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The quest for ever-shorter optical pulses has been ongoing for over half a century. Although few-cycle pulses have been generated for nearly 40 years, pulse lengths below the single-cycle limit have remained an elusive goal for a long time. For this purpose, optical waveform synthesizers, generating high-energy, high-average-power pulses via coherent combination of multiple pulses covering different spectral regions, have been recently developed. They allow unprecedented control over the generated optical waveforms, spanning an extremely broad spectral range from ultraviolet to infrared. Such control allows for steering strong-field interactions with increased degrees of freedom. When driving high-harmonic generation, tailored waveforms can produce bright attosecond pulse trains and even isolated attosecond pulses with tunable spectra up to the soft X-ray range. In this paper recent progress on parametric and hollow-core fiber waveform synthesizers is discussed. Newly developed seeding schemes; absolute, relative, and spectral phase measurement; and control techniques suitable for synthesizers are described. The progress on serial and parallel waveform synthesis based on Ti:sapphire and Ytterbium laser systems and their latest applications in high-harmonic generation in gaseous and solid media, attosecond science, and laser wakefield acceleration is discussed. [ABSTRACT FROM AUTHOR]

We have generated high-power deep ultraviolet laser pulses from a Yb-doped rod-type fiber laser through sum-frequency generation (SFG) based on lithium triborate (LBO) crystals. The 1030 nm nanosecond laser pulses from the fiber laser are frequency-tripled to 343 nm pulses with two LBO crystals, and the residual 1030 nm fundamental laser pulses are mixed with the 343 nm pulses in an LBO crystal for the generation of 257 nm pulses. The maximum 257 nm radiation power is 12 W with a repetition rate of 500 kHz, and the conversion efficiency from 1030 nm to 257 nm is 12%. It is observed that the power of 257 nm radiation is limited by the temperature increase in the LBO crystal used for the SFG. [ABSTRACT FROM AUTHOR]

We report the effect of the aluminum oxide substrate on the emission of monolithic AlGaAs-on-insulator nonlinear nanoantennas. By coupling nonlinear optical measurements with electron diffraction and microscopy observations, we find that the oxidation-induced stress causes negligible crystal deformation in the AlGaAs nanostructures and only plays a minor role in the polarization state of the harmonic field. This result highlights the reliability of the wet oxidation of thick AlGaAs optical substrates and further confirms the bulk χ(2) origin of second harmonic generation at 1.55 μm in these nanoantennas, paving the way for the development of AlGaAs-on-insulator monolithic metasurfaces.

• A careful choice of phase shifts applied to the polarization modulators connected in series parallel configuration ensures 2 strong 8th order sidebands spaced 160 GHz apart. • Bias free operation of Polarization modulators makes this scheme unique in comparison with MZM based techniques. • Due to a higher Frequency multiplication factor, requirement of high frequency Local oscillator is eliminated. • The key features of the manuscript include higher Optical and Electrical sideband suppression ratios and the frequency tunability that enables signal generation in the 60 GHz range. In this paper, a novel filter free method of generating 16 tupled millimeter wave signal using polarization property is proposed. Phase modulators utilized in this scheme are independent of bias voltage deviation and have higher extinction ratio thus ensuring higher quality of generated mm wave signal. A large frequency tunable range is achievable, as no optical filter is used. A 160 GHz microwave signal is obtained at the polarizer output when a 10GHz RF local oscillator is employed, which is 16 folds the RF drive signal. The simulation results show that the generated signal offers an Optical Sideband Suppression Ratio (OSSR) of 63dB and Radio Frequency Spurious Suppression Ratio (RFSSR) of 51dB. The sideband suppression ratios obtained in the proposed scheme is higher than the previously suggested schemes on microwave signal generation using polarization property. Theoretical computations validate the simulation results. Various parameters affecting system performance are analyzed in later sections. [ABSTRACT FROM AUTHOR]

