Fig. 5: A supercontinuum enhances phase linearity and enables the measurement of lower free electron number densities.
From: Quantitative single-shot Supercontinuum-Enhanced Terahertz Spectroscopy (SETS)
Simulated results demonstrating that (A) the supercontinuum increases the usable bandwidth (Δω) compared to a chirped pulse of the same duration (Tc), ensuring phase linearity without null frequencies (i.e., non-linearity) over a broader range of frequencies. B The supercontinuum allows Tc extension while maintaining a lower β compared to a chirped pulse, facilitating the detection of phase shifts (Δϕ) at lower frequencies. C Consequently, lower electron number densities (\({\widetilde{{\mbox{n}}}}_{{\mbox{e}}}\)) can be measured, determining the minimum measurable (or \({\widetilde{{\mbox{n}}}}_{{\mbox{e}}}\) resolution). As the ratio of plasma frequency (ωp) to collision frequency (\({\nu}_{{\mbox{en}}}\)) decreases, \({\widetilde{{\mbox{n}}}}_{{\mbox{e}}}\) resolution plateaus with increasing Tc, as Δϕ from the Drude model reaches a global maximum at higher frequencies, preventing further growth of Δϕ at lower frequencies. A longer Tc also provides access to lower frequency data, enabling the measurement of properties such as \({\nu}_{{\mbox{en}}}\). The \({\widetilde{{\mbox{n}}}}_{{\mbox{e}}}\) used in simulating the results shown in (A, B) is \(1\times {10}^{17}{{\mbox{m}}}^{-3}\). Pathlength, L, is 10 cm. The minimum measurable Δϕ of 0.1 rad was from the phase measurement uncertainty of the experimental setup (SI Sec. 8).
