Introduction

Narrow linewidth laser sources operating at short/visible wavelengths are highly desirable for achieving the high spectral purity required in precision atomic, molecular and optical (AMO) physics and associated applications1,2,3,4,5,6,7,8,9, including atomic clocks, quantum computing, atomic and molecular spectroscopy, and quantum sensing. Traditionally, such narrow-linewidth lasers have been realized using tabletop systems stabilized to vapor cells or large optical reference cavities10,11,12,13, achieving the ultrahigh spectral purity necessary for probing narrow optical clock transitions in cold atoms. However, as AMO experiments grow in complexity and as efforts intensify toward portable or even autonomous optical clocks, these bulky laboratory-scale systems face critical limitations in size, robustness and scalability. There is a growing demand for compact, reliable short/visible wavelength lasers capable of supporting increasing experimental complexity, including multiple operating wavelengths and a larger number of atoms or molecules involved. Photonic integration offers a promising solution by enabling chip-scale laser systems while improving reliability and reducing sensitivity to environmental disturbances within a chip-scale footprint and with low power consumption14,15,16.

On-chip stimulated Brillouin scattering (SBS) lasers leverage the narrow-bandwidth Brillouin gain of sub-100 megahertz and the ultrahigh optical Q factor of the on-chip resonators to realize ultra-low phase noise performance17,18,19,20,21,22,23,24,25,26,27. These on-chip stimulated Brillouin lasers have been predominantly demonstrated in material platforms which lack second-order nonlinearity χ(2) in the telecom band17,18,19,20,21,22,23,24,25,26,27, where they exhibit ultra-narrow fundamental linewidth lasing and support stable Kerr-Brillouin comb formation. It is noted that the operating wavelengths of stimulated Brillouin lasers could be expanded to visible and mid-infrared spectral ranges28,29. To date, bi-chromatic light emissions spanning both the telecom and short/visible bands are still out of reach in a single on-chip Brillouin laser resonator. Benefited from the wavelength diversity, such a bi-chromatic microlaser with narrow linewidth characteristics would enable advanced sensing and spectroscopic applications. Key bottleneck problems of such bi-chromatic microlaser include the lack of ultrahigh-Q χ(2) resonators in the short/visible band, and the difficulties in satisfying concurrent phase matching for both processes26,30,31,32 and triple-resonance of the pump, SBL, and SHG signals.

In this work, we demonstrate a bi-chromatic Brillouin microlaser operating simultaneously in the telecom and visible bands, realized in an on-chip thin-film lithium niobate (TFLN) microdisk. This Brillouin-quadratic microlaser demonstration relies on advances in precise dispersion engineering and low-loss fabrication of χ(2) microresonator with small mode volume. The suspended TFLN microdisk, with a compact diameter of 117 μm, is dispersion engineered to enable phase-matched Stokes Brillouin lasing (SBL) via strong optomechanical coupling with two orthogonally polarized fundamental cavity modes, while simultaneously generating the second harmonic of the SBL via modal phase match. The designed TFLN microdisk is fabricated with ultra-smooth surface by photolithography-assisted chemo-mechanical etching method33, featuring ultrahigh optical Q factors of 4.0 × 106 in the telecom band and 1.3 × 106 at its second harmonic, facilitating significant linewidth narrowing, reduced lasing threshold, and high-efficiency SHG. Consequently, SBL at 1559.718 nm is demonstrated with a 1.81 mW optical threshold, and a short-term integral linewidth of 254 Hz, under 1559.632 nm optical pump. Meanwhile, SHG of this SBL signal was observed at 779.859 nm with an intrinsic linewidth of 864 Hz and a normalized conversion efficiency of 3.61%/mW. This Brillouin-quadratic microlaser demonstration paves the way toward compact and multifunctional quantum and atomic photonic systems.

Results

Demonstration of visible-light Brillouin-quadratic microlasers in on-chip lithium niobate microresonators

The on-chip TFLN microdisk was fabricated on a Z-cut thin-film lithium niobate on insulator (LNOI) using femtosecond laser photolithography-assisted chemo-mechanical etching (PLACE) technique33. Further details for the fabrication are provided in the Supplementary Note 1. An optical microscope image of the fabricated TFLN microdisk supported by a small silicon dioxide pillar to suppress the leakage of the acoustic modes to the substrate34,35,36, is depicted in the inset of Fig. 1. The TFLN microdisk has a diameter of ~117 μm and a thickness of ~800 nm, dispersion engineered to simultaneously fulfill phase matching and energy conservation conditions for both the backward SBL25 and its SHG (i.e., Brillouin-quadratic microlaser formation) processes, thereby enabling bi-chromatic Brillouin microlaser operation.

