Hyperspectral z-scan: Measurement of spectrally resolved nonlinear optical properties.

Broadband hyperspectral z-scan using a supercontinuum light source is a convenient technique to obtain spectrally resolved nonlinear optical properties of the materials under investigation. Post-processing and segregation of the data obtained from the supercontinuum based hyperspectral z-scan measurement aids in determining the nonlinear optical properties with high spectral resolution. However, few data models exist to store and represent the large amount of information acquired from the hyperspectral z-scan measurement. In this paper, a 3D data model for representing the data obtained from broadband z-scan measurements and analysis is presented. This method would help in the quick characterization of spectrally resolved nonlinear optical properties of materials from a single z-scan measurement. The proposed model is used for obtaining the spectrally resolved nonlinear optical properties of rhodamine 6G.

Amyloid Beta42 (Aß42) Peptide Functionalized Iron Oxide Nanoparticles for Specific Targeting of SH-SY5Y Neuroblastoma Cells.

One of the most severe diseases threatening the ageing population is Alzheimer's disease (AD). Recent studies found that the cellular uptake of extracellular amyloid beta (Aß) peptides can lead to a build-up of intracellular Aß in certain neuronal cells, which consequently lead to the onset of AD pathogenesis. It is therefore hypothesized that the detection of cells that are involved in such Aß uptake could facilitate the early diagnosis of AD. In this work, a magnetofluorescent nanoprobe was prepared conjugating dye-labeled Aß42 peptides with iron oxide nanoparticles (IONPs). When incubated with SH-SY5Y cells, the cellular uptake of Aß42-IONPs was enhanced, compared to that of bare IONPs. Further, by labelling SH-SY5Y and HCT-116 cells, it was found that the Aß42-IONPs are selectively targeting the neuronal cells. This enhanced and specific neuronal targeting is attributed to the cellular uptake of extracellular amyloid by SH-SY5Y cells. In addition, the MR relaxivities of the Aß42-IONPs are preserved after the peptides functionalization. The results suggest that the Aß42 functionalized magnetofluorescent IONPs can be used as a bimodal probe to interrogate the cellular uptake of amyloid peptides.

Noninvasive and Noncontact Sequential Imaging of the Iridocorneal Angle and the Cornea of the Eye.

Purpose: High-resolution imaging of the critical anatomic structures of the eye, especially of the anterior chamber, in vivo, remains a challenge, even with currently available state-of-the-art medical imaging techniques. This study aims for the noninvasive and noncontact sequential imaging of the iridocorneal angle, especially the trabecular meshwork (TM) and the cornea of the eye in high-resolution using a newly developed imaging platform. Methods: Bessel beam scanned light sheet fluorescence microscopy is used to attain high-resolution images of the TM. The ability of the Bessel beam to self-reconstruct around obstacles increases the image contrast at the TM region inside eye by reducing scattering and shadow artifacts. With minimal modifications, the excitation arm of the developed imaging system is adapted for noncontact, high-resolution corneal imaging. Results: High-resolution images of the TM structures and cellular-level corneal structures are obtained in ex vivo porcine eyes, and subsequently in New Zealand white rabbit, in vivo. The spatial resolution of the developed system is 2.19 µm and has a noncontact working distance of 20 mm. Conclusions: A high-resolution imaging platform for noncontact sequential imaging of the TM and the cornea of the eye is developed. This imaging system is expected to be of potential interest in the evaluation and diagnosis of glaucoma and corneal diseases. Translational Relevance: The developed prototype offers the plausibility of in vivo, noncontact, and high-resolution imaging of the iridocorneal angle and cornea of the eye that will aid clinicians in diagnosing open-angle glaucoma and corneal diseases better.

Enhanced absorption in a graphene embedded 1D guided-mode-resonance structure without back-reflector and interferometrically written gratings.

A theoretical model based on the coupled mode theory is presented to calculate the absorption in a graphene embedded 1D guided-mode-resonance (GMR) structure that does not require a back reflector. The optimized graphene-GMR structure can absorb up to 70% of the incident light which far exceeds the already reported results without using any back-metal reflector or Bragg mirror. The theoretical analysis is valid for binary gratings and pyramidal gratings which are patterned using an interference lithography system. We experimentally validate our theoretical results and analyze the influence of the geometrical parameters to achieve critical coupling for the enhanced absorption.

