Optical metamaterials and transformation optics
Recently the advent of artificial electromagnetic (EM) materials, referred to as metamaterials, has opened up many new ways for us to interact with or control EM waves. EM phenomena that don't exist in nature have been demonstrated experimentally. While it is not a problem to engineer individual metamaterial, utilizing metamaterials in the appropriate way to achieve a desired macroscopic optical phenomenon remains a challenge. The recently proposed transformation optics method provides an ideal recipe for solving such a design problem. Our research efforts are thus focused on designing optical metamaterial with novel functionalities, in particular, with the assistance of transformation optics.
Plasmonics
Surface plasmon resonance (SPR) is the interaction between electromagnetic radiation and electrons at metallic interfaces or in small metallic nanostructures,
leading to an enhanced optical near field of sub-wavelength dimension. SPRs are being explored for their potential use in optical sensors, extraordinary transmission through sub-wavelength holes, sub-wavelength waveguiding, and local field enhancement. Our research concerns sub-wavelength or nanoscale plasmonic optical elements with more functionalities, with the characteristics of non-constraint of diffraction limits, novel dispersions, and local field enhancement. We are also aiming at the hybrid integration of dielectric components and plasmonic components, and low loss plasmonic devices.
Silicon photonics and silicon-based photonic crystal devices
Research on silicon photonics is booming due to its compatibility with the silicon technology platform used in the electronics industry. The main challenges of silicon photonics are integration of massive number of functional devices in one single chip and to generate and exploit novel functionalities. Our aims are to develop a comprehensive silicon photonics platform, to demonstrate silicon photonic devices (including photonic crystal devices), to integrate and incorporate silicon photonic devices in the high capacity photonic communication infrastructure, and to bringing forward novel research topics.
Listed below are some recent research highlights from our group.
2009
Achieving perfect imaging beyond passive and active obstacles
W. Yan, M. Yan, and M. Qiu, "Achieving perfect imaging beyond passive and active obstacles by a transformed bilayer lens", Phys. Rev. B, 79, 161101(R), 2009.
A bilayer lens is proposed based on transformation optics. It is shown that Pendry's perfect lens, perfect bilayer lens made of indefinite media, and the concept of compensated media are well unified under the scope of the proposed bilayer lens. Using this concept, we also demonstrate how one is able to achieve perfect imaging beyond passive objects or active sources which are present in front of the lens.
Broadband high-efficiency surface-plasmon-polariton coupler with silicon-metal interface
J. Tian, S.Q. Yu, W. Yan, and M. Qiu, "Broadband high-efficiency surface-plasmon-polariton coupler with silicon-metal interface", Appl. Phys. Lett., 95, 013504, 2009.
A high efficiency surface plasmonic coupler composed of a tapered silicon strip waveguide and a subwavelength scale metal gap waveguide is experimentally demonstrated. By tuning the parameters of the taper and the metal gap, the theoretical coupling efficiencies can be as high as 88% for a wide wavelength range. A silicon-gold plasmonic coupler is then fabricated, demonstrating 35% coupling efficiency per facet. Our experimental demonstration is a crucial step for hybrid integration of plasmonic components with conventional dielectric components.
2008
Direct characterization of focusing light by negative refraction in a photonic crystal flat lens
J. Tian, M. Yan, M. Qiu, C. G. Ribbing, Y.Z. Liu, D.Z. Zhang, and Z.Y. Li, "Direct characterization of focusing light by negative refraction in a photonic crystal flat lens", Appl. Phys. Lett., 93, 191114 (2008).
We report experimental measurements of the field distribution of the light spot focused by a two-dimensional photonic crystal flat lens at wavelengths 1.51-1.58 µm. The photonic crystal slab is fabricated on a silicon-on-insulator substrate by focused-ion-beam direct milling. We confirm the light focusing by the photonic crystal slab through direct observation of the light spot entering into free space at the end facet of the slab lens. The beam profiles as the function of lateral position are measured and the minimal full width half maximum of the beam at 1.2 µm (0.77 λ) is obtained.
Cylindrical superlens by a coordinate transformation
M. Yan, W. Yan, and M. Qiu, "Cylindrical superlens by a coordinate transformation", Phys. Rev. B, 78, 125113, 2008.
Cylinder-shaped perfect lens deduced from the coordinate transformation method is proposed. The previously reported perfect slab lens is noticed to be a limiting form of the cylindrical lens when the inner radius approaches infinite with respect to the lens thickness. Connaturality between a cylindrical lens and a slab lens is affirmed by comparing their eigen-field transfer functions. We numerically confirm the subwavelength focusing capability of such a cylindrical lens with consideration of material imperfection. Compared to a slab lens, a cylindrical lens has several advantages, including finiteness in structure, and ability in lensing with magnification or demagnification. Immediate applications of such a cylindrical lens can be in high-resolution imaging and lithography technologies. In addition, its invisibility property suggests that it may be valuable for non-invasive electromagnetic probing.
Dense wavelength conversion and multicasting in a resonance-split silicon microring
Q. Li, Z.Y. Zhang, F.F. Liu, M. Qiu, and Y.K. Su, "Dense wavelength conversion and multicasting in a resonance-split silicon microring", Appl. Phys. Lett., 93, 081113 (2008).
We experimentally demonstrate all-optical wavelength conversions in a 10 μm radius resonance-split silicon microring resonator based on free carrier dispersion effect. The split resonance is caused by the mutual coupling between the two countertraveling modes inside the ring resonator. Dense wavelength conversions are performed at data rates from 500 Mbytes/s to 5 Gbytes/s and a dual-channel wavelength multicasting is realized at a data rate of 1.25 Gbytes/s. The resonance splitting phenomenon opens up opportunities to convert more closely spaced wavelengths, thus effectively increasing the system capacity.
Non-magnetic simplified cylindrical cloak with suppressed zeroth order scattering
W. Yan, M. Yan, and M. Qiu, "Non-magnetic simplified cylindrical cloak with suppressed zeroth order scattering", Appl. Phys. Lett., 93, 021909 (2008).
A type of simplified cloaks with matched exterior boundaries is proposed. The cloak uses nonmagnetic material for the TM polarization and can function with a relatively thin thickness. It is shown that the zeroth order scattering of such cloak is dominant among all cylindrical scattering terms. A gap is added at the cloak's inner surface to eliminate the zeroth order scattering through the mechanism of scattering resonance. The reduction in scattering is relatively smooth, indicating that the proposed scattering reduction method has good tolerance to perturbations. Numerical simulations also confirm that the proposed structure has very low scattering.