Fig. 1: Schematic of the setup for the generation of a Brillouin-quadratic microlaser in the TFLN microdisk.
Fig. 1: Schematic of the setup for the generation of a Brillouin-quadratic microlaser in the TFLN microdisk.The alternative text for this image may have been generated using AI.
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PC polarization controller, VOA variable optical attenuator, SMF single-mode fiber, CCD charge-coupled device, OSA optical spectrum analyzer, PD photodetector, ESA electrical spectrum analyzer, PNA phase noise analyzer. Inset (middle left): Optical microscope image of the fabricated microdisk. Inset (bottom): Power measurement and polarization analysis of the Brillouin-quadratic lasing (BQL) signal scattering from the edge of the microdisk. WGP wire grating polarizer.

The experimental setup for the formation of the Brillouin-quadratic microlaser in the suspended LNOI microdisk resonator is illustrated in Fig. 1. A continuous wave (CW) tunable laser (New Focus Inc., Model TLB6728) in the telecom band was used as the pump laser source to excite the cascaded nonlinear processes in the microdisk resonator. Before the light from the CW laser was launched in the optical circulator, it propagates through an in-line fiber polarization controller (FPC562, Thorlabs Inc.) and an erbium-doped fiber amplifier (EDFA), followed by a variable optical attenuator (VOA) to modify the polarization, and achieve variable pump power. The pump light was then sent into the microdisk resonator via port 2 of the circulator using an optical tapered-fiber with a waist of 2 μm. It should be emphasized that the tapered-fiber was carefully placed on the edge of the microdisk resonator and was in direct contact with the resonator to couple light into and out of the resonator. The coupling position of the tapered-fiber is very crucial for achieving modal phase matching for both SBL and SHG processes26. The generated backward SBL signal was collected by the circulator, and then detected by an optical spectrum analyzer (AQ6370D, Yokogawa Inc.) with 0.02-nm resolution for spectral analysis. And a high-speed photodetector connected with a real-time electrical spectrum analyzer (RAS5115B Tektronix Inc., and FSPN26, Rohde & Schwarz Inc.) was used to measure the beat note microwave signal by beating the pump light and the SBL. The intrinsic linewidth of the SBL was measured by a home-built delayed self-heterodyne interferometer method based on a short-delayed self-heterodyne method37 with a 5.5-km-long optical fiber. As for integral linewidth estimation, short-term linewidth of the backward SBL was further measured with a commercially available laser phase noise analyzer (PNA) based on the correlated self-heterodyne method38 with 1-ms integration time. The details for the measurement of the Brillouin-quadratic lasing signal, including spectroscopic analysis, polarization analysis, power measurement, and linewidth characterization, can be found in the Methods section.

When the pump light injected into the microdisk resonator was tuned to 1559.632 nm and the pump power exceeded the threshold of SBL, two distinct nonlinear phenomena were concurrently observed, including a backward-propagating SBL signal and its SHG signal. The backward-propagating SBL signal was detected at 1559.718 nm, as shown in Fig. 2a. The wavelength interval between the pump light and the SBL signal was ~0.08 nm, corresponding to a Stokes Brillouin shift ΩB of ~10 GHz. And this Brillouin shift ΩB was further confirmed by the detected radio-frequency (RF) beat note microwave signal with finer resolution by means of the optical heterodyne method using the electrical spectrum analyzer (ESA). This RF signal was centered at 10.17-GHz frequency, as shown in Fig. 2b. Moreover, the output power of the backward SBL signal varied with different pump powers was also recorded to characterize the threshold behavior of the backward SBL, as plotted in Fig. 2c. When the pump power exceeds the threshold, the output power of the backward SBL (black dots) increases linearly with the pump power. Through linear fitting (red line), the threshold of the backward SBL is determined to be 1.81 mW, and the conversion efficiency reaches 19.27%. Figure 2d shows the measured frequency noise spectrum obtained by the short-delayed self-heterodyne method37. From the white noise floor Nwfn of 0.06 Hz2/Hz, we obtain an intrinsic linewidth LIL of the SBL as narrow as 0.18 Hz (LIL = Nwfn × π).

