Fs Laser Irradiation Research Articles

Micro- and nanostructures inside sapphire by fs-laser irradiation and selective etching

The fabrication of microchannels and self-assembled nanostructures in the volume of sapphire was performed by femtosecond laser irradiation followed by chemical etching with aqueous solution of HF acid. Depending on the focusing conditions self-organized nanostructures or elliptical microchannels are produced. While the dimensions in two directions are on a micro- respectively nanoscale, feature lengths of up to 1 mm are achieved. This comes out to aspect ratios of more than 1000. This fabrication technique is potentially usable for photonic crystal based integrated optical elements or microfluidic devices for applications in life science, biology or chemistry.

Risk estimation of skin damage due to ultrashort pulsed, focused near-infrared laser irradiation at 800 nm

New imaging techniques using near-infrared (NIR) femtosecond lasers (fs-lasers) in multiphoton laser scanning microscopy (MPLSM) have great potential for in vivo applications, particularly in human skin. However, little is known about possible risks. In order to evaluate the risk, a "biological dosimeter" was used. We irradiated fresh human skin samples with both an fs-laser and a solar simulator UV source (SSU). DNA damage introduced in the epidermis was evaluated using fluorescent antibodies against cyclobutane-pyrimidin-dimers (CPDs) in combination with immunofluorescence image analysis. Four fs-irradiation regimes (at 800-nm wavelength) were evaluated differing in laser power and step width of horizontal scans. Fs-irradiation did not give CPDs at 15-mW or 30-mW irradiation power using 10 horizontal scans every 5 microns. CPDs could be seen at 60-mW laser power and 5-microm step size and at 35-mW using 1-micron step width. Quantitative comparison of SSU-induced CPDs showed that the 60-mW laser irradiation regime is comparable to UV-irradiation, giving 0.6 minimal erythemal dose (MED). The 1-micron irradiation regime was comparable to 0.45 MED. Under these experimental conditions, the risk of DNA damage due to fs-laser irradiation on skin is in the range of natural UV-exposure.

Femtosecond laser irradiation-induced phase transformation on titanium dioxide crystal surface

Titanium dioxide (TiO2) rutile single crystal was irradiated by infrared femtosecond (fs) laser pulses with repetition rate of 250kHz and phase transformation of rutile TiO2 was observed. Micro-Raman spectra show that the intensity of Eg Raman vibrating mode of rutile phase increases and that of A1g Raman vibrating mode decreases apparently within the ablation crater after fs laser irradiation. With increasing of irradiation time at laser average power of 300mW, the Raman vibrating modes of anatase phase emerged. Rutile phase of TiO2 single crystal is partly transformed into anatase phase. The anatase phase content transformed from rutile phase increased to a maximum and then decreased with increasing of fs pulse laser irradiation time. The line mapping of micro-Raman spectrum at the crater, irradiated by fs pulse laser for 100s at average power of 300mW, indicates more rutile phase are transformed into anatase phase at the center.

Raman study of phase transformation of TiO 2 rutile single crystal irradiated by infrared femtosecond laser

Titanium dioxide (TiO 2) rutile single crystal was irradiated by infrared femtosecond (fs) laser pulses with repetition rate of 250 kHz and phase transformation of rutile TiO 2 was observed. Micro-Raman spectra show that the intensity of E g Raman vibrating mode of rutile phase increases and that of A 1g Raman vibrating mode decreases apparently within the ablation crater after fs laser irradiation. With increasing of irradiation time, the Raman vibrating modes of anatase phase emerged. Rutile phase of TiO 2 single crystal is partly transformed into anatase phase. The anatase phase content transformed from rutile phase increased to a constant with increasing of fs pulse laser irradiation time. The study indicates the more stable rutile phase is transformed into anatase phase by the high pressure produced by fs pulse laser irradiation.

