Characterizations of Ultrafast Pulse via Monolayer TMDs at Low Light Level
Yen-Chun Chen1*, Yu-Ling Chen1, Han Yeh1, Chih-Wai Luo1, Wen-Hao Chang1
1Department of Electrophysics, National Chiao Tung University, Hsinchu, Taiwan
* Presenter:Yen-Chun Chen,
Spectral phase of ultrafast pulses plays a critical role in optimizing ultrafast laser performances [1], quantum coherence controls [2] and complex pulse generations [3]. Second harmonic generation frequency-resolved optical grating (SHG-FROG) is a standard technique to characterize the amplitude and phase of ultrafast pulses by measuring the spectrally-resolved intensity autocorrelation traces generated from nonlinear crystals [4]. Replacing the conventional nonlinear crystals (such as β-BBO, LBO and KDP) by monolayer transition metal dichalcogenides (TMDs) can relax phase-matching requirement of SHG and avoid pulse broadening (caused by group velocity dispersion), since the atomically-short interaction length. However, the atomically-short interaction length and poor light coupling efficiency limits the TMDs’ performance in the SHG-FROG technique. Here we increased SHG efficiency by enhancing the light coupling efficiency and second-order nonlinear susceptibility χ(2) using the interference induced by the multiple reflections of the substrate [5] and excitonic resonance [6], respectively. We demonstrated that the SH intensity is enhanced by a factor up to ~ 880 when monolayer WSe2 is placed on a designed distributed Bragg reflector (DBR) and excited by a wavelength of 1480 nm (two-photon energy in resonance with A-exciton energy state). Based on this scheme, we can characterize ultrafast pulse with the same result retrieved by BBO when a time-averaged fundamental power density down to 0.96 W/cm2. We demonstrate that monolayer TMDs are ideal nonlinear media for characterizing ultrafast optical pulses using SHG-FROG.

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Keywords: Transition metal dichalcogenides, Ultrafast optics, Second harmonic generations, Frequency-resolved optical gratings, Substrate interferences