Photonic integrated circuits (PICs) are essential building blocks for the advancement of data communications and computing technologies, offering unparalleled capabilities for processing and transmitting information with high speed, high integration, low energy consumption, and immunity to electromagnetic interference^[1–9]. On-chip microlasers and amplifiers are critical components of PICs^[10–16]. On-chip microlasers serve as light signal sources that provide optical signals for data transmission and processing, whereas waveguide amplifiers increase the strength of these signals to ensure efficient and reliable operation. Thin-film tantalum pentoxide (Ta2O5) has attracted significant attention as a PIC platform because of its excellent optical properties, such as high refractive index (2.05), low optical loss, broad transparency window (300 nm to 8 µm), high nonlinear refractive index (7.2×10−19m2/W), as well as compatibility with CMOS technology, making it ideal for various integrated photonics applications^[17–20]. Therefore, the thin-film Ta2O5 in photonic devices enables efficient light confinement, enhances light–matter interactions, and improves device performance. To date, thin-film Ta2O5 has been utilized to achieve a supercontinuum spectrum, stimulated four-wave parametric conversion, soliton frequency combs, and all-optical modulators^[21–26]. Additionally, Ta2O5 serves as a good host material for rare-earth ion (REI) doping and introduces unique optical properties, such as tailored emission spectrum, enhanced light–matter interactions, and improved optical amplification capabilities^[27–33]. These thin-film REI-doped Ta2O5 materials have been utilized in the development of integrated lasers and optical amplifiers, highlighting their versatility and promising performance in photonics^[29–33].

In this work, we coated a thin film of erbium-doped Ta2O5 (Er:Ta2O5) on an insulator wafer via the sputtering method and fabricated on-chip microdisk resonator lasers and waveguide amplifiers on a thin-film Er:Ta2O5 via femtosecond direct-laser-writing-assisted chemical mechanical etching. The diameter of the microdisk resonator is 36 µm, and multimode lasing emission is observed at a pump power threshold of 225 µW. The fabricated Er:Ta2O5 rib waveguide is cladded with SiO2 and achieves an on-chip gain of 8.8 dB/cm at a 1550-nm wavelength. Thin-film Er:Ta2O5 microlasers and waveguide amplifiers with enhanced functionality and performance offer exciting opportunities for advanced photonic technologies.

Er:Ta2O5 thin films were prepared via radio-frequency (RF) magnetron sputter deposition (ULVAC SC-200Z). As shown in Fig. 1(a), a mixed Er2O3/Ta2O5 disc (100 mm diameter) was used as a sputtering target, and the concentration of Er2O3 was 1% (mass fraction) (2.5×1020erbiumions/cm3). As shown in Fig. 1(a), the mixed target appears slightly pink, which is caused mainly by erbium doping^[34]. A 4 inch (1 inch = 2.54 cm) 500-µm-thick silicon/2-µm-thick silicon dioxide (SiO2) wafer was placed above the target for depositing erbium-doped tantalum oxide thin films. During the coating process, the wafer substrate was rotated at a constant speed to ensure the uniformity of the coating. The parameters optimized for the deposition are the substrate temperature, distance from the target to the substrate, magnetron power, and ratio of argon (Ar) to oxygen (O2). The deposition of Er:Ta2O5 thin films was carried out in a vacuum chamber pumped to a base pressure of 10−5Pa and backfilled with 80 sccm Ar and 5 sccm O2, and the chamber pressure was maintained at a constant value of 0.2 Pa. The substrate temperature was 200°C. The distance from the target to the substrate was 115 mm. The supplied RF magnetron power was 350 W. After deposition, the films were annealed in a furnace in O2 ambient at 600°C for 10 h to reduce the number of defects and activate Er3+ ions. Figure 1(b) shows a fabricated 4-inch Er:Ta2O5 thin-film wafer. The surface profile of the deposited Er:Ta2O5 film is examined by a thin-film mapper (F54-XY-200), and an average film thickness of 700 nm with a ±70nm fluctuation is achieved.

