High Repetition Rate Femtosecond Pulses Research Articles

783 MHz fundamental repetition rate all-fiber ring laser mode-locked by carbon nanotubes

We demonstrate a 783 MHz fundamental repetition rate mode-locked Er-doped all-fiber ring laser with a pulse width of 623 fs. By using carbon nanotubes saturable absorber, a relatively low self-starting pump threshold of 108 mW is achieved. The laser has a very compact footprint less than 10 cm × 10 cm, benefiting from the all-active-fiber cavity design. The robust mode-locking is confirmed by the low relative intensity noise and a long-term stability test. We propose a new scheme for generating high repetition rate femtosecond optical pulses from a compact and stable all-active-fiber ring oscillator.

Minimized thermal effects and strong two-photon absorption of aluminum, silver doped platinum bi-metal nanoparticles

Herein, for the first time, we have investigated the strong nonlinear optical (NLO) properties aided with minimal thermo-optical effects in silver-platinum (Ag-Pt) nanoparticles (NPs) using Z-scan technique with high repetition rate femtosecond pulses at 700 nm. The NPs were synthesized using ultrafast laser ablation of Ag/Al targets immersed in the Pt colloidal solutions in isopropyl alcohol (IPA). We observed reverse saturable absorption (RSA)/two-photon absorption (2PA) behavior in Ag-Pt NPs (with 0.5 mM concentration of Pt), W-shaped switching behavior in Ag-Pt NPs (with 1 mM concentration of Pt), and RSA/2PA behavior in Ag-Pt NPs (with 3 mM concentration of Pt). The multiple-stage nonlinear absorption phenomenon is attributed to the interchange of pure 2PA or excited-state absorption, and 2PA saturation effects. Additionally, the broadband third-order NLO properties of aluminum-doped Pt (Al-Pt) bi-metallic NPs were obtained using spectral-dependent Z-scan studies at 700 nm, 800 nm, and 900 nm. The results revealed that Al-Pt bi-metallic NPs exhibited strong saturable absorption (SA), 2PA, and switching from SA to RSA. It is also revealed from the closed aperture Z-scan data that Ag-Pt and Al-Pt bi-metallic NPS exceed thermal contribution with strong 2PA coefficients unprecedentedfor single metal NPs. In the case of Ag-Pt NPs, strong 2PA saturation effects are attributed to the filling of Ag-O anti-bonding states by the photo-excited electrons and then charge transfer to the Pt plane. In the case of Al-Pt NPs, strong 2PA phenomena are ascribed to the enhancement of charge carriers in the conduction band and, hence, the strong surface plasmon resonances. The 2PA coefficients were evaluated to be ∼10−7 cm/W. The superior and switching NLO effects indicate that these materials might be utilized for potential applications, including optical modulators, and all-optical switching devices.

Multi-Scale Hierarchical Micro/Nano Surface Structures Induced by high Repetition rate femto-second Laser Pulses on Ti6Al4V in Ambient air

Multi-Scale Hierarchical Micro/Nano Surface Structures Induced by high Repetition rate femto-second Laser Pulses on Ti6Al4V in Ambient air

Influence of Heat Accumulation on Morphology Debris Deposition and Wetting of LIPSS on Steel upon High Repetition Rate Femtosecond Pulses Irradiation

The fabrication of laser-induced periodic surface structures (LIPSS) over extended areas at high processing speeds requires the use of high repetition rate femtosecond lasers. It is known that industrially relevant materials such as steel experience heat accumulation when irradiated at repetition rates above some hundreds of kHz, and significant debris redeposition can take place. However, there are few studies on how the laser repetition rate influences both the debris deposition and the final LIPSS morphology. In this work, we present a study of fs laser-induced fabrication of low spatial frequency LIPSS (LSFL), with pulse repetition rates ranging from 10 kHz to 2 MHz on commercially available steel. The morphology of the laser-structured areas as well as the redeposited debris was characterized by scanning electron microscopy (SEM) and µ-Raman spectroscopy. To identify repetition rate ranges where heat accumulation is present during the irradiations, we developed a simple heat accumulation model that solves the heat equation in 1 dimension implementing a Forward differencing in Time and Central differencing in Space (FTCS) scheme. Contact angle measurements with water demonstrated the influence of heat accumulation and debris on the functional wetting behavior. The findings are directly relevant for the processing of metals using high repetition rate femtosecond lasers, enabling the identification of optimum conditions in terms of desired morphology, functionality, and throughput.

