Stimulated Brillouin scattering (SBS) is a nonlinear optical process involving the interaction of light waves with acoustic waves in materials and has important applications in both fundamental science and engineering technology. SBS can be used for low-noise single-frequency lasers, high-sensitivity sensors, signal processing in microwave photonics, optical isolation in optical communication, optical storage, and other signal processing technologies.
Utilizing whispering-gallery mode (WGM) optical resonators can achieve low-threshold stimulated Brillouin scattering, or Brillouin lasing. Brillouin lasers have been realized on various material platforms of WGM resonators and successfully applied in narrow linewidth lasers, low-noise microwave signals, and Brillouin laser gyroscopes, becoming a research hotspot in the Brillouin laser field in recent years. However, these studies mainly focus on the application of Brillouin lasers, with little attention paid to the optimization of the Brillouin laser's intrinsic gain bandwidth.
Traditional stimulated Brillouin scattering has a very narrow gain bandwidth, for example, only about ten megahertz for fluoride crystal materials, so this characteristic poses high requirements on the accuracy of Brillouin laser devices and limits their applications. Associate Professor Lin Guoping's research team on crystalline whispering-gallery resonators at School of Science, Harbin Institute of Technology, Shenzhen proposed a scheme to form a wide Brillouin gain by taking advantage of the changing acoustic velocity along the resonator curved surface, for the first time using a quality factor of one billion Z-cut magnesium fluoride (MgF2) whispering-gallery resonators to achieve a wide range of stimulated Brillouin scattering, demonstrating an approximately two orders of magnitude increase in backward Brillouin gain and subsequently followed by the generation of Brillouin Kerr optical frequency combs. The related research results were published in Photonics Research June 2023 issue (Guoping Lin, Jingyi Tian, Tang Sun, Qinghai Song, Yanne K. Chembo. Hundredfold increase of stimulated Brillouin-scattering bandwidth in whispering-gallery mode resonators[J]. Photonics Research, 2023, 11(6): 917).
As shown in Figure 1(a), due to the propagation of light waves at the curved surface of the whispering-gallery resonator, the acoustic waves interacting with them depend on the angle between the position and the crystal axis. Thus, when the light wave travels around one round, the acoustic velocity undergoes periodic changes, and theoretical calculations show that the stimulated Brillouin scattering gain envelope produced by one round of Z-cut magnesium fluoride whispering-gallery resonators is nearly 100 times wider than the traditional crystal envelope. Figure 1(b) shows the experimental measurement results in two Z-cut resonators with rich optical whispering-gallery modes, with the observed wide-range stimulated Brillouin scattering frequency shift covering from 11.73 GHz to 14.47 GHz. As shown in Figure 1(c), the Brillouin frequency shifts with sufficient data reflect the theoretical gain envelope.
As shown in Fig.1(a), the setup consisted of a single-beam optical trap, triaxial position detection and parametric feedback scheme, electric driving, and field measurement circuit. By scanning the electric field distribution between parallel electrodes, the three-dimensional electric field mapping capability of the system was demonstrated, as shown in Fig.1(b). Its measuring spatial resolution depends on the motion amplitude of the nanoparticle in the equilibrium position and the manipulation accuracy of the equilibrium position, which can reach the order of nanometers.
The electric field detection sensitivity is limited by thermal noise and the amount of net charge carried by the nanoparticle. The measured noise equivalent electric intensity at resonant frequency reached 7.5μV⋅cm−1⋅Hz−1∕2 at 1.4 × 10−7 mbar, and the bandwidth within 3 dB above the thermodynamic limit was 1.1kHz, as shown in Fig.1(c). Linearity analysis near resonance shows a linear range of more than 4 orders of magnitude with a largest detectable electric field of 65.5kV/m, which was merely limited by the maximum output of the test equipment.
Figure 1(a) Left: Schematic diagram of stimulated Brillouin scattering at the curved surface of a whispering-gallery resonator; Middle: Theoretical curve of the acoustic velocity change in Z-cut magnesium fluoride whispering-gallery resonators; Right: Theoretical comparison of traditional Brillouin gain and broadened Brillouin gain; (b) Experimental measurement data of stimulated Brillouin scattering in Z-cut magnesium fluoride whispering-gallery resonators; (c) Comparison of experimental data and theoretical curves.
It should also be mentioned that by combining whispering-gallery resonators with crystal materials, the direction of the crystal axis can be adjusted to control the variation of phonon propagation speed in the resonator, thereby achieving the control of SBS gain bandwidth, widening it by a hundred times and reaching a range of several GHz, reducing the accuracy requirements for Brillouin lasers. In addition, this study also demonstrated the generation of Brillouin Kerr optical frequency combs in whispering-gallery resonators by combining them with magnesium fluoride crystals, with the wideband spectrum of the frequency combs spanning more than 120 nm and possessing over 400 frequency lines.
This research presents a new approach and method for efficient control of SBS gain bandwidth, which is not limited to whispering-gallery resonators but can also be applied to other optical devices such as fibers and waveguides. This method provides a new idea and approach to the optical field, with broad application prospects. It has been verified experimentally, proving its feasibility in practical applications.
