An intense desire for high-performance photonic integrated devices such as electro-optic modulators^[1–3], optical frequency conversion^[4–11], quantum light sources^[12–15], optical frequency combs^[16–22], and ultrafast pulse generation^[23], rapidly motivates the photonic integration platforms to lithium-niobate-on-insulator (LNOI) wafer^[24–27], owing to the outstanding material properties of lithium niobate, such as a broad transparency window (350 nm to 5 µm), a large linear electro-optic, second-order nonlinear, acoustic-optic, and piezo-electric coefficients^[24–28]. All the above-mentioned applications have been successfully demonstrated on passive LNOI wafers with unparalleled performance due to the accessible highly confined photonic structures with ultralow loss due to the rapid developments in ion-slicing technique and LNOI nanofabrication technology. The development of scalable photonic integrated circuits also raises the interest in doping rare-earth ions into LNOI photonic structures to add functionalities enabled by the active ions, for example, microlasers^[29–34], optical waveguide amplifiers^[35–37], and quantum photonic devices^[38]. Among these devices, single-mode microlasers have been demonstrated in erbium ion-doped microcavities^{[29–34,39–41]}, serving as coherent light sources operating in the telecom waveband. A threshold as low as 25 µW has been reported^[30,42], and the highest conversion efficiency reached 1.8×10−3^[33]. However, there is still plenty of room for improvement in the conversion efficiency and threshold of single-mode microlasers.

To improve the optical gain of erbium ions, ytterbium ions have been often used as sensitizers to enhance the excitation rates as well as to reduce the concentration quenching of erbium ions via resonant energy transfer, facilitating an improved pumping efficiency in the 980-nm band^[43,44]. Due to the difficulty in the growth of the co-doped lithium niobate crystal and the fabrication of co-doped thin-film LNOI, erbium and ytterbium ions have only recently been co-doped in LNOI devices, leading to multimode microlasers of low threshold^[45] and optical waveguide amplifiers with a net small-signal gain as high as 27 dB^[46]. However, because there are a large number of whispering gallery modes (WGMs) within optical gain bandwidth, single-mode microlasers have not been reported on the co-doped LNOI platform.

In this work, single-mode microlasers are demonstrated in an erbium, ytterbium co-doped LNOI microdisk with a diameter of 40 µm. Weak perturbation was introduced into the circular microdisk to organize the traditional WGMs as polygon modes^[30,47]. When the polygon mode in the weakly perturbed microdisk was excited around 980 nm wavelength, a single-mode lasing signal was observed at 1530 nm wavelength, benefiting from the significant suppression of the large number of WGMs within the gain bandwidth. A laser threshold as low as 1 µW was measured, which is enabled by the high optical gain of co-doping and the excitation of a high-Q polygon mode. The conversion efficiency reaches 4.06×10−3. Both these values are the state-of-the-art results in single-mode active LNOI microlasers in the telecom band^[42].

The fabrication of erbium-ytterbium co-doped Z-cut LNOI microdisks begins from the growth of the co-doped lithium niobate bulk crystal using the Czochralski method. The doping concentrations of 0.1% (erbium) and 0.1% (ytterbium) were adopted to provide high optical gain. Then ion implantation, silica deposition, bonding, chemomechanical polishing (CMP), and annealing were carried out to produce the co-doped thin-film LNOI wafer. The LNOI wafer consists of a 500-nm-thick co-doped lithium niobate thin film, a 2-µm-thick silica layer, and a 500-µm-thick silicon handle. The microdisks were fabricated on the co-doped LNOI wafer by the home-built photolithography-assisted chemomechanical polishing (PLACE) technique^[48]. To fabricate the microdisk by PLACE, first, a chromium (Cr) layer with a thickness of 200 nm was deposited on the surface of LNOI by the magnetron sputtering method. Subsequently, a microdisk-shaped pattern was produced in the Cr layer using spatial selective femtosecond laser ablation with a resolution of ∼200nm, which serves as a hard mask in the next etching step. Next, the CMP process was conducted to etch the exposed lithium niobate underneath the Cr mask. Therefore, the pattern was transferred from the Cr layer to the thin-film lithium niobate layer. Third, the sample was immersed in a Cr etching solution for 20 min to remove the Cr layer. Fourth, a secondary CMP process was carried out to improve the smoothness of the fabricated LNOI microdisk. Finally, the silica underneath the lithium niobate microdisk was partially undercut by chemical wet etching. The scanning electron microscopy (SEM) images of the fabricated microdisk are shown in Fig. 1, where the diameter of the microdisk was ~44 µm.

