University of Texas at Austin

Research Topics


Engineering Valley Excitons of van der Waals Materials

Dr. Wei-Ting Hsu studied van der Waals materials and heterostructures through light-matter interactions such as interband transition, phonon scattering, nonlinear and ultrafast time-resolved measurements. In recent works, Dr. Hsu demonstrated the Coulomb engineering of exciton binding energy, the symmetry-dictated interlayer electronic coupling, the layer-hybridized valley exciton, and the pressure-tuning of interlayer coupling in transition metal dichalcogenides.

Hsu et al., “Dielectric Impact on Exciton Binding Energy and Quasiparticle Bandgap in Monolayer WS2 and WSe2”, 2D Mater. 6, 025028 (2019).

Hsu et al., “Tailoring Excitonic States of van der Waals Bilayers through Stacking Configuration, Band Alignment and Valley Spin”, Science Adv. 5, eaax7407 (2019).

Hsu et al., “Engineering K-valley Couplings in Transition Metal Dichalcogenides through Controls of Interlayer Spacing”, Science Adv. (submitted).


Growth and characterization of superconducting nanostructures

We can grow atomic thin superconducting nanostructures on semiconductor/insulator substrate using molecular beam epitaxy. Characterization of such systems through STM/STS and double coil mutual inductance measurement helps us learn the electronic behavior near superconducting phase transition at both local and global scales.

Nam, H., Chen, H., Adams, P. W., Guan, S. Y., Chuang, T. M., Chang, C. S., ... & Shih, C. K. (2018). Nature communications, 9(1), 1-6.

Nam, H., Chen, H., Liu, T., Kim, J., Zhang, C., Yong, J., ... & Adams, P. W. (2016). Proceedings of the National Academy of Sciences, 113(38), 10513-10517.


Plasmonic Nanolaser

A nanolaser is a key component for on-chip optical communications and computing systems. In the past, we demonstrated on the low-threshold, continuous-wave operation of a subdiffraction nanolaser based on surface plasmon amplification by stimulated emission of radiation (SPASER). The plasmonic nanocavity is formed between an atomically smooth epitaxial silver film and a single optically pumped nanorod consisting of an epitaxial gallium nitride shell and an indium gallium nitride core acting as gain medium. The atomic smoothness of the metallic film is crucial for reducing the modal volume and plasmonic losses (Fig. 1). Moreover, we also realized all-color lasing in subdiffraction plasmonic resonators which is achieved via a novel mechanism based on a property of weak size dependence inherent in SPASER (Fig. 2).

Fig. 1 Plasmonic Nanolaser Using Epitaxially Grown Silver Film. Yu-Jung Lu et al. Science 337, 450 (2012)

Fig. 2 All-Color Plasmonic Nanolasers with Ultralow Thresholds: Autotuning Mechanism for Single-Mode Lasing Yu-Jung Lu et al. Nano Lett. 14, 4381-4388 (2014)


Facilities

ARPES

Our ARPES system consists of SCIENTA R3000 analyzer, home-built sample manipulator, UHV Prep chamber and pumps for UHV. Home-built sample manipulator has 6 axes degree of freedom and a cooling ability upto near liquid nitrogen temperature. Prep chamber is made up of a heater, thermocouple and ion gauge so that it is possible to treat samples before measurement. Furthermore, we are using a helium discharge lamp as a beam source.

Low temperature STM, LEED, CVD, and MBE cluster

This cluster of instruments enables growth and in-situ measurement capabilities. The MBE chamber is used for growth of atomically flat, epitaxial Ag(111), as well as Al capping layers. The MBE also equipped with a RHEED for in-situ characterization. There is a UHV prep chamber for transferring samples from atmosphere. The LEED chamber is also includes a sputter gun, an e-beaming stage, and growth capabilities of PVD of hBN and MBE of BeO. Finally, a low temperature STM is used for in-situ characterize the samples. This entire cluster was home-built, and the system is used for a variety of studies characterizing the structure and electronic properties of 2D materials.

Home-built low temperature STM & MBE growth chamber

MBE chamber can be easily connected to STM chamber, which makes in-situ measurement quick and simple. Project on this system mainly focus on 2D metallic thin film and superconductivity. Please see our RSI paper for more information:
Kim, J., et al. (2015). "Compact low temperature scanning tunneling microscope with in-situ sample preparation capability." Review of Scientific Instruments 86(9).

Molecular Beam Expitaxial

This home-built MBE system has a capability to grow different TMDs, topological insulators and metal thin films. The system equipped with a RHEED for monitoring growth status and different pumps for UHV.

Ultrafast mode-locked Ti:Sapphire laser systems

Ultrafast lasers provide short laser pulses of 20-160 fs with tunable wavelength from 700 nm to 950 nm. These lasers are excitation sources for time-resolved measurements and nonlinear optics such as time-resolved PL, transient reflection and SHG spectroscopies.

Room-temperature back-scattering optical microscope

It is capable of PL, Raman, DR and SHG measurements at room temperature.

Vacuum transfer system

It is a homebuilt facility. We use this system to prepare vertical van der Waals heterostructures in vacuum for avoiding the air bubble.

Low temperature microscope spectroscopy with closed-cycle cryostat

We can measure photoluminescence, Raman, nonlinear optics spectroscopy and et. al. at 4K environment.

Chemical vapor deposition

We use this facility to grow transition metal dichalcogenide monolayers (such as WS2, MoS2, Wse2 and MoSe2) or heterostructure.