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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 (Aβ) peptides can lead to a build-up of intracellular Aβ 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 Aβ uptake could facilitate the early diagnosis of AD. In this work, a magnetofluorescent nanoprobe was prepared conjugating dye-labeled Aβ42 peptides with iron oxide nanoparticles (IONPs). When incubated with SH-SY5Y cells, the cellular uptake of Aβ42-IONPs was enhanced, compared to that of bare IONPs. Further, by labelling SH-SY5Y and HCT-116 cells, it was found that the Aβ42-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 Aβ42-IONPs are preserved after the peptides functionalization. The results suggest that the Aβ42 functionalized magnetofluorescent IONPs can be used as a bimodal probe to interrogate the cellular uptake of amyloid peptides.

Dense plasma focus (DPF) device is conventionally operated in focus mode, to achieve pinch plasma with highest possible temperature and density to maximize the soft and hard x-rays and charged particles. In this paper, we report the first ever application of non-focus mode of DPF device, which is free of magneto-hydrodynamics (MHD) instabilities, for the deposition of zinc oxide thin films (ZnO TFs) on silicon substrates for various number (5, 10, 15 and 20) of non-focused deposition shots (NFDS). The X-ray diffraction (XRD) patterns of as-deposited ZnO TFs confirms the growth along (0 0 2) orientation only. The ZnO TFs are then annealed at 600 °C temperature for 2 h. The XRD patterns of annealed ZnO-TFs confirm the wurtzite phase of ZnO with (1 0 0), (0 0 2) and (1 0 0) planes with improved crystallinity. The up and down shifting of ZnO (0 0 2) diffraction plane indicates the presence of residual stresses which are reduced in annealed ZnO TF. The surface morphology, like shape, size and the distribution of rounded nano-particles, is strongly associated with increasing number of NFDS. Raman analysis shows the development of downshifted E2 (high) and upshifted A1 longitudinal optical (LO) modes centered at 430 cm−1 and 580 cm−1 compared to bulk ZnO (430 and 575 cm−1) indicating the presence of tensile residual stress due to mismatch of thermal expansion coefficient of ZnO TF and Si substrate and due to the presence of oxygen vacancies and Zn interstitials, respectively. The XPS analysis confirms the presence of Zn, Zn–O, C–O, and Zn–OH bonds. The energy band gap and refractive index of annealed ZnO-TF are found to be 3.30 eV and 1.88, respectively. A new method of high quality ZnO TFs synthesis using MHD instability free non-focusing mode of DPF device will open a new alternative synthesis technique.

Near Infra-Red Surface Enhanced Raman Spectroscopy (NIR SERS) has gained huge attention in recent years as the conventional visible SERS suffers from overwhelming fluorescence background from the fluorophore resulting in the masking of Raman signals. In this paper, we propose a novel multi-layered SERS substrate- (Cu2O - Au) - Graphene - Au - for efficient NIR SERS applications. The proposed structure has a monolayer of Cu2O - Au core-shell particles on a Au substrate with 1 nm thick graphene spacer layer. Mie simulations are used to optimize the aspect ratios of core-shell particles to shift their plasmon resonances to NIR region using MieLab software. Further, Finite Difference Time Domain (FDTD) simulations using Lumerical software are used for the design of the multiparticle layered SERS substrate as MieLab software works only for single particle systems. Designed structure is shown to provide high field enhancement factor of the order of 108 at an excitation of 1064 nm thus ensuring the possibility of using the proposed structure as efficient NIR SERS substrate which could probably be used for various NIR sensing applications. Economic Development Board (EDB) Ministry of Education (MOE) Published version The authors acknowledge NTU, COLE-EDB and Ministry of Education, MoE Singapore (RG 192/17) for the financial support.

