Second-harmonic Generation Of Ultrashort Pulses Research Articles

Second-harmonic generation of ultrashort pulses in refractive index linearly modulating quantum dot structure

While other works use segments of nonlinear crystal to compensate for walk-off, this work utilizes a quantum dot (QD) nanostructure for such compensation. Such nanocrystal structures allow two benefits: the high nonlinear susceptibility in these nanocrystals and the high surface density of QDs in the structure. Good results are obtained for pre-chirping and modulation parameters. The results are computed at 10 and 100 kV/cm applied electric fields. Compared to ordinary nonlinear crystals, the pulse width, intensity, and efficiency of the second harmonic (SH) pulse are all higher in QD nanocrystals, even with a low electric field applied. Higher applied fields give higher results.

Second harmonic generation of ultrashort pulses in refractive-index-linear-modulating nonlinear crystals

We present a theoretical model to deal with second harmonic generation (SHG) of ultrashort pulses in refractive-index-linear-modulating (RILM) nonlinear crystals. New coupled-wave equations were derived based on plane wave propagation. Under the undepleted-pump approximation, the analytic solutions for SHG of ultrashort pulses were obtained. The second harmonic (SH) pulse width and the efficiency were optimized in terms of the RILM parameter and the chirp coefficient of fundamental pulses. The results show that modulation of the refractive index (RI) in nonlinear crystals leads to a spectral localization of SHG that compensates the group velocity mismatch (GVM) and therefore compresses SH pulses. The modulation complicates the SHG process, which determines the pulse width, and the conversion efficiency varies with length and bandwidth. Especially, an overshooting compression phenomenon was found, which arose mainly from internal interference of the carrier frequency component in a SH pulse.

Efficient second-harmonic generation of ultrashort pulses with simultaneous quasi-phase matching and group velocity matching in periodic 90° domain structures of potassium niobate

Triple-line-shaped electrodes were devised to fabricate a periodically poled 90° domain structure of KNbO 3 (PP90KN) of 20.25 μm period with uniform periodicity and reproducibility. Using the fundamental pulse of 141 fs duration at 1619 nm wavelength, an efficient broadband type-I quasi-phase-matched second-harmonic generation was realized in an 8.3 mm long PP90KN by satisfying both phase matching and group-velocity-matching in the telecommunication L-band at room temperature. Second-harmonic pulses showed no significant temporal and spectral distortion and had a pulse duration of about 144 fs, which agreed well with the theoretical value of 142 fs. The conversion efficiency of the second-harmonic generation was 28.7% at the fundamental pulse energy of 0.92 nJ, which is about 60% of the theoretical value.

Analysis of Possible Group-Velocity-Matched Broadband Second-Harmonic Generation in Various Periodically Poled Nonlinear Crystals

For the efficient second-harmonic generation of ultrashort pulses in wide spectral ranges, we theoretically studied all possible group velocity matched broadband frequency conversion processes in different periodically poled materials, using quasi-phase matching. Important parameters such as the poling period, type of interaction, spectral bandwidth and conversion efficiency are estimated in detail. The results show that some materials can be efficiently applied to the generation of sub-100 fs pulses near the wavelengths of a widely used Ti:sapphire laser.

Analysis of ultrashort-pulse second-harmonic generation in both phase- and group-velocity-matched structures

Second-harmonic generation (SHG) of ultrashort pulse in both phase- and group-velocity mismatch structures is studied theoretically in temporal and frequency domains taking account of the effects of group-velocity dispersion (GVD) and high-order phase mismatch. Under the plane-wave and pump undepletion approximation, a set of simple analytical expressions is derived in frequency domain for Gaussian pulse. Furthermore, a formula describing the SH pulse is obtained under pump depletion condition. The obtained expressions are valid in large range of interaction length and have features such as explicit form, high accuracy, and less computing time. Meanwhile, the influences of intensity, pulse duration, chirp coefficient and shape of pump pulses, as well as the linear loss of material, on the pulsewidth preservation length are investigated by numerical method. In the case of both phase- and group-velocity mismatch structures, the conversion efficiency larger than 45% could be obtained without pulse distortion for weak input ultrashort pulse.

Femtosecond Z-scan measurements of nonlinear refraction in amino acid solutions

Abstract We report an investigation of third-order optical nonlinearities in several amino acid solutions using the Z-scan technique with femtosecond laser pulses at 775 nm. The amino acid solutions studied are usually employed for crystal growth, which can be used for ultrashort-pulse second-harmonic generation. The nonlinear refractive index n 2 in these aqueous solutions has been determined to be in the range from 10 −17 to 10 −16 cm 2 /W (second hyperpolaribality γ ranging from 0.9×10 −36 to 3.3×10 −36 esu). No two-photon absorption was observed. The experimental second molecular hyperpolarizabilities obtained for each amino acid are in good agreement with semi-empirical AM1 calculations.

