Abstract-Ultrafast THz modulators with WSe2 thin films [Invited]

Prashanth Gopalan, Ashish Chanana, Sriram Krishnamoorthy, Ajay Nahata, Michael A. Scarpulla, and Berardi Sensale-Rodriguez

Fig. 1 (a) XRD scans indicating highly crystalline WSe2 films showing only the (002) family of planes. (b) Raman mode at 251.2 cm−1, which agrees with reports in [24] for multilayer films. (c) UV-vis absorption measurements of WSe2 thin films indicating a strong excitonic absorption peak at ~740 nm. Based on this, optical excitation for OPTP and TRTS were chosen at 800 nm and 400 nm. (d) SEM image of WSe2 thin film shows grain size of ~1 µm.

https://www.osapublishing.org/ome/abstract.cfm?uri=ome-9-2-826

Tungsten diselenide (WSe2) thin films exhibit ultrafast carrier recombination lifetimes, which makes them promising candidates for high speed modulators. With pulsed optical excitation, they could be used to realize all-optical, frequency agile, terahertz devices. Looking into the potential of this material for such applications, time-resolved terahertz spectroscopy can provide significant insight into its free carrier and exciton dynamics such as recombination lifetimes, photo-induced conductivity and decay pathways. In this study, we measure transient terahertz conductivity and photo-generated carrier lifetimes in custom-grown large-area WSe2thin-films. We discuss its dependence on grain size and number of layers. By analyzing the tradeoffs between carrier-lifetimes, photo-generated conductivity, grain size, and the number of layers, we show that the response of these films can be tailored by controlling the growth parameters. Customizing the film terahertz response can enable large modulation without the need for integration with bulk semiconductors, as widely reported in the literature, thereby achieve high terahertz photoconductivity and high-speed operation. Across samples, our measurements show carrier decay timescale on the order ~10 to 100 ps and a transient conductivity that shows non-Drude behavior. This deviation from a Drude response is dominant within the first few picoseconds (<10 ps) before changing into a Drude like free-carrier response at longer delays. Based on our grown films, we experimentally demonstrate a metamaterial terahertz modulator with WSe2 as the only active element, attaining ~40% modulation depth.

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