Optical Coherence Tomography (OCT) is a valuable tool for label-free imaging with micrometer resolution. However, conventional approaches to OCT typically image a sample from a single direction or angle, which limits its ability to image structures placed behind strongly attenuating material or deep inside the object. Here we introduce a solution to this problem by adding a small chamber to a spectral-domain OCT setup. Acoustic actuation enables a contact-free levitation and stepwise reorientation of samples such as zebra-fish larvae and tumor spheroids in a controlled and reproducible manner. We further developed a model-based compressive algorithm, which is able to exploit the diverse multi-angle OCT volumes for a 3D-reconstruction with isotropic resolution and estimation of refractive index values. We demonstrate and validate our approach on zebrafish larvae. We believe that our approach represents a powerful enabling tool for developmental biology and for organoid and cancer spheroid research.
Light plays a central role in many applications. The key to unlocking its versatility lies in shaping it into the most appropriate form for the task at hand. Specifically tailored refractive index modifications, directly manufactured inside glass using a short pulsed laser, enable an almost arbitrary control of the light flow. However, the stringent requirements for quantitative knowledge of these modifications, as well as for fabrication precision, have so far prevented the fabrication of light-efficient aperiodic photonic volume elements (APVEs). Here, we present a powerful approach to the design and manufacturing of light-efficient APVEs. We optimize application-specific three-dimensional arrangements of hundreds of thousands of microscopic voxels and manufacture them using femtosecond direct laser writing inside millimeter-sized glass volumes. We experimentally achieve unprecedented diffraction efficiencies up to 80%, which is enabled by precise voxel characterization and adaptive optics during fabrication. We demonstrate APVEs with various functionalities, including a spatial mode converter and combined intensity shaping and wavelength multiplexing. Our elements can be freely designed and are efficient, compact, and robust. Our approach is not limited to borosilicate glass but is potentially extendable to other substrates, including birefringent and nonlinear materials, giving a preview of even broader functionalities, including polarization modulation and dynamic elements.
Combining optical tweezers with acoustic trapping in one platform allows us to trap and manipulate sub-millimeter sized biological samples in suspension in a contact-less and flexible manner. The acoustic radiation forces levitate and trap the sample and steerable holographic optical tweezers give us an additional means of manipulation. We have implemented 3D acoustic trapping on a microfluidic chip, with three independent MHz transducers in three orthogonal directions; two side-transducers and one transparent top-transducer facilitating optical access for optical trapping and imaging. We can reorient the samples, or induce sustained rotations to gain access to multiple viewing angles of the object.
We present a tomographic reconstruction algorithm that is able to reconstruct waveguide profiles from a set of intensity images taken at different illumination angles. Very recently, such algorithms have become the state of the art in the community of bio imaging, but have never been applied to direct laser written structures such as waveguides.
We adapt the algorithm to our application of characterizing translation-invariant structures and extend it to jointly estimate optical aberrations introduced by the imaging system. We show that a correct estimation of these aberrations is necessary for making effective use of high-angle tomographic data.
Furthermore, we present a novel method for cross-validating our RI reconstructions by comparing en-face widefield images of thin waveguide sections with matching simulations based on the retrieved RI profile.
We present a generally applicable holographic force measurement method to simultaneously measure all components of the individual forces applied to a specific particle in a trapped ensemble, or to a specific site of an extended object. We show individual force measurements for up to ten traps. As this method does not require information about size, shape, or optical properties of the particles, it is well suited to study biological specimen. We demonstrate disentangling the individual forces on a red blood cell stretched by four optical traps. The detailed information (amplitude, phase, and polarization) about the scattered light field, which we gain by our method, allows us to also uncover the optical torque exerted on trapped particles. We experimentally confirm that our method delivers accurate results in various settings.
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