IR Femtosecond Pulses Research Articles

Defect-assisted optical breakdown in synthetic diamonds irradiated by IR femtosecond pulses

A comparative study of optical breakdown in several CVD and HPHT single crystals has been performed with IR femtosecond pulses, which are widely used for diamond bulk processing. The breakdown thresholds for the diamond surface and bulk are measured and then compared with the morphology of the laser-modified regions formed during the breakdown process. The highest level of the bulk breakdown threshold is found to be typical for the HPHT crystals, whereas for the CVD crystals breakdown threshold varies greatly. It is assumed that certain defects assembled in microbunches of various sizes may decrease considerably the diamond breakdown threshold. The nature of these defects and the mechanism of the threshold reduction are under study.

Sub-Threshold Fabrication of Laser-Induced Periodic Surface Structures on Diamond-like Nanocomposite Films with IR Femtosecond Pulses.

In the paper, we study the formation of laser-induced periodic surface structures (LIPSS) on diamond-like nanocomposite (DLN) a-C:H:Si:O films during nanoscale ablation processing at low fluences—below the single-pulse graphitization and spallation thresholds—using an IR fs-laser (wavelength 1030 nm, pulse duration 320 fs, pulse repetition rate 100 kHz, scanning beam velocity 0.04–0.08 m/s). The studies are focused on microscopic analysis of the nanostructured DLN film surface at different stages of LIPSS formation and numerical modeling of surface plasmon polaritons in a thin graphitized surface layer. Important findings are concerned with (i) sub-threshold fabrication of high spatial frequency LIPSS (HSFL) and low spatial frequency LIPSS (LSFL) under negligible surface graphitization of hard DLN films, (ii) transition from the HSFL (periods of 140 ± 30 and 230 ± 40 nm) to LSFL (period of 830–900 nm) within a narrow fluence range of 0.21–0.32 J/cm2, (iii) visualization of equi-field lines by ablated nanoparticles at an initial stage of the LIPSS formation, providing proof of larger electric fields in the valleys and weaker fields at the ridges of a growing surface grating, (iv) influence of the thickness of a laser-excited glassy carbon (GC) layer on the period of surface plasmon polaritons excited in a three-layer system “air/GC layer/DLN film”.

Ultrafast intraband Auger process in self-doped colloidal quantum dots

Summary Investigating the separate dynamics of electrons and holes has been challenging, although it is critical for the fundamental understanding of semiconducting nanomaterials. n-Type self-doped colloidal quantum dots (CQDs) with excess electrons occupying the low-lying state in the conduction band (CB) have attracted a great deal of attention because of not only their potential applications to infrared optoelectronics but also their intrinsic system that offers a platform for investigating electron dynamics without elusive contributions from holes in the valence band. Here, we show an unprecedented ultrafast intraband Auger process, electron relaxation between spin-orbit coupling states, and exciton-to-ligand vibrational energy transfer process that all occur exclusively in the CB of the self-doped β-HgS CQDs. The electron dynamics obtained by femtosecond mid-infrared spectroscopy will pave the way for further understanding of the blinking phenomenon, disproportionate charging in light-emitting diodes, and hot electron dynamics in higher quantum states coupled to surface states of CQDs.

Generation of high-power few-cycle femtosecond IR pulses by double-chirp parametric amplification

We present an energy scalable configuration for the generation of high-power few-cycle femtosecond IR pulses based on the sequential parametric amplification of chirped radiation pulses at the idler (0.86 μm) and signal (1.39 μm) wavelengths from the multi-terawatt femtosecond laser system made at the Institute of Laser Physics, Siberian Branch of the Russian Academy of Sciences (ILP SB RAS). The configuration under consideration involves the sequential production of radiation pulses with centre wavelengths of 2.24, 3.56, and 7.25 μm. The conditions are determined for the generation of few-cycle pulses with a terawatt peak power in the 3.56 μm (15.8 fs, 1.5 cycles) and 7.25 μm (17.2 fs, shorter than one cycle) domains. The phase transfer between signal and idler waves in the double-chirp parametric amplification is investigated for the first time.

Femtosecond-pulse inscribed FBGs for mode selection in multimode fiber lasers

Abstract Reflection properties of a fiber Bragg grating (FBG) inscribed in a multimode fiber strongly depend on the transverse profile of FBG. Point-by-point FBG fabrication by IR femtosecond pulses with highly nonlinear refraction index variation as a function of intensity within the focal spot offers an opportunity to tailor the FBG transverse profile with high spatial resolution. In this paper we review our recent results on the femtosecond inscription of FBGs in multimode fibers, study of their spectral and mode-selective properties as well as applications for mode selection in multimode and multicore fiber lasers.

