Broadband Femtosecond Laser Pulses Research Articles

Temporal characterization of broadband femtosecond laser pulses by surface third-harmonic dispersion scan with ptychographic retrieval.

We present a new, to the best of our knowledge, variant of dispersion scan (d-scan) based on surface third-harmonic generation (STHG) and a ptychographic algorithm tailored for full retrieval (amplitude and phase) of broadband laser pulses. We demonstrate the technique by temporally measuring and compressing few-cycle pulses with 7 fs and 2.5 nJ from a Ti:sapphire oscillator, using a sapphire window as the nonlinear medium. The results are in very good agreement with standard second-harmonic d-scan measurements based on a nonlinear crystal. The intrinsically broadband and phase-matching-independent nature of STHG make this technique very suitable for the characterization of ultrashort laser pulses over a broad wavelength range extending into the mid-infrared.

A comparison of strategies for state‐selective coherent Raman excitation

AbstractWe report on the theoretical and experimental evaluation of state‐selective bond activation for coherent Raman spectroscopy and control of chemical reactivity. We compare six different strategies for state‐selective activation using shaped broadband femtosecond laser pulses. Results for a pair of long pulses with different wavelengths are also considered. For each of these approaches, we report on their theoretical and experimental ability to excite a desired vibrational mode while minimizing excitation of other modes including harmonics. We find that the double parabola spectral phase function gives the most intense Raman excitation with 10 cm−1 selectivity with the lowest background. The sinusoidal phase function produces intense Raman excitation with high selectivity but gives a series of harmonic excitations which could hinder selectivity. Implications of this study on several fields ranging from sensing, to controlling chemical reactions, and selective excited state excitation by X‐ray pulses are discussed.

Generation of tunable ultraviolet femtosecond pulse in MgO crystal by cascaded four wave mixing

Ultraviolet femtosecond laser pulse is an important tool in studying ultrafast chemical and physical processes. Realizing broadband ultraviolet laser pluses with a wide tunable range would significantly facilitate the study of ultrafast processes. As an effective and convenient method, the cascaded four-wave mixing (CFWM) has been widely adopted to generate broadband and tunable ultraviolet femtosecond laser pulses. In this work, we carry out CFWM in MgO crystal by using two 400-nm pulses to generate tunable ultraviolet femtosecond pulse. The MgO crystal is chosen due to its high third-order nonlinear susceptibility, large band gap and high transmittance in the ultraviolet region. In the experiment, nine frequency up-converted and five frequency down-converted sidebands are observed. The measured wavelength and scattering angle of each sideband are consistent with the CFWM theory predictions. The wavelength range of the sidebands covers 350–450 nm. The total conversion efficiency of the ultraviolet sidebands is 1.2%, which is higher than the reported values with visible/near infrared driven lasers. Meanwhile, the spectra of the high-order sidebands present a Gaussian profile and can support a Fourier-transform-limited pulse duration of less than 50 fs. Besides, the central wavelengths of the sidebands can be effectively tuned by adjusting the time-delay between the two pre-chirped pump pulses. Our study provides an efficient and convenient scheme to generate short ultraviolet femtosecond pulses with a wide tunable range.

Fourier transform second harmonic generation for high-resolution nonlinear spectroscopy

In this communication we demonstrate a new characterization technique combining Fourier transform (FT) spectroscopy and second harmonic generation (SHG) that enables high spectral resolution with broadband femtosecond laser pulses. The strong and narrow exciton resonances of the wide band gap semiconductor ZnO were chosen for demonstrating the capabilities of the method. FT-SHG offers high reproducibility and high spectral resolution within the bandwidth of the input pulse.

Simultaneous temporally and spectrally resolved Raman coherences with single-shot fs/ns rotational CARS

A novel technique for studies of the dynamics of molecular coherences has been developed. The concept is based on prompt excitation using broadband femtosecond (fs) laser pulses, whereupon a narrowband nanosecond (ns) laser pulse probes the fast dynamics of the coherences. Detection of the spectrally dispersed coherent signal using a streak camera allows simultaneous spectrally and temporally resolved studies of all excited coherences in a single-shot acquisition. Here we demonstrate the high capacity and versatility of this hybrid fs/ns coherent anti-Stokes Raman scattering (CARS) technique through some illustrative examples, all single-shot measurements, namely time-resolved studies of rotational Raman coherences in nitrogen and air, dynamics of Stark effect on rotational lines, and beating phenomena originating from close-lying rotational lines.

