arXiv subjects

Junjun Jia

Publications and source records attributed to Junjun Jia.

6 recordsLinked to original sources

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↗

arXiv2024

Dynamic tuning of ENZ wavelength in conductive polymer films via polaron excitation

Traditional metal and n-type doped semiconductor materials serve as emerging epsilon-near-zero (ENZ) materials, showcasing great potential for nonlinear photonic applications. However, a significant limitation for such materials is the lack of versatile ENZ wavelength tuning, and thus dynamic tuning of the ENZ wavelength remains a technical challenge, thereby restricting their potential applications, such as multi-band communications. Here, dynamic tuning of the ENZ wavelength in p-type organic PEDOT: PSS films is achieved through a reversible change in hole concentrations originated from the polaron formation/decoupling following optical excitation, and a tunable ENZ wavelength shift up to 150 nm is observed. Experimental investigations about ultrafast dynamics of polaron excitation reveal an approximately 80 fs time constant for polaron buildup and an approximately 280 fs time constant for polaron decoupling, indicating the potential of reversal ultrafast switching for the ENZ wavelength within subpicosecond time scale. These findings suggest that $p$--type organic semiconductors can serve as a novel platform for dynamically tuning the ENZ wavelength through polaron excitation, opening new possibilities for ENZ--based nonlinear optical applications in flexible optoelectronics.

physics.optics↗

arXiv2022

Organic metallic epsilon-near-zero materials with large ultrafast optical nonlinearity

Epsilon-near-zero (ENZ) materials have shown significant potential for nonlinear optical applications due to their ultrafast hot carriers and consequent optical nonlinearity enhancement. Modified poly(3,4-ethylenedioxythiophene) (PEDOT) films show metallic characteristics and a resultant ENZ wavelength near 1550nm through polar solvent treatment and annealing. The metallic PEDOT film exhibits an intrinsic optical nonlinear response that is comparable to gold and 100-fold higher than typical inorganic semiconductor ENZ materials due to {\pi}-conjugated delocalized electrons. Hot carriers generate a 22-fold increase in the optical nonlinearity coefficient of metallic PEDOT films at 1550 nm. Hot holes in metallic PEDOT films have a smaller enhancement multiple of carrier temperature and a longer relaxation time than hot electrons in inorganic ENZ materials due to the larger imaginary permittivity and hot-phonon bottleneck for carrier cooling. Our findings suggest that {\pi}-conjugated ENZ polymer may have unique ultrafast and nonlinear optical properties compared to inorganic ENZ materials, enabling new possibilities in on-chip nanophotonic devices, nonlinear optics, and plasmonics.

physics.optics↗

arXiv2022

Electromechanical coupling in Yb-substituted III-V nitride alloys

Group-III nitride alloys are currently used in various microwave communication applications because of the giant enhancement in electromechanical coupling after alloying with rocksalt nitrides such as YbN or ScN. Herein, the Yb-substitution induced enhancement for electromechanical coupling in wurtzite III-V nitrides is studied via theoretical calculations and experiments. The substitution induced mechanical softening and local strain can enhance electromechanical coupling. The mechanical softening induced by Yb substitution shows less dependence on the parent AlN or GaN, which is caused by the Yb-Yb pair interaction in the c-axis direction, and the difference of electromechanical coupling between the GaN- and AlN-based alloys mainly comes from their enhancement effect of Yb substitution for piezoelectric response. The largest change in piezoelectric response relative to the parent nitride is observed in GaN-based alloy, which is mainly considered as a consequence of small piezoelectric constant of the parent GaN. Our calculations also reveal that the substitutional element with a closer ionic size to the host cation is easier to substitute into the host nitride, and produces a larger internal strain to partly contribute to the enhancement in piezoelectric response. This can serve as a simple guideline to identify alloying components in a search for a massive increase in electromechanical coupling.

cond-mat.mtrl-sci↗

arXiv2021

Ultrafast dynamics of electronic structure in InN thin film

Simultaneous measurements of transient transmission and reflectivity were performed in the unintentionally doped InN film to reveal ultrafast optical bleaching and its recovery behavior under intense laser irradiation. The optical bleaching is attributed to Pauli blocking due to the occupation of photoexcited electrons at the probing energy level. The time constant for the transition from the excitation state to the conduction band edge is $\sim$260 fs. The interplay between band filling and band gap renormalization caused by electron-hole and electron-electron interactions gives rise to complex spectral characteristics of transient reflectivity, from which the time constants of photoexcited electron-hole direct recombination and band edge recombination are extracted as $\sim$60 fs and 250$\sim$400 fs, respectively. Our results also reveal that the electron-electron interaction suppresses band edge recombination, and mitigates the recovery process. Our experiments highlight the controllability of the band structure of semiconductors by intense laser irradiation.

physics.optics↗

arXiv2021

Origin of Enhanced Electromechanical Coupling in (Yb,Al)N Nitride Alloys

Our experiments demonstrate that alloying the cubic-phase YbN into the wurtzite-phase AlN results in clear mechanical softening and enhanced electromechanical coupling of AlN. First-principle calculations reproduce experimental results well, and predict a maximum 270% increase in electromechanical coupling coefficient caused by (1) an enhanced piezoelectric response induced by the local strain of Yb ions and (2) a structural flexibility of the (Yb,Al)N alloy. Extensive calculations suggest that the substitutional neighbor Yb-Yb pairs in wurtzite AlN are energetically stable along $c$ axis, and avoid forming on the basal plane of wurtzite structure due to the repulsion between them, which explains that (Yb,Al)N films with high Yb concentrations are difficult to fabricate in our sputtering experiments. Moreover, the neighbor Yb-Yb pair interactions also promote structural flexibility of (Yb,Al)N, and are considered a cause for mechanical softening of (Yb,Al)N.

cond-mat.mtrl-sci↗