Investigating the effects of intermolecular interactions on nonlinear optical properties of binary mixtures with high repetition rate femtosecond laser pulses

Measurements of nonlinear optical (NLO) properties of different binary mixtures having carbon disulfide (CS2) as the common component, namely CS2-acetone, CS2-cyclopentanone, CS2-toluene, and CS2-carbon tetrachloride (CCl4), are carried out by using the z-scan technique. Open-aperture z-scan (OAZS) and close-aperture z-scan (CAZS) experiments are performed to determine the nonlinear absorption coefficient (β) and nonlinear refractive index (n2) of all binary liquid mixtures at various compositions of the components by employing a pulsed, high repetition rate (HRR) femtosecond laser. Also, we were able to use the flowing liquid to measure NLO properties in the CS2-acetone binary mixture to remove the cumulative thermal effects produced due to the pulsed HRR laser light. Nonlinear refractive index (n2) values are found to be influenced by the weak dipole-induced dipole intermolecular interactions between the nonpolar CS2 and polar acetone as well as cyclopentanone of the respective binary mixtures. On the contrary n2 values are not found to be affected by the intermolecular interactions in CS2-toluene and CS2-CCl4 binary mixtures. In comparison, the nonlinear absorption coefficient (β) values are not found to be affected by the same in all different sets of binary mixtures.

Suppression of spallation induced nanoparticles by high repetition rate femtosecond laser pulses: realization of precise laser material processing with high throughput

This paper reports a mechanism to suppress nanoparticle (NP) generation during femtosecond laser processing of 64FeNi alloy (Invar) to realize high precision fine metal masks. Nanoparticle redeposition during processing can reduce precision and ablation efficiency. Since Gaussian laser beams have spatially distributed fluence, NP types can vary even within a laser spot. Surface areas irradiated by the beam center with high peak fluence can be decomposed into vapor and liquid droplets by phase explosion; whereas positions irradiated by the beam edge, where fluence is close to ablation threshold, can be decomposed by stress confinement under the surface, known as spallation. Spallation characteristics were verified from target surfaces covered with exfoliation and fragments. It occurred above a certain number of pulses, indicating a significant incubation effect. Spallation induced NPs, i.e., agglomerated fragments, distort micro-hole size and shape, but were effectively suppressed by increasing repetition rate, due to increased surface temperature, i.e., heat accumulation. Suppression also occurred from direct sample heating using a hot plate. Thus, thermal energy can relax stress confinement and inhibit spallation induced NPs. Numerical simulation for heat accumulation also confirmed that suppression arises from thermal effects. Increasing repetition rate also helped to increase productivity.

Copper circuits fabricated on flexible polymer substrates by a high repetition rate femtosecond laser-induced selective local reduction of copper oxide nanoparticles.

In this work, copper circuits were fabricated on flexible polyimide (PI) substrates by high repetition rate femtosecond laser-induced selective local reduction of copper oxide nanoparticles (CuO NPs). The effects of laser pulse energy and laser scanning velocity on the quality of the copper circuit were studied. By optimizing laser processing parameters, we prepared a Cu circuit of a line width of 5.5 µm and an electrical resistivity of 130.9 µΩ·cm. The Cu/O atomic ratio of the Cu circuit reaches ∼10.6 and the proportion of Cu is 91.42%. We then studied the formation mechanism of the copper circuit by simulating the temperature field under the irradiation of high repetition rate femtosecond laser pulses. The results show that the thermochemical reduction reaction induced by the high repetition rate femtosecond laser reduces CuO NPs into Cu NPs. Under the thermal effect of the high repetition rate femtosecond laser, Cu NPs agglomerate and grow to form a uniform and continuous Cu circuit.

