Second Harmonic Generation Pulse Research Articles

Gapless detection of broadband terahertz pulses using a metal surface in air based on field-induced second-harmonic generation

We investigate second-harmonic generation (SHG) light from a Pt surface in atmosphere under terahertz (THz) pulses. THz pulse-modulated SHG intensity, ΔI2ω, shows a clear time profile of the THz field, which is similar to that of the conventional electro-optic sampling. The result can be explained by interference between THz field-induced second-harmonic light from air molecules in an optical path and a local oscillator from a Pt surface, whereby heterodyne detection of the THz waveform can be achieved. Using numerical calculations of a wave equation, we discuss the contribution of the Gouy phases of all the pulses, including near-infrared, SHG, and THz pulses, and identification of effective nonlinear susceptibility of the Pt surface. Our method, simply using a polished metal surface and air molecules, does not suffer from phonons or phase matching in solid-state optics and does not require any power supply, bias voltage, or fabrication process, but it offers a simple and gapless sampling method for broadband THz pulses. Here, we demonstrate the gapless detection of a broadband THz pulse in the region of 0.2–20 THz using this method.

Frequency-doubled Q-switched 4 × 4 multicore fiber laser system.

Frequency doubling of a Q-switched Yb-doped rod-type 4 × 4 multicore fiber (MCF) laser system is reported. A second harmonic generation (SHG) efficiency of up to 52% was achieved with type I non-critically phase-matched lithium triborate (LBO), with a total SHG pulse energy of up to 17 mJ obtained at 1 kHz repetition rate. The dense parallel arrangement of amplifying cores into a shared pump cladding enables a significant increase in the energy capacity of active fibers. The frequency-doubled MCF architecture is compatible with high-repetition-rate and high-average-power operation and may provide an efficient alternative to bulk solid-state systems as pump sources for high-energy titanium-doped sapphire lasers.

Spatial–temporal evolution of ultrashort laser pulse second harmonic generation in β-barium borate (β-BBO) crystal

We establish an effective and efficient theoretical approach called the spatial–temporal broadband nonlinear coupled wave theory (ST-BNCWT) to evaluate the spatial and temporal evolution behaviors of femtosecond laser pulse second harmonic generation (SHG) in a nonlinear medium. In this method, all the frequency components comprising the fundamental-wave laser pulse participate in a series of complicated three-wave mixing nonlinear coupling, including sum-frequency generation, difference-frequency generation, and the usual SHG, to create a second harmonic wave pulse. The contribution from each three-wave mixing process is strongly influenced by the corresponding phase matching or mismatching. We have used this method to analyze systematical transmission evolution characteristics of ultrashort laser pulses in a β-barium borate crystal and disclose the variation of a number of critical physical quantities. Our ST-BNCWT methodology can greatly facilitate the deep understanding of the SHG and various pulse transmission characteristics of ultrashort laser pulses and provide great support and guidance for experimental prediction. Moreover, our scheme opens up a promising path to explore and visualize novel nonlinear optical interactions in solid-state materials spanning the spatial and temporal domain, which are very helpful for building versatile ultrafast lasers in various spectral windows via powerful nonlinear frequency conversion technology against a basic high-performance ultrafast laser such as a Ti:sapphire femtosecond laser.

Numerical study of sum frequency ultrashort pulse compression in borate crystals

Second harmonic generation (SHG) of picosecond pulses in type II phase-matched nonlinear optical crystals is not widely employed because of the lower efficiencies compared to the type I phase-matching scheme. The limited efficiencies come from the difference between group velocities (GV difference) of the ordinary and extraordinary polarized input pulses. However, if the input pulses are delayed before the nonlinear crystal, a short second harmonic pulse can be generated with a slowly widening temporal overlap. Furthermore, this GV difference can be controlled by the tilting of the pulse fronts, and optimal GV difference can be obtained to achieve powerful output pulses with durations an order of magnitude lower than those of the input pulses. In this work, we present numerical results of SHG pulse compression in a beta barium borate (BBO) nonlinear crystal, which is ideal for SHG of high-power 1030 nm thin-disk lasers. The pulse compression is controlled by predelay and tilting of the pulse fronts. We find optimal parameters to achieve five-fold increase in output power and 20-fold pulse compression of 1.7 ps input pulses. Finally, we consider the experimental aspects of the group velocity control.

Temperature gradient engineered to maintain pulse shape and efficiency of a 532 nm picosecond second-harmonic generation

We introduced a theoretical model to preserve the quality and the efficiency of a picosecond second-harmonic generation (SHG) in a 30-mm-long LBO crystal which is surrounded by a thermal oven. It is proposed to arrange a temperature gradient throughout the crystal length for mitigating the undesirable consequences of dispersion and group velocity mismatch. To optimize the pulse shape, a distance parameter is introduced and in addition, the SHG efficiency and pulsewidth are included in a merit function. By setting the temperature gradient as the variable the merit function is optimized. The results show that for temperature gradient of 60 °C, an SHG pulse with mostly Gaussian shape with 80.7% efficiency and with 4.47 ps pulsewidth is attainable. Furthermore, the effect of the chirping on the SHG pulse is investigated. Final results have turned the proposed model into an excellent candidate for obtaining an efficient SHG conversion with an appreciable pulse quality.

