Conventional Dielectric Waveguides Research Articles

Analysis of surface emitting distributed-feedback quantum cascade laser based on a surface-plasmon waveguide

An analysis of a surface emitting distributed-feedback quantum cascade laser (DFB QCL) based on a surface-plasmon waveguide is presented. The second-order grating realized by the sole patterning of the top metal provides strong feedback. The analysis is based on a coupled-mode theory derived from exact Floquet-Bloch solutions of the infinite periodic structure. The surface outcoupling efficiency and threshold gain for the optimized device design are 43% and 12 cm −1, respectively, which represent great improve ments on the conventional dielectric waveguide based DFB QCL with typical values of 17.5% and 20 cm−1.

Excitation of short range surface plasmon polariton mode based on integrated hybrid coupler

The short range surface plasmon polariton (SRSPP) mode, which has an antisymmetric field profile on the two sides of a thin metal film, has been excited efficiently based on an integrated vertical hybrid coupler. The coupler is composed of an Au (SRSPP) waveguide and a SiNx (dielectric) waveguide. Highly efficient coupling between the SRSPP mode and conventional dielectric waveguide mode was demonstrated. A compact (less than 90 μm long) polarizer with a low TE insertion loss and high TM extinction ratio up to 30 dB was realized by utilizing different characteristics of the TE and TM modes in the hybrid coupler.

Asymmetric plasmonic-dielectric coupler with short coupling length, high extinction ratio, and low insertion loss

Asymmetric directional coupling between a hybrid plasmonic waveguide with subwavelength field confinement and a conventional dielectric waveguide is investigated. The proposed hybrid coupler features short coupling length, high coupling efficiency, high extinction ratio, and low insertion loss; it can also be integrated into a silicon-based platform. This coupler can be potentially adopted for signal routing between plasmonic waveguides and dielectric waveguides in photonic integrated circuits. Furthermore, it can be exploited to efficiently excite hybrid plasmonic modes with conventional dielectric modes.

Structurally-tolerant vertical directional coupling between metal-insulator-metal plasmonic waveguide and silicon dielectric waveguide

Vertical directional coupling between a metal-insulator-metal (MIM) plasmonic waveguide and a conventional dielectric waveguide is investigated. The coupling length, extinction ratio, insertion loss and coupling efficiency of the hybrid coupler are analyzed. As an example, when the separation between the two waveguides is 250 nm, a maximum coupling efficiency of 73%, an insertion loss of -1.4 dB and an extinction ratio of 16 dB can be achieved at a coupling length of 4.5 microm at 1.55 microm wavelength. A particular feature of this hybrid coupler is that it is highly tolerant to the structural parameters of the plasmonic waveguide and the misalignment between the two waveguides. The performance of this hybrid coupler as a TM polarizer is also analyzed and a maximum extinction ratio of 44 dB and an insertion loss of -0.18 dB can be obtained. The application of this hybrid coupler includes the signal routing between plasmonic waveguides and dielectric waveguides in photonic integrated circuits and the polarization control between TE and TM modes. In addition, it provides an approach for efficiently exciting MIM plasmonic modes with conventional dielectric modes.

Guided modes near the Dirac point in negative-zero-positive index metamaterial waveguide

Motivated by the realization of the Dirac point (DP) with a double-cone structure for optical field in the negative-zero-positive index metamaterial (NZPIM), we make theoretical investigations of the guided modes in the NZPIM waveguide near the DP by using the graphical method. Due to the linear Dirac dispersion, the fundamental mode is absent when the angular frequency is smaller than the DP, while the behaviors of NZPIM waveguide are similar to the conventional dielectric waveguide when the angular frequency is larger than the DP. The unique properties of the guided modes are analogous to the propagation of electron waves in graphene waveguide [Appl. Phys. Lett., 94, 212105 (2009)], corresponding to the classical motion and the Klein tunneling. These results suggest that many exotic phenomena in graphene can be simulated by the relatively simple optical NZPIM.

Enhanced optical forces in 2D hybrid and plasmonic waveguides

We investigate the optical gradient force in 2D hybrid and plasmonic waveguides. By comparing with conventional dielectric waveguides, we show that the optical force can be enhanced by at least 1 order of magnitude in the hybrid and plasmonic waveguides due to strongly enhanced optical fields at the waveguide surfaces. We compare coupled plasmonic waveguides with different geometries, including rectangular, circular, and triangular cross sections and find that the rectangular waveguides provide the strongest force. We also show that the plasmonic enhancement is nonresonant and thus can be used for a broad range of wavelengths.

Blue-shifted contra-directional coupling between a periodic and conventional dielectric waveguides

The interaction between a periodic and conventional dielectric waveguides is investigated theoretically for a two-dimensional model system. A modified coupled-mode theory is formulated for the considered system and found to agree well with rigorous numerical calculations. It is shown that in a certain wavelength range the contra-directional coupling between the two waveguides can be achieved with high efficiency. But the spectrum of the coupling efficiency is blue-shifted and thus the strongest coupling does not occur in the case when two individual waveguides have the same propagation constant. For such a contra-directional coupling system, the coupling efficiency grows with the coupling length and it tends to 100% (excluding insertion loss) when the coupling length is larger than a certain value, and the coupling window can be largely broaden by reducing the distance between the coupled waveguides.

