Use Of Ultrashort Pulses Research Articles

Nano/Microstructuring of Nickel Electrodes by Combining Direct Laser Interference Patterning and Polygon Scanner Processing for Efficient Hydrogen Production

The present work utilizes direct laser interference patterning in conjunction with a polygon scanner system to fabricate micro‐ and nanostructures on the surface of nickel foils. A picosecond pulsed laser source with an average power of ≈470 W is used to pattern line‐like structures having spatial periods of 11.0 and 25.0 μm and a maximum structure depth of up to 15 μm. The scanning speed and the number of consecutive scans, ranging from 5 to 500, are varied in order to assess their influence on the structure formation process. Furthermore, the use of ultrashort pulses permits the production of hierarchical surface structures having up to four feature size levels because of incubation effects and growth of different types of laser‐induced periodic surface structures. 2D‐fast Fourier transforms are applied to scanning electron microscopy images for analyzing the evolution of features from the nano‐ to microscale. The method proposed here permits the reduction of 22% in the overpotential of the hydrogen evolution reaction, which linearly correlates to the drop in energy required to drive this chemical reaction.

Enhanced Nanobubble Formation: Gold Nanoparticle Conjugation to Qβ Virus-like Particles.

Plasmonic gold nanostructures are a prevalent tool in modern hypersensitive analytical techniques such as photoablation, bioimaging, and biosensing. Recent studies have shown that gold nanostructures generate transient nanobubbles through localized heating and have been found in various biomedical applications. However, the current method of plasmonic nanoparticle cavitation events has several disadvantages, specifically including small metal nanostructures (≤10 nm) which lack size control, tuneability, and tissue localization by use of ultrashort pulses (ns, ps) and high-energy lasers which can result in tissue and cellular damage. This research investigates a method to immobilize sub-10 nm AuNPs (3.5 and 5 nm) onto a chemically modified thiol-rich surface of Qβ virus-like particles. These findings demonstrate that the multivalent display of sub-10 nm gold nanoparticles (AuNPs) caused a profound and disproportionate increase in photocavitation by upward of 5-7-fold and significantly lowered the laser fluency by 4-fold when compared to individual sub-10 nm AuNPs. Furthermore, computational modeling showed that the cooling time of QβAuNP scaffolds is significantly extended than that of individual AuNPs, proving greater control of laser fluency and nanobubble generation as seen in the experimental data. Ultimately, these findings showed how QβAuNP composites are more effective at nanobubble generation than current methods of plasmonic nanoparticle cavitation.

Ultrafast, all-optical control of modal phases in a few-mode fiber for all-optical switching.

The phase differences between the transverse modes of an optical fiber can be altered all-optically by intermodal cross-phase modulation. In this Letter, we experimentally demonstrate this effect with ultrashort laser pulses. An ultrashort probe pulse, guided in both modes of a two-mode fiber, is co-propagating and temporally overlapping with an ultrashort control pulse, guided in the fundamental mode only and centered at a separate wavelength. The use of ultrashort pulses allows for a notable phase shift at a 33-fold reduced control pulse energy and a 173-fold reduced fiber length, compared to previous experiments. A total phase shift of 0.285π between the two probe modes was achieved at a 9 nJ control pulse energy in a 19 cm long two-mode graded-index fiber. Additionally, the capability of this scheme to switch ultrashort pulses in an all-optical manner was investigated. A modulation depth of 50% was achieved, limited by temporal nonlinear effects.

Frequency-resolved optical gating measurement of ultrashort pulses by using single nanowire

The use of ultrashort pulses for fundamental studies and applications has been increasing rapidly in the past decades. Along with the development of ultrashort lasers, exploring new pulse diagnositic approaches with higher signal-to-noise ratio have attracted great scientific and technological interests. In this work, we demonstrate a simple technique of ultrashort pulses characterization with a single semiconductor nanowire. By performing a frequency-resolved optical gating method with a ZnO nanowire coupled to tapered optical microfibers, the phase and amplitude of a pulse series are extracted. The generated signals from the transverse frequency conversion process can be spatially distinguished from the input, so the signal-to-noise ratio is improved and permits lower energy pulses to be identified. Besides, since the nanometer scale of the nonlinear medium provides relaxed phase-matching constraints, a measurement of 300-nm-wide supercontinuum pulses is achieved. This system is highly compatible with standard optical fiber systems, and shows a great potential for applications such as on-chip optical communication.