Fs-laser-induced Fabrication of Polymeric Optical and Fluidic Microstructures

In this paper first experimental results are presented on the fabrication of integrated-optical microstructures by femtosecond-laser irradiation at the surface or in the volume of a polymeric substrate. Thereby the refractive index or the optical absorption coefficient of the polymer material is locally modified in a controllable way due to nonlinear optical effects like multi-photon absorption. By this way surface relief grating structures, volume grating structures and linear waveguiding structures are generated. The surface relief gratings have diffraction efficiencies about 0.2 (first diffraction order), the diffraction efficiencies of the volume gratings are approximately smaller by one order of magnitude. Besides experiments have been performed on the modification of the polymeric surface by fs-laser irradiation followed by subsequent development of the irradiated area by an etching liquid generating linear groove structures. By this way simple linear microstructures are generated into the polymer substrate. The results of these experiments are relevant for the fabrication of three-dimensional structures in the polymeric substrate in the future. All the gratings and the linear microstructures can be considered as basic components for the fabrication of complex micro-optical and fluidic systems in a polymeric substrate (photonic lab-on-the-chip).

Femtosecond-laser-encoded distributed-feedback color center laser in lithium fluoride single crystal

Focused infrared femtosecond laser pulses (wavelength ∼800nm, emission pulse duration 100fs) were employed to fabricate optoelectronic devices such as waveguides, micro-gratings and laser active centers in LiF crystals. F2 color centers of about 2×1018cm−3 and refractive index change of about 1% at 633nm were induced by the fs-laser irradiation. This technique was applied to fabricate a distributed-feedback (DFB) F2 color center laser structure inside LiF single crystal. The LiF DFB laser exhibited laser oscillation at 707nm at room temperature. The slope efficiency of ∼10% and beam divergence of ∼20mrad were achieved.

PHOTONIC PATTERN FABRICATION IN FUSED SILICA, BK7, ANDGe-DOPED BOROPHOSPHOSILICA GLASS BY A FEMTOSECOND LASER

Photonic patterns were fabricated in fused silica, BK7, and Ge -doped borophosphosilica glass ( Ge -BPSG) using a focused femtosecond (fs)-laser beam. By focusing tens to hundreds of μJ fs-laser beam with a 10x microscope objective, we inscribed the semi-circular cavity patterns on the fused silica and the BK7. The inscribed hole diameters are 28 μm (fused silica) and 11 μm (BK7) at an input fluence of 71 J/cm2. This circular-cavity patterning is ascribed to the ablation via the multi-photon absorption process. For the application to functional devices, the surface relief gratings (SRGs) were made in fused silica and BK7 by focusing the fs-laser beam on the glass surface with a cylindrical lens and by translating the sample in the direction perpendicular to the focus line. The first-order diffraction efficiencies of the prepared SRGs are 34% (fused silica) and 14% (BK7). A refractive-index grating was also fabricated in the Ge -BPSG by using the two-beam interference method. The maximum index modulation of 2.5 × 10-3was obtained for 20,000 laser shots of 73 mJ/cm2per pulse. It is thought that the index modification occurs through the defect formation by the fs-laser irradiation.

Laser-based photoemission micro-spectroscopy for occupied and unoccupied states of inhomogeneous surfaces

Performance of our micro-spot photoemission spectrometer based on fs-laser radiation has been applied to observe (a) surface images due to an unoccupied electronic state and (b) inhomogeneous electronic structure of a thin organic film. Two-photon photoemission spectroscopy was performed with lateral resolution of 0.6 μm for a polycrystalline copper plate. We have observed lateral distribution of the unoccupied image-potential state of Cu(1 1 1) surface. In addition to μm-scale information from the surface image, the high-energy resolution spectroscopy provides insight on the step size of nm-scale. By employment of focused VUV light of 8.86 eV, one-photon photoemission spectroscopy was performed for a copper phthalocyanine film grown on the polycrystalline copper plate with a lateral resolution of 0.3 μm and an energy resolution of 30 meV. The photoemission band due to the highest occupied molecular orbital peaked at the binding energy of either 1.6 or 1.2 eV depending on the sample positions. Polarization dependence of the band showed that molecules are poorly oriented even on the (1 1 1) surface. The results demonstrate that a large inhomogeneity exists in the interface electronic structure.

