Low-energy Femtosecond Laser Pulses Research Articles

Characterization of X-ray radiation from solid Sn target irradiated by femtosecond laser pulses in the presence of air plasma sparks

AbstractThe emission of X-rays from solid tin targets irradiated by low-energy (few mJ) femtosecond laser pulses propagated through air plasma sparks is investigated. The aim is that to better understand the X-ray emission mechanism and to show the possibility to produce proper radiation for spectroscopic and imaging applications with a table-top laser system. The utilization of a controlled ultrashort prepulse is found necessary to optimize the conversion efficiency of laser energy into Lα radiation. The optimum contrast between the main pulse and the controlled prepulse is found about 102. A correlation between the laser contrast value and the laser near-infrared spectra at the exit of the plasma spark is observed.

Annealing of High Temperature Stable Hydrogen Loaded Fiber Bragg Gratings

We compare the isothermal and isochronal annealing response of ultrafast laser-induced first-order gratings to that of the third-order gratings up to 1000 °C. Both devices are written in $H_{2}$ -loaded SMF-28 fiber with low-energy femtosecond laser pulses. With the first-order gratings, we observed a regeneration process that allows for a final peak index change of roughly 4% of the original grating. A high-temperature stable peak reflectivity of 39% is obtained (−2.14-dB transmission loss). The third-order gratings do not anneal out completely (they do not regenerate) and roughly 26% of the device persists at 1000 °C corresponding to a final peak reflectivity of 24% (−1.17-dB transmission loss). Due to their similar fabrication conditions, these findings suggest a strong link between the regenerated first-order gratings and the stable third-order devices. We estimate that both first- and third-order gratings of $\sim 4.5$ mm in length are good candidates for the development of high-temperature stable fiber sensors.

New intrastromal corneal reshaping procedure using high-intensity femtosecond laser pulses

A minimally invasive keratorefractive procedure using high-intensity, low-energy femtosecond laser pulses to perform intrastromal ablation is described. Because of the low pulse energy and the ultrashort duration, tissue in the corneal stroma can be ablated with almost no heat or shockwave generation. This technique obviates the need for the laser in situ keratomileusis (LASIK) flap but retains the advantages of the LASIK procedure. In the technique, a series of femtosecond laser pulses create temporary microchannels in the stroma, oriented perpendicular to the eye's optical axis. After the microchannels are created, a second series of femtosecond pulses directly ablate the desired amount of stromal tissue in a controlled fashion. The ablated material is ejected from the microchannels so the surface layer above the ablated regions collapses, with a consequent change in the refractive power of the cornea. No author has a financial or proprietary interest in any material or method mentioned.

Femtosecond Laser Pulse Induced Ultrafast Demagnetization in Fe/GaAs Thin Films

As the magnetic storage density approaches 1 TB/in <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{2}$</tex></formula> , a grand challenge is looming as how to read/write such a huge amount of data within a reasonable time. In this study, we demonstrate femtosecond demagnetization in 10-nm epitaxially grown Fe thin films by applying low-energy femtosecond laser pulses. We used time-resolved pump-probe Magneto Kerr Effect spectroscopy to record ultrafast laser induced demagnetization and its recovery. The demagnetization time was found to be 70 fs from the time-resolved hysteresis loops. The time scale is much shorter than the phonon thermalization time. Our results show that the demagnetization can be completed before electron-phonon equilibration is achieved, and therefore indicate the feasibility of ultrafast optical control of demagnetization responses for next generation femtosecond-switching magnetic storage devices.

