Enhancement Of Harmonic Generation Research Articles

Second harmonic generation enhancement at the percolation threshold

We systematically investigate the second harmonic generated (SHG) signal created by metallic semi-continuous thin films as a function of the effective layer thickness for a large number of samples. The control of the film thickness allows us to precisely map the region around the percolation threshold. We obtain a sharp and intense SHG peak at the critical thickness where the near static permittivity diverges. This happens in the regime of almost touching particles while the thin film does not behave as a metal yet. This large SHG signal is obtained with samples fabricated using a thin film deposition technique that is easily wafer scalable.

Band-gap nonlinear optical generation: The structure of internal optical field and the structural light focusing

A novel approach for the enhancement of nonlinear optical effects inside globular photonic crystals (PCs) is proposed and systematically studied via numerical simulations. The enhanced optical harmonic generation is associated with two- and three-dimensional PC pumping with the wavelength corresponding to different PC band-gaps. The interactions between light and the PC are numerically simulated using the finite-difference time-domain technique for solving the Maxwell's equations. Both empty and infiltrated two-dimensional PC structures are considered. A significant enhancement of harmonic generation is predicted owing to the highly efficient PC pumping based on the structural light focusing effect inside the PC structure. It is shown that a highly efficient harmonic generation could be attained for both the empty and infiltrated two- and three-dimensional PCs. We are demonstrating the ability for two times enhancement of the parametric decay efficiency, one order enhancement of the second harmonic generation, and two order enhancement of the third harmonic generation in PC structures in comparison to the nonlinear generations in appropriate homogenous media. Obviously, the nonlinear processes should be allowed by the molecular symmetry. The criteria of the nonlinear process efficiency are specified and calculated as a function of pumping wavelength position towards the PC globule diameter. Obtained criterion curves exhibit oscillating characteristics, which indicates that the highly efficient generation corresponds to the various PC band-gap pumping. The highest efficiency of nonlinear conversions could be reached for PC pumping with femtosecond optical pulses; thus, the local peak intensity would be maximized. Possible applications of the observed phenomenon are also discussed.

Enhancement of Second Harmonic Signal in Nanofabricated Cones

Geometrical effects in optical nanostructures on nanoscale can lead to interesting phenomena such as inhibition of spontaneous emission,1,2 high-reflecting omnidirectional mirrors, structures that exhibit low-loss-waveguiding,3 and light confinement.4,5 Here, we demonstrate a similar concept of exploiting the geometrical effects on nanoscale through precisely fabricating lithium niobate (LiNbO3) nanocones arrays devices. We show a strong second harmonic generation (SHG) enhancement, shape and arrangement dependent, up to 4 times bigger than the bulk one. These devices allow below diffraction limited observation, being perfect platforms for single molecule fluorescence microscopy6 or single cell endoscopy.7 Nanocones create a confined illumination volume, devoid from blinking and bleaching, which can excite molecules in nanocones proximity. Illumination volume can be increased by combining the SH enhancement effect with plasmon resonances, excited thanks to a gold plasmonic shell deposited around the nanostructures. This results in a local further enhancement of the SH signal up to 20 times. The global SH enhancement can be rationally designed and tuned through the means of simulations.

Phase distortions of attosecond pulses produced by resonance-enhanced high harmonic generation

Resonant enhancement of high harmonic generation can be obtained in plasmas containing ions with strong radiative transitions resonant with harmonic orders. The mechanism for this enhancement is still debated. We perform the first temporal characterization of the attosecond emission from a tin plasma under near-resonant conditions for two different resonance detunings. We show that the resonance considerably changes the relative phase of neighboring harmonics. For very small detunings, their phase locking may even be lost, evidencing strong phase distortions in the emission process and a modified attosecond structure. These features are well reproduced by our simulations, allowing their interpretation in terms of the phase of the recombination dipole moment.

Plasmonic enhancement of High Harmonic Generation revisited: Predominance of Atomic Line Emission

We demonstrate nanostructure-enhanced extreme ultraviolet fluorescence from noble gases driven by low-energy, few-cycle light pulses. Despite suffi cient local inten- sities, plasmon-enhanced high harmonic generation is not observed, which follows from the small, nanometer-size coherent source volume.

Enhancement of high harmonic generation by confining electron motion in plasmonic nanostrutures

We study high-order harmonic generation (HHG) resulting from the illumination of plasmonic nanostructures with a short laser pulse of long wavelength. We demonstrate that both the confinement of the electron motion and the inhomogeneous character of the laser electric field play an important role in the HHG process and lead to a significant increase of the harmonic cutoff. In particular, in bow-tie nanostructures with small gaps, electron trajectories with large excursion amplitudes experience significant confinement and their contribution is essentially suppressed. In order to understand and characterize this feature, we combine the numerical solution of the time-dependent Schrödinger equation (TDSE) with the electric fields obtained from 3D finite element simulations. We employ time-frequency analysis to extract more detailed information from the TDSE results and classical tools to explain the extended harmonic spectra. The spatial inhomogeneity of the laser electric field modifies substantially the electron trajectories and contributes also to cutoff increase.