Additive manufacturing provides flexibility in making structures according to specific geometry requirements. Advances in this technology, including sophisticated laser-based stereolithography (SLA) technique, have paved the way to fabricate structures with layer resolutions up to 25 μm. Although the feasibility of random lasing is shown in 3D-printed structures, a study on varying lasing properties using SLA printing technique with continuous wave laser pumping is not studied yet in depth. Herein, SLA is used to fabricate structures conducive for random lasing. Randomly distributed vertical cylindrical microchannels are fabricated on a 1 cm × 1 cm photoresin chip of thickness 2 mm. The geometry of structures makes it viable for liquid optical gain media to be injected into the cylindrical channels. The random lasing characteristics are demonstrated including threshold and linewidths. Tunability in peak lasing wavelength, required threshold, and minimum achievable linewidth are demonstrated by changing the diameter of the microchannels. Further, it is shown that by changing the hole diameter from 260 to 470 μm, a wavelength tunability of approximately 22 nm is achieved. This structure expects to significantly contribute for its use as an on-chip photonic sensor, wherein liquid analytes achieved can be injected and sensed using optical parameters, and for on-chip spectroscopy applications. Ministry of Education (MOE) Submitted/Accepted version The authors acknowledge the financial support received through COLE funding and MOE-AcRF Tier 1 (No.: RG119/21), Ministry of Education, Singapore.

Hybrid optical elements, which combine refractive and diffractive optical components to enhance optical performance by taking advantage of the optical characteristics of the individual components, have enormous potential for next-generation optical devices. However, there have not been many reports on the simulation methodology to characterize such hybrid optical systems. Here, we present a method for simulating a hybrid optical element realized by attaching an ultra-thin, flexible diffractive optics array onto a refractive optical element. The ultra-thin diffractive optical element is fabricated by direct-laser-writing using a femtosecond pulsed laser as the light source. A systematic investigation of the proposed simulation method, which does not require extensive hardware resources or computational time, but retains resolution and accuracy, is presented. The proposed scheme is validated by comparing simulation and experimental results. The simulation and experimental results on the spot size and focal length for the diffractive Fresnel zone plate (FZP) match well, with typical errors of less than 6%. The aspect ratio of the focal spot sizes at the compound and FZP focal planes of the hybrid optical system from the simulation and experiment also match quite well, with typical errors below 7%. This simulation scheme will expedite the designs for novel hybrid optical systems with optimal optical performances for specific applications, such as microfluidics and aberration-controlled optics. Nanyang Technological University This work was financially supported by a research collaboration agreement by Panasonic Factory Solutions Asia Pacific (PFSAP) and Singapore Centre for 3D Printing (RCA15/027); National Research Foundation of the Republic of Korea (NRF-2012R1A3A1050386, 2020R1A2C2102338, 2021R1A4A1031660); Korea Forest Service (Korea Forestry Promotion Institute) through the R & D Program for Forest Science Technology (2020229C10-2022-AC01); and Basic Research Program funded by the Korea Institute of Machinery and Materials (NK224C).

Corneal diseases, along with glaucoma and cataract, are the leading causes of blindness worldwide. While corneal surgery offers good success rates, early diagnosis and disease prevention is the preferred choice over expensive and complex surgical procedures. Non-contact, high-resolution imaging modalities for imaging the corneal structures are highly desirable for clinical examinations for early diagnosis of corneal diseases. The currently used clinical standard is the in vivo confocal laser scanning microscopy, which is a contact based method. In this context, an in-house developed, non-contact, Gaussian epi-illumination based imaging system was used to image corneal injury in preclinical animal models in vivo. New Zealand white rabbits and Wistar rats were used as the preclinical models for the investigations. The lateral resolution of the developed system is 1.1 μm. The images obtained with the developed system, as well as the dimensional information of the corneal structures, are in good agreement with the data previously reported using standard methods. The proposed system offers a simple, easy operable, real-time, non-contact imaging modality for the evaluation of corneal structures in high-resolution. Agency for Science, Technology and Research (A*STAR) Nanyang Technological University National Medical Research Council (NMRC) The authors acknowledge financial support received through A*STAR-MIG project (BMRC1619077002), SERI-NTU ADVANCED OCULAR ENGINEERING (STANCE) Program Fund, NMRC Singapore Translational Research Investigator Award (NMRC/STaR/0023/2014), NMRC Transition Award (NMRC/TA/0040/2015), and NMRC Transition Award (NMRC/TA/0057/2017) for pursuing parts of the contents presented in this paper, and the research manpower and facilities provided through funding at COLE, NTU.

