Third-harmonic Pulses Research Articles

Ultraviolet supercontinuum generation using a differentially-pumped integrated glass chip

We investigate the generation of ultrabroadband femtosecond ultraviolet (UV) radiation via third-order harmonic generation in highly confined gas media. A dual-stage differential-pumping scheme integrated into a glass microfluidic chip provides an exceptional gas confinement up to several bar and allows the apparatus to be operated under high-vacuum environment. UV pulses are generated both in argon and neon with up to ∼0.8 μJ energy and 0.2% conversion efficiency for spectra that cover the UVB and UVC regions between 200 and 325 nm. Numerical simulations based on the unidirectional pulse propagation equation reveal that ionization plays a critical role for extending the spectral bandwidth of the generated third-harmonic pulse beyond the tripled 800 nm driving laser pulse bandwidth. By delivering UV supercontinua supporting Fourier transform limits below 2 fs, as well as comparable pulse energies with respect to capillary-based techniques that typically provide high spectral tunability but produce narrower bandwidths, our compact device makes a step forward towards the production and application of sub-fs UV pulses for the investigation of electron dynamics in neutral molecules.

Hybrid nanowires for phase-matching of third-harmonic generation in hyperbolic metamaterial.

We propose a phase-matching technique for third-harmonic generation, called hyperbolic phase matching, that possibly can be achieved by optimal designing and engineering dispersion of hybrid-nanowire hyperbolic metamaterial. We demonstrate phase-matched conditions for two different third-harmonic interacting configurations, which can be created at two optimal incident angles of the pump field. Moreover, each composed hybrid nanowire can enhance third-harmonic generation by using strong field confinement along the metal/dielectric interface due to plasmonic resonance. Finally, conversion efficiencies of transmitted and reflected third-harmonic pulses as a function of incident angle and input pulse intensity are examined by numerical integration of nonlinear birefringent coupled-mode equations. The numerical results validate the idea that, using a combination of phase-matched conditions and pump field confinement, we can achieve a dramatic enhancement of conversion efficiencies of third-harmonic generation.

Very high brightness and power LCLS-II hard X-ray pulses.

The feasibility of generating X-ray pulses in the 4-8 keV fundamental photon energy range with 0.65 TW peak power, 15 fs pulse duration and 9 × 10-5 bandwidth using the LCLS-II copper linac and hard X-ray (HXR) undulator is shown. In addition, third-harmonic pulses with 8-12 GW peak power and narrow bandwidth are also generated. High-power and small-bandwidth X-rays are obtained using two electron bunches separated by about 1 ns, one to generate a high-power seed signal, the other to amplify it through the process of the HXR undulator tapering. The bunch delay is compensated by delaying the seed pulse with a four-crystal monochromator. The high-power seed leads to higher output power and better spectral properties, with more than 94% of the X-ray power within the near-transform-limited bandwidth. Some of the experiments made possible by X-ray pulses with these characteristics are discussed, such as single-particle imaging and high-field physics.

Backward Third-Harmonic Pulse Generation in a One-Dimensional PIM/NIM Structure

In this work, a set of couple-mode equations for describing a backward third harmonic generation (BTHG) in a one-dimensional periodic structure of positive-index material (PIM) layers and third-order nonlinear negative-index material (NIM) layers was analysed using multiple-scale method. Due to the negative-index phase matching and band-edge field enhancement the intensities of backward third-harmonic pulsed generated from the PIM/NIM periodic structure is increased for 100 time of those generated from a single NIM medium.

Quasibirefringent phase-matching technique for third-harmonic pulse generation from multilayered metamaterials

A phase-matched third-harmonic (TH) generation from multilayered metamaterials (MMs) with a third-order nonlinearity is numerically investigated in the ultrashort pulse regime. To overcome the phase-mismatch problem due to natural material dispersion, we propose a solution based on engineered dispersion provided by MMs with hyperbolic dispersion, i.e., with anisotropic materials possessing a positive and negative dielectric permittivity. We analyzed selected material choices and demonstrated quasibirefringent phase-matched conditions for TH conversion, which could be achieved by optimizing the fill-factor of metamaterials. We study the conversion efficiencies of transmitted and reflected TH pulses as a function of incident angles and input fundamental-frequency intensity; the maximum efficiencies are obtained for optimal incident angles.

