Femtosecond Laser Fabrication Research Articles

Heuristic modelling of laser written mid-infrared LiNbO3 stressed-cladding waveguides.

Mid-infrared lithium niobate cladding waveguides have great potential in low-loss on-chip non-linear optical instruments such as mid-infrared spectrometers and frequency converters, but their three-dimensional femtosecond-laser fabrication is currently not well understood due to the complex interplay between achievable depressed index values and the stress-optic refractive index changes arising as a function of both laser fabrication parameters, and cladding arrangement. Moreover, both the stress-field anisotropy and the asymmetric shape of low-index tracks yield highly birefringent waveguides not useful for most applications where controlling and manipulating the polarization state of a light beam is crucial. To achieve true high performance devices a fundamental understanding on how these waveguides behave and how they can be ultimately optimized is required. In this work we employ a heuristic modelling approach based on the use of standard optical characterization data along with standard computational numerical methods to obtain a satisfactory approximate solution to the problem of designing realistic laser-written circuit building-blocks, such as straight waveguides, bends and evanescent splitters. We infer basic waveguide design parameters such as the complex index of refraction of laser-written tracks at 3.68 µm mid-infrared wavelengths, as well as the cross-sectional stress-optic index maps, obtaining an overall waveguide simulation that closely matches the measured mid-infrared waveguide properties in terms of anisotropy, mode field distributions and propagation losses. We then explore experimentally feasible waveguide designs in the search of a single-mode low-loss behaviour for both ordinary and extraordinary polarizations. We evaluate the overall losses of s-bend components unveiling the expected radiation bend losses of this type of waveguides, and finally showcase a prototype design of a low-loss evanescent splitter. Developing a realistic waveguide model with which robust waveguide designs can be developed will be key for exploiting the potential of the technology.

The Infrared Photodiode of Textured Silicon Irradiated Under Mixed Gas by Femtosecond Laser

Supersaturated S-doped black silicon (Si) samples are produced by femtosecond laser fabrication in mixed atmosphere of sulfur hexafluoride and oxygen (SF6/O2). The surface morphology with cone-microstructures has been observed in black Si samples fabricated in mixed atmosphere. After thermal annealing, the absorption below bandgap of black Si is obviously reduced. Moreover, the contents of sulfur in black Si samples obtained in mixture are also decreased by the lead in of oxygen. In contrast, an obvious increase of valent state ratio of Si4+ (SiO2) to Si0 (Si) is observed after the implantation of oxygen due to the formation of SiO2 in S-doped Si. Based on the passivation effect of SiO2, we have produced near-infrared black Si photodiode devices and showed photo-response properties of the devices. A notable photo-response of 11 mA/W at −3 V (26.6 mA/W at −5 V) for near-infrared 1.31- $\mu \text{m}$ detection light is obtained for the photodiode fabricated in mixture, which outweigh the performance of photodiode fabricated in pure SF6 atmosphere.

Mechanism and experimental study on three-dimensional facula shaping in femtosecond laser micromachining

Because of the laser beam waist and diffraction effect of the lens, the focal spot light field in femtosecond laser microprocessing has an ellipsoidal spatial distribution. This leads to the gap between two processing layers increasing along the axial direction, and the distribution density of processing points decreasing along the horizontal direction. This directly reduces the resolution of the microprocessing, and badly affects the machining accuracy and surface quality. We established a mathematical model for three-dimensional (3-D) laser beam shaping based on the Fresnel diffraction theory and designed a kind of four-ring complex amplitude transmittance phase plate by using a global optimization algorithm and genetic algorithm to simultaneously realize transverse and axial 3-D shaping. We numerically showed that the transverse and axial gains of the focal facula after 3-D shaping are 0.77 and 0.68, respectively, where the corresponding peak energy ratio is 0.36, the transverse and axial sidelobe energies are 0.28 and 0.62, respectively, and the defocusing amount is − 0.08 . We also constructed a confocal/two-photon microscope system to experimentally achieve a better shaping effect in the case of femtosecond laser fabrication at a point on the thin film of a photochromic material.

