Phase gratings with tunable diffraction efficiency.

We present a new principle for tuning the diffraction efficiency of an optical grating and its implementation in a micro-optical device. The overlap of two phase gratings is used to vary the effective phase shift and hence the diffraction efficiency. We study the working principle using Fourier Optics to simulate the diffraction pattern in the far field and design and realize a device based on integrated piezo actuation. We find good agreement between simulation and experiment and observe a suppression of the first diffraction order intensity by more than 97% and response times of less than 3 ms.

Volumetric HiLo microscopy employing an electrically tunable lens.

Electrically tunable lenses exhibit strong potential for fast motion-free axial scanning in a variety of microscopes. However, they also lead to a degradation of the achievable resolution because of aberrations and misalignment between illumination and detection optics that are induced by the scan itself. Additionally, the typically nonlinear relation between actuation voltage and axial displacement leads to over- or under-sampled frame acquisition in most microscopic techniques because of their static depth-of-field. To overcome these limitations, we present an Adaptive-Lens-High-and-Low-frequency (AL-HiLo) microscope that enables volumetric measurements employing an electrically tunable lens. By using speckle-patterned illumination, we ensure stability against aberrations of the electrically tunable lens. Its depth-of-field can be adjusted a-posteriori and hence enables to create flexible scans, which compensates for irregular axial measurement positions. The adaptive HiLo microscope provides an axial scanning range of 1 mm with an axial resolution of about 4 µm and sub-micron lateral resolution over the full scanning range. Proof of concept measurements at home-built specimens as well as zebrafish embryos with reporter gene-driven fluorescence in the thyroid gland are shown.

Bio-inspired variable imaging system simplified to the essentials: modelling accommodation and gaze movement.

A combination of an aspherical hybrid diffractive-refractive lens with a flexible fluidic membrane lens allows the implementation of a light sensitive and wide-aperture optical system with variable focus. This approach is comparable to the vertebrate eye in air, in which the cornea offers a strong optical power and the flexible crystalline lens is used for accommodation. Also following the natural model of the human eye, the decay of image quality with increasing field position is compensated, in the optical system presented here, by successively addressing different tilting angles which mimics saccadic eye-movements. The optical design and the instrumental implementation are presented and discussed, and the working principle is demonstrated.

Axial scanning in confocal microscopy employing adaptive lenses (CAL).

In this paper we analyze the capability of adaptive lenses to replace mechanical axial scanning in confocal microscopy. The adaptive approach promises to achieve high scan rates in a rather simple implementation. This may open up new applications in biomedical imaging or surface analysis in micro- and nanoelectronics, where currently the axial scan rates and the flexibility at the scan process are the limiting factors. The results show that fast and adaptive axial scanning is possible using electrically tunable lenses but the performance degrades during the scan. This is due to defocus and spherical aberrations introduced to the system by tuning of the adaptive lens. These detune the observation plane away from the best focus which strongly deteriorates the axial resolution by a factor of ~2.4. Introducing balancing aberrations allows addressing these influences. The presented approach is based on the employment of a second adaptive lens, located in the detection path. It enables shifting the observation plane back to the best focus position and thus creating axial scans with homogeneous axial resolution. We present simulated and experimental proof-of-principle results.

Variable optofluidic slit aperture.

The shape of liquid interfaces can be precisely controlled using electrowetting, an actuation mechanism which has been widely used for tunable optofluidic micro-optical components such as lenses or irises. We have expanded the considerable flexibility inherent in electrowetting actuation to realize a variable optofluidic slit, a tunable and reconfigurable two-dimensional aperture with no mechanically moving parts. This optofluidic slit is formed by precisely controlled movement of the liquid interfaces of two highly opaque ink droplets. The 1.5 mm long slit aperture, with controllably variable discrete widths down to 45 µm, may be scanned across a length of 1.5 mm with switching times between adjacent slit positions of less than 120 ms. In addition, for a fixed slit aperture position, the width may be tuned to a minimum of 3 µm with high uniformity and linearity over the entire slit length. This compact, purely fluidic device offers an electrically controlled aperture tuning range not achievable with extant mechanical alternatives of a similar size.