Fresnel incoherent correlation holography with single camera shot

Fresnel incoherent correlation holography(FINCH)is a self-interference based super-resolution three-dimensional imaging technique.FINCH in inline configuration requires an active phase modulator to record at least three phase-shifted camera shots to reconstruct objects without twin image and bias terms.In this study,FINCH is realized using a randomly multiplexed bifocal binary diffractive Fresnel zone lenses fabricated using electron beam lithography.The object space is calibrated by axially scanning a point object along the optical axis and recording the corresponding point spread holograms(PSHs).An object is mounted within the calibrated object space,and the object hologram was recorded under identical experimental conditions used for recording the PSHs.The image of the object at different depths was reconstructed by a cross-correlation between the object hologram and the PSHs.Application potential including bio-medical optics is discussed.

Beyond Lambertian light trapping for large-area silicon solar cells:fabrication methods

Light trapping photonic crystal(PhC)patterns on the surface of Si solar cells provides a novel opportunity to approach the theoretical efficiency limit of 32.3%,for light-to-electrical power conversion with a single junction cell.This is beyond the efficiency limit implied by the Lambertian limit of ray trapping~29%.The interference and slow light effects are harnessed for collecting light even at the long wavelengths near the Si band-gap.We compare two different methods for surface patterning,that can be extended to large area surface patterning:1)laser direct write and 2)step-&-repeat 5×reduction projection lithography.Large area throughput limitations of these methods are compared with the established elec-tron beam lithography(EBL)route,which is conventionally utilised but much slower than the presented methods.Spec-tral characterisation of the PhC light trapping is compared for samples fabricated by different methods.Reflectance of Si etched via laser patterned mask was~7%at visible wavelengths and was comparable with Si patterned via EBL made mask.The later pattern showed a stronger absorbance than the Lambertian limit6.

Single-shot mid-infrared incoherent holography using Lucy-Richardson-Rosen algorithm

In recent years,there has been a significant transformation in the field of incoherent imaging with new possibilities of compressing three-dimensional(3D)information into a two-dimensional intensity distribution without two-beam interference(TBI).Most of the incoherent 3D imagers without TBI are based on scattering by a random phase mask exhibiting sharp autocorrelation and low cross-correlation along the depth.Consequently,during reconstruction,high lateral and axial resolutions are obtained.Imaging based on scattering requires an astronomical photon budget and is therefore precluded in many power-sensitive applications.In this study,a proof-of-concept 3D imaging method without TBI using deterministic fields has been demonstrated.A new reconstruction method called the Lucy-Richardson-Rosen algorithm has been developed for this imaging concept.We believe that the proposed approach will cause a paradigm-shift in the current state-of-the-art incoherent imaging,fluorescence microscopy,mid-infrared fingerprinting,astronomical imaging,and fast object recognition applications.

Subtle Variations in Surface Properties of Black Silicon Surfaces Influence the Degree of Bactericidal Efficiency

One of the major challenges faced by the biomedical industry is the development of robust synthetic surfaces that can resist bacterial colonization. Much inspiration has been drawn recently from naturally occurring mechano-bactericidal surfaces such as the wings of cicada(Psaltoda claripennis) and dragonfly(Diplacodes bipunctata) species in fabricating their synthetic analogs. However,the bactericidal activity of nanostructured surfaces is observed in a particular range of parameters reflecting the geometry of nanostructures and surface wettability. Here,several of the nanometer-scale characteristics of black silicon(bSi) surfaces including the density and height of the nanopillars that have the potential to influence the bactericidal efficiency of these nanostructured surfaces have been investigated. The results provide important evidence that minor variations in the nanoarchitecture of substrata can substantially alter their performance as bactericidal surfaces.

Top-down control of bottom-up material synthesis@nanoscale

In a recently published paper in Opto-Electronic Advances,Mangirdas Malinauskas and his colleagues at Vilnius University report on a new high resolution inorganic material synthesis pathway using 3D laser printing and calcination.3D metamaterials(beyond what exist in nature)emerge from research in high-resolution 3D polymerisation/printing where proportions of inorganic parts of a composit can be widely tuned1.By adding a high temperature annealing(HTA)post-fabrication step of sintering/calcination in air/oxygen flow2−5 after the 3D polymerisation,a new glass and/or ceramic 3D structure with nanoscale features and large down-sizing can be produced6 as shown in Fig.1.

