Femtosecond Pulse Train Research Articles

Ultrafast dynamics control on ablation of Cu using shaped femtosecond pulse trains

The ablation processes of Cu film are investigated using temporal shaped femtosecond pulse trains. The depth is modulated by changing the number and interval of the sub-pulses. The underlying ultrafast dynamic processes are discussed based on plasma shielding and electron multiple heating mechanisms. When the sub-pulse interval is less than 0.4 ps electron multiple heating is the dominant mechanism, while the plasma shielding dominates the subsequent ablation processes when the sub-pulse interval is larger than 0.4 ps. The curve of depth obtained by three pulse trains shows more significant oscillation as the function of sub-pulse interval under the low-fluence. We propose that the oscillation of depth is due to the coherent phonon oscillation excited by the pulse train. The study provides a basis for giving insight into the ultrafast dynamics for improving micromachining and nano-fabrications using shaped femtosecond pulse trains.

Postfilament supercontinuum on 100 m path in air.

Pulses at 744 nm with 90 fs duration, 6 mJ energy, and a weakly divergent wavefront propagate for more than 100 m and generate a filament followed by an unprecedently long high intensity (≥1TW/cm2) light channel. Over a 20 m long sub-section of this channel, the pulse energy is transferred continuously to the infrared wing, forming spectral humps that extend up to 850 nm. From 3D+time carrier-resolved simulations of 100 m pulse propagation, we show that spectral humps indicate the formation of a train of femtosecond pulses appearing at a predictable position in the propagation path.

Neural Stimulation In Vitro and In Vivo by Photoacoustic Nanotransducers

Summary Neuromodulation is an invaluable approach for the study of neural circuits and clinical treatment of neurological diseases. Here, we report semiconducting polymer nanoparticles based photoacoustic nanotransducers (PANs) for neural stimulation in vitro and in vivo. Our PANs strongly absorb the nanosecond pulsed laser in the near-infrared second window (NIR-II) and generate localized acoustic waves. PANs are shown to be surface modified and selectively bind onto neurons. PAN-mediated activation of primary neurons in vitro is achieved with ten 3-ns laser pulses at 1,030 nm over a 3-ms duration. In vivo neural modulation of mouse brain activities and motor activities is demonstrated by PANs directly injected into brain cortex. With submillimeter spatial resolution and negligible heat deposition, PAN stimulation is a new non-genetic method for precise control of neuronal activities, opening up potentials in non-invasive brain modulation.

400 MHz ultrafast optical coherence tomography

An ultrafast time-stretched swept source with a sweep rate of 400 MHz is demonstrated based on the buffering of a 100 MHz femtosecond laser pulse train. To the best of our knowledge, this is the highest sweep rate of swept sources for optical coherence tomography (OCT) that has been reported. With a 10 dB sweep range of ∼100nm, an axial resolution of 19 µm is obtained in the OCT. OCT imaging of high-speed rotating disks is demonstrated. A composite complex apodization method is proposed and demonstrated to enhance the signal to noise ratio in the OCT imaging.

Generation of Supercontinuum in Filamentation Regime in a Water Droplet Containing Silver Nanoparticles at Low Temperature

Generation of supercontinuum (SC) was studied experimentally both in water droplets containing silver nanoparticles (NP) in the temperature range from 2 to 22°C and in droplet ice floes frozen to –15°C. It is established that intensity of SC emission exponentially decays along the droplet diameter following excitation by a train of femtosecond laser pulses and linearly increases with increase in NP concentration. Spectrum of SC emission generated in a water droplet containing NPs is investigated in the presence of localized plasmons producing fluorescence in the vicinity of 430 nm. Propagation of a thermal wave along the diameter (d = 1.0 mm) of a small frozen droplet at a speed of 190 mm/s accompanied by exponentially decaying SC emission is discovered. Mathematical model of heat-transfer processes in an ice floe upon thermal-wave formation is proposed.

