Generation of stable and breathing flat-top solitons via Raman assisted four wave mixing in microresonators

Flat-top-soliton (or platicon) dynamics in coherently pumped normal dispersion microresonators is important for both fundamental nonlinear physics and microcomb generation in the visible band. Here we numerically investigate the platicon generation that is initiated via Raman assisted four wave mixing instead of mode interaction. To show the possibility of generating coherent combs in the visible band, we design an aluminum nitride (AlN) microresonator with normal dispersion and investigate the comb generation dynamics in simulations. Stable platicon Kerr combs can be generated in this AlN microresonator using a 780-nm pump. Moreover, we also observe a breather platicon dynamics induced by the narrow Raman gain spectrum of crystalline AlN, which shows distinct dynamics from the dark soliton breathers reported in previous work that are dominated by Kerr effect. A phase diagram bearing the influence of the pump detuning and pump power on the breathing dynamics of the breather platicon is also computed. Furthermore, a transition to chaotic breathing is numerically observed.

Figure 1

(a) Schematic illustration of the AlN microresonator and the Kerr-Raman combs generation process. Signal and idler are generated by nondegenerate FWM between pump and single mode Raman laser. (b) Scheme for RFWM: the angular frequency offset between pump and central frequency is $\mathrm{\Omega }$, and the angular frequency offset between idler and central frequency is $\omega$.

Figure 2

Simulated group velocity dispersion (GVD) of the fundamental TM mode in the AlN waveguide with different geometrical parameters. The dispersion parameter is calculated with a finite-element mode solver. (a) Dependence of GVD on waveguide width ($w$) when the height is 800 nm. (b) Dependence of GVD on waveguide width ($h$) when the width is 1400 nm.

Figure 3

Dynamics of Kerr-Raman comb generation in an AlN microresonator, the waveguide width of which is 1400 nm and the height of which is 800 nm. (a) Evolution of the intracavity power when the pump is boosted to 660 mW and tuned into the resonance. The red dashed line shows the tuning of the detuning. In the red shaded (left) region the pump laser is boosted to 660 mW with a detuning of −0.0072. In the blue shaded (middle) region the pump is linearly tuned into the resonance with detuning linearly increasing from −0.0072 to 0.0636. In the gray shaded (right) region the pump is kept stable. (b) Temporal waveform and (c) optical spectrum for different positions I-IV in (a).

Figure 4

Dynamics of the Raman induced breather platicon. (a) Evolution of the temporal intensity profile and the corresponding evolution of pulse peak power, pulse width, and intracavity power. (b) Temporal waveform and (c) optical spectrum of the exact moment marked by the dashed line in (a).

Figure 5

Power exchange between pump area and Raman area of the platicon breather. (a) Detailed evolution of the temporal intensity and (b) the corresponding power transfer between pump area and Raman area near the negative trajectory slope regime. (c) Temporal waveform and (d) optical spectra of different times during the power transfer. The exact times of I-V are 2.465, 7.395, 9.663, 12.325, and 19.72 ns. The blue (orange) shaded regions illustrate the pump (Raman) area, corresponding to 760–800 (800–840) nm of the optical spectrum.

Figure 6

Raman induced platicon breather dynamics on pump condition. (a) Dependence of breathing frequency and breathing depth on the pump detuning. (b) Phase diagram in the (pump power, detuning) plane representing different pump parameters leading to various breathing dynamics [red triangle, one-period breather; green square, multiperiod breather; blue diamond, chaotic breather; blank region, homogeneous steady state (HSS)]. (c) Evolution of the temporal intensity profile and the corresponding evolution of intracavity power of the period-doubling state when detuning is 0.0621. (d) Evolution of the temporal intensity profile and the corresponding evolution of intracavity power of the chaotic breathing state when detuning is 0.06.