Focused Femtosecond Laser Research Articles

Synthesis of Carbon Microparticles in Silicone Oil Using Focused Femtosecond Laser

Synthesis of Carbon Microparticles in Silicone Oil Using Focused Femtosecond Laser

Characterising Spatio-Temporal Coupling of Tightly Focused Femtosecond Laser Pulses in Micro-Structured Dispersive Materials

Characterising Spatio-Temporal Coupling of Tightly Focused Femtosecond Laser Pulses in Micro-Structured Dispersive Materials

Self-Focusing and Self-Phase Modulation of a Focused Femtosecond Laser Beam in Fused Silica at Near-Critical Peak Power

Self-focusing and self-phase modulation of focused femtosecond laser pulse were studied in bulk fused silica. Depending on pulse energy magnitude, three different regimes were observed in the experiment: (1) linear regime, when the self-focusing effect is insignificant, characterized by linear spectral broadening; (2) transition regime, when the self-focusing effect notably reduces a laser beam spot in a focal plane without considerable nonlinear absorption and beam collapse, characterized by quick increase in spectral broadening due to self-phase modulation; (3) nonlinear regime, filamentation, characterized by near-to- linear spectral broadening due to self-phase modulation. The slope for the linear and nonlinear regimes was the same within the measurement error bars. The numerical simulation, using the ray transfer matrix method, envisioned the self-phase modulation spectral broadening effect in all these regimes.

Tightly Focused Femtosecond Laser Radiation Induces DNA Double-Strand Breaks in Human Tumor Cells.

We studied the formation of double-strand DNA breaks (DNA DSB) induced by femtosecond laser radiation in A549 human lung adenocarcinoma cells using immunocytochemical staining of the resulting tracks of a specific DSB marker protein phosphorylated ATM kinase (phospho-ATM). Additionally, colocalization of phospho-ATM tracks with γH2AX protein tracks was studied. The results of immunocytochemical analysis showed that 30 min after irradiation of cells with femtosecond pulses with energies of 1 and 2 nJ (radiation power density 2×1011 and 4×1011 W×cm-2, respectively), the formation of tracks consisting of phospho-ATM and γH2AX proteins located in sites where the laser beam passes through the cell nuclei was observed. The presence of phospho-ATM tracks co-localized with γH2AX allows us to conclude that exposure to focused femtosecond infrared laser radiation with a pulse energy of 1-2 nJ leads to the formation of DNA DSB in irradiated cells.

Immunocytochemical Localization of XRCC1 and γH2AX Foci Induced by Tightly Focused Femtosecond Laser Radiation in Cultured Human Cells.

To assess the prospects for using intense femtosecond laser radiation in biomedicine, it is necessary to understand the mechanisms of its action on biological macromolecules, especially on the informational macromolecule—DNA. The aim of this work was to study the immunocytochemical localization of DNA repair protein foci (XRCC1 and γH2AX) induced by tightly focused femtosecond laser radiation in human cancer A549 cells. The results showed that no XRCC1 or γH2AX foci tracks were observed 30 min after cell irradiation with femtosecond pulses of 1011 W∙cm−2 peak power density. An increase in the pulse power density to 2 × 1011 W∙cm−2 led to the formation of linear tracks consisting both of XRCC1 and γH2AX protein foci localized in the places where the laser beam passed through the cell nuclei. A further increase in the pulse power density to 4 × 1011 W∙cm−2 led to the appearance of nuclei with total immunocytochemical staining for XRCC1 and γH2AX on the path of the laser beam. Thus, femtosecond laser radiation can be considered as a tool for local ionization of biological material, and this ionization will lead to similar effects obtained using ionizing radiation.