Femtosecond laser fabrication of highly hydrophobic stainless steel surface with hierarchical structures fabricated by combining ordered microstructures and LIPSS

In this work we have developed hierarchical structures that consist of micro-patterned surfaces covered by nanostructures with a femtosecond laser. The first part of this work is a study to determine the microscale modifications produced on a stainless steel alloy (AISI304) surface at high pulse energy, different velocities, and number of overscans in order to obtain microstructures with a selected depth of around 10μm and line widths of 20μm. The second part of the work is focused on finding the optimal irradiation parameters to obtain the nanostructure pattern. Nanostructures have been defined by means of Laser Induced Periodical Surface Structures (LIPSS) around 250nm high and a period of 580nm, which constitute the nanostructure pattern. Finally, dual scale gratings of 50mm2 were fabricated with different geometries and their effect on the measured contact angle. Combining the micro-pattern with the LIPSS nano-pattern, highly hydrophobic surfaces have been developed with measured static contact angles higher than 150° on a stainless steel alloy.

Gradient induced liquid motion on laser structured black Si surfaces

This letter reports on the femtosecond laser fabrication of gradient-wettability micro/nano-patterns on Si surfaces. The dynamics of directional droplet spreading on the surface tension gradients developed is systematically investigated and discussed. It is shown that microdroplets on the patterned surfaces spread at a maximum speed of 505 mm/s, which is the highest velocity demonstrated so far for liquid spreading on a surface tension gradient in ambient conditions. The application of the proposed laser patterning technique for the precise fabrication of surface tension gradients for open microfluidic systems, liquid management in fuel cells, and drug delivery is envisaged.

Femtosecond-laser fabrication of cyclic structures in the bulk of transparent dielectrics

We report the results of the experiments on developing precision micromachining technology, obtained under the conditions of focusing the pulses of a femtosecond (FS) laser into the volume of a transparent material, which is important, particularly, in the processing of biomaterials in ophthalmology. The implementation conditions and some characteristic features of the special regime of micromachining are determined, when at a definite relation between the sample scanning velocity and the repetition rate of FS pulses the region, destroyed by the laser radiation, is shifted along the optical axis towards the objective and back, forming cyclic patterns inside the sample. It is supposed that the main causes of the damage region shift are the induced modification of the refractive index and the reduction of the damage threshold due to the change in the material density and structure in the microscopic domain, adjacent to the boundary of the cavity produced by the previous pulse. The results of the performed study with the above regime taken into account were used in the technology of precision cutting of crystals, glasses and polymers. The best quality of the cut surface is achieved under the conditions, eliminating the appearance of the cyclic regime. In the samples of polycarbonate, polymethyl methacrylate and fused silica the cylindrical cavities were obtained with the aspect ratio higher than 200, directed along the laser beam, and microcapillaries with the diameter in the direction, perpendicular to this beam.

Femtosecond laser fabrication of waveguides in Rhodamine B-doped GPTS/TEOS-derived organic/silica monolithic xerogel

This paper reports on the fabrication and characterization of waveguides inside of a dye doped-organic/inorganic bulk material using femtosecond laser microfabrication. Rhodamine B-doped GPTS/TEOS-derived organic/silica monolithic xerogels with excellent optical quality were prepared by sol–gel method. The influence of the dye concentration on the samples optical properties was also investigated in order to choose the proper one to be used for producing the waveguides. After investigation of parameters to fabrication in xerogels, such as, scan speed effects and pulse energy, we produced waveguides in bulks doped with 0.5mmol/L of Rhodamine B. Propagation losses in the single mode waveguides, at 632.8nm wavelength, were obtained.

Three-dimensional femtosecond laser fabrication of waveguide beam splitters in LiNbO_3 crystal

We report on the fabrication of waveguide beam splitters in z-cut LiNbO3 crystal by direct femtosecond laser writing. The guidance is valid only for TM polarization due to the Type I modification of extraordinary refractive index (ne) induced by femtosecond laser pulses. With single scan of femtosecond laser beams over the crystal bulk, the structures with channel geometry have been produced. In this work, such waveguide configurations were created as one-dimensional (1D) straight waveguide, two-dimensional (2D) 1 × 2 and three-dimensional (3D) 1 × 4 waveguide beam splitters. The waveguide beam splitters are characterized at the wavelength of 632.8 nm and 1064 nm both experimentally and numerically. This work opens the way for laser-written 3D LiNbO3 waveguide beam splitters as novel 3D nonlinear photonic devices.

