Nowadays virtually all properties of light can be controlled by nano-optical elements, such as plasmonic filters for wavelength selection, computer generated holograms (CGHs) for managing the propagation direction or wire grid polarizers and nano-optical wave plates for polarization control. Integrating these nano-optical elements with photo sensors arrays enables the realization of parallelized and strongly miniaturized optical systems. We focus on the design and fabrication of polarization measurement systems based on nano-optical wave plates and wire grid polarizers.
Nano-optical wave plates and polarizers are basing on grating structures with periods much smaller than the applied wavelength to avoid diffraction. In these structures form birefringence leads to a different effective complex refractive index for light with a electrical field vector parallel to the grating lines than for the orthogonal component. For polarizers, absorbing respectively metallic grating materials are used. Thus, a high absorption or reflection is achieved for polarized light oscillating parallel to the grating ridges. For perpendicular oscillating light the transmission is much higher, resulting in an unequal transmission ratio for the polarization components of light. This ratio is called polarization contrast and is an important characteristic of polarizers. In figure 1a a scanning-electron microscope cross-section image of a metallic wire grid polarizer displayed. For grating-based nano-optical wave plates non-absorbing materials with high refractive indices, such as titanium dioxide, are used. The phase shift is induced via retardation of the orthogonal polarization components of the incident light with respect to each other due to the refractive index difference of the grating structure. The amplitude ratio of the polarization components should be equal to avoid polarization effects. This ratio and the phase shift over the wavelength are important characteristics of wave plates.

Figure 1: a) wire grid polarizer b) encapsulated diffraction grating c) encapsulated wire grid polarizer.
(rights: IAP)

For the fabrication of nano-optical polarization elements we use a self-aligned double patterning process in combination with character projection electron beam lithography. The final structures are subsequently encapsulated for protection and to facilitate the fabrication of the next functional layer. For the encapsulation different techniques are available. In case of one-dimensional aligned gratings we use reactive ion beam deposition of silicon dioxide under oblique angle (see figure 1c). For structured samples we use a patented process to encapsulate the gratings (figure 1b). Onto this capping layer further nano-optical (e.g. polarizers, wave plates, colour filters) or micro-optical elements (e.g. pinholes, lenses, diffraction gratings) are applied. All these elements can be split in segments with different properties and orientations (figure 2a) to realize complex, miniaturized systems (see figure 2b).

02_WGP_Legende 03_Skizze_Retarder_Polarisator_Stapel
Figure 2: a) pixelated wire grid polarizer b) sketch of stacked nano-optical elements for polarization measurements.

Finally, these complex, stacked systems are aligned and bonded onto a photo detector array. An active setup containing an illumination, sample mounting and six-axis-positioning device is used (see figure 3). During the alignment specific structures on the elements are illuminated to determine the relative position between sensor and sample by evaluation of the readout from the detector array. Depending on the required positioning precision, different elements can be used, e.g. simple pin holes or complex elements based on computer generated holograms for sub-pixel alignment in all six degrees of freedom. Finally, the nano-optical element and the photo sensor are bonded permanently together by using an index-matched UV curing adhesive.

Figure 3: Setup and working principle of the used active alignment setup.

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