Next-generation hybrid optics will provide superior performances over traditional optics by combining the advantages of refractive, reflective, and diffractive optics and metasurfaces. Hybrid optics have been realized by integrating diffractive optical structures to the top surface of traditional bulk refractive or reflective optics. However, high-resolution manufacturing requirement of diffractive patterns on top of free-form refractive or reflective optical surfaces have hindered the wide-spread dissemination of hybrid optics. In this paper, we demonstrate a transferable ultra-thin micro-optics having multi-level transmittance and phase profiles which are arbitrarily patterned by tunable photoreduction and photoablation of graphene oxides (GO) using femtosecond (fs) direct laser writing. A 5 × 5 array of multi-level ultra-thin micro diffractive lens having a focal length of 15 mm was exemplarily patterned with real-time laser power control; the resulting spot size was smaller than 14 μm with the suppression of diffractive side peaks by 14.9% at the first order and 10.8% at the second order ones. This laser-patterned diffractive lens array was successfully transferred to the surface of a refractive cylindrical lens via polydimethylsiloxane (PDMS) as the flexible/stretchable substrate; the resulting optical performance agrees well with the theoretical simulation result. This new fabrication method will pave a way to novel hybrid optical systems. Accepted version

Malfunctions in printed circuit boards (PCBs) are often caused by damaged copper traces. Printing materials such as metal nanoparticles, conductive polymers, and graphene along with novel printing methods are being actively explored for repairing the conductive connections in PCBs. Because of its high-resolution capability, direct writing of conductive traces gets significant attention, especially with the widespread use of flexible PCBs. Graphene is an ideal material for such applications due to its excellent electrical and mechanical properties. However, there have been limited reports on graphene-based methods for the facile fabrication of conductive traces. A novel method of femtosecond laser direct writing of graphene traces by the photoreduction of graphene oxide (GO) to conductive reduced GO (rGO) for repair and modification of legacy PCBs is reported. A trace-width resolution of 28.4 μm is achieved over a large patterning area of 100 mm × 100 mm. The rGO thickness is found to be tunable from 0.6 to 4.4 μm, while the sheet resistance is minimized to 100 Ω sq−1. The system capability is demonstrated by printing conductive traces on top of a flexible substrate to form a closed path for turning on a light-emitting diode, as well as, by repairing a commercial PCB. The authors acknowledge funding received from research collaboration agreement by Panasonic Factory Solutions Asia Pacific (PFSAP) and Singapore Centre for 3D Printing (RCA-15/027). Y.-J.K. acknowledges funding from National Research Foundation of the Republic of Korea (NRF-2012R1A3A1050386, NRF-2020R1A2C2102338, NRF2021R1A4A1031660), Korea Forest Service (Korea Forestry Promotion Institute) through the R&D Program for Forest Science Technology (2020229C10-2022-AC01), KAIST UP Program, and Basic Research Program (NK224C) funded by the Korea Institute of Machinery and Materials.

Non-contact, cellular-level imaging of the corneal structures of the eye is challenging. This paper in this context, proposes and demonstrates the use of random laser illumination for artefact-free imaging of corneal structures.

Superior scattering properties of plasmonic nano-urchins are studied using FDTD simulations. Along with the many-fold increase in the scattering cross-section, they offer significant enhancement in the field intensity as compared to nanospheres.

