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

Bakker, H. J.; Joosen, W.; Noordam, L. D.

doi:10.1103/physreva.45.5126

Cited by 8 publications

(7 citation statements)

References 16 publications

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“…1 /2 > 0. Note that an analytical result for the detuned SH frequency was calculated previously [1,2] taking into account only GVM (i.e. accurate to 1. order only).…”

Section: Phase-matched Sidebands Theorymentioning

confidence: 99%

“…The soliton exists in the self-defocusing regime of the cascaded nonlinear interaction and in the normal dispersion regime of the crystal, and needs high input intensities to become excited.PACS numbers: 42.65. Re, 42.65.Tg, 42.65.Ky, 42.65.Sf A common observation in second-harmonic generation (SHG) of broadband laser pulses in thick crystals, is that when a phase mismatch ∆k is imposed, the second harmonic (SH) spectrum is dominated by a spectrally compressed peak that is wavelength-tunable through ∆k [1][2][3][4][5][6][7][8]. Figure 1(a) shows data from an experiment we performed using a thick β-barium borate (BBO) crystal.…”

mentioning

confidence: 99%

“…Thus, the excellent agreement between the nonlocal resonance wavelengths and the experimental data indirectly proves that this is a soliton-induced nonlocal resonance. Conversely, the historical measurements [1][2][3][4][5][6][7][8] used too low intensities for soliton formation: in this case the FW is dispersive and the phase-matched sideband theory prevails.…”

mentioning

confidence: 99%

“…A common observation in second-harmonic generation (SHG) of broadband laser pulses in thick crystals, is that when a phase mismatch ∆k is imposed, the second harmonic (SH) spectrum is dominated by a spectrally compressed peak that is wavelength-tunable through ∆k [1][2][3][4][5][6][7][8]. Figure 1(a) shows data from an experiment we performed using a thick β-barium borate (BBO) crystal.…”

mentioning

confidence: 99%

“…This is a completely new way of confirming the presence of a soliton in this elusive resonant nonlocal regime. The historical experiments [1][2][3][4][5][6][7][8] instead employed low intensities so solitons could not be excited. Thus the FW remained dispersive and therefore the traditional theory of phasematched sidebands could accurately explain the peaks.…”

mentioning

confidence: 99%

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Soliton-induced nonlocal resonances observed through high-intensity tunable spectrally compressed second-harmonic peaks

2014

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Experimental data of femtosecond thick-crystal second-harmonic generation shows that when tuning away from phase matching, a dominating narrow spectral peak appears in the second harmonic that can be tuned over 100's of nm by changing the phase-mismatch parameter. Traditional theory explains this as phase matching between a sideband in the broadband pump to its second-harmonic. However, our experiment is conducted under high input intensities and instead shows excellent quantitative agreement with a nonlocal theory describing cascaded quadratic nonlinearities. This theory explains the detuned peak as a nonlocal resonance that arises due to phase-matching between the pump and a detuned second-harmonic frequency, but where in contrast to the traditional theory the pump is assumed dispersion-free. As a soliton is inherently dispersion-free, the agreement between our experiment and the nonlocal theory indirectly proves that we have observed a soliton-induced nonlocal resonance. The soliton exists in the self-defocusing regime of the cascaded nonlinear interaction and in the normal dispersion regime of the crystal, and needs high input intensities to become excited. A common observation in second-harmonic generation (SHG) of broadband laser pulses in thick crystals, is that when a phase mismatch ∆k is imposed, the second harmonic (SH) spectrum is dominated by a spectrally compressed peak that is wavelength-tunable through ∆k [1][2][3][4][5][6][7][8]. Figure 1(a) shows data from an experiment we performed using a thick β-barium borate (BBO) crystal. The input fundamental wave (FW, center frequency ω 1 ) was an intense femtosecond pulse loosely focused and collimated at the crystal entrance to avoid diffraction. The tuning around ∆k = 0 was achieved by rotating the external angle of the crystal. A striking wavelength tunability over 100's of nanometers is possible, and the peak is also strongly compressed compared to the ideal thin-crystal bandwidth (in this case about 40 nm FWHM). The compressed SH peak pertains for large negative tuning angles, while it disappears for large positive tuning angles (here +5• ). The spectral compression is traditionally explained by a phase-matched sidebands theory, which uses the classical result that the SH efficiency ∝ sinc 2 [∆kL/2]: this explains the decreasing bandwidth in a thick crystal, and the frequency dependence of ∆k(ω) = k 2 (ω) − 2k 1 (ω/2) explains how a SH sideband frequency strongly detuned from the degenerate SH frequency ω 2 = 2ω 1 can become phase matched when ∆k(ω 2 ) = 0. Figure 1(c) shows the predicted phasematching wavelengths by the phase-matched sidebands theory, and remarkably it cannot explain the experimental data for large positive tuning angles.Instead an alternative nonlocal theory, shown in Fig. 1(b), predicts "resonance" wavelengths in the nonlocal response function that up to +4• are in excellent agreement with the experimental peak positions. At +5• the phase-matching condition behind this resonance is no longer fulfilled, yielding a broadband, non-...

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“…1 /2 > 0. Note that an analytical result for the detuned SH frequency was calculated previously [1,2] taking into account only GVM (i.e. accurate to 1. order only).…”

Section: Phase-matched Sidebands Theorymentioning

confidence: 99%