Harmonic Conversion Efficiency Research Articles

Ablation of boron carbide for high-order harmonic generation of ultrafast pulses in laser-produced plasma

We demonstrate the generation of harmonics up to the 27th order (λ=29.9nm) of 806nm radiation in the boron carbide plasma. We analyze the advantages and disadvantages of this target compared with the ingredients comprising B4C (solid boron and graphite) by studying the plasma emission and harmonic spectra from three species. We compare different schemes of the two-color pump of B4C plasma, particularly using the second harmonics of 806nm laser and optical parametric amplifier (1310nm) as the assistant fields, as well as demonstrate the sum and difference frequency generation using the mixture of the wavelengths of two laser sources. These studies showed the advantages of the two-color pump of B4C plasma leading to the stable harmonic generation and the growth of harmonic conversion efficiency. We also show that the coincidence of harmonic and plasma emission wavelengths in most cases does not cause the enhancement or decrease of the conversion efficiency of this harmonic. Our spatial characterization of harmonics shows their on-axis modification depending on the conditions of frequency conversion.

Growth and characterization of morpholinium dihydrogenphosphate single crystal

Morpholinium dihydrogenphosphate (MDP) single crystals were synthesized, and were subsequently grown by controlled evaporation technique at room temperature for nonlinear optical applications. The grown crystal, which belongs to the monoclinic system with the space group P21, was subjected to single crystal X-ray diffraction to confirm the structure. UV–vis–NIR spectroscopy was done on the grown crystal and it showed good optical transparency in the entire visible region with a minimum cut-off wavelength of 289nm. The optical band gap was computed as a function of photon energy using Tauc’s plot. The refractive index of the grown crystal was determined using a Metricon Prism Coupler. The thermogravimetric (TG) and differential thermal analysis (DTA) traces disclosed the thermal stability of the compound. The mechanical strength of the crystal was investigated by a Vickers microhardness tester. Dielectric constant and dielectric loss were calculated and plotted as a function of frequency at different temperatures. The second harmonic conversion efficiency was determined using the Kurtz–Perry powder technique, and the efficiency was found to be 1.2 times greater than that of standard KDP.

无周期光学超晶格中耦合三次谐波转换效率的提高

无周期光学超晶格中耦合三次谐波转换效率的提高

沉积速率对直流脉冲溅射钼薄膜微结构与光学性能的影响

Based on the mass fraction of 65%deuterated DKDP crystals,the fourth harmonic conversion of 1053nm laser is realized under the condition of noncritical phase matching(NCPM).The experimentally measured crystal temperature is 29.4℃and the acceptance angle width of the crystals under the NCPM condition is around 55mrad. Effects of the deuterated amount and the temperature of the DKDP crystals in the process of NCPM on the fourth harmonic conversion are theoretically analyzed.The relationship between the deuterated amount and temperature and the changing of the fourth harmonic conversion efficiency versus the double frequency light intensity are numerically simulated.These results provide a theoretical basis for the application of fourth harmonic laser in the high power laser system.

Localized and propagating surface plasmon resonances in aperture-based third harmonic generation.

We investigate the influence of localized and propagating surface plasmons on third harmonic generation from rectangular apertures in metal films. We designed optimal aperture array structures by using finite-difference time-domain simulations with nonlinear scattering theory. From this design space, we fabricated and measured the third harmonic in the region of maximal performance. We find the highest third harmonic conversion efficiency when the localized resonance is tuned to the fundamental wavelength and the propagating (Bragg) resonance is tuned to the third harmonic; this is 2.5 times larger than the case where the both localized and propagating are tuned to the fundamental wavelength. The two remaining configurations where also investigated with much lower conversion efficiency. When the Bragg resonance is tuned to the third harmonic, directivity improves the collection of third harmonic emission. On the other hand, due to the inherent absorption of gold at the third harmonic, tuning the localized surface plasmon resonance to the third harmonic is less beneficial. All cases showed quantitative agreement with the original theoretical analysis. This work points towards an optimal design criterion for harmonic generation from thin plasmonic metasurfaces.