The use of optical chopper increases the efficiency of femtosecond laser-induced cell fusion

Artificial cell fusion is a widely used approach in cell biology and biomedicine. Femtosecond laser-induced cell fusion is considered to be a precise and low-invasive tool for the cell fusion. However, the percentage of somatic cell fusion remains not very high, and the use of polyethylene glycol is often required. In our research we propose the use of rotating optical chopper to produce ultra-short trains of femtosecond pulses (up to 1 ms) for decreasing the laser impact. It helps to control the appearance and size of gas-vapor bubbles, avoiding cell destruction. We achieved the cell fusion efficiency of 45% without the use of polyethylene glycol, but only in freshly thawed cells. Continuously cultured cells completely failed to fuse.

Dynamics of a dielectric microsphere inside a nonlinear laser trap

The 2018 Nobel Prize in Physics was awarded for the invention of optical trapping and generation of ultrashort pulses, which revolutionized many areas of modern science and technology. However, physics of optical trapping under ultra-short pulsed excitation has not been explored much. The nonlinear nature of optical trapping force/potential under ultra-short pulsed excitation was theoretically investigated, however, without any direct experimental demonstration and development of any generalized theory independent of the particle size. In this work, we present a methodology to numerically estimate trapping force/potential including optical as well as thermal nonlinearity under ultra-short pulsed excitation and implement a variety of detection modalities to capture the particle's real-time trajectories. We show how highly asymmetric nonlinear axial potential, created by a femtosecond pulse-train, can be mapped from the dynamics of the trapped particle. Considering fine-tuning of trap-stiffness by changing nonlinearity, we envision far-reaching applications of using ultra-short pulsed excitation in laser trapping and manipulation.

Generation of femtosecond pulse train by pulse splitting in a large mode area fiber at 2 μm wavelength

Abstract We numerically demonstrate the generation of a very high peak power femtosecond pulse train by splitting a 1 ps, 100 kW pulse of central wavelength 2 μm using a few meters length of a large mode area fiber. The input 1 ps pulse during the propagation splits into multiple femtosecond pulses of very high peak powers. Concurrently, the spectral domain of the pulse also witnesses multiple peak formation. The used fiber is a W-type large mode area fiber, whose effective mode area can be varied from ~700 μm2 to ~1700 μm2 at 2 μm wavelength by varying the fiber parameters. The fiber design parameters were optimized for the efficient formation of femtosecond pulse train of peak powers ranging from ~100 kW to ~600 kW and FWHM ranging from ~100 fs to ~450 fs.

Reconfigurable time-stretched swept laser source with up to 100 MHz sweep rate, 100 nm bandwidth, and 100 mm OCT imaging range

The sweep rate, sweep range, and coherence length of swept sources, respectively, determine the acquisition rate, axial resolution, and imaging range of optical coherence tomography (OCT). In this paper, we demonstrate a reconfigurable high-speed and broadband swept laser by time stretching of a flat spectrum femtosecond pulse train with over 100 nm bandwidth and a repetition rate of 100 MHz. By incorporating an optical modulator and utilizing appropriate dispersive modules, the reconfiguration of the swept source is demonstrated with sweep rates of 25 and 2.5 MHz. The 2.5 MHz swept source enables an imaging range of > 110 mm with 6 dB sensitivity roll-off in OCT, which is the longest imaging range ever reported for megahertz OCT.

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.

Self-ordering and cavity cooling using a train of ultrashort pulses

A dilute atomic gas in an optical resonator exhibits a phase transition from a homogeneous density to crystalline order when laser illuminated orthogonal to the resonator axis. We study this well-known self-organization phenomenon for a generalized pumping scheme using a femtosecond pulse train with a frequency spectrum spanning a large bandwidth covering many cavity modes. We show that due to simultaneous scattering into adjacent longitudinal cavity modes the induced light forces and the atomic dynamics becomes nearly translation-invariant along the cavity axis. In addition the laser bandwidth introduces a new correlation length scale within which clustering of the atoms is energetically favorable. Numerical simulations allow us to determine the self-consistent ordering threshold power as function of bandwidth and atomic cloud size. We find strong evidence for a change from a second order to a first order self-ordering phase transition with growing laser bandwidth when the size of the atomic cloud gets bigger than the clustering length. An analysis of the cavity output reveals a corresponding transition from a single to a double pulse traveling within the cavity. This doubles the output pulse repetition rate and creates extra substructures in close analogy to a time crystal formation in the cavity output. Simulations also show that multi-mode operation significantly improves cavity cooling generating lower kinetic temperatures at a much faster cooling rate.