Fig. 2: Backward Stokes Brillouin lasing (SBL).
Fig. 2: Backward Stokes Brillouin lasing (SBL).The alternative text for this image may have been generated using AI.
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a Optical spectrum of the SBL signal. b Microwave beat note signal generated by beating the pump wave and the SBL signal. c Output power of the SBL vs. the on-chip pump power. d Frequency noise spectrum of the SBL obtained by the short-delayed self-heterodyne method, showing an intrinsic linewidth of 0.18 Hz. e Frequency noise spectrum of the pump laser obtained by the correlated delayed self-heterodyne method, showing a short-term linewidth of 733 Hz. f Frequency noise spectrum of the SBL obtained by the correlated delayed self-heterodyne method, showing a short-term linewidth of 254 Hz.

Furthermore, the integral linewidth of the SBL was measured by characterizing the frequency noise using the correlated self-heterodyne method24,38, as depicted in Fig. 2f. The white-frequency-noise floor of the backward SBL was quantified as 81 Hz2/Hz, revealing a short-term integral linewidth of 254 Hz. While the short-term linewidth of the pump laser was measured as 733 Hz, as depicted in Fig. 2e, the obtained SBL exhibited a clear linewidth narrowing, which is attributed to the narrow Brillouin gain bandwidth of TFLN, ultrahigh optical Q factors of the microresonator and the long lifetime of the Brillouin phonon17,18,19.

Simultaneously, only one shortwave signal was observed at 779.859 nm with the high spectral resolution 0.02 nm of the optical spectrum analyzer (OSA), as shown in Fig. 3a. This wavelength is precisely equal to the second harmonic of the SBL signal. As a result, this signal was ascribed to the SHG of the SBL signal. The side-view optical microscope image of the microdisk is depicted in the inset of Fig. 3b, showing obvious visible-light emission leaked from both edges of the microdisk. These light spots exhibit spatial high-order mode characteristics. Moreover, the light spots at the left edge are much brighter than the right ones, revealing that the counter-clockwise visible-light signal circulated in the microdisk is much stronger than the clockwise one. The clockwise SHG signal was believed to be presented due to cavity-enhanced Rayleigh scattering in the ultrahigh-Q microdisk39,40. We recorded the output power of the SHG signal by collecting the scattered light from the left edge of the microdisk at different pump powers, so as to determine the power dependence behavior. Figure 3b shows a quadratic dependence of the SHG signal on the SBL power. The measured SHG conversion efficiency (black dots) increases linearly with the SBL power, as shown in Fig. 3b. By linear fitting (red line), the normalized SHG conversion efficiency is determined to be 3.61%/mW. The maximum output power of the SHG signal reaches an output power of 2.8 μW with an absolute conversion efficiency of 1.01% when the SBL power was 0.3 mW (corresponding to an on-chip pump level of ~3 mW). It is worth noting that no SHG signal of the pump light was detected, which was attributed to the absence of phase matching and the difficulty in satisfying the dual-resonance condition.

Fig. 3: Visible Brillouin-quadratic microlaser.
Fig. 3: Visible Brillouin-quadratic microlaser.The alternative text for this image may have been generated using AI.
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a Optical spectrum of the forward SHG of the Brillouin lasing signal by intracavity Rayleigh backscattering. b Conversion efficiency of the SHG as a function of the on-chip SBL power. Inset: Side-view optical microscope image of the scattered light spots from both the edges of the microdisk, showing high-order spatial modes, with the dashed box denoting the cross-sectional profile of the microdisk (the vertical dimension of the box is enlarged by 6.8 times). c Frequency noise spectrum of the reference narrow-linewidth fiber laser obtained by the correlated delayed self-heterodyne method, showing a white noise level of 2.9 Hz2/Hz. d Frequency noise spectrum of the Brillouin-quadratic laser heterodyned with the reference fiber laser, showing a white noise level of 275 Hz2/Hz.

Measurement of the intrinsic linewidth of the visible-light Brillouin-quadratic microlaser

According to the linewidth theory, the linewidth of the SHG generated by a laser should be twice that of the pump laser41. The theoretically estimated short-term linewidth of the Brillouin-quadratic laser should be approximately 508 Hz. Direct linewidth measurement of the Brillouin-quadratic laser has been conducted by heterodyning this laser signal with a tunable narrow-linewidth reference fiber laser to generate an RF beat note signal to validate this estimation. The frequency noise of the reference laser was measured by the correlated delayed self-heterodyned method38 using a 780-nm commercially available laser phase noise analyzer, exhibiting a white noise level of only 2.9 Hz2/Hz, as shown in Fig. 3c. The measured frequency noise of the RF beat note signal indicates a white-noise floor of 275 Hz2/Hz, which is much larger than that of the reference laser, as plotted in Fig. 3d. As a result, the frequency noise of the RF beat note signal is mainly contributed from the Brillouin-quadratic laser, and the intrinsic linewidth of the Brillouin-quadratic laser was determined as 864 Hz, which is closed to the theoretically estimated value. This result robustly confirms the ultra-narrow linewidth of the visible laser.

