The recent development
of optical technology has enabled the practical
use of a carrier-envelope phase-controlled monocycle electric field
in the terahertz (THz) regime. By combining this technique with metal
nanostructures such as nanotips, which induce near-field enhancement,
the development of novel applications is anticipated. In particular,
THz scanning tunneling microscopy (THz-STM) is a promising technique
for probing ultrafast dynamics with the spatial resolution of STM.
However, the modulation of the THz waveform is generally accompanied
by an enhancement of the electric field, which is unknown in actual
measurement environments. Here, we present a method enabling direct
evaluation of the enhanced near field in the tunnel junction in THz-STM
in the femtosecond range, which is essential for the use of the THz
near field. In the tunneling regime, it was also demonstrated that
the transient electronic state excited by an optical pulse can be
evaluated using the THz-STM, and the ultrafast carrier dynamics in
2H-MoTe2 excited by an optical pulse was reproducibly probed.
The generation of high-order harmonics from hybrid organic–inorganic perovskites (HOIPs) is demonstrated by the excitation with a strong mid-infrared laser pulse. We prepare three types of HOIP polycrystalline thin film samples by solution processes (MAPbX3; MA = CH3NH3+; X = I, Br, Cl). The high-order harmonics from the sample (MAPbBr3) are more than tenfold stronger than those from the well-studied GaSe crystal despite their comparable bandgap energies, implying that the stronger band-to-band transition of the HOIPs causes the higher yields.
We investigate high-order harmonic generation (HHG) in graphene with a quantum master equation approach. The simulations reproduce the observed enhancement in HHG in graphene under elliptically polarized light [N. Yoshikawa et al, Science 356, 736 (2017)]. On the basis of a microscopic decomposition of the emitted high-order harmonics, we find that the enhancement in HHG originates from an intricate nonlinear coupling between the intraband and interband transitions that are respectively induced by perpendicular electric field components of the elliptically polarized light. Furthermore, we reveal that contributions from different excitation channels destructively interfere with each other. This finding suggests a path to potentially enhance the HHG by blocking a part of the channels and canceling the destructive interference through band-gap or chemical potential manipulation.
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