Fotonsko-mikrovalna konverzija u poluvodičima kontrolom vala nosioca u optičkom području

The ultrafast carrier dynamics in the optically excited semiconductors is studied by observing the THz radiation. We developed the pump and probe THz beam generation system with variable sample temperature control, and employed it to examine the ultrafast carrier scattering processes. The results proved that the THz beam generation, especially pump and probe method, is a powerful tool to study the ultrafast phenomena. We propose the new model to explain the ultrafast carrier dynamics just after photon arrivals in low-temperature-grown GaAs, which includes the intervalley scattering process.Proučavana je dinamika ultrabrzog nosioca u optički pobuđenim poluvodičima promatranjem generiranja zraka u THz području. Razvijen je sustav za generiranje THz zrake pumpe i probe s temperaturnom kontrolom varijabilnim uzorkom. Sustav je korišten za ispitivanje procesa raspršenja ultrabrzog nosioca. Rezultati ukazuju na činjenicu da je generiranje THz zraka, a posebno metoda pumpe i probe, snažan alat za proučavanje ultrabrzih pojava. Predložen je novi model za objašnjenje dinamike ultrabrzog nosioca, upravo nakon upada fotona GaAs dobiven pomoću niskotemperaturnog procesa. Model sadrži proces raspršenja među energetskim pojasevima

Terahertz radiation by ultrafast spontaneous polarization modulation in multiferroic BiFeO3_3 thin films

Terahertz (THz) radiation has been observed from multiferroic BiFeO$_3$ thin films via ultrafast modulation of spontaneous polarization upon carrier excitation with illumination of femtosecond laser pulses. The radiated THz pulses from BiFeO$_3$ thin films were clarified to directly reflect the spontaneous polarization state, giving rise to a memory effect in a unique style and enabling THz radiation even at zero-bias electric field. On the basis of our findings, we demonstrate potential approaches to ferroelectric nonvolatile random access memory with nondestructive readability and ferroelectric domain imaging microscopy using THz radiation as a sensitive probe.Comment: 4 pages, 4 figures, submitted to Physical Review Letter

Ultrafast spatiotemporal photocarrier dynamics near GaN surfaces studied by terahertz emission spectroscopy

Gallium nitride (GaN) is a promising wide-bandgap semiconductor, and new characterization tools are needed to study its local crystallinity, carrier dynamics, and doping effects. Terahertz (THz) emission spectroscopy (TES) is an emerging experimental technique that can probe the ultrafast carrier dynamics in optically excited semiconductors. In this work, the carrier dynamics and THz emission mechanisms of GaN were examined in unintentionally doped n-type, Si-doped n-type, and Mg-doped p-type GaN films. The photocarriers excited near the surface travel from the excited-area in an ultrafast manner and generate THz radiation in accordance with the time derivative of the surge drift current. The polarity of the THz amplitude can be used to determine the majority carrier type in GaN films through a non-contact and non-destructive method. Unique THz emission excited by photon energies less than the bandgap was also observed in the p-type GaN film

Sub-diffraction thin-film sensing with planar terahertz metamaterials

Planar metamaterials have been recently proposed for thin dielectric film sensing in the terahertz frequency range. Although the thickness of the dielectric film can be very small compared with the wavelength, the required area of sensed material is still determined by the diffraction-limited spot size of the terahertz beam excitation. In this article, terahertz near-field sensing is utilized to reduce the spot size. By positioning the metamaterial sensing platform close to the sub-diffraction terahertz source, the number of excited resonators, and hence minimal film area, are significantly reduced. As an additional advantage, a reduction in the number of excited resonators decreases the inter-cell coupling strength, and consequently the resonance Q factor is remarkably increased. The experimental results show that the resonance Q factor is improved by 113%. Moreover, for a film with a thickness of \lambda/375 the minimal area can be as small as 0.2\lambda by 0.2\lambda. The success of this work provides a platform for future metamaterial-based sensors for biomolecular detection.Comment: 8 pages, 6 figure

Enhanced luminescence efficiency in Eu-doped GaN superlattice structures revealed by terahertz emission spectroscopy

Eu-doped Gallium nitride (GaN) is a promising candidate for GaN-based red light-emitting diodes, which are needed for future micro-display technologies. Introducing a superlattice structure comprised of alternating undoped and Eu-doped GaN layers has been observed to lead to an order-of-magnitude increase in output power; however, the underlying mechanism remains unknown. Here, we explore the optical and electrical properties of these superlattice structures utilizing terahertz emission spectroscopy. We find that ~0.1% Eu doping reduces the bandgap of GaN by ~40 meV and increases the index of refraction by ~20%, which would result in potential barriers and carrier confinement within a superlattice structure. To confirm the presence of these potential barriers, we explored the temperature dependence of the terahertz emission, which was used to estimate the barrier potentials. The result revealed that even a dilutely doped superlattice structure induces significant confinement for carriers, enhancing carrier recombination within the Eu-doped regions. Such an enhancement would improve the external quantum efficiency in the Eu-doped devices. We argue that the benefits of the superlattice structure are not limited to Eu-doped GaN, which provides a roadmap for enhanced optoelectronic functionalities in all rare-earth-doped semiconductor systems.Murakami F., Takeo A., Mitchell B., et al. Enhanced luminescence efficiency in Eu-doped GaN superlattice structures revealed by terahertz emission spectroscopy. Communications Materials 4, 100 (2023); https://doi.org/10.1038/s43246-023-00428-6