Harmonic Generation Of Ultrashort Pulses Research Articles

Intermodal effective phase matching for second harmonic generation of ultrashort pulses in high index contrast optical fibers

Second harmonic generation (SHG) in glass optical fibers calls for creating a second order susceptibility in the fiber glass and for achieving phase matching between the pump and the second harmonic signal. The latter is very challenging when using ultrashort pulses, given that the group velocities of the pump and the second harmonic should also be matched. We have shown in previous work that it is possible to achieve simultaneous modal phase matching (MPM) and group velocity matching (GVM) when the pump and the second harmonic are propagating in the LP01 and LP02 modes, respectively, in high GeO2-content double-clad optical fibers. However, simultaneous MPM and GVM can only be obtained in optical fibers with dedicated designs and within very tight geometrical tolerances. In this paper, we show that instead of considering the matching of phase and group velocities separately, we can consider a more general or “effective” phase matching approach, in which we consider all the dispersion terms up to the second order in the expressions of the propagation constants of the pump and second harmonic signals. This allows introducing the pulse duration as a controllable parameter that helps to enforce the said effective phase matching in fibers with designs that deviate by as much as 10% from the target, while providing for temporal walk-off lengths in excess of several centimeters. The impact of this finding goes beyond SHG only and can be applied to other ultrashort laser pulse-based nonlinear optical processes in fibers and waveguides.

Involvement of small carbon clusters in the enhancement of high-order harmonic generation of ultrashort pulses in the plasmas produced during ablation of carbon-contained nanoparticles

Various carbon-based nanoparticles ablated at the conditions suitable for efficient harmonic generation during propagation of ultrashort pulses through the laser-produced plasmas were studied. The transmission electron microscopy of ablated debris and the time-of-flight mass-spectroscopy studies of plasmas are presented. The conditions of laser ablation of the carbon-contained nanoparticles (fullerenes, graphene, carbon nanotubes, carbon nanofibers, and diamond nanoparticles) were varied to define the impeding processes restricting the harmonic yield from such laser-produced plasmas. These studies show that the enhancement of harmonics during ablation of nanoparticle targets was related with the appearance of small carbon clusters at the moment of propagation of the ultrashort laser pulses though such plasmas.

Involvement of small carbon clusters in the enhancement of high-order harmonic generation of ultrashort pulses in the plasmas produced during ablation of carbon-contained nanoparticles, "Оптика и спектроскопия"

Involvement of small carbon clusters in the enhancement of high-order harmonic generation of ultrashort pulses in the plasmas produced during ablation of carbon-contained nanoparticles, "Оптика и спектроскопия"

Application of organic compounds for high-order harmonic generation of ultrashort pulses

The studies of the high-order nonlinear optical properties of a few organic compounds (polyvinyl alcohol, polyethylene, sugar, coffee, and leaf) are reported. Harmonic generation in the laser-produced plasmas containing the molecules and large particles of above materials is demonstrated. These studies showed that the harmonic distributions and harmonic cutoffs from organic compound plasmas were similar to those from the graphite ablation. The characteristic feature of observed harmonic spectra was the presence of bluesided lobes near the lower-order harmonics.

Double-pulse induced harmonic generation in laser-produced plasmas

We report the studies of the metals, non-metals, powders, and nanoparticles as the targets for laser ablation induced high-order harmonic generation of ultrashort pulses using the double-pulse technique. The proposed technique demonstrates the attractiveness as the method for the studies of the high-order nonlinear optical properties of various materials. The comparative analysis of the harmonic generation using different targets showed that the species allowing easier ablation (powders, nanoparticles) produce stronger harmonic yield in the extreme ultraviolet range.

Electron density measurements using high-order harmonic generation in laser-produced plasmas

We demonstrate the method of the measurements of electron density in the low-dense, low-ionized laser-produced plasma using the nonlinear optical process of high-order harmonic generation of ultrashort pulses at the conditions of the quasi-phase-matching of driving and harmonic waves. Electron density was defined using the relations of quasi-phase-matching. We show the results of our measurements of electron density in the range of 1016–1017 cm−3 using the multi-jet silver plasma produced for the harmonic generation.

Harmonic generation from partially ionized plasma [Invited

The latest achievements in high-order harmonic generation of ultrashort pulses in plasma plumes are reviewed. These studies demonstrate the attractiveness of this advanced approach of coherent extreme ultraviolet radiation generation. We analyze early studies in this field and review the application of quasi-phase matching, extended plasma plumes, cluster-containing ablation, etc., for improvement of the harmonic generation from partially ionized plasmas.

Morphology of laser-produced carbon nanoparticle plasmas and high-order harmonic generation of ultrashort pulses in clustered media

We analyse the morphology of the laser plasmas produced on surfaces containing various carbon clusters (nanotubes, nanofibres, fullerenes, and nanoparticles). It is shown that the presence of C5–C25 nanoparticles in the laser plasmas coincides with the enhancement of harmonics in the spectral range of 40–80 nm. We suggest that these nanoparticles, rather than larger clusters, play a decisive role in the observed growth of harmonic yield.

