通过飞秒激光泵浦探测，在多层二硫化钼与生物水凝胶复合界面上，实现了GHz超高频声波的全光产生与时间分辨探测。进一步，采用频谱分析与理论解析手段，获取了生物水凝胶的声速和杨氏模量等力学参数。研究结果为生物表界面力学参数提供了一种全光无损测量方法，可为基于二维半导体的新型光声换能器构建、生物表界面力学参数的成像和超高时空分辨探测技术发展提供理论和实验参考。

The mechanical properties and parameters of bio-surface/interface are very important in fundamental researches and applications, such as constructing organ-on-a-chip in order to realize vitro culture of human cells and tissues. To acquire mechanical parameters of bio-surface/interface for real human cells, Atomic Force Microscopy (AFM) is usually employed. The young's modulus of human cells' surface/interface can be obtained by measuring the stress and strain of human cells induced by AFM tip. Obviously, this conventional method is invasive, which might not only cause damage of bio-surface/interface such as cell membrane, but also accompany a low speed for sensing and imaging applications. Besides, ultrasound elastography imaging has been developed to obtain three-dimensional distribution of mechanical parameters for bio-tissues. Unfortunately, the MHz ultrasound waves emitted by conventional ultrasound transducer limit its spatial resolution to micrometer. Hence, developing an accurate, in-situ, noninvasive and quantitative measuring method with high spatiotemporal resolutions to evaluate the mechanical performances has been highly desired. In recent years, layered Two-dimensional (2D) semiconductors, such as Transition Metal Dichalcogenides (TMDs) have presented extraordinary fundamental physical properties as well as good biocompatibility. Van der Waals bondings facilitate their facile integrations with other materials, including bio-materials, to form hetero-interfaces. Most importantly, previous studies have shown that GHz Coherent Acoustic Phonon (CAP) oscillations can be generated under femtosecond laser excitations. Although TMDs show great potential as novel optoacoustic transducers, the effective generation of CAP pulse and experimental measurements of mechanical parameters of bio-surface/interface still need further investigations.In this paper, employing layered 2D semiconductors as GHz optoacoustic transducer, we report a new all-optical technique to noninvasively, accurately, and swiftly measure mechanical parameters of bio-surface/interface based on femtosecond laser pump-probe. To demonstrate our technique, 2D optoacoustic transducer/bio-material hetero-interface formed by integrating multilayer MoS₂ samples with PEGDA hydrogels are comprehensively investigated. Initially, through deformation potential mechanism, one femosecond pump pulse absorbed by multilayer MoS₂ can induce GHz CAP oscillations, which is usually called interlayer breathing mode. Then, MoS₂ lattice will periodically strike the surface of the PEGDA hydrogel and a CAP pulse can be emitted into PEGDA hydrogels by interfacial coupling of mechanical energy. Emitted CAP pulse will propagate in PEGDA at the speed of acoustic velocity. Since light speed is about five order of magnitude larger than acoustic velocity, to track the spatiotemproal propagations of emitted CAP pulse, another femtosecond probe pulse is employed. As the propagation of the emitted CAP pulse can induce a strain of PEDGA hydrogel, optical refractive index of PEGDA hydrogel will be changed so that by monitoring the differential reflection of probe laser as function of time delay with respect to the pump pulse, one can record the spatiotemporal propagation of emitted CAP pulse. As a result, the differential reflection signal of the probe laser contains exponential decay component originating from photocarrier relaxation of MoS₂ and the damped oscillation components originating from CAP oscillation of MoS₂ lattice as well as CAP pulse propagation in PEGDA hydrogel. To decouple the signal of CAP pulse propagation in PEGDA hydrogel, a curve fitting procedure is performed. At last, from frequency spectra obtained by fast Fourier transformations of the fitted time-resolved damped oscillation components, two different resonant frequency peaks are obtained. The higher resonant peak centered around 30.0 GHz is corresponding to CAP oscillations of MoS₂ lattice, while the lower resonant peak below 10.0 GHz is caused by spatiotemporal propagation of the emitted CAP pulse in PEGDA hydrogel. Based on the model of Brillouin oscillation for CAP, mechanical parameters, such as acoustic velocity and Young's modulus of PEGDA hydrogel, are calculated. Last but not the least, we investigate five different positions of MoS₂/PEGDA hydrogel interface. The spatial dependence of the mechanical properties of PEGDA hydrogel is discussed. In brief, physical principles, theoretical models, experimental systems, data analysis and calculation methods of the reported all-optical technique have been clearly demonstrated. Our results set a solid foundation for understanding CAP dynamics of hetero-interface, developing novel optoacoustic transducers for bio-surface/interface and realizing interfacial imaging of mechanical parameters with ultrahigh spatiotemporal resolutions.