Measurement of surface roughness over a large area is a very challenging task due to the limitations with the existing techniques. Surface roughness measurement techniques including stylus and microscopy are limited by point-by-point data acquisition and a small field of view (FOV). In effect, any solution that would subdue these limitations would be characterized by its full-field nature, large FOV, and the ability to acquire and process data at high speeds. To meet these requirements, large area speckle imaging has been used to obtain areal surface roughness parameters through the processing of spectrally correlated speckle images. An automated optical system is developed for surface roughness evaluation of components with large and curved surface areas. In order to extract areal surface roughness parameters from the captured set of images, processing algorithms are developed. The methodology is first validated using a comparator plate containing areas having an average surface roughness (Ra) ranging between 0.2 µm and 0.6 µm. Further, statistical significance tests are conducted to determine the main factors affecting system performance. The measurement results are compared and validated using a 3D optical microscope. The results obtained from the blind tests performed on aerospace component surfaces as large as 450mm×210mm are also presented. Economic Development Board (EDB) Ministry of Education (MOE) National Research Foundation (NRF) Accepted version This work was conducted within the Rolls-Royce@NTU Corporate Lab MRT 4.2 project with support from the National Research Foundation (NRF) Singapore under the Corp Lab@University Scheme. The authors are also grateful for the support from COLE EDB funding. One of the authors, Patinharekandy Prabhathan contributed to this work during his role as a research fellow at Rolls-Royce@NTU Corporate Lab. Also, Aswin Haridas acknowledges NTU Singapore for the research scholarship received through NTU under the RSS scheme.

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.

Early stage detection of neurodegenerative diseases such as Alzheimer's disease (AD) is of utmost importance, as it has become one of the leading causes of death of millions of people. The gradual intellectual decline in AD patients is an outcome of fibrillation of amyloid beta 1–42 (Aβ1–42) peptides in the brain. In this paper, we present localized surface plasmon resonance (LSPR) based sensing of Aβ1–42 fibrillation using Au nano-urchins. Strongly localized field confinement at the spiky nanostructures of nano-urchin surfaces enables them to detect very low concentrations of Aβ1–42. In addition, the LSPR peak of Au nano-urchins, which is very sensitive to ambient conditions, shows significant responses at different fibrillation stages of Aβ1–42. Reduction in LSPR peak intensity with an increase in the fibrillation is chosen as the sensing parameter here. This paper in this context provides LSPR based highly sensitive, label-free and real-time sensing of Aβ1–42 fibrillation that is highly advantageous compared to the existing techniques which require binding additives or fluorescent biomarkers. Economic Development Board (EDB) Ministry of Education (MOE) Published version The authors acknowledge NTU, COLE-EDB, and Ministry of Education, MoE Singapore (RG 192/17) for financial support.

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. Nanyang Technological University Submitted/Accepted version The authors acknowledge NTU-India Connect programme and COLE-EDB, NTU for funding and research manpower support.

The use of counterfeit banknotes increases crime rates and cripples the economy. New countermeasures are required to stop counterfeiters who use advancing technologies with criminal intent. Many countries started adopting polymer banknotes to replace paper notes, as polymer notes are more durable and have better quality. The research on authenticating such banknotes is of much interest to the forensic investigators. Hyperspectral imaging can be employed to build a spectral library of polymer notes, which can then be used for classification to authenticate these notes. This is however not widely reported and has become a research interest in forensic identification. This paper focuses on the use of hyperspectral imaging on polymer notes to build spectral libraries, using a pushbroom hyperspectral imager which has been previously reported. As an initial study, a spectral library will be built from three arbitrarily chosen regions of interest of five circulated genuine polymer notes. Principal component analysis is used for dimension reduction and to convert the information in the spectral library to principal components. A 99% confidence ellipse is formed around the cluster of principal component scores of each class and then used as classification criteria. The potential of the adopted methodology is demonstrated by the classification of the imaged regions as training samples.

