Low-loss Lithium Niobate on Insulator (LNOI) Waveguides of a 10 cm-length and a Sub-nanometer Surface Roughness

We develop a technique for realizing lithium niobate on insulator (LNOI) waveguides of a multi-centimeter-length with a propagation loss as low as 0.027 dB/cm. Our technique relies on patterning a chromium (Cr) thin film coated on the top surface of LNOI into a hard mask with a femtosecond laser followed by the chemo-mechanical polishing for structuring the LNOI into the waveguides. The surface roughness on the waveguides is determined to be 0.452 nm with an atomic force microscope (AFM). The approach is compatible with other surface patterning technologies such as optical and electron beam lithographies or laser direct writing, enabling high-throughput manufacturing of large-scale LNOI-based photonic integrated circuits.


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Recently, a revolutionary approach for building high performance PICs has been 55 emerging enabled by the successful demonstration of high quality lithium niobate on insulator 56 (LNOI) nanophotonic structures. The first experimental proof of this approach is provided by 57 first patterning the LNOI into the designated geometries using a femtosecond laser. The draft 58 structures obtained after the femtosecond laser patterning, which has a relatively high sidewall 59 roughness on the order of tens of nanometers, are then polished with a focused ion beam (FIB) 60 milling to smoothen the sidewall [11]. This concept was soon extended to incorporate with 61 other lithographic technologies such as optical lithography and electron beam writing (EBW) 62 for defining the planar patterns on LNOI substrates followed by reactive ion etching to 63 complete the nanostructuring of the LNOI [12,13]. The initial focus was mainly placed on 64 optical microresonators [11][12][13][14][15][16][17][18][19][20][21], and other devices such as waveguides and photonic crystals

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[22-28] appeared shortly, taking the advantage of high surface smoothness of the sidewalls as 66 a result of the ion dry etching. So far, the propagation loss in the LNOI waveguides has reached 67 0.04 dB/cm which opens the avenue toward large-scale PIC applications [29].

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It is noteworthy that the ion etching step which is necessary for achieving the high quality 69 sidewalls on the LNOI nanophotonic structures inherently leaves a low but non-negligible 70 surface roughness which is difficult to be completely removed [29]. Moreover, the use of FIB

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In our experiment, the LNOI waveguides were produced on a commercially available X-

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The LN thin film with a thickness of 400 nm is bonded to a 2 μm thick SiO2 layer grown on a 91 LN substrate. The fabrication process includes four steps, as schematically illustrated in Figure   92 1. First, a thin layer of chromium (Cr) with a thickness of 600 nm was deposited on the surface

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The CM polishing process was conducted using a wafer polishing machine (NUIPOL802,

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Kejing, Inc.). In the CM polishing process, we used a piece of velvet polishing cloth, and the polishing slurry (MasterMet, 60 nm amorphous colloidal silica suspension) was provided by 119 Buehler, Ltd. The soft velvet cloth allows not only the Cr film but also the exposed LNOI to be 120 accessed by the polishing slurry. Since the Cr film is of a higher hardness than that of the LNOI, 121 the exposed LNOI could be preferentially removed in the CM polishing process before the

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To characterize the propagation loss in the LNOI waveguide, we constructed a whispering

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However, the sidewall roughness on the Cr mask will only be transferred to the underneath 202 LNOI near the top surface, thus it can be completely suppressed with an additional polishing 203 process for thinning the LNOI substrate after the removal of the Cr mask (see, Figure 1d).

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It should be mentioned that the propagation loss obtained by measuring the Q factor of 205 the ring resonator may have been underestimated for the straight segments in the LNOI 206 waveguides as presented in Figure 3d -f due to a higher radiative loss in the ring resonator.

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Ultimately, the propagation loss in the LNOI waveguides is limited by the absorption in 208 crystalline LN which is well known to be on the order of ~10 -3 dB/cm. Our measured loss is still 209 one order of magnitude away from the theoretical limit, which could be attribute to several

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Thus, to realize LNOI waveguides with propagation losses on the order of 10 -3 dB/cm, a lot of 215 refinements should be systematically investigated in the future.

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The fabrication resolution of femtosecond laser direct writing is typically on the order of 217