Femtosecond Laser Interference Research Articles

A Submicrosecond-Response Ultrafast Microwave Ranging Method Based on Optically Generated Frequency-Modulated Pulses.

An ultrafast microwave ranging method based on optically generated frequency-modulated microwave pulses is proposed in this study. The theoretical analysis demonstrated that nanosecond-scale linear frequency modulation microwave pulse can be obtained by femtosecond laser interference under the condition of unbalanced dispersion, which can be used to achieve a high temporal resolution of the displacement change in the measurement by the principle of frequency modulation continuous wave (FMCW) radar. The proof-of-principle experiment successfully measured the displacement change with an error of 2.5 mm and a range of 0.6 m, with a response time of 468 ns. Compared to existing microwave ranging technologies, the temporal resolution was improved by two orders of magnitude, which greatly improves the temporal resolution of distance measurement in the field of microwave FMCW radar.

Laser-Induced Transfer of Noble Metal Nanodots with Femtosecond Laser-Interference Processing.

Noble metal nanodots have been applied to plasmonic devices, catalysts, and highly sensitive detection in bioinstruments. We have been studying the fabrications of them through a laser-induced dot transfer (LIDT) technique, a type of laser-induced forward transfer (LIFT), in which nanodots several hundred nm in diameter are produced via a solid–liquid–solid (SLS) mechanism. In the previous study, an interference laser processing technique was applied to LIDT, and aligned Au nanodots were successfully deposited onto an acceptor substrate in a single shot of femtosecond laser irradiation. In the present experiment, Pt thin film was applied to this technique, and the deposited nanodots were measured by scanning electron microscopy (SEM) and compared with the Au nanodots. A typical nanodot had a roundness and circularity . Compared to the previous experiment using Au thin film, the size distribution was more diffuse, and it was difficult to see the periodic alignment of the nanodots in the parameter range of this experiment. This method is promising as a method for producing large quantities of Pt particles with diameters of several hundred nm.

Fast laser surface texturing of spherical samples to improve the frictional performance of elasto-hydrodynamic lubricated contacts

Textured surfaces offer the potential to promote friction and wear reduction by increasing the hydrodynamic pressure, fluid uptake, or acting as oil or debris reservoirs. However, texturing techniques often require additional manufacturing steps and costs, thus frequently being not economically feasible for real engineering applications. This experimental study aims at applying a fast laser texturing technique on curved surfaces for obtaining superior tribological performances. A femtosecond pulsed laser (Ti:Sapphire) and direct laser interference patterning (with a solid-state Nd:YAG laser) were used for manufacturing dimple and groove patterns on curved steel surfaces (ball samples). Tribological tests were carried out under elasto-hydrodynamic lubricated contact conditions varying slide-roll ratio using a ball-on-disk configuration. Furthermore, a specific interferometry technique for rough surfaces was used to measure the film thickness of smooth and textured surfaces. Smooth steel samples were used to obtain data for the reference surface. The results showed that dimples promoted friction reduction (up to 20%) compared to the reference smooth specimens, whereas grooves generally caused less beneficial or detrimental effects. In addition, dimples promoted the formation of full film lubrication conditions at lower speeds. This study demonstrates how fast texturing techniques could potentially be used for improving the tribological performance of bearings as well as other mechanical components utilised in several engineering applications.

Large-area commercial-grating-quality subwavelength periodic ripples on silicon efficiently fabricated by gentle ablation with femtosecond laser interference via two cylindrical lenses