Enhancement of backward third-harmonic generation in a one-dimensional PIM/NIM periodic structure

A numerical simulation is reported for backward third-harmonic generation (BTHG) in a one-dimensional periodic structure containing a stack composed of positive-index material (PIM) and negative-index material (NIM). A multiple-scale method is used to formulate a completed set of coupled-mode equations with inhomogeneous higher-order nonlinear terms for both electric and magnetic fields. The coupled equations are numerically solved to simulate output third-harmonic frequency pulses and their conversion efficiencies by a fast Fourier transform–pulse propagation method. Finally, the numerical results validate the idea that using a combination of the negative-index phase-matched condition, which is created by engineering the dispersive property of the NIM layer, and local field enhancement, which is created by arranging PIM and NIM layers in an optimal periodic fashion, yields a dramatic enhancement of BTHG conversion efficiency.

Laser structuring of the diamond surface in the nanoablation regime

The possibility of using nanoablation (graphitisation-free photoinduced etching) for precise micro- and nanostructuring of the surface of a diamond single crystal has been experimentally investigated. The processing has been performed by femtosecond third-harmonic pulses of a Ti : sapphire laser . The specific features of the formation of a surface nanorelief (regularity of the newly formed structures and reduced nanoablation rate near craters) are discussed. The possibility of fabricating a diamond phase diffraction grating with a relief depth of is demonstrated and the diffraction pattern recorded at is compared with the results of theoretical analysis.

Spectral modulation of third-harmonic generation by molecular alignment and preformed plasma**Project supported by the National Key Scientific Instrument Project, China (Grant No. 2012YQ150092), the National Basic Research Program of China (Grant No. 2011CB808105), the National Natural Science Foundation of China (Grant No. 11434005), China Postdoctoral Science Foundation (Grant No. 2014M560348), the National Natural Science Foundation of China (Grant No. 11504237),

We demonstrate spectral modulation of third-harmonic generation from molecular alignment effects. The third harmonic spectrum is broadened or narrowed under different influences of cross-phase modulations originating from various molecular alignment revivals. Furthermore, the spectrum and spatial distribution of the generated third harmonic pulse change dramatically in the presence of a preformed plasma. Under the influence of a preformed plasma, a narrower third harmonic spectrum is observed, and the conical third-harmonic pulse increases while the axial part decreases. The investigation provides an effective method to modulate the spectral characteristic and spatial distribution of third-harmonic generation from intense femtosecond filament.

Effect of pulse slippage on density transition-based resonant third-harmonic generation of short-pulse laser in plasma

The resonant third-harmonic generation of a self-focusing laser in plasma with a density transition was investigated. Because of self-focusing of the fundamental laser pulse, a transverse intensity gradient was created, which generated a plasma wave at the fundamental wave frequency. Phase matching was satisfied by using a Wiggler magnetic field, which provided additional angular momentum to the third-harmonic photon to make the process resonant. An enhancement was observed in the resonant third-harmonic generation of an intense short-pulse laser in plasma embedded with a magnetic Wiggler with a density transition. A plasma density ramp played an important role in the self-focusing, enhancing the third-harmonic generation in plasma. We also examined the effect of the Wiggler magnetic field on the pulse slippage of the third-harmonic pulse in plasma. The pulse slippage was due to the group-velocity mismatch between the fundamental and third-harmonic pulses.

The extended “ladder” transition controlled by two harmonic pulses

The extended “ladder” transition controlled by two harmonic pulses is investigated by using a time-dependent quantum wave packet method. The molecular population transfers from the state|0, 0> angle to the states|5, 0> angle and|5, 2> angle induced by the fundamental and second-harmonic pulses, and to the states|5, 3> angle and |6, 2> angle induced by the fundamental and third-harmonic pulses. The calculated results show that the two harmonic pulses can induce a nearly 100% of population to transfer to the target state. The relative phase of two pulses can affect the population distribution. The variation of population has a period of π for the fundamental and second-harmonic pulses, and a period of 2π for the fundamental and third-harmonic pulses.

Phase control of two-color filamentation

An original way to control the nonlinear propagation of an intense pulse is presented. The co-propagation of a weak third-harmonic pulse with an intense laser pulse experiencing filamentation allows control of the nonlinear propagation of the latter. Because of quantum interference during the two-color ionisation process, the latter can be significantly enhanced or suppressed by a simple tuning of the relative phase between the two fields. As a first application, we demonstrate the production and control of an axially modulated plasma channel. Finally, an analytical formula describing the two-color ionisation rate as a function of the relative phase and intensity of the two fields is presented and tested in a propagation code. The numerical results successfully reproduce the experimental ones.

