Exploiting ultrafast optical pulses: probing excitonic properties of 2D materials and utilizing the Kerr effect for all-optical switching

Abstract

Ultrafast optics technology involves the development and application of femtosecond optical pulses. The work presented in this thesis consists of two research objectives which exploit these aspects of ultrafast optics technology.

In Part I of this work, we implement a method for probing the excitonic response of 2D MoS2. Excitonic properties of samples are investigated through photoluminescence (PL) mapping, which we demonstrate as a valuable characterization method. PL mapping is used to determine the number of layers present in samples of 2D MoS2. Monolayer MoS2 exhibits a PL peak intensity ∼ 5 times that of bi-layer, with a blue-shift of 18 ± 3 meV. We demonstrate the influence of MoS2 sample growth and preparation mechanisms on its excitonic response. The Exciton A peak of mechanically-exfoliated MoS2 (at 1.8958 ± 0.0006 eV, with FWHM 46 ± 2 meV) is narrower and blue-shifted compared to that of a sample grown via chemical vapour deposition (at 1.845±0.005 eV, with FWHM 110±5 meV). We also observe evidence of an increase in total PL intensity for regions of localized rips and tears in exfoliated samples, and a decrease in trion activity for samples isolated via hBN-encapsulation. Finally, we present initial measurements of ultrafast exciton lifetimes in monolayer MoS2 (0.99 ± 0.18 ps and 14.78 ± 1.94 ps) via femtosecond excitation correlation (FEC) spectroscopy.

In Part II of this work, we demonstrate the “carving out” of polarization-switched ultrafast optical pulses (424 fs in duration) from a continuous beam of light, through the optical Kerr effect. In addition to establishing this as a reliable technique for measuring intrinsic switching speed, we demonstrate the configurability of our switch. Our switch is capable of generating ultrashort pulses (with tunability in spectral bandwidth of 1.6–2.9 nm) and switched 2- and 4-pulse trains (with 2.2 ps temporal separation). Our all-optical switch holds potential in a number of technologies, particularly in optical communication and as a new ultrafast light source.