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

TL;DR

This work investigates ultrafast optical switching via transient Pauli blocking in degenerate InN thin films. The authors combine broadband pump–probe transient transmittance measurements with first-principles band-structure calculations and a quasi-equilibrium Fermi–Dirac model to describe hot-electron dynamics. They extract an electron–phonon coupling constant of $g_{ep}=1.0\times10^{17}\ \mathrm{W\,m^{-3}\,K^{-1}}$ and an electronic specific heat coefficient in the range $\gamma=1.52$–$2.02\ \mathrm{mJ\,mol^{-1}\,K^{-2}}$, enabling prediction of the spectral switching window across roughly $1.2$–$2.3$ eV. Importantly, they show that transient Pauli blocking can be induced by a laser-driven rise in electronic temperature without substantial conduction-band population, offering design principles for ultrafast optical modulators and photonic devices.

Abstract

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.

Figures (7)

• Figure 1: Schematic illustration of ultrafast optical switching in semiconductors via transient Pauli blocking. Under intensive laser irradiation with photon energy exceeding the band gap $E_g$, electrons are excited into conduction band and rapidly thermalize through electron--electron scattering ($e-e$ scattering), forming a transient quasi--equilibrium Fermi-Dirac distribution. The resulting electronic occupation transiently blocks optical absorption, enabling an ultrafast switching from an opaque to a transparent state. This mechanism offers opportunities for laser pulse modulation and other advanced optical applications. CB: conduction band, VB: valence band.
• Figure 2: Electronic band structure and optical band gap of wurtzite InN. (a) Calculated band structure of wurtzite InN using the HSE hybrid functional with spin--orbit coupling interaction included. The inset shows the high--symmetry points in the first Brillouin zone. The dashed line represents the Fermi level, estimated based on the unintentional carrier density obtained from Hall effect measurements. (b) Tauc plot used to extract the optical band gap from the experimental data (shown as circles).
• Figure 3: HS--UPS spectrum of the InN thin film. HS--UPS spectrum obtained using the monochromatic Xe I$\alpha$ line (photon energy of 8.437 eV), plotted on a logarithmic scale. The dash--dotted line near 0 eV represents the fitting curve based on the FD distribution. The inset shows the same spectrum on a linear scale. The labels "$E$", "$E_F$", and "$E_V$" indicate the energy, Fermi level, and the valence band maximum, respectively.
• Figure 4: Time--resolved transient transmittance spectra of InN. Transient transmittance spectra measured at a pump fluence of 63.8 J/m$^2$, using a pump laser with photon energy of 1.55 eV. The probe photon energies range from 1.31 to 2.07 eV.
• Figure 5: Band structures and optical transition matrix elements of wurtzite InN. Band structures illustrating possible optical transitions from the valence bands ($v_1$, $v_2$, and $v_3$) to the lowest conduction band $c_1$ along three high--symmetrical paths ((a): $\Gamma\rightarrow$ A, (b): $\Gamma\rightarrow$ K, and (c): $\Gamma\rightarrow$ M). The upper panels show the corresponding optical matrix elements, which are proportional to the transition probability. The dash--dotted lines denote the highest excitation level provided by a 1.55 eV pump laser, while the dotted lines indicate the Fermi level at equilibrium before photoexcitation. Shaded gray regions represent the energy window accessible to the probe light.