Femtosecond Optical Pulses Research Articles

Magnetization dynamics in epitaxial films induced by femtosecond optical pulses near the absorption edge

Peculiarities of the magnetization dynamics induced in iron garnet films by laser pulses with a frequency detuning near the absorption edge have been studied experimentally. It has been found that the dependence of the observed signal amplitude on the pumping energy becomes nonmonotonic with an increase in the pumping frequency. At the same time, the pumping energy corresponding to the maximum amplitude of the signal and this maximum signal amplitude decrease. Moreover, the signal amplitude starts to decrease with an increase in the pumping energy at frequencies within the absorption band. The observed phenomena are possibly caused by generation of magnetostatic spin waves and the effect of ultrafast optically induced demagnetization.

Conservation laws and rogue waves for a higher-order nonlinear Schrödinger equation with variable coefficients in the inhomogeneous fiber

Subpicosecond or femtosecond optical pulse propagation in the inhomogeneous fiber can be described by a higher-order nonlinear Schrödinger equation with variable coefficients, which is investigated in the paper. Via the Ablowitz-Kaup-Newell-Segur system and symbolic computation, the Lax pair and infinitely-many conservation laws are deduced. Based on the Lax pair and a modified Darboux transformation technique, the first- and second-order rogue wave solutions are constructed. Effects of the groupvelocity dispersion and third-order dispersion on the properties of the first- and second-order rouge waves are graphically presented and analyzed: The groupvelocity dispersion and third-order dispersion both affect the ranges and shapes of the first- and second-order rogue waves: The third-order dispersion can produce a skew angle of the first-order rogue wave and the skew angle rotates counterclockwise with the increase of the groupvelocity dispersion, when the groupvelocity dispersion and third-order dispersion are chosen as the constants; When the groupvelocity dispersion and third-order dispersion are taken as the functions of the propagation distance, the linear, X-shaped and parabolic trajectories of the rogue waves are obtained.

Dynamical Stability Limit for the Charge Density Wave in K_{0.3}MoO_{3}.

We study the response of the one-dimensional charge density wave in K_{0.3}MoO_{3} to different types of excitation with femtosecond optical pulses. We compare direct excitation of the lattice at midinfrared frequencies with injection of quasiparticles across the low energy charge density wave gap and with charge transfer excitation in the near infrared. For all three cases, we observe a fluence threshold above which the amplitude-mode oscillation frequency is softened and the mode becomes increasingly damped. We show that all the data can be collapsed onto a universal curve in which the melting of the charge density wave occurs abruptly at a critical lattice excursion. These data highlight the existence of a universal stability limit for a charge density wave, reminiscent of the Lindemann criterion for the melting of a crystal lattice.

Generation of femtosecond optical vortex pulse in fiber based on an acoustically induced fiber grating.

We proposed a method for generation of a femtosecond optical vortex pulse in a two-mode fiber based on an acoustically induced fiber grating (AIFG) driven by a radio frequency source. Theoretical analysis and experimental results demonstrated that the left- and right-handed circular polarization fundamental modes of the femtosecond optical pulse could be converted to the linearly polarized ±1-order optical vortex modes through the AIFG with the mode conversion efficiency of ∼95%. The off-axial interference experiment and the polarization angle-dependent intensity examination were performed to verify the topological charge and the polarization state of the femtosecond optical vortex, respectively.

Ultrafast Demagnetization After Femtosecond Laser Pulses: Transfer of Angular Momentum from the Electronic System to Magnetoelastic Spin-Phonon Modes

When exciting a ferromagnetic film with a femtosecond optical laser pulse, then there is a partial demagnetization within a few hundred fs. This means that the angular momentum of the electronic system is transferred to the lattice. This transfer is calculated by describing the lattice degrees of freedom as spin-phonon eigenmodes which have a sharp angular momentum. Possible other lattice degrees of freedom are mentioned but not considered explicitly. For Ni and Fe the calculated amount of transfer of angular momentum to the spin-phonon eigenmodes is rather small.

