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Absorption spectroscopy is widely used to detect samples with spectral specificity. Here, we propose and demonstrate a method for enhancing the sensitivity of absorption spectroscopy. Exploiting multiple light scattering generated by a boron nitride (h-BN) scattering cavity, the optical path lengths of light inside a diffusive reflective cavity are significantly increased, resulting in more than ten times enhancement of sensitivity in absorption spectroscopy. We demonstrate highly sensitive spectral measurements of low concentrations of malachite green and crystal violet aqueous solutions. Because this method only requires the addition of a scattering cavity to existing absorption spectroscopy, it is expected to enable immediate and widespread applications in various fields, from analytical chemistry to environmental sciences.

Random lasers are promising in the spectral regime, wherein conventional lasers are unavailable, with advantages of low fabrication costs and applicability of diverse gain materials. However, their practical application is hindered by high threshold powers, low power efficiency, and difficulties in light collection. Here, we demonstrate a power-efficient easy-to-fabricate non-resonant laser using a deep hole on a porous gain material. The laser action in this counterintuitive cavity was enabled by non-resonant feedback from strong diffuse reflections on the inner surface. Additionally, significant enhancements in slope efficiency, threshold power, and directionality were obtained from cavities fabricated on a porous Nd:YAG ceramic.

Given the low absorption contrast of X-rays, phase shift has been playing an important role as an alternative source of contrast in X-ray nanoimaging. Numerous phase-measuring techniques have been proposed, most of which, however, are based on significant assumptions or sample translations. In this study, we propose the application of Kramers–Kronig (KK) relations in the spatial domain as a solution to allow the X-ray quantitative phase image to be directly calculated from the measured intensity image without any additional requirements. Based on this straightforward principle, we have presented KK nanotomography by introducing a spatial-frequency cutoff filter into a conventional tomographic setup. The robustness and versatility of the proposed method were experimentally verified based on various sample tomograms. We expect KK nanotomography to be widely adopted as a powerful and easy-to-adapt phase quantification solution for X-ray microscopes.

In coherent diffraction experiments, knowledge of the incident field is important to extract pure sample-induced data from measured diffraction patterns. However, this becomes challenging in x-ray free-electron lasers (XFELs), as incident fields vary from pulse to pulse due to the stochastic nature of self-amplified stimulated emission. Despite various proposed field retrieval methods, single-shot characterization of each pulse remains elusive. To address this, we propose the speckle-correlation scattering matrix as a solution. Our method directly reconstructs the complex field without sample constraints or multiple acquisitions by introducing a designed diffuser before the detector. We demonstrate the robustness of our approach through successful field reconstructions in various experimental configurations. Based on the retrieved field results, pulse-to-pulse variations in pulse intensity, position, illumination angle, and shape were analyzed. We believe our method can readily serve as an on-field and real-time pulse diagnostic tool at XFELs and improve the overall quality of all experiments performed at XFELs.

Optical diffraction tomography (ODT) is a three-dimensional (3D) label-free imaging technique. The 3D refractive index distribution of a sample can be reconstructed from multiple two-dimensional optical field images via ODT. Herein, we introduce a temporally low-coherence ODT technique using a ferroelectric liquid crystal spatial light modulator (FLC SLM). The fast binary-phase modulation provided by the FLC SLM ensures the high spatiotemporal resolution. To reduce coherent noise, a superluminescent light-emitting diode is used as an economic low-coherence light source. We demonstrate the performance of the proposed system using various samples, including colloidal microspheres and live epithelial cells.

