A high-speed, hopping-free polarization modulation and particle rotation control technology based on optical heterodyne
The polarization states of light field can be temporally controlled by adjusting the phase difference of different polarization components. Many active phase modulation devices, such as liquid crystal phase retarders, photo elastic modulators and photoelectric modulators, have been developed to modulate polarizations for different applications. Limited by the phase modulation bandwidth of these devices, phase wrapping and hopping are inevitable in the modulation process of polarization. Quasi continuous polarization modulation has been realized by high-speed phase modulation of saw-tooth waveforms in which the hopping process can be controlled in very short time. Nevertheless, the effect of the hopping process cannot be ignored in applications that are sensitive to polarization evolution process, such as light induced rotation of nanoparticles. To promote the development of these applications, it is of great significance to develop a hopping-free polarization modulation technology. Continuous and unwrapping phase modulation method of the polarization state is the key for achieving hopping-free and continuous polarization modulation.
Optical heterodyne method, where two coherent beams with small frequency differences and same polarization are superimposed, is widely used in optical sensing. A low-frequency beat signal of light intensity is generated due to the continuous accumulated phase difference of two beams. By introducing frequency difference into two orthogonal polarized beams, the beat phenomenon of polarization state will be generated because of the phase difference continuously accumulated between two beams. The monotonic and continuously accumulated phase difference in optical heterodyne has greats prospects for continuous and hopping-free modulation of polarization state.
The research group of Prof. Kebin Shi from the School of Physics, Peking University, proposed a robust and high-speed linear polarization rotation control method by using a novel linear polarization synthesis based on optical heterodyne interference. Light field with continuously rotated polarization direction can be generated by interference of two circularly polarized lights with opposite handedness and a small frequency difference. The promising application in robust and high-speed particle rotation manipulation was also experimentally demonstrated. The modulation speed and stability of particle rotation can potentially reach MHz scale with sub-Hz accuracy.
Two orthogonally polarized beams will be frequency shifted respectively after passing through two acousto-optic crystals driven by RF signals with a small frequency difference. The angle between the optical axis of a quarter wave plate and the light polarization direction is 45°, which can be used to transfer two beams into circularly polarized light beams with opposite handedness. The phase difference between the two circularly polarized beams can directly determine the phase difference between the two circularly polarized beams. The continuous accumulation of the phase difference of the two beams will induce continuous and hopping free rotation of polarization direction. The rotation frequency of the polarization direction is equal to the heterodyne frequency in their configuration. Utilizing a passive liquid crystal vortex half wave plate, the dynamic linearly polarized light can be transformed into a dynamic cylindrical vector beam switching between radially and azimuthally polarized beams continuously. Combining with a polarizer and screen, the continuous and hopping-free rotation process of the polarization state is directly observed. The polarization modulation technique can be used for precisely controlling the particles rotation. The rotation speed of birefringent particles can be electrically controlled by adjusting the frequency difference of RF signals. Comparing with the traditional light induced rotation method, the rotation driven by the synthesized dynamic linearly polarized beam is insensitive to the fluctuation of light intensity and the ambient viscosity. The proposed polarization modulation method will be also beneficial for many research fields, such as micro-pumping, viscosity sensing, rotational manipulation in biophysical studies, measurements of torque, polarization sensitive imaging and spectroscopy. This work is published in Opto-Electronic Advances Vol. 8 2020 entitled “Robust and high-speed rotation control in optical tweezers by using polarization synthesis based on heterodyne interference”.
About The Group
Professor Kebin Shi’s group focuses on spectral and biophotonic imaging based on ultrafast / nonlinear optical principle, ultrafast fiber laser and optical frequency comb precision measurement research. His group has been funded by the key projects of NSFC and the national major instrument research and development project of the Ministry of science and technology. The research progresses have been published in scientific journals including Physical Review letters, Laser & Photonics Reviews, PNAS and Optics Letters.
Liu W, Dong D S, Yang H, Gong Q H, Shi K B. Robust and high-speed rotation control in optical tweezers by using polarization synthesis based on heterodyne interfer-ence. Opto-Electron Adv 3, 200022 (2020).