Ultrashort-pulse second-harmonic generation with longitudinally nonuniform quasi-phase-matching gratings: pulse compression and shaping: errata

Ultrashort-pulse second-harmonic generation with longitudinally nonuniform quasi-phase-matching gratings: pulse compression and shaping: errata

Femtosecond Z-scan measurements of nonlinear refraction in nonlinear optical crystals

We report an investigation of third-order optical nonlinearities in several nonlinear optical crystals using the Z-scan technique with femtosecond laser pulses at 780 nm wavelength. The crystals studied include LiNbO 3:MgO, KTiOAsO 4, KTiOPO 4, β-BaB 2O 4 and LiB 3O 5, which are extensively used for ultrashort-pulse second-harmonic generation and optical parametric oscillation. The nonlinear refractive index n 2 in these crystals has been determined to be in the range from 10 −16 to 10 −15 cm 2/W. No two-photon absorption has been observed. The experimental results are compared with the two-band model for the bound electronic Kerr nonlinearity. It is shown that the measured n 2 values in β-BaB 2O 4 and LiB 3O 5 are one order of magnitude smaller than those of LiNbO 3:MgO, KTiOAsO 4, KTiOPO 4, which is in agreement with the theoretical prediction.

Ultrashort-pulse second-harmonic generation with longitudinally nonuniform quasi-phase-matching gratings: pulse compression and shaping

We present a theory of ultrashort-pulse second-harmonic generation (SHG) in materials with longitudinally nonuniform quasi-phase-matching (QPM) gratings. We derive an expression for the output second-harmonic field generated in an arbitrary QPM grating from an arbitrary fundamental field, valid for arbitrary material dispersion in the undepleted-pump approximation. In the case when group-velocity dispersion can be neglected, a simple transfer-function relationship describes the SHG process. This SHG transfer function depends only on material properties and on the QPM grating design. We use this SHG transfer function to show that nonuniform QPM gratings can be designed to generate nearly arbitrarily shaped second-harmonic output pulses. We analyze in detail a technologically important example of pulse shaping: the generation of compressed second-harmonic pulses from linearly chirped fundamental input pulses. The efficiency of these interactions as well as the limits imposed by higher-order material dispersion are discussed.

Ultrashort-pulse second-harmonic generation II Non-transform-limited fundamental pulses

Ultrashort-pulse second-harmonic generation (USP SHG) is examined for fundamental pulses that are not transform limited but still possess coherent spectra. Pulses whose spectra contain various orders of nonlinear phases are considered, and spectral phase compensation (compression) in either the fundamental pulse (FP) or the second harmonic (SH) is investigated with regard to optimizing final SH pulse duration and overall conversion efficiency. For Gaussian shaped or similarly smooth spectra with only quadratic phase it is found that such a phase maps proportionally over to the spectrum of the SH, and the compressed SH pulse shapes and durations are independent of whether the compensation is performed in the FP or in the SH. The optimum compensation configuration and the conversion efficiency depend on the nonlinear interaction length. For a FP with multiple peaked spectra, compensation in the SH gives poor results, accentuating the structure inherent in the pulses. FP’s with higher-order spectral phases were found to give different results for the two different compensation schemes. In particular, it was found that second-plus fourth-order phases are easier to compensate than second- plus third-order. In all the cases examined, the optimum nonlinear interaction length is one to three times the pulse-width-preservation length. These results are useful for both understanding and experimental optimization of USP SHG with state-of-the-art systems and pulses 10 fs or shorter in duration.

Ultrashort-pulse second-harmonic generation I Transform-limited fundamental pulses

Ultrashort-pulse second-harmonic generation I Transform-limited fundamental pulses

Ultrashort-pulse second-harmonic generation in quasi-phase-matched dispersive media

Ultrashort-pulse (<50-fs) second-harmonic generation in quasi-phase-matched dispersive media is analyzed in the regime of weak conversion with phase mismatch, group-velocity mismatch, and linear absorption accounted for. In addition to the expected increase in conversion efficiency, the quasi-phase-matched structure is shown to counteract pulse distortions.

Observation of frequency modulation in second-harmonic generation of ultrashort pulses.

We study both experimentally and theoretically the intensity and frequency profile of the pulse that is generated by second-harmonic generation with phase mismatch of a pulse with a duration of 50 fs. Experimentally, we determine the shape and the frequency of the generated pulse by correlating it with a second fundamental pulse and by measuring the spectra of the generated pulse. Theoretically, we solve numerically the coupled differential equations that describe the nonlinear interaction