Intact optical information of an object delivered through an imaging system is deteriorated by imperfect optical elements and unwanted defects. Image deconvolution has been widely exploited as a recovery technique due to its practical feasibility, and operates by assuming linear shift-invariant property of the imaging system. However, shift invariance does not rigorously hold in all imaging situations and is not a necessary condition for solving an inverse problem of light propagation. Several improved deconvolution techniques exploiting spatially variant point spread functions have been proposed in previous studies. However, the full characterization of an optical imaging system for compensating aberrations has not been considered. Here, we present a generalized method to solve the linear inverse problem of coherent light propagations without any regularization method or constraint on shift invariance by fully measuring the transmission matrix of the imaging system. Our results show that severe aberrations produced by a tilted lens or an inserted disordered layer can be corrected properly only by the proposed generalized image deconvolution. This work generalizes the theory of image deconvolution, and enables distortion-free imaging under general imaging condition.

Holographic displays generate realistic 3D images that can be viewed without the need for any visual aids. They operate by generating carefully tailored light fields that replicate how humans see an actual environment. However, the realization of high-performance, dynamic 3D holographic displays has been hindered by the capabilities of present wavefront modulator technology. In particular, spatial light modulators have a small diffraction angle range and limited pixel number limiting the viewing angle and image size of a holographic 3D display. Here, we present an alternative method to generate dynamic 3D images by controlling volume speckle fields significantly enhancing image definition. We use this approach to demonstrate a dynamic display of micrometre-sized optical foci in a volume of 8 mm × 8 mm × 20 mm. Control of speckle fields yields high-definition 3D holographic images.

Advances in diverse areas such as inspection, imaging, manufacturing, telecommunications, and information processing have been stimulated by novel optical devices. Conventional material ingredients for these devices are typically made of homogeneous refractive or diffractive materials and require sophisticated design and fabrication, which results in practical limitations related to their form and functional figures of merit. To overcome such limitations, recent developments in the application of disordered materials as novel optical elements have indicated great potential in enabling functionalities that go beyond their conventional counterparts, while the materials exhibit potential advantages with respect to reduced form factors. Combined with wavefront shaping, disordered materials enable dynamic transitions between multiple functionalities in a single active optical device. Recent progress in this field is summarized to gain insight into the physical principles behind disordered optics with regard to their advantages in various applications as well as their limitations compared to conventional optics.

Holographic displays can provide a 3D visual experience to multiple users without requiring special glasses. By precisely tailoring light fields, holographic displays could resemble realistic 3D scenes with full motion parallax and continuous depth cues. However, available holographic displays are unable to generate such scenes given practical limitations in wavefront modulation. In fact, the limited diffraction angle and small number of pixels of current wavefront modulators derive into a 3D scene with small size and narrow viewing angle. We propose a flat-panel wavefront modulator capable of displaying large dynamic holographic images with wide viewing angle. Specifically, an ultrahigh-capacity non-periodic photon sieve, which diffracts light at wide angles, is combined with an off-the-shelf liquid crystal display panel to generate holographic images. Besides wide viewing angle and large screen size, the wavefront modulator provides multi-colour projection and a small form factor, which suggests the possible implementation of holographic displays on thin devices., 3D holographic displays still face many challenges such as small viewing angle and image size. Here the authors present a scalable method for displaying large holographic images with a wide viewing angle using a wide-angle diffracting element in a flat panel; a non-periodic photon sieve.