In recent years, additive manufacturing, also known as three-dimensional (3D) printing, has emerged as an environmentally friendly green manufacturing technology which brings great benefits, such as energy saving, less material consumption, and efficient production. These advantages are attributed to the successive material deposition at designated target areas by delivering the energy on it. In this regard, lasers are the most effective energy source in additive manufacturing since the laser beam can transfer a large amount of energy into micro-scale focal region instantaneously to solidify or cure materials in air, therefore enabling high-precision and high-throughput manufacturing for a wide range of materials. In this paper, we introduce laser-based additive manufacturing methods and review the types of lasers widely used in 3D printing machines. Important laser parameters relevant to additive manufacturing will be analyzed and general guidelines for selecting suitable lasers for additive manufacturing will be provided. Discussion on future prospects of laser technologies for additive manufacturing will be finally covered.

For powering wearable electronics, extensive research has been directed toward microscale flexible and stretchable energy-storage devices. Microsupercapacitors, though promising candidates, remain limited in terms of design flexibility, scalability, reusability, and compatibility with general substrates. This paper reports a high-performance sticker-type flexible microsupercapacitor using highly swollen reduced-graphene-oxide electrodes fabricated by an ultrashort-pulse laser to promote full active-site and durability of the electrodes. Our sticker-type flexible micropseudocapacitor provides a comparable volumetric energy density of 1.08 mWh cm−3 and 13 times higher volumetric power density of 83.5 mW cm−3 compared to conventional lithium thin-film batteries. Bio-inspired surface modifications are additionally applied to the reduced-graphene-oxide electrodes, which provides a six-fold increase (10.38 mF cm−2) of the areal capacitance. A 6 × 2 micropseudocapacitor array embedded in a sub-millimeter thin PDMS film adheres to safety goggles and successfully powers a μ-LED. The total capacitance of the array is maintained at ~97% of its original value after 200 repetitive attachments and detachments showing good durability. In addition, the sticker-type micropseudocapacitor array shows a stable performance under repeated deformation, and up to ~99% of capacitance retention after 200 bending cycles. This novel re-attachable flexible micropseudocapacitor will expedite the widespread use of flexible and wearable devices. National Research Foundation (NRF) Accepted version This work was supported by the Research and Development Program of the Korea Institute of Energy Research (KIER) [grant number B9-2461-02, B9-2434-01, B9-2434-03) (to H. Y.); and the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2018R1C1B5085456) (to Y. C.). Y.-J. Kim acknowledges financial support by an NRF Fellowship (NRF-NRFF2015-02) from the Singapore National Research Foundation and by a research collaboration agreement by Panasonic Factory Solutions Asia Pacific (PFSAP) and Singapore Centre for 3D Printing (RCA-15/027). S. K. acknowledges financial support from the Creative Materials Discovery Program (NRF-2017M3D1A1039287) and the Basic Research Lab Program (NRF-2018R1A4A1025623) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning.

The electro-optically interrogated surface plasmons device consists of a heterolayer structure formed by Au, graphene, and an ion-gel layer. Tuning of the reflectance was achieved by applying an external electric field across the heterolayer.

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. NRF (Natl Research Foundation, S’pore) MOE (Min. of Education, S’pore) EDB (Economic Devt. Board, S’pore) Accepted version

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. Published version

The design flexibility of aerospace components has been revolutionized with the recent advancements in the manufacturing of composite structures and 3D printed components. Since the manufacturing process is dependent on the control environment, variations from the nominal conditions can result in the degraded surface quality of the manufactured part. It is critical to image these surface features at high-resolutions to enable process feedback, therefore, improving the overall efficiency and process control. In this context, we develop and demonstrate a structured illumination optical fiber probe with embedded speckles for high-resolution surface feature imaging of 3D printed and composite samples. The improved sectioning capabilities by modifying the structured illumination patterns is validated using a plane mirror sample. This method is envisaged for high-resolution surface feature imaging to improve the overall manufacturing process efficiency of critical components.

