Novel Devices for Dynamically Shaping the Wavefront and Polarization of Light
She, Alan Jenting
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CitationShe, Alan Jenting. 2018. Novel Devices for Dynamically Shaping the Wavefront and Polarization of Light. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
AbstractLight, which provides the human eye with the sense of vision, is the basis for many key technologies used in everyday life, and the advanced control of which also benefits technological progress in other areas. The wavefront and polarization are two important properties of light, or, in general, electromagnetic radiation, that dictate its behavior. The ability to precisely control these properties as a function of time is highly desirable, as it is essential for numerous powerful applications, including imaging, high tech manufacturing, and communications, to name a few. There has been a bulk of work on the manipulation of these properties, but there remains much to be improved in terms of extreme compactification, reduction of power, and increase in speed and precision, as use cases are driven towards untethered, lightweight, and high performance devices. In this thesis, novel devices are presented that dynamically shape wavefront and polarization of light, using new principles of operation and design methods.
Wavefront shaping is achieved using metasurfaces, in which, recently, there has been significant scientific and technological interest due to their inherent multifunctionality and potential to dramatically reduce the thickness of the optics in a wide range of applications. The challenge, however, in making practical devices with metasurfaces is the difficulty in fabricating them with large areas. One of the key obstacles in this endeavor is the enormous data density required by the subwavelength resolution criterion imposed over large areas, resulting in giant file sizes for the layout design files describing structures greater than a few hundred microns in diameter. We present a scalable metasurface layout compression algorithm that exponentially reduce the design file size of (by 3 orders of magnitude for a centimeter diameter metalens, and even greater gains in compression for larger sizes) and a route to mass manufacturing of metasurface lenses (metalenses) using stepper photolithography with extremely large areas, up to (but not limited to) centimeters in diameter. Because of what is demonstrated here, the claim that chipmakers will be making lenses of the future (as well as the chips), i.e. the unification of two industries: semiconductor manufacturing and lens-making, is now closer to reality. However, metasurface devices are by themselves static, such that additional steps are required in order to introduce the feature of tunability, such as in focal length or magnification control. To take advantage of the thinness and planarity of metalenses, electrical tuning of lateral motion is essential for focal length and magnification control, which in conventional optical systems is performed by longitudinal mechanical motion along the optical axis. A large area, integrated optical device, no more than 30 microns thick, is presented, which imprints a strain field onto the optical wavefront, by way of a soft metasurface intermediary, enabling simultaneous control over focal length, astigmatism, and image shift. These combined capabilities have so far been possible only in electron optics, and has been made possible by combining metalenses, artificial muscles (i.e., dielectric elastomer actuators (DEAs)), and carbon nanotube-based stretchable transparent electrodes. This is the first step of many that seeks to exert arbitrary control over the strain profile of a metasurface: transforming a transformation optic.
Polarization shaping is achieved using a new architecture. Previously, the general problem of generating arbitrary time-varying states of polarization (SOP) has always been mathematically formulated by a series of linear transformations, i.e. a product of matrices, imposing a serial architecture, such as a series of rotating wave plates. An alternative parallel architecture is presented, which is described by a sum, rather than a product, of matrices. The theory is experimentally demonstrated by modulating spatially-separated polarization components of a laser using a digital micromirror device (DMD) that are subsequently beam combined. This method greatly expands the parameter space for engineering devices that control polarization. Performance characteristics, such as speed, stability, and spectral range, are entirely dictated by the technologies of optical intensity modulation – absorption, reflection, emission, and scattering, any of which may be used.
Our results have the potential of opening up a wealth of future new applications, including electrical control over all major optical aberrations and polarization, which could lead to major advances in optical microscopes and imaging systems as well as portable and wearable devices. Our results demonstrate the possibility of future optical microscopes, which operate fully electronically, as well as compact optical systems that rapidly probe various polarization states or employ the principles of adaptive optics to correct many orders of aberrations, simultaneously.
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