In coherent imaging systems, parasitic fringes and concentric patterns could commonly be found due to the unwanted multiple reflections [1]. One fundamental solution for the coherent noise is to use a temporally incoherent light source. However, maintaining the full-field interference fringe contrast using temporally incoherent light is not straightforward for interferometric imaging techniques such as quantitative phase imaging (QPI). Fortunately, several brilliant incoherent QPI techniques have been realized for wide-field imaging mainly through the common-path interferometer geometries [2-4]. However, it has been more difficult to implement incoherent-light-based optical diffraction tomography (ODT) due to the additional angle-scanning illumination unit that induces severe decoherence over the camera field-of-view [5]. Here, we suggest a temporally low-coherence optical diffraction tomography by angle-scanning broadband illumination based on general Mach-Zehnder interferometric geometry. We have designed an angle-scanning unit composed of two digital micromirror devices (DMDs) to maintain interference fringe contrast across the whole field of view during the angle-scanning sequence. Further, we have developed the theoretical framework for ODT reconstruction using incoherent light. In the light of our recent developments, we will discuss the theoretical and practical constraints, and suggest the best degree of incoherency for incoherent ODT. We will also demonstrate the incoherent optical diffraction tomography of plastic microspheres, human blood cells and rat pheochromocytoma cells. References 1. I. Choi, K. Lee, and Y. Park, "Compensation of aberration in quantitative phase imaging using lateral shifting and spiral phase integration," Opt. Express 25, 30771-30779 (2017). 2. Z. Wang, L. Millet, M. Mir, H. Ding, S. Unarunotai, J. Rogers, M. U. Gillette, and G. Popescu, "Spatial light interference microscopy (SLIM)," Opt. Express 19, 1016-1026 (2011). 3. B. Bhaduri, H. Pham, M. Mir, and G. Popescu, "Diffraction phase microscopy with white light," Opt. Lett. 37, 1094-1096 (2012). 4. Y. Baek, K. Lee, J. Yoon, K. Kim, and Y. Park, "White-light quantitative phase imaging unit," Opt. Express 24, 9308-9315 (2016). 5. M. Rinehart, Y. Zhu, and A. Wax, "Quantitative phase spectroscopy," Biomed. Opt. Express 3, 958-965 (2012).

© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Temporally low-coherent optical diffraction tomography (ODT) is proposed and demonstrated based on angle-scanning Mach-Zehnder interferometry. Using a digital micromirror device based on diffractive tilting, the full-field interference of incoherent light is successfully maintained during every angle-scanning sequences. Further, current ODT reconstruction principles for temporally incoherent illuminations are thoroughly reviewed and developed. Several limitations of incoherent illumination are also discussed, such as the nondispersive assumption, optical sectioning capacity and illumination angle limitation. Using the proposed setup and reconstruction algorithms, low-coherent ODT imaging of plastic microspheres, human red blood cells and rat pheochromocytoma cells is experimentally demonstrated.

Optical design and fabrication techniques are crucial for making optical elements. From conventional lenses to diffractive optical elements, and to recent metasurfaces, various types of optical elements have been proposed to manipulate light where optical materials are fabricated into desired structures. Here, we propose a scattering optical element (SOE) which exploits multiple light scattering and wavefront shaping. Instead of fabricating optical materials, the SOE consists of a disordered medium and a photopolymer-based wavefront recorder, with shapes the wavefront of impinging light on demand. With the proposed stand-alone SOEs, we experimentally demonstrate control of various properties of light, including intensity, polarisation, spectral frequency and near field. Due to the tremendous freedom brought about by disordered media, the proposed approach will provide unexplored routes to manipulate arbitrary optical fields in stand-alone optical elements.

The development of optical and computational techniques has enabled imaging without the need for traditional optical imaging systems. Modern lensless imaging techniques overcome several restrictions imposed by lenses, while preserving or even surpassing the capability of lens-based imaging. However, existing lensless methods often rely on a priori information about objects or imaging conditions. Thus, they are not ideal for general imaging purposes. The recent development of the speckle-correlation scattering matrix (SSM) techniques facilitates new opportunities for lensless imaging and sensing. In this review, we present the fundamentals of SSM methods and highlight recent implementations for holographic imaging, microscopy, optical mode demultiplexing, and quantification of the degree of the coherence of light. We conclude with a discussion of the potential of SSM and future research directions.

