Wideband and low dispersion slow-light waveguide based on a photonic crystal with crescent-shaped air holes

We present a procedure to generate slow light with a large group index, wideband, and low dispersion in our suggested photonic crystal waveguide. By modulation of the declinations in the first two rows of air holes, the group index, the bandwidth, and the dispersion can be tuned effectively. Utilizing the two-dimensional plane wave expansion method (PWE) and the finite-difference time-domain method (FDTD), we demonstrate slow light with the group indices of 23, 35, and 45, respectively, while restricting the group-index variation within a 10% range. We accordingly attain an available bandwidth of 40.7, 23.7, and 5.1?nm, respectively. Meanwhile, the normalized delay-bandwidth product stays around 0.45, with minimal dispersion less than 0.2 (ps²/m) for all the cases.     

Metal-insulator-metal plasmonic waveguide for low-distortion slow light at telecom frequencies

We propose metal-insulator-metal (MIM) plasmonic waveguide, in which metal is aluminum-doped zinc oxide (AZO) and insulator is air. The real part of relative dielectric constant is negative that is similar to the metal. Therefore, we call the AZO as metal. Owing to cutoff frequency of TM₀ model for the MIM waveguide being slightly larger than 193.5?THz, the MIM waveguide shows obviously slow light effect around the commonly used telecom frequency. Exploiting the dispersion relationship of MIM waveguide, we analyze the effect of core layer width for the MIM waveguide on slow group velocity, second-, and third-order dispersion. The core layer width of MIM waveguide is determined to simultaneously realize obviously slow light effect and the lower distortion for the pulse with width 200?fs, which is confirmed by simulation used the finite-difference time-domain method. The MIM waveguide potentially applied in optical delay lines is easily fabricated.     

TM and TE propagating modes of photonic crystal waveguide based on honeycomb lattices

The waveguide based on the honeycomb photonic crystal has propagating modes for both the TE and TM polarizations. The group index-normalized frequency curves are U-shaped for the two polarizations. The average group index of the TE mode is approximately 3, while the average group index of the TM mode is over 10, which implies that the TM mode is a slow light mode. With the shift value 0 ≤ δx ≤ 0.025a, the group index is over 10 and the normalized delay-bandwidth product is from 0.316 to 0.349, which is ideal for the slow light mode of the TM polarization. In the group velocity dispersion of the waveguide, there is a very large "zero" dispersion region for both the TM and TE modes, which is far larger than that of other photonic crystal waveguides. The TM mode of this kind of waveguide structure is a slow light mode with wide bandwidth and a large "zero" dispersion region.     

Slow light with large group index – bandwidth product in lattice-shifted photonic crystal waveguides

Abstract

This study presents a systematic optimization procedure to generate slow light with large group index, wideband, and low dispersion in an lattice-shifted photonic crystal waveguide. The waveguide is based on triangular lattice photonic crystal imposed by selectively altering the locations of the holes adjacent to the line defect. Under a constant group index criterion of ± 10% variation, when group indices are nearly constants of 24, 33, 46, 57, and 66, their corresponding bandwidths of flat band reach 24.2, 17.6, 12.8, 10.1 and 8.6 nm around 1550 nm, respectively. A nearly constant large group index – bandwidth product (GBP) of 0.37 is achieved for all cases. Low dispersion slow light propagation is confirmed by studying the relative temporal pulse-width spreading with the 2-D finite-difference time-domain method.     

Design of subwavelength corrugated metal waveguides for slow waves at terahertz frequencies

A subwavelength corrugated metal waveguide is studied and designed to slow down the light at terahertz frequencies. The waveguide consists of two parallel thin metal slabs with periodic corrugations on their inner boundaries. Compared with structures based on engineered surface plasmons, the proposed structure has smaller group velocity dispersion and lower propagation loss. The origin of the slow wave is also explained.     

