Optical pulse compression to 5 fs at a 1-MHz repetition rate

Passive magnetic-free broadband optical isolator based on unidirectional self-induced transparency

Achieving a broadband nonreciprocal device without gain and any external bias is very challenging and highly desirable for modern photonic technologies and quantum networks. Here we theoretically propose a passive and magnetic-free all-optical isolator for a femtosecond laser pulse by exploiting a new mechanism of unidirectional self-induced transparency, obtained with a nonlinear medium followed by a normal absorbing medium at one side. The transmission contrast between the forward and backward directions can reach 14.3 dB for a 2π - 5 fs laser pulse. The 20 dB bandwidth is about 56 nm, already comparable with a magneto-optical isolator. This work provides a new mechanism which may benefit non-magnetic isolation of ultrashort laser pulses.

Recent advances in ultrafast plasmonics: from strong field physics to ultraprecision spectroscopy

Surface plasmons, the collective oscillation of electrons, enable the manipulation of optical fields with unprecedented spatial and time resolutions. They are the workhorse of a large set of applications, such as chemical/biological sensors or Raman scattering spectroscopy, to name only a few. In particular, the ultrafast optical response configures one of the most fundamental characteristics of surface plasmons. Thus, the rich physics about photon-electron interactions could be retrieved and studied in detail. The associated plasmon-enhanced electric fields, generated by focusing the surface plasmons far beyond the diffraction limit, allow reaching the strong field regime with relatively low input laser intensities. This is in clear contrast to conventional optical methods, where their intrinsic limitations demand the use of large and costly laser amplifiers, to attain high electric fields, able to manipulate the electron dynamics in the non-linear regime. Moreover, the coherent plasmonic field excited by the optical field inherits an ultrahigh precision that could be properly exploited in, for instance, ultraprecision spectroscopy. In this review, we summarize the research achievements and developments in ultrafast plasmonics over the last decade. We particularly emphasize the strong-field physics aspects and the ultraprecision spectroscopy using optical frequency combs.

Modulation instability in nonlinear metamaterials modeled by a cubic-quintic complex Ginzburg-Landau equation beyond the slowly varying envelope approximation

Considering the theory of electromagnetic waves from the Maxwell's equations, we introduce a (3+1)-dimensionsal cubic-quintic complex Ginzburg-Landau equation describing the dynamics of dissipative light bullets in nonlinear metamaterials. The model equation, which is derived beyond the slowly varying envelope approximation, includes the effects of diffraction, dispersion, loss, gain, cubic, and quintic nonlinearities, as well as cubic and quintic self-steepening effects. The modulational instability of the plane waves is studied both theoretically, using the linear stability analysis, and numerically, using direct simulations of the Fourier space of the proposed nonlinear wave equation, based on the Drude model. The linear theory predicts instability for any amplitude of the primary wave. Also, in the linear stability analysis, self-steepening effects of different orders are confronted and one discusses their effects on the behavior of the gain spectrum under both normal and anomalous group-velocity dispersion regimes. Analytical results are equally confronted to direct numerical simulations and fully agree with the predictions from the gain spectra. Modulational instability is manifested by clusters of solitons and multihump and dromion-like structures, whose emergence and features depend not only on system parameters, such as the cubic and quintic self-steepening coefficients, but also on the propagation distance under a suitable balance between nonlinear and dispersive effects.

Generation of 3.9-cycle pulses from the coherent synthesis of two continuous-wave injection seeded optical parametric amplifiers at 53 MHz

A high repetition-rate, few-cycle light pulse is of great importance due to its potential for a variety of applications, including two-dimensional infrared spectroscopy and time-resolved imaging of molecular structures, which benefit from its ultrabroadband spectrum and ultrashort pulse duration. The generation of an ultrabroadband coherent spectrum is one of the frontiers of ultrafast optics, and accessing such few-cycle pulses is presently under active exploration. Here, we demonstrate a simple yet effective pulse synthesizer. It is based on two continuous-wave (cw) injection-seeded high-repetition-rate optical parametric amplification systems and the following self-phase-modulation dominated spectra-broadening processes. The combined spectrum spans from 1250 to 1670 nm, and a near Fourier-transform-limited 3.9-cycle (19.2 fs) synthesized pulse with a central wavelength of 1470 nm is obtained accordingly.

