Fabrication of high quality factor lithium niobate double-disk using a femtosecond laser

Currently, high-Q WGMs are mostly fabricated with e-beam or photo lithography, thermal reflow as well as mechanical milling and polishing technologies [11][12][13] . The mechanical approach lacks the potential for monolithic integration of multiple WGMs for on-chip applications while the lithography and reflow ones are limited by materials used in WGMs. Therefore, developing efficient fabrication techniques for producing on-chip crystalline WGMs is still challenging due to the incompatibility of a large number of crystalline materials and optical lithography. Nevertheless, it has recently been shown that femtosecond laser micromachining provides a promising approach to fabricating high-Q WGMs on various materials including glasses [14][15][16] , polymers [17,18] , and crystals [19][20][21] . It is noteworthy that the successful demonstrations of high-Q WGMs on crystalline substrates open the door for miniaturized nonlinear optics applications.

In the past few years, significant nonlinear optical phenomena and efficient electro-optic tuning effects have been experimentally demonstrated in lithium niobate (LN) WGMs [22][23][24][25][26] . In addition, the capability of monolithic integration of LN microresonators with various nanophotonic structures has been reported [25][26][27] . To date, only single-layered LN micro-disks have been fabricated using the LN thin film substrate. It is known that silica double-disk is one of the unique structures which displays strong optomechanical effects due to the large optical gradient force provided by the strong interaction of optical fields between the top and bottom disks [28][29][30][31][32] . Here, we fabricate double-disks on LN platform by utilizing a femtosecond laser in combination with focused ion beam as the LN crystal has advantageous nonlinear optical, mechanical and electro-optical properties compared to SiO2. The unique physical properties of LN can influence the optomechanical responses in WGMs and in turn provide opportunities to new findings and applications.

We designed the double-layer X-cut LN thin film substrate as illustrated in Fig. 1(a). The top and bottom LN thin films of 300 nm in thickness, respectively, are separated by a thin layer of SiO2 with a thickness of 200 nm. The double-layer LN thin film is bonded a 2-μm-thick SiO2 substrate, which is bonded to the 500-μm-thick LN substrate. Following our design, the wafer was produced by NANOLN, Jinan Jingzheng Electronics Co., Ltd. A Ti: sapphire femtosecond laser source (Coherent, Inc., center wavelength: 800 nm, pulse width: 40 fs, repetition rate: 1 kHz) was used for fabricating the on-chip double-disk LN microresonator. A variable neutral density filter was used to tune the average power of the laser beam. In the femtosecond laser direct writing process, an objective lens (100× / NA 0.80) was used to focus the beam down to a ~1 μm-diameter focal spot. The sample could be arbitrarily translated in 3D space at a resolution of 1 m using a PC-controlled XYZ stage combined with a nano-positioning stage. A charged coupled device (CCD) connected to the computer was installed to monitor the fabrication process in real time.

The procedures of fabricating the LN double-disk WGM are schematically illustrated in Fig. 1. First, the LN substrate was immersed in water and ablated with tightly focused femtosecond laser pulses, as shown in Fig. 1(a). The ablation in water can help reduce the debris and cracks in the fabricated structure. The height of the cylindrical microstructure patterned with femtosecond laser ablation is ~5 μm, as shown in Fig. 1(b). The femtosecond laser fabrication took about 1 hour. Next, the periphery of the LN double-disk WGM were smoothed using focused ion beam (FIB) milling, as illustrated in Fig. 1(c). In the FIB milling, a 30-kV ion beam with a beam current of 1 nA was used. The FIB milling was completed in 10 min. Finally, chemical wet etching, which selectively removes the SiO2 layers underneath the LN thin films to form freestanding double LN micro-disks, was performed in a solution of 2% hydrofluoric (HF) for 8 minutes, as shown in Fig. 1(e). The SiO2 layer was partially preserved to support the double-disk LN microresonator. The diameter of the LN micro-disk is 30 μm. It took about 1.5 hrs in total to produce the LN double-disk WGM.

The optical micrograph in Fig. 2 2(b), we determine that the thicknesses of top and bottom LN disks are 291 nm and 312 nm, respectively. An air gap of 138 nm is created between the top and bottom disks after the silica layer is partially removed by the chemical etching. In particular, from the left inset in Fig. 2(b), the sidewall of the double-disk displays a tilt angle of

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