High-quality-factor Optical Microresonators Fabricated on Lithium Niobite Thin Film with an Electro-optical Tuning Range Spanning Over One Free Spectral Range

We demonstrate high quality (intrinsic Q factor ~2.8×106) racetrack microresonators fabricated on lithium niobate (LN) thin film with a free spectral range (FSR) of ~86.38 pm. By integrating microelectrodes alongside the two straight arms of the racetrack resonator, the resonance wavelength around the 1550 nm can be red shifted by 92 pm when the electric voltage is raised from -100 V to 100 V. The microresonators of the tuning range spanning over a full FSR is fabricated using photolithography assisted chemo-mechanical etching (photolithography assisted chemo-mechanical etching, PLACE).


Introduction
Whispering gallery mode (WGM) optical microresonators play a crucial role in both photonic research and applications owing to the strong confinement of light fields resulting from the characteristic high quality (Q) factors [1]. Currently, most optical microresonators Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 28 July 2020 doi:10.20944/preprints202007.0685.v1 fabricated on chip have a footprint below 1 mm. Such small microresonators can be fabricated on various types of materials such as silica [2], semiconductors [3], crystals [4], and polymers [5] using lithographic techniques. The integrated microresonators have enabled a broad range of functionalities including filtering, wavelength conversion, sensing, optomechanics, and comb generation [6,7]. However, for some applications such as Brillouin lasers [8,9], opticalfrequency synthesizer [10], microwave photonics [11], spectroscopy [12], gyroscope sensors [13], optical atomic clock [14], and high-resolution spectrometers [15], WGM microresonators with perimeters up to centimeter level are desirable. Fabrication of such large microresonators using lithographic techniques is challenging. Usually, ultra-violet (UV) lithography cannot directly define such large microresonators, thus stitching is frequently employed to meet the size requirement [16]. Nevertheless, stitching can inevitably cause fabrication errors, which will spoil the Q. On the other hand, electron beam lithography (EBL) and focused ion beam (FIB) milling are both low-throughput fabrication techniques as compared to optical lithography technology [17,18]. It is time consuming to fabricate the large WGM microresonators using the EBL and FIB although high fabrication resolutions are readily achievable with the two techniques. For these reasons, advanced lithographic techniques should be developed to meet the requirements on both fabrication resolution and efficiency.
Here, we demonstrate high quality optical microresonators fabricated on lithium niobate (LN) thin film with an electro-optical (EO) tuning range spanning over one free spectral range.
The advantage of choosing LN as the substrate material is the strong electro-optic property. In particular, our fabrication technique based on photolithography assisted chemo-mechanical etching (PLACE) allows to define the mask pattern of a racetrack WGM resonator with a footprint size of 1.2 cm×5.2 cm in only ~1 hr. The fabricated device shows an intrinsic Q factor of 2.8×10 6 . We also integrate microelectrodes alongside the two straight arms of the racetrack resonator. We examine the EO tunability and tuning range of the microresonator. We observe that when the electric voltage is raised from -100 V to 100 V, the resonance wavelength around the 1550 nm can be red tuned by 92 pm, which exceeds the free spectral range (FSR~86.38 pm) of the fabricated microresonator.