Femtosecond laser inscription of Bragg gratings on a thin GaN film grown on a sapphire substrate

Bragg gratings were inscribed with a NIR femtosecond laser, cylindrical lens and a phase mask on the surface of a thin GaN film grown on a sapphire substrate. The phase mask was used to produce stable interference patterns from the focused femtosecond laser to form the gratings on the samples. The intensity threshold for the Bragg grating inscription was found to be ∼2.5·1012 W/cm2. By optimizing the inscription parameters, we successfully produced grating structures with uniform periods of ∼1.07 µm on the thin GaN film surface. We compared the inscription threshold to that of sapphire and fused silica. It was found that the threshold for Bragg grating inscription on GaN film is nearly an order of magnitude lower than that on sapphire and fused silica. This can be attributed to the lower-order nonlinear absorption process in GaN compared to that in sapphire or fused silica glass, originating from the smaller energy band gap of GaN.

Formation of Permanent Brown-colored Patterns in Transparent BK7 Glass upon Irradiation with a Tightly Focused Femtosecond Laser

Transparent borosilicate glass (BK7) is extensively used in optics applications. Herein, we demonstrate that a brown-colored pattern is generated inside BK7 glass upon irradiation with a near-infrared Ti:sapphire femtosecond laser and reveal the importance of maintaining the laser power at values well below the BK7 damage threshold for color center formation. The use of low laser power prevents the occurrence of mechanical damage (e.g., micro-crack or thread formation) in the generated colored area, and Gaussian fitting of the absorbance spectrum of BK7 shows that this material exhibits a band gap of 4.21 eV with three absorption edges. Thus, the developed patterning method is expected to find numerous applications in the future color processing of lines and points in glass.

Visible to Infrared Diamond Photonics Enabled by Focused Femtosecond Laser Pulses

Diamond’s nitrogen-vacancy (NV) centers show great promise in sensing applications and quantum computing due to their long electron spin coherence time and because they can be found, manipulated, and read out optically. An important step forward for diamond photonics would be connecting multiple diamond NVs together using optical waveguides. However, the inertness of diamond is a significant hurdle for the fabrication of integrated optics similar to those that revolutionized silicon photonics. In this work, we show the fabrication of optical waveguides in diamond, enabled by focused femtosecond high repetition rate laser pulses. By optimizing the geometry of the waveguide, we obtain single mode waveguides from the visible to the infrared. Additionally, we show the laser writing of individual NV centers within the bulk of diamond. We use µ-Raman spectroscopy to gain better insight on the stress and the refractive index profile of the optical waveguides. Using optically detected magnetic resonance and confocal photoluminescence characterization, high quality NV properties are observed in waveguides formed in various grades of diamond, making them promising for applications such as magnetometry, quantum information systems, and evanescent field sensors.

Characteristics and Applications of Spatiotemporally Focused Femtosecond Laser Pulses

Simultaneous spatial and temporal focusing (SSTF) of femtosecond laser pulses gives rise to strong suppression of nonlinear self-focusing during the propagation of the femtosecond laser beam. In this paper, we begin with an introduction of the principle of SSTF, followed by a review of our recent experimental results on the characterization and application of the spatiotemporally focused pulses for femtosecond laser micromachining. Finally, we summarize all of the results and give a future perspective of this technique.

Intricate Plasma-Scattered Images and Spectra of Focused Femtosecond Laser Pulses.

We report on some interesting phenomena in the focusing and scattering of femtosecond laser pulses in free space that provide insights on intense laser plasma interactions. The scattered image in the far field is analyzed and the connection with the observed structure of the plasma at the focus is discussed. We explain the physical mechanisms behind the changes in the colorful and intricate image formed by scattering from the plasma for different compressions, as well as orientations of plano-convex lens. The laser power does not show significant effect on the images. The pulse repetition rate above 500 Hz can affect the image through slow dynamics The spectrum of each color in the image shows oscillatory peaks due to interference of delayed pulse that correlate with the plasma length. Spectral lines of atomic species are identified and new peaks are observed through the white light emitted by the plasma spot. We find that an Ar gas jet can brighten the white light of the plasma spot and produce high resolution spectral peaks. The intricate image is found to be extremely sensitive and this is useful for applications in sensing microscale objects.