Glaucoma, one of the leading causes of blindness, is an eye disease caused by irregularities in the ocular aqueous outflow system causing an elevated intraocular pressure. High resolution imaging of the aqueous outflow system comprising trabecular meshwork is immensely valuable to vision analysts and clinicians in comprehending the disease state for the efficacious analysis and treatment of glaucoma. Currently available ocular imaging devices are unable to deliver high resolution images for the visualization of the trabecular meshwork. A method to obtain high resolution (sub-micrometer) images of the trabecular meshwork using Bessel-Gauss beam scanned light sheet fluorescence microscopy is presented and the optical sectioning capability of this technique to obtain three-dimensional volumetric images of the trabecular meshwork of an intact eye without any physical dissection is demonstrated. Figure: Three-dimensional visualization of trabecular meshwork of porcine eye.

Plasmonic random lasers continue to generate significant interest in the fields of optics and photonics due to their improved performance. This owes to the greater scattering strength and enhanced local field offered by the plasmonic scatterers. However, absorption losses and fluorescence quenching are the major setbacks to plasmonic random lasers. Design of plasmonic random lasers with low threshold and enhanced emission intensity continues to be a challenging area of research. This paper demonstrates that the use of one-dimensional anisotropic plasmonic structures can help to overcome these limitations to a great extent by manipulating the spectral overlap between the scatterers and gain medium. A low threshold random lasing system has been experimentally realized in this context, using Ag nanorods with aspect ratio tuned to give optimal scattering and field enhancement. The optimal aspect ratios were deduced using FDTD simulations and incoherent random lasing was experimentally demonstrated at a low threshold of 116 µJ/cm or 0.023 MW/cm . We also demonstrate through spatial coherence measurements that the bright emission from the plasmonic random laser enables speckle contrast reduction upto 0.034 with single pulse illumination. Economic Development Board (EDB) Ministry of Education (MOE) DST- SERB (EMR/2016/003183); Ministry of Education Singapore (MOE) (RG 192/17); COLE-EDB.

Porous graphene (PG) devices fabricated in situ from polyimide (PI) adhered onto paper substrates provide a cost-effective and recycling-friendly alternative to re-engineer paper for liquid-based power sources and sensors. However, paper is generally damaged due to heating during the fabrication of PG devices. Here integrated graphene-paper electronics with exceptional thermal control through the proposed thermally localized laser graphitization (LLG) process is demonstrated, employing optimized ultrafast laser writing. LLG enables in situ fabrication of localized porous graphene (LPG) devices (>1775 K) on 65 µm thick PI tape adhered to paper without heating above 348 K. Laser parameters for LLG are predicted using an analytical temperature model and validated experimentally. The LLG is demonstrated by fabricating liquid electrolyte LPG micro-supercapacitors and humidity sensors on liquid susceptible paper. It is envisaged that the scientific concepts proposed and demonstrated here will expedite the development of low-cost, scalable, and chemically robust LPG devices on thermally sensitive substrates. Ministry of Education (MOE) Funding: This work is supported under the research collaboration agreement by Panasonic Factory Solutions Asia Pacific (PFSAP) and Singapore Centre for 3D Printing (SC3DP) (RCA-15/027). V.M.M. also acknowledges the financial support received through COLE-EDB, and MOE Tier 1 Grant RG192/17. S.W.L. acknowledges the support by Academic Research Fund Tier 2 from Ministry of Education, Singapore under ref. no. 2018-T2-1-045.

Photoacoustic spectroscopy has been used to measure optical absorption coefficient and the application of tens of wavelength bands in photoacoustic spectroscopy was reported. Using optical methods, absorption-related information is, generally, derived from reflectance or transmittance values. Hence measurement accuracy is limited for highly absorbing samples where the reflectance or transmittance is too low to give reasonable signal-to-noise ratio. In this context, this paper proposes and illustrates a hyperspectral photoacoustic spectroscopy system to measure the absorption-related properties of highly absorbing samples directly. The normalized optical absorption coefficient spectrum of the highly absorbing iris is acquired using an optical absorption coefficient standard. The proposed concepts and the feasibility of the developed diagnostic medical imaging system are demonstrated using fluorescent microsphere suspensions and porcine eyes as test samples.