Antimicrobial susceptibility test (AST) is widely used to provide the minimum inhibitory concentration of bacteria, and crucial to provide appropriate uses of antibiotics and to address the issue of drug-resistance bacteria. However, ASTs require the time-consuming incubation about 16-20 h for the visual determination of the growth of bacterial colonies, which has been a major obstacle to on-site applications of ASTs. In this study, we propose a rapid and non-invasive method based on laser speckles to evaluate the bacterial growth movements in real time, thus reducing the time for the agar dilution method. With a simple configuration compatible with conventional agar plates, the analysis of laser speckle from samples enables the early detection of the presence of growth as well as its detailed history of the colony-forming movement on agar plates. Using the samples prepared through the same procedure as the agar dilution method, we obtained the AST results at least 4-8 hours earlier than the conventional method without compromising the accuracy. This technique does not require for the use of exogenous agents, but works for most bacteria regardless of their species. Furthermore, the distinctive responses of several species to microbial agents were revealed through the present technique supporting a comprehensive analysis of the effect of the antibiotics. The findings suggest that this new method could be a useful tool for rapid, simple, and low-cost ASTs in addition to providing the historical information of the bacterial growth on agar plates.

We present a flat-panel wavefront modulator capable of generating holographic images with a large viewing angle. Specifically, a non-periodic photon sieve, which diffracts light at a wide angle, is combined with a commercial LCD panel to generate dynamic holographic images.

High-resolution imaging at long working distances is very important in optical microscopy, providing great freedom in sample and imaging conditions. However, this is challenging for a conventional refraction-based optical lens, due to optical aberrations. The authors propose a microscopic technique based on multiple scattering of light instead of refractive optics, where the complex amplitude of light is retrieved using a scattering layer. This approach will impact engineering solutions for high-resolution long-working-distance imaging in optical microscopy, and can be extended to holography for $e.g.$ particle tracking or cell pathology.

A novel optical holographic technique is presented to simultaneously measure both the real and imaginary components of the complex refractive index (CRI) of a protein solution over a wide visible wavelength range. Quantitative phase imaging was employed to precisely measure the optical field transmitted from a protein solution, from which the CRIs of the protein solution were retrieved using the Fourier light scattering technique. Using this method, we characterized the CRIs of the two dominant structural states of a photoactive yellow protein solution over a broad wavelength range (461–582 nm). The significant CRI deviation between the two structural states was quantified and analysed. The results of both states show the similar overall shape of the expected rRI obtained from the Kramers–Kronig relations.

We propose and experimentally demonstrate a method of polarization-sensitive quantitative phase imaging using two photodetectors and a digital micromirror device. Instead of recording wide-field interference patterns, finding the modulation patterns maximizing focused intensities in terms of the polarization states enables polarization-dependent quantitative phase imaging without the need for a reference beam and an image sensor. The feasibility of the present method is experimentally validated by reconstructing Jones matrices of several samples including a polystyrene microsphere, a maize starch granule, and a mouse retinal nerve fiber layer. Since the present method is simple and sufficiently general, we expect that it may offer solutions for quantitative phase imaging of birefringent materials.

We demonstrate a flat-panel wavefront modulator for generating dynamic holographic images. A nonperiodic photon sieve is combined with a liquid crystal display panel to generate holographic images with a large area and wide viewing angle.

We found an error in Fig. 1 of our article “White-light Quantitative Phase Imaging Unit.” Here we publish the revised figure.

We present a method of reference-free single-point holographic imaging by exploiting the time-reversal nature of optical phase conjugation, without the need for an image sensor and a reference beam.

We propose an optical diffuser as a holographic lens exploiting the speckle-correlation scattering matrix with the spatially random property of diffused light. Using the proposed concept, we experimentally demonstrate a holographic camera by simply placing a commercial diffuser in front of an image sensor. We also perform numerical refocusing based upon a single captured hologram of a real object. Our holographic method is simple and compact; and does not require any assumptions, additional reference light, nor multiple measurements, which is the closest realization of the ideal holographic camera.

In digital 3D holographic displays, the generation of realistic 3D images has been hindered by limited viewing angle and image size. Here we demonstrate a digital 3D holographic display using volume speckle fields produced by scattering layers in which both the viewing angle and the image size are greatly enhanced. Although volume speckle fields exhibit random distributions, the transmitted speckle fields have a linear and deterministic relationship with the input field. By modulating the incident wavefront with a digital micro-mirror device, volume speckle patterns are controlled to generate 3D images of micrometer-size optical foci with 35° viewing angle in a volume of 2 cm × 2 cm × 2 cm.