光斑重叠率对飞秒激光硅材料表面着色的影响

The influence of overlapping rate of focused femtosecond laser spot on colorization of silicon surfaces is investigated.Experimental results indicate that the generation of periodic nanoscale ripple structures induced by femtosecond laser pulses is responsible for the silicon surfaces colorization.In the case of a certain femtosecond laser power,the colorization cannot be realized with too high or too low overlapping rate of focused laser spots.For a special range of laser power,the denser overlapping rate of focused laser spots,the more colors are observed,and the higher colorization efficiency is achieved.In addition,the measurement results of reflection indicate that the refection rate of colorized samples reduces about 50%in the visible light range.The window parameters for silicon surfaces colorization by femtosecond laser are obtained,which paves the way for femtosecond laser semiconductor colorizing technology.

Processing grating structures on surfaces of wide-bandgap semiconductors using femtosecond laser and phase mask

Fabrication of grating structures on surfaces of wide-bandgap semiconductors, namely silicon carbide (SiC) and gallium nitride (GaN), was achieved using a femtosecond laser and a phase mask. The phase mask was used to produce stable interference patterns from the focused femtosecond laser to form the grating struc- tures on the bulk materials. The effects of the irradiation power and time on the Bragg grating morphology that was formed on the SiC surface were studied. By optimizing the fabrication parameters, we successfully pro- duced grating structures with uniform periods of 1.07 μm on SiC and GaN. The threshold powers necessary for grating structure formation on wide-bandgap semiconductors were investigated. It was found that the thresh- old powers for SiC and GaN were much smaller than those for silica glass. The reason for this difference is that the absorption of the incident laser light in SiC and GaN is a lower-order nonlinear absorption process compared to that in silica glass. © 2015 Society of Photo-Optical Instrumentation Engineers (SPIE) (DOI: 10.1117/1.OE.54.12.126106)

Ionization-induced self-compression of tightly focused femtosecond laser pulses.

As lasers become progressively higher in power, optical damage thresholds will become a limiting factor. Using the nonlinear optics of plasma may be a way to circumvent these limits. Here, we present a new self-compression mechanism for high-power, femtosecond laser pulses based on geometrical focusing and three dimensional spatiotemporal reshaping in an ionizing plasma. By propagating tightly focused, 10-mJ femtosecond laser pulses through a 100-μm gas jet, the interplay between ionization gradients, focusing, and diffraction of the light pulse leads to stable and uniform self-compression of the pulse, while maintaining a high-energy throughput and excellent refocusability. Self-compression down to 16fs from an original 36-fs pulse is measured using second-harmonic-generation frequency-resolved optical gating. Using this mechanism, we are able to maintain a high transmission (>88%) such that the pulse peak power is doubled. Three-dimensional numerical simulations are performed to support our interpretation of the experimental observations.

Nonlinear optical collagen cross-linking and mechanical stiffening: a possible photodynamic therapeutic approach to treating corneal ectasia

In this study we test the hypothesis that nonlinear optical (NLO) multiphoton photoactivation of riboflavin using a focused femtosecond (FS) laser light can be used to induce cross-linking (CXL) and mechanically stiffen collagen as a potential clinical therapy for the treatment of keratoconus and corneal ectasia. Riboflavin-soaked, compressed collagen hydrogels are cross-linked using a FS laser tuned to 760 nm and set to either 100 mW (NLO CXL I) or 150 mW (NLO CXL II) of laser power. FS pulses are focused into the hydrogel using a 0.75 NA objective lens, and the hydrogel is three-dimensionally scanned. Measurement of hydrogel stiffness by indentation testing show that the calculated elastic modulus (E) values are significantly increased over twofold following NLO CXL I and II compared with baseline values (P<0.05). Additionally, no significant differences are detected between NLO CXL and single photon, UVA CXL (P>0.05). This data suggests that NLO CXL has a comparable effect to conventional UVA CXL in mechanically stiffening collagen and may provide a safe and effective approach to localize CXL at different regions and depths within the cornea.

