Simultaneous Characterization of Two Ultrashort Optical Pulses at Different Frequencies Using a WS₂ Monolayer

Abstract

The precise characterization of ultrashort laser pulses has been of interest to the scientific community for many years. Frequency-resolved optical gating (FROG) has been extensively used to retrieve the temporal and spectral field distributions of ultrashort laser pulses. In this work, we exploit the high, broad-band nonlinear optical response of a WS 2 monolayer to simultaneously characterize two ultrashort laser pulses with different frequencies. The relaxed phase-matching conditions in a WS 2 monolayer enable the simultaneous acquisition of the spectra resulting from both four-wave mixing (FWM) and sum-frequency generation (SFG) nonlinear processes while varying the time delay between the two ultrashort pulses. Next, we introduce an adjusted double-blind FROG algorithm, based on iterative fast Fourier transforms between two FROG traces, to extract the intensity distribution and phase of two ultrashort pulses from the combination of their FWM and SFG FROG traces. Using this algorithm, we find an agreement between the computed and observed FROG traces for both the FWM and SFG processes. Exploiting the broad-band nonlinear response of a WS 2 monolayer, we additionally characterize one of the pulses using a second-harmonic generation (SHG) FROG trace to validate the pulse shapes extracted from the combination of the FWM and SFG FROG traces. The retrieved pulse shape from the SHG FROG agrees well with the pulse shape retrieved from our nondegenerate cross-correlation FROG measurement. In addition to the nonlinear parametric processes, we also observe a nonlinearly generated photoluminescence (PL) signal emitted from the WS 2 monolayer. Because of its nonlinear origin, the PL signal can also be used to obtain complementary autocorrelation and cross-correlation traces.

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

“…Such processes lead to a higher conversion efficiency compared to their higher order counterparts (i.e., THG in graphene) and consequently require a lower laser intensity to generate light; thus, these are less susceptible to damage the nonlinear 2D medium. Several degenerate and nondegenerate second-order nonlinear mechanisms have been studied in a variety of 2D materials − and exploited in FROG systems to characterize laser pulses with pulse durations above 100 fs and bandwidths of ∼10 nm. , In addition, 2D-layered transition metal dichalcogenides (TMDCs) present a broad spectral range of transparency, , which can help to minimize reabsorption of the nonlinearly generated signals. Therefore, these materials offer an attractive alternative for spectral regions in which nonlinear crystals may not be available.…”