Ultrawideband air-core plasmonic slow-light waveguide with ultralow high-order dispersion

We propose a surface plasmonic waveguide that consists of a metal-dielectric-metal structure and an air-core which are sandwiched in both metals and dielectric material. Numerical results show that the waveguide is able to confine the surface plasmonic modes in a very small air area and achieve slow light with a group velocity of 0.0086?c and cancelled even-orders dispersion over the ultrawideband of 21?THz.     

Intermode light diffusion in multimode optical waveguides with rough surfaces

A theoretical analysis of incoherent intermode light power diffusion in multimode dielectric waveguides with rough (corrugated) surfaces is presented. The correlation length a of the surface-profile variations is assumed to be sufficiently large (a less less than lambda/2pi) to permit light scattering into the outer space only from the modes close to the critical angles of propagation and yet sufficiently small (a less less than d, where d is the average width of the waveguide) to permit direct interaction between a given mode and a large number of neighboring ones. The cases of a one-dimensional (1D) slab waveguide and a two-dimensional cylindrical waveguide (optical fiber) are analyzed, and we find that in both cases the partial differential equations that govern the evolution of the angular light power profile propagating along the waveguide are 1D and of the diffusion type. However, whereas in the former case the effective conductivity coefficient proves to be linearly dependent on the transverse-mode wave number, in the latter one the linear dependence is for the effective diffusion coefficient. The theoretical predictions are in reasonable agreement with experimental results for the intermode power diffusion in multimode (700 x 700) optical fibers with etched surfaces. The characteristic length of dispersion of a narrow angular power profile evaluated from the correlation length and standard deviation of heights of the surface profile proved to be in good agreement with the experimentally observed changes in the output angular power profiles.     

Two-dimensional model for three-dimensional index-guided multimode plasmonic waveguides and the design of ultrasmall multimode interference splitters

We demonstrate that a three-dimensional (3D) index-guided multimode plasmonic waveguide can be approximated to a two-dimensional (2D) lossy slab waveguide by using an effective-index method. It is found that this 2D approximation is more accurate when the width of the multimode waveguide increases. Such a 2D approximation can be used for a quicker and more efficient design of complicated multimode plasmonic devices. 1 x N ultrasmall multimode interference splitters based on multimode surface plasmon waveguides are designed by using this 2D model and the designs are validated with a 3D finite-difference time-domain method.     

Manipulating wavelength-selective emission with heterogeneous photonic crystals

The usual near-field radiation profile of a light beam emanating from a photonic crystal waveguide (PCW) has a main lobe at the center line of the waveguide. However, a centrally symmetric profile for the emission pattern with two sidelobes can be required in some applications, e.g., Y-type power dividers, wavelength multiplexers, and semiconductor lasers. With such motivations in mind, we present the design of a compact structure that deflects the beam propagation direction in this manner. The idea utilizes the manipulation of the dispersion diagram of cascaded photonic crystals by exploiting the bandgap and self-collimation properties. The waveguide mode in the PCW can be transformed from a propagating mode into a diffusive one by altering the filling factor, which, in turn, leads to off-axis light emission. By using the finite-difference time-domain method, we show that the emission takes place into free space at the inclined output surfaces of the PCW with deviation angles of ±45°.     

The effect of a gap between dielectric and waveguide on the dispersion relation of the ?erenkov device

We present a method for calculating efficiently the dispersion curve of a cylindrical ?erenkov device having a small gap between the dielectric liner and the guide wall. In any practical realisation, such a gap cannot be avoided and the results obtained here indicate that the dispersion curve is very sensitive to its thickness. Using a liner with a higher dielectric constant increases the sensitivity of the dispersion to the size of the gap. If a very high-energy electron beam is employed, having an electron velocity almost equal to c (speed of light in a free space) then synchronism occurs close to the intersection of the dispersion curve and the vacuum light line where the effect of the gap is small; in fact, a large gap could be left between the dielectric liner and the waveguide if this was advantageous for construction purposes. However, at lower electron energies, typical of those used in microwave devices, the effect of even a relatively small gap can be considerable on the synchronism condition.     

