Jun Yuan

Structural, optical, and electrical properties of (Zn,Al)O films over a wide range of compositions

(Zn,Al)O thin films have been prepared by a dc reactive magnetron sputtering system with the Al contents in a wide range of 0–50at.%. The structural, optical, and electrical properties of (Zn,Al)O films were detailedly and systematically studied. The amount of Al in the film was nearly the same as, but often lower than, that in the sputtering target. The growth rate of films monotonically decreased as the Al content increased. In a low Al content region (<10at.%), Al-doped ZnO (AZO) thin films could be obtained at 400°C in an Ar–O2 ambient with good properties. The optimal results of n-type AZO films were obtained at an Al content of 4at.%, with low resistivity ∼10−4Ωcm, high transmittance ∼90% in the visible region, and acceptable crystal quality with a high c-axis orientation. The band gap could be widened to 3.52eV at 4at.% Al due to the Burstein-Moss shift [E. Burstein, Phys. Rev. 93, 632 (1954)] modulated by many-body effects. An appropriate Al-doping concentration served effectively to release the r...

Control of p- and n-type conductivities in Li-doped ZnO thin films

Li-doped ZnO films were prepared by pulsed laser deposition. The carrier type could be controlled by adjusting the growth conditions. In an ionized oxygen atmosphere, p-type ZnO was achieved, with the hole concentration of 6.04×1017cm−3 at an optimal Li content of 0.6at.%, whereas ZnO exhibited n-type conductivity in a conventional O2 growth atmosphere. At a Li content of more than 1.2at.% only high-resistivity ZnO was obtained. The amount of Li introduced into ZnO and the relative concentrations of such defects as Li substitutions and interstitials could play an important role in determining the conductivity of films.

High-performance terahertz wave absorbers made of silicon-based metamaterials

Electromagnetic (EM) wave absorbers with high efficiency in different frequency bands have been extensively investigated for various applications. In this paper, we propose an ultra-broadband and polarization-insensitive terahertz metamaterial absorber based on a patterned lossy silicon substrate. Experimentally, a large absorption efficiency more than 95% in a frequency range of 0.9–2.5 THz was obtained up to a wave incident angle as large as 70°. Much broader absorption bandwidth and excellent oblique incidence absorption performance are numerically demonstrated. The underlying mechanisms due to the combination of a waveguide cavity mode and impedance-matched diffraction are analyzed in terms of the field patterns and the scattering features. The monolithic THz absorber proposed here may find important applications in EM energy harvesting systems such as THz barometer or biosensor.

Three-dimensional magnetic cloak working from d.c. to 250 kHz

Invisible cloaking is one of the major outcomes of the metamaterial research, but the practical potential, in particular for high frequencies (for example, microwave to visible light), is fatally challenged by the complex material properties they usually demand. On the other hand, it will be advantageous and also technologically instrumental to design cloaking devices for applications at low frequencies where electromagnetic components are favourably uncoupled. In this work, we vastly develop the bilayer approach to create a three-dimensional magnetic cloak able to work in both static and dynamic fields. Under the quasi-static approximation, we demonstrate a perfect magnetic cloaking device with a large frequency band from 0 to 250u2009kHz. The practical potential of our device is experimentally verified by using a commercial metal detector, which may lead us to having a real cloaking application where the dynamic magnetic field can be manipulated in desired ways.

Modulation of far-infrared light transmission by graphene-silicon Schottky junction

Tunable conductive properties of graphene in terahertz and far-infrared regimes provide a prominent way to control electromagnetic waves. In this paper, we explore the photon-electric properties of the graphene-silicon heterostructure and its application in modulating the transmission of far-infrared light. Experimentally, we show this structure will give rise to different transmission modulation ratios with amplitudes varying from tens to few percentages, dependent on the operation wavelength. The modulation effect gradually decreases and saturates within wavenumbers 1000-2000 cm−1 influenced by the pump light power. The diode transmission behavior is explicitly interpreted in terms of the Schottky junction formed between the graphene-silicon interface. The results give a further deep understanding of the electromagnetic behavior of graphene in the far-infrared region that may be integrated for potential applications.

