Recent developments in nonequilibrium and nonlinear terahertz (THz) spectroscopies have significantly advanced the understanding of collective excitations in superconductors. However, there is ongoing debate regarding the identification of Higgs, Leggett modes, and BCS charge fluctuations in the two-band superconductor ${\mathrm{MgB}}_2$. To address this issue, we have demonstrated the selective excitation of collective modes in ${\mathrm{MgB}}_2$ by systematically adjusting THz pump pulse characteristics. Using multicycle THz pump-probe spectroscopy, we distinguish various nonlinear processes through two-dimensional scans. The $\pi$-band Higgs mode is identified through its characteristic resonant enhancement at the temperatures where the pump frequency ${\omega }_{\mathrm{p}}$ matches the $\pi$-band superconductivity gap $2{\mathrm{\Delta }}_{\pi }(\mathrm{T})$. In contrast, employing single-cycle THz pulses that satisfy nonadiabatic excitation conditions reveals a heavily damped oscillation corresponding to the Leggett mode. Our observed mode-selective responses in ${\mathrm{MgB}}_2$ under tailored excitation provide a clear experimental route to discriminate between the modes, helping to reconcile existing controversies while highlighting the critical role of interband coupling in collective mode observation for multiband superconductors.

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• Multiband superconductivity
• Superconductivity
• Pump-probe spectroscopy
• Terahertz time-domain spectroscopy
• Two-dimensional coherent spectroscopy