Abstract This paper presents a new strategy for efficiently fabricating large-area subwavelength periodic ripples with commercial grating quality on a silicon surface. Two laser beams of 800 nm, 50 fs at a repeat frequency of 1 kHz were focused on a silicon wafer via two cylindrical lenses. The spacing of the interference fringes was adjusted to excite a coherent resonance-enhanced (CRE) surface plasmon polariton (SPP). The CRE SPP efficiently induced homogeneous and extremely straight periodic ripples through gentle ablation, which avoids substantial debris and reduces the roughness of the periodic ripples. At the two edges of the ablation band after single scanning, the ripples were shallow, spaced, and exceptionally straight, which is greatly advantageous in the preparation of large-area gratings line by line. The subwavelength periodic ripples were fabricated on a silicon wafer with a diameter of 100 mm by scanning at a speed of 2 mm/s and a spacing of 1.55 mm. Compared with the alternating amorphous-crystalline ripples induced by a single laser beam, the fabrication efficiency was enhanced by 2–3 orders. Measurements with a confocal optical microscope and white light diffraction demonstrate that the periodic ripples were extremely straight, smooth, and homogeneous, similar to the gold gratings. The diffraction resolution of the nanogratings of subwavelength periodic ripples was nearly as narrow as that of commercial gratings.

Interference femtosecond laser stamping of micro-grating structures and time-resolved observation of its dynamics.

Selective slicing on a 100 nm thick ZnO film deposited on a Si substrate is achieved by an interference femtosecond (fs) laser stamping. A micro-grating structure with a period of ∼5 µm is completely ablated by an energy-optimized single pulse in one step. The elemental mappings demonstrate complete slice removals of the irradiated areas from the substrate without impurities mixed into the thin film. A calculation of the energy transmitted to the substrate and the characterization of the ablated Si channels infer that the irradiated slices are detached from the substrate by the selective ablation of the thin film and the counterforce of the Si substrate. The temporal and spatial evolution of the grating formation is investigated through a pump-probe microscope using the white light continuum (WLC) as the illumination probe. It is found that the extinctive constructive fringes occur at a delay of 8 picosecond (ps) caused by the increase of electron density. The irradiated slices initially bulge at the delay of 10-12 ps, then subsequently swell until strong material ejections at 800 ps. This study provides an opportunity to advance the understanding of micro-grating fabrications and thin film removals on heterostructures using fs lasers.

Nanodot array deposition via single shot laser interference pattern using laser-induced forward transfer

Laser-induced forward transfer (LIFT) is a direct-writing technique capable of depositing a single dot smaller than the laser wavelength at small shot energy through the laser-induced dot transfer (LIDT) technique. To deposit a single nanodot in a single shot of laser irradiation, a liquid nanodrop is transferred from donor to receiver and finally solidified via a solid–liquid–solid (SLS) process. In conventional LIDT experiments, multi-shots with step scanning have been used to form array structures. However, interference laser processing can achieve an arrayed process and generate a periodic structure in a single shot. In this study, a femtosecond laser interference pattern was first applied to LIDT, and an array of nanodots was successfully deposited in a single shot, producing the following unit structures: a single dot, adjoining dots, and stacking dots. The diameter of the smallest nanodot was 355 nm, and the narrowest gap between two adjoining nanodots was 17.2 nm. The LIDT technique produces high-purity, catalyst-free that do not require post-cleaning or alignment processes. Given these significant advantages, LIDT can expand the usability of nanodots in a wide range of fields.

Tailoring of Silver Nanoparticle Size Distributions in Hydrogenated Amorphous Diamond‐Like Carbon Nanocomposite Thin Films by Direct Femtosecond Laser Interference Patterning

Patterning of nanocomposites by conventional lithography processes is problematic due to the different physical properties of matrix and filler. Direct laser interference patterning (DLIP) promises rapid micromachining of virtually any material with subwavelength resolution. A control of intense laser light modifies the filler particle size distribution in nanocomposite films. Herein, DLIP of periodic patterns with half‐pitch <1 μm in reactive magnetron‐sputtered hydrogenated amorphous diamond‐like carbon nanocomposite thin films with embedded silver nanoparticles (DLC:Ag) is demonstrated. Periodic patterns of regularly repeating areas with modified size distributions of silver nanoparticles are obtained by a variation of fluences and number of pulses of 515 nm wavelength femtosecond laser. Depending on the silver content in nanocomposite films, nanoparticle size distributions are either bimodal (for 14.1 at% Ag containing films) or unimodal (7.9 at%) with effective average diameters shifting from 13 to 69 nm, depending on the laser processing parameters. The importance of localized surface plasmon absorption and localized field enhancement at the DLC–Ag interface for lowering the ablation threshold of DLC:Ag nanocomposites below 6 mJ cm−2 at 1000 pulses is demonstrated. This is at least four times lower than the threshold for pure DLC obtained using the same DLIP setup.