We propose and demonstrate a system for wavefront shaping, which generates optical foci through complex disordered media and achieves an enhancement factor of greater than 100,000. To exploit the 1 megapixel capacity of a digital micro-mirror device and its fast frame rate, we developed a fast and efficient method to handle the heavy matrix algebra computation involved in optimizing the focus. We achieved an average enhancement factor of 101,391 within an optimization time of 73 minutes with amplitude control. This unprecedented enhancement factor may open new possibilities for realistic image projection and the efficient delivery of energy through scattering media. (C) 2017 Optical Society of America

One of the fundamental limitations in photonics is the lack of a bidirectional transducer that can convert optical information into electronic signals or vice versa. In acoustics or at microwave frequencies, wave signals can be simultaneously measured and modulated by a single transducer. In optics, however, optical fields are generally measured via reference-based interferometry or holography using silicone-based image sensors, whereas they are modulated using spatial light modulators. Here, we propose a scheme for an optical bidirectional transducer using a spatial light modulator. By exploiting the principle of time-reversal symmetry of light scattering, two-dimensional reference-free measurement and modulation of optical fields are realized. We experimentally demonstrate the optical bidirectional transducer for optical information consisting of 128 x 128 spatial modes at visible and short-wave infrared wavelengths.

Due to its time-reversal nature, optical phase conjugation generates a monochromatic light wave which retraces its propagation paths. Here, we demonstrate the regeneration of a subwavelength optical focus by phase conjugation. Monochromatic light from a subwavelength source is scattered by random nanoparticles, and the scattered light is phase conjugated at the far-field region by coupling its wavefront into a single-mode optical reflector using a spatial light modulator. Then the conjugated beam retraces its propagation paths and forms a refocus on the source at the subwavelength scale. This is the first direct experimental realisation of subwavelength focusing beyond the diffraction limit with far-field time reversal in the optical domain.

We present white light quantitative phase imaging unit (WQIU) as a practical realization of quantitative phase imaging. WQPIU which consists of a liquid crystal retarder, birefringent crystals and polarizers, enables quantitative phase imaging at conventional microscopes with incoherent illumination

Optical measurements of the morphological and biochemical imaging of phytoplankton are presented. Employing quantitative phase imaging techniques, 3-D refractive index maps and high-resolution 2-D quantitative phase images of individual live phytoplankton are simultaneously obtained without exogenous labeling agents. In addition, biochemical information of individual phytoplankton including volume, mass, and density of individual phytoplankton are also quantitatively obtained from the measured refractive index distributions. We expect the present method to become a powerful tool for the study of phytoplankton.

Modern optical imaging possesses a huge information capacity whose corresponding space-bandwidth product (SBP) reaches tens of megapixels. However, despite the advances in optical and electronic devices, the SBP of an optical microscope is greatly limited, resulting in a reduced field of view or resolution of an image. In this paper, we exploit the Kramers–Kronig relations in digital holography to achieve high SBP imaging, demonstrating a complex amplitude image that can surpass the SBP of a bright-field image. The capability of the proposed method is demonstrated by imaging static samples and biological tissues. We successfully measure a 4.2-megapixel complex amplitude image whose bright-field counterpart exhibits 16.7 megapixels.

A cellular-level study of the pathophysiology is crucial for understanding the mechanisms behind human diseases. Recent advances in quantitative phase imaging (QPI) techniques show promises for the cellular-level understanding of the pathophysiology of diseases. To provide important insight on how the QPI techniques potentially improve the study of cell pathophysiology, here we present the principles of QPI and highlight some of the recent applications of QPI ranging from cell homeostasis to infectious diseases and cancer.