Second-harmonic generation by relativistic self-focusing of cosh-Gaussian laser beam in underdense plasma

AbstractThis paper presents theoretical investigation of effect of relativistic self-focusing of cosh-Gaussian (ChG) laser beam on second-harmonic generation in an underdense plasma. Steep transverse density gradients are produced in the plasma by the electron plasma wave excited by relativistic self-focusing of ChG laser beam. The generated plasma wave interacts with the pump beam to produce its second harmonics. Following Jeffrey Wentzel Kramers Brillouin (J.W.K.B) approximation and moment theory the differential equation governing the evolution of spot size of laser beam with distance of propagation has been derived. The differential equation so obtained has been solved numerically by the Runge–Kutta method to investigate the effect of decentered parameter, intensity of laser beam as well as density of plasma on self-focusing of the ChG laser beam, and generation of its second harmonics. It has been observed that the peak intensity of the laser beam shifts in the transverse direction by changing the decentered parameter and a noticeable change is observed on focusing of the laser beam as well as on conversion efficiency of second harmonics.

Quasi-phase-matched frequency conversion in ridge waveguides fabricated by ion implantation and diamond dicing of MgO:LiNbO(3) crystals.

We report on the fabrication and second harmonic generation from a periodically-poled MgO-doped lithium niobate ridge waveguide within the telecommunication L-band. The ridge waveguide is fabricated by carbon ion implantation and the following diamond blade dicing method. A normalized second harmonic conversion efficiency of 20.3%W-1cm-2 was obtained with a total insertion loss of 4.3dB at wavelength of 1612.7nm. Our analysis shows that at least ~70% of the second-order nonlinearity was preserved after the implantation and thermal annealing process.

Impact of BaB2O4 growth method on frequency conversion to the deep ultra-violet

In this article, we report how the growth method used for barium beta-borate β-BaB2O4 (BBO) impacts its high power second harmonic generation properties in the deep-UV. We compared a BBO crystal grown by flux (Top Seeded Solution Growth or TSSG) and a BBO crystal grown by the Czochralski (CZ) method. We first characterized their transparency properties, then we measured their single-pass second harmonic conversion efficiencies with both a low average power and a high average power nanosecond pulsed lasers. We show that both crystals have comparable linear absorption and conversion efficiencies at low power, whereas in a high power experiment, the CZ-grown BBO yields higher conversion efficiency than the TSSG grown BBO. With a 30 W, 150 kHz, 8 ns green laser, the use of a CZ BBO led at best to a 40% increase in available average output power at 257 nm.

The effect of air flow on the temperature distribution and the harmonic conversion efficiency of the ADP crystal with large aperture in the temperature control scheme

This paper presented a temperature control scheme for ammonium dihydrogen phosphate (ADP) crystal of Ф80mm in diameter, and the influence of the air flow was also studied. This research aims to obtain the high energy, high frequency laser with large aperture under the non-critical phase matching (NCPM). Firstly, thermal analysis was carried out to investigate the air flow property in the cavity, as well as the effect of ambient temperature was analyzed. Secondly, the temperature distributions of air flow were achieved using the Finite Volume Method (FVM), and this prediction was validated by the experiment results. Finally, the effect of air flow in the cavity was obtained from the heating method, and the variation of harmonic conversion efficiency caused by the ambient temperature was also highlighted.

Nonlinear Duffing oscillator model for third harmonic generation

We employ a classical, nonlinear Lorentz–Duffing oscillator model to predict third harmonic conversion efficiencies in the ultrafast regime, from a variety of metal nanostructures, including smooth, isolated metal layers, a metal–dielectric photonic band gap structure, and a metal grating. As expected, the plasmonic grating yields the largest narrow-band conversion efficiencies. However, interference phenomena at play within the multilayer stack yield comparable, broadband conversion. The method includes both linear and nonlinear material dispersions that in turn sensitively depend on linear oscillator parameters. Concurrently, and unlike other techniques, the integration scheme is numerically stable. By design, one thus avoids the introduction of explicit, third-order nonlinear coefficients and also takes into account linear and nonlinear material dispersions simultaneously, elements that are often necessary to fully understand many of the subtleties of the interaction of light with matter.

