Fabrication of high-Q lithium niobate microresonators using femtosecond laser micromachining

We report on fabrication of high-Q lithium niobate (LN) whispering-gallery-mode (WGM) microresonators suspended on silica pedestals by femtosecond laser direct writing followed by focused ion beam (FIB) milling. The micrometer-scale (diameter ~82 μm) LN resonator possesses a Q factor of ~2.5 × 105 around 1550 nm wavelength. The combination of femtosecond laser direct writing with FIB enables high-efficiency, high-precision nanofabrication of high-Q crystalline microresonators.

difficult to achieve a Q-factor higher than 10 5 , to the best of our knowledge 14,16 . Our technique opens the new route for fabricating novel crystalline microresonators for on-chip photonics applications.

Results and Discussion
Characterization LN microresonator. The details on the fabrication of LN microresonator can be found in Methods. The thicknesses of the LN thin film and silica layer were 700 nm and 2 mm, respectively. Briefly speaking, we first fabricated a cylindrical post with a diameter of ,59 mm and a total height of ,15 mm (corresponding to a cutting depth of 12 mm into the LN substrate beneath the silica), which is shown in Fig. 2(a), by femtosecond laser micromachining. Since the roughness on the laser ablated sidewall of the cylindrical post is on the order of a few tens of nanometers, which is too poor for high-Q microresonator application, FIB milling was used to smooth the periphery of the cylindrical post. The FIB milling was operated twice, beginning with a coarse milling and followed with a fine one. In the coarse milling, a 30-keV ion beam with a beam current of 4 nA was used to polish the periphery; whereas in the fine milling, the beam current was reduced to 1 nA. The milling was stopped at a depth of 3 mm from the top surface. The total FIB milling process took ,15 min. After the FIB milling, the diameter of the microresonator was reduced to 55 mm. Most importantly, the LN microresonator shows a highly smooth edge, as evidenced in its scanning electron microscope (SEM) image ( Fig. 2(b)). Figure 2(c) shows another cylindrical post with a smaller diameter of 33 mm after the FIB milling. In principle, microresonators of diameters below 10 mm can be fabricated using our technique. It should be noted that the FIB process frequently induces the creation of lattice defects (i.e., vacancies and atomic nuclei), leading to the formation of amorphous material due to keV ion beam side dose or lateral ion straggle at the periphery of the microresonator. However, such defects are not critical in our experiments, since no free carrier is involved in the nonlinear generation process.
To form the freestanding microresonator (i.e., a thin disk sitting on top of a micro-pedestal), the silica layer sandwiched between the LN thin film and the LN substrate needs to be selectively etched to form the micro-pedestal under the LN microdisk. Therefore, the sample was immersed in a solution of 5% HF diluted with water for 8 min to form the silica pedestal. The upper LN microdisk suspended on the pedestal serves as the high-Q microresonator.
At last, a thermal annealing (500uC for 2 hrs in air) was applied to further smooth the rim of the LN thin disk. It should be mentioned that since the top and bottom surfaces of the LN thin film naturally possess an ultra-high smoothness with a surface roughness as low as 0.35 nm 17 , a high Q factor of the LN microresonator can be ensured after smoothing its rim. As shown by the side view of the freestanding microresonator in the inset in Fig. 2(d), an air gap between the LN microdisk and LN substrate can clearly be seen.
To measure the Q-factor of the fabricated LN microresonators, an evanescent fiber taper coupling method was employed (for details, see Methods). The transmitted spectrum measured from the output end of the fiber taper showed a series of sharp dips at the WGM resonant wavelengths. In these measurements, coarse scans over a   wide wavelength span were first performed to decide the WGM resonant wavelengths. As an example, the transmission spectrum obtained with the coarse scan with a step size of 1 nm for the 55 mm LN microresonator without annealing is shown in Fig. 3(a). After the coarse scan, fine scans with a step size of 0.5 pm around the resonant wavelengths were performed to measure the linewidths of the dips indicated by the Lorentzian fit.
The transmission spectra of the fiber taper coupled to the 55 mm microresonators before and after the annealing are shown in Figs. 3 (b)-(c). The resonance at 1554.28 nm wavelength showed a loaded Q factor of 5.2 3 10 4 ( Fig. 3(b)) of the microresonator before the annealing. After the annealing, the Q factor of the same microresonator was significantly improved to 1.6 3 10 5 around the resonant wavelength of 1554.90 nm, as evidenced in Fig. 3(c). Furthermore, the Q factor of the microresonator with a diameter of 82 mm was measured to be 2.5 3 10 5 after the annealing process, as indicated in Fig. 3(d). The result shows that the Q factor increases with the increasing diameter of the fabricated LN microresonator, as a result of radiation loss at the curved surface which has been well described by A. M. Armani et al 22 .
In conclusion, we demonstrate the fabrication of high-Q on-chip LN microresonators on single crystal LN thin film wafer by femtosecond laser 3D micromachining. The Q factor is measured to be 2.5 3 10 5 around 1550 nm wavelength. Since our technique uses high precision ablation of materials with femtosecond laser pulses, it is intrinsically material insensitive. In fact, in the past few years, we have fabricated high-Q optical microresonators in various materials such as fused silica 23 , an active Nd: glass 19 , and CaF 2 crystal 20 . It should be noticed that although femtosecond laser micromachining allows for efficient and precision ablation of dielectric materials, the surface roughness is typically too high for high-Q microresonator applications. The incorporation of FIB milling provides an ideal solution without spoiling the flexibility in femtosecond laser microfabrication. We envisage that our technique can be extended for fabricating high-Q on-chip microresonators on various types of dielectric materials.