Femtosecond laser fabrication of long period fiber gratings by a transversal-scanning inscription method and the research of its orientational bending characteristics

Abstract In order to make the whole fiber core modified, an improved point-by-point inscription method (called a transversal-scanning inscription method) is proposed to fabricate long period fiber gratings (LPFGs) by using femtosecond laser. LPFGs with an attenuation depth of 16 dB are achieved within the wavelength range of 1580–1680 nm. We have compared the bending properties of LPFGs fabricated by the line-by-line scanning inscription method and the transversal-scanning inscription method at the orthogonal directions. It is found that increasing scanning area using the transversal-scanning method can enhance the bend sensitivity obviously. The resulting curvature sensitivities are −4.82 nm/m −1 and −1.63 nm/m −1 at the 0° and 90° bend orientations respectively, within the curvature range from 1.75 m −1 to 3 m −1 in the experiment. The LPFG-I fabricated by the line-by-line scanning inscription method experiences red-shift, while the LPFG-II fabricated by the transversal-scanning inscription method experiences blue-shift.

Fiber optic stress-independent helical torsion sensor.

Femtosecond laser-fabricated waveguides have been formed into helical paths throughout the cladding of single-mode optical fibers to demonstrate a strain-independent fiber torsion sensor. A comparison between a Bragg grating sensor and a Mach-Zehnder based on helical waveguides (HWs) showed a much weaker twist sensitivity of 1.5 pm/(rad/m) for the grating in contrast with a value of 261 pm/(rad/m) for the interferometer. The HW geometry provided an unambiguous determination of the rotational direction of the twist while facilitating a convenient and efficient means for optical coupling into the single-mode core of the fiber. The flexible three-dimensional writing by the femtosecond laser fabrication method enabled the direct inscription of compact and robust optical cladding devices without the need for combining or splicing multiple-fiber segments.

Gold-film coating assisted femtosecond laser fabrication of large-area, uniform periodic surface structures.

A simple, repeatable approach is proposed to fabricate large-area, uniform periodic surface structures by a femtosecond laser. 20nm gold films are coated on semiconductor surfaces on which large-area, uniform structures are fabricated. In the case study of silicon, cross-links and broken structures of laser induced periodic surface structures (LIPSSs) are significantly reduced on Au-coated silicon. The good consistency between the scanning lines facilitates the formation of large-area, uniform LIPSSs. The diffusion of hot electrons in the Au films increases the interfacial carrier densities, which significantly enhances interfacial electron-phonon coupling. High and uniform electron density suppresses the influence of defects on the silicon and further makes the coupling field more uniform and thus reduces the impact of laser energy fluctuations, which homogenizes and stabilizes large-area LIPSSs.

Femtosecond laser fabrication of optofluidic devices for single cell manipulation

In this work we fabricate and validate two optofludic devices for the manipulation and analysis of single cells. The chips are fabricated by femtosecond laser micromachining exploiting the 3D capabilities of the technique and the inherent perfect alignment between microfluidic channels and optical networks. Both devices have been validated by probing the mechanical properties of different cancer cell lines, which are expected to show different elasticity because of their different metastatic potential.

Correction: Vertical sidewall electrodes monolithically integrated into 3D glass microfluidic chips using water-assisted femtosecond-laser fabrication for in situ control of electrotaxis

Correction for ‘Vertical sidewall electrodes monolithically integrated into 3D glass microfluidic chips using water-assisted femtosecond-laser fabrication for in situ control of electrotaxis’ by Jian Xu et al., RSC Adv., 2015, 5, 24072–24080.

Vertical sidewall electrodes monolithically integrated into 3D glass microfluidic chips using water-assisted femtosecond-laser fabrication for in situ control of electrotaxis

Novel sidewall metal patterning with high flexibility enables facile integration of vertical electrodes in microchannels forin situcontrol of electrotaxis.

Femtosecond laser fabrication of monolithically integrated microfluidic sensors in glass.

Femtosecond lasers have revolutionized the processing of materials, since their ultrashort pulse width and extremely high peak intensity allows high-quality micro- and nanofabrication of three-dimensional (3D) structures. This unique capability opens up a new route for fabrication of microfluidic sensors for biochemical applications. The present paper presents a comprehensive review of recent advancements in femtosecond laser processing of glass for a variety of microfluidic sensor applications. These include 3D integration of micro-/nanofluidic, optofluidic, electrofluidic, surface-enhanced Raman-scattering devices, in addition to fabrication of devices for microfluidic bioassays and lab-on-fiber sensors. This paper describes the unique characteristics of femtosecond laser processing and the basic concepts involved in femtosecond laser direct writing. Advanced spatiotemporal beam shaping methods are also discussed. Typical examples of microfluidic sensors fabricated using femtosecond lasers are then highlighted, and their applications in chemical and biological sensing are described. Finally, a summary of the technology is given and the outlook for further developments in this field is considered.

