Dielectric Metasurface Enabled Compact, Single-Shot Digital Holography for Quantitative Phase Imaging.

Quantitative phase imaging (QPI) enables nondestructive, real-time, label-free imaging of transparent specimens and can reveal information about their fundamental properties such as cell size and morphology, mass density, particle dynamics, and cellular fluctuations. Development of high-performance and low-cost quantitative phase imaging systems is thus required in many fields, including on-site biomedical imaging and industrial inspection. Here, we propose an ultracompact, highly stable interferometer based on a single-layer dielectric metasurface for common path off-axis digital holography and experimentally demonstrate quantitative phase imaging. The interferometric imaging system leveraging an ultrathin multifunctional metasurface captures image plane holograms in a single shot and provides quantitative phase information on the test samples for extraction of its physical properties. With the benefits of planar engineering and high integrability, the proposed metasurface-based method establishes a stable miniaturized QPI system for reliable and cost-effective point-of-care devices, live cell imaging, 3D topography, and edge detection for optical computing.

Saturable absorption assisted nonlinear structured illumination microscopy.

We propose a novel, to the best of our knowledge, super-resolution technique, namely saturable absorption assisted nonlinear structured illumination microscopy (SAN-SIM), by exploring the saturable absorption property of a material. In the proposed technique, the incident sinusoidal excitation is converted into a nonlinear illumination by propagating through a saturable absorbing material. The effective nonlinear illumination possesses higher harmonics which multiply fold high frequency components within the passband and hence offers more than two-fold resolution improvement over the diffraction limit. The theoretical background of the technique is presented, supported by the numerical results. The simulation is performed for both symmetric as well as random samples where the raw moiré frames are processed through a blind reconstruction approach developed for the nonlinear SIM. The results demonstrate the super-resolution capability of the proposed technique.

Characterization of the olfactory system of the giant honey bee, Apis dorsata.

Apis dorsata is an open-nesting, undomesticated, giant honey bee found in southern Asia. We characterized a number of aspects of olfactory system of Apis dorsata and compared it with the well-characterized, western honeybee, Apis mellifera, a domesticated, cavity-nesting species. A. dorsata differs from A. mellifera in nesting behavior, foraging activity, and defense mechanisms. Hence, there can be different demands on its olfactory system. We elucidated the glomerular organization of A. dorsata by creating a digital atlas for the antennal lobe and visualized the antennal lobe tracts and localized their innervations. We showed that the neurites of Kenyon cells with cell bodies located in a neighborhood in calyx retain their relative neighborhoods in the pedunculus and the vertical lobe forming a columnar organization in the mushroom body. The vertical lobe and the calyx of the mushroom body were found to be innervated by extrinsic neurons with cell bodies in the lateral protocerebrum. We found that the species was amenable to olfactory conditioning and showed good learning and memory retention at 24 h after training. It was also amenable to massed and spaced conditioning and could distinguish trained odor from an untrained novel odor. We found that all the above mentioned features in A. dorsata are very similar to those in A. mellifera. We thereby establish A. dorsata as a good model system, strikingly similar to A. mellifera despite the differences in their nesting and foraging behavior.

Method for single-shot fabrication of chiral woodpile photonic structures using phase-controlled interference lithography.

In this report, we propose a large-area, scalable and reconfigurable single-shot optical fabrication method using phase-controlled interference lithography (PCIL) to realize submicrometer chiral woodpile photonic structures. This proposed technique involves a 3 + 3 double-cone geometry with beams originated from a computed phase mask displayed on a single spatial light modulator. Simulation studies show the filtering response of such structures for linearly polarized plane wave illumination, with structural features tunable through a single parameter of interference angle. Further, these single chiral woodpile structures show dual chirality on illumination with both right circularly and left circularly polarized light through simulation. Experimentally fabricated patterns on photoresist show resemblance to the desired chiral woodpile structures.

High azimuthal angle tolerant dual-channel wavelength filter from visible to NIR using conically mounted guided mode resonance structures.

We present a concept to design narrow linewidth dual-channel wavelength filters using the principle of wavelength tuning under conical mounting of guided mode resonance structure. The general procedure for the design of such filters from visible to NIR wavelength range is presented and validated experimentally. We show that already fabricated guided mode resonance structures that do not show dual wavelength filtering at these wavelengths in classical mounting can exhibit dual wavelength filtering in conical mounting. Using this principle, we design high azimuthal angle tolerant guided mode resonance dual wavelength filters at C-band communication wavelengths (1310 and 1550 nm) that are insensitive to azimuthal angle over a range of up to 20 deg, achieved in expense of a tolerance in the angle of incidence that is less than 3 deg.