Methods
Fabrication of LN microresonator. A Ti: sapphire femtosecond laser source (Coherent, Inc., center wavelength: ,800 nm, pulse width: ,40 fs, repetition rate: 1 kHz) was used for fabricating LN microresonators. A variable neutral density filter was used to carefully adjust the average power. An objective lens with a numerical aperture (NA) of 0.80 was used to produce the tightly focused spot with a diameter of ,1 mm. The laser beam then focused into the LN thin film sample immersed in water. The sample was mounted on a computer-controlled XYZ translation stage with 1-mm resolution. A charged coupled device (CCD) connecting with the computer was installed above the objective lens to monitor the fabrication process in real time.
A layer-by-layer annular ablation from the top surface to internal substrate with 1 mm interval between the adjacent layers was adopted, so that the ablation always occurred at the interface between the water and the material. In this manner, the ablation debris can be more efficiently removed with the assistance of water. The laser power was chosen to be 0.35 mW for ablation in both the LN thin film and the LN substrate beneath the silica layer, whereas the laser power was raised to 1 mW for ablation in silica layer, because the ablation thresholds of LN crystal and silica glass are different.
Measurement of Q-factor. The Q factors were measured by the fiber taper coupling method. An external cavity continuous wave tunable laser diode (New Focus, Model: 6528-LN; output power: 0.08 mW; wavelength range: 1510 nm , 1620 nm; step resolution: 0.1 pm) was coupled into the fiber taper. The fiber taper with a diameter of ,1 mm was home made by heating and stretching a section of a commercial optical fiber (Corning, SMF-28). A transient optical power detector was used to measure the transmission spectra at the output end of the fiber taper. The detector can record the light signal over a 100 nm-wavelength-span with 0.5 pm wavelength resolution and 0.015 dB power accuracy in less than 1 second. A piezo-stage was used to control the relative position between the microresonator and the fiber taper waist. The critical coupling between the WGMs and the evanescent field of the fiber taper can be achieved by carefully adjusting the gap between the fiber taper and the microresonator. We used two CCD cameras whose optical axes are arranged to be perpendicular and parallel to the equatorial plane of the microresonators to simultaneously acquire both the side-view and top-view images of the microresonators coupled with the fiber taper. In the fine scan for measuring the Q factor, a slow scan speed of 2 nm/s was chosen.