Colour hologram projection with an SLM by exploiting its full phase modulation range

Multi-color complex spatial light modulation with a single digital micromirror device

Spatial light modulators enabling complex light field manipulation has opened up many opportunities in biomedical imaging, holographic display, and adaptive optics. However, traditional spatial light modulators do not allow multi-color operations simultaneously due to their physical constraints, while multi-color modulations are highly desirable in many applications. To overcome this limitation, we demonstrate a multi-color spatial complex light field modulation with a single binary hologram on digital micromirror devices (DMD). This method combines several neighboring micro-mirror pixels into a giant single superpixel, in which the light field's amplitude and phase can be individually determined by internal pixel combinations, and the dynamic range of phase modulation can exceed 2π for the single wavelength. As a result, this extra phase modulation range offers an additional degree of freedom for independent multi-wavelength light modulation. Based on this scheme, multi-color light modulations have been demonstrated in a 2D plane as well as in multiple 3D holographic planes. Moreover, a dual-colored Airy beam has been realized using the same technique. These results bring complex light modulation into a multi-color regime, paving the way for practical applications in information display, imaging, and optical trapping.

Compact Metasurface-Based Optical Pulse-Shaping Device

Dispersion is present in every optical setup and is often an undesired effect, especially in nonlinear-optical experiments where ultrashort laser pulses are needed. Typically, bulky pulse compressors consisting of gratings or prisms are used to address this issue by precompensating the dispersion of the optical components. However, these devices are only able to compensate for a part of the dispersion (second-order dispersion). Here, we present a compact pulse-shaping device that uses plasmonic metasurfaces to apply an arbitrarily designed spectral phase delay allowing for a full dispersion control. Furthermore, with specific phase encodings, this device can be used to temporally reshape the incident laser pulses into more complex pulse forms such as a double pulse. We verify the performance of our device by using an SHG-FROG measurement setup together with a retrieval algorithm to extract the dispersion that our device applies to an incident laser pulse.

Improved phase hologram generation of multiple 3D objects

We demonstrate the generation of phase holograms of multiple 3D objects at different axial positions without cross talk and significant improvements in performance over conventional methods. We first obtain the phase hologram of two 3D objects, each one comprising 50 layers, using the global Gerchberg-Saxton algorithm. Then, we discuss and demonstrate a propagation approach based on the singular value decomposition of the Fresnel impulse response function that enables fast computation of small distance propagations. Finally, we propose a new iterative hologram generation algorithm, to the best of our knowledge, that takes advantage of this propagation approach and use it to make the hologram of the same scene previously obtained with the global Gerchberg-Saxton algorithm. We perform numerical and experimental reconstructions to compare both methods, demonstrating that our proposal achieves 4 times faster computation, as well as improved reconstruction quality.

Holographic beam shaping of partially coherent light

We present an algorithmic approach for holographic shaping of partially coherent light, which is described by a mode expansion containing thousands of individual modes. Using gradient descent and algorithmic differentiation, our algorithm is able to find a set of axially separated phase patterns such that each mode undergoes an individually optimized transformation with respect to the formation of a user-defined target intensity distribution. We demonstrate numerically and experimentally that a tandem of two phase patterns can achieve any intensity profile transformation with good accuracy.

Time-averaged image projection through a multimode fiber

Many disciplines, ranging from lithography to opto-genetics, require high-fidelity image projection. However, not all optical systems can display all types of images with equal ease. Therefore, the image projection quality is dependent on the type of image. In some circumstances, this can lead to a catastrophic loss of intensity or image quality. For complex optical systems, it may not be known in advance which types of images pose a problem. Here we show a new method called Time-Averaged image Projection (TAP), allowing us to mitigate these limitations by taking the entire image projection system into account despite its complexity and building the desired intensity distribution up from multiple illumination patterns. Using a complex optical setup, consisting of a wavefront shaper and a multimode optical fiber illuminated by coherent light, we succeeded to suppress any speckle-related background. Further, we can display independent images at multiple distances simultaneously, and alter the effective sharpness depth through the algorithm. Our results demonstrate that TAP can significantly enhance the image projection quality in multiple ways. We anticipate that our results will greatly complement any application in which the response to light irradiation is relatively slow (one microsecond with current technology) and where high-fidelity spatial distribution of optical power is required.

