Judit Budai

Understanding the role of holes in energy transport mechanisms and determining electron intervalley scattering times is crucial for optimising Ge-based optoelectronic devices such as transistors, photodetectors, power devices, and light emitters. By employing our recently developed, ultrafast 20-fs resolution ellipsometer, we demonstrate that the absorption ( ɛ 2 ) in various types of Ge continues to decrease in the first picosecond after optical excitation, proving that the revival of the absorption is limited by the intrabandlike relaxation of holes (near the Γ point), instead of the intervalley scattering of the electrons. The observed decay times depend on the probing photon energies attributed to the different lifetimes of the holes in the light, split-off and heavy-hole bands. With gradually decreasing degree of crystallinity of our single crystalline, semicrystalline and amorphous Ge samples, the recovery times also decrease, which is related to the appearance and intensification of real-space scattering events over reciprocal space scattering. There is excellent correspondence for electron relaxation times for different samples with our two-temperature model, which is thoroughly analyzed via band-structure calculations. Our results contribute to the understanding of the excitation and decay processes in not only crystalline semiconductors, but also semicrystalline and amorphous semiconductors, resolving corresponding debates.

Since the discovery of carbon nanotubes (CNTs), extensive and comprehensive research has been conducted in many areas of materials science. Due to their structural and chemical properties, they can be an important part of electronic devices and structural materials that surround us. In this work, we focused on the preparation and basic analysis of vertically aligned CNTs. An aluminum oxide carrier layer and bimetallic iron–cobalt catalyst layers of different compositions were fabricated on the surface of a silicon substrate using a pulsed laser deposition method. Then, vertically aligned CNTs were grown using a catalytic chemical vapor deposition method based on the thermal decomposition of ethylene. During the experiments, the effect of water vapor and hydrogen gas was investigated on the structure of as-prepared carbon nanotubes. CNT forest samples were characterized by scanning electron microscopy and Raman spectroscopy. One of the most important findings of this research is that the presence of hydrogen gas in the CCVD system is essential, but high-quality vertically aligned CNTs can be produced on silicon substrates even without water vapor.

Thin flexible composite materials made of cellulose acetate and gold nanoparticles are synthesized by a modified Turkevich method. Extensive assessment of their properties suggests suitability for applications in optics, coatings or biomedicine.

Oriented electric currents in metals are routinely driven by applying an external electric potential. Although the response of electrons to the external electric fields occurs within attoseconds, conventional electronics do not use this speed potential. Ultrashort laser pulses with controlled shapes of electric fields that switch direction at petahertz frequencies open perspectives for driving currents in metals. Light field–driven currents were demonstrated in various media including dielectrics, semiconductors, and topological insulators. Now, our research question is whether we can drive and control orders of magnitude more charge carriers in metals enabling ultrafast switching with practically low-energy, picojoule-level pulses. Here, we demonstrate the interaction of light with nanometer-thick metallic layers, which leads to a generation of light field–controlled electric currents. We show that the implantation of metallic layers into a dielectric matrix leads to up to 40 times increase of the sensitivity in contrast to a bare dielectric, decreasing the intensity threshold for lightwave electronics.

Oriented electric currents in metals are routinely driven by applying an external electric potential. Although the response of electrons to the external electric fields occurs within attoseconds, conventional electronics do not utilize this speed potential. Ultrafast laser technology, which delivers laser pulses with controlled shapes of electric fields that switch direction at PHz frequencies, opens new perspectives for driving electric currents in metals. Here, we demonstrate an interaction of light with nm-thick metallic layers, that leads to a generation of PHz-bandwidth electric currents, providing evidence that currents in metals can be optically controlled. We show that the implantation of metallic layers into a dielectric matrix leads up to 40 times increase of the sensitivity in contrast to bare dielectric, decreasing the intensity threshold for the lightwave electronics. We establish a link between electronic and optical properties by identifying an empirical relationship between the index of nonlinear susceptibility X⁽³⁾ and the generated current. We provide an intraband-motion model elucidating the origin of the measured current.

Non-thermal and thermal carrier populations in plasmonic systems raised significant interest in contemporary fundamental and applied physics. Although the theoretical description predicts not only the energies but also the location of the generated carriers, the experimental justification of these theories is still lacking. Here, we demonstrate experimentally that upon the optical excitation of surface plasmon polaritons, a non-thermal electron population appears in the topmost domain of the plasmonic film directly coupled to the local fields. The applied all-optical method is based on spectroscopic ellipsometric determination of the dielectric function, allowing us to obtain in-depth information on surface plasmon induced changes of the directly related electron occupancies. The ultrahigh sensitivity of our method allows us to capture the signatures of changes induced by electron-electron scattering processes with ultrafast decay times. These experiments shed light on the build-up of plasmonic hot electron population in nanoscale media.

We demonstrate the nanoscale control of field enhancement of nanorods illuminated with linearly or circularly polarized femtosecond laser pulses. Simulations show that excitation and phase relations of different plasmon modes determines the achievable field enhancement.

We performed an on-air measurement of CEP spatial distribution of focused few-cycle laser oscillator beam. The on-chip probe exploits the CEP sensitivity of electric currents in dielectrics exposed to the strong field.

We performed an on-air measurement of CEP spatial distribution of focused few-cycle laser oscillator beam. The on-chip probe exploits the CEP sensitivity of electric currents in dielectrics exposed to the strong field.