Zhengbiao Ouyang

Full controlling of Fano resonances in metal-slit superlattice

Controlling of the lineshape of Fano resonance attracts much attention recently due to its wide capabilities for lasing, biosensing, slow-light applications and so on. However, the controllable Fano resonance always requires stringent alignment of complex symmetry-breaking structures and thus the manipulation could only be performed with limited degrees of freedom and narrow tuning range. Furthermore, there is no report so far on independent controlling of both the bright and dark modes in a single structure. Here, we semi-analytically show that the spectral position and linewidth of both the bright and dark modes can be tuned independently and/or simultaneously in a simple and symmetric metal-slit superlattice, and thus allowing for a free and continuous controlling of the lineshape of both the single and multiple Fano resonances. The independent controlling scheme is applicable for an extremely large electromagnetic spectrum range from optical to microwave frequencies, which is demonstrated by the numerical simulations with real metal and a microwave experiment. Our findings may provide convenient and flexible strategies for future tunable electromagnetic devices.

Photonic-crystal structures with polarized-wave-guiding property and their applications in the mid and far infrared wave bands

Photonic crystal (PhC) structures with polarized-wave-guiding property (PhC polarization waveguides) are proposed, demonstrated and applied to construct several new kinds of compact and efficient micro polarization devices in the mid and far infrared wave bands, including TE polarizers, TM polarizers, TE-downward T-shaped polarization-beam splitters (PBSs), TM-downward T-shaped PBSs and lying-T-shaped PBSs. Theoretical models for the operating mechanism of the structures are presented. The polarization devices built as applications of the PhC polarization waveguides are demonstrated by the finite-element method with the dispersion of materials being considered. Furthermore, optimized parameters are obtained by investigating the extinction ratio (EXR), the degree of polarization (DOP) and insertion loss. Moreover, structures based on PhC slabs derived from the 2D ones, together with woodpile PhC covers and substrates are suggested for the 3D version of the proposed devices for implementation. An example of the 3D-version structures shows a performance as good as that of the 2D structure. The devices proposed have relatively wide ranges of operating wavelength. Meanwhile, they are very compact in their structures and convenient for connection or coupling of signals among different optical elements, so they have the potential for wide applications in mid-and-far infrared optical devices or circuits, which are useful in remote sensing, image and vision, positioning and communications with infrared waves. Furthermore, the principle can be applied to build polarizers and PBSs in other wave bands.

Omnidirectional and multi-channel filtering by photonic quantum wells with negative-index materials

We propose a type of photonic quantum well made of two different photonic crystals with negative- and positive-index materials. It is demonstrated by transfer matrix method that, omnidirectional and multichannel filtering can be achieved. Resonance tunneling modes, or the multi-channel filtering modes, are found to exist when a passband of the well photonic crystal is located inside the gap of the barrier photonic crystals. And for each passband of the well photonic crystal in the photonic bandgap of the barrier photonic crystal, the number of modes is the same as the number of periods in the well photonic crystals. Moreover, the modes are insensitive to the incident angle from 0 to 85 degrees and the scaling of the barrier photonic crystals at a certain range. Such structures are useful for all-direction receiving, sending, or linking-up of multi-channel signals in wireless-communication networks. And they can be applied in signal-detection systems to enhance signal-detection sensitivity.

Single-TM-mode Bragg fibers made of magnetic materials.

Single-mode fibers are advantageous over multi-mode fibers in many aspects, e.g., much smaller loss, much longer transmission distance, much greater bandwidth, and higher bit rates. We propose a kind of single-TM-mode Bragg fiber in which magnetic materials are introduced. The idea for designing this kind of Bragg fiber comes from the symmetry of TE modes and TM modes when permittivity and permeability are replaced by each other. Through the transfer matrix method, we demonstrated a special kind of single-TM-mode Bragg fiber in a wide frequency range. Guiding modes may be in the bandgaps, at the edges of bandgaps, and in some region in conduction bands, but much more strongly confined guiding TM modes are inside the bandgaps. In addition, the optimization of the structure is also discussed.

A combined cavity for high sensitivity THz signal detection

Detection of signals in the THz frequency region is important for applications of THz waves in many areas, such as in medical imaging, forbidden-combined sensing, weapon monitoring, and wireless communications. Cooling system operating under very low temperature, for eliminating the unwanted background THz radiation that exists everywhere in room temperature, sets an obstacle for applications of conventional THz signal detecting systems. We present a combined cavity that can pick out the useful signal in high sensitivity, while the influence of the background THz radiation can be neglected in the detection. The combined cavity consists of a point-defect photonic-crystal resonator and a photonic-crystal WGR. The two resonators are coupled together through optical tunneling to form the combined cavity. Under proper operating parameters, the two resonators are in simultaneous resonance, and the field intensity in the point-defect resonator can be thousands of times of that of an incoming THz signal for a given frequency, so that the sensitivity of detection can be very high. Since the background THz radiation is not in resonance with the cavity, the influence of it to the detection of THz signals wanted can be neglected, and thus cooling systems can be omitted. Plane wave expansion method is used to determine the resonance wavelengths and mode patterns of the cavity. Finite-difference- time-domain method is used to find the quality factors and to simulate the resonance process. Parameter optimization and the conditions for simultaneous resonance of the two cavities are studied.

