Bogdan Voisiat

Two complementary ways of thin-metal-film patterning using laser beam interference and direct ablation

Interference of several identical laser beams allows for the fabrication of periodical structures over large areas. We present two complementary techniques for the patterning of thin films using the laser beam interference and direct ablation of material. The first one is based on precise control of the laser fluence relative to the ablation threshold of the film. Additional means for pattern management are provided by separate control of the intensity and phase of the interfering laser beams. Interference of four or six laser beams permits fabrication of periodical arrays of circular holes. Flexibility and sensitivity to the process parameters of the method beyond of simple hole arrays are shown for the thin Cr film ablation. Another versatile means for pattern generation in the films is the scanning of the periodic laser beam distribution over a workpiece in a sub-period area. By translation of the sample in small steps between laser shots, the structures with their shape independent of the interfering beam intensity distribution can be fabricated by overlapping the ablated holes. Slit- and cross-shaped periodical structures were fabricated using direct structuring with interfering nanosecond and femtosecond pulses. The thin-film structures fabricated by both techniques can be used as spectrally-selective elements for terahertz and infrared radiation.

Fabrication of periodic micro-structures by holographic lithography

The principles of the holographic lithography technique are reviewed, and the ability in the formation of structures with different periods by the holographic lithography technique in SZ2080 is demonstrated. The influence of laser exposure, phase shift between interfering laser beams, and the used laser wavelength on the structures fabricated by the four-beam interference technique has been studied. It is shown that by using the interfering beams with a different phase is it possible to reduce the period of the structure √ − 2 times compared to the four interfering beams of the same phase. Fabrication of micro-structures by the holographic lithography technique can be realized via multi-photon and single-photon polymerization processes. The structures created by these two techniques are compared. Finally, in the experiments with the five-beam interference, the doubleperiod effect was observed by adding the zero-order beam with a low intensity. The fabricated periodic microstructures have the potential to be used as a scaffold for cell growth and micro-optics.

Spectroscopic Terahertz Imaging at Room Temperature Employing Microbolometer Terahertz Sensors and Its Application to the Study of Carcinoma Tissues

A terahertz (THz) imaging system based on narrow band microbolometer sensors (NBMS) and a novel diffractive lens was developed for spectroscopic microscopy applications. The frequency response characteristics of the THz antenna-coupled NBMS were determined employing Fourier transform spectroscopy. The NBMS was found to be a very sensitive frequency selective sensor which was used to develop a compact all-electronic system for multispectral THz measurements. This system was successfully applied for principal components analysis of optically opaque packed samples. A thin diffractive lens with a numerical aperture of 0.62 was proposed for the reduction of system dimensions. The THz imaging system enhanced with novel optics was used to image for the first time non-neoplastic and neoplastic human colon tissues with close to wavelength-limited spatial resolution at 584 GHz frequency. The results demonstrated the new potential of compact RT THz imaging systems in the fields of spectroscopic analysis of materials and medical diagnostics.

Direct laser beam interference patterning technique for fast high aspect ratio surface structuring

New results on development of the Direct Laser Interference Patterning (DLIP) technique using the interference of several beams to directly ablate the material are presented. The method is capable of producing sub-wavelength features not limited by a beam spot size and is an effective method of forming two-dimensional periodic structures on relatively large area with just a single laser shot. Surface texturing speed of DLIP method and the direct laser writing was compared. Fabrication time reduction up to a few orders of magnitude using DLIP was evaluated. The sub-period scanning technique was applied for formation of the complex periodic structures. A new method of laser scanning for fabrication of periodic structures on large areas without any visible stitching signs between laser irradiation spots was tested.

Reflective terahertz imaging with the TEM01 mode laser beam.

Reflective terahertz imaging with a first-order Hermite-Gaussian laser beam was experimentally investigated. High spatial resolution targets prepared by direct laser microprocessing were used to evaluate the performance. The reflection imaging system at 2.524 THz frequency demonstrated up to diffraction limited resolution using the single focusing mirror with the numerical aperture not smaller than 0.6. The TEM(01) mode laser beam was also applied for practical samples such as silicon solar cell terahertz (THz) imaging. It is shown that usage of appropriate optics enables us to obtain high-quality THz images with the multimode laser beam.

On-chip integration solutions of compact optics and detectors in room-temperature terahertz imaging systems

On-chip integrated solutions employing properties of Fresnel zone plates with integrated band-pass filters for the room temperature terahertz imaging systems are discussed. Finite-difference time-domain simulations were used to predict properties of conventional zone plates and ones with resonant filter areas as flat optics components. They are produced employing the laser direct writing and characterized by electronic THz sources and an optically pumped terahertz laser. It was shown that more than one order of magnitude detection enhancement can be observed of bow-tie-shaped InGaAs-based terahertz detectors by on-chip incorporation of the secondary diffractive optics.