We present a comparative study on frequency-doubling characteristics of femtosecond laser pulses in thick nonlinear crystals with different temporal and spatial walk-off parameters. Using single-pass second harmonic generation (SHG) of 260 fs pulses at 1064 nm from a high-average-power femtosecond Yb-fiber laser in 5-mm-long crystals of β-BaB2O4 (BBO) and BiB3O6 (BIBO), we find that for comparable values of temporal and spatial walk-off parameters in each crystal, the optimum focusing condition for SHG is more strongly influenced by spatial walk-off than temporal walk-off. It is also observed that under such conditions, the Boyd and Kleinman theory commonly used to define the optimum focusing condition for frequency-doubling of cw and long-pulse lasers is also valid for SHG of ultrafast lasers. We also investigate the effect of focusing on the spectral, temporal, and spatial characteristics of the second harmonic (SH) radiation, as well as angular acceptance bandwidth for the SHG process, under different temporal and spatial walk-off conditions in the two crystals.

Nonlinear frequency conversion is the important method to get coherent radiation in all-optical waveband. The intentional manipulation of nonlinear harmonic wave, which has some special phase, amplitude, polarization, and so on, is also important to realize all kinds of optical micro-manipulation, optical micro-fabrication, and optical communication in all-optical waveband. There are three methods to manipulate such nonlinear harmonic wave, which are tailoring the functional facet, the structure of crystals, and the structure of the incident light. In this review, we will systematically introduce the fundamental principle and latest advances in this field and summarize the respective characteristics of these three methods. [ABSTRACT FROM AUTHOR]

Single-shot carrier envelope phase (CEP) measurement is a challenge in the research field of ultrafast optics. We theoretically investigate how an intense terahertz pulse modulates second harmonic emission (SH) from a gas plasma induced by a few-cycle laser pulse (FCL). Results show that the modulation quantity of SH intensity has a cosinoidal dependence on the CEP of FCL pulses, based on which we propose a low energy, all-optical method for single-shot CEP measurements via using a known intense terahertz pulse. Moreover, we propose an experimental realization. [ABSTRACT FROM AUTHOR]

We demonstrate a high efficiency and high power picosecond ultraviolet source at 355 nm with stable output by sum frequency generation from a Nd:YAG laser using a type-I critically phase matched K 3 B 6 O 10 Br crystal as nonlinear optical material. Conversion efficiency as high as 30.8% was achieved using a 25 ps laser at 1064 nm operated at 10 Hz. Similar work is done by using a 35 W 10 ps laser at 1064 nm as the pump source with a repetition rate of 80 MHz, and the highest average output power obtained was up to 5.3 W. In addition, the power stability of the 355 nm output power measurement shows that the standard deviation fluctuations of the average power are ± 0.69% and ± 0.91% at 3.0 W and 3.5 W, respectively. [ABSTRACT FROM AUTHOR]

We propose the asymmetrical plasmonic slot waveguide (APSW) design for third-harmonic generation (THG) from 2.25 μm. In this configuration, the phase-matching condition is fulfilled between the zeroth-order mode at fundamental frequency (FF) and the first-order mode at third-harmonic frequency (THF). Due to the asymmetrical geometry, the mode overlap between the two involved modes is significantly enhanced, leading to an efficient THG process. According to the numerical calculation, the conversion efficiency is predicted up to 1.4% with 1-W pump power. The proposed APSW has the potential to realize an integrated efficient THG device in nanometer scale.

Second harmonic generation (SHG) is an important nonlinear process which is critical for applications, such as optical integrated circuit, nonlinear microscopy, laser, etc. Many challenges remain in the improvement of nonlinear conversion efficiency, since the typical value is of only 10−5 in nanostructures. Here, we theoretically demonstrate a periodic structure consisting of a lithium niobate (LN) bar and an LN disk, on a nanoscale (~300 nm) thin-film platform, which is proposed for a highly efficient SHG. By breaking the structure symmetry, a Fano resonance with a high Q, up to 2350 and a strong optical field enhancement reaching forty-two folds is achieved, which yields a high conversion efficiency, up to 3.165 × 10−4. In addition to its strong second harmonic (SH) signal, we also demonstrate that by applying only 0.444 V on the planar electrode configurations of the nanostructured LN, the wavelength of SH can be tuned within a 1 nm range, while keeping its relatively high conversion efficiency. The proposed structure with the high nonlinear conversion efficiency can be potentially applied for a single-molecule fluorescence imaging, high-resolution nonlinear microscopy and active compact optical device.