Highly compact circulators in square-lattice photonic crystal waveguides.

We propose, demonstrate and investigate highly compact circulators with ultra-low insertion loss in square-lattice- square-rod-photonic-crystal waveguides. Only a single magneto- optical square rod is required to be inserted into the cross center of waveguides, making the structure very compact and ultra efficient. The square rods around the center defect rod are replaced by several right-angled-triangle rods, reducing the insertion loss further and promoting the isolations as well. By choosing a linear-dispersion region and considering the mode patterns in the square magneto-optical rod, the operating mechanism of the circulator is analyzed. By applying the finite-element method together with the Nelder-Mead optimization method, an extremely low insertion loss of 0.02 dB for the transmitted wave and ultra high isolation of 46 dB∼48 dB for the isolated port are obtained. The idea presented can be applied to build circulators in different wavebands, e.g., microwave or Tera-Hertz.

Trimeric metasurfaces for independent control of bright and dark modes of Fano resonances

In this paper, we present a simple trimeric metasurface consisting of three dipolar resonators in each unit cell, to achieve the independent controlling over both the broad bright mode and the sharp dark mode of Fano resonances. Through both the finite difference time domain simulation and microwave experiment, we find that spectral positions of the bright and dark modes are linearly dependent on, respectively, the global spacing between adjacent unit cells and the local spacing between adjacent dipoles within each unit cell. The dependence of the spectral position of bright (dark) mode on the global (local) spacing is independent without mutual influence, which provides a facile pathway to control the Fano resonance with large flexibility. Our proposed scheme to control Fano resonance is highly desired in various fields including lasing spaser and biosensing with improved performance.

All-optical sensitive phase shifting based on nonlinear out-of-plane coupling through 1-D slab photonic crystal with a layer of graphene

We realize all-optical sensitive phase shifting based on nonlinear out-of-plane coupling to a slab waveguide through Fano resonance of a slab 1-D photonic crystal (PhC). We use a graphene layer as the nonlinear material and change its refractive index by the input light intensity through Kerr nonlinear effect to obtain a shift in the Fano resonance frequency. The Fano resonance and self-focusing effect lead to light-intensity enhancement on the graphene in the PhC, reinforcing the nonlinear effect of refractive index in the graphene. Through finite-difference time-domain simulation, we demonstrate that the phase changing sensitivity obtained can be 4 orders higher than that by a single graphene under the same input light intensity. Moreover the threshold pump intensity for all-optical sensitive phase shifting in the coupled light to the waveguide is as low as ~4 MW per square centimeter. The results are applicable in micro optical integrated circuits for phase shifters, phase modulators, power limiters, and phase logic elements for optical computation, digital phase shift keying in communication systems, and non-contact sensitive signal detectors.

Coupled photonic crystal micro-cavities with ultra-low threshold power for stimulated Raman scattering.

We propose coupled cavities to realize a strong enhancement of the Raman scattering. Five sub cavities are embedded in the photonic crystals. Simulations through finite-difference time-domain (FDTD) method demonstrate that one cavity, which is used to propagate the pump beam at the optical-communication wavelength, has a Q factor as high as 
1.254×10⁸ and modal volume as small as 0.03 μm3 (0.3192(λ/n)3). These parameters result in ultra-small threshold lasing power~17.7 nW and 2.58 nW for Stokes and anti-Stokes respectively. The cavities are designed to support the required Stokes and anti-Stokes modal spacing in silicon. The proposed structure has the potential for sensor devices, especially for biological and medical diagnoses.

Plasmonic Spectral Splitting in Ring/Rod Metasurface

We report spectral splitting behaviors based on Fano resonances in a novel simple planar metasurface composed of gold nanobars and nanorings. Multiple plasmonic modes and sharp Fano effects are achieved in a broadband transmittance spectrum by exploiting the rotational symmetry of the metasurface. The transmission properties are effectively modified and tuned by modulating the structural parameters. The highest single side Q-factor and FoM which reaches 196 and 105 are observed at Fano resonances. Our proposed design is relatively simple and can be applied for various applications such as multi-wavelength highly sensitive plasmonic sensors, switching, and slow light devices.