Femtosecond Laser Pulse Shaping Research Articles

Manipulation of terahertz pulse generation in ZnTe crystal by shaping femtosecond laser pulses with a square phase modulation

We theoretically demonstrate that the terahertz pulse generation via optical rectification in ZnTe crystal can be effectively controlled by making use of the shaped femtosecond laser pulses with a square phase modulation. We show that, by precisely controlling these parameters characterizing the square phase modulation, the terahertz spectrum is periodically modulated and the terahertz pulses are split into multiple subpulses, and the modulation period of the terahertz spectrum and the relative intensity and time interval of these terahertz subpulses can be continuously tuned. Furthermore, we utilize the power spectrum of the shaped femtosecond laser pulses to explain the physical control mechanism of the shaped and tunable terahertz pulse generation.

Elimination of two‐photon excited fluorescence using a single‐beam coherent anti‐Stokes Raman scattering setup

In this report, we demonstrate how to obtain Raman spectra from single‐beam CARS measurements under the condition of strong overlapping two‐photon excited fluorescence (2PEF). Our approach is based on the use of a narrowband phase gate implemented via femtosecond laser pulse shaping from a single broadband laser source, without any use of polarization or amplitude modulation. The decisive quantity regarding the creation of 2PEF with shaped laser pulses is the second harmonic power spectrum, which shows a different dependence on the excitation phase from the CARS signal generation. By shifting the phase or the position of the gate in an appropriate way, it is possible to keep the second harmonic power spectrum and 2PEF constant, paving the way for an elimination of it. This allows to reconstruct the Raman signal and to retrieve quantitative information of resonances even when the fluorescence is about 160 times larger than the CARS signal. Copyright © 2013 John Wiley & Sons, Ltd.

Machining of Biocompatible Polymers with Shaped Femtosecond Laser Pulses

Article Machining of Biocompatible Polymers with Shaped Femtosecond Laser Pulses was published on September 7, 2013 in the journal Biomedical Engineering / Biomedizinische Technik (volume 58, issue SI-1-Track-C).

Control of resonance enhanced multi-photon ionization photoelectron spectroscopy by phase-shaped femtosecond laser pulse

In this paper, we theoretically demonstrate that the (2+1+1) resonance enhanced multi-photon ionization photoelectron spectroscopy in sodium atom can be effectively controlled by shaping femtosecond laser pulse with a π phase step modulation in weak laser field, involving its total photoelectron energy, maximal photoelectron intensity, and spectroscopic bandwidth. Our results show that the total photoelectron energy can be suppressed but not enhanced, the maximal photoelectron intensity can be enhanced and also suppressed, and the photoelectron spectroscopy can be tremendously narrowed. These theoretical results can provide a feasible scheme to achieve the high-resolution photoelectron spectroscopy and study the excited state structure in atomic and molecular systems.

Coherent control of multiphoton-ionization passage of excited-state rubidium atoms

We have investigated multiphoton-ionization passages of rubidium atoms initiated from the excited $5{P}_{3/2}$ energy state. For this we used coherent control schemes based on femtosecond laser pulse shaping applied to cold atoms spatially isolated in a magneto-optical trap. With programed laser pulses of spectral $\ensuremath{\pi}$-phase step, of which location was varied within the laser spectrum, the sequential two-photon-ionization passage along the $5{P}_{3/2}$-$5D$ continuum was probed in terms of trap-loss spectroscopy. The coherent control two-photon-ionization method unveiled not only the resonantly enhanced two-photon ionization but also the asymmetric nature of the ionization profile structure given as a function of the spectral phase step location. Experimental results show good agreement with the second-order perturbation calculation of the constituent possible ionization passages.

High-throughput rear-surface drilling of microchannels in glass based on electron dynamics control using femtosecond pulse trains

This study proposes a rear-surface ablation enhancement approach to fabricate high-aspect-ratio microchannels by temporally shaping femtosecond laser pulse trains. In the case study of K9 glass, enhancements of up to a 56 times higher material removal rate and a three times greater maximum drilling depth are obtained by the proposed method, as compared with conventional femtosecond laser drilling at the same processing parameters. The improvements are due to the changes of photon-electron interactions by shaping femtosecond pulse train, which can effectively adjust the photon absorption and localized transient material properties by changing electron dynamics such as free electron densities.