Studies on the growth, optical, thermal and mechanical properties of l-glutamic acid hydrobromide single crystals

A semiorganic crystal, l-glutamic acid hydrobromide (LGHB), was synthesized from aqueous solution by slow evaporation technique. The lattice parameters of the grown crystal were determined using single crystal X-ray diffraction analysis. The functional groups were identified using Fourier transforms infrared spectroscopy. UV–vis–NIR spectral analysis showed excellent transparency in the visible and infrared regions. Thermo gravimetric and differential thermal analysis showed that the grown crystals are thermally stable up to 218°C. The second harmonic generation conversion efficiency was investigated using Kurtz Perry technique. The mechanical strength of LGHB crystal was tested by Vicker's hardness tester. The grown crystal exhibits reverse indentation size effect (RISE) as the hardness value increases with increase in load. The elastic stiffness constant is calculated from the hardness value.

Multiple shape light sources generated in LiNbO3 nonlinear photonic crystals with Sierpinski fractal superlattices

LiNbO3 nonlinear photonic crystals with two kinds of Sierpinski fractal superlattice structures, in which their domain walls are circle and square cylindrical surfaces for the second order, respectively, are successfully fabricated. Quasi-phase matching and nonlinear Čerenkov harmonics under pulse laser beams are measured. The spot- and line-shaped light sources are realized by quasi-phase matching harmonics. Ten kinds of spot-shaped outputs can be accomplished, in which the quasi-phase matching harmonic conversion efficiency for the fundamental wavelength 1.352 μm reaches 27%. A green line-shaped light source with expansion angle of about 3° is observed, which originates from the coexistence of imperfect collinear and non-collinear quasi-phase matching processes. In addition, the ring-shaped light sources of continuous wavebands from 425 nm to 675 nm are obtained by nonlinear Čerenkov harmonics. The optical properties of quasi-phase matching and Čerenkov harmonics in two kinds of crystals are highly similar to each other.

Synthesis, growth, vibrational spectral investigations and structure–property relationship of an organic NLO crystal: 3,4-Dimethoxy chalcone

3,4-Dimethoxy chalcone, a nonlinear optical material, was synthesized and noncentro symmetric single crystals were grown in solution by slow evaporation technique. Thermal analysis such as DSC revealed a good thermal stability of 3,4-dimethoxy chalcone. The FT-IR and FT-Raman spectral studies were carried out to investigate the structural properties of the title compound. The molecular orbitals and natural bond analysis of this material were calculated by Density Functional Theory calculations. The second harmonic conversion efficiency was determined using the Kurtz powder technique, which is 8 times that of urea.

Strong Enhancement of Second Harmonic Emission by Plasmonic Resonances at the Second Harmonic Wavelength.

We perform second harmonic spectroscopy of aluminum nanoantenna arrays that exhibit plasmonic resonances at the second harmonic wavelength between 450 and 570 nm by focusing sub-30 fs laser pulses tunable from 900 to 1140 nm onto the nanoantenna arrays. We find that a plasmonic resonance at the second harmonic wavelength boosts the overall nonlinear process by more than an order of magnitude. In particular, in the measurement the resonant second harmonic polarization component is a factor of about 70 stronger when compared to the perpendicular off-resonant second harmonic polarization. Furthermore, the maximum of the second harmonic conversion efficiency is found to be slightly blue-shifted with respect to the peak of the linear optical far-field spectrum. This fact can be understood from a simple model that accounts for the almost off-resonant absorption at the fundamental wavelength and the resonant emission process at the second harmonic.

High-order harmonic generation enhanced by coherent population return

We present computations of high-order harmonic generation in atoms by using the coherent population return technique. This approach consists in two steps: firstly, the active medium is prepared in a coherent superposition of two bound states using a relatively long laser pulse which has a photon energy close to the energy gap between the states and then, the system is driven by an ultrashort infrared laser pulse. We demonstrate, by employing the numerical solution of the time-dependent Schrodinger equation in one and three dimensions, that using this scheme it is possible to enhance the harmonic conversion efficiency by several orders of magnitude. Our numerical results show that, on one hand the increase in ionization efficiency is responsible of an enhancement in the harmonic signal and on the other hand, the coherent preparation of the medium produces a much richer structure of the harmonic emission.

Advanced properties of extended laser-produced plasmas for efficient generation of the high-order harmonics of ultrashort laser pulses

The review of the studies showing the advanced properties of extended plasma plumes (5 mm) for efficient high-order harmonic generation of laser radiation compared with the short lengths of plasmas (~0.3–0.5 mm) used in previous studies is presented. The harmonic conversion efficiency quadratically increased with the growth of plasma length at the optimal conditions of plasma formation. The studies of this process using the long plasma jets produced on various metal surfaces, particularly including the resonance-enhanced harmonic generation, two-color pump, and extended nanoparticle plasma, are discussed.

