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Register a new userUltralong Lifetime Plasmons on Picosecond Time Scale Enabled by Hybrid Plasmon-Phonon Polaritons
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Graphene plasmonics is of great interest for compact optical devices working in broad frequency domains with ultrahigh speed and very low energy consumption. However, graphene plasmons damp out quickly on most substrates mainly due to scattering loss from substrate surface phonons and impurities. Here we discover a new hybridized plasmon-phonon polariton mode in graphene/h-BN van der Waals heterostructures, which enables ultralong hybrid plasmon lifetime up to 1.6 picosecond, the longest plasmon lifetime ever demonstrated. Such remarkably long lifetime arises from the coupling of long-lifetime h-BN transverse optical phonon with graphene plasmons, which uniquely exists in monolayer heterostructures. Our findings and understanding of this unexploited hybrid mode offer a novel approach to tune the plasmon behaviours in the frequency, time and space domains. This can potentially introduce a new paradigm to generate highly-confined plasmons with ultra-long lifetime for various applications, such as deep-subwavelength metamaterials, ultra-low-loss waveguides, and ultrafast optical switches.
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The valley degree of freedom in two-dimensional (2D) crystals recently emerged as a novel information carrier in addition to spin and charge. The intrinsic valley lifetime in 2D transition metal dichalcoginides (TMD) is expected to be remarkably long due to the unique spin-valley locking behavior, where the inter-valley scattering of electron requires simultaneously a large momentum transfer to the opposite valley and a flip of the electron spin. The experimentally observed valley lifetime in 2D TMDs, however, has been limited to tens of nanoseconds so far. Here we report efficient generation of microsecond-long lived valley polarization in WSe2/MoS2 heterostructures by exploiting the ultrafast charge transfer processes in the heterostructure that efficiently creates resident holes in the WSe2 layer. These valley-polarized holes exhibit near unity valley polarization and ultralong valley lifetime: we observe a valley-polarized hole population lifetime of over 1 us, and a valley depolarization lifetime (i.e. inter-valley scattering lifetime) over 40 us at 10 Kelvin. The near-perfect generation of valley-polarized holes in TMD heterostructures with ultralong valley lifetime, orders of magnitude longer than previous results, opens up new opportunities for novel valleytronics and spintronics applications.
We present measurements at millikelvin temperatures of the microwave-frequency acoustic properties of a crystalline silicon nanobeam cavity incorporating a phononic bandgap clamping structure for acoustic confinement. Utilizing pulsed laser light to excite a co-localized optical mode of the nanobeam cavity, we measure the dynamics of cavity acoustic modes with single-phonon sensitivity. Energy ringdown measurements for the fundamental $5$~GHz acoustic mode of the cavity shows an exponential increase in phonon lifetime versus number of periods in the phononic bandgap shield, increasing up to $tau approx 1.5$~seconds. This ultralong lifetime, corresponding to an effective phonon propagation length of several kilometers, is found to be consistent with damping from non-resonant two-level system defects on the surface of the silicon device. Potential applications of these ultra-coherent nanoscale mechanical resonators range from tests of various collapse models of quantum mechanics to miniature quantum memory elements in hybrid superconducting quantum circuits.
Optical control of electronic spins is the basis for ultrafast spintronics: circularly polarized light in combination with spin-orbit coupling of the electronic states allows for spin manipulation in condensed matter. However, the conventional approach is limited to spin orientation along one particular orientation that is dictated by the direction of photon propagation. Plasmonics opens new capabilities, allowing one to tailor the light polarization at the nanoscale. Here, we demonstrate ultrafast optical excitation of electron spin on femtosecond time scales via plasmon to exciton spin conversion. By time-resolving the THz spin dynamics in a hybrid (Cd,Mn)Te quantum well structure covered with a metallic grating, we unambiguously determine the orientation of the photoexcited electron spins which is locked to the propagation direction of surface plasmon-polaritons. Using the spin of the incident photons as additional degree of freedom, one can orient the photoexcited electron spin at will in a two-dimensional plane.
We report on strong coupling of the charge carrier plasmon $omega_{PL}$ in graphene with the surface optical phonon $omega_{SO}$ of the underlying SiC(0001) substrate with low electron concentration ($n=1.2times 10^{15}$ $cm^{-3}$) in the long wavelength limit ($q_parallel rightarrow 0$). Energy dependent energy-loss spectra give for the first time clear evidence of two coupled phonon-plasmon modes $omega_pm$ separated by a gap between $omega_{SO}$ ($q_parallel rightarrow 0$) and $omega_{TO}$ ($q_parallel >> 0$), the transverse optical phonon mode, with a Fano-type shape, in particular for higher primary electron energies ($E_0 ge 20eV$). A simplified model based on dielectric theory is able to simulate our energy - loss spectra as well as the dispersion of the two coupled phonon-plasmon modes $omega_pm$. In contrast, Liu and Willis [1] postulate in their recent publication no gap and a discontinuous dispersion curve with a one-peak structure from their energy-loss data.
We present an ultrafast route for a controlled, toggle switching of magnetic vortex cores with ultrashort unipolar magnetic field pulses. The switching process is found to be largely insensitive to extrinsic parameters, like sample size and shape, and it is faster than any field-driven magnetization reversal process previously known from micromagnetic theory. Micromagnetic simulations demonstrate that the vortex core reversal is mediated by a rapid sequence of vortex-antivortex pair-creation and annihilation sub-processes. Specific combinations of field pulse strength and duration are required to obtain a controlled vortex cores reversal. The operational range of this reversal mechanism is summarized in a switching diagram for a 200 nm Permalloy disk.
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