Probing time-resolved emission in laser-driven electron-multirescattering in a high-order harmonic generation

We present an ab initio study of quantitative extraction of the time–frequency spectra of multiple rescattering processes of the electrons driven by the mid-infrared laser field in a high-order harmonic generation (HHG). The HHG is calculated by solving the three-dimensional time-dependent Schrödinger equation by means of the time-dependent generalized pseudospectral method. We extend a synchrosqueezed transform (SST) technique to extract the individual contributions of multiple rescattering processes in HHG. Combining with an extended semiclassical analysis and the SST time–frequency spectrum, the role of quantum trajectories in multiple rescattering processes in HHG is clarified. We find that the SST allows us to distinguish the individual contribution of multiple rescattering processes in HHG and show the details of the spectral and temporal fine structures of the HHG, which provides an important tool for a deep understanding to the dynamics of multiple rescattering processes in HHG.     

Estimating the plasmonic field enhancement using high-order harmonic generation: the role of the field inhomogeneity

In strong field laser physics it is a common practice to use the high-order harmonic cutoff to estimate the laser intensity of the pulse that generates the harmonic radiation. Based on semiclassical arguments it is possible to find a direct relationship between the maximum value of the photon energy and the laser intensity. This approach is only valid if the laser electric field driving the high-order harmonic radiation is spatially homogeneous. In laser–matter processes driven by plasmonics fields, the enhanced fields present a spatial dependence that strongly modifies the electron motion and consequently all the associated laser driven phenomena. As a result, this method should be revised in order to more realistically estimate the intensity of the laser field. In this work, we demonstrate how the inhomogeneity of the enhanced plasmonic fields will affect this estimation. Furthermore, by employing both quantum mechanical and classical calculations, we show how one can obtain a better estimation for the intensity of the enhanced field in plasmonic nanostructures.     

Below-threshold harmonic generation from strong non-uniform fields

Strong-field photoemission below the ionization threshold is a rich/complex region where atomic emission and harmonic generation may coexist. We studied the mechanism of below-threshold harmonics (BTH) from spatially non-uniform local fields near the metallic nanostructures. Discrete harmonics are generated due to the broken inversion symmetry, suggesting enriched coherent emission in the vuv frequency range. Through the numerical solution of the time-dependent Schrödinger equation, we investigate wavelength and intensity dependence of BTH. Wavelength dependence identifies counter-regular resonances; individual contributions from the multi-photon emission and channel-closing effects due to quantum path interferences. In order to understand the underlying mechanism of BTH, we devised a generalized semi-classical model, including the influence of Coulomb and non-uniform field interactions. As in uniform fields, Coulomb potential in non-uniform fields is the determinant of BTH; we observed that the generation of BTH are due to returning trajectories with negative energies. Due to large distance effectiveness of the non-uniformity, only long trajectories are noticeably affected.     

Hole trapping in polydiacetylene field effect transistor studied by optical second harmonic generation

Hole trapping in polydiacetylene field effect transistor (PDA-FET) was studied by the electric field induced second harmonic generation (EFISHG). Response of SHG signal from PDA-FET with an application of external voltage was monitored. Applying positive voltage to source and drain electrodes with respect to gate electrode, SHG signal was not observed during bias application, whereas the signal was enhanced after turning off the bias. Since positive bias promotes hole injection from source and drain electrodes, electric field formed by trapped holes in PDA layer activated the SHG signal. Microscopic SHG measurement implies that the trapped holes are concentrated around source and drain electrodes.     

Theoretical modeling on resonating the second harmonic for ultraviolet laser generation

The resonance of the second harmonic was proposed to improve conversion efficiency in ultraviolet laser. A schematic to generate the fourth harmonic laser with a compound cavity was modeled based on plane-wave approximation. The simulated results show that the mode-matching condition of the multi-mode lasers depends strongly on both frequency-mode spacing and resonator loss. In the case of the same cavity lengths and losses, the tolerance on the optical cavity length should be maintained on tens or hundreds of nanometer scales. Meanwhile, the TEM₀₀ beam can be achieved due to the best mode-matching at resonance and the beam size increases arising from the cavity detune.     