Temporal pulse shaping effects on aluminium and boron ablation plumes generated by ultrashort pulsed laser ablation and analyzed by time- and space-resolved optical spectroscopy

This paper focuses on the effect of temporal shaping of femtosecond laser pulses on laser ablation of aluminium and boron. The influence on plasma composition is deduced from optical emission investigations. Irradiation generated by the temporal tailoring of an incident laser pulse into double pulses or longer pulses is examined and compared to results deduced from hydrodynamic simulations of plasma temperature profiles. Above the ion formation threshold, increasing the delay in the double pulses sequence or increasing the pulse duration leads to an increase of the ion emission of the plasma plume of aluminium or boron. The observed increase with a saturation of ion emission at long delays or long pulse durations result from an interaction between the shaped laser pulse and the expanding matter after the onset of the plasma. Irradiation of boron with long pulses or double pulses reduces the production of nanoparticles, compared to the case of short pulses. Temporal shaping does not significantly affect the nanoparticle temperature, in spite of significant variations in the quantity of the ejected nanoparticles.

Heterodyne coherent anti-Stokes Raman scattering by the phase control of its intrinsic background

We demonstrate the use of femtosecond laser pulse shaping for precise control of the interference between the coherent anti-Stokes Raman scattering (CARS) signal and the coherent nonresonant background generated within the same sample volume. Our technique is similar to heterodyne detection with the coherent background playing the role of the local oscillator field. In our experiment, we first apply two ultrashort (near-transform-limited) femtosecond pump and Stokes laser pulses to excite coherent molecular oscillations within a sample. After a short and controllable delay, we then apply a laser pulse that scatters off of these oscillations to produce the CARS signal. By making fine adjustments to the probe field spectral profile, we vary the relative phase between the Raman-resonant signal and the nonresonant background, and we observe a varying spectral interference pattern. These controlled variations of the measured pattern reveal the phase information within the Raman spectrum.

Field-free molecular alignment by shaping femtosecond laser pulse with cubic phase modulation

We propose a scheme to enhance the field-free molecular alignment by shaping the femtosecond laser pulse with cubic phase modulation. We show that the molecular alignment can be obtained adiabatically during the shaped pulse and can be revived at the full rotational periods with the same degree as that at the peak of the shaped pulse. The behavior of the molecular alignment by the shaped pulse is the same as that by the slow turn-on and rapid turn-off laser pulse. Furthermore, we show that the shaped pulse with cubic phase modulation has a slight advantage in enhancing the field-free molecular alignment by comparing it with the slow turn-on and rapid turn-off laser pulse under the same laser intensity.

Phase-cycling coherent anti-Stokes Raman scattering using shaped femtosecond laser pulses

We demonstrate a homodyne coherent anti-Stokes Raman scattering (CARS) technique based on femtosecond laser pulse shaping. This technique utilizes fast phase cycling to extract nonlinear Raman signatures with a self-generated reference signal acting as a local oscillator. The local oscillator is generated at the focus and is intrinsically stable relative to the Raman signal even in highly scattering samples. We can therefore retrieve phase information from the Raman signal and can suppress the ubiquitous non-resonant background.

Optimal control of the strong-field ionization of silver clusters in helium droplets

Optimal control techniques combined with femtosecond laser pulse shaping are applied to steer and enhance the strong-field induced emission of highly charged atomic ions from silver clusters embedded in helium nanodroplets. With light fields shaped in amplitude and phase we observe a substantial increase of the Ag$^{q+}$ yield for $q>10$ when compared to bandwidth-limited and optimally stretched pulses. A remarkably simple double-pulse structure, containing a low-intensity prepulse and a stronger main pulse, turns out to produce the highest atomic charge states up to Ag$^{20+}$. A negative chirp during the main pulse hints at dynamic frequency locking to the cluster plasmon. A numerical optimal control study on pure silver clusters with a nanoplasma model converges to a similar pulse structure and corroborates, that the optimal light field adapts to the resonant excitation of cluster surface plasmons for efficient ionization.