Enhancement of high-order harmonic generation in the presence of noise

We report on our simulations of the generation of high-order harmonics from atoms driven by an intense femtosecond laser field in the presence of noise. We numerically solve the non-perturbative stochastic time-dependent Schrödinger equation and observe how varying noise levels affect the frequency components of the high harmonic spectrum. Our calculations show that when an optimum amount of noise is present in the driving laser field, roughly a factor of 45 net enhancement can be achieved in high-order harmonic yield, especially, around the cut-off region. We observe that, for a relatively weak noise, the enhancement mechanism is sensitive to the carrier-envelope phase. We also investigate the possibility of generating ultra-short intense attosecond pulses by combining the laser field and noise and observe that a roughly four orders of magnitude enhanced isolated attosecond burst can be generated.

透射共振对二次谐波产生的增强

In order to increase the second harmonic generation conversion efficiency,the periodic optical superlattice was sandwiched with nonpoled and nonlinear media with finite and variable width,which can bring about the significant modulation for the fundamental wave with changing the thickness of the additional media.It is found that: the conversion efficiency increases or reduces when the energy of fundamental wave increases or reduces;it can be enhanced significantly when transmission resonance(the rate of transmission obtains 1) takes place;compared with the conventional sample(the optical superlattice embedded in infinite and nonlinear media),the second harmonic generation conversion efficiency is enhanced 1.5 times at most;for the practical sample(the optical superlattice embedded in air),it can be enhanced 3.3 times at most.So the second harmonic generation conversion efficiency can be enhanced apparently by the transmission resonance.

Multi-reflected enhancement of fourth harmonic DUV laser generation at 266 nm

The stable compact operation in the deep ultraviolet laser at 266 nm is reported from a diode-end-pumped acoustic-optic Q-switched Nd:YVO4/KTP/BBO laser. Fourth harmonic is generated by employing multiple reflections of 532 nm light for taking full advantage of the green beam. An ultraviolet laser at 266 nm with average power of 388.5 mW is obtained with a repetition-rate of 20 kHz, a pulse width of about 59.80 ns, and the corresponding optical to optical conversion efficiency of 11.9% from green to UV. High conversion efficiency and a circular spot of 266 nm laser beam have been achieved in an unfocused beam arrangement.

Second Harmonic Generation enhancement by use of annular aperture arrays embedded into silver and filled by lithium Niobate

We propose and theoretically study a metallo-dielectric photonic crystal (MDPhC) based on metallic annular aperture arrays (AAA) associated to a nonlinear material (LiNbO(3)) for the second harmonic generation (SHG). An optimal structure design can be found thanks to the relations that link the geometrical parameters to the operating point namely the wavelength of the fundamental and SHG signals. A slow light phenomenon, which occurs at the cut-off frequency of the guided mode through the annular cavities, is at the origin of the SHG signal enhancement. The benefit of the AAA is demonstrated through a comparison with cylindrical aperture arrays.

Influence of C60 morphology on high-order harmonic generation enhancement in fullerene-containing plasma

The morphologies of the fullerene targets and the ablated fullerenes to determine the optimal conditions of excitation of the C60-containing targets have been analyzed. The optimization of fullerene-containing plasma conditions allowed the enhanced harmonic generation in these plasmas using laser radiation of different wavelengths, pulse durations, and phase modulation. A comparison between the harmonic generation in single-atom/ion-containing plasmas (using bulk carbon, silver, and indium targets) and fullerene-rich plasma plumes showed better conversion efficiency for the latter medium. The influence of phase modulation of the fundamental radiation in fullerene plasmas on the spectral properties of harmonics has been studied.

Enhancement of high-order harmonic generation using a two-color pump in plasma plumes

The results of enhanced high-order harmonic generation (HHG) in laser-produced plasmas using a two-color laser pump (98% of 800 nm and 2% of 400 nm) are presented. Even (up to the 38th order) and odd (up to the 45th order) harmonics of 800 nm radiation were generated in the case of a two-color orthogonal polarization pump of various plasmas. The comparison between HHGs using parallel and orthogonally polarized 400 and 800 nm radiations showed better conversion efficiency for the latter case. We have also used circularly polarized 800 nm pump for second-harmonic generation and HHG. We also present the results of the influence of the chirp and phase modulation of the fundamental radiation in laser plasma on the spectral properties of odd and even harmonics.

Tunable high-order harmonic generation and the role of the folded quantum path

We theoretically demonstrate the selective enhancement of high-order harmonic generation (HHG) in two-color laser fields consisting of a single-cycle fundamental wave ($800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wavelength) and a multicycle subharmonic wave ($2400\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ wavelength). By performing time-frequency analyses based on a single-active-electron model, we reveal that such an enhancement is a result of the modified electron trajectories in the two-color field. Furthermore, we show that selectively enhanced HHG gives rise to a bandwidth-controllable extreme ultraviolet supercontinuum in the plateau region, facilitating the generation of intense single isolated attosecond pulses.

