Photorefractive Phase-Conjugating Mirror for Guiding of Light through Sub-Wavelength Metal Holes

该项目的目标是在光折变铌酸锂晶体中使用全息位相共轭，以便将光聚焦和限制在至少与传统光学技术所能达到的尺寸一样小的尺寸，例如显微镜物镜：从铌酸锂晶体顶部金属膜的纳米孔中出现的球面波与这种光折变材料中的参考波相干扰。结果，光感应电流产生空间电荷场，并且通过电光效应记录折射率图案。这就是位相共轭全息图：通过位相共轭读出，光应该被送回纳米孔，希望能以高效率返回。这种限制对于增强光与物质的相互作用是非常有希望的。小于光波长的金属薄膜中的孔径可能显示出反常的高透过率，这是一个引起广泛兴趣的主题。在过去的十年里，越来越多的人研究了各种形状和阵列的纳米孔，但我们仍然没有对这种现象做出令人满意的解释。此外，实验、模拟和理论还没有结合起来。用位相共轭的方法解决这种空穴提供了新的见解，光学近场显然在接近亚波长光圈时起着关键的作用。通过上面概述的全息方法，所有近场也将被全息地记录和检索。光折变晶体，如铌酸锂，提供了足够高的空间分辨率。在我们的实验中，金属膜厚度、孔直径以及光照条件(强度、偏振、波长)都会发生变化。实验结果将与一种新的理论和定制的数值模拟结果进行比较，这些数值模拟考虑了亚波长金属空穴中的波导现象。

The goal of this project is to use holographic phase conjugation in photorefractive lithium niobate crystals in order to focus and confine light to dimensions at least as small as reached by conventional optics, such as microscope objectives: A spherical wave emerging from a nanohole in a metal film on top of a lithium niobate crystal interferes with a reference wave in this photorefractive material. As a result light-induced currents generate a space charge field and, by the electrooptic effect, a refractive index pattern is recorded. This serves as the phase conjugating hologram: By phase-conjugate read-out the light should be sent back through the nano-hole, hopefully with high efficiency. Such confinement is very promising for enhanced light-matter interaction.Apertures in metal films smaller than the wavelength of light may show anomalously high transmission that is a subject of broad interest, Within the last decade, a variety of nanohole shapes and arrays have been investigated by a growing community, yet we still remain without a satisfactory explanation of the phenomenon. Furthermore, experiment, simulation, and theory have not been combined, yet. Addressing such holes by phase conjugation provides new insights.Optical near-fields play evidently a key role close to sub-wavelength apertures. By the holographic approach outlined above also all near-fields will be holographic ally recorded and retrieved. Photorefractive crystals, such as lithium niobate, offer a sufficiently high spatial resolution. Parameters such as the metal film thickness and the hole diameter as well as the illumination conditions (intensity, polarization, wavelength) will be varied in our experiments. Experimental results will be compared with predictions from a novel theory and from customized numerical simulations considering wave-guiding phenomena in sub-wavelength metal holes.