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Pure and Ag-doped ZnO nanofilms with four morphologies, namely nanocone, nanocrystal, nanochips and nanoleaves, have been successfully synthesized by magnetron sputtering method. The composition of the Ag-doped ZnO (AZO) nanostructures was explored by X-ray diffraction (XRD) and energy-dispersive spectroscopy (EDS). The XRD patterns revealed that the samples exhibited the hexagonal wurtzite structure with preferential orientation along the (002) crystal plane, although the c-axis as preferred orientation decreased with Ag doping. Moreover, we report the photoluminescence, field-emission (FE) and ultrafast nonlinear optical behavior of AZO films. A blue-shift was demonstrated in the UV region of photoluminescence results for the AZO nanostructures. The resulting FE showed that the superior FE properties were attributed to the better morphologies. The ultrafast time resolved optical Kerr gate experiment results showed that AZO was a good candidate for an optical Kerr gate medium due to its ultrafast response and large third-order nonlinear susceptibility. The result of time-resolved photoluminescence showed that the first decay time was about several picoseconds for films, and the slow relaxation process had the lifetime longer than hundred picoseconds. By analyzing the experimental results of the time-resolved photoluminescence experiment, we found the fast decay process could be attributed to the exciton migration and the slow decay to the recombination process of the relaxed excitons. [ABSTRACT FROM AUTHOR]

Eight wavelengths (465, 485, 508, 532, 559, 588, 621, and 658 nm) were synchronously generated by a single-pass potassium gadolinium tungstate (KGW) crystal Raman arrangement, which was pumped by a 532 nm, picosecond Nd: YAG laser with high peak power. To the best of our knowledge, this single-pass arrangement synchronously generated the highest-order Stokes and anti-Stokes among all known KGW Raman lasers. The first- to seventh-order Stokes conversion efficiency was calculated using Raman coupled equations. Further, the numerical results of third-order Stokes (621 nm) conversion efficiency were in good agreement with the experimental ones. We also analyze and discuss the factors because of which higher-order Stokes were not generated. [ABSTRACT FROM AUTHOR]

Numerical simulations demonstrate that mid-infrared pulses are arbitrarily shaped during the differential frequency mixing of two femtosecond near-infrared pulses propagating in an engineered quasi-periodic poled medium with optical nonlinearity and group velocity dispersion. Shaped pulses, including linearly chirped pulses and pulse trains, are generated with high conversion efficiencies. [ABSTRACT FROM AUTHOR]

We experimentally demonstrate compact and efficient single-pulse coherent anti-Stokes Raman spectroscopy (CARS) via spectral notch shaping implemented with a fiber Bragg grating. We show that a fiber Bragg grating can serve as a narrowband notch filtering component on a 90 nm broadband femtosecond pulsed laser without spectral distortion. Finally, we obtain CARS spectra of various samples in the fingerprint region of molecular vibrations. This scheme has potential for compact implementations of all-fiber single-pulse multiplex CARS due to its compatibility with fiber optics. [ABSTRACT FROM AUTHOR]

A detailed theoretical analysis of reverse saturable absorption (RSA) to saturable absorption (SA) transition has been presented in Ru complex [{Ru (bipy)}L][PF], with picosecond laser pulses at 532 nm wavelength. Interestingly, it is shown that an increase in pulse intensity leads to transition from RSA to SA, with increased contrast, due to strong excited-state absorption. Theoretical results are in good agreement with reported experimental results. The effect of pulse intensity, pulse width, frequency and concentration has been studied to optimize the RSA to SA transition, which has been used to design all-optical picosecond NOT, AND, OR, and the universal NAND and NOR logic gates. The designs are the first application of Ru dioxolene complex based all-optical logic gates with picosecond switch off/on time. These results show the potential of transition-metal complexes for photonic applications. [ABSTRACT FROM AUTHOR]

Coherent anti-Stokes Raman scattering is a powerful nonlinear optical spectroscopic technique primarily used in chemistry and biology for detection and diagnostics by excitation and probing of coherent molecular vibrations. Despite many advantages, there are certain limitations to the application of this technique due to the existence of the non-resonant four-wave mixing contamination. This issue is addressed here by offering the time-resolved exact solutions with all Gaussian input pulses. The paper explains signal enhancement with a suppression of the background at the threshold delay time of the probe and presents a one-to-one comparison between the solutions and experimental observations that provides a reconstruction of the pure coherent Raman resonant contribution. These findings add to the deep understanding of coherent Raman spectroscopic experimental data as a hands-on interpreting tool. Copyright © 2016 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

