39 research outputs found

    Giant Thermomechanical Bandgap Engineering in Quasi-two-dimensional Tellurium

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    Mechanical straining-induced bandgap modulation in two-dimensional (2D) materials has been confined to volatile and narrow modulation due to substrate slippage and poor strain transfer. We report the thermomechanical modulation of the inherent bandgap in quasi-2D tellurium nanoflakes (TeNFs) via non-volatile strain induction during hot-press synthesis. We leveraged the coefficient of thermal expansion (CTE) mismatch between TeNFs and growth substrates by maintaining a high-pressure enforced non-slip condition during thermal relaxation (623 to 300K) to achieve the optimal biaxial compressive strain of -4.6 percent in TeNFs/sapphire. This resulted in an enormous bandgap modulation of 2.3 eV, at a rate of up to ~600 meV/%, which is two-fold larger than reported modulation rate. Strained TeNFs display robust band-to-band radiative excitonic blue photoemission with an intrinsic quantum efficiency (IQE) of c.a. 79.9%, making it promising for energy efficient blue LEDs and nanolasers. Computational studies reveal that biaxial compressive strain inhibits exciton-exciton annihilation by evading van-Hove singularities, hence promoting radiative-recombination. Bandgap modulation by such nonvolatile straining is scalable to other 2D semiconductors for on-demand nano(opto)-electronics

    Vacancy-Engineered Phonon Polaritons in α\alpha-MoO3_3

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    Low-symmetry van der Waals materials have enabled strong confinement of mid-infrared light through hyperbolic phonon polaritons (HPhPs) at the nanoscale. Yet, the bottleneck persists in manipulating the intrinsic polaritonic dispersion to drive further progress in phonon-polaritonics. Here, we present a thermomechanical strategy to manipulate the HPhPs in α\alpha-MoO3_3 using high pressure and temperature treatment. The hot pressing engineers the stoichiometry of α\alpha-MoO3_3 by controllably introducing oxygen vacancy defects (OVDs), which cause a semiconductor-to-semimetal transition. Our density functional theory and finite-difference time-domain results, combined with experimental studies show that the OVDs induce a metastable metallic state by reducing the bandgap while modifying the intrinsic dielectric permittivity of α\alpha-MoO3_3. Photo-induced force microscopy confirms an average dielectric permittivity tunability of Δε/ε0.35|\Delta\varepsilon/\varepsilon|\approx0.35 within a Reststrahlen band of α\alpha-MoO3_3, resulting in drastic shifts in the HPhP dispersion. The polariton lifetimes for pristine and hot-pressed flakes were measured as 0.92±0.060.92 \pm 0.06 and 0.86±0.110.86 \pm 0.11 ps, respectively, exhibiting a loss of only 7%, while the group velocity exhibited an increase of 38.8±0.238.8 \pm 0.2%. The OVDs in α\alpha-MoO3_3 provide a low-loss platform that enables active tuning of mid-infrared HPhPs and have a profound impact on applications in super-resolution imaging, nanoscale thermal manipulation, boosted molecular sensing, and on-chip photonic circuits.Comment: 42 pages, 13 figures, supporting informatio

    Intensity-dependent reflectance modulation of femtosecond laser pulses in GaAs nanocylinders with magnetic resonances

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    Abstract We experimentally demonstrate modulation of reflectance in periodic arrays of subwavelength gallium arsenide nanocylinders with Mie-type resonances due to absorption saturation and changes in the refractive index of the semiconductor material of metasurface. The intensity-dependent reflectance modulation of up to 30% in the vicinity of the magnetic dipole resonance at a low laser fluence below 200 μ J/cm 2 is shown by I-scan measurements
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