Ultrafast spectroscopy of novel materials

Ultrafast spectroscopy is a powerful technique for investigating transient dynamics of photocarriers. It provides not only fundamental insights into electronic/optical properties of materials, but also of relevance to device design and optimizations. Our group is interested in studying the carrier dynamics in emerging materials from three-dimensional (3D) topological Dirac semimetals to low-dimensional materials such as transition metal dichalcogenides and single-wall carbon nanotubes, with a particular focus on identifying methods to control the relaxation characteristics. Currently, we have pump-probe systems covering a broad wavelength range, from 400 nm – 6,000 nm. The following are some exemplary research projects in the group.

3D Dirac semimetals: Recently, bulk Dirac fermions in 3D Dirac semimetals have proved to be both robust and scalable. The inter-Dirac-band excitation provides an excellent potential platform for infrared photonic devices. Our group is among the first to investigate the ultrafast carrier dynamics of this emerging functional material. In particularly, by engineering the band structure of Cd3As2 through an element-doping strategy, we have successfully demonstrated that the recombination time of the system can be flexibly tuned in broad mid-infrared wavelength range (2–6 μm band). Such a capability makes Dirac semimetal thin film a capable mid-infrared optical switch with unprecedented tunability, a long sought device for mid-infrared pulsed laser development.

Schematic diagram showing tunable photocarrier relaxation time achieved by band structure engineering. [LaserFocusWorld News]

2D semiconductors: Since the successful isolation of graphene in 2004, a number of atomically thin 2D crystals with intriguing properties have been uncovered. Among them, 2D semiconductors such as TMDs and black phosphorus, have drawn considerable attention and are considered candidates for future optoelectronics/ spintronics. When the thickness of these materials is reduced to atomic scale, they exhibit completely different properties with respect to their bulk counterparts, owing to quantum confinement effects. Our group is currently working on revealing the intrinsic and broadband optical properties of TMDs as well as related 2D heterostructures.

1D single-wall carbon nanotubes (SWNTs): SWNTs exhibit fascinating physical properties that are highly relevant to optoelectronic and photonic applications. We are working on both the ultrafast carrier dynamics of SWNTs and their photonics applications. In one study, by combining broadband pump-probe measurement with nonlinear absorption characterization, we have for the first time provided a broadband ‘map’ of the ultrafast nonlinear properties of SWNTs. This has led to the identification of a novel transient bandgap renormalization effect as well as the demonstration of nanotube enabled pulsed laser in the visible band.    

Transient spectra for SWNT samples with degenerate pump-probe configuration and excitation wavelengths from 1200 nm to 2400 nm. (a) Relaxation curves, (b) peak ΔT/T0 and (c) relaxation times.

Related group publications:
Nature Communications 8, 14111 (2017)
Appl. Phys. Lett. 111, 091101 (2017)
Appl. Phys. Lett. 111, 031906 (2017)
Appl. Phys. Lett.112, 031108 (2018)
Appl. Phys. Lett. 112, 171112 (2018)
Nanoscale 9, 18546 (2017)
Nanoscale 8, 9304 (2016)
Nanoscale 8, 1066 (2016)
Optics Letters 42, 671 (2017)
Scientific Reports 7, 11221 (2017)




School of Electronic Science and Engineering, Nanjing University
163 Xianlin Avenue, 210023 Nanjing, China

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