Band Slow Light Research Articles

Improving Low-Dispersion Bandwidth of the Silicon Photonic Crystal Waveguides for Ultrafast Integrated Photonics

We design a novel slow-light silicon photonic crystal waveguide which can operate over an extremely wide flat band for ultrafast integrated nonlinear photonics. By conveniently adjusting the radii and positions of the second air-holes rows, a flat slow-light low-dispersion band of 50 nm is achieved numerically. Such a slow-light photonic crystal waveguide with large flat low-dispersion wideband will pave the way for governing the femtosecond pulses in integrated nonlinear photonic platforms based on CMOS technology.

Trapping and releasing bidirectional rainbow at terahertz frequencies

Rainbow trapping arisen from slow light effect has been attracted lots of attention for the past two decades. However, until now only unidirectional rainbow trapping has been proposed, which relies on a complicated configuration or technique. Here we propose and investigate a metal–dielectric-semiconductor-dielectric-metal structure, where the semiconductor layer is applied with an external DC magnetic field. We show that in such a waveguide configuration there exist branches of dispersion curves showing slow-light bands at finite propagation constants. The positions of the slow-light bands can be tuned by varying the thickness of the semiconductor or the dielectric layers in the waveguide. By tapering the waveguide, we first demonstrate bidirectional rainbow trapping in such a simple waveguide configuration, and more importantly the trapped rainbow can be easily released by changing the external magnetic field.

Dispersion engineering and thermo-optic tuning in mid-infrared photonic crystal slow light waveguides on silicon-on-insulator.

In this Letter, the design, fabrication, and characterization of slow light devices using photonic crystal waveguides (PhCWs) in the mid-infrared wavelength range of 3.9-3.98μm are demonstrated. The PhCWs are built on the silicon-on-insulator platform without undercut to leverage its well-developed fabrication process and strong mechanical robustness. Lattice shifting and thermo-optic tuning methods are utilized to manipulate the slow light region for potential spectroscopy sensing applications. Up to 20nm wavelength shift of the slow light band edge is demonstrated. Normalized delay-bandwidth products of 0.084-0.112 are obtained as a result of dispersion engineering. From the thermo-optic characterization results, the slow light enhancement effect of thermo-optic tuning efficiency is verified by the proportional relationship between the phase shift and the group index. This work serves as a proof of concept that the slow light effect can strengthen light-matter interaction and thereby improve device performance in sensing and nonlinearity applications.

Plasmonic slow light waveguide with hyperbolic metamaterials claddings

Plasmonic waveguides with an insulator core sandwiched between hyperbolic metamaterials (HMMs) claddings, i.e. HIH waveguide, are investigated for achieving wide slow-light band with adjustable working wavelength. The transfer matrix method and the finite-difference-time-domain simulation are employed to study waveguide dispersion characteristics and pulse propagation. By selecting proper silver filling ratios for HMMs, the hetero-HIH waveguide presents a slow-light band with a zero group velocity dispersion wavelength of 1.55 μm and is capable of buffering pulses with pulse width as short as ∼20 fs. This type of waveguides might be applicable for ultrafast slow-light application.

Mode selection and dispersion engineering in Bragg-like slot photonic crystal waveguides for hybrid light–matter interactions

We introduce a family of slot photonic crystal waveguides (SPhCWs) for the hybrid integration of low-index active materials in silicon photonics with energy-confinement factors of ∼30% in low-index regions. The proposed approach, which is based on a periodic indentation of the etched slot in the middle of the SPhCW, makes it possible to reconcile a simultaneously narrow and wide slot for exploiting the two modes of even symmetry of a SPhCW. The resulting mode-selection mechanism allows a flexible choice of the modes to be used. Furthermore, the proposed structure offers tremendous flexibility for adjusting the dispersive properties of the slot-confined modes, in particular of their slow-light effects. Flat band slow light in a bandwidth of about 60 nm with a group velocity dispersion factor |β2| below 1 ps2/mm is numerically demonstrated by this approach, corresponding to a normalized delay bandwidth product of around 0.4. These results, obtained from hollow-core periodic waveguides that are directly designed in view of hybrid integration of active materials in mechanically robust structures (not based on free-standing membranes) could pave the way for the realization of on-chip slow-light bio-sensing, active hybrid-silicon optoelectronic devices, or all-optical hybrid-silicon nonlinear functionalities.