We theoretically investigate the high-order harmonic generation (HHG) and the isolated attosecond pulse (IAP) with tunable ellipticity from the aligned molecule. It is found that the ellipticity of harmonic depends on the internuclear distance R. Further analysis presents that the nonzero ellipticity of harmonic generated at R = 6 a.u. is attributed to the 2 p σ u state of H 2 +. The 2 p σ u state with nonvanishing angular momentum originates from a resonant transition from the 1 s σ g state induced by the fundamental pulse. With specific intensity of the orthogonally polarized two-color (OTC) laser fields, an IAP with high ellipticity can be obtained. To further illustrate the origin of the high ellipticity, we present the electron trajectories and electron currents induced by the electron wavepackets of H 2 + , which shows a clear picture of the dynamic process of HHG. Moreover, the polarization of the IAP varies from left to right elliptical polarization by synthesizing the harmonic spectrum with different orders. Such an IAP may serve as a potential tool for probing and manipulating the ultrafast magnetic and chiral dynamics. [ABSTRACT FROM AUTHOR]

We demonstrate and analyze a series of experiments in traditional and soft condensed matter using coherent optical spectroscopy and microscopy with ultrafast time resolution. We show the capabilities of resolving both real and imaginary parts of the third-order nonlinearity in the vicinity of Raman resonances from a medium probed within microscopic volumes with an equivalent spectral resolution of better than 0.1 cm−1. We can differentiate between vibrations of various types within unit cells of crystals, as well as perform targeted probes of areas within biological tissue. Vibrations within the TiO6 octahedron and the ones for the Ti-O-P intergroup were studied in potassium titanyl phosphate crystal to reveal a multiline structure within targeted phonon modes with closely spaced vibrations having distinctly different damping rates (~0.5 ps−1 versus ~1.1 ps−1). We also detected a 1.7–2.6 ps−1 decay of C-C stretching vibrations in fat tissue and compared that with the corresponding vibration in oil.

We report on the design and performance of a velocity map imaging (VMI) spectrometer optimized for experiments using high-intensity extreme ultraviolet (XUV) sources such as laser-driven high-order harmonic generation (HHG) sources and free-electron lasers (FELs). Typically exhibiting low repetition rates and high single-shot count rates, such experiments do not easily lend themselves to coincident detection of photo-electrons and -ions. In order to obtain molecular frame or reaction channel-specific information, one has to rely on other correlation techniques, such as covariant detection schemes. Our device allows for combining different photo-electron and -ion detection modes for covariance analysis. We present the expected performance in the different detection modes and present the first results using an intense high-order harmonic generation (HHG) source.

Nonlinear dynamics of free carriers in direct bandgap semiconductors at terahertz (THz) frequencies is studied using the intense few-cycle source available at the Advanced Laser Light Source (ALLS). Techniques such as Z-scan and optical-pump/THz-probe are employed to explore nonlinear interactions in an n-doped InGaAs thin film and a photoexcited GaAs sample, respectively. The physical mechanism that gives rise to such interactions is found to be intervalley scattering. A simple Drude-based mathematical model that incorporates the intervalley scattering process is developed and agrees well with the THz response of free carriers in semiconductors.

Broadband optical parametric amplification (OPA) in the near-infrared region (1.3–1.8 μm) is demonstrated in YCOB crystal pumped by 1 kHz Ti:sapphire based femtosecond laser at near critical wavelength degeneracy phase matching condition at first time. The gain bandwidth is closed to BIBO or BBO crystal OPA gain bandwidth. The energy obtained with 3.5 mm-thick YCOB reached 20 μJ with RMS 1.9%. After second harmonic generation, 17.6 fs pulse is obtained, and the pulse compressibility is demonstrated. [ABSTRACT FROM AUTHOR]