Generation of 30 fs pulses from a diode-pumped graphene mode-locked Yb:CaYAlO4 laser

Stable 30 fs pulses centered at 1068 nm (less than 10 optical cycles) are demonstrated in a diode pumped Yb:CaYAlO4 laser by using high-quality chemical vapor deposited monolayer graphene as the saturable absorber. The mode-locked 8.43 optical-cycle pulses have a spectral bandwidth of ∼50  nm and a pulse repetition frequency of ∼113.5  MHz. To the best of our knowledge, this is the shortest pulse ever reported for graphene mode-locked lasers and mode-locked Yb-doped bulk lasers. Our experimental results demonstrate that graphene mode locking is a very promising practical technique for directly generating few-cycle optical pulses from a laser oscillator.

Dependence of Two-Photon eGFP Bleaching on Femtosecond Pulse Spectral Amplitude and Phase

Photobleaching is a key limitation in two-photon imaging of fluorescent proteins with femtosecond pulsed excitation. We present measurements of the dependence of eGFP photobleaching on the spectral amplitude and phase of the pulses used. A strong dependence on the excitation wavelength was confirmed and measured over a 800-950 nm range. A fiber continuum light source and pulse shaping techniques were used to investigate photobleaching with broadband, 15 fs transform limited, pulses with differing spectral amplitude and phase. Narrow band pulses, >150 fs transform limited, typical of femtosecond laser sources used in two-photon imaging applications, were also investigated for their photobleaching dependence on pulse dispersion and bandwidth. The bleach rate for broadband pulses was found to be primarily determined by the second harmonic spectrum of the excitation light. On the other hand, for narrow band excitation pulses with similar center wavelengths improvement in bleach rate was found to be mostly dependent on reducing the pulse length. A simple model to predict the relative bleach rates for broadband pulses is presented and compared to the experimental data.

Doppler effects in the propagation of a few-cycle pulse through a dense medium

This numerical study demonstrates that Doppler redshift exists in the reflected spectrum of a few-cycle pulse, propagating through a dense medium. It manifests itself in two different forms, a sharp low-frequency spike (LFS) located at the red edge of the reflected spectrum and a relatively broader redshift near the carrier frequency. With the variation of the laser and medium parameters, the dominant reflection mechanism changes between bulk generation of backwards propagation waves and nonlinear reflection near the front face. This leads to the manifestation of Doppler effect changing accordingly between the two different forms. This study unifies the physical mechanism behind the LFS and dynamic nonlinear optical skin effect, which enriches the theoretical explanation of the spectral redshift of few-cycle pulse propagation beyond the intrapulse four-wave mixing.

Ultrashort optical waveguide excitations in uniaxial silica fibers: elastic collision scenarios

In this work, we investigate the dynamics of an uniaxial silica fiber under the viewpoint of propagation of ultimately ultrashort optical waveguide channels. As a result, we unveil the existence of three typical kinds of ultrabroadband excitations whose profiles strongly depend upon their angular momenta. Looking forward to surveying their scattering features, we unearth some underlying head-on scenarios of elastic collisions. Accordingly, we address some useful and straightforward applications in nonlinear optics through secured data transmission systems, as well as laser physics and soliton theory with optical soliton dynamics.

Nonlinear polarization dynamics in a weakly birefringent all-normal dispersion photonic crystal fiber: toward a practical coherent fiber supercontinuum laser

Dispersion-flattened dispersion-decreased all-normal dispersion (DFDD-ANDi) photonic crystal fibers have been identified as promising candidates for high-spectral-power coherent supercontinuum (SC) generation. However, the effects of the unintentional birefringence of the fibers on the SC generation have been ignored. This birefringence is widely present in nonlinear non-polarization maintaining fibers with a typical core size of 2 µm, presumably due to the structural symmetry breaks introduced in the fiber drawing process. We find that an intrinsic form-birefringence on the order of 10(-5) profoundly affects the SC generation in a DFDD-ANDi photonic crystal fiber. Conventional simulations based on the scalar generalized nonlinear Schrödinger equation (GNLSE) fail to reproduce the prominent observed features of the SC generation in a short piece (9-cm) of this fiber. However, these features can be qualitatively or semi-quantitatively understood by the coupled GNLSE that takes into account the form-birefringence. The nonlinear polarization effects induced by the birefringence significantly distort the otherwise simple spectrotemporal field of the SC pulses. We therefore propose the fabrication of polarization-maintaining DFDD-ANDi fibers to avoid these adverse effects in pursuing a practical coherent fiber SC laser.

