Femtosecond Laser Pulse Shaping Research Articles

Theoretical Study on the Ultrafast Selective Excitation of Surface-Enhanced Coherent Anti-Stokes Raman Scattering Based on Fano Resonance of Disk-Ring Nanostructures by Shaped Femtosecond Laser Pulses

The enhancement and selective excitation of coherent anti-Stokes Raman scattering (CARS) and the suppression of background noise are very important problems for real-time detection at the single-molecule level. Optimizing the plasmonic substrate to ensure that all the hot spots of the pump, probe, Stokes, and anti-Stokes light are at the same position is the key to increasing the CARS signal to reach the level of single-molecule detection. The selective excitation of the target CARS peak and the suppression of the other peaks are the key to improving the signal-to-noise ratio. In this paper, we present a theoretical study to control the selective excitation and enhancement of any one of the three CARS peaks using the Fano resonance of a disk-ring structure. By optimizing the modulation of the pump, Stokes, and probe pulse, one CARS peak is maximized, while the other two are suppressed to zero. Fano resonance is applied to simultaneously enhance the four surface plasmon modes of the pump, probe, Stokes, and anti-Stokes light and to ensure that all the hot spots are located at the same position by adjusting the size of the disk-ring structure. The hot spots of the four pulses are concentrated in the disk-ring gap with a deviation distance of less than 2 nm, and the intensity of the CARS is enhanced by 1.43 × 1012 times, which is much higher than the requirement of single-molecule detection. The time, frequency, and phase distribution of the input and the response of the four pulses are studied in detail. It was found that the selective excitation and the spectra of CARS are both well preserved.

Control of electron recollision and molecular nonsequential double ionization

Intense laser pulses lasting a few optical cycles, are able to ionize molecules via different mechanisms. One such mechanism involves a process whereby within one optical period an electron tunnels away from the molecule, and is then accelerated and driven back as the laser field reverses its direction, colliding with the parent molecule and causing correlated non-sequential double ionization (NSDI). Here we report control over NSDI via spectral-phase pulse shaping of femtosecond laser pulses. The measurements are carried out on ethane molecules using shaped pulses. We find that the shaped pulses can enhance or suppress the yield of dications resulting from electron recollision by factors of 3 to 6. This type of shaped pulses is likely to impact all phenomena stemming from electron recollision processes induced by strong laser fields such as above threshold ionization, high harmonic generation, attosecond pulse generation, and laser-induced electron diffraction.

Optimal spectral phase control of femtosecond laser-induced up-conversion luminescence in Sm3+:NaYF4 glass

The spectral phase of the femtosecond laser field is an important parameter that affects the upconversion (UC) luminescence efficiency of dopant lanthanide ions. In this work, we report an experimental study on controlling the UC luminescence efficiency in Sm3+:NaYF4 glass by 800-nm femtosecond laser pulse shaping using spectral phase modulation. The optimal phase control strategy efficiently enhances or suppresses the UC luminescence intensity. Based on the laser-power dependence of the UC luminescence intensity and its comparison with the luminescence spectrum under direct 266-nm femtosecond laser irradiation, we propose herein an excitation model combining non-resonant two-photon absorption with resonance-mediated three-photon absorption to explain the experimental observations.

Ultrafast selective excitation of surface-enhanced Raman scattering from a single molecule by shaping pump and Stokes pulses