Infrared-laser-pulse-enhanced ultrafast fragmentation of N22+ following Auger decay: Mixed quantum-classical simulations

We employ mixed quantum-classical molecular dynamics simulations to investigate the fragmentation of ${\mathrm{N}}_{2}$ molecules after core-level photoionization by an x-ray laser, subsequent Auger decay, and followed by a femtosecond IR pulse that interacts with ${{\mathrm{N}}_{2}}^{2+}$. The delayed IR pulse couples the dissociative electronic states of ${{\mathrm{N}}_{2}}^{2+}$ with electronic states that can support long-lived vibrational resonances. We compare our simulations with previous quantum dynamics calculations in a quasidiabatic representation, which employed a small number of electronic states. Good agreement for both the Auger spectrum as well as the influence of the delayed IR pulse is found. By employing the mixed quantum-classical treatment, we can greatly reduce the computational cost to simulate the fragmentation dynamics compared to the quantum dynamics simulations. Furthermore, we reinvestigate the title process by employing an extended set of adiabatic potential energy surfaces and also investigate the role of nonadiabatic coupling in the process. The use of the full set of adiabatic potentials increases the dissociation probability and changes the details of the interaction with the IR pulse, but no effect due to the nonadiabatic coupling is found.

Ag2S quantum dots in the fields of picosecond and femtosecond UV and IR pulses: optical limiting, nonlinear absorption and refraction properties

We demonstrate strong optical nonlinearities of silver sulfide (Ag2S) quantum dots (QDs) in the ultraviolet range. The 4-nm Ag2S QDs were prepared by chemical method and analyzed using picosecond (800 nm and 400 nm, 200 ps) and femtosecond (800 nm and 400 nm, 60 fs) laser pulses. Our Z-scan measurements show that these QDs have large nonlinear absorption coefficient (~ 10−3 cm W−1) at 400 nm. We also demonstrate the transient absorption and optical limiting in Ag2S QDs. Variations of the signs of nonlinear refractive indices and nonlinear absorption coefficients of QDs are demonstrated by changing pulse width and wavelength of probe radiation.

Formation of Light Bullets and the Generation of a Broadband Supercontinuum upon the Filamentation of Femtosecond IR Pulses

The formation of light bullets and the generation of a broadband supercontinuum with the minimum wavelength in the UV region are observed upon the filamentation of a femtosecond laser pulse in transparent dielectrics under conditions of an anomalous dispersion of group velocity. The duration of a light bullet, measured via laser coloration, is around one period of the light field’s oscillation, and its diameter is less than 10 μm. Numerically calculated parameters of the light bullet are in good agreement with experimentally measured values.

High-resolution broadband sum frequency generation vibrational spectroscopy using intrapulse interference.

We theoretically propose a new approach to obtain high-spectral-resolution sum frequency generation (SFG) vibrational spectra using intrapulse interference. By introducing a π-step phase modulation to the broadband 800 nm pulse, the broadband 800 nm laser pulse splits into two distinguishable pulses in the time domain with a fixed time delay. The resolution of the intrapulse interference SFG can be better than 1 cm-1 and is limited only by the spectral resolution of the spectrometer. This approach can accurately retrieve the amplitude and the relative phase of vibrational peaks. Additionally, the sensitivity of SFG is enhanced by adopting femtosecond IR and femtosecond visible pulses.

Angularly resolved RABBITT using a second harmonic pulse

Processes in atoms or molecules on the attosecond timescale have been measured using XUV attosecond and IR femtosecond pulses overlapping in time and controlled with attosecond accuracy. Within this general framework, many strategies have been developed using the harmonics of the fundamental pulse. In this paper, we focus on a specific configuration where the attosecond pulse train is composed by odd harmonics and is dressed by the second harmonic of the fundamental light. Measuring the angularly resolved photoelectron spectrum as a function of the delay between the pulses, a clear oscillation of the anisotropy parameters appears revealing attosecond controlled interferences. This process, is assigned to interferences between two quantum paths involving one XUV photon, on one path, and a XUV+UV photons on the other path. The XUV-UV delay dependent up–down asymmetry can be interpreted following the usual RABBITT formalism, where the dressing photon energy corresponds to the energy separation between the XUV photons of the attosecond pulse train. This approach allows an intuitive analysis of the interference and provides a well suited method for the study of complex valence band systems thanks to the limited congestion of the resulting spectrum.

