Dry‐etching‐assisted femtosecond laser machining

Femtosecond laser machining has been widely used for fabricating arbitrary 2.5 dimensional (2.5D) structures. However, it suffers from the problems of low fabrication efficiency and high surface roughness when processing hard materials. To solve these problems, we propose a dry‐etching‐assisted femtosecond laser machining (DE‐FsLM) approach in this paper. The fabrication efficiency could be significantly improved for the formation of complicated 2.5D structures, as the power required for the laser modification of materials is lower than that required for laser ablation. Furthermore, the surface roughness defined by the root‐mean‐square improved by an order of magnitude because of the flat interfaces of laser‐modified regions and untreated areas as well as accurate control during the dry‐etching process. As the dry‐etching system is compatible with the IC fabrication process, the DE‐FsLM technology shows great potential for application in the device integration processing industry.

     

Reduction of optical damage effects in LiNbO3 and LiTaO3

Optical damage effects in electro-optic crystals such as LiNbO₃ and LiTaO₃ result from light-induced refractive index changes which bring about decollimation and scattering of light beams. The origin of the index changes is discussed. Although it is currently impossible to avoid undesired optical damage completely, various possibilities are proposed to reduce the effects.     

Laser induced memory bits in photorefractive LiNbO3 and LiTaO3

We study experimentally the formation of refractive index voxels (volume elements) in photorefractive LiNbO₃ and LiTaO₃ crystals illuminated with high irradiance femtosecond laser pulses. We used 150 fs pulses at 800 nm wavelength (energy 6–50 nJ) tightly focused inside the crystals in a single shot regime. This resulted in a formation of a micrometer size region of elevated refractive index, which may be used as memory bits in information storage/retrieval application. The maximum refractive index change of 5×10⁻⁴ was recorded in undoped LiNbO₃ at an average light intensity of ∼TW/cm² that is close to the breakdown threshold. A simple setup for photorefractive recording and in situ monitoring of the refractive index changes has been proposed. M. Sūdžius leaves from: the Institute of Materials Science and Applied Research of Vilnius University, Lithuania.     

An EXAFS study of dopant substitution in LiNbO3 and LiTaO3

Abstract

We report an EXAFS study of Co and Fe doped LiNbO₃ and LiTaO₃. Our results show that the dopant ions occupy Li sites in both materials.     

3D atom probe assisted by femtosecond laser pulses

The 3DAP allows to image a material in 3D on a nearly atomic scale. It is based on the field evaporation occurring at the surface of a biased tip like shape specimen with an end radius of 50 nm. Surface atoms are removed one by one from the tip by means of fs laser pulses so that the physical process involved in this laser enhanced field evaporation might correspond to the very early stages of the ablation process. This technique makes possible to distinguish between different regimes of material removal such as thermal evaporation or in the case of metals or semiconductors an evaporation assisted by the rectification of the optical field at the surface. In this paper the principle of the 3DAP is presented and the underlying physics involved in the field evaporation assisted by femtosecond laser pulses is discussed.     

Three-dimensional photoprecipitation of oriented LiNbO3-like crystals in silica-based glass with femtosecond laser irradiation

We demonstrate crystals (LiNbO(3)-like) that were space-selectively nucleated and grown in the bulk of silica-based glass by femtosecond laser irradiation at a high repetition rate (typ. 300 kHz). Oriented crystals with their polar axis mostly aligned with or perpendicular to the laser scanning direction have been fabricated by manipulation of the temperature gradient in adjusting the laser parameters. The mechanism for the orientation of femtosecond laser-induced crystallization is briefly discussed.     

Room temperature femtosecond optical parametric generation in MgO-doped stoichiometric LiTaO3

We demonstrate room temperature femtosecond optical parametric generation with high average output power in periodically poled MgO-doped stoichiometric LiTaO₃. Direct pumping with 725-fs pulses from a passively mode-locked thin disk laser at 1030 nm resulted in stable 1.5 W average signal power at 1484 nm at the full laser repetition rate of 59 MHz. With this demonstration we achieved a significant simplification of our recently presented red-green-blue laser source because no temperature stabilization of any nonlinear crystal is required. PACS 42.65.Yj; 42.70.Mp; 42.79.Nv     

Development of portable laser machining system for laser writing applications

This study presents a portable laser machining system that consists of a fiber-optic diode laser source with a wavelength of 808 nm, optic/opto-mechanical components, a laser scanning module, and a laser energy control module. The laser beam quality was measured at different operation frequencies during system evaluation. The experimental results of beam profile evaluation indicate that the enlarged collimated beam was the TEM₀₀ mode with a roundness of approximately of 96%. The output laser power level increased as the pulse frequency increased during laser power evaluation. To control the rotating angle of the galvanometric scanning system, the deflective angle was adjusted using a 0.192 voltage to obtain a deflective value of 1mm and the maximum scan field of 100 × 100mm². The laser source operated at different frequencies, with pulse widths ranging from 530 to 48 μs. Finally, the proposed machine can also be used for black thick paper laser writing applications.     