Evolutionarily conserved anatomical and physiological properties of olfactory pathway through fourth-order neurons in a species of grasshopper (Hieroglyphus banian).

Olfactory systems of different species show variations in structure and physiology despite some conserved features. We characterized the olfactory circuit of the grasshopper Hieroglyphus banian of family Acrididae (subfamily: Hemiacridinae) and compared it to a well-studied species of locust, Schistocerca americana (subfamily: Cyrtacanthacridinae), also belonging to family Acrididae. We used in vivo electrophysiological, immunohistochemical, and anatomical (bulk tract tracing) methods to elucidate the olfactory pathway from the second-order neurons in antennal lobe to the fourth-order neurons in ß-lobe of H. banian. We observe conserved anatomical and physiological characteristics through the fourth-order neurons in the olfactory circuit of H. banian and S. americana, though they are evolutionarily divergent (~ 57 million years ago). However, we found one major difference between the two species-there are four antennal lobe tracts in H. banian, while only one is reported in S. americana. Besides, we have discovered a new class of bilateral neurons which respond weakly to olfactory stimuli, even though they innervate densely downstream of Kenyon cells.

Evidence for absence of bilateral transfer of olfactory learned information in Apis dorsata and Apis mellifera.

The capacity and condition under which the lateral transfer of olfactory memory is possible in insects is still debated. Here, we present evidence in two species of honeybees, Apis mellifera and Apis dorsata, consistent with the lack of ability to transfer olfactory associative memory in a proboscis extension response (PER) associative conditioning paradigm, where the untrained antenna is blocked by an insulating coat. We show that the olfactory system on each side of the bee can learn and retrieve information independently and the retrieval using the antenna on the side contralateral to the trained one is not affected by the training. Using the setup in which the memory on the contralateral side has been reported at 3 h after training, we see that the memory is available on the contralateral side immediately after training. In the same setup, coating the antenna with an insulator on the training side does not prevent learning, pointing to a possible insufficiency of the block of odor stimuli in this setup. Moreover, the behavior of the bee as a whole can be predicted if the sides are assumed to learn and store independently, and the organism as a whole is able to retrieve the memory if either of the sides have the memory.

Study of electric fields of diffraction from spatial light modulator: discussion.

A complete study of electric field vectors and efficiencies of diffraction orders for a phase pattern addressed to a pixelated spatial light modulator (SLM) is discussed here. General mathematical expressions of electric field vectors from SLM are explored here analytically for an arbitrary pattern on SLM with a given input electric field. Using the general expression, we calculate orientations of the electric fields of diffraction for sinusoidal and binary patterns of varying duty cycles. The patterns result in diffracted beams of various orders with different efficiencies calculated analytically and matching well with the experimental results. The binary pattern with 50% duty cycle delivers significant distribution of energies where all the even orders are absent, and the diffraction efficiency of the first-order beam can be close to 40% using an appropriately chosen modulation depth. Using the general expression, it is possible to obtain fields and efficiencies of diffraction for any arbitrary pattern on SLM. The discussion might help researchers using SLM in standard optics experiments.

Metamaterial structures of variable and gradient basis orientations embedded with periodic linear defects: phase engineered design, single step optical realization, and applications.

Metamaterial structures of different basis shapes and orientations and with gradient refractive index variations are applicable in integrated photonics, miniaturized optoelectronics, diffraction limited focusing, and super-resolution imaging. We present design and experimental realizations of gradient metamaterial structures embedded with linear periodic defects and propose its applications in on-substrate color filtering through a simulation-based study. A combination of phase engineered plane beams in double cone geometry and an axial plane beam are interfered to obtain different gradient basis metamaterial structures with linear defects in two dimensions and three dimensions, respectively. The defect size and spatial gradient amplitude modulations can be controlled computationally through shifts in the interference angle for some of the plane beams in double cone geometry without changing any optical components in the experiment. The designed and realized metamaterial structures upon transferring to certain materials will find application in optical circuits and metalenses for enhanced light matter interactions.

Cytoskeletal Mechanisms of Axonal Contractility.

