Metal-Organic Hybrid Metamaterials for Spectral-Band Selective Active Terahertz Modulators

Optically controlled spectral-band selective terahertz (THz) modulators based on metal-organic hybrid metamaterials were investigated. An artificially structured material, which consists of two single split-ring resonators put together on the split gap side, was patterned on a silicon substrate to generate frequency-selective properties. An active layer of an organic thin film (fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester, also called PCBM) was deposited on the metamaterial-silicon structure for modulating the transmission of incident THz radiation. The metal-organic hybrid metamaterials enabled active control of spectral bands present in the transmission spectra of THz waves. In addition, the changes in the photo-excited carrier density due to the transfer of charges between the layers were quantitatively analyzed by simulation results.

Ultrafast photo-induced enhancement of electron-phonon coupling in metal-halide perovskites

In metal-halide perovskites (MHPs), the nature of organic cations affects both, the perovskite’s structure and its optoelectronic properties. Using ultrafast pump-probe spectroscopy, we demonstrate that in state-of-the-art mixed-cation MHPs ultrafast photo-induced bandgap narrowing occurs, and linearly depends on the excited carrier density in the range from 1016 cm− 3 to above 1018 cm− 3. Furthermore, time-domain terahertz (td-THz) photoconductivity measurements reveal that the majority of carriers are localized and that the localization increases with the carrier density. Both observations, the bandgap narrowing and carrier localization, can be rationalized by ultrafast (sub-2ps) photo-induced enhancement of electron-phonon coupling, originating from dynamic disorder, as clearly evidenced by the presence of a Debye relaxation component in the terahertz photoconductivity spectra. The observation of photo-induced enhancement of electron-phonon coupling and dynamic disorder not only provides specific insight into the polaron-strain distribution of excited states in MHPs, but also adds to the development of a concise picture of the ultrafast physics of this important class of semiconductors.

Ultrafast transient sub-bandgap absorption of monolayer MoS2

The light–matter interaction in materials is of remarkable interest for various photonic and optoelectronic applications, which is intrinsically determined by the bandgap of the materials involved. To extend the applications beyond the bandgap limit, it is of great significance to study the light–matter interaction below the material bandgap. Here, we report the ultrafast transient absorption of monolayer molybdenum disulfide in its sub-bandgap region from ~0.86 µm to 1.4 µm. Even though this spectral range is below the bandgap, we observe a significant absorbance enhancement up to ~4.2% in the monolayer molybdenum disulfide (comparable to its absorption within the bandgap region) due to pump-induced absorption by the excited carrier states. The different rise times of the transient absorption at different wavelengths indicate the various contributions of the different carrier states (i.e., real carrier states in the short-wavelength region of ~<1 µm, and exciton states in the long wavelength region of ~>1 µm). Our results elucidate the fundamental understanding regarding the optical properties, excited carrier states, and carrier dynamics in the technologically important near-infrared region, which potentially leads to various photonic and optoelectronic applications (e.g., excited-state-based photodetectors and modulators) of two-dimensional materials and their heterostructures beyond their intrinsic bandgap limitations.

Excited Carrier Dynamics in Two-dimensional Materials

When electrons in materials are excited, they undergo several dynamic processes such as carrier thermalization, transfer, and recombination. These fundamental excited state processes are crucial to understanding the microscopic principles at work in electronic and optoelectronic devices. This article introduces the excited carrier dynamics in a two-dimensional van der Waals material and reveals several interesting phenomena that do not occur in bulk materials. Particularly, the focus will be two dynamic processes: carrier multiplication and ultrafast charge transfer.

Broadband nonlinear optical response in GeSe nanoplates and its applications in all-optical diode

Germanium selenide nanoplates (GeSe NPs) are considered to have broadband nonlinear optical (NLO) properties and great potential for applications in nanophotonic devices. In this work, we systematically studied the NLO response of GeSe NPs by the open-aperture Z-scan technique. GeSe NPs exhibit strong saturable absorption at wavelengths of 400, 800, and 1064 nm with different pulse durations. Furthermore, we investigated the excited carrier dynamics of GeSe NPs by the non-degenerate pump-probe technique. The fast and slow relaxation times at different wavelengths of 800, 871, 1064, and 1100 nm were components with lifetimes of about 0.54–1.08 and 52.4–167.2 ps, respectively. The significant ultrafast NLO properties of GeSe NPs imply their potential in the development of nanophotonic devices. Here, we designed and fabricated the all-optical diode by means of the GeSe/C60 tandem structure and demonstrated that the saturable absorption behavior of GeSe NPs can be used to fabricate a photonic diode, which exhibits nonreciprocal transmission of light similar to that of an electron diode.