Refraction and diffraction by a metal-dielectric multilayered structure

An optical refraction prism consisting of metal and dielectric, subwavelength, periodic multilayered thin films has been proposed. The multilayered structure of metal and dielectric thin films has a cylindrical dispersion surface for TM polarized light. The light behaviors are very different from those of conventional glass prisms and photonic crystal superprisms. Refraction and diffraction of the light wave for the metal-dielectric multilayered prism has been investigated by numerical simulations and graphical representation based on the dispersion surface. A prism with 0.2 microm period had an angular dispersion of 0.20 degrees /nm for approximately 0.8 microm wavelength light. The finite thick metal-dielectric multilayered structure acted as a slab waveguide.     

Reproduction in free space of fields confined by 3-dimensional optical waveguides

Abstract

It is shown by using the Fresnel-Kirchhoff diffraction theory and the method of images that a scalar field confined by a 3-dimensional optical waveguide can be generated in free space by a suitable light source. In the method the boundaries of a waveguide are replaced by virtual sources. This allows one to present the wave guiding as the interference and diffraction of multiple light beams produced in free space by the guide equivalent source (Fresnel waveguide). Thus, the scalar optics of a 3-dimensional waveguide is presented as the free-space beam optics. The method is illustrated by construction of the Fresnel sources for the triangular, rectangular and hexagonal waveguides. The numerical examples demonstrate the diffraction-free and self-imaging propagation in the free-space of the eigenmodes of the Fresnel rectangular-waveguide.     

A potential candidate design for nanosecond-order delay based on high-group index four rows optimized air holes line-defect photonic crystal waveguide

In order to meet the demand of ultra-high optical delay within a limited transmission distance, a high-group index line-defect photonic crystal waveguide (PCW) is designed. By adjusting the parameters of holes adjacent to the defect, a high constant group index about 680 and the corresponding flat bandwidth approximately 0.4 nm are obtained. Group velocity dispersion in the flat-band slow light region is in the order of 10⁹ ps²/km, which is non-negligible. Thus, the pulse broadening caused by dispersion that limits the transmission distance of the pulse in PCW is evaluated through broaden factor (BF). Considering the limitation of BF, a highest time delay up to 70.9 ps for 24 μm delay line is obtained. Assuming return-to-zero modulation format, the above optimized delay line supports a bit rate of the signal up to 12.9 bit/s. The study provides an important approach for realizing large time delay with small size using line-defect PCW.     

Trapping and releasing light by mechanical implementation in metamaterial waveguides

We show that light trapping and releasing can be switched by a mechanic tuning effect in metamaterial waveguides. The transition mechanism between the trapping and releasing states relies on the synergetic effect of the local Bragg reflection and cavity resonance in the waveguides. As a proof-of-concept demonstration, a heterostructured metamaterial waveguide comprised of dielectric claddings and a tapered metamaterial core formed by arrays of metal slats is analytically and numerically investigated. The spatial separation of the trapped light with various frequencies and the transition between the trapping and releasing states can be predicted by a "rainbow equation." The proposed light trapping and releasing scheme based on the mechanical implementation of waveguide geometrical parameters can be exploited to develop opto-mechanical devices for slow light technology.     

Grating-assisted coupling of terahertz waves into a dielectric waveguide studied by terahertz time-domain spectroscopy

We report on the efficient coupling of terahertz (THz) waves into a dielectric waveguide by means of a diffraction grating engraved at the top of the waveguide. The waveguide is made of a 201-microm-thick high-resistivity silicon wafer. The transmission of the device, measured versus frequency by terahertz time-domain spectroscopy, shows usual m lines when a frequency component of the THz pulse spectrum satisfies the phase-matching condition and is coupled into the waveguide. The experimental data are well modeled with the differential electromagnetic method to compute the diffraction pattern of the grating device. The dispersion curve of the first four modes of propagation is determined from the frequency position of the m lines recorded for different angles of incidence of the THz beam. The waveguide exhibits a weak group velocity dispersion at high frequencies.     