As a consequence of the general experimental challenge to detect signals in mid-IR range, taking dispersive chirped near-IR laser pulses as the injected signal source seems to be an artistic route avoiding the daunting mid-IR stretcher and constantly was applied in moderate energy mid-IR optical parametric chirped-pulse amplifications (OPCPA) systems. In this paper we study the wavelength effect on pulse stretching and compressing in detail. Beginning with the theoretical analysis on each dispersion term of grating pairs, we evaluate the residual dispersions when pulse stretcher and compressor work at distinct wavelengths, which shows that this wavelength effect will result in poorly compressed pulses far from transform-limited. Via proof-of-principle experiments based on mid-IR OPCPAs and corresponding numerical simulations, we show that this artful configuration led to un-compressible pulses of ∼2 ps with a time-bandwidth product of ∼ 10 when the chirped-pulse duration is ∼400 ps. To overcome this effect, we demonstrate a simple design of pulse stretcher and compressor. The presented design consisted of a reflection grism-pair compressor can simultaneously cancel the quadric and cubic dispersions of conventional grating based stretcher, showing a potential ability of supporting high-contrast, sub-100-fs pulse-duration and 10,000× of pulse expansion. [ABSTRACT FROM AUTHOR]

We present an optical system based on two toroidal mirrors in a Wolter configuration to focus broadband extreme ultraviolet (XUV) radiation. Optimization of the focusing optics alignment is carried out with the aid of an XUV wavefront sensor. Back-propagation of the optimized wavefront to the focus yields a focal spot of 3.6 × 4.0 µm2 full width at half maximum, which is consistent with ray-tracing simulations that predict a minimum size of 3.0 × 3.2 µm2. This work is important for optimizing the intensity of focused high-order harmonics in order to reach the nonlinear interaction regime.

[Display omitted] • A type I MoS 2 @ZnO heterostructure was constructed by heterogeneous interface engineering strategy. • The interlayer exciton pumping efficiency and NLA of MoS 2 /ZnO heterostructure can be adjusted. • The photoelectric functional performances and mechanism of the samples are studied. The semiconductor heterostructure was proposed and prepared by the wide-band gap zinc oxide (ZnO) and narrow-band gap molybdenum disulfide (MoS 2) materials. The interlayer exciton pumping efficiency and saturable/reverse saturable nonlinear absorption (NLA) characteristics of MoS 2 /ZnO heterostructure can be flexibly adjusted, revealing its important potential in exploring new optoelectronic devices. In this work, based on magnetron sputtering and chemical vapor deposition, the MoS 2 nanosheets, unmodified ZnO nanowires and MoS 2 @ZnO heterostructure were successfully synthesized. The depending relationship of the laser intensity, pulse width and wavelength on the NLA of samples has been studied using Z-scan technique. The ultrafast dynamics process was researched using two-color pump probe technology with the excitation wavelength at 325, 380 and 400 nm. The fast and slow components characters of the ultrafast electron transfer process were discussed to analyzing its producing mechanism in detail. The photoelectric functional performances and physical mechanisms are studied in theory and experiment. This paper will provide a theoretical basis and guidance for manipulation and improvement of the nonlinear optical properties in semiconductor micro/nano-material, as well as the corresponding application in optoelectronic devices. [ABSTRACT FROM AUTHOR]

Experimental and numerical study on chirped transient stimulated Raman scattering in ethanol have been performed. Negatively chirped pump pulses yielded much higher conversion efficiency than positively chirped pump pulses. The calculated results reveal that the group velocity mismatch between the pump and Stokes pulses can rapidly degrade the conversion efficiency when the pump laser generates chirped pulses. However, the combined effects of group velocity dispersion and self-phase modulation offset the negative chirps during propagation, yielding higher conversion efficiency. This is a novel mechanism for controlling transient stimulated Raman scattering and is likely to be of importance for various applications. [ABSTRACT FROM AUTHOR]

The generation of sub-optical-cycle, carrier-envelope phase-stable light pulses is one of the frontiers of ultrafast optics. The two key ingredients for sub-cycle pulse generation are bandwidths substantially exceeding one octave and accurate control of the spectral phase. These requirements are very challenging to satisfy with a single laser beam, and thus intense research activity is currently devoted to the coherent synthesis of pulses generated by separate sources. In this review we discuss the conceptual schemes and experimental tools that can be employed for the generation, amplification, control, and combination of separate light pulses. The main techniques for the spectrotemporal characterization of the synthesized fields are also described. We discuss recent implementations of coherent waveform synthesis: from the first demonstration of a single-cycle optical pulse by the addition of two pulse trains derived from a fiber laser, to the coherent combination of the outputs from optical parametric chirped-pulse amplifiers. [ABSTRACT FROM AUTHOR]

