Ultrasound-induced reorientation for multi-angle optical coherence tomography.

Organoid and spheroid technology provide valuable insights into developmental biology and oncology. Optical coherence tomography (OCT) is a label-free technique that has emerged as an excellent tool for monitoring the structure and function of these samples. However, mature organoids are often too opaque for OCT.

Efficient and accurate intensity diffraction tomography of multiple-scattering samples.

Optical Diffraction Tomography (ODT) is a label-free method to quantitatively estimate the 3D refractive index (RI) distributions of microscopic samples. Recently, significant efforts were directed towards methods to model multiple-scattering objects. The fidelity of reconstructions rely on accurately modelling light-matter interactions, but the efficient simulation of light propagation through high-RI structures over a large range of illumination angles is still challenging.

Optofluidic adaptive optics in multi-photon microscopy.

Adaptive optics, in combination with multi-photon techniques, is a powerful approach to image deep into a specimen. Remarkably, virtually all adaptive optics schemes today rely on wavefront modulators that are reflective, diffractive or both. This, however, can pose a severe limitation for applications.

Generally Applicable Holographic Torque Measurement for Optically Trapped Particles.

We present a method to measure the optical torque applied to particles of arbitrary shape such as micrometer-sized micro-organisms or cells held in an optical trap, inferred from the change of angular momentum of light induced by the particle. All torque components can be determined from a single interference pattern recorded by a camera in the back focal plane of a high-NA condenser lens provided that most of the scattered light is collected. We derive explicit expressions mapping the measured complex field in this plane to the torque components.

Simultaneous scattering compensation at multiple points in multi-photon microscopy.

The two-photon fluorescence imaging depth has been significantly improved in recent years by compensating for tissue scattering with wavefront correction. However, in most approaches the wavefront corrections are valid only over a small sample region on the order of 1 to 10 µm. In samples where most scattering structures are confined to a single plane, sample conjugate correction geometries can increase the observable field to a few tens of µm.

Fast holographic scattering compensation for deep tissue biological imaging.

Scattering in biological tissues is a major barrier for in vivo optical imaging of all but the most superficial structures. Progress toward overcoming the distortions caused by scattering in turbid media has been made by shaping the excitation wavefront to redirect power into a single point in the imaging plane. However, fast, non-invasive determination of the required wavefront compensation remains challenging.

Controlled orientation and sustained rotation of biological samples in a sono-optical microfluidic device.

In cell biology, recently developed technologies for studying suspended cell clusters, such as organoids or cancer spheroids, hold great promise relative to traditional 2D cell cultures. There is, however, growing awareness that sample confinement, such as fixation on a surface or embedding in a gel, has substantial impact on cell clusters. This creates a need for contact-less tools for 3D manipulation and inspection.

High-NA two-photon single cell imaging with remote focusing using a diffractive tunable lens.

Fast, volumetric structural and functional imaging of cellular and sub-cellular dynamics inside the living brain is one of the most desired capabilities in the neurosciences, but still faces serious challenges. Specifically, while few solutions for rapid 3D scanning exist, it is generally much easier to facilitate fast in-plane scanning than it is to scan axially at high speeds. Remote focusing in which the imaging plane is shifted along the optical axis by a tunable lens while maintaining the position of the sample and objective is a promising approach to increase the axial scan speed, but existing techniques often introduce severe optical aberrations in high-NA imaging systems, eliminating the possibility of diffraction-limited single-cell imaging.

Diffractive tunable lens for remote focusing in high-NA optical systems.

Remote focusing means to translate the focus position of an imaging system along the optical axis without moving the objective lens. The concept gains increasing importance as it allows for quick 3D focus steering in scanning microscopes, leaves the sample region unperturbed and is compatible with conjugated adaptive optics. Here we present a novel remote focusing approach that can be used in conjunction with high numerical aperture optics.

Model-based compensation of pixel crosstalk in liquid crystal spatial light modulators.

Spatial light modulators (SLMs) based on liquid crystals are widely used for wavefront shaping. Their large number of pixels allows one to create complex wavefronts. The crosstalk between neighboring pixels, also known as fringing field effect, however, can lead to strong deviations.

Multi-color operation of tunable diffractive lenses.

Rotationally tunable diffractive optical elements (DOEs) consist of two stacked diffractive optical elements which are rotated with respect to each other around their central optical axis. The combined diffractive element acts as a highly efficient diffractive lens, which changes its optical power as a function of the mutual rotation angle. Here we show that the principle can be extended to produce polychromatic tunable lenses, i.

Modified Alvarez lens for high-speed focusing.

We present a modified configuration of a tunable Alvarez lens with a refocusing frequency of 1 kHz or more. In contrast to the classic Alvarez lens, the approach does not utilize a translational motion of two sub-lenses with respect to each other, but uses a 4f-setup to image two diffractive sub-lenses onto each other. Hereby focus tuning is achieved by rotating a galvo-mirror which affects the overlap of the two sub-lenses which together form an effective lens of refractive power which depends on the rotation angle of the galvo-mirror.

Vortex-free phase profiles for uniform patterning with computer-generated holography.