Influence of the temporal shape of femtosecond pulses on silicon micromachining

The influence of femtosecond laser pulse shaping on silicon wafer micromachining is explored. Surface second harmonic generation provides in situ pulse characterization of the laser pulses, and plasma and atomic emissions were identified as valuable indicators of the micromachining process. The ablation threshold was found to decrease as the bandwidth of the pulses increases, as well as for shorter pulses. Dependence of atomic and plasma emissions on temporal shape of the pulses confirmed that emission preceded ablation and has a threshold as well. The morphology of micromachined holes was observed to be dependent upon pulse duration.

Control of Ionization Processes in High Band Gap Materials

An overview on femtosecond laser pulse shaping techniques applied to control of the initial photo-physical steps involved in materials processing is presented. First, pulse shaping methodology in frequency domain is introduced and examples of shaped pulses relevant to laser microfabrication are discussed. Then, the use of tailored femtosecond pulses to control the initial steps of laser processing of high band gap materials is demonstrated. In particular, control on basic ionization proc-esses acting as the initial photo-physical step of the ablation dynamics is exerted by highly asymmetric femtosecond laser pulse shapes generated by Third Order Dispersion (TOD).

Pulse shaping and its characterization of mid-infrared femtosecond pulses: Toward coherent control of molecules in the ground electronic states

We have examined a technique of complex shaping of mid-infrared femtosecond laser pulses towards accurate and precise control of rovibrational wave packets of molecules in the ground electronic state. A Germanium acousto-optics modulator was used as a device for the pulse shaping. In order to characterize the shaped pulses precisely, sum-frequency cross-correlation frequency-resolved optical gating was introduced for the analysis of both amplitude and phase of the electric fields. The mid-infrared pulses were shaped and characterized with a frequency resolution better than 4.5 cm −1. Such a resolution is supposed to be sufficient for the realization of quantum gate operations with high fidelity, which is one of the most challenging applications of rovibrational wave packet manipulation of molecules. In order to demonstrate the precision of our method of shaping and its characterization, we have generated shaped pulses that will realize Hadamard and NOT quantum gates with rovibrational states of a CO molecule.

Experimental implementation of ultrashort laser pulses in the von Neumann picture

The characterization and interpretation of ultrashort laser pulses is most intuitive in the joint time–frequency domain, where structures like multiple pulses become immediately apparent. For practical reasons, ultrafast femtosecond laser pulse shaping, however, is commonly carried out in the frequency domain. Here we implement pulse shaping of optical fields defined in the von Neumann representation, which is a joint time–frequency distribution with complex-valued Gaussian basis functions. We discuss the feasibility as well as the principal limitations of this technique, show illustrative examples, and propose possible applications in coherent control.

Towards the adaptive optimization of field-free molecular alignment

We theoretically report the optimization of field-free molecular alignment by spectral phase shaping of femtosecond laser pulses. Optimal pulse shapes are designed iteratively by an evolutionary algorithm in conjunction with a non-perturbative regime calculation. The investigation is conducted in O2 and N2 under realistic conditions of intensity, temperature and pulse shaping. We demonstrate that specific tailored pulses can provide significant maximization of field-free alignment compared to the Fourier transform limited pulses of the same energy. The underlying control mechanism is discussed. The effect of pulse energy and temperature is analysed leading to the identification of a general criteria for a successful optimization. Finally, the optimal spectral phase learned from the algorithm is rather smooth and can be described by a representation in terms of a sigmoidal function. We show that the use of a low-dimensional parametrization of the phase yields an efficient optimization of the alignment within a highly reduced convergence time.