Listening to Cells: A Non-Contact Optoacoustic Nanoprobe

Cells have a complex composition that yields an intricate rheological behavior, appealing for measurements over a wide frequency range. However the existing techniques cannot exceed the kHz range, and rely for most on injected or contacting functionalized microprobes. Here, we report on an innovative non-contact optoacoustic probing of the mechanical properties of single cells at GHz acoustic frequencies under physiological conditions. We culture cells on a biocompatible Ti6Al4V metal alloy. Low-energy femtosecond laser pulses are focused at the cell-Ti6Al4V interface to a sub-µm spot. The ensuing ultrafast thermal dilatation of the metal launches a high-frequency sound pulse in the cell. Acoustic propagation is measured remotely with an ultrafast laser probe through Brillouin light scattering. This all-optical non-invasive technique offers a broad frequency range, extending up to 1 THz, a sub-µm lateral resolution and a nanometer in-depth resolution. For illustration, we here concentrate on the cell nucleus. Using the laser-generated GHz acoustic waves, we probe the stiffness and viscosity of the nuclei of various cell types. We demonstrate for the first time that, at GHz frequencies, the nucleus stiffness tends to a unique value, much larger than that observed at kHz frequencies. We demonstrate that this universal stiffness reflects the compressional dynamics of the nucleus components. We also show that the evolution of the mechanical properties of the nucleus during cell differentiation is correlated with a specific gene expression pattern. In the frame of soft glass rheology, we project the GHz mechanical properties of the differentiated nuclei to the kHz range. Comparison with the kHz data obtained by alternative techniques suggests the existence of a new absorption process appearing at GHz frequencies. This innovative technique defines a new class of experiments to enlighten cell mechanics in physiological conditions at a subcell scale.

Light-Induced Inhibition of Photoluminescence Emission of Core/Shell Semiconductor Nanorods and Its Application for Optical Data Storage

In this work, we demonstrate that the photoluminescence emission of CdSe/CdS spherical core/rod like shell nanorods can be completely inhibited by using low-energy femtosecond laser pulses at 720 nm. A combined analysis of optical confocal microscopy images and transmission electron microscope micrographs reveals the presence of different power regimes in the nanocrystals photodegradation process. The photoluminescence inhibition of the nanorods is found to start at a power range in which no apparent structural damage occurs to the nanorods after irradiation. This suggests the presence of a photochemical transformation of the nanocrystals that is of potential interest for application in optical data storage. This is because in recording systems based on photochemical processes the photoexcited volume can be effectively confined within the diffraction-limited laser focus. We indeed demonstrate that the recorded mark can be scaled down to a few nanorods, or even to a single nanorod, without crosstalk betwee...

Experimentelle Hornhautbildgebung und Hornhautchirurgie mit nicht verstärkten Femtosekundenlaserpulsen

Non-amplified femtosecond laser was used to induce multiphoton effects for corneal tissue imaging and for tissue ablation. A non-amplified titanium-sapphire laser was coupled to a laser scanning microscope in order to examine human and porcine cornea. Tissue was subjected to imaging and lesions were created using identical optical pathways at pulse energies below 2 nJ. Cellular components and the extracellular matrix were selectively imaged by applying autofluorescence and second harmonic generation at submicron resolution. Intrastromal linear scanning at higher power resulted in luminescent plasma along the scanning line. Lesion width decreased with increasing tissue depth and increased with increasing laser power at the target. Light microscopy showed intact stromal tissue around the area of the lesion. High-resolution images as well as high precision tissue lesions were created in the cornea using low energy femtosecond laser pulses. Easy switching between tissue imaging and ablation seems to be suitable for diagnostic and therapeutic applications.

Non-thermal excitation and control of magnetization in Fe/GaAs film by ultrafast laser pulses

We present our recent study of non-thermal excitation and coherent control of spin reorientation in 10-nm epitaxially grown Fe thin films by low-energy femtosecond laser pulses. The magnetization dynamics and hysteresis curves were recorded by pump-probe differential magnetic Kerr (DMK) spectroscopy using linearly polarized laser beams. A sharp switching in DMK signal is observed when we rotated the pump polarization. This result indicates a non-thermal origin of magnetization excitation and reorientation in Fe films. We reveal that spins can interact coherently with the polarization induced by the pulsed laser field in magnetic metals. Such opto-magnetic interactions are instantaneous and are only limited in time by the properties of laser pulses. Our results suggest the feasibility of ultrafast optical control of both the magnetization and the demagnetization responses in magnetic films.

Evaluation of Yields of γ-Rays Produced by Electrons from Gas Jets Irradiated by Low-Energy Laser Pulses: Towards “Virtual Radioisotopes”

Electron generation from a gas jet irradiated by low-energy femtosecond laser pulses is studied as a promising source of ∼1 MeV radiation for radioisotope-free γ-ray imaging systems: “virtual radioisotopes”. The yield of γ-rays in the 0.5–2 MeV range produced by low-average-power lasers and gas targets exceeds the yields from solid tape targets up to 2 orders of magnitude; it can be competitive with the yield from conventional radioisotopes used in industrial applications.