The second harmonic generation (SHG) of a femtosecond optical frequency comb (FOFC) has been studied. This work focuses on the SHG frequencies that are generated by the mixing of even-numbered frequency components from the original comb with odd-numbered components. It is observed that the generation of those frequencies is the reason the original FOFC and FOFC-based SHG signal have the same repetition frequency. The theoretical derivation agrees with the result of an optical experiment. Our results may be of use with the high-harmonic-generation process and FOFC-based SHG applications, including high-resolution spectroscopy, attosecond pulse generation, and precision length measurement. [ABSTRACT FROM AUTHOR]

A new method for increasing the speed of all-optical Mach-Zehnder Interferometric (MZI) switching with a bulk Semiconductor Optical Amplifier (SOA), using chirped control signals, is suggested and theoretically analyzed. For 125fs input and chirped control pulses, we show acceleration of the gain recovery process using the cross phase modulation (XPM) effect. Our method depicts that Tb/s switching speeds, using bulk SOA-MZI with a proper Q-factor, is feasible. For the first time, we reach operation capability at 2Tb/s with a Q-factor of more than 10. The new scheme also improves the extinction and amplitude ratio of the output power, as well as increasing the contrast ratio of the switched signal. We use a finite difference beam propagation method for MZI analysis, taking into account all nonlinear effects of SOA, such as Group Velocity Dispersion (GVD), Kerr effect, Two Photon Absorption (TPA), Carrier Heating (CH) and Spectral Hole Burning (SHB). [ABSTRACT FROM AUTHOR]

The state of the art of ultrafast Er:fiber technology is reviewed. Such lasers are increasingly used for generation of ultrabroadband and widely tunable pulse trains. Er:fiber sources prove to be flexible, compact and robust with important applications in fundamental and interdisciplinary sciences. After a short overview of different oscillator and amplifier designs the discussion focuses on coherent and tailored supercontinuum generation in highly nonlinear germanosilicate fibers. This approach enables a tuning range spanning from the visible to the mid infrared, synthesis of single-cycle light pulses and passive locking of the carrier-envelope phase. [ABSTRACT FROM AUTHOR]

The spectrum of terahertz (THz) emission in gases via ionizing two-color femtosecond pulses is analyzed by means of a semi-analytic model and numerical simulations in 1D, 2D and 3D geometries taking into account propagation effects of both pump and THz fields. We show that produced THz signals interact with free electron trajectories and thus significantly influence further THz generation upon propagation, i.e., make the process inherently nonlocal. This self-action contributes to the observed strong spectral broadening of the generated THz field. We show that diffraction of the generated THz radiation is the limiting factor for the co-propagating low frequency amplitudes and thus for the self-action mechanism in 2D and 3D geometries.

Five-cycle (50 fs) mid-IR pulses at 80-MHz repetition rate are produced using a degenerate (subharmonic) optical parametric oscillator (OPO), synchronously pumped by an ultrafast 1560-nm fiber laser. The effects of cavity dispersion and the length of a periodically poled lithium niobate (PPLN) gain element on the output spectrum and pulse duration are investigated by taking advantage of a very broad (∼ 1000 cm−1) gain bandwidth near the 3.1-μm OPO degeneracy point. A new method of assessing the total OPO group delay dispersion across its entire spectrum is proposed, based on measuring spectral signatures of trace amounts of molecular gases injected into the OPO cavity. [ABSTRACT FROM AUTHOR]

Abstract: We numerically study the propagation dynamics and compression of ultrashort laser pulses in a hollow-core fiber (HCF) filled with noble gases at different carrier wavelengths from 1.8μm to 3.9μm. In the optimal parameter range, single-cycle or even sub-cycle pulses with clean spatial and temporal profiles can be obtained due to chirp compensation beyond 3rd order by bulk material. It is found that the intensity-dependent group velocity sets the upper limits on both the gas pressures and input pulse energies if a clean and well-compressed pulse is required only through compression with bulk materials. In order to use higher gas pressures and larger input energies, two ways are demonstrated to deal with the limitations imposed by the intensity-dependent group velocity. [Copyright &y& Elsevier]