Multiplexed PSF Engineering for Three-Dimensional Multicolor Particle Tracking

Three-dimensional spatiotemporal tracking of microscopic particles in multiple colors is a challenging optical imaging task. Existing approaches require a trade-off between photon efficiency, field of view, mechanical complexity, spectral specificity, and speed. Here, we introduce multiplexed point-spread-function engineering that achieves photon-efficient, 3D multicolor particle tracking over a large field of view. This is accomplished by first chromatically splitting the emission path of a microscope to different channels, engineering the point-spread function of each, and then recombining them onto the same region of the camera. We demonstrate our technique for simultaneously tracking five types of emitters in vitro as well as colocalization of DNA loci in live yeast cells.

See-through holographic display with randomly distributed partial computer generated holograms

Holographic displays have the feature to show images out of the plane of the device itself, which is especially favored for augmented reality (AR) applications where the images need to be merged with the real world. In existing cases of AR holographic display, a combiner is used to converge the light path of the display image and surrounding scene toward the viewer's eye. In this paper, the idea of combining the holographic device and the combiner has been proposed, resulting in a see-through holographic display. In order to maintain the see-through quality of the device, the concept of partial hologram has been introduced, which means only a part of the area on the device has the holographic fringe pattern while leaving the rest fully transparent. Experiment and theoretical investigation shows that an evenly yet randomly distributed partial hologram provides the best holographic image quality assuming a fixed percentage of the holographic area on the device. A passive computer generated hologram (CGH) with two phase levels has been designed and fabricated for the verification. With partial hologram sharing 25% of the whole area, the CGH exhibits 90.9% of total transmission and 72.2% of parallel transmission. The demonstration shows a high see-through quality while providing a clear holographic image.

Recent advances in point spread function engineering and related computational microscopy approaches: from one viewpoint

This personal hybrid review piece, written in light of my recipience of the UIPAB 2020 young investigator award, contains a mixture of my scientific biography and work so far. This paper is not intended to be a comprehensive review, but only to highlight my contributions to computation-related aspects of super-resolution microscopy, as well as their origins and future directions.

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. The realized wavefront may deviate significantly from the prediction based on the idealized assumption that the response across a pixel is uniform and independent of its neighbors. Detailed numerical simulations of the SLM response based on a full 3D physical model accurately match the measured response and properly model the pixel crosstalk. The full model is then used to validate a simplified model that enables much faster crosstalk evaluation and pattern optimization beyond standard performance. General conclusions on how to minimize crosstalk in liquid crystal on silicon (LCoS) SLM systems are derived, as well as a readily accessible estimation of the amount of fringing in a given SLM.

Encoding lenses with focal lengths lower than the Nyquist limit using high phase-modulation displays

Liquid crystal displays allow the easy implementation of diffractive optical elements. However, the shortest focal lengths for lenses are limited by Nyquist conditions. In this work, we show that focal lengths much lower than this Nyquist limit can be encoded onto devices having a large phase-dynamic range. Experimental results are included with a display showing 10π phase modulation reducing the Nyquist limit by a factor of about 1/10.

Electrowetting-actuated multifunctional optofluidic lens to improve the quality of computer-generated holography

This paper presents an electrowetting-actuated multifunctional optofluidic (EAMO) lens to improve the quality of computer-generated holography (CGH). A unique structure of the EAMO lens based on electrowetting effect is designed. When the electrodes of the EAMO lens are applied on different voltages, the functions of focal length change and aperture change can be achieved. Then the proposed lens is used in the reproduction system of the CGH due to the multiple functions. The experimental results show that the CGH with zoom function can be realized and undesirable light can be eliminated due to the unique structure of the EAMO lens. The focal length changes can be varied from 11.6 cm to + ∞ and -∞ to -150.6 cm. The aperture size changes can be varied from 10.1 cm to 6.7 cm. By using the proposed EAMO lens, high-quality CGH can be realized without moving the position of any components mechanically, while the setup of the CGH is greatly simplified.

Method of chromatic aberration elimination in holographic display based on zoomable liquid lens

In this paper, we propose a method of chromatic aberration elimination in holographic display based on a zoomable liquid lens. The liquid lens is filled with two immiscible liquids and developed by using the principle of electrowetting. The shape at the liquid-liquid interface changes with the voltage applied to the liquid lens, so the focal length can be adjusted by changing the voltage. By using the liquid lens in the holographic display system, the position of the reconstructed image can be controlled. When three color lasers illuminate the corresponding holograms and the focal length of the liquid lens changes accordingly, three color images can coincide in the same location clearly. The experimental results verify its feasibility.

