Ultra-compact Structure Research Articles

Nonreciprocal perfect absorber based on an ultra-compact nonsymmetry cavity structure

In this paper, the transmission and absorption properties of nonsymmetrical cavity structure with metal material have been studied through transfer matrix method and electromagnetic (EM) field simulations. The absorption is found to be dependent on the cavity resonance effect. For a nonsymmetrical structure, the EM wave can be totally absorbed from one incidence direction, at the same time the EM wave is totally reflected from the opposite incidence direction. The reasons of the nonreciprocal absorption are analyzed in detail.

Nonreciprocal transmission in a nonlinear photonic-crystal Fano structure with broken symmetry

Nanostructures that feature nonreciprocal light transmission are highly desirable building blocks for realizing photonic integrated circuits. Here, a simple and ultracompact photonic-crystal structure, where a waveguide is coupled to a single nanocavity, is proposed and experimentally demonstrated, showing very efficient optical diode functionality. The key novelty of the structure is the use of cavity-enhanced material nonlinearities in combination with spatial symmetry breaking and a Fano resonance to realize nonreciprocal propagation effects at ultralow power and with good wavelength tunability. The nonlinearity of the device relies on ultrafast carrier dynamics, rather than the thermal effects usually considered, allowing the demonstration of nonreciprocal operation at a bit-rate of 10 Gbit s−1 with a low energy consumption of 4.5 fJ bit−1.

Multispectral Sharp Plasmon Resonances for Polarization-Manipulated Subtractive Polychromatic Filtering and Sensing

Compared with the conventional color filter with the only one fixed output color filtering, developing active multicolor filters with controllable color output can lead to more compact and sophisticated color filter-based devices. We propose and demonstrate a plasmon scheme consisting of a nonclose-packed cross-shaped metallic antenna array for active subtractive multicolor filtering and high-quality sensing. Multiband antireflection spectrum with ultra-narrow bandwidths (<10 nm) in the optical region is achieved, which could be tuned quasi-quantitatively by controlling the incidence polarization state. The multispectral optical features are with high scalability via properly tuning the geometry parameters or the crystal symmetry. In addition, high-performance sensing is realized in such ultra-compact nanoplasmonic structure (~λ/4) and suggests one more way to artificially modify the polychromatic color filtering. The results presented by such a scheme open up a new approach for the multispectral nanophotonic device platform that combines the unique merits of multiband, sharp resonances with polarization-adjusting and plasmonic-integrated photonics.

High-resolution demultiplexers based on coupled-resonator effect in cross plasmonic-waveguide structure

An ultracompact wavelength demultiplexing structure based on the resonator-coupled effect is proposed and demonstrated numerically. The structure consists of a cross plasmonic-waveguide structure with baffles in the output channels, each of which functions as resonator. Due to the strong couplings of different resonators in the structure, a series of continuous sharp transmission spectra occur. This considerably increases the wavelength resolution of wavelength demultiplexing, which is obviously narrower than the full width of the isolated resonator. The proposed structure could be utilized to design of ultracompact wavelength-division multiplexing (WDM) systems for large-scale photonic integration.

Tunable filter and optical buffer based on dual plasmonic ring resonators

We demonstrate the realization of on chip plasmon-induced transparency using dual ring resonators coupling to metal–dielectric–metal bus waveguide. The theoretical results agree well with the finite-difference time-domain simulative ones. Moreover, by adjusting the radius, width, as well as the coupling distance can efficiently operate the wavelengths and bandwidths of our filter. In theory, we propose a feasible method to improve the trade-off between transmission and quality factor. Finally, the ultra-compact structure possesses slow light effect and manifests a low group velocity, which provides a guideline to control the light and has potential application in optical filter and optical buffer.

Narrowband Light Total Antireflection and Absorption in Metal Film–Array Structures by Plasmonic Near-Field Coupling

We study the cooperative effects between plasmon gap modes and optical cavity modes of a novel triple-layer structure consisting of double continuous gold films separated by a gold nanosphere array. Narrowband near-perfect antireflection of optical field is achieved for the first time due to the strong near-field light–matter interaction within the deep sub-wavelength gaps between adjacent nanospheres combined with the spatial field confinement effects of the optical cavity built by the double gold films. The coexistence cooperation of near-field dipole plasmon resonances and spatial optical field confinement presents more efficient light modification than that of the individual subsystem and may open a new approach to manage light flow. By varying the period of nanosphere array, the diameter of nanospheres, and the distance between the array and the film, optical behaviors of the proposed structure can be tuned in a wide range. High environmental sensitivity and large figure of merit factor are obtained using this structure as the detecting substrate. Furthermore, ultra-compact structure and high conduction suggest the proposed structure being a good candidate for potential applications in highly integrated optoelectronic devices, such as plasmonic filters and sensors.