FS laser processing of bio-polymer thin films for studying cell-to-substrate specific response

The use of ultra-short pulses for nanoengineering of biomaterials opens up possibilities for biological, medical and tissue engineering applications. Structuring the surface of a biomaterial into arrays with micro- and nanoscale features and architectures, defines new roadmaps to innovative engineering of materials. Thin films of novel collagen/elastin composite and gelatin were irradiated by Ti:sapphire fs laser in air at central wavelength 800nm, with pulse durations in the range of 30fs. The size and shape as well as morphological forms occurring in the resulted areas of interaction were analyzed as a function of irradiation fluence and number of pulses by atomic force microscopy (AFM). The fs interaction regime allows generation of well defined micro porous surface arrays.In this study we examined a novel composite consisting of collagen and elastin in order to create a biodegradable matrix to serve as a biomimetic surface for cell attachment. Confocal microscopy images of modified zones reveal formation of surface fringe patterns with orientation direction alongside the area of interaction. Outside the crater rim a wave-like topography pattern is observed. Structured, on a nanometer scale, surface array is employed for cell-culture experiments for testing cell’s responses to substrate morphology. Mice fibroblasts migration was monitored after 3days cultivation period using FESEM. We found that fibroblasts cells tend to migrate and adhere along the laser modified zones. The performed study proved that the immobilized collagen based biofilms suite as a template for successful fibroblasts cell guidance and orientation. Fs laser induced morphological modification of biomimetic materials exhibit direct control over fibroblasts behaviour due to induced change in their wettability state.

Laser surface structuring with 100 W of average power and sub-ps pulses

High throughput still represents a key factor for industrial use of ultrashort pulses in the field of surface structuring. Reliable systems with average powers up to 100 W are today available. It has already been proved that metals, especially steel having a low threshold fluence, can be machined with excellent surface quality at average powers of more than 40 W and a spot radius of about 25 μm, if a polygon line scanner, offering fast scanning speeds, is used. A further scale-up into the 100 W regime should be possible for metals showing a threshold fluence of about 0.2 J/cm2 or higher. But, it will lead to problems with heat accumulation in the case of steel and a straight forward scale-up is not possible. In order to keep a good surface quality, the machining strategy has to be adapted. A maximum flexibility can be obtained with an “interlaced” mode by using very high marking speeds of several 100 m/s and repetition rates of several tenths of MHz. As this is at the edge of today available technologies, alternative strategies are additionally investigated. Enlarging the spot size represents the most simple approach to reduce the heat accumulation in the case of steel but also multispots represent an attractive alternative.

Sub-10 nm near-field localization by plasmonic metal nanoaperture arrays with ultrashort light pulses.

Near-field localization by ultrashort femtosecond light pulses has been investigated using simple geometrical nanoapertures. The apertures employ circular, rhombic, and triangular shapes to localize the distribution of surface plasmon. To understand the geometrical effect on the localization, aperture length and period of the nanoapertures were varied. Aperture length was shown to affect the performance more than aperture period due mainly to intra-aperture coupling of near-fields. Triangular apertures provided the strongest spatial localization below 10 nm in size as well as the highest enhancement of field intensity by more than 7000 times compared to the incident light pulse. Use of ultrashort pulses was found to allow much stronger light localization than with continuous-wave light. The results can be used for super-localization sensing and imaging applications where spatially localized fields can break through the limits in achieving improved sensitivity and resolution.

Large volume ablation of Sapphire with ultra-short laser pulses

The superior optical and mechanical properties of Sapphire (Al2O3) are highly desirable in various opto-electronics and micro-mechanical applications. However, Sapphire's intrinsic hardness and resistance to most chemicals result in significant processing difficulties. Laser micro-machining is emerging as a promising technology, in particular, the use of ultra-short pulses for material ablation. In this work we investigate and characterize experimentally large volume ablation of Sapphire with femtosecond pulses, and compare the results to previously reported drilling and cutting experiments. We manage to identify optimized parameters for overcoming deleterious thermal effects and debris scattering, and demonstrate high quality 180μm-deep ablation of 1mm×15mm area in Sapphire.

Basic considerations on coherent combining of ultrashort laser pulses

Coherent combining is a novel approach to scale the performance of laser amplifiers. The use of ultrashort pulses in a coherent combining setup results in new challenges compared to continuous wave operation or to pulses on the nanosecond timescale, because temporal and spectral effects such as self-phase modulation, dispersion and the optical path length difference between the pulses have to be considered. In this paper the impact of these effects on the combining process has been investigated and simple analytical equations for the evaluation of this impact have been obtained. These formulas provide design guidelines for laser systems using coherent combining. The results show that, in spite of the temporal and spectral effects mentioned above, for a carefully adjusted and stabilized system an excellent efficiency of the combining process can still be achieved.

