Method And Setup Development For Imaging The Vibration Mode Of Two-dimensional Membrane

Since the discovery of graphene,two-dimensional materials have brought many new research hotspots due to their excellent properties,including nanomechanical systems based on suspended two-dimensional materials.The suspended two-dimensional film has a very small mass,so it can be vibrated by a very small force.The detection of the vibration of a two-dimensional membrane can quantitatively image its vibration mode.Identifying the vibration mode of the membrane will make it possible to measure the force driving the membrane to vibrate.The optical method is one of the commonly used methods for detecting two-dimensional membrane vibration.When the laser is perpendicularly incident on the substrate,the reflected light will couple with the incident light to form a standing wave,and the simple resonance of the two-dimensional membrane in the standing wave electric field will modulate the reflected light intensity.The fluctuation of the reflected light intensity appears as a peak in the frequency spectrum,and the peak value depends on the amplitude of the membrane at the position where the laser is incident.Therefore,the vibration mode of the film can be imaged by measuring the distribution of the peak value of the reflected light intensity's frequency spectrum in the plane of the membrane.We built a scanning system for quantitative imaging of the vibration mode of the membrane.The scanning system will move the whole vacuum enclosure containing the sample by using the micromechanical platform to realize the scanning test.In order to prove the performance of the scanning setup,we did some one-dimensional stability tests and some two-dimensional scanning accuracy tests.The test results show that the scanning setup can bear the weight of the whole vacuum enclosure,has a submicron moving step and a scanning stroke of more than 50 microns,and has high scanning stability and accuracy.The performance of the scanning system has reached the expected goal and meets the needs of the imaging film vibration mode.Based on the scanning setup,we present a new method for measuring the radius of the focused laser beam:by moving the sample,the boundary between the gold microstructure and the substrate gradually passes through the cross section of the laser beam,and the power of the reflected laser is measured at the same time.The calculated Gaussian function is used to fit the measurement data to obtain the radius of the laser beam.The minimum beam radius measured by this method agrees with the theoretical calculation value.