Sustained coherent spin wave emission using frequency combs

We demonstrate sustained coherent emission of spin waves in NiFe films using rapid demagnetization from high repetition rate femtosecond laser pulse trains. As the pulse separation is shorter than the magnon decay time, magnons having a frequency equal to a multiple of the 1 GHz repetition-rate are coherently amplified. Using scanning micro-Brillouin Light Scattering (BLS) we observe this coherent amplification as strong peaks spaced 1 GHz apart. The BLS counts vs. laser power exhibit a stronger than parabolic dependence consistent with counts being proportional to the square of the magnetodynamic amplitude, and the demagnetization pulse strength being described by a Bloch law. Spatial spin wave mapping demonstrates how both localized and propagating spin waves can be excited, and how the propagation direction can be directly controlled. Our results demonstrate the versatility of BLS spectroscopy for rapid demagnetization studies and enable a new platform for photo-magnonics where sustained coherent spin waves can be utilized.

Rapid programmable pulse shaping of femtosecond pulses at the MHz repetition rate

We have shown experimentally the successful engineering of femtosecond pulse shaping at a 76 MHz repetition rate input pulse with an acousto-optic modulator (AOM). High repetition rate (HRR) femtosecond laser pulse shaping using an AOM in the Fourier plane was incomprehensible because of its intrinsic 100 kHz acoustic update limit. We demonstrate an effective way of pulse selection and a calibration routine (Fourier shift theorem), which enables generation of ∼10 MHz shaped output pulses from the HRR input pulse train. We have generated a temporally shifted rectangular shaped pulse profile by applying modulation on both the phase and amplitude of the ‘sinc’ RF modulation function.

Biomimetic surface structures in steel fabricated with femtosecond laser pulses: influence of laser rescanning on morphology and wettability.

The replication of complex structures found in nature represents an enormous challenge even for advanced fabrication techniques, such as laser processing. For certain applications, not only the surface topography needs to be mimicked, but often also a specific function of the structure. An alternative approach to laser direct writing of complex structures is the generation of laser-induced periodic surface structures (LIPSS), which is based on directed self-organization of the material and allows fabrication of specific micro- and nanostructures over extended areas. In this work, we exploit this approach to fabricate complex biomimetic structures on the surface of steel 1.7131 formed upon irradiation with high repetition rate femtosecond laser pulses. In particular, the fabricated structures show similarities to the skin of certain reptiles and integument of insects. Different irradiation parameters are investigated to produce the desired structures, including laser repetition rate and laser fluence, paying special attention to the influence of the number of times the same area is rescanned with the laser. The latter parameter is identified to be crucial for controlling the morphology and size of specific structures. As an example for the functionality of the structures, we have chosen the surface wettability and studied its dependence on the laser processing parameters. Contact angle measurements of water drops placed on the surface reveal that a wide range of angles can be accessed by selecting the appropriate irradiation parameters, highlighting also here the prominent role of the number of scans.

Nonlinear absorption and refraction studies of truncated CuNb3O8 with high-repetition rate femtosecond pulses

Copper niobate with CuNb3O8 composition was prepared via one-step solid state reaction (750 °C, 12 h). Powder X-ray diffraction (XRD) confirmed the formation of parental CuNb2O6 phase with traces of CuNb3O8 on initial sintering (till 9 h), which was then transformed into pure CuNb3O8 on higher sintering (12 h). Field emission scanning electron microscope studies (FESEM) illustrates the formation of truncated cubes due to unification of layered structure upon prolonged heating. Third-order nonlinear optical properties of truncated CuNb3O8 was studied by Z-scan technique using Ti: Sapphire laser (800 nm, 150 fs, 80 MHz). Pure CuNb3O8 possessed strong nonlinear absorption (two-photon absorption coefficient of 5.3 × 10−10 m/W) and nonlinear refractive index n2 (2.1 × 10−16 m2/W) compared to mixed copper niobate (CuNb2O6 - CuNb3O8). The third-order nonlinear optical susceptibility of CuNb3O8 was ∼10−11 esu and was higher than the known nonlinear optical (NLO) systems such as lithium niobate and sodium niobate measured at similar pulse duration. Enhancement in nonlinearity arises due to the influence of morphology and here truncated cubes with the platonic solids offer high polarizability as the charges are concentrated in the edges of the system. The truncated cube CuNb3O8 exhibited strong optical limiting action with a low limiting threshold (34.6 μJ/cm2), thus making it appealing for ultrafast optical limiter devices towards photosensitive component protection from Infrared (IR) laser damage.