As shown in Fig. 2, we fabricated a microdisk resonator and waveguide on a thin-film Er:Ta2O5 using femtosecond direct-laser-writing-assisted chemical mechanical etching. The preparation process for the Er:Ta2O5 microdisk resonator and waveguide involves two main steps. Take the preparation of microdisk resonator as an example. Initially, a 600 nm-thick chromium (Cr) film and an approximately 1 µm-thick Er:Ta2O5 layer were deposited sequentially on the SiO2 buffer layer through magnetron sputtering. This yielded an advantageous optical confinement in the Er:Ta2O5 thin film due to the high refractive index contrast ratio (∼0.6) between the SiO2 buffer layer and the Er:Ta2O5 thin film. Subsequently, a spatially selective direct laser writing process was employed to shape the Cr layer into circular patterns through subtractive manufacturing. A femtosecond pulsed laser with a 250 kHz repetition rate and a 190 fs pulse width was used at an average power of 0.08 mW for etching, with the sample positioned on an X–Y air-flotation motorized translation stage moving at 1 mm/s to enable precise patterning with a resolution of approximately 200 nm. The Er:Ta2O5 layer remained intact during the direct laser writing process due to its higher damage threshold power (0.24 mW). Following this, chemo-mechanical polishing (CMP) selectively etched the exposed Er:Ta2O5 thin film, facilitated by Cr’s superior hardness over Er:Ta2O5, enabling the pattern transfer from the Cr layer to the Er:Ta2O5 thin film. Subsequent steps involved chemical etching to eliminate the Cr mask, a secondary CMP process to refine the surface smoothness of the Er:Ta2O5 microdisk, and chemical wet etching to partially undercut the SiO2 beneath the Er:Ta2O5 microdisk, creating a SiO2 pedestal for supporting the suspended Ta2O5 microdisk. Further details on the fabrication of the waveguide can be found in our previous work^[35]. Figures 2(a)–2(c) show optical micrographs and scanning electron microscope (SEM) images of the fabricated suspended Er:Ta2O5 microdisk. The diameter is approximately 36 µm, with a thickness of approximately 700 nm. Notably, the surfaces and sidewalls of the microdisks are very smooth, according to the SEM images. The smoothness of the total internal reflection interface and the sidewalls plays a crucial role in achieving low-threshold laser operation. Additionally, as shown in Figs. 2(a) and 2(c), the Er:Ta2O5 microdisk periphery is separated from the rough boundary of the SiO2 pillar by approximately 12 µm.When the microdisk is too close to the SiO2 pillar boundary, the evanescent field interacts strongly with surface imperfections, leading to scattering losses that degrade the Q factor. By maintaining a 12 µm gap, the intensity of the evanescent field at the pillar boundary decays exponentially, effectively suppressing scattering losses. If the suspended portion of the microdisk becomes too large, collapse of the microdisk may occur. 12 µm represents a balance between minimizing scattering losses and maintaining structural robustness. The average surface roughness of the original Er:Ta2O5 thin film was measured to be 0.29 nm, with the atomic force microscope (AFM) image of the microdisk shown in the inset of Fig. 2(c). The combination of ultrasmooth total reflection interfaces and the significant distance between the periphery of the microdisk and the pillar boundary contributes to the formation of low-threshold multimode lasing within the microdisk. As shown in Figs. 2(e) and 2(f), the fabricated straight 5-mm-long Er:Ta2O5 waveguides have a rib structure with a top width of 1 µm, a bottom width of 5 µm, and an etched thickness of 500 nm. To reduce optical propagation loss, the Er:Ta2O5 waveguides are clad with 1.5-µm-thick SiO2 via PECVD (OXFORD PlasmaPro 100 PECVD) at 300°C. Figure 2(f) is the false color SEM image of the fabricated Er:Ta2O5 waveguide end face; the Er:Ta2O5 core is shown in yellow, and the SiO2 cladding is shown in green.

