The interaction of macroscopic mechanical object with electron charge and spin plays a vital role in today’s information technology and fundamental studies of the quantum-classical boundary. Recently emerged valleytronics encodes information to the valley degree-of-freedom and promises exciting applications in communication and computation. Exploring the interplay between the valley physics and macroscopic mechanics will bring new perspectives for valley information processing and exploration of the quantum-classical boundary.
Recently discovered two-dimensional (2D) transition metal dichalcogenides (TMDs) provide an appealing platform to explore valley-mechanical interaction. Their honeycomb lattice supports two valleys (namely K and K’) and thus forms a spin-like binary system called valley-pseudospin. The broken inversion symmetry together with the strong spin-orbit coupling give rise to a unique optical selection rule, which provides a powerful tool for optical generation and manipulation of the valley. On the other hand, nano-mechanical systems made of 2D materials have shown high mechanical strength, high quality-factor, and extraordinary mass and force sensitivities. Their extremely small mass also leads to large quantum zero-point motion and thus facilitates its use in quantum system.
Because of the broken inversion symmetry in monolayer TMDs, electrons in the K and K’ valleys possess total magnetic moments that are equal in magnitude but opposite in sign. When a magnetic field gradient is applied perpendicular to the monolayer, it experiences a net force whose direction depends on which valley is populated and therefore allows transduction of the valley information (K or K’) into the mechanical motion (upward or downward displacement).
We fabricate the valley-resonators by dry-transferring an exfoliated monolayer MoS2 onto pre-patterned square hole structures, which are conformally coated with a film of high permeability permalloy (Ni/Fe). Under an external magnetic field, the permalloy film distorts the field and generates a strong local magnetic field gradient. For an applied magnetic field of 26 mT, the magnetic field gradient in the central region of the suspended MoS2 reaches ~4000 T/m. The monolayer nature of the MoS2 membrane is confirmed by Raman and photoluminescence (PL) spectroscopy. A PL scan across the sample shows that the emission from the suspended regions is much stronger than that from the metal substrate, which confirms that the membrane is freestanding. The square structure has a lateral dimension of 5.2 × 5.2 um^2 and the resonator has an effective mass of 21 fg. The mechanical resonance frequency is ~35.7 MHz and the quality factor is 22,000 at 30 K.
We observe the valley-mechanical actuation of the monolayer MoS2 at low temperature. We detect the resonator motion by an optical interferometric scheme using a probe laser (654 nm) and excite the K and K’ valleys alternatingly by modulating the polarization of the pump laser (633 nm) between left- and right-circular (LCP and RCP) while keeping the optical intensity constant. This exerts an oscillating push-pull force to drive the resonator. The measured displacement of the resonator shows a clear Lorentzian response. Meanwhile, a linearly-polarized pump light shows no driving effect because it equally populates both valleys and thus results in zero net force. The mechanical displacement shows an opposite phase when the pump laser polarization is switched to opposite helicity, which confirms that the excitation of carriers at different valleys exerts opposite forces onto the monolayer. In the measurement, the polarization modulation frequency is close to the mechanical resonance (~35.7 MHz) which is much slower than the decay of the valley carriers, whose timescale is in the range of picoseconds to a few nanoseconds. Therefore, the valley carrier population adiabatically follows the polarization modulation. The probe light is kept linearly-polarized throughout the measurement to eliminate its effect on the net valley population.
Utilizing the fact that the direction of the force depends on which valley is populated, we demonstrate transduction of the valley information into the mechanical state of the nano-resonator. We examine the mechanical quadratures of the device when it is resonantly driven by opposite valley population. Two distinct mechanical states of opposite phase are clearly resolved within the measurement uncertainty and the confidence level of differentiating the two states is close to 100%. This result demonstrates that the valley information of the monolayer is unambiguously transferred into the mechanical states.
In conclusion, our experiment demonstrates direct transduction of valley information to mechanical states of a MoS2 monolayer resonator. The valley-mechanical interaction lays the foundation for a new class of valley-controlled mechanical devices. It also facilitates hybridization of valley pseudospin with other quantum information carriers such as two-level qubits and microwave photons.
The formation of half-light half-matter quasiparticle exciton polariton and its condensation in semiconductor microcavities are striking phenomena for the macroscopically quantum coherence at elevated temperature. The matter constituent of exciton polariton dictates the interacting behaviors of these bosonic particles primarily via exciton-exciton interactions. However, these interactions are all limited to the ground state exciton, although they are expected to be much stronger at Rydberg states with higher principal numbers. Here, for the first time, we observe the spontaneous formed Rydberg exciton polaritons (REPs) in a high quality Fabry-Perot cavity embedded with single crystal inorganic perovskite. Such REPs exhibit strong nonlinear behavior and anisotropic, enabling an anomalous dynamic process that leads to a coherent polariton condensate with prominent blue shift. This discovery presents a unique platform to study quantum coherent many-body physics, and enables unprecedented manipulation of these Rydberg states by new means such as chemical composition engineering, structural phase control, and external gauge fields. The solid state REP and its condensates also hold great potential for important applications, such as sensing, communication, and quantum computing.
Perhaps the most successful application of plasmonics to date has been in sensing, where the interaction of a nanoscale localized field with analytes leads to high-sensitivity detection in real time and in a label-free fashion. However, all previous designs have been based on passively excited surface plasmons, in which sensitivity is intrinsically limited by the low quality factors induced by metal losses. It has recently been proposed theoretically that surface plasmon sensors with active excitation (gain-enhanced) can achieve much higher sensitivities due to the amplification of the surface plasmons. Here, we experimentally demonstrate an active plasmon sensor that is free of metal losses and operating deep below the diffraction limit for visible light. Loss compensation leads to an intense and sharp lasing emission that is ultrasensitive to adsorbed molecules. We validated the efficacy of our sensor to detect explosives in air under normal conditions and have achieved a sub-part-per-billion detection limit, the
lowest reported to date for plasmonic sensors with 2,4-dinitrotoluene and ammonium nitrate. The selectivity
between 2,4-dinitrotoluene, ammoniumnitrate and nitrobenzene is on a par with other state-of-the-art explosives detectors. Our results show that monitoring the change of the lasing intensity is a superior method than monitoring the wavelength shift, as is widely used in passive surface plasmon sensors. We therefore envisage that nanoscopic sensors that make use of plasmonic lasing could become an important tool in security screening and biomolecular diagnostics.