Optical Field Modulation Realized Via Liquid Crystal Photoalignment Technology

In recent years,with the continuous progress of laser technology and development of optics,more and more attention has been paid to optical field modulation.On the one hand,through the modulation of the phase and amplitude of Gaussian beams,we can obtain special optical beams with novel structures and unique properties like optical vortex and Airy beam,which are widely used in optical manipulation,quantum computation and storage,astronomical observation,high resolution microscopic imaging and so on.On the other hand,modulating the polarization parameter of the incident light to achieve depolarization effects also plays an important role in fiber communication and optical detection.Liquid crystals(LCs)can be used to implement efficient modulation of optical fields with their excellent electro-optical characteristics.As a new way to orient LCs,photoalignment technology shows great advantages in the fabrication of LC elements with complex structures.In this thesis,via SD1 based LC photoalignemt technique and digital micro-mirror device based microlithography system,we design and fabricate different kinds of LC elements for optical field modulation.The major results are summarized as follows:1.We fabricated various LC fork gratings with different orientation modes and structures by controlling LC athimuzal director for the first time,realizing the generation and modulation of a series of optical vortices.74%high efficiency of Gaussian beam being converted to ±1 order diffracted optical vortices was achieved via the hybrid aligned nematic LC fork gratings and the switch between on and off states of optical vortices was accomplished with low voltage 1.7 V and 15 V applied.By optimizing the parameters of the LC elements,a fast response on/off time of 300?s/570?s was obtained respectively.The LC fork gratings exhibit extraordinary features of polarization independency and wide operating spectrum range,and the rewritability of SD1 additionally endows optical vortices with reconfigurability.These advantages increase the modulating flexibility of optical vortices,expanding their application prospects.2.On the basis of the traditional cubic phase mask,we introduced the concept of geometric phase,and fabricated a polarization Airy mask characterized by spatially variant LC azimuthal director through a partly-overlapping multi-step exposure technique.The modulation of Gaussian beam to single or dual Airy beams was then carried out through a lens to perform the Fourier transformation.By adjusting the polarization state of the incident light,the switching between these two Airy beams was realized,and the arbitrary proportion of their optical energy could be controlled at the same time.The transverse acceleration and nondiffraction properties were verified by measuring the propagation dynamics of dual Airy beams.This LC polarization Airy mask reveals good tolerance to incident light wavelength and can withstand at least 600 pulses(0.5 J/cm2,1064 nm,10 ns,1 Hz)illumination.The design provides a convenient and effective way to the generation and modulation of high quality and versatile Airy beams.3.By introducing the q-plate technique to produce optical vortex,we modulated the phase and amplitude of the Gaussian beam simultaneously with the combination of LC polarization Airy mask,and generated vortex Airy beams of different modes.We analyzed the propagation dynamics of these vortex Airy beams,and the characteristics of topological charges,nondiffraction,self-healing and transverse acceleration of the optical vortex embedded in Airy beam were experimentally verified.Mode switch among Gaussian,vortex,vector,Airy beams and their combinations could be easily acquired by the tuning of external electric field or the rotation of the quarter-wave plate in the optical setup,supplying a flexible and efficient manner for optical field modulation and beam shaping.The proposed high-quality vortex Airy beam and the optical vortex with self-healing,quasi-nondiffraction and transverse acceleration properties pave their way to the applications of multi-dimensional particles manipulation,lossless transmission in quantum communications,higher-resolution biological observation,etc.4.Based on the principle of half-wave plate modulating linearly polarized light,a novel LC depolarizer was demonstrated via step-by-step exposure.It consisted of 24×18 homogeneous square domains,in which LC molecules were randomly aligned.A degree of polarization of less than 5%was obtained for any linearly polarized incident light in the telecom band without the need of external electric field,which displayed outstanding polarization insensitivity.For circularly and elliptically polarized light,the same depolarization effect could also be achieved by electrically tuning the phase retardation of the sample.The depolarizer was not only suitable for 1550 nm monochromatic light,but also functioned well for wide range 1520 nm-1610 nm polychromatic light,and the minimum degree of polarization obtained were only 0.384%and 0.156%accordingly.The overall performance of the LC depolarizer is better than that of commercial products.It has great values in laser processing,astronomy instruments,optical fiber communications and other electro-optical systems.