High-energy continuum generation in an array of thin plates pumped by tunable femtosecond IR pulses

We report the generation of octave-spanning supercontinuum pulses in an arrangement of thin transparent plates pumped by 60 fs infrared pulses tunable from 1.2 to 1.75 $$\upmu \mathrm{m}$$ . We demonstrate that the multiple plate technique allows scaling up the energy of the continuum to the level of hundreds of $$\upmu \mathrm{J}$$ while avoiding optical damage and beam breakup due to multiple filamentation. We investigate the spatial and temporal structures of the continuum by performing XFROG and frequency-resolved angular spectra measurements and confirm that the continuum wave packet is well-behaved in time and space and possesses an estimated spectral energy density in excess of 10 nJ/nm throughout the range, which exceeds that of continua generated in bulk solids by more than two orders of magnitude. Finally, we demonstrate for the first time that the carrier-envelope phase stability of the multi-plate continuum is comparable to that of a continuum generated in a bulk medium.

Nonlinear optical waveguides in As_2S_3-Ag_2S chalcogenide glass thin films

Chalcogenide glasses (ChGs) are promising materials for nonlinear optical applications due to their elevated third-order optical susceptibility. Nonetheless, oxidation and photorefractive effects make the characterization and processing of this class of materials an intricate process. Employing ultrashort laser pulses, we were able to investigate the optical nonlinearities of As2S3 thin films doped with Ag2S nanocrystals, as well as produce channel waveguides. The results showed that the nonlinear refractive indexes of these films are in the order of 2.3-5.2 x 10−18 m2/W at wavelengths in the range of 775-1600 nm. Such film revealed to be proper for the development of nonlinear optical devices, since we have demonstrated the functionality of an 8.7 mm long waveguide for spectral broadening femtosecond IR pulses.

Attosecond quantum-beat spectroscopy in helium

The evolution of electron wavepackets determines the course of many physical and chemical phenomena, and attosecond spectroscopy aims to measure and control such dynamics in real time. Here, we investigate radial electron wavepacket motion in helium by using an XUV attosecond pulse train to prepare a coherent superposition of excited states and a delayed femtosecond IR pulse to ionize them. Quantum-beat signals observed in the high resolution photoelectron spectrogram allow us to follow the field-free evolution of the bound electron wavepacket and determine the time-dependent ionization dynamics of the low-lying state.

Electronic dynamics by ultrafast pump photoelectron detachment probed by ionization: a dynamical simulation of negative-neutral-positive in LiH(-).

The control of electronic dynamics in the neutral electronic states of LiH before the onset of significant nuclei motion is investigated using a negative-neutral-positive (NeNePo) ultrafast IR pump-attoescond pulse train (APT) probe scheme. Starting from the ground state of the anion (LiH(-)), multiphoton ultrafast electron detachment and subsequent excitation of the neutral by a few femtosecond intense IR pulse produces a non-equilibrium electronic density in neutral LiH. The coherent electronic wave packet is then probed by angularly resolved photoionization to the cation by an APT generated from a replica of the pump IR pulse at several time delays. Realistic parameters for the pump and the APT are used. Several NeNePo schemes are simulated using different IR carrier frequencies, showing that the delay between the successive attosecond pulses in the train can be used as a filter to probe the different pairs of states present in the coherent electronic wave packet produced by the pump pulse. The dynamical simulations include the pump and the probe pulses to all orders by solving the time-dependent Schrödinger equation using a coupled equation scheme for the manifolds of the anion, neutral, and cation subspaces. We show that an incomplete molecular orientation of the molecule in the laboratory frame does not prevent probing the electronic density localization by angularly resolved photoelectron maps.

Plasma channels under filamentation of infrared and ultraviolet double femtosecond laser pulses

The influence of a plasma channel, created by a filament of focused UV or IR femtosecond laser pulse (λ = 248 or 740 nm), on the characteristics of another plasma channel formed by a femtosecond pulse at the same wavelength following the first one with varied nanosecond time delays was experimentally studied. The dependence of the optical transparency of the first channel and the plasma density of the second channel on the time delay was demonstrated to be quite different for such double UV and IR femtosecond pulses.