Femtosecond laser application for high capacity optical data storage

A femtosecond (fs) laser application for multi-layer optical recording is investigated. Information patterns at different layer depths were written inside a transparent glass substrate due to micro-void formation by fs laser ablation, which causes re-distribution in glass materials and a refractive index modification. The information bits recorded in a single layer can be retrieved clearly without interference from the neighboring layers. A fs laser irradiation of a transparent polymer matrix (doped with fluorescent materials for use as low-cost recording media) is also studied. A fs laser induced photo-chemical reaction changes the chemical properties of the fluorescent materials and records information bits inside the matrix. With an ultra-fast laser as a new light source, 3D optical recording can be available for high capacity data storage up to 1 TB per disc.

Composition influence of non-oxidic ceramics on self-assembled nanostructures due to fs-laser irradiation

The interaction of femtosecond laser pulses in the near-infrared region with non-oxidic ceramics such as silicon carbide, SiC, a composite compound SiC–TiC–TiB2, and aluminium nitride, AlN, was investigated with respect to morphological changes. Multi-pulse irradiation resulted in negligible melting and debris. Most interesting was the occurrence of periodic structures (ripples). These self-assembled nanostructures showed an orientation perpendicular to the plane of polarization. All materials exhibited at least two different periodicities. Energy dispersive X-ray analysis (EDX) allowed correlating the chemical composition of the ceramic compound with the morphological nature of these ripples. Phenomena near the ablation threshold are discussed in terms of non-thermal melting processes.

High-resolution X-ray imaging spectroscopy diagnostic of hollow ions in dense plasmas

The formation of hollow ions in dense Z-pinch and laser produced plasmas is investigated by means of high-resolution X-ray spectroscopy and atomic data calculations. Dense plasma effects are found to result in highly populated excited states which open up excitation channels not accessible in low-density plasma sources. This makes fs-laser radiation interacting with matter an important tool for modern atomic physics investigations.

Laser Ablation of Transparent Materials by UV fs-Laser Irradiation.

We have investigated the laser ablation of UV transparent materials of PTFE, FEP, quartz and sapphire with a femtosecond-pulsed UV laser at the wavelength of 268nm. Upon laser irradiation, a fine etching without debris was obtained on the surfaces of PTFE and quartz. The laser-ablated surfaces were analyzed by optical microscopy. The ablation mechanism is discussed in terms of multi-photon absorption and thermal diffusion induced by a ultra-short pulse of the laser.

Revisiting the thermal-spike concept in ion-surface interactions

In recent years many groups have advocated a thermal-spike model to explain a variety of experimental results in ion-irradiation of solids, as for example sputtering, mixing, compositional change, structural change, and track formation. The latter include crystal-to-amorphous transitions as well as track formation due to MeV/u particles. In this paper we reconsider the phenomena occurring during ion impact of solids looking at the time scale generally indicated as relevant for thermal-spike effects, namely a picosecond scale as shown by molecular dynamics. Sputtering, mixing, and track formation, however, will be analyzed in more detail. We consider first ion-beam sputtering and reiterate (as is already well-known) that yields which increase with the bulk temperature most often indicate merely the onset of normal vaporization. Indeed, only simulations appear to be capable of giving insight even if the information is sometimes tentative. In mixing, ballistic transport is important but not dominant. It is often argued that the additional transport is provided by thermal spikes but it is noted that such an assumption is normally not required by the experimental results. What is more relevant is a role for residual defects such that the total diffusion flux includes (if the defects are chemically guided) a modified Darken factor, or (if the defects are not chemically guided) simply an increased diffusivity. The time scale (min), distances (well beyond the collision cascade), temperature sensitivity (changes of as little as 75 K are relevant), and correlation with vacancy properties (thence with the solid rather than liquid state) which are relevant to these residual defects are not understandable in terms of thermal spikes. We finally consider track formation. Recent work claiming that track formation in solids, irradiated with heavy ions, may be understood in terms of thermal spikes is reconsidered to show that the thermal-spike model is utilized without considering all the relevant phenomena included in irradiation-induced heating and phase transitions. For example, a comparison of fs-laser pulse irradiation of Si with swift heavy-ion irradiation, shows that melting is possible in the first case since the excited electrons have a low and more or less restricted energy while in the case of swift ion-irradiation, the motion of the excited electrons includes a ballistic component which does not favour the localization of the thermal energy necessary to induce lattice melting. It is concluded that track formation is better understandable in a more general framework of defect-induced processes in solids.