Measurement of the mode structures of the microresonator

To evaluate the mode structures of the microdisk, two continuously tunable lasers around 1560 and 780 nm wavelengths were used for exciting the involved modes by scanning the wavelength cross the modes participated in the visible Brillouin-quadratic lasing, respectively. The pump power injected into the microdisk resonator was set as low as 5 μW, and the transmission spectra of the microdisk resonator were obtained, as shown in Fig. 4. The transverse-electric (TE) pump mode and the transverse-magnetic (TM) SBL mode are depicted in Fig. 4a, showing a wavelength of 0.086 nm, agreeing well with the aforementioned results. The Lorenz fitting (red curves) shows that the loaded Q factors of the pump mode and the SBL mode were determined to be 4.00 × 106 and 3.27 × 106, as shown in Fig. 4b, c, respectively. Meanwhile, the mode structure around the SHG mode of the Brillouin-quadratic lasing signal is shown in Fig. 4d. There is actually a high-order mode resonant with the SHG of the Brillouin-quadratic lasing signal at 779.859 nm, and the loaded Q factor was measured as high as 1.35 × 106. The ultrahigh optical Q factors guarantee a considerable increase in the intracavity built-up pump power within the small microdisk and boost high conversion efficiencies, which are crucial for low-threshold SBL and efficient Brillouin-quadratic laser generation. And the influence of the relatively rough silicon dioxide pillar supporting the microdisk on the SBL is discussed in the Supplementary Note 2 (Supplementary Fig. 1). It is worth noting that there is no mode resonant with the SHG of the pump light or the sum-frequency generation of the pump light and the Brillouin lasing signal.

Fig. 4: Mode structures of the microdisk.
Fig. 4: Mode structures of the microdisk.The alternative text for this image may have been generated using AI.
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a Transmission spectrum of the pump mode and the SBL mode. b Loaded Q factor of the pump mode. c Loaded Q factor of the SBL mode. d Transmission spectrum around 779.86 nm, with the red box denoting the SHG mode of the SBL signal, and there is no mode resonant with the SHG of the pump light.

Analysis of phase-matched Brillouin-quadratic lasing in the microresonator

It is necessary to reveal phase matching schemes for the Brillouin-quadratic lasing process. Here, the cross-polarized SBS process involved with the fundamental TM optical mode (TM0,421 as the pump mode), fundamental TE optical mode (TE0,−454 as the SBL mode) and shear mechanical mode, was leveraged in the small microdisk, ensuring a strong optomechanical coupling rate26. The subscript numbers (n, m) of the modes denote the radial-mode number and the azimuthal-mode number, respectively. The mode distributions of the pump mode and the backward-propagating SBL mode were obtained through numerical simulations, as shown in Fig. 5. Moreover, we also performed numerical simulations showing the optical field distribution of the Brillouin-quadratic lasing mode, which is determined to be TM4,-908 mode, as illustrated in Fig. 6. This cross-polarized configuration of the SHG process could utilize the second-order nonlinear coefficient of d31 of lithium niobate. And the SHG process of the pump light, and the sum-frequency generation of the pump light and the Brillouin lasing signal are prohibited due to the absence of double-resonance and triple-resonance conditions, respectively.

Fig. 5: Electric field distribution of the optical modes.
Fig. 5: Electric field distribution of the optical modes.The alternative text for this image may have been generated using AI.
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Electric field distribution of the optical modes for a the pump light (TM0-polarized, m = 421 @ 1559.632 nm, where 421 is the azimuthal-mode number) and b the SBL modes (TE0-polarized, m = −454 @ 1559.718 nm). The distributions of the electric field components, \({E}_{r}\) and \({E}_{z}\), are shown for both the pump and the SBL modes.

Fig. 6: Optical mode profile distribution of the Brillouin-quadratic lasing mode.
Fig. 6: Optical mode profile distribution of the Brillouin-quadratic lasing mode.The alternative text for this image may have been generated using AI.
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(TM4-polarized, m = −908 @ 779.859 nm). Normalized electric field components a \(E\), b \({E}_{r}\) and c \({E}_{z}\) of the optical mode.