Control of ablation morphology on Cu film with tailored femtosecond pulse trains

The ablation morphology of Cu film is investigated using tailored femtosecond pulse trains. The width, depth, and structure of ablation craters are systematically analyzed as the function of sub-pulse interval. Two distinct ablation sub-structures are observed. The gentle slope irradiated by low fluence emerges from the peripheral region of the ablation crater, and the steep slope formed in the central region is irradiated by high fluence. The ablation morphology can be controlled by manipulating the shaped pulse trains. We demonstrate that the ablation depth modulation is significant with the ablation width almost unchanged. By fitting the depth curves, we define a coefficient K to characterize the plasma shielding effect between sub-pulses with the increase in sub-pulse interval. For three fluences, the coefficient K using two kinds of pulse trains are extracted experimentally. The results reveal that the plasma shielding effect can be modulated by changing sub-pulse interval, and the corresponding physical interpretation is proposed.

Coherently controlled generation of single-cycle terahertz pulses from a thin layer of nonlinear medium with low-frequency resonances

We propose a novel scheme to generate single-cycle terahertz (THz) pulses via reflection of an optical femtosecond pulse train from a thin layer of nonlinear resonant medium. Our method is based on a coherent control of low-frequency oscillations and free induction decay in the medium. The specific single-cycle shape of generated THz pulses requires a plane wavefront and detection in the near field. Our theoretical results pave the way to a new, simple and high-efficiency way to generate single-cycle waveshape-tunable terahertz pulses.

Extremely nonlinear Raman interaction of an ultrashort nitrogen ion laser with an impulsively excited molecular wave packet

We report generation of cascaded rotational Raman scattering up to 58th orders in coherently excited CO_2 molecules. The high-order Raman scattering, which produces a quasiperiodic frequency comb with more than 600 sidebands, is obtained using an intense femtosecond laser to impulsively excite rotational coherence and the femtosecond-laser-induced N_2^+ lasing to generate cascaded Raman signals. The novel configuration allows this experiment to be performed with a single femtosecond laser beam at free-space standoff locations. It is revealed that the efficient spectral extension of Raman signals is attributed to the specific spectra-temporal structures of N_2^+ lasing, the ideal spatial overlap of femtosecond laser and N2+ lasing, and the guiding effect of molecular alignment. The Raman spectrum extending above 2000 cm^-1 naturally corresponds to a femtosecond pulse train due to the periodic revivals of molecular rotational wavepackets.

Controllable photon energy deposition efficiency in laser processing of fused silica by temporally shaped femtosecond pulse: Experimental and theoretical study

Photon energy deposition plays a crucial role in femtosecond laser irradiation followed by chemical etching processing, which is an emerging technique for the better control of micro/nano structures on fused silica. In this study, we have experimentally and theoretically studied the controllable photon energy deposition in laser irradiation of fused silica by temporally shaped femtosecond laser pulse trains. The experimental result shows that the photon energy deposition efficiency could be either reduced or enhanced by adjusting the total fluence of shaped pulse trains. Furthermore, the sub-pulse interval and intensity ratio of temporally shaped pulse trains were revealed to play a critical role in photon energy deposition. The corresponding experimental observations are qualitatively explained by a plasma model that considers the free electron generation processes and corresponding feedback on the photon energy deposition.

Carrier-Envelope Offset Frequency Stabilization of a Thin-Disk Laser Oscillator via Depletion Modulation

We present a novel concept for the stabilization of the carrier-envelope offset (CEO) frequency of femtosecond pulse trains from thin-disk laser oscillators by exploiting gain depletion modulation in the active gain region. We shine a small fraction of the laser output power back onto the thin disk allowing the population inversion in the gain medium to be controlled. We employ this technique in our home-built Kerr-lens mode-locked Yb:YAG thin-disk laser and benchmark the performance against the proven technique of pump current modulation for CEO stabilization, showing that the two techniques have equivalent performance. The new method which only requires an additional AOM demonstrates a scalable and cost-effective method for CEO stabilization of high-power laser oscillators.

