Femtosecond-time-resolved imaging of the dielectric function of ZnO in the visible to near-IR spectral range

Abstract

We demonstrate micrometer-resolved imaging of the transient dielectric function of a c-ZnO thin film with femtosecond resolution in the visible to near-IR spectral range measured by pump-probe ellipsometry at five different probe photon-energies. The spatial profile of the real part of the dielectric function broadens drastically with increasing time delay, which we associate with the combined effect of carrier cooling and fast carrier transport with an effective diffusion coefficient of (1.1±0.1)×104 cm2/s. A ring structure is detected in the image after a few picoseconds, which can be explained by a random-walk model including ballistic transport due to the thermal gradient induced by the hot-phonon effect.

Local Optical Properties in CVD-Grown Monolayer WS2 Flakes

“…To perform ISE, we used state-of-the-art instrumentation that allows us to reduce the lateral resolution of ellipsometry down to ∼1 to 2 μm (details in the Methods section). Indeed, ISE is a recent and powerful technical advancement of spectral ellipsometry, which has only recently begun to prove its value in the analysis of 2D systems − and is generally well suited to probe local variations in the optical properties of materials. , The ISE characterization consisted of 11 (Δ, Ψ) maps acquired between 1.94 and 2.85 eV, each map comprising ∼57,000 pixels. The ISE data at 2.0 eV (Figure b,c) were particularly relevant since that energy value was very close to the maximum excitonic absorption of WS 2 .…”

Fluence-Dependent Transient Reflectance of Stainless Steel Investigated by Ultrafast Imaging Pump–Probe Reflectometry

“…The ballistic electrons diffuse from the center to the outer area without scattering. , To induce the annular shape of Δ R / R within the first few hundreds of femtoseconds, electron velocities close to the speed of light would be required, which is highly improbable, since ballistic electrons are usually assumed to move with the corresponding Fermi velocity being in the range of approximately 10 6 m/s.…”

“…The increase of ΔR/R for H 0 > 1.0 J/cm 2 causing the annular shape of the spatially resolved ΔR/R (Figure 3) is not reproducible by the two-temperature model considering the optical parameters in the literature. 38,43 To explain the spatial annular shape of ΔR/ R at t < t 0 + 1 ps (Figure 3), ballistic electron diffusion, 7,15 the formation of a highly dense electron gas, 49−51 and transient electron transitions 20,35,43,44 should be considered. Ballistic Electrons.…”

“…The rising demands on laser processing in terms of precision and efficiency require a deep comprehension of the physical processes involved in the interaction of ultrashort pulsed laser radiation and matter. Pump–probe metrology has become an established technique for observing the ultrafast dynamics by measuring the optical response of the material with a temporal resolution at femtosecond time scales. − …”

Linear and Nonlinear Excitation of P3HT Induced by Spectral-Shaped Ultrafast Mid-IR Laser Radiation

“…The electron temperature T e is approximated for a free electron gas by with the absorbed energy density of the laser radiation considering the temporal evolution of the ponderomotive potential from eq needed to excite the current electron density n e . The collision frequency of electrons in disordered organic materials might be smaller than in a crystalline inorganic semiconductor, − and thus the collision time is in the range of femtoseconds. ,, …”