Creating electron phase holograms using femtosecond laser interference processing.

Recently, electron beams with structured phase fronts, such as electron vortex beams, have attracted considerable interest. Herein, we present a novel method of fabricating electron phase holograms using a femtosecond laser interference processing. A 35-nm-thick silicon membrane, corresponding to a phase shift of π for 200-keV electrons, was processed using single-shot laser irradiation, whereas processing such thin membranes with a focused ion beam milling technique would be very difficult. This rapid and efficient technique is expected to produce phase diffraction elements for practical applications in a wide range of electron optics fields.

Local Melting of Gold Thin Films by Femtosecond Laser-Interference Processing to Generate Nanoparticles on a Source Target.

Shape- and size-controlled metallic nanoparticles are very important due to their wide applicability. Such particles have been fabricated by chemosynthesis, chemical-vapor deposition, and laser processing. Pulsed-laser deposition and laser-induced dot transfer use ejections of molten layers and solid-liquid-solid processes to fabricate nanoparticles with a radius of some tens to hundreds of nm. In these processes, the nanoparticles are collected on an acceptor substrate. In the present experiment, we used laser-interference processing of gold thin films, which deposited nanoparticles directly on the source thin film with a yield ratio. A typical nanoparticle had roundness and circularity , and the radius was controllable between 69 and 188 nm. The smallest radius was 82 nm on average, and the smallest standard deviation was 3 nm. The simplicity, high yield, and ideal features of the nanoparticles produced by this method will broaden the range of applications of nanoparticles in fields such as plasmonics.

Three-dimensional chiral microstructures fabricated by structured optical vortices in isotropic material

Optical vortices, a type of structured beam with helical phase wavefronts and ‘doughnut’-shaped intensity distributions, have been used to fabricate chiral structures in metals and spiral patterns in anisotropic polarization-dependent azobenzene polymers. However, in isotropic polymers, the fabricated microstructures are typically confined to non-chiral cylindrical geometry due to the two-dimensional ‘doughnut’-shaped intensity profile of the optical vortices. Here we develop a powerful strategy to realize chiral microstructures in isotropic material by coaxial interference of a vortex beam and a plane wave, which produces three-dimensional (3D) spiral optical fields. These coaxial interference beams are generated by designing contrivable holograms consisting of an azimuthal phase and an equiphase loaded on a liquid-crystal spatial light modulator. In isotropic polymers, 3D chiral microstructures are achieved under illumination using coaxial interference femtosecond laser beams with their chirality controlled by the topological charge. Our further investigation reveals that the spiral lobes and chirality are caused by interfering patterns and helical phase wavefronts, respectively. This technique is simple, stable and easy to perform, and it offers broad applications in optical tweezers, optical communications and fast metamaterial fabrication.