Polarization multiplexing in large-mode-area waveguides and its application to signal enhancement in multiphoton microscopy

Soliton self-frequency shift (SSFS) is an efficient technique of generating broadband tunable femtosecond optical pulses. By using large-mode-area (LMA) waveguides such as photonic-crystal (PC) rods or LMA fibers, SSFS is capable of generating solitons with tens of or even >100 nJ pulse energy, enabling deep-tissue multiphoton microscopy (MPM) with the unprecedented imaging depth. MPM signals are proportional to the repetition rate of the laser. Here, we demonstrate an efficient technique of enhancing MPM signals in LMA waveguides, through polarization multiplexing, in both a PC rod with no polarization-maintaining (PM) structure and a PM LMA fiber. The collinear output soliton pulses with orthogonal linear polarizations show similar pulse energy, pulse width, and spectrum. We also demonstrate the application of this polarization multiplexing technique to MPM signal enhancement in biological tissues. Compared with single-polarization soliton excitation, excitation with polarization-multiplexed solitons can efficiently boost MPM signals in different modalities of MPM.

Self-group-velocity modulation and AM-to-PM conversion in optical rectification

Optical rectification as a detection technique does not suffer from the nonlinearity, saturation and large amplitude-to-phase modulation (AM-to-PM) conversion effects exhibited by semiconductor carrier-based photodiodes. In this paper we analyze the effect of self-group-velocity modulation as a mechanism for AM-to-PM conversion in traveling-wave optical rectification detectors (ORDs) and derive a closed-form expression for the AM-to-PM conversion gain. For a small signal amplitude modulation index m, we find the induced phase modulation index βi on the nonlinear polarization wave due to a train of femtosecond optical pulses in a bulk LiNbO3 traveling-wave ORD to be βi<3×10−8 m or βi/m≈−151 dB. Cascaded χ(2) processes as well as the thermo-optic effect are also considered with the former producing conversion gain comparable to direct χ(3) while the latter is four orders of magnitude smaller.

Many Exact Solutions for a Higher-Order Nonlinear Schrödinger Equation with Non-Kerr Terms Describing the Propagation of Femtosecond Optical Pulses in Nonlinear Optical Fibers

In this article, we apply two powerful methods, namely the first integral method and a direct algebraic method for constructing many exact solutions for the higher-order nonlinear Schrodinger equation with non-Kerr terms that describes the propagation of femtosecond optical pulses in nonlinear optical fibers. Using a simple transformation, we reduce the given equation to a nonlinear ordinary differential equation (ODE). Various solutions of the resulting nonlinear ODE are obtained by using the above two methods. A comparison between our recent results and the well-known results is given.

Generation of intense femtosecond optical vortex pulses with blazed-phase grating in chirped-pulse amplification system of Ti:sapphire laser

We demonstrate the generation of an intense femtosecond optical vortex (OV) pulse by employing an OV converter set between two laser amplifiers in a chirped-pulse amplification (CPA) system of a Ti:sapphire laser. The OV converter is composed of a liquid-crystal spatial light modulator (LC-SLM) exhibiting a blazed-phase computer-generated hologram, a concave mirror, and a flat mirror in the 4f setup. Owing to the intrinsic nature of the 4f setup, the OV converter is free from chromatic and topological-charge dispersions, which are always induced in a spiral phase plate conventionally used to convert an intense Gaussian laser pulse to an OV pulse, while we can avoid damage to the LC-SLM by the irradiation of a low-energy pulse before the second amplifier. We have increased the throughput of the OV converter to 42% by systematically investigating the diffraction efficiency of the blazed-phase hologram on the LC-SLM, which relaxes the gain condition required for the second amplifier. The combination of the high-throughput OV converter and the two-stage amplification enables us to generate OV pulses with an energy of 1.63 mJ and a pulse duration of 60 fs at a wavelength of ~720 nm, at which the gain of the Ti:sapphire laser is only 60% of the peak gain around 800 nm.

Free-electron mechanisms of low-frequency radiation generation on metal surfaces.

We present a time-domain solution of the equations describing low-frequency radiation generation when a metal surface is irradiated by a non-damaging femtosecond optical pulse of s-polarized radiation. Ponderomotive force, drag current, and temperature gradient along the surface are found to be sources of the terahertz radiation. A comparison of these sources under typical experimental conditions reveals that both the drag current and the thermal gradient give similar contribution to the terahertz signal. The ponderomotive force effect is several times less than these two and exists only during the incident pulse.

