Fig. 4: Sound excitation with different modulation sequences.

a, c \({{\Delta }}\bar{n}(x,t)\) data at ω_ex = 0.61ω_r, ΔU = 0.3U₀ and \({\left({k}_{{{{{{{{\rm{F}}}}}}}}}a\right)}^{-1}=(1.61\pm 0.05)\) for a modulation of 1.5 cycles and of 1 cycle, respectively. The excitation pulse excites both, first and second sound waves. b, d \({{\Delta }}\bar{n}(x,t)\) from numerical c-field simulations. Top row: False color images of \({{\Delta }}\bar{n}(x,t)\). First and second sound waves are marked with red and orange arrows, respectively. The inset in (d) is an enlargement, showing how the calculated bright first sound wave (black arrow) is damped when it crosses the second sound wave (orange arrow). Mid row: Shown is \({{\Delta }}\bar{n}\) for tω_x/2π = 0 (a, b) and for tω_x/2π = 0.15 (c, d). Bottom row: Applied excitation scheme.