Mr. Li He Profile

Li He

at Univ of Minnesota

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Proceedings Article | 10 March 2020 Presentation

Chiral interaction between photon angular momentum and surface electrons in topological insulators (Conference Presentation)

Mo Li, Li He, Seokhyeong Lee

Proceedings Volume 11282, 112820O (2020) https://doi.org/10.1117/12.2552970

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The propagation of both electrons and photons becomes chiral when their momentum and spin are correlated in forms such as spin-momentum locking. For the surface electrons in three-dimensional topological insulators (TIs), their spin is locked to the transport direction. For photons in optical fibers and photonic waveguides, they carry transverse spin angular momentum (SAM) which is also locked to the propagation direction. A direct connection between chiral electrons and chiral photons occurs in Tis with lifted spin degeneracy, which leads to spin-dependent selection rules of optical transition and results in phenomena such as circular photogalvanic effect (CPGE). Here, we demonstrate an optoelectronic device that integrates a TI with a chiral photonic waveguide. Interaction between the photons in the transverse-magnetic (TM) mode of the waveguide, which carries transverse SAM, and the surface electrons in a Bi2Se3 layer generates a directional, spin-polarized photocurrent. Because of

Proceedings Article | 14 March 2018 Presentation

Measurement of photon torques with torsional nano-optomechanical systems (Conference Presentation)

Li He, Huan Li, Mo Li

Proceedings Volume 10535, 1053519 (2018) https://doi.org/10.1117/12.2294530

KEYWORDS: Photon polarization, Integrated optics, Optomechanical design, Optical tweezers, Integrated photonics, Light-matter interactions, Laser optics, Nanophotonics, Light wave propagation, Waveguides

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Photons carry linear momentum, and spin angular momentum when circularly or elliptically polarized. During light-matter interaction, transfer of linear momentum leads to optical forces, while angular momentum transfer induces optical torque. Optical forces including radiation pressure and gradient forces have long been utilized in optical tweezers and laser cooling. In nanophotonic devices optical forces can be significantly enhanced, leading to unprecedented optomechanical effects in both classical and quantum regimes. In contrast, to date, the angular momentum of light and the optical torque effect remain unexplored in integrated photonics. Here, we demonstrate the measurement of the spin angular momentum of photons propagating in a birefringent waveguide and the use of optical torque to actuate rotational motion of an optomechanical device. We show that the sign and magnitude of the optical torque are determined by the photon polarization states that are synthesized on the chip. Our study reveals the mechanical effect of photon’s polarization degree of freedom and demonstrates its control in integrated photonic devices. Exploiting optical torque and optomechanical interaction with photon angular momentum can lead to torsional cavity optomechanics and optomechanical photon spin-orbit coupling, as well as applications such as optomechanical gyroscope and torsional magnetometry.

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