Discussion and conclusion

Conventionally, on platforms such as Si3N4 and SiO2 that lack second-order nonlinearity, SBL and its SHG have to be realized separately. The comparison of on-chip SBLs is shown in Supplementary Note 3 (Supplementary Table 1). This integration of SBL and SHG within a single compact TFLN platform surpasses traditional setups by eliminating separate optical components, which significantly reduces system complexity, cost, and alignment challenges and enables more versatile and power-efficient nonlinear frequency conversion.

Another key advantage lies in the spectral purity and narrow linewidth characteristics of the dual-wavelength outputs. The SBL process inherently delivers ultra-narrow linewidths, giving rise to the SHG in the visible-light spectrum with similarly high purity (~864 Hz). Notably, achieving SHG with a narrow linewidth is impractical without leveraging the high spectral purity of SBL as the fundamental wave. Moreover, this narrow-linewidth synergy boosts the SHG efficiency through high spectral purity. Benefited from the wavelength diversity, such a bi-chromatic laser with narrow linewidth characteristics would enable various applications, particularly for simultaneous monitoring of multiple parameters or targets42,43. For example, in biomedical imaging, the infrared SBL signal enables deeper penetration into materials or tissues, while the visible SHG signal captures surface-level optical responses, facilitating discriminative and interference-free measurements. In addition, in comparison to the generation of visible-light SBL by direct optical pump28, here, since the frequency interval between the pump light and Brillouin-quadratic signal is large, the separation of these two signals becomes much easier.

As a proof-of-concept demonstration of the Brillouin-quadratic microlaser, a suspended LNOI microdisk coupled with a tapered-fiber nanowaveguide was employed so far. However, there is strong potential for full integration. For instance, the Brillouin-quadratic laser could be realized using an unsuspended LNOI microdisk side-coupled to a bus waveguide, as demonstrated in recent work44. This approach would enable monolithic integration without compromising performance. To further enhance linewidth narrowing and phase noise performance, the pump laser should be replaced with a stabilized source employing the Pound-Drever-Hall (PDH) technique, which improves pump stability and reduces phase noise transferred to the Brillouin laser17. Moreover, elevating the intrinsic optical Q factors of the resonators to the order of 108 would further optimize performance18,45. These advancements could position our approach at the forefront of ultra-stable laser design, facilitating the realization of on-chip bi-chromatic ultra-stable lasers.

In summary, benefiting from the χ(2) nonlinearity and strong photon-phonon interaction along with careful dispersion engineering, a Brillouin-quadratic microlaser is demonstrated in the high-Q LNOI microdisk resonator. The backward SBL is generated with a record-low threshold of only 1.81 mW on the TFLN platform so far26,27, accompanied by efficient SHG with a normalized conversion efficiency of 3.61%/mW. By demonstrating the coexistence of Brillouin lasing and its SHG with high spectral purity within a single high-Q LNOI microresonator, we establish an efficient paradigm for the design of efficient and compact nonlinear photonic systems, which facilitates the significant reduction of system complexity and enables diverse photonic applications.

Methods

Measurement of the Brillouin-quadratic laser

Since the limited detectable bandwidth of the circulator, the SHG of the backward SBL does not propagate through the circulator. Consequently, it is difficult to detect the spectrum of the backward propagating SHG directly. However, because of strong Rayleigh backscattering of the backward SHG signal circulating within the high-Q microdisk39,40, there was a weak forward propagating SHG signal, and was detected by the OSA. Meanwhile, an objective lens with a numerical aperture of 0.25 was used to collect the scattered SHG from the edge of the microdisk for power measurement. Optical filters and an optical polarizer were inserted before a power meter for blocking the infrared signals and polarization analysis, respectively. To measure the intrinsic linewidth, the Brillouin-quadratic laser was heterodyned with a narrow-linewidth reference laser with a 3 dB fiber-coupler and a fast visible-light photodetector connected with the real-time ESA. A benchtop 780-nm tunable fiber laser with a short-term linewidth of 9 Hz was used as the reference laser. An optical polarization controller was used to control the polarization state of the fiber laser to be the same as Brillouin-quadratic laser. When the wavelength interval between the tunable fiber laser and the Brillouin-quadratic laser was tuned within the bandwidth of the photodetector (125 MHz), an RF beat note signal would appear in the real-time ESA. The frequency noise of the RF beat note signal was analyzed.