Ultrafast volume holography for stretchable photonic structures.

Stretchability and flexibility are two key requirements for manipulating the propagation of light in compact and high-performance lab-on-a-chip systems. These requirements are best met by embedding stretchable and flexible tuning elements such as volume phase gratings (VPGs) in polydimethylsiloxane (PDMS), making them attractive alternatives to conventional rigid optical elements. However, fabrication of these PDMS VPGs is a challenge, requiring extensive modifications to PDMS or complex multi-step processes that require long processing times. In this context, we propose the concept of "ultrafast volume holography" for the fabrication of stretchable photonic structures such as tunable VPGs directly in unmodified PDMS. Our concept translates insights in heat regulation via fs repetition rate control into volumetric patterning, forming periodic refractive index modulation of 1.95 × 10-4 in the PDMS without post-processing. VPGs formed are further demonstrated as active beam steering units and tunable spectroscopic optical elements.

Breaking diffraction limit of far-field imaging via structured illumination Bessel beam microscope (SIBM).

Breaking the diffraction limit in imaging microscopes with far-field imaging options has always been the thrust challenge for optical engineers and biologists over the years. Although structured illumination microscopy and Bessel beam assisted imaging has shown the capability of imaging with sub-diffraction resolutions, they rely on the use of objective lenses with large numerical apertures (NA). Hence, they fail to sustain resolutions at larger working distances. In this context, we demonstrate a method for nanoscale resolution imaging at longer working distances, named as Structured Illumination Bessel Microscopy (SIBM). The proposed method is envisaged for both biological and engineering applications that necessitate high imaging resolutions at large working distances.

A targeted illumination optical fiber probe for high resolution fluorescence imaging and optical switching.

An optical imaging probe with targeted multispectral and spatiotemporal illumination features has applications in many diagnostic biomedical studies. However, these systems are mostly adapted in conventional microscopes, limiting their use for in vitro applications. We present a variable resolution imaging probe using a digital micromirror device (DMD) with an achievable maximum lateral resolution of 2.7 µm and an axial resolution of 5.5 µm, along with precise shape selective targeted illumination ability. We have demonstrated switching of different wavelengths to image multiple regions in the field of view. Moreover, the targeted illumination feature allows enhanced image contrast by time averaged imaging of selected regions with different optical exposure. The region specific multidirectional scanning feature of this probe has facilitated high speed targeted confocal imaging.

Note: Design considerations and characterization of a flexible snapshot hyperspectral probe.

Hyperspectral imaging is a combination of imaging and spectroscopy to give detailed spectral information for each spatial point in the imaged scene. Using the concept of integral field spectroscopy, a custom fabricated two-dimensional to one-dimensional fiber bundle has recently been reported. It is used as a flexible snapshot hyperspectral probe, which can be used as an endoscope for biomedical applications. This paper reports on the design considerations of the fiber bundle as the flexible probe in the snapshot hyperspectral imaging system. The physical characterization of the custom fabricated fiber bundle and lateral resolution of the developed hyperspectral imaging system are also analyzed and described.

Preclinical imaging of iridocorneal angle and fundus using a modified integrated flexible handheld probe.

A flexible handheld imaging probe consisting of a [Formula: see text] charge-coupled device camera, light-emitting diode light sources, and near-infrared laser source is designed and developed. The imaging probe is designed with specifications to capture the iridocorneal angle images and posterior segment images. Light propagation from the anterior chamber of the eye to the exterior is considered analytically using Snell's law. Imaging of the iridocorneal angle region and fundus is performed on ex vivo porcine samples and subsequently on small laboratory animals, such as the New Zealand white rabbit and nonhuman primate, in vivo. The integrated flexible handheld probe demonstrates high repeatability in iridocorneal angle and fundus documentation. The proposed concept and methodology are expected to find potential application in the diagnosis, prognosis, and management of glaucoma.

Characterization and optimization of illumination vector for contouring surface form and feature using DSPI.

