Full Temporal Characterization of Ultrabroadband Few-Cycle Laser Pulses Using Atomically Thin WS₂

Nowadays, the accurate and full temporal characterization of ultrabroadband few-cycle laser pulses with pulse durations below 7 fs is of great importance in fields of science that investigate ultrafast dynamic processes. There are several indirect methods that use nonlinear optical signals to retrieve the complex electric field of femtosecond lasers. However, the precise characterization of few-cycle femtosecond laser sources with an ultrabroadband spectrum presents additional difficulties, such as reabsorption of nonlinear signals, partial phase matching, and spatiotemporal mismatches. In this work, we combine the dispersion scan (d-scan) method with atomically thin WS2 flakes to overcome these difficulties and fully characterize ultrabroadband laser pulses with a pulse duration of 6.9 fs and a spectrum that ranges from 650 to 1050 nm. Two-dimensional WS2 acts as a remarkably efficient nonlinear medium that offers a broad transparency range and allows for achieving relaxed phase-matching conditions due to its atomic thickness. Using mono- and trilayers of WS2, we acquire d-scan traces by measuring the second-harmonic generation (SHG) signal, originated via laser-WS2 interaction, as a function of optical dispersion (i.e., glass thickness) and wavelength. Our retrieval algorithm extracts a pulse duration at full-width half-maximum of 6.9 fs and the same spectral phase function irrespective of the number of layers. We benchmark and validate our results obtained using WS2 by comparing them with those obtained using a 10-μm-thick BBO crystal. Our findings show that atomically thin media can be an interesting alternative to micrometer-thick bulk crystals to characterize ultrabroadband femtosecond laser pulses using SHG-d-scan with an error below 100 as (attoseconds).