Current trend of the average laser power increase follows Moore's law. The average power of ultra-short lasers in 2000 was 1 W and now is 1 kW following trend of doubling per year: 2^(20years/2) = 1024. This trend can be harnessed for large area patterning. Here we show application of direct laser writing for processing of surface of solar cells.
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-times reduction projection lithography.
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.
Photonic crystal patterns for light trapping in Si solar cells are developed via different process flows: 1) step-and-repeat projection lithography with lift-off or 2) laser ablation and subsequent etching dry or/and wet. Both methods are amenable for large area (2×2 cm2) fabrication and can be used to break the ray-optics light trapping limit. This is required to surpass the record high efficiency ∼ 26% of solar-to-electrical power conversion of Si solar cells and approach the theoretical limit of ∼ 30%. Also, standard electron beam lithography (EBL) was used to define Si3N4 or Cr masks for wet KOH etching on silicon-on-insulator (SOI) and Si wafers. Direct laser writing of the etch mask by ablation (10 nJ, 515 nm, 230 fs pulses) has an advantage due to its scalability. The large area patterning is important for industrial application of direct laser writing of light trapping patterns in solar cells and absorbers/emitters for the IR spectral range.
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.
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.
Laser fabrication with ultra-short laser pulses (sub-1 ps) have the ability for precise energy delivery to target materials for ablation, spallation or polymerisation down to sub-wavelength resolution. We show, that by applying electrical and magnetic fields, the electron-ion ablation plasma can be controlled following the Lorentz force exerted on to the plasma F = eE + e[v ×B], where v is velocity of charge e, E is the applied electrical bias and B is the magnetic flux density. The vectorial nature of the Lorentz force was investigated using the ablation of silicon. The application potential for ablation debris control and mass, charge spectroscopes of ablated materials is discussed.
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