Surface defect or damage is one of the critical factors leading to the failure of engineering materials and structures. The methodologies for the measurement of surface shape and feature or defect have been extensively explored and developed over the past few decades, including both contact and non-contact methods. Speckle pattern interferometry, as a non-contact optical method, has been demonstrated to effectively contour the surface shape through adjusting the illumination vector. However, few studies have been made to investigate the effect of the initial position of the illumination source as well as the source translation direction. In this paper, we report to carry out a study of measuring the surface form and feature using digital speckle pattern interferometry system via a slight translation of illumination source. Through theoretically analyzing the sensitivity factor along with the experimental validation, it is shown that the contouring fringe is more sensitive to the surface height with an off-axis illumination than the paraxial illumination. It is also found that translating the source along axial and lateral direction can be both used for the surface shape re-construction.

High resolution iridocorneal angle imaging system by axicon lens assisted gonioscopy.

Direct visualization and assessment of the iridocorneal angle (ICA) region with high resolution is important for the clinical evaluation of glaucoma. However, the current clinical imaging systems for ICA do not provide sufficient structural details due to their poor resolution. The key challenges in achieving high quality ICA imaging are its location in the anterior region of the eye and the occurrence of total internal reflection due to refractive index difference between cornea and air. Here, we report an indirect axicon assisted gonioscopy imaging probe with white light illumination. The illustrated results with this probe shows significantly improved visualization of structures in the ICA including TM region, compared to the current available tools. It could reveal critical details of ICA and expected to aid management by providing information that is complementary to angle photography and gonioscopy.

A four-dimensional snapshot hyperspectral video-endoscope for bio-imaging applications.

Hyperspectral imaging has proven significance in bio-imaging applications and it has the ability to capture up to several hundred images of different wavelengths offering relevant spectral signatures. To use hyperspectral imaging for in vivo monitoring and diagnosis of the internal body cavities, a snapshot hyperspectral video-endoscope is required. However, such reported systems provide only about 50 wavelengths. We have developed a four-dimensional snapshot hyperspectral video-endoscope with a spectral range of 400-1000 nm, which can detect 756 wavelengths for imaging, significantly more than such systems. Capturing the three-dimensional datacube sequentially gives the fourth dimension. All these are achieved through a flexible two-dimensional to one-dimensional fiber bundle. The potential of this custom designed and fabricated compact biomedical probe is demonstrated by imaging phantom tissue samples in reflectance and fluorescence imaging modalities. It is envisaged that this novel concept and developed probe will contribute significantly towards diagnostic in vivo biomedical imaging in the near future.

Spatial-scanning hyperspectral imaging probe for bio-imaging applications.

The three common methods to perform hyperspectral imaging are the spatial-scanning, spectral-scanning, and snapshot methods. However, only the spectral-scanning and snapshot methods have been configured to a hyperspectral imaging probe as of today. This paper presents a spatial-scanning (pushbroom) hyperspectral imaging probe, which is realized by integrating a pushbroom hyperspectral imager with an imaging probe. The proposed hyperspectral imaging probe can also function as an endoscopic probe by integrating a custom fabricated image fiber bundle unit. The imaging probe is configured by incorporating a gradient-index lens at the end face of an image fiber bundle that consists of about 50,000 individual fiberlets. The necessary simulations, methodology, and detailed instrumentation aspects that are carried out are explained followed by assessing the developed probe's performance. Resolution test targets such as United States Air Force chart as well as bio-samples such as chicken breast tissue with blood clot are used as test samples for resolution analysis and for performance validation. This system is built on a pushbroom hyperspectral imaging system with a video camera and has the advantage of acquiring information from a large number of spectral bands with selectable region of interest. The advantages of this spatial-scanning hyperspectral imaging probe can be extended to test samples or tissues residing in regions that are difficult to access with potential diagnostic bio-imaging applications.

Individual speckle diffraction based 1D and 2D Random Grating Fabrication for detector and solar energy harvesting applications.

Laser speckles and speckle patterns, which are formed by the random interference of scattered waves from optically rough surfaces, have found tremendous applications in a wide range of metrological and biomedical fields. Here, we demonstrate a novel edge diffraction phenomenon of individual speckle for the fabrication of 1D and 2D micron and sub-micron size random gratings. These random gratings exhibit broadband response with interesting diffusive diffraction patterns. As an immediate application for solar energy harvesting, significant reduction in transmission and enhanced absorption in thin "Si-random grating-Si" sandwich structure is demonstrated. This work has multifaceted significance where we exploited the individual speckle diffraction properties for the first time. Besides the solar harvesting applications, random gratings are suitable structures for fabrication of theoretically proposed random quantum well IR detectors and hence expected that this work will augur well for such studies in the near future.