A comprehensive model of gain recovery due to unipolar electron transport after a short optical pulse in quantum cascade lasers

We have developed a comprehensive model of gain recovery due to unipolar electron transport after a short optical pulse in quantum cascade lasers (QCLs) that takes into account all the participating energy levels, including the continuum, in a device. This work takes into account the incoherent scattering of electrons from one energy level to another and quantum coherent tunneling from an injector level to an active region level or vice versa. In contrast to the prior work that only considered transitions to and from a limited number of bound levels, this work include transitions between all bound levels and between the bound energy levels and the continuum. We simulated an experiment of S. Liu et al., in which 438-pJ femtosecond optical pulses at the device's lasing wavelength were injected into an In0.653Ga0.348As/In0.310Al0.690As QCL structure; we found that approximately 1% of the electrons in the bound energy levels will be excited into the continuum by a pulse and that the probability that these electrons will be scattered back into bound energy levels is negligible, ∼10−4. The gain recovery that is predicted is not consistent with the experiments, indicating that one or more phenomena besides unipolar electron transport in response to a short optical pulse play an important role in the observed gain recovery.

Generation of a femtosecond vacuum ultraviolet optical pulse by four-wave Raman mixing

Generation of a femtosecond vacuum ultraviolet optical pulse by four-wave Raman mixing

Laser-initiated magnetization reversal and correlated morphological effects visualized within situFresnel transmission electron microscopy

Laser-initiated switching of magnetization direction in ferrimagnetic rare-earth--transition-metal (RE-TM) alloys---whether laser induced or photothermal via compensation point---is being vigorously pursued owing to the promise of extending operating frequencies of magnetic devices into the terahertz regime. Despite intense interest, however, the effects of repeated laser exposure on the film structure and subsequent switching behavior have yet to be investigated. In order to better understand the correlated effects of femtosecond-laser irradiation on both the magnetic response and photoinduced morphological variations of RE-TM alloys, we performed in situ Fresnel transmission electron microscopy (TEM) on $\mathrm{T}{\mathrm{b}}_{23}\mathrm{C}{\mathrm{o}}_{77}$ thin films with Ta protecting layers. Via optical access to the specimen in a modified TEM, we irradiated the thin films in situ with both individual and series of femtosecond optical pulses, and correlated laser-induced changes in magnetic domain-wall formation and growth with photothermal crystal formation and accompanying pinned magnetic sites. We find that, for a range of applied laser fluences and numbers of individual pulses, several distinct regions are formed displaying varied magnetic behavior (switchable, nonswitchable, demagnetized) and morphological features (small-to-large crystal-grain variations). Through a series of systematic studies, we quantified these linked magnetic and morphological properties as a function of laser fluence, number of pulse-train cycles, and number of individual femtosecond-laser pulses and the duration between each. Our results show how the sensitive connection between magnetic behavior and morphological structure can emerge in magneto-optic experiments across several parameters, thus illustrating the need for rigorous characterization so that potential operating regimes may be universally identified.

High power multi-color OPCPA source with simultaneous femtosecond deep-UV to mid-IR outputs.

Many experimental investigations demand synchronized pulses at various wavelengths, ideally with very short pulse duration and high repetition rate. Here we describe a femtosecond multi-color optical parametric chirped pulse amplifier (OPCPA) with simultaneous outputs from the deep-UV to the mid-IR with optical synchronization. The high repetition rate of 160 kHz is well suited to compensate for low interaction probability or low cross section in strong-field interactions. Our source features high peak powers in the tens to hundreds of MW regime with pulse durations below 110 fs, which is ideal for pump-probe experiments of nonlinear and strong-field physics. We demonstrate its utility by strong-field ionization experiments of xenon in the near- to mid-IR.

Femtosecond wavelength-tunable OPCPA system based on picosecond fiber laser seed and picosecond DPSS laser pump.

We present a compact and stable femtosecond wavelength-tunable optical parametric chirped pulse amplification (OPCPA) system. A novel OPCPA front-end was constructed using a multi-channel picosecond all-in-fiber source for seeding DPSS pump laser and white light supercontinuum generation. Broadband chirped pulses were parametrically amplified up to 1 mJ energy and compressed to less than 40 fs duration. Pulse wavelength tunability in the range from 680 nm to 930 nm was experimentally demonstrated.