Scanning thermal microscopy and Raman analysis of bulk fused silica exposed to lowenergy femtosecond laser pulses

Low energy femtosecond laser pulses locally increase the refractive index and the hydro-fluoric acid etching rate of fused silica. These phenomena form the basis of a direct-write method to fabricate integrated glass devices that are of particular interest for optofluidics and optomechanical applications. Yet the underlying physical mechanism behind these effects remains elusive, especially the role of the laser polarization. Using Scanning Thermal Microscope and Raman spectrometer we observe in laser affected zones, a localized sharp decrease of the thermal conductivity correlated with an increased presence of low-number SiO(2) cycles. In addition, we find that a high correlation exists between the amount of structural changes and the decrease of thermal conductivity. Furthermore, sub-wavelength periodic patterns are detected for high peak power exposures. Finally, our findings indicate that, to date, the localized densification induced by femtosecond laser pulses remains well below the theoretical value achievable in mechanically densified silica.

Intratissue Refractive Index Shaping (IRIS) of the Cornea and Lens Using a Low-Pulse-Energy Femtosecond Laser Oscillator

To assess the optical effect of high-repetition-rate, low-energy femtosecond laser pulses on lightly fixed corneas and lenses. Eight corneas and eight lenses were extracted postmortem from normal, adult cats. They were lightly fixed and stored in a solution that minimized swelling and opacification. An 800-nm Ti:Sapphire femtosecond laser oscillator with a 27-fs pulse duration and 93-MHz repetition rate was used to inscribe gratings consisting of 20 to 40 lines, each 1-microm wide, 100-microm long, and 5-microm apart, 100 mum below the tissue surface. Refractive index changes in the micromachined regions were calculated immediately and after 1 month of storage by measuring the intensity distribution of diffracted light when the gratings were irradiated with a 632.8-nm He-Ne laser. Periodic gratings were created in the stromal layer of the corneas and the cortex of the lenses by adjusting the laser pulse energy until visible plasma luminescence and bubbles were no longer generated. The gratings had low scattering loss and could only be visualized using phase microscopy. Refractive index changes measured 0.005 +/- 0.001 to 0.01 +/- 0.001 in corneal tissue and 0.015 +/- 0.001 to 0.021 +/- 0.001 in the lenses. The gratings and refractive index changes were preserved after storing the micromachined corneas and lenses for 1 month. These pilot experiments demonstrate a novel application of low-pulse-energy, MHz femtosecond lasers in modifying the refractive index of transparent ocular tissues without apparent tissue destruction. Although it remains to be verified in living tissues, the stability of this effect suggests that the observed modifications are due to long-term molecular and/or structural changes.

Optic cavitation in an ultrasonic field

Cavitation bubbles are generated in water by low-energy femtosecond laser pulses in the presence of an ultrasonic field. Bubble dynamics and cavitation luminescence are investigated by CCD photography and photomultiplier measurements in dependence on the phase of the acoustic cycle at which the bubbles are generated. The experimental results demonstrate that the initially small laser-generated bubbles can be expanded significantly by the sound field and that weak cavitation luminescence can be observed in two small intervals of the seeding phase. The luminescence yield sensitively depends on the degree of sphericity of bubble collapse.

Thermal conductivity contrast measurement of fused silica exposed to low-energy femtosecond laser pulses

Femtosecond laser irradiation has various noticeable effects on fused silica. Of particular interest, pulses with energy levels below the ablation threshold can locally increase the refractive index and the material etching selectivity to hydrofluoric acid. The mechanism responsible for these effects is not yet fully understood. In this letter, the authors report on local thermal conductivity mapping of laser-affected zones. It is found that these zones exhibit a lower thermal conductivity at room temperature.

Nanoindentation and birefringence measurements on fused silica specimen exposed to low-energy femtosecond pulses

Femtosecond laser pulses used in a regime below the ablation threshold have two noticeable effects on Fused Silica (a-SiO2): they locally increase the material refractive index and modify its HF etching selectivity. The nature of the structural changes induced by femtosecond laser pulses in fused silica is not fully understood. In this paper, we report on nanoindentation and birefringence measurements on fused silica exposed to low-energy femtosecond laser pulses. Our findings further back the hypothesis of localized densification effect even at low energy regime.

