, we demonstrate an ultra-high $Q$ microring resonator in LNOI without ion-induced lattice damage by CMP. The intrinsic $Q$ factor above $10^{8}$ was experimentally demonstrated at the 1550 nm wavelength band, while the waveguide propagation loss is only $0.0034 \pm 0.0001 dB / cm$ . 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.

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