Generation of plasma X-ray sources via high repetition rate femtosecond laser pulses

In this study, we present the development and characterization of Cu plasma X-ray source driven by 20 W average power high repetition rate femtosecond laser in ambient atmosphere environment. The peak Cu-Kα photon flux of 2.3 × 109 photons/s into full solid angle is demonstrated (with a process conversion efficiency of ~ 10−7), using pulses with peak intensity of 4.65 × 1014 W/cm2. Such Cu-Kα flux is significantly larger than others found in comparable experiments, performed in air environment. The effects of resonance plasma absorption process, when optimized, are shown to increase measured flux by the factor of 2–3. The relationship between X-ray photon flux and plasma-driving pulse repetition rate is quasi-linear, suggesting that fluxes could further be increased to 1010 photons/s using even higher average powers of driving radiation. These results suggest that to fully utilize the potential of high repetition rate laser sources, novel target material delivery systems (for example, jet-based ones) are required. On the other hand, this study demonstrates that high energy lasers currently used for plasma X-ray sources can be conveniently and efficiently replaced by high average power and repetition rate laser radiation, as a way to increase the brightness of the generated X-rays.

Extracting third order optical nonlinearities of Mn(III)-Phthalocyanine chloride using high repetition rate femtosecond pulses

Third order nonlinearities of Mn(III)-Phthalocyanine chloride in dimethyl-sulphoxide under 50 fs pulses, operating at 94 MHz, by eliminating cumulative thermal effects have been investigated and reported by us. Modifications were done in data acquisition during Z-scan experiment, which included recording of time evolution waveform traces in an oscilloscope and not collection of Z versus transmission and utilization of a chopper of a suitable duty cycle. Time evolution traces were further processed analytically through MatLab® programming, which yielded Z-scan traces similar to what was obtained with single shot 50 fs pulse. We observed reverse saturable absorption at 800 nm owing to excited state absorption. We show that the nonlinear refractive index (γ) and nonlinear absorption coefficient (β) are over estimated almost 100 times, when MHz pulses are used compared to a situation, where thermo-optical nonlinearities are accounted. Illumination and dark periods are carefully set in a way, so that the sample is able to completely recover its initial temperature before arrival of the next pulse. Magnitudes of γ and β were found to be −(6.5–4.9) × 10−16 m2/W and (5.4–6.2) × 10−10 m/W under the MHz condition, whereas they were −(0.18−2.2) × 10−18 m2/W and (9.5–15) × 10−12 m/W under the thermally managed condition, respectively. To reveal the associated fast nonlinearity, femtosecond transient absorption experiment was performed, which inferred excited state absorption and ground state bleaching across the 450–780 nm region. Dynamics associated with these processes are reported along with fluorescence lifetime obtained through the TCSPC technique. Structure optimization using TDDFT calculations and HOMO-LUMO gaps with orbital pictures are also shown.

Periodic disruptions induced by high repetition rate femtosecond pulses on magnesium-oxide-doped lithium niobate surfaces

In this paper, we demonstrate the periodic disruption formation on magnesium-oxide-doped lithium niobate surfaces by a femtosecond fiber laser system with wavelength and repetition rate of 1040 nm and 52 MHz, respectively. Three main experimental conditions, laser average power, scanning speed, and orientation of sample were systematically studied. In particular, the ablation morphologies of periodic disruptions under different crystal orientations were specifically researched. The result shows that such disruptions consisting of a bamboo-like inner structure appears periodically for focusing on the surface of X-, Y- and Z-cut wafers, which are formed by a rapid quenching of the material. Meanwhile, due to the anisotropic property, the bamboo-like inner structures consist of a cavity only arise from X- and Z-cut orientation.

PH Effect on Two-Photon Cross Section of Highly Fluorescent Dyes Using Femtosecond Two-Photon Induced Fluorescence.

Effect of solution pH on two-photon absorption cross-section of highly fluorescent Coumarin and Rhodamine dyes with high repetition rate femtosecond laser pulses at 780nm is presented using two-photon induced fluorescence technique. A correspondence in the measured two-photon and single-photon cross-section values is seen when the pH changes from acidic to basic conditions (pH=2-10) for solutions in 1:1 water-ethanol binary mixture. By plotting changes in the single-photon and two-photon fluorescence in this pH range, the excited state pKa values are found. The ground state pKa values are also affected by the protonation deprotonation equilibrium as a result of variation in pH from acidic to basic, which are characterized by changes in absorbance spectra. Most of these single-photon and two-photon induced fluorescence spectra show characteristic blue shifts. Different fluorescence quantum yields calculated at each pH reflect a change in structure corresponding to their associated properties as a result of acid base equilibrium.