Materials and Methods
In our experiment, the on-chip LN racetrack resonator integrated with Cr electrodes was fabricated on a commercially available X-cut LN on insulator (LNOI) wafer (NANOLN, Jinan Jingzheng Electronics Co. Ltd.). The LN thin film with a thickness of 900 nm was bonded onto a silica layer with a thickness of ~2 µm, and the silica layer was grown on a 0.5 mm-thick LN substrate. A 600-nm-thick chromium (Cr) film was further deposited on the top surface of LNOI by magnetron sputtering. The fabrication process includes four steps, as illustrated in Fig. 1. Firstly, the Cr film on the LNOI sample was patterned into a stripe-shaped mask using space-selective femtosecond laser (PHAROS, LIGHT CONVERSION Inc.). Subsequently, the chemo-mechanical polishing (CMP) process was performed to fabricate the LN waveguide using a wafer polishing machine (NUIPOL802, Kejing Inc.). The LN thin film protected by the Cr mask was preserved after the CMP process whereas the remaining LN in the opening area was completely removed. This step allows to create LN waveguides with extremely smooth sidewalls, ensuring high Q factors for the fabricated microresonators [19][20][21]. Next, the femtosecond laser ablation was carried out again to remove the Cr mask left behind on the LN Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 28 July 2020 doi:10.20944/preprints202007.0685.v1 waveguides. Finally, a post-CMP process was performed for thinning the LN disk and smoothing the top surface. In the current experiment, the fabricated ridge waveguides have a top width of ~2 μm, and the Cr electrodes on the two sides of the waveguide are separated by a distance of ~20 μm. It takes about 1 hr in total to produce the whole racetrack microresonator as shown in Fig. 2(a). The microelectrodes were fabricated by patterning the Cr film using the femtosecond laser micromachining.        Fig. 3(c). In this case, the spatial mode profiles in the two waveguides can most efficiently overlap. Thus, as the fundamental mode is excited in the upper waveguide with the lensed fiber, the same fundamental mode will be excited in the racetrack resonator as well. As shown in Fig. 4(b), the FSR of the microresonator was determined to be 86.38 nm. One of the WGM at the resonant wavelength of 1547.93 nm was chosen for the measurement of the loaded quality factor QL. By fitting with the Lorentz function, QL is determined to be 1.4×10 6 , which corresponds to an intrinsic quality factor Qi~2.8×10 6 as evidenced by the critical coupling characteristic (i.e., the very deep dips in the transmission spectrum) in Fig. 4(b).  The linearly fitting reveals an electrical tuning efficiency of ~0.46pm/V and indicates that the tuning range spans over a full FSR.

Results
Finally, we carried out the real-time tuning of the racetrack microresonator by adding a tunable electric voltage on the Cr microelectrodes. Figure 6(a) shows a group of transmission spectra recorded near the 1550 nm wavelength at various electric voltages in the range of -100 V and +100 V with a voltage tuning step of 20 V. We observe that by increasing the electric voltage from -100 V to +100 V, the resonance wavelength can be continuously red tuned by ~92 pm, which spans over one FSR (~86.38pm). The fitting line in Fig. 6(b) indicates a linear dependence of the resonance wavelength on the applied electric voltage and a tuning rate of ~0.46 pm/V.

Discussions
This is the first attempt of fabricating a large WGM microresonator on LNIOI using the PLACE fabrication technique. Before, we have shown that freestanding microdisks of diameters of ~100 μm fabricated on the LNOI substrates using the PLACE technique can easily Preprints (www.preprints.org) | NOT PEER-REVIEWED | Posted: 28 July 2020 doi:10.20944/preprints202007.0685.v1 achieve Q factors above 10 7 [19,21]. Here, the racetrack microresonator shows a Q factor nearly one order of magnitude lower. Most likely, this is caused by the scratches and defects which can be more easily generated on large photonic structures during the chemo-mechanical polishing. This suggests that more stringent control on the polishing and sample cleaning processes is necessary for maintaining the high Qs in the fabrication of the large microresonators. In principle, the sub-nanometer surface roughness provided by the PLACE fabrication technique is sufficient to support Q factors above 10 7 for the large microresonators.
We also notice that the EO tuning efficiency of 0.46 pm/V is lower in comparison with the results reported in the recent literatures [22,23]. This is because the waveguide designed in this work has been optimized for dispersion engineering, which features a relatively broad width at the bottom of the waveguide (~7 μm). Thus, the two electrodes on the two sides of the waveguides are separated from each other by ~20 μm, which is the main cause of the relatively low tuning efficiency. However, in the current work, our goal is to achieve the large tuning range covering the full FSR but not the high-speed EO modulation. The demonstrated tuning efficiency is sufficiently high for such application.

Conclusions
To conclude, we have demonstrated high Q optical microresonators fabricated on LN thin film with an EO tuning range spanning over a full FSR. The EO tunable racetrack resonator of the perimeter above 1 cm features an FSR of ~86.38 nm, an intrinsic Q factor of ~2.8×10 6 , and an EO tuning efficiency of 0.46 pm/V. The device provides a technological platform for a range of applications such as filtering, microwave photonics, sensing, and information processing.