Fabrication of Quantum Photonic Integrated Circuits by Means of Femtosecond Laser Pulses

Femtosecond laser microfabrication has emerged in the last decade as a powerful technique for direct inscription of low loss optical waveguides in practically any transparent dielectric substrate, showing outstanding versatility. Prototyping of new devices is made rapid, cheap and easy: optical circuits are written directly buried in the substrate, using the laser beam as an optical pen, without any need of costly masks as required by conventional photolithography. Many proof-of-principle demonstrations of integrated optics can be obtained, including splitters, directional couplers, and Mach–Zehnder interferometers. Actually, the road towards applications has just been opened, and the unique capabilities of femtosecond laser micromachining will enable achievements inconceivable with other technologies. In this work, the femtosecond laser fabrication technique is discussed, together with its application to the realization of integrated photonic quantum circuits.

Femtosecond laser fabrication of silver plasmonic structures for application as single particle SERS detectors

This work demonstrates the feasibility of fabricating silver nanoparticles (NPs) into sub-micron and micron-sized core-satellite structures by exposure to femtosecond laser radiation. We find that the size and shape of these structures can be tailored by adjusting laser fluence (1.8–10.5 J cm−2) and irradiation time (20–60 min), along with the concentration of NPs in aqueous solution (0.01–0.05 mM). Raman measurements of adenine molecules adsorbed on these sub-micron substrates indicate that core-satellite structures having complex shapes are effective as sensitive surface enhanced Raman spectroscopy (SERS) substrates. The sensitivity is such that these structures are potentially useful as single particle SERS substrates for bio-sensing. Finite difference time domain (FDTD) simulation results indicate that these structures can be generated by the joining of NPs in response to localized surface plasmon induced hotspots.

Femtosecond laser fabrication of waveguides in DR13-doped PMMA

This work demonstrated the fabrication of tubular waveguides in bulk samples of PMMA doped with Disperse Red 13 (DR13) by oscillator only fs-laser micromachining. We studied the influence of the incident pulse energy on the diameter and quality of the fabricated waveguides by analyzing optical microscopy images. HeNe laser (632.8 nm) was coupled into the fabricated waveguides, revealing an annular intensity distribution resulting from the superposition of propagation modes with azimuthal symmetry. The averaged total loss of the fabricated waveguides was estimated as 0.8 dB/mm. Residual birefringence was observed in the produced waveguides, probably generated during the fabrication process, which prevented determining optically induced birefringence owing to the presence of the azochromophore DR13.

Assembling of gold nanorods by femtosecond laser fabrication

Metal nanoparticle is of great importance for the study of surface plasmon resonance, and its functional assembly can show more excellent overall collaborative performance. We propose an efficient assembly of gold nanoparticles (GNP) through femtosecond laser fabrication without introducing additional modifier, which can not only retains the particle’s surface plasma resonance characteristic, but also realizes the assembling pattern of arbitrary subtle shape. We apply the assembly in microfluidic chip for surface-enhanced Raman scattering detection and achieve a very good enhancement. It provides a new approach for the preparation of plasma devices.

Femtosecond laser fabrication of sub-diffraction nanoripples on wet Al surface in multi-filamentation regime: High optical harmonics effects?

Relief ripples with sub-diffraction periods (≈λlas/3, λlas/4) were produced on a aluminum surface immersed in water and irradiated in a multi-filamentation regime by focused 744nm femtosecond laser pulses with highly supercritical, multi-GW peak powers. For the VUV (8.5eV) surface plasmon resonance on the wet aluminum surface, such small-scale surface nanogratings can be produced by high – second and third – optical harmonics, coming to the surface from the optical filaments in the water layer. Then, the sub-diffraction surface ripples may appear through interference of their transverse electric fields with the longitudinal electric fields of their counterparts, scattered on the surface roughness and appeared as the corresponding high-energy, high-wavenumber surface polaritons.