The experimental setup for characterizing the Er:Ta2O5 microdisk resonator laser is shown in Fig. 3(a). A tunable laser (TLB-6719, New Focus) is used as the pump light source, with a wavelength scanning range from 940 to 985 nm. A fiber polarization controller (PC) is employed to adjust the polarization state of the tunable laser. The light is coupled into the Er:Ta2O5 microdisk resonator through a tapered fiber with a waist diameter of 2 µm to excite the whispering gallery modes (WGMs) in the microdisk. The coupling position between the microdisk and the tapered fiber is adjusted by an XYZ piezoelectric stage with a resolution of 20 nm. An optical microscope imaging system is positioned above the microdisk to monitor the position of the tapered fiber coupled to the microdisk. As shown in Fig. 3(b), the microdisk resonator emits intense green upconversion fluorescence when pumped by a 980-nm laser. A power meter (PM) constantly monitors the power of the input pump light. The signals are coupled out of the microdisk through the same tapered fiber and then sent to an optical spectrum analyzer (OSA: AQ6370D, YOKOGAWA Inc.) for spectrum analysis. The signals are also sent to a photodetector (PD) and an oscilloscope (OSC) to characterize the quality (Q) factor of the Er:Ta2O5 microdisk resonator. To measure the Q factor of the modes, a triangular-wave signal generated by a signal generator (SG) is applied to modulate the tunable laser, enabling precise wavelength scanning near the resonance wavelength in the microdisk resonator. Figures 3(c) and 3(e) display the transmission spectrum of the mode at wavelengths of approximately 951 and 1566 nm. By performing Lorentz fitting of the typical resonant dip in the transmission spectrum, the loaded quality factors (QL) of 5.1×105 at 951 nm and 3.1×104 at 1566 nm were determined. The intrinsic quality factors (Qi) are 1.89×106 at 951 nm and 2.95×105 at 1566 nm. The propagation loss can be calculated as α=−10·lg(e−2πngQiλ0)=10·(2πngQiλ0)·lge,where ng is the group refractive index, λ0 is the resonant wavelength of the mode, and ng is measured as 1.8464 at 951 nm and 1.7327 at 1566 nm. Therefore, we calculated the propagation loss at these two wavelengths as 0.28 dB/cm for the 980 nm band and 1.022 dB/cm for the 1550 nm band. When the microdisk resonator is pumped by a 980-nm laser, Er3+ ions in the Er:Ta2O5 thin film absorb the pump photons and are excited from the ground state (I415/2) to the higher energy state (I411/2). The excited Er3+ ions then undergo nonradiative relaxation to the metastable state (I413/2), where they accumulate because of the relatively long lifetime of this state. This process leads to a population inversion between the I413/2 and I415/2 states. Initially, the excited Er3+ ions in the I413/2 state decay spontaneously to the ground state (I415/2), emitting photons at around 1550 nm. These spontaneously emitted photons have random directions and phases. Some of these spontaneously emitted photons are trapped within the microdisk resonator due to total internal reflection at the boundary between the high-refractive-index Er:Ta2O5 (n≈2.05) and the surrounding low-refractive-index SiO2 (n≈1.45). This confinement enables the photons to circulate within the microdisk, forming WGMs. As the pump power increases, the population inversion grows, and the probability of stimulated emission increases. When a photon circulating in a WGM interacts with an excited Er3+ ion, it stimulates the emission of another photon with the same phase, direction, and wavelength. This stimulated emission process amplifies the light within the microdisk resonator. The optical feedback provided by the WGMs ensures that the light circulates multiple times, leading to further amplification. The laser threshold is reached when the gain from stimulated emission balances the total losses in the microdisk resonator, including scattering, absorption, and coupling losses. In our experiment, the threshold pump power was measured to be 225 µW. Above the threshold, stimulated emission dominates spontaneous emission, leading to coherent laser oscillation. The laser emission is characterized by narrow-linewidth peaks corresponding to the WGMs of the microdisk resonator. Because of the relatively large diameter of the microdisk (36 µm), multiple WGMs can be supported within the gain bandwidth of the Er3+ ions. This results in multimode lasing, as observed in the emission spectrum [Fig. 3(d)], where multiple narrow-linewidth peaks are separated by the free spectral range (FSR) of the resonator. Figure 3(e) shows the emission spectrum collected in the range of 1450 to 1650 nm for different launched pump laser powers. The laser emission is highly multimode, consisting of approximately 14 narrow-linewidth peaks. The spacing between adjacent peaks in the laser emission spectrum is approximately 11 nm, which closely matches the FSR of the Er:Ta2O5 microdisk resonator. As shown in Fig. 3(f), the laser output power is depicted at various launched pump powers, revealing a lasing threshold of 225 µW.