Monitoring long-term evolution of molecular vibrational wave packet using high-order harmonic generation

We present a method for probing ultrafast nuclear dynamics in molecules using transient enhancement of high-order harmonic generation (HHG). The method exploits strong dependence of the overall harmonic yield in a fixed spectral range on the nuclear separation in a molecule, which is shown to take place for both aligned and randomly oriented molecules. Our numerical simulations show that this method is capable of monitoring long-term evolution of the nuclear vibrational wave packets in molecules, even in light-weight ones, with very high time resolution. By the example of D2+ vibrational wave packet launched via tunnelling ionization of D2 and probed by a time-delayed 8 fs laser pulse with λ = 800 nm and the peak intensity of 1014 W cm-2, we show that the time-delay dependence of the high harmonic signal exhibits pronounced features, which are due to the collapses and revivals of the nuclear wave packet. The time-frequency analysis of the pump–probe signal reveals structures with a periodicity down to 6 fs, which correspond to the fractional revivals of orders 1/5 and 1/10. With the laser parameters used, the deuteron motion during the probe pulse is shown to have almost negligible effect on the resulting signal.

Enhancement of Relativistic Harmonic Generation by an Optically Preformed Periodic Plasma Waveguide

Enhancement of relativistic third-harmonic generation by using an optically preformed periodic plasma waveguide was achieved. Resonant dependence of harmonic intensity on plasma density and density modulation parameters was observed, which is a distinct characteristic of quasi-phase-matching. The results demonstrate the potential of a modulated plasma waveguide in high-field applications.

Enhancement of high-order harmonic generation by a two-color field: Influence of propagation effects

Recent calculations of the response of a single atom subjected to a two-color laser pulse with the higher frequency being resonant with an excitation of the target atom revealed a significant enhancement of photoionization as well as high-order harmonic generation [K. Ishikawa, Phy. Rev. Lett. 91, 043002 (2003)]. We investigate the problem in the framework a fully quantum-mechanical pulse propagation algorithm and perform calculations for rare gases in the single-active-electron approximation. The enhancement of harmonic output compared to the corresponding one-color pulse remains intact for short propagation lengths, promising the feasibility of experimental realization. We also study weak second colors resonant via a two-photon transition where significant enhancements in harmonic yields can be observed as well.

Significant Enhancement of the Optical Second Harmonic Generation in a Poled Azopolymer Thin Grating

Nonlinear optical properties of an electrically poled surface relief grating inscribed on a thin film of azopolymer have been investigated. The linear and nonlinear optical far-field diffraction patterns of the grating are compared, and they show a clear angular separation of the fundamental (lambda = 1.064 mum) and second harmonic generated beams that are diffracted at different angles. The intensity of the zero order transmitted second harmonic generation (SHG) beam from inscribed surface relief grating (SRG) areas has been recorded using the Maker fringe technique and compared to the response from flat areas that were only poled. Poled gratings exhibit a sharp second harmonic generation enhancement for coupling angles of theta(i) = +/-52 degrees due to a quasi-phase matching process. A simple phenomenological model allows one to explain both the second harmonic intensity generated from the thin polar film and the dispersion curve observed under "pp" polarization due to the mismatch of the angular coupling condition: this gives us a good indication of the intensity enhancement of the SHG beam in the forward direction.

Enhancement of Bichromatic High-Order Harmonic Generation by aStrong Laser Field and Its Third Harmonic

We investigate high-order harmonic generation (HHG) in a linearly polarizedbichromatic field composed of a fundamental laser field withfrequency ω and an additional laser field with frequency 3ω. Thenumerical results show that it is possible to enhance the intensity ofmost high harmonics in orders of magnitude. A most striking feature inthe enhancement is that the intensity of several special high harmonicsis practically impaired as compared with that in the monochromatic case.The qualitative explanation to the great enhancement is that theadditional high-frequency field can provide new transition paths forelectrons to reach the continuum. The relative phase between thefundamental field and its third harmonic field also affects theintensity of high-order harmonics near the cutoff efficiently.

Enhancement of harmonic generation by Fresnel-lensing effects

We present experimental results of enhanced second- and third-harmonic generation efficiency from Fresnel-lensing effects. We obtained enhancement by a factor of 6 in the third-harmonic yield by simply limiting the laser beam with an iris. Higher enhancement factors (up to 16) were obtained with a Fresnel zone plate. The experimental findings are in good agreement with the results of a simple theoretical analysis.

Second Harmonic Generation of C60Incorporated in Alkali Metal Ion Zeolites and Mesoporous MCM-41 Silica

Different loadings of C60 have been incorporated in mesoporous MCM-41 aluminosilicate as well as proton and alkali metal ion zeolites. These samples show non-linear optics activity whose efficiency increases upon C60 content, size of the alkali metal ion, and laser exposure. The later unprecedented second harmonic generation (SHG) enhancement effect can lead up to 2 orders of magnitude efficiency increase. After having irradiated the solid, the SHG enhancement decays upon standing in the dark toward the initial value of fresh and unexposed samples. EPR measurements and alternative generation of zeolite electron−holes ions pairs have ruled out triplet excited states and radical ions as responsible for this enhancement upon irradiation. One proposal at the moment is that this SHG activity enhancement arises from unstable photoproducts such as photoadduct or dimers. However, attempts to isolate any C60-derived photoproduct have met with failure so far.