Design and characterization of spatial light modulator optical systems and geometrical phase elements for the generation of structured light

Abstract

This thesis develops optical systems for the generation and control of structured light. These light beams with spatial control of their amplitude, phase and polarization are receiving much attention in the scientific community because of their applications in areas such as microscopy, materials processing, optical communications and polarimetry, among others. Two main kinds of techniques exist to generate these beams. One method is based on geometrical phase (GP) elements, which are advanced diffractive optical components consisting of microstructured retarders, where the spatial distribution of the optical axis allows the encoding of any phase function. The other technique employs spatial light modulators (SLMs), which are pixelated displays that allow the dynamic encoding of arbitrary phase functions. The thesis is presented as a compendium of five publications where we designed and characterized optical systems based on GP elements and on SLMs to generate light beams with structured polarization (vector beams) and optical vortices. The work is focused on the experimental realization of such vector and vortex beams with these technologies. Thus, it includes an important part devoted to the characterization and evaluation of the components and devices used for this purpose. However, the generation of such structured light requires a deep understanding of the properties of the superposition of light beams with different polarization states. Therefore, the thesis also includes a comprehensive synthesis of the formalism required to describe such control of polarization and on the design of diffractive optical elements used to generate the proper superposition. The first work consists in a spectral characterization of the retardance and the birefringence colors of a tunable liquid-crystal commercial q-plate operative in the visible and near-IR range. This device enables the realization of vector beams in compact and simple optical setups and, to our knowledge, is the only tunable q-plate model that is commercially available. The retardance tunability makes it very convenient to manipulate vector beams in a wide spectral range from 400 nm to 1600 nm. In addition, we demonstrated that placing the q-plate between crossed linear polarizers and illuminating the system with broadband white light is a simple and fast method to determine the voltage values where it behaves as a first-order retarder. Also, the method allows a rough estimation of the wavelength where the device has pi retardance. This tunable device was employed to generate pure and hybrid vector beams with exotic intensity and polarization patterns. Second, we presented the design of a liquid-crystal geometric-phase diffraction grating that generates a two-dimensional array of optical vortices with different topological charges. The key aspect of the work is that its design relies on the optimal triplicator phase profile, which ensures the maximum theoretical diffraction efficiency achievable with a pure-phase function. In addition, since it is built as a geometrical phase element, it is a flat and thin element that may be easily incorporated in optical systems where compactness is required. The obtained experimental results have proven that the designed grating can also serve as a topological charge vortex detection system. The third work presents a technique to efficiently generate vector beams with liquid-crystal on silicon (LCOS) SLMs. SLM-based optical systems are the most versatile way to generate structured light since they are programmable devices. But because of their pixelated structure they present diffraction losses, that make their optical efficiency lower than systems based on geometrical phase elements. In addition, SLMs make the system bulkier, typically requiring the use of beam splitters, and they can present a phase fluctuation effect caused by flicker. All these aspects significantly reduce the efficiency of the system generating vector beams. In this work, we developed an efficient system that uses two LCOS SLMs using a common-path geometry. The employed SLMs were free from flicker and, therefore, they could be used to apply a complex amplitude encoding technique that generated the desired beam on axis. As a whole, the optical system represents a very efficient and versatile way to generate vector modes. The fourth work that constitutes this thesis is a new technique for measuring the backplane deformation of a SLM. This is another major defect that can be present in SLMs and must be taken into account to achieve good results. The technique is based on a self-interference effect that occurs inside the SLM when being illuminated with light of wavelength far from its operation spectral range. In this situation, the anti-reflection coating of the device does not work properly and we can exploit this a priori undesired effect to measure and then correct the backplane deformation. Since the effect is produced by the SLM itself, no external interferometric arrangements are required, thus making this technique robust and stable. Finally, we used a SLM to generate beams with tailored spatial variation of its degree of polarization across its transverse section. The depolarization spatial variation was generated by imaging a time-varying SLM phase pattern. We generated exotic beams with a spirally-shaped distribution of its degree of polarization, analogous to the spiral phase variation in an optical vortex. The technique might be of interest in testing imaging polarimeters and in the emulation of structured light exhibiting a spatial variation of the degree of polarization as an additional parameter. These publications represent an advance in the field of the techniques employed to generate vortex and vector light beams, thus making several contributions to the efficient control of polarized light with diffractive elements.