The experimental setup for characterizing Er:Ta2O5 waveguide amplifiers is shown in Fig. 4(a). Two 980-nm LDs are used for bidirectional pumping through lensed fibers to couple into the Er:Ta2O5 waveguide amplifier. A continuously tunable laser (CTL 1550, TOPTICA Photonics, Inc.) generates a tunable laser signal at approximately 1550 nm, which also couples into the Er:Ta2O5 waveguide amplifier via 980/1550 wavelength division multiplexers (WDMs). The polarization states of both the pump and signal lasers are adjusted via FPCs. The output amplified signal spectra were measured by an OSA. Here, two lensed fibers were used to couple the pump light and signal light into the waveguide through butt coupling. The simplified Er3+ energy level diagram in Fig. 4(a) shows a simplified energy diagram of Er:Ta2O5 with the pump at 980 and 1480 nm, as well as the signal at 1550 nm. Figure 4(b) shows an optical image of a pumped Er:Ta2O5 waveguide amplifier chip but coupled with lensed fibers. Figures 4(c) and 4(e) show the measured spectra with different launched pump powers for a signal wavelength of 1550 nm pumped by the two 980-nm LDs and two 1480-nm LDs, respectively. Figures 4(d) and 4(f) show the on-chip gain of the Er:Ta2O5 waveguide amplifier as a function of the launched pump power for a signal wavelength at 1550 nm pumped by the two 980-nm LDs and two 1480-nm LDs, respectively. In both cases, a rapid increase in gain values following increasing pump power is first observed, which is followed by a slow gain saturation at higher pump powers (>15mW). Specifically, the on-chip gain pumped by the 980-nm LD approaches the maximum gain of ∼7.8dB/cm, and the on-chip gain pumped by the 1480-nm LD approaches the maximum gain of ∼8.8dB/cm.

In conclusion, we have successfully achieved on-chip multimode lasers with a threshold of 225 µW and on-chip optical waveguide amplifiers with a gain of 8.8 dB/cm on a thin-film Er:Ta2O5 platform. Silicon-based platforms cannot directly incorporate REI doping within the material. They require bonding with III–V materials, which involves complex and highly demanding processes. Although SiO2 material allows direct REI doping, Ta2O5 offers higher refractive index and better integration capabilities^[10–12]. High-quality thin-film rare-earth doped silicon nitride requires low-pressure chemical vapor deposition and ion implantation processes. Similarly, thin-film rare-earth doped lithium niobate platforms also require smart-cut techniques^[34,36]. In comparison, thin-film Er:Ta2O5 can be deposited using a mixed Er2O3/Ta2O5 disc as a sputtering target, which significantly reduces costs while achieving similar functionality gains. Compared to the previously reported Er:Ta2O5 of Ref. 30, we use a mixed Er2O3/Ta2O5 disc as a sputtering target, whereas Ref. 30 required dual targets with two RF sources. Furthermore, we fabricated Er:Ta2O5 lasers and amplifiers using femtosecond direct-laser-writing-assisted chemical mechanical etching, while previous methods utilized electron beam lithography and ion etching. Our process not only reduces costs but also allows for larger writing fields and higher efficiency.

In the future, further reduction of the optical absorption loss of Er:Ta2O5 will be achieved through coating and annealing, enabling higher-power light sources. Additionally, different REI dopants and the nonlinear optical properties of Ta2O5 can be utilized to realize light sources at other wavelength bands.