arXiv subjects

Ryotaro Nakazawa

Publications and source records attributed to Ryotaro Nakazawa.

3 recordsLinked to original sources

arXiv2026

Determining the density of in-gap states in organic semiconductors: A pitfall of photoelectron yield spectroscopy

Accurate determination of low-density electronic states in the bandgap (in-gap states) is crucial for optimizing the performance of organic optoelectronic devices. Derivative photoelectron yield spectroscopy (PYS) is employed to estimate the density of states (DOS) of in-gap states. However, low-energy photons in PYS can generate excitons and anions in organic semiconductors, raising questions about whether derivative PYS spectra truly represent the DOS. We revealed that PYS signals originate from the single-quantum external photoelectron effect (SQEPE) of in-gap states, SQEPE of the singly occupied molecular orbital (SOMO) of anions, and the biphotonic electron emission (BEE) effect via exciton fusion. Because BEE signals mask the DOS contribution, derivative PYS misestimates the DOS of in-gap states. In contrast, constant final state yield spectroscopy (CFS-YS) reliably determines the DOS by separating these components. For a tris(8-hydroxyquinoline) aluminum (Alq3) film, CFS-YS revealed the DOS of in-gap and SOMO states over six orders of magnitude, clarifying why the Alq3 layer works effectively in organic light-emitting diodes. In the devices, BEE can act as carrier-generation and degradation processes, and CFS-YS can also probe it. We provide the practical guidelines of low-energy photon measurements for DOS determination, such as measurements of photon-flux dependency.

cond-mat.mtrl-sci↗

arXiv2026

Peak separation methods for inverse photoelectron spectra: Comparing second derivative, curve fitting, and deconvolution analyses

Inverse photoelectron spectroscopy (IPES) is a powerful technique for probing the unoccupied electronic states of materials. It can be regarded as the inversion process of photoelectron spectroscopy (PES), which examines the occupied states. Recently developed low-energy inverse photoelectron spectroscopy (LEIPS) can significantly advance the study of unoccupied states, owing to minimal sample damage and suppressed dark counts compared to conventional IPES. However, the instrumental resolution remains at 0.2 eV, which is one order of magnitude lower than that of PES. Spectral broadening caused by the low instrumental resolution often results in overlapping peaks. Peak separation is therefore crucial in the analysis of LEIPS spectra. In this study, we compared three peak separation methods-second derivative, curve fitting, and deconvolution. These methods were applied to modeled and experimental LEIPS spectra of the lowest unoccupied molecular orbital-derived band of pentacene, which consists of two splitting peaks due to the two inequivalent molecules in the unit cell. We systematically and quantitatively evaluated the performance of each method in terms of analysis parameters and discussed its robustness to noise as well as its peak separation capabilities as a function of peak energy spacing and intensity ratio. This work offers a practical framework for peak separation in LEIPS, with extensions to PES and a wide range of spectroscopies.

physics.ins-det↗

arXiv2026

Transient Pauli blocking in a InN film as a mechanism for broadband ultrafast optical switching

The transient Pauli blocking effect offers a promising route for achieving ultrafast optical switching in semiconductors, enabling a rapid switching from an initially opaque state to a relatively transparent state upon photoexcitation. Herein, we demonstrate broadband ultrafast optical switching in degenerate InN thin films, spanning the visible to near-infrared spectral range, using pump-probe transient transmittance measurements. To elucidate the underlying physical mechanism, we perform probe-energy-resolved analysis for ultrafast dynamics, and develop a theoretical model based on a quasi-equilibrium Fermi-Dirac distribution. The model successfully captures the experimental transients and yields an electron-phonon coupling constant of $1.0\times10^{17}\,\mathrm{W\,m^{-3}\,K^{-1}}$, along with an electronic specific heat coefficient ranging from 1.52 to 2.02 $\mathrm{mJ\,mol^{-1}\,K^{-2}}$, which allow direct prediction of the spectral switching window. Notably, we demonstrate that the Pauli blocking effect can be induced solely by a laser-excitation driven rise in electronic temperature, without requiring significant carrier injection into the conduction band in degenerate semiconductors. These findings offer new insights for designing ultrafast optical modulators, shutters, and photonic devices for next-generation communication and computing technologies.

physics.optics↗