Femtosecond CPA hybrid laser system with pulse-on-demand operation.

In this manuscript we present a true pulse-on-demand concept of a hybrid CPA laser system, consisting of a chirped-pulse fiber amplifier and an additional solid-state amplifier, capable of generating femtosecond pulses on demand without an external optical modulator/shutter. Pulse-on-demand operation is achieved by introducing idler pulses with a few nanoseconds duration and selectively switching between the femtosecond and idler pulses. The idler pulses are used to maintain a constant population inversion in the fiber amplifier as well as in the solid-state amplifier. Second harmonic generation (SHG) unit then effectively filters out the idler pulses due to their low peak power, leaving only a stable femtosecond pulse train. This concept is demonstrated on a CPA hybrid system that can generate pulses with up to 200 µJ at 515 nm with a pulse duration under 450 fs. As there is no optical modulator at the laser output, the presented concept also enables further power scaling.

Multi-frame interferometric imaging with a femtosecond stroboscopic pulse train for observing irreversible phenomena.

We describe a high-speed single-shot multi-frame interferometric imaging technique enabling multiple interferometric images with femtosecond exposure time over a 50 ns event window to be recorded, following a single laser-induced excitation event. The stroboscopic illumination of a framing camera is made possible through the use of a doubling cavity that produces a femtosecond pulse train that is synchronized to the gated exposure windows of the individual frames of the camera. The imaging system utilizes a Michelson interferometer to extract phase and ultimately displacement information. We demonstrate the method by monitoring laser-induced deformation and the propagation of high-amplitude acoustic waves in a silicon nitride membrane. The method is applicable to a wide range of fast irreversible phenomena such as crack branching, shock-induced material damage, cavitation, and dielectric breakdown.

Three-dimensional laser writing inside silicon using THz-repetition-rate trains of ultrashort pulses

Three-dimensional laser writing inside silicon remains today inaccessible with the shortest infrared light pulses unless complex schemes are used to circumvent screening propagation nonlinearities. Here, we explore a new approach irradiating silicon with trains of femtosecond laser pulses at repetition rates up to 5.6 THz. This extremely high repetition rate is faster than laser energy dissipation from microvolume inside silicon, thus enabling unique capabilities for pulse-to-pulse accumulation of free carriers generated by nonlinear ionization, as well as progressive thermal bandgap closure before any diffusion process comes into play. By space-resolved measurements of energy delivery inside silicon, we evidence a net increase on the level of space-time energy localization. The improvement is also supported by experiments demonstrating an apparent decrease of the energy threshold for modification and drastic improvements on the repeatability, uniformity, and symmetricity of the produced features. The unique benefits of THz bursts can provide a new route to meet the challenge of 3D inscription inside narrow bandgap materials.

Ultrafast Laser Writing Deep inside Silicon with THz-Repetition-Rate Trains of Pulses.

Three-dimensional laser writing inside silicon remains today inaccessible with the shortest infrared light pulses unless complex schemes are used to circumvent screening propagation nonlinearities. Here, we explore a new approach irradiating silicon with trains of femtosecond laser pulses at repetition rates up to 5.6 THz that is order of magnitude higher than any source used for laser processing so far. This extremely high repetition rate is faster than laser energy dissipation from microvolume inside silicon, thus enabling unique capabilities for pulse-to-pulse accumulation of free carriers generated by nonlinear ionization, as well as progressive thermal bandgap closure before any diffusion process comes into play. By space-resolved measurements of energy delivery inside silicon, we evidence changes in the interplay between detrimental nonlinearities and accumulation-based effects. This leads to a net increase on the level of space-time energy localization. The improvement is also supported by experiments demonstrating high performance for 3D laser writing inside silicon. In comparison to repeated single pulses, irradiation with trains of only four-picosecond pulses with the same total energy leads to an apparent decrease of the energy threshold for modification and drastic improvements on the repeatability, uniformity, and symmetricity of the produced features. The unique benefits of THz bursts can provide a new route to meet the challenge of 3D inscription inside narrow bandgap materials.