Dark pulses and harmonic mode locking in graphene-based passively mode-locked Yb3+-doped fiber laser with all-normal dispersion cavity

Graphene has recently been proposed as an attractive material in saturable absorption (SA) applications due to its broad operation range, low saturation power, easy fabrication, high reliability, and quick recovery time. In this paper, we use laser-induced deposition to prepare graphene saturable absorber, and apply it in a mode-locked all-normal-dispersion (ANDi) Yb-doped fiber laser to experimentally investigate different operational states. By adjusting a polarization controller (PC) and the pump power, bright pulses, dark-bright pulse pairs and their second-harmonic pulses, as well as dark pulses and their second, third-harmonic pulses can all be obeserved. In particular, it is the first time to our knowledge to report on the formation of dark-bright pulse pairs, dark pulses and their harmonic mode locking (HML) counterparts in graphene-based passively mode-locked Yb-doped fiber laser with ANDi cavity. Accoding to simulation, the main causes of these pulses are different cavity nonlinear effects which result from the fiber mode-lock members including graphene. Bright pulses, dark pulses and dark-bright pulse pairs are determined both by the laser structure and their own initial signals. Bright pulse harmonic generation is ascribed to the noise gains which form new components. However, it is found that the multiple-time repetition rate of dark pulses is a result of square pulse splitting of each component. This consequence may be of potential application in new type mode-locked fiber lasers.

Multidimensional resonance Raman spectroscopy by six-wave mixing in the deep UV.

Two-dimensional (2D) resonance Raman spectroscopies hold great potential for uncovering photoinduced relaxation processes in molecules but are not yet widely applied because of technical challenges. Here, we describe a newly developed 2D resonance Raman experiment operational at the third-harmonic of a Titanium-Sapphire laser. High-sensitivity and rapid data acquisition are achieved by combining spectral interferometry with a background-free (six-pulse) laser beam geometry. The third-harmonic laser pulses are generated in a filament produced by the fundamental and second-harmonic pulses in neon gas at pressures up to 35 atm. The capabilities of the setup are demonstrated by probing ground-state wavepacket motions in triiodide. The information provided by the experiment is explored with two different representations of the signal. In one representation, Fourier transforms are carried out with respect to the two experimentally controlled delay times to obtain a 2D Raman spectrum. Further insights are derived in a second representation by dispersing the signal pulse in a spectrometer. It is shown that, as in traditional pump-probe experiments, the six-wave mixing signal spectrum encodes the wavepacket's position by way of the (time-evolving) emission frequency. Anharmonicity additionally induces dynamics in the vibrational resonance frequency. In all cases, the experimental signals are compared to model calculations based on a cumulant expansion approach. This study suggests that multi-dimensional resonance Raman spectroscopies conducted on systems with Franck-Condon active modes are fairly immune to many of the technical issues that challenge off-resonant 2D Raman spectroscopies (e.g., third-order cascades) and photon-echo experiments in the deep UV (e.g., coherence spikes). The development of higher-order nonlinear spectroscopies operational in the deep UV is motivated by studies of biological systems and elementary organic photochemistries.

Third- and fifth-harmonic generation in transparent solids with few-optical-cycle midinfrared pulses

We report an experimental and numerical investigation of third- and fifth-harmonic generation in a CaF${}_{2}$ crystal with 20 fs (three-optical-cycle), 2 $\ensuremath{\mu}$m driving pulses. The double-peaked temporal profile of the third-harmonic pulse and its propagation dynamics was captured by means of the cross-correlation technique, showing that the third-harmonic pulse naturally consists of free and driven components propagating with different group velocities, and which occur without the splitting of the driving pulse at the fundamental frequency. Relevant characteristics of the harmonics generation process, such as the harmonics spectra, energy oscillations, and conversion efficiency, were measured as functions of propagation length and input-pulse energy and intensity. Our results demonstrate that the fifth harmonic is generated solely via cascaded four-wave mixing between the fundamental and third-harmonic frequency pulses due to cubic nonlinearity, without any detectable contribution of six-wave mixing due to quintic nonlinearity in the process.