Quantum wavepacket exploration of isolated high-order harmonic attosecond pulse generation: from two-color scheme to three-color scheme

Isolated attosecond generation under the two-color scheme and three-color scheme are investigated theoretically by use of wave-packet dynamics calculation. Optimized relative phase under the two-color field is obtained and comparison is taken between the second-harmonic control and half-harmonic control. A further exploration of the three-color scheme by adding a weak second-harmonic pulse and a half-harmonic pulse onto 8×10¹⁴?W?cm^?2 6?fs/800?nm pulse shows the potential for generating the 337?eV continuum supporting creation of an isolated 11 as pulse.     

Probing and modeling of carrier motion in organic devices by optical second harmonic generation

We here report a novel optical second harmonic generation (SHG) measurement that allows an electric field formed in organic solid to be probed. We examined the SHG intensity profile that changes depending on a space charge field caused by carrier injection. Experiments making use of time-resolved SHG technique has revealed dynamic changes of SHG intensity profiles arising from pentacene, and that carrier transport in OFET was diffusion-like. Calculations using drift-diffusion equation well accounted for the visualized carrier motion probed by time-resolved SHG. That is, interface charge propagation process dominates the carrier transport, which is regulated by applied gate-source voltage.     

Numerical simulation of cascaded third harmonic generation for both phase matched and mismatched schemes

Numerical solutions to the nonlinear coupling-wave equations of second harmonic (SH) and third harmonic (TH) generators are investigated for both phase matched and phase mismatched configurations. For phase mismatched TH generation, several kinds of schemes (the phase mismatch either in second harmonic generation (SHG) or sum-frequency generation (SFG) process) are considered and analyzed. The physical nature corresponding to the different ratios of the coupling coefficients is discussed.     

Electron trajectory control of odd and even order harmonics in high harmonic generation using an orthogonally polarised second harmonic field

We investigate quantum trajectory control in high-order harmonic generation using an additional orthogonally polarised second harmonic field. By controlling the relative phase between this field and the fundamental, we are able to suppress and enhance particular electron trajectories which results in a modulation of the harmonic emission. We observe a phase shift of the modulation between the short and long trajectories that is different for adjacent odd and even harmonics. These results show qualitative agreement with a full propagation calculation where the single atom response was obtained from the strong field approximation and the main results are explained by consideration of a single atom quantum orbit picture.     

Optical second harmonic generation measurements for investigating electron injection into a pentacene field effect transistor with Au source and drain electrodes

The pentacene field effect transistors (FETs)' operation for the injection carrier was revealed by means of the drain current-elapsed time (I_ds-t) and optical second harmonic generation (SHG) measurements. The charge carriers forming the conducting channel of pentacene FETs were mainly holes injected from the Au source electrode. Carrier injection from source and drain electrodes was followed by the carrier trapping, and the SHG signal modulated by the change in the electric field distribution between Au the source and drain electrodes was shown. In particular, at the off state of the FET, electron injection and succeeding trapping were suggested. Furthermore, hole injection assisted by trapped electrons was also suggested.     

Second harmonic generation from a ferroelectric film

We derive an expression for transmittivity (T_SHG) of second harmonic generation (SHG) signals from a ferroelectric (FE) film. Intensities of up and down fields in the medium are investigated in relation to T_SHG. The derivations are made based on undepletion of input fields and nonlinear wave equation derived from the Maxwell equations. We present two cases: film without mirrors and with partial mirrors. Expressions for the newly derived nonlinear susceptibility coefficients of SHG for real crystal symmetry [J. Opt. Soc. Am. B 19 (2002) 2007] are used to get more realistic results. Variations in T_SHG with respect to film thickness are illustrated.     

Large enhancement of second harmonic generation in highly cerium doped lead-phosphate glass

The influence of lead and cerium concentration on photoinduced second harmonic generation (SHG) in lead-phosphate glasses has been studied. It has been shown that SHG can be considerably enhanced by increasing of lead and cerium concentration. We also have found that cerium greatly extends the induced nonlinearity lifetime. Physical mechanism of the phenomenon is discussed.     

Optical second harmonic generation at platinum phthalocyanine-modified platinum electrodes

Optical second harmonic generation (SHG) has been used to observe changes within electronically conducting platinum phthalocyanine (PtPc) films deposited on polycrystalline platinum electrodes as the film undergoes electrochemical modification. PtPc-modified electrodes produced enhanced SHG responses over bare platinum surfaces. This is proposed to be due to an electric quadrupole contribution from the PtPc molecule. Examination of the polarization dependence of the SHG response reveals that electroactive PtPc molecules exist in environments both parallel and perpendicular to the electrode surface. It has been possible to observe redox processes occurring within the films by monitoring the magnitude of the SHG response with variations in potential. The decrease in SHG signal has been shown empirically to be proportional to the charge removed from the film during oxidation of the phthalocyanine (Pc) ring.     

