Surface Modifications of Poly(methyl methacrylate) Induced by Controlled Electronic and Molecular Vibrational Excitation Applying Ultrafast Mid-IR Laser Radiation

Abstract

Optimizing the processing of organic photovoltaic devices by laser radiation requires a fundamental understanding of the excitation processes during laser radiation−matter interaction. Spatially and temporally resolved pump−probe ellipsometry on poly(methyl methacrylate) (PMMA), excited by single-pulsed ultrafast mid-IR laser radiation, reveals electronic and vibrational excitation as competing excitation processes. Mid-IR laser radiation in the femtosecond regime induces mainly wavelength independent nonlinear excitation of electrons, as predicted by a theoretical model of the induced free charge carrier density. In contrast, mid-IR laser radiation in the picosecond regime enables linear vibrational excitation, provided the laser radiation frequency corresponds to a resonance frequency of PMMA. Thereby, a wavelength selective processing of organic materials may enable faster processing of multilayer systems depending on the polymer specific vibrational modes.

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 .…”

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

“…Thereby, ω L represents the angular frequency of the laser radiation, m red the reduced effective electron mass, e the elementary charge, c 0 the speed of light in vacuum, I the intensity of the radiation, ϵ 0 the vacuum permittivity, and n = n (ω L ) the refractive index of the medium. It should be noted that, although the excitation from the valence into the conduction band is often referred to as multiphoton ionization and tunnel ionization, , the absorption of photons in disordered organic materials is given by exciting electrons from the highest occupied molecular orbital (HOMO) into excitonic states or extended states of the lowest unoccupied molecular orbital (LUMO). ,, These excited electrons cannot be considered as ionized nearly free electrons, for which reason in this article the terms multiphoton excitation (MPE) and tunnel excitation (TE) are more appropriate . The density of the excited electrons n e induced by MPE and TE is approximated by the rate equation , applying the following substitutions with the elliptic integrals of the first and second kind and and the Dawson integral Φ.…”

“…Although the excited electrons in disordered organic matter are not nearly free, a free electron gas is assumed as a first-order approximation, and therefore, the collision time τ c is approximated for a dense plasma by , Thereby, Z represents the charge per atom, k B the Boltzmann constant, and Λ the Coulomb logarithm , given by with the reduced Planck constant ℏ and the electron temperature T e . 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. ,, …”

“…There are no laser sources providing a tunable wavelength at those various specific vibrational resonances, but optical parametric amplifiers allow tuning the wavelength of ultrafast laser radiation in this large spectral range. 10 However, by using ultrafast laser radiation, the probability for nonlinear excitation of the electrons by tunneling increases. 11,12 This work presents results on the investigation on the linear vibrational 13,14 and nonlinear electronic excitation 15−18 of the polymer poly(3-hexylthiophene) (P3HT) after irradiation by single-pulsed ultrafast mid-IR laser radiation.…”

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

“…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. − …”