We numerically and experimentally investigated the Cherenkov-type second harmonic generation of structured fundamental wave, whose phase was periodically modulated, in periodically poled nonlinear crystals. The Cherenkov-type second harmonic generation with different parameters of the structured fundamental wave was investigated. The experimental results are in good agreement with the theoretical analysis. This study provides a method of dynamically tailoring the Cherenkov-type second harmonic wave and also has potential application in other nonlinear frequency conversion processes. [ABSTRACT FROM AUTHOR]

We develop a novel theoretical model for the study of harmonic generation in cylindrical structures of finite height. Our technique is based on the decomposition of the electromagnetic field over the complete set of modes of the corresponding infinite cylinder. Differently from previous works, in our model no constraints are given neither on the cross section nor on the refractive index of the cylinder. We then apply our approach in the special case of frequency doubling in tall AlGaAs nanodisks, where radiative modes can be neglected. Our results open new perspectives for optimization and engineering of the radiation pattern in harmonic generation processes at the nanoscale. [ABSTRACT FROM AUTHOR]

Temporal characteristics of a cavity enhancement second harmonic (SH) generation for picosecond laser pulse are investigated. We experimentally measured pulse width changes that were indued by group velocity mismatching (GVM), SH process, and enhancement cavity. It indicates that the generated pulse width is a combined effect of the GVM and SH process. Meanwhile, the effect of the enhancement cavity can be avoided by controlling its free spectrum range. A interferometric autocorrelator with a KBBF-PCD as nonlinear crystal is also composed and this extends the measurement light wavelength below 410 nm. [ABSTRACT FROM AUTHOR]

We propose an internal asymmetric plasmonic slot waveguide (IAPSW) containing two different materials in the slot region for third harmonic generation (THG) from 2.25 μm. In the IAPSW, the required phase matching condition is satisfied between the 0th-order mode at the fundamental frequency and 1st-order mode at the third harmonic frequency. By choosing an appropriate slot geometry and materials, the third harmonic electric field distribution can be engineered to significantly enhance the nonlinear overlap coefficient for THG. According to our simulation, a conversion efficiency up to 0.67 % with 1 W pump power is predicted within a ∼10 μm IAPSW. Additionally, the waveguide shows large fabrication tolerance in terms of geometry parameters. The proposed waveguide can find potential applications for high-speed all-optical signal processing. [ABSTRACT FROM AUTHOR]

We propose and demonstrate a spatial beam shaping method to achieve high-quality near-field for a high-power frequency tripling laser system by using a liquid crystal spatial light modulator (SLM). Considering the nonlinear relationship between the output 3 ω intensity and the input 1 ω intensity of the frequency conversion system and the transmittance nonuniformity of the whole laser system, we introduce an efficient spatial beam shaping method that improves the output near-field beam quality of frequency tripling laser dramatically. Results show that the near-field peak-to-mean value of the frequency tripling laser improves from 1.83:1 to 1.42:1 after spatial beam shaping within four shots. This method provides effective guidance for spatial beam shaping of high-power frequency tripling laser systems. [ABSTRACT FROM AUTHOR]

We demonstrate an efficient third harmonic generation (THG) with the effect of nonlinear loss based on a proposed double-hybrid plasmonic slot waveguide structure. Silicon nanocrystal is integrated into the two parallel slots acting as the main interactive nonlinear material. To achieve phase matching condition, we analyze the relation between the guided mode dispersions and the waveguide geometrical parameters. The effect of nonlinear loss, which is contributed by the two-photon-absorption (TPA) effect, on the THG conversion efficiency is investigated numerically. The simulation results reveal that the THG conversion efficiency can be significantly enhanced by the field confinement and enhancement, whereas it is greatly impeded in the presence of the TPA effect (a decrement of 17.6% on the THG conversion efficiency with a pump power of 4 W). According to our calculation, the THG conversion efficiency of 0.91% without the consideration of the TPA effect and 0.75% with the TPA effect under a 4-W input pump power condition is achieved. [ABSTRACT FROM PUBLISHER]