To demonstrate a single-mode microlaser from polygon modes, the Q factors of the pump mode and lasing mode were characterized. The experimental setup for Q-factor measurement and lasing is illustrated in Fig. 2(a). A tunable laser (TLB-6719, New Focus Inc.) with linewidth <200kHz connected with an inline polarization controller (PC) was used as the pump light, allowing a large wavelength scanning range from 940 to 985 nm. The pump light was coupled into the microdisk through a tapered fiber with a waist of 2.0 µm. The tapered fiber was placed in close contact with the top surface of the circular microdisk at the position that was ∼18.5µm far from the disk center to introduce weak perturbation for the formation of polygon modes resonant with both the lasing and pump wavelengths. The coupled position was controlled by a 3D piezo-electric stage with a resolution of 20 nm. A microscope imaging system consisting of an objective lens with numerical aperture of 0.28 and a charge-coupled device was mounted above the weakly perturbed microdisk to monitor and capture the optical field distribution in the microdisk. The generated signal was coupled out of the microdisk by the same tapered fiber, and separately sent to an optical spectrum analyzer (OSA, AQ6370D, Yokogawa Inc.) and a photodetector (1611-FC, New Focus Inc.) connected with an oscilloscope (MDO3104, Tektronix Inc.) for optical spectrum analysis and transmission spectrum measurement, respectively. To measure the Q factor of the pump polygon mode, a ramp signal was sent to the tunable laser for fine wavelength scanning across the resonant wavelength. A weak input power as low as 0.5 µW was chosen to avoid the thermal-optic effect and lasing. And the transmission spectrum of the tapered fiber was recorded by the oscillation during wavelength scanning. To measure the Q factor of the lasing polygon mode, another tunable laser (TLB-6728, New Focus Inc.) replaced the above-mentioned laser, working in the telecom band. Once the pump wavelength was adjusted to resonant with the polygon pump mode with above-threshold pump power, lasing signals were detected and recorded by the OSA.

Sharp dips will appear in the transmission spectrum when the scanning wavelength is resonant with the modes. The Q factors of the pump and lasing polygon modes are plotted in Figs. 2(b) and 2(c), respectively. The spatial distribution of the upconversion fluorescence of the pump light is plotted in the inset of Fig. 2(a), showing a hexagon pattern. Since the upconversion fluorescence is excited by the pump light, it records the optical intensity distribution of the pump mode, showing that the pump mode possesses hexagon-patterned spatial distribution^[30]. And under polygon mode pumping, only the polygon modes other than the WGMs possess enough gain for lasing^[30], since the spatial mode overlap between the polygon lasing modes and the polygon pump mode is much higher than that of lasing WGMs. The loaded Q factor QL of the pump mode at 974.79 nm was measured as 4.79×105 through Lorentz fitting. The coupling efficiency of the pump light was determined as 22.1%. The loaded Q factor QL of the lasing mode at 1531.40 nm was measured to 1.28×106 through Lorentz fitting. The coupling efficiency of the pump light was determined to be 40.0%. Compared with ultrahigh Q-loaded factors >107 in the cold cavity^[30], the co-doping of erbium and ytterbium ions will increase absorption loss and degrade the crystal quality, resulting in a reduction of the loaded Q factors. In spite of this, both the polygon pump and lasing modes possess high enough Q factors, resulting from the ultrasmooth surface of the fabricated microdisk and the very weak perturbation.

A stable single-mode lasing signal was observed at 1531.40 nm when the microdisk was under optical pump at 974.79 nm wavelength with on-chip pump power higher than 1 µW, as shown in Fig. 3. The evolution of the output power of the microlaser under different pump powers is plotted in Fig. 3(b), showing a linear growth. The saturated output power of the microlaser reaches 0.977 µW, corresponding to the side mode suppression ratio of 32.4 dB when the pump power was more than 6 mW. Figure 3(d) plots the output power of the microlaser as a function of the pump power. The output power of the microlaser grows linearly with the increasing pump power, which agrees well with the nature of lasing. The threshold was determined to be ∼1.04µW, which is one order of magnitude lower than the best results reported in single-mode LNOI microlasers^[30,42]. A conversion efficiency as high as 4.06×10−3 was measured. Both these two values are the best results reported in single-mode active LNOI microlasers, benefiting from the erbium, ytterbium ions co-doping and the excitation of high-Q polygons, which provide high spatial mode overlap between the pump and lasing modes.

To conclude, we have demonstrated a stable single-mode microdisk laser in the telecom band with an ultralow threshold based on an erbium-ytterbium co-doped LNOI chip. Thanks to the erbium-ytterbium co-doping providing higher optical gain, the ultralow loss nanostructuring by PLACE, and the excitation of high-Q polygon modes allowing high spatial mode overlap and suppression of multimode lasing^[30,47,49], a threshold as low as 1 µW was achieved. A conversion efficiency up to 4.06×10−3 was reported at room temperature. Such a low-threshold single-mode microlaser will promote the development of the scalable photonic integrated circuits on LNOI platforms^[50–52].