Laser safety aspects for refractive eye surgery with femtosecond laser pulses

Abstract We report on laser safety aspects for near infrared femtosecond laser refractive surgery. In particular, the transmittance of microjoule laser pulses at 1040 nm through the cornea during flap procedures based on femtosecond laser induced multiphoton ionization and photodisruption has been determined. When using focusing optics with a numerical aperture of 0.3, more than 20% of the incident NIR photons are propagating towards the retina. In addition, self-focusing, white light and second harmonic generation, and destructive photodisruptive side effects have to be considered when using such high energy laser pulses of amplified laser systems. Microjoule femtosecond laser pulses in combination with low NA objectives have the potential to induce destructive intraocular side effects. Further studies are required to evaluate the damage potential of the transmitted photons absorbed by the retinal pigment epithelium and other intraocular compartments. Because of the fact that flaps can be also generated with low nanojoule energy femtosecond laser pulses of non-amplified MHz lasers in combination with high NA objectives, a compromise between procedure time, pulse energy and numerical apertures has to be found for safe ocular femtosecond laser surgery.

Ablation of cytoskeletal filaments and mitochondria in live cells using a femtosecond laser nanoscissor.

Analysis of cell regulation requires methods for perturbing molecular processes within living cells with spatial discrimination on the nanometer-scale. We present a technique for ablating molecular structures in living cells using low-repetition rate, low-energy femtosecond laser pulses. By tightly focusing these pulses beneath the cell membrane, we ablate cellular material inside the cell through nonlinear processes. We selectively removed sub-micrometer regions of the cytoskeleton and individual mitochondria without altering neighboring structures or compromising cell viability. This nanoscissor technique enables non-invasive manipulation of the structural machinery of living cells with several-hundred-nanometer resolution. Using this approach, we unequivocally demonstrate that mitochondria are structurally independent functional units, and do not form a continuous network as suggested by some past studies.

Phase-matched high-order harmonics by inter-action of Ar atoms with high-repetition-rate low-energy femtosecond laser pulses

Phase-matched high-order harmonic generation in Ar gas-filled cell was investigated experimentally. We obtained phase-matched 27th order harmonic driven by a commercially available solid-state femtosecond laser system at 0.55 mJ/pulse energy level and 1 kHz repetition rate. To our knowledge, this is the lowest driving laser energy used to obtain phase-matched 27th order harmonic in a static gas cell. High-order harmonic generation at different gas density was studied systematically. Spectral blueshift and broadening of high harmonics under different pressure were analyzed. We found that the source size and spatial distribution of high-order harmonics are quite different under the phase-matching condition from those of the phase-mismatching case.

Light confinement and supercontinuum generation switching in photonic-molecule modes of a microstructure fiber

The modes guided in a ring system of microstructure-integrated fibers are shown to have much in common with electron wave functions in a two-dimensional polyatomic cyclic molecule. This photonic-molecule analogy provides, in particular, an illustrative and physically clear model of dispersion properties and the mode structure of an electromagnetic field in microstructure fibers of the considered type. A high degree of light confinement in waveguide modes of such a photonic molecule enhances nonlinear-optical processes, permitting an octave spectral broadening to be achieved for low-energy femtosecond laser pulses.

How Does a Gold Nanorod Melt?

Structural transformation of gold nanorods are investigated by high-resolution transmission electron microscopy after they have been exposed to low-energy femtosecond and nanosecond laser pulses in colloidal solution. The pulse energies were below the gold nanorod melting threshold, but allowed early stage shape transformation processes. It is found that while the as-prepared nanorods are defect-free, laser-irradiation induces point and line defects. The defects are dominated by (multiple) twins and stacking faults (planar defects), which are the precursor that drives the nanorods to convert their {110} facets into the more stable {100} and {111} facets and hence minimize their surface energy. These observations suggest that short-laser pulsed photothermal melting begins with the creation of defects inside the nanorods followed by surface reconstruction and diffusion, in contrast with the thermal melting of the rods or the bulk material, where the melting starts at the surface.