Abstract: We numerically investigated the interior connection dynamics of separated sub-filaments generated by the inner beam of double synchronous and coaxial beams. We demonstrated the critical differences from the case of sequential connection of two filaments generated by two asynchronous pulses. It shows that the background reservoir formed by the co-propagation beams has a secondary function of generation of wave-guide interlayer, resulting in a long continuous plasma channel. [Copyright &y& Elsevier]

Abstract: A femtosecond supercontinuum generating system is presented by employing a microstructured fiber within a synchronously pumped ring resonator. Via the ring resonator, optical feedback is introduced, which leads to the creation of a highly nonlinear oscillator and to the formation of nonlinear dynamics like period multiplication, limit cycle, and chaos. The effect of delayed feedback on supercontinuum generation is studied numerically and experimentally, using the input pulse power and the resonator length detuning as control parameters. In combination, these two control parameters allow for a systematical adjustment of the regimes of nonlinear dynamics, which leads to an alteration of the pattern of pulse trains. Changing the resonator length on a sub-wavelength scale enables manipulation of the feedback phase, which can be used to adjust shape and bandwidth of the optical spectrum. Furthermore, a possibility to tailor the modes of the optical frequency comb is demonstrated, and the influence of the feedback phase on the noise properties of the supercontinuum generating system is discussed. [Copyright &y& Elsevier]

Abstract: Chromium doped zinc oxide thin solid films were deposited on soda–lime glass substrates. The photoconductivity of the material and its influence on the optical behavior was evaluated. A non-alkoxide sol–gel synthesis approach was used for the preparation of the samples. An enhancement of the photoluminescence response exhibited by the resulting photoconductive films with embedded chromium nanoclusters is presented. The modification in the photoconduction induced by a 445nm wavelength was measured and then associated with the participation of the optical absorptive response. In order to investigate the third order optical nonlinearities of the samples, a standard time-resolved Optical Kerr Gate configuration with 80fs pulses at 830nm was used and a quasi-instantaneous pure electronic nonlinearity without the contribution of nonlinear optical absorption was observed. We estimate that from the inclusion of Cr nanoclusters into the sample results a strong optical Kerr effect originated by quantum confinement. The large photoluminescence response and the important refractive nonlinearity of the photoconductive samples seem to promise potential applications for the development of multifunctional all-optical nanodevices. [Copyright &y& Elsevier]

We report on optimization of the intensity of THz signals generated and detected by optical rectification and electro-optic sampling in dispersive, nonlinear media. Addition of a negative prechirp to the femtosecond laser pulses used in the THz generation and detection processes in 1-mm thick ZnTe crystals leads to an increase of the THz intensity of more than 30% at low laser intensity and up to 60% at higher laser intensity. Dispersion compensation in the ZnTe crystal, which becomes significant for laser pulses with durations much less than 100 fs, is responsible for the enhanced generation and detection efficiency at low excitation intensity, and our simulations indicate that two-photon absorption is responsible for the increased efficiency enhancement observed at high excitation intensity. [ABSTRACT FROM AUTHOR]

Abstract: The triggering and guiding of high voltage discharge by plasma filaments generated by 400nm and 800nm femtosecond laser were studied experimentally. The detailed diagnostics for characteristics of filaments, such as amount of free electrons, diameter and electron density, were performed using sonographic method, fluorescence imaging and resistivity measurement. A significant reduction of the breakdown voltage threshold due to the pre-ionization of the air gap by laser pulse filaments was observed. It is shown that the 400nm laser pulses demonstrated stronger triggering ability than 800nm laser pulse under same pulse energy. This behavior of 400nm femtosecond pulse is connected to the rich population of highly excited particles in filaments. [Copyright &y& Elsevier]

Abstract: A detailed theoretical analysis of femtosecond transition from saturable (SA) to reverse saturable absorption (RSA) has been carried out in Copper-Phthalocyanine (CuPc)-doped polymethylmethacrylate (PMMA) thin films. The transition due to fifth-order effect of excited-state absorption induced two-photon process has been optimized with respect to intensity, concentration and nonlinear coefficients to design various all-optical logic gates, namely, OR and AND at lower intensities (SA region), XOR at the transition intensity, and the universal NAND and NOR at higher intensities (RSA region). The advantages of ultrafast operation, simplicity, tunability, high contrast, stability of CuPc-doped PMMA thin film, and the possibility to control and realize various logic operations in the same film at the same wavelength by only controlling the pulse intensity, instead of a pump-probe configuration, make them attractive for practical implementation. [Copyright &y& Elsevier]