Plateau-Rayleigh instability triggered transformation in thin chromium film on glass substrate under nanosecond laser irradiation

Metal films on transparent substrates are widely applied for mask production in lithography, and lasers are frequently used for their patterning. Quality of the patterning is limited by fundamental phenomena taking place close to edges of the laser ablated area. We experimentally and numerically investigated transformations in metal films during their irradiation with the nanosecond laser beam with fluence above the ablation threshold. Ridges of the resolidified metal with non-uniform thickness were always formed on edges of the cleaned area. Instabilities during the ablation process forced the molten metal in the ridges to break up into droplets with the periodicity predicted by the Plateau–Rayleigh instability. The droplets on ridges were starting points for formation of self-organized lines of metal film by irradiation with partially overlapping laser pulses. The initial droplets and later the self-organized parallel lines of chromium metal were heat sinks that cooled down the metal in their close proximity. Temperature modulation along the laser irradiation spot was high enough to initiate the Marangoni effect which resulted in movement of the molten metal from hot to colder areas.

Structuring of functional thin films and surfaces with picosecond-pulsed lasers

During the recent few years picosecond lasers have been proved as a reliable tool for microfabrication of diverse materials. We present results of our research on structuring of thin films and surfaces using the direct laser writing and the laser beam interference ablation techniques. The processes of micro-pattering were developed for metallic, dielectric films as well as complex multi-layer structures of thin-film solar cells as a way to manufacture frequency-selective surfaces, fine optical components and integrated series interconnects for photovoltaics. Technologies of nano-structuring of surfaces of advanced technical materials such as tungsten carbide were developed using picosecond lasers as well. Experimental work was supported by modeling and simulation of energy coupling and dissipation inside the layers. Selectiveness of the ablation process is defined by optical and mechanical properties of the materials, and selection of the laser wavelength facilitated control of the structuring process. Implementation of the technologies required fine adjustment of spatial distribution of laser irradiation, therefore both techniques are benefiting from shaping the laser beam with diffractive optical elements. Utilization of the whole laser energy included beam splitting and multi-beam processing.

Compact solutions for spectroscopic solid-state-based terahertz imaging systems

Convenience in use of room-temperature terahertz (THz) imaging systems, reduction of their dimensions and presence of on-chip solutions remains one of prime interests for direct implementation aims. Solid-state-based solutions in miniaturization of spectroscopic THz imaging systems including novel semiconductor nanostructures bias-free emitters, diffractive THz optics components and their on-chip integration with THz detectors are discussed. In particular, pulsed optoelectronic terahertz emitter based on a δ-doped p-i-n-i GaAs/AlxGa1−xAs heterostructure was studied and it is demonstrated that the heterostructure can serve as efficient antenna- and bias-free surface emitter. Diffractive optics elements – Fresnel zone plates –with integrated band-pass filters were simulated employing Finite-difference time domain method. Structures were fabricated using the laser direct writing and investigated using electronic THz sources and an optically pumped terahertz laser. Advantages of on-chip integration of diffractive optics and bow-tie-shaped InGaAs-based terahertz detectors are revealed via detection enhancement. Bow-tie diodes properties in frequency scale and detection sensitivity are considered and compared for different materials. Homodyne detection and imaging of low-absorbing objects at 0.6 THz are demonstrated and discussed.

Laser processing for precise fabrication of the THz optics

Zone plates with integrated band-pass filters and binary Fresnel lenses designed for the THz spectral range were fabricated by direct laser ablation in metal films and the silicon substrate. Results on the process performance and quality of the products are reviewed. The focusing performance was measured using the THz source that produces the 580 GHz radiation. The beam was directed to the centre of the fabricated optical elements. Zone plates with integrated band-pass filters have shown the double performance in focusing and spectral selection. The dependence of ablation rate and surface roughness on the laser process parameters was thoroughly investigated on the silicon. The depth of the ablated grooves linearly depends on the number of laser scans number with a particular slope for each scanning speed. The process regime with the 125 mm/s scanning speed provided the most precise control over the ablation depth. The topography measurements of the laser fabricated multilevel phase zone plates (Fresnel lenses) with the 10 mm focal length showed good agreement with the calculated topography. The intensity distribution of the focus spots using the laser fabricated 2, 4 and 8 level binary Fresnel lenses showed better focusing performance when more depth levels were applied in the lens production.