Design of mechanically robust metasurface lenses for RGB colors

In recent years, dielectric rod based metasurface lenses have been particularly investigated for their potential applications in replacing the traditional bulky lens with high efficiency. However, the isolated granular structure may lead to robustness and chromatic aberration concerns. In this work, a low refractive-index embedding medium was applied to solve the structural stability problem that also enables the device to be transferable to desired position or surface. Based on this, a compound metasurface lens composed of randomly interleaved frequency-selective zone sectors is proposed to broaden the bandwidth of the two-dimensional device. The validities of the proposed method and the application potentials for the multifunctional lens that can manipulate RGB three colors have been numerically examined and verified. The current results are of essence in guiding the future design of metasurface lenses for real practice.

Plasmon–phonon coupling in graphene–hyperbolic bilayer heterostructures*

Polar dielectrics are important optical materials enabling the subwavelength manipulation of light in infrared due to their capability to excite phonon polaritons. In practice, it is highly desired to actively modify these hyperbolic phonon polaritons (HPPs) to optimize or tune the response of the device. In this work, we investigate the plasmonic material, a monolayer graphene, and study its hybrid structure with three kinds of hyperbolic thin films grown on SiO2 substrate. The inter-mode hybridization and their tunability have been thoroughly clarified from both the band dispersions and the mode patterns numerically calculated through a transfer matrix method. Our results show that these hybrid multilayer structures are of strong potentials for applications in plasmonic waveguides, modulators and detectors in infrared.

Controlling the plasmonic surface waves of metallic nanowires by transformation optics

In this letter, we introduce the technique of using transformation optics to manipulate the mode states of surface plasmonic waves of metallic nanowire waveguides. As examples we apply this technique to design two optical components: a three-dimensional (3D) electromagnetic mode rotator and a mode convertor. The rotator can rotate the polarization state of the surface wave around plasmonic nanowires by arbitrarily desired angles, and the convertor can transform the surface wave modes from one to another. Full-wave simulation is performed to verify the design and efficiency of our devices. Their potential application in photonic circuits is envisioned.

Photonic spin Hall effect with controlled transmission by metasurfaces

Photonic spin Hall effect (PSHE) with high efficiency and controllable transmission is widely pursued. In the paper, we designed a dielectric metasurface that could yield spin splitting effect with energy transfer efficiency up to 84%. Switching the output photonic spin states is realized by controlling the polarization azimuthal angle of the incident light. Introducing a background phase gradient, we also propose a high-efficiency asymmetric PSHE metasurface that could give rise to arbitrary transmission angles for the left and right circularly-polarized light. These results are numerically validated, which are of the significance in guiding the design of high-performance photonic spin manipulation devices.

Hight efficient waveplates and microlens arrays designed by metasurface

Metasurface has gained increasing research attention due to their remarkable abilities in light manipulation, versatility, ease of on-chip fabrication and integration owing to their planar profiles. In this talk, we will introduce our recent progress on this subject by focusing on two novel applications: one is for waveplates (polarization manipulation) and the other is for compact lenses. An ultrathin, broadband half-wave plate in the near-infrared range using a plasmonic metasurface will be first presented. The simulated results show that the linear polarization conversion efficiency is over 97% with over 90% reflectance across an 800-nm bandwidth, which was experimentally verified over a wide range of incident angles. A plate with supercell design was also realized to achieve reflection-phase gradient for the cross-polarized beam that could separate the anomalous (cross-polarized) and the normal (co-polarized) reflected beams. In the second part, a metasurface lens made of silicon rod arrays will be presented with transitivity larger than 80% at the light communication wavelength. We will mainly discuss its application potential in microlens array. In addition, we will also talk how to design metasurface lens that could deal with evanescent waves.