Fine 3D control of THz emission in air with dual femtosecond laser pre-pulses at tunnelling ionisation regime

Emission of THz radiation from air breakdown at focused ultra-short fs-laser pulses(800 nm/35 fs)was investigated for the 3D spatio-temporal control where two pre-pulses are used before the main-pulse.The laser pulse induced air breakdown forms a~120μm-long focal volume generate shockwaves which deliver a denser air into the focal region of the main pulse for enhanced generation of THz radiation at 0.1-2.5 THz spectral window.The intensity of 162 pre-and main-pulses was at the tunnelling ionisation intensities(1-3)×10 W/cm and corresponded to sub-critical(transparent)plasma formation in air.Polarisation analysis of THz radiation revealed that orientation of the air density gradients generated by pre-pulses and their time-position locations defined the ellipticity of the generated THz electrical field.The rotational component of electric current is the origin of THz radiation.

Ultrafast laser processing of materials: from science to industry

Processing of materials by ultrashort laser pulses has evolved significantly over the last decade and is starting to reveal its scientific,technological and industrial potential.In ultrafast laser manufacturing,optical energy of tightly focused femtosecond or picosecond laser pulses can be delivered to precisely defined positions in the bulk of materials via two-/multi-photon excitation on a timescale much faster than thermal energy exchange between photoexcited electrons and lattice ions.Control of photoionization and thermal processes with the highest precision,inducing local photomodification in sub-100-nm-sized regions has been achieved.State-of-the-art ultrashort laser processing techniques exploit high 0.1–1μm spatial resolution and almost unrestricted three-dimensional structuring capability.Adjustable pulse duration,spatiotemporal chirp,phase front tilt and polarization allow control of photomodification via uniquely wide parameter space.Mature opto-electrical/mechanical technologies have enabled laser processing speeds approaching meters-per-second,leading to a fast lab-to-fab transfer.The key aspects and latest achievements are reviewed with an emphasis on the fundamental relation between spatial resolution and total fabrication throughput.Emerging biomedical applications implementing micrometer feature precision over centimeter-scale scaffolds and photonic wire bonding in telecommunications are highlighted.

Plasmonic nano-printing: large-area nanoscale energy deposition for efficient surface texturing

The lossy nature of plasmonic wave due to absorption is shown to become an advantage for scaling-up a large area surface nanotexturing of transparent dielectrics and semiconductors by a self-organized sub-wavelength energy deposition leading to an ablation pattern—ripples—using this plasmonic nano-printing.Irreversible nanoscale modifications are delivered by surface plasmon polariton(SPP)using:(i)fast scan and(ii)cylindrical focusing of femtosecond laser pulses for a high patterning throughput.The mechanism of ripple formation on ZnS dielectric is experimentally proven to occur via surface wave at the substrate–plasma interface.The line focusing increase the ordering quality of ripples and facilitates fabrication over wafer-sized areas within a practical time span.Nanoprinting using SPP is expected to open new applications in photo-catalysis,tribology,and solar light harvesting via localized energy deposition rather scattering used in photonic and sensing applications based on re-scattering of SPP modes into far-field modes.

O-FIB:far-field-induced near-field breakdown for direct nanowriting in an atmospheric environment

Nanoscale surface texturing,drilling,cutting,and spatial sculpturing,which are essential for applications,including thin-film solar cells,photonic chips,antireflection,wettability,and friction drag reduction,require not only high accuracy in material processing,but also the capability of manufacturing in an atmospheric environment.Widely used focused ion beam(FIB)technology offers nanoscale precision,but is limited by the vacuum-working conditions;therefore,it is not applicable to industrial-scale samples such as ship hulls or biomaterials,e.g.,cells and tissues.Here,we report an optical far-field-induced near-field breakdown(O-FIB)approach as an optical version of the conventional FIB technique,which allows direct nanowriting in air.The writing is initiated from nanoholes created by femtosecondlaser-induced multiphoton absorption,and its cutting“knife edge”is sharpened by the far-field-regulated enhancement of the optical near field.A spatial resolution of less than 20 nm(λ/40,withλbeing the light wavelength)is readily achieved.O-FIB is empowered by the utilization of simple polarization control of the incident light to steer the nanogroove writing along the designed pattern.The universality of near-field enhancement and localization makes O-FIB applicable to various materials,and enables a large-area printing mode that is superior to conventional FIB processing.

Biomimetic sapphire windows enabled by inside-out femtosecond laser deep-scribing

Femtosecond laser machining of biomimetic micro/nanostructures with high aspect ratio(larger than 10)on ultrahard materials,such as sapphire,is a challenging task,because the uncontrollable surface damage usually results in poor surface structures,especially for deep scribing.Here,we report an inside-out femtosecond laser deep scribing technology in combination with etching process for fabricating bio-inspired micro/nanostructures with high-aspect-ratio on sapphire.To effectively avoid the uncontrollable damage at the solid/air interface,a sacrificial layer of silicon oxide was employed for surface protection.High-quality microstructures with an aspect ratio as high as 80:1 have been fabricated on sapphire surface.As a proof-of-concept application,we produced a moth-eye inspired antireflective window with sub-wavelength pyramid arrays on sapphire surface,by which broadband(3-5μm)and high transmittance(98%at 4μm,the best results reported so far)have been achieved.The sacrificial layer assisted inside-out femtosecond laser deep scribing technology is effective and universal,holding great promise for producing micro/nanostructured optical devices.

