Ultrafast dynamics of hot charge carriers in an oxide semiconductor probed by femtosecond spectroscopic ellipsometry

Richter, Steffen; Herrfurth, Oliver; Espinoza, Shirly; Rębarz, Mateusz; Kloz, Miroslav; Leveillee, Joshua; Schleife, André; Zollner, Stefan; Grundmann, Marius; Andreasson, Jakob; Schmidt‐Grund, Rüdiger

doi:10.1088/1367-2630/aba7f3

Cited by 27 publications

(27 citation statements)

References 109 publications

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“…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.…”

Section: ■ Discussionmentioning

confidence: 99%

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

et al. 2021

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The ablation efficiency during laser processing strongly depends on the initial and transient reflectance of the irradiated material surface. This article reports on the transient relative change of the reflectance ΔR/R of stainless steel during and after ultrashort pulsed laser excitation (800 nm, 40 fs) by spatially resolved pump–probe reflectometry. The spatial resolution of the setup in combination with the spatial Gaussian intensity distribution of the pump radiation enables a fluence-resolved detection of ΔR/R. Within the first picosecond after irradiation with a peak fluence of 2 J/cm2, the spatially resolved ΔR/R of stainless steel evolves into an annular shape, in which the center almost remains at its initial reflectance, whereas the outer region features a decreased reflectance. The decreasing trend of ΔR/R is qualitatively supported by applying a two-temperature model, considering the transient optical properties of stainless steel from the literature. At larger fluences and thus higher electron temperatures, the experimental data deviate from the transient reflectance given in the literature. A drastically decreased occupation of the states below the Fermi energy and the subsequent excitation of electrons into these new vacant states by the probe radiation are considered being the most probable origin for this behavior at high fluences.

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Section: ■ Discussionmentioning

confidence: 99%

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

et al. 2021

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“…( 1), the measured reflectance-difference spectra are applied to a reference steady-state spectra by a Müller-matrix formalism for each photon energy and delay-time. A more detailed description of the experimental setup and method can be found elsewhere [8][9][10]. Ellipsometric angles Ψ and ∆ were measured in Polarizer-Sample-Compensator-Analyzer (PSCA) configuration (see Fig.…”

Section: Methodsmentioning

confidence: 99%

“…Furthermore, measurement techniques that are able to accurately investigate those effects are necessary. In recent years, time-resolved pump-probe spectroscopic ellipsometry (trSE) proved to be an excellent contestant for this with many possibilities [8][9][10][11]. Spectroscopic ellipsometry is a predestined method for obtaining optical and material properties of semiconductors as well as many other materials, due to its high accuracy and sensitivity.…”

Section: Introductionmentioning

confidence: 99%

Femtosecond pump-probe absorption edge spectroscopy of cubic GaN

Baron¹,

Goldhahn²,

Espinoza³

et al. 2022

Preprint

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Time-dependent femtosecond pump-probe spectroscopic ellipsometry studies on zincblende gallium-nitride (zb-GaN) are performed and analyzed between 2.9-3.7eV. An ultra-fast change of the absorption onset (3.23eV for zb-GaN) is observed by investigating the imaginary part of the dielectric function. The 266nm (4.66eV) pump pulses induce a large free-carrier concentration up to 4 × 10 20 cm −3 , influencing the transition energy between conduction and valence bands due to many-body effects, like band filling and band gap renormalization, up to ≈500meV. Additionally, the absorption of the pump-beam creates a free-carrier profile within the 605nm zb-GaN layer. This leads to varying optical properties from sample surface to substrate, which are taken into account by grading analysis for an accurate description of the experimental data. A temporal resolution of 100fs allows in-depth investigations of occurring ultra-fast relaxation and recombination processes. We provide a quantitative description of the free-carrier concentration and absorption onset at the sample surface as a function of relaxation, recombination, and diffusion yielding a characteristic relaxation time of 0.19ps and a recombination time of 26.1ps.

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“…In Figure 5a, it can be observed that after band gap excitation at 3.44 eV, a slowly recovered bleaching signal in the visible region appears. 63,64 The observed bleaching signal is caused by the trap state filling from the relaxation of the photoexcited hot electrons to the conduction band minimum. 65 When the excitation energy is decreased below 3.10 eV, it turns into a fast absorption signal which decays in several picoseconds.…”

Section: Identification Of Excited Polaronic States From Shallow Boundmentioning

confidence: 99%

Observation of the Polaron Excited State in a Single-Crystal ZnO

et al. 2021

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Elucidating the property of the polaronic state at ultrafast temporal resolution is of great significance, especially for photocatalytic and solar-energy materials. In this work, femtosecond transient absorption (TA) spectroscopy with state selectivity is utilized to monitor the dynamics of the polaron generated by exciting the mid-gap states from the oxygen vacancy into the polaronic state in a single-crystal ZnO. The photoexcited polaronic state is verified by the observed coherent acoustic phonon with an oscillation on the kinetics at the initial several picoseconds at a frequency of about 50 GHz. We found that the excited state of polaron is located inside the conduction band (CB), which injects an electron into the CB within about 1 ps observed via a broad-band TA spectrum ranging from 400 to 4000 nm. Compared with that of free carriers generated by direct band gap excitation, the lifetime of the carriers resulted from the electron injection from the polaronic states is significantly prolonged, an important feature beneficial to photocatalysts.

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Ultrafast dynamics of hot charge carriers in an oxide semiconductor probed by femtosecond spectroscopic ellipsometry

Cited by 27 publications

References 109 publications

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

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

Femtosecond pump-probe absorption edge spectroscopy of cubic GaN

Observation of the Polaron Excited State in a Single-Crystal ZnO

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