Multicomponent control via shaped, strong laser fields mass spectrometry

The fragmentation processes of polyatomic molecules induced by an intense laser field exhibit sensitive dependence on the laser characteristics such as intensity, pulse duration, wavelength, and shape of the temporal pulse envelope. Adaptive laser pulse shaping can control the fragmentation of methyl/methoxy groups in dimethyl methylphosphonate (DMMP), a simulant for nerve agent sarin. The exploitation of the sensitivity of molecular fragmentation to laser pulse shapes represents a new way to discriminate molecular identity. Here we have shown manipulation of branching ratio M-(OCH3)+/M-(CH3)+, M-2(CH3)+/M-(CH3)+ and M-(OCH3)+/M-2(CH3)+ fragment ion ratios for DMMP in the presence of complex background in the extraction region of a TOF spectrometer using tailored femtosecond laser pulses. We suggest that the use of adaptive femtosecond laser pulse shaping coupled to TOF mass spectrometry is an accurate way to identify complex airborne organophosphate molecules similar to nerve agents.

Adaptive subwavelength control of nano-optical fields

Adaptive shaping of the phase and amplitude of femtosecond laser pulses has been developed into an efficient tool for the directed manipulation of interference phenomena, thus providing coherent control over various quantum-mechanical systems. Temporal resolution in the femtosecond or even attosecond range has been demonstrated, but spatial resolution is limited by diffraction to approximately half the wavelength of the light field (that is, several hundred nanometres). Theory has indicated that the spatial limitation to coherent control can be overcome with the illumination of nanostructures: the spatial near-field distribution was shown to depend on the linear chirp of an irradiating laser pulse. An extension of this idea to adaptive control, combining multiparameter pulse shaping with a learning algorithm, demonstrated the generation of user-specified optical near-field distributions in an optimal and flexible fashion. Shaping of the polarization of the laser pulse provides a particularly efficient and versatile nano-optical manipulation method. Here we demonstrate the feasibility of this concept experimentally, by tailoring the optical near field in the vicinity of silver nanostructures through adaptive polarization shaping of femtosecond laser pulses and then probing the lateral field distribution by two-photon photoemission electron microscopy. In this combination of adaptive control and nano-optics, we achieve subwavelength dynamic localization of electromagnetic intensity on the nanometre scale and thus overcome the spatial restrictions of conventional optics. This experimental realization of theoretical suggestions opens a number of perspectives in coherent control, nano-optics, nonlinear spectroscopy, and other research fields in which optical investigations are carried out with spatial or temporal resolution.

Control in Dissociative Ionization of Ethanol Molecules and Direct Visualization of Vibrational Nuclear Wavepacket Evolution with Shaped Femtosecond Laser Pulses

Under the intense laser-field (>1012W/cm2), the structure of the potential energy surface of the molecule is modified and thus, the time evolution of the vibrational nuclear wave packet could be manipulated by the carrier frequency, amplitude or phase of the excitation laser pulse. In this paper, we describe how the pulse shaping of intense ultrashort laser pulses affects dissociative ionization reaction of ethanol molecules. Influence of the laser pulsewidth on the non-adiabatic transition is clearly demonstrated. Moreover, the dynamics of vibrational nuclear wavepacket generated with intense femtosecond laser pulse in repulsive potential curves is monitored via Coulomb explosion induced by delayed probe laser pulses.

Selective excitation of CARS by adaptive pulse shaping based on genetic algorithm

Selective excitation of coherent anti-Stokes Raman scattering (CARS) from the benzene solution is experimentally demonstrated by shaping femtosecond laser pulses based on genetic algorithm. Second harmonic generation frequency-resolved optical gating (SHG-FROG) technique is adopted to characterize the original and optimal laser pulses, and the mechanisms for the coherently selective excitation of coherent anti-Stokes Raman scattering (CARS) is discussed. The results show that two-pulse coherent anti-Stokes Raman scattering (CARS) has good signal-to-background ratio and high sensitivity, and it has attractive potential applications in the complicated molecular system, such as biomolecules and materials.