Simulation of two photon absorption in silicon wire waveguide for implementation of all optical logic gates

All optical switching action of silicon wire waveguide for the design of the proposed logic gates is simulated. This is one possible building block of the future all optical computer or photonic devices. All optical logic gates NOT, NAND and AND gates using two photon absorption in silicon wire waveguide are presented. Use of ultra short pulse has negligible free carrier absorption effect; hence the operating speed of the gates is very high and has potential application in photonic processing. NAND gate is universal one and thus one can perform any logical operation using this. The device (Si wire WG) requires low energy pulse and is ultrafast one.

A method of implementation of all optical logic gates based on two photon absorption in silicon wire wave guide

All optical logic gates NOT, NAND and AND gates using two photon absorption in silicon wire waveguide is proposed. Due to use of ultra short pulse there is negligible free carrier absorption effect and hence the operating speed of the gates is very high and has potential application in photonic processing. NAND gate is universal one and thus one can perform any logical operation using this. The device silicon wire waveguide (Si wire WG) requires low energy pulse (few PJ) and is ultra fast one. Key words: Silicon wire waveguide, two photon absorption, logic gate, photonic integration.

Orthogonal frequency coded filters for use in ultra-wideband communication systems

The use of ultra-short pulses, producing very wide bandwidths and low spectral power density, are the widely accepted approach for ultra-wideband (UWB) communication systems. This approach is simple and can be implemented with current digital signal processing technologies. However, surface acoustic wave (SAW) devices have the capability of producing complex signals with wide bandwidths and relatively high frequency operation. This approach, using SAW based correlators, eliminates many of the costly components that are needed in the IF block in the transmitter and receiver, and reduces many of the signal processing requirements. This work presents the development of SAW correlators using orthogonal frequency coding (OFC) for use in UWB spread spectrum communication systems. OFC and pseudonoise (PN) coding provide a means for UWB spreading of data. The use of OFC spectrally spreads a PN sequence beyond that of code division multiple access (CDMA) because of the increased bandwidth providing an improvement in processing gain. The transceiver approach is still very similar to that of a CDMA but provides greater code diversity. Experimental results of a SAW filter designed with OFC transducers are presented. The SAW correlation filter was designed using seven contiguous chip frequencies within the transducer. SAW correlators with a 29% fractional bandwidth were fabricated on lithium niobate (LiNbO3) having a center frequency of 250 MHz. A coupling-of-modes (COM) model is used to predict the SAW filter response experimentally and is compared to the measured data. Good correlation between the predicted COM responses and the measured device data is obtained. Discussion of the design, analysis, and measurements are presented. The experimental matched filter results are shown for the OFC device and are compared to the ideal correlation. The results demonstrate the OFC SAW device concept for UWB communication transceivers.

Simplified setup for high-resolution spectroscopy that uses ultrashort pulses.

Using a broadband femtosecond laser and a simple optical setup, we demonstrate narrow-bandwidth-tunable excitation of vibrational modes in CCl4 and CHBr3 liquids. The resolution obtained is 80 times higher than the laser bandwidth. A pair of time-shifted, linearly chirped pulses is created by use of a high-order dispersion-compensated prism-interferometer setup. We use this pulse pair to selectively excite Raman-active transitions. Our setup represents a significant simplification with improved resolution, of previous approaches to the use of ultrashort pulses for chemically selective spectroscopy.

Detection and quantification of aromatic contaminants in water and soil samples by means of laser desorption laser mass spectrometry.

The combination of laser desorption of untreated soil samples and subsequent selective laser ionization followed by time-of-flight mass analysis results in an ultrafast technique for the quantitative detection of aromatic contaminants in soil samples. The method allows for high sample throughput, because the complete measurement is finished within about 1 min. Although the different types of soil investigated (sand, humus, clay) showed differences in the desorption efficiency, none of them produced mass spectrometric interferences when an ionization laser wavelength of 266 nm was used. Quantification was carried out by relative measurement with respect to an internal standard and gave satisfactory results over 4 orders of magnitude of analyte concentration. Although the detection of polycyclic aromatic hydrocarbons could successfully be carried out using nanosecond laser pulses, the quality of the mass spectra obtained for labile substances, for example, nitrotoluenes, could be greatly improved by the use of ultrashort pulses in the subpicosecond range. With the preliminary setup, detection limits in the low micrograms-per-gram range were achieved. The identical setup can be used for the analysis of liquids, in particular, water, when the soil sample is replaced by a solid, porous adsorber medium onto which the sample is applied. Activated carbon proved to be a useful adsorber for IR laser desorption, whereas for the UV, granular clay or lime/sand mixtures are preferable.