Conduction band population in graphene in ultrashort strong laser field: Case of massive Dirac particles

The Dirac-like quasiparticles in honeycomb graphene lattice are taken to possess a non-zero effective mass. The charge carriers involve to interact with a femtosecond strong laser pulse. Due to the scattering time of electrons in graphene ([Formula: see text]10–100 fs), the one femtosecond optical pulse is used to establish the coherence effect and, consequently, it can be realized to use the time-dependent Schrödinger equation for electron coupled with strong electromagnetic field. Generalized wave vector of relativistic electrons interacting with electric field of laser pulse causes to obtain a time-dependent electric dipole matrix element. Using the coupled differential equations of a two-state system of graphene, the density of probability of population transition between valence (VB) and conduction bands (CB) of gapped graphene is calculated. In particular, the effect of bandgap energy on dipole matrix elements at the Dirac points and resulting CB population (CBP) is investigated. The irreversible electron dynamics is achieved when the optical pulse end. Increasing the energy gap of graphene results in decreasing the maximum CBP.

Solitary wave solutions of the cubic-quintic-septic nonlinear Schrödinger equation in fiber Bragg gratings

We consider the nonlinear coupled mode equations with competing cubic-quintic-septic nonlinearities, describing propagation of femtosecond optical pulses through fiber Bragg grating structure. By means of the multiple scale analysis, the model is reduced into the perturbed nonlinear Schrödinger equation with non-Kerr terms and higher-order dispersions. The complex envelope function ansatz of Li is adopted to investigate general analytic solitary wave solutions and derive explicit bright and dark solitons for the resulting nonlinear Schrödinger type equation. The physical parameters of the shape preserving solutions such as power and pulse width are calculated in terms of the dependent model coefficients. The constraint conditions for the existence of exact solutions are presented.

Saturable absorption of femtosecond optical pulses in multilayer turbostratic graphene.

We investigate the nonlinear transmission of a ~280-layer turbostratic graphene sheet for near-infrared amplifier laser pulses (775 nm, Ti:sapphire laser) with a duration of 150-fs and 20-fs. Saturable absorption is observed in both cases, however it is not very strong, amounting to ~13% transmittance change for the 20-fs (150-fs) pulses at a peak intensity of 30 GW/cm2 (4 GW/cm2). The dependence on incident peak intensity is reproduced well using a theoretical model for the time-dependent saturable absorption, where the excited carriers vacate the photo-excited energy range within 3-5 fs, which we attribute to energy redistribution due to carrier-carrier scattering. This is also supported by spectrally resolved measurements for the 20-fs pulses, which show a marked dependence of the degree of saturation on the photon energy. A key result is that the shorter pulses do not yield a lower saturation fluence, due to the combined effects of the broader excitation bandwidth, and the rapid and broad energy redistribution. We also predict the potential performance of multilayer graphene samples for removing pedestal and pre-pulse structure from ultrafast high-energy pulses.

III–V Semiconductor Nanoresonators—A New Strategy for Passive, Active, and Nonlinear All‐Dielectric Metamaterials

We demonstrate 2D and multilayer dielectric metamaterials made from III–V semiconductors using a monolithic fabrication process. The resulting structures could be used to recompress chirped femtosecond optical pulses and in a variety of other optical applications requiring low loss. Moreover, these III–V all-dielectric metamaterials could enable novel active applications such as efficient nonlinear frequency converters, light emitters, detectors, and modulators.

New optical gating technique for detection of electric field waveforms with subpicosecond resolution.

The new optical gating technique uses a femtosecond optical laser pulses for the photoconductive detection of short pulses of terahertz (THz) radiation. This technique reproduces the shape of the THz pulse and after pulse plasmonic response of the two-dimensional electron gas in a short channel high electron mobility transistor (HEMT). The results are in excellent agreement with the electro-optic effect measurements and with the simulation results obtained in the frame of a two-dimensional hydrodynamic model. The femtosecond optical laser pulse time is delayed with respect to the THz pulse and generates a large concentration of the electron-hole pairs in the AlGaAs/InGaAs HEMT. This drastically increases the channel conductivity on the femtosecond scale and effectively shorts the device quenching the transistor response. The achieved time resolution is better than 250 femtoseconds and could be improved using shorter femtosecond laser pulses. The spatial resolution of this technique is on the order of tens of nanometers or even smaller. It could be applied for studying the electron transport in a variety of electronic devices ranging from silicon MOSFETs to heterostructure bipolar transistors.