Surface-enhanced Raman scattering (SERS) has attracted much attention as an important technique for studying and monitoring the behavior of molecules. Coherent control by shaping femtosecond pulses is an efficient method for controlling molecular vibration state, rotation state, ionization state, and other molecule dynamics. In this paper, we propose a theoretical scheme of selective excitation of SERS from a single porphycene molecule with two Raman peaks at 327[Formula: see text]cm[Formula: see text] and 367[Formula: see text]cm[Formula: see text] placed in the gap between silver dimer nanospheres. Based on the theory of quantum coherent control, selective excitation of the peak at 327[Formula: see text]cm[Formula: see text] and depression of another at 367[Formula: see text]cm[Formula: see text] are demonstrated by properly shaping femtosecond laser pulses with central frequency of 12500[Formula: see text]cm[Formula: see text] and a full width at half maximum (FWHM) of 300[Formula: see text]cm[Formula: see text]. Ultrafast dynamics of the response of dimer nanospheres to the shaped pump and Stokes pulses are simulated by finite difference time domain (FDTD) method followed by Fourier transform. The enhanced and selected single molecule SERS is realized simultaneously. The SERS probability is enhanced by more than six orders in magnitude compared with the case in air environment, and the FWHMs of the enhanced and the depressed Raman peaks are both larger than 17[Formula: see text]cm[Formula: see text] despite the narrow gap of only 40[Formula: see text]cm[Formula: see text] between the two Raman peaks.

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.

基于整形飞秒激光脉冲的三维微纳制备

基于整形飞秒激光脉冲的三维微纳制备

Control of periodic surface structures on silicon by combined temporal and polarization shaping of femtosecond laser pulses

We demonstrate the capability to exercise advanced control on the laser-induced periodic surface structures (LIPSS) on silicon by combining the effect of temporal shaping, via tuning the interpulse temporal delay between double femtosecond laser pulses, along with the independent manipulation of the polarization state of each of the individual pulses. For this, cross-polarized (CP) as well as counter-rotating (CR) double circularly polarized pulses have been utilized. The pulse duration was 40 fs and the central wavelength of 790 nm. The linearly polarized double pulses are generated by a modified Michelson interferometer allowing the temporal delay between the pulses to vary from Δτ = −80 ps to Δτ = +80 ps with an accuracy of 0.2 fs. We show the significance of fluence balance between the two pulse components and its interplay with the interpulse delay and with the order of arrival of the individually polarized pulse components of the double pulse sequence on the final surface morphology. For the case of CR pulses we found that when the pulses are temporally well separated the surface morphology attains no axial symmetry. But strikingly, when the two CP pulses temporally overlap, we demonstrate, for the first time in our knowledge, the detrimental effect that the phase delay has on the ripple orientation. Our results provide new insight showing that temporal pulse shaping in combination with polarization control gives a powerful tool for drastically controlling the surface nanostructure morphology.

Valence state manipulation of Sm^3+ ions via a phase-shaped femtosecond laser field

The ability to manipulate the valence state conversion of rare-earth ions is crucial for their applications in color displays, optoelectronic devices, laser sources, and optical memory. The conventional femtosecond laser pulse has been shown to be a well-established tool for realizing the valence state conversion of rare-earth ions, although the valence state conversion efficiency is relatively low. Here, we first propose a femtosecond laser pulse shaping technique for improving the valence state conversion efficiency of rare-earth ions. Our experimental results demonstrate that the photoreduction efficiency from Sm3+ to Sm2+ in Sm3+-doped sodium aluminoborate glass using a π phase step modulation can be comparable to that using a transform-limited femtosecond laser field, while the peak laser intensity is decreased by about 63%, which is very beneficial for improving the valence state conversion efficiency under the laser-induced damage threshold of the glass sample. Furthermore, we also theoretically develop a (2+1) resonance-mediated three-photon absorption model to explain the modulation of the photoreduction efficiency from Sm3+ to Sm2+ under the π-shaped femtosecond laser field.

Optimal control of photoelectron emission by realistic waveforms

Recent experimental techniques in multicolor waveform synthesis allow the temporal shaping of strong femtosecond laser pulses with applications in the control of quantum mechanical processes in atoms, molecules, and nanostructures. Prediction of the shapes of the optimal waveforms can be done computationally using quantum optimal control theory. In this work we demonstrate the control of above-threshold photoemission of one-dimensional hydrogen model with pulses feasible for experimental waveform synthesis. By mixing different spectral channels and thus lowering the intensity requirements for individual channels, the resulting optimal pulses can extend the cutoff energies by at least up to 50% and bring up the electron yield by several orders of magnitude. Insights into the electron dynamics for optimized photoelectron emission are obtained with a semiclassical two-step model.