Development of a multiplex fast-scan system for ultrafast time-resolved spectroscopy

A fast-scan method was developed to obtain time-resolved signals with femtosecond resolution over a picosecond range on the fly and in real time. Traditional fast-scan methods collect data at each probe wavelength one by one, which is time consuming and thus not possible for the study of photofragile materials. In this work, we have developed a system that performs fast scans with multiplex detection. Ultrafast time-resolved spectroscopy was demonstrated using the newly developed system. Femtosecond laser pulses have been used for pump-probe studies of ultrafast processes in various materials, and both electronic relaxation and vibrational dynamics have been studied. However, experiments have been limited in sensitivity and reliability because they are affected by the long-term instability of the ultrashort laser pulses and by the fragility of the samples. The instability of the sources hinders precise determination of electronic decay dynamics and introduces systematic errors. The fragility of the samples reduces their amount or concentration, and can lead to contamination of the materials even if they were pure before the measurement. These effects make it difficult to obtain reproducible and reliable experimental data. In the present work, we have developed a fast-scan pump-probe spectroscopic system that can complete a set of measurements in less than 2 min. Quantitative estimates of the signal reproducibility demonstrate that these measurements provide higher reproducibility and reliability than conventional measurements.

Soliton-effect pulse compression in the single-cycle regime in broadband dielectric-coated metallic hollow waveguides

We study soliton-effect pulse compression in the single-cycle region in dielectric-coated metallic hollow waveguides filled with a noble gas exploiting the broad region of anomalous dispersion in these waveguides. We predict the compression of a 20-fs pulse to a FWHM duration of 1.7 fs with an energy of 6 muJ and study the physical factors determining the limitations on shortest pulses in the single-cycle regime.

Sub-two-cycle light pulses at 1.6 microm from an optical parametric amplifier

We generate ultrabroadband pulses, spanning the 1200-2100 nm wavelength range, from an 800 nm pumped optical parametric amplifier (OPA) working at degeneracy. We compress the microjoule-level energy pulses to nearly transform-limited 8.5 fs duration by an adaptive system employing a deformable mirror. To our knowledge, these are the shortest light pulses generated at 1.6 microm.

Evolution of the unidirectional electromagnetic pulses in an anisotropic two-level medium

We find a general integrable system of reduced Maxwell-Bloch equations describing the interaction of a two-component electromagnetic field with the dipole transition of a two-level medium. The anisotropy of the anisotropic dipole momentum of the transition as a permanent dipole momentum is taken into account. A solution of the model is found for a particular case by using the inverse scattering transform. The method is based on a solution of the Riemann-Hilbert problem taking into account the symmetry properties of the corresponding fundamental solutions.

Soliton-effect compression of supercontinuum to few-cycle durations in photonic nanowires

By exploiting the broad region of anomalous group-velocity dispersion (GVD) and the large e.ective nonlinearity of photonic nanowires, we demonstrate soliton-e.ect self-compression of 70-fs pulses down to 6.8 fs. Under suitable conditions, simulations predict that self-compression down to single-cycle duration is possible.

Quasiperiodic Raman technique for ultrashort pulse generation

We report the experimental demonstration of a new Raman technique that produces 200 sidebands, ranging in wavelength from 3 microm to 195 nm. By studying multiphoton ionization of nitric oxide (NO) molecules, we show mutual phase coherence among 15 visible sidebands covering 0.63 octaves of bandwidth.

Tunable optimal compression of ultrabroadband pulses by cross-phase modulation

We show how cross-phase modulation between two pulses, combined with optimal pulse shaping at the input of a dielectric medium, can be used to generate nearly single-cycle pulses that are tunable from the ultraviolet to the mid-infrared at the output of the medium, precompensating for dispersion to all orders.

FAST CARS: engineering a laser spectroscopic technique for rapid identification of bacterial spores

Airborne contaminants, e.g., bacterial spores, are usually analyzed by time-consuming microscopic, chemical, and biological assays. Current research into real-time laser spectroscopic detectors of such contaminants is based on e.g., resonance fluorescence. The present approach derives from recent experiments in which atoms and molecules are prepared by one (or more) coherent laser(s) and probed by another set of lasers. However, generating and using maximally coherent oscillation in macromolecules having an enormous number of degrees of freedom is challenging. In particular, the short dephasing times and rapid internal conversion rates are major obstacles. However, adiabatic fast passage techniques and the ability to generate combs of phase-coherent femtosecond pulses provide tools for the generation and utilization of maximal quantum coherence in large molecules and biopolymers. We call this technique FAST CARS (femtosecond adaptive spectroscopic techniques for coherent anti-Stokes Raman spectroscopy), and the present article proposes and analyses ways in which it could be used to rapidly identify preselected molecules in real time.