Quantized structuring of transparent films with femtosecond laser interference

The confinement of laser interactions inside transparent materials assisted by tight optical focusing and short-pulsed nonlinear interactions has driven many high-resolution patterning and probing applications in science and technology. In thin transparent films, laser interactions confined to the film/substrate interface have underpinned blistering and ejection processes for nanofluidic channel fabrication, film patterning and cell catapulting. Here, we harness femtosecond lasers to drive nonlinear interactions within Fabry–Perot interference fringes to define narrow nanolength scale zones for highly resolved internal structuring of a film of refractive index, nfilm, at fringe maxima separated by λ/2nfilm. This novel interaction internally cleaves the film to open subwavelength internal cavities and form thin membranes at single or multiple depths from which follow significant opportunities for writing multilevel nanofluidic channels inside the film, as well as ejecting nanodisks at quantized film depths for coloring and three-dimensional surface patterning that promise new compact types of lab-in-film devices. High-resolution structuring of thin transparent films using a femtosecond laser can aid the fabrication of various devices. Kitty Kumar and co-workers from the University of Toronto in Canada say that interference from reflections at the film’s boundaries causes the laser light to create a series of periodic plasma disks in the dielectric film that eject material. The resulting laser-generated nanovoids could be useful for fabricating miniature cavities and fluidic waveguides or performing nanoscale texturing to improve light harvesting in solar cells or light extraction from LEDs. The researchers tested their scheme with a stream of 200 fs pulses from a 522 nm frequency-doubled fibre laser operating at a repetition rate of 100 kHz. They focused the pulses onto an SiNx film measuring 20–1,545 nm thick.

Fabrication of Periodic Micro-Structures by Multi-Photon Polymerization Using the Femtosecond Laser and Four-Beam Interference

Multi-photon polymerization by interference is a promising technique for mass fabrication of 3D periodic micro-structures over large areas, which can be used in various fields such as biomedicine (cell growing), photonics (photonics crystals), etc. As direct laser writing approach is a relatively slow process, fabrication of periodic structures by interference field of a few laser beams can be a significantly faster alternative route of parallel processing. The periodic structures are produced by the single laser exposure over the whole irradiated area. In this paper we present examples how periodic micro-structures can be fabricated over a large area by multi-photon polymerization in a negative photopolymer SZ2080 (ORMOSIL) using four-beam interference of a femtosecond laser.http://dx.doi.org/10.5755/j01.ms.17.3.587

Fabrication of Nanocrystalline Silicon Gratings Embedded within a Silicon Nitride Matrix by Femtosecond Laser-Induced Crystallization

Nanocrystalline silicon gratings were fabricated by applying both femtosecond-laser-interference crystallization and post thermal annealing to amorphous silicon (a-Si) nanoclusters embedded within a silicon nitride matrix. Catalytic chemical vapor deposition was used to fabricate the embedded a-Si nanoclusters, and the formation of a-Si nanoclusters was confirmed by photoluminescence spectroscopy. The femtosecond laser interference technique was employed to produce a seed pattern for the spatially-selected crystallization of a-Si nanoclusters. Micro-Raman spectroscopy and selected-area electron diffraction, together with high-resolution transmission-electron microscopy, show that nanocrystalline silicon gratings were formed through an amorphous-to-crystalline transformation with femtosecond laser pulses, and that the degree of crystallization was enhanced by applying post thermal annealing to the seed gratings.

Direct fabrication of periodic patterns with hierarchical sub-wavelength structures on poly(3,4-ethylene dioxythiophene)–poly(styrene sulfonate) thin films using femtosecond laser interference patterning

A simple optical interference method for the fabrication of simply periodic and periodic with a substructure on poly(3,4-ethylene dioxythiophene)–poly(styrene sulfonate) using femtosecond laser interference patterns is demonstrated. The femtosecond laser pulse was split by a diffractive beam splitter and overlapped with two lenses. Homogeneous periodic arrays could be fabricated even using a single laser pulse. In addition, multipulse irradiation resulted in reproducible sub-wavelength ripples oriented perpendicularly to the laser polarization with spatial period from 170 to 220 nm (around one-fourth of the laser wavelength). In addition, the observed size of the spatial period was not affected by the number of incident laser pulses or accumulated energy density. Using high energy pulses it was possible to completely remove the PEDOT:PSS layer without inducing damage to the underneath substrate.