Shaping of ultraviolet femtosecond laser pulses by Fourier domain harmonic generation.

We present a method to finely tailor ultraviolet femtosecond laser pulses using a pulse shaper with ability in the infrared/visible spectral range. We have developed to that end a frequency doubling module in which the up-conversion mechanism is carried out in the Fourier plane of a 4 f -line. The pulse shaper is used to imprint a spectral phase and/or amplitude onto the fundamental pulse. The shaped pulse is then frequency doubled through the module which transfers the applied spectral shaping to the second harmonic field in a predictable manner. The relevance of the method is demonstrated by synthesizing and characterizing shaped pulses at a central wavelength of 400 nm. The results demonstrate a full control over the spectral phase and amplitude of the harmonic field. The experimental setup is simple and features interesting prospects for the polarization shaping of ultraviolet pulses and the production of shaped ultraviolet pulses requested for the seeding of free-electron lasers.

High-aspect-ratio, high-quality microdrilling by electron density control using a femtosecond laser Bessel beam

We propose an efficient microdrilling method of high-aspect-ratio, high-quality microholes in polymethyl methacrylate by controlling localized transient spatial electron density using single-pulse femtosecond laser Bessel beams. The microholes fabricated with diameters of 1.5–2.4 μm are taper-free, which are of much better quality in the entrances and sidewalls, as compared with those fabricated by Gaussian beams. The aspect ratio of the microholes is up to 330:1. It takes 42 min to fabricate a 501 × 501 microhole array (with 251,001 holes in total, about 100 holes per second under a repetition rate of 100 Hz) in a 1 cm × 1 cm area, which is very uniform in size and shape. For single-pulse drilling of a microhole array, the number of ultrahigh-aspect-ratio microholes processed per second is theoretically determined by the repetition rate. The liquid infiltration method and cross-sectional profile tests confirm hollow microhole drillings rather than material modifications. The theoretical simulation of optical intensity distribution and intensified charge-coupled device detection shows that fabricating such thin, long, uniform microholes using Bessel beams is attributed to electron density control by spatially shaping femtosecond laser pulses.

Control of grating-coupled ultrafast surface plasmon pulse and its nonlinear emission by shaping femtosecond laser pulse

Spatiotemporal nanofocusing of ultrafast surface plasmon polaritons (SPPs) coupled on a metal Au tapered tip with a curvature radius of a few tens of nanometers is deterministically controlled based on the measured plasmon response function. We control the SPP pulse shape and the second harmonic generation at the apex of the Au tapered tip by shaping the excitation femtosecond laser pulses based on the response function. We also adapted a similar control scheme for coherent anti-Stokes Raman scattering (CARS) and achieved selective CARS excitation of a single Raman mode of carbon nanotubes with only a single excitation laser pulse at the apex of the tip.

Controllable high-throughput high-quality femtosecond laser-enhanced chemical etching by temporal pulse shaping based on electron density control.

We developed an efficient fabrication method of high-quality concave microarrays on fused silica substrates based on temporal shaping of femtosecond (fs) laser pulses. This method involves exposures of fs laser pulse trains followed by a wet etching process. Compared with conventional single pulses with the same processing parameters, the temporally shaped fs pulses can enhance the etch rate by a factor of 37 times with better controllability and higher quality. Moreover, we demonstrated the flexibility of the proposed method in tuning the profile of the concave microarray structures by changing the laser pulse delay, laser fluence, and pulse energy distribution ratio. Micro-Raman spectroscopy was conducted to elucidate the stronger modification induced by the fs laser pulse trains in comparison with the single pulses. Our calculations show that the controllability is due to the effective control of localized transient free electron densities by temporally shaping the fs pulses.

Femtosecond laser pulse shaping at megahertz rate via a digital micromirror device.