Abstract: We report the observation of periodic modulations in the high order harmonic radiation from diatomic molecules in a semi-infinitive gas cell when a preceding femtosecond laser beam, which is off-axis to the high-harmonic generation beam, modifies the phase-matching condition through the field-free alignment of the molecules. The observed modulations of the high harmonic radiation versus the time delay between the pulses result from periodic changes in the nonlinear refractive index associated with rotational Raman coherence. This opens up a new potential technique for studying rotational coherence dynamics in the ground states of molecules. [Copyright &y& Elsevier]

We theoretically investigate the generation of ultra broadband supercontinuum from helium atoms exposed to a linearly polarized mid-infrared field. By adopting a UV trigger pulse to the mid-infrared field, the continuous harmonic yields are significantly enhanced by 3.5 orders, and a supercontinuum with the width of 230 eV is observed. The spectrum can support a sub-20 as pulse, which is below one atomic unit of time (24 as). The short quantum path is selected by adjusting the time delay between the UV pulse and the mid-infrared pulse, then broadband single 70 as pulses with tunable central wavelengths are obtained, which can be extended to the 'water window' region (284-543 eV). [ABSTRACT FROM AUTHOR]

The development of new sources in the terahertz (THz) spectral region has attracted much attention over the past 20 years. In particular, the last three years have seen a surge of new laser-based techniques for generating intense, few-cycle THz pulses in the microjoule energy range, thus paving the way to the study of the nonlinear optical properties of various materials at THz frequencies. Simultaneously, innovative solutions for broad-band THz detection were found, allowing one to sense matter in the THz range with an unprecedented time resolution. In this paper, we will attempt to give a review of the properties and characteristics of the recently developed intense THz sources, with a particular eye on their potential application in ultrafast THz nonlinear spectroscopy. [ABSTRACT FROM PUBLISHER]

The ultrafast dynamics of gain and phase in bulk semiconductor optical amplifiers are numerically investigated over a broad band of about 200 nm based on the semiclassical method of density matrix equations. The gain dynamics and phase dynamics are compared in four different bands for 200 fs Gaussian pump pulse. It is predicted that the recovery time of phase may be longer or shorter than that of gain depending on probe wavelengths. Furthermore, phase recovery overshoot may appear at wavelengths shorter than the pump wavelength. These interesting features result from the competition between spectral-hole burning and carrier heating over the whole band. [ABSTRACT FROM AUTHOR]

We demonstrate the interference phenomenon in the White Light Continuum (WLC) generated by a single femtosecond laser beam. Different kinds of spatial interference patterns of the WLC generation under various conditions were investigated. The spatial patterns were attributed to interference between the filaments in the WLC generated by the fundamental laser beam yielding the diffraction effect from spatial confinement. Simulations of different patterns were performed. By comparing the results of simulation with those of experiments, the distances of several micrometers between the neighboring filaments can be derived, which agree with the literature values from direct measurements. [ABSTRACT FROM AUTHOR]

Abstract: We firstly demonstrate a simple system of fast light generation using a soliton pulse circulating in the integrated micro ring devices. Using such a system, the attosecond pulse and beyond can be easily generated. Simulation results obtained have shown that the generation of a very narrow full-width at half maximum (FWHM) and sharp tip are achieved. In principle, light pulse known as a soliton pulse is generated and input into a three-stage micro ring resonator, where the ring radii are within the ranges from 5 to 15μm. With some selected parameters such as ring radii, coupling coefficients and nonlinear refractive indices, the extremely short pulse is generated, which means fast light is generated from ns to zs and beyond by using the simple system. [Copyright &y& Elsevier]

Abstract: A 120fs Ti–sapphire laser was used to fabricate waveguides in YAG crystal. A 7mm long waveguide was written at a position of 100μm below the surface, which shows multimode propagation at 633nm with optical attenuation of about 0.2dB/mm. The light guiding occurs in the region around the visible laser-damaged region, indicating that the light guiding area is induced by stress. The waveguide exhibited strong birefringence property with maximum magnitude of about 1.5×10−5. Infrared and Raman spectroscopy analysis indicate there is no change in chemical composition in laser-modified zone. [Copyright &y& Elsevier]