Phase modulation of ultrashort light pulses using molecular rotational wave packets

We demonstrate experimentally how the time-dependent phase modulation induced by molecular rotational wave packets can manipulate the phase and spectral content of ultrashort light pulses. Using impulsively excited rotational wave packets in CO2, we increase the bandwidth of a probe pulse by a factor of 9, while inducing a negative chirp. This chirp is removed by propagation through a fused silica window, without the use of a pulse compressor. This is a very general technique for optical phase modulation that can be applied over a broad spectral region from the IR to the UV.

Pulse compression over a 170-THz bandwidth in the visible by use of only chirped mirrors

We report on double-chirped mirrors with custom-tailored dispersion characteristics over a bandwidth of 170 THz in the visible. The mirrors are used in a prismless compressor for a noncollinear optical parametric amplifier in the visible. The compressed pulses, characterized for the what is believed to be first time by use of the spectral phase interferometry for direct electric field reconstruction technique, display a nearly flat phase from 510 to 710 nm and have a duration of 5.7 fs.

Femtosecond light source for phase-controlled multiphoton ionization

We describe a femtosecond Raman light source with more than an octave of optical bandwidth. We use this source to demonstrate phase control of multiphoton ionization under conditions where ionization requires eleven photons of the lowest frequency of the spectrum or five photons of the highest frequency. The nonlinearity of the photoionization process allows us to characterize the light source. Experiment-to-theory comparison implies generation of a near single-cycle waveform.

Evolution of ultrashort light pulses in a two-level medium visualized with the finite-difference time domain technique

The finite-difference time-domain technique is employed to examine the evolution of the amplitude, duration, waveform, and phase of ultrashort light pulses propagating in a medium of two-level atoms or molecules. The results of these numerical simulations agree reasonably well with predictions of the McCall-Hahn analysis for the evolution of the amplitude and the phase of short pulses in a two-level medium until the pulse duration becomes less than the duration of a single optical cycle. Noticeable deviations from the McCall-Hahn scenario were observed for pulses with durations shorter than the duration of a single field cycle.

Generation of one-cycle laser pulses by use of high-amplitude plasma waves

The dynamics of a short laser pulse located in the density trough of a background plasma wave is investigated and a scheme is proposed to compress the pulse duration by use of a high-amplitude plasma wave. The threshold amplitude of the plasma wave, at which the compressing effect just balances the dispersive spreading of the laser pulse, is estimated for certain pulse profiles. Numerical simulations are conducted with particle-in-cell codes, where a pump pulse is used to generate a high-amplitude plasma wave and a signal pulse copropagates behind. It is shown that the signal pulse can be compressed by the plasma wave from ten laser cycles to about one cycle within a millimeter in tenuous plasma only a few percent of the critical density.

Phase relations, quasicontinuous spectra and subfemtosecond pulses in high-order stimulated raman scattering with short-pulse excitation

High-order stimulated Raman scattering for pumping by ps and sub-ps pulses is studied in the frame of a time-domain approach without the slowly varying envelope approximation. Formation of pulse trains with sub-fs durations in the ps-pump regime is demonstrated using external phase compensation. For sub-ps excitation a novel broadening mechanism of Raman lines is predicted that leads to a quasicontinuous spectrum and permits one to generate single sub-fs pulses. We predict also essential shortening of a delayed fs-probe pulse by the Raman oscillations without phase control.

Five-optical-cycle pulse generation in the mid infrared from an optical parametric oscillator based on aperiodically poled lithium niobate

We describe an optical parametric oscillator based on aperiodically poled lithium niobate that generates nearly transform-limited 53-fs duration pulses at a center wavelength of 3mum, corresponding to only 5 optical cycles. Results are presented illustrating the effect of pump- and grating-period chirp on the idler pulses, and a configuration capable of producing idler bandwidths in excess of 700 nm is discussed.

