A five-dimensional imaging technique to resolve objects in three-dimensional space, spectrum and time has been developed using a single diffractive optical element consisting of a quasi-random array of pinholes. The object space and spectrum were sampled by axially scanning a point object with different wavelengths along the optical axis and recording the corresponding spatio-spectral signatures. The recorded spatio-spectral signatures were catalogued and stored in a library. A thick object emitting light at different wavelengths was placed in the object space within the axial boundaries of the library and illuminated by light sources within the spectral boundaries of the spatio-spectral library and a single object intensity pattern was recorded. The spatio-spectral images of the object were reconstructed by processing the object intensity pattern and the spatio-spectral library in the computer. Depth-wavelength reciprocity was exploited to see colour from depth and depth from colour. A computational approach utilizing the scaling factors in image formation was used to synthesize the spatio-spectral signatures. The application of depth-wavelength reciprocity and the synthesis of spatio-spectral signatures accelerated the training procedure leading to a faster multidimensional multispectral imaging technology in comparison to the existing coded aperture holography techniques. The developed technology was implemented for spatio-spectral-temporal imaging of a fast transient event such as a spark with a temporal resolution of 40 μs. A single camera shot of the spark recorded by a monochrome high-speed camera was converted into intensity cube data along depth for different wavelengths.
Fresnel incoherent correlation holography (FINCH) is a super-resolution incoherent imaging technique built on the principle of self-interference. In FINCH, the object wave is split into two, modulated by diffractive lenses with different focal distances and interfered to generate the hologram. At least three holograms with different phase-shifts between the two object waves were recorded and combined to synthesize a complex hologram. The different planes of the object were reconstructed by numerically propagating the complex hologram to one of the focal planes of the diffractive lenses. FINCH, in general, has a higher lateral resolution but lower axial and temporal resolutions than direct imaging. A new approach inspired by guide-star holography techniques is applied to FINCH. In this study, two diffractive lens functions with different focal lengths have been spatially multiplexed into one using a binary quasi-random function and fabricated using electron beam lithography. In the first step, the point spread hologram library was recorded by axially scanning a point object along the optical axis. In the next step, an object hologram was recorded. The three-dimensional image of the object was reconstructed using a cross-correlation between the point spread hologram library and the object hologram. The modified approach increased the time resolution of FINCH by three times, improved the axial resolving power and enabled the development of the most compact version of FINCH. We believe that FINCH can be applied for monitoring bacterial cell shape during contact with mechanically biocidal nanotextured surface and light matter interactions.
Infrared (IR) microspectroscopy is a powerful molecular fingerprinting tool widely used for the identification of structural and functional composition of biological and chemical samples. The IR microspectroscopy beamline at the Australian Synchrotron can be operated either with a single-point narrow-band mercury cadmium telluride (MCT) detector or a focal plane array (FPA) imaging detector with 64 × 64 pixels. For the implementation of indirect nonscanning imaging technology, the system was operated with the FPA detector. In this study, we propose an indirect IR imaging technique based on the principles of correlation optics using diffractive optical elements such as random pinhole array (RPA) and Fresnel zone plate (FZP). The spatial and spectral variations of point spread functions (PSFs) of the RPA and FZP were simulated for the synchrotron configuration. Intensity responses for 2D objects were simulated using the same simulation conditions and reconstructed using Lucy-Richardson algorithm. Fabrication of diffractive elements for IR wavelengths is often a challenging task as the IR transparent material substrates, such as barium fluoride and calcium fluoride, are highly susceptible to thermal shocks and brittle by nature. The diffractive elements were fabricated by ablating directly on a 100 nm thick gold coated substrate using femtosecond laser pulses. The simulation results and the fabrication outcomes demonstrate the feasibility of indirect imaging at the synchrotron IR beamline.
Even though graphene is a gapless material, it demonstrates strong interband absorption from a broad range of wavelengths between VIS and NIR. Recent photocurrent graphene-based detectors demonstrated strong photoresponse signal near the graphene/metal boundaries. To increase the response time of photodetectors, the use of low thermal capacity materials and structures are required. SiN membranes are good candidates due to their high-quality factor (up to 106-107), low mass and excellent optical properties. The motivation for this study was based on a lack of any suitable solution for nano-dimension form factor detector that could be integrated into 3D photonic bandgap structures for real-time internal characterization.
Novel optical absorbers are made using titanium metal and silicon dioxide thin films in specific layer thicknesses that create a low fractional reflectance and transmittance material. The absorbers have high absorption fraction measured from 200 - 1400 nm at normal incidence and specific reflectance band peaks. Layer thicknesses affect the wavelength at which light is reflected or absorbed and opens up tuneability for specific wavelengths within the visible spectrum depending on applications such as for solar cells and light modulation.
Light harvesting using photonic crystal (PhC) surface patterns provides an opportunity to surpass the ray-optics defined light trapping and to approach thermodynamic ShockleyQueisser limit of solar cell efficiency, which for a single junction Si solar cell is ~ 32%. For an industry amenable nano-patterning of Si solar cells, we used laser direct write and stepper lithography based approaches for defining a large area (1 cm2) light trapping PhC patterns on silicon. Nanoholes of ~ 500 nm in diameter were fabricated by direct laser writing in a thin layer of chromium to act as a mask for subsequent reactive plasma etching to fabricate the nanostructures forming a PhC surface over a square centimeter. Surface area fabrication throughput was improved by more than order of magnitude as compared with electron beam lithography required to achieve sub-1 μm resolution.
Nano-texturing of surface by self-organised ablation ripples as well as modifications of internal volume of materials, transparent at the wavelength of laser irradiation, is gaining interest due to simplicity of direct laser writing/printing. With ultra-short laser pulses (τp < 1 ps) a wider range of structuring morphologies is accessible, namely, sub-wavelength ripples. The surface wave formed on the plasma-dielectric (air or substrate) explains difference of the formed pattern. These corresponding front- and back-side (in respect to the incoming laser beam) modes of laser structuring accounts for the ripple formation inside transparent materials, where a skin-layer plasma is formed. Emerging applications of nano-textured surfaces for bio-medical field are discussed.
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