Properties of nanoparticles generated during femtosecond laser machining in air and water

Femtosecond laser ablation is used to generate nanoparticle aerosols and colloids from solid targets of various materials (Ti, Ag, Au, Co, etc.) in air and water ambience. We determine the influence of different laser parameters (pulse energy, pulse overlap) and properties of media (air, airbrush, water) on the rate of production and size distribution of the laser-generated nanoparticles. It is shown that the pulse overlap and laser fluence are the parameters determining the nanoparticle size. At optimum conditions the nanoparticle productivity can be increased by 150–300%. The generation of multimaterial nanoparticle dispersions is demonstrated. Being free of toxic impurities, the laser-produced nanoparticles may be promising for biomedical applications. PACS 79.20.Ds; 81.16.Mk; 81.16.-c; 52.38.Ph; 06.60.Jn     

Femtosecond laser micromachined optical waveguides in LiTaO3 crystal

By using femtosecond laser micromachining, optical wave guides in both depressed cladding and dual‐line configurations have been produced in LiTaO₃ crystal. The guiding properties and the thermal stability have been investigated for both geometries, which exhibit different performance. Depressed cladding waveguides support guidance along both extraordinary and ordinary index polarizations, while dual‐line waveguides support only extraordinary index polarization. Thermal annealing has been proved to be an effective method to reduce the propagation losses. For the cladding waveguide, the lowest propagation loss was as low as 0.38 dB/cm after the annealing treatment at 400 °C. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Birefringent Fresnel zone plates in silica fabricated by femtosecond laser machining

We demonstrate maskless, single-step fabrication of strongly birefringent Fresnel zone plates by focusing of femtosecond laser pulses deep within silica substrates. The process allows us to produce alternate zone rings directly by inducing a local refractive-index modification of the order of n~10(-2) . The embedded zone plates shown in this Letter exhibit efficiencies that vary by as much as a factor of ~6 for orthogonal polarizations. Focal lengths of primary and secondary foci are shown to compare well with theory.     

A comparison of the performance of LiNbO3, and LiTaO3, traveling-wave phase modulators

A comparison of the phase modulators made by Ti:LiNbO₃ and Ti:LiTaO₃ optical waveguides is presented. Of particular interest is their halfwave voltages at the wavelength 0.6328 μm and the frequency responses for the same electrode structure. For the Ti:LiNbO₃ phase modulator, a halfwave voltage of 6.6 V and a band-width of 13 GHz are obtained. Whereas, the Ti:LiTaO₃, a 6.8 V halfwave voltage and 12 GHz bandwidth are obtained.     

A comparison of the performance of LiNbO3, and LiTaO3, traveling-wave phase modulators

Abstract

A comparison of the phase modulators made by Ti:LiNbO₃ and Ti:LiTaO₃ optical waveguides is presented. Of particular interest is their halfwave voltages at the wavelength 0.6328 μm and the frequency responses for the same electrode structure. For the Ti:LiNbO₃ phase modulator, a halfwave voltage of 6.6 V and a band-width of 13 GHz are obtained. Whereas, the Ti:LiTaO₃, a 6.8 V halfwave voltage and 12 GHz bandwidth are obtained.     

Rapid communicationPhotoelectron emission in femtosecond laser assisted scanning tunneling microscopy

Direct illumination of the tunneling gap in an ultrahigh vacuum scanning tunneling microscope with ultrashort pump-probe laser pulses may offer ultimate spatial and temporal resolution in surface experiments. The electronic bandwidth of the tunneling gap ( 1 THz) does not limit the time resolution. Our experiments show that multiphoton photoelectron emission from the sample limits the application of this detection scheme at high laser fluence. However, a substrate specific pump-probe effect in the photoelectron yield with femtosecond transients is observed on Tantalum and on GaAs(110) surfaces. Received: 5 November 1996