Parametrically Tunable Soliton-Induced Resonant Radiation by Three-Wave Mixing

We show that a temporal soliton can induce resonant radiation by three-wave mixing nonlinearities. This constitutes a new class of resonant radiation whose spectral positions are parametrically tunable. The experimental verification is done in a periodically poled lithium niobate crystal, where a femtosecond near-IR soliton is excited and resonant radiation waves are observed exactly at the calculated soliton phase-matching wavelengths via the sum- and difference-frequency generation nonlinearities. This extends the supercontinuum bandwidth well into the mid IR to span 550–5000 nm, and the mid-IR edge is parametrically tunable over 1000 nm by changing the three-wave mixing phase-matching condition. The results are important for the bright and broadband supercontinuum generation and for the frequency comb generation in quadratic nonlinear microresonators.

Figure 1

Supercontinuum recorded for ${\lambda }_0=1.75\mu \mathrm{m}$ and $I_0=150$, using a 10 mm PPLN with $\mathrm{\Lambda }=30.0\mu \mathrm{m}$; the experimental data (thick red) are directly compared to a numerical simulation (thin light red) and calculated higher-order quasiphase matching resonances (dashed lines). The power-spectral densities (PSDs) are normalized to the peak input PSD so that the average output power matches the input power. The top plot shows the theoretical RR phase-matching conditions to the soliton using ${\lambda }_s=1.68\mu \mathrm{m}$. ZDW: zero-dispersion wavelength ($1.92\mu \mathrm{m}$).

Figure 2

False-color representation of the experimental supercontinua for various intensities with the same parameters as Fig. 1. Dashed lines: theoretical phase-matching wavelengths using the extracted soliton wavelengths (black line). Bottom: details of the visible and short-wavelength near-IR range, including calculated QPM resonances.

Figure 3

(a) Calculated tuning curves for SPM, DFG, and SFG RR waves (using a fixed ${\lambda }_s=1.68\mu \mathrm{m}$), plotted with data from the same experiment as Fig. 1, but where the QPM pitch was varied. Inset: details around the experimental data. (b) Experimental mid-IR spectra showing the DFG-RR peaks, recorded with a 20 mm PPLN with 10 different QPM pitch values, using ${\lambda }_0=1.75\mu \mathrm{m}$ and $I_0=55\mathrm{GW}/{\mathrm{cm}}^2$; note the linear $y$ axis. Inset: typical transverse beam profile of the long-pass filtered mid-IR RR wave.