Computer-generated holography enables efficient light pattern generation through phase-only wavefront modulation. While perfect patterning usually requires control over both phase and amplitude, iterative Fourier transform algorithms (IFTA) can achieve phase-only approximations which maximize light efficiency at the cost of uniformity. The phase being unconstrained in the output plane, it can vary abruptly in some regions leading to destructive interferences.

Programmable freeform optical elements.

Modern liquid crystal spatial light modulators (SLMs) are capable of shifting the optical path length by some microns, which corresponds to phase shifts of several multiples of 2π. We use this capability to display freeform optical elements (FOEs) on a SLM, as largely smooth phase variations with only a small number of wrapping lines. These FOEs can be programmed to generate so-called caustic intensity distributions, which may be real images reconstructed at a selected position in front of the SLM surface.

Assessing various Infrared (IR) microscopic imaging techniques for post-mortem interval evaluation of human skeletal remains.

Due to the influence of many environmental processes, a precise determination of the post-mortem interval (PMI) of skeletal remains is known to be very complicated. Although methods for the investigation of the PMI exist, there still remains much room for improvement. In this study the applicability of infrared (IR) microscopic imaging techniques such as reflection-, ATR- and Raman- microscopic imaging for the estimation of the PMI of human skeletal remains was tested.

Orbital angular momentum light in microscopy.

Light with a helical phase has had an impact on optical imaging, pushing the limits of resolution or sensitivity. Here, special emphasis will be given to classical light microscopy of phase samples and to Fourier filtering techniques with a helical phase profile, such as the spiral phase contrast technique in its many variants and areas of application.This article is part of the themed issue 'Optical orbital angular momentum'.

Colored point spread function engineering for parallel confocal microscopy.

Using the color selectivity of a spatial light modulator (SLM) for both, tailoring the excitation beam at one wavelength, and multiplexing the image at the red-shifted fluorescence wavelength, it is possible to parallelize confocal microscopy, i.e. to simultaneously detect an axial stack (z-stack) of a sample.

High-resolution confocal Raman microscopy using pixel reassignment.

We present a practical modification of fiber-coupled confocal Raman scanning microscopes that is able to provide high confocal resolution in conjunction with high light collection efficiency. For this purpose, the single detection fiber is replaced by a hexagonal lenslet array in combination with a hexagonally packed round-to-linear multimode fiber bundle. A multiline detector is used to collect individual Raman spectra for each fiber.

Acoustic force mapping in a hybrid acoustic-optical micromanipulation device supporting high resolution optical imaging.

Many applications in the life-sciences demand non-contact manipulation tools for forceful but nevertheless delicate handling of various types of sample. Moreover, the system should support high-resolution optical imaging. Here we present a hybrid acoustic/optical manipulation system which utilizes a transparent transducer, making it compatible with high-NA imaging in a microfluidic environment.

Tilt-effect of holograms and images displayed on a spatial light modulator.

We show that a liquid crystal spatial light modulator (LCOS-SLM) can be used to display amplitude images, or phase holograms, which change in a pre-determined way when the display is tilted, i.e. observed under different angles.

Combined holographic optical trapping and optical image processing using a single diffractive pattern displayed on a spatial light modulator.

We demonstrate simultaneous holographic optical trapping and optical image processing using a single-phase diffraction pattern displayed on a liquid crystal spatial light modulator (SLM). The ability of modern SLMs to provide multiorder phase shifts represents a degree of freedom that allows the calculation of diffraction patterns that act in precisely defined but different ways on light beams of different wavelengths. We exploit this property to calculate a single-phase hologram that shapes multiple optical traps at 785 nm while performing double-helix point spread function engineering at 532 nm.

Direct measurement of axial optical forces.

Direct measurement of optical forces based on recording the change of momentum between the in- and outgoing light does not have specific requirements on particle size or shape, or on beam shape. Thus this approach overcomes many of the limitations of force measurements based on position measurements, which require frequent calibration. In this work we validate the achievable accuracy for direct force measurements in the axial direction for a single beam optical tweezers setup, based on numerical simulations and experimental investigations of situations, where the true force is known.

Adjustable diffractive spiral phase plates.

We report on the fabrication and the experimental demonstration of Moiré diffractive spiral phase plates with adjustable helical charge. The proposed optical unit consists of two axially stacked diffractive elements of conjugate structure. The joint transmission function of the compound system corresponds to that of a spiral phase plate where the angle of mutual rotation about the central axis enables continuous adjustment of the helical charge.

How to use a phase-only spatial light modulator as a color display.

We demonstrate that a parallel aligned liquid crystal on silicon (PA-LCOS) spatial light modulator (SLM) without any attached color mask can be used as a full color display with white light illumination. The method is based on the wavelength dependence of the (voltage controlled) birefringence of the liquid crystal pixels. Modern SLMs offer a wide range over which the birefringence can be modulated, leading (in combination with a linear polarizer) to several intensity modulation periods of a reflected light wave as a function of the applied voltage.

Cortical contractility triggers a stochastic switch to fast amoeboid cell motility.

3D amoeboid cell migration is central to many developmental and disease-related processes such as cancer metastasis. Here, we identify a unique prototypic amoeboid cell migration mode in early zebrafish embryos, termed stable-bleb migration. Stable-bleb cells display an invariant polarized balloon-like shape with exceptional migration speed and persistence.