Review of Fresnel incoherent correlation holography with linear and non-linear correlations

Fresnel incoherent correlation holography(FINCH)is a well-established incoherent imaging technique.In FINCH,three selfinterference holograms are recorded with calculated phase differences between the two interfering,differently modulated object waves and projected into a complex hologram.The object is reconstructed without the twin image and bias terms by a numerical Fresnel back propagation of the complex hologram.A modified approach to implement FINCH by a single camera shot by pre-calibrating the system involving recording of the point spread function library and reconstruction by a nonlinear cross correlation has been introduced recently.The expression of the imaging characteristics from the modulation functions in original FINCH and the modified approach by pre-calibration in spatial and polarization multiplexing schemes are reviewed.The study reveals that a reconstructing function completely independent of the function of the phase mask is required for the faithful expression of the characteristics of the modulating function in image reconstruction.In the polarization multiplexing method by non-linear cross correlation,a partial expression was observed,while in the spatial multiplexing method by non-linear cross correlation,the imaging characteristics converged towards a uniform behavior.

Holographic laser fabrication of 3D artificial compound μ-eyes

The demand for fast optical image acquisition without movable optical elements(e.g.,for self-driving car technology)can be met using bioinspired 3D compound eyes.3D laser processing strategies enable designable 3D structuring but suffer from low fabrication efficiency,which significantly limits their applications in producing complex 3D optical devices.Herein,we demonstrate a versatile yet simple wet-etching-assisted holographic laser fabrication method for the development of 3D compound eyes.Artificial compoundμ-eyes can be readily fabricated by programming a 3D spot array for the parallel ablation of a curved fused silica surface,followed by controllable etching in a hydrofluoric(HF)acid solution.A 3D-concave-lens array made on a curved surface over an area of 100μm cross-section with each lenslet of 10μm radius was fabricated with high fidelity and excellent imaging/focusing quality.The resultant 3D-concave-lens can serve as a hard template for the mass production of soft compound eyes through soft lithography.Additionally,using a generative adversarial network(GAN)-based deep learning algorithm,image restoration was conducted for each lenslet,which retained a large field of view and significantly improved image quality.This method provides a simple solution to the requirements of compoundμ-eyes required by Industry 4.0.

Review of engineering techniques in chaotic coded aperture imagers

Coded aperture imaging(CAI)is a technique to image three-dimensional scenes with special controlled abilities.In this review,we survey several recently proposed techniques to control the parameters of CAI by engineering the aperture of the system.The prime architectures of these indirect methods of imaging are reviewed.For each design,we mention the relevant application of the CAI recorders and summarize this overview with a general perspective on this research topic.

Laser polymerized photonic wire bonds approach 1 Tbit/s data rates

Microelectronics has solved the challenge of packaging different functional elements with integrated chips(ICs)in modern computing and communication by wire bonding.Miniaturization was a trend guided by the requirements for faster,more portable and less expensive(smaller amount of materials)solutions,where wire bonding evolved to accommodate increasingly more complex 3D architectures of chips and printed circuit boards.

Light Enhancement in Surface-Enhanced Raman Scattering at Oblique Incidence

Surface enhanced Raman scattering （SERS） measurements have been carried out at different focusing conditions using objective lenses of different numerical apertures. The experimentally observed dependence of SERS intensity of thiophenol-coated Ag nano-islands shows a close-to-linear scaling with the collection aperture. The linear relationship breaks down for large numerical apertures, which suggests that the scattering is anisotropic. Numerical simulations of realistically shaped Ag nano-islands were carried out, and the spatial distribution of hot-spots has been revealed at different heights near the nano-islands. Local field enhancements of up to 100 times were estimated. The simulation also suggests an explanation for the anisotropy in the scattering observed for larger numerical aperture objectives. This appears to be due to a reduction in the local field enhancement as the electric field vector component in the plane of the shallow metal islands reduces at larger angles of incidence.

Deep-UV fluorescence lifetime imaging microscopy

A novel fluorescence lifetime imaging microscopy(FLIM) working with deep UV 240–280 nm wavelength excitations has been developed. UV-FLIM is used for measurement of defect-related fluorescence and its changes upon annealing from femtosecond laser-induced modifications in fused silica. This FLIM technique can be used with microfluidic and biosamples to characterize temporal characteristics of fluorescence upon UV excitation, a capability easily added to a standard microscope-based FLIM. UV-FLIM was tested to show annealing of the defects induced by silica structuring with ultrashort laser pulses. Frequency-domain fluorescence measurements were converted into the time domain to extract long fluorescence lifetimes from defects in silica.