Second harmonic generation using an electrically controlled asymmetric plasmonic waveguide

ABSTRACT

In this study, it was shown that field enhancement in the nonlinear metal-insulator-metal (MIM) plasmonic waveguides can result in a large enhancement of the second harmonic generation (SHG) magnitude as compared with values reported in the literature. The proposed structure has two metals at the top and two metals at the bottom of the crystal. In this structure, a voltage is applied on metals to produce a SHG electrically. Hence, the metals that define the cavity also serve as electrodes capable of generating high direct current electric fields across the nonlinear material. The frequency of a fundamental wave at 458 nm was doubled and modulated in intensity by applying a moderate external voltage to the electrodes, yielding a voltage-dependent nonlinear generation with a higher coupling efficiency. All the simulations here have been calculated by using the finite-element-based commercial COMSOL software.     

Harmonic generation in laser-produced plasmas containing atoms,ions and clusters: a review

A review of studies of the high-order harmonic generation of laser radiation in laser-produced plasma revealed recent developments in this field. These include new approaches in application of two-colour pumps, generation of extremely broadened harmonics, further developments in harmonic generation in clusters (fullerenes, carbon nanotubes), destructive interference of harmonics from different emitters, new approaches in resonance-induced enhancement of harmonics, applications of high pulse repetition rate lasers for the enhancement of average power of generating harmonics and observation of quantum path signatures, etc. We show that this method of frequency conversion of laser radiation towards the extreme ultraviolet range became mature during multiple sets of studies carried out in many laboratories worldwide and demonstrated new approaches in the generation of strong coherent short-wavelength radiation for various applications.     

Second harmonic generation from electrically polarized CuCl-microcrystallite-doped glass

Second harmonic generation has been observed in electrically polarized CuCl-microcrystallite-doped glass. The glass was prepared by means of melting–quenching method and poled at 200°C for 1 h under applied dc field of 4 kV. The incident angular variation of SHG intensity for the polarized glass was measured. To explain the effect, it is proposed that the second-order nonlinear polarization results from the dipolar orientation in the glass.     

An efficient method for analyzing second harmonic generation with the consideration of pump depletion

We propose a simple method to analyze second harmonic generation (SHG) in general quasi-phase-matching media with the consideration of pump depletion. It allows us to conveniently predict precise result from undepleted pump approximation solution. Based on this method, multi-wavelength generation with arbitrary target efficiency and wavelength spacing is engineered. The effect of pump depletion on the performance of constructed multi-wavelength generator is analyzed. We demonstrate that it is efficient to design nonlinear device for multi-wavelength SHG by proposed method.     

Laser-parameter effects on the generation of ultrabroad harmonic and ultrashort attosecond pulse in a long-plus-short scheme

By numerically solving the time-dependent Schrödinger equation for helium gas in a special two-color laser field, which is synthesized by a long (9?fs) driving pulse and a short (6?fs) controlling pulse, we discuss the influence of the carrier-envelope phase, frequency, and the intensity of the controlling pulse on the generation of harmonic spectra and isolated attosecond pluses. In the cutoff region, two or three plateaus can be controlled by optimizing these laser parameters, and an ultrabroad supercontinuum harmonic spectrum with a bandwidth of 800?eV can be produced, which can support an ultrashort isolated 4.5 as pulse generation by Fourier transformation. Furthermore, using classical ionizing and returning energy maps, time–frequency analyses are presented to explain the underlying physical mechanisms.     

Quasi-phase matching of third harmonic generation in corrugated discharge capillary

Quasi-phase matching third harmonic of Ti:sapphire laser was measured by propagation through corrugated discharge capillaries. The 1 cm long capillary with periodical varying inner radius was used with corrugation frequency of 200 μm. The longitudinal plasma density was periodically varied by a discharge current ablating the inner capillary walls, altering the refractive index accordingly. Average plasma density measured was 10¹⁸ cm⁻³ at minimum corrugation radius. Peak laser intensity was 10¹⁰ W/cm² allowing enough pondermotive energy to the electron for high harmonic generation.