Mechanotransduction is likely to be an important mechanism of signaling in thin, elongated cells such as neurons. Maintenance of prestress or rest tension may facilitate mechanotransduction in these cells. In recent years, functional roles for mechanical tension in neuronal development and physiology are beginning to emerge, but the cellular mechanisms regulating neurite tension remain poorly understood. Active contraction of neurites is a potential mechanism of tension regulation. In this study, we have explored cytoskeletal mechanisms mediating active contractility of neuronal axons. We have developed a simple assay in which we evaluate contraction of curved axons upon trypsin-mediated detachment. We show that curved axons undergo contraction and straighten upon deadhesion. Axonal straightening was found to be actively driven by actomyosin contractility, whereas microtubules may subserve a secondary role. We find that although axons show a monotonous decrease in length upon contraction, subcellularly, the cytoskeleton shows a heterogeneous contractile response. Further, using an assay for spontaneous development of tension without trypsin-induced deadhesion, we show that axons are intrinsically contractile. These experiments, using novel experimental approaches, implicate the axonal cytoskeleton in tension homeostasis. Our data suggest that although globally, the axon behaves as a mechanical continuum, locally, the cytoskeleton is remodeled heterogeneously.

Fabrication of helical photonic structures with submicrometer axial and spatial periodicities following "inverted umbrella" geometry through phase-controlled interference lithography.

In this Letter we report for the first time, to the best of our knowledge, a phase spatial light modulator (SLM)-based interference lithography (IL) approach for the realization of hexagonally packed helical photonic structures with a submicrometer scale spatial, as well as axial, periodicity over a large area. A phase-only SLM is used to electronically generate six phase-controlled plane beams. These six beams from the front side and a direct central backside beam are used together in an "inverted umbrella" geometry setup to realize the desired submicrometer axial periodic chiral photonic structures through IL. The realized structures with 650 nm spatial and 353 nm axial periodicities on negative photoresist can be used as an optical filter and refractive index sensor, as evidenced from the FDTD-based simulation study on its optical properties. Further, the fabricated templates can be transferred to metals such as silver or aluminum for the realization of a metamaterial-based broadband circular polarizer ranging from 1 to 3.5 µm of near-infrared spectra.

Design and fabrication of woodpile photonic structures through phase SLM-based interference lithography for omnidirectional optical filters.

In this Letter, we report a large-area and single-step optical fabrication technique based on phase engineering interference lithography that is scalable and reconfigurable for the realization of submicrometer scale periodic face-centered cubic inverse woodpile photonic structures. The realized inverse woodpile structure on positive having four number axial layers with 740 nm spatial and 1046 nm axial periodicities shows 10% reflectance and 90% transmittance at 776 nm wavelength that can further be improved for the addition of axial layers. The realized structure can be transferred to crystalline silicon for realizing a bandpass/rejection near-infrared filter in a reflection/transmission mode. Further, woodpile structures based on low-contrast silicon nitride (Si3N4) are designed as selective narrow frequency filters at 1310 and 1550 nm wavelengths for telecommunication applications and omnidirectional red-green-blue filters for display devices by tuning the design parameters.

ZnO nanowire-enabled light funneling effect for antireflection and light convergence applications.

We present a new light trapping technique to reduce reflection loss, as well as for light, focusing at submicron scales for solar cell and image sensing applications. We have fabricated hexagonal arrays of ZnO funnel-like structures on Si substrate by the patterned growth of ZnO nanowires in a hydrothermal growth process. The funnels are optimized so that the effective refractive index along the vertical direction decreases gradually from the Si surface to the top of funnel to reduce Fresnel reflection at a device-air interface. Finite difference time domain simulation is used for optimization of the minimum reflectivity and to analyze optical properties such as angle dependency, polarization dependency, and funneling effect. The structures function similar to a GRIN lens in light trapping and convergence. An optimized structure reduces the average reflectivity close to 3% in the wavelength range of 300-1200 nm with the possibility of confining incident light to a few hundreds of nanometers.

Influence of periodic texture profile and parameters for enhanced light absorption in amorphous silicon ultra-thin solar cells.

We have investigated the antireflection and light trapping properties of two-dimensional grating arrays in the hexagonal symmetry with various texture morphologies. Optical simulation based on finite-difference time-domain (FDTD) analysis is carried out to understand the role of the structure profile for different periodicities and heights to achieve enhanced light trapping. The considered active medium of interest is 200-nm-thick hydrogenated amorphous silicon. Although the considered texture profiles possess an incremental change of refractive index from incident medium to active medium, a parabolic-shaped front side texture provides better antireflection effects owing to its high diffraction efficiencies in the higher-order modes as compared to other pattern morphologies. In the back side texture, the parabolic-shaped pattern also dominates with better light trapping efficiencies due to its ability to distribute a major amount of diffracted energy in the higher-order modes. The average reflection calculations in the wavelength range of 300-800 nm confirm that in both side textures, a periodicity of 500 nm with a height of 200 nm can be preferentially recommended for less reflection loss and improved scattering in oblique angles. The quantum efficiency calculation verifies that a device designed with these optimized parameters can offer improved efficiency for ultra-thin solar cells.