Application of carbon cluster-contained extended plasmas for the high-order harmonic generation of ultrashort pulses

We present the studies of the high-order harmonic generation of 802-nm, 64-fs pulses in the extended plasmas contained the carbon-based clusters (fullerenes, nanofibers, nanoparticles, and nanotubes). Our studies show that the 11th–19th harmonics in the 15–33 eV range may originate from the low-sized nanoparticles produced during ablation of large clusters and agglomerates. The conversion efficiency of the 11th harmonic generated from the 5-mm-long fullerene plasma was estimated to be 2 × 10−5.

Second harmonic generation of ultrashort pulses in refractive-index-linear-modulating nonlinear crystals

We present a theoretical model to deal with second harmonic generation (SHG) of ultrashort pulses in refractive-index-linear-modulating (RILM) nonlinear crystals. New coupled-wave equations were derived based on plane wave propagation. Under the undepleted-pump approximation, the analytic solutions for SHG of ultrashort pulses were obtained. The second harmonic (SH) pulse width and the efficiency were optimized in terms of the RILM parameter and the chirp coefficient of fundamental pulses. The results show that modulation of the refractive index (RI) in nonlinear crystals leads to a spectral localization of SHG that compensates the group velocity mismatch (GVM) and therefore compresses SH pulses. The modulation complicates the SHG process, which determines the pulse width, and the conversion efficiency varies with length and bandwidth. Especially, an overshooting compression phenomenon was found, which arose mainly from internal interference of the carrier frequency component in a SH pulse.

Theory of second harmonic generation of ultrashort pulses for collinear achromatic phase matching

Achromatic phase matching (APM), a favorable method in increasing phase matching bandwidth, is explored for second harmonic generation of ultrashort pulses based on a typical grating telescope system. A set of coupled equations incorporating angular dispersion is constructed in the space–time domain. An analytic solution with a pump of tilting pulsed Gaussian beam to the equations is given under the undepleted pump approximation. With the aid of matrix formalism, some properties of the conversion are demonstrated. Though a maximal phase matching bandwidth is obtained, angular dispersion makes the conversion active only around the geometrical focus. Theory shows that APM does not require an overall pulse-front matching in the conversion process.

Properties of periodic multicrystal configurations in walk-off-compensating second harmonic generation of ultrashort pulses

This work designs a four-platelet periodic multicrystal configuration in the second harmonic generation of ultrashort pulses as a new walk-off-compensating device. It theoretically investigates a proposed active and a typical passive compensating scheme with the undepleted-pump approximation. The result shows that the angular and spectral band-widths are proportional to the number of crystal pairs as expected, but the temperature tunability is basically unaltered owing to inter-plate pulse interference. At the same time, an analysis reveals that a misuse of the phase mismatch factor is responsible for a historic controversy about pulse interference. A real design of an ultraviolet second harmonic generation (262.5 nm) is considered in a passive periodic β-Barium Borate-calcite configuration, where the inter-plate pulse interference is found to form an azimuthal tuning restriction and to lower plate length tolerance. A subsequent numerical simulation with pump depletion is in good accordance with theoretical prediction.

Cavity behavior of second and third harmonic inhomogeneous solutions of Maxwell's equations

We study second and third harmonic generation of ultrashort pulses in a cavity environment and focus on the role of a phase locking phenomenon that causes the pump to trap and drag the generated harmonic fields. The results reveal strong field localisation and resonance conditions for the phase locked components, which are tuned in a spectral region of huge nominal material absorption. This unique behavior suggests the possibility of new optical phenomena and new applications for opaque materials (i.e. semiconductors) in the visible and UV ranges, such as new laser sources with relatively high conversion efficiencies.

Efficient self phase matched third harmonic generation of ultrashort pulses in a material with positive dispersion

An ultrafast transient refractive index grating, produced in barium fluoride, a material with positive dispersion, promotes very efficient third harmonic generation. The significant enhancement of the generation up to a conversion efficiency of about 3% is due to self phase matching, involving the instantaneous grating. At the same time several diffraction orders of the third harmonic signal are observed behind the sample.

Ultrashort pulse second harmonic generation in quasi-phase-matched structures

Second harmonic generation of ultrashort pulses in first- and higher-order quasi-phase-matched nonlinear structures is investigated theoretically. It is shown that using such structures high conversion efficiency can be achieved while preserving the ultrashort pulse duration. Using periodically poled bulk lithium niobate as an example, it is found that nearly complete conversion of a 900 nm wavelength 100 fs pulse to the second harmonic may be possible in a first-order structure without any broadening of the pulse width.