Selective bond breaking mediated by state specific vibrational excitation in model HOD molecule through optimized femtosecond IR pulse: A simulated annealing based approach

The selective control of O-H/O-D bond dissociation in reduced dimensionality model of HOD molecule has been explored through IR+UV femtosecond pulses. The IR pulse has been optimized using simulated annealing stochastic approach to maximize population of a desired low quanta vibrational state. Since those vibrational wavefunctions of the ground electronic states are preferentially localized either along the O-H or O-D mode, the femtosecond UV pulse is used only to transfer vibrationally excited molecule to the repulsive upper surface to cleave specific bond, O-H or O-D. While transferring from the ground electronic state to the repulsive one, the optimization of the UV pulse is not necessarily required except specific case. The results so obtained are analyzed with respect to time integrated flux along with contours of time evolution of probability density on excited potential energy surface. After preferential excitation from [line]0, 0> ([line]m, n> stands for the state having m and n quanta of excitations in O-H and O-D mode, respectively) vibrational level of the ground electronic state to its specific low quanta vibrational state ([line]1, 0> or [line]0, 1> or [line]2, 0> or [line]0, 2>) by using optimized IR pulse, the dissociation of O-D or O-H bond through the excited potential energy surface by UV laser pulse appears quite high namely, 88% (O-H ; [line]1, 0>) or 58% (O-D ; [line]0, 1>) or 85% (O-H ; [line]2, 0>) or 59% (O-D ; [line]0, 2>). Such selectivity of the bond breaking by UV pulse (if required, optimized) together with optimized IR one is encouraging compared to the normal pulses.

Correlated electron and nuclear dynamics in strong field photoionization of H(2)(+).

We present a theoretical study of H(2)(+) ionization under strong IR femtosecond pulses by using a method designed to extract correlated (2D) photoelectron and proton kinetic energy spectra. The results show two distinct ionization mechanisms-tunnel and multiphoton ionization-in which electrons and nuclei do not share the energy from the field in the same way. Electrons produced in multiphoton ionization share part of their energy with the nuclei, an effect that shows up in the 2D spectra in the form of energy-conservation fringes similar to those observed in weak-field ionization of diatomic molecules. In contrast, tunneling electrons lead to fringes whose position does not depend on the proton kinetic energy. At high intensity, the two processes coexist and the 2D plots show a very rich behavior, suggesting that the correlation between electron and nuclear dynamics in strong field ionization is more complex than one would have anticipated.

Filamentation of IR and UV femtosecond pulses upon focusing in air

The filamentation of IR and UV laser pulses has been studied numerically and experimentally for different initial beam focusing geometries, and linear electron density profiles along the plasma channel of filaments have been obtained. The results demonstrate that changes in laser beam focusing have a stronger effect on filament and plasma channel parameters for UV radiation thanfor IR radiation. Focusing causes individual high fluence regions produced by refocusing to merge to form a continuous extended filament with a continuous plasma channel.

Neutral-Fragmentation Paths of Methane Induced by Intense Ultrashort IR Laser Pulses: Ab Initio Molecular Orbital Approach

Instantaneous (laser-field-dependent) potential energy curves leading to neutral fragmentations of methane were calculated at several laser intensities from 1.4 × 10(13) to 1.2 × 10(14) W/cm(2) (from 1.0 × 10(10) to 3.0 × 10(10) V/m) using ab initio molecular orbital (MO) methods to validate the observation of neutral fragmentations induced by intense femtosecond IR pulses (Kong et al. J. Chem. Phys. 2006, 125, 133320). Two fragmentation paths, CH(2) + 2H and CH(2) + H(2), in (1)T(2) superexcited states that are located in the energy range of 12-16 eV were considered as the reaction paths because these states are responsible for Jahn-Teller distortion opening up reaction paths during ultrashort pulses. As field intensity increased, the low-lying excited (1)A(1) states originated from the Jahn-Teller (1)T(2) states were substantially stabilized along the neutral-fragment path CH(4) → CH(2) + 2H and were located below the ionization threshold. On the other hand, the low-lying excited (1)B(1) states, which also originate from the Jahn-Teller (1)T(2) states, were embedded on the ionized state along the dissociation path to CH(2) + H(2). This indicates that ionic fragments, rather than neutral ones, are produced along the CH(2) + H(2) path. The computational results support neutral fragmentations through superexcited states proposed by Kong et al.

On the biphoton excitation of the fluorescence of the bacteriochlorophyll molecules of purple photosynthetic bacteria by powerful near IR femto-picosecond pulses

The authors of a number of experimental works detected nonresonance biphoton excitation of bacteriochlorophyll molecules, which represent the main pigment in the light-absorbing natural “antenna” complexes of photosynthesizing purple bacteria, by femtosecond IR pulses (1250–1500 nm). They believe that IR quanta excite hypothetic forbidden levels of the pigments of these bacteria in the double frequency range 625–750 nm. We propose and ground an alternative triplet mechanism to describe this phenomenon. According to our hypothesis, the mechanism of biphoton excitation of molecules by IR quanta can manifest itself specifically, through high triplet levels of molecules in the high fields induced by femtosecond-picosecond laser pulses.