Measurement of the pulse-front distortion in high-numerical-aperture optics

Using ultrashort laser pulses in combination with laser scanning microscopes offers the fascinating possibility to investigate samples with high spatial and temporal resolution. On the other hand the use of ultrashort pulses leads to pulse broadening and pulse-front distortion due to the dispersion of the optical elements. Three different experimental techniques are presented which are able to determine the pulse-front distortion for high-numerical-aperture objectives due to chromatic and spherical aberration. Results for a number of widely used objectives are listed.

Preliminary electron density profile and fluctuation measurements on GAMMA 10 using ultrashort-pulse reflectometry

The use of ultrashort pulses (full width at half-maximum=1–100 ps) as a source of broadband, time-resolved radiation has high potential for application in microwave diagnostics for fusion plasmas. Here we report on the ultrashort-pulse reflectometry system developed for electron density profile and fluctuation measurements on the GAMMA 10 tandem mirror. The four-channel system (selectable frequencies of 8–17 GHz at 1 GHz intervals) measures the double-pass time-of-flight from the vacuum window to the cutoff layer. The hardware has an uncertainty of ±40 ps and can be operated at a repetition rate of 400 kHz. The system is operational, however, data from plasma experiments is unavailable at the moment. Results from a single-channel O-mode system show time-of-flight measurements consistent with values calculated from profiles obtained by a scannable single-channel interferometer. Fluctuation measurements are also consistent with results from a Fraunhofer diffraction diagnostic. These results suggest that with further refinements, ultrashort- pulse reflectometry will yield a simple yet reliable diagnostic for fusion devices.

Vibrational quantum beats observed by Raman-induced optical Kerr effect

Ultra fast molecular motion in liquids may be studied in the time domain with the pump/probe optical heterodyne-detected (OHD) Raman-induced optical Kerr effect (RIKE) technique, which makes use of ultra short pulses from a mode-locked laser. The oscillatory signal observed is attributed to vibrational quantum interference, and a detailed theoretical account is provided. In the framework of linear response theory, the theoretical development of this work follows two different approaches: the density matrix formalism in the Schrödinger representation and the Hellwarth formalism involving the evaluation of the time correlation of linear susceptibility tensors. An unambiguous explanation of the decay, in addition to the oscillation, observed in OHD-RIKE is also provided.

Tunable-laser-induced gratings for the measurement of ultrafast phenomena

Optically induced gratings form the basis of a class of optical frequency-domain techniques for the measurement of ultrafast phenomena with demonstrated temporal resolution on the order of tens of femtoseconds without the use of ultrashort pulses. We describe these techniques and calculate experimental line shapes. We discuss experimental strategies and some experimental problems that have plagued early experiments, but which have been solved. In addition, we present results obtained for triphenyl methane dyes and the optical Kerr liquid, carbon disulfide, and we discuss the interpretation of these results.

Application of time-resolved resonance Raman spectroscopy to the study of rapid intramolecular radiationless transitions

It is shown that the decay rate γ 2 of the excited state of a molecule can be determined by measuring the decrease of the resonance Raman intensity on a time scale much longer than γ −1 2, provided that the time needed for repopulation of the ground state, following rapid intramolecular radiationless transitions from the excited state to a molecular quasicontinuum, is itself much longer than γ −1 2. Thus, it should be possible to avoid the use of ultrashort pulses in the study of very fast intramolecular decay process.

Two-photon absorption of ultrashort pulses in a gas

Two-photon absorption of a continuous train of ultrashort pulses in a gas gives rise to resonances when the sum of the frequencies of two locked modes coincides with the transition frequency in the gas. It is shown that the width of these resonances is not governed by the pulse duration but by the width of the twophoton transition. Resonance intensities are of the same order of magnitude as in the single-frequency regime at the same power level. It is worth noting an important feature of the use of ultrashort pulses in studies of two-photon resonances due to the 1S-2S transition in the hydrogen atom: there is a strong increase in the efficiency of conversion of dye laser radiation to the ultraviolet range.