Modelling of Polarization-Dependent Loss in Plasmonic Nanowire Waveguides

We have investigated the potential of using gold nanowires embedded in a dielectric cladding environment as polarization-independent long-range surface plasmon polariton waveguides at telecom wavelengths. We performed finite-element analysis on various symmetric and close-to-symmetric cross-sectional geometries and evaluated the effects of cladding thickness on the propagation and coupling loss. The calculations confirm that fabrication of polarization-independent waveguides with reasonable tolerances is feasible and that straight-waveguide insertion losses around 1.5 dB for short (0.5 mm) devices can be realized when coupling to and from conventional dielectric waveguide geometries.

Backward Guiding of Terahertz Radiation in Periodic Dielectric Waveguides

The guiding properties of periodic dielectric waveguides (PDWGs) are investigated theoretically at terahertz frequencies for both two- and three-dimensional model systems. It is shown that in a PDWG there may exist several bound modes, and among them often occur backward modes with antiparallel phase velocity and energy flow. The backward guiding behavior of the PDWG is demonstrated by its contra-directional coupling with a conventional dielectric waveguide (CDWG). For the coupler formed by a PDWG and a CDWG, the coupling direction of energy flow is selectable, since the PDWG supports either a forward or a backward mode at different frequencies.

Performance-enhanced superluminescent diode with surface plasmon waveguide

Super luminescent Diode (SLD) with a new structure is proposed in which light is guided by surface plasmon waveguide (SPWG) rather than by the conventional dielectric waveguide. This results in a great increase of the spontaneous emission coupling. Other parameters important to the device operation such as the confinement factor, waveguide loss and waveguide facets reflectivities are also considered. It is shown that the new design outperforms the conventional ones using dielectric waveguides in both the output power and optical spectral width.

Vertical coupling between short range surface plasmon polariton mode and dielectric waveguide mode

Coupling performance between a short range surface plasmon polariton (SRSPP) mode and a conventional dielectric waveguide mode is demonstrated numerically. Simulation results show that the coupling length, as short as tens of microns, can be realized because the field of SRSPP extremely concentrates to the metal surface. SRSPP-based hybrid coupler provides not only an approach to realize highly compact functional devices, such as the TE-pass polarizer with high performance, but also an integratable route for efficiently exciting SRSPP mode, which is very useful in the SRSPP-based biosensor or SRSPP-assisted emission enhancement devices.

Light wheel confined in a purely dielectric composite waveguide

A properly designed composite waveguide consisting of a one-dimensional photonic crystal waveguide and a conventional dielectric waveguide is proposed for the realization of a localized "light wheel". Light confinedly rotating between the two waveguides is numerically demonstrated and explained physically in detail. A delocalized "light wheel" is found at the band gap edge caused by contra-directional coupling between the two waveguides. Because of this delocalized "light wheel" , the composite waveguide can be used to trap light as a cavity, and a quality factor of 9 x 10(3) is achieved as an example. The present structure is completely dielectric and thus easy to realize with a low loss.

MODAL DISPERSION CHARACTERISTICS OF A BRAGG FIBER HAVING PLASMA IN THE CLADDING REGIONS

The modal dispersion relation of electromagnetic waves in a Bragg fiber having plasma in the cladding regions is investigated analytically. The proposed Bragg fiber consists of a low index central region having air surrounded by a large number of periodic cladding layers of alternating high and low refractive indices of dielectric and plasma respectively. The modal dispersion relation is obtained by solving Maxwell wave equations using a simple boundary matching method. The analysis shows that the normalized frequency parameter (also called V -number) is frequency independent. This indicates that the proposed Bragg fiber may be used for single mode operation without high frequency limitation as well as with little loss of energy compared to the conventional dielectric waveguide.

Hybrid Three-Arm Coupler Consisted of Long Range Surface Plasmon Polariton and Dielectric Waveguides

The hybrid three-arm coupler, which consists of the middle long range surface plasmon polariton (LRSPP) waveguide and two outside conventional dielectric waveguides, is proposed and analyzed numerically with finite element method. The characteristics, including coupling length, coupling loss, extinction ratio and frequency dependence, are discussed in detail with different structure parameters. Compared with the two-arm hybrid coupler, the coupling loss of the three-arm hybrid coupler based devices can be reduced, especially with wider middle LRSPP waveguide. Extra transmission loss on the metal strip can be avoided because of the lossless input and output arms. Furthermore, different from conventional three-arm dielectric coupler, we find that the three-arm hybrid coupler with asymmetric structure can reach a much higher extinction ratio by introducing a middle arm offset. Since the middle arm can only transfer the TM mode from one dielectric arm to the other one within the TM mode coupling length, this structure is promising for realizing integrated polarization splitter or other functional devices.

