In this communication, we consider principles of design and assembling of nonparaxial THz imaging systems based on silicon diffractive optics components. The investigation is dedicated to lensless photonic setups comprising high-resistivity silicon-based DOEs such as Fresnel zone plates, Fibonacci lenses, Bessel axicons, and Airy zone plates, all fabricated from a high-resistance 500 μm thick silicon substrate by femtosecond laser ablation. The exploration underlines the significance of structuring both the illumination and light-collection schemes as well as assembly principles of silicon diffractive optical elements in compact THz imaging.
This study introduces a stainless steel-based complementary C-shaped single split ring resonator (CSRR) metasurface designed for Terahertz (THz) imaging applications. The CSRR metasurface was created from a 25 μm thick stainless-steel foil using laser ablation, and serves as a zone plate, allowing precise manipulation of 100 GHz radiation. The investigation involved beam-shaping characterization of the metasurface comprising CSRRs of different geometrical parameters and examining the bending influence on the metasurface's functionality, revealing a minimal reduction in beam intensity. Additionally, the proposed metasurface demonstrated the ability to control polarization by sweeping its rotational angle, enhancing THz polarization-resolved imaging capabilities. Practical demonstrations showcase the metasurface's suitability for real-life scenarios, highlighting its value in THz imaging systems.
Micromachining of transparent materials with high-power lasers demands new approaches to utilize most of the laser power without reduction of the quality. The usually exploited Gaussian beams become absolute when specific beams can be utilized to do the set task. For instance, the Bessel beams can be used to drill very high aspect ratio holes with a single laser pulse, which increases the speed of manufacturing by many orders. In our research, we have even more versatility in the beam shapes as we use the beam shaping techniques which withstand high-power laser pulses. Therefore, higher-order, vectorial Bessel beams, and flattop beams were used to micromachine transparent materials and results will be shown.
We analyze the concept of using customized spatially variable waveplates for beam tailoring towards enhanced various glass microwelding process. These elements work as precise flat optical elements that have very high diffraction efficiency (<90%), high optical damage threshold, and can be freely customized for specific needs that transform the spatial intensity profile into tailored beam shape. In this work, we investigate custom made flat-top beam and "C" shaped beams for deep microwelding purposes. By using numerical simulations and experimental research we compare the performance of such beams and demonstrate thin glass deep microwelding capabilities using custom beam shaping elements.
During the last decade the zeroth order Bessel-Gauss laser beam has found many uses in the transparent material processing. The high aspect ratio channels can be created that slice through various thin transparent materials and increase the efficiency of cutting. However, the generation of high-quality Bessel-Gauss beam remains a challenge due to imperfection of glass axicon manufacturing, i.e. rounded tip, not smooth surface etc. These imperfections generate intensity modulation along propagation axis or even modify transversal central core intensity distribution, that results in worsening of micro-machining quality. The diffractive optical element (DOE) is a great alternative that do not suffer from previously mentioned problems. In this study we show the possibility of generating high quality Bessel-type beams with geometric phase optical elements (GPOEs) (manufactured by Workshop of Photonics). These elements act as precise flat DOEs that have very high diffraction efficiency (>90%), high optical damage threshold and can be freely customized for specific needs. Therefore, with the use of high-power laser they can be applied to process transparent materials. In this work, controllable phase shifts are implemented in axicon phase masks to create unique and fanciful Bessel-type beams as well as asymmetric core beams for thin glass modification/cutting application. Using numerical simulations and experimental data we compare performance of GPOEs and demonstrate thin glass processing using powerful laser with reshaped intensity distribution by GPOE.
Long focal lines with transverse spot sizes as small as a few wavelengths are called optical needles. A zeroth order Bessel beam being a good example is widely used in such applications as laser micromachining. In practice Bessel beam generated with an axicon has a peak in axial intensity distribution and is not only due to aberrations caused by planar dielectric material interface. Here, we investigate optical needles with controlled axial intensity distribution via intensity modulation of the incoming beam. We have chosen to generate constant axial intensity Bessel beam and propose spatial transmission mask to do so. Experimental verification is presented using diffractive optics elements based on Pancharatnam-Berry phase. We demonstrate a flattening of the axial intensity profile of the Bessel beam without the alteration to the optical needle diameter.
Beam profile engineering, where a desired optical intensity distribution is generated by phase shifting and/or amplitude changing elements, is a promising approach in various laser-related applications. For example, vector geometrical phase elements enable various flat special optical elements such as top-hat converters. We present a study on engineering efficient top-hat converters inscribed in the glass by femtosecond laser pulses. We start with an amplitude encoded top hat converter and demonstrate how its efficiency can be further increased by introduction of phase masks and by the polarization of the incident beam. Experimental verification of the concept is also presented.
Nondiffracting beams are known for their long line of focus, which has various applications in laser materials processing. Zeroth order Bessel beam is usually generated with an axicon and has a distinct circular spatial spectra. The nature of higher order Bessel beams, elliptical and parabolic nondiffracting beams is also conical and their spatial spectra have their own azimuthal modulation. We study numerically and verify experimentally generation of vortical Bessel beams, their superpositions along with elliptical and parabolical beams using an axicon. Laser induced modifications in glasses for various durations and beam powers using generated pulsed beams are analyzed.
Zeroth order Bessel beams are widely used in laser micromachining of transparent materials. The small diameter of central core and elongated focus enables to generate high aspect ratio voids. The simplest way to generate this beam is to induce a conical shape phase with an axicon. However, the quality of the axicon tip is very crucial to generate smooth Bessel beams since it is known that a blunt axicon tip induces large intensity modulation in propagation direction. Alternative Bessel beam generation method is to use a Diffractive Optical Elements (DOEs) that do not suffer from previously mentioned problem. In this work we demonstrate generation of a zeroth order Bessel beam with Geometric Phase Optical Elements (GPOEs) (manufactured by Workshop of Photonics) acting as a diffractive beam shaping element. Having absolute control of induced beam phase, we have modified mask phase so that half of it had additional phase shift or spatial transposition resulting in creation of fanciful induced beam phase patterns. With the use of laser beam propagation numerical modeling we show that these new phase masks can create various beam transverse intensity patterns such as asymmetrical central core, generation of multiple peaks or even large rings that are highly demanded for various laser micromachining applications. We have chosen couple of most perspective beam shapes and manufactured GPOEs to generate them. The experimentally generated beams were compared to numerical simulations. As the GPOEs are able to work with high power pulses we have also investigated induced transparent material modifications.
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