Investigating Two-Photon-Induced Fluorescence in Rhodamine-6G in Presence of Cetyl-Trimethyl-Ammonium-Bromide.

We investigate the effect of cetyl-trimethyl-ammonium-bromides (CTAB) concentration on the fluorescence of Rhodamine-6G in water. This spectroscopic study of Rhodamine-6G in presence of CTAB was performed using two-photon-induced-fluorescence at 780nm wavelength using high repetition rate femtosecond laser pulses. We report an increment of ∼10% in the fluorescence in accordance with ∼12% enhancement in the absorption intensity of the dye molecule around the critical micellar concentration. We discuss the possible mechanism for the enhancement in the two-photon fluorescence intensity and the importance of critical micellar concentration.

Chromatic annuli formation and sample oxidation on copper thin films by femtosecond laser.

We report an experimental investigation on the irradiation of copper thin films with high repetition rate femtosecond laser pulses (1040 nm, 50 MHz), in ambient air and liquid water. We observe a novel, striking phenomenon of chromatic copper oxides (CuO and Cu2O) annuli generation. The characteristic features of the chromatic copper oxide annuli are studied by exploiting micro-Raman spectroscopy, optical and scanning electron microscopies. In the case of irradiation in water, the seldom investigated effects of the immersion time, tw, after irradiation with a fixed number of pulses are analyzed, and an intriguing dependence of the color of the chromatic annuli on tw is observed. This remarkable behavior is explained by proposing an interpretation scenario addressing the various processes involved in the process. Our experimental findings show that Cu2O nanoparticles (size of ≈20 nm) and Cu2O nanocubes (nanocube edges of ≈30, ≈60 nm) can be effectively generated by exploiting high repetition rate laser-assisted oxidation.

Efficient neutron generation from solid-nanoparticle explosions driven by DPSSL-pumped high-repetition rate femtosecond laser pulse

We propose novel neutron source using high-intensity laser based on the cluster fusion scheme. We developed DPSSL-pumped high-repetition-rate 20-TW laser system and solid nanoparticle target for neutron generation demonstration. In our neutron generation experiment, high-energy deuterons were generated from coulomb explosion of CD solid- nanoparticles and neutrons were generated by DD fusion reaction. Efficient and stable neutron generation was obtained by irradiating an intense femtosecond laser pulse of >2×1018 W/cm2. A yield of ∼105 neutrons per shot was stably observed during 0.1-1 Hz continuous operation.

Direct writing of sub-wavelength ripples on silicon using femtosecond laser at high repetition rate

The near sub-wavelength and deep sub-wavelength ripples on monocrystalline silicon were formed in air by using linearly polarized and high repetition rate femtosecond laser pulses (f=76MHz, λ=800nm, Ͽ=50fs). The effects of laser pulse energy, direct writing speed and laser polarization on silicon surface morphology are studied. When the laser pulse energy is 2nJ/pulse and the direct writing speed varies from 10 to 25mm/s, the near sub-wavelength ripples (NSRs) with orientation perpendicular to the laser polarization are generated. While the direct writing speed reaches 30mm/s, the direction of the obtained deep sub-wavelength ripples (DSRs) suddenly changes and becomes parallel to the laser polarization, rarely reported so far for femtosecond laser irradiation of silicon. Meanwhile, we extend the Sipe⿿Drude interference theory by considering the thermal excitation, and numerically calculate the efficacy factor for silicon irradiated by femtosecond laser pulses. The revised Sipe⿿Drude interference theoretical results show good agreement with the periods and orientations of sub-wavelength ripples.

Grating-assisted generation of regular two-dimensional multicolored arrays in a tellurite glass.

A grating structure was inscribed in a tellurite glass after irradiation with high-repetition rate femtosecond laser pulses. High diffraction efficiency was obtained due to the large refractive index change, which was caused by the precipitation of Te crystals in the laser modified region. Two-dimensional multicolored arrays were generated by cascaded four-wave mixing (CFWM) together with the prefabricated grating structure, which showed much more superior than those induced by beam breakup.