A novel fast zooming scheme for direct-driven laser fusion

Illumination uniformity of a target is highly required to achieve central ignition in inertial confinement fusion based on direct-driven fusion. However, the shell may be compressed due to its interaction with laser beams during the laser pulse duration, and may further result in the degradation of illumination uniformity and the enhancement of the cross-beam energy transfer (CBET). Hence a novel fast zooming scheme for direct-driven laser fusion is proposed, i.e., by introducing the variable wavefront to the laser beam provided by a special-designed electro-optic crystal, thus the location and size of the focal spot could be controlled in real time, achieving the aim of improving the illumination uniformity and alleviating the CBET. This model of fast zooming has been built up, and variations of spot size and the radius of curvature of the additional spherical wavefront with time have been numerically simulated and analyzed based on the analysis of the interaction of the laser beam and the target. Results indicate that the fast zooming scheme could achieve effective and real-time control on the radius ratio between the focal spot and the target. Moreover, the fast zooming scheme has a little impact on the spatial filter capability and the third harmonic frequency conversion efficiency.

Theoretical model of phase-matching angles for KDP crystals and its verification analysis

In final optics assembly of high-power solid-state laser, in order to improve the third harmonic generation efficiency, the accurate assembly and calibration of ultra-thin KH2PO4 (KDP) crystal with large-aperture is one of the key technologies to realize inertial confinement fusion. In order to meet the requirements for high efficiency and precision crystal of online installation, it is necessary to measure crystalline phase matching angle for achieving the highest third harmonic conversion efficiency of high power laser. In this paper, for the third harmonic conversion by ultra-thin type Ⅰ/Ⅱ KDP crystals with large-aperture, the relationship between phase matching angles at different locations on the crystal is obtained according to the nonlinear optical properties of the crystal. Based on the analysis of the propagation path of the laser beam in the crystal, the relationship among the crystal surface shape, the phase matching angle and the best deflection angle is given. On this basis, the theoretical model for phase-matching angle of type Ⅰ/Ⅱ KDP crystal is proposed, and verified by the experimental results. The results show that the difference in phase matching angle between the prediction values and the experimental results is within 10.0 rad, showing that the theoretical model for phase-matching angles of type Ⅰ/Ⅱ KDP crystals is valid. This model provides a simple and efficient prediction method to obtain the phase matching angle distribution in full aperture of KDP crystal.

Exploitation of an external unstable multi-pass cavity to enhance the second harmonic conversion efficiency

Attempts to overcome the low conversion efficiency often encountered in second harmonic generation process by increasing the interaction length through usage of a number of crystals in tandem or in parallel or a single crystal in the intra-cavity or external resonant enhanced configuration have been made although with mixed success. We report here on the gainful exploitation of an unstable cavity to greatly enhance nonlinear conversion efficiency by integrating it with the stable pump cavity. This integration allowed transport of the pump radiation into the external cavity that at the same time exhibited high ‘Q’, prerequisite for multi-pass of the unabsorbed pump through the crystal. The unstable cavity not only ensured longer interaction length in the crystal without exposing it to high optical flux thereby safeguarding the crystal even in the pulsed operation but also eliminated the feedback problem. This scheme is demonstrated in the mid-IR region by frequency doubling of pulsed CO2 laser emission but its advantage should persist across UV, visible, and near-IR regions too.

Growth, thermal, mechanical, structural and optical properties of organic NLO crystals of novel cis-2,6-bis(2-chlorophenyl)-3,3-dimethylpiperidin-4-one

An organic NLO material viz., cis-2,6-bis(2-chlorophenyl)-3,3-dimethylpiperidin-4-one (2C3DMPO), has been synthesized and a slow evaporation technique was applied to produce a single crystal. X-ray diffraction study on the single crystal 2C3DMPO reveals a non-centro symmetric crystal, possessing a monoclinic space group P21 and prefers to adopt a chair conformation. The crystal has been characterised using UV, FT-IR and NMR spectral studies. Solubility study and mechanical study using micro hardness methods have also been carried out. Furthermore, the thermal stability of the crystal was established by TG/DTA. The second harmonic conversion efficiency of the crystal was determined using the Kurtz and Perry powder technique and the activity observed was 3.83 times greater than that of KDP.