Correction of Fabry-Pérot interference effects in phase and amplitude pulse shapers based on liquid crystal spatial light modulators

Broadband femtosecond laser pulses manipulated by pulse shapers based on a liquid crystal spatial light modulator (LC-SLM) inevitably experience periodic spectral distortions due to Fabry-Perot interference effects within the LC-SLM. We present a method, applicable to phase and amplitude pulse shapers based on dual LC-SLMs, that enables the calibration and suppression of the undesired spectral intensity modulations in a non-iterative fashion. We demonstrate that the method considerably improves the amplitude shaping fidelity of phase and amplitude pulse shapers without compromising the phase shaping properties.

Efficient and tunable spectral compression using frequency-domain nonlinear optics.

A key requirement in the field of ultrafast vibrational spectroscopy is to efficiently generate intense tunable narrowband picosecond laser pulses synchronized to a broadband femtosecond laser source. Current nonlinear methods for picosecond pulse generation suffer from complexities in both experimental implementation and pulse frequency tunability. We present here a straightforward method for spectral bandwidth compression that produces frequency tunable picosecond pulses with efficient power conversion. Broadband femtosecond laser pulses are compressed to narrowband picosecond pulses using frequency domain sum-frequency generation of spatially chirped pulses, achieving spectral bandwidths of <20 cm-1 and power conversion efficiency of ∼18%. The experimental design of the bandwidth compressor is presented and its application to stimulated Raman spectroscopy is demonstrated.

Electric-field induced second-harmonic generation of femtosecond pulses in atmospheric air

Electric-field induced second harmonic generation with a femtosecond Ti:sapphire pump laser in atmospheric air is investigated. Phase matching between fundamental and second harmonic waves is achieved via quasi-phase matching, employing periodic electrode arrangements on printed circuit boards with a total length of about 0.5 m. The weak chromatic dispersion of the atmospheric air enables broadband phase matching conditions and makes this method tailorable for frequency doubling broadband femtosecond laser pulses. A maximum energy of 0.13 nJ is observed in a 20 nm bandwidth around 395 nm.

Pulse-shaping based two-photon FRET stoichiometry.

Förster Resonance Energy Transfer (FRET) based measurements that calculate the stoichiometry of intermolecular interactions in living cells have recently been demonstrated, where the technique utilizes selective one-photon excitation of donor and acceptor fluorophores to isolate the pure FRET signal. Here, we present work towards extending this FRET stoichiometry method to employ two-photon excitation using a pulse-shaping methodology. In pulse-shaping, frequency-dependent phases are applied to a broadband femtosecond laser pulse to tailor the two-photon excitation conditions to preferentially excite donor and acceptor fluorophores. We have also generalized the existing stoichiometry theory to account for additional cross-talk terms that are non-vanishing under two-photon excitation conditions. Using the generalized theory we demonstrate two-photon FRET stoichiometry in live COS-7 cells expressing fluorescent proteins mAmetrine as the donor and tdTomato as the acceptor.

Coherent anti-Stokes Raman spectroscopy in the presence of strong resonant signal from background molecules

Spectroscopy of molecular mixtures with broadband femtosecond laser pulses often involves simultaneous excitation of multiple molecular species with close resonance frequencies. Interpreting the collective optical response typically requires Fourier analysis of time-resolved signals. We propose and demonstrate an alternative method of separating coherent Raman scattering signals from two molecular components with neighboring vibrational modes. We exploit "silence windows," which arise from coupling of the vibrational and rotational degrees of freedom in molecular dynamics. In silence windows, the detected signal stems solely from the minority species (here, CO2), while the strong resonant background from the dominant species (O2) is greatly suppressed.

Achieving high-resolution photoelectron spectroscopy from a broadband femtosecond laser pulse

Femtosecond-induced resonance-enhanced multiphoton-ionization photoelectron spectroscopy (REMPI-PS) suffers from poor spectral resolution due to the large laser spectral bandwidth. In this paper, we present a scheme to achieve high-resolution REMPI-PS in the potassium atom by shaping the ultrashort femtosecond laser pulse. Our results show that the REMPI-PS bandwidth can be narrowed by several tens of times with a cubic spectral phase modulation, and thus high-resolution REMPI-PS and the fine structure of the energy-level diagram for multiple excited states can be obtained in spite of the broadband femtosecond laser pulse. Furthermore, we explain the physical control mechanism of the narrowband REMPI-PS by considering the two-photon power spectrum.