Coupled-Resonator-Induced Fano Resonances for Plasmonic Sensing with Ultra-High Figure of Merits

Fano resonances are numerically predicted in an ultracompact plasmonic structure, comprising a metal-isolator-metal (MIM) waveguide side-coupled with two identical stub resonators. This phenomenon can be well explained by the analytic model and the relative phase analysis based on the scattering matrix theory. In sensing applications, the sensitivity of the proposed structure is about 1.1 × 103 nm/RIU and its figure of merit is as high as 2 × 105 at λ = 980 nm, which is due to the sharp asymmetric Fano line-shape with an ultra-low transmittance at this wavelength. This plasmonic structure with such high figure of merits and footprints of only about 0.2 μm2 may find important applications in the on-chip nano-sensors.

Ultra-compact and fabrication-tolerant polarization rotator based on a bend asymmetric-slab waveguide

We propose and analyze a polarization rotator based on a bend asymmetric-slab waveguide on the silicon-on-insulator platform. The device can be fabricated using standard complementary metal-oxide-semiconductor process involving only two dry etching steps. Compared with the formerly reported polarization rotators based on two-step etching, our introduced device demonstrates a significant improvement for fabrication tolerance. Furthermore, an ultra compact structure of ~5 μm conversion length, an insertion loss of only 0.5 dB, and an extinction ratio of >40 dB for both TE to TM polarization conversion and TM to TE polarization conversion are exhibited. Operation wavelength and the influence of environmental temperature on our device are also discussed.

Multiple plasmon-induced transparencies in coupled-resonator systems

Multiple plasmon-induced transparencies are numerically predicted in an ultracompact plasmonic structure, comprising series of stub resonators side-coupled with a metal-isolator-metal waveguide. Because of the phase-coupled effect, electromagnetically induced transparency (EIT)-like spectral response occurs between two adjacent stub resonators with detuned resonant wavelengths. In this approach, multiple EIT-like spectral responses, with bandwidths of the order of several nanometers, are obtained in the plasmonic structure with a small footprint of about 0.6 μm2. An analytic model and the relative phase analysis based on the scattering matrix theory are used to explain this phenomenon.

A theoretical study of a nano-opto-mechanical sensor using a photonic crystal-cantilever cavity

In this simulation study, integration of a nanocantilever inside a two-dimensional (2D) photonic crystal (PC) cavity resulted in a unique photonic crystal-cantilever cavity (PC3), where the cantilever served as a tunable mechanical defect of the PC slab. Strong nano-opto-mechanical interactions between the cantilever and the defect-mode field inside the PC3 gave rise to a high sensitivity of the resonance wavelength to surface stress-induced cantilever deflection. Mechanical and optical responses of the PC3 to surface stress changes on the cantilever surface were studied by using a finite-element method (FEM) and a finite-difference time-domain (FDTD) method, respectively. Theoretical analysis revealed that the devised PC3 sensor could resolve a conservative minimum surface stress at the level of ∼0.8 mN m−1, representing state-of-the-art cantilever sensor performance. Also, the PC3 sensor design used an ultracompact structure with an on-chip optical length of only several microns, while a conventional reflected laser beam detection scheme requires a ∼1 m long free-space optical path.

Design of ultra-compact wavelength splitter based on lithium niobate nanowire optical waveguides

A wavelength splitter with ultra-compact and simple structure is proposed and analyzed by using both plane wave expansion (PWE) method and finite difference time domain (FDTD) method. The device is based on directional coupling between two parallel lithium niobate (LiNbO3, LN) nanowire optical waveguides. The wavelength splitter with a coupling region length of 5 m can separate 1.31μm and 1.55μm wavelengths for corresponding outputs with transmittance higher than 97%.

Anomalous Propagation in Low Index Contrast Metamaterials: Assessment of the Beam Collimation Condition

Anomalous beam propagation in low index-contrast metamaterials has been analyzed. The condition for a well-collimated beam is found to be depending on the beam width and the pertinent Fourier component of the dielectric function. Guided by this condition, an ultra-compact metamaterial structure is designed to deflect a light beam at a wide angle. The structure is tolerant to structural parameter deviation and has a wide bandwidth.

Ultra-compact resonator with orthogonally polarized dual-wavelength output based on layered epsilon-negative and anisotropic materials

We propose a resonator based on a sandwich structure composed of the epsilon-negative and anisotropic materials. In view of the numerical method, we study its transmission properties. This ultra-compact structure can generate orthogonally polarized dual-wavelength output with a perfect transmittance. The resonance wavelengths can be adjusted changing the layer thickness or the angle of incidence.