We demonstrate that simultaneous application of optical clearing agents (OCAs) and complex wavefront shaping in optical coherence tomography (OCT) can provide significant enhancement of penetration depth and imaging quality. OCA reduces optical inhomogeneity of a highly scattering sample, and the wavefront shaping of illumination light controls multiple scattering, resulting in an enhancement of the penetration depth and signal-to-noise ratio. A tissue phantom study shows that concurrent applications of OCA and wavefront shaping successfully operate in OCT imaging. The penetration depth enhancement is further demonstrated for

The word ‘holography' means a drawing that contains all of the information for light—both amplitude and wavefront. However, because of the insufficient bandwidth of current electronics, the direct measurement of the wavefront of light has not yet been achieved. Though reference-field-assisted interferometric methods have been utilized in numerous applications, introducing a reference field raises several fundamental and practical issues. Here we demonstrate a reference-free holographic image sensor. To achieve this, we propose a speckle-correlation scattering matrix approach; light-field information passing through a thin disordered layer is recorded and retrieved from a single-shot recording of speckle intensity patterns. Self-interference via diffusive scattering enables access to impinging light-field information, when light transport in the diffusive layer is precisely calibrated. As a proof-of-concept, we demonstrate direct holographic measurements of three-dimensional optical fields using a compact device consisting of a regular image sensor and a diffusor., Holographic techniques store and retrieve complete optical information, but the requirement of a reference beam can make the process complicated and sensitive to noise. Here, the authors develop a reference-free method that harnesses self-interference in a diffusive scattering medium.

Sickle cell disease (SCD) is common across Sub-Saharan Africa. However, the investigation of SCD in this area has been significantly limited mainly due to the lack of research facilities and skilled personnel. Here, we present optical measurements of individual red blood cells from healthy individuals and individuals with SCD and sickle cell trait in Tanzania using the quantitative phase imaging technique. By employing a quantitative phase imaging unit, an existing microscope in a clinic is transformed into a powerful quantitative phase microscope providing measurements on the morphological, biochemical, and biomechanical properties of individual cells. The present approach will open up new opportunities for cost-effective investigation and diagnosis of several diseases in low resource environments.

When a light propagates through highly disordered medium, its optical parameters such as amplitude, phase and polarization states are completely scrambled because of multiple scattering events. Since the multiple scattering is a fundamental optical process that contains extremely high degrees of freedom, optical information of a transmitted light is totally mingled. Until recently, the presence of multiple scattering in an inhomogeneous medium is considered as a major obstacle when manipulating a light transmitting through the medium. However, a recent development of wavefront shaping techniques enable us to control the propagation of light through turbid media; a light transmitting through a turbid medium can be effectively controlled by modulating the spatial profile of the incident light using spatial light modulator. In this work, stand-alone scattering optical device is proposed; a holographic photopolymer film, which is much economic compared to the other digital spatial light modulators, is used to record and reconstruct permanent wavefront to generate optical field behind a scattering medium. By employing our method, arbitrary optical field can be generated since the scattering medium completely mixes all the optical parameters which allow us to access all the optical information only by modulating spatial phase profile of the impinging wavefront. The method is experimentally demonstrated in both the far-field and near-field regime where it shows promising fidelity and stability. The proposed stand-alone scattering optical device will opens up new avenues for exploiting the randomness inherent in disordered medium.

We investigate energy leakage induced by incomplete measurements of the scattering matrices of complex media. Owing to the limited numerical apertures of an optical system, it is experimentally challenging to access theoretically predicted perfect transmission channels in the diffusive regime. By conducting numerical simulations on scattering matrices, we demonstrate that energy leakage contributed from uncollected transmission in the transmission matrices provides an energy transmission that is more enhanced than that predicted by measurement. On the other hand, energy leakage originating from the uncollected reflection in the partial measurement of a reflection matrix strongly suppresses the energy transmission through a zero-reflection channel, restricting the transmission enhancement to no more than a fivefold enhancement in limited optical systems. Our study provides useful insights into the effective control of energy delivery through scattering media and its ultimate limitation in practical schemes.