The automatic calibration in Fourier-domain optical coherence tomography (FD-OCT) systems allows for high resolution imaging with precise depth ranging functionality in many complex imaging scenarios, such as microsurgery. However, the accuracy and speed of the existing automatic schemes are limited due to the functional approximations and iterative operations used in their procedures. In this paper, we present a new real-time automatic calibration scheme for swept source-based optical coherence tomography (SS-OCT) systems. The proposed automatic calibration can be performed during scanning operation and does not require an auxiliary interferometer for calibration signal generation and an additional channel for its acquisition. The proposed method makes use of the spectral component corresponding to the sample surface reflection as the calibration signal. The spectral phase function representing the non-linear sweeping characteristic of the frequency-swept laser source is determined from the calibration signal. The phase linearization with improved accuracy is achieved by normalization and rescaling of the obtained phase function. The fractional-time indices corresponding to the equidistantly spaced phase intervals are estimated directly from the resampling function and are used to resample the OCT signals. The proposed approach allows for precise calibration irrespective of the path length variation induced by the non-planar topography of the sample or galvo scanning. The conceived idea was illustrated using an in-house-developed SS-OCT system by considering the specular reflection from a mirror and other test samples. It was shown that the proposed method provides high-performance calibration in terms of axial resolution and sensitivity without increasing computational and hardware complexity.

We report our results on a plasmonic random laser with three-dimensional (3D) gold nano-urchins as scatterers distributed in Rhodamine 6G dye doped polymer film. The performance of anisotropic urchin scatterers is first studied using electromagnetic simulations for absorption/scattering cross-section and local field enhancement. This is compared to gold nanoparticles of similar size. The simulation results indicate a two-fold local field enhancement, a higher scattering cross-section, and a low absorption cross-section in the 400 nm-570 nm spectral region of interest for nano-urchins. The effective scattering mean free path for urchins is calculated to be 90 µm less than nanoparticles. This suggests nano-urchins to be efficient scatterers over conventional nanospheres for random lasing. Random laser is then experimentally demonstrated using gold nano-urchin scatterers. Incoherent random lasing is observed for very low urchin number density of order ~108 cm-3. A three-fold increase in scatterer concentration is shown to reduce the threshold energy from 0.8 mJ to 0.28 mJ per pulse. This is accompanied by a linewidth decrease from the rhodamine 6G emission bandwidth of 58 nm to up to 3 nm. The random feedback mechanism has been validated using a different spot pump scheme and angular measurement of emission. With low gold nano-urchin concentration, being incoherent and in film form, this plasmonic random laser could be an economical solution for speckle-free imaging applications. Ministry of Education (MOE) Accepted version V S Gummaluri thanks COLE-EDB for funding, R Gayathri thanks NTU-India connect for the joint degree program. V M Murukeshan acknowledges the Ministry of Education, MOE Singapore (RG 192/17) for financial support.