Slow light in narrow-core hollow optical waveguide with low loss and large bandwidth.

A narrow-core hollow waveguide with low loss is proposed that exhibits slow light characteristics. The slow light is guided in air between the top and bottom mirrors, each based on high-index-contrast gratings. The proposed design shows a low propagation loss of 1.8 dB/cm at a 1-μm-thick narrow air core, and the loss remains low for a broad range of wavelengths from 1200 to 1600 nm. Also, the flat band slow light is realized at a grating period of 0.8 μm in 1-μm-thick narrow air core. Further design analysis reveals a large fabrication tolerance of the proposed hollow structure with respect to the grating period.

Wide band dispersionless slow light in hetero-MIM plasmonic waveguide.

A flat slow-light band over a wide frequency range is obtained in the hetero-MIM (metal-insulator-metal) waveguide with zero group velocity dispersion (GVD). The zero GVD originates from dispersion compensation by the photonic mode and the plasmonic mode, the mechanism of which does not exist in the homo-MIM structure. By changing dielectric permittivity of the insulator or the difference of two different metallic plasma frequencies, the group index and the bandwidth can be tuned. The dispersionless slow light characteristic in the hetero-MIM waveguide may be useful in the new design of plasmonic devices.

Broad self-trapped and slow light bands based on negative refraction and interference of magnetic coupled modes

We propose a new mechanism to achieve light localization and slow light. Through the study on the coupling of two magnetic surface modes, we find a special convex band that takes on a negative refraction effect. The negative refraction results in an energy flow concellation effect from two degenerated modes on the convex band. The energy flow concellation effect leads to forming of the self-trapped and slow light bands. In the self-trapped band light is localized around the source without reflection wall in the waveguide direction, whereas in the slow light band, light becomes the standing-waves and moving standing-waves at the center and the two sides of the waveguide, respectively.

Tunable Multi-switching in Plasmonic Waveguide with Kerr Nonlinear Resonator.

We propose a nanoplasmonic waveguide side-coupled with bright-dark-dark resonators in our paper. A multi-oscillator theory derived from the typical two-oscillator model, is established to describe spectral features as well as slow-light effects in bright-dark-dark structures, and confirmed by the finite-difference time domain (FDTD). That a typical plasmon induced transparency (PIT) turns to double PIT spectra is observed in this waveguide structure. At the same time, multi-switching effects with obvious double slow-light bands based on double PIT are also discovered in our proposed structure. What’s more, dynamically tuning the multi-switching is achieved by means of filling Fabry-Perot resonators with the Kerr nonlinear material Ag-BaO. These results may have applications in all-optical devices, moreover, the multi-oscillator theory may play a guiding role in designing plasmonic devices.

Flat band slow light in silicon photonic crystal waveguide with large delay bandwidth product and low group velocity dispersion

Flat band slow light is achieved in a silicon photonic crystal waveguide with large normalised delay bandwidth product (NDBP) while maintaining smaller values of group velocity dispersion. By altering the waveguide width through shifting the position of two rows of air holes adjacent to the waveguide, it is possible to achieve a high NDBP value. The NDBP values of 0.40, 0.41 and 0.43 over bandwidths of 11, 14 and 18 nm are, respectively, reported. Towards the applicability of the proposed slow light waveguide for buffering applications on silicon, buffering capacity as high as 140 bits can be achieved with the proposed design approach.z

Highly directed emission from self-assembled quantum dots into guided modes in disordered photonic-crystal waveguides