Single-step optical realization of bio-inspired dual-periodic motheye and gradient-index-array photonic structures.

This Letter demonstrates a single-step optical realization method for hexagonal and square lattice-based dual periodic motheye and gradient-index-array photonic structures over large areas. Computed phase mask of gradient interference patterns are used as inputs to a phase-only spatial light modulator (SLM), and the first-order diffracting beams are coherently superposed with the help of a 2f-2f Fourier filtering setup to avoid complex optical geometry for generation and control of individual beams. The simulated interference patterns are verified experimentally through a CMOS camera. The fabricated micro-structures on a positive photoresist are shown to have a major periodicity of 638 µm and minor periodicity of 25.2 µm, with the air hole diameter varying from 22.7 to 6.9 µm along the X and Y axes. The depth of the fabricated structure gradually varies from 4.203 µm at the center to 1.818 µm at the corner. These structures may be scaled down to submicron features that can show improved anti-reflection properties for solar energy harvesting and GRIN lens for optical wavelength region.

Submicrometer photonic structure fabrication by phase spatial-light-modulator-based interference lithography.

We present a large-area and single-step fabrication approach based on phase spatial light modulator (SLM)-assisted interference lithography for the realization of submicrometer photonic structures on photoresist. A multimirror beam steering unit is used to reflect the SLM-generated phase-engineered beams leading to a large angle between interfering beams while also preserving the large area of the interfering plane beams. Both translational and rotational periodic submicrometer structures are experimentally realized. This approach increases the flexibility of interference lithography to fabricate more complex submicrometer photonic structures and photonic metamaterial structures for future applications.

High sensitivity refractive index sensor based on simple diffraction from phase grating.

We present a technique for refractive index sensing using a phase grating structure. A grating under normal incidence can be designed such that the first-order diffracted light travels at a diffraction angle of 90° with respect to the zeroth order. The diffracted light, which is along the direction of periodicity, can further be diffracted from the grating and interfere with the zeroth-order light. Under this condition, the π phase difference that arises between the two interfering beams results in a transmission dip. We can tune this dip wavelength for senor applications, based on the grating equation. This Letter presents both simulation and experimental data that show good agreement with each other.

N-single-helix photonic-metamaterial based broadband optical range circular polarizer by induced phase lags between helices.

In this work, we have designed a photonic-metamaterial based broadband circular polarizer using N=4 phase-lagged aluminum single helices arranged in a square array as a unit cell. The effect of phase differences between the helices in an array on the optical performance of the structure is studied, and a comparative study is done with that of multi-intertwined helices. It is observed that the proposed metamaterial structure shows circular polarization sensitivity over a broad optical wavelength range (≈450-900 nm), with improved optical performance in average extinction ratio and broad positive circular dichroism in comparison to multiple intertwined helices. The induced phase lag between the helices in a square-array based unit cell reduces the linear birefringence and leads to the recovery of circular space symmetry in the structure.

Generating a hexagonal lattice wave field with a gradient basis structure.

We present a new, single-step approach for generating a hexagonal lattice wave field with a gradient local basis structure. We incorporate this by coherently superposing two (or more) hexagonal lattice wave fields, which differ in their basis structures. The basis of the resultant lattice wave field is highly dependent on the relative strengths of constituent wave fields, and a desired spatial modulation of basis structure is thus obtained by controlling the spatial modulation of relative strengths of constituent wave fields. The experimental realization of gradient lattice is achieved by using a phase-only spatial light modulator (SLM) in an optical 4f Fourier filter setup where the SLM is displayed with a numerically calculated gradient phase mask. The presented method is wavelength independent and is completely scalable, making it promising for microfabrication of corresponding structures.

Digitally reconfigurable complex two-dimensional dual-lattice structure by optical phase engineering.

We present a method to combine two periodic lattice wave fields to generate a complex dual-lattice wave field which could be employed for microfabrication of corresponding two-dimensional dual-lattice structures. Since the addition of two periodic lattice wave fields is coherent in nature, the resultant dual-lattice structure is highly dependent on the relative phase difference between constituent wave fields. We show that it is possible to have control over the dual-lattice pattern by precisely controlling this relative phase difference. This control is enabled by making use of digitally addressable phase-only spatial light modulator (SLM). We provide the computational method for calculation of the corresponding phase mask to be displayed on the SLM and also verify the results experimentally by employing a simple 4f Fourier filter-based geometry. The method is completely scalable and reconfigurable in terms of the choice of periodic lattice wave fields and has the potential to form gradient phase masks which could be useful for fabrication of graded-index optical components.