Transient Stress Imaging after Irradiation with a Focused Femtosecond Laser Pulse inside a Single Crystal

A focused femtosecond laser pulse inside a single crystal induces anisotropic structural changes, which depend on the atomic arrangement of the crystal. The transient stress after the photoexcitation should be responsible for the structural changes. In this study, we developed a novel transient stress observation system by time-resolved birefringence analysis and observed the relative amplitude and azimuth of the transient stresses after the photoexcitation inside MgO and LiF crystals. The principle and advantage of the observation system are described in this paper by comparing the system with a conventional stress observation method, a crossed-Nicols imaging method.

Transient Stress Imaging after Irradiation with a Focused Femtosecond Laser Pulse inside a Single Crystal

A focused femtosecond laser pulse inside a single crystal induces anisotropic structural changes, which depend on the atomic arrangement of the crystal. The transient stress after the photoexcitation should be responsible for the structural changes. In this study, we developed a novel transient stress observation system by time-resolved birefringence analysis and observed the relative amplitude and azimuth of the transient stresses after the photoexcitation inside MgO and LiF crystals. The principle and advantage of the observation system are described in this paper by comparing the system with a conventional stress observation method, a crossed-Nicols imaging method.

Rapid Fabrication of Three-Dimensional Woodpile PhotonicCrystals by Means of Two-Photon Photopolymerization

Two-photon photopolymerization (TPP) of femtosecond laser is apromising method to fabricate three-dimensional woodpile photoniccrystals (PCs). We build micro-fabrication system based on theprinciple of TPP. Three-dimensional woodpile PCs consisting of in-planerod distances ranging from 1000 nm to 2000 nm are fabricated by focusingfemtosecond laser in photosensitive liquid resin ORMOCER. Theproperties of the PCs are also discussed, and fundamental photonic bandgaps in middle-infrared range are measured, whose in-plane roddistances are 1500 nm and 2000 nm. Three-dimensional woodpile PC deviceswith desired defects, such as cross-waveguide and micro-laserstructures, are introduced easily by TPP. We fabricate thethree-dimensional woodpile PCs in the liquid resin at the fast scanningspeed of 120 μm/s.

Self-Assembled Quasi-Periodic Voids in Glass Induced by aTightly Focused Femtosecond Laser

Multiple refocusing of a tightly focused femtosecond laser due to thedynamic transformation between self-focusing and self-defocusing isemployed to provide a novel method to produce quasi-periodic voids inglass. It is found that the diameter or the interval of the periodicvoids increases with the increasing pulse energy of the laser. Thedetailed course for producing periodic voids is discussed by analysingthe damaged track induced by the tightly focused femtosecond laserpulses. It is suggested that this periodic structure has potentialapplications in fabrication of three-dimensional optical devices.

Dynamic Recompartmentalization of Supported Lipid Bilayers Using Focused Femtosecond Laser Pulses

H ow the presence,distributions,and dynam -ics of diffusionalbarriers influence B row nian m otions of lipids andproteins, however, remains poorly understood. To systematicallystudy these processes, model systems which exhibit dynamicmembrane compartmentalization are highly desirable.In a parallel line of research, ultrafast laser pulses (e.g.,fem tosecond) are proving to be valuable in inducing highly localizedphotomodifications in many soft materials including glasses,polymers, and cells.

Fabrication of Gallium Nitride Grating by Interferometric Irradiation Using Focused Femtosecond Laser

We report the optical modification of gallium nitride by femtosecond laser irradiation. A 5-nm-band-gap shift, observed by spectral ellipsometry, results a reduction of 0.01 in the refractive index at a wavelength of 400 nm. An indistinct circular pattern was also observed using nano-beam diffraction. These results suggest that femtosecond-laser irradiation induces stress–strain, rather than slight amorphization. We also observed that the angle of first-order diffraction from the grating with a 1.0 µm period, fabricated by interferometric irradiation using a femtosecond laser without any damage, corresponds to the calculated angle. The first-order diffraction efficiency for a wavelength of 403 nm was estimated to be 4.0×10-3%.