Holographic zoom micro-projection system based on three spatial light modulators

In this paper, we propose a holographic zoom micro-projection system based on three spatial light modulators (SLMs). Three color lasers, three filters, and three solid lenses form the system's collimated light sources. Three beam splitters and a prism are used in the system for beam deflection. The SLMs are used as the micro-displays in order to realize phase modulation. A liquid lens, which consists of a circular hole in the center of the middle substrate and several holes around the center, is developed in the system and it. A receiving screen is located behind the liquid lens. When the voltage applied to the liquid lens is changed, the focal length changes accordingly due to electrowetting effect. Three color holograms are loaded on the SLMs, respectively. When three color lasers are used to illuminate the corresponding holograms, the position and size of each color reproduction image can be adjusted by changing the focal length of the liquid lens and holograms loaded onto the SLMs. Therefore, three color images can be reconstructed together perfectly. The proposed system can realize function of zoom micro-projection without chromatic aberration. The experimental results verify its feasibility.

Generation of Color Images by Utilizing a Single Composite Diffractive Optical Element

This paper presents an approach that is capable of producing a color image using a single composite diffractive optical element (CDOE). In this approach, the imaging function of a DOE and the spectral deflection characteristics of a grating were combined together to obtain a color image at a certain position. The DOE was designed specially to image the red, green, and blue lights at the same distance along an optical axis, and the grating was designed to overlay the images to an off-axis position. We report the details of the design process of the DOE and the grating, and the relationship between the various parameters of the CDOE. Following the design and numerical simulations, a CDOE was fabricated, and imaging experiments were carried out. Both the numerical simulations and the experimental verifications demonstrated a successful operation of this new approach. As a platform based on coaxial illumination and off-axis imaging, this system is featured with simple structures and no cross-talk of the light fields, which has huge potentials in applications such as holographic imaging.

Interactive Holographic Display Based on Finger Gestures

In this paper, we demonstrate an interactive, finger-sensitive system which enables an observer to intuitively handle electro-holographic images in real time. In this system, a motion sensor detects finger gestures (swiping and pinching) and translates them into the rotation and enlargement/reduction of the holographic image, respectively. By parallelising the hologram calculation using a graphics processing unit, we realised the interactive handling of the holographic image in real time. In a demonstration of the system, we used a Leap Motion sensor and a phase modulation-type spatial light modulator with 1,920 × 1,080 pixels and a pixel pitch of 8.0 µm × 8.0 µm. The constructed interactive finger-sensitive system was able to rotate a holographic image composed of 4,096 point light sources using a swiping gesture and enlarge or reduce it using a pinching gesture in real time. The average calculation speed was 27.6 ms per hologram. Finally, we extended the constructed system to a full-colour reconstruction system that generates a more realistic three-dimensional image. The extended system successfully allowed the handling of a full-colour holographic image composed of 1,709 point light sources with a calculation speed of 22.6 ms per hologram.

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. For this purpose, two diffractive patterns, one steering the excitation light, and the other manipulating the emission light, are combined within the same area of the SLM, which acts as a pure phase modulator. A recently demonstrated technique allows one to combine the patterns with high diffraction efficiency and low crosstalk, using the extended phase shifting capability of the SLM, which covers multiples of 2π at the respective wavelengths. For a first demonstration we compare standard confocal imaging with simultaneous image acquisition in two separate sample planes, which shows comparable results.

Diffractive fan-out elements for wavelength-multiplexing subdiffraction-limit spot generation in three dimensions

Wavelength-multiplexing generation of subdiffraction-limit spots in three dimensions using propagating light was demonstrated and evaluated. Our previous design algorithm [Opt. Express22, 25196 (2014)OPEXFF1094-408710.1364/OE.22.025196] was extended to consider multiple output planes and multiple wavelengths by integrating modulation distributions for individual wavelengths. A diffractive fan-out element that generates subdiffraction-limit spot arrays with two wavelengths on two planes was demonstrated. Spot sizes were reduced to 79% of that of the diffraction-limit spot on average. Numerical calculations showed that seven-wavelength multiplexing is achievable, and the cross-talk suppression conditions are effective for cross-talk suppression between wavelengths.

Color holographic display with white light LED source and single phase only SLM

This work presents color holographic display, which is based on a single phase only spatial light modulator (SLM). In the display entire area of the SLM is illuminated by an on-axis white light beam generated by a single large LED. The holographic display fully utilizes SLM bandwidth and has capability of full-color, full frame rate imaging of outstanding quality. This is achieved through: (i) optimal use of the source coherence volume, (ii) application of the single white light LED source, (iii) a development of a novel concept of color multiplexing technique with color filter mask in Fourier plane of the SLM, (iv) and a complex coding with improved diffraction efficiency. Within experimental part of the paper we show single color, full-color holographic 2D and 3D images generated for reconstruction depth exceeding 10 cm.