Speckle lithography for fabricating Gaussian, quasi-random 2D structures and black silicon structures.

Laser speckle pattern is a granular structure formed due to random coherent wavelet interference and generally considered as noise in optical systems including photolithography. Contrary to this, in this paper, we use the speckle pattern to generate predictable and controlled Gaussian random structures and quasi-random structures photo-lithographically. The random structures made using this proposed speckle lithography technique are quantified based on speckle statistics, radial distribution function (RDF) and fast Fourier transform (FFT). The control over the speckle size, density and speckle clustering facilitates the successful fabrication of black silicon with different surface structures. The controllability and tunability of randomness makes this technique a robust method for fabricating predictable 2D Gaussian random structures and black silicon structures. These structures can enhance the light trapping significantly in solar cells and hence enable improved energy harvesting. Further, this technique can enable efficient fabrication of disordered photonic structures and random media based devices.

Pushbroom hyperspectral imaging system with selectable region of interest for medical imaging.

A spatial-scanning pushbroom hyperspectral imaging (HSI) system incorporating a video camera (VC) which is not only used for direct video imaging but also for the selection of the region of interest within the VC's full field-of-view is presented. Using a VC for these two applications brings many benefits to a pushbroom HSI system, such as a minimized data acquisition time and smaller data storage requirement. A detailed description of the system followed by the methods and formulas used for calibration and electronic hardware interfacing were discussed and analyzed using United States Air Force resolution chart, chicken breast tissue, and fluorescent targets as test samples. The proposed concepts and developed system can find potential biomedical imaging applications and can be extended to endoscopic imaging applications as well.

Integrated flexible handheld probe for imaging and evaluation of iridocorneal angle.

An imaging probe is designed and developed by integrating a miniaturized charge-coupled diode camera and light-emitting diode light source, which enables evaluation of the iridocorneal region inside the eye. The efficiency of the prototype probe instrument is illustrated initially by using not only eye models, but also samples such as pig eye. The proposed methodology and developed scheme are expected to find potential application in iridocorneal angle documentation, glaucoma diagnosis, and follow-up management procedures.

Integrated photoacoustic, ultrasound and fluorescence platform for diagnostic medical imaging-proof of concept study with a tissue mimicking phantom.

The structural and molecular heterogeneities of biological tissues demand the interrogation of the samples with multiple energy sources and provide visualization capabilities at varying spatial resolution and depth scales for obtaining complementary diagnostic information. A novel multi-modal imaging approach that uses optical and acoustic energies to perform photoacoustic, ultrasound and fluorescence imaging at multiple resolution scales from the tissue surface and depth is proposed in this paper. The system comprises of two distinct forms of hardware level integration so as to have an integrated imaging system under a single instrumentation set-up. The experimental studies show that the system is capable of mapping high resolution fluorescence signatures from the surface, optical absorption and acoustic heterogeneities along the depth (>2cm) of the tissue at multi-scale resolution (<1µm to <0.5mm).

Interferometric lithography for nanoscale feature patterning: a comparative analysis between laser interference, evanescent wave interference, and surface plasmon interference.

In this paper, we experimentally demonstrate and compare single-exposure multiple-beam interference lithography based on conventional laser interference, evanescent wave interference, and surface plasmon interference. The proposed two-beam and four-beam interference approaches are carried out theoretically and verified experimentally, employing the proposed configurations so as to realize the patterning of one- and two-dimensional periodic features on photoresists. A custom-fabricated grating is employed in the configuration in order to achieve two- and four-beam interference.

Excitation of gap modes in a metal particle-surface system for sub-30 nm plasmonic lithography.

In this Letter, a near-field optical excitation of gap modes in a metal particle-surface system for patterning periodic nanostructure is proposed and numerically demonstrated using the finite-difference time-domain method. It is observed that high-density sub-30 nm periodic structures were achievable by employing an aluminium nanosphere-silver surface system. A 2D resist profile cross section using the modified cellular automata model, which was obtained through this proposed configuration, is also presented.