We explore the dynamics and directionality of spontaneous emission from self-assembled In(Ga)As quantum dots into TE-polarised guided modes in GaAs two-dimensional photonic crystal waveguides. The local group velocity of the guided waveguide mode is probed, with values as low as $\sim 1.5\%\times c$ measured close to the slow-light band edge. By performing complementary continuous wave and time-resolved measurements with detection along, and perpendicular to the waveguide axis we probe the fraction of emission into the waveguide mode ($\beta$-factor). For dots randomly positioned within the unit cell of the photonic crystal waveguide our results show that the emission rate varies from $\geq 1.55$ $ns^{-1}$ close to the slow-light band edge to $\leq 0.25$ $ns^{-1}$ within the two-dimensional photonic bandgap. We measure an average Purcell-factor of $\sim 2\times$ for dots randomly distributed within the waveguide and maximum values of $\beta\sim 90 \%$ close to the slow light band edge. Spatially resolved measurements performed by exciting dots at a well controlled distance $0-45$ {\mu}m from the waveguide facet highlight the impact of disorder on the slow-light dispersion. Although disorder broadens the spectral width of the slow light region of the waveguide dispersion from $\delta E_{d}\leq 0.5$ meV to $>6$ meV, we find that emission is nevertheless primarily directed into propagating waveguide modes. The ability to control the rate and directionality of emission from isolated quantum emitters by placing them in a tailored photonic environment provides much promise for the use of slow-light phenomena to realise efficient single photon sources for quantum optics in a highly integrated setting.

Improved design of slow light interferometer and its application in FBG displacement sensor

An optical fiber Mach–Zehnder interferometer (MZI) based on slow light in polymer-infiltrated PCW (PI-PCW) was proposed to enhance the demodulation sensitivity of fiber Bragg grating (FBG) displacement sensor. By optimizing the structure of PI-PCW, slow light with high group index of 110 was realized, which is advantageous to develop high-sensitive MZI. And thanks to electro-optic effect of the infiltrated polymer, the working wavelength of flat band slow light, as well as the demodulation range of MZI, could be flexibility changed and enlarged by tuning external driving voltage. At last, differential and orthogonal method was used to demodulate the interference spectrum of MZI with high stability and good linearity. The FBG was pasted on an Omega-like beam, thus the displacement variation of the free end of the Omega-like beam could be measured by monitoring the output phase of MZI. Finally, the proposed displacement sensor demonstrated a high sensitivity of 1.035rad/mm with good linearity and wide measurement range of 55.6mm.

Direct eigenmode analysis of plasmonic modes in metal nanoparticle chain with layered medium

Using the dyadic Green function (GF) with a multilayer medium, we propose an eigendecomposition (ED) analysis of a plasmonic system composed of a one-dimensional periodic metal nanoparticle chain and planar layered structure. An effective eigenpolarizability involving the collective effects of both the chain and the layered structure is well defined to characterize the dispersion relation and the mode quality of the plasmonic modes. Applying this method, we demonstrate that the interplay between the surface plasmon polaritons (SPPs) at the metal-dielectric interface and the localized plasmon in the chain enables strong mode splitting. In particular, for the polarization perpendicular to layer surface, high-quality modes can be present inside the light cone even if the chain is open to the surrounding air. A slow-light band is also predicted to exist as long as the layered medium supports a SPP mode that can couple to the chain mode.

Tunable flat band slow light in reconfigurable photonic crystal waveguides based on magnetic fluids

A kind of two-dimensional photonic crystal line-defect waveguide with 45°-rotated square lattice is proposed to present slow light phenomena. Infiltrating the photonic crystal waveguide with appropriate magnetic fluids can generate very wide flat bands of guided modes, which give rise to the excellent slow light properties. The bandwidth centered at λ0=1550nm of the designed W1 waveguide is considerably large (around 54nm). The obtained group velocity dispersion β2 within the bandwidth is ultralow (varying from −2118a/2πc2 to 1845a/2πc2, where a and c are the period of the lattice and the light speed in vacuum, respectively). Simultaneously, the normalized delay-bandwidth product is relatively large compared with other works. Reconfiguring the photonic crystal waveguide with magnetic fluids of different concentrations can remarkably tune the slow light parameters and the trade-off between them, while the type of magnetic nanoparticles constituting the magnetic fluids negligibly affect the slow light properties. The explicit linear variation of the slow light parameters with the magnetic fluid concentration is convenient for the practical tuning.

Slow light property in ring-shape-hole slotted photonic crystal waveguide

Slow light in photonic crystal waveguide is now being heavily investigated for applications in optical devices. A ring-shape-hole slotted PhCW (RSPhCW) is designed in the paper. Band structures and group indices are calculated through plane wave expansion method. Flat band slow light property is observed, which provides low group velocity dispersion and avoids high speed optics signal from distortion in transmission. A method is proposed by tuning the positions of the first and second rows with the ring-shape-holes adjacent to the slot, which can tune the group index and the bandwidth of the guided band at the same time. This method is used to optimize the slow light property of the RSPhCW. The bandwidths ranging from 1.8nm to 17.3nm for group indices from 15 to 67 are obtained. From the point of view of reality, this method has its advantage of fabricating than tuning the outer and inner radii of the ring-shape-holes.

