Abstract-Ultrafast Photoconductivity and Terahertz Vibrational Dynamics in Double-Helix SnIP Nanowires

 

David N. Purschke, Markus R. P. Pielmeier, Ebru Üzer, Claudia Ott, Charles Jensen, Annabelle Degg, Anna Vogel, Naaman Amer, Tom Nilges, Frank A. Hegmann, 

https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.202100978

Tin iodide phosphide (SnIP), an inorganic double-helix material, is a quasi-1D van der Waals semiconductor that shows promise in photocatalysis and flexible electronics. However, the understanding of the fundamental photophysics and charge transport dynamics of this new material is limited. Here, time-resolved terahertz (THz) spectroscopy is used to probe the transient photoconductivity of SnIP nanowire films and measure the carrier mobility. With insight into the highly anisotropic electronic structure from quantum chemical calculations, an electron mobility as high as 280 cm2  V−1s−1 along the double-helix axis and a hole mobility of 238 cm2  V−1 s−1 perpendicular to the double-helix axis are detected. Additionally, infrared-active (IR-active) THz vibrational modes are measured, which shows excellent agreement with first-principles calculations, and an ultrafast photoexcitation-induced charge redistribution is observed that reduces the amplitude of a twisting mode of the outer SnI helix on picosecond timescales. Finally, it is shown that the carrier lifetime and mobility are limited by a trap density greater than 1018 cm−3. The results provide insight into the optical excitation and relaxation pathways of SnIP and demonstrate a remarkably high carrier mobility for such a soft and flexible material, suggesting that it could be ideally suited for flexible electronics applications.

Abstract-Terahertz Excitonics in Carbon Nanotubes: Exciton Autoionization and Multiplication

Filchito Bagsican, Michael Wais, Natsumi Komatsu, Weilu Gao, Weilu Gao, Lincoln W. Weber, Kazunori Serita,  Hironaru Murakami, Karsten. Held, Frank A. Hegmann, Masayoshi Tonouchi, Junichiro Kono, Iwao Kawaya, Marco Battiato

https://pubs.acs.org/doi/10.1021/acs.nanolett.9b05082

Excitons play major roles in optical processes in modern semiconductors, such as single-wall carbon nanotubes (CNTs), transition metal dichalcogenides, and 2D perovskite quantum wells. They possess extremely large binding energies (>100 meV), dominating absorption and emission spectra even at high temperatures. The large binding energies imply that they are stable, that is, hard to ionize, rendering them seemingly unsuited for optoelectronic devices that require mobile charge carriers, especially terahertz emitters and solar cells. Here, we have conducted terahertz emission and photocurrent studies on films of aligned single-chirality semiconducting CNTs and find that excitons autoionize, i.e., spontaneously dissociate into electrons and holes. This process naturally occurs ultrafast (<1 ps) while conserving energy and momentum. The created carriers can then be accelerated to emit a burst of terahertz radiation when a dc bias is applied, with promising efficiency in comparison to standard GaAs-based emitters. Furthermore, at high bias, the accelerated carriers acquire high enough kinetic energy to create secondary excitons through impact exciton generation, again in a fully energy and momentum conserving fashion. This exciton multiplication process leads to a nonlinear photocurrent increase as a function of bias. Our theoretical simulations based on nonequilibrium Boltzmann transport equations, taking into account all possible scattering pathways and a realistic band structure, reproduce all of our experimental data semiquantitatively. These results not only elucidate the momentum-dependent ultrafast dynamics of excitons and carriers in CNTs but also suggest promising routes toward terahertz excitonics despite the orders-of-magnitude mismatch between the exciton binding energies and the terahertz photon energies.

Abstract-Dynamical Control over Terahertz Electromagnetic Interference Shielding with 2D Ti3C2Ty MXene by Ultrafast Optical Pulses

Guangjiang Li,  Naaman Amer,  Hassan A. Hafez, Shuohan Huang, Dmitry Turchinovich, Vadym N. Mochalin,  Frank A. Hegmann,  Lyubov V. Titova,

https://pubs.acs.org/doi/pdf/10.1021/acs.nanolett.9b04404 

High electrical conductivity and strong absorption of electromagnetic radiation in the terahertz (THz) frequency range by metallic 2D MXene Ti3C2Ty make it a promising material for electromagnetic interference shielding, THz detectors, and transparent conducting electrodes. Here, we demonstrate that ultrafast optical pulses with wavelengths straddling the visible range (400 and 800 nm) induce transient broad-band THz transparency in the MXene that persists for nanoseconds. We demonstrate that optically induced transient THz transparency is independent of temperature from 95 to 290 K. This discovery opens new possibilities for development of switchable electromagnetic interference shielding materials and devices that can be rendered partially transparent on demand for transmitting THz signals, or for designing new THz devices such as sensitive optically gated detectors.