Adjustable liquid aperture to eliminate undesirable light in holographic projection

In this paper, we propose an adjustable liquid aperture to eliminate the undesirable light in a holographic projection. The aperture is based on hydrodynamic actuation. A chamber is formed with a cylindrical tube. A black droplet is filled in the sidewall of the cylinder tube and the outside space is the transparent oil which is immiscible with the black droplet. An ultrathin glass sheet is attached on the bottom substrate of the device and a black shading film is secured to the central area of the glass sheet. By changing the volume of the black droplet, the black droplet will move to the middle or sidewall due to hydrodynamic actuation, so the device can be used as an adjustable aperture. A divergent spherical wave and a solid lens are used to separate the focus planes of the reconstructed image and diffraction beams induced by the liquid crystal on silicon in the holographic projection. Then the aperture is used to eliminate the diffraction beams by adjusting the size of the liquid aperture and the holographic projection does not have undesirable light.

Multicolour localization microscopy by point-spread-function engineering

Super-resolution microscopy has revolutionized cellular imaging in recent years^1-4. Methods relying on sequential localization of single point emitters enable spatial tracking at ~10-40 nm resolution. Moreover, tracking and imaging in three dimensions is made possible by various techniques, including point-spread-function (PSF) engineering^5-9 -namely, encoding the axial (z) position of a point source in the shape that it creates in the image plane. However, a remaining challenge for localization-microscopy is efficient multicolour imaging - a task of the utmost importance for contextualizing biological data. Normally, multicolour imaging requires sequential imaging^{10, 11}, multiple cameras¹², or segmented dedicated fields of view^{13, 14}. Here, we demonstrate an alternate strategy, the encoding of spectral information (colour), in addition to 3D position, directly in the image. By exploiting chromatic dispersion, we design a new class of optical phase masks that simultaneously yield controllably different PSFs for different wavelengths, enabling simultaneous multicolour tracking or super-resolution imaging in a single optical path.

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. This is similar to the tilt-effect (also called "latent image effect") known from various security elements ("kinegrams") on credit cards or bank notes. The effect is achieved without any specialized optical components, simply by using the large phase shifting capability of a "thick" SLM, which extends over several multiples of 2π, in combination with the angular dependence of the phase shift. For hologram projection one can use the fact that the phase of a monochromatic wave is only defined modulo 2π. Thus one can design a phase pattern extending over several multiples of 2π, which transforms at different readout angles into different 2π-wrapped phase structures, due to the angular dependence of the modulo 2π operation. These different beams then project different holograms at the respective readout angles. In amplitude modulation mode (with inserted polarizer) the intensity of each SLM pixel oscillates over several periods when tuning its control voltage. Since the oscillation period depends on the readout angle, it is possible to find a certain control voltage which produces two (or more) selectable gray levels at a corresponding number of pre-determined readout angles. This is done with all SLM pixels individually, thus constructing different images for the selected angles. We experimentally demonstrate the reconstruction of multiple (Fourier- and Fresnel-) holograms, and of different amplitude images, by readout of static diffractive patterns in a variable angular range between 0° and 60°.

Multi-wavelength holography with a single spatial light modulator for ultracold atom experiments

We demonstrate a method to independently and arbitrarily tailor the spatial profile of light of multiple wavelengths and we show possible applications to ultracold atoms experiments. A single spatial light modulator is programmed to create a pattern containing multiple spatially separated structures in the Fourier plane when illuminated with a single wavelength. When the modulator is illuminated with overlapped laser beams of different wavelengths, the position of the structures is wavelength-dependent. Hence, by designing their separations appropriately, a desired overlap of different structures at different wavelengths is obtained. We employ regional phase calculation algorithms and demonstrate several possible experimental scenarios by generating light patterns with 670 nm, 780 nm and 1064 nm laser light which are accurate to the level of a few percent. This technique is easily integrated into cold atom experiments, requiring little optical access.

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. Because of dispersion, the oscillation period strongly depends on the wavelength. Thus each voltage applied to an SLM pixel corresponds to another reflected color spectrum. For SLMs with a sufficiently broad tuning range, one obtains a color palette (i.e., a "color lookup-table"), which allows one to display color images. An advantage over standard liquid crystal displays (LCDs), which use color masks in front of the individual pixels, is that the light efficiency and the display resolution are increased by a factor of three.