In this Letter, we present a scanner and digital micromirror device (DMD)-based ultrafast pulse shaper, i.e., S-DUPS, for programmable ultrafast pulse modulation, achieving a shaping rate of 2 MHz. To our knowledge, the S-DUPS is the fastest programmable pulse shaper reported to date. In the S-DUPS, the frequency spectrum of the input pulsed laser is first spread horizontally, and then mapped to a thin stripe on the DMD programmed with phase modulation patterns. A galvanometric scanner, synchronized with the DMD, subsequently scans the spectrum vertically on the DMD to achieve a shaping rate up to 10 s MHz. A grating pair and a cylindrical lens in front of the DMD compensate for the temporal and spatial dispersion of the system. To verify the concept, experiments were conducted with the DMD and the galvanometric scanner operated at 2 kHz and 1 kHz, respectively, achieving a 2 MHz speed for continuous group velocity dispersion tuning, as well as 2% efficiency. Up to 5% efficiency of S-DUPS can be expected with high efficiency gratings and optical components of proper coatings.

Graphene Near-Degenerate Four-Wave Mixing for Phase Characterization of Broadband Pulses in Ultrafast Microscopy

We investigate near-degenerate four-wave mixing in graphene using femtosecond laser pulse shaping microscopy. Intense near-degenerate four-wave mixing signals on either side of the exciting laser spectrum are controlled by amplitude and phase shaping. Quantitative signal modeling for the input pulse parameters shows a spectrally flat phase response of the near-degenerate four-wave mixing due to the linear dispersion of the massless Dirac Fermions in graphene. Exploiting these properties we demonstrate that graphene is uniquely suited for the intrafocus phase characterization and compression of broadband laser pulses, circumventing disadvantages of common methods utilizing second or third harmonic light.

Femtosecond pulse shaping with plasmonic crystals

The temporal shaping of femtosecond laser pulses reflected from a one-dimensional plasmonic crystal using a commercially available polymer grating coated with a silver film is experimentally demonstrated by timeresolved measurements of the intensity correlation function. Shaping is achieved by the excitation of surface plasmon-polaritons with a lifetime comparable to the 130 fs laser pulse duration. The variety of data obtained demonstrate the flexible shaping of fs-pulses by delaying, advancing, splitting, broadening, compressing, and changing the topological properties of the pulse with the plasmonic crystals under study.

Full vector-field control of ultrashort laser pulses utilizing a single dual-layer spatial light modulator in a common-path setup

We present a design for shaping femtosecond laser pulses in amplitude, phase, and polarization by manipulating two perpendicular polarization components in two arms of an interferometric common-path setup. A thin-film polarizer is used for polarization splitting and recombination to avoid angular dispersion and phase variations across the beam profile. We demonstrate how the optimal design parameters can be found by numerical calculations and present an on-the-fly phase reduction and stabilization routine to automatically compress the pulse and improve the long-term stability of the setup. Examples of deterministically generated polarization-shaped multipulse sequences prove the capabilities of the setup.

Femtosecond laser induced damage characterization of transmission volume phase gratings

A procedure to characterize the induced damage and the incubation effects in volume transmission gratings under femtosecond laser pulse train illumination is presented. It was also developed a formalism that explains the damage processes. Our proposal was employed on glass gratings to show the effectiveness of the method and its potential to design transmission gratings with enhanced laser induced damage threshold. This procedure is able to be extended to any transmission grating composed by chemically non-uniform material, opening up new perspectives to femtosecond laser pulse shaping.

飞秒激光整形脉冲激发金膜的超快热弛豫特性

飞秒激光整形脉冲激发金膜的超快热弛豫特性

Femtosecond pulse shaping technology and its applications

By controlling the spectral amplitude, phase and polarization of the femtosecond laser pulse in the frequency domain, a shaped femtosecond laser pulse with almost arbitrary pulse shape in time domain can be obtained, and this femtosecond laser pulse shaping technique provides a new experimental tool to study the nonlinear interaction between light and atoms or molecules. In this paper, we introduce the development history, technical method, control technique and relevant applications of the femtosecond laser pulse shaping technique, and also carry out a prospect on the research trends of this technique.