Ab-initio design of nanophotonic waveguides for tunable, bidirectional optical forces

We propose a set of principles to tailor and enhance optical forces between parallel, periodic dielectric waveguides by molding the eigen-mode field distribution via the combined effects of highly symmetric slow light modes and waveguide morphology. The geometries here considered are amenable to standard lithographic techniques and possess strong repulsive and attractive optical forces that can be enhanced via slow-light band edge modes. This new methodology should enable the fabrication of optomechanical devices for applications in sensing, switching and nano-optomechanical systems.

Great optical buffering capacity for optical delay line and extraordinary optical reflection and mode conversion with extremely weak dielectric perturbations based on circular Bragg resonators

Circular Bragg resonators (CBRs) are analyzed in both the frequency domain and the time domain based on the scattering matrix method and the numerical model. The CBR with the same size as a dielectric ring can be designed to have denser resonant mode distributions in the frequency domain, and the expansion of the slow light band is imposed by the combination of multiresonant modes. Thus the expansion is independent of group velocity and is not limited by the delay-bandwidth product constraint in static photonic structures, which is deduced for a single resonant mode. Hence, the CBR can store more bits than a dielectric ring. For certain parameters, clockwise (CW) and counterclockwise (CCW) modes in the CBR are quite sensitive to dielectric perturbations, which are weak enough that they have little effect on the CW mode and CCW mode in a dielectric ring. When light propagates along a line waveguide coupled with the CBR, and if there are weak dielectric perturbations in the CBRs, extraordinary reflections could be produced and there exists strong coupling and conversion between CW and CCW modes in the CBR. The optical property indicates that extremely weak dielectric perturbations in the CBR play an important role in mode conversion. These unique properties of CBRs may find applications in the design of practical optical delay line buffers, and they also provide a new method to achieve light control by mode conversion in passive optical resonators.

Slow-light dispersion properties of multiatomic multiband coupled-resonator optical waveguides

In this paper, we investigate the dispersion properties of a multiatomic coupled-resonator optical waveguide (CROW), and show the existence of band-dependent group velocities in its slow-light bands. By including the next-nearest-neighbor coupling terms in a coupled-mode theory (CMT) analysis for the structure, we explain the physical origin of the band-dependent group velocities in terms of the modification of molecular mode-coupling strengths, and also derive the criteria for complete band separation and perfect intermode intensity overlap. Our results imply that when estimating the performance of a multiwave slow-light device, the band dependency of group velocities must also be considered in addition to the conventional CROW dispersion. Numerical analysis with a photonic crystal platform shows excellent agreement with theory.

Flat Band Slow Light With High Coupling Efficiency in One-Dimensional Grating Waveguides

Flat band slow light with high coupling efficiency is achieved in a simple one-dimensional grating waveguide. An ideal flat band indicating slow light with group index of 18.3 and bandwidth of 10.3 nm is obtained by plane wave expansion calculation. This result is also verified by inputting a Gaussian pulse into a 48.7-μm -long waveguide in a finite-difference time-domain simulation, in which optical pulse delay of 2.85 ps and group index of 17.54 can be observed. Meanwhile, by introducing a step taper, the coupling efficiency could be improved remarkably from below 20% for butt coupling to 70%. The proposed structure is an ideal candidate for wideband and compact low loss slow light delay lines or optical buffers in on-chip signal processing.

Subwavelength slow-light waveguides based on a plasmonic analogue of electromagnetically induced transparency

We introduce a plasmonic waveguide system, based on a plasmonic analogue of electromagnetically induced transparency, which supports a subwavelength slow-light mode, and exhibits a small group velocity dispersion. The system consists of a periodic array of two metal-dielectric-metal (MDM) stub resonators side-coupled to a MDM waveguide. Decreasing the frequency spacing between the two resonances increases the slowdown factor and decreases the bandwidth of the slow-light band. We also show that there is a trade-off between the slowdown factor and the propagation length of the slow-light mode.