We experimentally present an acousto-optic Q-switched, side-pumped laser generating the watt-level deep-ultraviolet (DUV) output at 266 nm. A quasi-enhanced cavity, containing a BBO crystal as the fourth harmonic generator, was employed in the compact DUV laser system. A DUV laser with the average power of 1.08 W was achieved after careful alignment of the resonator. Our experimental results demonstrated that the DUV power was very sensitive to the misalignment of a cavity mirror. It was attributed to the thermal effect in BK7 mirror induced by high green power, by which the beam radius of green light was compressed in the compound resonator. [ABSTRACT FROM AUTHOR]

A new method which is a combination of the harmonic balance and finite difference techniques (HBFD) is proposed for complete time-harmonic solution of the nonlinear wave equation. All interactions between different harmonics up to an arbitrary order can be incorporated. The effect of higher order harmonics on two important nonlinear optical phenomena, namely, the second harmonic generation (SHG) and frequency mixing is investigated by this method and the results are compared with well-known analytical solutions. The method is quite general and can be used to study wave propagation in all nonlinear media. [ABSTRACT FROM AUTHOR]

We numerically studied nonlocal effects that affect enhancement of second harmonic generation (SHG) in a metal nanofilm plasmonic structure. The hydrodynamical Drude model was implemented for metal film in time-domain simulations that combine nonlocal effects and nonlinear effects. It is found that the nonlocal effect on the second harmonic wave is more severe than that on the fundamental wave. The SHG enhancement is thus weakened in the nonlocal response compared with the usual local response described by the Drude model, and the reduction becomes appreciable for the metal film thickness down to nanomenter scale. [ABSTRACT FROM AUTHOR]

We theoretically propose an asymmetric plasmonic slot waveguide (APSW) with a thin silicon nanocrystal (Si-nc) layer and a thinner silicon layer that fills the bottom and the top of the metallic slot region to increase third-harmonic generation (THG) efficiency. The needed modal phase-matching condition is satisfied with properly mode dispersion engineering by exploiting the waveguide geometrical parameters. Combining the asymmetric waveguide structure and the high third-order susceptibility of the nonlinear materials, efficient phase-matched THG from mid-infrared (IR) to near-IR regions is realized. Then, the THG performance was further improved by increasing the silicon slot width wherein the THG nonlinear coefficient as large as 21452 \m^-1 \cdot \W^-1 is achieved. The corresponding THG efficiency comes up to 0.5% at a waveguide length of 9.3 \mu\m with a pump power of 1 W. [ABSTRACT FROM PUBLISHER]

We report on frequency doubling of 1.55-μm fiber oscillator-amplifier laser with conversion efficiency of 27% by using off-the-shelf nonlinear crystal. The system generates 135-femto-second (fs) pulses with average power of 110 mW at 80 MHz repetition rate. Frequency doubling efficiency as a function of pulse pre-chirp in front of a single mode erbium fiber amplifier was investigated. [ABSTRACT FROM AUTHOR]

Abstract: A computational study of the uni-directional second-harmonic generation in a one-dimensional dual photonic crystal structure made of GaAs, AlAs and SiO2 with quadratic optical nonlinearity and material dispersion is presented. The computational approach uses a shooting method to solve nonlinear wave equations for coupled fundamental and second-harmonic fields and the invariant imbedding method to obtain the linear transmittance and group index spectra. The dual structure consists of two substructures, the conversion structure creating a strongly enhanced second-harmonic signal and the filter structure blocking the fundamental frequency field by Bragg reflection while permitting the passage of the second-harmonic field. The conversion structure is built with an elementary cell consisting of four sublayers whose thicknesses are systematically varied. Doubly resonant second-harmonic generation with very high conversion efficiency is achieved for light incident from the conversion structure side by choosing the geometrical parameters of the elementary cell optimally and controlling the band structure. A new mechanism to enhance second-harmonic generation by controlling the energy flow between the fundamental frequency and second-harmonic fields has also been found. [Copyright &y& Elsevier]

