Santiago M. Olaizola

Formation of laser-induced periodic surface structures on niobium by femtosecond laser irradiation

The surface morphology of a Niobium sample, irradiated in air by a femtosecond laser with a wavelength of 800 nm and pulse duration of 100 fs, was examined. The period of the micro/nanostructures, parallel and perpendicularly oriented to the linearly polarized fs-laser beam, was studied by means of 2D Fast Fourier Transform analysis. The observed Laser-Induced Periodic Surface Structures (LIPSS) were classified as Low Spatial Frequency LIPSS (periods about 600 nm) and High Spatial Frequency LIPSS, showing a periodicity around 300 nm, both of them perpendicularly oriented to the polarization of the incident laser wave. Moreover, parallel high spatial frequency LIPSS were observed with periods around 100 nm located at the peripheral areas of the laser fingerprint and overwritten on the perpendicular periodic gratings. The results indicate that this method of micro/nanostructuring allows controlling the Niobium grating period by the number of pulses applied, so the scan speed and not the fluence is the key parameter of control. A discussion on the mechanism of the surface topology evolution was also introduced.

Fabrication of high quality sub-micron Au gratings over large areas with pulsed laser interference lithography for SPR sensors

Metallic gratings were fabricated using high energy laser interference lithography with a frequency tripled Nd:YAG nanosecond laser. The grating structures were first recorded in a photosensitive layer and afterwards transferred to an Au film. High quality Au gratings with a period of 770 nm and peak-to-valley heights of 20-60 nm exhibiting plasmonic resonance response were successfully designed, fabricated and characterized.

Photonic structures in diamond based on femtosecond UV laser induced periodic surface structuring (LIPSS)

We study the fabrication of photonic surface structures in single crystal diamond by means of highly controllable direct femtosecond UV laser induced periodic surface structuring. By appropriately selecting the excitation wavelength, intensity, number of impinging pulses and their polarization state, we demonstrate emerging high quality and fidelity diamond grating structures with surface roughness below 1.4 nm. We characterize their optical properties and study their potential for the fabrication of photonic structure anti-reflection coatings for diamond Raman lasers in the near-IR.

Quality inspection of nanoscale patterns produced by Laser Interference Lithography using image analysis techniques

This paper introduces the quality inspection of nanoscale patterns produced by the Laser Interference Lithography (LIL) technology using image analysis techniques. In this paper, patterns of two-beam and four-beam interferences are considered. Image analysis techniques based on the Hough transform (HT) and Maximum Likelihood Estimation (MLE) have been applied to detect and estimate various quality parameters for the two types of textures. The HT and a modified grey-scaled HT are introduced as a global approach for the analysis of the two-beam interference patterns. Surface parameters, such as the period, depth, and their uniformities, can be obtained directly without a priori knowledge of the textures. Due to different pattern structures and strong noise effects, the four-beam patterns are dealt with a different approach, using a statistical method based on the likelihood function to estimate each circles center and shape. Taking into consideration of noises and defects, another further rejection step is introduced to filter out noises and defects. Results from experimental samples are presented.

System Requirement Analysis of Laser Interference Nanolithography

This paper presents a system requirement analysis of multi-beam laser interference nanolithography for nanoscale structuring of materials including seven sections: introduction, formation of multi-beam laser interference patterns, user requirements, system architecture, experiments, discussions and conclusions. Analytical expressions were obtained for the spatial distribution of radiation of the interfering beams as a function of their amplitudes, phases, angles of incidence on the sample, and polarization planes with computer simulation and experimental results. The environmental effect and technological potential were also discussed.

Enhancement of surface area and wettability properties of boron doped diamond by femtosecond laser-induced periodic surface structuring

We demonstrate the formation of laser-induced periodic surface structures (LIPSS) in boron-doped diamond (BDD) by irradiation with femtosecond near-IR laser pulses. The results show that the obtained LIPSS are perpendicular to the laser polarization, and the ripple periodicity is on the order of half of the irradiation wavelength. The surface structures and their electrochemical properties were characterized using Raman micro-spectroscopy, in combination with scanning electron and atomic force microscopies. The textured BDD surface showed a dense and large surface area with no change in its structural characteristics. The effective surface area of the textured BDD electrode was approximately 50% larger than that of a planar substrate, while wetting tests showed that the irradiated area becomes highly hydrophilic. Our results indicate that LIPSS texturing of BDD is a straightforward and simple technique for enhancing the surface area and wettability properties of the BDD electrodes, which could enable higher current efficiency and lower energy consumption in the electrochemical oxidation of toxic organics.

Ultrafast laser inscription of volume phase gratings with low refractive index modulation and self-images of high visibility

Ultrafast laser inscription of volume phase gratings with low index contrast and self-images with visibility of 0.96 is demonstrated. It is also demonstrated that phase differences of π/2 for visible light are achievable with only one layer of structures induced in bulk borosilicate glass by direct laser writing. The fabrication method avoids the stitching of several layers of structures and significantly reduces the time of process. The increment of visibility with the induced phase difference is proved and results are compared with the expected for planar phase gratings.

Interference lithography processes with high-power laser pulses

Laser interference lithography (LIL) is concerned with the use of interference patterns generated from two or several coherent beams of laser radiation for the structuring of materials. This paper presents the work on the processes based on resists and direct writing with laser interference lithography. In the work, a four-beam laser interference system was used as a submicrometer structuring tool in which a high-energy pulsed, frequency-tripled and TM polarized Nd:YAG laser (355 nm) with a coherent length of 3 m, energy power up to 320 mJ/cm2, pulse duration of 8 ns and 10 Hz repetition rate was used as a light source. The experimental results were achieved with 2-beam and 4-beam interference patterning. The processes can be used to define submicron surface relieves in large areas for use in the field of MEMS.

Formation of 4-beam laser interference patterns for nanolithography

This paper presents a theoretical analysis of formation of 4-beam laser interference patterns for nanolithography. Parameters of 4-beam interference patterns including the pattern amplitude, period, orientation and uniformity were discussed. Analytical expressions were obtained for the spatial distribution of radiation of the interfering beams as a function of their amplitudes, phases, angles of incidence on the sample, and polarization planes with computer simulation and experimental results.

Current developments and applications using multi-beam laser interference lithography for nanoscale structuring of materials

Multi-beam laser interference lithography (MB-LIL) is a rapid and cost-effective maskless optical lithography technique to parallelly pattern periodic or quasi- periodic micro/nano-structured material over large areas more than square centimetres. An interference pattern between two or more coherent laser beams is set up and recorded in a recording material of substrate. This interference pattern consists of a periodic series of geometries representing intensity minima and maxima. The patterns that can be formed depend on the number and configuration of laser beams. This review introduces the development and application of MB-LIL system for fabrication of micro/nano-structured material. At first, it surveys various types of MB-LIL methods by classifying different beam configurations. Then the paper shows some application results for fabrication 2D/3D micro/nano structure arrays by means of interference patterns with multi-exposed or directly ablation technique. The patterend micro/nano-structure arrays include crossed diffraction grating array in photoresist, 3D pattern in polymetric photonic crystals, and magnetic nanoarrays in thin film. Finally, an innovative four-beam LIL system is introduced, which is being developed within the EC-granted project DELILA.