High efficiency second harmonic generation of nanojoule-level femtosecond pulses in the visible based on BiBO

We demonstrate high efficiency second harmonic generation (SHG) of near infrared femtosecond pulses using a $\text{BiB}_{3}\text{O}_{6}$ crystal in a single-pass tight focusing geometry setup. A frequency doubling efficiency of $63\%$ is achieved, which is, to the best of our knowledge, the highest value ever reported in the femtosecond regime for such low energy (nJ-level) pumping pulses. Theoretical analyses of the pumping scheme focusing waist and the SHG efficiency are performed, by numerically solving the three wave mixing coupled equations in the plane-wave scenario and by running simulations with a commercial full 3D code. Simulations show a good agreement with the experimental data regarding both the efficiency and the pulse spectral profile. The simulated SHG pulse temporal profile presents the characteristic features of the group velocity mismatch broadening in a ‘thick’ crystal.

Blue Femtosecond Pulse Generated in Periodically Poled Lithium Tantalite

Femtosecond second harmonic generation (SHG) in periodically poled lithium tantalite (PPLT) crystal under conditions of third-order quasi-phase matching is investigated. SHG is easily obtained at average input power of only a few milliwatts. Blue femtosecond pulse is generated by using a PPLT crystal in a simple, single-pass extracavity geometry. The temporal and spectral characteristics of the SHG pulses are presented.

Non-degenerate fs pump-probe study on InGaN with multi-wavelength second-harmonic generation

Non-degenerate fs pump-probe experiments in the UV-visible range for ultrafast carrier dynamics study of InGaN with adjustable pump and probe photon energies are implemented with simultaneously multiwavelength second-harmonic generation (SHG) of a 10 fs Ti:sapphire laser. The multi-wavelength SHG is realized with two beta-barium borate crystals of different cutting angles. The full-widths at half-maximum of the SHG pulses are around 150 fs, which are obtained from the cross-correlation measurement with a reverse-biased 280-nm light-emitting diode as the twophoton absorption photo-detector. Such pulses are used to perform nondegenerate pump-probe experiments on an InGaN thin film, in which indium-rich nano-clusters and compositional fluctuations have been identified. Relaxation of carriers from the pump level to the probe one through the scattering-induced local thermalization (<1 ps) and then the carrier-transport-dominating global thermalization (in several ps) processes is observed.

Second-harmonic generation pulse splitting in quartz observed by frequency-domain interferometry

Second-harmonic generation from a thin quartz wedge is studied using short laser pulses. Under phase mismatched condition a temporal splitting of the second-harmonic field occurs due to the different group velocities of the free and forced modes. By measuring the temporal splitting as a function of the quartz crystal thickness using spectral interference a non-zero time-difference between the two second-harmonic pulses at the incident surface is predicted. This is interpreted as a phase-difference between the homogeneous and the inhomogeneous solution of the wave equation at the incident surface.

Second-harmonic generation from regeneratively amplified femtosecond laser pulses in BBO and LBO crystals

The spectral and temporal characteristics and optical-conversion efficiency of ∼150-fs laser pulses at 400 nm generated by second-harmonic generation (SHG) of a regeneratively amplified mode-locked Ti:sapphire laser were investigated both theoretically and experimentally. The theoretical investigation was done by taking into account cubic nonlinearity, pulse walk-off, group-velocity dispersion, Kerr nonlinearity, quadratic broadening, frequency chirping of the fundamental pulse, and higher-order nonlinear mixing such as backconversion and optical parametric processing. The experimental studies of the effects of crystal length and pumping intensity on the pulse duration, the spectrum, and the optical-conversion efficiency of the SHG were carried out in BBO and LBO crystals of various thicknesses and compared with the theory. It was found that in a non-transform-limited pulse, the most significant contribution to the temporal and spectral distortion of the ∼150-fs SHG pulses is mainly due to the chirping of the fundamental beam and self-phase modulation at high pumping intensity and long crystal length. The optimum crystal length and pumping intensity for obtaining a high optical-conversion efficiency and a pure spectrum in SHG are also calculated and experimentally investigated. It was found that a transform-limited fundamental pulse is essential to obtain a high conversion efficiency and to preserve the temporal profile of the second-harmonic pulse. It is also found that for a non-transform-limited ∼150-fs pulse, a 0.5–0.6-mm BBO crystal and a modest pumping intensity of ∼40 GW/cm2 are the most suitable for SHG.

SECOND HARMONIC GENERATION IN LiNbO3 BY PICOSECOND PULSES

Short pulses emitted by a Nd:glass laser were passed through LiNbO3 crystals to produce SHG (second harmonic generation) of tunable duration. The duration of the SHG pulse was tuned between 1.5 psec and 9 psec by varying the length of the LiNbO3 crystals.