Two-dimensional array self-assembled quantum dot sub-diffraction waveguides with low loss and low crosstalk

We model and demonstrate the behavior of two-dimensional (2D) self-assembled quantum dot (QD) sub-diffraction waveguides. By pumping the gain-enabled semiconductor nanoparticles and introducing a signal light, energy coupling of stimulated photons from the QDs enables light transmission along the waveguide. Monte Carlo simulation with randomized inter-dot separation reveals that the optical gain necessary for unity transfer is 3.1 × 10(7)?m(-1) for a 2D (2?μm length by 500?nm width) array compared to 11.6 × 10(7)?m(-1) for a 1D (2?μm length) given 8?nm diameter quantum dots. The theoretical results are borne out in experiments on 2D arrays by measurement of negligible crosstalk component with as little as 200?nm waveguide separation and is indicative of near-field optical coupling behavior. The transmission loss for 500?nm wide structures is determined to be close to 3?dB/4?μm, whereas that for 100?nm width is 3?dB/2.3?μm. Accordingly, higher pump power and gain would be necessary on the narrower device to create similar throughput. Considering existing nanoscale propagation methods, which commonly use negative dielectric materials, our waveguide shows an improved loss characteristic with comparable or smaller dimensions. Thus, the application of QDs to nanophotonic waveguiding represents a promising path towards ultra-high density photonic integrated circuits.     

Large polarization response of planarized optical waveguide functionalized with 2D material overlays

ABSTRACT

We propose a planarized optical waveguide structure functionalized with 2D materials that exhibits strong polarization dependent light interaction. Numerical study on both TE – and TM-polarized lights propagation in the 2D material coated optical waveguide is carried out. The effective index, n_eff of optical waveguide increases with the increase of refractive index of the 2D material overlay but decreases when its extinction coefficient is increased. On the other hand, the effective extinction coefficient, k_eff of 2D material coated waveguide increases with the increase of both the real and imaginary parts of 2D material refractive index. It is found that TE-mode has a stronger interaction with 2D material overlay compared to TM-mode, giving rise to potential application like waveguide polarizer. By optimizing the coating thickness, TM-pass waveguide polarizer with high polarization extinction ratio of 78.5 and 97.7?dB can be produced using black phosphorus and graphene oxide, respectively, with a coating length of only 1?mm.     

Elastic waves in multiferroic cylinders of sectorial cross-section

The propagation of elastic waves in multiferroic cylindrical waveguides of sectorial cross-section is analyzed based on the linear theory of electro-magneto-elasticity. A wave potential method is adopted to derive analytically the characteristic equations, from which the dispersion relations are readily obtained. Representative examples are performed to investigate the essential characteristics of the waves by looking into the real, imaginary and complex branches of full dispersion spectra. It is found that phase velocities and cutoff frequencies depend considerably on the waveguide’s angular measure, which should be a significant factor to control the dispersion characteristics of the multiferroic waveguide of specific materials. Furthermore, a two-layer model is also presented to compare with the single model.     

Elastic waves in multiferroic cylinders of sectorial cross-section

The propagation of elastic waves in multiferroic cylindrical waveguides of sectorial cross-section is analyzed based on the linear theory of electro-magneto-elasticity. A wave potential method is adopted to derive analytically the characteristic equations, from which the dispersion relations are readily obtained. Representative examples are performed to investigate the essential characteristics of the waves by looking into the real, imaginary and complex branches of full dispersion spectra. It is found that phase velocities and cutoff frequencies depend considerably on the waveguide’s angular measure, which should be a significant factor to control the dispersion characteristics of the multiferroic waveguide of specific materials. Furthermore, a two-layer model is also presented to compare with the single model.     

Chromatic dispersion correction in planar waveguide using one-layer volume holograms based on three-step exposure

We propose a novel method to correct the chromatic dispersion in a planar waveguide with volume holograms fabricated by the three-step exposure technique. The 532 nm green laser is used to illuminate the holographic plate in three groups of different angles for achieving the desired holograms. When it is used in the planar waveguide, the chromatic dispersion of the original display can be corrected and an image with the real color can be obtained. The experiments are performed, and the results are in good agreement with the theory. It is believed that this technique is a good way to correct the chromatic problems in the display systems in the future.