Abstract-Modulated Photoluminescence of Germanium via Intense Terahertz Pulse Electric Fields

Mary A. B. Narreto, Chenxi Huang, David N. Purschke,  Frank A. Hegmann,

https://www.osapublishing.org/abstract.cfm?uri=LS-2018-JW3A.33

We show a nonlinear effect of intense terahertz pulse electric fields (>200 kV/cm) on the photoluminescence of bulk germanium. Terahertz-pulse-induced direct-gap photoluminescence quenching is observed.

© 2018 The Author(s)

Abstract-Terahertz-Pulse-Induced Patterning on the Nanoscale with Terahertz Scanning Tunneling Microscopy

Vedran Jelic, Daniel Mildenberger, Peter H. Nguyen, Tianwu Wang, and Frank A. Hegmann

https://www.osapublishing.org/abstract.cfm?uri=cleo_si-2018-SW3D.1&origin=search

We demonstrate that ultrafast terahertz pulses focused onto the tip of scanning tunneling microscope (THz-STM) can be used to pattern structures on a silicon surface via field-assisted removal of surface atoms.

© 2018 The Author(s)

Abstract-Ultrafast imaging of terahertz Cherenkov waves and transition-like radiation in LiNbO3

Ultrafast imaging of terahertz Cherenkov waves and transition-like radiation in LiNbO3 Zhenyou Wang, FuHai Su, and Frank A. Hegmann  »View Author Affiliations

Optics Express, Vol. 23, Issue 6, pp. 8073-8086 (2015)
http://dx.doi.org/10.1364/OE.23.008073

http://www.opticsinfobase.org/oe/abstract.cfm?uri=oe-23-6-8073

View Full Text Article

Enhanced HTML    Acrobat PDF (7315 KB)

We use ultrafast phase-contrast imaging to directly observethe cone-like terahertz (THz) Cherenkov wave generated by optical rectification of femtosecond laser pulses focused into bulk lithium niobate (LiNbO₃) single crystals. The transverse imaging geometry allows the Cherenkov angle, THz wave velocity, and optical pump pulse group velocity to be measured. Furthermore, transition-like THz radiation generated by the femtosecond laser pulse at the air-crystal boundary is observed. The effect of optical pump pulse polarization on the generation of THz Cherenkov waves and transition-like radiation in LiNbO₃is also investigated.

© 2015 Optical Society of America

Abstract-Effect of intense THz pulses on expression of genes associated with skin cancer and inflammatory skin conditions

Lyubov V. Titova, Ayesheshim K. Ayesheshim, David Purschke, Frank A. Hegmann

Univ. of Alberta (Canada)

Andrey Golubov, Rocio Rodriguez-Juarez, Rafal Woycicki, Olga Kovalchuk

Univ. of Lethbridge (Canada)

Proc. SPIE 8941, Optical Interactions with Tissue and Cells XXV; and Terahertz for Biomedical Applications, 89411G (March 13, 2014); doi:10.1117/12.2044167

http://proceedings.spiedigitallibrary.org/proceeding.aspx?articleid=1850014

The growing experimental evidence suggests that broadband, picosecond-duration THz pulses may influence biological systems and functions. While the mechanisms by which THz pulse-induced biological effects are not yet known, experiments using in vitro cell cultures, tissue models, as well as recent in vivo studies have demonstrated that THz pulses can elicit cellular and molecular changes in exposed cells and tissues in the absence of thermal effects. Recently, we demonstrated that intense, picosecond THz pulses induce phosphorylation of H2AX, indicative of DNA damage, and at the same time activate DNA damage response in human skin tissues. We also find that intense THz pulses have a profound impact on global gene expression in human skin. Many of the affected genes have important functions in epidermal differentiation and have been implicated in skin cancer and inflammatory skin conditions. The observed THzinduced changes in expression of these genes are in many cases opposite to disease-related changes, suggesting possible therapeutic applications of intense THz pulses. © (2014) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.