Characterization of sub-10-fs pulse focusing with high-numerical-aperture microscope objectives

Dispersion precompensation with a prism sequence and a third-order dispersion mirror resulted in negligible broadening of sub-10-fs pulses at subwavelength spot sizes when the pulses were focused with microscope objectives and moderate apertures. At larger apertures, lens chromaticity and spherical aberration led to an effective pulse broadening of up to 1.3x , depending on the aperture size and the detector position. The data suggest that intensities exceeding 10(14) W/cm(2) can be produced directly from femtosecond pulse oscillators.

Sub-two-cycle pulses from a Kerr-lens mode-locked Ti:sapphire laser

Pulses shorter than two optical cycles with bandwidths in excess of 400 nm have been generated from a Kerr-lens mode-locked Ti:sapphire laser with a repetition rate of 90 MHz and an average power of 200 mW. Low-dispersion prisms and double-chirped mirrors provide broadband controlled dispersion and high reflectivity. These pulse durations are to our knowledge the shortest ever generated directly from a laser oscillator.

Amplitude and chirp characterization of high-power laser pulses in the 5-fs regime

Frequency-resolved optical gating (FROG) based on second-harmonic generation has been demonstrated to be capable of high-fidelity measurement of the electric-field envelope and of the temporal evolution of the instantaneous carrier frequency of 0.1-TW 5-fs pulses without the need for any correction for systematic experimental errors. At a 1-kHz repetition rate, pulse energies of a few microjoules are sufficient for reliable FROG characterization of pulses with durations down to the single-cycle regime. The results obtained reveal that carefully designed hollow-fiber chirped-mirror compressors are able to deliver high-power sub-10-fs pulses with a smooth Gaussianlike leading edge that has an intensity contrast of approximately 10(-2) .

Generation of intense 8-fs pulses at 400 nm

Frequency-doubled pulses from a sub-40-fs, 1-kHz Ti:sapphire amplifier system are spectrally broadened in an argon-filled hollow waveguide. Compression of the self-phase-modulated pulses is implemented with chirped mirrors and a prism pair, yielding 8-fs, 15-muJ pulses in the violet spectral range.

ULTRAFAST SOLVATION DYNAMICS EXPLORED BY FEMTOSECOND PHOTON ECHO SPECTROSCOPIES

Chemical reaction and optical dynamics in the liquid phase are strongly affected by specific solute-solvent interactions. The dynamical part of this coupling leads to energy fluctuations. The associated energy gap dynamics can be probed by using various nonlinear optical spectroscopies. We discuss various forms of photon echo--time-integrated, time-gated, and heterodyne-detected photon echo--as well as Fourier transform spectral interferometry. It is shown that for solutions of the dye molecule DTTCI, a system-bath correlation function can be acquired that provides a quantitative description of all (non)linear spectroscopic experiments. The deduced correlation function is projected onto the multimode Brownian oscillator model, which allows for a physical interpretation of the multiple-time correlation function and a determination of the spectral density relevant to the solvation process. The following applications of photon echo to condensed phase dynamics are discussed: enhanced vibrational mode suppression, Liouville pathways interference, and dynamical Stokes shift. Recent results of echo-peak shift experiments on the hydrated electron are also presented. The review concludes that photon echo should be useful as a novel tool to explore transition state dynamics.

Amplitude and phase characterization of 4.5-fs pulses by frequency-resolved optical gating

The technique of second-harmonic generation frequency-resolved optical gating is applied to measure the intensity and the phase of 4.5-fs pulses resulting from the fiber-compressed output of a cavity-dumped Ti:sapphire laser. Characterization of even shorter optical pulses by this method should also be feasible.

Sub-8-fs pulses from an ultrabroadband optical parametric amplifier in the visible

Pulses with 180-THz bandwidth and 2-microJ energy were generated by a noncollinear optical parametric amplifier in the visible, pumped by the second harmonic of a Ti:sapphire laser. A portion of the amplified pulse spectrum was compressed to 7.2 fs by use of a thin prism sequence.

Continuous-wave mode-locked Ti:sapphire laser focusable to 5 x 10(13) W/cm(2)

Generation of sub-10-fs pulses with an average power of 1 W and a peak of 1.5 MW from a Kerr-lens mode-locked mirror-dispersion-controlled Ti:sapphire laser is demonstrated. A specially designed lens triplet focuses the output of this compact all-solid-state source to a peak intensity in excess of 5x10(13) W/cm (2) . Nonperturbative nonlinear optics is now becoming feasible by use of the output of a cw mode-locked laser.