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The authors investigate bulk 2H-NbSe$_2$ across its 33 K charge-density-wave (CDW) transition using far-field terahertz time-domain spectroscopy alongside angle-resolved photoemission spectroscopy and numerical modeling. Resolving these coupled collective excitations is critical for disentangling the microscopic interplay between electronic order parameters, lattice degrees of freedom, and defect pinning. Below the transition temperature, the optical response exhibits a distinct ~1.5 THz amplitude mode accompanied by longer-lived sub-THz defect-pinned phase oscillations, both of which suppress sharply upon warming into the normal state.
Terahertz collective excitations in 2H-NbSe$_2$ decouple into distinct ~1.5 THz amplitude modes and sub-THz defect-pinned phase oscillations, establishing a non-invasive optical fingerprint for phase-transition dynamics in correlated quantum materials.
Charge-density-wave (CDW) formation in 2H-NbSe$_2$ modifies the low-energy electronic structure and gives rise to collective excitations coupled to the lattice. Here, we investigate bulk 2H-NbSe$_2$ single crystals using far-field terahertz time-domain spectroscopy (THz-TDS) in reflection geometry across the CDW transition at $T_{\mathrm{CDW}} \approx 33$~K. Below $T_{\mathrm{CDW}}$, the THz response shows a pronounced high-frequency feature near 1.5~THz together with longer-lived sub-THz oscillations. Both responses progressively weaken upon warming and are strongly suppressed across the CDW transition, supporting their association with the CDW state. Using time-dependent Ginzburg--Landau simulations, we reproduce the main features of the experimental THz response, associating the high-frequency response mainly with CDW amplitude dynamics and the sub-THz response with defect-pinned phase dynamics. The $\sim1.5$~THz feature lies close to the frequency range reported for the Raman CDW amplitude mode, while coupling to lattice degrees of freedom may also influence its spectral position. We also performed complementary angle-resolved photoemission spectroscopy measurements, which reveal momentum-selective redistribution of near-Fermi-level spectral weight across the transition. Together, these results show that far-field THz spectroscopy provides a sensitive probe of collective CDW dynamics in bulk 2H-NbSe$_2$.