Third-harmonic pulse generation in one-dimensional photonic crystal structures

Enhanced third-harmonic generation in a one-dimensional photonic crystal doped with third-order nonlinear medium was numerically investigated using the multiple-scale method and the split-step Fourier transform. The optimal fundamental frequency for third-harmonic wave generation was determined from the transmission spectrum. The third-harmonic pulse intensities grow, depending on the structure thickness and the fundamental-frequency detuning parameter, which determines the band-edge phase matching condition. Furthermore, the total energy output of third-harmonic pulses, depending on the fundamental-frequency pulse width, may be more than 1000 times the energy produced by a phase-matched bulk medium. A narrow pulse with bandwidth less than the band-edge transmission peak enables high conversion efficiency. The maximum conversion efficiency of the forward component may be 12 to 13 orders of magnitude greater than that of the backward component.

Three Multiple-Pulse Operation States of an All-Normal-Dispersion Dissipative Soliton Fiber Laser

Multiple-pulse operation states of an all-normal-dispersion Yb-doped double-clad dissipative soliton fiber laser are investigated in this paper. The proposed laser can deliver harmonic mode-locked pulses, bound states of dissipative solitons, and dual-wavelength dual-pulses. Stable second-harmonic and third-harmonic mode-locked pulse trains are obtained with the output power of 1.39 W and 1.46 W, respectively, and the corresponding single pulse energies are 12.1 nJ and 8.5 nJ. With the adjustment of pump power and the wave plates, the fiber laser generates bound states of two or three dissipative solitons. Moreover, a dual-wavelength dual-pulse state is presented, where the output pulses from the nonlinear polarization rotation rejection port consists of the leading and trailing edges of the pulses circulating in the cavity.

Effect of plasma channel non-uniformity on resonant third harmonic generation

AbstractWe study the generation of resonant third harmonic laser radiation in a density non-uniform rippled plasma channel. An introduction of plasma channel non-uniformity strongly enhances the self-focusing and compression of main laser pulse at lower powers. In a deeper plasma channel, self-focusing is less sensitive to laser amplitude variation but increases compression. Plasma density ripple ‘nq’ leading to resonant third harmonic generation when kq = 4ω2p/3meω0cγ0, where ‘ω’p is electron plasma frequency, ‘ω0’ is laser frequency, and ‘γ0’ is the electron Lorentz factor. Third harmonic is produced through the beating of ponderomotive force induced second harmonic density oscillations and the oscillatory velocity of electrons at main laser frequency. The self-focusing and compression of the fundamental pulse periodically enhances the intensity of the third-harmonic pulse at lower powers of main laser. In a deeper plasma channel, the third harmonic power is less effective by self-focusing and the compression of main laser, and increase with main laser pulse power.

Two-dimensional plasma grating by non-collinear femtosecond filament interaction in air

We experimentally demonstrated that two-dimensional (2D) plasma gratings could be generated in air by nonlinear interaction of three femtosecond filaments. The intensity interference of non-collinearly overlapped filaments was self-projected along a relatively long distance and accompanied with a wavelength-scale periodic change of the refractive index in the encircling air due to periodic plasma density modulation. The 2D plasma gratings supported 2D diffraction of the generated third-harmonic pulses. By using in-line time-resolved holographic imaging and time-delayed diffraction, the 2D plasma gratings were evidenced to last a few tens picoseconds after the excitation pulses.

Generation of high-energy, sub-20-fs pulses in the deep ultraviolet by using spectral broadening during filamentation in argon

Generation of sub-20-fs UV pulses with more than 300 μJ energy at 268 nm is reported. First, the UV pulses are produced by successive second-harmonic and third-harmonic (TH) generation of 805 nm pulses of a 1 kHz Ti:sapphire laser amplifier. The spectral broadening of TH pulses is realized in a filament, generated in argon. The produced pulses are compressed in a simple double-pass prism-pair compressor. Starting from 100 fs pulses, we achieve a fivefold pulse shortening.

Frequency doubling and tripling in a Q-switched fiber laser

The frequency doubling and tripling in a Q-switched all-fiber laser are studied. It is demonstrated that the main limitations on the efficiency of the harmonic generation are related to the random polarization that is nonuniform with respect to axes, the asymmetric pulse shape with a flat trailing edge, and the significant spectral broadening of the multimode radiation of the fiber master oscillator in the fiber amplifier. The methods to increase the efficiency are proposed. For an IR pulse energy of about 0.3 mJ, duration of about 40 ns, and repetition rate of 1 kHz, the second- and third-harmonic pulse energies are greater than 60 and about 10 μJ, respectively.