Unraveling Ultrafast Dynamics in Photoexcited Aniline

A combination of ultrafast time-resolved velocity map imaging (TR-VMI) methods and complete active space self-consistent field (CASSCF) ab initio calculations are implemented to investigate the electronic excited-state dynamics in aniline (aminobenzene), with a perspective for modeling (1)πσ* mediated dynamics along the amino moiety in the purine derived DNA bases. This synergy between experiment and theory has enabled a comprehensive picture of the photochemical pathways/conical intersections (CIs), which govern the dynamics in aniline, to be established over a wide range of excitation wavelengths. TR-VMI studies following excitation to the lowest-lying (1)ππ* state (1(1)ππ*) with a broadband femtosecond laser pulse, centered at wavelengths longer than 250 nm (4.97 eV), do not generate any measurable signature for (1)πσ* driven N-H bond fission on the amino group. Between wavelengths of 250 and >240 nm (<5.17 eV), coupling from 1(1)ππ* onto the (1)πσ* state at a 1(1)ππ*/(1)πσ* CI facilitates ultrafast nonadiabatic N-H bond fission through a (1)πσ*/S(0) CI in <1 ps, a notion supported by CASSCF results. For excitation to the higher lying 2(1)ππ* state, calculations reveal a near barrierless pathway for CI coupling between the 2(1)ππ* and 1(1)ππ* states, enabling the excited-state population to evolve through a rapid sequential 2(1)ππ* → 1(1)ππ* → (1)πσ* → N-H fission mechanism, which we observe to take place in 155 ± 30 fs at 240 nm. We also postulate that an analogous cascade of CI couplings facilitates N-H bond scission along the (1)πσ* state in 170 ± 20 fs, following 200 nm (6.21 eV) excitation to the 3(1)ππ* surface. Particularly illuminating is the fact that a number of the CASSCF calculated CI geometries in aniline bear an exceptional resemblance with previously calculated CIs and potential energy profiles along the amino moiety in guanine, strongly suggesting that the results here may act as an excellent grounding for better understanding (1)πσ* driven dynamics in this ubiquitous genetic building block.

Two-photon excited fluorescence enhancement with broadband versus tunable femtosecond laser pulse excitation

The inverse relationship between two-photon excited fluorescence (TPEF) and laser pulse duration suggests that two-photon microscopy (TPM) performance may be improved by decreasing pulse duration. However, for ultrashort pulses of sub-10 femtosecond (fs) in duration, its spectrum contains the effective gain bandwidth of Ti:Sapphire and its central wavelength is no longer tunable. An experimental study was performed to explore this apparent tradeoff between untuned sub-10 fs transform-limited pulse (TLP) and tunable 140 fs pulse for TPEF. Enhancement factors of 1.6, 6.7, and 5.2 are measured for Indo-1, FITC, and TRITC excited by sub-10 fs TLP compared with 140 fs pulse tuned to the two-photon excitation (TPE) maxima at 730 nm, 800 nm, and 840 nm, respectively. Both degenerate (v(1) = v(2)) and nondegenerate (v(1) ≠ v(2)) mixing of sub-10 fs TLP spectral components result in its broad second-harmonic (SH) power spectrum and high spectral density, which can effectively compensate for the lack of central wavelength tuning and lead to large overlap with dye TPE spectra for TPEF enhancements. These pulse properties were also exploited for demonstrating its potential applications in multicolor imaging with TPM.

Strong-field effects in Rabi oscillations between a single state and a superposition of states

Rabi oscillations of quantum population are known to occur in two-level systems driven by spectrally narrow laser fields. In this work we study Rabi oscillations induced by shaped broadband femtosecond laser pulses. Due to the broad spectral width of the driving field, the oscillations are initiated between a ground state and a coherent superposition of excited states, or a "wavepacket", rather than a single excited state. Our experiments reveal an intricate dependence of the wavepacket phase on the intensity of laser field. We confirm numerically that the effect is associated with the strong-field nature of the interaction, and provide a qualitative picture by invoking a simple theoretical model.