A hybrid plasmonic waveguide for subwavelength confinement and long-range propagation

The emerging field of nanophotonics1 addresses the critical challenge of manipulating light on scales much smaller than the wavelength. However, very few feasible practical approaches exist at present. Surface plasmon polaritons2,3 are among the most promising candidates for subwavelength optical confinement3,4,5,6,7,8,9,10. However, studies of long-range surface plasmon polaritons have only demonstrated optical confinement comparable to that of conventional dielectric waveguides, because of practical issues including optical losses and stringent fabrication demands3,11,12,13. Here, we propose a new approach that integrates dielectric waveguiding with plasmonics. The hybrid optical waveguide consists of a dielectric nanowire separated from a metal surface by a nanoscale dielectric gap. The coupling between the plasmonic and waveguide modes across the gap enables ‘capacitor-like’ energy storage that allows effective subwavelength transmission in non-metallic regions. In this way, surface plasmon polaritons can travel over large distances (40–150 µm) with strong mode confinement (ranging from λ2/400 to λ2/40). This approach is fully compatible with semiconductor fabrication techniques and could lead to truly nanoscale semiconductor-based plasmonics and photonics. Xiang Zhang and colleagues from the University of California, Berkeley, propose a new approach for confining light on scales much smaller than the wavelength of light. Using hybrid waveguides that incorporate dielectric and plasmonic waveguiding techniques, they are able to confine surface plasmon polaritons very strongly over large distances. The advance could lead to truly nanoscale plasmonics and photonics.

Numerical Analysis of Coupling between Two-Dimensional Conventional Dielectric Waveguides and Photonic Crystal Waveguide

A problem of coupling between conventional dielectric waveguides and a two-dimensional photonic crystal waveguide is analyzed, using the Fourier series expansion method combined with the absorbing boundary condition of a perfectly matched layer (PML). A novel structure for the input and output ends of the photonic crystal waveguide is proposed to increase the coupling efficiency. Numerical examples are demonstrated and discussed to validate the effectiveness of the proposed structure.

High order plasmonic Bragg reflection in the metal-insulator-metal waveguide Bragg grating

High order plasmonic Bragg reflection in the metal-insulatormetal (MIM) waveguide Bragg grating (WBG) and its applications are proposed and demonstrated numerically. With the effective index method and the standard transfer matrix method, we reveal that there exist high order plasmonic Bragg reflections in MIM WBG and corresponding Bragg wavelengths can be obtained. Contrary to the high order Bragg wavelengths in the case of the conventional dielectric slab waveguide, the results of the MIM WBG exhibit red shifts of tens of nanometers. We also propose a method to design a MIM WBG to have high order plasmonic Bragg reflection at a desired wavelength. The MIM WBG operating in visible spectral regime, which requires quite accurate fabrication process with grating period of 100 to 200 nm for the fundamental Bragg reflection, can be implemented by using the higher order plasmonic Bragg reflection with grating period of 400 to 600 nm . It is shown that the higher order plasmonic Bragg reflection can be em ployed to implement a narrow reflection bandwidth as well. We also address the dependence of the filling factor upon the bandgap and discuss the quarter-wave stack condition and the second bandgap closing.

EXPERIMENTAL CONFIRMATION OF GUIDANCE PROPERTIES USING PLANAR ANISOTROPIC LEFT-HANDED METAMATERIAL SLABS BASED ON S-RING RESONATORS

We experimentally studied the guidance properties of the S-shaped metamaterial slabs. A peek of transmitted power due to the bulk guidance modes is observed in the negative band of the metamaterial,which is larger than a conventional dielectric waveguide made of FR4. The peek transmission frequency is shown related with the change of the negative band of the S-shaped metamaterial slab. Our results show good agreement with the theoretical predictions.

Design and Analysis of 2D Photonic Crystal Waveguides for High Coupling Efficiency

In this paper we have proposed a photonic crystal structure to enhance the coupling efficiency between photonic crystal waveguide (PCW) and the conventional dielectric waveguide. The proposed waveguide characterized with employing the taper structure and varying the holes’ radius in the taper. Two-dimensional (2D) finite-difference time-domain (FDTD) method has been used to analyze the structure. The simulation results show that the coupling efficiency can be achieved as high as 81.6% at the wavelength of 1.55um.

Subwavelength Discrete Solitons in Nonlinear Metamaterials

We present the first study of subwavelength discrete solitons in nonlinear metamaterials: nanoscaled periodic structures consisting of metal and nonlinear dielectric slabs. The solitons supported by such media result from a balance between tunneling of surface plasmon modes and nonlinear self-trapping. The dynamics in such systems, arising from the threefold interplay between periodicity, nonlinearity, and surface plasmon polaritons, is substantially different from that in conventional nonlinear dielectric waveguide arrays. We expect these phenomena to inspire fundamental studies as well as potential applications of nonlinear metamaterials, particularly in subwavelength nonlinear optics.