Extraordinary transmittance and focusing of light in quasi far field region using miniaturized optical devices is a daunting task. A polarization independent, broadband, planar metallic transmissive micro aperture capable of achromatically focusing visible light in quasi far field region is proposed. The calculated enhancement factor of transmission efficiency was about ~2.2. The total transmission after the aperture is about 60%. This high throughput focusing device will open new avenues for focusing electromagnetic energy in the wide area of sensors and energy concentration.
Ultrafast, femtosecond laser pulse interaction with dielectric materials has shown them to have significantly higher laser fluence threshold requirements, as compared to metals and semiconductors, for laser material modification, such as laser ablation. Examples of dielectrics are crystalline materials such as quartz and sapphire, and amorphous glasses. The interaction between femtosecond laser pulses, at a wavelength with negligible linear absorption, and a dielectric has been found to be weak, and multiple pulse irradiation is therefore typically used in order to see significant and quantifiable effects. In this study the dielectric is the crystalline, layered, natural mineral muscovite, a mica with formula ( ) ( ) . Muscovite, newly cleaved, is used in a wide range of technological and scientific applications including as an insulating material in electronics and as an ultra-flat and ultra-clean substrate. A single, ~800 nm wavelength, ~6 micron spotsize, ~150 fs laser pulse is found to lead to a systematic range of laser modification topologies, as a function of the fluence of the single laser pulse, including bulk removal of material. The fs laser pulse/material interaction is greater than expected for a standard dielectric at a given fluence. Optical surface profiling and FESEM are used to characterise the topologies. Contrasting the results of the two techniques supports the use of optical surface profiling to characterise the material modification despite its limitations in lateral resolution as compared to FESEM. The interlayer mineral water content of natural muscovite is proposed as the primary reason that mica behaves differently to a standard dielectric when irradiated with a single 800 nm fs laser pulse.
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