Two-Wave Mixing Amplification of Femtosecond Laser Pulses at 800 nm in Rh:BaTiO3Photorefractive Crystal

We report the photorefractive (PR) two-wave mixing amplification (TWMA) of broadband femtosecond laser pulses in a Rh:BaTiO3 crystal by one-photon absorption. Two-wave mixing (TWM) is performed in the crystal using 30 fs laser pulses at an 800 nm wavelength. A maximum two-wave mixing amplification gain coefficient of ∼11/cm was achieved using a chirped volume grating written at a grating spacing of 5 µm. The broadband spectrum and the original phase characteristics of the signal pulse are also maintained. Beam fanning for the femtosecond pulse laser is less pronounced than that for the cw laser, facilitating a higher two-wave mixing gain for femtosecond pulse lasers.

Two-Wave Mixing Amplification of Femtosecond Laser Pulses at 800 nm in Rh:BaTiO3 Photorefractive Crystal

We report the photorefractive (PR) two-wave mixing amplification (TWMA) of broadband femtosecond laser pulses in a Rh:BaTiO3 crystal by one-photon absorption. Two-wave mixing (TWM) is performed in the crystal using 30 fs laser pulses at an 800 nm wavelength. A maximum two-wave mixing amplification gain coefficient of ∼11/cm was achieved using a chirped volume grating written at a grating spacing of 5 µm. The broadband spectrum and the original phase characteristics of the signal pulse are also maintained. Beam fanning for the femtosecond pulse laser is less pronounced than that for the cw laser, facilitating a higher two-wave mixing gain for femtosecond pulse lasers.

Influence of a half-wave plate–polarizer attenuator on broadband femtosecond laser pulses

Spectral and temporal influence of a conventional half-wave plate–polarizer attenuator on broadband 50-fs laser pulses is experimentally investigated. Strong spectral modulation, pulse narrowing, and pulse shape variation of the transmitted femtosecond laser beam are recorded at maximum attenuation together with substantial depolarization effect. A simplified theoretical model is developed to take account of the wavelength dependence of the wave plate, which provides satisfactory explanations to the experimental observations.

Harmonic generation in the discrete spectral region of xenon using broadband femtosecond laser pulses

The conversion of 34-fs Ti:sapphire laser pulses into the wavelength region 105–210 nm has been studied in xenon for laser intensities up to 5×1013 W/cm2. A strongly structured, pressure-dependent emission spectrum is observed. Radiation is detected in regions expected for the 5th and 7th harmonics but also in regions in between. In the resonance region (λ<147 nm), self-phase-modulation processes in the conversion medium together with phase-matched, efficient harmonic generation in negative-dispersive spectral regions explain the observations. Broadband emission is detected in the resonance-free, positive-dispersive spectral region 155–210 nm. Emission from the xenon dimer is superimposed on a ‘direct’ 5th-harmonic signal.

Mode-focusing in molecules by feedback-controlled shaping of femtosecond laser pulses

The feasibility of mode-selective excitation with broadband femtosecond laser pulses is demonstrated for toluene in liquid phase. A learning-loop optimal control scheme was applied to a stimulated Raman excitation process. Modifications of the phase shape of one of the exciting pulses resulted in dramatic changes of the mode distribution reflected in coherent anti-Stokes Raman spectra. An evolutionary algorithm guided the coherent excitation process to a selective enhancement or suppression of one or more vibrational modes over the complete coherence lifetime spanning several picoseconds. New ways of spectral filtering as well as exciting possibilities of mode-selective studying of chemical reaction dynamics are indicated.

Spectral filtering of femtosecond laser pulses by interference filters

Phase properties of optical thin film interference filters are discussed from the aspect of their usage for phase-error free wavelength separation of broadband femtosecond laser pulses. It is shown that both transmissive or reflective interference filters with high contrast ratios exhibit high cubic phase shifts on transmission or reflection, respectively, causing intolerable distortion in the temporal pulse shape. We show, however, that high efficiency wavelength separation of broadband femtosecond laser pulses can be achieved by using low contrast, properly designed reflective optical interference filters directly built into the cavity of the broad spectrum, femtosecond pulse lasers or parametric oscillators. For demonstrative purposes, we implemented the idea for a Kerr-lens mode-locked Ti:sapphire laser, and obtained two-color, inherently synchronized, unchirped, femtosecond pulse outputs from a single laser oscillator.