Complete spatial and temporal locking in phase-mismatched second-harmonic generation (original) (raw)
Related papers
2008
We experimentally demonstrate simultaneous phase and group velocity locking of fundamental and generated second harmonic pulses in Lithium Niobate, under conditions of material phase mismatch. In phasemismatched, pulsed second harmonic generation in addition to a reflected signal two forward-propagating pulses are also generated at the interface between a linear and a second order nonlinear material: the first pulse results from the solution of the homogeneous wave equation, and propagates at the group velocity expected from material dispersion; the second pulse is the solution of the inhomogeneous wave equation, is phaselocked and trapped by the pump pulse, and follows the pump trajectory. At normal incidence, the normal and phase locked pulses simply trail each other. At oblique incidence, the consequences can be quite dramatic. The homogeneous pulse refracts as predicted by material dispersion and Snell's law, yielding at least two spatially separate second harmonic spots at the medium's exit. We thus report the first experimental results showing that, at oblique incidence, fundamental and phase-locked second harmonic pulses travel with the same group velocity and follow the same trajectory. This is direct evidence that, at least up to first order, the effective dispersion of the phase-locked pulse is similar to the dispersion of the pump pulse.
Induced group-velocity dispersion in phase-mismatched second-harmonic generation
Journal of the Optical Society of America B, 2006
We show theoretically and experimentally that, in phase-mismatched second-harmonic generation, an effective group-velocity dispersion is induced at the second-harmonic frequency. Experimental results agree quite well with the theoretical predictions. Although large phase mismatch is required to induce appreciable groupvelocity dispersion in birefringent nonlinear crystals, similar effects can be observed in quasi-phase-matched structures at much lower phase mismatch. Some implications of the induced dispersion for pulse propagation in quadratic media are discussed.
Third-harmonic generation in isotropic media by focused pulses
Physical Review A, 2004
For focused pulses of light in isotropic nonlinear media, third-harmonic generation can be strongly affected by group-velocity mismatch between the fundamental and third-harmonic. There is a characteristic time determined by the group-velocity mismatch and the Rayleigh range of the focused pulse. The dynamics depend on two dimensionless quantities, namely the ratio of the characteristic time to the pulse duration and the phase-velocity mismatch times the Rayleigh range. Pulses shorter than the characteristic time have physics described by simple analytic formulas. Pulses near the characteristic time have an intermediate behavior given by an explicit but more complicated formula. Pulses longer than the characteristic time tend to the continuouswave case.
Applied Physics Letters, 2003
We experimentally demonstrate the second harmonic generation (SHG) of infrared femtosecond pulses using a BIBO crystal placed in an external ring cavity, synchronized with an input mode-locked laser at 78 MHz. A frequency doubling efficiency of 53% is achieved which is, to the best of our knowledge, the highest value ever reported for a low energy input beam of 1.4 nJ/pulse. Theoretical analysis of cavity related issues such as design, fundamental mode characteristics and fidelity against misalignments are also presented. The modeling of SHG cavity enables us to estimate the cavity losses and the mode matching visibility. Such synchronized SHG cavities in pulse domain, having higher SHG conversion efficiencies compared to their continuous wave counterparts, may find potential applications in scientific areas such as in photonics, and in quantum optics.
Ultrashort-pulse second-harmonic generation. I. Transform-limited fundamental pulses
JOSA B, 1995
A general theory of second-harmonic generation, including all the effects of group-velocity dispersion, is given for coherent ultrashort pulses with arbitrary shapes and carrier chirps. Ultrashort-pulse second-harmonic generation is analyzed for transform-limited fundamental pulses. The effects of intrapulse group-velocity dispersion (IGVD) on the second-harmonic (SH) pulse shape are investigated for parameters representative of popular phase-matchable crystals and wavelength, including Ti:sapphire lasers. In phase-matched structures IGVD at the SH cannot be neglected for pulses approaching 10 fs. It results in a spectral quadratic phase on the SH and in some cases can shorten the pulse. External dispersive shaping of the SH pulses distorted by group-velocity mismatch (GVM) is examined, and some pulse shortening is found possible. It is shown that the effect of IGVD at the SH wavelength on the pulse is similar to that of the spectral quadratic phase provided by an external pulse shaper. Group-velocity-matched configurations are also investigated. IGVD at both the fundamental and the SH wavelengths is found to limit the optimum thickness of the nonlinear medium. A measure of the interaction length in which the pulse width of the fundamental pulse is preserved in the SH is introduced. It is defined in terms of the GVM and the pulse bandwidth for phase-matched structures and in terms of the IGVD and pulse bandwidths for group-velocity-matched configurations.
Metamaterials, 2008
We simulate and discuss novel spatio-temporal propagation effects that relate specifically to pulsed, phase-mismatched second harmonic generation in a negative index material having finite length. Using a generic Drude model for the dielectric permittivity and magnetic permeability, the fundamental and second harmonic frequencies are tuned so that the respective indices of refraction are negative for the pump and positive for the second harmonic signal. A phase-locking mechanism causes part of the second harmonic signal generated at the entry surface to become trapped and dragged along by the pump
Optics Letters, 1997
We demonstrate the use of an aperiodic quasi-phase-matching (QPM) grating to generate second-harmonic pulses that are stretched or compressed relative to input pulses at the fundamental frequency. We frequency doubled an externally chirped erbium-doped fiber laser generating 17-ps (FWHM) pulses at 1560 nm to produce near-transform-limited 110-fs (FWHM) pulses at 780 nm by use of a 5-cm-long lithium niobate crystal poled with a QPM grating chirped from an 18.2-to a 19.8-mm period.
Second harmonic pulse distortion by imperfect phase matching
Optics Communications, 2000
We report on the distortion of second harmonic pulses generated in non-phase-matched conditions by highly efficient nonlinear crystals. The effect depends strongly on the temporal and spatial profiles of the incident fundamental pulse and on the phase mismatch. A Q-switched and mode-locked Nd:YAG laser was employed to demonstrate several temporal and spatial shapes for the second harmonic pulse train produced by a KTP crystal. q 0030-4018r00r$ -see front matter q 2000 Elsevier Science B.V. All rights reserved.
Journal of the Optical Society of America B, 2002
We report studies of second-harmonic generation (SHG) of femtosecond pulses in long periodically poled lithium niobate waveguides under large conversion conditions. Strong saturation of the SHG efficiency was observed, accompanied by spectral and temporal distortion of the pump pulse. Our simulation studies suggest that the pulse distortions may be caused by the interaction of the phase-matched SHG process and an additional cascaded (2) process or processes, leading to a large nonlinear phase modulation. Such additional cascaded (2) processes could be caused by the existence of multiple transverse modes in the nonlinear waveguide. These phenomena, which to our knowledge have not been reported previously, may have a significant effect on studies of high-power short-pulse parametric process in waveguide devices and on the design of novel nonlinear optical waveguide devices for such applications.
Optics Letters, 1997
We demonstrate that traveling-wave second-harmonic generation produces amplitude-squeezed light at both the fundamental and the harmonic frequencies. Quasi-phase-matched second-harmonic conversion efficiencies approaching 60% were obtained in a 26-mm-long single-mode LiNbO 3 waveguide with pulses from a modelocked laser at 1.53 mm. The amplitude noise of the transmitted fundamental field was measured to be 0.8 dB below the shot-noise level, and the generated 0.765-mm harmonic light was measured to be amplitude squeezed by 0.35 dB. The conversion-efficiency dependence of the observed squeezing at both wavelengths agrees with theoretical predictions. Waveguide losses appear to degrade the squeezing, but the maximum observed squeezing is currently limited only by the available input power.