Lu Zhao

Intrinsic half-metallic BN―C nanotubes

Using spin-polarized density functional theory calculations, we demonstrate that hybrid BN–C nanotubes (BN-CNTs) have diverse electronic and magnetic properties depending on their percentage of carbon and BN components. Typically, a BN-CNT is converted from a nonmagnetic semiconductor to a spin-polarized metal and then to a nonmagnetic semiconductor by increasing the ratio of BN component. The intrinsic half-metallicity could be achieved when the percentage of carbon component in the tube is within a certain ratio, and is insensitive to the tube curvature. Our findings suggest that BN-CNTs may offer unique opportunities for developing nanoscale spintronic materials.

Design of strain-engineered quantum tunneling devices for topological surface states

Strain-dependent charge and spin transport on a topological insulator (TI) surface are investigated by combining first-principles calculations with quantum tunneling theory. It is shown that the Dirac point of helical surface states can be significantly shifted by applying compressive uniaxial strain. As an example of strain engineering applications based on this effect, a strain-induced quantum tunneling nanostructure is designed, where the tunneling conductance and the spin texture of surface states can be sensitively modulated by strain. Our work suggests that various local strain patterns can be integrated to manipulate surface states in all-TI-based spintronic nanodevices.

Investigation of Ramsey spectroscopy in a lin-par-lin Ramsey coherent population trapping clock with dispersion detection

We demonstrate that the lin-par-lin Ramsey coherent population trapping 87Rb clock using a dispersion detection technique has a promising performance. We theoretically and experimentally investigate the signal-to-noise ratio of the Ramsey spectrum signal by varying the relative angle of the polarizer and analyzer as well as the magnetic field. Based on the experimental results, the optimized relative angle and magnetic field are determined. This kind of atomic clock is attractive for the development of compact, high performance vapor clock based on CPT.

All-optical Fresnel lens in coherent media: controlling image with image

We theoretically explore an all-optical method for generating tunable diffractive Fresnel lenses in coherent media based on electromagnetically induced transparency. In this method, intensity-modulated images in coupling light fields can pattern the coherent media to induce the desired modulo-2π quadratic phase profiles for the lenses to diffract probe light fields. We characterize the focusing and imaging properties of the induced lenses. In particular, we show that the images in coupling fields can flexibly control the images in probe fields by diffraction, where large focal length tunability from 1 m to infinity and high output (∼ 88% diffraction efficiency) can be achieved. Additionally, we also find that the induced Fresnel lenses can be rapidly modulated with megahertz refresh rates using image-bearing square pulse trains in coupling fields. Our proposed lenses may find a wide range of applications for multimode all-optical signal processing in both the classical and quantum regimes.

Manipulating stored images with phase imprinting at low light levels

Coherent manipulation of stored images is performed at low light levels based on enhanced cross-Kerr nonlinearity in a four-level N-type electromagnetically induced transparency (EIT) system. Using intensity masks in the signal pulse, quadratic phase shifts with low nonlinear absorption can be efficiently imprinted on the Fraunhofer diffraction patterns already stored in the EIT system. Fast-Fourier-transform-based numerical simulations clearly demonstrate that the far-field images of the retrieved probe light can be flexibly modulated by applying different signal fields. Our studies could help advance the goals of nonlinear all-optical processing for spatial information coherently stored in EIT systems.

Slow-light-enhanced codirectional couplers with negative index materials.

Optical codirectional coupling structures consisting of two parallel planar waveguides with negative index materials (NIMs) are systematically studied in different configurations using coupled-mode theory under the weak-coupling condition. As a result, we find that the coupling strength between copropagating optical modes can be enhanced in such structures. More importantly, both our analytical derivations and numerical simulations clearly indicate that the slow-light effect in the waveguides with NIMs plays an essential role in such enhancement. The configuration with two conventional positive-index-material cores embedded in NIM claddings (or vice versa) can lead to the strongest enhancement because it can give rise to the slowest light in our scheme. Therefore, as well as offering a fundamental understanding of the slow-light effect in codirectional coupling structures with NIMs for constructing compact photonic devices, our investigations suggest a useful guideline for optimizing the design of codirectional couplers using slow-light systems for both the classical and quantum information processing and communication networks.