Precision Measurement Method Of Beam Waist Based On In-plane Photonic Spin Splitting

The beam waist is the decisive factor of the laser beam quality factor,which has a huge impact on laser technology and imaging systems.In recent years,with the rapid development of laser technology,the precision measurement of laser beam waist is particularly important in many laser applications.The traditional methods of measuring the laser beam waist include: scanning pinhole method,scanning knife edge method,scanning slit method,CCD method and so on.Although the operation of these traditional measurement methods is simple,it is easy to produce errors in the measurement process.Therefore,it is of great and far-reaching significance to develop a simple and high-precision method to detect the beam waist.The appearance of the photonic spin Hall effect provides the possibility to solve the above scientific problems.It appears as when a linearly polarized light is reflected or refracted at the interface of different media,the spin splitting of left-handed and right-handed photons occurs in the direction perpendicular to and parallel to the incident plane,which is called the transverse and the in-plane spin splitting,respectively.So far,researchers combine the quantum weak measurement technology to improve the sensitivity of the transverse spin displacement to the physical parameters by several orders of magnitude,and carry out a series of applied studies on precision measurement,such as determining the number of graphene layers,observing the optical rotation of chiral molecules and the optical conductivity of Two-dimensional atomic crystal and so on.However,the existing research on in-plane spin splitting is focus on theoretical calculation and experimental detection,and seldom pays attention to its application.In fact,the in-plane spin splitting and the transverse spin displacement have similar properties and are also very sensitive to the changes of physical parameters.Moreover,the value of the in-plane spin splitting is larger than that of the transverse spin displacement under certain special angles.Thus,this paper proposes to introduce in-plane spin splitting into practical applications to precisely detect the beam waist size.The details are as follows:(1)We use the in-plane spin splitting as the probe to determine the size of the beam waist.We reveal the relationship between the in-plane spin splitting and beam waist when an linearly polarized light is reflected at the air-glass interface.It is found that when the incident angle is near Brewster angle,the in-plane spin splitting is extremely sensitive to the change of beam waist.We can utilize this sensitive property to establish a highly sensitive transmission model of in-plane spin splitting and beam waist.(2)Based on the highly sensitive transmission model,we combine quantum weak measurement technology to improve measurement accuracy and conduct experimental verification.It is shown that the amplified shifts can reach 1500 microns,the variation of beam waist can be accurately detected even by propagation amplification alone.The research provide an effective method for precise measurement of beam waist.