Ultracompact all-optical XOR logic gate in a slow-light silicon photonic crystal waveguide

We demonstrate an ultracompact, chip-based, all-optical exclusive-OR (XOR) logic gate via slow-light enhanced four-wave mixing (FWM) in a silicon photonic crystal waveguide (PhCWG). We achieve error-free operation (<10⁻⁹) for 40 Gbit/s differential phase-shift keying (DPSK) signals with a 2.8 dB power penalty. Slowing the light to vg = c/32 enables a FWM conversion efficiency, η, of -30 dB for a 396 μm device. The nonlinear FWM process is enhanced by 20 dB compared to a relatively fast mode of vg = c/5. The XOR operation requires ≈ 41 mW, corresponding to a switching energy of 1 pJ/bit. We compare the slow-light PhCWG device performance with experimentally demonstrated XOR DPSK logic gates in other platforms and discuss scaling the device operation to higher bit-rates. The ultracompact structure suggests the potential for device integration.

Micro-disks embedded microring for optical filter

We propose a novel ultra compact structure of micro-disks embedded microring filter (MDEMR) and several parameters which impact the performance of MDEMR are analyzed. The filter is shown to exhibit much smaller size and better spectrum than traditional microring one. The number and radius of the inner disks are found to mainly influence the resonance frequency of the micro-disks, and the valley of transmittivity. The resonance between microring and micro-disks is decrease with increasing the gap between microring and micro-disks, which also leads to the extinction ratio improved significantly. The Q factor is found to be improved from 300 to 3000 with the gap increased from 0.2 μm to 0.4 μm, while the transmission peak affected slightly.

Ultra-compact spatial filtering with both wide spatial-frequency bandwidth and high transmittance using single-negative material

A new application of multilayer structures containing single-negative material is proposed as spatial filter. The spatial filter has an ultra-compact structure (only two periods) and is based on that the transmittance of waves through the structure is not sensitive to the incident angle. Through optimised structure parameters, the filter permits waves propagating within a wide spatial-frequency bandwidth and with perfect transmittance. The spatial-frequency bandwidth is tunable by adjusting structure parameter and frequency.

Achieving optical bistability through ultra-compact metal-dielectric structure

The transmission spectra of two-period metal-dielectric structure are studied through the transfer matrix method. When the dielectric is nonlinear material, an optical bistability has been found. The upper threshold value is insensitive to incident frequency, while the lower threshold value increases with the incident frequency decreasing. The effect of loss from the metal layers on the optical bistability is also considered.

Direction and frequency filter basing on an ultra-compact structure consisting of dielectric films

The transmission properties of a two-period structure consisting of dielectric layers and air layers are studied by numeric methods. On the condition of total reflection from dielectric to air layer, the coupling of the evanescent waves in air layers and propagation waves in dielectric layers forms some special transmission bands. Given proper structure parameters, one incident direction only allows one transmission frequency; on the other hand, one transmission frequency only allows one incident direction. This structure can simultaneously achieve frequency filtering and direction filtering.

A novel proposal for DWDM demultiplexer design using modified-T photonic crystal structure

We propose an ultra compact structure to realize demultiplexing operation for Dense Wavelength Division Multiplexing (DWDM) communication systems using resonant cavity in modified-T Photonic Crystal (PC) structure. To the best of our knowledge, this is for the first time that a PC-based demultiplexer has been achieved with 1 nm channel spacing and 0.45 nm mean value of bandwidth without using either specific materials or complexities in fabrication process. Designs offering improvement of channel spacing and bandwidth of the proposed demultiplexer is our aim in this work. The attained characteristics are approximately in the range of the DWDM communication systems. Accurate resonant cavities have been used in terms of location and size of holes in the proposed structure in order for them to capture desired wavelengths in optical telecommunication range. Our simulations indicate the average amount of crosstalk (Xt) and the average quality factor (Q) to be −21.1 dB and 3488, respectively. Two-dimensional (2D) Finite-Difference-Time-Domain (FDTD) is chosen for simulation of the proposed structure. The footprint of the structure is approximately 536 μm 2 and can be fabricated and integrated densely and easily.

Tunable Refractive Index Sensor with Ultracompact Structure Twisted by Poly(trimethylene terephthalate) Nanowires

We report a tunable refractive index sensor with ultracompact structure in a 2 x 2 poly(trimethylene terephthalate) (PTT) nanowire coupling splitter assembled by twisting two flexible PTT nanowires. The sensor consists of two input branches, a twisted coupling region, and two output branches. The changes of optical power caused due to variations in the surrounding medium around the twisted coupling region were measured in the output branches. The highest measured sensitivity of the sensor is 26.96 mW/RIU (refractive index unit) and the maximum detection limit on refractive index change is 1.85 x 10(-7). The average tunabilities observed are 1.2 mW per RIU per twisted turn and 1.8 mW per RIU per 5 degrees branching angle change.