Group-IV monochalcogenides belong to a family of 2D layered materials. Monolayers of group-IV monochalcogenides GeS, GeSe, SnS and SnSe have been theoretically predicted to exhibit a large shift current owing to a spontaneous electric polarization at room temperature. Using THz emission spectroscopy, we find that above band gap photoexcitation with ultrashort laser pulses results in emission of nearly single-cycle THz pulses due to a surface shift current in multi-layer, sub-μm to few- μm thick GeS and GeSe, as inversion symmetry breaking at the crystal surface enables THz emission by the shift current. Experimental demonstration of THz emission by the surface shift current puts this layered group-IV monochalcogenides forward as a candidate for next generation shift current photovoltaics, nonlinear photonic devices and THz sources.
We have observed emission of terahertz radiation from photoexcited GeS nanosheets without external bias. We attribute the origin of terahertz pulse emission to the shift current resulting from inversion symmetry breaking in ferroelectric single- or few-layer GeS nanosheets. We find that the direction of the shift current, and the corresponding polarity of the emitted THz pulses is determined by the spontaneous polarization in the ferroelectric GeS nanosheets. Experimental observation of zero-bias photocurrents puts GeS nanosheets forth as a promising candidate material for applications in third generation photovoltaics based on shift current, or bulk photovoltaic effect.
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.
We detail a new ultrafast scanning tunneling microscopy technique called THz-STM that uses terahertz (THz) pulses coupled to the tip of a scanning tunneling microscope (STM) to directly modulate the STM bias voltage over subpicosecond time scales . In doing so, THz-STM achieves ultrafast time resolution via a mode complementary to normal STM operation, thus providing a general ultrafast probe for stroboscopic pump-probe measurements. We use THz-STM to image ultrafast carrier trapping into a single InAs nanodot and demonstrate simultaneous nanometer (2 nm) spatial resolution and subpicosecond (500 fs) temporal resolution in ambient conditions. Extending THz-STM to vacuum and low temperature operation has the potential to enable studies of a wide variety of subpicosecond dynamics on materials with atomic resolution.
We have recently developed an ultrafast terahertz-pulse-coupled scanning tunneling microscope (THz-STM) that can
image nanoscale dynamics with simultaneous 0.5 ps temporal resolution and 2 nm spatial resolution under ambient
conditions. Broadband THz pulses that are focused onto the metallic tip of an STM induce sub-picosecond voltage
transients across the STM junction, producing a rectified current signal due to the nonlinear tunnel junction currentvoltage
(I-V) relationship. We use the Simmons model to simulate a tunnel junction I-V curve whereby a THz pulse
induces an ultrafast voltage transient, generating milliamp-level rectified currents over sub-picosecond timescales. The
nature of the ultrafast field emission tunneling regime achieved in the THz-STM is discussed.
Pulsed terahertz (THz) imaging has been suggested as a novel high resolution, noninvasive medical diagnostic tool.
However, little is known about the influence of pulsed THz radiation on human tissue, i.e., its genotoxicity and effects on
cell activity and cell integrity. We have carried out a comprehensive investigation of the biological effects of THz
radiation on human skin tissue using a high power THz pulse source and an in vivo full-thickness human skin tissue
model. We have observed that exposure to intense THz pulses causes DNA damage and changes in the global gene
expression profile in the exposed skin tissue. Several of the affected genes are known to play major roles in human
cancer. While the changes in the expression levels of some of them suggest possible oncogenic effects of pulsed THz
radiation, changes in the expression of the other cancer-related genes might have a protective influence. This study may
serve as a roadmap for future investigations aimed at elucidating the exact roles that all the affected genes play in skin
carcinogenesis and in response to pulsed THz radiation.
Nonlinear dynamics of free-carriers in direct bandgap semiconductors at terahertz (THz) frequencies is studied using
intense few-cycle pulses. Techniques as Z-scan, THz-pump / THz-probe, and optical-pump/ THz-probe are employed to
explore nonlinear interactions in both n-doped and photoexcited systems. The physical mechanism that gives rise to such
interactions is found to be intervalley scattering.