The fluctuation effects in the ideal quasi-phase-matching have already been studied in detail from the point of view of periodicity and wavelength. In this paper, the length fluctuation on each segment of periodicity poled nonlinear crystals (PPNC) in the second harmonic generation has been studied. By adding the linear crystals in PPNC which is positive, zero, negative and zero (PZNZ) configuration, (+, 0, -, 0), random length errors affect its output efficiency. It has also been revealed that in disordered structures, the output efficiency of the nonlinear generation of light is increased in PZNZ and in these conditions, the efficiency in PZNZ is better than that of PPNC. [ABSTRACT FROM AUTHOR]

Abstract: We show that topological phase-defects are spontaneously generated from noise fluctuations in the degenerate configuration of the parametric interaction. These localized coherent structures are shown to affect the coherence properties of the parametrically generated field. It is shown that the emergence of coherence in the fundamental field relies on a previously unrecognized process of mutual annihilation of pairs of neighboring phase-defects. More precisely, the density of phase-defects N, and the time correlation of the generated field, are shown to exhibit a power-law behavior with the propagation length, i.e., . [Copyright &y& Elsevier]

Photonic crystal waveguides (PCW) have shown great potential to generate second harmonic (SH), but the available schemes with cavity or defect suffer from a lack of multiple wavelengths operation and excessive phase-matching requirement. Here we propose a 2D PCW structure comprised of a modulated single line defect, which is capable of independently controlling the fundamental frequency (FF) and SH modes. Therefore, two near-flat group velocity dispersions can be achieved for these modes, and they simultaneously satisfy phase matching conditions. Based on multiply scattering method, we theoretically demonstrate that the SH resonant modes originated from the nonradiative near-flat dispersion have high quality factors (nearly 6. 5 × 1 0 4 for the maximum value). And the maximum SH conversion efficiency for these resonant modes are 2 orders of magnitude higher than that of off-resonant modes. In addition, benefitting from the matching of the group velocities of the second harmonic and the fundamental, the central wavelength of SH can be modulated within wavelength range ∼ 3 nm. These results can provide potential applications in on-chip integration of photonics and quantum optics. [ABSTRACT FROM AUTHOR]

A potassium dihydrogen phosphate (KDP) crystal is set to be a vital factor in inertial confinement fusion (ICF). This paper focuses on how to reduce the gravitational distortion and increase the second-harmonic generation (SHG) efficiency of the KDP crystal. Multipoint support mounting configurations are proposed. Their influence on distortion and SHG efficiency is analyzed. Mechanical and optical models are established, and the distortion of the KDP crystal is calculated using the finite-element methods. In addition, phase mismatch caused by distortion and SHG efficiency is calculated and analyzed. The numerical results reveal that multipoint support is a suitable mounting configuration as it has little distortion and increases SHG efficiency. Multipoint support mounting configuration is well applied in the ICF facility. [ABSTRACT FROM AUTHOR]

Second harmonic generation (SHG) is an important nonlinear process which is critical for applications, such as optical integrated circuit, nonlinear microscopy, laser, etc. Many challenges remain in the improvement of nonlinear conversion efficiency, since the typical value is of only 10−5 in nanostructures. Here, we theoretically demonstrate a periodic structure consisting of a lithium niobate (LN) bar and an LN disk, on a nanoscale (~300 nm) thin-film platform, which is proposed for a highly efficient SHG. By breaking the structure symmetry, a Fano resonance with a high Q, up to 2350 and a strong optical field enhancement reaching forty-two folds is achieved, which yields a high conversion efficiency, up to 3.165 × 10−4. In addition to its strong second harmonic (SH) signal, we also demonstrate that by applying only 0.444 V on the planar electrode configurations of the nanostructured LN, the wavelength of SH can be tuned within a 1 nm range, while keeping its relatively high conversion efficiency. The proposed structure with the high nonlinear conversion efficiency can be potentially applied for a single-molecule fluorescence imaging, high-resolution nonlinear microscopy and active compact optical device. [ABSTRACT FROM AUTHOR]