An efficient pulse shaping method using counter-propagating pulses in the femtosecond regime is proposed and investigated. The effects of pump pulse power and pulsewidth on probe pulse are studied in counter-propagation scheme. It is shown that, with the proposed method, output probe pulse temporal and spectral peak shift due to femtosecond nonlinearities can be compensated, while the output pulse is amplified sufficiently. Furthermore, in relatively high power regime, the probe pulsewidth and time-bandwidth product are improved using counter-propagating pump pulse. [ABSTRACT FROM AUTHOR]

Abstract: Two types of nanophotonic technologies—two-dimensional photonic crystal (2D PC) slab waveguides (WGs) and quantum dots (QDs)—were developed for key photonic device structures in the future. For an ultrafast digital photonic network, an ultrasmall and ultrafast symmetrical Mach–Zehnder (SMZ)-type all-optical switch (PC-SMZ) and an optical flip–flop device (PC-FF) have been developed. To realize these devices, one method is to develop a selective-area molecular beam epitaxial growth QD technique by employing a metal mask method. Another method is to establish a new design method, i.e., topology optimization of the 2DPC WG with a wide and flat bandwidth, high transmittance, and low reflectivity. We also fabricated an optical microcavity in a photonic crystal slab embedded with GaAs QDs by droplet epitaxy. The Purcell effect on the exciton emission of GaAs QDs was confirmed by microphotoluminescence and lifetime measurements. [Copyright &y& Elsevier]

Based on the semiclassical density-matrix equations, ultrafast probe gain and phase dynamics in bulk semiconductor optical amplifiers excited by a 200 fs Gaussian pump pulse centered at 1,558 nm are numerically investigated and compared over a wide material gain region (1,460–1,650 nm). It is predicted that the recovery time of phase may be longer or shorter than that of gain depending on probe wavelengths. Besides, as functions of probe wavelength, the dynamic gain range and the dynamic phase range have different shapes. Moreover, the wavelength dependence of phase recovery overshoot is illustrated. At last, the probe chirp dynamics and the wavelength dependence of the chirp characteristics are described. These interesting features result from a restriction of the Kramers–Kronig relations and different ways of the competition or cooperation among the effects of spectral-hole burning, carrier heating and two-photon absorption on subpicosecond timescale over the whole concerned band. [ABSTRACT FROM AUTHOR]

The characteristics of the filaments generated by propagating a femtosecond Gaussian beam in a 2 m long gas cell with continuously varying pressures at different focal distances are numerically investigated. The simulated results indicate that the onset distance and the length of the filaments depend sensitively on the variation of pressure. These effects are enhanced with the increase of the focal length for the initial power P i n = 5 P c r . The main reason is that some parameters (the group velocity dispersion, the nonlinear refractive index, multiphoton ionization rate and the neutral atom density) are variational with the gaseous pressure. And the stability of the filament is determined by the combined effect of the pressure and the focal distance. In addition, we discuss the differences of the filament characteristics between the fixed pressure and continuously varying pressures. Through the analyses of the temporal dynamics and supercontinuum spectra, we find that maintaining a large pressure (1 atm) and changing to a large pressure (such as 0. 3 ∼ 1 atm) are beneficial to the propagation of the filament and the broadening of the spectra. It is interesting that, for fixing the initial energy E i n = 1 mJ, although the critical power of laser pulse is higher at the low pressure, the filament length at the continuously varying pressure p = 0. 3 ∼ 1 atm is still longer than that of p = 1 ∼ 0. 3 atm as the focal distance increases. This research is of great significance to remotely detect atmospheric pollutant components. • The characteristics of the filaments generated by propagating a femtosecond Gaussian beam in a 2 m long gas cell with continuously varying pressures at different focal distances are numerically investigated. • The simulated results indicate that the onset distance and the length of the filaments depend sensitively on the variation of pressure. These effects are enhanced with the increase of the focal length for the initial power P in =5P cr. • The main reason is that some parameters (the group velocity dispersion, the nonlinear refractive index, multiphoton ionization rate and the neutral atom density) are variational with the gaseous pressure. And the stability of the filament is determined by the combined effect of the pressure and the focal distance. In addition, we discuss the differences of the filament characteristics between the fixed pressure and continuously varying pressures. • Through the analyses of the temporal dynamics and supercontinuum spectra, we find that maintaining a large pressure (1 atm) and changing to a large pressure (such as 0.3 ∼ 1 atm) are beneficial to the propagation of the filament and the broadening of the spectra. • It is interesting that, for fixing the initial energy E in =1 mJ, although the critical power of laser pulse is higher at the low pressure, the filament length at the continuously varying pressure p=0.3 ∼ 1 atm is still longer than that of p=1 ∼ 0.3 atm as the focal distance increases. This research is of great significance to remotely detect atmospheric pollutant components. [ABSTRACT FROM AUTHOR]