Effect of Post Thermal Annealing on Femtosecond LaserCrystallization of 500-nm-thick Amorphous Silicon Films

Femtosecond laser interference crystallization was studied in 500-nm-thick amorphous silicon (a-Si) films prepared on glass by using plasma-enhanced chemical-vapor deposition. The efficient crystallization of 500-nm-thick a-Si films was found to require post thermal annealing as well as laser annealing. Femtosecond laser interference technique was used to produce the seed pattern for the spatially-selected crystallization of a-Si. Post thermal annealing of the seed pattern was performed at 550 ◦C for 20 hours under a nitrogen atmosphere. By applying post thermal annealing to laser-crystallized silicon, the degree of crystallization was enhanced. The femtosecond laser-induced grating can be regarded as a pattern of alternating a-Si and μc-Si (microcrystalline silicon) bands with a period of about 2 μm. Probe-beam diffraction, micro-Raman spectroscopy, and transmission electron microscopy were used to investigate the diffraction behavior and to confirm the spatiallyselected crystallization of a-Si.

Fabrication of Periodic Microstructures in Pentaerythritol Triacrylate Through Femtosecond Laser Interference Two‐Photon Polymerization

Fabrication of periodic, two-dimensional microstructures in pentaerythritol triacrylate (PETIA) is demonstrated through femtosecond laser interference patterning. Planar arrayed structures of different periodicities and geometries were fabricated using three, four, and five interfering beams from an 800 nm ultrafast femtosecond laser. The periodic topography can be varied from isolated conical pillars to interconnected cellular structures by simple control of the interference pattern in combination with energy dose.

Fabrication of micro-gratings on Au–Cr thin film by femtosecond laser interference with different pulse durations

We encoded surface relief micro-gratings on Au–Cr thin films using two-beam interference of femtosecond laser pulses with the durations from 25 fs to 70 fs. The dependence of the fabrication quality on the pulse duration has been investigated both numerically and experimentally. The results revealed that the shorter pulses were preferable to prepare periodical microstructures with minimal ablation fringe width and satisfied fabrication quality. This work has potential applications on periodic functional microstructures fabrication for ultra-fine processing and modification on various materials, especially for intractable materials.

Nano-Sized and Periodic Structures Generated by Interfering Femtosecond Laser

Top down technique of interfering femtosecond laser processing was applied to generate new nanomaterials such as nanocrown, nanomesh, nanobelt, and dual periodic structure. In the case of nanocrown, whiskers are standing at the edge of a nanohole. The width of a whisker was just 80 nm. A nanohole array structure generated by interfering 4 beams was applied to a spatial filter as a base material for the growth of photo-polymerizing polymer, and polymer nanobump array was generated. Dual periodic structures were generated by multiple shots, and bimetallic nanobelt was generated from bilayer thin film. Processing of fiber film was tested.

Formation of periodic magnetic structure by localized amorphization of crystalline Co2MnSi using femtosecond laser

Femtosecond laser-interference induced amorphization (FLIA) was used to form a spatially periodic magnetic structure by selectively amorphizing the surface of bulk Co2MnSi. Regularly spaced alternating lines with a periodicity of 2μm were produced by FLIA. Magnetic force microscopy of the samples clearly revealed one-dimensional periodic magnetic domains resulting from the modulated surface structure which was confirmed by transmission electron microscopy. The periodicity of the modulated structure can be further decreased or the process can be modified to write two-dimensional pattern so that the resulting magnetic structure can be potentially utilized for a permanent magnetic identification.

Fabrication of Optoelectronic Devices in Lithium Fluoride Crystals by Interfering Femtosecond Laser Pulses

Fine-pitched microgratings either on or beneath surface of bulk lithium fluoride (LiF) are holographically fabricated by interfering with the second harmonic (400 nm) of a mode-locked Ti:sapphire oscillator–amplifier laser. The laser-active F2 and F3+ color centers in LiF are excellent candidates for producing visible laser action from the green-to-red spectral range when excited with a single wavelength. Here a green distributed feedback (DFB) laser action with a narrower oscillating linewidth is demonstrated by utilizing simultaneous formation of the F3+ color centers and waveguide with the microgratings encoded by interference of 400 nm femtosecond laser pulses. In addition, the possibility of a dual-beam DFB laser based on these color centers in LiF is discussed.