Glaucoma is one of the leading cause of blindness characterized by increased intra ocular pressure (IOP), visual field defects and irreversible loss of vision. Remedial intervention of glaucoma primarily aims at the reduction of IOP and subsequent examination concerning the related anomalies in the aqueous outflow system (AOS) especially with newer angle procedures. Thus, high resolution imaging of the iridocorneal angle (ICA) region comprising trabecular meshwork (TM) is extremely valuable to clinicians and vision analysts in comprehending the disease state for the efficacious analysis and treatment of glaucoma. Imaging of the AOS inside the eye using the digitally scanned Bessel-Gauss beam light sheet microscopy has been used in this study to obtain high resolution optical sections with minimal phototoxicity and photobleaching. This paper investigates the effect of long working distance objectives in obtaining high resolution TM images while offering non-contact and non-invasive approach in imaging. A series of experiments were conducted to optimize various imaging parameters using porcine eyes as test samples. Investigations carried out by illuminating both the anterior segment region and limbal region resulted in promising results. A delineated network of collagen fibers in a meshwork fashion can be clearly seen in the obtained images of the TM. The optical sectioning capability of this technique is demonstrated and the structural features match well with previous literature reports. Agency for Science, Technology and Research (A*STAR) Accepted version The authors acknowledge the financial support received through A*STAR-MIG project (BMRC1619077002) and the facilities and research manpower provided through COLE-EDB funding.

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. EDB (Economic Devt. Board, S’pore) MOE (Min. of Education, S’pore) Published version

In this study, we present diffractive optical elements printed on reduced graphene oxide (rGO) thin film using a femtosecond (fs) laser as the direct laser writing source. Graphene oxide (GO) is an interesting optical material providing unique properties such as the large light absorption modulation level during its laser reduction process; after the photoreduction, the transparent GO become opaque rGO. In order to fabricate the diffractive optical pattern of GO and rGO with sub-micrometer resolution, we use femtosecond laser pulses to induce the photoreduction of GO resulting in the ultrathin binary diffractive optical elements such as a light diffraction grating and a Fresnel lens. In addition, the rGO can play a role as a complaint electrode so that the rGO pattern can be applied to a dielectric elastomer actuation (DEA) after the pattern is transferred to a dielectric elastomer substrate with the few tens of micrometer thickness. As a high voltage is applied to the flexible substrate, it can be stretched, which leads to the expansion of the diffractive element; it means the light diffraction pattern of the element can be tunable as well. Therefore, we could control the optical specifications of the elements, such as grating period and the focal length of the Fresnel lens. Because of the simple fabrication process and arbitrary patterning capability of the femtosecond laser direct writing will provide ultrathin, tunable, compact, cost-effective, and highly efficient optical components over conventional optics.

Light transport and the physical phenomena related to light propagation in random media are very intriguing, they also provide scope for new paradigms of device functionality, most of which remain unexplored. Here we demonstrate, experimentally and by simulation, a novel kind of asymmetric light transmission (diffusion) in a stack of random media (SRM) with graded transport mean free path. The structure is studied in terms of transmission, of photons propagated through and photons generated within the SRM. It is observed that the SRM exhibits asymmetric transmission property with a transmission contrast of 0.25. In addition, it is shown that the SRM works as a perfect optical low-pass filter with a well-defined cutoff wavelength at 580 nm. Further, the photons generated within the SRM found to exhibit functionality similar to an optical diode with a transmission contrast of 0.62. The basis of this functionality is explained in terms of wavelength dependent photon randomization and the graded transport mean free path of SRM. MOE (Min. of Education, S’pore) Accepted version

Surface plasmon polariton (SPP) waveguide-coupled back reflector geometry is proposed for efficient light trapping and broadband absorption enhancement in thin-film silicon solar cells. The proposed geometry takes advantage of the localized surface plasmon (LSP) enhancement, Fabry-Perot (FP) resonance, and strong electric field confinement resulting from the SPP interference in a metal waveguide. It is shown that the designed light trapping structures contribute to significant light trapping and enhancement in the red to near-infrared part of the solar spectrum. For a thin-film silicon solar cell of 220-nm thickness, an absorption enhancement of 153 % is obtained when compared to a bare silicon solar cell. In comparison to other SPP-excited back reflection geometries, such as nano-gratings and nano-grooves, the proposed configuration shows a higher absorption enhancement factor and uniform field distribution inside the silicon layer. These results are expected to introduce new directions in the design of optimized nanoscale back reflectors in thin-film silicon solar cells. MOE (Min. of Education, S’pore) Accepted Version