We report a compact femtosecond optical parametric oscillator (OPO) with a tunable repetition rate that is harmonically pumped using a 52.5 MHz Yb-doped fiber laser system. By reducing the OPO cavity length, the repetition rate can be adjusted successively from 1.935 to 2.95 GHz at 52.5 MHz intervals. The multiplication of the repetition rate is achieved by placing a nonlinear crystal in a compact linear cavity, and the minimum cavity length is 47.8 mm. The maximum signal power (205.2 mW) at 2.04 GHz is obtained at an input pump power of 2.35 W and a central wavelength of 1472 nm. The central wavelength of the signal pulses can be tuned from 1383 to 1596 nm. To the best of our knowledge, 2.95 GHz is the highest repetition rate achieved using a femtosecond OPO based on an ultrashort cavity with a length that is N times (N is an integer) shorter than that of the pump cavity. • A maximum repetition rate of 2.95 GHz is obtained, which is the highest repetition rate achieved from a femtosecond OPO based on a compact linear cavity design. • The repetition rate can be adjusted successively from 1.935 to 2.95 GHz at 52.5 MHz intervals by reducing the OPO cavity length. • By replacing the concave mirror in the V-cavity with a lens, the OPO cavity is more compact and easier to align. • The signal wavelength can be tuned by changing the grating period of the crystal, covering a range from 1350 to 1625 nm. [ABSTRACT FROM AUTHOR]

We propose a new system of an attosecond pulse generation by using the integrated nonlinear microring devices. A soliton light pulse with wavelength of 1550 nm is entered into the multistage microring resonators, where the ring radii of the recent integrated device product are within the range from 5 to 10 microns. Results obtained have shown that the ultrashort pulse width in the attoscale regime and beyond can be easily generated. When the input peak power of 12 W is applied into the system, the generated pulse with pulse width of 50 is achieved. © 2008 Wiley Periodicals, Inc. Microwave Opt Technol Lett 50: 3108–3111, 2008; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mop.23888 [ABSTRACT FROM AUTHOR]

Abstract: Simultaneous spatial and temporal focusing (SSTF), when combined with nonlinear microscopy, can improve the axial excitation confinement of wide-field and line-scanning imaging. Because two-photon excited fluorescence depends inversely on the pulse width of the excitation beam, SSTF decreases the background excitation of the sample outside of the focal volume by broadening the pulse width everywhere but at the geometric focus of the objective lens. This review theoretically describes the beam propagation within the sample using Fresnel diffraction in the frequency domain, deriving an analytical expression for the pulse evolution. SSTF can scan the temporal focal plane axially by adjusting the GVD in the excitation beam path. We theoretically define the axial confinement for line-scanning SSTF imaging using a time-domain understanding and conclude that line-scanning SSTF is similar to the temporally-decorrelated multifocal multiphoton imaging technique. Recent experiments on the temporal focusing effect and its axial confinement, as well as the axial scanning of the temporal focus by tuning the GVD, are presented. We further discuss this technique for axial-scanning multiphoton fluorescence fiber probes without any moving parts at the distal end. The temporal focusing effect in SSTF essentially replaces the focusing of one spatial dimension in conventional wide-field and line-scanning imaging. Although the best axial confinement achieved by SSTF cannot surpass that of a regular point-scanning system, this trade-off between spatial and temporal focusing can provide significant advantages in applications such as high-speed imaging and remote axial scanning in an endoscopic fiber probe. [Copyright &y& Elsevier]